Internet DRAFT - draft-ietf-pcn-psdm-encoding
draft-ietf-pcn-psdm-encoding
Congestion and Pre Congestion M. Menth
Internet-Draft University of Tuebingen
Intended status: Historic J. Babiarz
Expires: September 13, 2012 3inova Networks Inc
T. Moncaster
University of Cambridge
B. Briscoe
BT
March 12, 2012
PCN Encoding for Packet-Specific Dual Marking (PSDM Encoding)
draft-ietf-pcn-psdm-encoding-02
Abstract
Pre-congestion notification (PCN) is a link-specific and load-
dependent packet re-marking mechanism and provides in Differentiated
Services networks feedback to egress nodes about load conditions
within the domain. It is used to support admission control and flow
termination decision in a simple way. This document proposes how PCN
marks can be encoded into the IP header for packet-specific dual
marking (PSDM). PSDM encoding provides three different codepoints:
not-ETM, not-ThM, and PM. Excess-traffic-marking may re-mark not-
ETM-packets to PM and threshold-marking may re-mark not-ThM-packets
to PM.
Status
Since its original publication, the baseline encoding (RFC5696) on
which this document depends has become obsolete. The PCN working
GRoup has chosen to publish this as a historical document to preserve
the details of the encoding and to allow it to be cited in other
documents.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 13, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Encoding for Packet-Specific Dual Marking . . . . . . . . . . 6
3.1. Proposed Encoding and Expected Node Behavior . . . . . . . 6
3.1.1. PCN Codepoints . . . . . . . . . . . . . . . . . . . . 6
3.1.2. Codepoint Handling by PCN Ingress Nodes . . . . . . . 6
3.1.3. Codepoint Handling by PCN Interfaces . . . . . . . . . 7
3.1.4. Codepoint Handling by PCN Egress Nodes . . . . . . . . 7
3.2. Reasons for the Proposed Encoding . . . . . . . . . . . . 7
3.2.1. Scarcity of DSCPs . . . . . . . . . . . . . . . . . . 7
3.2.2. Problems with Tunneling . . . . . . . . . . . . . . . 7
3.2.3. Problems with the ECN Field . . . . . . . . . . . . . 8
3.3. Handling of ECN Traffic . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
protect the quality of service (QoS) of inelastic flows within a
Diffserv domain, in a simple, scalable, and robust fashion. Two
mechanisms are used: admission control (AC), to decide whether to
admit or block a new flow request, and (in abnormal circumstances)
flow termination (FT) to decide whether to terminate some of the
existing flows. To achieve this, the overall rate of PCN-traffic is
metered on every link in the 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 to boundary nodes about overloads before any congestion
occurs (hence "pre-congestion notification").
The level of marking allows boundary nodes to make decisions about
whether to admit or terminate. 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 lower
reference rate [RFC5670]. These marks are monitored by the egress
nodes of the PCN domain.
This document proposes how PCN marks can be encoded into the IP
header when packet-specific dual marking (PSDM) is used to re-mark
packets [Menth09f]. That means, both excess-traffic-marking and
threshold-marking are activated on the links within a PCN domain, but
packets are subject to re-marking by only one of them. The encoding
of unmarked PCN packets indicates whether they are subject to either
excess-traffic-marking (not-ETM) or threshold-marking (not-ThM) and
they may be re-marked to PCN-marked (PM).
PSDM encoding can be applied in networks implementing
o only AC based on threshold-marking (reference rate = PCN-
admissible-rate),
o only FT based on excess-traffic-marking (reference rate = PCN-
supportable-rate),
o both AC and FT based on excess-traffic-marking (reference rate =
PCN-admissible-rate)
o Probe-based AC based on threshold-marking (reference rate = PCN-
admissible-rate) and FT based on excess-traffic-marking (reference
rate = PCN-supportable-rate)[Menth09f].
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The motivation for PSDM encoding is that probe packets are subject
only to threshold-marking and that data packets are subject only to
excess-traffic-marking. Nevertheless, routers should not need to
differentiate explicitly between probe and data packets since packets
are a priori marked with an appropriate codepoint (not-ETM, not-ThM)
indicating the marking mechanism applying to them.
Following the publication of new rules relating to the tunnelling of
ECN marks [RFC6040], the PCN workign group decided to obsolete
[RFC5696] in favour of the 3-in-1 encoding
[I-D.ietf-pcn-3-in-1-encoding]. A side-effect of this decision was
to make the PSDM encoding obsolete. However the PCN working group
feels it is useful to have a formal historical record of this
encoding. This ensures details of the encoding are not lost and also
allows it to be cited in other documents.
1.1. 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].
