Network Working Group P. Eardley, Ed.
Request for Comments: 5670 BT
Category: Standards Track November 2009
Metering and Marking Behaviour of PCN-Nodes
Abstract
The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain
in a simple, scalable, and robust fashion. This document defines the
two metering and marking behaviours of PCN-nodes. Threshold-metering
and -marking marks all PCN-packets if the rate of PCN-traffic is
greater than a configured rate ("PCN-threshold-rate"). Excess-
traffic-metering and -marking marks a proportion of PCN-packets, such
that the amount marked equals the rate of PCN-traffic in excess of a
configured rate ("PCN-excess-rate"). The level of marking allows
PCN-boundary-nodes to make decisions about whether to admit or
terminate PCN-flows.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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modifications of such material outside the IETF Standards Process.
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Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................4
1.1.1. Requirements Language ...............................5
2. Specified PCN-Metering and -Marking Behaviours ..................5
2.1. Behaviour Aggregate Classification Function ................5
2.2. Dropping Function ..........................................5
2.3. Threshold-Meter Function ...................................6
2.4. Excess-Traffic-Meter Function ..............................6
2.5. Marking Function ...........................................7
3. Security Considerations .........................................7
4. Acknowledgements ................................................8
5. References ......................................................8
5.1. Normative Reference ........................................8
5.2. Informative References .....................................8
Appendix A. Example Algorithms ...................................11
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 ........................15
B.4. Dropping ..................................................15
B.5. Threshold-Metering ........................................17
B.6. Excess-Traffic-Metering ...................................18
B.7. Marking ...................................................19
1. Introduction
The objective of Pre-Congestion Notification (PCN) 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 to decide whether to admit or block a new flow
request, and (in abnormal circumstances) flow termination 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
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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. Within the domain,
PCN-traffic is forwarded in a prioritised Diffserv traffic class
[RFC2475].
This document defines the two metering and 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-metering and -marking: to mark all PCN-packets (with a
"threshold-mark") when the bit rate of PCN-traffic is greater than
its configured reference rate ("PCN-threshold-rate").
o Excess-traffic-metering and -marking: when the bit rate of PCN-
packets is greater than its configured reference rate ("PCN-
excess-rate"), to mark PCN-packets (with an "excess-traffic-mark")
at a rate equal to the difference between the rate of PCN-traffic
and the PCN-excess-rate.
Note that although [RFC3168] defines a broadly RED-like (Random Early
Detection) default congestion marking behaviour, it allows
alternatives to be defined; this document defines such an
alternative.
Section 2 below describes the functions involved, which in outline
(see Figure 1) are:
o Behaviour aggregate (BA) classification: decide whether or not an
incoming packet is a PCN-packet.
o Dropping (optional): drop packets if the link is overloaded.
o Threshold-meter: determine whether the bit rate of PCN-traffic
exceeds its configured reference rate (PCN-threshold-rate). The
meter operates on all PCN-packets on the link, and not on
individual flows.
o Excess-traffic-meter: measure by how much the bit rate of PCN-
traffic exceeds its configured reference rate (PCN-excess-rate).
The meter operates on 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.
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+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+----------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream | | | | | | | Stream
+----------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+
Figure 1: Schematic of PCN-interior-node functionality
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.
The general architecture for PCN is described in [RFC5559], whilst
[Menth10] is an overview of PCN.
1.1. Terminology
In addition to the terminology defined in [RFC5559] 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 (only PCN-packets
can be PCN-marked).
Note: In general, it is not advised to have any competing-non-PCN-
traffic.
Note: There is likely to be traffic (such as best effort) that is
forwarded at lower priority than PCN-traffic; although it shares
the link with PCN-traffic, it doesn't compete for forwarding
bandwidth, and hence it is not competing-non-PCN-traffic. See
Appendix B.1 for further discussion about competing-non-PCN-
traffic.
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o Metered-packet: a packet that is metered by the metering functions
specified in Sections 2.3 and 2.4. 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.
1.1.1. 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].
2. Specified PCN-Metering and -Marking Behaviours
This section defines the two PCN-metering and -marking behaviours.
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
Differentiated Services Code Point (DSCP) and Explicit Congestion
Notification (ECN) fields correspond to a PCN-enabled codepoint, as
defined in the encoding scheme applicable to the PCN-domain (for
example, [RFC5696] defines the baseline encoding). 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. Dropping Function
Note: If the PCN-node's queue overflows, then naturally packets are
dropped. This section describes additional action.
