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Network Working Group P. Eardley, Ed.
Request for Comments: 5559 BT
Category: Informational June 2009
Pre-Congestion Notification (PCN) Architecture
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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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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RFC 5559 PCN Architecture June 2009
Table of Contents
1. Introduction ....................................................3
1.1. Overview of PCN ............................................3
1.2. Example Use Case for PCN ...................................4
1.3. Applicability of PCN .......................................7
1.4. Documents about PCN ........................................8
2. Terminology .....................................................9
3. High-Level Functional Architecture .............................11
3.1. Flow Admission ............................................13
3.2. Flow Termination ..........................................14
3.3. Flow Admission and/or Flow Termination When There Are Only
Two PCN Encoding States ...................................15
3.4. Information Transport .....................................16
3.5. PCN-Traffic ...............................................16
3.6. Backwards Compatibility ...................................17
4. Detailed Functional Architecture ...............................18
4.1. PCN-Interior-Node Functions ...............................19
4.2. PCN-Ingress-Node Functions ................................19
4.3. PCN-Egress-Node Functions .................................20
4.4. Admission Control Functions ...............................21
4.5. Flow Termination Functions ................................22
4.6. Addressing ................................................22
4.7. Tunnelling ................................................23
4.8. Fault Handling ............................................25
5. Operations and Management ......................................25
5.1. Fault Operations and Management ...........................25
5.2. Configuration Operations and Management ...................26
5.2.1. System Options .....................................27
5.2.2. Parameters .........................................28
5.3. Accounting Operations and Management ......................30
5.4. Performance and Provisioning Operations and Management ....30
5.5. Security Operations and Management ........................31
6. Applicability of PCN ...........................................32
6.1. Benefits ..................................................32
6.2. Deployment Scenarios ......................................33
6.3. Assumptions and Constraints on Scope ......................35
6.3.1. Assumption 1: Trust and Support of PCN -
Controlled Environment .............................36
6.3.2. Assumption 2: Real-Time Applications ...............36
6.3.3. Assumption 3: Many Flows and Additional Load .......37
6.3.4. Assumption 4: Emergency Use Out of Scope ...........37
6.4. Challenges ................................................37
7. Security Considerations ........................................40
8. Conclusions ....................................................41
9. Acknowledgements ...............................................41
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RFC 5559 PCN Architecture June 2009
10. References ....................................................42
10.1. Normative References .....................................42
10.2. Informative References ...................................42
Appendix A. Possible Future Work Items ...........................48
A.1. Probing .................................................50
A.1.1. Introduction ....................................50
A.1.2. Probing Functions ...............................50
A.1.3. Discussion of Rationale for Probing, Its
Downsides and Open Issues .......................51
1. Introduction
1.1. Overview of PCN
The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain
in a simple, scalable, and robust fashion. Two mechanisms are used:
admission control, to decide whether to admit or block a new flow
request, and (in abnormal circumstances) flow termination, to decide
whether to terminate some of the existing flows. To achieve this,
the overall rate of PCN-traffic is metered on every link in the
domain, and PCN packets are appropriately marked when certain
configured rates are exceeded. These configured rates are below the
rate of the link, thus providing notification to boundary nodes about
overloads before any congestion occurs (hence, "Pre-Congestion
Notification"). The level of marking allows boundary nodes to make
decisions about whether to admit or terminate.
Within a PCN-domain, PCN-traffic is forwarded in a prioritised
Diffserv traffic class. Every link in the PCN-domain is configured
with two rates (PCN-threshold-rate and PCN-excess-rate). If the
overall rate of PCN-traffic on a link exceeds a configured rate, then
a PCN-interior-node marks PCN-packets appropriately. The PCN-egress-
nodes use this information to make admission control and flow
termination decisions. Flow admission control determines whether a
new flow can be admitted without any impact, in normal circumstances,
on the QoS of existing PCN-flows. However, in abnormal circumstances
(for instance, a disaster affecting multiple nodes and causing
traffic re-routes), the QoS on existing PCN-flows may degrade even
though care was exercised when admitting those flows. The flow
termination mechanism removes sufficient traffic in order to protect
the QoS of the remaining PCN-flows. All PCN-boundary-nodes and PCN-
interior-nodes are PCN-enabled and are trusted for correct PCN
operation. PCN-ingress-nodes police arriving packets to check that
they are part of an admitted PCN-flow that keeps within its agreed
flowspec, and hence they maintain per-flow state. PCN-interior-nodes
meter all PCN-traffic, and hence do not need to maintain any per-flow
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state. Decisions about flow admission and termination are made for a
particular pair of PCN-boundary-nodes, and hence PCN-egress-nodes
must be able to identify which PCN-ingress-node sent each PCN-packet.
1.2. Example Use Case for PCN
This section outlines an end-to-end QoS scenario that uses the PCN
mechanisms within one domain. The parts outside the PCN-domain are
out of scope for PCN, but are included to help clarify how PCN could
be used. Note that this section is only an example -- in particular,
there are other possibilities (see Section 3) for how the PCN-
boundary-nodes perform admission control and flow termination.
As a fundamental building block, each link of the PCN-domain operates
the following. Please refer to [Eardley09] and Figure 1.
o A threshold meter and marker, which marks all PCN-packets if the
rate of PCN-traffic is greater than a first configured rate, the
PCN-threshold-rate. The admission control mechanism limits the
PCN-traffic on each link to *roughly* its PCN-threshold-rate.
o An excess-traffic meter and marker, which marks a proportion of
PCN-packets such that the amount marked equals the traffic rate in
excess of a second configured rate, the PCN-excess-rate. The flow
termination mechanism limits the PCN-traffic on each link to
*roughly* its PCN-excess-rate.
