Congestion and Pre Congestion T. Moncaster
Internet-Draft BT
Intended status: Experimental B. Briscoe
Expires: October 10, 2009 BT & UCL
M. Menth
University of Wuerzburg
April 8, 2009
A PCN encoding using 2 DSCPs to provide 3 or more states
draft-ietf-pcn-3-state-encoding-00
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Abstract
Pre-congestion notification (PCN) is a mechanism designed to protect
the Quality of Service of inelastic flows within a controlled domain.
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 experimental encoding scheme specifies how three encoding states
can be carried in the IP header using a combination of two DSCPs and
the ECN bits. The Basic scheme only allows for three encoding
states. The Full scheme additionally provides limited end-to-end
support for 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 draft-moncaster-pcn-3-state-encoding-01:
o Changed to WG draft. Title changed from "A three state extended
PCN encoding scheme"
o Imposed new structure on document. This structure is intended to
be followed by all extensions to the baseline PCN encoding scheme.
o Extensive changes throughout to ensure consistency with the
baseline PCN encoding scheme.
From 00 to 01:
o Checked terminology for consistency with
[I-D.ietf-pcn-baseline-encoding]
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o Minor editorial changes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. The Requirement for Three PCN Encoding States . . . . . . . . 5
5. Adding Limited End-to-End ECN Support to PCN . . . . . . . . . 6
6. Encoding Three PCN States in IP . . . . . . . . . . . . . . . 6
6.1. Basic Three State Encoding . . . . . . . . . . . . . . . . 7
6.2. Full Three State Encoding . . . . . . . . . . . . . . . . 7
6.3. Valid and invalid codepoint transitions at
PCN-ingress-nodes . . . . . . . . . . . . . . . . . . . . 8
6.4. Valid and invalid codepoint transitions at
PCN-interior-nodes . . . . . . . . . . . . . . . . . . . . 8
6.5. Forwarding traffic out of the PCN-domain . . . . . . . . . 9
7. PCN-domain support for the PCN extension encoding . . . . . . 10
7.1. End-to-End transport behaviour compliant with the PCN
extension encoding . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
13.1. Normative References . . . . . . . . . . . . . . . . . . . 12
13.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The objective of Pre-Congestion Notification (PCN)
[I-D.ietf-pcn-architecture] is to protect the quality of service
(QoS) of inelastic flows within a Diffserv domain, in a simple,
scalable and robust fashion. The overall rate of the PCN-traffic is
metered on every link in the PCN-domain, and PCN-packets are
appropriately marked when certain configured rates are exceeded.
These configured rates are below the rate of the link thus providing
notification before any congestion occurs (hence "pre-congestion
notification"). The level of marking allows the boundary nodes to
make decisions about whether to admit or block a new flow request,
and (in abnormal circumstances) whether to terminate some of the
existing flows, thereby protecting the QoS of previously admitted
flows.
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 addition 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 Section 5.
As in the baseline encoding, this extension encoding re-uses the ECN
bits within the IP header within a controlled PCN-domain. This
extension requires the use of two DSCPs as described later in this
document. This experimental scheme is one of three that are being
proposed within the PCN working group. The aim is to allow
implementors to decide which scheme is most suitable for possible
future standardisation.
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].
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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.
o Not-marked (xxx), where xxx represents a standard ECN codepoint -
packets that are PCN capable but carry no PCN mark. Abbreviated
as 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 Marking Behaviours document [I-D.ietf-pcn-marking-behaviour]
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
codepoint.
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 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. Note that there are other
experimental encoding schemes being considered which only use one
DSCP but require either alternative tunnel semantics
([I-D.briscoe-pcn-3-in-1-encoding]) or additional signalling
([I-D.menth-pcn-psdm-encoding])in order to work.
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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
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-
boundary-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 end-to-end ECN to be
carried across the PCN-domain.
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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.
+--------+--------------+-------------+-------------+---------+
| 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
In order to comply with this encoding all the nodes within the PCN-
domain MUST be configured with this encoding scheme. However there
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may be operators who choose not to be fully compliant with the
scheme. If an operator chooses to leave some PCN-interior-nodes that
only support two marking states (the base encoding), then they must
be aware of the following: Ideally such nodes would be configured to
indicate pre-congestion or congestion using the ETM state since this
would ensure they could notify worst-case congestion, however this is
not possible since it requires the packets to be re-marked to DSCP 2
(hence altering the baseline encoding). This means that such nodes
will only be able to indicate ThM traffic.
