One document matched: draft-ietf-pcn-architecture-11.xml
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<front>
<title abbrev="PCN Architecture">Pre-Congestion Notification (PCN)
Architecture</title>
<author fullname="Philip Eardley" initials="Philip"
surname="Eardley (Editor)">
<organization>BT</organization>
<address>
<postal>
<street>B54/77, Sirius House Adastral Park Martlesham Heath</street>
<city>Ipswich</city>
<code>IP5 3RE</code>
<region>Suffolk</region>
<country>United Kingdom</country>
</postal>
<email>philip.eardley@bt.com</email>
</address>
</author>
<date day="7" month="April" year="2009" />
<area>Transport</area>
<workgroup>Congestion and Pre-Congestion Notification Working
Group</workgroup>
<keyword>Quality of Service</keyword>
<keyword>QoS</keyword>
<keyword>Congestion Control</keyword>
<keyword>Differentiated Services</keyword>
<keyword>Admission Control</keyword>
<keyword>Termination</keyword>
<abstract>
<t>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.</t>
</abstract>
<!-- ================================================================ -->
<note title="Status">
<t></t>
</note>
</front>
<middle>
<!-- ================================================================ -->
<section title="Introduction">
<t></t>
<section title="Overview of PCN">
<t>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.</t>
<t>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, then 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
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.</t>
</section>
<section title="Example use case for PCN ">
<t>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 the section is only an example – in particular
there are other possibilities (see later) for how the
PCN-boundary-nodes perform admission control and flow termination.</t>
<t>As a fundamental building block, each link of the PCN-domain
operates a <xref target="PCN08-2"></xref> (Figure 1): <list
style="symbols">
<t>Threshold meter and marker, which marks all PCN-packets if the
PCN traffic rate 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.</t>
<t>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.</t>
</list>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, <xref target="RFC3168"></xref>).
Note that the link only meters the bulk PCN-traffic (and not per
flow). <figure>
<preamble></preamble>
<artwork><![CDATA[ == 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
|
]]></artwork>
<postamble>Figure 1: Example of how the PCN admission control and
flow termination mechanisms operate as the rate of PCN-traffic
increases.</postamble>
</figure></t>
<t>The two forms of PCN-marking are indicated by setting of the ECN
and DSCP (Differentiated Services Codepoint <xref
target="RFC2474"></xref>) 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.</t>
<t>For admission control, we assume end-to-end RSVP signalling
(Resource Reservation Protocol) <xref target="RFC2205"></xref>) 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, <xref
target="RFC3246"></xref>. Hence the overall solution is in line with
the "IntServ over Diffserv" framework defined in <xref
target="RFC2998"></xref>, as shown in Figure 2. <figure>
<preamble></preamble>
<artwork><![CDATA[ ___ ___ _______________________________________ ____ ___
| | | | | 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--------------->
]]></artwork>
<postamble>Figure 2: Example of possible overall QoS
architecture</postamble>
</figure></t>
<t>A source wanting to start a new QoS flow sends an RSVP PATH
message. Normal hop-by-hop IntServ <xref target="RFC1633"></xref> is
used outside the PCN-domain (we assume successfully). The PATH message
travels across the PCN-domain; the PCN-egress-node reads the PHOP
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 contract. 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.</t>
<t>The flow termination mechanism may happen when some abnormal
circumstances 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 it can block any further traffic on the
terminated flow.</t>
</section>
<section title="Applicability of PCN">
<t>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
<xref target="RFC1633"></xref>, PCN only requires per flow state at
the PCN-ingress-nodes. Compared with the Diffserv architecture <xref
target="RFC2475"></xref>, 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 they 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.</t>
<t>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: <list style="symbols">