Overall, the aim is to give an "early warning" of potential
congestion before there is any significant build-up of PCN-packets in
the queue on the link; we term this "Pre-Congestion Notification" by
analogy with ECN (Explicit Congestion Notification, [RFC3168]). Note
that the link only meters the bulk PCN-traffic (and not per flow).
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RFC 5559 PCN Architecture June 2009
== Metering & ==
==Marking behaviour== ==PCN mechanisms==
^
Rate of ^
PCN-traffic on |
bottleneck link |
|
| 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: Example of how the PCN admission control and flow
termination mechanisms operate as the rate of PCN-traffic increases.
The two forms of PCN-marking are indicated by setting the ECN and
DSCP (Differentiated Services Codepoint [RFC2474]) fields to known
values, which are configured for the domain. Thus, the PCN-egress-
nodes can monitor the PCN-markings in order to measure the severity
of pre-congestion. In addition, the PCN-ingress-nodes need to set
the ECN and DSCP fields to that configured for an unmarked PCN-
packet, and the PCN-egress-nodes need to revert to values appropriate
outside the PCN-domain.
For admission control, we assume end-to-end RSVP (Resource
Reservation Protocol) [RFC2205]) signalling in this example. The
PCN-domain is a single RSVP hop. The PCN-domain operates Diffserv,
and we assume that PCN-traffic is scheduled with the expedited
forwarding (EF) per-hop behaviour [RFC3246]. Hence, the overall
solution is in line with the "IntServ over Diffserv" framework
defined in [RFC2998], as shown in Figure 2.
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___ ___ _______________________________________ ____ ___
| | | | | PCN- PCN- PCN- | | | | |
| | | | |ingress interior egress| | | | |
| | | | | -node -nodes -node | | | | |
| | | | |-------+ +-------+ +-------+ +------| | | | |
| | | | | | | PCN | | PCN | | | | | | |
| |..| |..|Ingress|..|meter &|..|meter &|..|Egress|..| |..| |
| |..| |..|Policer|..|marker |..|marker |..|Meter |..| |..| |
| | | | |-------+ +-------+ +-------+ +------| | | | |
| | | | | \ / | | | | |
| | | | | \ / | | | | |
| | | | | \ PCN-feedback-information / | | | | |
| | | | | \ (for admission control) / | | | | |
| | | | | --<-----<----<----<-----<-- | | | | |
| | | | | PCN-feedback-information | | | | |
| | | | | (for flow termination) | | | | |
|___| |___| |_______________________________________| |____| |___|
Sx Access PCN-domain Access Rx
End Network Network End
Host Host
<---- signalling across PCN-domain--->
(for admission control & flow termination)
<-------------------end-to-end QoS signalling protocol--------------->
Figure 2: Example of possible overall QoS architecture.
A source wanting to start a new QoS flow sends an RSVP PATH message.
Normal hop-by-hop IntServ [RFC1633] is used outside the PCN-domain
(we assume successfully). The PATH message travels across the PCN-
domain; the PCN-egress-node reads the PHOP (previous RSVP hop) object
to discover the specific PCN-ingress-node for this flow. The RESV
message travels back from the receiver, and triggers the PCN-egress-
node to check what fraction of the PCN-traffic from the relevant PCN-
ingress-node is currently being threshold-marked. It adds an object
with this information onto the RESV message, and hence the PCN-
ingress-node learns about the level of pre-congestion on the path.
If this level is below some threshold, then the PCN-ingress-node
admits the new flow into the PCN-domain. The RSVP message triggers
the PCN-ingress-node to install two normal IntServ items: five-tuple
information, so that it can subsequently identify data packets that
are part of a previously admitted PCN-flow, and a traffic profile, so
that it can police the flow to within its reservation. Similarly,
the RSVP message triggers the PCN-egress-node to install five-tuple
and PHOP information so that it can identify packets as part of a
flow from a specific PCN-ingress-node.
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The flow termination mechanism may happen when some abnormal
circumstance causes a link to become so pre-congested that it excess-
traffic-marks (and perhaps also drops) PCN-packets. In this example,
when a PCN-egress-node observes such a packet, it then, with some
probability, terminates this PCN-flow; the probability is configured
low enough to avoid over termination and high enough to ensure rapid
termination of enough flows. It also informs the relevant PCN-
ingress-node so that it can block any further traffic on the
terminated flow.
1.3. Applicability of PCN
Compared with alternative QoS mechanisms, PCN has certain advantages
and disadvantages that will make it appropriate in particular
scenarios. For example, compared with hop-by-hop IntServ [RFC1633],
PCN only requires per-flow state at the PCN-ingress-nodes. Compared
with the Diffserv architecture [RFC2475], an operator needs to be
less accurate and/or conservative in its prediction of the traffic
matrix. The Diffserv architecture's traffic-conditioning agreements
are static and coarse; they are defined at subscription time and are
used (for instance) to limit the total traffic at each ingress of the
domain, regardless of the egress for the traffic. On the other hand,
PCN firstly uses admission control based on measurements of the
current conditions between the specific pair of PCN-boundary-nodes,
and secondly, in case of a disaster, PCN protects the QoS of most
flows by terminating a few selected ones.
PCN's admission control is a measurement-based mechanism. Hence, it
assumes that the present is a reasonable prediction of the future:
the network conditions are measured at the time of a new flow
request, but the actual network performance must be acceptable during
the call some time later. Hence, PCN is unsuitable in several
circumstances:
o If the source adapts its bit rate dependent on the level of pre-
congestion, because then the aggregate traffic might become
unstable. The assumption in this document is that PCN-packets
come from real-time applications generating inelastic traffic,
such as the Controlled Load Service [RFC2211].
o If a potential bottleneck link has capacity for only a few flows,
because then a new flow can move a link directly from no pre-
congestion to being so overloaded that it has to drop packets.
