draft-ietf-rmcat-coupled-cc-09.txt   rfc8699.txt 
RTP Media Congestion Avoidance Techniques (rmcat) S. Islam Internet Engineering Task Force (IETF) S. Islam
Internet-Draft M. Welzl Request for Comments: 8699 M. Welzl
Intended status: Experimental S. Gjessing Category: Experimental S. Gjessing
Expires: February 23, 2020 University of Oslo ISSN: 2070-1721 University of Oslo
August 22, 2019 January 2020
Coupled congestion control for RTP media Coupled Congestion Control for RTP Media
draft-ietf-rmcat-coupled-cc-09
Abstract Abstract
When multiple congestion controlled Real-time Transport Protocol When multiple congestion-controlled Real-time Transport Protocol
(RTP) sessions traverse the same network bottleneck, combining their (RTP) sessions traverse the same network bottleneck, combining their
controls can improve the total on-the-wire behavior in terms of controls can improve the total on-the-wire behavior in terms of
delay, loss and fairness. This document describes such a method for delay, loss, and fairness. This document describes such a method for
flows that have the same sender, in a way that is as flexible and flows that have the same sender, in a way that is as flexible and
simple as possible while minimizing the amount of changes needed to simple as possible while minimizing the number of changes needed to
existing RTP applications. It specifies how to apply the method for existing RTP applications. This document also specifies how to apply
the Network-Assisted Dynamic Adaptation (NADA) congestion control the method for the Network-Assisted Dynamic Adaptation (NADA)
algorithm, and provides suggestions on how to apply it to other congestion control algorithm and provides suggestions on how to apply
congestion control algorithms. it to other congestion control algorithms.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for examination, experimental implementation, and
evaluation.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document defines an Experimental Protocol for the Internet
and may be updated, replaced, or obsoleted by other documents at any community. This document is a product of the Internet Engineering
time. It is inappropriate to use Internet-Drafts as reference Task Force (IETF). It represents the consensus of the IETF
material or to cite them other than as "work in progress." community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on February 23, 2020. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8699.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions
3. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Limitations
4. Architectural overview . . . . . . . . . . . . . . . . . . . 5 4. Architectural Overview
5. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Roles
5.1. SBD . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. SBD
5.2. FSE . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.2. FSE
5.3. Flows . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.3. Flows
5.3.1. Example algorithm 1 - Active FSE . . . . . . . . . . 9 5.3.1. Example Algorithm 1 - Active FSE
5.3.2. Example algorithm 2 - Conservative Active FSE . . . . 10 5.3.2. Example Algorithm 2 - Conservative Active FSE
6. Application . . . . . . . . . . . . . . . . . . . . . . . . . 11 6. Application
6.1. NADA . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1. NADA
6.2. General recommendations . . . . . . . . . . . . . . . . . 11 6.2. General Recommendations
7. Expected feedback from experiments . . . . . . . . . . . . . 12 7. Expected Feedback from Experiments
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9. Security Considerations
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13 10. References
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 10.1. Normative References
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 10.2. Informative References
11.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Application to GCC
Appendix A. Application to GCC . . . . . . . . . . . . . . . . . 16 Appendix B. Scheduling
Appendix B. Scheduling . . . . . . . . . . . . . . . . . . . . . 16 Appendix C. Example Algorithm - Passive FSE
Appendix C. Example algorithm - Passive FSE . . . . . . . . . . 16 C.1. Example Operation (Passive)
C.1. Example operation (passive) . . . . . . . . . . . . . . . 19 Acknowledgements
Appendix D. Change log . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses
D.1. draft-welzl-rmcat-coupled-cc . . . . . . . . . . . . . . 23
D.1.1. Changes from -00 to -01 . . . . . . . . . . . . . . . 23
D.1.2. Changes from -01 to -02 . . . . . . . . . . . . . . . 23
D.1.3. Changes from -02 to -03 . . . . . . . . . . . . . . . 23
D.1.4. Changes from -03 to -04 . . . . . . . . . . . . . . . 24
D.1.5. Changes from -04 to -05 . . . . . . . . . . . . . . . 24
D.2. draft-ietf-rmcat-coupled-cc . . . . . . . . . . . . . . . 24
D.2.1. Changes from draft-welzl-rmcat-coupled-cc-05 . . . . 24
D.2.2. Changes from -00 to -01 . . . . . . . . . . . . . . . 24
D.2.3. Changes from -01 to -02 . . . . . . . . . . . . . . . 24
D.2.4. Changes from -02 to -03 . . . . . . . . . . . . . . . 24
D.2.5. Changes from -03 to -04 . . . . . . . . . . . . . . . 24
D.2.6. Changes from -04 to -05 . . . . . . . . . . . . . . . 25
D.2.7. Changes from -05 to -06 . . . . . . . . . . . . . . . 25
D.2.8. Changes from -06 to -07 . . . . . . . . . . . . . . . 25
D.2.9. Changes from -07 to -08 . . . . . . . . . . . . . . . 25
D.2.10. Changes from -08 to -09 . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
When there is enough data to send, a congestion controller attempts When there is enough data to send, a congestion controller attempts
to increase its sending rate until the path's capacity has been to increase its sending rate until the path's capacity has been
reached. Some controllers detect path capacity by increasing the reached. Some controllers detect path capacity by increasing the
sending rate further, until packets are ECN-marked [RFC8087] or sending rate further, until packets are ECN-marked [RFC8087] or
dropped, and then decreasing the sending rate until that stops dropped, and then decreasing the sending rate until that stops
happening. This process inevitably creates undesirable queuing delay happening. This process inevitably creates undesirable queuing delay
when multiple congestion-controlled connections traverse the same when multiple congestion-controlled connections traverse the same
network bottleneck, and each connection overshoots the path capacity network bottleneck, and each connection overshoots the path capacity
as it determines its sending rate. as it determines its sending rate.
The Congestion Manager (CM) [RFC3124] couples flows by providing a The Congestion Manager (CM) [RFC3124] couples flows by providing a
single congestion controller. It is hard to implement because it single congestion controller. It is hard to implement because it
requires an additional congestion controller and removes all per- requires an additional congestion controller and removes all per-
connection congestion control functionality, which is quite a connection congestion control functionality, which is quite a
significant change to existing RTP based applications. This document significant change to existing RTP-based applications. This document
presents a method to combine the behavior of congestion control presents a method to combine the behavior of congestion control
mechanisms that is easier to implement than the Congestion Manager mechanisms that is easier to implement than the Congestion Manager
[RFC3124] and also requires less significant changes to existing RTP [RFC3124] and also requires fewer significant changes to existing
based applications. It attempts to roughly approximate the CM RTP-based applications. It attempts to roughly approximate the CM
behavior by sharing information between existing congestion behavior by sharing information between existing congestion
controllers. It is able to honor user-specified priorities, which is controllers. It is able to honor user-specified priorities, which is
required by rtcweb [I-D.ietf-rtcweb-overview] [RFC7478]. required by WebRTC [RTCWEB-OVERVIEW] [RFC7478].
The described mechanisms are believed safe to use, but are The described mechanisms are believed safe to use, but they are
experimental and are presented for wider review and operational experimental and are presented for wider review and operational
evaluation. evaluation.
2. Definitions 2. Definitions
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", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in RFC 2119 [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Available Bandwidth: Available Bandwidth:
The available bandwidth is the nominal link capacity minus the The available bandwidth is the nominal link capacity minus the
amount of traffic that traversed the link during a certain time amount of traffic that traversed the link during a certain time
interval, divided by that time interval. interval, divided by that time interval.
Bottleneck: Bottleneck:
The first link with the smallest available bandwidth along the The first link with the smallest available bandwidth along the
path between a sender and receiver. path between a sender and receiver.
Flow: Flow:
A flow is the entity that congestion control is operating on. A flow is the entity that congestion control is operating on.
It could, for example, be a transport layer connection, or an It could, for example, be a transport-layer connection or an
RTP stream [RFC7656], whether or not this RTP stream is RTP stream [RFC7656], regardless of whether or not this RTP
multiplexed onto an RTP session with other RTP streams. stream is multiplexed onto an RTP session with other RTP
streams.
Flow Group Identifier (FGI): Flow Group Identifier (FGI):
A unique identifier for each subset of flows that is limited by A unique identifier for each subset of flows that is limited by
a common bottleneck. a common bottleneck.
Flow State Exchange (FSE): Flow State Exchange (FSE):
The entity that maintains information that is exchanged between The entity that maintains information that is exchanged between
flows. flows.
Flow Group (FG): Flow Group (FG):
A group of flows having the same FGI. A group of flows having the same FGI.
Shared Bottleneck Detection (SBD): Shared Bottleneck Detection (SBD):
The entity that determines which flows traverse the same The entity that determines which flows traverse the same
bottleneck in the network, or the process of doing so. bottleneck in the network or the process of doing so.
3. Limitations 3. Limitations
Sender-side only: Sender-side only:
Shared bottlenecks can exist when multiple flows originate from Shared bottlenecks can exist when multiple flows originate from
the same sender, or when flows from different senders reach the the same sender or when flows from different senders reach the
same receiver (see [RFC8382], section 3). Coupled congestion same receiver (see Section 3 of [RFC8382]). Coupled congestion
control as described here only supports the former case, not control, as described here, only supports the former case, not
the latter, as it operates inside a single host on the sender the latter, as it operates inside a single host on the sender
side. side.
