ECN alternate semantics: Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field

Network Working Group                                           S. Floyd
Request for Comments: 4774                                          ICIR
BCP: 124                                                   November 2006
Category: Best Current Practice


                  Specifying Alternate Semantics for
            the Explicit Congestion Notification (ECN) Field

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

   There have been a number of proposals for alternate semantics for the
   Explicit Congestion Notification (ECN) field in the IP header RFC
   3168.  This document discusses some of the issues in defining
   alternate semantics for the ECN field, and specifies requirements for
   a safe coexistence in an Internet that could include routers that do
   not understand the defined alternate semantics.  This document
   evolved as a result of discussions with the authors of one recent
   proposal for such alternate semantics.

Table of Contents

   1. Introduction ....................................................2
   2. An Overview of the Issues .......................................3
   3. Signalling the Use of Alternate ECN Semantics ...................4
      3.1. Using the Diffserv Field for Signalling ....................5
   4. Issues of Incremental Deployment ................................6
      4.1. Option 1:  Unsafe for Deployment in the Internet ...........7
      4.2. Option 2:  Verification that Routers Understand the
           Alternate ..................................................8
      4.3. Option 3:  Friendly Coexistence with Competing Traffic .....8
   5. Evaluation of the Alternate ECN Semantics ......................10
      5.1. Verification of Feedback from the Router ..................10
      5.2. Coexistence with Competing Traffic ........................11
      5.3. Proposals for Alternate ECN with Edge-to-Edge Semantics ...12
      5.4. Encapsulated Packets ......................................12
      5.5. A General Evaluation of the Alternate ECN Semantics .......12
   6. Security Considerations ........................................12
   7. Conclusions ....................................................13
   8. Acknowledgements ...............................................13
   9. Normative References ...........................................13
   10. Informative References ........................................13

1.  Introduction

   [RFC3168], a Proposed Standard document, defines the ECN field in the
   IP header, and specifies the semantics for the codepoints for the ECN
   field.  However, end nodes could specify the use of alternate
   semantics for the ECN field, e.g., using codepoints in the diffserv
   field of the IP header.

   There have been a number of proposals in the IETF and in the research
   community for alternate semantics for the ECN codepoint.  One such
   proposal, [BCF05], proposes alternate ECN semantics for real-time
   inelastic traffic such as voice, video conferencing, and multimedia
   streaming in DiffServ networks.  In this proposal, the alternate ECN
   semantics would provide information about two levels of congestion
   experienced along the path [BCF05].  Another research proposal,
   [XSSK05], proposes a low-complexity protocol, Variable-structure
   congestion Control Protocol (VCP), that uses the two bits in the ECN
   field to indicate low-load, high-load, and overload (congestion),
   where transport protocols can increase more rapidly during the low-
   load regime.  Some of the proposals for alternate ECN semantics are
   for when ECN is used in an edge-to-edge context between gateways at
   the edge of a network region, e.g., for pre-congestion notification
   for admissions control [BESFC06].  Other proposals for alternate ECN
   semantics are listed on the ECN Web Page [ECN].

   The definition of multiple semantics for the ECN field could have
   significant implications on both host and router implementations.
   There is a huge base of installed hosts and routers in the Internet,
   and in other IP networks, and updating these is an enormous and
   potentially expensive undertaking.  Some existing devices might be
   able to support the new ECN semantics with only a software upgrade
   and without significant degradation in performance.  Some other
   equipment might be able to support the new semantics, but with a
   degradation in performance -- which could range from trivial to
   catastrophic.  Some other deployed equipment might be able to support
   the new ECN semantics only with a hardware upgrade, which, in some
   cases, could be prohibitively expensive to deploy on a very wide
   scale.  For these reasons, it would be difficult and would take a
   significant amount of time to universally deploy any new ECN
   semantics.  In particular, routers can be difficult to upgrade, since
   small routers sometimes are not updated frequently, and large routers
   commonly have specialized forwarding paths to facilitate high
   performance.

   This document describes some of the technical issues that arise in
   specifying alternate semantics for the ECN field, and gives
   requirements for a safe coexistence in a world using the default ECN
   semantics (or using no ECN at all).

2.  An Overview of the Issues

   In this section, we discuss some of the issues that arise if some of
   the traffic in a network consists of alternate-ECN traffic (i.e.,
   traffic using alternate semantics for the ECN field).  The issues
   include the following: (1) how routers know which ECN semantics to
   use with which packets; (2) incremental deployment in a network where
   some routers use only the default ECN semantics or do not use ECN at
   all; (3) coexistence of alternate-ECN traffic with competing traffic
   on the path; and (4) a general evaluation of the alternate ECN
   semantics.

