draft-ietf-tsvwg-udp-guidelines-06.txt   draft-ietf-tsvwg-udp-guidelines-07.txt 
Transport Area Working Group L. Eggert Transport Area Working Group L. Eggert
Internet-Draft Nokia Internet-Draft Nokia
Intended status: BCP G. Fairhurst Intended status: BCP G. Fairhurst
Expires: October 5, 2008 University of Aberdeen Expires: November 22, 2008 University of Aberdeen
April 3, 2008 May 21, 2008
UDP Usage Guidelines for Application Designers Guidelines for Application Designers on Using Unicast UDP
draft-ietf-tsvwg-udp-guidelines-06 draft-ietf-tsvwg-udp-guidelines-07
Status of this Memo Status of this Memo
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aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
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This Internet-Draft will expire on October 5, 2008. This Internet-Draft will expire on November 22, 2008.
Abstract Abstract
The User Datagram Protocol (UDP) provides a minimal, message-passing The User Datagram Protocol (UDP) provides a minimal, message-passing
transport that has no inherent congestion control mechanisms. transport that has no inherent congestion control mechanisms.
Because congestion control is critical to the stable operation of the Because congestion control is critical to the stable operation of the
Internet, applications and upper-layer protocols that choose to use Internet, applications and upper-layer protocols that choose to use
UDP as an Internet transport must employ mechanisms to prevent UDP as an Internet transport must employ mechanisms to prevent
congestion collapse and establish some degree of fairness with congestion collapse and establish some degree of fairness with
concurrent traffic. This document provides guidelines on the use of concurrent traffic. This document provides guidelines on the use of
UDP for the designers of such applications and upper-layer protocols. UDP for the designers of unicast applications and upper-layer
Congestion control guidelines are a primary focus, but the document protocols. Congestion control guidelines are a primary focus, but
also provides guidance on other topics, including message sizes, the document also provides guidance on other topics, including
reliability, checksums and middlebox traversal. message sizes, reliability, checksums and middlebox traversal.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. UDP Usage Guidelines . . . . . . . . . . . . . . . . . . . . . 4 3. UDP Usage Guidelines . . . . . . . . . . . . . . . . . . . . . 5
3.1. Congestion Control Guidelines . . . . . . . . . . . . . . 5 3.1. Congestion Control Guidelines . . . . . . . . . . . . . . 6
3.2. Message Size Guidelines . . . . . . . . . . . . . . . . . 10 3.2. Message Size Guidelines . . . . . . . . . . . . . . . . . 11
3.3. Reliability Guidelines . . . . . . . . . . . . . . . . . . 11 3.3. Reliability Guidelines . . . . . . . . . . . . . . . . . . 12
3.4. Checksum Guidelines . . . . . . . . . . . . . . . . . . . 12 3.4. Checksum Guidelines . . . . . . . . . . . . . . . . . . . 13
3.5. Middlebox Traversal Guidelines . . . . . . . . . . . . . . 13 3.5. Middlebox Traversal Guidelines . . . . . . . . . . . . . . 14
3.6. Programming Guidelines . . . . . . . . . . . . . . . . . . 15 3.6. Programming Guidelines . . . . . . . . . . . . . . . . . . 16
3.7. ICMP Guidelines . . . . . . . . . . . . . . . . . . . . . 16 3.7. ICMP Guidelines . . . . . . . . . . . . . . . . . . . . . 18
4. Security Considerations . . . . . . . . . . . . . . . . . . . 17 4. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 20 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 21 8.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . . . 26 Intellectual Property and Copyright Statements . . . . . . . . . . 27
1. Introduction 1. Introduction
The User Datagram Protocol (UDP) [RFC0768] provides a minimal, The User Datagram Protocol (UDP) [RFC0768] provides a minimal,
unreliable, best-effort, message-passing transport to applications unreliable, best-effort, message-passing transport to applications
and upper-layer protocols (both simply called "applications" in the and upper-layer protocols (both simply called "applications" in the
remainder of this document). Compared to other transport protocols, remainder of this document). Compared to other transport protocols,
UDP and its UDP-Lite variant [RFC3828] are unique in that they do not UDP and its UDP-Lite variant [RFC3828] are unique in that they do not
establish end-to-end connections between communicating end systems. establish end-to-end connections between communicating end systems.
UDP communication consequently does not incur connection UDP communication consequently does not incur connection
establishment and teardown overheads and there is minimal associated establishment and teardown overheads and there is minimal associated
end system state. Because of these characteristics, UDP can offer a end system state. Because of these characteristics, UDP can offer a
very efficient communication transport to some applications. very efficient communication transport to some applications.
A second unique characteristic of UDP is that it provides no inherent A second unique characteristic of UDP is that it provides no inherent
congestion control mechanisms. On many platforms, applications can congestion control mechanisms. On many platforms, applications can
send UDP messages at the line rate of the link interface, which is send UDP datagrams at the line rate of the link interface, which is
often much greater than the available path capacity, and doing so often much greater than the available path capacity, and doing so
contributes to congestion along the path. [RFC2914] describes the contributes to congestion along the path. [RFC2914] describes the
best current practice for congestion control in the Internet. It best current practice for congestion control in the Internet. It
identifies two major reasons why congestion control mechanisms are identifies two major reasons why congestion control mechanisms are
critical for the stable operation of the Internet: critical for the stable operation of the Internet:
1. The prevention of congestion collapse, i.e., a state where an 1. The prevention of congestion collapse, i.e., a state where an
increase in network load results in a decrease in useful work increase in network load results in a decrease in useful work
done by the network. done by the network.
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multiple flows to share the capacity of a path reasonably multiple flows to share the capacity of a path reasonably
equitably. equitably.
Because UDP itself provides no congestion control mechanisms, it is Because UDP itself provides no congestion control mechanisms, it is
up to the applications that use UDP for Internet communication to up to the applications that use UDP for Internet communication to
employ suitable mechanisms to prevent congestion collapse and employ suitable mechanisms to prevent congestion collapse and
establish a degree of fairness. [RFC2309] discusses the dangers of establish a degree of fairness. [RFC2309] discusses the dangers of
congestion-unresponsive flows and states that "all UDP-based congestion-unresponsive flows and states that "all UDP-based
streaming applications should incorporate effective congestion streaming applications should incorporate effective congestion
avoidance mechanisms." This is an important requirement, even for avoidance mechanisms." This is an important requirement, even for
applications that do not use UDP for streaming. For example, an applications that do not use UDP for streaming. In addition,
application that generates five 1500-byte UDP messages in one second congestion-controlled transmission is of benefit to an application
can already exceed the capacity of a 56 Kb/s path. For applications itself, because it can reduce self-induced packet loss, minimize
that can operate at higher, potentially unbounded data rates, retransmissions and hence reduce delays. Congestion control is
congestion control becomes vital to prevent congestion collapse and essential even at relatively slow transmission rates. For example,
establish some degree of fairness. Section 3 describes a number of an application that generates five 1500-byte UDP datagrams in one
simple guidelines for the designers of such applications. second can already exceed the capacity of a 56 Kb/s path. For
applications that can operate at higher, potentially unbounded data
rates, congestion control becomes vital to prevent congestion
collapse and establish some degree of fairness. Section 3 describes
a number of simple guidelines for the designers of such applications.
A UDP message is carried in a single IP packet and is hence limited A UDP datagram is carried in a single IP packet and is hence limited
to a maximum payload of 65,507 bytes for IPv4 and 65,527 bytes for to a maximum payload of 65,507 bytes for IPv4 and 65,527 bytes for
IPv6. The transmission of large IP packets usually requires IP IPv6. The transmission of large IP packets usually requires IP
fragmentation. Fragmentation decreases communication reliability and fragmentation. Fragmentation decreases communication reliability and
efficiency and should be avoided. IPv6 allows the option of efficiency and should be avoided. IPv6 allows the option of
transmitting large packets ("jumbograms") without fragmentation when transmitting large packets ("jumbograms") without fragmentation when
all link layers along the path support this [RFC2675]. Some of the all link layers along the path support this [RFC2675]. Some of the
guidelines in Section 3 describe how applications should determine guidelines in Section 3 describe how applications should determine
appropriate message sizes. appropriate message sizes. Other sections of this document provide
guidance on reliability, checksums and middlebox traversal.
This document provides guidelines and recommendations. Although most
unicast UDP applications are expected to follow these guidelines,
there do exist valid reasons why a specific application may decide
not to follow a given guideline. In such cases, it is RECOMMENDED
that the application designers document the rationale for their
design choice in the technical specification of their application or
protocol.