The assumption in this document is that this isn't a problem.
o If there is the danger of a "flash crowd", in which many admission
requests arrive within the reaction time of PCN's admission
mechanism, because then they all might get admitted and so
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overload the network. The assumption in this document is that, if
it is necessary, then flash crowds are limited in some fashion
beyond the scope of this document, for instance by rate-limiting
QoS requests.
The applicability of PCN is discussed further in Section 6.
1.4. Documents about PCN
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. This document describes the PCN
architecture at a high level (Section 3) and in more detail
(Section 4). It also defines some terminology, and provides
considerations about operations, management, and security. Section 6
considers the applicability of PCN in more detail, covering its
benefits, deployment scenarios, assumptions, and potential
challenges. The Appendix covers some potential future work items.
Aspects of PCN are also documented elsewhere:
o Metering and marking: [Eardley09] standardises threshold metering
and marking and excess-traffic metering and marking. A PCN-packet
may be marked, depending on the metering results.
o Encoding: the "baseline" encoding is described in [Moncaster09-1],
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, or not PCN-marked), for instance, see
[Moncaster09-2]. (There may be further encoding states as
suggested in [Westberg08].) Section 3.6 considers the backwards
compatibility of PCN encoding with ECN.
o PCN-boundary-node behaviour: how the PCN-boundary-nodes convert
the PCN-markings into decisions about flow admission and flow
termination, as described in Informational documents such as
[Taylor09] and [Charny07-2]. The concept is that the standardised
metering and marking by PCN-nodes allows several possible PCN-
boundary-node behaviours. A number of possibilities are outlined
in this document; detailed descriptions and comparisons are in
[Charny07-1] and [Menth09-2].
o Signalling between PCN-boundary-nodes: signalling is needed to
transport PCN-feedback-information between the PCN-boundary-nodes
(in the example above, this is the fraction of traffic, between
the pair of PCN-boundary-nodes, that is PCN-marked). The exact
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details vary for different PCN-boundary-node behaviours, and so
should be described in those documents. It may require an
extension to the signalling protocol -- standardisation is out of
scope of the PCN WG.
o The interface by which the PCN-boundary-nodes learn identification
information about the admitted flows: the exact requirements vary
for different PCN-boundary-node behaviours and for different
signalling protocols, and so should be described in those
documents. They will be similar to those described in the example
above -- a PCN-ingress-node needs to be able to identify that a
packet is part of a previously admitted flow (typically from its
five-tuple) and each PCN-boundary-node needs to be able to
identify the other PCN-boundary-node for the flow.
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 that in principle can, through PCN-marking packets,
influence decisions about flow admission and termination for the
PCN-domain; includes the PCN-egress-nodes, which measure these
PCN-marks, and the PCN-ingress-nodes.
o PCN-boundary-node: a PCN-node that connects one PCN-domain to a
node either in another PCN-domain or in a non-PCN-domain.
o PCN-interior-node: a node in a PCN-domain that is not a PCN-
boundary-node.
o PCN-node: a PCN-boundary-node or a PCN-interior-node.
o PCN-egress-node: a PCN-boundary-node in its role in handling
traffic as it leaves a PCN-domain.
o PCN-ingress-node: a PCN-boundary-node in its role in handling
traffic as it enters a PCN-domain.
o PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of
different Diffserv behaviour aggregates (BAs) [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.
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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 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. Such a packet is
then called PCN-marked.
o Threshold-metering: a metering behaviour that, if the PCN-traffic
exceeds the PCN-threshold-rate, indicates that all PCN-traffic is
to be threshold-marked.
o PCN-threshold-rate: the reference rate of a threshold-meter, which
is configured for each link in the PCN-domain and which is lower
than the PCN-excess-rate.
o Threshold-marking: the setting of the header in a PCN-packet to a
specific encoding, based on indications from the threshold-meter.
Such a packet is then called threshold-marked.
o Excess-traffic-metering: a metering behaviour that, if the PCN-
traffic exceeds the PCN-excess-rate, indicates that the amount of
PCN-traffic to be excess-traffic-marked is equal to the amount in
excess of the PCN-excess-rate.
o PCN-excess-rate: the reference rate of an excess-traffic-meter,
which is a configured for each link in the PCN-domain and which is
higher than the PCN-threshold-rate.
o Excess-traffic-marking: the setting of the header in a PCN-packet
to a specific encoding, based on indications from the excess-
traffic-meter. Such a packet is then called excess-traffic-
marked.
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
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.
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o Ingress-egress-aggregate: The collection of PCN-packets from all
PCN-flows that travel in one direction between a specific pair of
PCN-boundary-nodes.
o PCN-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. 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 are they 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 is not in 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:
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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, not per flow. There are
two algorithms: one for threshold-metering and one for excess-
traffic-metering. The meters trigger PCN-marking as necessary.
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 [Moncaster09-1], 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 [Moncaster09-2]) and/or set the ECN
field to values other than 11 (eg, as in [Menth08-2]).
In a PCN-domain, the operator may have two or three encoding states
available. The baseline encoding provides two encoding states (not
PCN-marked and PCN-marked), whilst extended encodings can provide
three encoding states (not PCN-marked, threshold-marked, and excess-
traffic-marked).
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. Three encoding states naturally allows
both flow admission and flow termination. If there are only two
encoding states, then there are several options -- see Section 3.3.
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 terminate existing flow(s). Sections
4.4 and 4.5 mention various possibilities for how the functionality
could be distributed.
PCN-metering and PCN-marking need 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 [Eardley09] for further
discussion.
