Congestion and Pre-Congestion Philip. Eardley (Editor)
Notification Working Group BT
Internet-Draft July 14, 2008
Intended status: Informational
Expires: January 15, 2009
Pre-Congestion Notification Architecture
draft-ietf-pcn-architecture-04
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Abstract
The purpose of this document is to describe 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.
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Status
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Deployment scenarios . . . . . . . . . . . . . . . . . . . . . 8
5. Assumptions and constraints on scope . . . . . . . . . . . . . 10
5.1. Assumption 1: Trust and support of PCN - controlled
environment . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Assumption 2: Real-time applications . . . . . . . . . . . 11
5.3. Assumption 3: Many flows and additional load . . . . . . . 12
5.4. Assumption 4: Emergency use out of scope . . . . . . . . . 12
6. High-level functional architecture . . . . . . . . . . . . . . 12
6.1. Flow admission . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Flow termination . . . . . . . . . . . . . . . . . . . . . 15
6.3. Flow admission and flow termination when there are
only two PCN encoding states . . . . . . . . . . . . . . . 16
6.4. Information transport . . . . . . . . . . . . . . . . . . 16
6.5. PCN-traffic . . . . . . . . . . . . . . . . . . . . . . . 17
6.6. Backwards compatibility . . . . . . . . . . . . . . . . . 17
7. Detailed Functional architecture . . . . . . . . . . . . . . . 18
7.1. PCN-interior-node functions . . . . . . . . . . . . . . . 19
7.2. PCN-ingress-node functions . . . . . . . . . . . . . . . . 19
7.3. PCN-egress-node functions . . . . . . . . . . . . . . . . 20
7.4. Other admission control functions . . . . . . . . . . . . 20
7.5. Other flow termination functions . . . . . . . . . . . . . 21
7.6. Addressing . . . . . . . . . . . . . . . . . . . . . . . . 22
7.7. Tunnelling . . . . . . . . . . . . . . . . . . . . . . . . 23
7.8. Fault handling . . . . . . . . . . . . . . . . . . . . . . 24
8. Design goals and challenges . . . . . . . . . . . . . . . . . 24
9. Operations and Management . . . . . . . . . . . . . . . . . . 27
9.1. Configuration OAM . . . . . . . . . . . . . . . . . . . . 27
9.1.1. System options . . . . . . . . . . . . . . . . . . . . 28
9.1.2. Parameters . . . . . . . . . . . . . . . . . . . . . . 29
9.2. Performance & Provisioning OAM . . . . . . . . . . . . . . 31
9.3. Accounting OAM . . . . . . . . . . . . . . . . . . . . . . 32
9.4. Fault OAM . . . . . . . . . . . . . . . . . . . . . . . . 32
9.5. Security OAM . . . . . . . . . . . . . . . . . . . . . . . 33
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
11. Security considerations . . . . . . . . . . . . . . . . . . . 34
12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 35
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
14. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 36
15. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
15.1. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 36
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15.2. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 37
15.3. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 38
15.4. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 39
16. Appendix A: Possible work items beyond the scope of the
current PCN WG Charter . . . . . . . . . . . . . . . . . . . . 40
17. Appendix B: Probing . . . . . . . . . . . . . . . . . . . . . 42
17.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 42
17.2. Probing functions . . . . . . . . . . . . . . . . . . . . 43
17.3. Discussion of rationale for probing, its downsides and
open issues . . . . . . . . . . . . . . . . . . . . . . . 43
18. Informative References . . . . . . . . . . . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 51
Intellectual Property and Copyright Statements . . . . . . . . . . 52
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1. Introduction
The purpose of this document is to describe a general architecture
for flow admission and termination based on (pre-) congestion
information in order to protect the quality of service of flows
within a DiffServ domain [RFC2475]. This document defines an
architecture for implementing two mechanisms to protect the quality
of service of established inelastic flows within a single DiffServ
domain, where all boundary and interior nodes are PCN-enabled and
trust each other for correct PCN operation. Flow admission control
determines whether a new flow should be admitted, in order to protect
the QoS of existing PCN-flows in normal circumstances. However, in
abnormal circumstances, for instance a disaster affecting multiple
nodes and causing traffic re-routes, then the QoS on existing PCN-
flows may degrade even though care was exercised when admitting those
flows. Therefore we also propose a mechanism for flow termination,
which removes enough traffic in order to protect the QoS of the
remaining PCN-flows.
As a fundamental building block to enable these two mechanisms, PCN-
interior-nodes generate, encode and transport pre-congestion
information towards the PCN-egress-nodes. Two rates, a PCN-
threshold-rate and a PCN-excess-rate, are associated with each link
of the PCN-domain. Each rate is used by a marking behaviour that
determines how and when PCN-packets are marked, and how the markings
are encoded in packet headers. Overall the 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 the queue.
PCN-boundary-nodes convert measurements of these PCN-markings into
decisions about flow admission and termination. The admission
control mechanism limits the PCN-traffic on each link to *roughly*
its PCN-threshold-rate and the flow termination mechanism limits the
PCN-traffic on each link to *roughly* its PCN-excess-rate.
This document describes the PCN architecture and outlines some
benefits, deployment scenarios, assumptions and terminology for PCN.
The behaviour of PCN-interior-nodes is standardised in three
documents, which are summarised in this
document.[I-D.eardley-pcn-marking-behaviour] 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, "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 second
configured rate, "PCN-excess-rate". 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.
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[I-D.moncaster-pcn-baseline-encoding] standardises two PCN encoding
states (PCN-marked and not PCN-marked) whilst
[I-D.moncaster-pcn-3-state-encoding] standardises an extended scheme
with three encoding states (threshold-marked, excess-traffic-marked,
not PCN-marked) but requires an extra DiffServ codepoint. PCN
therefore defines semantics for the ECN field different from the
default semantics of [RFC3168]; PCN's encoding has been chosen to
meet the guidelines of BCP124, [RFC4774]. The behaviour of PCN-
boundary-nodes is described in Informational documents. Several
possibilities are outlined in this document; detailed descriptions
and comparisons are in [I-D.charny-pcn-comparison] and [Menth08].
2. Terminology
o PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled
nodes that perform DiffServ scheduling; the complete set of PCN-
nodes whose PCN-marking can in principle influence decisions about
flow admission and termination for the PCN-domain, including the
PCN-egress-nodes which measure these PCN-marks.
o PCN-boundary-node: a PCN-node that connects one PCN-domain to a
node either in another PCN-domain or in a non PCN-domain.
o PCN-interior-node: a node in a PCN-domain that is not a PCN-
boundary-node.
o PCN-node: a PCN-boundary-node or a PCN-interior-node
o PCN-egress-node: a PCN-boundary-node in its role in handling
traffic as it leaves a PCN-domain.
o PCN-ingress-node: a PCN-boundary-node in its role in handling
traffic as it enters a PCN-domain.
o PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of
different DiffServ behaviour aggregates (BAs) [RFC2475]. The
PCN-BA uses the PCN mechanisms to carry PCN-traffic and the
corresponding packets are PCN-packets. The same network will
carry traffic of other DiffServ BAs. The PCN-BA is distinguished
by a combination of the DiffServ codepoint (DSCP) and ECN fields;
note that a packet that shares the same DSCP as PCN-traffic but
its ECN field is 00 (Not ECT) is not part of the PCN-BA.
o PCN-flow: the unit of PCN-traffic that the PCN-boundary-node
admits (or terminates); the unit could be a single microflow (as
defined in [RFC2475]) or some identifiable collection of
microflows.
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o Ingress-egress-aggregate: The collection of PCN-packets from all
PCN-flows that travel in one direction between a specific pair of
PCN-boundary-nodes.
o PCN-threshold-rate: a reference rate configured for each link in
the PCN-domain, which is lower than the PCN-excess-rate. It is
used by a marking behaviour that determines whether a packet
should be PCN-marked with a first encoding, "threshold-marked".
It's roughly the rate up to which PCN admission control should
accept new flows.
o PCN-excess-rate: a reference rate configured for each link in the
PCN-domain, which is higher than the PCN-threshold-rate. It is
used by a marking behaviour that determines whether a packet
should be PCN-marked with a second encoding, "excess-traffic-
marked". It's roughly that rate down to which flow termination
should, if necessary, terminate already admitted flows.
o Threshold-marking: a PCN-marking behaviour with the objective that
all PCN-traffic is marked if the PCN-traffic exceeds the PCN-
threshold-rate.
o Excess-traffic-marking: a PCN-marking behaviour with the objective
that the amount of PCN-traffic that is PCN-marked is equal to the
amount that exceeds the PCN-excess-rate.
o Pre-congestion: a condition of a link within a PCN-domain in which
the PCN-node performs PCN-marking, in order to provide an "early
warning" of potential congestion before there is any significant
build-up of PCN-packets in the real queue. (Hence, by analogy
with ECN we call our mechanism Pre-Congestion Notification.)
o PCN-marking: the process of setting the header in a PCN-packet
based on defined rules, in reaction to pre-congestion; either
threshold-marking or excess-traffic-marking.
o PCN-feedback-information: information signalled by a PCN-egress-
node to a PCN-ingress-node or central control node, which is
needed for the flow admission and flow termination mechanisms.
3. Benefits
We believe that the key benefits of the PCN mechanisms described in
this document are that they are simple, scalable, and robust because:
o Per flow state is only required at the PCN-ingress-nodes
("stateless core"). This is required for policing purposes (to
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prevent non-admitted PCN traffic from entering the PCN-domain) and
so on. It is not generally required that other network entities
are aware of individual flows (although they may be in particular
deployment scenarios).
o Admission control is resilient: PCN's QoS is decoupled from the
routing system; hence in general admitted flows can survive
capacity, routing or topology changes without additional
signalling, and they don't have to be told (or learn) about such
changes. The PCN-threshold-rate on each PCN-node can be chosen
small enough that admitted traffic can still be carried after a
rerouting in most failure cases [Menth]. This is an important
feature as QoS violations in core networks due to link failures
are more likely than QoS violations due to increased traffic
volume [Iyer].
o The PCN-marking behaviours only operate on the overall PCN-traffic
on the link, not per flow.
o The information of these measurements is signalled to the PCN-
egress-nodes by the PCN-marks in the packet headers, ie "in-band".
No additional signalling protocol is required for transporting the
PCN-marks. Therefore no secure binding is required between data
packets and separate congestion messages.
o The PCN-egress-nodes make separate measurements, operating on the
aggregate PCN-traffic from each PCN-ingress-node, ie not per flow.
Similarly, signalling by the PCN-egress-node of PCN-feedback-
information (which is used for flow admission and termination
decisions) is at the granularity of the ingress-egress-aggregate.
An alternative approach is that the PCN-egress-nodes monitor the
PCN-traffic and signal PCN-feedback-information (which is used for
flow admission and termination decisions) at the granularity of
one (or a few) PCN-marks.
o The admitted PCN-load is controlled dynamically. Therefore it
adapts as the traffic matrix changes, and also if the network
topology changes (eg after a link failure). Hence an operator can
be less conservative when deploying network capacity, and less
accurate in their prediction of the PCN-traffic matrix.
o The termination mechanism complements admission control. It
allows the network to recover from sudden unexpected surges of
PCN-traffic on some links, thus restoring QoS to the remaining
flows. Such scenarios are expected to be rare but not impossible.