Shared bottlenecks do not change quickly: Shared bottlenecks do not change quickly:
As per the definition above, a bottleneck depends on cross As per the definition above, a bottleneck depends on cross
traffic, and since such traffic can heavily fluctuate, traffic, and since such traffic can heavily fluctuate,
bottlenecks can change at a high frequency (e.g., there can be bottlenecks can change at a high frequency (e.g., there can be
oscillation between two or more links). This means that, when oscillation between two or more links). This means that, when
flows are partially routed along different paths, they may flows are partially routed along different paths, they may
quickly change between sharing and not sharing a bottleneck. quickly change between sharing and not sharing a bottleneck.
For simplicity, here it is assumed that a shared bottleneck is For simplicity, here it is assumed that a shared bottleneck is
valid for a time interval that is significantly longer than the valid for a time interval that is significantly longer than the
interval at which congestion controllers operate. Note that, interval at which congestion controllers operate. Note that,
for the only SBD mechanism defined in this document for the only SBD mechanism defined in this document
(multiplexing on the same five-tuple), the notion of a shared (multiplexing on the same five-tuple), the notion of a shared
bottleneck stays correct even in the presence of fast traffic bottleneck stays correct even in the presence of fast traffic
fluctuations: since all flows that are assumed to share a fluctuations; since all flows that are assumed to share a
bottleneck are routed in the same way, if the bottleneck bottleneck are routed in the same way, if the bottleneck
changes, it will still be shared. changes, it will still be shared.
4. Architectural overview 4. Architectural Overview
Figure 1 shows the elements of the architecture for coupled Figure 1 shows the elements of the architecture for coupled
congestion control: the Flow State Exchange (FSE), Shared Bottleneck congestion control: the Flow State Exchange (FSE), Shared Bottleneck
Detection (SBD) and Flows. The FSE is a storage element that can be Detection (SBD), and Flows. The FSE is a storage element that can be
implemented in two ways: active and passive. In the active version, implemented in two ways: active and passive. In the active version,
it initiates communication with flows and SBD. However, in the it initiates communication with flows and SBD. However, in the
passive version, it does not actively initiate communication with passive version, it does not actively initiate communication with
flows and SBD; its only active role is internal state maintenance flows and SBD; its only active role is internal state maintenance
(e.g., an implementation could use soft state to remove a flow's data (e.g., an implementation could use soft state to remove a flow's data
after long periods of inactivity). Every time a flow's congestion after long periods of inactivity). Every time a flow's congestion
control mechanism would normally update its sending rate, the flow control mechanism would normally update its sending rate, the flow
instead updates information in the FSE and performs a query on the instead updates information in the FSE and performs a query on the
FSE, leading to a sending rate that can be different from what the FSE, leading to a sending rate that can be different from what the
congestion controller originally determined. Using information congestion controller originally determined. Using information
about/from the currently active flows, SBD updates the FSE with the about/from the currently active flows, SBD updates the FSE with the
correct Flow Group Identifiers (FGIs). correct Flow Group Identifiers (FGIs).
This document describes both active and passive versions. While the This document describes both active and passive versions. While the
passive algorithm works better for congestion controls with RTT- passive algorithm works better for congestion controls with RTT-
independent convergence, it can still produce oscillations on short independent convergence, it can still produce oscillations on short
time scales. The passive algorithm, described in Appendix C, is time scales. The passive algorithm, described in Appendix C, is
therefore considered as highly experimental and not safe to deploy therefore considered highly experimental and not safe to deploy
outside of testbed environments. Figure 2 shows the interaction outside of testbed environments. Figure 2 shows the interaction
between flows and the FSE, using the variable names defined in between flows and the FSE using the variable names defined in
Section 5.2. Section 5.2.
------- <--- Flow 1 ------- <--- Flow 1
| FSE | <--- Flow 2 .. | FSE | <--- Flow 2 ..
------- <--- .. Flow N ------- <--- .. Flow N
^ ^
| | | |
------- | ------- |
| SBD | <-------| | SBD | <-------|
------- -------
skipping to change at page 6, line 15 skipping to change at line 227
Flow#1(cc) FSE Flow#2(cc) Flow#1(cc) FSE Flow#2(cc)
---------- --- ---------- ---------- --- ----------
#1 JOIN ----register--> REGISTER #1 JOIN ----register--> REGISTER
REGISTER <--register-- JOIN #1 REGISTER <--register-- JOIN #1
#2 CC_R(1) ----UPDATE----> UPDATE (in) #2 CC_R(1) ----UPDATE----> UPDATE (in)
#3 NEW RATE <---FSE_R(1)-- UPDATE (out) --FSE_R(2)-> #3 NEW RATE #3 NEW RATE <---FSE_R(1)-- UPDATE (out) --FSE_R(2)-> #3 NEW RATE
Figure 2: Flow-FSE interaction Figure 2: Flow-FSE interactions
Since everything shown in Figure 1 is assumed to operate on a single Since everything shown in Figure 1 is assumed to operate on a single
host (the sender) only, this document only describes aspects that host (the sender) only, this document only describes aspects that
have an influence on the resulting on-the-wire behavior. It does have an influence on the resulting on-the-wire behavior. It does
not, for instance, define how many bits must be used to represent not, for instance, define how many bits must be used to represent
FGIs, or in which way the entities communicate. FGIs or in which way the entities communicate.
Implementations can take various forms: for instance, all the Implementations can take various forms; for instance, all the
elements in the figure could be implemented within a single elements in the figure could be implemented within a single
application, thereby operating on flows generated by that application application, thereby operating on flows generated by that application
only. Another alternative could be to implement both the FSE and SBD only. Another alternative could be to implement both the FSE and SBD
together in a separate process which different applications together in a separate process that different applications
communicate with via some form of Inter-Process Communication (IPC). communicate with via some form of Inter-Process Communication (IPC).
Such an implementation would extend the scope to flows generated by Such an implementation would extend the scope to flows generated by
multiple applications. The FSE and SBD could also be included in the multiple applications. The FSE and SBD could also be included in the
Operating System kernel. However, only one type of coupling Operating System kernel. However, only one type of coupling
algorithm should be used for all flows. Combinations of multiple algorithm should be used for all flows. Combinations of multiple
algorithms at different aggregation levels (e.g., the Operating algorithms at different aggregation levels (e.g., the Operating
System coupling application aggregates with one algorithm, and System coupling application aggregates with one algorithm, and
applications coupling their flows with another) have not been tested applications coupling their flows with another) have not been tested
and are therefore not recommended. and are therefore not recommended.
5. Roles 5. Roles
This section gives an overview of the roles of the elements of This section gives an overview of the roles of the elements of
coupled congestion control, and provides an example of how coupled coupled congestion control and provides an example of how coupled
congestion control can operate. congestion control can operate.
5.1. SBD 5.1. SBD
SBD uses knowledge about the flows to determine which flows belong in SBD uses knowledge about the flows to determine which flows belong in
the same Flow Group (FG), and assigns FGIs accordingly. This the same Flow Group (FG) and assigns FGIs accordingly. This
knowledge can be derived in three basic ways: knowledge can be derived in three basic ways:
1. From multiplexing: it can be based on the simple assumption that 1. From multiplexing: It can be based on the simple assumption that
packets sharing the same five-tuple (IP source and destination packets sharing the same five-tuple (IP source and destination
address, protocol, and transport layer port number pair) and address, protocol, and transport-layer port number pair) and
having the same values for the Differentiated Services Code Point having the same values for the Differentiated Services Code Point
(DSCP) and the ECN field in the IP header are typically treated (DSCP) and the ECN field in the IP header are typically treated
in the same way along the path. This method is the only one in the same way along the path. This method is the only one
specified in this document: SBD MAY consider all flows that use specified in this document; SBD MAY consider all flows that use
the same five-tuple, DSCP and ECN field value to belong to the the same five-tuple, DSCP, and ECN field value to belong to the
same FG. This classification applies to certain tunnels, or RTP same FG. This classification applies to certain tunnels or RTP
flows that are multiplexed over one transport (cf. flows that are multiplexed over one transport (cf.
[transport-multiplex]). Such multiplexing is also a recommended [TRANSPORT-MULTIPLEX]). Such multiplexing is also a recommended
usage of RTP in rtcweb [rtcweb-rtp-usage]. usage of RTP in WebRTC [RTCWEB-RTP-USAGE].
2. Via configuration: e.g. by assuming that a common wireless uplink 2. Via configuration: e.g., by assuming that a common wireless
is also a shared bottleneck. uplink is also a shared bottleneck.
3. From measurements: e.g. by considering correlations among 3. From measurements: e.g., by considering correlations among
measured delay and loss as an indication of a shared bottleneck. measured delay and loss as an indication of a shared bottleneck.
The methods above have some essential trade-offs: e.g., multiplexing The methods above have some essential trade-offs. For example,
is a completely reliable measure, however it is limited in scope to multiplexing is a completely reliable measure, but it is limited in
two end points (i.e., it cannot be applied to couple congestion scope to two endpoints (i.e., it cannot be applied to couple
controllers of one sender talking to multiple receivers). A congestion controllers of one sender talking to multiple receivers).
measurement-based SBD mechanism is described in [RFC8382]. A measurement-based SBD mechanism is described in [RFC8382].