This document provides guidelines to designers of applications that This document provides guidelines to designers of applications that
use UDP for unicast transmission. A special class of applications use UDP for unicast transmission, which is the most common case.
uses UDP for IP multicast transmissions. Congestion control, flow Specialized classed of applications use UDP for IP multicast
control or reliability for multicast transmissions is more difficult [RFC1112], broadcast [RFC0919], or anycast [RFC1546] transmissions.
to establish than for unicast transmissions, because a single sender The design of such specialized applications requires expertise that
may transmit to multiple receivers across potentially very goes beyond the simple, unicast-specific guidelines given in this
heterogeneous paths at the same time. Designing multicast document. Multicast and broadcast senders may transmit to multiple
applications requires expertise that goes beyond the simple receivers across potentially very heterogeneous paths at the same
guidelines given in this document. The IETF has defined a reliable time, which significantly complicates congestion control, flow
control and reliability mechanisms. The IETF has defined a reliable
multicast framework [RFC3048] and several building blocks to aid the multicast framework [RFC3048] and several building blocks to aid the
designers of multicast applications, such as [RFC3738] or [RFC4654]. designers of multicast applications, such as [RFC3738] or [RFC4654].
Anycast senders must be aware that successive messages sent to the
same anycast address may be delivered to different underlying IP
addresses, i.e., arrive at different locations in the topology. It
is not intended that the guidelines in this document apply to
multicast, broadcast or anycast applications that use UDP.
Finally, although this document specifically refers to unicast
applications that use UDP, the spirit of some of its guidelines also
applies to other message-passing applications and protocols
(specifically on the topics of congestion control, message sizes and
reliability). Examples include signaling or control applications
that choose to run directly over IP by registering their own IP
protocol number with IANA. This document may provide useful
background reading to the designers of such applications and
protocols.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119]. [RFC2119].
3. UDP Usage Guidelines 3. UDP Usage Guidelines
Internet paths can have widely varying characteristics, including Internet paths can have widely varying characteristics, including
transmission delays, available bandwidths, congestion levels, transmission delays, available bandwidths, congestion levels,
reordering probabilities, supported message sizes or loss rates. reordering probabilities, supported message sizes or loss rates.
Furthermore, the same Internet path can have very different Furthermore, the same Internet path can have very different
conditions over time. Consequently, applications that may be used on conditions over time. Consequently, applications that may be used on
the Internet MUST NOT make assumptions about specific path the Internet MUST NOT make assumptions about specific path
characteristics. They MUST instead use mechanisms that let them characteristics. They MUST instead use mechanisms that let them
operate safely under very different path conditions. Typically, this operate safely under very different path conditions. Typically, this
requires conservatively probing the Internet path to establish a requires conservatively probing the current conditions of the
transmission behavior that it can sustain and that is reasonably fair Internet path they communicate over to establish a transmission
to other traffic sharing the path. behavior that it can sustain and that is reasonably fair to other
traffic sharing the path.
These mechanisms are difficult to implement correctly. For most These mechanisms are difficult to implement correctly. For most
applications, the use of one of the existing IETF transport protocols applications, the use of one of the existing IETF transport protocols
is the simplest method of acquiring the required mechanisms. is the simplest method of acquiring the required mechanisms.
Consequently, the RECOMMENDED alternative to the UDP usage described Consequently, the RECOMMENDED alternative to the UDP usage described
in the remainder of this section is the use of an IETF transport in the remainder of this section is the use of an IETF transport
protocol such as TCP [RFC0793], SCTP [RFC4960] and SCTP-PR [RFC3758], protocol such as TCP [RFC0793], SCTP [RFC4960] and SCTP-PR [RFC3758],
or DCCP [RFC4340] with its different congestion control types or DCCP [RFC4340] with its different congestion control types
[RFC4341][RFC4342][I-D.ietf-dccp-ccid4]. [RFC4341][RFC4342][I-D.ietf-dccp-ccid4].
If used correctly, these more fully-featured transport protocols are If used correctly, these more fully-featured transport protocols are
not as "heavyweight" as often claimed. For example, TCP's "Nagle" not as "heavyweight" as often claimed. For example, the TCP
algorithm [RFC0896] can be disabled, improving communication latency algorithms have been continuously improved over decades, and have
at the expense of more frequent - but still congestion-controlled - reached a level of efficiency and correctness that custom
packet transmissions. Another example is the TCP SYN Cookie application-layer mechanisms will struggle to easily duplicate. In
mechanism [RFC4987], which is available on many platforms. TCP with addition, many TCP implementations allow connections to be tuned by
SYN Cookies does not require a server to maintain per-connection an application to its purposes. For example, TCP's "Nagle" algorithm
state until the connection is established. TCP also requires the end [RFC0896] can be disabled, improving communication latency at the
that closes a connection to maintain the TIME-WAIT state that expense of more frequent - but still congestion-controlled - packet
prevents delayed segments from one connection instance to interfere transmissions. Another example is the TCP SYN Cookie mechanism
with a later one. Applications that are aware of and designed for [RFC4987], which is available on many platforms. TCP with SYN
this behavior can shift maintenance of the TIME-WAIT state to Cookies does not require a server to maintain per-connection state
conserve resources by controlling which end closes a TCP connection until the connection is established. TCP also requires the end that
[FABER]. Finally, TCP's built-in capacity-probing and awareness of closes a connection to maintain the TIME-WAIT state that prevents
the maximum transmission unit supported by the path (PMTU) results in delayed segments from one connection instance to interfere with a
efficient data transmission that quickly compensates for the initial later one. Applications that are aware of and designed for this
connection setup delay, for transfers that exchange more than a few behavior can shift maintenance of the TIME-WAIT state to conserve
messages. resources by controlling which end closes a TCP connection [FABER].
Finally, TCP's built-in capacity-probing and awareness of the maximum
transmission unit supported by the path (PMTU) results in efficient
data transmission that quickly compensates for the initial connection
setup delay, for transfers that exchange more than a few segments.
3.1. Congestion Control Guidelines 3.1. Congestion Control Guidelines
If an application or upper-layer protocol chooses not to use a If an application or upper-layer protocol chooses not to use a
congestion-controlled transport protocol, it SHOULD control the rate congestion-controlled transport protocol, it SHOULD control the rate
at which it sends UDP messages to a destination host, in order to at which it sends UDP datagrams to a destination host, in order to
fulfill the requirements of [RFC2914]. It is important to stress fulfill the requirements of [RFC2914]. It is important to stress
that an application SHOULD perform congestion control over all UDP that an application SHOULD perform congestion control over all UDP
traffic it sends to a destination, independently from how it traffic it sends to a destination, independently from how it
generates this traffic. For example, an application that forks generates this traffic. For example, an application that forks
multiple worker processes or otherwise uses multiple sockets to multiple worker processes or otherwise uses multiple sockets to
generate UDP messages SHOULD perform congestion control over the generate UDP datagrams SHOULD perform congestion control over the
aggregate traffic. aggregate traffic.
The remainder of this section discusses several approaches for this The remainder of this section discusses several approaches for this
purpose. Not all approaches discussed below are appropriate for all purpose. Not all approaches discussed below are appropriate for all
UDP-transmitting applications. Section 3.1.1 discusses congestion UDP-transmitting applications. Section 3.1.1 discusses congestion
control options for applications that perform bulk transfers over control options for applications that perform bulk transfers over
UDP. Such applications can employ schemes that sample the path over UDP. Such applications can employ schemes that sample the path over
several subsequent RTTs during which data is exchanged, in order to several subsequent RTTs during which data is exchanged, in order to
determine a sending rate that the path at its current load can determine a sending rate that the path at its current load can
support. Other applications only exchange a few UDP messages with a support. Other applications only exchange a few UDP datagrams with a
destination. Section 3.1.2 discusses congestion control options for destination. Section 3.1.2 discusses congestion control options for
such "low data-volume" applications. Because they typically do not such "low data-volume" applications. Because they typically do not
transmit enough data to iteratively sample the path to determine a transmit enough data to iteratively sample the path to determine a
safe sending rate, they need to employ different kinds of congestion safe sending rate, they need to employ different kinds of congestion
control mechanisms. Finally, Section 3.1.3 discusses congestion control mechanisms. Section 3.1.3 discusses congestion control
control considerations when UDP is used as a tunneling protocol. considerations when UDP is used as a tunneling protocol.
It is important to note that congestion control should not be viewed It is important to note that congestion control should not be viewed
as an add-on to a finished application. Many of the mechanisms as an add-on to a finished application. Many of the mechanisms
discussed in the guidelines below require application support to discussed in the guidelines below require application support to
operate correctly. Application designers need to consider congestion operate correctly. Application designers need to consider congestion
control throughout the design of their application, similar to how control throughout the design of their application, similar to how
they consider security aspects throughout the design process. they consider security aspects throughout the design process.
