Congestion and Pre-Congestion Philip. Eardley (Editor)
Notification Working Group BT
Internet-Draft October 20, 2008
Intended status: Informational
Expires: April 23, 2009
Pre-Congestion Notification (PCN) Architecture
draft-ietf-pcn-architecture-08
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Abstract
This 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.
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Status
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Deployment scenarios . . . . . . . . . . . . . . . . . . . . . 9
5. Assumptions and constraints on scope . . . . . . . . . . . . . 12
5.1. Assumption 1: Trust and support of PCN - controlled
environment . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Assumption 2: Real-time applications . . . . . . . . . . . 13
5.3. Assumption 3: Many flows and additional load . . . . . . . 13
5.4. Assumption 4: Emergency use out of scope . . . . . . . . . 14
6. High-level functional architecture . . . . . . . . . . . . . . 14
6.1. Flow admission . . . . . . . . . . . . . . . . . . . . . . 16
6.2. Flow termination . . . . . . . . . . . . . . . . . . . . . 16
6.3. Flow admission and/or flow termination when there are
only two PCN encoding states . . . . . . . . . . . . . . . 17
6.4. Information transport . . . . . . . . . . . . . . . . . . 18
6.5. PCN-traffic . . . . . . . . . . . . . . . . . . . . . . . 19
6.6. Backwards compatibility . . . . . . . . . . . . . . . . . 20
7. Detailed Functional architecture . . . . . . . . . . . . . . . 20
7.1. PCN-interior-node functions . . . . . . . . . . . . . . . 21
7.2. PCN-ingress-node functions . . . . . . . . . . . . . . . . 21
7.3. PCN-egress-node functions . . . . . . . . . . . . . . . . 22
7.4. Admission control functions . . . . . . . . . . . . . . . 23
7.5. Flow termination functions . . . . . . . . . . . . . . . . 23
7.6. Addressing . . . . . . . . . . . . . . . . . . . . . . . . 24
7.7. Tunnelling . . . . . . . . . . . . . . . . . . . . . . . . 25
7.8. Fault handling . . . . . . . . . . . . . . . . . . . . . . 27
8. Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9. Operations and Management . . . . . . . . . . . . . . . . . . 29
9.1. Configuration OAM . . . . . . . . . . . . . . . . . . . . 29
9.1.1. System options . . . . . . . . . . . . . . . . . . . . 30
9.1.2. Parameters . . . . . . . . . . . . . . . . . . . . . . 31
9.2. Performance & Provisioning OAM . . . . . . . . . . . . . . 33
9.3. Accounting OAM . . . . . . . . . . . . . . . . . . . . . . 34
9.4. Fault OAM . . . . . . . . . . . . . . . . . . . . . . . . 34
9.5. Security OAM . . . . . . . . . . . . . . . . . . . . . . . 35
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
11. Security considerations . . . . . . . . . . . . . . . . . . . 36
12. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 37
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 37
14. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 38
15. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
15.1. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 38
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15.2. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 38
15.3. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 38
15.4. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 39
15.5. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 40
15.6. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 41
15.7. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 42
15.8. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 43
16. Appendix: Possible work items beyond the scope of the
current PCN WG charter . . . . . . . . . . . . . . . . . . . . 44
16.1. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 46
16.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . 46
16.1.2. Probing functions . . . . . . . . . . . . . . . . . . 47
16.1.3. Discussion of rationale for probing, its downsides
and open issues . . . . . . . . . . . . . . . . . . . 47
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
17.1. Normative References . . . . . . . . . . . . . . . . . . . 50
17.2. Informative References . . . . . . . . . . . . . . . . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 55
Intellectual Property and Copyright Statements . . . . . . . . . . 57
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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 are
trusted 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. In a PCN-domain with
both threshold marking and excess traffic marking enabled, then 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.
Other scenarios are discussed later.
The behaviour of PCN-interior-nodes is standardised in other
documents, which are summarised in this document:
o Marking behaviour: threshold marking and excess traffic marking
[I-D.ietf-pcn-marking-behaviour]. 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".
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o Encoding: a combination of the DSCP field and ECN field in the IP
header indicates that a packet is a PCN-packet and whether it is
PCN-marked. The "baseline" encoding is standardised in
[I-D.ietf-pcn-baseline-encoding], which standardises two PCN
encoding states (PCN-marked and not PCN-marked), whilst
(experimental) extensions to the baseline encoding can provide
three encoding states (threshold-marked, excess-traffic-marked,
not PCN-marked, or perhaps further encoding states as suggested in
[I-D.westberg-pcn-load-control]). PCN encoding therefore defines
semantics for the ECN field different from the default semantics
of [RFC3168], and so its encoding needs to meet the guidelines of
BCP 124 [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].