Measurements can never be 100% reliable, in particular because they Measurements can never be 100% reliable, in particular because they
are based on the past but applying coupled congestion control means are based on the past, but applying coupled congestion control
to make an assumption about the future; it is therefore recommended involves making an assumption about the future; it is therefore
to implement cautionary measures, e.g. by disabling coupled recommended to implement cautionary measures, e.g., by disabling
congestion control if enabling it causes a significant increase in coupled congestion control if enabling it causes a significant
delay and/or packet loss. Measurements also take time, which entails increase in delay and/or packet loss. Measurements also take time,
a certain delay for turning on coupling (refer to [RFC8382] for which entails a certain delay for turning on coupling (refer to
details). Using system configuration to decide about shared [RFC8382] for details). When this is possible, it can be more
bottlenecks can be more efficient (faster to obtain) than using efficient to statically configure shared bottlenecks (e.g., via a
measurements, but it relies on assumptions about the network system configuration or user input) based on assumptions about the
environment. network environment.
5.2. FSE 5.2. FSE
The FSE contains a list of all flows that have registered with it. The FSE contains a list of all flows that have registered with it.
For each flow, it stores the following: For each flow, the FSE stores the following:
o a unique flow number f to identify the flow. * a unique flow number f to identify the flow.
o the FGI of the FG that it belongs to (based on the definitions in * the FGI of the FG that it belongs to (based on the definitions in
this document, a flow has only one bottleneck, and can therefore this document, a flow has only one bottleneck and can therefore be
be in only one FG). in only one FG).
o a priority P(f), which is a positive number, greater than zero. * a priority P(f), which is a number greater than zero.
o The rate used by the flow in bits per second, FSE_R(f). * The rate used by the flow in bits per second, FSE_R(f).
o The desired rate DR(f) of flow f. This can be smaller than * The desired rate DR(f) of flow f. This can be smaller than
FSE_R(f) if the application feeding into the flow has less data to FSE_R(f) if the application feeding into the flow has less data to
send than FSE_R(f) would allow, or if a maximum value is imposed send than FSE_R(f) would allow or if a maximum value is imposed on
on the rate. In the absence of such limits DR(f) must be set to the rate. In the absence of such limits, DR(f) must be set to the
the sending rate provided by the congestion control module of flow sending rate provided by the congestion control module of flow f.
f.
Note that the absolute range of priorities does not matter: the Note that the absolute range of priorities does not matter; the
algorithm works with a flow's priority portion of the sum of all algorithm works with a flow's priority portion of the sum of all
priority values. For example, if there are two flows, flow 1 with priority values. For example, if there are two flows, flow 1 with
priority 1 and flow 2 with priority 2, the sum of the priorities is priority 1 and flow 2 with priority 2, the sum of the priorities is
3. Then, flow 1 will be assigned 1/3 of the aggregate sending rate 3. Then, flow 1 will be assigned 1/3 of the aggregate sending rate,
and flow 2 will be assigned 2/3 of the aggregate sending rate. and flow 2 will be assigned 2/3 of the aggregate sending rate.
Priorities can be mapped to the "very-low", "low", "medium" or "high" Priorities can be mapped to the "very-low", "low", "medium", or
priority levels described in [I-D.ietf-rtcweb-transports] by simply "high" priority levels described in [WEBRTC-TRANS] by simply using
using the values 1, 2, 4 and 8, respectively. the values 1, 2, 4, and 8, respectively.
In the FSE, each FG contains one static variable S_CR which is the In the FSE, each FG contains one static variable, S_CR, which is the
sum of the calculated rates of all flows in the same FG. This value sum of the calculated rates of all flows in the same FG. This value
is used to calculate the sending rate. is used to calculate the sending rate.
The information listed here is enough to implement the sample flow The information listed here is enough to implement the sample flow
algorithm given below. FSE implementations could easily be extended algorithm given below. FSE implementations could easily be extended
to store, e.g., a flow's current sending rate for statistics to store, e.g., a flow's current sending rate for statistics
gathering or future potential optimizations. gathering or future potential optimizations.
5.3. Flows 5.3. Flows
Flows register themselves with SBD and FSE when they start, Flows register themselves with SBD and FSE when they start,
deregister from the FSE when they stop, and carry out an UPDATE deregister from the FSE when they stop, and carry out an UPDATE
function call every time their congestion controller calculates a new function call every time their congestion controller calculates a new
sending rate. Via UPDATE, they provide the newly calculated rate and sending rate. Via UPDATE, they provide the newly calculated rate
optionally (if the algorithm supports it) the desired rate. The and, optionally (if the algorithm supports it), the desired rate.
desired rate is less than the calculated rate in case of application- The desired rate is less than the calculated rate in case of
limited flows; otherwise, it is the same as the calculated rate. application-limited flows; otherwise, it is the same as the
calculated rate.
Below, two example algorithms are described. While other algorithms Below, two example algorithms are described. While other algorithms
could be used instead, the same algorithm must be applied to all could be used instead, the same algorithm must be applied to all
flows. Names of variables used in the algorithms are explained flows. Names of variables used in the algorithms are explained
below. below.
o CC_R(f) - The rate received from the congestion controller of flow CC_R(f) The rate received from the congestion controller of flow f
f when it calls UPDATE. when it calls UPDATE.
o FSE_R(f) - The rate calculated by the FSE for flow f. FSE_R(f) The rate calculated by the FSE for flow f.
o DR(f) - The desired rate of flow f. DR(f) The desired rate of flow f.
o S_CR - The sum of the calculated rates of all flows in the same S_CR The sum of the calculated rates of all flows in the same
FG; this value is used to calculate the sending rate. FG; this value is used to calculate the sending rate.
o FG - A group of flows having the same FGI, and hence sharing the FG A group of flows having the same FGI and hence, sharing the
same bottleneck. same bottleneck.
o P(f) - The priority of flow f which is received from the flow's P(f) The priority of flow f, which is received from the flow's
congestion controller; the FSE uses this variable for calculating congestion controller; the FSE uses this variable for
FSE_R(f). calculating FSE_R(f).
o S_P - The sum of all the priorities. S_P The sum of all the priorities.
o TLO - The total leftover rate: the sum of rates that could not be TLO The total leftover rate; the sum of rates that could not be
assigned to flows that were limited by their desired rate. assigned to flows that were limited by their desired rate.
o AR - The aggregate rate that is assigned to flows that are not AR The aggregate rate that is assigned to flows that are not
limited by their desired rate. limited by their desired rate.
5.3.1. Example algorithm 1 - Active FSE 5.3.1. Example Algorithm 1 - Active FSE
This algorithm was designed to be the simplest possible method to This algorithm was designed to be the simplest possible method to
assign rates according to the priorities of flows. Simulations assign rates according to the priorities of flows. Simulation
results in [fse] indicate that it does however not significantly results in [FSE] indicate that it does not, however, significantly
reduce queuing delay and packet loss. reduce queuing delay and packet loss.
(1) When a flow f starts, it registers itself with SBD and the FSE. (1) When a flow f starts, it registers itself with SBD and the FSE.
FSE_R(f) is initialized with the congestion controller's initial FSE_R(f) is initialized with the congestion controller's initial
rate. SBD will assign the correct FGI. When a flow is assigned rate. SBD will assign the correct FGI. When a flow is assigned
an FGI, it adds its FSE_R(f) to S_CR. an FGI, it adds its FSE_R(f) to S_CR.
(2) When a flow f stops or pauses, its entry is removed from the (2) When a flow f stops or pauses, its entry is removed from the
list. list.
(3) Every time the congestion controller of the flow f determines a (3) Every time the congestion controller of the flow f determines a
new sending rate CC_R(f), the flow calls UPDATE, which carries new sending rate CC_R(f), the flow calls UPDATE, which carries
out the tasks listed below to derive the new sending rates for out the tasks listed below to derive the new sending rates for
all the flows in the FG. A flow's UPDATE function uses three all the flows in the FG. A flow's UPDATE function uses three
local (i.e. per-flow) temporary variables: S_P, TLO and AR. local (i.e., per-flow) temporary variables: S_P, TLO, and AR.
(a) It updates S_CR. (a) It updates S_CR.
S_CR = S_CR + CC_R(f) - FSE_R(f) S_CR = S_CR + CC_R(f) - FSE_R(f)
(b) It calculates the sum of all the priorities, S_P, and (b) It calculates the sum of all the priorities, S_P, and
initializes FSE_R. initializes FSE_R.
S_P = 0 S_P = 0
for all flows i in FG do for all flows i in FG do
S_P = S_P + P(i) S_P = S_P + P(i)
FSE_R(i) = 0 FSE_R(i) = 0
end for end for
(c) It distributes S_CR among all flows, ensuring that each (c) It distributes S_CR among all flows, ensuring that each
flow's desired rate is not exceeded. flow's desired rate is not exceeded.
TLO = S_CR TLO = S_CR
while(TLO-AR>0 and S_P>0) while(TLO-AR>0 and S_P>0)
AR = 0 AR = 0
for all flows i in FG do for all flows i in FG do
if FSE_R[i] < DR[i] then if FSE_R[i] < DR[i] then
if TLO * P[i] / S_P >= DR[i] then if TLO * P[i] / S_P >= DR[i] then
TLO = TLO - DR[i] TLO = TLO - DR[i]
FSE_R[i] = DR[i] FSE_R[i] = DR[i]
S_P = S_P - P[i] S_P = S_P - P[i]
else else
FSE_R[i] = TLO * P[i] / S_P FSE_R[i] = TLO * P[i] / S_P
AR = AR + TLO * P[i] / S_P AR = AR + TLO * P[i] / S_P
end if end if
end if end if
end for end for
end while end while
(d) It distributes FSE_R to all the flows. (d) It distributes FSE_R to all the flows.
for all flows i in FG do for all flows i in FG do
send FSE_R(i) to the flow i send FSE_R(i) to the flow i
end for end for
5.3.2. Example algorithm 2 - Conservative Active FSE 5.3.2. Example Algorithm 2 - Conservative Active FSE
This algorithm changes algorithm 1 to conservatively emulate the This algorithm changes algorithm 1 to conservatively emulate the
behavior of a single flow by proportionally reducing the aggregate behavior of a single flow by proportionally reducing the aggregate
rate on congestion. Simulations results in [fse] indicate that it rate on congestion. Simulation results in [FSE] indicate that it can
can significantly reduce queuing delay and packet loss. significantly reduce queuing delay and packet loss.