On all links in the PCN-domain, dropping MAY be done by first
metering all metered-packets to determine if the rate of metered-
traffic on the link is greater than the rate allowed for such
traffic; 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.
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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 reference rate (bits per second). The amount of tokens in
the token bucket is termed F_tm. Tokens are added at the reference
rate (PCN-threshold-rate), to a maximum value BS_tm. Tokens are
removed equal to the size in bits of the metered-packet, to a minimum
F_tm = 0. (Explanation of abbreviations: F is short for Fill of the
token bucket, BS for bucket size, and tm for threshold-meter.)
The token bucket has a configured intermediate depth, termed
threshold. If F_tm < 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 PCN-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. The excess-
traffic-meter SHOULD indicate packets to be excess-traffic-marked,
independent of their size ("packet size independent marking"); if
"packet size independent marking" is not implemented, then the
excess-traffic-meter MUST use the "classic" metering behaviour.
For the "classic" metering behaviour, the excess-traffic-meter has
behaviour functionally equivalent to the following.
The meter acts like a token bucket, which is sized in bits and has a
configured reference rate (bits per second). The amount of tokens in
the token bucket is termed F_etm. Tokens are added at the reference
rate (PCN-excess-rate), to a maximum value BS_etm. Tokens are
removed equal to the size in bits of the metered-packet, to a minimum
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F_etm = 0. If the token bucket is empty (F_etm = 0), then the meter
indicates to the marking function that the packet is to be excess-
traffic-marked. (Explanation of abbreviations: F is short for Fill
of the token bucket, BS for bucket size, and etm for excess-traffic-
meter.)
For "packet size independent marking", the excess-traffic-meter has
behaviour functionally equivalent to the following.
The meter acts like a token bucket, which is sized in bits and has a
configured reference rate (bits per second). The amount of tokens in
the token bucket is termed F_etm. Tokens are added at the reference
rate (PCN-excess-rate), to a maximum value BS_etm. If the token
bucket is not negative, then tokens are removed equal to the size in
bits of the metered-packet (and the meter does not indicate to the
marking function that the packet is to be excess-traffic-marked). If
the token bucket is negative (F_etm < 0), then the meter indicates to
the marking function that the packet is to be excess-traffic-marked
(and no tokens are removed). (Explanation of abbreviations: F is
short for Fill of the token bucket, BS for bucket size, and etm for
excess-traffic-meter.)
Otherwise, the meter MUST NOT indicate marking.
2.5. Marking Function
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. A
consistent choice of encoding scheme MUST be made throughout a PCN-
domain.
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.
Note: Although competing-non-PCN-packets MAY be metered, they MUST
NOT be PCN-marked.
3. Security Considerations
It is assumed that all PCN-nodes are PCN-enabled and are trusted for
truthful PCN-metering and PCN-marking. If this isn't the case, then
there are numerous potential attacks. For instance, a rogue PCN-
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interior-node could PCN-mark all packets so that no flows were
admitted. Another possibility is that it doesn't PCN-mark any
packets, even when it is pre-congested.
Note that PCN-interior-nodes are not flow-aware. This prevents some
security attacks where an attacker targets specific flows in the data
plane -- for instance, for Denial-of-Service (DoS) or eavesdropping.
As regards Security Operations and Management, PCN adds few specifics
to the general good practice required in this field [RFC4778]. For
example, it may be sensible for a PCN-node to raise an alarm if it is
persistently PCN-marking.
Security considerations are further discussed in [RFC5559].
4. Acknowledgements
This document is the result of extensive collaboration within the PCN
WG. Amongst the most active other contributors to the development of
the ideas specified in this document have been Jozef Babiarz, Bob
Briscoe, Kwok-Ho Chan, Anna Charny, Georgios Karagiannis, Michael
Menth, Toby Moncaster, Daisuke Satoh, and Joy Zhang. Appendix A is
based on text from Michael Menth.
This document is a development of [Briscoe06-2]. Its authors are
therefore also contributors to this document: Jozef Babiarz, Attila
Bader, Bob Briscoe, Kwok-Ho Chan, Anna Charny, Stephen Dudley, Philip
Eardley, Georgios Karagiannis, Francois Le Faucheur, Vassilis
Liatsos, Dave Songhurst, and Lars Westberg.
