draft-ietf-taps-minset-08.txt   draft-ietf-taps-minset-09.txt 
TAPS M. Welzl TAPS M. Welzl
Internet-Draft S. Gjessing Internet-Draft S. Gjessing
Intended status: Informational University of Oslo Intended status: Informational University of Oslo
Expires: March 9, 2019 September 5, 2018 Expires: March 17, 2019 September 13, 2018
A Minimal Set of Transport Services for End Systems A Minimal Set of Transport Services for End Systems
draft-ietf-taps-minset-08 draft-ietf-taps-minset-09
Abstract Abstract
This draft recommends a minimal set of Transport Services offered by This draft recommends a minimal set of Transport Services offered by
end systems, and gives guidance on choosing among the available end systems, and gives guidance on choosing among the available
mechanisms and protocols. It is based on the set of transport mechanisms and protocols. It is based on the set of transport
features in RFC 8303. features in RFC 8303.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 9, 2019. This Internet-Draft will expire on March 17, 2019.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The Minimal Set of Transport Features . . . . . . . . . . . . 5 3. Deriving the minimal set . . . . . . . . . . . . . . . . . . 5
3.1. ESTABLISHMENT, AVAILABILITY and TERMINATION . . . . . . . 5 4. The Reduced Set of Transport Features . . . . . . . . . . . . 6
3.2. MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. CONNECTION Related Transport Features . . . . . . . . . . 7
3.2.1. Connection groups . . . . . . . . . . . . . . . . . . 8 4.2. DATA Transfer Related Transport Features . . . . . . . . 8
3.2.2. Individual connections . . . . . . . . . . . . . . . 10 4.2.1. Sending Data . . . . . . . . . . . . . . . . . . . . 8
3.3. DATA Transfer . . . . . . . . . . . . . . . . . . . . . . 10 4.2.2. Receiving Data . . . . . . . . . . . . . . . . . . . 9
3.3.1. Sending Data . . . . . . . . . . . . . . . . . . . . 10 4.2.3. Errors . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2. Receiving Data . . . . . . . . . . . . . . . . . . . 11 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Sending Messages, Receiving Bytes . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 5.2. Stream Schedulers Without Streams . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 5.3. Early Data Transmission . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.4. Sender Running Dry . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12 5.5. Capacity Profile . . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13 5.6. Security . . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Deriving the minimal set . . . . . . . . . . . . . . 14 5.7. Packet Size . . . . . . . . . . . . . . . . . . . . . . . 13
A.1. Step 1: Categorization -- The Superset of Transport 6. The Minimal Set of Transport Features . . . . . . . . . . . . 14
Features . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1. ESTABLISHMENT, AVAILABILITY and TERMINATION . . . . . . . 14
A.1.1. CONNECTION Related Transport Features . . . . . . . . 17 6.2. MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . 17
A.1.2. DATA Transfer Related Transport Features . . . . . . 33 6.2.1. Connection groups . . . . . . . . . . . . . . . . . . 18
A.2. Step 2: Reduction -- The Reduced Set of Transport 6.2.2. Individual connections . . . . . . . . . . . . . . . 19
Features . . . . . . . . . . . . . . . . . . . . . . . . 38 6.3. DATA Transfer . . . . . . . . . . . . . . . . . . . . . . 20
A.2.1. CONNECTION Related Transport Features . . . . . . . . 39 6.3.1. Sending Data . . . . . . . . . . . . . . . . . . . . 20
A.2.2. DATA Transfer Related Transport Features . . . . . . 40 6.3.2. Receiving Data . . . . . . . . . . . . . . . . . . . 21
A.3. Step 3: Discussion . . . . . . . . . . . . . . . . . . . 41 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
A.3.1. Sending Messages, Receiving Bytes . . . . . . . . . . 41 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
A.3.2. Stream Schedulers Without Streams . . . . . . . . . . 42 9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
A.3.3. Early Data Transmission . . . . . . . . . . . . . . . 43 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.3.4. Sender Running Dry . . . . . . . . . . . . . . . . . 44 10.1. Normative References . . . . . . . . . . . . . . . . . . 22
A.3.5. Capacity Profile . . . . . . . . . . . . . . . . . . 44 10.2. Informative References . . . . . . . . . . . . . . . . . 22
A.3.6. Security . . . . . . . . . . . . . . . . . . . . . . 45 Appendix A. The Superset of Transport Features . . . . . . . . . 24
A.3.7. Packet Size . . . . . . . . . . . . . . . . . . . . . 45 A.1. CONNECTION Related Transport Features . . . . . . . . . . 25
Appendix B. Revision information . . . . . . . . . . . . . . . . 46 A.2. DATA Transfer Related Transport Features . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47 A.2.1. Sending Data . . . . . . . . . . . . . . . . . . . . 41
A.2.2. Receiving Data . . . . . . . . . . . . . . . . . . . 45
A.2.3. Errors . . . . . . . . . . . . . . . . . . . . . . . 46
Appendix B. Revision information . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction 1. Introduction
Currently, the set of transport services that most applications use Currently, the set of transport services that most applications use
is based on TCP and UDP (and protocols that are layered on top of is based on TCP and UDP (and protocols that are layered on top of
them); this limits the ability for the network stack to make use of them); this limits the ability for the network stack to make use of
features of other transport protocols. For example, if a protocol features of other transport protocols. For example, if a protocol
supports out-of-order message delivery but applications always assume supports out-of-order message delivery but applications always assume
that the network provides an ordered bytestream, then the network that the network provides an ordered bytestream, then the network
stack can not immediately deliver a message that arrives out-of- stack can not immediately deliver a message that arrives out-of-
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delay. delay.
By exposing the transport services of multiple transport protocols, a By exposing the transport services of multiple transport protocols, a
transport system can make it possible for applications to use these transport system can make it possible for applications to use these
services without being statically bound to a specific transport services without being statically bound to a specific transport
protocol. The first step towards the design of such a system was protocol. The first step towards the design of such a system was
taken by [RFC8095], which surveys a large number of transports, and taken by [RFC8095], which surveys a large number of transports, and
[RFC8303] as well as [RFC8304], which identify the specific transport [RFC8303] as well as [RFC8304], which identify the specific transport
features that are exposed to applications by the protocols TCP, features that are exposed to applications by the protocols TCP,
MPTCP, UDP(-Lite) and SCTP as well as the LEDBAT congestion control MPTCP, UDP(-Lite) and SCTP as well as the LEDBAT congestion control
mechanism. This memo is based on these documents and follows the mechanism. LEDBAT was included as the only congestion control
same terminology (also listed below). Because the considered mechanism in this list because the "low extra delay background
transport protocols conjointly cover a wide range of transport transport" service that it offers is significantly different from the
features, there is reason to hope that the resulting set (and the typical service provided by other congestion control mechanisms.
reasoning that led to it) will also apply to many aspects of other This memo is based on these documents and follows the same
transport protocols that may be in use today, or may be designed in terminology (also listed below). Because the considered transport
the future. protocols conjointly cover a wide range of transport features, there
is reason to hope that the resulting set (and the reasoning that led
to it) will also apply to many aspects of other transport protocols
that may be in use today, or may be designed in the future.
By decoupling applications from transport protocols, a transport By decoupling applications from transport protocols, a transport
system provides a different abstraction level than the Berkeley system provides a different abstraction level than the Berkeley
sockets interface. As with high- vs. low-level programming sockets interface [POSIX]. As with high- vs. low-level programming
languages, a higher abstraction level allows more freedom for languages, a higher abstraction level allows more freedom for
automation below the interface, yet it takes some control away from automation below the interface, yet it takes some control away from
the application programmer. This is the design trade-off that a the application programmer. This is the design trade-off that a
transport system developer is facing, and this document provides transport system developer is facing, and this document provides
guidance on the design of this abstraction level. Some transport guidance on the design of this abstraction level. Some transport
features are currently rarely offered by APIs, yet they must be features are currently rarely offered by APIs, yet they must be
offered or they can never be used. Other transport features are offered or they can never be used. Other transport features are
offered by the APIs of the protocols covered here, but not exposing offered by the APIs of the protocols covered here, but not exposing
them in an API would allow for more freedom to automate protocol them in an API would allow for more freedom to automate protocol
usage in a transport system. The minimal set presented in this usage in a transport system. The minimal set presented here is an
document is an effort to find a middle ground that can be recommended effort to find a middle ground that can be recommended for transport
for transport systems to implement, on the basis of the transport systems to implement, on the basis of the transport features
features discussed in [RFC8303]. discussed in [RFC8303].
Applications use a wide variety of APIs today. The transport Applications use a wide variety of APIs today. The transport
features in the minimal set in this document must be reflected in features in the minimal set in this document must be reflected in
*all* network APIs in order for the underlying functionality to *all* network APIs in order for the underlying functionality to
become usable everywhere. For example, it does not help an become usable everywhere. For example, it does not help an
application that talks to a library which offers its own application that talks to a library which offers its own
communication interface if the underlying Berkeley Sockets API is communication interface if the underlying Berkeley Sockets API is
extended to offer "unordered message delivery", but the library only extended to offer "unordered message delivery", but the library only
exposes an ordered bytestream. Both the Berkeley Sockets API and the exposes an ordered bytestream. Both the Berkeley Sockets API and the
library would have to expose the "unordered message delivery" library would have to expose the "unordered message delivery"
transport feature (alternatively, there may be ways for certain types transport feature (alternatively, there may be ways for certain types
of libraries to use this transport feature without exposing it, based of libraries to use this transport feature without exposing it, based
on knowledge about the applications -- but this is not the general on knowledge about the applications -- but this is not the general
case). In most situations, in the interest of being as flexible and case). Similarly, transport protocols such as SCTP offer multi-
efficient as possible, the best choice will be for a library to streaming, which cannot be utilized, e.g., to prioritize messages
between streams, unless applications communicate the priorities and
the group of connections upon which these priorities should be
applied. In most situations, in the interest of being as flexible
and efficient as possible, the best choice will be for a library to
expose at least all of the transport features that are recommended as expose at least all of the transport features that are recommended as
a "minimal set" here. a "minimal set" here.
This "minimal set" can be implemented "one-sided" over TCP. This This "minimal set" can be implemented "one-sided" over TCP. This
means that a sender-side transport system can talk to a standard TCP means that a sender-side transport system can talk to a standard TCP
receiver, and a receiver-side transport system can talk to a standard receiver, and a receiver-side transport system can talk to a standard
TCP sender. If certain limitations are put in place, the "minimal TCP sender. If certain limitations are put in place, the "minimal
set" can also be implemented "one-sided" over UDP. set" can also be implemented "one-sided" over UDP. While the
possibility of such "one-sided" implementation may help deployment,
it comes at the cost of limiting the set to services that can also be
provided by TCP (or, with further limitations, UDP). Thus, the
minimal set of transport features here is applicable for many, but
not all, applications: some application protocols have requirements
that are not met by this "minimal set".
Note that, throughout this document, protocols are meant to be used
natively. For example, when transport features of UDP, or
"implementation over" UDP is discussed, this refers to native usage
of UDP.
2. Terminology 2. Terminology
Transport Feature: a specific end-to-end feature that the transport Transport Feature: a specific end-to-end feature that the transport
layer provides to an application. Examples include layer provides to an application. Examples include
confidentiality, reliable delivery, ordered delivery, message- confidentiality, reliable delivery, ordered delivery, message-
versus-stream orientation, etc. versus-stream orientation, etc.
Transport Service: a set of Transport Features, without an Transport Service: a set of Transport Features, without an
association to any given framing protocol, which provides a association to any given framing protocol, which provides a
complete service to an application. complete service to an application.
Transport Protocol: an implementation that provides one or more Transport Protocol: an implementation that provides one or more
different transport services using a specific framing and header different transport services using a specific framing and header
format on the wire. format on the wire.
Transport Service Instance: an arrangement of transport protocols Application: an entity that uses a transport layer interface for
with a selected set of features and configuration parameters that end-to-end delivery of data across the network (this may also be
implements a single transport service, e.g., a protocol stack (RTP an upper layer protocol or tunnel encapsulation).
over UDP).
Application: an entity that uses the transport layer for end-to-end
delivery data across the network (this may also be an upper layer
protocol or tunnel encapsulation).
Application-specific knowledge: knowledge that only applications Application-specific knowledge: knowledge that only applications
have. have.
Endpoint: an entity that communicates with one or more other End system: an entity that communicates with one or more other end
endpoints using a transport protocol. systems using a transport protocol. An end system provides a
Connection: shared state of two or more endpoints that persists transport layer interface to applications.
across messages that are transmitted between these endpoints. Connection: shared state of two or more end systems that persists
across messages that are transmitted between these end systems.
Connection Group: a set of connections which share the same Connection Group: a set of connections which share the same
configuration (configuring one of them causes all other configuration (configuring one of them causes all other
connections in the same group to be configured in the same way). connections in the same group to be configured in the same way).
We call connections that belong to a connection group "grouped", We call connections that belong to a connection group "grouped",
while "ungrouped" connections are not a part of a connection while "ungrouped" connections are not a part of a connection
group. group.
Socket: the combination of a destination IP address and a Socket: the combination of a destination IP address and a
destination port number. destination port number.