Finally, in the past, the IETF has investigated integrated congestion In the past, the IETF has also investigated integrated congestion
control mechanisms that act on the traffic aggregate between two control mechanisms that act on the traffic aggregate between two
hosts, i.e., across all communication sessions active at a given time hosts, i.e., across all communication sessions active at a given time
independent of specific transport protocols, such as the Congestion independent of specific transport protocols, such as the Congestion
Manager [RFC3124]. Such mechanisms have failed to see deployment, Manager [RFC3124]. Such mechanisms have failed to see deployment,
but would otherwise also fulfill the congestion control requirements but would otherwise also fulfill the congestion control requirements
in [RFC2914], because they provide congestion control for UDP in [RFC2914], because they provide congestion control for UDP
sessions. sessions.
3.1.1. Bulk Transfer Applications 3.1.1. Bulk Transfer Applications
Applications that perform bulk transmission of data to a peer over Applications that perform bulk transmission of data to a peer over
UDP, i.e., applications that exchange more than a small number of UDP, i.e., applications that exchange more than a small number of UDP
messages per RTT, SHOULD implement TCP-Friendly Rate Control (TFRC) datagrams per RTT, SHOULD implement TCP-Friendly Rate Control (TFRC)
[RFC3448], window-based, TCP-like congestion control, or otherwise [RFC3448], window-based, TCP-like congestion control, or otherwise
ensure that the application complies with the congestion control ensure that the application complies with the congestion control
principles. principles.
TFRC has been designed to provide both congestion control and TFRC has been designed to provide both congestion control and
fairness in a way that is compatible with the IETF's other transport fairness in a way that is compatible with the IETF's other transport
protocols. TFRC is currently being updated protocols. TFRC is currently being updated
[I-D.ietf-dccp-rfc3448bis], and application designers SHOULD always [I-D.ietf-dccp-rfc3448bis], and application designers SHOULD always
evaluate whether the latest published specification fits their needs. evaluate whether the latest published specification fits their needs.
If an application implements TFRC, it need not follow the remaining If an application implements TFRC, it need not follow the remaining
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results in bandwidth use that competes fairly with TCP within an results in bandwidth use that competes fairly with TCP within an
order of magnitude. [RFC3551] suggests that applications SHOULD order of magnitude. [RFC3551] suggests that applications SHOULD
monitor the packet loss rate to ensure that it is within acceptable monitor the packet loss rate to ensure that it is within acceptable
parameters. Packet loss is considered acceptable if a TCP flow parameters. Packet loss is considered acceptable if a TCP flow
across the same network path under the same network conditions would across the same network path under the same network conditions would
achieve an average throughput, measured on a reasonable timescale, achieve an average throughput, measured on a reasonable timescale,
that is not less than that of the UDP flow. The comparison to TCP that is not less than that of the UDP flow. The comparison to TCP
cannot be specified exactly, but is intended as an "order-of- cannot be specified exactly, but is intended as an "order-of-
magnitude" comparison in timescale and throughput. magnitude" comparison in timescale and throughput.
Finally, some bulk transfer applications chose not to implement any Finally, some bulk transfer applications may choose not to implement
congestion control mechanism and instead rely on transmitting across any congestion control mechanism and instead rely on transmitting
reserved path capacity. This might be an acceptable choice for a across reserved path capacity. This might be an acceptable choice
subset of restricted networking environments, but is by no means a for a subset of restricted networking environments, but is by no
safe practice for operation in the Internet. When the UDP traffic of means a safe practice for operation in the Internet. When the UDP
such applications leaks out on unprovisioned Internet paths, it can traffic of such applications leaks out on unprovisioned Internet
significantly degrade the performance of other traffic sharing the paths, it can significantly degrade the performance of other traffic
path and even result in congestion collapse. Applications that sharing the path and even result in congestion collapse.
support an uncontrolled or unadaptive transmission behavior SHOULD Applications that support an uncontrolled or unadaptive transmission
NOT do so by default and SHOULD instead require users to explicitly behavior SHOULD NOT do so by default and SHOULD instead require users
enable this mode of operation. to explicitly enable this mode of operation.
3.1.2. Low Data-Volume Applications 3.1.2. Low Data-Volume Applications
When applications that exchange only a small number of messages with When applications that exchange only a small number of UDP datagrams
a destination at any time implement TFRC or one of the other with a destination at any time implement TFRC or one of the other
congestion control schemes in Section 3.1.1, the network sees little congestion control schemes in Section 3.1.1, the network sees little
benefit, because those mechanisms perform congestion control in a way benefit, because those mechanisms perform congestion control in a way
that is only effective for longer transmissions. that is only effective for longer transmissions.
Applications that exchange only a small number of messages with a Applications that exchange only a small number of UDP datagrams with
destination at any time SHOULD still control their transmission a destination at any time SHOULD still control their transmission
behavior by not sending on average more than one UDP message per behavior by not sending on average more than one UDP datagram per
round-trip time(RTT) to a destination. Similar to the recommendation round-trip time (RTT) to a destination. Similar to the
in [RFC1536], an application SHOULD maintain an estimate of the RTT recommendation in [RFC1536], an application SHOULD maintain an
for any destination with which it communicates. Applications SHOULD estimate of the RTT for any destination with which it communicates.
implement the algorithm specified in [RFC2988] to compute a smoothed Applications SHOULD implement the algorithm specified in [RFC2988] to
RTT (SRTT) estimate. They SHOULD also detect packet loss and compute a smoothed RTT (SRTT) estimate. They SHOULD also detect
exponentially back-off their retransmission timer when a loss event packet loss and exponentially back-off their retransmission timer
occurs. When implementing this scheme, applications need to choose a when a loss event occurs. When implementing this scheme,
sensible initial value for the RTT. This value SHOULD generally be applications need to choose a sensible initial value for the RTT.
as conservative as possible for the given application. TCP uses an This value SHOULD generally be as conservative as possible for the
initial value of 3 seconds [RFC2988], which is also RECOMMENDED as an given application. TCP uses an initial value of 3 seconds [RFC2988],
initial value for UDP applications. SIP [RFC3261] and GIST which is also RECOMMENDED as an initial value for UDP applications.
[I-D.ietf-nsis-ntlp] use an initial value of 500 ms, and initial SIP [RFC3261] and GIST [I-D.ietf-nsis-ntlp] use an initial value of
timeouts that are shorter than this are likely problematic in many 500 ms, and initial timeouts that are shorter than this are likely
cases. It is also important to note that the initial timeout is not problematic in many cases. It is also important to note that the
the maximum possible timeout - the RECOMMENDED algorithm in [RFC2988] initial timeout is not the maximum possible timeout - the RECOMMENDED
yields timeout values after a series of losses that are much longer algorithm in [RFC2988] yields timeout values after a series of losses
than the initial value. that are much longer than the initial value.
Some applications cannot maintain a reliable RTT estimate for a Some applications cannot maintain a reliable RTT estimate for a
destination. The first case is applications that exchange too few destination. The first case is applications that exchange too few
messages with a peer to establish a statistically accurate RTT UDP datagrams with a peer to establish a statistically accurate RTT
estimate. Such applications MAY use a pre-determined transmission estimate. Such applications MAY use a pre-determined transmission
interval that is exponentially backed-off when packets are lost. TCP interval that is exponentially backed-off when packets are lost. TCP
uses an initial value of 3 seconds [RFC2988], which is also uses an initial value of 3 seconds [RFC2988], which is also
RECOMMENDED as an initial value for UDP applications. SIP [RFC3261] RECOMMENDED as an initial value for UDP applications. SIP [RFC3261]
and GIST [I-D.ietf-nsis-ntlp] use an interval of 500 ms, and shorter and GIST [I-D.ietf-nsis-ntlp] use an interval of 500 ms, and shorter
values are likely problematic in many cases. As in the previous values are likely problematic in many cases. As in the previous
case, note that the initial timeout is not the maximum possible case, note that the initial timeout is not the maximum possible
timeout. timeout.
A second class of applications cannot maintain an RTT estimate for a A second class of applications cannot maintain an RTT estimate for a
destination, because the destination does not send return traffic. destination, because the destination does not send return traffic.
Such applications SHOULD NOT send more than one UDP message every 3 Such applications SHOULD NOT send more than one UDP datagram every 3
seconds, and SHOULD use an even less aggressive rate when possible. seconds, and SHOULD use an even less aggressive rate when possible.