Step (a) of the UPDATE function is changed as described below. This Step (a) of the UPDATE function is changed as described below. This
also introduces a local variable DELTA, which is used to calculate also introduces a local variable DELTA, which is used to calculate
the difference between CC_R(f) and the previously stored FSE_R(f). the difference between CC_R(f) and the previously stored FSE_R(f).
To prevent flows from either ignoring congestion or overreacting, a To prevent flows from either ignoring congestion or overreacting, a
timer keeps them from changing their rates immediately after the timer keeps them from changing their rates immediately after the
common rate reduction that follows a congestion event. This timer is common rate reduction that follows a congestion event. This timer is
set to 2 RTTs of the flow that experienced congestion because it is set to two RTTs of the flow that experienced congestion because it is
assumed that a congestion event can persist for up to one RTT of that assumed that a congestion event can persist for up to one RTT of that
flow, with another RTT added to compensate for fluctuations in the flow, with another RTT added to compensate for fluctuations in the
measured RTT value. measured RTT value.
(a) It updates S_CR based on DELTA. (a) It updates S_CR based on DELTA.
if Timer has expired or was not set then if Timer has expired or was not set then
DELTA = CC_R(f) - FSE_R(f) DELTA = CC_R(f) - FSE_R(f)
if DELTA < 0 then // Reduce S_CR proportionally if DELTA < 0 then // Reduce S_CR proportionally
S_CR = S_CR * CC_R(f) / FSE_R(f) S_CR = S_CR * CC_R(f) / FSE_R(f)
Set Timer for 2 RTTs Set Timer for 2 RTTs
else else
S_CR = S_CR + DELTA S_CR = S_CR + DELTA
end if end if
end if end if
6. Application 6. Application
This section specifies how the FSE can be applied to specific This section specifies how the FSE can be applied to specific
congestion control mechanisms and makes general recommendations that congestion control mechanisms and makes general recommendations that
facilitate applying the FSE to future congestion controls. facilitate applying the FSE to future congestion controls.
6.1. NADA 6.1. NADA
Network-Assisted Dynamic Adapation (NADA) [I-D.ietf-rmcat-nada] is a Network-Assisted Dynamic Adaptation (NADA) [RFC8698] is a congestion
congestion control scheme for rtcweb. It calculates a reference rate control scheme for WebRTC. It calculates a reference rate r_ref upon
r_ref upon receiving an acknowledgment, and then, based on the receiving an acknowledgment and then, based on the reference rate,
reference rate, it calculates a video target rate r_vin and a sending calculates a video target rate r_vin and a sending rate for the
rate for the flows, r_send. flows, r_send.
When applying the FSE to NADA, the UPDATE function call described in When applying the FSE to NADA, the UPDATE function call described in
Section 5.3 gives the FSE NADA's reference rate r_ref. The Section 5.3 gives the FSE NADA's reference rate r_ref. The
recommended algorithm for NADA is the Active FSE in Section 5.3.1. recommended algorithm for NADA is the Active FSE in Section 5.3.1.
In step 3 (c), when the FSE_R(i) is "sent" to the flow i, this means In step 3 (d), when the FSE_R(i) is "sent" to the flow i, r_ref
updating r_ref(r_vin and r_send) of flow i with the value of (r_vin and r_send) of flow i is updated with the value of FSE_R(i).
FSE_R(i).
6.2. General recommendations 6.2. General Recommendations
This section provides general advice for applying the FSE to This section provides general advice for applying the FSE to
congestion control mechanisms. congestion control mechanisms.
Receiver-side calculations: Receiver-side calculations:
When receiver-side calculations make assumptions about the rate When receiver-side calculations make assumptions about the rate
of the sender, the calculations need to be synchronized or the of the sender, the calculations need to be synchronized, or the
receiver needs to be updated accordingly. This applies to TFRC receiver needs to be updated accordingly. This applies to TCP
[RFC5348], for example, where simulations showed somewhat less Friendly Rate Control (TFRC) [RFC5348], for example, where
favorable results when using the FSE without a receiver-side simulations showed somewhat less favorable results when using
change [fse]. the FSE without a receiver-side change [FSE].
Stateful algorithms: Stateful algorithms:
When a congestion control algorithm is stateful (e.g., TCP, When a congestion control algorithm is stateful (e.g., during
with Slow Start, Congestion Avoidance and Fast Recovery), these the TCP slow start, congestion avoidance, or fast recovery
states should be carefully considered such that the overall phase), these states should be carefully considered such that
state of the aggregate flow is correct. This may require the overall state of the aggregate flow is correct. This may
sharing more information in the UPDATE call. require sharing more information in the UPDATE call.
Rate jumps: Rate jumps:
The FSE-based coupling algorithms can let a flow quickly The FSE-based coupling algorithms can let a flow quickly
increase its rate to its fair share, e.g. when a new flow joins increase its rate to its fair share, e.g., when a new flow
or after a quiescent period. In case of window-based joins or after a quiescent period. In case of window-based
congestion controls, this may produce a burst which should be congestion controls, this may produce a burst that should be
mitigated in some way. An example of how this could be done mitigated in some way. An example of how this could be done
without using a timer is presented in [anrw2016], using TCP as without using a timer is presented in [ANRW2016], using TCP as
an example. an example.
7. Expected feedback from experiments 7. Expected Feedback from Experiments
The algorithm described in this memo has so far been evaluated using The algorithm described in this memo has so far been evaluated using
simulations covering all the tests for more than one flow from simulations covering all the tests for more than one flow from
[I-D.ietf-rmcat-eval-test] (see [IETF-93], [IETF-94]). Experiments [RMCAT-PROPOSALS] (see [IETF-93] and [IETF-94]). Experiments should
should confirm these results using at least the NADA congestion confirm these results using at least the NADA congestion control
control algorithm with real-life code (e.g., browsers communicating algorithm with real-life code (e.g., browsers communicating over an
over an emulated network covering the conditions in emulated network covering the conditions in [RMCAT-PROPOSALS]). The
[I-D.ietf-rmcat-eval-test]. The tests with real-life code should be tests with real-life code should be repeated afterwards in real
repeated afterwards in real network environments and monitored. network environments and monitored. Experiments should investigate
Experiments should investigate cases where the media coder's output cases where the media coder's output rate is below the rate that is
rate is below the rate that is calculated by the coupling algorithm calculated by the coupling algorithm (FSE_R(i) in algorithms 1
(FSE_R(i) in algorithms 1 and 2, section 5.3). Implementers and (Section 5.3.1) and 2 (Section 5.3.2)). Implementers and testers are
testers are invited to document their findings in an Internet draft. invited to document their findings in an Internet-Draft.
8. Acknowledgements
This document has benefitted from discussions with and feedback from
Andreas Petlund, Anna Brunstrom, Colin Perkins, David Hayes, David
Ros (who also gave the FSE its name), Ingemar Johansson, Karen
Nielsen, Kristian Hiorth, Mirja Kuehlewind, Martin Stiemerling,
Spencer Dawkins, Varun Singh, Xiaoqing Zhu, and Zaheduzzaman Sarker.
The authors would like to especially thank Xiaoqing Zhu and Stefan
Holmer for helping with NADA and GCC, and Anna Brunstrom as well as
Julius Flohr for helping us correct the active algorithm for the case
of application-limited flows.
This work was partially funded by the European Community under its
Seventh Framework Programme through the Reducing Internet Transport
Latency (RITE) project (ICT-317700).
9. IANA Considerations 8. IANA Considerations
This memo includes no request to IANA. This document has no IANA actions.
10. Security Considerations 9. Security Considerations
In scenarios where the architecture described in this document is In scenarios where the architecture described in this document is
applied across applications, various cheating possibilities arise: applied across applications, various cheating possibilities arise,
e.g., supporting wrong values for the calculated rate, the desired e.g., supporting wrong values for the calculated rate, desired rate,
rate, or the priority of a flow. In the worst case, such cheating or priority of a flow. In the worst case, such cheating could either
could either prevent other flows from sending or make them send at a prevent other flows from sending or make them send at a rate that is
rate that is unreasonably large. The end result would be unfair unreasonably large. The end result would be unfair behavior at the
behavior at the network bottleneck, akin to what could be achieved network bottleneck, akin to what could be achieved with any UDP-based
with any UDP based application. Hence, since this is no worse than application. Hence, since this is no worse than UDP in general,
UDP in general, there seems to be no significant harm in using this there seems to be no significant harm in using this in the absence of
in the absence of UDP rate limiters. UDP rate limiters.