6.3. Valid and invalid codepoint transitions at PCN-ingress-nodes
A PCN-ingress-node operating the Basic version of the 3-State
Encoding scheme MUST set the Not-marked codepoint on any arriving
packet that belongs to a PCN-flow. It MUST set the not-PCN codepoint
on any other packet.
A PCN-ingress-node operating the Full version of the 3-State Encoding
scheme MUST establish whether a packet is a member of a PCN-enabled-
ECN-flow. If it is, the PCN-ingress-node MUST set the appropriate
NM(xxx) codepoint depending on the value carried in the ECN field of
that packet. To be clear:
o A packet carrying the not-ECT codepoint in the ECN field MUST be
assigned the NM(not-ECT) codepoint
o A packet carrying the ECT(0) codepoint in the ECN field MUST be
assigned the NM(ECT(0)) codepoint
o A packet carrying the ECT(1) codepoint in the ECN field MUST be
assigned the NM(ECT(1)) codepoint
o A packet carrying the CE codepoint in the ECN field MUST be
assigned the NM(CE) codepoint
If it is not a member of such a flow then the behaviour MUST be the
same as for the Basic version of the Encoding scheme.
6.4. Valid and invalid codepoint transitions at PCN-interior-nodes
A PCN-interior-node MUST obey the following rules:
o It MUST NOT change the not-PCN codepoint to any other codepoint.
o It MAY change any Not-marked codepoint to either the Threshold-
marked or Excess-traffic-marked codepoints.
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o It MUST NOT change a Not-marked codepoint to the not-PCN
codepoint.
o A Not-marked codepoint MUST NOT be changed to any other Not-marked
codepoint.
o It MAY change the ThM codepoint to the ETM codepoint but it MUST
NOT change the ThM codepoint to any other codepoint.
o It MUST NOT change the ETM codepoint to any other codepoint.
Obviously in every case a codepoint can remain unchanged. The
precise rules governing which valid transition to use are set out in
[I-D.ietf-pcn-marking-behaviour]
6.5. 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 rules dictate how the ECN field should be
reset:
o A packet carrying the not-PCN codepoint MUST be given the not-ECT
codepoint.
o A packet carrying the NM(not-ECT) codepoint MUST be assigned the
not-ECT codepoint.
o A packet carrying the NM(ECT(0)) codepoint MUST be assigned the
ECT(0) codepoint.
o A packet carrying the NM(ECT(1)) codepoint MUST be assigned the
ECT(1) codepoint.
o A packet carrying the NM(CE) codepoint MUST be assigned the CE
codepoint.
o A packet carrying the ThM codepoint MUST be assigned CE codepoint.
o A packet carrying the ETM codepoint MUST be assigned CE codepoint.
If the packet is part of a PCN-flow then it MUST be assigned the not-
ECT codepoint regardless of which PCN-codepoint it carried.
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
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mechanisms may allow the DSCP that packets entered the PCN-domain
with to be reinstated.
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.
7.1. End-to-End transport behaviour compliant with the PCN extension
encoding
Transports wishing to use both PCN 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.
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.
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.
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 IANA will be requested in a later document to assign
a dedicated DiffServ codepoint from pool 1 for standards use.
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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.
10. Conclusions
This document describes an extended encoding scheme for PCN that
provides for three encoding states as well as optional 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
(Section to be removed by RFC_Editor) 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
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13.1. Normative References
[I-D.ietf-pcn-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-02 (work in progress),
March 2009.
[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.briscoe-pcn-3-in-1-encoding]
Briscoe, B., "PCN 3-State Encoding Extension in a single
DSCP", draft-briscoe-pcn-3-in-1-encoding-00 (work in
progress), October 2008.
[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),
March 2009.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", draft-ietf-pcn-architecture-11 (work in
progress), April 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-03 (work in progress),
April 2009.
[I-D.ietf-tsvwg-ecn-tunnel]
Briscoe, B., "Tunnelling of Explicit Congestion
Notification", draft-ietf-tsvwg-ecn-tunnel-02 (work in
progress), March 2009.
[I-D.menth-pcn-psdm-encoding]
Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
"PCN Encoding for Packet-Specific Dual Marking (PSDM)",
draft-menth-pcn-psdm-encoding-00 (work in progress),
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July 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.
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
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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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