Internet Research Task Force (IRTF) M. Welzl
Request for Comments: 5783 University of Oslo
Category: Informational W. Eddy
ISSN: 2070-1721 MTI Systems
February 2010 Congestion Control in the RFC Series
This document is an informational snapshot taken by the IRTF's
Internet Congestion Control Research Group (ICCRG) in October 2008.
It provides a survey of congestion control topics described by
documents in the RFC series. This does not modify or update the
specifications or status of the RFC documents that are discussed. It
may be used as a reference or starting point for the future work of
the research group, especially in noting gaps or open issues in the
current IETF standards.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the Internet
Congestion Control Research Group of the Internet Research Task Force
(IRTF). Documents approved for publication by the IRSG are not a
candidate for any level of Internet Standard; see Section 2 of RFC
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
In this document, we define congestion control as the feedback-based
adjustment of the rate at which data is sent into the network.
Congestion control is an indispensable set of principles and
mechanisms for maintaining the stability of the Internet. Congestion
control has been closely associated with TCP since 1988 [Jac88], but
there has also been a great deal of congestion control work outside
of TCP (e.g., for real-time multimedia applications, multicast, and
router-based mechanisms). Several such proposals have been produced
within the IETF and published as RFCs, along with RFCs that give
architectural guidance (e.g., by pointing out the importance of
performing some form of congestion control). Several of these
mechanisms are in use within the Internet.
When designing a new Internet transport protocol, it is therefore
important to not only understand how congestion control works in TCP
but also have a broader understanding of the other congestion control
RFCs -- some give guidance, some of them describe mechanisms that may
have a direct influence on a newly designed protocol, and some of
them may only be "related work" worth knowing about. The purpose of
this document is to facilitate and encourage this search for
knowledge by providing an overview of RFCs related to congestion
control that have been published thus far. This document is a
product of the IRTF's Internet Congestion Control Research Group
(ICCRG). It was developed because a strong grasp of the existing
literature should benefit further ICCRG work. The ICCRG developed
consensus on the content of this document during a two-year
development period based on review comments and ICCRG mailing list
discussions. A list of the main review contributors is contained in
the Acknowledgements section of this document.
While the ICCRG agreed to the document's production, any opinions
expressed are the authors' own, and as this document is not an IETF
publication, it does not update or modify the status of any published
RFCs. The format of this document is similar to an annotated
bibliography. Although host and router requirements for congestion
control functions are discussed, this is only an informational
document and does not contain any formal standards bearing of its
Congestion control is a large and active topic, and so the scope of
this document is limited to published RFCs and a small number of
current working group drafts. This allows the document to focus on
congestion control principles and mechanisms that are among the most
well-supported, well-accepted, or widely used. Significant
contributions to this subject also exist in both the academic
literature and in the form of Internet-Drafts; however, we exclude
these from this study. In many cases the RFC describing some
mechanism will contain references to relevant academic publications
in journals or conference proceedings that presented the research and
validation of the mechanism. For instance, RFC 2581 cites Jacobson's
1988 SIGCOMM paper that has a less standards-oriented but more
illustrative treatment and explanation of some of the mechanisms in
The majority of the documents discussed here pertain to end-host-
based congestion control. Many network-based mechanisms, such as a
number of queue management algorithms, do not require any protocol
exchanges between elements, but merely operate within a single host
or router. Thus, network-based congestion control mechanisms have
often not been described in any RFC, as they generally fall under the
domain of implementation details that do not influence
There are many RFCs related to Quality of Service (QoS), especially
within the Integrated Services and Differentiated Services frameworks
[RFC1633] [RFC2475] [RFC2998]. These QoS RFCs themselves deserve a
similar bibliography to the one that this document provides for
congestion control. We specifically do not include the vast amount
of QoS work into the scope of this document, as it is a full field in
its own right, and deals with issues that are mostly orthogonal to
end-host congestion control and router queue management. Although
there can certainly be interactions between QoS and congestion
control mechanisms, scheduling mechanisms used to implement QoS (on
either a per-flow or an aggregate basis), for instance, can be used
independently of the end-host congestion control and queue management
functions also in use. Similar arguments can be made for traffic-
shaping, admission control, and other functions that are intended for
QoS and are only side-notes for congestion control.
A similar argument can be made for excluding consideration of the
media access control (MAC) layer protocols used by the links
throughout a path. Although the MAC protocols implement various
forms of resolving contention for shared links (and sometimes offer
QoS services), these are also distinct from end-to-end congestion
control. Furthermore, MAC protocols are not typically discussed in
the RFC series, but they are defined in outside documents (e.g., IEEE
standards), since the IETF does not generally work on link layers
themselves. Few, if any, of the RFCs that describe mappings of IP
onto various link layers directly discuss congestion control.
To organize the subject matter in this document, the content is
classified into several broad categories. First, we list documents
relating to Internet architecture and general architectural concepts
in Section 2. Next, the congestion control algorithms used in the
TCP transport protocol are discussed in Section 3. Interactions
between link properties and mechanisms with the kinds of algorithms
and heuristics used within end-to-end congestion control are covered
in Section 4. One method that has been developed by the IETF (and
deployed to some extent) for allowing network-based and host-based
congestion control to interact without dropping packets is the
subject of Section 5.1. The congestion control algorithms used by
unicast transport protocols other than TCP are described in
Section 6. Work on congestion control for multicast transports and
applications is listed in Section 7. RFCs that give guidance to
developers of new algorithms are discussed in Section 8. Finally,
documents that have historic significance, but perhaps not current
direct technical application, have been classified into Section 9.