This document describes the PCN architecture at a high level (Section
6) and in more detail (Section 7). It also defines some terminology
and outlines some benefits, deployment scenarios, and assumptions of
PCN (Sections 2-5). Finally it outlines some challenges, operations
and management, and security considerations, and some potential
future work items (Sections 8, 9, 11 and Appendix).
2. Terminology
o PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled
nodes that perform DiffServ scheduling [RFC2474]; 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.
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o PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of
different DiffServ behaviour aggregates (BAs) [RFC2474]. 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.
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 [RFC2474]) or some identifiable collection of
microflows.
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".
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".
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 such
that 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-colouring: the process of setting the header in a PCN-packet
by a PCN-boundary-node; performed by a PCN-ingress-node so that
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PCN-nodes can easily identify PCN-packets; performed by a PCN-
egress-node so that the header is appropriate for nodes beyond the
PCN-domain.
o PCN-feedback-information: information signalled by a PCN-egress-
node to a PCN-ingress-node (or a central control node), which is
needed for the flow admission and flow termination mechanisms.
o PCN-admissible-rate: the rate of PCN-traffic on a link up to which
PCN admission control should accept new PCN-flows.
o PCN-supportable-rate: the rate of PCN-traffic on a link down to
which PCN flow termination should, if necessary, terminate already
admitted PCN-flows.
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
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: with PCN QoS is decoupled from the
routing system. Hence in general admitted flows can survive
capacity, routing or topology changes without additional
signalling. The PCN-admissible-rate on each link 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 [Style]
"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.
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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
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-supportable-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, which statically define the parameters of the traffic
that will be accepted from a customer, and so the operator has to
verify provision is sufficient each time a new customer is added
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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). One difference is that PCN-
traffic has better QoS guarantees than normal DiffServ traffic,
because the PCN mechanisms better protect the QoS of admitted flows.
Another difference may occur in the rare circumstance when there is a
failure: 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.
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.
A PCN-domain may have three encoding states (or pedantically, an
operator may choose to use up three encoding states for PCN): not
PCN-marked, threshold-marked, excess-traffic-marked. Then both PCN
admission control and flow termination can be supported. As
illustrated in Figure 1, admission control accepts new flows until
the PCN-traffic rate on the bottleneck link rises above the PCN-
threshold-rate, whilst if necessary the flow termination mechanism
terminates flows down to the PCN-excess-rate on the bottleneck link.
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==Marking behaviour== ==PCN mechanisms==
Rate of ^
PCN-traffic on |
bottleneck link | (as below and also)
| (as below) Drop some PCN-pkts
|
scheduler rate -|------------------------------------------------
(for PCN-traffic) |
| Some pkts Terminate some
| excess-traffic-marked admitted flows
| & &
| Rest of pkts Block new flows
| threshold-marked
|
PCN-excess-rate -|------------------------------------------------
(=PCN-supportable-rate)|
| All pkts Block new flows
| threshold-marked
|
PCN-threshold-rate -|------------------------------------------------
(=PCN-admissible-rate)|
| No pkts Admit new flows
| PCN-marked
|
Figure 1: Schematic of how the PCN admission control and flow
termination mechanisms operate as the rate of PCN-traffic increases,
for a PCN-domain with three encoding states.
On the other hand, a PCN-domain may have two encoding states (as in
[I-D.ietf-pcn-baseline-encoding]) (or pedantically, an operator may
choose to use up two encoding states for PCN): not PCN-marked, PCN-
marked. Then there are three possibilities, as discussed in the
following paragraphs (see also Section 6.3).
First, an operator could just use 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 sufficient sessions have ended to
relieve the congestion, all PCN-flows as well as lower priority
services will be adversely affected.
Second, 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
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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.
Third, both admission control and flow termination can be triggered
from the single type of PCN-marking; the main downside is that
admission control is less accurate [I-D.charny-pcn-single-marking].
Within the PCN-domain there is some flexibility about how the
decision making functionality is distributed. These possibilities
are outlined in Section 7.4 and also discussed elsewhere, such as in
[Menth08].
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-
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 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 possible ways to do this are
to tunnel all PCN-packets across the PCN-domain, so that the ECN
marks are carried transparently across the PCN-domain, or to use an
encoding like [I-D.moncaster-pcn-3-state-encoding]. Tunnelling is
discussed further in Section 7.7.