Moreover, throughout the document, the protocol name "UDP(-Lite)" is Moreover, throughout the document, the protocol name "UDP(-Lite)" is
used when discussing transport features that are equivalent for UDP used when discussing transport features that are equivalent for UDP
and UDP-Lite; similarly, the protocol name "TCP" refers to both TCP and UDP-Lite; similarly, the protocol name "TCP" refers to both TCP
and MPTCP. and MPTCP.
3. The Minimal Set of Transport Features 3. Deriving the minimal set
Based on the categorization, reduction, and discussion in Appendix A, We assume that applications have no specific requirements that need
knowledge about the network, e.g. regarding the choice of network
interface or the end-to-end path. Even with these assumptions, there
are certain requirements that are strictly kept by transport
protocols today, and these must also be kept by a transport system.
Some of these requirements relate to transport features that we call
"Functional".
Functional transport features provide functionality that cannot be
used without the application knowing about them, or else they violate
assumptions that might cause the application to fail. For example,
ordered message delivery is a functional transport feature: it cannot
be configured without the application knowing about it because the
application's assumption could be that messages always arrive in
order. Failure includes any change of the application behavior that
is not performance oriented, e.g. security.
"Change DSCP" and "Disable Nagle algorithm" are examples of transport
features that we call "Optimizing": if a transport system
autonomously decides to enable or disable them, an application will
not fail, but a transport system may be able to communicate more
efficiently if the application is in control of this optimizing
transport feature. These transport features require application-
specific knowledge (e.g., about delay/bandwidth requirements or the
length of future data blocks that are to be transmitted).
The transport features of IETF transport protocols that do not
require application-specific knowledge and could therefore be
utilized by a transport system on its own without involving the
application are called "Automatable".
We approach the construction of a minimal set of transport features
in the following way:
1. Categorization (Appendix A): the superset of transport features
from [RFC8303] is presented, and transport features are
categorized as Functional, Optimizing or Automatable for later
reduction.
2. Reduction (Section 4): a shorter list of transport features is
derived from the categorization in the first step. This removes
all transport features that do not require application-specific
knowledge or would result in semantically incorrect behavior if
they were implemented over TCP or UDP.
3. Discussion (Section 5): the resulting list shows a number of
peculiarities that are discussed, to provide a basis for
constructing the minimal set.
4. Construction (Section 6): Based on the reduced set and the
discussion of the transport features therein, a minimal set is
constructed.
Following [RFC8303] and retaining its terminology, we divide the
transport features into two main groups as follows:
1. CONNECTION related transport features
- ESTABLISHMENT
- AVAILABILITY
- MAINTENANCE
- TERMINATION
2. DATA Transfer related transport features
- Sending Data
- Receiving Data
- Errors
4. The Reduced Set of Transport Features
By hiding automatable transport features from the application, a
transport system can gain opportunities to automate the usage of
network-related functionality. This can facilitate using the
transport system for the application programmer and it allows for
optimizations that may not be possible for an application. For
instance, system-wide configurations regarding the usage of multiple
interfaces can better be exploited if the choice of the interface is
not entirely up to the application. Therefore, since they are not
strictly necessary to expose in a transport system, we do not include
automatable transport features in the reduced set of transport
features. This leaves us with only the transport features that are
either optimizing or functional.
A transport system should be able to communicate via TCP or UDP if
alternative transport protocols are found not to work. For many
transport features, this is possible -- often by simply not doing
anything when a specific request is made. For some transport
features, however, it was identified that direct usage of neither TCP
nor UDP is possible: in these cases, even not doing anything would
incur semantically incorrect behavior. Whenever an application would
make use of one of these transport features, this would eliminate the
possibility to use TCP or UDP. Thus, we only keep the functional and
optimizing transport features for which an implementation over either
TCP or UDP is possible in our reduced set.
The following list contains the transport features from Appendix A,
reduced using these rules. The "minimal set" derived in this
document is meant to be implementable "one-sided" over TCP, and, with
limitations, UDP. In the list, we therefore precede a transport
feature with "T:" if an implementation over TCP is possible, "U:" if
an implementation over UDP is possible, and "T,U:" if an
implementation over either TCP or UDP is possible.
4.1. CONNECTION Related Transport Features
ESTABLISHMENT:
o T,U: Connect
o T,U: Specify number of attempts and/or timeout for the first
establishment message
o T: Configure authentication
o T: Hand over a message to reliably transfer (possibly multiple
times) before connection establishment
o T: Hand over a message to reliably transfer during connection
establishment
AVAILABILITY:
o T,U: Listen
o T: Configure authentication
MAINTENANCE:
o T: Change timeout for aborting connection (using retransmit limit
or time value)
o T: Suggest timeout to the peer
o T,U: Disable Nagle algorithm
o T,U: Notification of Excessive Retransmissions (early warning
below abortion threshold)
o T,U: Specify DSCP field
o T,U: Notification of ICMP error message arrival
o T: Change authentication parameters
o T: Obtain authentication information
o T,U: Set Cookie life value
o T,U: Choose a scheduler to operate between streams of an
association
o T,U: Configure priority or weight for a scheduler
o T,U: Disable checksum when sending
o T,U: Disable checksum requirement when receiving
o T,U: Specify checksum coverage used by the sender
o T,U: Specify minimum checksum coverage required by receiver
o T,U: Specify DF field
o T,U: Get max. transport-message size that may be sent using a non-
fragmented IP packet from the configured interface
o T,U: Get max. transport-message size that may be received from the
configured interface
o T,U: Obtain ECN field
o T,U: Enable and configure a "Low Extra Delay Background Transfer"
TERMINATION:
o T: Close after reliably delivering all remaining data, causing an
event informing the application on the other side
o T: Abort without delivering remaining data, causing an event
informing the application on the other side
o T,U: Abort without delivering remaining data, not causing an event
informing the application on the other side
o T,U: Timeout event when data could not be delivered for too long
4.2. DATA Transfer Related Transport Features
4.2.1. Sending Data
o T: Reliably transfer data, with congestion control
o T: Reliably transfer a message, with congestion control
o T,U: Unreliably transfer a message
o T: Configurable Message Reliability
o T: Ordered message delivery (potentially slower than unordered)
o T,U: Unordered message delivery (potentially faster than ordered)
o T,U: Request not to bundle messages
o T: Specifying a key id to be used to authenticate a message
o T,U: Request not to delay the acknowledgement (SACK) of a message
4.2.2. Receiving Data
o T,U: Receive data (with no message delimiting)
o U: Receive a message
o T,U: Information about partial message arrival
4.2.3. Errors
This section describes sending failures that are associated with a
specific call to in the "Sending Data" category (Appendix A.2.1).
o T,U: Notification of send failures
o T,U: Notification that the stack has no more user data to send
o T,U: Notification to a receiver that a partial message delivery
has been aborted
5. Discussion
The reduced set in the previous section exhibits a number of
peculiarities, which we will discuss in the following. This section
focuses on TCP because, with the exception of one particular
transport feature ("Receive a message" -- we will discuss this in
Section 5.1), the list shows that UDP is strictly a subset of TCP.
We can first try to understand how to build a transport system that
can run over TCP, and then narrow down the result further to allow
that the system can always run over either TCP or UDP (which
effectively means removing everything related to reliability,
ordering, authentication and closing/aborting with a notification to
the peer).
Note that, because the functional transport features of UDP are --
with the exception of "Receive a message" -- a subset of TCP, TCP can
be used as a replacement for UDP whenever an application does not
need message delimiting (e.g., because the application-layer protocol
already does it). This has been recognized by many applications that
already do this in practice, by trying to communicate with UDP at
first, and falling back to TCP in case of a connection failure.
5.1. Sending Messages, Receiving Bytes
For implementing a transport system over TCP, there are several
transport features related to sending, but only a single transport
feature related to receiving: "Receive data (with no message
delimiting)" (and, strangely, "information about partial message
arrival"). Notably, the transport feature "Receive a message" is
also the only non-automatable transport feature of UDP(-Lite) for
which no implementation over TCP is possible.
To support these TCP receiver semantics, we define an "Application-
Framed Bytestream" (AFra-Bytestream). AFra-Bytestreams allow senders
to operate on messages while minimizing changes to the TCP socket
API. In particular, nothing changes on the receiver side - data can
be accepted via a normal TCP socket.
In an AFra-Bytestream, the sending application can optionally inform
the transport about message boundaries and required properties per
message (configurable order and reliability, or embedding a request
not to delay the acknowledgement of a message). Whenever the sending
application specifies per-message properties that relax the notion of
reliable in-order delivery of bytes, it must assume that the
receiving application is 1) able to determine message boundaries,
provided that messages are always kept intact, and 2) able to accept
these relaxed per-message properties. Any signaling of such
information to the peer is up to an application-layer protocol and
considered out of scope of this document.
For example, if an application requests to transfer fixed-size
messages of 100 bytes with partial reliability, this needs the
receiving application to be prepared to accept data in chunks of 100
bytes. If, then, some of these 100-byte messages are missing (e.g.,
if SCTP with Configurable Reliability is used), this is the expected
application behavior. With TCP, no messages would be missing, but
this is also correct for the application, and the possible
retransmission delay is acceptable within the best-effort service
model (see [RFC7305], Section 3.5). Still, the receiving application
would separate the byte stream into 100-byte chunks.
Note that this usage of messages does not require all messages to be
equal in size. Many application protocols use some form of Type-
Length-Value (TLV) encoding, e.g. by defining a header including
length fields; another alternative is the use of byte stuffing
methods such as COBS [COBS]. If an application needs message
numbers, e.g. to restore the correct sequence of messages, these must
also be encoded by the application itself, as the sequence number
related transport features of SCTP are not provided by the "minimum
set" (in the interest of enabling usage of TCP).
5.2. Stream Schedulers Without Streams
We have already stated that multi-streaming does not require
application-specific knowledge. Potential benefits or disadvantages
of, e.g., using two streams of an SCTP association versus using two
separate SCTP associations or TCP connections are related to
knowledge about the network and the particular transport protocol in
use, not the application. However, the transport features "Choose a
scheduler to operate between streams of an association" and
"Configure priority or weight for a scheduler" operate on streams.
Here, streams identify communication channels between which a
scheduler operates, and they can be assigned a priority. Moreover,
the transport features in the MAINTENANCE category all operate on
assocations in case of SCTP, i.e. they apply to all streams in that
assocation.
With only these semantics necessary to represent, the interface to a
transport system becomes easier if we assume that connections may be
not only a transport protocol's connection or association, but could
also be a stream of an existing SCTP association, for example. We
only need to allow for a way to define a possible grouping of
connections. Then, all MAINTENANCE transport features can be said to
operate on connection groups, not connections, and a scheduler
operates on the connections within a group.
To be compatible with multiple transport protocols and uniformly
allow access to both transport connections and streams of a multi-
streaming protocol, the semantics of opening and closing need to be
the most restrictive subset of all of the underlying options. For
example, TCP's support of half-closed connections can be seen as a
feature on top of the more restrictive "ABORT"; this feature cannot
be supported because not all protocols used by a transport system
(including streams of an association) support half-closed
connections.
5.3. Early Data Transmission
There are two transport features related to transferring a message
early: "Hand over a message to reliably transfer (possibly multiple
times) before connection establishment", which relates to TCP Fast
Open [RFC7413], and "Hand over a message to reliably transfer during
connection establishment", which relates to SCTP's ability to
transfer data together with the COOKIE-Echo chunk. Also without TCP
Fast Open, TCP can transfer data during the handshake, together with
the SYN packet -- however, the receiver of this data may not hand it
over to the application until the handshake has completed. Also,
different from TCP Fast Open, this data is not delimited as a message
by TCP (thus, not visible as a ``message''). This functionality is
commonly available in TCP and supported in several implementations,
even though the TCP specification does not explain how to provide it
to applications.
A transport system could differentiate between the cases of
transmitting data "before" (possibly multiple times) or "during" the
handshake. Alternatively, it could also assume that data that are
handed over early will be transmitted as early as possible, and
"before" the handshake would only be used for messages that are
explicitly marked as "idempotent" (i.e., it would be acceptable to
transfer them multiple times).
The amount of data that can successfully be transmitted before or
during the handshake depends on various factors: the transport
protocol, the use of header options, the choice of IPv4 and IPv6 and
the Path MTU. A transport system should therefore allow a sending
application to query the maximum amount of data it can possibly
transmit before (or, if exposed, during) connection establishment.
5.4. Sender Running Dry
The transport feature "Notification that the stack has no more user
data to send" relates to SCTP's "SENDER DRY" notification. Such
notifications can, in principle, be used to avoid having an
unnecessarily large send buffer, yet ensure that the transport sender
always has data available when it has an opportunity to transmit it.
This has been found to be very beneficial for some applications
[WWDC2015]. However, "SENDER DRY" truly means that the entire send
buffer (including both unsent and unacknowledged data) has emptied --
i.e., when it notifies the sender, it is already too late, the
transport protocol already missed an opportunity to send data. Some
modern TCP implementations now include the unspecified
"TCP_NOTSENT_LOWAT" socket option that was proposed in [WWDC2015],
which limits the amount of unsent data that TCP can keep in the
socket buffer; this allows to specify at which buffer filling level
the socket becomes writable, rather than waiting for the buffer to
run empty.