Note that the use of the term "historic" here has nothing to do with
the IETF's formal classification of documents as having "Historic"
2. Architectural Documents
Some documents in this section contain architectural guidance and
concerns, while others specify congestion-control-related mechanisms
that are broadly applicable and have impacts on more than a single
class of congestion control techniques. Some of these documents are
direct products of the Internet Architecture Board (IAB), giving
their guidance on specific aspects of congestion control in the
RFC 1122: "Requirements for Internet Hosts -- Communication Layers"
[RFC1122] formally mandates that hosts perform congestion control.
For TCP, several congestion control features are described and
listed as required elements of conforming implementations, and for
UDP, RFC 1122 leaves congestion control as an issue for higher-
layered protocols. Although sending and reacting to ICMP Source
Quench packets is no longer recommended [RFC1812] [Gont10], the
rest of the congestion control guidance in this RFC is still a
basis for several current practices in TCP implementations.
RFC 1812: "Requirements for IP Version 4 Routers" (June 1995)
Numerous issues relevant to router behavior are discussed in
[RFC1812], and requirements for routers to support are prescribed
within the document. Portions of RFC 1812 that are particularly
relevant to congestion control include the directive that routers
SHOULD NOT originate ICMP Source Quench messages, discussion of
precedence in queueing, and Section 5.3.6 titled "Congestion
Control" that recommends sizing buffers as a function of the
product of the bandwidth of the link times the path delay of the
flows using the link, and advises on the implementation of active
queue management techniques.
RFC 1958: "Architectural Principles of the Internet" (June 1996)
Several guidelines for network systems design that have proven
useful in the evolution of the Internet are sketched in [RFC1958].
Congestion control is not specifically mentioned or alluded to,
but the general principles apply to congestion control. For
instance, performing end-to-end functions at end nodes, lack of
centralized control, heterogeneity, scalability, simplicity,
avoiding options and parameters, etc., are all valid concerns in
the design and assessment of congestion control schemes for the
RFC 2140: "TCP Control Block Interdependence" (April 1997)
[RFC2140] suggests that TCP connections between the same endpoints
might share some information, including their congestion control
state. To some degree, this is done in practice by a few current
operating systems; for example, Linux currently has a destination
cache with this information, but this behavior is not yet formally
standardized or recognized as a best practice by the IETF.
RFC 2309: "Recommendations on Queue Management and Congestion
Avoidance in the Internet" (April 1998)
[RFC2309] briefly discusses the history of congestion and the
origin of congestion control in the Internet. The focus is mainly
on network- or router-based queue management algorithms. This RFC
recommends to test, standardize, and deploy Active Queue
Management (AQM) in routers; it provides an overview of one such
mechanism, Random Early Detection (RED), and explains how and why
AQM mechanisms can improve the performance of the Internet.
Finally, this document explains the danger of a possible
"congestion collapse" from unresponsive flows and makes a strong
recommendation to develop and eventually deploy router mechanisms
to protect the Internet from such traffic.
Today, the advice in this document has been followed to some
extent. Hardware and software vendors have been receptive, and
AQM techniques are widely available in many popular dedicated
commercial router products and even in more general operating
systems that are sometimes used as routers. However, AQM
techniques may not be enabled in default configurations of these
systems, and it is often left to users and network engineers to
enable and configure AQM mechanisms when desired. In some cases,
enabling QoS mechanisms on a device also enables AQM mechanisms by
default. The number of production routers that actually have
these AQM features enabled is an open question.
RFC 2914 (BCP 41): "Congestion Control Principles" (September 2000)
[RFC2914] is an explanation of the principles of congestion
control, and the IETF's Best Current Practice for congestion
control design. It points out that there are an increasing number
of applications that do not use TCP, and elaborates on the
importance of performing congestion control for such traffic in
order to prevent congestion collapse. The TCP Reno congestion
control mechanisms are described as an example of end-to-end
congestion control within transport protocols.
SCTP is one example of a non-TCP transport protocol that
implements congestion control based on these principles. The
developments of TFRC [RFC3448] and DCCP [RFC4340] are attempts to
provide useful tools implementing those principles for
applications with needs similar to streaming media, where TCP's
reactions are too fast. It would be beneficial for users and the
Internet itself if these carefully designed tools become widely
deployed in place of other ad hoc schemes that may not be well-
grounded in the congestion control principles. This replacement
process is ongoing and not yet complete. Appropriate and usable
congestion control schemes for non-TCP flows continue to be an
open research area.
RFC 3124: "The Congestion Manager" (June 2001)
[RFC3124] specifies the Congestion Manager, an end-system service
that realizes congestion control on a per-host-pair rather than a
per-connection basis, which may be a more appropriate way to carry
out congestion control. Using the Congestion Manager, multiple
streams between two hosts (which may include TCP flows) can adapt
to network congestion in a unified fashion.
This proposal is related to RFC 2140, discussed above, but with a
wider scope than TCP. Because some pieces of its supporting
architecture have not yet been specified, the Congestion Manager's
techniques are not commonly used today and have not been widely
implemented and deployed yet beyond experimental stacks. Sharing
of congestion and path information between individual connections
continues to be an open research area with branches in detecting
shared bottlenecks when using multiple paths, caching of old state
for faster startup, and sharing of current state and feedback.