They can be caused by large network failures that redirect lots of
admitted PCN-traffic to other links, or by malfunction of the
measurement-based admission control in the presence of admitted
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flows that send for a while with an atypically low rate and then
increase their rates in a correlated way.
o Flow termination can also enable an operator to be less
conservative when deploying network capacity. It is an
alternative to running links at low utilisation in order to
protect against link or node failures. This is especially the
case with SRLGs (shared risk link groups, which are links that
share a resource, such as a fibre, whose failure affects all those
links [RFC4216]. A requirement to fully protect traffic against a
single SRLG failure requires low utilisation (~10%) of the link
bandwidth on some links before failure [PCN-email-SRLG].
o The PCN-excess-rate may be set below the maximum rate that PCN-
traffic can be transmitted on a link, in order to trigger
termination of some PCN-flows before loss (or excessive delay) of
PCN-packets occurs, or to keep the maximum PCN-load on a link
below a level configured by the operator.
o Provisioning of the network is decoupled from the process of
adding new customers. By contrast, with the DiffServ architecture
[RFC2475] operators rely on subscription-time Service Level
Agreements that statically define the parameters of the traffic
that will be accepted from a customer, and so the operator has to
run the provisioning process each time a new customer is added to
check that the Service Level Agreement can be fulfilled. A PCN-
domain doesn't need such traffic conditioning.
4. Deployment scenarios
Operators of networks will want to use the PCN mechanisms in various
arrangements, for instance depending on how they are performing
admission control outside the PCN-domain (users after all are
concerned about QoS end-to-end), what their particular goals and
assumptions are, how many PCN encoding states are available, and so
on.
From the perspective of the outside world, a PCN-domain essentially
looks like a DiffServ domain. PCN-traffic is either transported
across it transparently or policed at the PCN-ingress-node (ie
dropped or carried at a lower QoS). A couple of differences are
that: PCN-traffic has better QoS guarantees than normal DiffServ
traffic (because PCN's mechanisms better protect the QoS of admitted
flows); and in rare circumstances (failures), on the one hand some
PCN-flows may get terminated, but on the other hand other flows will
get their QoS restored. Non PCN-traffic is treated transparently, ie
the PCN-domain is a normal DiffServ domain.
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An operator may choose to deploy either admission control or flow
termination or both. Although designed to work together, they are
independent mechanisms, and the use of one does not require or
prevent the use of the other.
For example, an operator could use just PCN's admission control,
solving heavy congestion (caused by re-routing) by 'just waiting' -
as sessions end, PCN-traffic naturally reduces, and meanwhile the
admission control mechanism will prevent admission of new flows that
use the affected links. So the PCN-domain will naturally return to
normal operation, but with reduced capacity. The drawback of this
approach would be that until PCN-traffic naturally departs to relieve
the congestion, all PCN-flows as well as lower priority services will
be adversely affected.
Another example is that an operator could just rely for admission
control on statically provisioned capacity per PCN-ingress-node
(regardless of the PCN-egress-node of a flow), as is typical in the
hose model of the DiffServ architecture [RFC2475]. Such traffic
conditioning agreements can lead to focused overload: many flows
happen to focus on a particular link and then all flows through the
congested link fail catastrophically. PCN's flow termination
mechanism could then be used to counteract such a problem.
The possibility of deploying just one of PCN's flow admission and
termination mechanisms is certainly an option when only two PCN
encoding states are available (PCN-marked and not PCN-marked), as in
[I-D.moncaster-pcn-baseline-encoding]. Another option in this
circumstance is to trigger both admission control and flow
termination from the single type of PCN-marking; the main downside is
that admission control is less accurate.
Within the PCN-domain there is some flexibility about where the
decision making functionality is located. For admission control, the
most natural place is the PCN-ingress-node. For flow termination,
whether the PCN-ingress-node or PCN-egress-node is more natural
depends on the mechanism used to convert packet markings into a flow
termination decision. These possibilities are outlined more later
and also discussed elsewhere, such as in [Menth08]. Another
possibility is that the decision making functionality is at some
central control node. This is briefly discussed in Appendix A and
described in [I-D.tsou-pcn-racf-applic].
The flow admission and termination decisions need to be enforced
through per-flow policing by the PCN-ingress-nodes. If there are
several PCN-domains on the end-to-end path then each needs to police
at its PCN-ingress-nodes. One exception is if the operator runs both
the access network (not a PCN-domain) and the core network (a PCN-
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domain); per flow policing could be devolved to the access network
and not done at the PCN-ingress-node. Note: to aid readability, the
rest of this draft assumes that policing is done by the PCN-ingress-
nodes.
PCN admission control has to fit with the overall approach to
admission control. For instance [I-D.briscoe-tsvwg-cl-architecture]
describes the case where RSVP signalling runs end-to-end. The PCN-
domain is a single RSVP hop, ie only the PCN-boundary-nodes process
RSVP messages, with RSVP messages processed on each hop outside the
PCN-domain, as in IntServ over DiffServ [RFC2998]. It would also be
possible for the RSVP signalling to be originated and/or terminated
by proxies, with application-layer signalling between the end user
and the proxy (eg SIP signalling with a home hub). A similar example
would use NSIS signalling is used instead of RSVP.
It is possible that a user wants its inelastic traffic to use the PCN
mechanisms but also react to ECN marking outside the PCN-domain
[I-D.sarker-pcn-ecn-pcn-usecases]. Two ways to do this are to tunnel
all PCN-packets across the PCN-domain, so that the ECN marks is
carried transparently across the PCN-domain, or to use the three
state PCN encoding [I-D.moncaster-pcn-3-state-encoding]. This is
discussed further in Section Section 7.
Some possible deployment models that are outside the current PCN WG
Charter are outlined in Appendix A.
5. Assumptions and constraints on scope
The scope of PCN is, at least initially (see Appendix A), restricted
by the following assumptions:
1. these components are deployed in a single DiffServ domain, within
which all PCN-nodes are PCN-enabled and trust each other for
truthful PCN-marking and transport
2. all flows handled by these mechanisms are inelastic and
constrained to a known peak rate through policing or shaping
3. the number of PCN-flows across any potential bottleneck link is
sufficiently large that stateless, statistical mechanisms can be
effective. To put it another way, the aggregate bit rate of PCN-
traffic across any potential bottleneck link needs to be
sufficiently large relative to the maximum additional bit rate
added by one flow. This is the basic assumption of measurement-
based admission control.
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4. PCN-flows may have different precedence, but the applicability of
the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.)
is out of scope.
5.1. Assumption 1: Trust and support of PCN - controlled environment
We assume that the PCN-domain is a controlled environment, ie all the
nodes in a PCN-domain run PCN and trust each other. There are
several reasons for proposing this assumption:
o The PCN-domain has to be encircled by a ring of PCN-boundary-
nodes, otherwise traffic could enter a PCN BA without being
subject to admission control, which would potentially degrade the
QoS of existing PCN-flows.
o Similarly, a PCN-boundary-node has to trust that all the PCN-nodes
mark PCN-traffic consistently. A node not doing PCN-marking
wouldn't be able to alert when it suffered pre-congestion, which
potentially would lead to too many PCN-flows being admitted (or
too few being terminated). Worse, a rogue node could perform
various attacks, as discussed in the Security Considerations
section.
One way of assuring the above two points is that the entire PCN-
domain is run by a single operator. Another possibility is that
there are several operators but they trust each other to a sufficient
level, in their handling of PCN-traffic.
Note: All PCN-nodes need to be trustworthy. However if it's known
that an interface cannot become pre-congested then it's not strictly
necessary for it to be capable of PCN-marking. But this must be
known even in unusual circumstances, eg after the failure of some
links.
5.2. Assumption 2: Real-time applications
We assume that any variation of source bit rate is independent of the
level of pre-congestion. We assume that PCN-packets come from real
time applications generating inelastic traffic, ie it sends packets
at the rate the codec produces them, regardless of the availability
of capacity [RFC4594]. For example, voice and video requiring low
delay, jitter and packet loss, the Controlled Load Service,
[RFC2211], and the Telephony service class, [RFC4594]. This
assumption is to help focus the effort where it looks like PCN would
be most useful, ie the sorts of applications where per flow QoS is a
known requirement. In other words we focus on PCN providing a
benefit to inelastic traffic (PCN may or may not provide a benefit to
other types of traffic). For instance, the impact of this assumption
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would be to guide simulations work.
As a consequence, it is assumed that PCN-marking is being applied to
traffic scheduled with the expedited forwarding per-hop behaviour,
[RFC3246], or traffic with similar characteristics.
5.3. Assumption 3: Many flows and additional load
We assume that there are many PCN-flows on any bottleneck link in the
PCN-domain (or, to put it another way, the aggregate bit rate of PCN-
traffic across any potential bottleneck link is sufficiently large
relative to the maximum additional bit rate added by one PCN-flow).
Measurement-based admission control assumes that the present is a
reasonable prediction of the future: the network conditions are
measured at the time of a new flow request, however the actual
network performance must be OK during the call some time later. One
issue is that if there are only a few variable rate flows, then the
aggregate traffic level may vary a lot, perhaps enough to cause some
packets to get dropped. If there are many flows then the aggregate
traffic level should be statistically smoothed. How many flows is
enough depends on a number of things such as the variation in each
flow's rate, the total rate of PCN-traffic, and the size of the
"safety margin" between the traffic level at which we start
admission-marking and at which packets are dropped or significantly
delayed.
We do not make explicit assumptions on how many PCN-flows are in each
ingress-egress-aggregate. Performance evaluation work may clarify
whether it is necessary to make any additional assumption on
aggregation at the ingress-egress-aggregate level.
5.4. Assumption 4: Emergency use out of scope
PCN-flows may have different precedence, but the applicability of the
PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc) is out
of scope for consideration by the PCN WG.
6. High-level functional architecture
The high-level approach is to split functionality between:
o PCN-interior-nodes 'inside' the PCN-domain, which monitor their
own state of pre-congestion and mark PCN-packets if appropriate.
They are not flow-aware, nor aware of ingress-egress-aggregates.
The functionality is also done by PCN-ingress-nodes for their
outgoing interfaces (ie those 'inside' the PCN-domain).
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o PCN-boundary-nodes at the edge of the PCN-domain, which control
admission of new PCN-flows and termination of existing PCN-flows,
based on information from PCN-interior-nodes. This information is
in the form of the PCN-marked data packets (which are intercepted
by the PCN-egress-nodes) and not signalling messages. Generally
PCN-ingress-nodes are flow-aware.
The aim of this split is to keep the bulk of the network simple,
scalable and robust, whilst confining policy, application-level and
security interactions to the edge of the PCN-domain. For example the
lack of flow awareness means that the PCN-interior-nodes don't care
about the flow information associated with the PCN-packets that they
carry, nor do the PCN-boundary-nodes care about which PCN-interior-
nodes its flows traverse. The objective is to standardise PCN-
marking behaviour, but potentially produce more than one
(informational) RFC describing how PCN-boundary-nodes react to PCN-
marks.
In order to generate information about the current state of the PCN-
domain, each PCN-node PCN-marks packets if it is "pre-congested".
Exactly when a PCN-node decides if it is "pre-congested" (the
algorithm) and exactly how packets are "PCN-marked" (the encoding)
are defined in separate standards-track documents, but at a high
level it is as follows:
o the algorithms: a PCN-node meters the amount of PCN-traffic on
each one of its outgoing (or incoming) links. The measurement is
made as an aggregate of all PCN-packets, and not per flow. There
are two algorithms, one for threshold-marking and one for excess-
traffic-marking.
o the encoding(s): a PCN-node PCN-marks a PCN-packet by setting the
ECN field to 11 and potentially altering the DSCP.