Thanks to those who've made comments on the document: Joe Babiarz,
Fred Baker, David Black, Bob Briscoe, Ken Carlberg, Anna Charny,
Ralph Droms, Mehmet Ersue, Adrian Farrel, Ruediger Geib, Wei Gengyu,
Fortune Huang, Christian Hublet, Ingemar Johansson, Georgios
Karagiannis, Alexey Melnikov, Michael Menth, Toby Moncaster, Dimitri
Papadimitriou, Tim Polk, Daisuke Satoh, and Magnus Westerlund.
5. References
5.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
5.2. Informative References
[Baker08] Baker, F., Polk, J., and M. Dolly, "DSCP for Capacity-
Admitted Traffic", Work in Progress, November 2008.
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[Briscoe06-1] Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur,
F., Charny, A., Babiarz, J., Chan, K., Dudley, S.,
Karagiannis, G., Bader, A., and L. Westberg, "An edge-
to-edge Deployment Model for Pre-Congestion
Notification: Admission Control over a DiffServ
Region", Work in Progress, October 2006.
[Briscoe06-2] Briscoe, B., Eardley, P., Songhurst, D., Le Faucheur,
F., Charny, A., Liatsos, V., Babiarz, J., Chan, K.,
Dudley, S., Karagiannis, G., Bader, A., and L.
Westberg, "Pre-Congestion Notification marking", Work
in Progress, October 2006.
[Briscoe08] Briscoe, B., "Byte and Packet Congestion
Notification", Work in Progress, August 2008.
[Charny07] Charny, A., Babiarz, J., Menth, M., and X. Zhang,
"Comparison of Proposed PCN Approaches", Work
in Progress, November 2007.
[Menth10] Menth, M., Lehrieder, F., Briscoe, B., Eardley, P.,
Moncaster, T., Babiarz, J., Chan, K., Charny, A.,
Karagiannis, G., Zhang, X., Taylor, T., Satoh, D., and
R. Geib, "A Survey of PCN-Based Admission Control and
Flow Termination", IEEE Communications Surveys and
Tutorials, 2010 (third issue), .
[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.
[RFC4778] Kaeo, M., "Operational Security Current Practices in
Internet Service Provider Environments", RFC 4778,
January 2007.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", RFC 5127, February 2008.
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[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, June 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
[Taylor09] Charny, A., Huang, F., Menth, M., and T. Taylor, "PCN
Boundary Node Behaviour for the Controlled Load (CL)
Mode of Operation", Work in Progress, March 2009.
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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 [Charny07] and
[Menth10].
There is no attempt to optimise the algorithms. The metering and
marking functions are implemented 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
overloaded. For excess-traffic-marking, "packet size independent
marking" applies.
A.1. Threshold-Metering and -Marking
A token bucket with the following parameters:
* PCN-threshold-rate: token rate of token bucket (bits/second)
* BS_tm: depth of token bucket (bits)
* threshold: marking threshold of token bucket (bits)
* lastUpdate: time the token bucket was last updated (seconds)
* F_tm: amount of tokens in token bucket (bits)
A PCN-packet has the following parameters:
* packet_size: the size of the PCN-packet (bits)
* packet_mark: the PCN encoding state of the packet
In addition there is the parameter:
now: the current time (seconds)
The following steps are performed when a PCN-packet arrives on a
link:
* F_tm = min(BS_tm, F_tm + (now - lastUpdate) * PCN-threshold-
rate); // add tokens to token bucket
* F_tm = max(0, F_tm - packet_size); // remove tokens from token
bucket
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* if ((F_tm < 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
* lastUpdate = now // Note: 'now' has the same value as in step 1
A.2. Excess-Traffic-Metering and -Marking
A token bucket with the following parameters:
* PCN-excess-rate: token rate of token bucket (bits/second)
* BS_etm: depth of TB in token bucket (bits)
* lastUpdate: time the token bucket was last updated (seconds)
* F_etm: amount of tokens in token bucket (bits)
A PCN-packet has the following parameters:
* packet_size: the size of the PCN-packet (bits)
* packet_mark: the PCN encoding state of the packet
In addition there is the parameter:
* now: the current time (seconds)
The following steps are performed when a PCN-packet arrives on a
link:
* F_etm = min(BS_etm, F_etm + (now - lastUpdate) * PCN-excess-
rate); // add tokens to token bucket
* if (packet_mark != excess-traffic-marked) then // do not meter
packets that are already excess-traffic-marked
+ if (F_etm < 0) then packet_mark = excess-traffic-marked; //
do excess-traffic-marking. The algorithm ensures this is
independent of packet size
+ else F_etm = F_etm - packet_size; // remove tokens from
token bucket if don't mark packet
* lastUpdate = now // Note: 'now' has the same value as in step 1
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Appendix B. Implementation Notes
Note: This Appendix is informative, not normative. It comments on
Section 2, including reasoning about whether MUSTs or SHOULDs are
required. For guidance on Operations and Management considerations,
please see [RFC5559].