Some possible deployment models that are outside the current PCN WG
charter are outlined in the Appendix.
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5. Assumptions and constraints on scope
The scope of PCN is, at least initially (see Appendix), restricted by
the following assumptions:
1. these components are deployed in a single DiffServ domain, within
which all PCN-nodes are PCN-enabled and are trusted 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.
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 are trusted. 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 performing 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 that trust each other in their handling
of PCN-traffic.
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Note: All PCN-nodes need to be trustworthy. However if it is known
that an interface cannot become pre-congested then it is 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 sending 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).
As a consequence, it is assumed that PCN-marking is being applied to
traffic scheduled with the expedited forwarding per-hop behaviour,
[RFC3246], or a per-hop behaviour 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 acceptable 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 factors 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
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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 as 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).
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 PCN-packets, nor do the
PCN-boundary-nodes care about which PCN-interior-nodes its ingress-
egress-aggregates traverse.
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)
will be 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.
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o the encoding(s): a PCN-node PCN-marks a PCN-packet by modifying a
combination of the DSCP and ECN fields. In the "baseline"
encoding [I-D.ietf-pcn-baseline-encoding], the ECN field is set to
11 and the DSCP is not altered. Extension encodings may be
defined that, at most, use a second DSCP (eg as in
[I-D.moncaster-pcn-3-state-encoding]) and/or set the ECN field to
values other than 11 (eg as in [I-D.menth-pcn-psdm-encoding]).
In a PCN-domain the operator may have two or three encoding states
available. The baseline encoding provides two encoding states (not
PCN-marked, PCN-marked), whilst extended encodings can provide three
encoding states (not PCN-marked, threshold-marked, excess-traffic-
marked).
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 distributed decision is made about whether to
admit a prospective new flow or whether to terminate existing
flow(s). Sections 7.4 and 7.5 mention various possibilities for how
the functionality could be distributed.
PCN-marking needs to be configured on all (potentially pre-congested)
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 point is that a consistent choice is made
across the PCN-domain to ensure that the PCN mechanisms protect all
links. See [I-D.ietf-pcn-marking-behaviour] for further discussion.
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.ietf-pcn-marking-behaviour].
All the presently proposed 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.
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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-
admissible-rate, by admitting or blocking prospective new flows, in
order to protect the QoS of existing PCN-flows. With three encoding
states available, the PCN-threshold-rate is configured by the
operator as equal to the PCN-admissible-rate on each link. It is 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 (others might be possible):
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
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 may be 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-supportable-rate, by
terminating some existing PCN-flows, in order to protect the QoS of
the remaining PCN-flows. With three encoding states available, the
PCN-excess-rate is configured by the operator as equal to the PCN-
supportable-rate on each link. It may be set lower than the traffic
rate at which the link becomes congested and the node drops packets.
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Exactly how the flow termination decision is made will be defined
separately in informational documents. At a high level several
approaches are proposed (others might be possible):
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, and communicates this
to the PCN-ingress-node. 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 less traffic than the previous bullet if some nodes are
dropping PCN-traffic.
o Another approach monitors PCN-packets and terminates some of the
PCN-flows that have an excess-traffic-marked packet. (If all such
flows were terminated, far too much traffic would be terminated,
so a random selection needs to be made from those with an excess-
traffic-marked packet, [I-D.menth-pcn-emft].)
Since flow termination is designed for "abnormal" circumstances, it
is quite likely that some PCN-nodes are congested and hence packets
are being dropped and/or significantly queued. The flow termination
mechanism must accommodate this.
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/or 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 is using the baseline encoding
[I-D.ietf-pcn-baseline-encoding], then an operator has three options
(others might be possible):
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.
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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 equal to the PCN-admissible-rate, as
shown in Figure 2. [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.
==Marking behaviour== ==PCN mechanisms==
Rate of ^
PCN-traffic on |
bottleneck link | Terminate some
| Further pkts admitted flows
| excess-traffic-marked &
| Block new flows
|
|
U*PCN-excess-rate -|------------------------------------------------
(=PCN-supportable-rate)|
| Some pkts Block new flows
| excess-traffic-marked
|
PCN-excess-rate -|------------------------------------------------
(=PCN-admissible-rate)|
| No pkts Admit new flows
| PCN-marked
|
Figure 2: Schematic of how the PCN admission control and flow
termination mechanisms operate as the rate of PCN-traffic increases,
for a PCN-domain with two encoding states and using the approach of
[I-D.charny-pcn-single-marking]. Note: U is a global parameter for
all links in the PCN-domain.
