PCN Working Group Philip. Eardley (Editor)
Internet-Draft BT
Intended status: Standards Track March 5, 2009
Expires: September 6, 2009
Marking behaviour of PCN-nodes
draft-ietf-pcn-marking-behaviour-02
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Abstract
This document standardises the two marking behaviours of PCN-nodes:
threshold marking and excess traffic marking. Threshold marking
marks all PCN-packets if the PCN traffic rate is greater than a
configured rate ("PCN-threshold-rate"). Excess traffic marking marks
a proportion of PCN-packets, such that the amount marked equals the
traffic rate in excess of a configured rate ("PCN-excess-rate").
Setting the configured rates below the physical link rates enables
PCN-nodes to provide information to support admission control and
flow termination in order to protect the quality of service of
established inelastic flows.
Requirements Language
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 RFC 2119 [RFC2119].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Specified PCN-marking behaviour . . . . . . . . . . . . . . . 6
2.1. Behaviour aggregate classification function . . . . . . . 6
2.2. Traffic conditioning function . . . . . . . . . . . . . . 6
2.3. Threshold meter function . . . . . . . . . . . . . . . . . 7
2.4. Excess traffic meter function . . . . . . . . . . . . . . 7
2.5. Marking function . . . . . . . . . . . . . . . . . . . . . 8
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Changes to -02 from -01 . . . . . . . . . . . . . . . . . 9
6.2. Changes to -01 from -00 . . . . . . . . . . . . . . . . . 9
6.3. Changes to -00 . . . . . . . . . . . . . . . . . . . . . . 9
7. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Example algorithms . . . . . . . . . . . . . . . . . 11
A.1. Threshold metering and marking . . . . . . . . . . . . . . 12
A.2. Excess traffic metering and marking . . . . . . . . . . . 13
Appendix B. Implementation notes . . . . . . . . . . . . . . . . 13
B.1. Competing-non-PCN-traffic . . . . . . . . . . . . . . . . 14
B.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 15
B.3. Behaviour aggregate classification . . . . . . . . . . . . 15
B.4. Traffic conditioning . . . . . . . . . . . . . . . . . . . 16
B.5. Threshold metering . . . . . . . . . . . . . . . . . . . . 17
B.6. Excess traffic metering . . . . . . . . . . . . . . . . . 18
B.7. Marking . . . . . . . . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
This document standardises the two marking behaviours of PCN-nodes.
Their aim is to enable PCN-nodes to give an "early warning" of
potential congestion before there is any significant build-up of PCN-
packets in their queues. In summary, their objectives are:
o threshold marking: its objective is to mark all PCN-packets (with
a "threshold-mark") whenever the rate of PCN-packets is greater
than its configured rate ("PCN-threshold-rate");
o excess traffic marking: whenever the rate of PCN-packets is
greater than its configured rate ("PCN-excess-rate"), its
objective is to mark PCN-packets (with an "excess-traffic-mark")
at a rate equal to the difference between the bit rate of PCN-
packets and the PCN-excess-rate.
[I-D.ietf-pcn-architecture] describes a general architecture for how,
in a particular DiffServ domain, PCN-boundary-nodes convert these
PCN-markings into decisions about flow admission and flow
termination. Other documents describe the wider per-domain behaviour
and how the PCN-markings are encoded in packet headers. PCN encoding
uses a combination of the DSCP field and ECN field in the IP header
to indicate that a packet is a PCN-packet and whether it is PCN-
marked. The baseline encoding [I-D.ietf-pcn-baseline-encoding]
standardises two encoding states (PCN-marked and not-marked), whilst
other documents define extended schemes with three encoding states
(eg [I-D.moncaster-pcn-3-state-encoding] defines PCN-threshold-
marked, PCN-excess-traffic-marked and not-marked). [RFC3168] defines
a broadly RED-like default congestion marking behaviour, but allows
alternatives to be defined; this document defines such an
alternative.
Section 2 below specifies the functions involved, which in outline
(see Figure 1) are:
o Behaviour aggregate classification: decide whether an incoming
packet is a PCN-packet or not.
o Traffic condition (optional): drop packets if the link is
overloaded.
o Threshold meter: determine whether the rate of PCN-packets is
greater than its configured PCN-threshold-rate. The meter
operates on the aggregate of all PCN-packets on the link, and not
on individual flows.
