Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Experimental B. Briscoe
Expires: September 10, 2009 BT & UCL
M. Menth
University of Wuerzburg
March 9, 2009
A three state extended PCN encoding scheme
draft-moncaster-pcn-3-state-encoding-01
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 10, 2009.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
Moncaster, et al. Expires September 10, 2009 [Page 1]
Internet-Draft 3 State PCN Encoding March 2009
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
Pre-congestion notification (PCN) is a mechanism designed to protect
the Quality of Service of inelastic flows. It does this by marking
packets when traffic load on a link is approaching or has exceeded a
threshold below the physical link rate. This baseline encoding
specified how two encoding states could be encoded into the IP
header. This document specified an extension to the baseline
encoding that enables three encoding states to be carried in the IP
header as well as enabling limited support for end-to-end ECN.
Status (to be removed by RFC Editor)
This memo is posted as an Internet-Draft with an intent to eventually
be published as an experimental RFC. The PCN Working Group will be
asked to adopt this memo as a Working Group document describing one
of several possible experimental PCN encoding schemes. The intention
is that the title of this document will change to avoid confusion
with the three state marking scheme.
Changes from previous drafts
From 00 to 01:
o Checked terminology for consistency with
[I-D.ietf-pcn-baseline-encoding]
o Minor editorial changes.
Moncaster, et al. Expires September 10, 2009 [Page 2]
Internet-Draft 3 State PCN Encoding March 2009
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. The Requirement for Three PCN Encoding States . . . . . . . . 5
5. Adding Limited End-to-End ECN Support to PCN . . . . . . . . . 5
6. Encoding Three PCN States in IP . . . . . . . . . . . . . . . 6
6.1. Basic Three State Encoding . . . . . . . . . . . . . . . . 6
6.2. Full Three State Encoding . . . . . . . . . . . . . . . . 7
6.2.1. Forwarding Traffic Out of the PCN-domain . . . . . . . 7
7. PCN domain support for the PCN extension encoding . . . . . . 8
7.1. End-to-End transport behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 8
7.2. PCN-boundary-node behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 9
7.2.1. Behaviour for packets belonging to a PCN-flow . . . . 9
7.2.2. Behaviour for packets belonging to a
PCN-enabled-ECN-flow . . . . . . . . . . . . . . . . . 9
7.3. PCN-interior-node behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 10
7.4. Behaviour of any PCN node compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1. Normative References . . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Moncaster, et al. Expires September 10, 2009 [Page 3]
Internet-Draft 3 State PCN Encoding March 2009
1. Introduction
Pre-congestion notification provides information to support admission
control and flow termination at the boundary nodes of a Diffserv
region in order to protect the quality of service (QoS) of inelastic
flows [I-D.ietf-pcn-architecture]. This is achieved by marking
packets on interior nodes according to some metering function
implemented at each node. Excess traffic marking marks PCN packets
that exceed a certain reference rate on a link while threshold
marking marks all PCN packets on a link when the PCN traffic rate
exceeds a higher reference rate. These marks are monitored by the
egress nodes of the PCN-domain.
The baseline encoding described in [I-D.ietf-pcn-baseline-encoding]
provides for deployment scenarios that only require two PCN encoding
states. This document describes an experimental extension to the
base-encoding in the IP header that adds two capabilities:
o the encoding of a third PCN encoding state in the IP header
o preservation of the end-to-end semantics of the ECN field even
though PCN uses the field within a PCN-region that interrupts the
end-to-end path
The second of these capabilities is optional and the reasons for
doing it are discussed in
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Terminology
Most of the terminology used in this document is defined either in
[I-D.ietf-pcn-architecture] or in [I-D.ietf-pcn-baseline-encoding].
The following additional terms are defined in this document:
o PCN-flow - a flow covered by a reservation but which hasn't
signalled that it requires end-to-end ECN support.
o PCN-enabled-ECN-flow - a flow covered by reservation and for which
the end-to-end transport has explicitly negotiated ECN support
from the PCN-boundary-nodes.