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
future documents about possible boundary mechanisms. The signalling
could be done by an extension of RSVP or NSIS, for instance; protocol
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work will be done by the relevant WG, but for example
[I-D.lefaucheur-rsvp-ecn] describes the extensions needed for RSVP.
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 It is not advised to have non PCN-traffic that competes for the
same capacity as PCN-traffic but, if there is such traffic, there
needs to be a mechanism to limit it. "Capacity" means the
forwarding bandwidth on a link; "competes" means that non PCN-
packets will delay PCN-packets in the queue for the link. Hence
more non PCN-traffic results in poorer QoS for PCN. Further, the
unpredictable amount of non PCN-traffic makes the PCN mechanisms
less accurate and so reduces PCN's ability to protect the QoS of
admitted PCN-flows
o Two examples of such non PCN-traffic (ie that competes for the
same capacity as PCN-traffic) are:
1. traffic that is priority scheduled over PCN (perhaps a particular
application or an operator's control messages).
2. traffic that is scheduled at the same priority as PCN (for
example if the Voice-Admit codepoint is used for PCN-traffic
[I-D.ietf-pcn-baseline-encoding] and there is non-PCN voice-admit
traffic in the PCN-domain).
o If there is such non PCN-traffic (ie that competes for the same
capacity as PCN-traffic), then PCN's mechanisms should take
account of it, in order to improve the accuracy of the decision
about whether to admit (or terminate) a PCN-flow. For example,
one mechanism is that such non PCN-traffic contributes to the PCN
meters (ie is metered by the threshold-marking and excess-traffic-
marking algorithms).
o There will be non PCN-traffic that doesn't compete for the same
capacity as PCN-traffic, because it is forwarded at lower
priority. Hence it shouldn't contribute to the PCN meters.
Examples are best effort and assured forwarding traffic. However,
a PCN-node should dedicate some capacity to lower priority traffic
so that it isn't starved.
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o The document assumes that the PCN mechanisms are applied to a
single behaviour aggregate in the PCN-domain. However, it would
also be possible to apply them independently to more than one
behaviour aggregate, which are distinguished by DSCP.
6.6. Backwards compatibility
PCN specifies semantics for the ECN field that differ from the
default semantics of [RFC3168]. A particular PCN encoding scheme
needs to describe how it meets the guidelines of BCP 124 [RFC4774]
for specifying alternative semantics for the ECN field. In summary
the approach is to:
o use a DSCP to allow PCN-nodes to distinguish PCN-traffic that uses
the alternative ECN semantics;
o define these semantics for use within a controlled region, the
PCN-domain;
o take appropriate action if ECN capable, non-PCN traffic arrives at
a PCN-ingress-node with the DSCP used by PCN.
For the baseline encoding [I-D.ietf-pcn-baseline-encoding], the
'appropriate action' is to block ECN-capable traffic that uses the
same DSCP as PCN 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 'appropriate action' is needed is that
the PCN-egress-node clears the ECN field to 00.
Extended encoding schemes may take different 'appropriate action'.
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
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2. functions needed at PCN-ingress-nodes
3. functions needed at PCN-egress-nodes
4. other functions needed for flow admission control
5. other functions needed for flow termination control
Note: Probing is covered in the Appendix.
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 Behaviour aggregate classification - determine whether an incoming
packet is a PCN-packet or not.
o Meter - measure the 'amount of PCN-traffic'. The measurement is
made as an aggregate of all PCN-packets, and not per flow.
o PCN-mark - algorithms determine whether to PCN-mark PCN-packets
and what packet encoding is used.
The functions are defined in [I-D.ietf-pcn-marking-behaviour] and the
baseline encoding in [I-D.ietf-pcn-baseline-encoding] (extended
encodings are to be defined in other documents).
7.2. PCN-ingress-node functions
Each ingress link of the PCN-domain is configured with the following
functionality:
o Packet classification - determine 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 Traffic conditioning - police, by dropping or downgrading, any
packets received with a DSCP indicating PCN transport that do not
belong to an admitted flow. (A prospective PCN-flow that is
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rejected could be blocked or admitted into a lower priority
behaviour aggregate.) Similarly, police packets that are part of
a previously admitted flow, to check that the flow keeps to the
agreed rate or flowspec (eg [RFC1633] for a microflow and its NSIS
equivalent).
o PCN-colour - set the DSCP and ECN fields appropriately for the
PCN-domain, for example as in [I-D.ietf-pcn-baseline-encoding].