<t>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, <xref target="RFC2211"></xref>.</t>
<t>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.</t>
<t>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 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.</t>
</list></t>
<t>The applicability of PCN is discussed further in Section 6.</t>
</section>
<section title="Documents about PCN">
<t>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 considerations about operations and 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.</t>
<t>Aspects of PCN are also documented elsewhere:<list style="symbols">
<t>Metering and marking: <xref target="PCN08-2"></xref>
standardises threshold metering and marking, and excess traffic
metering and marking. A PCN-packet may be marked, depending on the
metering results.</t>
<t>Encoding: the "baseline" encoding is described in <xref
target="PCN08-1"></xref>, which standardises two PCN encoding
states (PCN-marked and not PCN-marked), whilst (experimental)
extensions to the baseline encoding can provide three encoding
states (threshold-marked, excess-traffic-marked, not PCN-marked,
or perhaps further encoding states as suggested in <xref
target="Westberg08"></xref>). Section 3.6 considers backwards
compatability of PCN encoding with ECN.</t>
<t>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. 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 <xref target="Charny07-1"></xref> and <xref
target="Menth08-3"></xref>.</t>
<t>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 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.</t>
<t>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.</t>
</list></t>
<t></t>
</section>
</section>
<section title="Terminology">
<t><list style="symbols">
<t>PCN-domain: a PCN-capable domain; a contiguous set of PCN-enabled
nodes that perform Diffserv scheduling <xref
target="RFC2474"></xref>; 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; the
PCN-domain includes the PCN-egress-nodes, which measure these
PCN-marks, and the PCN-ingress-nodes.</t>
<t>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.</t>
<t>PCN-interior-node: a node in a PCN-domain that is not a
PCN-boundary-node.</t>
<t>PCN-node: a PCN-boundary-node or a PCN-interior-node</t>
<t>PCN-egress-node: a PCN-boundary-node in its role in handling
traffic as it leaves a PCN-domain.</t>
<t>PCN-ingress-node: a PCN-boundary-node in its role in handling
traffic as it enters a PCN-domain.</t>
<t>PCN-traffic, PCN-packets, PCN-BA: a PCN-domain carries traffic of
different Diffserv behaviour aggregates (BAs) <xref
target="RFC2474"></xref>. 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.</t>
<t>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 <xref target="RFC2474"></xref>) or some identifiable
collection of microflows.</t>
<t>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.)</t>
<t>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.</t>
<t>PCN-threshold-rate: a reference rate configured for each link in
the PCN-domain, which is lower than the PCN-excess-rate. It is used
by a metering behaviour that determines whether a packet should be
PCN-marked with a first encoding, "threshold-marked".</t>
<t>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.</t>
<t>Threshold-marking: the setting of the header in a PCN-packet to a
specific encoding, based on indications from the
threshold-meter.</t>
<t>PCN-excess-rate: a reference rate configured for each link in the
PCN-domain, which is higher than the PCN-threshold-rate. It is used
by a metering behaviour that determines whether a packet should be
PCN-marked with a second encoding, "excess-traffic-marked".</t>
<t>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 PCN-marked is equal to the amount in excess of
the PCN-excess-rate.</t>
<t>Excess-traffic-marking: the setting of the header in a PCN-packet
to a specific encoding, based on indications from the
excess-traffic-meter.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>PCN-admissible-rate: the rate of PCN-traffic on a link up to
which PCN admission control should accept new PCN-flows.</t>
<t>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.</t>
</list></t>
</section>
<section title="High-level functional architecture">
<t anchor="high-level-arch">The high-level approach is to split
functionality between:</t>
<t><list style="symbols">
<t>PCN-interior-nodes 'inside' the PCN-domain, which monitor their
own state of pre-congestion and mark PCN-packets as appropriate.