Moncaster, et al. Expires September 10, 2009 [Page 4]
Internet-Draft 3 State PCN Encoding March 2009
o Not-Marked (xxx), where xxx represents a standard ECN codepoint -
packets that are PCN capable but carry no PCN mark. Also NM(xxx).
The (xxx) represents the ECN codepoint that the packet arrived
with at the PCN-ingress-node e.g. NM(CE) represents a PCN capable
packet that has no PCN marking but which arrived with the ECN bits
set to congestion experienced.
4. The Requirement for Three PCN Encoding States
The PCN architecture [I-D.ietf-pcn-architecture] describes proposed
PCN schemes that require traffic to be metered and marked using both
Threshold and Excess Traffic schemes. In order to achieve this it is
necessary to allow for three PCN encoding states. The constraints
imposed by the way tunnels process the ECN field severely limit how
to encode these states as explained in
[I-D.ietf-pcn-baseline-encoding] and [I-D.ietf-tsvwg-ecn-tunnel].
The obvious way to provide one more encoding state than the base
encoding is through the use of an additional PCN-compatible DiffServ
codepoints. One aim of this document is to allow for experiments to
show whether such schemes are better than those that only employ two
PCN encoding states. As such, the additional DSCP will be taken from
as the EXP/LU pools defined in [RFC2474]. If the experiments
demonstrate that PCN schemes employing three encoding states are
significantly better than those only employing two then at a later
date IANA might be asked to assign a new PCN enabled DSCP from pool
1.
5. Adding Limited End-to-End ECN Support to PCN
[I-D.sarker-pcn-ecn-pcn-usecases] suggests a number of use-cases
where explicit preservation of end-to-end ECN semantics might be
needed across a PCN domain. One of the use-cases suggests that the
end-nodes might be running rate-adaptive codecs that would respond to
ECN marks by reducing their transmission rate. If the sending
transport sets the ECT codepoint, the setting of the ECN field as it
arrives at the PCN ingress node will need to be re-instated as it
leaves the PCN egress node.
If a PCN region is starting to suffer pre-congestion then it may make
sense to expose marks generated within the PCN region by forwarding
CE marks from the PCN egress to such a rate-adaptive endpoint. They
would be in addition to any CE marks generated elsewhere on the end-
to-end path. This would allow the endpoints to reduce the traffic
rate. This will in turn help to alleviate the pre-congestion,
potentially averting any need for call blocking or termination.
However, the 'leaking' of CE marks out of the PCN region is
Moncaster, et al. Expires September 10, 2009 [Page 5]
Internet-Draft 3 State PCN Encoding March 2009
potentially dangerous and could violate [RFC4774] if the end hosts
don't understand ECN (see section 18.1.4 of [RFC3168]).
Therefore, a PCN region can only support end-to-end ECN if the PCN
edge nodes are sure that the end-to-end transport is ECN-capable.
That way the PCN egress nodes can ensure that they only expose CE
marks to those receivers that will correctly interpret them as a
notification of congestion. The end-points may indicate they are
ECN-capable through some higher-layer signalling process that sets up
their reservation with the PCN boundary nodes. The exact process of
negotiation is beyond the scope of this document but is likely to
involve explicit two way signalling between the end-host and the PCN-
domain.
In the absence of such signalling the default behaviour of the PCN
egress node will be to clear the ECN field to 00 as in the baseline
PCN encoding [I-D.ietf-pcn-baseline-encoding].
6. Encoding Three PCN States in IP
The three state PCN encoding scheme is based closely on that defined
in [I-D.ietf-pcn-baseline-encoding] so that there will be no
compatibility issues if a PCN-domain changes from using the baseline
encoding scheme to the experimental scheme described here. There are
two versions of the scheme. The basic three state scheme allows for
carrying both Threshold Marked (ThM) and Excess traffic MArked (ETM)
traffic. The full scheme additionally allows us to carry end-to-end
ECN.