The objective of threshold-marking, as triggered by the threshold-
metering algorithm, is to threshold-mark all PCN-packets whenever the
bit rate of PCN-packets is greater than some configured rate, the
PCN-threshold-rate. The objective of excess-traffic-metering, as
triggered by the excess-traffic-marking algorithm, is to excess-
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RFC 5559 PCN Architecture June 2009
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
algorithms rather than their 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
[Eardley09].
Admission and termination approaches are detailed and compared in
[Charny07-1] and [Menth09-2]. The discussion below is just a brief
summary. Sections 3.1 and 3.2 assume there are three encoding states
available, whilst Section 3.3 assumes there are two encoding states
available.
From the perspective of the outside world, a PCN-domain essentially
looks like a Diffserv domain, but without the Diffserv architecture's
traffic-conditioning agreements. 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.
3.1. 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. This document describes two
approaches (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
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admitted; if the fraction is above the threshold value, then it is
blocked. The fraction could be measured as an EWMA (exponentially
weighted moving average), which has sometimes been called 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-setup packet (eg,
RSVP PATH). Another is that after one (or several) threshold-
marks, 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.
3.2. 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 at which the node drops
packets.
Exactly how the flow termination decision is made will be defined
separately in Informational documents. This document describes
several approaches (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. The difference represents 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 approach, if some nodes
are dropping PCN-traffic.
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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 [Menth08-1].)
Since flow termination is designed for "abnormal" circumstances, it
is quite likely that some PCN-nodes are congested and, hence, that
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.
3.3. 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
[Moncaster09-1], then an operator has three options (others might be
possible):
o admission control only: PCN-marking means threshold-marking, ie,
only the threshold-metering algorithm triggers PCN-marking. Only
PCN admission control is available.
o flow termination only: PCN-marking means excess-traffic-marking,
ie, only the excess-traffic-metering algorithm triggers PCN-
marking. Only PCN termination control is available.
o both admission control and flow termination: only the excess-
traffic-metering algorithm triggers PCN-marking; however, the
configured rate (PCN-excess-rate) is set equal to the PCN-
admissible-rate, as shown in Figure 3. [Charny07-2] describes how
both admission control and flow termination can be triggered in
this case and also gives some pros and cons of this approach. The
main downside is that admission control is less accurate.
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RFC 5559 PCN Architecture June 2009
== Metering & ==
==Marking behaviour== ==PCN mechanisms==
^
Rate of ^
PCN-traffic on |
bottleneck link | Terminate some
| admitted flows
| &
| Block new flows
|
| Some pkts
U*PCN-excess-rate -| excess-traffic-marked -----------------
(=PCN-supportable-rate)|
| Block new flows
|
|
PCN-excess-rate -|------------------------------------------------
(=PCN-admissible-rate)|
| No pkts Admit new flows
| PCN-marked
|
Figure 3: 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
[Charny07-2]. Note: U is a global parameter for all links in the
PCN-domain.
3.4. 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 -- 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 (Next Steps in Signalling), for instance; [Lefaucheur06]
describes the extensions needed for RSVP.
3.5. 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.
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o It is not advised to have competing-non-PCN-traffic but, if there
is such traffic, there needs to be a mechanism to limit it.
"Competing-non-PCN-traffic" means traffic that shares a link with
PCN-traffic and competes for its forwarding bandwidth. Hence,
more competing-non-PCN-traffic results in poorer QoS for PCN.
Further, the unpredictable amount of competing-non-PCN-traffic
makes the PCN mechanisms less accurate and so reduces PCN's
ability to protect the QoS of admitted PCN-flows.
o Two examples of such competing-non-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
[Moncaster09-1] and there is non-PCN, voice-admit traffic in
the PCN-domain).
o If there is such competing-non-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 competing-non-PCN-traffic
contributes to the PCN-meters (ie, is metered by the threshold-
marking and excess-traffic-marking algorithms).
o There will be other non-PCN-traffic that doesn't compete for the
same forwarding bandwidth 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.
o This 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.
3.6. 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;
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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 [Moncaster09-1], 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 need to take different "appropriate
action".
4. 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 the normal router
functions for PCN-nodes. Then, it describes the further
functionality needed for both flow admission control and flow
termination; these are signalling and decision-making functions, and
there are various possibilities for where the functions are
physically located. The section is split into:
1. functions needed at PCN-interior-nodes
2. functions needed at PCN-ingress-nodes
3. functions needed at PCN-egress-nodes
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
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4.1. PCN-Interior-Node Functions
Each link of the PCN-domain is configured with the following
functionality:
o Behaviour aggregate classification - determine whether or not an
incoming packet is a PCN-packet.
o PCN-meter - measure the "amount of PCN-traffic". The measurement
is made on the overall PCN-traffic, not per flow. Algorithms
determine whether to indicate to the PCN-marking functionality
that packets should be PCN-marked.
o PCN-mark - as triggered by indications from the PCN-meter
functionality; if necessary, PCN-mark packets with the appropriate
encoding.
o Drop - if the queue overflows, then naturally packets are dropped.
In addition, the link may be configured with a maximum rate for
PCN-traffic (below the physical link rate), above which PCN-
packets are dropped.
The functions are defined in [Eardley09] and the baseline encoding in
[Moncaster09-1] (extended encodings are to be defined in other
documents).
+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+----------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream | | | | | | | Stream
+----------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+
Figure 4: Schematic of PCN-interior-node functionality.
4.2. PCN-Ingress-Node Functions
Each ingress link of the PCN-domain is configured with the following
functionality:
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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, port numbers, and
protocol).
o Police - police, by dropping any packets received with a DSCP
indicating PCN transport that do not belong to an admitted flow.
(A prospective PCN-flow that is 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, see
[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 [Moncaster09-1].
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 [Briscoe06] for an
example using RSVP). PCN-colouring allows the rest of the PCN-domain
to recognise PCN-packets.