They are not flow-aware, nor aware of ingress-egress-aggregates. The
functionality is also done by PCN-ingress-nodes for their outgoing
interfaces (ie those 'inside' the PCN-domain).</t>
<t>PCN-boundary-nodes at the edge of the PCN-domain, which control
admission of new PCN-flows and termination of existing PCN-flows,
based on information from PCN-interior-nodes. This information is in
the form of the PCN-marked data packets (which are intercepted by
the PCN-egress-nodes) and not signalling messages. Generally
PCN-ingress-nodes are flow-aware.</t>
</list></t>
<t>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.</t>
<t>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:</t>
<t><list style="symbols">
<t>the algorithms: a PCN-node meters the amount of PCN-traffic on
each one of its outgoing (or incoming) links. The measurement is
made as an aggregate of all PCN-packets, and not per flow. There are
two algorithms, one for threshold-metering and one for
excess-traffic-metering. The meters trigger PCN-marking as
necessary.</t>
<t>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
<xref target="PCN08-1"></xref>, 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 <xref
target="Moncaster08"></xref>) and/or set the ECN field to values
other than 11 (eg as in <xref target="Menth08-2"></xref>).</t>
</list></t>
<t>In a PCN-domain the operator may have two or three encoding states
available. The baseline encoding provides two encoding states (not
PCN-marked, PCN-marked), whilst extended encodings can provide three
encoding states (not PCN-marked, threshold-marked,
excess-traffic-marked).</t>
<t>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.</t>
<t></t>
<t>The PCN-boundary-nodes monitor the PCN-marked packets in order to
extract information about the current state of the PCN-domain. Based on
this monitoring, a distributed decision is made about whether to admit a
prospective new flow or whether to terminate existing flow(s). Sections
4.4 and 4.5 mention various possibilities for how the functionality
could be distributed.</t>
<t>PCN-metering and PCN-marking needs to be configured on all
(potentially pre-congested) links in the PCN-domain to ensure that the
PCN mechanisms protect all links. The actual functionality can be
configured on the outgoing or incoming interfaces of PCN-nodes - or one
algorithm could be configured on the outgoing interface and the other on
the incoming interface. The important point is that a consistent choice
is made across the PCN-domain to ensure that the PCN mechanisms protect
all links. See <xref target="PCN08-2"></xref> for further
discussion.</t>
<t>The objective of threshold-marking, as triggerd by the
threshold-metering algorithm, is to threshold-mark all PCN-packets
whenever the rate of PCN-packets is greater than some configured rate,
the PCN-threshold-rate. The objective of excess-traffic-metering, as
triggered by the excess-traffic-marking algorithm, is to
excess-traffic-mark PCN-packets at a rate equal to the difference
between the bit rate of PCN-packets and some configured rate, the
PCN-excess-rate. Note that this description reflects the overall intent
of the 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 <xref
target="PCN08-2"></xref>.</t>
<t>Admission and termination approaches are detailed and compared in
<xref target="Charny07-1"></xref> and <xref target="Menth08-3"></xref>.
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.</t>
<t>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.</t>
<section title="Flow admission">
<t>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.</t>
<t>Exactly how the admission control decision is made will be defined
separately in informational documents. This document describes two
approaches (others might be possible):</t>
<t><list style="symbols">
<t>the PCN-egress-node measures (possibly as a moving average) the
fraction of the PCN-traffic that is threshold-marked. The fraction
is measured for a specific ingress-egress-aggregate. If the
fraction is below a threshold value then the new flow is admitted,
and if the fraction is above the threshold value then it is
blocked. The fraction could be measured as an EWMA (exponentially
weighted moving average), which has sometimes been called the
"congestion level estimate".</t>
<t>the PCN-egress-node monitors PCN-traffic and if it receives one
(or several) threshold-marked packets, then the new flow is
blocked, otherwise it is admitted. One possibility may be to react
to the marking state of an initial flow set-up packet (eg RSVP
PATH). Another is that after one (or several) threshold-marks then
all flows are blocked until after a specific period of no
congestion.</t>
</list></t>
<t>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.</t>
</section>
<t></t>
<section title="Flow termination">
<t>The objective of PCN's flow termination mechanism is to limit the
PCN-traffic on each link to *roughly* its PCN-supportable-rate, by