6.1. Basic Three State Encoding
The following table shows how to encode the three PCN states in IP.
The authors spent some time trying to establish which way round to
put the two marked states before settling on this. Because it is
envisaged that DSCP 2 will be of lower priority than DSCP 1 the
change in marking from Threshold to Excess Traffic involves
downgrading the traffic which seems to be consistent with the
requirement that such changes should not be reversed.
Moncaster, et al. Expires September 10, 2009 [Page 6]
Internet-Draft 3 State PCN Encoding March 2009
+--------+--------------+-------------+-------------+---------+
| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
+--------+--------------+-------------+-------------+---------+
| DSCP 1 | Not-PCN | NM | CU | ThM |
| DSCP 2 | Not-PCN | CU | CU | ETM |
+--------+--------------+-------------+-------------+---------+
Where DSCP 1 is a PCN-compatible DiffServ codepoint (see
[I-D.ietf-pcn-baseline-encoding]) and DSCP 2 is a PCN-compatible DSCP
from the EXP/LU pools as defined in [RFC2474]
Table 1: Encoding three PCN states in IP
6.2. Full Three State Encoding
Table 2 shows how to additionally carry the end-to-end ECN state in
the IP header.
+--------+--------------+-------------+-------------+---------+
| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
+--------+--------------+-------------+-------------+---------+
| DSCP 1 | Not-PCN | NM(Not-ECT) | NM(CE) | ThM |
| DSCP 2 | Not-PCN | NM(ECT(0)) | NM(ECT(1)) | ETM |
+--------+--------------+-------------+-------------+---------+
Where DSCP 1 is a PCN-compatible DiffServ codepoint (see
[I-D.ietf-pcn-baseline-encoding]) and DSCP 2 is a PCN-compatible DSCP
from the EXP/LU pools as defined in [RFC2474]
Table 2: Encoding three PCN states in IP
The four different Not Marked (NM) states allow for the addition of
limited end-to-end ECN support as explained in the previous section.
Warning
6.2.1. Forwarding Traffic Out of the PCN-domain
As each packet exits the PCN-domain, the PCN-egress-node MUST check
whether it belongs to a PCN-enabled-ECN-flow. If it belongs to such
a flow then the following table shows how the ECN field should be re-
set. In addition all packets should have their DSCP reset to the
appropriate DSCP for the next hop. If the next hop is not another
PCN region this will not be a PCN-compatible DSCP, and by default
will be the best-efforts DSCP. Alterntively higher layer signalling
mechanisms may allow the DSCP that packets entered the PCN-domain
with to be re-instated.
Moncaster, et al. Expires September 10, 2009 [Page 7]
Internet-Draft 3 State PCN Encoding March 2009
+-------+-------------+-----------------+-----------------+---------+
| DSCP | 00 | 10 | 01 | 11 |
+-------+-------------+-----------------+-----------------+---------+
| DSCP | Not PCN --> | NM(Not-ECT) --> | NM(CE) --> CE | ThM --> |
| 1 | Not ECT | not-ECT | | CE |
| DSCP | Not PCN --> | NM(ECT(0)) --> | NM(ECT(1)) --> | ETM --> |
| 2 | Not ECT | ECT(0) | ECT(1) | CE |
+-------+-------------+-----------------+-----------------+---------+
Where each cell gives the incoming PCN state and the outgoing ECN
state.
Table 3: Egress rules for resetting ECN field for PCN Enabled ECN
Flows
For packets belonging to a PCN-flow the ECN field MUST be reset to
not-ECT (00) as defined in [I-D.ietf-pcn-baseline-encoding].