The 3-second interval was chosen based on TCP's retransmission The 3-second interval was chosen based on TCP's retransmission
timeout when the RTT is unknown [RFC2988], and shorter values are timeout when the RTT is unknown [RFC2988], and shorter values are
likely problematic in many cases. Note that the initial timeout likely problematic in many cases. Note that the sending rate in this
interval must be more conservative than in the two previous cases, case must be more conservative than in the two previous cases,
because the lack of return traffic prevents the detection of packet because the lack of return traffic prevents the detection of packet
loss, i.e., congestion events, and the application therefore cannot loss, i.e., congestion events, and the application therefore cannot
perform exponential back-off to reduce load. perform exponential back-off to reduce load.
Applications that communicate bidirectionally SHOULD employ Applications that communicate bidirectionally SHOULD employ
congestion control for both directions of the communication. For congestion control for both directions of the communication. For
example, for a client-server, request-response-style application, example, for a client-server, request-response-style application,
clients SHOULD congestion control their request transmission to a clients SHOULD congestion control their request transmission to a
server, and the server SHOULD congestion-control its responses to the server, and the server SHOULD congestion-control its responses to the
clients. Congestion in the forward and reverse direction is clients. Congestion in the forward and reverse direction is
uncorrelated and an application SHOULD independently detect and uncorrelated and an application SHOULD independently detect and
respond to congestion along both directions. respond to congestion along both directions.
3.1.3. UDP Tunnels 3.1.3. UDP Tunnels
One increasingly popular use of UDP is as a tunneling protocol, where One increasingly popular use of UDP is as a tunneling protocol, where
a tunnel endpoint encapsulates the packets of another protocol inside a tunnel endpoint encapsulates the packets of another protocol inside
UDP messages and transmits them to another tunnel endpoint, which UDP datagrams and transmits them to another tunnel endpoint, which
decapsulates the UDP messages and forwards the original packets decapsulates the UDP datagrams and forwards the original packets
contained in the payload. Tunnels establish virtual links that contained in the payload. Tunnels establish virtual links that
appear to directly connect locations that are distant in the physical appear to directly connect locations that are distant in the physical
Internet topology, and can be used to create virtual (private) Internet topology, and can be used to create virtual (private)
networks. Using UDP as a tunneling protocol is attractive when the networks. Using UDP as a tunneling protocol is attractive when the
payload protocol is not supported by middleboxes that may exist along payload protocol is not supported by middleboxes that may exist along
the path, because many middleboxes support UDP transmissions. the path, because many middleboxes support UDP transmissions.
Well-implemented tunnels are generally invisible to the endpoints Well-implemented tunnels are generally invisible to the endpoints
that happen to transmit over a path that includes tunneled links. On that happen to transmit over a path that includes tunneled links. On
the other hand, to the routers along the path of a UDP tunnel, i.e., the other hand, to the routers along the path of a UDP tunnel, i.e.,
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traffic sharing the path, and specific congestion control mechanism traffic sharing the path, and specific congestion control mechanism
for the tunnel are not necessary. for the tunnel are not necessary.
However, if the IP traffic in the tunnel is known to not be However, if the IP traffic in the tunnel is known to not be
congestion-controlled, additional measures are RECOMMENDED in order congestion-controlled, additional measures are RECOMMENDED in order
to limit the impact of the tunneled traffic on other traffic sharing to limit the impact of the tunneled traffic on other traffic sharing
the path. the path.
The following guidelines define these possible cases in more detail: The following guidelines define these possible cases in more detail:
1. Tunnel generates UDP traffic at a volume that corresponds to the 1. A tunnel generates UDP traffic at a volume that corresponds to
volume of payload traffic, and the payload traffic is IP-based the volume of payload traffic, and the payload traffic is IP-
and hence assumed to be congestion-controlled. based and hence assumed to be congestion-controlled.
This is arguably the most common case for Internet tunnels. In This is arguably the most common case for Internet tunnels. In
this case, the UDP tunnel SHOULD NOT employ its own congestion this case, the UDP tunnel SHOULD NOT employ its own congestion
control mechanism, because congestion losses of tunneled traffic control mechanism, because congestion losses of tunneled traffic
will already trigger an appropriate congestion response at the will already trigger an appropriate congestion response at the
original senders of the tunneled traffic. original senders of the tunneled traffic.
Note that this guideline is built on the assumption that most IP- Note that this guideline is built on the assumption that most IP-
based communication is congestion-controlled. If a UDP tunnel is based communication is congestion-controlled. If a UDP tunnel is
used for IP-based traffic that is known to not be congestion- used for IP-based traffic that is known to not be congestion-
controlled, the next set of guidelines applies: controlled, the next set of guidelines applies:
2. Tunnel generates UDP traffic at a volume that corresponds to the 2. A tunnel generates UDP traffic at a volume that corresponds to
volume of payload traffic, and the payload traffic is not known the volume of payload traffic, and the payload traffic is not
to be IP-based or is known to be IP-based, but not congestion- known to be IP-based, or is known to be IP-based but not
controlled. congestion-controlled.
This can be case, for example, when some link-layer protocols are This can be the case, for example, when some link-layer protocols
encapsulated within UDP (but not all link-layer protocols; some are encapsulated within UDP (but not all link-layer protocols;
are congestion-controlled.) Because it is not known that some are congestion-controlled.) Because it is not known that
congestion losses of tunneled non-IP traffic will trigger an congestion losses of tunneled non-IP traffic will trigger an
appropriate congestion response at the senders, the UDP tunnel appropriate congestion response at the senders, the UDP tunnel
SHOULD employ an appropriate congestion control mechanism. SHOULD employ an appropriate congestion control mechanism.
Because tunnels are usually bulk-transfer applications as far as Because tunnels are usually bulk-transfer applications as far as
the intermediate routers are concerned, the guidelines in the intermediate routers are concerned, the guidelines in
Section 3.1.1 apply. Section 3.1.1 apply.
3. Tunnel generates UDP traffic at a volume that does not correspond 3. A tunnel generates UDP traffic at a volume that does not
to the volume of payload traffic, independent of whether the correspond to the volume of payload traffic, independent of
payload traffic is IP-based or congestion-controlled. whether the payload traffic is IP-based or congestion-controlled.
Examples of this class include UDP tunnels that send at a Examples of this class include UDP tunnels that send at a
constant rate, increase their transmission rates under loss, for constant rate, increase their transmission rates under loss, for
example, due to increasing redundancy when forward-error- example, due to increasing redundancy when forward-error-
correction is used, or are otherwise constrained in their correction is used, or are otherwise constrained in their
transmission behavior. These specialized uses of UDP for transmission behavior. These specialized uses of UDP for
tunneling go beyond the scope of the general guidelines given in tunneling go beyond the scope of the general guidelines given in
this document. The implementer of such specialized tunnels this document. The implementer of such specialized tunnels
SHOULD carefully consider congestion control in the design of SHOULD carefully consider congestion control in the design of
their tunneling mechanism. their tunneling mechanism.
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delivered. This fundamental issue with fragmentation exists for both delivered. This fundamental issue with fragmentation exists for both
IPv4 and IPv6. In addition, some NATs and firewalls drop IP IPv4 and IPv6. In addition, some NATs and firewalls drop IP
fragments. The network address translation performed by a NAT only fragments. The network address translation performed by a NAT only
operates on complete IP packets, and some firewall policies also operates on complete IP packets, and some firewall policies also
require inspection of complete IP packets. Even with these being the require inspection of complete IP packets. Even with these being the
case, some NATs and firewalls simply do not implement the necessary case, some NATs and firewalls simply do not implement the necessary
reassembly functionality, and instead choose to drop all fragments. reassembly functionality, and instead choose to drop all fragments.
Finally, [RFC4963] documents other issues specific to IPv4 Finally, [RFC4963] documents other issues specific to IPv4
fragmentation. fragmentation.
Due to these issues, an application SHOULD NOT send UDP messages that Due to these issues, an application SHOULD NOT send UDP datagrams
result in IP packets that exceed the MTU of the path to the that result in IP packets that exceed the MTU of the path to the
destination. Consequently, an application SHOULD either use the path destination. Consequently, an application SHOULD either use the path
MTU information provided by the IP layer or implement path MTU MTU information provided by the IP layer or implement path MTU
discovery itself [RFC1191][RFC1981][RFC4821] to determine whether the discovery itself [RFC1191][RFC1981][RFC4821] to determine whether the
path to a destination will support its desired message size without path to a destination will support its desired message size without
fragmentation. fragmentation.