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o Excess traffic meter: measure by how much the rate of PCN-packets
is greater than its configured PCN-excess-rate. The meter
operates on the aggregate of all PCN-packets on the link, and not
on individual flows.
o PCN-mark: actually mark the PCN-packets, if the meter functions
indicate to do so.
+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+---------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classify |==>|Condition|==?================>|Marker|==> Packet
Stream | | | | | | | Stream
+---------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+
Figure 1: Schematic of functions for PCN-marking
Appendix A gives an example of algorithms that fulfil the
specification of Section 2, and Appendix B provides some explanations
of and comments on Section 2. Both the Appendices are informative.
1.1. Terminology
In addition to the terminology defined in [I-D.ietf-pcn-architecture]
and [RFC2474], the following terms are defined:
o Competing-non-PCN-packet: a non PCN-packet that shares a link with
PCN-packets and competes with them for its forwarding bandwidth.
Competing-non-PCN-packets MUST NOT be PCN-marked (ie only PCN-
packets can be PCN-marked). Note: In general it is not advised to
have any competing-non-PCN-traffic.
o Metered-packet: a packet that is metered by the metering functions
specified below. A PCN-packet MUST be treated as a metered-packet
(with the minor exception noted below in Section 2.4). A
competing-non-PCN-packet MAY be treated as a metered-packet.
o Note: "Excess-traffic-marked" means a packet that is marked as a
result of the excess traffic meter function; "threshold-marked"
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means a packet that is marked as a result of the threshold meter
function. [I-D.ietf-pcn-baseline-encoding] defines only two PCN
encoding states available (PCN-marked and not-marked); the
deployment MUST choose whether PCN-marked is interpreted as
excess-traffic-marked or threshold-marked; a consistent choice
MUST be made throughout a PCN-domain.
2. Specified PCN-marking behaviour
This section specifies the PCN-marking behaviour. The descriptions
are functional and are not intended to restrict the implementation.
The informative Appendices supplement this section.
2.1. Behaviour aggregate classification function
A PCN-node MUST classify a packet as a PCN-packet if the value of its
DSCP and ECN fields correspond to a PCN-enabled codepoint, as defined
in the encoding scheme applicable to the PCN-domain. Otherwise the
packet MUST NOT be classified as a PCN-packet.
A PCN-node MUST classify a packet as a competing-non-PCN-packet if it
is not a PCN-packet and it competes with PCN-packets for its
forwarding bandwidth on a link.
2.2. Traffic conditioning function
Note: if the PCN-node's queue overflows then naturally packets are
dropped; traffic conditioning is action additional to this.
On all links in the PCN-domain, traffic conditioning MAY be done by:
o metering all metered-packets to determine if the rate of metered-
traffic is greater than its scheduling rate (ie determine if any
packets are out-of-profile. Metering is "the process of measuring
the temporal properties (eg rate) of a traffic stream" [RFC2475].)
o if the rate of metered-traffic is too high, then drop metered-
packets.
If the PCN-node drops PCN-packets then:
o PCN-packets that arrive at the PCN-node already excess-traffic-
marked SHOULD be preferentially dropped;
o the PCN-node's Excess traffic Meter SHOULD NOT meter the PCN-
packets that it drops.
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2.3. Threshold meter function
A PCN-node MUST implement a Threshold Meter that has behaviour
functionally equivalent to the following.
The meter acts like a token bucket, which is sized in bits and has a
configured bit rate, termed PCN-threshold-rate. The amount of tokens
in the token bucket is termed TBthreshold.fill. Tokens are added at
the PCN-threshold-rate, to a maximum value TBthreshold.max. Tokens
are removed equal to the size in bits of the metered-packet, to a
minimum TBthreshold.fill=0.
The token bucket has a configured intermediate depth, termed
TBthreshold.threshold. If TBthreshold.fill < TBthreshold.threshold,
then the meter indicates to the Marking function that the packet is
to be threshold-marked; otherwise it does not.
2.4. Excess traffic meter function
A PCN-packet SHOULD NOT be metered (by this excess traffic meter
function) in the following two cases:
o If the packet is already excess-traffic-marked on arrival at the
PCN-node;
o If this PCN-node drops the packet.