7. PCN domain support for the PCN extension encoding
PCN traffic MUST be marked with a DiffServ codepoint that indicates
PCN is enabled. To comply with the PCN extension encoding, this
codepoint is either a PCN-compatible DSCP assigned by IANA for use
with the baseline PCN encoding [I-D.ietf-pcn-baseline-encoding] or a
DSCP from pools 2 or 3 for experimental and local use [RFC2474]. The
exact choice of DSCP may vary between PCN-domains but MUST be fixed
within each PCN-domain.
All nodes within a PCN-domain MUST understand and support the three
PCN states of the PCN extension coding. Therefore if any PCN-node
does not support three PCN encoding states, any node in the same PCN-
domain MUST NOT be configured to use three PCN encoding states as
defined here.
7.1. End-to-End transport behaviour compliant with the PCN extension
encoding
Transports wishing to use both a reservation and end-to-end ECN MUST
establish that their path supports this combination. Support of end-
to-end ECN by PCN boundary nodes is OPTIONAL. Therefore transports
MUST check with both the PCN-ingress-node and PCN-egress-node for
each flow. The sending of such a request MUST NOT be taken to mean
the request has been granted. The PCN-boundary-nodes MAY choose to
inform the end-node of a successful request. The exact mechanism for
such negotiation is beyond the scope of this document. A transport
that receives no response or a negative response to a request to
support end-to-end ECN within a flow reservation MUST set the ECN
Moncaster, et al. Expires September 10, 2009 [Page 8]
Internet-Draft 3 State PCN Encoding March 2009
field of all subsequent packets in that flow to Not-ECT if it wishes
to guarantee that the flow will receive PCN treatment.
If a domain wishes to use the full scheme described in Table 2 all
nodes in that domain MUST be configured to understand the full
scheme.
7.2. PCN-boundary-node behaviour compliant with the PCN extension
encoding
o If both the PCN ingress and egress nodes support end-to-end ECN,
and the transport has successfully requested end-to-end ECN the
flow becomes a PCN-enabled-ECN-flow.
o If either of a PCN ingress-egress pair does not support end-to-end
ECN or if the end-to-end transport does not request support for
end-to-end ECN then the PCN-boundary-nodes MUST assume the packet
belongs to a PCN-flow.
7.2.1. Behaviour for packets belonging to a PCN-flow
o If a packet belongs to a PCN-flow arrives at the PCN-ingress-node
with its ECN field already marked as CE or ECT, it SHOULD be
dropped. Alternatively it MAY be downgraded to a lower (non-PCN)
service class or MAY be tunnelled through the PCN region. It MUST
NOT be admitted to the PCN region directly.
o When a packet belonging to a PCN-flow carrying the not-ECT
codepoint arrives at the PCN-ingress-node, the ECN field MUST be
set to ECT(0) (10) and the DiffServ field set to DSCP 1.
o When a packet belonging to a PCN-flow leaves the PCN-domain
through the PCN-egress-node, the ECN bits MUST be set to not-ECT
(00).
7.2.2. Behaviour for packets belonging to a PCN-enabled-ECN-flow
o When a packet belonging to a PCN-enabled-ECN-flow arrives at the
PCN-ingress-node, then the ECN field and DSCP MUST be set to the
appropriate NM(xxx) setting as shown in Table 2.
o When a packet belonging to a PCN-enabled-ECN-flow leaves the PCN-
region through a PCN-egress-node, the ECN bits MUST be set
according to Table 3 and the DSCP MUST be set to the appropriate
DSCP for the next hop as discussed in Section 6.2.1 above.
Moncaster, et al. Expires September 10, 2009 [Page 9]
Internet-Draft 3 State PCN Encoding March 2009
7.3. PCN-interior-node behaviour compliant with the PCN extension
encoding
o If a PCN interior node indicates that a packet is to be threshold
marked then the ThM codepoint MUST be set by changing the ECN bits
to 11 and ensuring the Diffserv field is set to DSCP1.
o If a PCN interior node indicates that a packet is to be excess
traffic marked then the EM codepoint MUST be set by changing the
ECN bits to 11 and ensuring the Diffserv field is set to DSCP2 as
defined above.