SCTP allows to configure the sender-side buffer too: the automatable
Transport Feature "Configure send buffer size" provides this
functionality, but only for the complete buffer, which includes both
unsent and unacknowledged data. SCTP does not allow to control these
two sizes separately. It therefore makes sense for a transport
system to allow for uniform access to "TCP_NOTSENT_LOWAT" as well as
the "SENDER DRY" notification.
5.5. Capacity Profile
The transport features:
o Disable Nagle algorithm
o Enable and configure a "Low Extra Delay Background Transfer"
o Specify DSCP field
all relate to a QoS-like application need such as "low latency" or
"scavenger". In the interest of flexibility of a transport system,
they could therefore be offered in a uniform, more abstract way,
where a transport system could e.g. decide by itself how to use
combinations of LEDBAT-like congestion control and certain DSCP
values, and an application would only specify a general "capacity
profile" (a description of how it wants to use the available
capacity). A need for "lowest possible latency at the expense of
overhead" could then translate into automatically disabling the Nagle
algorithm.
In some cases, the Nagle algorithm is best controlled directly by the
application because it is not only related to a general profile but
also to knowledge about the size of future messages. For fine-grain
control over Nagle-like functionality, the "Request not to bundle
messages" is available.
5.6. Security
Both TCP and SCTP offer authentication. TCP authenticates complete
segments. SCTP allows to configure which of SCTP's chunk types must
always be authenticated -- if this is exposed as such, it creates an
undesirable dependency on the transport protocol. For compatibility
with TCP, a transport system should only allow to configure complete
transport layer packets, including headers, IP pseudo-header (if any)
and payload.
Security is discussed in a separate document
[I-D.ietf-taps-transport-security]. The minimal set presented in the
present document excludes all security related transport features
from Appendix A: "Configure authentication", "Change authentication
parameters", "Obtain authentication information" and and "Set Cookie
life value" as well as "Specifying a key id to be used to
authenticate a message".
5.7. Packet Size
UDP(-Lite) has a transport feature called "Specify DF field". This
yields an error message in case of sending a message that exceeds the
Path MTU, which is necessary for a UDP-based application to be able
to implement Path MTU Discovery (a function that UDP-based
applications must do by themselves). The "Get max. transport-message
size that may be sent using a non-fragmented IP packet from the
configured interface" transport feature yields an upper limit for the
Path MTU (minus headers) and can therefore help to implement Path MTU
Discovery more efficiently.
6. The Minimal Set of Transport Features
Based on the categorization, reduction, and discussion in Section 3,
this section describes a minimal set of transport features that end this section describes a minimal set of transport features that end
systems should offer. The described transport system can be systems should offer. Any configuration based the described minimum
implemented over TCP. Elements of the system that are not marked set of transport feature can always be realized over TCP but also
with "!UDP" can also be implemented over UDP. gives the transport system flexibility to choose another transport if
implemented. In the text of this section, "not UDP" is used to
indicate elements of the system that cannot be implemented over UDP.
Conversely, all elements of the system that are not marked with "not
UDP" can also be implemented over UDP.
The arguments laid out in Appendix A.3 ("discussion") were used to The arguments laid out in Section 5 ("discussion") were used to make
make the final representation of the minimal set as short, simple and the final representation of the minimal set as short, simple and
general as possible. There may be situations where these arguments general as possible. There may be situations where these arguments
do not apply -- e.g., implementers may have specific reasons to do not apply -- e.g., implementers may have specific reasons to
expose multi-streaming as a visible functionality to applications, or expose multi-streaming as a visible functionality to applications, or
the restrictive open / close semantics may be problematic under some the restrictive open / close semantics may be problematic under some
circumstances. In such cases, the representation in Appendix A.2 circumstances. In such cases, the representation in Section 4
("reduction") should be considered. ("reduction") should be considered.
As in Appendix A, Appendix A.2 and [RFC8303], we categorize the As in Section 3, Section 4 and [RFC8303], we categorize the minimal
minimal set of transport features as 1) CONNECTION related set of transport features as 1) CONNECTION related (ESTABLISHMENT,
(ESTABLISHMENT, AVAILABILITY, MAINTENANCE, TERMINATION) and 2) DATA AVAILABILITY, MAINTENANCE, TERMINATION) and 2) DATA Transfer related
Transfer related (Sending Data, Receiving Data, Errors). Here, the (Sending Data, Receiving Data, Errors). Here, the focus is on
focus is on connections that the transport system offers as an connections that the transport system offers as an abstraction to the
abstraction to the application, as opposed to connections of application, as opposed to connections of transport protocols that
transport protocols that the transport system uses. the transport system uses.
3.1. ESTABLISHMENT, AVAILABILITY and TERMINATION 6.1. ESTABLISHMENT, AVAILABILITY and TERMINATION
A connection must first be "created" to allow for some initial A connection must first be "created" to allow for some initial
configuration to be carried out before the transport system can configuration to be carried out before the transport system can
actively or passively establish communication with a remote endpoint. actively or passively establish communication with a remote end
All configuration parameters in Section 3.2 can be used initially, system. All configuration parameters in Section 6.2 can be used
although some of them may only take effect when a connection has been initially, although some of them may only take effect when a
established with a chosen transport protocol. Configuring a connection has been established with a chosen transport protocol.
connection early helps a transport system make the right decisions. Configuring a connection early helps a transport system make the
For example, grouping information can influence the transport system right decisions. For example, grouping information can influence the
to implement a connection as a stream of a multi-streaming protocol's transport system to implement a connection as a stream of a multi-
existing association or not. streaming protocol's existing association or not.
For ungrouped connections, early configuration is necessary because For ungrouped connections, early configuration is necessary because
it allows the transport system to know which protocols it should try it allows the transport system to know which protocols it should try
to use. In particular, a transport system that only makes a one-time to use. In particular, a transport system that only makes a one-time
choice for a particular protocol must know early about strict choice for a particular protocol must know early about strict
requirements that must be kept, or it can end up in a deadlock requirements that must be kept, or it can end up in a deadlock
situation (e.g., having chosen UDP and later be asked to support situation (e.g., having chosen UDP and later be asked to support
reliable transfer). As an example description of how to correctly reliable transfer). As an example description of how to correctly
handle these cases, we provide the following decision tree (this is handle these cases, we provide the following decision tree (this is
derived from Appendix A.2.1 excluding authentication, as explained in derived from Section 4.1 excluding authentication, as explained in
Section 6): Section 9):
- Will it ever be necessary to offer any of the following? - Will it ever be necessary to offer any of the following?
* Reliably transfer data * Reliably transfer data
* Notify the peer of closing/aborting * Notify the peer of closing/aborting
* Preserve data ordering * Preserve data ordering
Yes: SCTP or TCP can be used. Yes: SCTP or TCP can be used.
- Is any of the following useful to the application? - Is any of the following useful to the application?
* Choosing a scheduler to operate between connections * Choosing a scheduler to operate between connections
in a group, with the possibility to configure a priority in a group, with the possibility to configure a priority
skipping to change at page 7, line 4 skipping to change at page 16, line 5
Yes: UDP-Lite is preferred. Yes: UDP-Lite is preferred.
No: UDP is preferred. No: UDP is preferred.
Note that this decision tree is not optimal for all cases. For Note that this decision tree is not optimal for all cases. For
example, if an application wants to use "Specify checksum coverage example, if an application wants to use "Specify checksum coverage
used by the sender", which is only offered by UDP-Lite, and used by the sender", which is only offered by UDP-Lite, and
"Configure priority or weight for a scheduler", which is only offered "Configure priority or weight for a scheduler", which is only offered
by SCTP, the above decision tree will always choose UDP-Lite, making by SCTP, the above decision tree will always choose UDP-Lite, making
it impossible to use SCTP's schedulers with priorities between it impossible to use SCTP's schedulers with priorities between
grouped connections. We caution implementers to be aware of the full grouped connections. Also, several other factors may influence the
set of trade-offs, for which we recommend consulting the list in decisions for or against a protocol -- e.g. penetration rates, the
Appendix A.2.1 when deciding how to initialize a connection. ability to work through NATs, etc. We caution implementers to be
aware of the full set of trade-offs, for which we recommend
consulting the list in Section 4.1 when deciding how to initialize a
connection.
To summarize, the following parameters serve as input for the To summarize, the following parameters serve as input for the
transport system to help it choose and configure a suitable protocol: transport system to help it choose and configure a suitable protocol:
o Reliability: a boolean that should be set to true when any of the o Reliability: a boolean that should be set to true when any of the
following will be useful to the application: reliably transfer following will be useful to the application: reliably transfer
data; notify the peer of closing/aborting; preserve data ordering. data; notify the peer of closing/aborting; preserve data ordering.
o Checksum coverage: a boolean to specify whether it will be useful o Checksum coverage: a boolean to specify whether it will be useful
to the application to specify checksum coverage when sending or to the application to specify checksum coverage when sending or
receiving. receiving.
skipping to change at page 7, line 36 skipping to change at page 16, line 40
application: hand over a message to reliably transfer (possibly application: hand over a message to reliably transfer (possibly
multiple times) before connection establishment; suggest timeout multiple times) before connection establishment; suggest timeout
to the peer; notification of excessive retransmissions (early to the peer; notification of excessive retransmissions (early
warning below abortion threshold); notification of ICMP error warning below abortion threshold); notification of ICMP error
message arrival. message arrival.
Once a connection is created, it can be queried for the maximum Once a connection is created, it can be queried for the maximum
amount of data that an application can possibly expect to have amount of data that an application can possibly expect to have
reliably transmitted before or during transport connection reliably transmitted before or during transport connection
establishment (with zero being a possible answer) (see establishment (with zero being a possible answer) (see
Section 3.2.1). An application can also give the connection a Section 6.2.1). An application can also give the connection a
message for reliable transmission before or during connection message for reliable transmission before or during connection
establishment (!UDP); the transport system will then try to transmit establishment (not UDP); the transport system will then try to
it as early as possible. An application can facilitate sending a transmit it as early as possible. An application can facilitate
message particularly early by marking it as "idempotent" (see sending a message particularly early by marking it as "idempotent"
Section 3.3.1); in this case, the receiving application must be (see Section 6.3.1); in this case, the receiving application must be
prepared to potentially receive multiple copies of the message prepared to potentially receive multiple copies of the message
(because idempotent messages are reliably transferred, asking for (because idempotent messages are reliably transferred, asking for
idempotence is not necessary for systems that support UDP). idempotence is not necessary for systems that support UDP).
After creation, a transport system can actively establish After creation, a transport system can actively establish
communication with a peer, or it can passively listen for incoming communication with a peer, or it can passively listen for incoming
connection requests. Note that active establishment may or may not connection requests. Note that active establishment may or may not
trigger a notification on the listening side. It is possible that trigger a notification on the listening side. It is possible that
the first notification on the listening side is the arrival of the the first notification on the listening side is the arrival of the
first data that the active side sends (a receiver-side transport first data that the active side sends (a receiver-side transport
system could handle this by continuing to block a "Listen" call, system could handle this by continuing to block a "Listen" call,
immediately followed by issuing "Receive", for example; callback- immediately followed by issuing "Receive", for example; callback-
based implementations could simply skip the equivalent of "Listen"). based implementations could simply skip the equivalent of "Listen").
This also means that the active opening side is assumed to be the This also means that the active opening side is assumed to be the
first side sending data. first side sending data.
A transport system can actively close a connection, i.e. terminate it A transport system can actively close a connection, i.e. terminate it
after reliably delivering all remaining data to the peer (if reliable after reliably delivering all remaining data to the peer (if reliable
data delivery was requested earlier (!UDP)), in which case the peer data delivery was requested earlier (not UDP)), in which case the
is notified that the connection is closed. Alternatively, a peer is notified that the connection is closed. Alternatively, a
connection can be aborted without delivering outstanding data to the connection can be aborted without delivering outstanding data to the
peer. In case reliable or partially reliable data delivery was peer. In case reliable or partially reliable data delivery was
requested earlier (!UDP), the peer is notified that the connection is requested earlier (not UDP), the peer is notified that the connection
aborted. A timeout can be configured to abort a connection when data is aborted. A timeout can be configured to abort a connection when
could not be delivered for too long (!UDP); however, timeout-based data could not be delivered for too long (not UDP); however, timeout-
abortion does not notify the peer application that the connection has based abortion does not notify the peer application that the
been aborted. Because half-closed connections are not supported, connection has been aborted. Because half-closed connections are not
when a host implementing a transport system receives a notification supported, when a host implementing a transport system receives a
that the peer is closing or aborting the connection (!UDP), its peer notification that the peer is closing or aborting the connection (not
may not be able to read outstanding data. This means that UDP), its peer may not be able to read outstanding data. This means
unacknowledged data residing a transport system's send buffer may that unacknowledged data residing in a transport system's send buffer
have to be dropped from that buffer upon arrival of a "close" or may have to be dropped from that buffer upon arrival of a "close" or
"abort" notification from the peer. "abort" notification from the peer.