In the case of a single-user system, it should also be in the In the case of a single-user system, it should also be in the
interest of any application programmer to give the user the best interest of any application programmer to give the user the best
possible experience by using reasonable flow priorities or even possible experience by using reasonable flow priorities or even
letting the user choose them. In a multi-user system, this interest letting the user choose them. In a multi-user system, this interest
may not be given, and one could imagine the worst case of an "arms may not be given, and one could imagine the worst case of an "arms
race" situation, where applications end up setting their priorities race" situation where applications end up setting their priorities to
to the maximum value. If all applications do this, the end result is the maximum value. If all applications do this, the end result is a
a fair allocation in which the priority mechanism is implicitly fair allocation in which the priority mechanism is implicitly
eliminated, and no major harm is done. eliminated and no major harm is done.
Implementers should also be aware of the Security Considerations Implementers should also be aware of the Security Considerations
sections of [RFC3124], [RFC5348], and [RFC7478]. sections of [RFC3124], [RFC5348], and [RFC7478].
11. References 10. References
11.1. Normative References
[I-D.ietf-rmcat-nada] 10.1. Normative References
Zhu, X., *, R., Ramalho, M., Cruz, S., Jones, P., Fu, J.,
and S. D'Aronco, "NADA: A Unified Congestion Control
Scheme for Real-Time Media", draft-ietf-rmcat-nada-11
(work in progress), July 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager", [RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, DOI 10.17487/RFC3124, June 2001, RFC 3124, DOI 10.17487/RFC3124, June 2001,
<https://www.rfc-editor.org/info/rfc3124>. <https://www.rfc-editor.org/info/rfc3124>.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008, RFC 5348, DOI 10.17487/RFC5348, September 2008,
<https://www.rfc-editor.org/info/rfc5348>. <https://www.rfc-editor.org/info/rfc5348>.
11.2. Informative References [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[anrw2016] [RFC8698] Zhu, X., Pan, R., Ramalho, M., and S. Mena, "Network-
Islam, S. and M. Welzl, "Start Me Up:Determining and Assisted Dynamic Adaptation (NADA): A Unified Congestion
Control Scheme for Real-Time Media", RFC 8698,
DOI 10.17487/RFC8698, January 2020,
<https://www.rfc-editor.org/info/rfc8698>.
10.2. Informative References
[ANRW2016] Islam, S. and M. Welzl, "Start Me Up: Determining and
Sharing TCP's Initial Congestion Window", ACM, IRTF, ISOC Sharing TCP's Initial Congestion Window", ACM, IRTF, ISOC
Applied Networking Research Workshop 2016 (ANRW 2016) , Applied Networking Research Workshop 2016 (ANRW 2016),
2016. DOI 10.1145/2959424.2959440, Proceedings of the 2016
Applied Networking Research Workshop Pages 52-54, July
2016, <https://doi.org/10.1145/2959424.2959440>.
[fse] Islam, S., Welzl, M., Gjessing, S., and N. Khademi, [FSE] Islam, S., Welzl, M., Gjessing, S., and N. Khademi,
"Coupled Congestion Control for RTP Media", ACM SIGCOMM "Coupled Congestion Control for RTP Media", ACM SIGCOMM
Capacity Sharing Workshop (CSWS 2014) and ACM SIGCOMM CCR Capacity Sharing Workshop (CSWS 2014) and ACM SIGCOMM CCR
44(4) 2014; extended version available as a technical 44(4) 2014, March 2014,
report from <http://safiquli.at.ifi.uio.no/paper/fse-tech-report.pdf>.
http://safiquli.at.ifi.uio.no/paper/fse-tech-report.pdf ,
2014.
[fse-noms]
Islam, S., Welzl, M., Hayes, D., and S. Gjessing,
"Managing Real-Time Media Flows through a Flow State
Exchange", IEEE NOMS 2016, Istanbul, Turkey , 2016.
[I-D.ietf-rmcat-eval-test] [FSE-NOMS] Islam, S., Welzl, M., Hayes, D., and S. Gjessing,
Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test "Managing real-time media flows through a flow state
Cases for Evaluating RMCAT Proposals", draft-ietf-rmcat- exchange", IEEE NOMS 2016, DOI 10.1109/NOMS.2016.7502803,
eval-test-10 (work in progress), May 2019. <https://doi.org/10.1109/NOMS.2016.7502803>.
[I-D.ietf-rmcat-gcc] [GCC-RTCWEB]
Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S. Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S.
Mascolo, "A Google Congestion Control Algorithm for Real- Mascolo, "A Google Congestion Control Algorithm for Real-
Time Communication", draft-ietf-rmcat-gcc-02 (work in Time Communication", Work in Progress, Internet-Draft,
progress), July 2016. draft-ietf-rmcat-gcc-02, 8 July 2016,
<https://tools.ietf.org/html/draft-ietf-rmcat-gcc-02>.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-19
(work in progress), November 2017.
[I-D.ietf-rtcweb-transports]
Alvestrand, H., "Transports for WebRTC", Internet-draft
draft-ietf-rtcweb-transports-17.txt, October 2016.
[IETF-93] Islam, S., Welzl, M., and S. Gjessing, "Updates on Coupled [IETF-93] Islam, S., Welzl, M., and S. Gjessing, "Updates on
Congestion Control for RTP Media", July 2015, 'Coupled Congestion Control for RTP Media'", RTP Media
Congestion Avoidance Techniques (rmcat) Working Group,
IETF 93, July 2015,
<https://www.ietf.org/proceedings/93/rmcat.html>. <https://www.ietf.org/proceedings/93/rmcat.html>.
[IETF-94] Islam, S., Welzl, M., and S. Gjessing, "Updates on Coupled [IETF-94] Islam, S., Welzl, M., and S. Gjessing, "Updates on
Congestion Control for RTP Media", November 2015, 'Coupled Congestion Control for RTP Media'", RTP Media
Congestion Avoidance Techniques (rmcat) Working Group,
IETF 94, November 2015,
<https://www.ietf.org/proceedings/94/rmcat.html>. <https://www.ietf.org/proceedings/94/rmcat.html>.
[RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real- [RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use Cases and Requirements", RFC 7478, Time Communication Use Cases and Requirements", RFC 7478,
DOI 10.17487/RFC7478, March 2015, DOI 10.17487/RFC7478, March 2015,
<https://www.rfc-editor.org/info/rfc7478>. <https://www.rfc-editor.org/info/rfc7478>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656, for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
skipping to change at page 15, line 38 skipping to change at line 651
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using [RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087, Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017, DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>. <https://www.rfc-editor.org/info/rfc8087>.
[RFC8382] Hayes, D., Ed., Ferlin, S., Welzl, M., and K. Hiorth, [RFC8382] Hayes, D., Ed., Ferlin, S., Welzl, M., and K. Hiorth,
"Shared Bottleneck Detection for Coupled Congestion "Shared Bottleneck Detection for Coupled Congestion
Control for RTP Media", RFC 8382, DOI 10.17487/RFC8382, Control for RTP Media", RFC 8382, DOI 10.17487/RFC8382,
June 2018, <https://www.rfc-editor.org/info/rfc8382>. June 2018, <https://www.rfc-editor.org/info/rfc8382>.
[rtcweb-rtp-usage] [RMCAT-PROPOSALS]
Sarker, Z., Singh, V., Zhu, X., and M. Ramalho, "Test
Cases for Evaluating RMCAT Proposals", Work in Progress,
Internet-Draft, draft-ietf-rmcat-eval-test-10, 23 May
2019, <https://tools.ietf.org/html/draft-ietf-rmcat-eval-
test-10>.
[RTCWEB-OVERVIEW]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", Work in Progress, Internet-
Draft, draft-ietf-rtcweb-overview-19, 11 November 2017,
<https://tools.ietf.org/html/draft-ietf-rtcweb-overview-
19>.
[RTCWEB-RTP-USAGE]
Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
Communication (WebRTC): Media Transport and Use of RTP", Communication (WebRTC): Media Transport and Use of RTP",
Internet-draft draft-ietf-rtcweb-rtp-usage-26.txt, March Work in Progress, Internet-Draft, draft-ietf-rtcweb-rtp-
2016. usage-26, 17 March 2016, <https://tools.ietf.org/html/
draft-ietf-rtcweb-rtp-usage-26>.
[transport-multiplex] [TRANSPORT-MULTIPLEX]
Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a
Single Lower-Layer Transport", Internet-draft draft- Single Lower-Layer Transport", Work in Progress, Internet-
westerlund-avtcore-transport-multiplexing-07.txt, October Draft, draft-westerlund-avtcore-transport-multiplexing-07,
2013. October 2013, <https://tools.ietf.org/html/draft-
westerlund-avtcore-transport-multiplexing-07>.
[WEBRTC-TRANS]
Alvestrand, H., "Transports for WebRTC", Work in Progress,
Internet-Draft, draft-ietf-rtcweb-transports-17, 26
October 2016, <https://tools.ietf.org/html/draft-ietf-
rtcweb-transports-17>.
Appendix A. Application to GCC Appendix A. Application to GCC
Google Congestion Control (GCC) [I-D.ietf-rmcat-gcc] is another Google Congestion Control (GCC) [GCC-RTCWEB] is another congestion
congestion control scheme for RTP flows that is under development. control scheme for RTP flows that is under development. GCC is not
GCC is not yet finalised, but at the time of this writing, the rate yet finalized, but at the time of this writing, the rate control of
control of GCC employs two parts: controlling the bandwidth estimate GCC employs two parts: controlling the bandwidth estimate based on
based on delay, and controlling the bandwidth estimate based on loss. delay and controlling the bandwidth estimate based on loss. Both are
Both are designed to estimate the available bandwidth, A_hat. designed to estimate the available bandwidth, A_hat.