Otherwise the PCN-packet MUST be treated as a metered-packet, that is
it is metered by the Excess traffic Meter.
A PCN-node MUST implement an Excess traffic Meter that has behaviour
functionally equivalent to the following.
The meter acts like a token bucket, which is sized in bits and has a
configured bit rate, termed PCN-excess-rate. The amount of tokens in
the token bucket is termed TBexcess.fill. Tokens are added at the
PCN-excess-rate, to a maximum value TBexcess.max. Tokens are removed
equal to the size in bits of the metered-packet, to a minimum
TBexcess-fill=0. If the token bucket is empty (TBexcess.fill = 0),
then the meter indicates to the Marking function that the packet is
to be excess-traffic-marked. The PCN-excess-rate is greater than (or
equal to) the PCN-threshold-rate.
In addition to the above, if the token bucket is within an MTU of
being empty, then the meter SHOULD indicate to the Marking function
that the packet is to be excess-traffic-marked; MTU means the maximum
size of PCN-packets on the link ("packet size independent marking").
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Otherwise the meter MUST NOT indicate marking.
2.5. Marking function
A PCN-node MUST NOT:
o PCN-mark a packet that is not a PCN-packet;
o change a non PCN-packet into a PCN-packet;
o change a PCN-packet into a non PCN-packet.
A PCN-packet MUST be marked to reflect the metering results by
setting its encoding state appropriately, as specified by the
specific encoding scheme that applies in the PCN-domain.
Note: In some deployment scenarios there may be only two PCN encoding
states available (PCN-marked and not-marked). In such scenarios, the
deployment MUST choose whether the Threshold meter function or the
Excess traffic meter function can trigger a packet to be PCN-marked;
a consistent choice MUST be made throughout a PCN-domain.
3. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
4. Security Considerations
See [I-D.ietf-pcn-architecture]
5. Acknowledgements
Michael Menth, Joe Babiarz, Anna Charny reviewed a preliminary
version of the draft-eardley-pcn-marking-behaviour-00 draft.
Thanks to those who've made comments on this draft: Michael Menth,
Joe Babiarz, Anna Charny, Ruediger Geib, Wei Gengyu, Fortune Huang,
Bob Briscoe, Toby Moncaster, Christian Hublet, Ingemar Johansson, Ken
Carlberg, Georgios Karagiannis.
All the work by many people in the PCN WG.
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6. Changes
6.1. Changes to -02 from -01
Updates as follows:
o added notes (end of S1.1 & 2.5) to clarify what "excess-traffic-
marked" means when there is only one encoding for PCN-marking
o added explanations for in Section B.4 and B.6 about why various
things are SHOULD or SHOULD NOT rather than MUST or MUST NOT.
o Deleted a couple of paragraphs about encoding states, as they are
relevant to encoding documents rather than this document.
6.2. Changes to -01 from -00
Updates as follows:
o corrected the term 'not PCN-marked' to 'not-marked' (throughout)
o re-phrased the definition of competing-non-PCN-packets
o corrected the definition of metered-packet
o delete most of Section 2.5 (marking function). The material
deleted belongs as part of [I-D.ietf-pcn-baseline-encoding]; other
encoding schemes would need to include similar material.
o deleted Appendix C (it was only a temporary archive of material
concerning per domain behaviour and PCN-boundary-node operation)
o clarifications throughout
o made all references Informative
6.3. Changes to -00
First version of WG draft, derived from
draft-eardley-pcn-marking-behaviour-01, with the following changes:
o Removed material concerning per domain behaviour and PCN-boundary-
node operation (temporarily archived to Appendix C)
o Removed mention of downgrading as an option for per-hop traffic
conditioning. In fact, downgrading is no longer allowed because S
2.6 now says "A PCN-node MUST NOT ...change a PCN-packet into a
non PCN-packet".
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o Traffic conditioning is now a MAY. Since in general flow
termination (not traffic conditioning) is PCN's method for
handling problems of too much traffic.
o Metered-packets: competing-non-PCN-packets now MAY be metered.
Since it is recommended that the operator doesn't allow any
competing-non-PCN-traffic, and (if there is) there are potentially
other ways of coping.
o No changes (outside traffic conditioning & metering of competing-
non-PCN-traffic) to the Normative sections of the draft.
o Appendix B.1 added about competing-non-PCN-traffic. Recommended
that there is no such traffic, but guidance given if there is.