4.3. 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. Alternatives
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-
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marked packets. For PCN-colouring, [Moncaster09-1] specifies that
the PCN-egress-node resets the ECN field to 00; other encodings may
define different behaviour.
4.4. 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 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", the decision is based on whether or not one
(or several) packets are PCN-marked (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 will 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 the amount of PCN-marks on the
ingress-egress-aggregate, whereas the PCN-ingress-node is the
policy enforcement point [RFC2753] that 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).
Note: Admission control functionality is not performed by normal PCN-
interior-nodes.
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4.5. Flow Termination Functions
As well as the functions covered above, other specific termination
control functions need to be performed (others might be possible):
o PCN-meter at PCN-egress-node - similarly to flow admission, there
are two types of possibilities: to "measure PCN-traffic" on the
ingress-egress-aggregate, or 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
[Briscoe06], 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 Section 4.4.
Note: Flow termination functionality is not performed by normal PCN-
interior-nodes.
4.6. Addressing
PCN-nodes may need to know the address of other PCN-nodes. Note that
PCN-interior-nodes don't need to know the address of other PCN-nodes
(except their next-hop neighbours for routing purposes).
At a minimum, the PCN-egress-node needs to know the address of the
PCN-ingress-node associated with a flow so that the PCN-ingress-node
can be informed of the admission decision (and any flow termination
decision) and enforce it through policing. There are various
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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, through the regular processing of an RSVP PATH message,
as the PCN-ingress-node is the previous RSVP hop (PHOP)
([Lefaucheur06]). Another option is that the PCN-ingress-node
could signal its address to the PCN-egress-node.
o Always tunnel PCN-traffic across the PCN-domain. Then the PCN-
ingress-node's address is simply the source address of the outer
packet header. The PCN-ingress-node needs to learn the address of
the PCN-egress-node, either by manual configuration or by one of
the automated tunnel endpoint discovery mechanisms (such as
signalling or probing over the data route, interrogating routing,
or using a centralised broker).
4.7. 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 both
this and the approach of [RFC4303] and [Briscoe09], 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],
with the "full-functionality option" of [RFC3168], and with
[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 that the order of increasing severity is: not PCN-marked,
threshold-marked, and excess-traffic-marked.
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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 4.3) 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, and, even if
ECMP (Equal Cost Multi-Path) is running, all PCN-packets on a
particular ingress-egress-aggregate follow the same path (for more on
ECMP, see Section 6.4). But such tunnelling also has drawbacks, for
example, the additional overhead in terms of bandwidth and processing
as well as 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 [Moncaster09-1], 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
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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 [Briscoe09].
4.8. 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
that new flows will not be admitted until enough existing flows have
departed. Re-routing may result in heavy (pre-)congestion, which
will cause the flow termination mechanism to 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, [Briscoe09] considers what happens if RSVP is the QoS
signalling protocol.
5. Operations and Management
This section considers operations and management issues, under the
FCAPS headings: Faults, Configuration, Accounting, Performance, and
Security. Provisioning is discussed with performance.
5.1. Fault Operations and Management
Fault Operations and Management 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 4 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
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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 5.2) to upgrade resources in danger of causing
service degradation or to trigger manual tracing of intermittent
incipient errors that would otherwise have gone unnoticed.
Finally, of course, many faults are caused by failings in the
management process ("human error"): a wrongly configured address in a
node, a wrong address given in a signalling protocol, a wrongly
configured parameter in a queueing algorithm, a node set into a
different mode from other nodes, and so on. Generally, a clean
design with few configurable options ensures this class of faults can
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 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.
5.2. Configuration Operations and Management
Threshold-metering and -marking and excess-traffic-metering and
-marking are standardised in [Eardley09]. However, more diversity in
PCN-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.
PCN-metering behaviour 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.
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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
(Simple Network Management Protocol) 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 (such as 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.
5.2.1. 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.
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
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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 their 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 Sections 3.1 and 3.2).
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 and by IP packet filter spec) and how
to take account of ECMP.
o If an equipment vendor provides a choice, there may be options for
selecting which smoothing algorithm to use for measurements.
5.2.2. Parameters
Like any Diffserv domain, every node within a PCN-domain will need to
be configured with the DSCP(s) used to identify PCN-packets. On each
interior link, the main configuration parameters are the PCN-
threshold-rate and PCN-excess-rate. A larger PCN-threshold-rate
enables more PCN-traffic to be admitted on a link, hence improving
capacity utilisation. A PCN-excess-rate set further above the PCN-
threshold-rate allows greater increases in traffic (whether due to
natural fluctuations or some unexpected event) before any flows are
terminated, ie, minimises the chances of unnecessarily triggering the
termination mechanism. For instance, an operator may want to design
their network so that it can cope with a failure of any single PCN-
node without terminating any flows.
Setting these rates on the 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 [Menth09-1]. But, once a domain is in
operation, a PCN design goal is to be able to determine growth in
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these configured rates much more simply, by monitoring PCN-marking
rates from actual rather than expected traffic (see Section 5.4 on
Performance and 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-metering requires a threshold token bucket depth to be
configured, excess-traffic-metering requires 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 is preferable to have
rules that set defaults for these parameters but to then allow
operators to change them -- for instance, if average traffic
characteristics change over time.
The PCN-egress-node may allow configuration of:
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.
One particular approach [Charny07-2] would require a global parameter
to be defined on all PCN-nodes, but would only need one PCN-marking
rate to be configured on each link. The global parameter is a
scaling factor between admission and termination (the rate of PCN-
traffic on a link up to which flows are admitted vs. the rate above
which flows are terminated). [Charny07-2] discusses in full the
impact of this particular approach on the operation of PCN.
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5.3. Accounting Operations and Management
Accounting is only done at trust boundaries so it is out of scope of
this document, 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.