The PCN-boundary-nodes monitor the PCN-marked packets in order to
extract information about the current state of the PCN-domain. Based
on this monitoring, a decision is made about whether to admit a
prospective new flow or whether to terminate existing flow(s).
PCN-marking needs to be configured on all links in the PCN-domain to
ensure that the PCN mechanisms protect all links. The actual
functionality can be configured on the outgoing or incoming
interfaces of PCN-nodes - or one algorithm could be configured on the
outgoing interface and the other on the incoming interface. The
important thing is that a consistent choice is made across the PCN-
domain to ensure that the PCN mechanisms protect all links. See
[I-D.eardley-pcn-marking-behaviour] for further discussion.
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The objective of the threshold-marking algorithm is to threshold-mark
all PCN-packets whenever the rate of PCN-packets is greater than some
configured rate, the PCN-threshold-rate. The objective of the
excess-traffic-marking algorithm is to excess-traffic-mark PCN-
packets at a rate equal to the difference between the bit rate of
PCN-packets and some configured rate, the PCN-excess-rate. Note that
this description reflects the overall intent of the algorithm rather
than its instantaneous behaviour, since the rate measured at a
particular moment depends on the detailed algorithm, its
implementation and the traffic's variance as well as its rate (eg
marking may well continue after a recent overload even after the
instantaneous rate has dropped). The algorithms are specified in
[I-D.eardley-pcn-marking-behaviour].
In a PCN-domain the operator may have two or three encoding states
available. In both cases the ECN field is set to 11 to indicate PCN-
marking. In the former case, one DSCP is used. In the latter case a
second DSCP is used, which allows distinct threshold-marks and
excess-traffic-marks. The encoding is specified in
[I-D.moncaster-pcn-baseline-encoding] and
[I-D.moncaster-pcn-3-state-encoding].
All the various admission and termination approaches are detailed and
compared in [I-D.charny-pcn-comparison] and [Menth08]. The
discussion below is just a brief summary. It initially assumes there
are three encoding states available.
6.1. Flow admission
The objective of PCN's flow admission control mechanism is to limit
the PCN-traffic on each link in the PCN-domain to *roughly* its PCN-
threshold-rate, by admitting or blocking prospective new flows, in
order to protect the QoS of existing PCN-flows. The PCN-threshold-
rate is a parameter that can be configured by the operator and will
be set lower than the traffic rate at which the link becomes
congested and the node drops packets.
Exactly how the admission control decision is made will be defined
separately in informational documents. At a high level two
approaches are proposed:
o the PCN-egress-node measures (possibly as a moving average) the
fraction of the PCN-traffic that is threshold-marked. The
fraction is measured for a specific ingress-egress-aggregate. If
the fraction is below a threshold value then the new flow is
admitted, and if the fraction is above the threshold value then it
is blocked. In [I-D.eardley-pcn-architecture] the fraction is
measured as an EWMA (exponentially weighted moving average) and
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termed the "congestion level estimate".
o the PCN-egress-node monitors PCN-traffic and if it receives one
(or several) threshold-marked packets, then the new flow is
blocked, otherwise it is admitted. One possibility is to react to
the marking state of an initial flow set-up packet (eg RSVP PATH).
Another is that after one (or several) threshold-marks then all
flows are blocked until after a specific period of no congestion.
Note that the admission control decision is made for a particular
pair of PCN-boundary-nodes. So it is quite possible for a new flow
to be admitted between one pair of PCN-boundary-nodes, whilst at the
same time another admission request is blocked between a different
pair of PCN-boundary-nodes.
6.2. Flow termination
The objective of PCN's flow termination mechanism is to limit the
PCN-traffic on each link to *roughly* its PCN-excess-rate, by
terminating some existing PCN-flows, in order to protect the QoS of
the remaining PCN-flows. The PCN-excess-rate is a parameter that can
be configured by the operator and may be set lower than the traffic
rate at which the link becomes congested and the node drops packets.
Exactly how the flow termination decision is made will be defined
separately in informational documents. At a high level several
approaches are proposed:
o In one approach the PCN-egress-node measures the rate of PCN-
traffic that is not excess-traffic-marked, which is the amount of
PCN-traffic that can actually be supported. Also the PCN-ingress-
node measures the rate of PCN-traffic that is destined for this
specific PCN-egress-node, and hence it can calculate the excess
amount that should be terminated.
o Another approach instead measures the rate of excess-traffic-
marked traffic and terminates this amount of traffic. This
terminates more traffic than the previous bullet if some nodes are
dropping PCN-traffic.
o Another approach monitors PCN-packets and terminates any PCN-flow
with an excess-traffic-marked packet. Compared with the
approaches above, PCN-marking needs to be done at a reduced rate
(every "s" bytes of excess traffic) otherwise far too much traffic
would be terminated.
Since flow termination is designed for "abnormal" circumstances, it
is quite likely that some PCN-nodes are congested and hence packets
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are being dropped and/or significantly queued. The flow termination
mechanism must bear this in mind.
Note also that the termination control decision is made for a
particular pair of PCN-boundary-nodes. So it is quite possible for
PCN-flows to be terminated between one pair of PCN-boundary-nodes,
whilst at the same time none are terminated between a different pair
of PCN-boundary-nodes.
6.3. Flow admission and flow termination when there are only two PCN
encoding states
If a PCN-domain has only two encoding states available (PCN-marked
and not PCN-marked), ie it's using the baseline encoding
[I-D.moncaster-pcn-baseline-encoding], then an operator has three
options:
o admission control only: PCN-marking means threshold-marking, ie
only the threshold-marking algorithm writes PCN-marks. Only PCN
admission control is available.
o flow termination only: PCN-marking means excess-traffic-marking,
ie only the excess-traffic-marking algorithm writes PCN-marks.
Only PCN termination control is available.
o both admission control and flow termination: only the excess-
traffic-marking algorithm writes PCN-marks, however the configured
rate (PCN-excess-rate) is set at the rate the admission control
mechanism needs to limit PCN-traffic to.
[I-D.charny-pcn-single-marking] describes how both admission
control and flow termination can be triggered in this case and
also gives some of the pros and cons of this approach. The main
downside is that admission control is less accurate.
6.4. Information transport
The transport of pre-congestion information from a PCN-node to a PCN-
egress-node is through PCN-markings in data packet headers, ie "in-
band": no signalling protocol messaging is needed. Signalling is
needed to transport PCN-feedback-information between the PCN-
boundary-nodes, for example to convey the fraction of PCN-marked
traffic from a PCN-egress-node to the relevant PCN-ingress-node.
Exactly what information needs to be transported will be described in
the future PCN WG document(s) about the boundary mechanisms. The
signalling could be done by an extension of RSVP or NSIS, for
instance; protocol work will be done by the relevant WG, but for
example [I-D.lefaucheur-rsvp-ecn] describes the extensions needed for
RSVP.
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6.5. PCN-traffic
The following are some high-level points about how PCN works:
o There needs to be a way for a PCN-node to distinguish PCN-traffic
from other traffic. This is through a combination of the DSCP
field and/or ECN field.
o The PCN mechanisms may be applied to more than one behaviour
aggregate which are distinguished by DSCP. However the current
PCN encodings, [I-D.moncaster-pcn-baseline-encoding] and
[I-D.moncaster-pcn-3-state-encoding], only allow one PCN-BA.
o There may be traffic that is more important than PCN, perhaps a
particular application or an operator's control messages. A PCN-
node may dedicate capacity to such traffic or priority schedule it
over PCN. In the latter case its traffic needs to contribute to
the PCN meters (ie be metered by the threshold-marking and excess-
traffic-marking algorithms).
o There may be other traffic that uses the same DSCP as PCN-traffic
but with the ECN field is 00 (Not ECT), and so not subject to PCN-
marking, nor PCN's admission control and flow termination
mechanisms.. To quote [I-D.moncaster-pcn-baseline-encoding]: "To
conserve DSCPs, DiffServ Codepoints SHOULD be chosen that are
already defined for use with admission controlled traffic, such as
the Voice-Admit codepoint defined in [voice-admit]." Since
scheduling behaviour is coupled with the DSCP only, therefore the
same scheduling and buffer management rules are applied to non-
PCN-traffic and PCN-traffic using the same PCN-enabled DSCP.
There may be no "non-PCN-traffic", but if there is it needs to
contribute to the PCN meters.
o There will be traffic less important than PCN. For instance best
effort or assured forwarding traffic. It will be scheduled at
lower priority than PCN, and use a separate queue or queues.
However, a PCN-node should dedicate some capacity to lower
priority traffic so that it isn't starved. Such traffic doesn't
contribute to the PCN meters.
6.6. Backwards compatibility
PCN specifies semantics for the ECN field that differ from the
default semantics of [RFC3168]. BCP124 [RFC4774] gives guidelines
for specifying alternative semantics for the ECN field. These are
discussed in the baseline encoding
[I-D.moncaster-pcn-baseline-encoding] and extended encoding
[I-D.moncaster-pcn-3-state-encoding] documents. In summary, PCN
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meets these guidelines by:
o using a DSCP (or two DSCPs in the extended encoding) to allow PCN-
nodes to distinguish PCN-traffic that uses the alternative ECN
semantics;
o defining these semantics for use within a controlled region, the
PCN-domain;
o taking appropriate action if ECN capable, non-PCN traffic arrives
at a PCN-ingress-node with the DSCP used by PCN.
The 'appropriate action' can differ in the case of baseline encoding
and extended encoding. In the former, ECN-capable traffic that uses
the same DSCP as PCN is blocked from entering the PCN-domain
directly. Blocking means it is dropped or downgraded to a lower
priority behaviour aggregate, or alternatively such traffic may be
tunnelled through the PCN-domain. The reason that blocking is needed
is that the PCN-egress-node clears the ECN field to 00. The extended
encoding adds support for end-to-end ECN, since the value of the ECN
field is preserved across the PCN-domain. However, PCN-packets that
get PCN-marked emerge from the PCN-domain with the ECN field set to
11 (CE). It may make sense to expose such marks to a rate adaptive
endpoint. However, it could violate [RFC4774] if the endpoint
doesn't understand ECN, and therefore the PCN-domain first needs to
ensure that the end-to-end transport is ECN capable (probably through
signalling).
7. Detailed Functional architecture
This section is intended to provide a systematic summary of the new
functional architecture in the PCN-domain. First it describes
functions needed at the three specific types of PCN-node; these are
data plane functions and are in addition to their normal router
functions. Then it describes further functionality needed for both
flow admission control and flow termination; these are signalling and
decision-making functions, and there are various possibilities for
where the functions are physically located. The section is split
into:
1. functions needed at PCN-interior-nodes
2. functions needed at PCN-ingress-nodes
3. functions needed at PCN-egress-nodes
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4. other functions needed for flow admission control
5. other functions needed for flow termination control
Note: Probing is covered in Appendix B.
The section then discusses some other detailed topics:
1. addressing
2. tunnelling
3. fault handling
7.1. PCN-interior-node functions
Each link of the PCN-domain is configured with the following
functionality:
o Packet classify - decide whether an incoming packet is a PCN-
packet or not.
o Packet condition - if the level if traffic is sufficiently high to
overload the PCN_BA, ie cause real congestion, then drop or
downgrade PCN-packets.
o Meter - measure the 'amount of PCN-traffic'. The measurement is
made as an aggregate of all PCN-packets, and not per flow.
o Mark - algorithms determine whether to PCN-mark PCN-packets and
what packet encoding is used.