7.4. Behaviour of any PCN node compliant with the PCN extension
encoding
o PCN nodes MUST NOT change not-PCN to another codepoint and they
MUST NOT change a PCN-Capable codepoint to not-PCN.
o ThM MUST NOT be changed to NM.
o ETM MUST NOT be changed to ThM or to NM.
8. IANA Considerations
This document asks IANA to assign one DiffServ codepoint from Pool 2
or Pool 3 (for experimental/local use)[RFC2474]. Should any of the
three encoding state experimental PCN schemes prove sufficiently
successful then, at a later date, IANA will be requested in a later
document to assign a dedicated DiffServ codepoint from pool 1 for
standards use.
9. Security Considerations
The security concerns relating to this extended PCN encoding are
essentially the same as those in [I-D.ietf-pcn-baseline-encoding].
This extension coding gives end-to-end support for the ECN nonce
[RFC3540], which is intended to protect the sender against the
receiver or against network elements concealing a congestion
experienced marking or a lost packet. PCN-based reservations
combined with end-to-end ECN are intended for partially inelastic
traffic using rate-adaptive codecs. Therefore the end-to-end
transport is unlikely to be TCP, but at this time the nonce has only
been defined for TCP transports.
Moncaster, et al. Expires September 10, 2009 [Page 10]
Internet-Draft 3 State PCN Encoding March 2009
10. Conclusions
This document describes an extended encoding scheme for PCN that
provides for three encoding states as well as support for end-to-end
ECN. The encoding scheme builds on the baseline encoding described
in [I-D.ietf-pcn-baseline-encoding]. Using this encoding scheme it
is possible for operators to conduct experiments to check whether the
addition of an extra encoding state will significantly improve the
performance of PCN. It will also allow experiments to determine
whether there is a need for end-to-end ECN support within the PCN-
domain (as against end-to-end ECN support through the use of IP-in-IP
tunnelling or by downgrading the traffic to a lower service class).
11. Acknowledgements
This document builds extensively on work done in the PCN working
group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Joe
Babiarz and others. Full details of alternative schemes that were
considered for adoption can be found in the document
[I-D.chan-pcn-encoding-comparison].
12. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF Transport Area working group mailing list
<tsvwg@ietf.org>, and/or to the authors.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
13.2. Informative References
[I-D.chan-pcn-encoding-comparison]
Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
Eardley, P., and B. Briscoe, "Pre-Congestion Notification
Encoding Comparison",
draft-chan-pcn-encoding-comparison-04 (work in progress),
Moncaster, et al. Expires September 10, 2009 [Page 11]
Internet-Draft 3 State PCN Encoding March 2009
February 2008.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", draft-ietf-pcn-architecture-09 (work in
progress), January 2009.
[I-D.ietf-pcn-baseline-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
draft-ietf-pcn-baseline-encoding-02 (work in progress),
February 2009.
[I-D.ietf-tsvwg-ecn-tunnel]
Briscoe, B., "Layered Encapsulation of Congestion
Notification", draft-ietf-tsvwg-ecn-tunnel-01 (work in
progress), October 2008.
[I-D.sarker-pcn-ecn-pcn-usecases]
Sarker, Z. and I. Johansson, "Usecases and Benefits of end
to end ECN support in PCN Domains",
draft-sarker-pcn-ecn-pcn-usecases-02 (work in progress),
November 2008.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
Moncaster, et al. Expires September 10, 2009 [Page 12]
Internet-Draft 3 State PCN Encoding March 2009
Authors' Addresses
Toby Moncaster
BT
B54/70, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 648734
Email: toby.moncaster@bt.com
URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/
Bob Briscoe
BT & UCL
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
Michael Menth
University of Wuerzburg
room B206, Institute of Computer Science
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49 931 888 6644
Email: menth@informatik.uni-wuerzburg.de
Moncaster, et al. Expires September 10, 2009 [Page 13]