5.4. Performance and Provisioning Operations and Management
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 that
is 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. However, there will usually be
sufficient physical capacity for the upgrade to go ahead as a simple
configuration change. Alternatively, [Songhurst06] describes 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.
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All the above processes require just three extra counters associated
with each PCN queue: threshold-markings, excess-traffic-markings, and
drops. 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.
5.5. Security Operations and Management
Security Operations and Management 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]. The correct functions of
the system should be monitored (Section 5.4) 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
mean opinion score) should raise an alarm. The following are
examples of symptoms that may be the result of innocent faults,
rather than attacks; however, until diagnosed, they should be logged
and should 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 that are 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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6. Applicability of PCN
6.1. Benefits
The key benefits of the PCN mechanisms 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 to
be small enough that admitted traffic can still be carried after a
re-routing in most failure cases [Menth09-1]. 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 [Iyer03].
o The PCN-metering behaviours only operate on the overall PCN-
traffic on the link, not per flow.
o The information of these measurements is signalled to the PCN-
egress-nodes by the PCN-marks in the packet headers, ie, "in-
band". No additional signalling protocol is required for
transporting the PCN-marks. Therefore, no secure binding is
required between data packets and separate congestion messages.
o The PCN-egress-nodes make separate measurements, operating on the
aggregate PCN-traffic from each PCN-ingress-node, ie, not per
flow. Similarly, signalling by the PCN-egress-node of PCN-
feedback-information (which is used for flow admission and
termination decisions) is at the granularity of the ingress-
egress-aggregate. An alternative approach is that the PCN-egress-
nodes monitor the PCN-traffic and signal PCN-feedback-information
(which is used for flow admission and termination decisions) at
the granularity of one (or a few) PCN-marks.
o The admitted PCN-load is controlled dynamically. Therefore, it
adapts as the traffic matrix changes. It also adapts 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.
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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 the malfunction of
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 links
in that group [RFC4216]). Fully protecting traffic against a
single SRLG failure requires low utilisation (~10%) of the link
bandwidth on some links before failure [Charny08].
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 the
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. This way, the operator has
to verify that provision is sufficient each time a new customer is
added to check that the Service Level Agreement can be fulfilled.
A PCN-domain doesn't need such traffic conditioning.
6.2. Deployment Scenarios
Operators of networks will want to use the PCN mechanisms in various
arrangements depending, for instance, 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.
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, and excess-traffic-marked. This way,
both PCN admission control and flow termination can be supported. As
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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.
On the other hand, a PCN-domain may have two encoding states (as in
[Moncaster09-1]) (or pedantically, an operator may choose to use up
two encoding states for PCN): not PCN-marked and PCN-marked. This
way, there are three possibilities, as discussed in the following
paragraphs (see also Section 3.3).
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; 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 on statically provisioned
capacity per PCN-ingress-node (regardless of the PCN-egress-node of a
flow) for admission control, as is typical in the hose model of the
Diffserv architecture [Kumar01]. 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 here is that
admission control is less accurate [Charny07-2]. This possibility is
illustrated in Figure 3.
Within the PCN-domain, there is some flexibility about how the
decision-making functionality is distributed. These possibilities
are outlined in Section 4.4 and are also discussed elsewhere, such as
in [Menth09-2].
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
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and not be done at the PCN-ingress-node. Note that, to aid
readability, the rest of this document 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, [Briscoe06] 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
(Next Steps in Signalling) [RFC3726] instead of RSVP.
It is possible that a user wants its inelastic traffic to use the PCN
mechanisms but also react to ECN markings outside the PCN-domain
[Sarker08]. 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
[Moncaster09-2]. Tunnelling is discussed further in Section 4.7.
Some further possible deployment models are outlined in the Appendix.
6.3. Assumptions and Constraints on Scope
The scope of this document is 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.
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4. PCN-flows may have different precedence, but the applicability of
the PCN mechanisms for emergency use (911, GETS (Government
Telecommunications Service), WPS (Wireless Priority Service),
MLPP (Multilevel Precedence and Premption), etc.) is out of
scope.
6.3.1. Assumption 1: Trust and Support of PCN - Controlled Environment
It is assumed 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 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 send an 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 Section 7.
One way of assuring the above two points are in effect is to have the
entire PCN-domain run by a single operator. Another way is to have
several operators that trust each other in their handling of PCN-
traffic.
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.
6.3.2. Assumption 2: Real-Time Applications
It is assumed 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]. Examples of such real-time
applications include 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
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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-metering and PCN-marking is
being applied to traffic scheduled with an expedited forwarding per-
hop behaviour [RFC3246] or with a per-hop behaviour with similar
characteristics.
6.3.3. Assumption 3: Many Flows and Additional Load
It is assumed 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, but 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.
No explicit assumptions are made about how many PCN-flows are in each
ingress-egress-aggregate. Performance-evaluation work may clarify
whether it is necessary to make any additional assumptions on
aggregation at the ingress-egress-aggregate level.
6.3.4. 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 this document.
6.4. Challenges
Prior work on PCN and similar mechanisms has led to a number of
considerations about PCN's design goals (things PCN should be good
at) and some issues that have been hard to solve in a fully
satisfactory manner. Taken as a whole, PCN represents a list of
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trade-offs (it is unlikely that they can all be 100% achieved) and
perhaps a list of evaluation criteria to help an operator (or the
IETF) decide between options.
The following are open issues. They are mainly taken from
[Briscoe06], 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.
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 [Hancock02].
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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: Such
scenarios 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 for 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
(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, if a big burst of admission requests
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potentially 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.
7. Security Considerations
Security considerations essentially come from the Trust Assumption
Section 6.3.1, ie, that all PCN-nodes are PCN-enabled and are trusted
for truthful PCN-metering and PCN-marking. 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 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.