2. Terminology
Most of the terminology used in this document is defined in
[RFC5559]. The following additional terms are defined in this
document:
o PCN-capable flow - a flow subject to PCN-based admission control
and/or flow termination
o PCN-enabled DSCP - DSCP indicating within a PCN domain that
packets possibly belong to a PCN-capable flow
o PCN-capable ECN codepoint (PCN codepoint) - DSCP set to a PCN-
enabled DSCP and ECN field set to a codepoint indicating that a
packet belongs to a PCN-capable flow (not-ThM, not-ETM, or PM,
explained below)
o PCN packet - a packet belonging to a PCN capable flow within a PCN
domain, must have a PCN-enabled DSCP and a PCN-capable ECN
codepoint
o not-PCN capable (not-PCN) - new ECN codepoint for packets of non-
PCN-capable flows when a PCN-enabled DSCP is set
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o not-excess-traffic-marked (not-ETM) - new ECN codepoint for
unmarked PCN packets that are subject to excess-traffic-marking
o not-threshold-marked (not-ThM) - new ECN codepoint for unmarked
PCN packets that are subject to threshold-marking
o PCN-marked (PM) - new ECN codepoint for re-marked PCN packets
regardless whether they were subject to excess-traffic-marking or
threshold-marking.
3. Encoding for Packet-Specific Dual Marking
In this section the encoding for packet-specific dual marking (PSDM)
is presented and the reasons for the proposed design are outlined.
3.1. Proposed Encoding and Expected Node Behavior
The encoding reuses a PCN-enabled DSCP to indicate packets of PCN-
capable flows within a PCN domain.
3.1.1. PCN Codepoints
The ECN field of packets with a PCN-enabled DSCP is interpreted
within a PCN domain as PCN codepoint while it is interpreted as ECN
codepoint outside PCN domains. Four new PCN codepoints are defined
in Figure 1. PSDM encoding can be seen as an extension of baseline
encoding [RFC5696]
+--------+----------------------------------------------------+
| | Codepoint in ECN field of IP header |
| DSCP | (RFC3168 codepoint name) |
| +--------------+-------------+-------------+---------+
| | 00 (Not-ECT) | 10 (ECT(0)) | 01 (ECT(1)) | 11 (CE) |
+--------+--------------+-------------+-------------+---------+
| DSCP n | not-PCN | not-ETM | not-ThM | PM |
+--------+--------------+-------------+-------------+---------+
Figure 1: PSDM encoding.
3.1.2. Codepoint Handling by PCN Ingress Nodes
When packets belonging to PCN flows arrive at the ingress router of
the PCN domain, the ingress router first drops all CE-marked packets.
Then, it sets the DSCP of the remaining PCN packets to an PCN-enabled
DSCP and re-marks the ECN field of all PCN packets that are subject
to threshold-marking to not-ThM (e.g. probe packets), and all PCN
packets that are subject to excess-traffic-marking to not-ETM (e.g.
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data packets). If packets with a PCN-enabled DSCP arrive that belong
to non-PCN flows, the PCN ingress node re-marks their ECN field to
not-PCN or re-marks their DSCP to a different one while preserving
the contents of the ECN field.
3.1.3. Codepoint Handling by PCN Interfaces
If the meter for excess-traffic-marking of a PCN node indicates that
a PCN packet should be re-marked, its ECN field is set to PCN-marked
(PM) only if it was not-ETM before. If the meter for threshold-
marking of a PCN node indicates that a PCN packet should be re-
marked, its ECN field is set to PCN-marked (PM) only if it was not-
ThM before.
3.1.4. Codepoint Handling by PCN Egress Nodes
If the egress node of a PCN domain receives a PM-packet, it infers
somehow whether the packet was not-ETM or not-ThM by the PCN ingress
node to interpret the marking. This can be done as probe packets
must be distinguishable from PCN data packets anyway. The egress
node resets the ECN field of all packets with PCN-enabled DSCPs to
not-ECT. This breaks the ECN capability for all flows with PCN-
enabled DSCPs, regardless whether they are PCN-capable or not.
Appropriate tunnelling across a PCN domain can preserve the ECN
marking of packets with PCN-enabled DSCPs and the ECN-capability of
their flows (see Section 3.3). When the DSCPs in the headers of
packets belonging to flows with PCN-enabled DSCPs have been changed
to another DSCP, the egress node should reverse that change.
3.2. Reasons for the Proposed Encoding
3.2.1. Scarcity of DSCPs
DSCPs are a scarce resource in the IP header so that at most one
should be used for PCN encoding.
3.2.2. Problems with Tunneling
The encoding scheme must cope with tunnelling within PCN domains.
However, various tunnelling schemes limit the persistence of ECN
marks in the top-most IP header to a different degree. Two IP-in-IP
tunnelling modes are defined in [RFC3168] and a third one in
[RFC4301] for IP-in-IPsec tunnels.
The limited-functionality option in [RFC3168] requires that the ECN
codepoint in the outer header is set to not-ECT so that ECN is
disabled for all tunnel routers, i.e., they drop packets instead of
mark them in case of congestion. The tunnel egress just decapsulates
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the packet and leaves the ECN codepoints of the inner packet header
unchanged.
o This mode protects the inner IP header from being PCN-marked upon
decapsulation. It can be used to tunnel ECN marks across PCN
domains such that PCN marking is applied to the outer header
without affecting the inner header.
o This mode is not useful to tunnel PCN traffic with PCN-enabled
DSCP and PCN-capable PCN-codepoints within PCN domain because the
ECN marking information from the outer ECN fields is lost upon
decapsulation.