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 has been
admitted and to ensure that the flow keeps to the flowspec agreed (eg
doesn't exceed an agreed maximum 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). PCN-
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 PCN-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. For PCN-colouring, [I-D.ietf-pcn-baseline-encoding]
specifies that the PCN-egress-node re-sets the ECN field to 00; other
encodings may define different behaviour.
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7.4. Admission control functions
As well as the functions covered above, other specific admission
control functions need to be performed (others might be possible):
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 (eg the RSVP PATH
message). In either case, the admission decision also takes
account of policy and application layer requirements [RFC2753].
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 could 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.
o The decision is recommended by the PCN-egress-node (admit or
block) but the decision is definitively made by the PCN-ingress-
node. The rationale is that the PCN-egress-node naturally has the
necessary information about PCN-marking on the ingress-egress-
aggregate, but the PCN-ingress-node is the policy enforcement
point [RFC2753], which polices incoming traffic to ensure it is
part of an admitted PCN-flow.
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; beyond
scope of current PCN WG charter).
Note: Admission control functionality is not performed by normal PCN-
interior-nodes.
7.5. Flow termination functions
As well as the functions covered above, other specific termination
control functions need to be performed (others might be possible):
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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
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 [RFC2753].
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.
There are various possibilities for how the functionality could be
distributed, similar to those discussed above in the Admission
control section.
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
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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).
7.7. Tunnelling
Tunnels may originate and/or terminate within a PCN-domain (eg IP
over IP, IP over MPLS). 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
Note: A tunnel will not provide this behaviour if it complies with
[RFC3168] tunnelling in either mode, but it will if it complies with
[RFC4301] IPSec tunnelling.
Similarly, in line with the "uniform conceptual model" of [RFC2983],
the "full-functionality option" of [RFC3168], and [RFC4301], 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.
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
PCN-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
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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 originates outside a PCN-domain and ends 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.
2. The tunnel originates inside a PCN-domain and ends 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 determine, the
characteristics of the other end of the tunnel as part of
establishing it.
Tunnelling constraints were a major factor in the choice of the
baseline encoding. As explained in [I-D.ietf-pcn-baseline-encoding],
with current tunnelling endpoints only the 11 codepoint of the ECN
field survives decapsulation, and hence the baseline encoding only
uses the 11 codepoint to indicate PCN-marking. Extended encoding
schemes need to explain their interactions with (or assumptions
about) tunnelling. 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].
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7.8. Fault handling
If a PCN-interior-node (or one of its links) fails, then lower layer
protection mechanisms or the regular IP routing protocol will
eventually re-route around 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 be responsible for taking appropriate action.
As an example [I-D.briscoe-tsvwg-cl-architecture] considers what
happens if RSVP is the QoS signalling protocol.
8. Challenges
Prior work on PCN and similar mechanisms has thrown up a number of
considerations about PCN's design goals (things PCN should be good
at) [I-D.chan-pcn-problem-statement] 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 is 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 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: a flow is terminated, but its path
doesn't travel through the (pre-)congested router(s). Since
flow termination is a 'last resort', which 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
will not be an issue. 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
difficult 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 microflows that should be admitted
(or terminated) as a pair - for instance a bi-directional voice
call only makes sense if microflows in both directions are
admitted. However, the PCN 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.
However, 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
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(or even terminate) flows on one ingress-egress-aggregate so that
more important flows through a different ingress-egress-aggregate
could be admitted. The problem may well be relatively
insignificant.
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 is 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 is 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
must not move the PCN-domain directly from a no congestion state
to overload. This 'vulnerability period' may have an 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
Threshold-marking and excess-traffic-marking are standardised in
[I-D.ietf-pcn-marking-behaviour]. However, more diversity in PCN-
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boundary-node behaviours 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.
A PCN marking behaviour (threshold-marking, excess-traffic-marking)
is enabled 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 equipment from different
vendors.
It may be possible for an operator to configure some PCN-interior-
nodes so that 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 In the case where only one PCN-marking is enabled, all nodes must
be configured to generate PCN-marks from the same meter (ie either
the threshold meter or the excess traffic meter).
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 interpret that marking (which
includes understanding, in a PCN-domain that uses only one type of
PCN-marking, whether they are generated by PCN-interior-nodes'
threshold meters or the excess traffic meters). Therefore all
PCN-boundary-nodes must be configured the same in this respect.
o Where flow admission and termination decisions are made: at PCN-
ingress-nodes or at PCN-egress-nodes (or at a centralised node,
see Appendix). 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.
o How PCN-markings are translated into admission control and flow
termination decisions (see Section 6.1 and Section 6.2).
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.
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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
single or multiple failures, and the expected load from other classes
relative to link capacities [Menth]. But once a domain is in
operation, 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.