3.2. MAINTENANCE 6.2. MAINTENANCE
A transport system must offer means to group connections, but it A transport system must offer means to group connections, but it
cannot guarantee truly grouping them using the transport protocols cannot guarantee truly grouping them using the transport protocols
that it uses (e.g., it cannot be guaranteed that connections become that it uses (e.g., it cannot be guaranteed that connections become
multiplexed as streams on a single SCTP association when SCTP may not multiplexed as streams on a single SCTP association when SCTP may not
be available). The transport system must therefore ensure that be available). The transport system must therefore ensure that
group- versus non-group-configurations are handled correctly in some group- versus non-group-configurations are handled correctly in some
way (e.g., by applying the configuration to all grouped connections way (e.g., by applying the configuration to all grouped connections
even when they are not multiplexed, or informing the application even when they are not multiplexed, or informing the application
about grouping success or failure). about grouping success or failure).
As a general rule, any configuration described below should be As a general rule, any configuration described below should be
carried out as early as possible to aid the transport system's carried out as early as possible to aid the transport system's
decision making. decision making.
3.2.1. Connection groups 6.2.1. Connection groups
The following transport features and notifications (some directly The following transport features and notifications (some directly
from Appendix A.2, some new or changed, based on the discussion in from Section 4, some new or changed, based on the discussion in
Appendix A.3) automatically apply to all grouped connections: Section 5) automatically apply to all grouped connections:
(!UDP) Configure a timeout: this can be done with the following (not UDP) Configure a timeout: this can be done with the following
parameters: parameters:
o A timeout value for aborting connections, in seconds o A timeout value for aborting connections, in seconds
o A timeout value to be suggested to the peer (if possible), in o A timeout value to be suggested to the peer (if possible), in
seconds seconds
o The number of retransmissions after which the application should o The number of retransmissions after which the application should
be notifed of "Excessive Retransmissions" be notifed of "Excessive Retransmissions"
Configure urgency: this can be done with the following parameters: Configure urgency: this can be done with the following parameters:
skipping to change at page 9, line 31 skipping to change at page 18, line 40
[I-D.ietf-tsvwg-rtcweb-qos]). [I-D.ietf-tsvwg-rtcweb-qos]).
o A buffer limit (in bytes); when the sender has less than the o A buffer limit (in bytes); when the sender has less than the
provided limit of bytes in the buffer, the application may be provided limit of bytes in the buffer, the application may be
notified. Notifications are not guaranteed, and it is optional notified. Notifications are not guaranteed, and it is optional
for a transport system to support buffer limit values greater than for a transport system to support buffer limit values greater than
0. Note that this limit and its notification should operate 0. Note that this limit and its notification should operate
across the buffers of the whole transport system, i.e. also any across the buffers of the whole transport system, i.e. also any
potential buffers that the transport system itself may use on top potential buffers that the transport system itself may use on top
of the transport's send buffer. of the transport's send buffer.
Following Appendix A.3.7, these properties can be queried: Following Section 5.7, these properties can be queried:
o The maximum message size that may be sent without fragmentation o The maximum message size that may be sent without fragmentation
via the configured interface. This is optional for a transport via the configured interface. This is optional for a transport
system to offer, and may return an error ("not available"). It system to offer, and may return an error ("not available"). It
can aid applications implementing Path MTU Discovery. can aid applications implementing Path MTU Discovery.
o The maximum transport message size that can be sent, in bytes. o The maximum transport message size that can be sent, in bytes.
Irrespective of fragmentation, there is a size limit for the Irrespective of fragmentation, there is a size limit for the
messages that can be handed over to SCTP or UDP(-Lite); because messages that can be handed over to SCTP or UDP(-Lite); because
the service provided by a transport system is independent of the the service provided by a transport system is independent of the
transport protocol, it must allow an application to query this transport protocol, it must allow an application to query this
value -- the maximum size of a message in an Application-Framed- value -- the maximum size of a message in an Application-Framed-
Bytestream (see Appendix A.3.1). This may also return an error Bytestream (see Section 5.1). This may also return an error when
when data is not delimited ("not available"). data is not delimited ("not available").
o The maximum transport message size that can be received from the o The maximum transport message size that can be received from the
configured interface, in bytes (or "not available"). configured interface, in bytes (or "not available").
o The maximum amount of data that can possibly be sent before or o The maximum amount of data that can possibly be sent before or
during connection establishment, in bytes. during connection establishment, in bytes.
In addition to the already mentioned closing / aborting notifications In addition to the already mentioned closing / aborting notifications
and possible send errors, the following notifications can occur: and possible send errors, the following notifications can occur:
o Excessive Retransmissions: the configured (or a default) number of o Excessive Retransmissions: the configured (or a default) number of
retransmissions has been reached, yielding this early warning retransmissions has been reached, yielding this early warning
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the conveyed ICMP message has arrived. the conveyed ICMP message has arrived.
o ECN Arrival (parameter: ECN value): a packet carrying the conveyed o ECN Arrival (parameter: ECN value): a packet carrying the conveyed
ECN value has arrived. This can be useful for applications ECN value has arrived. This can be useful for applications
implementing congestion control. implementing congestion control.
o Timeout (parameter: s seconds): data could not be delivered for s o Timeout (parameter: s seconds): data could not be delivered for s
seconds. seconds.
o Drain: the send buffer has either drained below the configured o Drain: the send buffer has either drained below the configured
buffer limit or it has become completely empty. This is a generic buffer limit or it has become completely empty. This is a generic
notification that tries to enable uniform access to notification that tries to enable uniform access to
"TCP_NOTSENT_LOWAT" as well as the "SENDER DRY" notification (as "TCP_NOTSENT_LOWAT" as well as the "SENDER DRY" notification (as
discussed in Appendix A.3.4 -- SCTP's "SENDER DRY" is a special discussed in Section 5.4 -- SCTP's "SENDER DRY" is a special case
case where the threshold (for unsent data) is 0 and there is also where the threshold (for unsent data) is 0 and there is also no
no more unacknowledged data in the send buffer). more unacknowledged data in the send buffer).
3.2.2. Individual connections 6.2.2. Individual connections
Configure priority or weight for a scheduler, as described in Configure priority or weight for a scheduler, as described in
[RFC8260]. [RFC8260].
Configure checksum usage: this can be done with the following Configure checksum usage: this can be done with the following
parameters, but there is no guarantee that any checksum limitations parameters, but there is no guarantee that any checksum limitations
will indeed be enforced (the default behavior is "full coverage, will indeed be enforced (the default behavior is "full coverage,
checksum enabled"): checksum enabled"):
o A boolean to enable / disable usage of a checksum when sending o A boolean to enable / disable usage of a checksum when sending
o The desired coverage (in bytes) of the checksum used when sending o The desired coverage (in bytes) of the checksum used when sending
o A boolean to enable / disable requiring a checksum when receiving o A boolean to enable / disable requiring a checksum when receiving
o The required minimum coverage (in bytes) of the checksum when o The required minimum coverage (in bytes) of the checksum when
receiving receiving
3.3. DATA Transfer 6.3. DATA Transfer
3.3.1. Sending Data 6.3.1. Sending Data
When sending a message, no guarantees are given about the When sending a message, no guarantees are given about the
preservation of message boundaries to the peer; if message boundaries preservation of message boundaries to the peer; if message boundaries
are needed, the receiving application at the peer must know about are needed, the receiving application at the peer must know about
them beforehand (or the transport system cannot use TCP). Note that them beforehand (or the transport system cannot use TCP). Note that
an application should already be able to hand over data before the an application should already be able to hand over data before the
transport system establishes a connection with a chosen transport transport system establishes a connection with a chosen transport
protocol. Regarding the message that is being handed over, the protocol. Regarding the message that is being handed over, the
following parameters can be used: following parameters can be used:
o Reliability: this parameter is used to convey a choice of: fully o Reliability: this parameter is used to convey a choice of: fully
reliable with congestion control (!UDP), unreliable without reliable with congestion control (not UDP), unreliable without
congestion control, unreliable with congestion control (!UDP), congestion control, unreliable with congestion control (not UDP),
partially reliable with congestion control (see [RFC3758] and partially reliable with congestion control (see [RFC3758] and
[RFC7496] for details on how to specify partial reliability) [RFC7496] for details on how to specify partial reliability) (not
(!UDP). The latter two choices are optional for a transport UDP). The latter two choices are optional for a transport system
system to offer and may result in full reliability. Note that to offer and may result in full reliability. Note that
applications sending unreliable data without congestion control applications sending unreliable data without congestion control
should themselves perform congestion control in accordance with should themselves perform congestion control in accordance with
[RFC2914]. [RFC8085].
o (!UDP) Ordered: this boolean parameter lets an application choose o (not UDP) Ordered: this boolean parameter lets an application
between ordered message delivery (true) and possibly unordered, choose between ordered message delivery (true) and possibly
potentially faster message delivery (false). unordered, potentially faster message delivery (false).
o Bundle: a boolean that expresses a preference for allowing to o Bundle: a boolean that expresses a preference for allowing to
bundle messages (true) or not (false). No guarantees are given. bundle messages (true) or not (false). No guarantees are given.
o DelAck: a boolean that, if false, lets an application request that o DelAck: a boolean that, if false, lets an application request that
the peer would not delay the acknowledgement for this message. the peer would not delay the acknowledgement for this message.
o Fragment: a boolean that expresses a preference for allowing to o Fragment: a boolean that expresses a preference for allowing to
fragment messages (true) or not (false), at the IP level. No fragment messages (true) or not (false), at the IP level. No
guarantees are given. guarantees are given.
o (!UDP) Idempotent: a boolean that expresses whether a message is o (not UDP) Idempotent: a boolean that expresses whether a message
idempotent (true) or not (false). Idempotent messages may arrive is idempotent (true) or not (false). Idempotent messages may
multiple times at the receiver (but they will arrive at least arrive multiple times at the receiver (but they will arrive at
once). When data is idempotent it can be used by the receiver least once). When data is idempotent it can be used by the
immediately on a connection establishment attempt. Thus, if data receiver immediately on a connection establishment attempt. Thus,
is handed over before the transport system establishes a if data is handed over before the transport system establishes a
connection with a chosen transport protocol, stating that a connection with a chosen transport protocol, stating that a
message is idempotent facilitates transmitting it to the peer message is idempotent facilitates transmitting it to the peer
application particularly early. application particularly early.
An application can be notified of a failure to send a specific An application can be notified of a failure to send a specific
message. There is no guarantee of such notifications, i.e. send message. There is no guarantee of such notifications, i.e. send
failures can also silently occur. failures can also silently occur.
3.3.2. Receiving Data 6.3.2. Receiving Data
A receiving application obtains an "Application-Framed Bytestream" A receiving application obtains an "Application-Framed Bytestream"
(AFra-Bytestream); this concept is further described in (AFra-Bytestream); this concept is further described in Section 5.1).
Appendix A.3.1). In line with TCP's receiver semantics, an AFra- In line with TCP's receiver semantics, an AFra-Bytestream is just a
Bytestream is just a stream of bytes to the receiver. If message stream of bytes to the receiver. If message boundaries were
boundaries were specified by the sender, a receiver-side transport specified by the sender, a receiver-side transport system
system implementing only the minimum set of transport services implementing only the minimum set of transport services defined here
defined here will still not inform the receiving application about will still not inform the receiving application about them (this
them (this limitation is only needed for transport systems that are limitation is only needed for transport systems that are implemented
implemented to directly use TCP). to directly use TCP).
Different from TCP's semantics, if the sending application has Different from TCP's semantics, if the sending application has
allowed that messages are not fully reliably transferred, or allowed that messages are not fully reliably transferred, or
delivered out of order, then such re-ordering or unreliability may be delivered out of order, then such re-ordering or unreliability may be
reflected per message in the arriving data. Messages will always reflected per message in the arriving data. Messages will always
stay intact - i.e. if an incomplete message is contained at the end stay intact - i.e. if an incomplete message is contained at the end
of the arriving data block, this message is guaranteed to continue in of the arriving data block, this message is guaranteed to continue in
the next arriving data block. the next arriving data block.
4. Acknowledgements 7. Acknowledgements
The authors would like to thank all the participants of the TAPS The authors would like to thank all the participants of the TAPS
Working Group and the NEAT and MAMI research projects for valuable Working Group and the NEAT and MAMI research projects for valuable
input to this document. We especially thank Michael Tuexen for help input to this document. We especially thank Michael Tuexen for help
with connection connection establishment/teardown, Gorry Fairhurst with connection connection establishment/teardown, Gorry Fairhurst
for his suggestions regarding fragmentation and packet sizes, and for his suggestions regarding fragmentation and packet sizes, and
Spencer Dawkins for his extremely detailed and constructive review. Spencer Dawkins for his extremely detailed and constructive review.
This work has received funding from the European Union's Horizon 2020 This work has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334 research and innovation programme under grant agreement No. 644334
(NEAT). (NEAT).
5. IANA Considerations 8. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
6. Security Considerations 9. Security Considerations
Authentication, confidentiality protection, and integrity protection Authentication, confidentiality protection, and integrity protection
are identified as transport features by [RFC8095]. As currently are identified as transport features by [RFC8095]. As currently
deployed in the Internet, these features are generally provided by a deployed in the Internet, these features are generally provided by a
protocol or layer on top of the transport protocol; no current full- protocol or layer on top of the transport protocol; no current full-
featured standards-track transport protocol provides all of these featured standards-track transport protocol provides all of these
transport features on its own. Therefore, these transport features transport features on its own. Therefore, these transport features
are not considered in this document, with the exception of native are not considered in this document, with the exception of native
authentication capabilities of TCP and SCTP for which the security authentication capabilities of TCP and SCTP for which the security
considerations in [RFC5925] and [RFC4895] apply. The minimum considerations in [RFC5925] and [RFC4895] apply. The minimum
requirements for a secure transport system are discussed in a requirements for a secure transport system are discussed in a
separate document (Section 5 on Security Features and Transport separate document (Section 5 on Security Features and Transport
Dependencies of [I-D.ietf-taps-transport-security]). Dependencies of [I-D.ietf-taps-transport-security]).