   (1) The first issue concerns how routers know which ECN semantics to
       use with which packets in the network:

       How does the connection indicate to the router that its packets
       are using alternate ECN semantics?  Is the specification of
       alternate-ECN semantics robust and unambiguous?  If not, is this
       a problem?

       As an example, in most of the proposals for alternate ECN
       semantics, a diffserv field is used to specify the use of
       alternate ECN semantics.  Do all routers that understand this
       diffserv codepoint understand that it uses alternate ECN
       semantics, or not?  Diffserv allows routers to re-mark DiffServ
       Code Point (DSCP) values within the network; what is the effect
       of this on the alternate ECN semantics?

       This is discussed in more detail in Section 3 below.

   (2) A second issue is that of incremental deployment in a network
       where some routers only use the default ECN semantics, and other
       routers might not use ECN at all.  In this document, we use the
       phrase "new routers" to refer to the routers that understand the
       alternate ECN semantics, and "old routers" to refer to routers
       that don't understand or aren't willing to use the alternate ECN
       semantics.

       The possible existence of old routers raises the following
       question:  How does the possible presence of old routers affect
       the performance of the alternate-ECN connections?

   (3) The possible existence of old routers also raises the question of
       how the presence of old routers affects the coexistence of the
       alternate-ECN traffic with competing traffic on the path.

       Issues (2) and (3) are discussed in Section 4 below.

   (4) A final issue is that of the general evaluation of the alternate
       ECN semantics:

       How well does the alternate-ECN traffic perform, and how well
       does it coexist with competing traffic on the path, in a "clean"
       environment with new routers and with the unambiguous
       specification of the use of alternate ECN semantics?

       These issues are discussed in Section 5.

3.  Signalling the Use of Alternate ECN Semantics

   This section discusses question (1) from Section 2:

   (1) How does the connection indicate to the router that its packets
       are using alternate ECN semantics?  Is the specification of
       alternate ECN semantics robust and unambiguous?  If not, is this
       a problem?

   The assumption of this document is that when alternate semantics are
   defined for the ECN field, a codepoint in the diffserv field is used
   to signal the use of these alternate ECN semantics to the router.
   That is, the end host sets the codepoint in the diffserv field to
   indicate to routers that alternate semantics to the ECN field are
   being used.  Routers that understand this diffserv codepoint would
   know to use the alternate semantics for interpreting and setting the
   ECN field.  Old ECN-capable routers that do not understand this
   diffserv codepoint would use the default ECN semantics in
   interpreting and setting the ECN field.

   In general, the diffserv codepoints are used to signal the per-hop
   behavior at router queues.  One possibility would be to use one
   diffserv codepoint to signal a per-hop behavior with the default ECN
   semantics, and a separate diffserv codepoint to signal a similar
   per-hop behavior with the alternate ECN semantics.  Another
   possibility would be to use a diffserv codepoint to signal the use of
   best-effort per-hop queueing and scheduling behavior, but with
   alternate ECN semantics.  A detailed discussion of these issues is
   beyond the scope of this document.

   We note that this discussion does not exclude the possibility of
   using other methods, including out-of-band mechanisms, for signalling
   the use of alternate semantics for the ECN field.  The considerations
   in the rest of this document apply regardless of the method used to
   signal the use of alternate semantics for the ECN field.

3.1.  Using the Diffserv Field for Signalling

   We note that the default ECN semantics defined in RFC 3168 are the
   current default semantics for the ECN field, regardless of the
   contents of any other fields in the IP header.  In particular, the
   default ECN semantics apply for more than best-effort traffic with a
   codepoint of '000000' for the diffserv field - the default ECN
   semantics currently apply regardless of the contents of the diffserv
   field.

   There are two ways to use the diffserv field to signal the use of
   alternate ECN semantics.  One way is to use an existing diffserv
   codepoint, and to modify the current definition of that codepoint,
   through approved IETF processes, to specify the use of alternate ECN
   semantics with that codepoint.  A second way is to define a new
   diffserv codepoint, and to specify the use of alternate ECN semantics
   with that codepoint.  We note that the first of these two mechanisms
   raises the possibility that some routers along the path will
   understand the diffserv codepoint but will use the default ECN
   semantics with this diffserv codepoint, or won't use ECN at all, and
   that other routers will use the alternate ECN semantics with this
   diffserv codepoint.