When applying the FSE to GCC, the UPDATE function call described in When applying the FSE to GCC, the UPDATE function call described in
Section 5.3 gives the FSE GCC's estimate of available bandwidth Section 5.3 gives the FSE GCC's estimate of available bandwidth
A_hat. The recommended algorithm for GCC is the Active FSE in A_hat. The recommended algorithm for GCC is the Active FSE in
Section 5.3.1. In step 3 (c), when the FSE_R(i) is "sent" to the Section 5.3.1. In step 3 (d) of this algorithm, when the FSE_R(i) is
flow i, this means updating A_hat of flow i with the value of "sent" to the flow i, A_hat of flow i is updated with the value of
FSE_R(i). FSE_R(i).
Appendix B. Scheduling Appendix B. Scheduling
When flows originate from the same host, it would be possible to use When flows originate from the same host, it would be possible to use
only one single sender-side congestion controller which determines only one sender-side congestion controller that determines the
the overall allowed sending rate, and then use a local scheduler to overall allowed sending rate and then use a local scheduler to assign
assign a proportion of this rate to each RTP session. This way, a proportion of this rate to each RTP session. This way, priorities
priorities could also be implemented as a function of the scheduler. could also be implemented as a function of the scheduler. The
The Congestion Manager (CM) [RFC3124] also uses such a scheduling Congestion Manager (CM) [RFC3124] also uses such a scheduling
function. function.
Appendix C. Example algorithm - Passive FSE Appendix C. Example Algorithm - Passive FSE
Active algorithms calculate the rates for all the flows in the FG and Active algorithms calculate the rates for all the flows in the FG and
actively distribute them. In a passive algorithm, UPDATE returns a actively distribute them. In a passive algorithm, UPDATE returns a
rate that should be used instead of the rate that the congestion rate that should be used instead of the rate that the congestion
controller has determined. This can make a passive algorithm easier controller has determined. This can make a passive algorithm easier
to implement; however, when round-trip times of flows are unequal, to implement; however, when round-trip times of flows are unequal,
shorter-RTT flows may (depending on the congestion control algorithm) flows with shorter RTTs may (depending on the congestion control
update and react to the overall FSE state more often than longer-RTT algorithm) update and react to the overall FSE state more often than
flows, which can produce unwanted side effects. This problem is more flows with longer RTTs, which can produce unwanted side effects.
significant when the congestion control convergence depends on the This problem is more significant when the congestion control
RTT. While the passive algorithm works better for congestion convergence depends on the RTT. While the passive algorithm works
controls with RTT-independent convergence, it can still produce better for congestion controls with RTT-independent convergence, it
oscillations on short time scales. The algorithm described below is can still produce oscillations on short time scales. The algorithm
therefore considered as highly experimental and not safe to deploy described below is therefore considered highly experimental and not
outside of testbed environments. Results of a simplified passive FSE safe to deploy outside of testbed environments. Results of a
algorithm with both NADA and GCC can be found in [fse-noms]. simplified passive FSE algorithm with both NADA and GCC can be found
in [FSE-NOMS].
In the passive version of the FSE, TLO (the Total Leftover Rate) is a In the passive version of the FSE, TLO (Total Leftover Rate) is a
static variable per FG which is initialized to 0. Additionally, S_CR static variable per FG that is initialized to 0. Additionally, S_CR
is limited to increase or decrease as conservatively as a flow's is limited to increase or decrease as conservatively as a flow's
congestion controller decides in order to prohibit sudden rate jumps. congestion controller decides in order to prohibit sudden rate jumps.
(1) When a flow f starts, it registers itself with SBD and the FSE. (1) When a flow f starts, it registers itself with SBD and the FSE.
FSE_R(f) and DR(f) are initialized with the congestion FSE_R(f) and DR(f) are initialized with the congestion
controller's initial rate. SBD will assign the correct FGI. controller's initial rate. SBD will assign the correct FGI.
When a flow is assigned an FGI, it adds its FSE_R(f) to S_CR. When a flow is assigned an FGI, it adds its FSE_R(f) to S_CR.
(2) When a flow f stops or pauses, it sets its DR(f) to 0 and sets (2) When a flow f stops or pauses, it sets its DR(f) to 0 and sets
P(f) to -1. P(f) to -1.
(3) Every time the congestion controller of the flow f determines a (3) Every time the congestion controller of the flow f determines a
new sending rate CC_R(f), assuming the flow's new desired rate new sending rate CC_R(f), assuming the flow's new desired rate
new_DR(f) to be "infinity" in case of a bulk data transfer with new_DR(f) to be "infinity" in case of a bulk data transfer with
an unknown maximum rate, the flow calls UPDATE, which carries an unknown maximum rate, the flow calls UPDATE, which carries
out the tasks listed below to derive the flow's new sending out the tasks listed below to derive the flow's new sending
rate, Rate(f). A flow's UPDATE function uses a few local (i.e. rate, Rate(f). A flow's UPDATE function uses a few local (i.e.,
per-flow) temporary variables, which are all initialized to 0: per-flow) temporary variables, which are all initialized to 0:
DELTA, new_S_CR and S_P. DELTA, new_S_CR, and S_P.
(a) For all the flows in its FG (including itself), it (a) For all the flows in its FG (including itself), it
calculates the sum of all the calculated rates, new_S_CR. calculates the sum of all the calculated rates, new_S_CR.
Then it calculates DELTA: the difference between FSE_R(f) Then, it calculates DELTA: the difference between FSE_R(f)
and CC_R(f). and CC_R(f).
for all flows i in FG do for all flows i in FG do
new_S_CR = new_S_CR + FSE_R(i) new_S_CR = new_S_CR + FSE_R(i)
end for end for
DELTA = CC_R(f) - FSE_R(f) DELTA = CC_R(f) - FSE_R(f)
(b) It updates S_CR, FSE_R(f) and DR(f). (b) It updates S_CR, FSE_R(f), and DR(f).
FSE_R(f) = CC_R(f) FSE_R(f) = CC_R(f)
if DELTA > 0 then // the flow's rate has increased if DELTA > 0 then // the flow's rate has increased
S_CR = S_CR + DELTA S_CR = S_CR + DELTA
else if DELTA < 0 then else if DELTA < 0 then
S_CR = new_S_CR + DELTA S_CR = new_S_CR + DELTA
end if end if
DR(f) = min(new_DR(f),FSE_R(f)) DR(f) = min(new_DR(f),FSE_R(f))
(c) It calculates the leftover rate TLO, removes the terminated (c) It calculates the leftover rate TLO, removes the terminated
flows from the FSE and calculates the sum of all the flows from the FSE, and calculates the sum of all the
priorities, S_P. priorities, S_P.
for all flows i in FG do for all flows i in FG do
if P(i)<0 then if P(i)<0 then
delete flow delete flow
else else
S_P = S_P + P(i) S_P = S_P + P(i)
end if end if
end for end for
if DR(f) < FSE_R(f) then if DR(f) < FSE_R(f) then
TLO = TLO + (P(f)/S_P) * S_CR - DR(f)) TLO = TLO + (P(f)/S_P) * S_CR - DR(f))
end if end if
(d) It calculates the sending rate, Rate(f). (d) It calculates the sending rate, Rate(f).
Rate(f) = min(new_DR(f), (P(f)*S_CR)/S_P + TLO) Rate(f) = min(new_DR(f), (P(f)*S_CR)/S_P + TLO)
if Rate(f) != new_DR(f) and TLO > 0 then if Rate(f) != new_DR(f) and TLO > 0 then
TLO = 0 // f has 'taken' TLO TLO = 0 // f has 'taken' TLO
end if end if
(e) It updates DR(f) and FSE_R(f) with Rate(f). (e) It updates DR(f) and FSE_R(f) with Rate(f).
if Rate(f) > DR(f) then if Rate(f) > DR(f) then
DR(f) = Rate(f) DR(f) = Rate(f)
end if end if
FSE_R(f) = Rate(f) FSE_R(f) = Rate(f)
The goals of the flow algorithm are to achieve prioritization, The goals of the flow algorithm are to achieve prioritization,
improve network utilization in the face of application-limited flows, improve network utilization in the face of application-limited flows,
and impose limits on the increase behavior such that the negative and impose limits on the increase behavior such that the negative
impact of multiple flows trying to increase their rate together is impact of multiple flows trying to increase their rate together is
minimized. It does that by assigning a flow a sending rate that may minimized. It does that by assigning a flow a sending rate that may
not be what the flow's congestion controller expected. It therefore not be what the flow's congestion controller expected. It therefore
builds on the assumption that no significant inefficiencies arise builds on the assumption that no significant inefficiencies arise
from temporary application-limited behavior or from quickly jumping from temporary application-limited behavior or from quickly jumping
to a rate that is higher than the congestion controller intended. to a rate that is higher than the congestion controller intended.
How problematic these issues really are depends on the controllers in How problematic these issues really are depends on the controllers in
use and requires careful per-controller experimentation. The coupled use and requires careful per-controller experimentation. The coupled
congestion control mechanism described here also does not require all congestion control mechanism described here also does not require all
controllers to be equal; effects of heterogeneous controllers, or controllers to be equal; effects of heterogeneous controllers, or
homogeneous controllers being in different states, are also subject homogeneous controllers being in different states, are also subject
to experimentation. to experimentation.
This algorithm gives all the leftover rate of application-limited This algorithm gives the leftover rate of application-limited flows
flows to the first flow that updates its sending rate, provided that to the first flow that updates its sending rate, provided that this
this flow needs it all (otherwise, its own leftover rate can be taken flow needs it all (otherwise, its own leftover rate can be taken by
by the next flow that updates its rate). Other policies could be the next flow that updates its rate). Other policies could be
applied, e.g. to divide the leftover rate of a flow equally among all applied, e.g., to divide the leftover rate of a flow equally among
other flows in the FGI. all other flows in the FGI.