RFC 3426: "General Architectural and Policy Considerations" (November
[RFC3426] lists a number of questions that can be answered for a
particular technical solution to determine its architectural
impact and desirability. These are valid for congestion control
mechanisms, and end-point congestion management is used as an
example case-study several times in RFC 3426. Two salient
questions that RFC 3426 advises asking about proposed mechanisms
are why they are needed in addition to existing protocols, and why
they are needed at a certain layer rather than at other layers.
These are particularly relevant for congestion control mechanisms
since several already exist and since they can span network,
transport, and application layers.
RFC 3439: "Some Internet Architectural Guidelines and Philosophy"
[RFC3439] supplements RFC 1958. Simplicity is stressed, as the
unpredictable results of complexity (due to amplification and
coupling) are described. Congestion control issues stemming from
layering interactions between transport and lower protocols are
presented, as well as other items relevant to congestion control,
including asymmetry and the "myth of over-provisioning".
RFC 3714: "IAB Concerns Regarding Congestion Control for Voice
Traffic in the Internet" (March 2004)
[RFC3714] can be seen as a follow-up to the concerns that were
discussed in RFC 2914. It expresses the IAB's concern over the
lack of effective end-to-end congestion control for best-effort
voice traffic, which is noted as being a current service with
growing demand. An example of a VoIP connection between Atlanta,
Georgia, USA, and Nairobi, Kenya, is given, where a single VoIP
call consumed more than half of the access link capacity (which is
normally shared across several different users). This example is
used as the basis for further discussion, making it clear that
using some form of congestion control for VoIP traffic is highly
3. TCP Congestion Control
The TCP specifications found in RFC 793 and its predecessors did not
contain any discussion of using or managing a congestion window.
Other than a simple retransmission timeout and flow control through
the advertised receive window, TCP implementations based only on RFC
793 do not contain congestion control. As several congestion
collapse events occurred on the Internet, it was later realized that
congestion control was needed. The host requirements in RFC 1122
require conforming TCP implementations to implement Jacobson's slow
start and congestion avoidance algorithms (later specified in RFC
2001 and then RFC 2581). RFC 1122 also recommends several other
behaviors that influence congestion control like the Nagle algorithm,
delayed acknowledgements, Jacobson's retransmission timeout (RTO)
estimation algorithm, and exponential backoff of the retransmission
Basic TCP congestion control is defined in RFC 2581, with many other
RFCs that specify ancillary modifications and enhancements. RFC 2581
obsoletes the first proposed standard for TCP congestion control in
RFC 2001. These two RFCs document the mechanisms that had already
been in common use by TCP implementations for many years. The reader
may refer to the TCP Roadmap [RFC4614] for more information on the
RFCs that specifically describe TCP congestion control, as this
material is not replicated here.
Recently, significant effort has been put into experimental TCP
congestion control modifications for obtaining high throughput with
reduced startup and recovery times. RFCs have been published on some
of these modifications, including HighSpeed TCP [RFC3649], and
Limited Slow-Start [RFC3742], but high-rate congestion control
mechanisms are still considered an open issue in congestion control
research. Other schemes have been published as Internet-Drafts or
have been discussed a little by the IETF, but much of the work in
this area has not been adopted within the IETF yet, so the majority
of this work is outside the RFC series and may be discussed in other
products of the ICCRG.
At the time of writing, the IETF's TCP Maintenance and Minor
Extensions (TCPM) Working Group was developing an update to RFC 2581
to incorporate small changes from other documents and advance TCP
congestion control mechanisms on the IETF Standards Track. The
update also clarifies and revises some points. These include the
definition of a duplicate ACK, initial congestion window and slow
start threshold values, behavior in response to retransmission
timeouts, the use of the limited transmit mechanism, and security
with regards to misbehaving receivers that practice ACK division.
4. Challenging Link and Path Characteristics
Links with large and/or variable bandwidth-delay products have
traditionally been problematic for congestion control schemes because
they can distort the properties of the feedback loop. Links that
either expose a high rate of packet losses to the upper layers, or
use highly-persistent retransmission mechanisms to prevent losses
also cause problems with some of the standard congestion control
mechanisms. The documents in this section discuss challenging link
characteristics; many of them were written by the Performance
Implications of Link Characteristics (PILC) Working Group.
While these documents often refer to specific problems with TCP, the
link characteristics that they describe can be expected to affect
other congestion control mechanisms too. In particular, interactions
between link properties and TCP congestion control will be shared by
other protocols that use the similar congestion control behavior,
such as SCTP [RFC4960] and DCCP with CCID 2 [RFC4341] (see
Section 6), and should be taken into consideration by designers of
congestion control mechanisms that utilize the same kind of feedback
Some RFCs only make recommendations regarding the implementation and
configuration of TCP based upon characteristics of special links. As
these RFCs are so closely connected to the specification of TCP
itself, they are not included in this document, but are listed in the
TCP Roadmap [RFC4614].
RFC 2488 (BCP 28): "Enhancing TCP Over Satellite Channels using
Standard Mechanisms" (January 1999)
The summary of recommendations in [RFC2488] came from the TCP over
Satellite (TCPSAT) Working Group, whose goal was to identify the
performance problems that TCP may have over satellite links and
suggest mitigations. The document explains several ways that
existing standards can be applied to improve the performance of
basic TCP congestion control over paths with characteristics
similar to those involving satellite links.