4.  Issues of Incremental Deployment

   This section discusses questions (2) and (3) posed in Section 2:

   (2) How does the possible presence of old routers affect the
       performance of the alternate-ECN connections?

   (3) How does the possible presence of old routers affect the
       coexistence of the alternate-ECN traffic with competing traffic
       on the path?

   When alternate semantics are defined for the ECN field, it is
   necessary to ensure that there are no problems caused by old routers
   along the path that don't understand the alternate ECN semantics.

   One possible problem is that of poor performance for the alternate-
   ECN traffic.  Is it essential to the performance of the alternate-ECN
   traffic that all routers along the path understand the alternate ECN
   semantics?  If not, what are the possible consequences, for the
   alternate-ECN traffic itself, when some old routers along the path
   don't understand the alternate ECN semantics?  These issues have to
   be answered in the context of each specific proposal for alternate
   ECN semantics.

   A second specific problem is that of possible unfair competition with
   other traffic along the path.  If there is an old router along the
   path that doesn't use ECN, that old router could drop packets from
   the alternate-ECN traffic, and expect the alternate-ECN traffic to
   reduce its sending rate as a result.  Does the alternate-ECN traffic
   respond to packet drops as an indication of congestion?

                                  |--------|
     Alternate-ECN traffic ---->  |        | ---> CE-marked packet
                                  |  Old   |
     Non-ECN traffic ---------->  | Router | ---> dropped packet
                                  |        |
     RFC-3168 ECN traffic ----->  |        | ---> CE-marked packet
                                  |--------|

    Figure 1: Alternate-ECN traffic, an old router, using RFC-3168 ECN,
     that is congested and ready to drop or mark the arriving packet.

   Similarly, what if there is an old router along the path that
   understands only the default ECN semantics from RFC 3168, as in
   Figure 1 above?  In times of congestion, the old default-ECN router
   could see an alternate-ECN packet with one of the ECN-Capable
   Transport (ECT) codepoints set in the ECN field in the IP header, as
   defined in RFC 3168, and set the Congestion Experienced (CE)
   codepoint in the ECN field as an alternative to dropping the packet.
   The router in this case would expect the alternate-ECN connection to
   respond, in terms of congestion control, as it would if the packet
   has been dropped.  If the alternate-ECN traffic fails to respond
   appropriately to the CE codepoint being set by an old router, this
   could increase the aggregate traffic arriving at the old router,
   resulting in an increase in the packet-marking and packet-dropping
   rates at that router, further resulting in the alternate-ECN traffic
   crowding out the other traffic competing for bandwidth on that link.

   Basically, there are three possibilities for avoiding scenarios where
   the presence of old routers along the path results in the alternate-
   ECN traffic competing unfairly with other traffic along the path:

   Option 1:  Alternate-ECN traffic is clearly understood as unsafe for
   deployment in the global Internet; or

   Option 2:  All alternate-ECN traffic deploys some mechanism for
   verifying that all routers on the path understand and agree to use
   the alternate ECN semantics for this traffic; or

   Option 3:  The alternate ECN semantics are defined in such a way as
   to ensure the fair and peaceful coexistence of the alternate-ECN
   traffic with best-effort and other traffic, even in environments that
   include old routers that do not understand the alternate ECN
   semantics.

   Each of these alternatives is explored in more detail below.

4.1.  Option 1:  Unsafe for Deployment in the Internet

   The first option specified above is for the alternate-ECN traffic to
   be clearly understood as only suitable for enclosed environments, and
   as unsafe for deployment in the global Internet.  Specifically, this
   would mean that it would be unsafe for packets using the alternate
   ECN semantics to be unleashed in the global Internet.  This
   restriction would prevent the alternate-ECN traffic from traversing
   an old router outside of the enclosed environment that didn't
   understand the alternate semantics.  This document doesn't comment on
   whether a mechanism would be required to ensure that the alternate
   ECN semantics would not be let loose on the global Internet.  This
   document also doesn't comment on the chances that this scenario would
   be considered acceptable for standardization by the IETF community.

