Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Experimental B. Briscoe
Expires: December 25, 2008 BT & UCL
M. Menth
University of Wuerzburg
June 23, 2008
A three state extended PCN encoding scheme
draft-moncaster-pcn-3-state-encoding-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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
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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
This memo is posted as an Internet-Draft with an intent to eventually
be published as an experimental RFC.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. The Requirement for Three PCN Encoding States . . . . . . . . 4
5. Adding Limited End-to-End ECN Support to PCN . . . . . . . . . 4
6. Encoding Three PCN States in IP . . . . . . . . . . . . . . . 5
6.1. Forwarding Traffic Out of the PCN-domain . . . . . . . . . 6
7. PCN domain support for the PCN extension encoding . . . . . . 6
7.1. End-to-End transport behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 7
7.2. PCN-boundary-node behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 7
7.2.1. Behaviour for packets belonging to a PCN-flow . . . . 7
7.2.2. Behaviour for packets belonging to a
PCN-enabled-ECN-flow . . . . . . . . . . . . . . . . . 8
7.3. PCN-interior-node behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 8
7.4. Behaviour of any PCN node compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
13.1. Normative References . . . . . . . . . . . . . . . . . . . 9
13.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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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 [PCN-arch]. 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 [PCN-base-encode] 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
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
[PCN-arch] or in [PCN-base-encode]. 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.
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
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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 [PCN-arch] 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 [PCN-base-encode]. The obvious way to provide
one more encoding state than the base encoding is through the use of
an additional PCN enabled 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
[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
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.
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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 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 [PCN-base-encode].
6. Encoding Three PCN States in IP
The three state PCN encoding scheme is based closely on that defined
in [PCN-base-encode] 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. The exact manner in which the
PCN encoding states are carried in the IP header is shown in Table 1.
In the following table ThM refers to packets that have been metered
and marked according to a Threshold Markins scheme and ETM refers to
packets that have been metered and marked according to an Excess
Traffic Marking scheme.
+--------+--------------+-------------+-------------+---------+
| 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-enabled DiffServ codepoint (see
[PCN-base-encode]) and DSCP 2 is a PCN-enabled-DSCP from the EXP/LU
pools as defined in [RFC2474]
Table 1: 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
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6.1. 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 enabled 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.
+-------+-------------+-----------------+-----------------+---------+
| 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 2: 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 [PCN-base-encode].
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 enabled DSCP assigned by IANA for use with
the baseline PCN encoding [PCN-base-encode] or a DSCP from pools 2 or
3 for experimental and local use [RFC2474]. The exact 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.
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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
field of all subsequent packets in that flow to Not-ECT if it wishes
to guarantee that the flow will receive PCN treatment.
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).
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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 1.
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 2 and the DSCP MUST be set to the appropriate
DSCP for the next hop as discussed in Section 6.1 above.
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 [PCN-base-encode].
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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.
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 [PCN-base-encode]. 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
[pcn-enc-compare].
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
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Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
13.2. Informative References
[PCN-arch]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-03 (work in progress),
February 2008.
[PCN-base-encode]
Moncaster, T., Briscoe, B., and M. Menth, "A three state
extended PCN encoding scheme",
draft-moncaster-pcn-baseline-encoding-01 (work in
progress), June 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.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[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-01 (work in progress),
May 2008.
[pcn-enc-compare]
Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
Eardley, P., and B. Briscoe, "Pre-Congestion Notification
Encoding Comparison",
draft-chan-pcn-encoding-comparison-03 (work in progress),
February 2008.
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Appendix A. Tunnelling Constraints
The rules that govern the behaviour of the ECN field for IP-in-IP
tunnels were defined in [RFC3168]. This allowed for two tunnel modes
to exist. The limited functionality mode sets the outer header to
Not ECT, regardless of the value of the inner header. The full
functionality mode copies the inner ECN field into the outer header
if the inner header is Not ECT or either of the 2 ECT codepoints. If
the inner header is CE then the outer header is set to ECT(0). On
decapsulation, if the CE codepoint is set on the outer header then
this is copied into the inner header. Otherwise the inner header is
left unchanged. The reason for blocking CE from being copied to the
outer header was to prevent this from being used as a covert channel
through IPSec tunnels.
The IPSec protocol [RFC4301] changed the ECN tunnelling rule to allow
IPSec tunnels to simply copy the inner header into the outer header.
This was because the security community had decided the available
bandwidth of the covert channel offered by ECN was too low to be a
significant threat. On decapsulation the outer header is discarded
and the ECN field is only copied down if it is set to CE. Because of
the possible existence of tunnels, only CE (11) can be used as a PCN
marking as it is the only mark that will survive decapsulation.
There is a further issue involving tunnelling. In [RFC3168], IP in
IP tunnels are expected to set the ECN field to ECT(0) if the inner
ECN field is set to CE. This leads to the possibility that some
packets within the PCN field that have already been marked may have
that mark concealed further into the region. This is undesirable for
many PCN schemes and thus standard IP in IP tunnels SHOULD NOT be
used within a PCN region.
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/
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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
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