RFC 3135: "Performance Enhancing Proxies Intended to Mitigate Link-
Related Degradations" (June 2001)
[RFC3135] is a survey of Performance Enhancing Proxies (PEPs)
often employed to improve degraded TCP performance caused by
characteristics of specific link environments, for example, in
satellite, wireless WAN, and wireless LAN environments. Different
types of PEPs are described as well as the mechanisms used to
improve performance. While there is a specific focus on TCP in
this document, PEPs can operate on any protocol, and the
performance enhancements that PEPs achieve are often closely
related to congestion control.
The use of PEPs has architectural implications as they sometimes
violate end-to-end assumptions and can add complexity to the inner
portions of a network. Certain types of PEPs are commonly used
today in satellite or long-distance networking because it is
easier to insert a small number of PEPs near problematic links
than to upgrade the TCP implementations on all the end hosts that
might use those links. One down-side is that their deployment
raises some issues when introducing new or updated congestion
control (CC) methods into these deployed networks, since the PEPs
may be operating with undocumented algorithms, making assumptions
about the end-host CC behavior, and/or altering packet fields that
will affect the end-host CC behavior.
RFC 3150 (BCP 48): "End-to-end Performance Implications of Slow
Links" (July 2001)
[RFC3150] makes performance-related recommendations for users of
network paths that traverse "very low bit-rate" links. It
includes a discussion of interactions between such links and TCP
RFC 3155 (BCP 50): "End-to-end Performance Implications of Links with
Errors" (August 2001)
Under the premise that several types of PEP have undesirable
implications, [RFC3155] recommends end-to-end alternatives for
improving TCP performance over paths with error-prone links.
RFC 3366 (BCP 62): "Advice to link designers on link Automatic Repeat
reQuest (ARQ)" (August 2002)
Link-layer ARQ techniques are a popular means to increase the
robustness of particular links to transmission errors via
retransmission and acknowledgement mechanisms. As [RFC3366]
explains, ARQ techniques on a link can interact poorly with TCP's
end-to-end congestion control if they lead to additional delay
variation or reordering. This RFC gives some advice on limiting
the extent of these types of problematic interactions. The proper
balance between end-to-end and link-layer reliability mechanisms
is still an open research issue that has been explored in many
academic papers outside the IETF.
RFC 3449 (BCP 69): "TCP Performance Implications of Network Path
Asymmetry" (December 2002)
[RFC3449] describes performance limitations of TCP when the
capacity of the ACK path is limited. Several techniques to aid
TCP in these circumstances are recommended as Best Current
Practices, particularly ACK congestion control and sender pacing
are relevant to other non-TCP congestion control schemes, outside
the scope of this document. For instance, in the design of the
Reliable Multicast Transport (RMT) protocols for multicast,
preventing ACK-implosion at multicast sources can be seen as a
form of ACK congestion control.
RFC 3481: "TCP over Second (2.5G) and Third (3G) Generation Wireless
Networks" (February 2003)
Among other issues, some mobile data systems exhibit delay spikes,
handovers, and bandwidth oscillation. [RFC3481] describes the
problems that these conditions cause for TCP congestion control
and how some TCP extensions can be used to mitigate them.
RFC 3819 (BCP 89): "Advice for Internet Subnetwork Designers" (July
Several issues in link design and optimization for carrying IP
traffic are discussed in [RFC3819], which recommends Best Current
Practices. Many of these principles are motivated by properties
of TCP, but most of them also apply to other transport-layer
congestion control techniques as well.
5. End-Host and Router Cooperative Signaling
Some RFCs define mechanisms that allow routers to add signaling
information to packets that makes the network's congestion state less
of a mystery to end-host congestion controllers. Routers supporting
these can signal information about the current congestion state to
flows in-band, providing faster and finer-grained information than
inference-based methods. Two examples of this are discussed in this
section; the first directs sources to slow down in order to avoid
losses, and the other assists in determining an appropriate starting
rate for new flows.
5.1. Explicit Congestion Notification
Traditionally, under congestion, IP routers enqueue packets until
some limit is reached, at which point packets are dropped. TCP, and
other IETF transport protocols, use a stream of acknowledgements to
infer these losses and take congestion control action. This section
describes a more advanced way to signal congestion to sources before
packet-dropping is required.
There are two Explicit Congestion Notification (ECN) bits in the IP
header that enable an AQM mechanism (see [RFC2309] or Section 2) to
convey congestion information to endpoints without dropping packets.
This can significantly reduce the losses experienced by transport
endpoints if they are responsive to ECN. While ECN is most
frequently discussed in the context of TCP (and therefore included in
the TCP Roadmap [RFC4614]), its applicability is broader, and ECN use
has also been specified for protocols such as DCCP and SCTP.
RFC 2481: "A Proposal to add Explicit Congestion Notification (ECN)
to IP" (January 1999) - Obsoleted by RFC 3168
[RFC2481] introduced ECN into the RFC series, describing when the
Congestion Experienced (CE) bit in the IP header should be set in
routers, and what modifications are needed to TCP to make it ECN-
capable. It includes a discussion of issues related to nodes and
routers that are non-compliant, IPsec tunnels, and dropped or
corrupted packets, as well as a summary of related work. Many of
these issues will also be faced by operators trying to deploy
other network-based congestion control methods. RFC 2481 has been
obsoleted by RFC 3168.
RFC 2884: "Performance Evaluation of Explicit Congestion Notification
(ECN) in IP Networks" (July 2000)
[RFC2884] presents a performance study of ECN as specified in
[RFC2481] using an implementation on the Linux operating system.