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 [RFC5559]. But if
there is competing-non-PCN-traffic, then:
1. There should be a mechanism to limit it, for example:
* limit 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 drop competing-non-PCN-
packets in excess of some rate.
* police competing-non-PCN-traffic at the PCN-ingress-nodes, as
in the Diffserv architecture, for example. However,
Diffserv's static traffic conditioning agreements risk a
focused overload of traffic from several PCN-ingress-nodes
onto one link.
* by design, it is known 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:
* competing-non-PCN-traffic contributes to the PCN-meters;
competing-non-PCN-packets are treated as metered-packets.
* each PCN-node, on its links: (1) reduces the reference rates
(PCN-threshold-rate and PCN-excess-rate), in order to allow
'headroom' for the competing-non-PCN-traffic; (2) limits 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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3. The operator should decide on appropriate action. Dropping is
discussed further in Appendix B.4.
One specific example of competing-non-PCN-traffic occurs if the PCN-
compatible Diffserv codepoint is one of those that [Baker08] defines
as suitable for use with admission control and there is such non-PCN-
traffic in the PCN-domain. A similar example could occur for
Diffserv codepoints of the Real-Time Treatment Aggregate [RFC5127].
In such cases, PCN-traffic and competing-non-PCN-traffic are
distinguished by different values of the ECN field [RFC5696].
Another example would occur if there is more than one PCN-compatible
Diffserv codepoint in a PCN-domain. For instance, suppose there are
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, for instance by the design of the network topology,
that some links can never be pre-congested (even in unusual
circumstances, such as after the failure of some links). There is
then no need to deploy the PCN-metering and -marking behaviour on
those links.
The meters can be implemented on the ingoing or outgoing interface of
a PCN-node. It may be that existing hardware can support only one
meter per ingoing interface and one per outgoing interface. Then,
for instance, threshold-metering could be run on all the ingoing
interfaces and excess-traffic-metering 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.
The baseline encoding [RFC5696] specifies only two encoding states
(PCN-marked and not-marked). In this case, "excess-traffic-marked"
means a packet that is PCN-marked as a result of the excess-traffic-
meter function, and "threshold-marked" means a packet that is PCN-
marked as a result of the threshold-meter function. As far as
terminology is concerned, this interpretation is consistent with that
defined in [RFC5559]. Note that a deployment needs to make a
consistent choice throughout the PCN-domain whether PCN-marked is
interpreted as excess-traffic-marked or threshold-marked.
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 to remove a configuration option
and associated complexity. Hardware with limited availability of
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token buckets could be configured to run only one of the meters, but
it must be possible to enable either meter. Although, in the
scenario with two encoding states, indications from one of the meters
are ignored by the marking function, 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 the
scope of this document.
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. For
instance, [RFC5696] defines the baseline encoding.
Configuration will also define what values of the DSCP and ECN fields
indicate a competing-non-PCN-packet in a particular PCN-domain.
B.4. Dropping
The objective of the dropping function 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.
If there is no competing-non-PCN-traffic, then it is not expected
that the dropping function is needed, since PCN's flow admission and
termination mechanisms limit the amount of PCN-traffic. Even so, it
still might be implemented as a back stop against misconfiguration of
the PCN-domain, for instance.
If there is competing-non-PCN-traffic, then the details of the
dropping function will depend on how the router's implementation
handles the two sorts of traffic:
1. a common queue for PCN-traffic and competing-non-PCN-traffic,
with a traffic conditioner for the competing-non-PCN-traffic; or
2. separate queues, in which 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 discussion here is based on that in [Baker08].)
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RFC 5670 PCN metering and marking November 2009
Note that only dropping of packets is allowed. Downgrading of
packets to a lower priority BA is not allowed (see Appendix B.7),
since it would lead to packet mis-ordering. Shaping ("the process of
delaying packets" [RFC2475]) is not suitable if the traffic comes
from real-time applications.