The full-functionality option in [RFC3168] requires that the ECN
codepoint in the outer header is copied from the inner header unless
the inner header codepoint is CE. In this case, the outer header
codepoint is set to ECT(0). This choice has been made to disable the
ECN fields of the outer header as a covert channel. Upon
decapsulation, the ECN codepoint of the inner header remains
unchanged unless the outer header ECN codepoint is CE. In this case,
the inner header codepoint is also set to CE. This preserves outer
header information if it is CE. However, the fact that CE marks of
the inner header are not visible in the outer header may be a problem
for excess-traffic-marking as it takes already marked traffic into
account and for some required packet drop policies.
Tunnelling with IPsec copies the inner header ECN field to the outer
header ECN field RFC4301, Sect. 5.1.2.1 [RFC4301] upon encapsulation.
Upon decapsulation, CE-marks of the outer header are copied into the
inner header, the other marks are ignored. With this tunnelling
mode, CE marks of the inner header become visible to all meters,
markers, and droppers for tunnelled traffic. In addition, limited
information from the outer header is propagated into the inner
header. Therefore, only IPsec tunnels should be used inside PCN
domains when ECN bits are reused for PCN encoding. Another
consequence is that CE is the only codepoint to which packets can be
re-marked along a tunnel within a PCN domain so that the changed
codepoint survives decapsulation.
3.2.3. Problems with the ECN Field
The guidelines in [RFC4774] describe how the ECN bits can be reused
while being compatible with [RFC3168]. A CE mark of a packet must
never be changed to another ECN codepoint. Furthermore, a not-ECT
mark of a packet must never be changed to one of the ECN-capable
codepoints ECT(0), ECT(1), or CE. Care must be taken that this rule
is enforced when PCN packets leave the PCN domain. As a consequence,
all CE-marked PCN packets must be dropped before entering a PCN
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domain and the ECN field of all PCN packets must be reset to not-ECT
when leaving a PCN domain.
3.3. Handling of ECN Traffic
ECN is intended to control elastic traffic as TCP reacts to ECN
marks. Inelastic real-time traffic is mostly not transmitted over
TCP such that this application of ECN is not appropriate. However,
there have been proposals made that would re-use the PCN signals for
rate adaptation. Therefore, two different options might be useful.
o preserve ECN marks from outside a PCN domain, i.e. CE-marked
packets should not be dropped. To handle this case, ECN packets
should be tunnelled through a PCN domain such that the ECN marking
is hidden from the PCN control and PCN marking is applied only to
the outer header.
o add PCN markings to the ECN field if applications wish to receive
the PCN markings for whatever purpose. In that case IPsec tunnels
should be used for tunnelling. This, however, must be done only
if end systems are ECN capable and signal that they wish to
receive this additional PCN marking information. If this is
useful, the required signalling needs to be defined.
Both options are an independent of the way how PCN marks are encoded.
Therefore, they are not in the scope of this document.
4. IANA Considerations
This document makes no request to IANA.
5. Security Considerations
{ToDo}
6. Conclusions
This document describes an encoding scheme with the following
benefits: {ToDo}
7. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF PCN working group mailing list <pcn@ietf.org>,
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and/or to the authors.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, June 2009.
[RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
Nodes", RFC 5670, November 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
8.2. Informative References
[I-D.ietf-pcn-3-in-1-encoding]
Briscoe, B., Moncaster, T., and M. Menth, "Encoding 3 PCN-
States in the IP header using a single DSCP",
draft-ietf-pcn-3-in-1-encoding-09 (work in progress),
March 2012.
[Menth09f]
Menth, M., Babiarz, J., and P. Eardley, "Pre-Congestion
Notification Using Packet-Specific Dual Marking",
Proceedings of the International Workshop on the Network
of the Future (Future-Net), IEEE, Dresden, Germany,
June 2009.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
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Authors' Addresses
Michael Menth
University of Tuebingen
Department of Computer Science
Sand 13
Tuebingen D-72076
Germany
Phone: +49 07071 29 70505
Email: menth@informatik.uni-tuebingen.de
URI: http://www.kn.inf.uni-tuebingen.de
Jozef Babiarz
3inova Networks Inc
CRC Innovation Centre
Bldg 94 Room 216D
3701 Carling Avenue
Ottawa K2H 8S2
Canada
Phone: +1-613-298-0438
Email: j.babiarz@3inovanetworks.com
Toby Moncaster
University of Cambridge
Computer Laboratory
JJ Thomson Avenue
Cambridge CB3 0FD
UK
Phone: +44 1223 763654
Email: toby@moncaster.com
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://www.bobbriscoe.net
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