The experiments focused on ECN for both bulk and transactional
transfers, showing that there is improvement in throughput over
TCP without ECN in the case of bulk transfers and substantial
improvement for transactional transfers. Studies like this help
to build the community's confidence that extensions like ECN are
both safe and valuable. Similar RFCs helped the community accept
larger initial windows for TCP [RFC2414] [RFC2415] [RFC2416].
RFC 3168: "The Addition of Explicit Congestion Notification (ECN) to
IP" (September 2001)
[RFC3168], which obsoletes [RFC2481], specifies the incorporation
of ECN into TCP and IP. One notable change in this significantly
extended specification is the definition of a bit combination that
was not defined in [RFC2481], which can be used to realize a nonce
that would prevent a receiver from falsely claiming that there was
no congestion. Potential issues related to ECN are discussed at
length, including those already included in [RFC2481] and
backwards compatibility with implementations that would follow the
specification in the obsoleted document.
ECN, as specified in RFC 3168, is implemented in several popular
router and end-host platforms. It is in active use, to at least
some extent. Problems with ECN "blackholes" (Internet routers
misconfigured to discard packets with ECN-capable bits set) were
discovered when ECN was enabled by default in some end-host
operating systems. Fears about the persisting presence of these
blackholes currently may be keeping ECN from being used by default
in many end-host operating systems even though it is implemented
as an option within them. Some measurements on ECN support and
usability are available [PF01] [MAF04] [MAF05].
RFC 3540: "Robust Explicit Congestion Notification (ECN) Signaling
with Nonces" (June 2003)
[RFC3540] specifies a nonce mechanism that uses an ECN bit
combination that is not used in [RFC2481], but that is specified
in [RFC3168] to allow a one-bit ECN nonce. This nonce mechanism
includes a Nonce Sum (NS) field in the TCP header so that senders
can ensure that ACKs that do not indicate congestion are credible.
The mechanism improves the robustness of congestion control by
preventing receivers from exploiting ECN to gain an unfair share
of network bandwidth.
This nonce technique is not understood to have been widely
implemented or deployed, and there has been some discussion as to
whether the mechanism is really effective or is the best use of
these bits (see emails to the IETF Transport Area Working Group
(TSVWG) mailing list, in the thread "ECN nonce snag in TCP ESTATS
MIB" from December 2006 - January 2007, or [MBJ07]).
RFC 4782: "Quick-Start for TCP and IP" (January 2007)
Quick-Start provides a way for hosts to ask routers to help them
select an initial sending rate, and use this rate rather than the
traditional small initial congestion window and slow-start
algorithm. [RFC4782] describes the Quick-Start mechanism and its
use with TCP. In addition to discussing the benefits of Quick-
Start, the document also discusses several limitations of the
Quick-Start technique with respect to some types of tunnels in use
over the Internet today and other potential costs of Quick-Start
including those related to router design. Analysis of the effects
of misbehaving entities and appendices containing design rationale
and related work are also notably present in this RFC.
Many of the issues discussed in RFC 4782, including router
architecture, network design / tunnels, and misbehaving agents are
all challenges relevant to other proposals that try to add router
assistance into the network. The consideration of these issues
can be illustrative for other protocol designers, even if they are
not interested in Quick-Start itself.
6. Non-TCP Unicast Congestion Control
In the past, TCP dominated Internet traffic, as it was used for many
of the popular applications (email, web browsing, file transfer,
remote login, etc.). The majority of early congestion control work
focused on TCP, and the introduction of congestion control into TCP
alone is often credited with saving the Internet from additional
congestion collapse events. Today, TCP has been joined by other
transport protocols (e.g., custom UDP-based protocols, SCTP, DCCP,
RTP over UDP [RFC3550], etc.), and so having properly functioning
congestion control within these other protocols is important for the
Internet's health (as explained in RFC 3714, for instance, or see the
discussion of the "congestion control arms race" scenario in RFC
2914). Documents that describe unicast congestion control methods
for non-TCP transport protocols have been grouped into this section.
RFC 4960: "Stream Control Transmission Protocol" (September 2007)
SCTP congestion control is very similar to TCP with Selective
Acknowledgements, but there are some differences, as described in
Section 7.1 of [RFC4960]. The major difference lies in the fact
that SCTP supports multihoming, whereas TCP does not. Thus, SCTP
keeps a different set of congestion control parameters for each
destination address within an association, whereas TCP only keeps
a single set of congestion control parameters per connection.
RFC 5348: "TCP Friendly Rate Control (TFRC): Protocol Specification"
[RFC5348], which obsoletes [RFC3448], specifies TCP-Friendly Rate
Control (TFRC), a rate-based congestion control mechanism for
unicast flows operating in a best-effort Internet environment
where flows are competing with standard TCP traffic. TFRC ensures
conformance with TCP by continuously calculating the rate that a
TCP sender would obtain under similar circumstances using a
slightly simplified version of the TCP Reno throughput equation in
[PFTK98]. Its sending rate is smoother than the rate of TCP,
making it suitable for multimedia applications. TFRC is not a
wire protocol but rather a mechanism that could, for instance, be
used within a UDP-based application, in a transport protocol such
as RTP, or in the context of endpoint congestion management
RFC 3550: "RTP: A Transport Protocol for Real-Time Applications"
[RFC3550] specifies the real-time transport protocol RTP along
with its control protocol RTCP. RTP/RTCP does not prescribe a
specific congestion control behavior, but it is recommended that
such a behavior be specified in each RTP profile (which is due to
the fact that the potential for reducing the sending rate is often
content dependent in the case of real-time streams).