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RFC 5559 PCN Architecture June 2009
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.
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 RSVP
protocol specifically, hop-by-hop authentication is in [RFC2747],
and [Behringer09] may also be useful.
Operational security advice is given in Section 5.5.
8. Conclusions
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. The main topic is the functional
architecture. This document also mentions other topics like the
assumptions and open issues associated with the PCN architecture.
9. Acknowledgements
This document is a revised version of an earlier individual working
draft authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R.
Geib, G. Karagiannis, M. Menth, and T. Tsou. They are therefore
contributors to this document.
Thanks to those who have made comments on this document: Lachlan
Andrew, Joe Babiarz, Fred Baker, David Black, Steven Blake, Ron
Bonica, Scott Bradner, Bob Briscoe, Ross Callon, Jason Canon, Ken
Carlberg, Anna Charny, Joachim Charzinski, Andras Csaszar, Francis
Dupont, Lars Eggert, Pasi Eronen, Adrian Farrel, Ruediger Geib, Wei
Gengyu, Robert Hancock, Fortune Huang, Christian Hublet, Cullen
Jennings, Ingemar Johansson, Georgios Karagiannis, Hein Mekkes,
Michael Menth, Toby Moncaster, Dimitri Papadimitriou, Dan Romascanu,
Daisuke Satoh, Ben Strulo, Tom Taylor, Hannes Tschofenig, Tina Tsou,
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RFC 5559 PCN Architecture June 2009
David Ward, Lars Westberg, Magnus Westerlund, and 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; their authors 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, and A. Bhargava.
The admission control mechanism evolved from the work led by Martin
Karsten on the Guaranteed Stream Provider developed in the M3I
project [Karsten02] [M3I], which in turn was based on the theoretical
work of Gibbens and Kelly [Gibbens99].
10. References
10.1. 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.
10.2. Informative References
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and
S. Jamin, "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC 2205,
September 1997.
[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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RFC 5559 PCN Architecture June 2009
[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.
[RFC3726] Brunner, M., "Requirements for Signaling Protocols",
RFC 3726, April 2004.
[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.
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RFC 5559 PCN Architecture June 2009
[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. 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.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label
Switching (MPLS) Label Stack Entry: "EXP" Field
Renamed to "Traffic Class" Field", RFC 5462,
February 2009.
[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.
[Babiarz06] Babiarz, J., Chan, K., Karagiannis, G., and P.
Eardley, "SIP Controlled Admission and Preemption",
Work in Progress, October 2006.
[Behringer09] Behringer, M. and F. Le Faucheur, "Applicability of
Keying Methods for RSVP Security", Work in Progress,
March 2009.
[Briscoe06] Briscoe, B., Eardley, P., Songhurst, D., Le
Faucheur, F., Charny, A., Babiarz, J., Chan, K.,
Dudley, S., Karagiannis, G., Bader, A., and L.
Westberg, "An edge-to-edge Deployment Model for Pre-
Congestion Notification: Admission Control over a
Diffserv Region", Work in Progress, October 2006.
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RFC 5559 PCN Architecture June 2009
[Briscoe08] Briscoe, B., "Emulating Border Flow Policing using
Re-PCN on Bulk Data", Work in Progress,
September 2008.
[Briscoe09] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", Work in Progress, March 2009.
[Bryant08] Bryant, S., Davie, B., Martini, L., and E. Rosen,
"Pseudowire Congestion Control Framework", Work
in Progress, May 2008.
[Charny07-1] Charny, A., Babiarz, J., Menth, M., and X. Zhang,
"Comparison of Proposed PCN Approaches", Work
in Progress, November 2007.
[Charny07-2] Charny, A., Zhang, X., Le Faucheur, F., and V.
Liatsos, "Pre-Congestion Notification Using Single
Marking for Admission and Termination", Work
in Progress, November 2007.
[Charny07-3] Charny, A., "Email to PCN WG mailing list",
November 2007, <http://www1.ietf.org/mail-archive/
web/pcn/current/msg00871.html>.
[Charny08] Charny, A., "Email to PCN WG mailing list",
March 2008, <http://www1.ietf.org/mail-archive/web/
pcn/current/msg01359.html>.
[Eardley07] Eardley, P., "Email to PCN WG mailing list",
October 2007, <http://www1.ietf.org/mail-archive/
web/pcn/current/msg00831.html>.
[Eardley09] Eardley, P., "Metering and marking behaviour of PCN-
nodes", Work in Progress, May 2009.
[Gibbens99] Gibbens, R. and F. Kelly, "Distributed connection
acceptance control for a connectionless network",
Proceedings International Teletraffic Congress
(ITC16), Edinburgh, pp. 941-952, 1999.
[Hancock02] Hancock, R. and E. Hepworth, "Slide 14 of 'NSIS: An
Outline Framework for QoS Signalling'", May 2002, <h
ttp://www-nrc.nokia.com/sua/nsis/interim/
nsis-framework-outline.ppt>.
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RFC 5559 PCN Architecture June 2009
[Iyer03] Iyer, S., Bhattacharyya, S., Taft, N., and C. Diot,
"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>.
[Karsten02] Karsten, M. and J. Schmitt, "Admission Control Based
on Packet Marking and Feedback Signalling --
Mechanisms, Implementation and Experiments", TU-
Darmstadt Technical Report TR-KOM-2002-03, May 2002,
<http://www.kom.e-technik.tu-darmstadt.de/
publications/abstracts/KS02-5.html>.
[Kumar01] Kumar, A., Rastogi, R., Silberschatz, A., and B.
Yener, "Algorithms for Provisioning Virtual Private
Networks in the Hose Model", Proceedings ACM SIGCOMM
(ITC16), , 2001.