C.1. Example operation (passive) C.1. Example Operation (Passive)
In order to illustrate the operation of the passive coupled In order to illustrate the operation of the passive coupled
congestion control algorithm, this section presents a toy example of congestion control algorithm, this section presents a toy example of
two flows that use it. Let us assume that both flows traverse a two flows that use it. Let us assume that both flows traverse a
common 10 Mbit/s bottleneck and use a simplistic congestion common 10 Mbit/s bottleneck and use a simplistic congestion
controller that starts out with 1 Mbit/s, increases its rate by 1 controller that starts out with 1 Mbit/s, increases its rate by 1
Mbit/s in the absence of congestion and decreases it by 2 Mbit/s in Mbit/s in the absence of congestion, and decreases it by 2 Mbit/s in
the presence of congestion. For simplicity, flows are assumed to the presence of congestion. For simplicity, flows are assumed to
always operate in a round-robin fashion. Rate numbers below without always operate in a round-robin fashion. Rate numbers below without
units are assumed to be in Mbit/s. For illustration purposes, the units are assumed to be in Mbit/s. For illustration purposes, the
actual sending rate is also shown for every flow in FSE diagrams even actual sending rate is also shown for every flow in FSE diagrams even
though it is not really stored in the FSE. though it is not really stored in the FSE.
Flow #1 begins. It is a bulk data transfer and considers itself to Flow #1 begins. It is a bulk data transfer and considers itself to
have top priority. This is the FSE after the flow algorithm's step have top priority. This is the FSE after the flow algorithm's step
1: 1:
skipping to change at page 19, line 42 skipping to change at line 860
Its congestion controller gradually increases its rate. Eventually, Its congestion controller gradually increases its rate. Eventually,
at some point, the FSE should look like this: at some point, the FSE should look like this:
----------------------------------------- -----------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 10 | 10 | 10 | | 1 | 1 | 1 | 10 | 10 | 10 |
----------------------------------------- -----------------------------------------
S_CR = 10, TLO = 0 S_CR = 10, TLO = 0
Now another flow joins. It is also a bulk data transfer, and has a Now, another flow joins. It is also a bulk data transfer and has a
lower priority (0.5): lower priority (0.5):
------------------------------------------ ------------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 10 | 10 | 10 | | 1 | 1 | 1 | 10 | 10 | 10 |
| 2 | 1 | 0.5 | 1 | 1 | 1 | | 2 | 1 | 0.5 | 1 | 1 | 1 |
------------------------------------------ ------------------------------------------
S_CR = 11, TLO = 0 S_CR = 11, TLO = 0
Now assume that the first flow updates its rate to 8, because the Now, assume that the first flow updates its rate to 8, because the
total sending rate of 11 exceeds the total capacity. Let us take a total sending rate of 11 exceeds the total capacity. Let us take a
closer look at what happens in step 3 of the flow algorithm. closer look at what happens in step 3 of the flow algorithm.
CC_R(1) = 8. new_DR(1) = infinity. CC_R(1) = 8. new_DR(1) = infinity.
3 a) new_S_CR = 11; DELTA = 8 - 10 = -2.
3 b) FSE_R(1) = 8. DELTA is negative, hence S_CR = 9; (3a) new_S_CR = 11; DELTA = 8 - 10 = -2.
DR(1) = 8.
3 c) S_P = 1.5. (3b) FSE_R(1) = 8. DELTA is negative, hence S_CR = 9; DR(1) = 8
3 d) new sending rate Rate(1) = min(infinity, 1/1.5 * 9 + 0) = 6.
3 e) FSE_R(1) = 6. (3c) S_P = 1.5.
(3d) new sending rate Rate(1) = min(infinity, 1/1.5 * 9 + 0) = 6.
(3e) FSE_R(1) = 6.
The resulting FSE looks as follows: The resulting FSE looks as follows:
------------------------------------------- -------------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 6 | 8 | 6 | | 1 | 1 | 1 | 6 | 8 | 6 |
| 2 | 1 | 0.5 | 1 | 1 | 1 | | 2 | 1 | 0.5 | 1 | 1 | 1 |
------------------------------------------- -------------------------------------------
S_CR = 9, TLO = 0 S_CR = 9, TLO = 0
The effect is that flow #1 is sending with 6 Mbit/s instead of the 8 The effect is that flow #1 is sending with 6 Mbit/s instead of the 8
Mbit/s that the congestion controller derived. Let us now assume Mbit/s that the congestion controller derived. Let us now assume
skipping to change at page 21, line 6 skipping to change at line 904
------------------------------------------- -------------------------------------------
S_CR = 9, TLO = 0 S_CR = 9, TLO = 0
The effect is that flow #1 is sending with 6 Mbit/s instead of the 8 The effect is that flow #1 is sending with 6 Mbit/s instead of the 8
Mbit/s that the congestion controller derived. Let us now assume Mbit/s that the congestion controller derived. Let us now assume
that flow #2 updates its rate. Its congestion controller detects that flow #2 updates its rate. Its congestion controller detects
that the network is not fully saturated (the actual total sending that the network is not fully saturated (the actual total sending
rate is 6+1=7) and increases its rate. rate is 6+1=7) and increases its rate.
CC_R(2) = 2. new_DR(2) = infinity. CC_R(2) = 2. new_DR(2) = infinity.
3 a) new_S_CR = 7; DELTA = 2 - 1 = 1.
3 b) FSE_R(2) = 2. DELTA is positive, hence S_CR = 9 + 1 = 10; (3a) new_S_CR = 7; DELTA = 2 - 1 = 1.
DR(2) = 2.
3 c) S_P = 1.5. (3b) FSE_R(2) = 2. DELTA is positive, hence S_CR = 9 + 1 = 10;
3 d) Rate(2) = min(infinity, 0.5/1.5 * 10 + 0) = 3.33. DR(2) = 2.
3 e) DR(2) = FSE_R(2) = 3.33.
(3c) S_P = 1.5.
(3d) Rate(2) = min(infinity, 0.5/1.5 * 10 + 0) = 3.33.
(3e) DR(2) = FSE_R(2) = 3.33.
The resulting FSE looks as follows: The resulting FSE looks as follows:
------------------------------------------- -------------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 6 | 8 | 6 | | 1 | 1 | 1 | 6 | 8 | 6 |
| 2 | 1 | 0.5 | 3.33 | 3.33 | 3.33 | | 2 | 1 | 0.5 | 3.33 | 3.33 | 3.33 |
------------------------------------------- -------------------------------------------
S_CR = 10, TLO = 0 S_CR = 10, TLO = 0
The effect is that flow #2 is now sending with 3.33 Mbit/s, which is The effect is that flow #2 is now sending with 3.33 Mbit/s, which is
close to half of the rate of flow #1 and leads to a total utilization close to half of the rate of flow #1 and leads to a total utilization
skipping to change at page 21, line 32 skipping to change at line 936
The effect is that flow #2 is now sending with 3.33 Mbit/s, which is The effect is that flow #2 is now sending with 3.33 Mbit/s, which is
close to half of the rate of flow #1 and leads to a total utilization close to half of the rate of flow #1 and leads to a total utilization
of 6(#1) + 3.33(#2) = 9.33 Mbit/s. Flow #2's congestion controller of 6(#1) + 3.33(#2) = 9.33 Mbit/s. Flow #2's congestion controller
has increased its rate faster than the controller actually expected. has increased its rate faster than the controller actually expected.
Now, flow #1 updates its rate. Its congestion controller detects Now, flow #1 updates its rate. Its congestion controller detects
that the network is not fully saturated and increases its rate. that the network is not fully saturated and increases its rate.
Additionally, the application feeding into flow #1 limits the flow's Additionally, the application feeding into flow #1 limits the flow's
sending rate to at most 2 Mbit/s. sending rate to at most 2 Mbit/s.
CC_R(1) = 7. new_DR(1) = 2. CC_R(1) = 7. new_DR(1) = 2.
3 a) new_S_CR = 9.33; DELTA = 1.
3 b) FSE_R(1) = 7, DELTA is positive, hence S_CR = 10 + 1 = 11; (3a) new_S_CR = 9.33; DELTA = 1.
DR(1) = min(2, 7) = 2.
3 c) S_P = 1.5; DR(1) < FSE_R(1), hence TLO = 1/1.5 * 11 - 2 = 5.33. (3b) FSE_R(1) = 7, DELTA is positive, hence S_CR = 10 + 1 = 11;
3 d) Rate(1) = min(2, 1/1.5 * 11 + 5.33) = 2. DR(1) = min(2, 7) = 2.
3 e) FSE_R(1) = 2.
(3c) S_P = 1.5; DR(1) < FSE_R(1), hence TLO = 1/1.5 * 11 - 2 = 5.33.
(3d) Rate(1) = min(2, 1/1.5 * 11 + 5.33) = 2.
(3e) FSE_R(1) = 2.
The resulting FSE looks as follows: The resulting FSE looks as follows:
------------------------------------------- -------------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 2 | 2 | 2 | | 1 | 1 | 1 | 2 | 2 | 2 |
| 2 | 1 | 0.5 | 3.33 | 3.33 | 3.33 | | 2 | 1 | 0.5 | 3.33 | 3.33 | 3.33 |
------------------------------------------- -------------------------------------------
S_CR = 11, TLO = 5.33 S_CR = 11, TLO = 5.33
Now, the total rate of the two flows is 2 + 3.33 = 5.33 Mbit/s, i.e.