The functions are specified in [I-D.eardley-pcn-marking-behaviour]
and the encodings in [I-D.moncaster-pcn-baseline-encoding] and
[I-D.moncaster-pcn-3-state-encoding].
7.2. PCN-ingress-node functions
Each ingress link of the PCN-domain is configured with the following
functionality:
o Packet classify - decide whether an incoming packet is part of a
previously admitted flow, by using a filter spec (eg DSCP, source
and destination addresses and port numbers).
o Police - police, by dropping or downgrading, any packets received
with a DSCP demanding PCN transport that do not belong to an
admitted flow. Similarly, police packets that are part of a
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previously admitted flow, to check that the flow keeps to the
agreed rate or flowspec (eg RFC1633 [RFC1633] for a microflow and
its NSIS equivalent).
o Packet colour - set the DSCP and ECN fields appropriately, see
[I-D.moncaster-pcn-baseline-encoding] or
[I-D.moncaster-pcn-3-state-encoding] as appropriate for the PCN-
domain.
o Meter - some approaches to flow termination require the PCN-
ingress-node to measure the (aggregate) rate of PCN-traffic
towards a particular PCN-egress-node.
The first two are policing functions, needed to make sure that PCN-
packets admitted into the PCN-domain belong to a flow that's been
admitted and to ensure that the flow keeps to the flowspec agreed (eg
doesn't go at a faster rate and is inelastic traffic). Installing
the filter spec will typically be done by the signalling protocol, as
will re-installing the filter, for example after a re-route that
changes the PCN-ingress-node (see [I-D.briscoe-tsvwg-cl-architecture]
for an example using RSVP). Packet colouring allows the rest of the
PCN-domain to recognise PCN-packets.
7.3. PCN-egress-node functions
Each egress link of the PCN-domain is configured with the following
functionality:
o Packet classify - determine which PCN-ingress-node a PCN-packet
has come from.
o Meter - "measure PCN-traffic" or "monitor PCN-marks".
o Packet colour - for PCN-packets, set the DSCP and ECN fields to
the appropriate values for use outside the PCN-domain.
The metering functionality of course depends on whether it is
targeted at admission control or flow termination. Alternative
proposals involve the PCN-egress-node "measuring" as an aggregate (ie
not per flow) all PCN-packets from a particular PCN-ingress-node, or
"monitoring" the PCN-traffic and reacting to one (or several) PCN-
marked packets.
7.4. Other admission control functions
As well as the functions covered above, other specific admission
control functions can be performed at a PCN-boundary-node (PCN-
ingress-node or PCN-egress-node) or at a centralised node, but not at
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normal PCN-interior-nodes. The functions are:
o Make decision about admission - based on the output of the PCN-
egress-node's PCN meter function. In the case where it "measures
PCN-traffic", the measured traffic on the ingress-egress-aggregate
is compared with some reference level. In the case where it
"monitors PCN-marks", then the decision is based on whether one
(or several) packets is (are) PCN-marked or not. In either case,
the admission decision also takes account of policy and
application layer requirements.
o Communicate decision about admission - signal the decision to the
node making the admission control request (which may be outside
the PCN-domain), and to the policer (PCN-ingress-node function)
for enforcement of the decision.
There are various possibilities for how the functionality can be
distributed (we assume the operator would configure which is used):
o The decision is made at the PCN-egress-node and the decision
(admit or block) is signalled to the PCN-ingress-node. This seems
most natural.
o The decision is made at the PCN-ingress-node, which requires that
the PCN-egress-node signals PCN-feedback-information to the PCN-
ingress-node. For example, it could signal the current fraction
of PCN-traffic that is PCN-marked.
o The decision is made at a centralised node (see Appendix A).
7.5. Other flow termination functions
Specific termination control functions can be performed at a PCN-
boundary-node (PCN-ingress-node or PCN-egress-node) or at a
centralised node, but not at normal PCN-interior-nodes. There are
various possibilities for how the functionality can be distributed,
similar to those discussed above in the Admission control section;
the flow termination decision could be made at the PCN-ingress-node,
the PCN-egress-node or at some centralised node. The functions are:
o PCN-meter at PCN-egress-node - similarly to flow admission, there
are two types of proposals: to "measure PCN-traffic" on the
ingress-egress-aggregate, and to "monitor PCN-marks" and react to
one (or several) PCN-marks.
o (if required) PCN-meter at PCN-ingress-node - make "measurements
of PCN-traffic" being sent towards a particular PCN-egress-node;
again, this is done for the ingress-egress-aggregate and not per
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flow.
o (if required) Communicate PCN-feedback-information to the node
that makes the flow termination decision. For example, as in
[I-D.briscoe-tsvwg-cl-architecture], communicate the PCN-egress-
node's measurements to the PCN-ingress-node.
o Make decision about flow termination - use the information from
the PCN-meter(s) to decide which PCN-flow or PCN-flows to
terminate. The decision takes account of policy and application
layer requirements.
o Communicate decision about flow termination - signal the decision
to the node that is able to terminate the flow (which may be
outside the PCN-domain), and to the policer (PCN-ingress-node
function) for enforcement of the decision.
7.6. Addressing
PCN-nodes may need to know the address of other PCN-nodes. Note: in
all cases PCN-interior-nodes don't need to know the address of any
other PCN-nodes (except as normal their next hop neighbours, for
routing purposes).
The PCN-egress-node needs to know the address of the PCN-ingress-node
associated with a flow, at a minimum so that the PCN-ingress-node can
be informed to enforce the admission decision (and any flow
termination decision) through policing. There are various
possibilities for how the PCN-egress-node can do this, ie associate
the received packet to the correct ingress-egress-aggregate. It is
not the intention of this document to mandate a particular mechanism.
o The addressing information can be gathered from signalling. For
example, regular processing of an RSVP Path message, as the PCN-
ingress-node is the previous RSVP hop (PHOP)
([I-D.lefaucheur-rsvp-ecn]). Or the PCN-ingress-node could signal
its address to the PCN-egress-node.
o Always tunnel PCN-traffic across the PCN-domain. Then the PCN-
ingress-node's address is simply the source address of the outer
packet header. The PCN-ingress-node needs to learn the address of
the PCN-egress-node, either by manual configuration or by one of
the automated tunnel endpoint discovery mechanisms (such as
signalling or probing over the data route, interrogating routing
or using a centralised broker).
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7.7. Tunnelling
Tunnels may originate and/or terminate within a PCN-domain. It is
important that the PCN-marking of any packet can potentially
influence PCN's flow admission control and termination - it shouldn't
matter whether the packet happens to be tunnelled at the PCN-node
that PCN-marks the packet, or indeed whether it's decapsulated or
encapsulated by a subsequent PCN-node. This suggests that the
"uniform conceptual model" described in [RFC2983] should be re-
applied in the PCN context. In line with this and the approach of
[RFC4303] and [I-D.briscoe-tsvwg-ecn-tunnel], the following rule is
applied if encapsulation is done within the PCN-domain:
o any PCN-marking is copied into the outer header
Similarly, in line with the "uniform conceptual model" of [RFC2983]
and the "full-functionality option" of [RFC3168], the following rule
is applied if decapsulation is done within the PCN-domain:
o if the outer header's marking state is more severe then it is
copied onto the inner header
o Note: the order of increasing severity is: not PCN-marked;
threshold-marking; excess-traffic-marking.
An operator may wish to tunnel PCN-traffic from PCN-ingress-nodes to
PCN-egress-nodes. The PCN-marks shouldn't be visible outside the
PCN-domain, which can be achieved by the PCN-egress-node doing the
packet colouring function (Section 7.3) after all the other (PCN and
tunnelling) functions. The potential reasons for doing such
tunnelling are: the PCN-egress-node then automatically knows the
address of the relevant PCN-ingress-node for a flow; even if ECMP is
running, all PCN-packets on a particular ingress-egress-aggregate
follow the same path. But it also has drawbacks, for example the
additional overhead in terms of bandwidth and processing, and the
cost of setting up a mesh of tunnels between PCN-boundary-nodes
(there is an N^2 scaling issue).
Potential issues arise for a "partially PCN-capable tunnel", ie where
only one tunnel endpoint is in the PCN domain:
1. The tunnel starts outside a PCN-domain and finishes inside it.
If the packet arrives at the tunnel ingress with the same
encoding as used within the PCN-domain to indicate PCN-marking,
then this could lead the PCN-egress-node to falsely measure pre-
congestion.
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2. The tunnel starts inside a PCN-domain and finishes outside it.
If the packet arrives at the tunnel ingress already PCN-marked,
then it will still have the same encoding when it's decapsulated
which could potentially confuse nodes beyond the tunnel egress.
In line with the solution for partially capable DiffServ tunnels in
[RFC2983], the following rules are applied:
o For case (1), the tunnel egress node clears any PCN-marking on the
inner header. This rule is applied before the 'copy on
decapsulation' rule above.
o For case (2), the tunnel ingress node clears any PCN-marking on
the inner header. This rule is applied after the 'copy on
encapsulation' rule above.
Note that the above implies that one has to know, or figure out, the
characteristics of the other end of the tunnel as part of setting it
up.
Tunnelling constraints were a major factor in the choice of encoding,
as explained in [I-D.moncaster-pcn-baseline-encoding] and
[I-D.moncaster-pcn-3-state-encoding]. A lengthy discussion of all
the issues associated with layered encapsulation of congestion
notification (for ECN as well as PCN) is in
[I-D.briscoe-tsvwg-ecn-tunnel].
7.8. Fault handling
If a PCN-interior-node fails (or one of its links), then lower layer
protection mechanisms or the regular IP routing protocol will
eventually re-route round it. If the new route can carry all the
admitted traffic, flows will gracefully continue. If instead this
causes early warning of pre-congestion on the new route, then
admission control based on pre-congestion notification will ensure
new flows will not be admitted until enough existing flows have
departed. Re-routing may result in heavy (pre-)congestion, when the
flow termination mechanism will kick in.
If a PCN-boundary-node fails then we would like the regular QoS
signalling protocol to take care of things. As an example
[I-D.briscoe-tsvwg-cl-architecture] considers what happens if RSVP is
the QoS signalling protocol.
8. Design goals and challenges
Prior work on PCN and similar mechanisms has thrown up a number of
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considerations about PCN's design goals (things PCN should be good
at) and some issues that have been hard to solve in a fully
satisfactory manner. Taken as a whole it represents a list of trade-
offs (it's unlikely that they can all be 100% achieved) and perhaps
as evaluation criteria to help an operator (or the IETF) decide
between options.
The following are key design goals for PCN (based on
[I-D.chan-pcn-problem-statement]):
o The PCN-enabled packet forwarding network should be simple,
scalable and robust
o Compatibility with other traffic (ie a proposed solution should
work well when non-PCN traffic is also present in the network)
o Support of different types of real-time traffic (eg should work
well with CBR and VBR voice and video sources treated together)
o Reaction time of the mechanisms should be commensurate with the
desired application-level requirements (eg a termination mechanism
needs to terminate flows before significant QoS issues are
experienced by real-time traffic, and before most users hang up).
o Compatibility with different precedence levels of real-time
applications (eg preferential treatment of higher precedence calls
over lower precedence calls, [ITU-MLPP]).
The following are open issues. They are mainly taken from
[I-D.briscoe-tsvwg-cl-architecture] which also describes some
possible solutions. Note that some may be considered unimportant in
general or in specific deployment scenarios or by some operators.