Threshold-marking requires a threshold token bucket depth to be
configured, excess-traffic-marking needs a value for the MTU (maximum
size of a PCN-packet on the link) and both require setting a maximum
size of their token buckets. It will be preferable for there to be
rules to set defaults for these parameters, 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 an admission control algorithm that is based on the fraction of
marked packets 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.
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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 (the
PCN-traffic rate on a link up to which flows are admitted vs the rate
above which flows are terminated). [I-D.charny-pcn-single-marking]
discusses in full the impact of this particular proposal on the
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 occurred 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
verify 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
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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 distinguished from regular
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.
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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
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 should be performed. PCN adds nothing
specific to this class of problems.
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 a 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 are trusted
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. There is nothing specific
to PCN.
o the PCN-ingress-nodes police packets to ensure a PCN-flow sticks
within its agreed limit, and to ensure that only PCN-flows that
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 are not flow-aware. This prevents some
security attacks where an attacker targets specific flows in the
data plane - for instance for DoS or eavesdropping.
o The PCN-boundary-nodes rely on correct PCN-marking by the PCN-
interior-nodes. 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
is 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.
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.
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o the signalling between the PCN-boundary-nodes 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.
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 have 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, Fortune Huang, Christian Hublet, Ingemar Johansson,
Georgios Karagiannis, Hein Mekkes, Michael Menth, Toby Moncaster,
Daisuke Satoh, 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
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.
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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 -07 to -08
Small changes from second WG last call:
o Section 2: added definition for PCN-admissible-rate and PCN-
supportable-rate. Small changes to use these terms as follows:
Section 3, bullets 2 & 9; S6.1 para 1; S6.2 para1; S6.3 bullet 3;
added to Figs 1 & 2.
o added the phrase "(others might be possible") before the list of
approaches in Section 6.3, 7.4 & 7.5.
o added references to RFC2753 (A framework for policy-based
admission control) in S7.4 & S7.5.
o throughout, updated references now that marking behaviour &
baseline encoding are WG drafts.
o a few typos corrected
15.2. Changes from -06 to -07
References re-formatted to pass ID nits. No other changes.
15.3. Changes from -05 to -06
Minor clarifications throughout, the least insignificant are as
follows:
o Section 1: added to the list of encoding states in an 'extended'
scheme: "or perhaps further encoding states as suggested in
draft-westberg-pcn-load-control"
o Section 2: added definition for PCN-colouring (to clarify that the
term is used consistently differently from 'PCN-marking')
o Section 6.1 and 6.2: added "(others might be possible)" before the
list of high level approaches for making flow admission
(termination) decisions.
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o Section 6.2: corrected a significant typo in 2nd bullet (more ->
less)
o Section 6.3: corrected a couple of significant typos in Figure 2
o Section 6.5 (PCN-traffic) re-written for clarity. Non PCN-traffic
contributing to PCN meters is now given as an example (there may
be cases where don't need to meter it).
o Section 7.7: added to the text about encapsulation being done
within the PCN-domain: "Note: A tunnel will not provide this
behaviour if it complies with [RFC3168] tunnelling in either mode,
but it will if it complies with [RFC4301] IPSec tunnelling."
o Section 7.7: added mention of [RFC4301] to the text about
decapsulation being done within the PCN-domain.
o Section 8: deleted the text about design goals, since this is
already covered adequately earlier eg in S3.
o Section 11: replaced the last sentence of bullet 1 by "There is
nothing specific to PCN."
o Appendix: added to open issues: possibility of automatically and
periodically probing.
o References: Split out Normative references (RFC2474 & RFC3246).
15.4. Changes from -04 to -05
Minor nits removed as follows:
o Further minor changes to reflect that baseline encoding is
consensus, standards track document, whilst there can be
(experimental track) encoding extensions
o Traffic conditioning updated to reflect discussions in Dublin,
mainly that PCN-interior-nodes don't police PCN-traffic (so
deleted bullet in S7.1) and that it is not advised to have non
PCN-traffic that shares the same capacity (on a link) as PCN-
traffic (so added bullet in S6.5)
o Probing moved into Appendix A and deleted the 'third viewpoint'
(admission control based on the marking of a single packet like an
RSVP PATH message) - since this isn't really probing, and in any
case is already mentioned in S6.1.