Specifically, [RFC3550] states: "For some profiles, it may be
sufficient to include an applicability statement restricting the
use of that profile to environments where congestion is avoided by
engineering. For other profiles, specific methods such as data
rate adaptation based on RTCP feedback may be required".
[RFC4585], which discusses RTCP feedback and adaptation
mechanisms, points out that RTCP feedback may operate on much
slower timescales than transport layer feedback mechanisms, and
that additional mechanisms are therefore required to perform
proper congestion control. One way to make use of such additional
mechanisms is to run RTP over DCCP.
RFC 4336: "Problem Statement for the Datagram Congestion Control
Protocol (DCCP)" (March 2006)
[RFC4336] provides the motivation leading to the design of DCCP.
In doing so, other possibilities of implementing similar
functionality are discussed, including unreliable extensions of
SCTP, RTP-based congestion control, and providing congestion
control above or below UDP.
RFC 4340: "Datagram Congestion Control Protocol" (March 2006)
[RFC4340] specifies DCCP, the Datagram Congestion Control
Protocol. This protocol provides bidirectional unicast
connections of congestion-controlled unreliable datagrams. It is
suitable for applications that can benefit from control over the
tradeoff between timeliness and reliability. The core DCCP
specification does not include a specific congestion control
behavior; rather, it functions as a framework for such mechanisms,
which can be selected via the Congestion Control Identifier
RFC 4341: "Profile for Datagram Congestion Control Protocol (DCCP)
Congestion Control ID 2: TCP-like Congestion Control" (March 2006)
[RFC4341] is the specification of TCP-like congestion control
within DCCP. This should be used by senders who would like to
take advantage of the available bandwidth in an environment with
rapidly changing conditions, and who are able to adapt to the
abrupt changes in the congestion window typical of TCP's Additive
Increase Multiplicative Decrease (AIMD) congestion control. ECN
is also supported within RFC 4341.
RFC 4342: "Profile for Datagram Congestion Control Protocol (DCCP)
Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)" (March
[RFC4342] is the specification of TFRC congestion control as
described in [RFC3448] for DCCP. This should be used by senders
who want a TCP-friendly sending rate, possibly with Explicit
Congestion Notification (ECN), while minimizing abrupt rate
7. Multicast Congestion Control
In the IETF, congestion control for multicast (one-to-many)
communication has primarily been tackled in the Reliable Multicast
Transport (RMT) Working Group. Except for [RFC2357] and [RFC3208],
all the documents in this section were written by this group. Since
a "one size fits all" protocol cannot meet the requirements of all
possible applications in this space, the approach taken is a modular
one, consisting of "protocol cores" and "building blocks". Multiple
congestion control building blocks have been defined, providing both
sender-driven and receiver-driven congestion control methods that
differ widely in their assumptions and behavior.
RFC 2357: "IETF Criteria for Evaluating Reliable Multicast Transport
and Application Protocols" (June 1998)
Some early multicast content dissemination proposals did not
incorporate proper congestion control; this is pointed out as
being a severe mistake in [RFC2357], as large-scale multicast
applications have the potential to do vast congestion-related
damage. This document clearly makes the case that congestion
control mechanisms should be developed and incorporated into
multicast content dissemination protocols intended for use over
RFC 2887: "The Reliable Multicast Design Space for Bulk Data
Transfer" (August 2000)
Several classes of potential congestion control schemes for
single-sender multicast protocols are briefly sketched as
possibilities, but no specific protocols are developed or selected
RFC 3048: "Reliable Multicast Transport Building Blocks for One-to-
Many Bulk-Data Transfer" (January 2001)
[RFC3048] discusses the building block approach to RMT protocols
and mentions that several different congestion control building
blocks may be required in order to deal with different situations.
Some of the possible interactions between building blocks for
congestion control and those for Forward Error Correction (FEC),
acknowledgement, and group management are also mentioned.
RFC 3208: "PGM Reliable Transport Protocol Specification" (December
Pragmatic General Multicast (PGM) is a reliable multicast
transport protocol for applications that require ordered or
unordered, duplicate-free, multicast data delivery from multiple
sources to multiple receivers. As discussed in [RFC3208]'s
Appendix B, a PGM protocol source can request congestion control
feedback from both network elements (routers) and receivers (end
hosts). These reports can indicate the load on the worst link in
a particular path, or the load on the worst path. The actual
procedure used in response to this feedback is not part of RFC
3208, but the notion of using multicast routers to assist in
congestion control is significant.
RFC 3450: "Asynchronous Layered Coding (ALC) Protocol Instantiation"
[RFC3450] specifies ALC, a rough header format using the RMT
building blocks, that can be used by multicast content
dissemination protocols. ALC is intended to use a multi-rate
congestion control building block, where the sender does not
require any feedback, but where multiple multicast groups with
different transmission rates are available within and ALC session,
and receivers control their rates by joining or leaving groups.
RFC 3738: "Wave and Equation Based Rate Control (WEBRC) Building
Block" (April 2004)
The WEBRC mechanism defined in [RFC3738] is a receiver-driven form
of congestion control, where each receiver in a multicast group
can determine the individual rate at which packets are delivered
to it. WEBRC senders create a base channel for control
information and several multicast channels for data transmission
that each send packets at a varying rate in the form of a wave.