[Lefaucheur06] Le Faucheur, F., Charny, A., Briscoe, B., Eardley,
P., Babiarz, J., and K. Chan, "RSVP Extensions for
Admission Control over Diffserv using Pre-congestion
Notification (PCN)", Work in Progress, June 2006.
[M3I] "M3I - Market Managed Multiservice Internet",
<http://www.m3iproject.org/>.
[Menth08-1] Menth, M., Lehrieder, F., Eardley, P., Charny, A.,
and J. Babiarz, "Edge-Assisted Marked Flow
Termination", Work in Progress, February 2008.
[Menth08-2] Menth, M., Babiarz, J., Moncaster, T., and B.
Briscoe, "PCN Encoding for Packet-Specific Dual
Marking (PSDM)", Work in Progress, July 2008.
[Menth09-1] Menth, M. and M. Hartmann, "Threshold Configuration
and Routing Optimization for PCN-Based Resilient
Admission Control", Computer Networks, 2009,
<http://dx.doi.org/10.1016/j.comnet.2009.01.013>.
[Menth09-2] Menth, M., Lehrieder, F., Briscoe, B., Eardley, P.,
Moncaster, T., Babiarz, J., Chan, K., Charny, A.,
Karagiannis, G., Zhang, X., Taylor, T., Satoh, D.,
and R. Geib, "A Survey of PCN-Based Admission
Control and Flow Termination", IEEE
Communications Surveys and Tutorials, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/
Publications/papers/Menth08-PCN-Overview.pdf>>.
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RFC 5559 PCN Architecture June 2009
[Moncaster09-1] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion
Information", Work in Progress, May 2009.
[Moncaster09-2] Moncaster, T., Briscoe, B., and M. Menth, "A PCN
encoding using 2 DSCPs to provide 3 or more states",
Work in Progress, April 2009.
[Sarker08] Sarker, Z. and I. Johansson, "Usecases and Benefits
of end to end ECN support in PCN Domains", Work
in Progress, November 2008.
[Songhurst06] Songhurst, DJ., Eardley, P., Briscoe, B., Di Cairano
Gilfedder, C., and J. Tay, "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>.
[Taylor09] Charny, A., Huang, F., Menth, M., and T. Taylor,
"PCN Boundary Node Behaviour for the Controlled Load
(CL) Mode of Operation", Work in Progress,
March 2009.
[Tsou08] Tsou, T., Huang, F., and T. Taylor, "Applicability
Statement for the Use of Pre-Congestion Notification
in a Resource-Controlled Network", Work in Progress,
November 2008.
[Westberg08] Westberg, L., Bhargava, A., Bader, A., Karagiannis,
G., and H. Mekkes, "LC-PCN: The Load Control PCN
Solution", Work in Progress, November 2008.
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Appendix A. Possible Future Work Items
This section mentions some topics that are outside the PCN WG's
current charter but that 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, or 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 described
in Section 6.3:
o A single PCN-domain encompasses several autonomous systems that do
not trust each other. A possible solution is a mechanism like re-
PCN [Briscoe08].
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
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 Appendix A.1 below.
o The PCN-domain extends to the end users. This scenario is
described in [Babiarz06]. The end users need to be trusted to do
their own policing. If there is sufficient traffic, then the
aggregation assumption may hold. A variant is that the PCN-domain
extends out as far as the LAN edge switch.
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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). Such
possibility 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 [Tsou08].
o Signalling extensions for specific protocols (eg, RSVP and 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 for coping
with failure of a PCN-boundary-node ([Briscoe06] considers what
happens if RSVP is the QoS signalling protocol) and for
establishing a tunnel across the PCN-domain if it is necessary to
carry ECN marks transparently.
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 [Bryant08].
o PCN for MPLS. [RFC3270] defines how to support the Diffserv
architecture in MPLS (Multiprotocol Label Switching) networks.
[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 [RFC5462] re-names 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
of the IETF's remit.
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A.1. Probing
A.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. This 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, a
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
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.
A.1.2. Probing Functions
The probing functions are:
o Make the decision that probing is needed. As described above,
this is when the ingress-egress-aggregate (or the ECMP path -- see
Section 6.4) carries no PCN-traffic. An alternative is to always
probe, ie, probe before admitting any PCN-flow.
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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-metering algorithm; for
example, an excess-traffic-metering algorithm triggers fewer PCN-
marks than a threshold-metering 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.
A.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
(eg, ECMP) is running in the PCN-domain. We now consider each in
turn.
The first viewpoint assumes the following:
o There is no PCN-traffic on the ingress-egress-aggregate (so a
normal admission decision cannot be made).
o Simply admitting the new flow has a significant risk of leading to
overload: packets dropped or flows terminated.
On the former bullet, [Eardley07] 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.
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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 (eg, 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:
* 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 of
blocking a flow that should be admitted.
Note: [Charny07-3] suggests unbalanced traffic is quite
possible, even with quite a large number of flows on a PCN-link
(eg, 1000), when Assumption 3 (aggregation) is likely to be
satisfied.
(2) If admission control is based on measurements of pre-congestion
on specific ECMP paths, then the viewpoint that probing is
useful assumes:
* There is no PCN-traffic on the ECMP path on which to base an
admission decision.
* Simply admitting the new flow has a significant risk of
leading to overload.
* The PCN-egress-node can match a packet to an ECMP path.
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 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-
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boundary-nodes. Tunnelling also solves the issue in the point
immediately above (which is otherwise hard to solve 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 these viewpoints 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?
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).
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Author's Address
Philip Eardley (editor)
BT
B54/77, Sirius House Adastral Park Martlesham Heath
Ipswich, Suffolk IP5 3RE
United Kingdom
EMail: philip.eardley@bt.com
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