7. Authors
Many people need to be added.
8. Informative References
[I-D.briscoe-tsvwg-byte-pkt-mark]
Briscoe, B., "Byte and Packet Congestion Notification",
draft-briscoe-tsvwg-byte-pkt-mark-02 (work in progress),
February 2008.
[I-D.briscoe-tsvwg-cl-architecture]
Briscoe, B., "An edge-to-edge Deployment Model for Pre-
Congestion Notification: Admission Control over a
DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04
(work in progress), October 2006.
[I-D.charny-pcn-comparison]
Charny, A., "Comparison of Proposed PCN Approaches",
draft-charny-pcn-comparison-00 (work in progress),
November 2007.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", draft-ietf-pcn-architecture-07 (work in
progress), September 2008.
[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-01 (work in progress),
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October 2008.
[I-D.ietf-tsvwg-admitted-realtime-dscp]
Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
Admitted Traffic",
draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
progress), February 2008.
[I-D.moncaster-pcn-3-state-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "A three state
extended PCN encoding scheme",
draft-moncaster-pcn-3-state-encoding-00 (work in
progress), June 2008.
[Menth] "Menth", 2008, <http://www3.informatik.uni-wuerzburg.de/
staff/menth/Publications/Menth08-PCN-Comparison.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
Appendix A. Example algorithms
Note: This Appendix is informative, not normative. It is an example
of algorithms that implement Section 2 and is based on
[I-D.charny-pcn-comparison] and [Menth].
There is no attempt to optimise the algorithms. It implements the
metering and marking functions together. It is assumed that three
encoding states are available (one for threshold-marked, one for
excess-traffic-marked and one for not-marked"). It is assumed that
all metered-packets are PCN-packets and that the link is never
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overloaded.
A.1. Threshold metering and marking
A token bucket with the following parameters:
o TBthreshold.PCN-threshold-rate: token rate of token bucket (bits/
second)
o TBthreshold.max: depth of token bucket (bits)
o TBthreshold.threshold: marking threshold of token bucket (bits)
o TBthreshold.lastUpdate: time the token bucket was last updated
(seconds)
o TBthreshold.fill: amount of tokens in token bucket (bits)
A PCN-packet has the following parameters:
o packet.size: the size of the PCN-packet (bits)
o packet.mark: the PCN encoding state of the packet
In addition there are the parameters:
o now: the current time (seconds)
The following steps are performed when a PCN-packet arrives on a
link:
o TBthreshold.fill = min(TBthreshold.max, TBthreshold.fill + (now -
TBthreshold.lastUpdate) * TBthreshold.PCN-threshold-rate); // add
tokens to token bucket
o TBthreshold.fill = max(0, TBthreshold.fill - packet.size); //
remove tokens from token bucket
o if ((TBthreshold.fill < TBthreshold.threshold) AND (packet.mark !=
excess-traffic-marked)) then packet.mark = threshold-marked; // do
threshold marking, but don't re-mark packets that are already
excess-traffic-marked
o TBthreshold.lastUpdate = now
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A.2. Excess traffic metering and marking
A token bucket with the following parameters:
o TBexcess.PCN-excess-rate: token rate of token bucket (bits/second)
o TBexcess.max: depth of TB in token bucket (bits)
o TBexcess.lastUpdate: time the token bucket was last updated
(seconds)
o TBexcess.fill: amount of tokens in token bucket (bits)
A PCN-packet has the following parameters:
o packet.size: the size of the PCN-packet (bits)
o packet.mark: the PCN encoding state of the packet
In addition there are the parameters:
o now: the current time (seconds)
o MTU: the maximum transfer unit of the link (or the known maximum
size of PCN-packets on the link) (bits)
The following steps are performed when a PCN-packet arrives on a
link:
o TBexcess.fill = min(TBexcess.max, TBexcess.fill + (now -
TBexcess.lastUpdate) * TBexcess.PCN-excess-rate); // add tokens to
token bucket
o if (packet.mark != excess-traffic-marked) then TBexcess.fill =
max(0, TBexcess.fill - packet.size); // remove tokens from token
bucket, but do not meter packets that are already excess-traffic-
marked
o if (TBexcess.fill < MTU) then packet.mark = excess-traffic-marked;
// do (packet size independent) excess traffic marking
o TBthreshold.lastUpdate = now
Appendix B. Implementation notes
Note: This Appendix is informative, not normative. It comments on
Section 2.