The receivers dynamically join and leave channels at chosen points
within the wave of sending rates to obtain the desired overall
receive rate based on an equation using the estimated loss
probability and round-trip time within an epoch. WEBRC is
compatible for use within ALC.
RFC 4654: "TCP-Friendly Multicast Congestion Control (TFMCC):
Protocol Specification" (August 2006)
TFMCC, as described in [RFC4654], is a sender-driven congestion
control mechanism, where the received rate for the entire
multicast group is determined by the worst-connected receiver.
TFMCC builds upon TFRC, but scales down the feedback to prevent
ACK-implosion effects by having receivers suppress their feedback
unless they perceive it to be the worst among the reception group.
8. Guidance for Developing and Analyzing Congestion Control Techniques
Some recently published RFCs discuss the properties of congestion
control protocols that are "safe" for Internet deployment, as well as
how to measure the properties of congestion control mechanisms and
transport protocols. These documents are particularly relevant to
the ICCRG as some of the group's activities involve reviewing
congestion control proposals that have been brought to the IETF for
RFC 5033 (BCP 133): "Specifying New Congestion Control Algorithms"
The concurrent development of multiple TCP modifications for high-
rate use and the deployments of these modifications on the
Internet prompted [RFC5033] to be written. RFC 5033 comes from
the Transport Area Working Group (TSVWG), and gives guidance on
the classes of Experimental RFC that can be published to document
algorithms that are either encouraged for investigation on the
Internet, and those that are only encouraged for experimentation
in less-critical environments. It has been described as a list of
things for people to think about when creating new congestion
control techniques that they are planning to widely deploy.
RFC 5166: "Metrics for the Evaluation of Congestion Control
Mechanisms" (March 2008)
The IRTF Transport Modeling Research Group (TMRG) produced
[RFC5166] to describe the set of metrics and related tradeoffs
between metrics that can be used to compare, contrast, and
evaluate congestion control techniques. This RFC gives an
overview of many such metrics, and gives references to their
9. Historic Interest
Early in the RFC series, there are many documents that represent an
author's thoughts on a subject or brief summaries from measurement
and experimentation, rather than the result of a long formal IETF
process. Some of the RFCs listed in this section have this
RFC 889: "Internet Delay Experiments" (December 1983)
Based on reported measurement experiments, changes to the TCP
retransmission timeout (RTO) calculation are suggested in
[RFC0889]. It is noted that the original TCP RTO calculation
leads to congestion when a delay spike occurs because it takes too
long for the RTO to adapt, leading to superfluous retransmissions.
RFC 896: "Congestion Control in IP/TCP Internetworks" (January 1984)
[RFC0896] is the first document known to the authors where the
term "congestion collapse" was used. Here, it refers to the
stable state that was observed when a sudden load on the net
caused the round-trip time to rise faster than the sending hosts
measured round-trip time could be updated. Two problems are
discussed: the "small-packet problem" (now commonly known by the
name "silly window syndrome") and the "source-quench problem",
which is about inappropriately deciding when to send and how to
react to ICMP Source Quench messages. Solutions for these
problems are presented.
RFC 970: "On Packet Switches with Infinite Storage" (December 1985)
Using a thought experiment based on a router with infinite
buffering capacity, [RFC0970] develops a different kind of
congestion collapse scenario, where few useful packet
transmissions occur due to the queue being longer than the time-
to-live of the packets within it. As described in RFC 970, this
scenario was also demonstrated using real equipment by the author.
The document also includes discussion of game-theoretic analysis
of congestion control and obtaining fairness between behaving and
non-behaving flows, by focusing on the order of scheduling packets
within the buffer rather than the actual allocation of buffer
space between flows.
RFC 1016: "Something a Host Could Do with Source Quench: The Source
Quench Introduced Delay (SQuID)" (July 1987)
[RFC1016] outlines a rate-based congestion control mechanism where
end-hosts use Source Quench packets from routers to adjust their
sending rates. RFC 1016 also suggests sending congestion
notifications before queues are actually full, at a rate that
increases with the current queue occupancy. This strategy has
been used in several other AQM mechanisms, notably RED [FJ93].
RFC 1254: "Gateway Congestion Control Survey" (August 1991)
[RFC1254] is a survey of congestion control approaches in routers
that first discusses general congestion control performance goals
(such as fairness), and then elaborates on the use of Source
Quench messages (which are now discouraged, as they have been
found ineffective), Random Drop (which would now be called "Active
Queue Management"), Congestion Indication (DEC Bit; an early form
of ECN), "Selective Feedback Congestion Indication" (one
particular method for applying ECN), and Fair Queuing. Finally,
end-system congestion control policies are discussed, including
Jacobson's well-known algorithms [Jac88] and their predecessor --
10. Security Considerations
This document introduces no new security considerations. Each RFC
listed in this document discusses the security considerations of the
specification it contains.
Several participants in the ICCRG contributed useful comments in the
development of this document, including Rex Buddenberg, Mitchell
Erblichs, Lachlan Andrew, Sally Floyd, Stephen Farrell, Gorry
Fairhurst, Lars Eggert, Mark Allman, and Juergen Schoenwaelder.
12. Informative References
[FJ93] Floyd, S. and V. Jacobson, "Random Early Detection
Gateways for Congestion Avoidance", IEEE/ACM Transactions
on Networking, volume 1, number 4, August 1993.