NOTE: Potential solutions are out of scope for this document.
o ECMP (Equal Cost Multi-Path) Routing: The level of pre-congestion
is measured on a specific ingress-egress-aggregate. However, if
the PCN-domain runs ECMP, then traffic on this ingress-egress-
aggregate may follow several different paths - some of the paths
could be pre-congested whilst others are not. There are three
potential problems:
1. over-admission: a new flow is admitted (because the pre-
congestion level measured by the PCN-egress-node is
sufficiently diluted by unmarked packets from non-congested
paths that a new flow is admitted), but its packets travel
through a pre-congested PCN-node
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2. under-admission: a new flow is blocked (because the pre-
congestion level measured by the PCN-egress-node is
sufficiently increased by PCN-marked packets from pre-
congested paths that a new flow is blocked), but its packets
travel along an uncongested path
3. ineffective termination: flows are terminated, however their
path doesn't travel through the (pre-)congested router(s).
Since flow termination is a 'last resort' that protects the
network should over-admission occur, this problem is probably
more important to solve than the other two.
o ECMP and signalling: It is possible that, in a PCN-domain running
ECMP, the signalling packets (eg RSVP, NSIS) follow a different
path than the data packets, which could matter if the signalling
packets are used as probes. Whether this is an issue depends on
which fields the ECMP algorithm uses; if the ECMP algorithm is
restricted to the source and destination IP addresses, then it
won't be. ECMP and signalling interactions are a specific
instance of a general issue for non-traditional routing combined
with resource management along a path [Hancock].
o Tunnelling: There are scenarios where tunnelling makes it hard to
determine the path in the PCN-domain. The problem, its impact and
the potential solutions are similar to those for ECMP.
o Scenarios with only one tunnel endpoint in the PCN domain may make
it harder for the PCN-egress-node to gather from the signalling
messages (eg RSVP, NSIS) the identity of the PCN-ingress-node.
o Bi-Directional Sessions: Many applications have bi-directional
sessions - hence there are two flows that should be admitted (or
terminated) as a pair - for instance a bi-directional voice call
only makes sense if flows in both directions are admitted.
However, PCN's mechanisms concern admission and termination of a
single flow, and coordination of the decision for both flows is a
matter for the signalling protocol and out of scope of PCN. One
possible example would use SIP pre-conditions; there are others.
o Global Coordination: PCN makes its admission decision based on
PCN-markings on a particular ingress-egress-aggregate. Decisions
about flows through a different ingress-egress-aggregate are made
independently. However, one can imagine network topologies and
traffic matrices where, from a global perspective, it would be
better to make a coordinated decision across all the ingress-
egress-aggregates for the whole PCN-domain. For example, to block
(or even terminate) flows on one ingress-egress-aggregate so that
more important flows through a different ingress-egress-aggregate
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could be admitted. The problem may well be second order.
o Aggregate Traffic Characteristics: Even when the number of flows
is stable, the traffic level through the PCN-domain will vary
because the sources vary their traffic rates. PCN works best when
there's not too much variability in the total traffic level at a
PCN-node's interface (ie in the aggregate traffic from all
sources). Too much variation means that a node may (at one
moment) not be doing any PCN-marking and then (at another moment)
drop packets because it's overloaded. This makes it hard to tune
the admission control scheme to stop admitting new flows at the
right time. Therefore the problem is more likely with fewer,
burstier flows.
o Flash crowds and Speed of Reaction: PCN is a measurement-based
mechanism and so there is an inherent delay between packet marking
by PCN-interior-nodes and any admission control reaction at PCN-
boundary-nodes. For example, potentially if a big burst of
admission requests occurs in a very short space of time (eg
prompted by a televote), they could all get admitted before enough
PCN-marks are seen to block new flows. In other words, any
additional load offered within the reaction time of the mechanism
mustn't move the PCN-domain directly from no congestion to
overload. This 'vulnerability period' may impact at the
signalling level, for instance QoS requests should be rate limited
to bound the number of requests able to arrive within the
vulnerability period.
o Silent at start: after a successful admission request the source
may wait some time before sending data (eg waiting for the called
party to answer). Then the risk is that, in some circumstances,
PCN's measurements underestimate what the pre-congestion level
will be when the source does start sending data.
9. Operations and Management
This Section considers operations and management issues, under the
FCAPS headings: OAM of Faults, Configuration, Accounting, Performance
and Security. Provisioning is discussed with performance.
9.1. Configuration OAM
This architecture document predates the detailed standards actions of
the PCN WG. Here we assume that only inter-operable PCN-marking
behaviours will be standardised, otherwise we would have to consider
how to avoid interactions between non inter-operable marking
behaviours. However, more diversity in PCN-boundary-node behaviours
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is expected, in order to interface with diverse industry
architectures. It may be possible to have different PCN-boundary-
node behaviours for different ingress-egress-aggregates within the
same PCN-domain.
PCN functionality is configured on either the egress or the ingress
interfaces of PCN-nodes. A consistent choice must be made across the
PCN-domain to ensure that the PCN mechanisms protect all links.
PCN configuration control variables fall into the following
categories:
o system options (enabling or disabling behaviours)
o parameters (setting levels, addresses etc)
One possibility is that all configurable variables sit within an SNMP
management framework [RFC3411], being structured within a defined
management information base (MIB) on each node, and being remotely
readable and settable via a suitably secure management protocol
(SNMPv3).
Some configuration options and parameters have to be set once to
'globally' control the whole PCN-domain. Where possible, these are
identified below. This may affect operational complexity and the
chances of interoperability problems between kit from different
vendors.
It may be possible for an operator to configure some PCN-interior-
nodes so they don't run the PCN mechanisms, if it knows that these
links will never become (pre-)congested.
9.1.1. System options
On PCN-interior-nodes there will be very few system options:
o Whether two PCN-markings (threshold-marked and excess-traffic-
marked) are enabled or only one. Typically all nodes throughout a
PCN-domain will be configured the same in this respect. However,
exceptions could be made. For example, if most PCN-nodes used
both markings, but some legacy hardware was incapable of running
two algorithms, an operator might be willing to configure these
legacy nodes solely for excess-traffic-marking to enable flow
termination as a back-stop. It would be sensible to place such
nodes where they could be provisioned with a greater leeway over
expected traffic levels.
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o what marking algorithm to use, if an equipment vendor provides a
choice.
PCN-boundary-nodes (ingress and egress) will have more system
options:
o Which of admission and flow termination are enabled. If any PCN-
interior-node is configured to generate a marking, all PCN-
boundary-nodes must be able to handle that marking. Therefore all
PCN-boundary-nodes must be configured the same in this respect.
o Where flow admission and termination decisions are made: at the
PCN-ingress-node, PCN-egress-node or at a centralised node (see
Section 7). Theoretically, this configuration choice could be
negotiated for each pair of PCN-boundary-nodes, but we cannot
imagine why such complexity would be required, except perhaps in
future inter-domain scenarios.
PCN-egress-nodes will have further system options:
o How the mapping should be established between each packet and its
aggregate, eg by MPLS label, by IP packet filterspec; and how to
take account of ECMP.
o If an equipment vendor provides a choice, there may be options to
select which smoothing algorithm to use for measurements.
9.1.2. Parameters
Like any DiffServ domain, every node within a PCN-domain will need to
be configured with the DSCP(s) used to identify PCN-packets. On each
interior link the main configuration parameters are the PCN-
threshold-rate and PCN-excess-rate. A larger PCN-threshold-rate
enables more PCN-traffic to be admitted on a link, hence improving
capacity utilisation. A PCN-excess-rate set further above the PCN-
threshold-rate allows greater increases in traffic (whether due to
natural fluctuations or some unexpected event) before any flows are
terminated, ie minimises the chances of unnecessarily triggering the
termination mechanism. For instance an operator may want to design
their network so that it can cope with a failure of any single PCN-
node without terminating any flows.
Setting these rates on first deployment of PCN will be very similar
to the traditional process for sizing an admission controlled
network, depending on: the operator's requirements for minimising
flow blocking (grade of service), the expected PCN traffic load on
each link and its statistical characteristics (the traffic matrix),
contingency for re-routing the PCN traffic matrix in the event of
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single or multiple failures and the expected load from other classes
relative to link capacities [Menth]. But once a domain is up and
running, a PCN design goal is to be able to determine growth in these
configured rates much more simply, by monitoring PCN-marking rates
from actual rather than expected traffic (see Section 9.2 on
Performance & Provisioning).
Operators may also wish to configure a rate greater than the PCN-
excess-rate that is the absolute maximum rate that a link allows for
PCN-traffic. This may simply be the physical link rate, but some
operators may wish to configure a logical limit to prevent starvation
of other traffic classes during any brief period after PCN-traffic
exceeds the PCN-excess-rate but before flow termination brings it
back below this rate.
Specific marking algorithms will also depend on further configuration
parameters. For instance, threshold-marking will require a threshold
queue depth and excess-traffic-marking may require a scaling
parameter. It will be preferable for each marking algorithm to have
rules to set defaults for these parameters relative to the reference
marking rate, but then allow operators to change them, for instance
if average traffic characteristics change over time. The PCN-egress-
node may allow configuration of the following:
o how it smooths metering of PCN-markings (eg EWMA parameters)
Whichever node makes admission and flow termination decisions will
contain algorithms for converting PCN-marking levels into admission
or flow termination decisions. These will also require configurable
parameters, for instance:
o any admission control algorithm will at least require a marking
threshold setting above which it denies admission to new flows;
o flow termination algorithms will probably require a parameter to
delay termination of any flows until it is more certain that an
anomalous event is not transient;
o a parameter to control the trade-off between how quickly excess
flows are terminated and over-termination.
One particular proposal, [I-D.charny-pcn-single-marking] would
require a global parameter to be defined on all PCN-nodes, but only
needs one PCN marking rate to be configured on each link. The global
parameter is a scaling factor between admission and termination, for
example the amount by which the PCN-excess-rate is implicitly assumed
to be above the PCN-threshold-rate. [I-D.charny-pcn-single-marking]
discusses in full the impact of this particular proposal on the
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operation of PCN.
9.2. Performance & Provisioning OAM
Monitoring of performance factors measurable from *outside* the PCN
domain will be no different with PCN than with any other packet-based
flow admission control system, both at the flow level (blocking
probability etc) and the packet level (jitter [RFC3393], [Y.1541],
loss rate [RFC4656], mean opinion score [P.800], etc). The
difference is that PCN is intentionally designed to indicate
*internally* which exact resource(s) are the cause of performance
problems and by how much.
Even better, PCN indicates which resources will probably cause
problems if they are not upgraded soon. This can be achieved by the
management system monitoring the total amount (in bytes) of PCN-
marking generated by each queue over a period. Given possible long
provisioning lead times, pre-congestion volume is the best metric to
reveal whether sufficient persistent demand has mounted up to warrant
an upgrade. Because, even before utilisation becomes problematic,
the statistical variability of traffic will cause occasional bursts
of pre-congestion. This 'early warning system' decouples the process
of adding customers from the provisioning process. This should cut
the time to add a customer when compared against admission control
provided over native DiffServ [RFC2998], because it saves having to
re-run the capacity planning process before adding each customer.