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o Minor changes to S9 Operations and management - mainly to reflect
that consensus on marking behaviour has simplified things so eg
there are fewer parameters to configure.
o A few terminology-related errors expunged, and two pictures added
to help.
o Re-phrased the claim about the natural decision point in S7.4
o Clarified that extended encoding schemes need to explain their
interactions with (or assumptions about) tunnelling (S7.7) and how
they meet the guidelines of BCP124 (S6.6)
o Corrected the third bullet in S6.2 (to reflect consensus about
PCN-marking)
15.5. Changes from -03 to -04
o Minor changes throughout to reflect the consensus 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.
o S6.5: updated the bullet about non-PCN-traffic that uses the same
DSCP as PCN-traffic.
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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.
15.6. 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).
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
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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.
15.7. 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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15.8. 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: 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 is 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 5):
o a single PCN-domain encompasses several autonomous systems that do
not trust each other, perhaps by using a mechanism like re-PCN,
[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 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 adapt 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 less accurate,
with a greater risk of over-admission for instance.
o the applicability of PCN mechanisms for emergency use (911, GETS,
WPS, MLPP, etc.)
Other possibilities include:
o Probing. This is discussed in Section 16.1 below.
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 the decision about admission (or termination). It may need
the centralised node and the PCN-boundary-nodes to be configured
with each other's addresses. The centralised case is described
further in [I-D.tsou-pcn-racf-applic].
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.
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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 (Multi-protocol label switching).
[RFC5129] describes how to add PCN for admission control of
microflows into a set of MPLS aggregates. PCN-marking is done in
MPLS's EXP field (which [I-D.ietf-mpls-cosfield-def] proposes to
re-name to the Class of Service (CoS) field).
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.
16.1. Probing
16.1.1. Introduction
Probing is a potential 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 a link that is
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.
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
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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.
16.1.2. 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.
16.1.3. Discussion of rationale for probing, its downsides and open
issues
It is an unresolved question whether probing is really needed, but
two viewpoints have been put forward as to why it is useful. The
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. We now consider each in turn.
The first viewpoint assumes the following:
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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 new flows start on many of the empty
ingress-egress-aggregates, which together overload 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.
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 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
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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 is. 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 issue in
the bullet immediately above (which is otherwise hard because an
ECMP routing decision is made independently on each node).
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 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, since they might confuse
the destination end node. This is non-trivial, since probe
packets are addressed to the destination end node, in order to
test the relevant ECMP path (ie they are not addressed to the PCN-
egress-node, unlike the first viewpoint above).
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), and
possible packet drops, cause applications?
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o Can the delay be alleviated by automatically and periodically
probing on the ingress-egress-aggregate? Or does this add too
much overhead?
o Are there other ways of dealing with the flash crowd scenario?
For instance, by limiting the rate at which new flows are
admitted; or perhaps by a PCN-egress-node blocking new flows on
its empty ingress-egress-aggregates when its non-empty ones are
pre-congested.
o (Second viewpoint only) How does the PCN-egress-node disambiguate
probe packets from data packets (so it can consume the former)?
The PCN-egress-node must match the characteristic setting of
particular bits in the probe packet's header or body - but these
bits must not be used by any PCN-interior-node's ECMP algorithm.
In the general case this isn't possible, but it should be possible
for a typical ECMP algorithm (which examines: the source and
destination IP addresses and port numbers, the protocol ID, and
the DSCP).
17. References
17.1. Normative References
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[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.
17.2. Informative References
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC2211] Wroclawski, J., "Specification of the Controlled-Load
Network Element Service", RFC 2211, September 1997.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
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[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
for Policy-based Admission Control", RFC 2753,
January 2000.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, October 2000.
[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.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[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.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002.
[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.
[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
August 2006.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
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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.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
[P.800] "Methods for subjective determination of transmission
quality", ITU-T Recommendation P.800, August 1996.
[Y.1541] "Network Performance Objectives for IP-based Services",
ITU-T Recommendation Y.1541, February 2006.
[I-D.ietf-mpls-cosfield-def]
Andersson, L. and R. Asati, ""EXP field" renamed to
"Traffic Class field"", draft-ietf-mpls-cosfield-def-05
(work in progress), October 2008.
[I-D.ietf-pcn-baseline-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
draft-ietf-pcn-baseline-encoding-01 (work in progress),
October 2008.
[I-D.ietf-pcn-marking-behaviour]
Eardley, P., "Marking behaviour of PCN-nodes",
draft-ietf-pcn-marking-behaviour-00 (work in progress),
October 2008.
[I-D.ietf-pwe3-congestion-frmwk]
Bryant, S., Davie, B., Martini, L., and E. Rosen,
"Pseudowire Congestion Control Framework",
draft-ietf-pwe3-congestion-frmwk-01 (work in progress),
May 2008.