Now, the total rate of the two flows is 2 + 3.33 = 5.33 Mbit/s, i.e.,
the network is significantly underutilized due to the limitation of the network is significantly underutilized due to the limitation of
flow #1. Flow #2 updates its rate. Its congestion controller flow #1. Flow #2 updates its rate. Its congestion controller
detects that the network is not fully saturated and increases its detects that the network is not fully saturated and increases its
rate. rate.
CC_R(2) = 4.33. new_DR(2) = infinity. CC_R(2) = 4.33. new_DR(2) = infinity.
3 a) new_S_CR = 5.33; DELTA = 1.
3 b) FSE_R(2) = 4.33. DELTA is positive, hence S_CR = 12; (3a) new_S_CR = 5.33; DELTA = 1.
DR(2) = 4.33.
3 c) S_P = 1.5. (3b) FSE_R(2) = 4.33. DELTA is positive, hence S_CR = 12; DR(2) =
3 d) Rate(2) = min(infinity, 0.5/1.5 * 12 + 5.33 ) = 9.33. 4.33.
3 e) FSE_R(2) = 9.33, DR(2) = 9.33.
(3c) S_P = 1.5.
(3d) Rate(2) = min(infinity, 0.5/1.5 * 12 + 5.33 ) = 9.33.
(3e) FSE_R(2) = 9.33, DR(2) = 9.33.
The resulting FSE looks as follows: The resulting FSE looks as follows:
------------------------------------------- -------------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 2 | 2 | 2 | | 1 | 1 | 1 | 2 | 2 | 2 |
| 2 | 1 | 0.5 | 9.33 | 9.33 | 9.33 | | 2 | 1 | 0.5 | 9.33 | 9.33 | 9.33 |
------------------------------------------- -------------------------------------------
S_CR = 12, TLO = 0 S_CR = 12, TLO = 0
Now, the total rate of the two flows is 2 + 9.33 = 11.33 Mbit/s. Now, the total rate of the two flows is 2 + 9.33 = 11.33 Mbit/s.
Finally, flow #1 terminates. It sets P(1) to -1 and DR(1) to 0. Let Finally, flow #1 terminates. It sets P(1) to -1 and DR(1) to 0. Let
skipping to change at page 22, line 30 skipping to change at line 990
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 1 | 1 | 1 | 2 | 2 | 2 | | 1 | 1 | 1 | 2 | 2 | 2 |
| 2 | 1 | 0.5 | 9.33 | 9.33 | 9.33 | | 2 | 1 | 0.5 | 9.33 | 9.33 | 9.33 |
------------------------------------------- -------------------------------------------
S_CR = 12, TLO = 0 S_CR = 12, TLO = 0
Now, the total rate of the two flows is 2 + 9.33 = 11.33 Mbit/s. Now, the total rate of the two flows is 2 + 9.33 = 11.33 Mbit/s.
Finally, flow #1 terminates. It sets P(1) to -1 and DR(1) to 0. Let Finally, flow #1 terminates. It sets P(1) to -1 and DR(1) to 0. Let
us assume that it terminated late enough for flow #2 to still us assume that it terminated late enough for flow #2 to still
experience the network in a congested state, i.e. flow #2 decreases experience the network in a congested state, i.e., flow #2 decreases
its rate in the next iteration. its rate in the next iteration.
CC_R(2) = 7.33. new_DR(2) = infinity. CC_R(2) = 7.33. new_DR(2) = infinity.
3 a) new_S_CR = 11.33; DELTA = -2.
3 b) FSE_R(2) = 7.33. DELTA is negative, hence S_CR = 9.33; (3a) new_S_CR = 11.33; DELTA = -2.
DR(2) = 7.33.
3 c) Flow 1 has P(1) = -1, hence it is deleted from the FSE. (3b) FSE_R(2) = 7.33. DELTA is negative, hence S_CR = 9.33; DR(2) =
S_P = 0.5. 7.33.
3 d) Rate(2) = min(infinity, 0.5/0.5*9.33 + 0) = 9.33.
3 e) FSE_R(2) = DR(2) = 9.33. (3c) Flow 1 has P(1) = -1, hence it is deleted from the FSE. S_P =
0.5.
(3d) Rate(2) = min(infinity, 0.5/0.5*9.33 + 0) = 9.33.
(3e) FSE_R(2) = DR(2) = 9.33.
The resulting FSE looks as follows: The resulting FSE looks as follows:
------------------------------------------- -------------------------------------------
| # | FGI | P | FSE_R | DR | Rate | | # | FGI | P | FSE_R | DR | Rate |
| | | | | | | | | | | | | |
| 2 | 1 | 0.5 | 9.33 | 9.33 | 9.33 | | 2 | 1 | 0.5 | 9.33 | 9.33 | 9.33 |
------------------------------------------- -------------------------------------------
S_CR = 9.33, TLO = 0 S_CR = 9.33, TLO = 0
Appendix D. Change log Acknowledgements
D.1. draft-welzl-rmcat-coupled-cc
D.1.1. Changes from -00 to -01
o Added change log.
o Updated the example algorithm and its operation.
D.1.2. Changes from -01 to -02
o Included an active version of the algorithm which is simpler.
o Replaced "greedy flow" with "bulk data transfer" and "non-greedy"
with "application-limited".
o Updated new_CR to CC_R, and CR to FSE_R for better understanding.
D.1.3. Changes from -02 to -03
o Included an active conservative version of the algorithm which
reduces queue growth and packet loss; added a reference to a
technical report that shows these benefits with simulations.
o Moved the passive variant of the algorithm to appendix.
D.1.4. Changes from -03 to -04
o Extended SBD section.
o Added a note about window-based controllers.
D.1.5. Changes from -04 to -05
o Added a section about applying the FSE to specific congestion
control algorithms, with a subsection specifying its use with
NADA.
D.2. draft-ietf-rmcat-coupled-cc
D.2.1. Changes from draft-welzl-rmcat-coupled-cc-05
o Moved scheduling section to the appendix.
D.2.2. Changes from -00 to -01
o Included how to apply the algorithm to GCC.
o Updated variable names of NADA to be in line with the latest
version.
o Added a reference to [I-D.ietf-rtcweb-transports] to make a
connection to the prioritization text there.
D.2.3. Changes from -01 to -02
o Minor changes.
o Moved references of NADA and GCC from informative to normative.
o Added a reference for the passive variant of the algorithm.
D.2.4. Changes from -02 to -03
o Minor changes.
o Added a section about expected feedback from experiments.
D.2.5. Changes from -03 to -04
o Described the names of variables used in the algorithms.
o Added a diagram to illustrate the interaction between flows and
the FSE.
o Added text on the trade-off of using the configuration based
approach.
o Minor changes to enhance the readability.
D.2.6. Changes from -04 to -05
o Changed several occurrences of "NADA and GCC" to "NADA", including
the abstract.
o Moved the application to GCC to an appendix, and made the GCC
reference informative.
o Provided a few more general recommendations on applying the
coupling algorithm.
D.2.7. Changes from -05 to -06
o Incorporated comments by Colin Perkins.
D.2.8. Changes from -06 to -07
o Addressed OPSDIR, SECDIR, GENART, AD and IESG comments.
D.2.9. Changes from -07 to -08
o Updated the algorithms in section 5 to support application-limited
flows. Moved definition of Desired Rate from appendix to section
5. Updated references.
D.2.10. Changes from -08 to -09 This document benefited from discussions with and feedback from
Andreas Petlund, Anna Brunstrom, Colin Perkins, David Hayes, David
Ros (who also gave the FSE its name), Ingemar Johansson, Karen
Nielsen, Kristian Hiorth, Martin Stiemerling, Mirja K├╝hlewind,
Spencer Dawkins, Varun Singh, Xiaoqing Zhu, and Zaheduzzaman Sarker.
The authors would like to especially thank Xiaoqing Zhu and Stefan
Holmer for helping with NADA and GCC, and Anna Brunstrom as well as
Julius Flohr for helping us correct the active algorithm for the case
of application-limited flows.
o Minor improvement of the algorithms in section 5. This work was partially funded by the European Community under its
Seventh Framework Program through the Reducing Internet Transport
Latency (RITE) project (ICT-317700).
Authors' Addresses Authors' Addresses
Safiqul Islam Safiqul Islam
University of Oslo University of Oslo
PO Box 1080 Blindern PO Box 1080 Blindern
Oslo N-0316 N-0316 Oslo
Norway Norway
Phone: +47 22 84 08 37 Phone: +47 22 84 08 37
Email: safiquli@ifi.uio.no Email: safiquli@ifi.uio.no
Michael Welzl Michael Welzl
University of Oslo University of Oslo
PO Box 1080 Blindern PO Box 1080 Blindern
Oslo N-0316 N-0316 Oslo
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
University of Oslo University of Oslo
PO Box 1080 Blindern PO Box 1080 Blindern
Oslo N-0316 N-0316 Oslo
Norway Norway
Phone: +47 22 85 24 44 Phone: +47 22 85 24 44
Email: steing@ifi.uio.no Email: steing@ifi.uio.no
 End of changes. 140 change blocks. 
529 lines changed or deleted 451 lines changed or added

This html diff was produced by rfcdiff 1.47. The latest version is available from http://tools.ietf.org/tools/rfcdiff/