PCN Working Group Philip. Eardley (Editor)
Internet-Draft BT
Intended status: Standards Track October 2, 2008
Expires: April 5, 2009
Marking behaviour of PCN-nodes
draft-ietf-pcn-marking-behaviour-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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 first
configured rate. Excess traffic marking marks a proportion of PCN-
packets, such that the amount marked equals the traffic rate in
excess of a second configured rate.
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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].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Specified PCN-marking behaviour . . . . . . . . . . . . . . . 4
2.1. Behaviour aggregate classification function . . . . . . . 5
2.2. Traffic conditioning function . . . . . . . . . . . . . . 5
2.3. Threshold meter function . . . . . . . . . . . . . . . . . 5
2.4. Excess traffic meter function . . . . . . . . . . . . . . 6
2.5. Marking function . . . . . . . . . . . . . . . . . . . . . 6
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Changes to -00 . . . . . . . . . . . . . . . . . . . . . . 8
7. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Example algorithms . . . . . . . . . . . . . . . . . 10
A.1. Threshold metering and marking . . . . . . . . . . . . . . 11
A.2. Excess traffic metering and marking . . . . . . . . . . . 12
Appendix B. Implementation notes . . . . . . . . . . . . . . . . 13
B.1. Competing-non-PCN-traffic . . . . . . . . . . . . . . . . 13
B.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 14
B.3. Behaviour aggregate classification . . . . . . . . . . . . 14
B.4. Traffic conditioning . . . . . . . . . . . . . . . . . . . 15
B.5. Threshold metering . . . . . . . . . . . . . . . . . . . . 16
B.6. Excess traffic metering . . . . . . . . . . . . . . . . . 17
B.7. Marking . . . . . . . . . . . . . . . . . . . . . . . . . 18
Appendix C. Per-domain Behaviour . . . . . . . . . . . . . . . . 18
C.1. (from Introduction) . . . . . . . . . . . . . . . . . . . 19
C.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 19
C.3. Classify . . . . . . . . . . . . . . . . . . . . . . . . . 20
C.4. Colour . . . . . . . . . . . . . . . . . . . . . . . . . . 20
C.5. Traffic conditioning . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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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 some configured rate ("PCN-threshold-rate");
o excess traffic marking: whenever the rate of PCN-packets is
greater than some 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 PCN-marked),
whilst other documents (eg [I-D.moncaster-pcn-3-state-encoding])
define extended schemes with three encoding states (PCN-threshold-
marked, PCN-excess-traffic-marked, not PCN-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 Condition: drop packets if the link is overloaded.
o Threshold meter: determine whether the rate of PCN-packets is
greater than the configured PCN-threshold-rate. The measurement
is made as an aggregate of all PCN-packets, and not per flow.
o Excess traffic meter: measure by how much the rate of PCN-packets
is greater than the configured PCN-excess-rate. The measurement
is made as an aggregate of all PCN-packets, and not per flow.
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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
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 competes for the
same capacity as PCN-traffic. "Capacity" means the forwarding
bandwidth on a link; "competes" means that competing-non-PCN-
packets will delay PCN-packets in the queue for the link.
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 (with the minor exception noted below in Section
2.5). A PCN-packet MUST be treated as a metered-packet. A
competing-non-PCN-packet MAY be treated as a metered-packet.
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 Appendixes supplement it.
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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 for the same capacity as PCN-
traffic. "Capacity" means the forwarding bandwidth on a link;
"competes" means that competing-non-PCN-packets will delay PCN-
packets in the queue for the 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 level of metered-
traffic is sufficiently high to overload the PCN behaviour
aggregate. (According to [RFC2475] metering is "the process of
measuring the temporal properties (eg rate) of a traffic stream".)
o if the level of metered-traffic is sufficiently 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.
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.
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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 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 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. The PCN-excess-rate is greater than (or equal to)
the PCN-threshold-rate.
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.
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. 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.
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A PCN-packet MUST be marked to reflect the metering results by
setting its encoding state appropriately, as specified below. The
encoding states are defined values of the DSCP and ECN fields, as
specified in the appropriate encoding document.
There are three possibilities, depending on how many encoding states
are available:
o if three encoding states are available (one for threshold-marked,
one for excess-traffic-marked and one for "not PCN-marked") then:
* the encoding state of a packet that has already been excess-
traffic-marked is not altered, whatever the meters indicate;
* Otherwise:
+ if both meters indicate marking, then the packet is excess-
traffic-marked;
+ if the threshold meter indicates marking and the excess
traffic meter doesn't, then threshold-marking is applied;
+ if the excess traffic meter indicates marking and the
threshold traffic meter doesn't, then excess-traffic-marking
is applied;
+ if neither meter indicates marking, then the packet's
encoding state is not altered.
o if two encoding states are available (one for threshold-marked and
one for "not PCN-marked") then:
* if the Threshold Meter indicates marking, then the packet is
threshold-marked;
* otherwise the packet's encoding state is not altered.
o if two encoding states are available (one for excess-traffic-
marked and one for "not PCN-marked") then:
* if the Excess traffic Meter indicates marking, then the packet
is excess-traffic-marked;
* otherwise the packet's encoding state is not altered.
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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.
6. Changes
6.1. 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".
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
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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. References
8.1. Normative References
[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-00 (work in progress),
September 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.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
8.2. 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
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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-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>.
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, April 2001.
[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].
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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 PCN-marked"). It is assumed
that all metered-packets are PCN-packets and that the link is never
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
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o TBthreshold.lastUpdate = now
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
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Appendix B. Implementation notes
Note: This Appendix is informative, not normative. It comments on
Section 2.
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 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
focussed 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.