[I-D.babiarz-pcn-sip-cap]
Babiarz, J., "SIP Controlled Admission and Preemption",
draft-babiarz-pcn-sip-cap-00 (work in progress),
October 2006.
[I-D.behringer-tsvwg-rsvp-security-groupkeying]
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Behringer, M. and F. Faucheur, "Applicability of Keying
Methods for RSVP Security",
draft-behringer-tsvwg-rsvp-security-groupkeying-01 (work
in progress), November 2007.
[I-D.briscoe-re-pcn-border-cheat]
Briscoe, B., "Emulating Border Flow Policing using Re-PCN
on Bulk Data", draft-briscoe-re-pcn-border-cheat-02 (work
in progress), September 2008.
[I-D.briscoe-tsvwg-cl-architecture]
Briscoe, B., "An edge-to-edge Deployment Model for Pre-
Congestion Notification: Admission Control over a
DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04
(work in progress), October 2006.
[I-D.briscoe-tsvwg-cl-phb]
Briscoe, B., "Pre-Congestion Notification marking",
draft-briscoe-tsvwg-cl-phb-03 (work in progress),
October 2006.
[I-D.briscoe-tsvwg-ecn-tunnel]
Briscoe, B., "Layered Encapsulation of Congestion
Notification", draft-briscoe-tsvwg-ecn-tunnel-01 (work in
progress), July 2008.
[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.charny-pcn-comparison]
Charny, A., "Comparison of Proposed PCN Approaches",
draft-charny-pcn-comparison-00 (work in progress),
November 2007.
[I-D.charny-pcn-single-marking]
Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
Congestion Notification Using Single Marking for Admission
and Termination", draft-charny-pcn-single-marking-03
(work in progress), November 2007.
[I-D.eardley-pcn-architecture]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-eardley-pcn-architecture-00 (work in progress),
June 2007.
[I-D.eardley-pcn-marking-behaviour]
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Internet-Draft PCN Architecture October 2008
Eardley, P., "Marking behaviour of PCN-nodes",
draft-eardley-pcn-marking-behaviour-01 (work in progress),
June 2008.
[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.menth-pcn-emft]
Menth, M., Lehrieder, F., Eardley, P., Charny, A., and J.
Babiarz, "Edge-Assisted Marked Flow Termination",
draft-menth-pcn-emft-00 (work in progress), February 2008.
[I-D.menth-pcn-psdm-encoding]
Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
"PCN Encoding for Packet-Specific Dual Marking (PSDM)",
draft-menth-pcn-psdm-encoding-00 (work in progress),
July 2008.
[I-D.moncaster-pcn-3-state-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "A three state
extended PCN encoding scheme",
draft-moncaster-pcn-3-state-encoding-00 (work in
progress), June 2008.
[I-D.moncaster-pcn-baseline-encoding]
Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
draft-moncaster-pcn-baseline-encoding-02 (work in
progress), July 2008.
[I-D.sarker-pcn-ecn-pcn-usecases]
Sarker, Z. and I. Johansson, "Usecases and Benefits of end
to end ECN support in PCN Domains",
draft-sarker-pcn-ecn-pcn-usecases-01 (work in progress),
May 2008.
[I-D.tsou-pcn-racf-applic]
Tsou, T. and T. Taylor, "Applicability Statement for the
Use of Pre-Congestion Notification in a Resource-
Controlled Network", draft-tsou-pcn-racf-applic-00 (work
in progress), February 2008.
[I-D.westberg-pcn-load-control]
Westberg, L., Bhargava, A., Bader, A., Karagiannis, G.,
and H. Mekkes, "LC-PCN: The Load Control PCN Solution",
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draft-westberg-pcn-load-control-04 (work in progress),
July 2008.
[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>.
[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>.
[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>.
[Menth08] "PCN-Based Admission Control and Flow Termination", 2008,
<http://www3.informatik.uni-wuerzburg.de/staff/menth/
Publications/Menth08-PCN-Comparison.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-SRLG]
"Email to PCN WG mailing list", March 2008, <http://
www1.ietf.org/mail-archive/web/pcn/current/msg01359.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>.
[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/
projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf>.
[Style] "Guardian Style", Note: This document uses the
abbreviations 'ie' and 'eg' (not 'i.e.' and 'e.g.'), as in
many style guides, eg, 2007,
<http://www.guardian.co.uk/styleguide/>.
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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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Internet-Draft PCN Architecture October 2008
Full Copyright Statement
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Eardley (Editor) Expires April 23, 2009 [Page 57]