Applications that choose to not adapt their transmit message size Applications that do not follow this recommendation to do PMTU
SHOULD NOT send UDP messages that would result in IP datagrams that discovery SHOULD still avoid sending UDP datagrams that would result
exceed the effective PMTU. In the absence of actual knowledge of the in IP packets that exceed the path MTU. Because the actual path MTU
minimum MTU along the path, the effective PMTU depends upon the IP is unknown, such applications SHOULD fall back to sending messages
version used for transmission. It is the smaller of 576 bytes and that are shorter that the default effective MTU for sending (EMTU_S
the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for IPv6 in [RFC1122]). EMTU_S is the smaller of 576 bytes and the first-hop
[RFC2460]. The effective PMTU for a directly connected destination MTU for IPv4 [RFC1122] and 1280 bytes for IPv6 [RFC2460]. The
(with no routers on the path) is the configured interface MTU, which effective PMTU for a directly connected destination (with no routers
could be less than the maximum link payload size. Transmission of on the path) is the configured interface MTU, which could be less
minimum-sized messages is inefficient over paths that support a than the maximum link payload size. Transmission of minimum-sized
larger PMTU, which is a second reason to implement PMTU discovery. UDP datagrams is inefficient over paths that support a larger PMTU,
which is a second reason to implement PMTU discovery.
To determine an appropriate UDP payload size, applications MUST To determine an appropriate UDP payload size, applications MUST
subtract the size of the IP header (which includes any IPv4 optional subtract the size of the IP header (which includes any IPv4 optional
headers or IPv6 extension headers) as well as the length of the UDP headers or IPv6 extension headers) as well as the length of the UDP
header (8 bytes) from the PMTU size. This size, known as the MMS_S, header (8 bytes) from the PMTU size. This size, known as the MMS_S,
can be obtained from the TCP/IP stack [RFC1122]. can be obtained from the TCP/IP stack [RFC1122].
Applications that do not send messages that exceed the effective PMTU Applications that do not send messages that exceed the effective PMTU
of IPv4 or IPv6 need not implement any of the above mechanisms. Note of IPv4 or IPv6 need not implement any of the above mechanisms. Note
that the presence of tunnels can cause an additional reduction of the that the presence of tunnels can cause an additional reduction of the
effective PMTU, so implementing PMTU discovery will still be effective PMTU, so implementing PMTU discovery will still be
beneficial in some cases. beneficial in some cases.
Applications that fragment an application-layer message into multiple
UDP datagrams SHOULD perform this fragmentation so that each datagram
can be received independently, and be independently retransmitted in
the case where an application implements its own reliability
mechanisms.
3.3. Reliability Guidelines 3.3. Reliability Guidelines
Application designers are generally aware that UDP does not provide Application designers are generally aware that UDP does not provide
any reliability, e.g., it does not retransmit any lost packets. any reliability, e.g., it does not retransmit any lost packets.
Often, this is a main reason to consider UDP as a transport. Often, this is a main reason to consider UDP as a transport.
Applications that do require reliable message delivery MUST implement Applications that do require reliable message delivery MUST implement
an appropriate mechanism themselves. an appropriate mechanism themselves.
UDP also does not protect against message duplication, i.e., an UDP also does not protect against datagram duplication, i.e., an
application may receive multiple copies of the same message. application may receive multiple copies of the same UDP datagram.
Application designers SHOULD verify that their application handles Application designers SHOULD verify that their application handles
message duplication gracefully, and may consequently need to datagram duplication gracefully, and may consequently need to
implement mechanisms to detect duplicates. Even if message reception implement mechanisms to detect duplicates. Even if UDP datagram
triggers idempotent operations, applications may want to suppress reception triggers idempotent operations, applications may want to
duplicate messages to reduce load. suppress duplicate datagrams to reduce load.
In addition, the Internet can significantly delay some packets with In addition, the Internet can significantly delay some packets with
respect to others, e.g., due to routing transients, intermittent respect to others, e.g., due to routing transients, intermittent
connectivity, or mobility. This can cause message reordering, where connectivity, or mobility. This can cause reordering, where UDP
UDP messages arrive at the receiver in an order different from the datagrams arrive at the receiver in an order different from the
transmission order. Applications that require ordered delivery MUST transmission order. Applications that require ordered delivery MUST
reestablish message ordering themselves. reestablish datagram ordering themselves.
Finally, it is important to note that delay spikes can be very large. Finally, it is important to note that delay spikes can be very large.
This can cause reordered packets to arrive many seconds after they This can cause reordered packets to arrive many seconds after they
were sent. [RFC0793] defines the the maximum delay a TCP segment were sent. [RFC0793] defines the the maximum delay a TCP segment
should experience - the Maximum Segment Lifetime (MSL) - as 2 should experience - the Maximum Segment Lifetime (MSL) - as 2
minutes. No other RFC defines an MSL for other transport protocols minutes. No other RFC defines an MSL for other transport protocols
or IP itself. This document clarifies that the MSL value to be used or IP itself. This document clarifies that the MSL value to be used
for UDP SHOULD be the same 2 minutes as for TCP. Applications SHOULD for UDP SHOULD be the same 2 minutes as for TCP. Applications SHOULD
be robust to the reception of delayed or duplicate packets that are be robust to the reception of delayed or duplicate packets that are
received within this 2-minute interval. received within this 2-minute interval.
Applications that require reliable and ordered message delivery Applications that require reliable and ordered message delivery
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was the intended destination of the packet, because it covers the IP was the intended destination of the packet, because it covers the IP
addresses, port numbers and protocol number, and it verifies that the addresses, port numbers and protocol number, and it verifies that the
packet is not truncated or padded, because it covers the size field. packet is not truncated or padded, because it covers the size field.
It therefore protects an application against receiving corrupted It therefore protects an application against receiving corrupted
payload data in place of, or in addition to, the data that was sent. payload data in place of, or in addition to, the data that was sent.
Applications SHOULD enable UDP checksums, although [RFC0768] permits Applications SHOULD enable UDP checksums, although [RFC0768] permits
the option to disable their use. Applications that choose to disable the option to disable their use. Applications that choose to disable
UDP checksums when transmitting over IPv4 therefore MUST NOT make UDP checksums when transmitting over IPv4 therefore MUST NOT make
assumptions regarding the correctness of received data and MUST assumptions regarding the correctness of received data and MUST
behave correctly when a message is received that was originally sent behave correctly when a UDP datagram is received that was originally
to a different destination or is otherwise corrupted. The use of the sent to a different destination or is otherwise corrupted. The use
UDP checksum is MANDATORY when applications transmit UDP over IPv6 of the UDP checksum is REQUIRED when applications transmit UDP over
[RFC2460]. IPv6 [RFC2460].
3.4.1. UDP-Lite 3.4.1. UDP-Lite
A special class of applications can derive benefit from having A special class of applications can derive benefit from having
partially damaged payloads delivered, rather than discarded, when partially damaged payloads delivered, rather than discarded, when
using paths that include error-prone links. Such applications can using paths that include error-prone links. Such applications can
tolerate payload corruption and MAY choose to use the Lightweight tolerate payload corruption and MAY choose to use the Lightweight
User Datagram Protocol (UDP-Lite) [RFC3828] variant of UDP instead of User Datagram Protocol (UDP-Lite) [RFC3828] variant of UDP instead of
basic UDP. Applications that choose to use UDP-Lite instead of UDP basic UDP. Applications that choose to use UDP-Lite instead of UDP
MUST still follow the congestion control and other guidelines should still follow the congestion control and other guidelines
described for use with UDP in Section 3. described for use with UDP in Section 3.
UDP-Lite changes the semantics of the UDP "payload length" field to UDP-Lite changes the semantics of the UDP "payload length" field to
that of a "checksum coverage length" field. Otherwise, UDP-Lite is that of a "checksum coverage length" field. Otherwise, UDP-Lite is
semantically identical to UDP. The interface of UDP-Lite differs semantically identical to UDP. The interface of UDP-Lite differs
from that of UDP by the addition of a single (socket) option that from that of UDP by the addition of a single (socket) option that
communicates a checksum coverage length value: at the sender, this communicates a checksum coverage length value: at the sender, this
specifies the intended checksum coverage, with the remaining specifies the intended checksum coverage, with the remaining
unprotected part of the payload called the "error insensitive part". unprotected part of the payload called the "error insensitive part".
If required, an application may dynamically modify this length value, If required, an application may dynamically modify this length value,
e.g., to offer greater protection to some messages. UDP-Lite always e.g., to offer greater protection to some messages. UDP-Lite always
verifies that a packet was delivered to the intended destination, verifies that a packet was delivered to the intended destination,
i.e., always verifies the header fields. Errors in the insensitive i.e., always verifies the header fields. Errors in the insensitive
part will not cause a UDP message to be discarded by the destination. part will not cause a UDP datagram to be discarded by the
Applications using UDP-Lite therefore MUST NOT make assumptions destination. Applications using UDP-Lite therefore MUST NOT make
regarding the correctness of the data received in the insensitive assumptions regarding the correctness of the data received in the
part of the UDP-Lite payload. insensitive part of the UDP-Lite payload.