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B.1. Competing-non-PCN-traffic
In general it is not advised to have any competing-non-PCN-traffic,
essentially because the unpredictable amount of competing-non-PCN-
traffic makes the PCN mechanisms less accurate and so reduces PCN's
ability to protect the QoS of admitted PCN-flows
[I-D.ietf-pcn-architecture]. But if there is competing-non-PCN-
traffic, then there needs to be:
1. a mechanism to limit it, for example:
* limiting the rate at which competing-non-PCN-traffic can be
forwarded on each link in the PCN-domain. One method for
achieving this is to queue competing-non-PCN-packets
separately from PCN-packets, and to limit the scheduling rate
of the former. Another method is to police (traffic
condition) the competing-non-PCN-traffic on each link, ie drop
competing-non-PCN-packets in excess of some rate.
* policing of competing-non-PCN-traffic at the PCN-ingress-
nodes. For example, as in the DiffServ architecture -
although its static traffic conditioning agreements risk a
focused overload of traffic from several PCN-ingress-nodes on
one link.
* design: it is known by design that the level of competing-non-
PCN-traffic is always very small (perhaps it consists of
operator control messages only)
2. In general PCN's mechanisms should take account of competing-non-
PCN-traffic (in order to improve the accuracy of the decision
about whether to admit (or terminate) a PCN-flow), for example
by:
* competing-non-PCN-traffic contributes to the PCN meters (ie
competing-non-PCN-packets are treated as metered-packets).
* each PCN-node reduces, on its links, the PCN-threshold-rate
and PCN-excess-rate, in order to allow 'headroom' for the
competing-non-PCN-traffic; also limiting the maximum
forwarding rate of competing-non-PCN-traffic to be less than
the 'headroom'. In this case competing-non-PCN-packets are
not treated as metered-packets.
It is left up to the operator to decide on appropriate action.
Traffic conditioning is discussed further in the separate section
below.
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One specific example of competing-non-PCN-traffic occurs if the PCN-
compatible Diffserv codepoint is the Voice-admit codepoint, and there
is voice-admit traffic in the PCN-domain.
Another example would occur if there was more than one PCN-compatible
Diffserv codepoint in a PCN-domain. For instance, suppose there were
two PCN-BAs treated at different priorities. Then as far as the
lower priority PCN-BA is concerned, the higher priority PCN-traffic
needs to be treated as competing-non-PCN-traffic.
B.2. Scope
It may be known, eg by the design of the network topology, that some
links can never be pre-congested (even in unusual circumstances, eg
after the failure of some links). There is then no need to deploy
PCN behaviour on those links.
The meter and marker can be implemented on the ingoing or outgoing
interface of a PCN-node. It may be that existing hardware can
support only one meter and marker per ingoing interface and one per
outgoing interface. Then for instance threshold metering and marking
could be run on all the ingoing interfaces and excess traffic
metering and marking on all the outgoing interfaces; note that the
same choice must be made for all the links in a PCN-domain to ensure
that the two metering behaviours are applied exactly once for all the
links.
Note that even if there are only two encoding states, it is still
required that both the meters are implemented, in order to ease
compatibility between equipment and remove a configuration option and
associated complexity. Hardware with limited availability of token
buckets could be configured to run only one of the meters, but it
must be possible to enable either meter. Although this scenario
means that the Marking function ignores indications from one of the
meters, they may be logged or acted upon in some other way, for
example by the management system or an explicit signalling protocol;
such considerations are out of scope of PCN.
B.3. Behaviour aggregate classification
Configuration of PCN-nodes will define what values of the DSCP and
ECN fields indicate a PCN-packet in a particular PCN-domain.
Configuration will also define what values of the DSCP and ECN fields
indicate a competing-non-PCN-packet in a particular PCN-domain.
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B.4. Traffic conditioning
If there is no competing-non-PCN-traffic, then it is not expected
that traffic conditioning is needed, since PCN's flow admission and
termination mechanisms limit the amount of PCN-traffic. Even so,
traffic conditioning still might be implemented as a back stop
against misconfiguration of the PCN-domain, for instance.