7. References 10. References
7.1. Normative References 10.1. Normative References
[I-D.ietf-taps-transport-security] [I-D.ietf-taps-transport-security]
Pauly, T., Perkins, C., Rose, K., and C. Wood, "A Survey Pauly, T., Perkins, C., Rose, K., and C. Wood, "A Survey
of Transport Security Protocols", draft-ietf-taps- of Transport Security Protocols", draft-ietf-taps-
transport-security-02 (work in progress), June 2018. transport-security-02 (work in progress), June 2018.
[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind, [RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095, Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017, DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>. <https://www.rfc-editor.org/info/rfc8095>.
[RFC8303] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of [RFC8303] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
Transport Features Provided by IETF Transport Protocols", Transport Features Provided by IETF Transport Protocols",
RFC 8303, DOI 10.17487/RFC8303, February 2018, RFC 8303, DOI 10.17487/RFC8303, February 2018,
<https://www.rfc-editor.org/info/rfc8303>. <https://www.rfc-editor.org/info/rfc8303>.
7.2. Informative References 10.2. Informative References
[COBS] Cheshire, S. and M. Baker, "Consistent Overhead Byte [COBS] Cheshire, S. and M. Baker, "Consistent Overhead Byte
Stuffing", IEEE/ACM Transactions on Networking Vol. 7, No. Stuffing", IEEE/ACM Transactions on Networking Vol. 7, No.
2, April 1999. 2, April 1999.
[I-D.ietf-tsvwg-rtcweb-qos] [I-D.ietf-tsvwg-rtcweb-qos]
Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP Jones, P., Dhesikan, S., Jennings, C., and D. Druta, "DSCP
Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb- Packet Markings for WebRTC QoS", draft-ietf-tsvwg-rtcweb-
qos-18 (work in progress), August 2016. qos-18 (work in progress), August 2016.
[LBE-draft] [LBE-draft]
Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)", Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
Internet-draft draft-tsvwg-le-phb-03, February 2018. Internet-draft draft-tsvwg-le-phb-03, February 2018.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, [POSIX] "IEEE Standard for Information Technology--Portable
RFC 2914, DOI 10.17487/RFC2914, September 2000, Operating System Interface (POSIX(R)) Base Specifications,
<https://www.rfc-editor.org/info/rfc2914>. Issue 7", IEEE Std 1003.1-2017 (Revision of IEEE Std
1003.1-2008), January 2018,
<http://www.opengroup.org/onlinepubs/9699919799/functions/
contents.html>.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. [RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP) Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758, Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004, DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>. <https://www.rfc-editor.org/info/rfc3758>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, [RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission "Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
skipping to change at page 14, line 20 skipping to change at page 23, line 39
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>. <https://www.rfc-editor.org/info/rfc7413>.
[RFC7496] Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto, [RFC7496] Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
"Additional Policies for the Partially Reliable Stream "Additional Policies for the Partially Reliable Stream
Control Transmission Protocol Extension", RFC 7496, Control Transmission Protocol Extension", RFC 7496,
DOI 10.17487/RFC7496, April 2015, DOI 10.17487/RFC7496, April 2015,
<https://www.rfc-editor.org/info/rfc7496>. <https://www.rfc-editor.org/info/rfc7496>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann, [RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the "Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", RFC 8260, Stream Control Transmission Protocol", RFC 8260,
DOI 10.17487/RFC8260, November 2017, DOI 10.17487/RFC8260, November 2017,
<https://www.rfc-editor.org/info/rfc8260>. <https://www.rfc-editor.org/info/rfc8260>.
[RFC8304] Fairhurst, G. and T. Jones, "Transport Features of the [RFC8304] Fairhurst, G. and T. Jones, "Transport Features of the
User Datagram Protocol (UDP) and Lightweight UDP (UDP- User Datagram Protocol (UDP) and Lightweight UDP (UDP-
Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018, Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018,
<https://www.rfc-editor.org/info/rfc8304>. <https://www.rfc-editor.org/info/rfc8304>.
[SCTP-stream-1]
Weinrank, F. and M. Tuexen, "Transparent Flow Mapping for
NEAT", IFIP NETWORKING Workshop on Future of Internet
Transport (FIT 2017), June 2017.
[SCTP-stream-2]
Welzl, M., Niederbacher, F., and S. Gjessing, "Beneficial
Transparent Deployment of SCTP", IEEE GlobeCom 2011,
December 2011.
[WWDC2015] [WWDC2015]
Lakhera, P. and S. Cheshire, "Your App and Next Generation Lakhera, P. and S. Cheshire, "Your App and Next Generation
Networks", Apple Worldwide Developers Conference 2015, San Networks", Apple Worldwide Developers Conference 2015, San
Francisco, USA, June 2015, Francisco, USA, June 2015,
<https://developer.apple.com/videos/wwdc/2015/?id=719>. <https://developer.apple.com/videos/wwdc/2015/?id=719>.
Appendix A. Deriving the minimal set Appendix A. The Superset of Transport Features
We approach the construction of a minimal set of transport features
in the following way:
1. Categorization (Appendix A.1): the superset of transport features
from [RFC8303] is presented, and transport features are
categorized for later reduction.
2. Reduction (Appendix A.2): a shorter list of transport features is
derived from the categorization in the first step. This removes
all transport features that do not require application-specific
knowledge or would result in semantically incorrect behavior if
they were implemented over TCP or UDP.
3. Discussion (Appendix A.3): the resulting list shows a number of
peculiarities that are discussed, to provide a basis for
constructing the minimal set.
4. Construction (Section 3): Based on the reduced set and the
discussion of the transport features therein, a minimal set is
constructed.
A.1. Step 1: Categorization -- The Superset of Transport Features
Following [RFC8303], we divide the transport features into two main
groups as follows:
1. CONNECTION related transport features
- ESTABLISHMENT
- AVAILABILITY
- MAINTENANCE
- TERMINATION
2. DATA Transfer related transport features
- Sending Data
- Receiving Data
- Errors
We assume that applications have no specific requirements that need
knowledge about the network, e.g. regarding the choice of network
interface or the end-to-end path. Even with these assumptions, there
are certain requirements that are strictly kept by transport
protocols today, and these must also be kept by a transport system.
Some of these requirements relate to transport features that we call
"Functional".
Functional transport features provide functionality that cannot be
used without the application knowing about them, or else they violate
assumptions that might cause the application to fail. For example,
ordered message delivery is a functional transport feature: it cannot
be configured without the application knowing about it because the
application's assumption could be that messages always arrive in
order. Failure includes any change of the application behavior that
is not performance oriented, e.g. security.
"Change DSCP" and "Disable Nagle algorithm" are examples of transport
features that we call "Optimizing": if a transport system
autonomously decides to enable or disable them, an application will
not fail, but a transport system may be able to communicate more
efficiently if the application is in control of this optimizing
transport feature. These transport features require application-
specific knowledge (e.g., about delay/bandwidth requirements or the
length of future data blocks that are to be transmitted).
The transport features of IETF transport protocols that do not
require application-specific knowledge and could therefore be
utilized by a transport system on its own without involving the
application are called "Automatable".
Finally, in three cases, transport features are aggregated and/or
slightly changed from [RFC8303] in the description below. These
transport features are marked as "ADDED". These do not add any new
functionality but just represent a simple refactoring step that helps
to streamline the derivation process (e.g., by removing a choice of a
parameter for the sake of applications that may not care about this
choice). The corresponding transport features are automatable, and
they are listed immediately below the "ADDED" transport feature.
In this description, transport services are presented following the In this description, transport features are presented following the
nomenclature "CATEGORY.[SUBCATEGORY].SERVICENAME.PROTOCOL", nomenclature "CATEGORY.[SUBCATEGORY].FEATURENAME.PROTOCOL",
equivalent to "pass 2" in [RFC8303]. We also sketch how functional equivalent to "pass 2" in [RFC8303]. We also sketch how functional
or optimizing transport features can be implemented by a transport or optimizing transport features can be implemented by a transport
system. The "minimal set" derived in this document is meant to be system. The "minimal set" derived in this document is meant to be
implementable "one-sided" over TCP, and, with limitations, UDP. implementable "one-sided" over TCP, and, with limitations, UDP.
Hence, for all transport features that are categorized as Hence, for all transport features that are categorized as
"functional" or "optimizing", and for which no matching TCP and/or "functional" or "optimizing", and for which no matching TCP and/or
UDP primitive exists in "pass 2" of [RFC8303], a brief discussion on UDP primitive exists in "pass 2" of [RFC8303], a brief discussion on
how to implement them over TCP and/or UDP is included. how to implement them over TCP and/or UDP is included.
We designate some transport features as "automatable" on the basis of We designate some transport features as "automatable" on the basis of
skipping to change at page 16, line 46 skipping to change at page 24, line 50
application-specific knowledge. This means that a connection that application-specific knowledge. This means that a connection that
is exhibited to an application could be implemented by using a is exhibited to an application could be implemented by using a
single stream of an SCTP association instead of mapping it to a single stream of an SCTP association instead of mapping it to a
complete SCTP association or TCP connection. This could be complete SCTP association or TCP connection. This could be
achieved by using more than one stream when an SCTP association is achieved by using more than one stream when an SCTP association is
first established (CONNECT.SCTP parameter "outbound stream first established (CONNECT.SCTP parameter "outbound stream
count"), maintaining an internal stream number, and using this count"), maintaining an internal stream number, and using this
stream number when sending data (SEND.SCTP parameter "stream stream number when sending data (SEND.SCTP parameter "stream
number"). Closing or aborting a connection could then simply free number"). Closing or aborting a connection could then simply free
the stream number for future use. This is discussed further in the stream number for future use. This is discussed further in
Appendix A.3.2. Section 5.2.
o All transport features that are related to using multiple paths or o All transport features that are related to using multiple paths or
the choice of the network interface were designated as the choice of the network interface were designated as
"automatable". Choosing a path or an interface does not depend on "automatable". Choosing a path or an interface does not depend on
application-specific knowledge. For example, "Listen" could application-specific knowledge. For example, "Listen" could
always listen on all available interfaces and "Connect" could use always listen on all available interfaces and "Connect" could use
the default interface for the destination IP address. the default interface for the destination IP address.
A.1.1. CONNECTION Related Transport Features Finally, in three cases, transport features are aggregated and/or
slightly changed from [RFC8303] in the description below. These
transport features are marked as "CHANGED FROM RFC8303". These do
not add any new functionality but just represent a simple refactoring
step that helps to streamline the derivation process (e.g., by
removing a choice of a parameter for the sake of applications that
may not care about this choice). The corresponding transport
features are automatable, and they are listed immediately below the
"CHANGED FROM RFC8303" transport feature.
A.1. CONNECTION Related Transport Features
ESTABLISHMENT: ESTABLISHMENT:
o Connect o Connect
Protocols: TCP, SCTP, UDP(-Lite) Protocols: TCP, SCTP, UDP(-Lite)
Functional because the notion of a connection is often reflected Functional because the notion of a connection is often reflected
in applications as an expectation to be able to communicate after in applications as an expectation to be able to communicate after
a "Connect" succeeded, with a communication sequence relating to a "Connect" succeeded, with a communication sequence relating to
this transport feature that is defined by the application this transport feature that is defined by the application
protocol. protocol.
skipping to change at page 17, line 27 skipping to change at page 25, line 41
Lite). Lite).
o Specify which IP Options must always be used o Specify which IP Options must always be used
Protocols: TCP, UDP(-Lite) Protocols: TCP, UDP(-Lite)
Automatable because IP Options relate to knowledge about the Automatable because IP Options relate to knowledge about the
network, not the application. network, not the application.
o Request multiple streams o Request multiple streams
Protocols: SCTP Protocols: SCTP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge (example implementations of using
Implementation: see Appendix A.3.2. multi-streaming without involving the application are described in
[SCTP-stream-1] and [SCTP-stream-2]).
Implementation: see Section 5.2.
o Limit the number of inbound streams o Limit the number of inbound streams
Protocols: SCTP Protocols: SCTP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Specify number of attempts and/or timeout for the first o Specify number of attempts and/or timeout for the first
establishment message establishment message
Protocols: TCP, SCTP Protocols: TCP, SCTP
Functional because this is closely related to potentially assumed Functional because this is closely related to potentially assumed
reliable data delivery for data that is sent before or during reliable data delivery for data that is sent before or during
connection establishment. connection establishment.
Implementation: Using a parameter of CONNECT.TCP and CONNECT.SCTP. Implementation: Using a parameter of CONNECT.TCP and CONNECT.SCTP.