The sending application SHOULD select the minimum checksum coverage The sending application SHOULD select the minimum checksum coverage
to include all sensitive protocol headers. For example, applications to include all sensitive protocol headers. For example, applications
that use the Real-Time Protocol (RTP) [RFC3550] will likely want to that use the Real-Time Protocol (RTP) [RFC3550] will likely want to
protect the RTP header against corruption. Applications, where protect the RTP header against corruption. Applications, where
appropriate, MUST also introduce their own appropriate validity appropriate, MUST also introduce their own appropriate validity
checks for protocol information carried in the insensitive part of checks for protocol information carried in the insensitive part of
the UDP-Lite payload (e.g., internal CRCs). the UDP-Lite payload (e.g., internal CRCs).
The receiver MUST set a minimum coverage threshold for incoming The receiver MUST set a minimum coverage threshold for incoming
packets that is not smaller than the smallest coverage used by the packets that is not smaller than the smallest coverage used by the
sender. This may be a fixed value, or may be negotiated by an sender. This may be a fixed value, or may be negotiated by an
application. UDP-Lite does not provide mechanisms to negotiate the application. UDP-Lite does not provide mechanisms to negotiate the
checksum coverage between the sender and receiver. checksum coverage between the sender and receiver.
Applications may still experience packet loss, rather than Applications may still experience packet loss, rather than
corruption, when using UDP-Lite. The enhancements offered by UDP- corruption, when using UDP-Lite. The enhancements offered by UDP-
Lite rely upon a link being able to intercept the UDP-Lite header to Lite rely upon a link being able to intercept the UDP-Lite header to
correctly identify the partial coverage required. When tunnels correctly identify the partial coverage required. When tunnels
and/or encryption are used, this can result in UDP-Lite messages and/or encryption are used, this can result in UDP-Lite datagrams
being treated the same as UDP messages, i.e., result in packet loss. being treated the same as UDP datagrams, i.e., result in packet loss.
Use of IP fragmentation can also prevent special treatment for UDP- Use of IP fragmentation can also prevent special treatment for UDP-
Lite messages, and is another reason why applications SHOULD avoid IP Lite datagrams, and is another reason why applications SHOULD avoid
fragmentation Section 3.2. IP fragmentation Section 3.2.
3.5. Middlebox Traversal Guidelines 3.5. Middlebox Traversal Guidelines
Network address translators (NATs) and firewalls are examples of Network address translators (NATs) and firewalls are examples of
intermediary devices ("middleboxes") that can exist along an end-to- intermediary devices ("middleboxes") that can exist along an end-to-
end path. A middlebox typically performs a function that requires it end path. A middlebox typically performs a function that requires it
to maintain per-flow state. For connection-oriented protocols, such to maintain per-flow state. For connection-oriented protocols, such
as TCP, middleboxes snoop and parse the connection-management traffic as TCP, middleboxes snoop and parse the connection-management traffic
and create and destroy per-flow state accordingly. For a and create and destroy per-flow state accordingly. For a
connectionless protocol such as UDP, this approach is not possible. connectionless protocol such as UDP, this approach is not possible.
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one side of them to be interior to the domain they serve, whereas the one side of them to be interior to the domain they serve, whereas the
partial path on their other side is defined to be exterior to that partial path on their other side is defined to be exterior to that
domain. Per-flow state is typically created when the first packet domain. Per-flow state is typically created when the first packet
crosses from the interior to the exterior, and while the state is crosses from the interior to the exterior, and while the state is
present, NATs and firewalls will forward return traffic. Return present, NATs and firewalls will forward return traffic. Return
traffic arriving after the per-flow state has timed out is dropped, traffic arriving after the per-flow state has timed out is dropped,
as is other traffic arriving from the exterior. as is other traffic arriving from the exterior.
Many applications that use UDP for communication operate across Many applications that use UDP for communication operate across
middleboxes without needing to employ additional mechanisms. One middleboxes without needing to employ additional mechanisms. One
example is the DNS, which has a strict request-response communication example is the Domain Name System (DNS), which has a strict request-
pattern that typically completes within seconds. response communication pattern that typically completes within
seconds.
Other applications may experience communication failures when Other applications may experience communication failures when
middleboxes destroy the per-flow state associated with an application middleboxes destroy the per-flow state associated with an application
session during periods when the application does not exchange any UDP session during periods when the application does not exchange any UDP
traffic. Applications SHOULD be able to gracefully handle such traffic. Applications SHOULD be able to gracefully handle such
communication failures and implement mechanisms to re-establish communication failures and implement mechanisms to re-establish
application-layer sessions and state. application-layer sessions and state.
For some applications, such as media transmissions, this re- For some applications, such as media transmissions, this re-
synchronization is highly undesirable, because it can cause user- synchronization is highly undesirable, because it can cause user-
perceivable playback artifacts. Such specialized applications MAY perceivable playback artifacts. Such specialized applications MAY
send periodic keep-alive messages to attempt to refresh middlebox send periodic keep-alive messages to attempt to refresh middlebox
state. It is important to note that keep-alive messages are NOT state. It is important to note that keep-alive messages are NOT
RECOMMENDED for general use - they are unnecessary for many RECOMMENDED for general use - they are unnecessary for many
applications and can consume significant amounts of system and applications and can consume significant amounts of system and
network resources. network resources.
An application that needs to employ keep-alives to deliver useful An application that needs to employ keep-alives to deliver useful
service in the presence of middleboxes SHOULD NOT transmit them more service over UDP in the presence of middleboxes SHOULD NOT transmit
frequently than once every 15 seconds and SHOULD use longer intervals them more frequently than once every 15 seconds and SHOULD use longer
when possible. No common timeout has been specified for per-flow UDP intervals when possible. No common timeout has been specified for
state for arbitrary middleboxes. For NATs, [RFC4787] requires a per-flow UDP state for arbitrary middleboxes. For NATs, [RFC4787]
state timeout of 2 minutes or longer. However, empirical evidence requires a state timeout of 2 minutes or longer. However, empirical
suggests that a significant fraction of the deployed middleboxes evidence suggests that a significant fraction of the deployed
unfortunately uses shorter timeouts. The timeout of 15 seconds middleboxes unfortunately uses shorter timeouts. The timeout of 15
originates with the Interactive Connectivity Establishment (ICE) seconds originates with the Interactive Connectivity Establishment
protocol [I-D.ietf-mmusic-ice]. Applications that operate in more (ICE) protocol [I-D.ietf-mmusic-ice]. Applications that operate in
controlled network environments SHOULD investigate whether the more controlled network environments SHOULD investigate whether the
environment they operate in allows them to use longer intervals, or environment they operate in allows them to use longer intervals, or
whether it offers mechanisms to explicitly control middlebox state whether it offers mechanisms to explicitly control middlebox state
timeout durations, for example, using MIDCOM [RFC3303], NSIS timeout durations, for example, using MIDCOM [RFC3303], NSIS
[I-D.ietf-nsis-nslp-natfw] or UPnP [UPNP]. [I-D.ietf-nsis-nslp-natfw] or UPnP [UPNP]. It is RECOMMENDED that
applications apply slight random variations ("jitter") to the timing
of keep-alive transmissions, in order to reduce the potential for
persistent synchronization between keep-alive transmissions from
different hosts.
It is important to note that sending keep-alives is not a substitute Sending keep-alives is not a substitute for implementing robust
for implementing robust connection handling. Like all UDP messages, connection handling. Like all UDP datagrams, keep-alives can be
keep-alives can be delayed or dropped, causing middlebox state to delayed or dropped, causing middlebox state to time out. In
time out. In addition, the congestion control guidelines in addition, the congestion control guidelines in Section 3.1 cover all
Section 3.1 cover all UDP transmissions by an application, including UDP transmissions by an application, including the transmission of
the transmission of middlebox keep-alives. Congestion control may middlebox keep-alives. Congestion control may thus lead to delays or
thus lead to delays or temporary suspension of keep-alive temporary suspension of keep-alive transmission.
transmission.