4.2.  Option 2:  Verification that Routers Understand the Alternate
      Semantics

   The second option specified above is for the alternate-ECN traffic to
   include a mechanism for ensuring that all routers along the path
   understand and agree to the use of the alternate ECN semantics for
   this traffic.  As an example, such a mechanism could consist of a
   field in an IP option that all routers along the path decrement if
   they agree to use the alternate ECN semantics with this traffic.  (A
   similar mechanism is proposed for Quick-Start, for verifying that all
   of the routers along the path understand the Quick-Start IP Option
   [QuickStart].)  Using such a mechanism, a sender could have
   reasonable assurance that the packets that are sent specifying the
   use of alternate ECN semantics only traverse routers that, in fact,
   understand and agree to use these alternate semantics for these
   packets.  Note, however, that most existing routers are optimized for
   IP packets with no options, or with only some very well-known and
   simple IP options.  Thus, the definition and use of any new IP option
   may have a serious detrimental effect on the performance of many
   existing IP routers.

   Such a mechanism should be robust in the presence of paths with
   multi-path routing, and in the presence of routing or configuration
   changes along the path while the connection is in use.  In
   particular, if this option is used, connections could include some
   form of monitoring for changes in path behavior and/or periodic
   monitoring that all routers along the path continue to understand the
   alternate ECN semantics.

4.3.  Option 3:  Friendly Coexistence with Competing Traffic

   The third option specified above is for the alternate ECN semantics
   to be defined so that traffic using the alternate semantics would
   coexist safely in the Internet on a path with one or more old routers
   that use only the default ECN semantics.  In this scenario, a
   connection sending alternate-ECN traffic would have to respond
   appropriately to a CE packet (a packet with the ECN codepoint "11")
   received at the receiver, using a conformant congestion control
   response.  Hopefully, the connection sending alternate-ECN traffic
   would also respond appropriately to a dropped packet, which could be
   a congestion indication from a router that doesn't use ECN.

   RFC 3168 defines the default ECN semantics as follows:

   "Upon the receipt by an ECN-Capable transport of a single CE packet,
   the congestion control algorithms followed at the end-systems MUST be
   essentially the same as the congestion control response to a *single*
   dropped packet.  For example, for ECN-Capable TCP the source TCP is
   required to halve its congestion window for any window of data
   containing either a packet drop or an ECN indication."

   The only conformant congestion control mechanisms currently
   standardized in the IETF are TCP [RFC2581] and protocols using TCP-
   like congestion control (e.g., SCTP [RFC2960], DCCP with CCID-2
   ([RFC4340], [RFC4341])), and TCP-Friendly Rate Control (TFRC)
   [RFC3448], and protocols with TFRC-like congestion control (e.g.,
   DCCP using CCID-3 [RFC4342]).  TCP uses Additive-Increase
   Multiplicative-Decrease congestion control, and responds to the loss
   or ECN-marking of a single packet by halving its congestion window.
   In contrast, the equation-based congestion control mechanism in TFRC
   estimates the loss event rate over some period of time, and uses a
   sending rate that would be comparable, in packets per round-trip-
   time, to that of a TCP connection experiencing the same loss event
   rate.

   So what are the requirements for alternate-ECN traffic to compete
   appropriately with other traffic on a path through an old router that
   doesn't understand the alternate ECN semantics (and therefore might
   be using the default ECN semantics)?  The first and second
   requirements below concern compatibility between traffic using
   alternate ECN semantics and routers using default ECN semantics.

   The first requirement for compatibility with routers using default
   ECN is that if a packet is marked with the ECN codepoint "11" in the
   network, this marking is not changed on the packet's way to the
   receiver (unless the packet is dropped before it reaches the
   receiver).  This requirement is necessary to ensure that congestion
   indications from a default-ECN router make it to the transport
   receiver.

   A second requirement for compatibility with routers using default ECN
   is that the end-nodes respond to packets that are marked with the ECN
   codepoint "11" in a way that is friendly to flows using IETF-
   conformant congestion control.  This requirement is needed because
   the "11"-marked packets might have come from a congested router that
   understands only the default ECN semantics, and that expects that
   end-nodes will respond appropriately to CE packets.  This requirement
   would ensure that the traffic using the alternate semantics does not
   `bully' competing traffic that it might encounter along the path, and
   that it does not drive up congestion on the shared link
   inappropriately.

   Additional requirements concern compatibility between traffic using
   default ECN semantics and routers using alternate ECN semantics.
   This situation could occur if a diffserv codepoint using default ECN
   semantics is redefined to use alternate ECN semantics, and traffic
   from an "old" source traverses a "new" router.  If the router "knows"
   that a packet is from a sender using alternate semantics (e.g.,
   because the packet is using a certain diffserv codepoint, and all
   packets with that diffserv codepoint use alternate semantics for the
   ECN field), then the requirements below are not necessary, and the
   rules for the alternate semantics apply.