Alternatively, before triggering an upgrade, the long term pre-
congestion volume on each link can be used to balance traffic load
across the PCN-domain by adjusting the link weights of the routing
system. When an upgrade to a link's configured PCN-rates is
required, it may also be necessary to upgrade the physical capacity
available to other classes. But usually there will be sufficient
physical capacity for the upgrade to go ahead as a simple
configuration change. Alternatively, [Songhurst] has proposed an
adaptive rather than preconfigured system, where the configured PCN-
threshold-rate is replaced with a high and low water mark and the
marking algorithm automatically optimises how physical capacity is
shared using the relative loads from PCN and other traffic classes.
All the above processes require just three extra counters associated
with each PCN queue: threshold-markings, excess-traffic-markings and
drop. Every time a PCN packet is marked or dropped its size in bytes
should be added to the appropriate counter. Then the management
system can read the counters at any time and subtract a previous
reading to establish the incremental volume of each type of
(pre-)congestion. Readings should be taken frequently, so that
anomalous events (eg re-routes) can be separated from regular
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fluctuating demand if required.
9.3. Accounting OAM
Accounting is only done at trust boundaries so it is out of scope of
the initial Charter of the PCN WG which is confined to intra-domain
issues. Use of PCN internal to a domain makes no difference to the
flow signalling events crossing trust boundaries outside the PCN-
domain, which are typically used for accounting.
9.4. Fault OAM
Fault OAM is about preventing faults, telling the management system
(or manual operator) that the system has recovered (or not) from a
failure, and about maintaining information to aid fault diagnosis.
Admission blocking and particularly flow termination mechanisms
should rarely be needed in practice. It would be unfortunate if they
didn't work after an option had been accidentally disabled.
Therefore it will be necessary to regularly test that the live system
works as intended (devising a meaningful test is left as an exercise
for the operator).
Section 7 describes how the PCN architecture has been designed to
ensure admitted flows continue gracefully after recovering
automatically from link or node failures. The need to record and
monitor re-routing events affecting signalling is unchanged by the
addition of PCN to a DiffServ domain. Similarly, re-routing events
within the PCN-domain will be recorded and monitored just as they
would be without PCN.
PCN-marking does make it possible to record 'near-misses'. For
instance, at the PCN-egress-node a 'reporting threshold' could be set
to monitor how often - and for how long - the system comes close to
triggering flow blocking without actually doing so. Similarly,
bursts of flow termination marking could be recorded even if they are
not sufficiently sustained to trigger flow termination. Such
statistics could be correlated with per-queue counts of marking
volume (Section 9.2) to upgrade resources in danger of causing
service degradation, or to trigger manual tracing of intermittent
incipient errors that would otherwise have gone unnoticed.
Finally, of course, many faults are caused by failings in the
management process ('human error'): a wrongly configured address in a
node, a wrong address given in a signalling protocol, a wrongly
configured parameter in a queueing algorithm, a node set into a
different mode from other nodes, and so on. Generally, a clean
design with few configurable options ensures this class of faults can
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be traced more easily and prevented more often. Sound management
practice at run-time also helps. For instance: a management system
should be used that constrains configuration changes within system
rules (eg preventing an option setting inconsistent with other
nodes); configuration options should also be recorded in an offline
database; and regular automatic consistency checks between live
systems and the database. PCN adds nothing specific to this class of
problems. By the time standards are in place, we expect that the PCN
WG will have ruthlessly removed gratuitous configuration choices.
However, at the time of writing, the WG is yet to choose between
multiple competing proposals, so the range of possible options in
Section 9.1 does seem rather wide compared to the original near-zero
configuration intent of the architecture.
9.5. Security OAM
Security OAM is about using secure operational practices as well as
being able to track security breaches or near-misses at run-time.
PCN adds few specifics to the general good practice required in this
field [RFC4778], other than those below. The correct functions of
the system should be monitored (Section 9.2) in multiple independent
ways and correlated to detect possible security breaches. Persistent
(pre-)congestion marking should raise an alarm (both on the node
doing the marking and on the PCN-egress-node metering it).
Similarly, persistently poor external QoS metrics such as jitter or
MOS should raise an alarm. The following are examples of symptoms
that may be the result of innocent faults, rather than attacks, but
until diagnosed they should be logged and trigger a security alarm:
o Anomalous patterns of non-conforming incoming signals and packets
rejected at the PCN-ingress-nodes (eg packets already marked PCN-
capable, or traffic persistently starving token bucket policers).
o PCN-capable packets arriving at a PCN-egress-node with no
associated state for mapping them to a valid ingress-egress-
aggregate.
o A PCN-ingress-node receiving feedback signals about the pre-
congestion level on a non-existent aggregate, or that are
inconsistent with other signals (eg unexpected sequence numbers,
inconsistent addressing, conflicting reports of the pre-congestion
level, etc).
o Pre-congestion marking arriving at an PCN-egress-node with
(pre-)congestion markings focused on particular flows, rather than
randomly distributed throughout the aggregate.
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10. IANA Considerations
This memo includes no request to IANA.
11. Security considerations
Security considerations essentially come from the Trust Assumption
(Section 5.1), ie that all PCN-nodes are PCN-enabled and trust each
other for truthful PCN-marking and transport. PCN splits
functionality between PCN-interior-nodes and PCN-boundary-nodes, and
the security considerations are somewhat different for each, mainly
because PCN-boundary-nodes are flow-aware and PCN-interior-nodes are
not.
o Because the PCN-boundary-nodes are flow-aware, they are trusted to
use that awareness correctly. The degree of trust required
depends on the kinds of decisions they have to make and the kinds
of information they need to make them. For example when the PCN-
boundary-node needs to know the contents of the sessions for
making the admission and termination decisions, or when the
contents are highly classified, then the security requirements for
the PCN-boundary-nodes involved will also need to be high.
o the PCN-ingress-nodes police packets to ensure a PCN-flow sticks
within its agreed limit, and to ensure that only PCN-flows which
have been admitted contribute PCN-traffic into the PCN-domain.
The policer must drop (or perhaps downgrade to a different DSCP)
any PCN-packets received that are outside this remit. This is
similar to the existing IntServ behaviour. Between them the PCN-
boundary-nodes must encircle the PCN-domain, otherwise PCN-packets
could enter the PCN-domain without being subject to admission
control, which would potentially destroy the QoS of existing
flows.
o PCN-interior-nodes aren't flow-aware. This prevents some security
attacks where an attacker targets specific flows in the data plane
- for instance for DoS or eavesdropping.
o PCN-marking by the PCN-interior-nodes along the packet forwarding
path needs to be trusted, because the PCN-boundary-nodes rely on
this information. For instance a rogue PCN-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's pre-congested. More subtly, the rogue PCN-interior-node
could perform these attacks selectively on particular flows, or it
could PCN-mark the correct fraction overall, but carefully choose
which flows it marked.
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o the PCN-boundary-nodes should be able to deal with DoS attacks and
state exhaustion attacks based on fast changes in per flow
signalling.
o the signalling between the PCN-boundary-nodes (and possibly a
central control node) must be protected from attacks. For example
the recipient needs to validate that the message is indeed from
the node that claims to have sent it. Possible measures include
digest authentication and protection against replay and man-in-
the-middle attacks. For the specific protocol RSVP, hop-by-hop
authentication is in [RFC2747], and
[I-D.behringer-tsvwg-rsvp-security-groupkeying] may also be
useful; for a generic signalling protocol the PCN WG document on
"Requirements for signalling" will describe the requirements in
more detail.
Operational security advice is given in Section 9.5.
12. Conclusions
The document 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 main topic is the functional architecture. It
also mentions other topics like the assumptions and open issues.
13. Acknowledgements
This document is a revised version of [I-D.eardley-pcn-architecture].
Its authors were: P. Eardley, J. Babiarz, K. Chan, A. Charny, R.
Geib, G. Karagiannis, M. Menth, T. Tsou. They are therefore
contributors to this document.
Thanks to those who've made comments on
[I-D.eardley-pcn-architecture] and on earlier versions of this draft:
Lachlan Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake,
Bob Briscoe, Jason Canon, Ken Carlberg, Anna Charny, Joachim
Charzinski, Andras Csaszar, Lars Eggert, Ruediger Geib, Wei Gengyu,
Robert Hancock, Ingemar Johansson, Georgios Karagiannis, Michael
Menth, Toby Moncaster, Ben Strulo, Tom Taylor, Hannes Tschofenig,
Tina Tsou, Lars Westberg, Magnus Westerlund, Delei Yu. Thanks to Bob
Briscoe who extensively revised the Operations and Management
section.
This document is the result of discussions in the PCN WG and
forerunner activity in the TSVWG. A number of previous drafts were
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presented to TSVWG: [I-D.chan-pcn-problem-statement],
[I-D.briscoe-tsvwg-cl-architecture], [I-D.briscoe-tsvwg-cl-phb],
[I-D.charny-pcn-single-marking], [I-D.babiarz-pcn-sip-cap],
[I-D.lefaucheur-rsvp-ecn], [I-D.westberg-pcn-load-control]. The
authors of them were: B, Briscoe, P. Eardley, D. Songhurst, F. Le
Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley, G. Karagiannis,
A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G. Liu, A. Bhargava.
14. Comments Solicited
Comments and questions are encouraged and very welcome. They can be
addressed to the IETF PCN working group mailing list <pcn@ietf.org>.
15. Changes
15.1. Changes from -03 to -04
o Minor changes throughout to reflect the consenus call about PCN-
marking (as reflected in [I-D.eardley-pcn-marking-behaviour]).
o Minor changes throughout to reflect the current decisions about
encoding (as reflected in [I-D.moncaster-pcn-baseline-encoding]and
[I-D.moncaster-pcn-3-state-encoding]).
o Introduction: re-structured to create new sections on Benefits,
Deployment scenarios and Assumptions.
o Introduction: Added pointers to other PCN documents.
o Terminology: changed PCN-lower-rate to PCN-threshold-rate and PCN-
upper-rate to PCN-excess-rate; excess-rate-marking to excess-
traffic-marking.
o Benefits: added bullet about SRLGs.
o Deployment scenarios: new section combining material from various
places within the document.
o S6 (high level functional architecture): re-structured and edited
to improve clarity, and reflect the latest PCN-marking and
encoding drafts.
o S6.4: added claim that the most natural place to make an admission
decision is a PCN-egress-node.
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o S6.5: updated the bullet about non-PCN-traffic that uses the same
DSCP as PCN-traffic.
o S6.6: added a section about backwards compatibility with respect
to [RFC4774].
o Appendix A: added bullet about end-to-end PCN.
o Probing: moved to Appendix B.
o Other minor clarifications, typos etc.
15.2. Changes from -02 to -03
o Abstract: Clarified by removing the term 'aggregated'. Follow-up
clarifications later in draft: S1: expanded PCN-egress-nodes
bullet to mention case where the PCN-feedback-information is about
one (or a few) PCN-marks, rather than aggregated information; S3
clarified PCN-meter; S5 minor changes; conclusion.
o S1: added a paragraph about how the PCN-domain looks to the
outside world (essentially it looks like a DiffServ domain).
o S2: tweaked the PCN-traffic terminology bullet: changed PCN
traffic classes to PCN behaviour aggregates, to be more in line
with traditional DiffServ jargon (-> follow-up changes later in
draft); included a definition of PCN-flows (and corrected a couple
of 'PCN microflows' to 'PCN-flows' later in draft)
o S3.5: added possibility of downgrading to best effort, where PCN-
packets arrive at PCN-ingress-node already ECN marked (CE or ECN
nonce)
o S4: added note about whether talk about PCN operating on an
interface or on a link. In S8.1 (OAM) mentioned that PCN
functionality needs to be configured consistently on either the
ingress or the egress interface of PCN-nodes in a PCN-domain.
o S5.2: clarified that signalling protocol installs flow filter spec
at PCN-ingress-node (& updates after possible re-route)
o S5.6: addressing: clarified
o S5.7: added tunnelling issue of N^2 scaling if you set up a mesh
of tunnels between PCN-boundary-nodes
o S7.3: Clarified the "third viewpoint" of probing (always probe).