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It is left up to the operator to decide on appropriate action.
Traffic conditioning is discussed further in the separate section
below.
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 might 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
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indicate a competing-non-PCN-packet in a particular PCN-domain.
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. Downgrading of
packets to a lower priority BA is left as a theoretical possibility
(beware of packet mis-ordering). Shaping ("the process of delaying
packets" [RFC2475]) is not suitable here as the traffic is expected
to come 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 policer 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". 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].
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
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particular instant.
Section 2.2 also specifies: "PCN-packets that are dropped
(downgraded) SHOULD NOT be metered by the Excess traffic Meter."
This avoids over-termination, with the CL/SM edge behaviour, in the
event of multiple bottlenecks [I-D.charny-pcn-comparison].
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
above. "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
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.
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* 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.
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].
Section 2.4 specifies: "A packet SHOULD NOT be metered (by this
excess traffic meter function) ... If the packet is already excess-
traffic-marked". 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.
Section 2.4 specifies: "A packet SHOULD NOT be metered (by this
excess traffic meter function) ... If this PCN-node drops
(downgrades) the packet because the link is overloaded." 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 substantially pre-congested and hence
PCN-marking all packets.)
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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. The bullet needs to be modified if the WG decides to allow such
traffic conditioning.
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.
In some environments encoding states may be scarce, for example MPLS
[RFC5129], and then it may be preferable to have only two encoding
states, as in the baseline encoding [I-D.ietf-pcn-baseline-encoding].
In other environments it will be possible to use one of the extension
encodings that specify three encoding states.
Section 2.5 states: "if three encoding states are available ... if
the threshold meter indicates marking and the excess traffic meter
doesn't, then threshold-marking is applied; if the excess traffic
meter indicates marking and the threshold traffic meter doesn't, then
excess-traffic-marking is applied". The latter case seems infeasible
but is possible for a short time - because the meters might react at
different speeds when the traffic rate changes.
Appendix C. Per-domain Behaviour
Note: This Appendix is informative, and to be deleted in next
version. It simply archives material from the previous draft which
concerns PCN-boundary-node behaviour and/or per-domain behaviour
[RFC3086].
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C.1. (from Introduction)
[ietf-pcn-architecture] describes a general architecture for flow
admission and termination based on pre-congestion information in
order to protect the quality of service of established inelastic
flows within a single DiffServ domain. The pre-congestion
information consists of specific markings of PCN-packets. The edge
nodes of the DiffServ domain read these markings and convert them
into flow admission and termination decisions.
So in a particular deployment the operator may have three encoding
states available (so allowing both threshold marking and excess
traffic marking) or may have only two encoding states (so allowing
either threshold marking and excess traffic marking). As described
in [I-D.ietf-pcn-architecture], flow termination is based on excess
traffic marked packets, whilst admission control can be based on
either threshold marked or excess traffic marked packets (the former
is more accurate, [I-D.charny-pcn-comparison]). This leads to the
following four use cases:
1. an operator requires both admission control and flow termination,
and has three encoding states available. Then admission control
is triggered from PCN-packets that are threshold-marked, and flow
termination from PCN-packets that are excess-traffic-marked.
2. an operator requires both admission control and flow termination,
and has only two encoding states available. Then both admission
control and flow termination are triggered from PCN-packets that
are excess-traffic-marked.
3. an operator requires only admission control. Then admission
control is triggered from PCN-packets that are threshold-marked
and only two encoding states are needed. (Flow termination may
be provided by a non PCN mechanism; this is out of scope.)
4. an operator requires only flow termination. Then flow
termination is triggered from PCN-packets that are excess-
traffic-marked and only two encoding states are needed.
(Admission control may be provided by a non PCN mechanism; this
is out of scope.)
C.2. Scope
The functions defined in the following sub-sections SHOULD be
implemented on all links in the PCN-domain.
There are three possibilities regarding encoding states:
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o three encoding states are available,
* one for threshold marks,
* one for excess rate marks
* one for "not PCN-marked";
o two encoding states are available,
* one for threshold marks
* one for "not PCN-marked";
o two encoding states are available,
* one for excess rate marks
* one for "not PCN-marked".
The same choice of encoding states MUST be used throughout a PCN-
domain.
C.3. Classify
Configuration action defines the values of DSCP & ECN fields
associated with PCN across the domain.
C.4. Colour
PCN-ingress-nodes MUST colour PCN-traffic (if necessary) by altering
the DSCP and ECN fields to values appropriate for the PCN-domain.
C.5. Traffic conditioning
In addition, PCN-ingress-nodes MUST police PCN-traffic by
[NOTE; discussion in Dublin: make this a SHOULD, as PCN may well be a
small % traffic and is prioritised]:
o metering PCN-packets that are part of a previously admitted PCN-
flow, to check that it keeps to the agreed rate or flowspec (eg
[RFC1633] for a microflow, and its NSIS equivalent).
o checking that any packets received that demand PCN treatment do
indeed belong to a previously admitted flow.
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o dropping or downgrading packets that fail the above checks.
In addition, PCN-ingress-nodes MUST police other-traffic by:
o metering other-traffic to check that it meets its traffic
conditioning agreement, which is the parameters of the traffic
that will be accepted from a customer. Typically it is statically
defined as part of the subscription-time service level agreement,
as in the DiffServ architecture [RFC2475].
o dropping or downgrading packets that fail the above check.
In addition, an operator MAY measure the amount of traffic entering
(or leaving) its network for accounting reasons. Consideration is
out of scope of this document.
Author's Address
Philip Eardley
BT
Adastral Park, Martlesham Heath
Ipswich IP5 3RE
UK
Email: philip.eardley@bt.com
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