Implementation over UDP: Do nothing (this is irrelevant in case of Implementation over UDP: Do nothing (this is irrelevant in case of
UDP because there, reliable data delivery is not assumed). UDP because there, reliable data delivery is not assumed).
skipping to change at page 19, line 10 skipping to change at page 27, line 25
Implementation over TCP: not possible (TCP does not offer this Implementation over TCP: not possible (TCP does not offer this
functionality). functionality).
Implementation over UDP: not possible (UDP does not offer this Implementation over UDP: not possible (UDP does not offer this
functionality). functionality).
o Request to negotiate interleaving of user messages o Request to negotiate interleaving of user messages
Protocols: SCTP Protocols: SCTP
Automatable because it requires using multiple streams, but Automatable because it requires using multiple streams, but
requesting multiple streams in the CONNECTION.ESTABLISHMENT requesting multiple streams in the CONNECTION.ESTABLISHMENT
category is automatable. category is automatable.
Implementation: via a parameter in CONNECT.SCTP. Implementation: controlled via a parameter in CONNECT.SCTP. One
possible implementation is to always try to enable interleaving.
o Hand over a message to reliably transfer (possibly multiple times) o Hand over a message to reliably transfer (possibly multiple times)
before connection establishment before connection establishment
Protocols: TCP Protocols: TCP
Functional because this is closely tied to properties of the data Functional because this is closely tied to properties of the data
that an application sends or expects to receive. that an application sends or expects to receive.
Implementation: via a parameter in CONNECT.TCP. Implementation: via a parameter in CONNECT.TCP.
Implementation over UDP: not possible (UDP does not provide Implementation over UDP: not possible (UDP does not provide
reliability). reliability).
skipping to change at page 20, line 6 skipping to change at page 28, line 20
AVAILABILITY: AVAILABILITY:
o Listen o Listen
Protocols: TCP, SCTP, UDP(-Lite) Protocols: TCP, SCTP, UDP(-Lite)
Functional because the notion of accepting connection requests is Functional because the notion of accepting connection requests is
often reflected in applications as an expectation to be able to often reflected in applications as an expectation to be able to
communicate after a "Listen" succeeded, with a communication communicate after a "Listen" succeeded, with a communication
sequence relating to this transport feature that is defined by the sequence relating to this transport feature that is defined by the
application protocol. application protocol.
ADDED. This differs from the 3 automatable transport features CHANGED FROM RFC8303. This differs from the 3 automatable
below in that it leaves the choice of interfaces for listening transport features below in that it leaves the choice of
open. interfaces for listening open.
Implementation: by listening on all interfaces via LISTEN.TCP (not Implementation: by listening on all interfaces via LISTEN.TCP (not
providing a local IP address) or LISTEN.SCTP (providing SCTP port providing a local IP address) or LISTEN.SCTP (providing SCTP port
number / address pairs for all local IP addresses). LISTEN.UDP(- number / address pairs for all local IP addresses). LISTEN.UDP(-
Lite) supports both methods. Lite) supports both methods.
o Listen, 1 specified local interface o Listen, 1 specified local interface
Protocols: TCP, SCTP, UDP(-Lite) Protocols: TCP, SCTP, UDP(-Lite)
Automatable because decisions about local interfaces relate to Automatable because decisions about local interfaces relate to
knowledge about the network and the Operating System, not the knowledge about the network and the Operating System, not the
application. application.
skipping to change at page 21, line 25 skipping to change at page 29, line 39
should therefore only allow to authenticate all chunk types. Key should therefore only allow to authenticate all chunk types. Key
material must be provided in a way that is compatible with both material must be provided in a way that is compatible with both
[RFC4895] and [RFC5925]. [RFC4895] and [RFC5925].
Implementation over UDP: not possible (UDP does not offer Implementation over UDP: not possible (UDP does not offer
authentication). authentication).
o Obtain requested number of streams o Obtain requested number of streams
Protocols: SCTP Protocols: SCTP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Limit the number of inbound streams o Limit the number of inbound streams
Protocols: SCTP Protocols: SCTP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Indicate (and/or obtain upon completion) an Adaptation Layer via o Indicate (and/or obtain upon completion) an Adaptation Layer via
an adaptation code point an adaptation code point
Protocols: SCTP Protocols: SCTP
Functional because it allows to send extra data for the sake of Functional because it allows to send extra data for the sake of
identifying an adaptation layer, which by itself is application- identifying an adaptation layer, which by itself is application-
specific. specific.
Implementation: via a parameter in LISTEN.SCTP. Implementation: via a parameter in LISTEN.SCTP.
Implementation over TCP: not possible (TCP does not offer this Implementation over TCP: not possible (TCP does not offer this
functionality). functionality).
skipping to change at page 25, line 36 skipping to change at page 34, line 9
rnext_key from a previously received segment. Key material must rnext_key from a previously received segment. Key material must
be provided in a way that is compatible with both [RFC4895] and be provided in a way that is compatible with both [RFC4895] and
[RFC5925]. [RFC5925].
Implementation over UDP: not possible (UDP does not offer Implementation over UDP: not possible (UDP does not offer
authentication). authentication).
o Reset Stream o Reset Stream
Protocols: SCTP Protocols: SCTP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Notification of Stream Reset o Notification of Stream Reset
Protocols: STCP Protocols: STCP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Reset Association o Reset Association
Protocols: SCTP Protocols: SCTP
Automatable because deciding to reset an association does not Automatable because deciding to reset an association does not
require application-specific knowledge. require application-specific knowledge.
Implementation: via RESET_ASSOC.SCTP. Implementation: via RESET_ASSOC.SCTP.
o Notification of Association Reset o Notification of Association Reset
Protocols: STCP Protocols: STCP
Automatable because this notification does not relate to Automatable because this notification does not relate to
application-specific knowledge. application-specific knowledge.
o Add Streams o Add Streams
Protocols: SCTP Protocols: SCTP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Notification of Added Stream o Notification of Added Stream
Protocols: STCP Protocols: STCP
Automatable because using multi-streaming does not require Automatable because using multi-streaming does not require
application-specific knowledge. application-specific knowledge.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Choose a scheduler to operate between streams of an association o Choose a scheduler to operate between streams of an association
Protocols: SCTP Protocols: SCTP
Optimizing because the scheduling decision requires application- Optimizing because the scheduling decision requires application-
specific knowledge. However, if a transport system would not use specific knowledge. However, if a transport system would not use
this, or wrongly configure it on its own, this would only affect this, or wrongly configure it on its own, this would only affect
the performance of data transfers; the outcome would still be the performance of data transfers; the outcome would still be
correct within the "best effort" service model. correct within the "best effort" service model.
Implementation: using SET_STREAM_SCHEDULER.SCTP. Implementation: using SET_STREAM_SCHEDULER.SCTP.
Implementation over TCP: do nothing (streams are not available in Implementation over TCP: do nothing (streams are not available in
skipping to change at page 27, line 19 skipping to change at page 35, line 33
TCP, but no guarantee is given that this transport feature has any TCP, but no guarantee is given that this transport feature has any
effect). effect).
Implementation over UDP: do nothing (streams are not available in Implementation over UDP: do nothing (streams are not available in
UDP, but no guarantee is given that this transport feature has any UDP, but no guarantee is given that this transport feature has any
effect). effect).
o Configure send buffer size o Configure send buffer size
Protocols: SCTP Protocols: SCTP
Automatable because this decision relates to knowledge about the Automatable because this decision relates to knowledge about the
network and the Operating System, not the application (see also network and the Operating System, not the application (see also
the discussion in Appendix A.3.4). the discussion in Section 5.4).
o Configure receive buffer (and rwnd) size o Configure receive buffer (and rwnd) size
Protocols: SCTP Protocols: SCTP
Automatable because this decision relates to knowledge about the Automatable because this decision relates to knowledge about the
network and the Operating System, not the application. network and the Operating System, not the application.
o Configure message fragmentation o Configure message fragmentation
Protocols: SCTP Protocols: SCTP
Automatable because fragmentation relates to knowledge about the Automatable because this relates to knowledge about the network
network and the Operating System, not the application. and the Operating System, not the application. Note that this
SCTP feature does not control IP-level fragmentation, but decides
on fragmentation of messages by SCTP, in the end system.
Implementation: by always enabling it with Implementation: by always enabling it with
CONFIG_FRAGMENTATION.SCTP and auto-setting the fragmentation size CONFIG_FRAGMENTATION.SCTP and auto-setting the fragmentation size
based on network or Operating System conditions. based on network or Operating System conditions.
o Configure PMTUD o Configure PMTUD
Protocols: SCTP Protocols: SCTP
Automatable because Path MTU Discovery relates to knowledge about Automatable because Path MTU Discovery relates to knowledge about
the network, not the application. the network, not the application.
o Configure delayed SACK timer o Configure delayed SACK timer
skipping to change at page 29, line 5 skipping to change at page 37, line 17
Functional because this is closely tied to properties of the data Functional because this is closely tied to properties of the data
that an application sends or expects to receive. that an application sends or expects to receive.
Implementation over TCP: not possible (TCP does not offer Implementation over TCP: not possible (TCP does not offer
identification of message boundaries). identification of message boundaries).
Implementation over UDP: not possible (UDP does not fragment Implementation over UDP: not possible (UDP does not fragment
messages). messages).
o Disable checksum when sending o Disable checksum when sending
Protocols: UDP Protocols: UDP
Functional because application-specific knowledge is necessary to Functional because application-specific knowledge is necessary to
decide whether it can be acceptable to lose data integrity. decide whether it can be acceptable to lose data integrity with
respect to random corruption.
Implementation: via SET_CHECKSUM_ENABLED.UDP. Implementation: via SET_CHECKSUM_ENABLED.UDP.
Implementation over TCP: do nothing (TCP does not offer to disable Implementation over TCP: do nothing (TCP does not offer to disable
the checksum, but transmitting data with an intact checksum will the checksum, but transmitting data with an intact checksum will
not yield a semantically wrong result). not yield a semantically wrong result).
o Disable checksum requirement when receiving o Disable checksum requirement when receiving
Protocols: UDP Protocols: UDP
Functional because application-specific knowledge is necessary to Functional because application-specific knowledge is necessary to
decide whether it can be acceptable to lose data integrity. decide whether it can be acceptable to lose data integrity with
respect to random corruption.
Implementation: via SET_CHECKSUM_REQUIRED.UDP. Implementation: via SET_CHECKSUM_REQUIRED.UDP.
Implementation over TCP: do nothing (TCP does not offer to disable Implementation over TCP: do nothing (TCP does not offer to disable
the checksum, but transmitting data with an intact checksum will the checksum, but transmitting data with an intact checksum will
not yield a semantically wrong result). not yield a semantically wrong result).
o Specify checksum coverage used by the sender o Specify checksum coverage used by the sender
Protocols: UDP-Lite Protocols: UDP-Lite
Functional because application-specific knowledge is necessary to Functional because application-specific knowledge is necessary to
decide for which parts of the data it can be acceptable to lose decide for which parts of the data it can be acceptable to lose
data integrity. data integrity with respect to random corruption.
Implementation: via SET_CHECKSUM_COVERAGE.UDP-Lite. Implementation: via SET_CHECKSUM_COVERAGE.UDP-Lite.
Implementation over TCP: do nothing (TCP does not offer to limit Implementation over TCP: do nothing (TCP does not offer to limit
the checksum length, but transmitting data with an intact checksum the checksum length, but transmitting data with an intact checksum
will not yield a semantically wrong result). will not yield a semantically wrong result).
Implementation over UDP: if checksum coverage is set to cover Implementation over UDP: if checksum coverage is set to cover
payload data, do nothing. Else, either do nothing (transmitting payload data, do nothing. Else, either do nothing (transmitting
data with an intact checksum will not yield a semantically wrong data with an intact checksum will not yield a semantically wrong
result), or use the transport feature "Disable checksum when result), or use the transport feature "Disable checksum when
sending". sending".
o Specify minimum checksum coverage required by receiver o Specify minimum checksum coverage required by receiver
Protocols: UDP-Lite Protocols: UDP-Lite
Functional because application-specific knowledge is necessary to Functional because application-specific knowledge is necessary to
decide for which parts of the data it can be acceptable to lose decide for which parts of the data it can be acceptable to lose
data integrity. data integrity with respect to random corruption.
Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite. Implementation: via SET_MIN_CHECKSUM_COVERAGE.UDP-Lite.
Implementation over TCP: do nothing (TCP does not offer to limit Implementation over TCP: do nothing (TCP does not offer to limit
the checksum length, but transmitting data with an intact checksum the checksum length, but transmitting data with an intact checksum
will not yield a semantically wrong result). will not yield a semantically wrong result).
Implementation over UDP: if checksum coverage is set to cover Implementation over UDP: if checksum coverage is set to cover
payload data, do nothing. Else, either do nothing (transmitting payload data, do nothing. Else, either do nothing (transmitting
data with an intact checksum will not yield a semantically wrong data with an intact checksum will not yield a semantically wrong
result), or use the transport feature "Disable checksum result), or use the transport feature "Disable checksum
requirement when receiving". requirement when receiving".
skipping to change at page 31, line 32 skipping to change at page 40, line 8
network, not the application. network, not the application.
o Obtain IP Options o Obtain IP Options
Protocols: UDP(-Lite) Protocols: UDP(-Lite)
Automatable because IP Options relate to knowledge about the Automatable because IP Options relate to knowledge about the
network, not the application. network, not the application.
o Enable and configure a "Low Extra Delay Background Transfer" o Enable and configure a "Low Extra Delay Background Transfer"
Protocols: A protocol implementing the LEDBAT congestion control Protocols: A protocol implementing the LEDBAT congestion control
mechanism mechanism
Optimizing because whether this service is appropriate or not Optimizing because whether this feature is appropriate or not
depends on application-specific knowledge. However, wrongly using depends on application-specific knowledge. However, wrongly using
this will only affect the speed of data transfers (albeit this will only affect the speed of data transfers (albeit
including other transfers that may compete with the transport including other transfers that may compete with the transport
system's transfer in the network), so it is still correct within system's transfer in the network), so it is still correct within
the "best effort" service model. the "best effort" service model.
Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP / Implementation: via CONFIGURE.LEDBAT and/or SET_DSCP.TCP /
SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [LBE-draft]. SET_DSCP.SCTP / SET_DSCP.UDP(-Lite) [LBE-draft].
Implementation over TCP: do nothing (TCP does not support LEDBAT Implementation over TCP: do nothing (TCP does not support LEDBAT
congestion control, but not implementing this functionality will congestion control, but not implementing this functionality will
not yield a semantically wrong behavior). not yield a semantically wrong behavior).
skipping to change at page 33, line 11 skipping to change at page 41, line 29
timeout. timeout.
o Timeout event when data could not be delivered for too long o Timeout event when data could not be delivered for too long
Protocols: TCP, SCTP Protocols: TCP, SCTP
Functional because this notifies that potentially assumed reliable Functional because this notifies that potentially assumed reliable
data delivery is no longer provided. data delivery is no longer provided.
Implementation: via TIMEOUT.TCP and TIMEOUT.SCTP. Implementation: via TIMEOUT.TCP and TIMEOUT.SCTP.
Implementation over UDP: do nothing (this event will not occur Implementation over UDP: do nothing (this event will not occur
with UDP). with UDP).
A.1.2. DATA Transfer Related Transport Features A.2. DATA Transfer Related Transport Features
A.1.2.1. Sending Data A.2.1. Sending Data
o Reliably transfer data, with congestion control o Reliably transfer data, with congestion control
Protocols: TCP, SCTP Protocols: TCP, SCTP
Functional because this is closely tied to properties of the data Functional because this is closely tied to properties of the data
that an application sends or expects to receive. that an application sends or expects to receive.
Implementation: via SEND.TCP and SEND.SCTP. Implementation: via SEND.TCP and SEND.SCTP.
Implementation over UDP: not possible (UDP is unreliable). Implementation over UDP: not possible (UDP is unreliable).
o Reliably transfer a message, with congestion control o Reliably transfer a message, with congestion control
Protocols: SCTP Protocols: SCTP
skipping to change at page 33, line 38 skipping to change at page 42, line 17
boundaries will not be identifiable by the receiver, because TCP boundaries will not be identifiable by the receiver, because TCP
provides a byte stream service. provides a byte stream service.
Implementation over UDP: not possible (UDP is unreliable). Implementation over UDP: not possible (UDP is unreliable).
o Unreliably transfer a message o Unreliably transfer a message
Protocols: SCTP, UDP(-Lite) Protocols: SCTP, UDP(-Lite)
Optimizing because only applications know about the time Optimizing because only applications know about the time
criticality of their communication, and reliably transfering a criticality of their communication, and reliably transfering a
message is never incorrect for the receiver of a potentially message is never incorrect for the receiver of a potentially
unreliable data transfer, it is just slower. unreliable data transfer, it is just slower.
ADDED. This differs from the 2 automatable transport features CHANGED FROM RFC8303. This differs from the 2 automatable
below in that it leaves the choice of congestion control open. transport features below in that it leaves the choice of
congestion control open.
Implementation: via SEND.SCTP or SEND.UDP(-Lite). Implementation: via SEND.SCTP or SEND.UDP(-Lite).
Implementation over TCP: use SEND.TCP. With SEND.TCP, messages Implementation over TCP: use SEND.TCP. With SEND.TCP, messages
will be sent reliably, and message boundaries will not be will be sent reliably, and message boundaries will not be
identifiable by the receiver. identifiable by the receiver.
o Unreliably transfer a message, with congestion control o Unreliably transfer a message, with congestion control
Protocols: SCTP Protocols: SCTP
Automatable because congestion control relates to knowledge about Automatable because congestion control relates to knowledge about
the network, not the application. the network, not the application.
skipping to change at page 34, line 33 skipping to change at page 43, line 12
configuration: based on the assumption of the best-effort service configuration: based on the assumption of the best-effort service
model, unnecessarily delivering data does not violate application model, unnecessarily delivering data does not violate application
expectations. Moreover, it is not possible to associate the expectations. Moreover, it is not possible to associate the
requested reliability to a "message" in TCP anyway. requested reliability to a "message" in TCP anyway.
Implementation over UDP: not possible (UDP is unreliable). Implementation over UDP: not possible (UDP is unreliable).
o Choice of stream o Choice of stream
Protocols: SCTP Protocols: SCTP
Automatable because it requires using multiple streams, but Automatable because it requires using multiple streams, but
requesting multiple streams in the CONNECTION.ESTABLISHMENT requesting multiple streams in the CONNECTION.ESTABLISHMENT
category is automatable. Implementation: see Appendix A.3.2. category is automatable. Implementation: see Section 5.2.
o Choice of path (destination address) o Choice of path (destination address)
Protocols: SCTP Protocols: SCTP
Automatable because it requires using multiple sockets, but Automatable because it requires using multiple sockets, but
obtaining multiple sockets in the CONNECTION.ESTABLISHMENT obtaining multiple sockets in the CONNECTION.ESTABLISHMENT
category is automatable. category is automatable.
o Ordered message delivery (potentially slower than unordered) o Ordered message delivery (potentially slower than unordered)
Protocols: SCTP Protocols: SCTP
Functional because this is closely tied to properties of the data Functional because this is closely tied to properties of the data
skipping to change at page 36, line 27 skipping to change at page 45, line 4
authentication). authentication).
o Request not to delay the acknowledgement (SACK) of a message o Request not to delay the acknowledgement (SACK) of a message
Protocols: SCTP Protocols: SCTP
Optimizing because only an application knows for which message it Optimizing because only an application knows for which message it
wants to quickly be informed about success / failure of its wants to quickly be informed about success / failure of its
delivery. delivery.
Implementation over TCP: do nothing (TCP does not offer this Implementation over TCP: do nothing (TCP does not offer this
functionality, but ignoring this request from the application will functionality, but ignoring this request from the application will
not yield a semantically wrong behavior). not yield a semantically wrong behavior).
Implementation over UDP: do nothing (UDP does not offer this Implementation over UDP: do nothing (UDP does not offer this
functionality, but ignoring this request from the application will functionality, but ignoring this request from the application will
not yield a semantically wrong behavior). not yield a semantically wrong behavior).
A.1.2.2. Receiving Data A.2.2. Receiving Data
o Receive data (with no message delimiting) o Receive data (with no message delimiting)
Protocols: TCP Protocols: TCP
Functional because a transport system must be able to send and Functional because a transport system must be able to send and
receive data. receive data.
Implementation: via RECEIVE.TCP. Implementation: via RECEIVE.TCP.
Implementation over UDP: do nothing (UDP only works on messages; Implementation over UDP: do nothing (UDP only works on messages;
these can be handed over, the application can still ignore the these can be handed over, the application can still ignore the
message boundaries). message boundaries).
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that an application sends or expects to receive. that an application sends or expects to receive.
Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite). Implementation: via RECEIVE.SCTP and RECEIVE.UDP(-Lite).
Implementation over TCP: not possible (TCP does not support Implementation over TCP: not possible (TCP does not support
identification of message boundaries). identification of message boundaries).
o Choice of stream to receive from o Choice of stream to receive from
Protocols: SCTP Protocols: SCTP
Automatable because it requires using multiple streams, but Automatable because it requires using multiple streams, but
requesting multiple streams in the CONNECTION.ESTABLISHMENT requesting multiple streams in the CONNECTION.ESTABLISHMENT
category is automatable. category is automatable.
Implementation: see Appendix A.3.2. Implementation: see Section 5.2.
o Information about partial message arrival o Information about partial message arrival
Protocols: SCTP Protocols: SCTP
Functional because this is closely tied to properties of the data Functional because this is closely tied to properties of the data
that an application sends or expects to receive. that an application sends or expects to receive.
Implementation: via RECEIVE.SCTP. Implementation: via RECEIVE.SCTP.
Implementation over TCP: do nothing (this information is not Implementation over TCP: do nothing (this information is not
available with TCP). available with TCP).
Implementation over UDP: do nothing (this information is not Implementation over UDP: do nothing (this information is not
available with UDP). available with UDP).
A.1.2.3. Errors A.2.3. Errors
This section describes sending failures that are associated with a This section describes sending failures that are associated with a
specific call to in the "Sending Data" category (Appendix A.1.2.1). specific call to in the "Sending Data" category (Appendix A.2.1).
o Notification of send failures o Notification of send failures
Protocols: SCTP, UDP(-Lite) Protocols: SCTP, UDP(-Lite)
Functional because this notifies that potentially assumed reliable Functional because this notifies that potentially assumed reliable
data delivery is no longer provided. data delivery is no longer provided.
ADDED. This differs from the 2 automatable transport features CHANGED FROM RFC8303. This differs from the 2 automatable
below in that it does not distinugish between unsent and transport features below in that it does not distinugish between
unacknowledged messages. unsent and unacknowledged messages.
Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(- Implementation: via SENDFAILURE-EVENT.SCTP and SEND_FAILURE.UDP(-
Lite). Lite).
Implementation over TCP: do nothing (this notification is not Implementation over TCP: do nothing (this notification is not
available and will therefore not occur with TCP). available and will therefore not occur with TCP).
o Notification of an unsent (part of a) message o Notification of an unsent (part of a) message
Protocols: SCTP, UDP(-Lite) Protocols: SCTP, UDP(-Lite)
Automatable because the distinction between unsent and Automatable because the distinction between unsent and
unacknowledged is network-specific. unacknowledged does not relate to application-specific knowledge.
o Notification of an unacknowledged (part of a) message o Notification of an unacknowledged (part of a) message
Protocols: SCTP Protocols: SCTP
Automatable because the distinction between unsent and Automatable because the distinction between unsent and
unacknowledged is network-specific. unacknowledged does not relate to application-specific knowledge.
o Notification that the stack has no more user data to send o Notification that the stack has no more user data to send
Protocols: SCTP Protocols: SCTP
Optimizing because reacting to this notification requires the Optimizing because reacting to this notification requires the
application to be involved, and ensuring that the stack does not application to be involved, and ensuring that the stack does not
run dry of data (for too long) can improve performance. run dry of data (for too long) can improve performance.
Implementation over TCP: do nothing (see the discussion in Implementation over TCP: do nothing (see the discussion in
Appendix A.3.4). Section 5.4).
Implementation over UDP: do nothing (this notification is not Implementation over UDP: do nothing (this notification is not
available and will therefore not occur with UDP). available and will therefore not occur with UDP).
o Notification to a receiver that a partial message delivery has o Notification to a receiver that a partial message delivery has
been aborted been aborted
Protocols: SCTP Protocols: SCTP
Functional because this is closely tied to properties of the data Functional because this is closely tied to properties of the data
that an application sends or expects to receive. that an application sends or expects to receive.
Implementation over TCP: do nothing (this notification is not Implementation over TCP: do nothing (this notification is not
available and will therefore not occur with TCP). available and will therefore not occur with TCP).
Implementation over UDP: do nothing (this notification is not Implementation over UDP: do nothing (this notification is not
available and will therefore not occur with UDP). available and will therefore not occur with UDP).
A.2. Step 2: Reduction -- The Reduced Set of Transport Features
By hiding automatable transport features from the application, a
transport system can gain opportunities to automate the usage of
network-related functionality. This can facilitate using the
transport system for the application programmer and it allows for
optimizations that may not be possible for an application. For
instance, system-wide configurations regarding the usage of multiple
interfaces can better be exploited if the choice of the interface is
not entirely up to the application. Therefore, since they are not
strictly necessary to expose in a transport system, we do not include
automatable transport features in the reduced set of transport
features. This leaves us with only the transport features that are
either optimizing or functional.
A transport system should be able to communicate via TCP or UDP if
alternative transport protocols are found not to work. For many
transport features, this is possible -- often by simply not doing
anything when a specific request is made. For some transport
features, however, it was identified that direct usage of neither TCP
nor UDP is possible: in these cases, even not doing anything would
incur semantically incorrect behavior. Whenever an application would
make use of one of these transport features, this would eliminate the
possibility to use TCP or UDP. Thus, we only keep the functional and
optimizing transport features for which an implementation over either
TCP or UDP is possible in our reduced set.
The "minimal set" derived in this document is meant to be
implementable "one-sided" over TCP, and, with limitations, UDP. In
the following list, we therefore precede a transport feature with
"T:" if an implementation over TCP is possible, "U:" if an
implementation over UDP is possible, and "TU:" if an implementation
over either TCP or UDP is possible.