Preferential dropping of competing-non-PCN-traffic:
In general, it is reasonable for competing-non-PCN-traffic to get
harsher treatment than PCN-traffic (that is, 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 dropper should not be needed for
the PCN-traffic.
Preferential dropping of excess-traffic-marked packets:
Section 2.2 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, with the "controlled load" edge behaviour
[Taylor09], this avoids over-termination in the event of multiple
bottlenecks in the PCN-domain [Charny07]. A fuller explanation is
as follows. The optimal dropping behaviour depends on the
particular edge behaviour [Menth10]. 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
terminated with the "controlled load" edge behaviour, in the event
of multiple bottlenecks in the PCN-domain [Charny07]). 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 [Menth10]; (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 [Menth10]. Effectively, it means that the dropping
function (if present) should be done before the meter functions --
which is natural.
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RFC 5670 PCN metering and marking November 2009
B.5. Threshold-Metering
The description is in terms of a 'token bucket with threshold' (which
[Briscoe06-1] views as a virtual queue). However, the description is
not intended to standardise implementation.
The reference rate of the threshold-meter (PCN-threshold-rate) is
configured at less than the rate allocated to the PCN-traffic class.
Also, the PCN-threshold-rate is less than, or possibly equal to, the
PCN-excess-rate.
Section 2.3 specifies, "If F_tm < threshold, then the meter indicates
to the marking function that the packet is to be threshold-marked;
otherwise, it does not." Note that a PCN-packet 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, for example, if either
precisely the same set of packets is marked or if the set is shifted
by one packet. 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 PCN-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 F_tm, then the precise change in F_tm is up to the
implementation. For instance:
* set F_tm = 0 and indicate threshold-mark to the marking
function.
* check whether F_tm < threshold and if it is, then indicate
threshold-mark to the marking function; then set F_tm = 0.
* leave F_tm unaltered and indicate threshold-mark to the marking
function.
o similarly, when the token bucket is very nearly full and a packet
arrives larger than (BS_tm - F_tm), then the precise change in
F_tm is up to the implementation.
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RFC 5670 PCN metering and marking November 2009
Note that all PCN-packets, even if already marked, are metered by the
threshold-meter function (unlike the excess-traffic-meter function),
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.
The reference rate of the excess-traffic-meter (PCN-excess-rate) is
configured at less than (or possibly equal to) the rate allocated to
the PCN-traffic class. Also, the PCN-excess-rate is greater than, or
possibly equal to, the PCN-threshold-rate.
As in Section B.5, "functionally equivalent" allows some
implementation flexibility, for example, the exact algorithm 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
[Charny07]. Note that an implementation could determine whether the
packet is already excess-traffic-marked as an integral part of its BA
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 [Menth10]. (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 threshold-marking all packets.)
It seems natural to perform the dropping function 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" -- excess-traffic-marking that is
independent of packet size -- is specified as a 'SHOULD' rather than
a 'MUST' in Section 2.4 because it may be slightly harder for some
equipment to implement, and the impact of not doing so is undesirable
but moderate (sufficient traffic is terminated, but flows with large
packets are more likely to be terminated). With the "classic"
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RFC 5670 PCN metering and marking November 2009
excess-traffic-meter behaviour, large packets are more likely to be
excess-traffic-marked than small packets (because packets are marked
if the number of tokens in the token bucket is smaller than the
packet size). This means that, with some edge behaviours, flows with
large packets are more likely to be terminated than flows with small
packets ([Briscoe08], [Menth10]). "Packet size independent marking"
can be achieved by a small modification of the "classic" excess-
traffic-meter. The number of tokens in the bucket can become
negative; if this number is negative at a packet's arrival, the
packet is marked; otherwise, the amount of tokens equal to the packet
size is removed from the bucket. Note that with "packet size
independent marking", either the packet is marked or tokens are
removed -- never both. Hence, the token bucket cannot become more
negative than the maximum packet size on the link. The algorithm
described in Appendix A implements this behaviour.
Note that BS_etm is independent of BS_tm, F_etm is independent of
F_tm (except in that a packet can change 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 is not allowed to
downgrade a PCN-packet into a lower priority Diffserv BA (hence,
downgrading is not allowed as an alternative to dropping).
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, 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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RFC 5670 PCN metering and marking November 2009
Author's Address
Philip Eardley (editor)
BT
Adastral Park, Martlesham Heath
Ipswich IP5 3RE
UK
EMail: philip.eardley@bt.com
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RFC, FYI, BCP