Internet DRAFT - draft-floyd-ecn-alternates
draft-floyd-ecn-alternates
Internet Engineering Task Force S. Floyd
INTERNET-DRAFT ICIR
draft-floyd-ecn-alternates-02.txt 18 August 2005
Expires: February 2006
Specifying Alternate Semantics for the Explicit Congestion
Notification (ECN) Field
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
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This Internet-Draft will expire on February 2006.
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
There have been a number of proposals for alternate semantics for
the ECN field in the IP header [ECN]. This document discusses some
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of the issues in defining alternate semantics for the ECN field, and
specifies requirements for a safe co-existence 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.
NOTE TO RFC EDITOR: PLEASE DELETE THIS NOTE UPON PUBLICATION.
Changes from draft-floyd-ecn-alternates-01.txt:
* Changed requirement for TCP friendliness, to a requirement of
friendliness with IETF-conformant congestion control. From email
from Mark Allman.
* Added to discussion of robustness to route changes. From email
from Mark Allman.
* Added an explicit note that the ECN nonce is agnostic to the
semantics of the other codepoints, and could be used with alternate
ECN semantics.
* Minor editing, from email from Mark Allman.
Changes from draft-floyd-ecn-alternates-00.txt:
* Added requirements for compatibility between traffic using default
ECN semantics and routers using alternate ECN semantics, to the
section on "Option 3: Friendly Co-existence with Competing
Traffic". From email from Gorry Fairhurst.
* Added to the discussion of using the diffserv code point to signal
alternate ECN semantics. From email from Gorry Fairhurst.
* Minor editing for clarity, also from email from Gorry Fairhurst.
END OF NOTE TO RFC EDITOR.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
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. . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Option 2: Verification that Routers Under-
stand the Alternate Semantics . . . . . . . . . . . . . . . . 8
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4.3. Option 3: Friendly Co-existence with Com-
peting Traffic. . . . . . . . . . . . . . . . . . . . . . . . 8
5. Evaluation of the Alternate-ECN Semantics . . . . . . . . . . 11
5.1. Verification of Feedback from the Router . . . . . . . . 11
5.2. Co-existence with Competing Traffic. . . . . . . . . . . 11
5.3. A General Evaluation of the Alternate-ECN
Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Who Wants to Use Alternate Semantics for the ECN
Codepoint? . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 12
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Normative References . . . . . . . . . . . . . . . . . . . . 13
11. Informative References . . . . . . . . . . . . . . . . . . . 13
IANA Considerations. . . . . . . . . . . . . . . . . . . . . . . 14
AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
RFC 3168, 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. This document describes some of the issues
that arise in specifying such alternate semantics for the ECN field,
and gives requirements for a safe co-existence 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 no ECN at
all; (3) co-existence 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-
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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 co-existence 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 co-exist 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 below.
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
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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
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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 co-
existence 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?
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|--------|
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.
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 co-existence 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.
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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. This would
mean that it would be unsafe for packets using the alternate ECN
semantics to be unleashed in the global Internet, in order to avoid
the chance of the alternate-ECN traffic traversing an old router
that don'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.
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 Co-existence 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
co-exist safely in the Internet on a path with one or more old
routers that use only the default ECN semantics. In this scenario,
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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, that 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
[DCCP]), and TCP-Friendly Rate Control (TFRC) and protocols with
TFRC-like congestion control (e.g., DCCP using CCID-3 [DCCP]). 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 in order 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
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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 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 diff-serv 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 diff-serv
codepoint, and all packets with that diff-serv 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.
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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 co-exist 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 incentives for lying 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 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 [RFC 3540]. 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. Co-existence with Competing Traffic
A second general issue concerns the co-existence of alternate-ECN
traffic with competing traffic along the path, in a clean
environment where all routers understand and are willing to use the
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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
co-existence of diffserv traffic with other traffic on the link;
this should be addressed in the specification of the diffserv
codepoint itself.
5.3. 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. Who Wants to Use Alternate Semantics for the ECN Codepoint?
There have been a number of proposals in the IETF and in the
research community for alternate semantics for the ECN codepoint
[ECN]. One such proposal, [BCF05], proposes an 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].
Some of the other proposals for alternate ECN semantics are listed
on the ECN Web Page [ECN].
7. Security Considerations
This document doesn't propose any new mechanisms for the Internet
protocol, and therefore doesn't introduce any new security
considerations.
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
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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. 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.
10. Normative References
11. Informative References
[BCF05] J. Babiarz, K. Chan, and V. Firoiu, Congestion Notification
Process for Real-Time Traffic, internet-draft draft-babiarz-tsvwg-
rtecn-04, work in progress, July 2005.
[DCCP] DCCP Web Page, URL "http://www.icir.org/kohler/dccp/".
[ECN] ECN Web Page, URL "www.icir.org/floyd/ecn.html".
[QuickStart] Quick-Start Web Page, URL
"http://www.icir.org/floyd/quickstart.html".
[RFC2581] M. Allman, V. Paxson, and W. Stevens, TCP Congestion
Control, RFC 2581, Proposed Standard, April 1999.
[RFC2914] S. Floyd, Congestion Control Principles, RFC 2914, Best
Current Practice, September 2000.
[RFC2960] R. Stewart et al, Stream Control Transmission Protocol,
RFC 2960, October 2000.
[RFC3168] Ramakrishnan, K.K., Floyd, S., and Black, D., The Addition
of Explicit Congestion Notification (ECN) to IP, RFC 3168, Proposed
Standard, September 2001.
[RFC3540] N. Spring, D. Wetherall, and D. Ely, Robust Explicit
Congestion Notification (ECN) Signaling with Nonces, RFC 3540,
Experimental, June 2003.
[RFC3714] S. Floyd and J. Kempf, Editors, IAB Concerns Regarding
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Congestion Control for Voice Traffic in the Internet, RFC 3714,
Informational, March 2004.
IANA Considerations
There are no IANA considerations in this document.
AUTHORS' ADDRESSES
Sally Floyd
Phone: +1 (510) 666-2989
ICIR (ICSI Center for Internet Research)
Email: floyd@icir.org
URL: http://www.icir.org/floyd/
Full Copyright Statement
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it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
Floyd [Page 14]
INTERNET-DRAFT Expires: February 2006 August 2005
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Floyd [Page 15]