   A requirement for compatibility with end-nodes using default ECN is
   that if a packet that *could* be using default semantics is marked
   with the ECN codepoint "00", this marking must not be changed to
   "01", "10", or "11" in the network.  This prevents the packet from
   being represented incorrectly to a default-ECN router downstream as
   ECN-Capable.  Similarly, if a packet that *could* be using default
   semantics is marked with the ECN codepoint "01", then this codepoint
   should not be changed to "10" in the network (and a "10" codepoint
   should not be changed to "01").  This requirement is necessary to
   avoid interference with the transport protocol's use of the ECN nonce
   [RFC3540].

   As discussed earlier, the current conformant congestion control
   responses to a dropped or default-ECN-marked packet consist of TCP
   and TCP-like congestion control, and of TFRC (TCP-Friendly Rate
   Control).  Another possible response considered in RFC 3714, but not
   standardized in a standards-track document, is that of simply
   terminating an alternate-ECN connection for a period of time if the
   long-term sending rate is higher than would be that of a TCP
   connection experiencing the same packet dropping or marking rates
   [RFC3714].  We note that the use of such a congestion control
   response to CE-marked packets would require specification of time
   constants for measuring the loss rates and for stopping transmission,
   and would require a consideration of issues of packet size.

5.  Evaluation of the Alternate ECN Semantics

   This section discusses question (4) posed in Section 2:

   (4) How well does the alternate-ECN traffic perform, and how well
       does it coexist with competing traffic on the path, in a "clean"
       environment with new routers and with the unambiguous
       specification of the use of alternate ECN semantics?

5.1.  Verification of Feedback from the Router

   One issue in evaluating the alternate ECN semantics concerns
   mechanisms to discourage lying from the transport receiver to the
   transport sender.  In many cases, the sender is a server that has an
   interest in using the alternate ECN semantics correctly, while the
   receiver has more incentive to lie about the congestion experienced
   along the path.

   In the default ECN semantics, two of the four ECN codepoints are used
   for ECN-Capable(0) and ECN-Capable(1).  The use of two codepoints for
   ECN-Capable, instead of one, permits the data sender to verify the
   receiver's reports that packets were actually received unmarked at
   the receiver.  In particular, the sender can specify that the
   receiver report to the sender whether each unmarked packet was
   received ECN-Capable(0) or ECN-Capable(1), as discussed in RFC 3540
   [RFC3540].  This use of ECN-Capable(0) and ECN-Capable(1) is
   independent of the semantics of the other ECN codepoints, and could
   be used, if desired, with alternate semantics for the other
   codepoints.

   If alternate semantics for the ECN codepoint don't include the use of
   two separate codepoints to indicate ECN-Capable, then the connections
   using those semantics have lost the ability to verify that the data
   receiver is accurately reporting the received ECN codepoint to the
   data sender.  In this case, it might be necessary for the alternate-
   ECN framework to include alternate mechanisms for ensuring that the
   data receiver is reporting feedback appropriately to the sender.  As
   one possibility, policers could be used in routers to ensure that end
   nodes are responding appropriately to marked packets.

5.2.  Coexistence with Competing Traffic

   A second general issue concerns the coexistence of alternate-ECN
   traffic with competing traffic along the path, in a clean environment
   where all routers understand and are willing to use the alternate ECN
   semantics for the traffic that specifies its use.

   If the traffic using the alternate ECN semantics is best-effort
   traffic, then it is subject to the general requirement of fair
   competition with TCP and other traffic along the path [RFC2914].

   If the traffic using the alternate ECN semantics is diffserv traffic,
   then the requirements are governed by the overall guidelines for that
   class of diffserv traffic.  It is beyond the scope of this document
   to specify the requirements, if any, for the coexistence of diffserv
   traffic with other traffic on the link; this should be addressed in
   the specification of the diffserv codepoint itself.

5.3.  Proposals for Alternate ECN with Edge-to-Edge Semantics

   RFC 3168 specifies the use of the default ECN semantics by an end-
   to-end transport protocol, with the requirement that "upon the
   receipt by an ECN-Capable transport of a single CE packet, the
   congestion control algorithms followed at the end-systems MUST be
   essentially the same as the congestion control response to a *single*
   dropped packet" ([RFC3168], Section 5).  In contrast, some of the
   proposals for alternate ECN semantics are for ECN used in an edge-
   to-edge context between gateways at the edge of a network region,
   e.g., [BESFC06].