The objective of traffic conditioning is to minimise the queueing
delay suffered by metered-traffic at a PCN-node, since PCN-traffic
(and perhaps competing-non-PCN-traffic) is expected to be inelastic
traffic generated by real time applications. In practice it would be
defined as exceeding a specific traffic profile, typically based on a
token bucket. The details will depend on how the router's
implementation handles the two sorts of traffic
[I-D.ietf-tsvwg-admitted-realtime-dscp]:
o a common queue for PCN-traffic and competing-non-PCN-traffic, and
a traffic conditioner for the competing-non-PCN-traffic;
o separate queues. In this case the amount of competing-non-PCN-
traffic can be limited by limiting the rate at which the scheduler
(for the competing-non-PCN-traffic) forwards packets.
The traffic conditioning action is to drop packets (which is often
called "policing"). Downgrading of packets to a lower priority BA is
not allowed (see B.7), since it wold lead to packet mis-ordering.
Shaping ("the process of delaying packets" [RFC2475]) is not suitable
if the traffic comes from real time applications. In general it is
reasonable for competing-non-PCN-traffic to get harsher treatment
than PCN-traffic (ie competing-non-PCN-packets are preferentially
dropped), because PCN's flow admission and termination mechanisms are
stronger than the mechanisms that are likely to be applied to the
competing-non-PCN-traffic. The PCN mechanisms also mean that a
traffic conditioner should not be needed for the PCN-traffic.
Preferential dropping of excess-traffic-marked packets: Section 2.3
specifies: "If the PCN-node drops PCN-packets then ... PCN-packets
that arrive at the PCN-node already excess-traffic-marked SHOULD be
preferentially dropped". In brief, the reason is that this avoids
over-termination, with the CL/SM edge behaviour, in the event of
multiple bottlenecks in the PCN-domain [I-D.charny-pcn-comparison].
A fuller explanation is as follows. The optimal dropping behaviour
depends on the particular edge behaviour [Menth]. A single dropping
behaviour is defined, as it is simpler to standardise, implement and
operate. The standardised dropping behaviour is at least adequate
for all edge behaviours (and good for some), whereas others are not
(for example with tail dropping far too much traffic may be
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terminated with the CL/SM edge behaviour, in the event of multiple
bottlenecks in the PCN-domain [I-D.charny-pcn-comparison]). The
dropping behaviour is defined as a 'SHOULD', rather than a 'MUST', in
recognition that other dropping behaviour may be preferred in
particular circumstances, for example: (1) with the marked flow
termination edge behaviour, preferential dropping of unmarked packets
may be better; (2) tail dropping may make PCN marking behaviour
easier to implement on current routers.
Exactly what "preferentially dropped" means is left to the
implementation. It is also left to the implementation what to do if
there are no excess-traffic-marked PCN-packets available at a
particular instant.
Section 2.2 also specifies: "the PCN-node's Excess traffic Meter
SHOULD NOT meter the PCN-packets that it drops." This avoids over-
termination [Menth]. Effectively it means that traffic conditioning
should be done before the meter functions - which is natural.
B.5. Threshold metering
The description is in terms of a 'token bucket with threshold' (which
[I-D.briscoe-tsvwg-cl-architecture] views as a virtual queue).
However the implementation is not standardised.
Section 2.3 defines: "If TBthreshold.fill < TBthreshold.threshold,
then the meter indicates to the Marking function that the packet is
to be threshold-marked; otherwise it does not." Note that the PCN-
packet (that causes the token bucket to cross TBthreshold.threshold)
is marked without explicit additional bias for the packet's size.
The behaviour must be functionally equivalent to the description in
Section 2.3. "Functionally equivalent" means the observable 'black
box' behaviour is the same or very similar. It is intended to allow
implementation freedom over matters such as:
o whether tokens are added to the token bucket at regular time
intervals or only when a packet is processed
o whether the new token bucket depth is calculated before or after
it is decided whether to mark the packet. The effect of this is
simply to shift the sequence of marks by one packet.
o when the token bucket is very nearly empty and a packet arrives
larger than TBthreshold.fill, then the precise change in
TBthreshold.fill is up to the implementation. A behaviour is
functionally equivalent if either precisely the same set of
packets is marked, or if the set is shifted by one packet. For
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instance, the following should all be considered as "functionally
equivalent":
* set TBthreshold.fill = 0 and indicate threshold-mark to the
Marking function.