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o S8.1: clarified that SNMP is only an example; added note that an
operator may be able to not run PCN on some PCN-interior-nodes, if
it knows that these links will never become (pre-)congested; added
note that it may be possible to have different PCN-boundary-node
behaviours for different ingress-egress-aggregates within the same
PCN-domain.
o Appendix: Created an Appendix about "Possible work items beyond
the scope of the current PCN WG Charter". Material moved from
near start of S3 and elsewhere throughout draft. Moved text about
centralised decision node to Appendix.
o Other minor clarifications.
15.3. Changes from -01 to -02
o S1: Benefits: provisioning bullet extended to stress that PCN does
not use RFC2475-style traffic conditioning.
o S1: Deployment models: mentioned, as variant of PCN-domain
extending to end nodes, that may extend to LAN edge switch.
o S3.1: Trust Assumption: added note about not needing PCN-marking
capability if known that an interface cannot become pre-congested.
o S4: now divided into sub-sections
o S4.1: Admission control: added second proposed method for how to
decide to block new flows (PCN-egress-node receives one (or
several) PCN-marked packets).
o S5: Probing sub-section removed. Material now in new S7.
o S5.6: Addressing: clarified how PCN-ingress-node can discover
address of PCN-egress-node
o S5.6: Addressing: centralised node case, added that PCN-ingress-
node may need to know address of PCN-egress-node
o S5.8: Tunnelling: added case of "partially PCN-capable tunnel" and
degraded bullet on this in S6 (Open Issues)
o S7: Probing: new section. Much more comprehensive than old S5.5.
o S8: Operations and Management: substantially revised.
o other minor changes not affecting semantics
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15.4. Changes from -00 to -01
In addition to clarifications and nit squashing, the main changes
are:
o S1: Benefits: added one about provisioning (and contrast with
DiffServ SLAs)
o S1: Benefits: clarified that the objective is also to stop PCN-
packets being significantly delayed (previously only mentioned not
dropping packets)
o S1: Deployment models: added one where policing is done at ingress
of access network and not at ingress of PCN-domain (assume trust
between networks)
o S1: Deployment models: corrected MPLS-TE to MPLS
o S2: Terminology: adjusted definition of PCN-domain
o S3.5: Other assumptions: corrected, so that two assumptions (PCN-
nodes not performing ECN and PCN-ingress-node discarding arriving
CE packet) only apply if the PCN WG decides to encode PCN-marking
in the ECN-field.
o S4 & S5: changed PCN-marking algorithm to marking behaviour
o S4: clarified that PCN-interior-node functionality applies for
each outgoing interface, and added clarification: "The
functionality is also done by PCN-ingress-nodes for their outgoing
interfaces (ie those 'inside' the PCN-domain)."
o S4 (near end): altered to say that a PCN-node "should" dedicate
some capacity to lower priority traffic so that it isn't starved
(was "may")
o S5: clarified to say that PCN functionality is done on an
'interface' (rather than on a 'link')
o S5.2: deleted erroneous mention of service level agreement
o S5.5: Probing: re-written, especially to distinguish probing to
test the ingress-egress-aggregate from probing to test a
particular ECMP path.
o S5.7: Addressing: added mention of probing; added that in the case
where traffic is always tunnelled across the PCN-domain, add a
note that he PCN-ingress-node needs to know the address of the
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PCN-egress-node.
o S5.8: Tunnelling: re-written, especially to provide a clearer
description of copying on tunnel entry/exit, by adding explanation
(keeping tunnel encaps/decaps and PCN-marking orthogonal),
deleting one bullet ("if the inner header's marking state is more
sever then it is preserved" - shouldn't happen), and better
referencing of other IETF documents.
o S6: Open issues: stressed that "NOTE: Potential solutions are out
of scope for this document" and edited a couple of sentences that
were close to solution space.
o S6: Open issues: added one about scenarios with only one tunnel
endpoint in the PCN domain .
o S6: Open issues: ECMP: added under-admission as another potential
risk
o S6: Open issues: added one about "Silent at start"
o S10: Conclusions: a small conclusions section added
16. Appendix A: Possible work items beyond the scope of the current PCN
WG Charter
This section mentions some topics that are outside the PCN WG's
current Charter, but which have been mentioned as areas of interest.
They might be work items for: the PCN WG after a future re-
chartering; some other IETF WG; another standards body; an operator-
specific usage that's not standardised.
NOTE: it should be crystal clear that this section discusses
possibilities only.
The first set of possibilities relate to the restrictions on scope
imposed by the PCN WG Charter (see Section 3):
o a single PCN-domain encompasses several autonomous systems that
don't trust each other (perhaps by using a mechanism like re-ECN,
[I-D.briscoe-re-pcn-border-cheat].
o not all the nodes run PCN. For example, the PCN-domain is a
multi-site enterprise network. The sites are connected by a VPN
tunnel; although PCN doesn't operate inside the tunnel, the PCN
mechanisms still work properly because the of the good QoS on the
virtual link (the tunnel). Another example is that PCN is
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deployed on the general Internet (ie widely but not universally
deployed).
o applying the PCN mechanisms to other types of traffic, ie beyond
inelastic traffic. For instance, applying the PCN mechanisms to
traffic scheduled with the Assured Forwarding per-hop behaviour.
One example could be flow-rate adaptation by elastic applications,
that adapts according to the pre-congestion information.
o the aggregation assumption doesn't hold, because the link capacity
is too low. Measurement-based admission control is then risky.
o the applicability of PCN mechanisms for emergency use (911, GETS,
WPS, MLPP, etc.)
Other possibilities include:
o The PCN-domain extends to the end users. The scenario is
described in [I-D.babiarz-pcn-sip-cap]. The end users need to be
trusted to do their own policing. This scenario is in the scope
of the PCN WG charter if there is sufficient traffic for the
aggregation assumption to hold. A variant is that the PCN-domain
extends out as far as the LAN edge switch.
o indicating pre-congestion through signalling messages rather than
in-band (in the form of PCN-marked packets)
o the decision-making functionality is at a centralised node rather
than at the PCN-boundary-nodes. This requires that the PCN-
egress-node signals PCN-feedback-information to the centralised
node, and that the centralised node signals to the PCN-ingress-
node about the decision about admission (or termination). It may
also need the centralised node and the PCN-boundary-nodes to know
each other's addresses. It would be possible for the centralised
node to be one of the PCN-boundary-nodes, when clearly the
signalling would sometimes be replaced by a message internal to
the node.
o Signalling extensions for specific protocols (eg RSVP, NSIS). For
example: the details of how the signalling protocol installs the
flowspec at the PCN-ingress-node for an admitted PCN-flow; and how
the signalling protocol carries the PCN-feedback-information.
Perhaps also for other functions such as: coping with failure of a
PCN-boundary-node ([I-D.briscoe-tsvwg-cl-architecture] considers
what happens if RSVP is the QoS signalling protocol); establishing
a tunnel across the PCN-domain if it is necessary to carry ECN
marks transparently. Note: There is a PCN WG Milestone on
"Requirements for signalling", which is potential input for the
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appropriate WGs.
o Policing by the PCN-ingress-node may not be needed if the PCN-
domain can trust that the upstream network has already policed the
traffic on its behalf.
o PCN for Pseudowire: PCN may be used as a congestion avoidance
mechanism for edge to edge pseudowire emulations
[I-D.ietf-pwe3-congestion-frmwk].
o PCN for MPLS: [RFC3270] defines how to support the DiffServ
architecture in MPLS networks. [RFC5129] describes how to add PCN
for admission control of microflows into a set of MPLS aggregates
(Multi-protocol label switching). PCN-marking is done in MPLS's
EXP field (which [I-D.andersson-mpls-expbits-def] proposes to re-
name to the Class of Service (CoS) bits).
o PCN for Ethernet: Similarly, it may be possible to extend PCN into
Ethernet networks, where PCN-marking is done in the Ethernet
header. NOTE: Specific consideration of this extension is outside
the IETF's remit.
.
17. Appendix B: Probing
17.1. Introduction
Probing is an optional mechanism to assist admission control.
PCN's admission control, as described so far, is essentially a
reactive mechanism where the PCN-egress-node monitors the pre-
congestion level for traffic from each PCN-ingress-node; if the level
rises then it blocks new flows on that ingress-egress-aggregate.
However, it's possible that an ingress-egress-aggregate carries no
traffic, and so the PCN-egress-node can't make an admission decision
using the usual method described earlier.
One approach is to be "optimistic" and simply admit the new flow.
However it's possible to envisage a scenario where the traffic levels
on other ingress-egress-aggregates are already so high that they're
blocking new PCN-flows, and admitting a new flow onto this 'empty'
ingress-egress-aggregate adds extra traffic onto the link that's
already pre-congested - which may 'tip the balance' so that PCN's
flow termination mechanism is activated or some packets are dropped.
This risk could be lessened by configuring on each link sufficient
'safety margin' above the PCN-threshold-rate.
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An alternative approach is to make PCN a more proactive mechanism.
The PCN-ingress-node explicitly determines, before admitting the
prospective new flow, whether the ingress-egress-aggregate can
support it. This can be seen as a "pessimistic" approach, in
contrast to the "optimism" of the approach above. It involves
probing: a PCN-ingress-node generates and sends probe packets in
order to test the pre-congestion level that the flow would
experience.
One possibility is that a probe packet is just a dummy data packet,
generated by the PCN-ingress-node and addressed to the PCN-egress-
node. Another possibility is that a probe packet is a signalling
packet that is anyway travelling from the PCN-ingress-node to the
PCN-egress-node (eg an RSVP PATH message travelling from source to
destination).
17.2. Probing functions
The probing functions are:
o Make decision that probing is needed. As described above, this is
when the ingress-egress-aggregate (or the ECMP path - Section 8)
carries no PCN-traffic. An alternative is always to probe, ie
probe before admitting every PCN-flow.
o (if required) Communicate the request that probing is needed - the
PCN-egress-node signals to the PCN-ingress-node that probing is
needed
o (if required) Generate probe traffic - the PCN-ingress-node
generates the probe traffic. The appropriate number (or rate) of
probe packets will depend on the PCN-marking algorithm; for
example an excess-traffic-marking algorithm generates fewer PCN-
marks than a threshold-marking algorithm, and so will need more
probe packets.
o Forward probe packets - as far as PCN-interior-nodes are
concerned, probe packets are handled the same as (ordinary data)
PCN-packets, in terms of routing, scheduling and PCN-marking.
o Consume probe packets - the PCN-egress-node consumes probe packets
to ensure that they don't travel beyond the PCN-domain.
17.3. Discussion of rationale for probing, its downsides and open
issues
It is an unresolved question whether probing is really needed, but
three viewpoints have been put forward as to why it is useful. The
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first is perhaps the most obvious: there is no PCN-traffic on the
ingress-egress-aggregate. The second assumes that multipath routing
ECMP is running in the PCN-domain. The third viewpoint is that
admission control is always done by probing. We now consider each in
turn.
