One document matched: draft-ietf-pcn-psdm-encoding-00.txt
Congestion and Pre Congestion M. Menth
Internet-Draft University of Wuerzburg
Intended status: Experimental J. Babiarz
Expires: December 28, 2009 Nortel Networks
T. Moncaster
BT
B. Briscoe
BT & UCL
June 26, 2009
PCN Encoding for Packet-Specific Dual Marking (PSDM)
draft-ietf-pcn-psdm-encoding-00
Status of this Memo
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Abstract
This document proposes how PCN marks can be encoded into the IP
header. The presented encoding reuses the ECN field of the Voice-
Admit DSCP in a single PCN domain. The encoding of unmarked PCN
packets indicates whether they are subject to either excess- or
exhaustive-marking. This is useful, e.g., when data and probe
packets require different marking mechanisms.
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
1.1. Requirements notation . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Encoding for Packet-Specific Dual Marking . . . . . . . . . . . 4
3.1. Proposed Encoding and Expected Node Behavior . . . . . . . 4
3.1.1. PCN Codepoints . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Codepoint Handling by PCN Ingress Nodes . . . . . . . . 5
3.1.3. Codepoint Handling by PCN Interfaces . . . . . . . . . 5
3.1.4. Codepoint Handling by PCN Egress Nodes . . . . . . . . 5
3.2. Reasons for the Proposed Encoding . . . . . . . . . . . . . 6
3.2.1. Problems with DSCPs . . . . . . . . . . . . . . . . . . 6
3.2.2. Problems with Tunneling . . . . . . . . . . . . . . . . 6
3.2.3. Problems with the ECN Field . . . . . . . . . . . . . . 7
3.3. Handling of ECN Traffic . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 exhaustive marking marks all PCN
packets on a link when the PCN traffic rate exceeds a reference rate
[PCN-marking-behaviour]. 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. The presented encoding reuses the ECN field of the Voice-
Admit DSCP in a single PCN domain. The encoding of unmarked PCN
packets indicates whether they are subject to either excess- or
exhaustive-marking. Therefore, we call this proposal encoding for
packet-specific dual marking (PSDM).
PSDM supports exhaustive marking and excess marking as long as
individual packets are subject to only one of them. It can be
applied in networks implementing
o only AC based on exhaustive marking (reference rate = admissible
rate),
o only FT based on excess marking (reference rate = supportable
rate),
o both AC and FT based on excess marking (reference rate =
admissible rate)
o Probe-based AC based on exhaustive marking (reference rate =
admissible rate) and FT based on excess marking (reference rate =
supportable rate).
Although the motivation for this encoding scheme is to exhaustive-
mark probe packets and to excess-mark data packets, routers do not
need to differentiate explicitly between probe and data packets since
packets are a priori marked with an appropriate codepoint indicating
the marking mechanism applying to them.
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].
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2. Terminology
Most of the terminology used in this document is defined in
[PCN-arch]. The following additional terms are defined in this
document:
o Exhaustive marking - generalization of threshold and ramp marking
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-ExM, not-EhM, or M,
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
o not-excess-marked (not-ExM) - new ECN codepoint for unmarked PCN
packets that are subject to excess marking
o not-exhaustive-marked (not-EhM) - new ECN codepoint for unmarked
PCN packets that are subject to exhaustive marking
o marked (M) - new ECN codepoint for marked PCN packets regardless
whether they were subject to excess or exhaustive marking.
3. Encoding for Packet-Specific Dual Marking
In this section the encoding for packet-specific single marking
(PSDM) is presented and the reasons for the proposed design are
outlined.
3.1. Proposed Encoding and Expected Node Behavior
The encoding reuses the Voice-Admit DSCP [voice-admit] as a PCN-
enabled DSCP to indicate packets of PCN-capable flows within a PCN
domain. So far, this is the only DSCP considered for that use, but
this encoding scheme is easily extensible towards multiple PCN-
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enabled DSCPs.
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 Table 1.
+------------------+---------+---------+---------+----+
| DSCP | 00 | 10 | 01 | 11 |
+------------------+---------+---------+---------+----+
| PCN-enabled DSCP | not-PCN | not-ExM | Not-EhM | M |
+------------------+---------+---------+---------+----+
Table 1: Mapping of PCN codepoints into the ECN field
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 exhaustive marking to not-EhM (e.g. probe packets), and all PCN
packets that are subject to excess marking to not-ExM (e.g. 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.
3.1.3. Codepoint Handling by PCN Interfaces
If the meter for excess marking of a PCN node indicates that a PCN
packet should be marked, its ECN field is set to marked (M) only if
it was not-ExM before. If the meter for exhaustive marking of a PCN
node indicates that a PCN packet should be marked, its ECN field is
set to marked (M) only if it was not-EhM before.
3.1.4. Codepoint Handling by PCN Egress Nodes
If the egress node of a PCN domain receives a marked PCN packet, it
infers somehow whether the packet was not-ExM or not-EhM by the PCN
ingress node to interpret the marking. This can be done as probe
packets must be distinguishable from PCN data packets. 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
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their flows (see Section 3.3).
3.2. Reasons for the Proposed Encoding
3.2.1. Problems with DSCPs
DSCPs are a scarce resource in the IP header such that at most one
should be used for PCN. To avoid the requirement for a new DSCP, the
Voice-Admit DSCP is reused. To differentiate pure Voice-Admit
traffic from PCN traffic within a PCN domain, pure Voice-Admit
traffic has its ECN field set to not-PCN within a PCN domain. The
encoding should be extensible towards different data plane priorities
for PCN traffic in PCN domains which requires different PCN-enabled
DSCPs, one for each priority level.
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 such 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
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
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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 marking as it takes already marked traffic into account
and for some required packet drop policies.
Tunnelling with IPSec copies the inner header ECN bits to the outer
header ECN bits 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 that can be used to
indicate a marked packet beyond tunnelling.
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 Voice-Admit packets must be dropped before entering a
PCN domain and the ECN field of all Voice-Admit packets must be set
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 are plans to reuse ECN signals for rate adapatation
[ecn-pcn-usecases]. 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
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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. It does however suggest a
change to the ([RFC3168]) behaviour for the ECN field for the Voice-
Admit [voice-admit] DSCP within a PCN domain.
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>,
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.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
8.2. Informative References
[PCN-arch]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-03 (work in progress),
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February 2008.
[PCN-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-eardley-pcn-marking-behaviour-01 (work in progress),
June 2008.
[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.
[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.
[voice-admit]
Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
Admitted Traffic",
draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
progress), February 2008.
Authors' Addresses
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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Jozef Babiarz
Nortel Networks
3500 Carling Avenue
Ottawa K2H 8E9
Canada
Phone: +1-613-763-6098
Email: babiarz@nortel.com
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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