A.2.1. CONNECTION Related Transport Features
ESTABLISHMENT:
o T,U: Connect
o T,U: Specify number of attempts and/or timeout for the first
establishment message
o T: Configure authentication
o T: Hand over a message to reliably transfer (possibly multiple
times) before connection establishment
o T: Hand over a message to reliably transfer during connection
establishment
AVAILABILITY:
o T,U: Listen
o T: Configure authentication
MAINTENANCE:
o T: Change timeout for aborting connection (using retransmit limit
or time value)
o T: Suggest timeout to the peer
o T,U: Disable Nagle algorithm
o T,U: Notification of Excessive Retransmissions (early warning
below abortion threshold)
o T,U: Specify DSCP field
o T,U: Notification of ICMP error message arrival
o T: Change authentication parameters
o T: Obtain authentication information
o T,U: Set Cookie life value
o T,U: Choose a scheduler to operate between streams of an
association
o T,U: Configure priority or weight for a scheduler
o T,U: Disable checksum when sending
o T,U: Disable checksum requirement when receiving
o T,U: Specify checksum coverage used by the sender
o T,U: Specify minimum checksum coverage required by receiver
o T,U: Specify DF field
o T,U: Get max. transport-message size that may be sent using a non-
fragmented IP packet from the configured interface
o T,U: Get max. transport-message size that may be received from the
configured interface
o T,U: Obtain ECN field
o T,U: Enable and configure a "Low Extra Delay Background Transfer"
TERMINATION:
o T: Close after reliably delivering all remaining data, causing an
event informing the application on the other side
o T: Abort without delivering remaining data, causing an event
informing the application on the other side
o T,U: Abort without delivering remaining data, not causing an event
informing the application on the other side
o T,U: Timeout event when data could not be delivered for too long
A.2.2. DATA Transfer Related Transport Features
A.2.2.1. Sending Data
o T: Reliably transfer data, with congestion control
o T: Reliably transfer a message, with congestion control
o T,U: Unreliably transfer a message
o T: Configurable Message Reliability
o T: Ordered message delivery (potentially slower than unordered)
o T,U: Unordered message delivery (potentially faster than ordered)
o T,U: Request not to bundle messages
o T: Specifying a key id to be used to authenticate a message
o T,U: Request not to delay the acknowledgement (SACK) of a message
A.2.2.2. Receiving Data
o T,U: Receive data (with no message delimiting)
o U: Receive a message
o T,U: Information about partial message arrival
A.2.2.3. Errors
This section describes sending failures that are associated with a
specific call to in the "Sending Data" category (Appendix A.1.2.1).
o T,U: Notification of send failures
o T,U: Notification that the stack has no more user data to send
o T,U: Notification to a receiver that a partial message delivery
has been aborted
A.3. Step 3: Discussion
The reduced set in the previous section exhibits a number of
peculiarities, which we will discuss in the following. This section
focuses on TCP because, with the exception of one particular
transport feature ("Receive a message" -- we will discuss this in
Appendix A.3.1), the list shows that UDP is strictly a subset of TCP.
We can first try to understand how to build a transport system that
can run over TCP, and then narrow down the result further to allow
that the system can always run over either TCP or UDP (which
effectively means removing everything related to reliability,
ordering, authentication and closing/aborting with a notification to
the peer).
Note that, because the functional transport features of UDP are --
with the exception of "Receive a message" -- a subset of TCP, TCP can
be used as a replacement for UDP whenever an application does not
need message delimiting (e.g., because the application-layer protocol
already does it). This has been recognized by many applications that
already do this in practice, by trying to communicate with UDP at
first, and falling back to TCP in case of a connection failure.
A.3.1. Sending Messages, Receiving Bytes
For implementing a transport system over TCP, there are several
transport features related to sending, but only a single transport
feature related to receiving: "Receive data (with no message
delimiting)" (and, strangely, "information about partial message
arrival"). Notably, the transport feature "Receive a message" is
also the only non-automatable transport feature of UDP(-Lite) for
which no implementation over TCP is possible.
To support these TCP receiver semantics, we define an "Application-
Framed Bytestream" (AFra-Bytestream). AFra-Bytestreams allow senders
to operate on messages while minimizing changes to the TCP socket
API. In particular, nothing changes on the receiver side - data can
be accepted via a normal TCP socket.
In an AFra-Bytestream, the sending application can optionally inform
the transport about message boundaries and required properties per
message (configurable order and reliability, or embedding a request
not to delay the acknowledgement of a message). Whenever the sending
application specifies per-message properties that relax the notion of
reliable in-order delivery of bytes, it must assume that the
receiving application is 1) able to determine message boundaries,
provided that messages are always kept intact, and 2) able to accept
these relaxed per-message properties. Any signaling of such
information to the peer is up to an application-layer protocol and
considered out of scope of this document.
For example, if an application requests to transfer fixed-size
messages of 100 bytes with partial reliability, this needs the
receiving application to be prepared to accept data in chunks of 100
bytes. If, then, some of these 100-byte messages are missing (e.g.,
if SCTP with Configurable Reliability is used), this is the expected
application behavior. With TCP, no messages would be missing, but
this is also correct for the application, and the possible
retransmission delay is acceptable within the best effort service
model (see [RFC7305], Section 3.5). Still, the receiving application
would separate the byte stream into 100-byte chunks.
Note that this usage of messages does not require all messages to be
equal in size. Many application protocols use some form of Type-
Length-Value (TLV) encoding, e.g. by defining a header including
length fields; another alternative is the use of byte stuffing
methods such as COBS [COBS]. If an application needs message
numbers, e.g. to restore the correct sequence of messages, these must
also be encoded by the application itself, as the sequence number
related transport features of SCTP are not provided by the "minimum
set" (in the interest of enabling usage of TCP).
A.3.2. Stream Schedulers Without Streams
We have already stated that multi-streaming does not require
application-specific knowledge. Potential benefits or disadvantages
of, e.g., using two streams of an SCTP association versus using two
separate SCTP associations or TCP connections are related to
knowledge about the network and the particular transport protocol in
use, not the application. However, the transport features "Choose a
scheduler to operate between streams of an association" and
"Configure priority or weight for a scheduler" operate on streams.
Here, streams identify communication channels between which a
scheduler operates, and they can be assigned a priority. Moreover,
the transport features in the MAINTENANCE category all operate on
assocations in case of SCTP, i.e. they apply to all streams in that
assocation.
With only these semantics necessary to represent, the interface to a
transport system becomes easier if we assume that connections may be
a transport protocol's connection or association, but could also be a
stream of an existing SCTP association, for example. We only need to
allow for a way to define a possible grouping of connections. Then,
all MAINTENANCE transport features can be said to operate on
connection groups, not connections, and a scheduler operates on the
connections within a group.
To be compatible with multiple transport protocols and uniformly
allow access to both transport connections and streams of a multi-
streaming protocol, the semantics of opening and closing need to be
the most restrictive subset of all of the underlying options. For
example, TCP's support of half-closed connections can be seen as a
feature on top of the more restrictive "ABORT"; this feature cannot
be supported because not all protocols used by a transport system
(including streams of an association) support half-closed
connections.
A.3.3. Early Data Transmission
There are two transport features related to transferring a message
early: "Hand over a message to reliably transfer (possibly multiple
times) before connection establishment", which relates to TCP Fast
Open [RFC7413], and "Hand over a message to reliably transfer during
connection establishment", which relates to SCTP's ability to
transfer data together with the COOKIE-Echo chunk. Also without TCP
Fast Open, TCP can transfer data during the handshake, together with
the SYN packet -- however, the receiver of this data may not hand it
over to the application until the handshake has completed. Also,
different from TCP Fast Open, this data is not delimited as a message
by TCP (thus, not visible as a ``message''). This functionality is
commonly available in TCP and supported in several implementations,
even though the TCP specification does not explain how to provide it
to applications.
A transport system could differentiate between the cases of
transmitting data "before" (possibly multiple times) or "during" the
handshake. Alternatively, it could also assume that data that are
handed over early will be transmitted as early as possible, and
"before" the handshake would only be used for messages that are
explicitly marked as "idempotent" (i.e., it would be acceptable to
transfer them multiple times).
The amount of data that can successfully be transmitted before or
during the handshake depends on various factors: the transport
protocol, the use of header options, the choice of IPv4 and IPv6 and
the Path MTU. A transport system should therefore allow a sending
application to query the maximum amount of data it can possibly
transmit before (or, if exposed, during) connection establishment.
A.3.4. Sender Running Dry
The transport feature "Notification that the stack has no more user
data to send" relates to SCTP's "SENDER DRY" notification. Such
notifications can, in principle, be used to avoid having an
unnecessarily large send buffer, yet ensure that the transport sender
always has data available when it has an opportunity to transmit it.
This has been found to be very beneficial for some applications
[WWDC2015]. However, "SENDER DRY" truly means that the entire send
buffer (including both unsent and unacknowledged data) has emptied --
i.e., when it notifies the sender, it is already too late, the
transport protocol already missed an opportunity to send data. Some
modern TCP implementations now include the unspecified
"TCP_NOTSENT_LOWAT" socket option that was proposed in [WWDC2015],
which limits the amount of unsent data that TCP can keep in the
socket buffer; this allows to specify at which buffer filling level
the socket becomes writable, rather than waiting for the buffer to
run empty.
SCTP allows to configure the sender-side buffer too: the automatable
Transport Feature "Configure send buffer size" provides this
functionality, but only for the complete buffer, which includes both
unsent and unacknowledged data. SCTP does not allow to control these
two sizes separately. It therefore makes sense for a transport
system to allow for uniform access to "TCP_NOTSENT_LOWAT" as well as
the "SENDER DRY" notification.
A.3.5. Capacity Profile
The transport features:
o Disable Nagle algorithm
o Enable and configure a "Low Extra Delay Background Transfer"
o Specify DSCP field
all relate to a QoS-like application need such as "low latency" or
"scavenger". In the interest of flexibility of a transport system,
they could therefore be offered in a uniform, more abstract way,
where a transport system could e.g. decide by itself how to use
combinations of LEDBAT-like congestion control and certain DSCP
values, and an application would only specify a general "capacity
profile" (a description of how it wants to use the available
capacity). A need for "lowest possible latency at the expense of
overhead" could then translate into automatically disabling the Nagle
algorithm.
In some cases, the Nagle algorithm is best controlled directly by the
application because it is not only related to a general profile but
also to knowledge about the size of future messages. For fine-grain
control over Nagle-like functionality, the "Request not to bundle
messages" is available.
A.3.6. Security
Both TCP and SCTP offer authentication. TCP authenticates complete
segments. SCTP allows to configure which of SCTP's chunk types must
always be authenticated -- if this is exposed as such, it creates an
undesirable dependency on the transport protocol. For compatibility
with TCP, a transport system should only allow to configure complete
transport layer packets, including headers, IP pseudo-header (if any)
and payload.
Security is discussed in a separate document
[I-D.ietf-taps-transport-security]. The minimal set presented in the
present document excludes all security related transport features:
"Configure authentication", "Change authentication parameters",
"Obtain authentication information" and and "Set Cookie life value"
as well as "Specifying a key id to be used to authenticate a
message".
A.3.7. Packet Size
UDP(-Lite) has a transport feature called "Specify DF field". This
yields an error message in case of sending a message that exceeds the
Path MTU, which is necessary for a UDP-based application to be able
to implement Path MTU Discovery (a function that UDP-based
applications must do by themselves). The "Get max. transport-message
size that may be sent using a non-fragmented IP packet from the
configured interface" transport feature yields an upper limit for the
Path MTU (minus headers) and can therefore help to implement Path MTU
Discovery more efficiently.
Appendix B. Revision information Appendix B. Revision information
XXX RFC-Ed please remove this section prior to publication. XXX RFC-Ed please remove this section prior to publication.
-02: implementation suggestions added, discussion section added, -02: implementation suggestions added, discussion section added,
terminology extended, DELETED category removed, various other fixes; terminology extended, DELETED category removed, various other fixes;
list of Transport Features adjusted to -01 version of [RFC8303] list of Transport Features adjusted to -01 version of [RFC8303]
except that MPTCP is not included. except that MPTCP is not included.
-03: updated to be consistent with -02 version of [RFC8303]. -03: updated to be consistent with -02 version of [RFC8303].
skipping to change at page 47, line 32 skipping to change at page 48, line 42
..). ..).
WG -05: addressed comments from Spencer Dawkins. WG -05: addressed comments from Spencer Dawkins.
WG -06: Fixed nits. WG -06: Fixed nits.
WG -07: Addressed Genart comments from Robert Sparks. WG -07: Addressed Genart comments from Robert Sparks.
WG -08: Addressed one more Genart comment from Robert Sparks. WG -08: Addressed one more Genart comment from Robert Sparks.
Authors' Addresses WG -09: Addressed comments from Mirja Kuehlewind, Alvaro Retana, Ben
Campbell, Benjamin Kaduk and Eric Rescorla.
Authors' Addresses
Michael Welzl Michael Welzl
University of Oslo University of Oslo
PO Box 1080 Blindern PO Box 1080 Blindern
Oslo N-0316 Oslo N-0316
Norway Norway
Phone: +47 22 85 24 20 Phone: +47 22 85 24 20
Email: michawe@ifi.uio.no Email: michawe@ifi.uio.no
Stein Gjessing Stein Gjessing
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