The first viewpoint assumes the following:
o There is no PCN-traffic on the ingress-egress-aggregate (so a
normal admission decision cannot be made).
o Simply admitting the new flow has a significant risk of leading to
overload: packets dropped or flows terminated.
On the former bullet, [PCN-email-traffic-empty-aggregates] suggests
that, during the future busy hour of a national network with about
100 PCN-boundary-nodes, there are likely to be significant numbers of
aggregates with very few flows under nearly all circumstances.
The latter bullet could occur if a new flow starts on many of the
empty ingress-egress-aggregates and causes overload on a link in the
PCN-domain. To be a problem this would probably have to happen in a
short time period (flash crowd) because, after the reaction time of
the system, other (non-empty) ingress-egress-aggregates that pass
through the link will measure pre-congestion and so block new flows,
and also flows naturally end anyway.
The downsides of probing for this viewpoint are:
o Probing adds delay to the admission control process.
o Sufficient probing traffic has to be generated to test the pre-
congestion level of the ingress-egress-aggregate. But the probing
traffic itself may cause pre-congestion, causing other PCN-flows
to be blocked or even terminated - and in the flash crowd scenario
there will be probing on many ingress-egress-aggregates.
The open issues associated with this viewpoint include:
o What rate and pattern of probe packets does the PCN-ingress-node
need to generate, so that there's enough traffic to make the
admission decision?
o What difficulty does the delay (whilst probing is done) cause
applications, eg packets might be dropped?
o Are there other ways of dealing with the flash crowd scenario?
For instance limit the rate at which new flows are admitted; or
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perhaps for a PCN-egress-node to block new flows on its empty
ingress-egress-aggregates when its non-empty ones are pre-
congested.
The second viewpoint applies in the case where there is multipath
routing (ECMP) in the PCN-domain. Note that ECMP is often used on
core networks. There are two possibilities:
(1) If admission control is based on measurements of the ingress-
egress-aggregate, then the viewpoint that probing is useful assumes:
o there's a significant chance that the traffic is unevenly balanced
across the ECMP paths, and hence there's a significant risk of
admitting a flow that should be blocked (because it follows an
ECMP path that is pre-congested) or blocking a flow that should be
admitted.
o Note: [PCN-email-ECMP] suggests unbalanced traffic is quite
possible, even with quite a large number of flows on a PCN-link
(eg 1000) when Assumption 3 (aggregation) is likely to be
satisfied.
(2) If admission control is based on measurements of pre-congestion
on specific ECMP paths, then the viewpoint that probing is useful
assumes:
o There is no PCN-traffic on the ECMP path on which to base an
admission decision.
o Simply admitting the new flow has a significant risk of leading to
overload.
o The PCN-egress-node can match a packet to an ECMP path.
o Note: This is similar to the first viewpoint and so similarly
could occur in a flash crowd if a new flow starts more-or-less
simultaneously on many of the empty ECMP paths. Because there are
several (sometimes many) ECMP paths between each pair of PCN-
boundary-nodes, it's presumably more likely that an ECMP path is
'empty' than an ingress-egress-aggregate. To constrain the number
of ECMP paths, a few tunnels could be set-up between each pair of
PCN-boundary-nodes. Tunnelling also solves the third bullet
(which is otherwise hard because an ECMP routing decision is made
independently on each node).
The downsides of probing for this viewpoint are:
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o Probing adds delay to the admission control process.
o Sufficient probing traffic has to be generated to test the pre-
congestion level of the ECMP path. But there's the risk that the
probing traffic itself may cause pre-congestion, causing other
PCN-flows to be blocked or even terminated.
o The PCN-egress-node needs to consume the probe packets to ensure
they don't travel beyond the PCN-domain (eg they might confuse the
destination end node). Hence somehow the PCN-egress-node has to
be able to disambiguate a probe packet from a data packet, via the
characteristic setting of particular bit(s) in the packet's header
or body - but these bit(s) mustn't be used by any PCN-interior-
node's ECMP algorithm. In the general case this isn't possible,
but it should be OK for a typical ECMP algorithm which examines:
the source and destination IP addresses and port numbers, the
protocol ID and the DSCP.
The third viewpoint assumes the following:
o Every admission control decision involves probing, using the
signalling set-up message as the probe packet (eg RSVP PATH).
o The PCN-marking behaviour is such that every packet is PCN-marked
if the flow should be blocked, hence only a single probing packet
is needed.
This viewpoint [I-D.draft-babiarz-pcn-3sm] has in particular been
suggested for the scenario where the PCN-domain reaches out towards
the end terminals (note that it's assumed the trust and aggregation
assumptions still hold), although it has also been suggested for
other scenarios.
18. Informative References
[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.briscoe-tsvwg-cl-phb]
Briscoe, B., "Pre-Congestion Notification marking",
draft-briscoe-tsvwg-cl-phb-03 (work in progress),
October 2006.
[I-D.babiarz-pcn-sip-cap]
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Babiarz, J., "SIP Controlled Admission and Preemption",
draft-babiarz-pcn-sip-cap-00 (work in progress),
October 2006.
[I-D.lefaucheur-rsvp-ecn]
Faucheur, F., "RSVP Extensions for Admission Control over
Diffserv using Pre-congestion Notification (PCN)",
draft-lefaucheur-rsvp-ecn-01 (work in progress),
June 2006.
[I-D.chan-pcn-problem-statement]
Chan, K., "Pre-Congestion Notification Problem Statement",
draft-chan-pcn-problem-statement-01 (work in progress),
October 2006.
[I-D.ietf-pwe3-congestion-frmwk]
"Pseudowire Congestion Control Framework", May 2008, <http
://www.ietf.org/internet-drafts/
draft-ietf-pwe3-congestion-frmwk-01.txt>.
[I-D.briscoe-tsvwg-ecn-tunnel]
"Layered Encapsulation of Congestion Notification",
July 2008, <http://www.ietf.org/internet-drafts/
briscoe-tsvwg-ecn-tunnel-01.txt>.
[I-D.charny-pcn-single-marking]
"Pre-Congestion Notification Using Single Marking for
Admission and Termination", November 2007, <http://
www.ietf.org/internet-drafts/
draft-charny-pcn-single-marking-03.txt>.
[I-D.eardley-pcn-architecture]
"Pre-Congestion Notification Architecture", June 2007, <ht
tp://www.ietf.org/internet-drafts/
draft-eardley-pcn-architecture-00.txt>.
[I-D.westberg-pcn-load-control]
"LC-PCN: The Load Control PCN Solution", February 2008, <h
ttp://www.ietf.org/internet-drafts/
draft-westberg-pcn-load-control-03.txt>.
[I-D.behringer-tsvwg-rsvp-security-groupkeying]
"Applicability of Keying Methods for RSVP Security",
November 2007, <http://www.watersprings.org/pub/id/
draft-behringer-tsvwg-rsvp-security-groupkeying-01.txt>.
[I-D.briscoe-re-pcn-border-cheat]
"Emulating Border Flow Policing using Re-ECN on Bulk
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Data", February 2008, <http://tools.ietf.org/id/
draft-briscoe-re-pcn-border-cheat-01.txt>.
[I-D.draft-babiarz-pcn-3sm]
"Three State PCN Marking", November 2007, <http://
www.watersprings.org/pub/id/draft-babiarz-pcn-3sm-01.txt>.
[RFC5129] "Explicit Congestion Marking in MPLS", RFC 5129,
January 2008.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
August 2006.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC2211] Wroclawski, J., "Specification of the Controlled-Load
Network Element Service", RFC 2211, September 1997.
[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework for Integrated Services Operation
over Diffserv Networks", RFC 2998, November 2000.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
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RFC 2983, October 2000.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002.
[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, November 2006.
[RFC4778] Kaeo, M., "Operational Security Current Practices in
Internet Service Provider Environments", RFC 4778,
January 2007.
[ITU-MLPP]
"Multilevel Precedence and Pre-emption Service (MLPP)",
ITU-T Recommendation I.255.3, 1990.
[Iyer] "An approach to alleviate link overload as observed on an
IP backbone", IEEE INFOCOM , 2003,
<http://www.ieee-infocom.org/2003/papers/10_04.pdf>.
[Y.1541] "Network Performance Objectives for IP-based Services",
ITU-T Recommendation Y.1541, February 2006.
[P.800] "Methods for subjective determination of transmission
quality", ITU-T Recommendation P.800, August 1996.
[Songhurst]
"Guaranteed QoS Synthesis for Admission Control with
Shared Capacity", BT Technical Report TR-CXR9-2006-001,
Feburary 2006, <http://www.cs.ucl.ac.uk/staff/B.Briscoe/
Eardley (Editor) Expires January 15, 2009 [Page 49]
Internet-Draft Document July 2008
projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf>.
[Menth] "PCN-Based Resilient Network Admission Control: The Impact
of a Single Bit"", Technical Report , 2007, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth07-PCN-Config.pdf>.
[PCN-email-ECMP]
"Email to PCN WG mailing list", November 2007, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg00871.html>.
[PCN-email-traffic-empty-aggregates]
"Email to PCN WG mailing list", October 2007, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg00831.html>.
[PCN-email-SRLG]
"Email to PCN WG mailing list", March 2008, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg01359.html>.
[I-D.eardley-pcn-marking-behaviour]
"Marking behaviour of PCN-nodes", June 2008, <http://
www.ietf.org/internet-drafts/
draft-eardley-pcn-marking-behaviour-01.txt>.
[I-D.moncaster-pcn-baseline-encoding]
"Baseline Encoding and Transport of Pre-Congestion
Information", July 2008, <http://www.ietf.org/
internet-drafts/
draft-moncaster-pcn-baseline-encoding-02.txt>.
[I-D.moncaster-pcn-3-state-encoding]
"A three state extended PCN encoding scheme", June 2008, <
http://www.ietf.org/internet-drafts/
draft-moncaster-pcn-3-state-encoding-00.txt>.
[I-D.charny-pcn-comparison]
"Pre-Congestion Notification Using Single Marking for
Admission and Termination", November 2007, <http://
www.watersprings.org/pub/id/
draft-charny-pcn-comparison-00.txt>.
[I-D.tsou-pcn-racf-applic]
"Applicability Statement for the Use of Pre-Congestion
Notification in a Resource-Controlled Network",
February 2008, <http://tools.ietf.org/id/
draft-tsou-pcn-racf-applic-00.txt>.
[I-D.sarker-pcn-ecn-pcn-usecases]
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"Usecases and Benefits of end to end ECN support in PCN
Domains", May 2008, <http://tools.ietf.org/id/
draft-sarker-pcn-ecn-pcn-usecases-01.txt>.
[I-D.andersson-mpls-expbits-def]
"MPLS EXP-bits definition", March 2008, <http://
tools.ietf.org/id/
draft-andersson-mpls-expbits-def-00.txt>.
[Menth08] "PCN-Based Admission Control and Flow Termination", 2008,
<http://www3.informatik.uni-wuerzburg.de/staff/menth/
Publications/Menth08-PCN-Comparison.pdf>.
[Hancock] "Slide 14 of 'NSIS: An Outline Framework for QoS
Signalling'", May 2002, <http://www-nrc.nokia.com/sua/
nsis/interim/nsis-framework-outline.ppt>.
Author's Address
Philip Eardley
BT
B54/77, Sirius House Adastral Park Martlesham Heath
Ipswich, Suffolk IP5 3RE
United Kingdom
Email: philip.eardley@bt.com
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