[Gont10] Gont, F., "ICMP attacks against TCP", Work in Progress,
[Jac88] Jacobson, V., "Congestion Avoidance and Control",
Proceedings of ACM SIGCOMM 1988, in ACM Computer
Communication Review, 18 (4), pp. 314-329.
[Jain86] Jain, R., "A Timeout-Based Congestion Control Scheme for
Window Flow-Controlled Networks", IEEE Journal on Selected
Areas in Communications, volume 4, number 7, October 1986.
[MAF04] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
Proceedings of the Internet Measurement Conference 2004,
[MAF05] Medina, A., Allman, M., and S. Floyd, "Measuring the
Evolution of Transport Protocols in the Internet", ACM
Computer Communications Review, volume 35, issue 2,
[MBJ07] Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
Allow Senders to Identify Receiver Non-Compliance", Work
in Progress, November 2007.
[PF01] Padhye, J. and S. Floyd, "On Inferring TCP Behavior",
Proceedings of ACM SIGCOMM 2001, August 2001.
[PFTK98] Padhye, J., Firoiu, V., Towsley, D., and J. Kurose,
"Modeling TCP Throughput: A Simple Model and its Empirical
Validation", Proceedings of ACM SIGCOMM 1998.
[RFC0889] Mills, D., "Internet delay experiments", RFC 889,
[RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks",
RFC 896, January 1984.
[RFC0970] Nagle, J., "On packet switches with infinite storage",
RFC 970, December 1985.
[RFC1016] Prue, W. and J. Postel, "Something a host could do with
source quench: The Source Quench Introduced Delay
(SQuID)", RFC 1016, July 1987.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1254] Mankin, A. and K. Ramakrishnan, "Gateway Congestion
Control Survey", RFC 1254, August 1991.
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1958] Carpenter, B., "Architectural Principles of the Internet",
RFC 1958, June 1996.
[RFC2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001,
[RFC2140] Touch, J., "TCP Control Block Interdependence", RFC 2140,
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998.
[RFC2357] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
[RFC2414] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 2414, September 1998.
[RFC2415] Poduri, K., "Simulation Studies of Increased Initial TCP
Window Size", RFC 2415, September 1998.
[RFC2416] Shepard, T. and C. Partridge, "When TCP Starts Up With
Four Packets Into Only Three Buffers", RFC 2416,
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2481] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
Congestion Notification (ECN) to IP", RFC 2481,
[RFC2488] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP
Over Satellite Channels using Standard Mechanisms",
BCP 28, RFC 2488, January 1999.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
Explicit Congestion Notification (ECN) in IP Networks",
RFC 2884, July 2000.
[RFC2887] Handley, M., Floyd, S., Whetten, B., Kermode, R.,
Vicisano, L., and M. Luby, "The Reliable Multicast Design
Space for Bulk Data Transfer", RFC 2887, August 2000.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, September 2000.
[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework for Integrated Services Operation
over Diffserv Networks", RFC 2998, November 2000.
[RFC3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
Floyd, S., and M. Luby, "Reliable Multicast Transport
Building Blocks for One-to-Many Bulk-Data Transfer",
RFC 3048, January 2001.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
RFC 3124, June 2001.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135, June 2001.
[RFC3150] Dawkins, S., Montenegro, G., Kojo, M., and V. Magret,
"End-to-end Performance Implications of Slow Links",
BCP 48, RFC 3150, July 2001.
[RFC3155] Dawkins, S., Montenegro, G., Kojo, M., Magret, V., and N.
Vaidya, "End-to-end Performance Implications of Links with
Errors", BCP 50, RFC 3155, August 2001.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3208] Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,
Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
Sumanasekera, R., and L. Vicisano, "PGM Reliable Transport
Protocol Specification", RFC 3208, December 2001.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
[RFC3426] Floyd, S., "General Architectural and Policy
Considerations", RFC 3426, November 2002.
[RFC3439] Bush, R. and D. Meyer, "Some Internet Architectural
Guidelines and Philosophy", RFC 3439, December 2002.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, December 2002.
[RFC3450] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
Instantiation", RFC 3450, December 2002.
[RFC3481] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A., and
F. Khafizov, "TCP over Second (2.5G) and Third (3G)
Generation Wireless Networks", BCP 71, RFC 3481,
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, December 2003.
[RFC3714] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet", RFC 3714,
[RFC3738] Luby, M. and V. Goyal, "Wave and Equation Based Rate
Control (WEBRC) Building Block", RFC 3738, April 2004.
[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
Congestion Windows", RFC 3742, March 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4336] Floyd, S., Handley, M., and E. Kohler, "Problem Statement
for the Datagram Congestion Control Protocol (DCCP)",
RFC 4336, March 2006.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
[RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap
for Transmission Control Protocol (TCP) Specification
Documents", RFC 4614, September 2006.
[RFC4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast
Congestion Control (TFMCC): Protocol Specification",
RFC 4654, August 2006.
[RFC4782] Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
Start for TCP and IP", RFC 4782, January 2007.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033, August 2007.
[RFC5166] Floyd, S., "Metrics for the Evaluation of Congestion
Control Mechanisms", RFC 5166, March 2008.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, September 2008.
University of Oslo
Department of Informatics
PO Box 1080 Blindern
N-0316 Oslo, Norway
Phone: +47 22 85 24 20
Wesley M. Eddy
NASA Glenn Research Center
21000 Brookpark Rd, MS 500-ASRC
Cleveland, OH 44135
Phone: (216) 433-6682