   When alternate ECN is defined with edge-to-edge semantics, this
   definition needs to ensure that the edge-to-edge semantics do not
   conflict with a connection using other ECN semantics end-to-end.  One
   way to avoid conflict would be for the edge-to-edge ECN proposal to
   include some mechanism to ensure that the edge-to-edge ECN is not
   used for connections that are using other ECN semantics (standard or
   otherwise) end-to-end.  Alternately, the edge-to-edge semantics could
   be defined so that they do not conflict with a connection using other
   ECN semantics end-to-end.

5.4.  Encapsulated Packets

   RFC 3168 has an extensive discussion of the interactions between ECN
   and IP tunnels, including IPsec and IP in IP.  Proposals for
   alternate ECN semantics might interact with IP tunnels differently
   than default ECN.  As a result, proposals for alternate ECN semantics
   must explicitly consider the issue of interactions with IP tunnels.

5.5.  A General Evaluation of the Alternate ECN Semantics

   A third general issue concerns the evaluation of the general merits
   of the proposed alternate ECN semantics.  Again, it would be beyond
   the scope of this document to specify requirements for the general
   evaluation of alternate ECN semantics.

6.  Security Considerations

   This document doesn't propose any new mechanisms for the Internet
   protocol, and therefore doesn't introduce any new security
   considerations.

7.  Conclusions

   This document has discussed some of the issues to be considered in
   the specification of alternate semantics for the ECN field in the IP
   header.

   Specifications of alternate ECN semantics must clearly state how they
   address the issues raised in this document, particularly the issues
   discussed in Section 2.  In addition, specifications for alternate
   ECN semantics must meet the requirements in Section 5.2 for
   coexistence with competing traffic.

8.  Acknowledgements

   This document is based in part on conversations with Jozef Babiarz,
   Kwok Ho Chan, and Victor Firoiu on their proposal for an alternate
   use of the ECN field in DiffServ environments.  Many thanks to
   Francois Le Faucheur for feedback recommending that the document
   include a section at the beginning discussing the potential issues
   that need to be addressed.  Thanks also to Mark Allman, Fred Baker,
   David Black, Gorry Fairhurst, and to members of the TSVWG working
   group for feedback on these issues.

9.  Normative References

   [RFC3168]    Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
                of Explicit Congestion Notification (ECN) to IP", RFC
                3168, September 2001.

10.  Informative References

   [BCF05]      Babiarz, J., Chan, K., and V. Firoiu, "Congestion
                Notification Process for Real-Time Traffic", Work in
                Progress, July 2005.

   [BESFC06]    Briscoe, B., et al., "An edge-to-edge Deployment Model
                for Pre-Congestion Notification: Admission Control over
                a DiffServ Region", Work in Progress, June 2006.

   [ECN]        ECN Web Page, URL <www.icir.org/floyd/ecn.html>.

   [QuickStart] S. Floyd, M. Allman, A. Jain, and P. Sarolahti, "Quick-
                Start for TCP and IP", Work in Progress, October 2006.

   [RFC2581]    Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
                Control", RFC 2581, April 1999.

   [RFC2914]    Floyd, S., "Congestion Control Principles", BCP 41, RFC
                2914, September 2000.

   [RFC2960]    Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
                Zhang, L., and V. Paxson, "Stream Control Transmission
                Protocol", RFC 2960, October 2000.

   [RFC3448]    Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
                Friendly Rate Control (TFRC): Protocol Specification",
                RFC 3448, January 2003.

   [RFC3540]    Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
                Congestion Notification (ECN) Signaling with Nonces",
                RFC 3540, June 2003.

   [RFC3714]    Floyd, S. and J. Kempf, "IAB Concerns Regarding
                Congestion Control for Voice Traffic in the Internet",
                RFC 3714, March 2004.

   [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, March 2006.

   [XSSK05]     Y. Xia,  L. Subramanian, I. Stoica, and S. Kalyanaraman,
                One More Bit Is Enough, SIGCOMM 2005, September 2005.

Author's Address

   Sally Floyd
   ICIR (ICSI Center for Internet Research)

   Phone: +1 (510) 666-2989
   EMail: floyd@icir.org
   URL: http://www.icir.org/floyd/

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