* check whether TBthreshold.fill < TBthreshold.threshold and if
it is then indicate threshold-mark to the Marking function;
then set TBthreshold.fill = 0.
* leave TBthreshold.fill unaltered and indicate threshold-mark to
the Marking function.
o similarly, when the token bucket is very nearly full and a packet
arrives large than (TBthreshold.max - TBthreshold.fill), then the
precise change in TBthreshold.fill is up to the implementation.
o Note that all packets, even if already marked, are metered by the
threshold meter function (unlike the excess traffic meter function
- see below) - because all packets should contribute to the
decision whether there is room for a new flow.
B.6. Excess traffic metering
The description is in terms of a token bucket, however the
implementation is not standardised.
As in Section B.3, "functionally equivalent" allows some
implementation flexibility when the token bucket is very nearly empty
or very nearly full.
Section 2.4 specifies: "A packet SHOULD NOT be metered (by this
excess traffic meter function) ... If the packet is already excess-
traffic-marked on arrival at the PCN-node". This avoids over-
termination (with some edge behaviours) in the event that the PCN-
traffic passes through multiple bottlenecks in the PCN-domain
[I-D.charny-pcn-comparison]. Note that an implementation could
determine whether the packet is already excess-traffic-marked as an
integral part of its Classification function. The behaviour is
defined as a 'SHOULD NOT', rather than a 'MUST NOT', because it may
be slightly harder to implement than a metering function that is
blind to previous packet markings.
Section 2.4 specifies: "A packet SHOULD NOT be metered (by this
excess traffic meter function) ... If this PCN-node drops the
packet." This avoids over-termination [Menth]. (A similar statement
could also be made for the threshold meter function, but is
irrelevant, as a link that is overloaded will already be
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substantially pre-congested and hence PCN-marking all packets.) It
seems natural to do traffic conditioning before the metering
functions, although for some equipment it may be harder to implement;
hence the behaviour is defined as a 'SHOULD NOT', rather than a 'MUST
NOT'.
Packet size independent marking is specified as a SHOULD in Section
2.4 ( "if the token bucket is within an MTU of being empty, then the
meter SHOULD indicate to the Marking function that the packet is to
be excess-traffic-marked; MTU means the maximum size of PCN-packets
on the link".) Without it, large packets are more likely to be
excess-traffic-marked than small packets and this means that, with
some edge behaviours, flows with large packets are more likely to be
terminated than flows with small packets
[I-D.briscoe-tsvwg-byte-pkt-mark] [Menth]. The behaviour is a
'SHOULD', rather than a 'MUST', because packet size independent
marking may be slightly harder for some equipment to implement, and
the impact of not doing it is moderate (sufficient traffic is
terminated, but flows with large packets are more likely to be
terminated).
Note that TBexcess.max is independent of TBthreshold.max;
TBexcess.fill is independent of TBthreshold.fill (except in that a
packet changes both); and the two configured rates, PCN-excess-rate
and PCN-threshold-rate are independent (except that PCN-excess-rate
>= PCN-threshold-rate).
B.7. Marking
Section 2.5 defines: "A PCN-node MUST NOT ...change a PCN-packet into
a non PCN-packet". This means that a PCN-node MUST NOT traffic
condition by downgrading a PCN-packet into a lower priority DiffServ
BA.
Section 2.5 defines: "A PCN-node MUST NOT ...PCN-mark a packet that
is not a PCN-packet". This means that in the scenario where
competing-non-PCN-packets are treated as metered-packets, a meter may
indicate a packet is to be PCN-marked, but the Marking function knows
it cannot be marked. It is left open to the implementation exactly
what to do in this case; one simple possibility is to mark the next
PCN-packet. Note that unless the PCN-packets are a large fraction of
all the metered-packets then the PCN mechanisms may not work well.
Although the metering functions are described separately from the
Marking function, they can be implemented in an integrated fashion.
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Author's Address
Philip Eardley
BT
Adastral Park, Martlesham Heath
Ipswich IP5 3RE
UK
Email: philip.eardley@bt.com
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