Keep-alive messages are NOT RECOMMENDED for general use. They are
unnecessary for many applications and can consume significant amounts
of system and network resources. For example, on battery-powered
devices, if an application needs to maintain connectivity for long
periods with little traffic, the frequency at which keep-alives are
sent can become the determining factor that governs power
consumption, depending on the underlying network technology. Because
many middleboxes are designed to require keep-alives for TCP
connections at a frequency that is much lower than that needed for
UDP, this difference alone can often be sufficient to prefer TCP over
UDP for these deployments. On the other hand, there is anecdotal
evidence that suggests that direct communication through middleboxes,
e.g., by using ICE [I-D.ietf-mmusic-ice], does succeed less often
with TCP than with UDP. The tradeoffs between different transport
protocols - especially when it comes to middlebox traversal - deserve
carful analysis.
3.6. Programming Guidelines 3.6. Programming Guidelines
The de facto standard application programming interface (API) for The de facto standard application programming interface (API) for
TCP/IP applications is the "sockets" interface [POSIX]. Although TCP/IP applications is the "sockets" interface [POSIX]. Although
this API was developed for UNIX in the early 1980s, a wide variety of this API was developed for UNIX in the early 1980s, a wide variety of
non-UNIX operating systems also implements it. The sockets API non-UNIX operating systems also implements it. The sockets API
supports both IPv4 and IPv6 [RFC3493]. The UDP sockets API differs supports both IPv4 and IPv6 [RFC3493]. The UDP sockets API differs
from that for TCP in several key ways. Because application from that for TCP in several key ways. Because application
programmers are typically more familiar with the TCP sockets API, the programmers are typically more familiar with the TCP sockets API, the
remainder of this section discusses these differences. [STEVENS] remainder of this section discusses these differences. [STEVENS]
provides usage examples of the UDP sockets API. provides usage examples of the UDP sockets API.
UDP messages may be directly sent and received, without any UDP datagrams may be directly sent and received, without any
connection setup. Using the sockets API, applications can receive connection setup. Using the sockets API, applications can receive
packets from more than one IP source address on a single UDP socket. packets from more than one IP source address on a single UDP socket.
Some servers use this to exchange data with more than one remote host Some servers use this to exchange data with more than one remote host
through a single UDP socket at the same time. When applications need through a single UDP socket at the same time. When applications need
to ensure that they receive packets from a particular source address, to ensure that they receive packets from a particular source address,
they MUST implement corresponding checks at the application layer or they MUST implement corresponding checks at the application layer or
explicitly request that the operating system filter the received explicitly request that the operating system filter the received
packets. packets.
If a client/server application executes on a host with more than one If a client/server application executes on a host with more than one
IP interface, the application SHOULD send any UDP responses in reply IP interface, the application SHOULD send any UDP responses in reply
to arriving UDP datagrams with an IP source address that matches the to arriving UDP datagrams with an IP source address that matches the
IP destination address of the datagram that carried the request. IP destination address of the datagram that carried the request (see
Many middleboxes expect this transmission behavior and drop replies [RFC1122], Section 4.1.3.5). Many middleboxes expect this
that are sent from a different IP address, as explained in transmission behavior and drop replies that are sent from a different
Section 3.5. IP address, as explained in Section 3.5.
A UDP receiver can receive a valid UDP datagram with a zero-length A UDP receiver can receive a valid UDP datagram with a zero-length
payload. Note that this is different from a return value of zero payload. Note that this is different from a return value of zero
from a read() socket call, which for TCP indicates the end of the from a read() socket call, which for TCP indicates the end of the
connection. connection.
Many operating systems also allow a UDP socket to be connected, i.e., Many operating systems also allow a UDP socket to be connected, i.e.,
to bind a UDP socket to a specific pair of addresses and ports. This to bind a UDP socket to a specific pair of addresses and ports. This
is similar to the corresponding TCP sockets API functionality. is similar to the corresponding TCP sockets API functionality.
However, for UDP, this is only a local operation that serves to However, for UDP, this is only a local operation that serves to
skipping to change at page 17, line 13 skipping to change at page 18, line 29
Applications SHOULD validate that the information in the ICMP message Applications SHOULD validate that the information in the ICMP message
payload, e.g., a reported error condition, corresponds to a UDP payload, e.g., a reported error condition, corresponds to a UDP
datagram that the application actually sent. Note that not all APIs datagram that the application actually sent. Note that not all APIs
have the necessary functions to support this validation, and some have the necessary functions to support this validation, and some
APIs already perform this validation internally before passing ICMP APIs already perform this validation internally before passing ICMP
information to the application. information to the application.
Any application response to ICMP error messages SHOULD be robust to Any application response to ICMP error messages SHOULD be robust to
temporary routing failures, i.e., transient ICMP "unreachable" temporary routing failures, i.e., transient ICMP "unreachable"
messages should not normally cause a communication abort. messages should not normally cause a communication abort.
Applications SHOULD appropriately respond to ICMP messages generated Applications SHOULD appropriately process ICMP messages generated in
in response to transmitted traffic. A correct response often response to transmitted traffic. A correct response often requires
requires context, such as local state about communication instances context, such as local state about communication instances to each
to each destination, that although readily available in connection- destination, that although readily available in connection-oriented
oriented transport protocols is not always maintained by UDP-based transport protocols is not always maintained by UDP-based
applications. applications.
4. Security Considerations 4. Security Considerations
UDP does not provide communications security. Applications that need UDP does not provide communications security. Applications that need
to protect their communications against eavesdropping, tampering, or to protect their communications against eavesdropping, tampering, or
message forgery SHOULD employ end-to-end security services provided message forgery SHOULD employ end-to-end security services provided
by other IETF protocols. by other IETF protocols.
One option of securing UDP communications is with IPsec [RFC4301], One option of securing UDP communications is with IPsec [RFC4301],
skipping to change at page 17, line 50 skipping to change at page 19, line 19
Although it is possible to use IPsec to secure UDP communications, Although it is possible to use IPsec to secure UDP communications,
not all operating systems support IPsec or allow applications to not all operating systems support IPsec or allow applications to
easily configure it for their flows. A second option of securing UDP easily configure it for their flows. A second option of securing UDP
communications is through Datagram Transport Layer Security (DTLS) communications is through Datagram Transport Layer Security (DTLS)
[RFC4347]. DTLS provides communication privacy by encrypting UDP [RFC4347]. DTLS provides communication privacy by encrypting UDP
payloads. It does not protect the UDP headers. Applications can payloads. It does not protect the UDP headers. Applications can
implement DTLS without relying on support from the operating system. implement DTLS without relying on support from the operating system.
Many other options for authenticating or encrypting UDP payloads Many other options for authenticating or encrypting UDP payloads
exist. These include IETF security frameworks such as GSS-API exist. For example, the GSS-API security framework [RFC2743] or
[RFC2743], SASL [RFC4422] and EAP [RFC3748], which are designed to Cryptographic Message Syntax (CMS) [RFC3852] could be used to protect
provide security services for network protocols. The IETF standard UDP payloads. The IETF standard for securing RTP [RFC3550] realtime
for securing RTP [RFC3550] realtime communication sessions over UDP communication sessions over UDP is SRTP [RFC3711]. In some
is SRTP [RFC3711]. For some other applications, S/MIME [RFC3851] or applications, a better solution is to protect larger standalone
PGP [RFC4880] might provide a better solution, because they provide objects, such as files or messages, instead of individual UDP
protection for larger standalone objects such as files or messages. payloads. In these situations, CMS [RFC3852], S/MIME [RFC3851] or
However, they generally involve public-key operations on an entire OpenPGP [RFC4880] could be used. In addition, there are many non-
object, which can have performance implications. In addition, there IETF protocols in this area.
are many non-IETF protocols in this area.
Like congestion control mechanisms, security mechanisms are difficult Like congestion control mechanisms, security mechanisms are difficult
to design and implement correctly. It is hence RECOMMENDED that to design and implement correctly. It is hence RECOMMENDED that
applications employ well-known standard security mechanisms such as applications employ well-known standard security mechanisms such as
DTLS or IPsec, rather than inventing their own. DTLS or IPsec, rather than inventing their own.
In terms of congestion control, [RFC2309] and [RFC2914] discuss the In terms of congestion control, [RFC2309] and [RFC2914] discuss the
dangers of congestion-unresponsive flows to the Internet. This dangers of congestion-unresponsive flows to the Internet. This
document provides guidelines to designers of UDP-based applications document provides guidelines to designers of UDP-based applications
to congestion-control their transmissions, and does not raise any to congestion-control their transmissions, and does not raise any
additional security concerns. additional security concerns.
5. Summary 5. Summary
This section summarizes the guidelines made in Section 3 and This section summarizes the guidelines made in Section 3 and
Section 4 in a tabular format in Table 1 for easy referencing. Section 4 in a tabular format in Table 1 for easy referencing.
+---------------------------------------------------------+---------+ +---------------------------------------------------------+---------+
| Recommendation | Section | | Recommendation | Section |
+---------------------------------------------------------+---------+ +---------------------------------------------------------+---------+
| MUST accommodate wide range of Internet path conditions | 3 | | MUST tolerate wide range of Internet path conditions | 3 |
| SHOULD use a full-featured transport (TCP, SCTP, DCCP) | | | SHOULD use a full-featured transport (TCP, SCTP, DCCP) | |
| | | | | |
| SHOULD control rate of transmission | 3.1 | | SHOULD control rate of transmission | 3.1 |
| SHOULD perform congestion control over all traffic | | | SHOULD perform congestion control over all traffic | |
| | | | | |
| for bulk transfers, | 3.1.1 | | for bulk transfers, | 3.1.1 |
| SHOULD consider implementing TFRC | | | SHOULD consider implementing TFRC | |
| else, SHOULD otherwise use bandwidth similar to TCP | | | else, SHOULD otherwise use bandwidth similar to TCP | |
| | | | | |
| for non-bulk transfers, | 3.1.2 | | for non-bulk transfers, | 3.1.2 |
| SHOULD measure RTT and transmit 1 message/RTT | | | SHOULD measure RTT and transmit 1 datagram/RTT | |
| else, SHOULD send at most 1 message every 3 seconds | | | else, SHOULD send at most 1 datagram every 3 seconds | |
| | | | | |
| SHOULD NOT send messages that exceed the PMTU, i.e., | 3.2 | | SHOULD NOT send datagrams that exceed the PMTU, i.e., | 3.2 |
| SHOULD discover PMTU or send messages < minimum PMTU | | | SHOULD discover PMTU or send datagrams < minimum PMTU | |
| | | | | |
| SHOULD handle message loss, duplication, reordering | 3.3 | | SHOULD handle datagram loss, duplication, reordering | 3.3 |
| SHOULD be robust to delivery delays up to 2 minutes | | | SHOULD be robust to delivery delays up to 2 minutes | |
| | | | | |
| SHOULD enable UDP checksum | 3.4 | | SHOULD enable UDP checksum | 3.4 |
| else, MAY use UDP-Lite with suitable checksum coverage | 3.4.1 | | else, MAY use UDP-Lite with suitable checksum coverage | 3.4.1 |
| | | | | |
| SHOULD NOT always send middlebox keep-alives | 3.5 | | SHOULD NOT always send middlebox keep-alives | 3.5 |
| MAY use keep-alives when needed (min. interval 15 sec) | | | MAY use keep-alives when needed (min. interval 15 sec) | |
| | | | | |
| MUST check IP source address | 3.6 | | MUST check IP source address | 3.6 |
| and, for client/server applications | | | and, for client/server applications | |
skipping to change at page 19, line 47 skipping to change at page 20, line 47
| | | | | |
| SHOULD use standard IETF security protocols when needed | 4 | | SHOULD use standard IETF security protocols when needed | 4 |
+---------------------------------------------------------+---------+ +---------------------------------------------------------+---------+
Table 1: Summary of recommendations. Table 1: Summary of recommendations.
6. IANA Considerations 6. IANA Considerations
This document raises no IANA considerations. This document raises no IANA considerations.
(Note to the RFC Editor: Please remove this section upon
publication.)
7. Acknowledgments 7. Acknowledgments
Thanks to Paul Aitken, Mark Allman, Francois Audet, Iljitsch van Thanks to Paul Aitken, Mark Allman, Francois Audet, Iljitsch van
Beijnum, Stewart Bryant, Remi Denis-Courmont, Wesley Eddy, Sally Beijnum, Stewart Bryant, Remi Denis-Courmont, Wesley Eddy, Pasi
Floyd, Jeffrey Hutzelman, Cullen Jennings, Tero Kivinen, Philip Eronen, Sally Floyd, Robert Hancock, Jeffrey Hutzelman, Cullen
Matthews, Joerg Ott, Colin Perkins, Tom Petch, Carlos Pignataro, Pasi Jennings, Tero Kivinen, Peter Koch, Jukka Manner, Philip Matthews,
Joerg Ott, Colin Perkins, Tom Petch, Carlos Pignataro, Pasi
Sarolahti, Pascal Thubert, Joe Touch and Magnus Westerlund for their Sarolahti, Pascal Thubert, Joe Touch and Magnus Westerlund for their
comments on this document. comments on this document.
The middlebox traversal guidelines in Section 3.5 incorporate ideas The middlebox traversal guidelines in Section 3.5 incorporate ideas
from Section 5 of [I-D.ford-behave-app] by Bryan Ford, Pyda Srisuresh from Section 5 of [I-D.ford-behave-app] by Bryan Ford, Pyda Srisuresh
and Dan Kegel. and Dan Kegel.
Lars Eggert is partly funded by [TRILOGY], a research project Lars Eggert is partly funded by [TRILOGY], a research project
supported by the European Commission under its Seventh Framework supported by the European Commission under its Seventh Framework
Program. Program.
8. References 8. References
8.1. Normative References 8.1. Normative References
[POSIX] IEEE Std. 1003.1-2001, "Standard for Information
Technology - Portable Operating System Interface (POSIX)",
Open Group Technical Standard: Base Specifications Issue
6, ISO/IEC 9945:2002, December 2001.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980. August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
skipping to change at page 21, line 42 skipping to change at page 22, line 42
[I-D.ietf-dccp-ccid4] [I-D.ietf-dccp-ccid4]
Floyd, S. and E. Kohler, "Profile for Datagram Congestion Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Control Protocol (DCCP) Congestion ID 4: TCP-Friendly
Rate Control for Small Packets (TFRC-SP)", Rate Control for Small Packets (TFRC-SP)",
draft-ietf-dccp-ccid4-02 (work in progress), draft-ietf-dccp-ccid4-02 (work in progress),
February 2008. February 2008.
[I-D.ietf-dccp-rfc3448bis] [I-D.ietf-dccp-rfc3448bis]
Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification",
draft-ietf-dccp-rfc3448bis-05 (work in progress), draft-ietf-dccp-rfc3448bis-06 (work in progress),
February 2008. April 2008.
[I-D.ietf-mmusic-ice] [I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT) (ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007. draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-nsis-nslp-natfw] [I-D.ietf-nsis-nslp-natfw]
Stiemerling, M., Tschofenig, H., Aoun, C., and E. Davies, Stiemerling, M., Tschofenig, H., Aoun, C., and E. Davies,
"NAT/Firewall NSIS Signaling Layer Protocol (NSLP)", "NAT/Firewall NSIS Signaling Layer Protocol (NSLP)",
draft-ietf-nsis-nslp-natfw-18 (work in progress), draft-ietf-nsis-nslp-natfw-18 (work in progress),
February 2008. February 2008.
[I-D.ietf-nsis-ntlp] [I-D.ietf-nsis-ntlp]
Schulzrinne, H. and R. Hancock, "GIST: General Internet Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", draft-ietf-nsis-ntlp-15 (work in Signalling Transport", draft-ietf-nsis-ntlp-15 (work in
progress), February 2008. progress), February 2008.
[POSIX] IEEE Std. 1003.1-2001, "Standard for Information
Technology - Portable Operating System Interface (POSIX)",
Open Group Technical Standard: Base Specifications Issue
6, ISO/IEC 9945:2002, December 2001.
[RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks", [RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks",
RFC 896, January 1984. RFC 896, January 1984.
[RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5,
RFC 919, October 1984.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. [RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, October 1993. Fixes", RFC 1536, October 1993.
[RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998. Internet", RFC 2309, April 1998.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999. RFC 2675, August 1999.
skipping to change at page 23, line 23 skipping to change at page 24, line 36
Video Conferences with Minimal Control", STD 65, RFC 3551, Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003. July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
[RFC3738] Luby, M. and V. Goyal, "Wave and Equation Based Rate [RFC3738] Luby, M. and V. Goyal, "Wave and Equation Based Rate
Control (WEBRC) Building Block", RFC 3738, April 2004. Control (WEBRC) Building Block", RFC 3738, April 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[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, May 2004. Partial Reliability Extension", RFC 3758, May 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89, Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004. RFC 3819, July 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification", Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004. RFC 3851, July 2004.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005. Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, [RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005. December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005. RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
skipping to change at page 24, line 18 skipping to change at page 25, line 30
Congestion Control", RFC 4341, March 2006. Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for [RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342, Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
March 2006. March 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006. Security", RFC 4347, April 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast [RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast
Congestion Control (TFMCC): Protocol Specification", Congestion Control (TFMCC): Protocol Specification",
RFC 4654, August 2006. RFC 4654, August 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007. Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", [RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007. RFC 4960, September 2007.
 End of changes. 65 change blocks. 
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