terminating some existing PCN-flows, in order to protect the QoS of
the remaining PCN-flows. With three encoding states available, the
PCN-excess-rate is configured by the operator as equal to the
PCN-supportable-rate on each link. It may be set lower than the
traffic rate at which the link becomes congested and the node drops
packets.</t>
<t>Exactly how the flow termination decision is made will be defined
separately in informational documents. This document describes several
approaches (others might be possible):<list style="symbols">
<t>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.</t>
<t>Another approach instead measures the rate of
excess-traffic-marked traffic and terminates this amount of
traffic. This terminates less traffic than the previous bullet if
some nodes are dropping PCN-traffic.</t>
<t>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, <xref
target="Menth08-1"></xref>.)</t>
</list>Since flow termination is designed for "abnormal"
circumstances, it is quite likely that some PCN-nodes are congested
and hence packets are being dropped and/or significantly queued. The
flow termination mechanism must accommodate this.</t>
<t>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.</t>
</section>
<section title="Flow admission and/or flow termination when there are only two PCN encoding states">
<t>If a PCN-domain has only two encoding states available (PCN-marked
and not PCN-marked), ie it is using the baseline encoding <xref
target="PCN08-1"></xref>, then an operator has three options (others
might be possible):</t>
<t><list style="symbols">
<t>admission control only: PCN-marking means threshold-marking, ie
only the threshold-metering algorithm triggers PCN-marking. Only
PCN admission control is available.</t>
<t>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.</t>
<t>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. <xref
target="Charny07-2"></xref> describes how both admission control
and flow termination can be triggered in this case and also gives
some of the pros and cons of this approach. The main downside is
that admission control is less accurate.</t>
</list></t>
</section>
<t><figure>
<preamble></preamble>
<artwork><![CDATA[ == 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
|
]]></artwork>
<postamble>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 <xref target="Charny07-2"></xref>. Note: U is a global
parameter for all links in the PCN-domain.</postamble>
</figure></t>
<section title="Information transport">
<t>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, for instance; <xref target="Lefaucheur06"></xref>
describes the extensions needed for RSVP.</t>
</section>
<section title="PCN-traffic">
<t>The following are some high-level points about how PCN works:</t>
<t><list style="symbols">
<t>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.</t>
<t>It is not advised to have non PCN-traffic that competes for the
same capacity as PCN-traffic but, if there is such traffic, there
needs to be a mechanism to limit it. “Capacity” means
the forwarding bandwidth on a link; “competes” means
that non PCN-packets will delay PCN-packets in the queue for the
link. Hence more non PCN-traffic results in poorer QoS for PCN.
Further, the unpredictable amount of non PCN-traffic makes the PCN
mechanisms less accurate and so reduces PCN’s ability to
protect the QoS of admitted PCN-flows</t>
<t>Two examples of such non PCN-traffic (ie that competes for the
same capacity as PCN-traffic) are:</t>
</list><list style="numbers">
<t>traffic that is priority scheduled over PCN (perhaps a
particular application or an operator's control messages).</t>
<t>traffic that is scheduled at the same priority as PCN (for
example if the Voice-Admit codepoint is used for PCN-traffic <xref
target="PCN08-1"></xref> and there is non-PCN voice-admit traffic
in the PCN-domain).</t>
</list><list style="symbols">
<t>If there is such non PCN-traffic (ie that competes for the same
capacity as PCN-traffic), then PCN’s mechanisms should take
account of it, in order to improve the accuracy of the decision
about whether to admit (or terminate) a PCN-flow. For example, one
mechanism is that such non PCN-traffic contributes to the PCN
meters (ie is metered by the threshold-marking and
excess-traffic-marking algorithms).</t>
<t>There will be non PCN-traffic that doesn’t compete for
the same capacity as PCN-traffic, because it is forwarded at lower
priority. Hence it shouldn’t contribute to the PCN meters.
Examples are best effort and assured forwarding traffic. However,
a PCN-node should dedicate some capacity to lower priority traffic
so that it isn't starved.</t>
<t>The document assumes that the PCN mechanisms are applied to a
single behaviour aggregate in the PCN-domain. However, it would
also be possible to apply them independently to more than one
behaviour aggregate, which are distinguished by DSCP.</t>
</list></t>
</section>
<section title="Backwards compatibility">
<t>PCN specifies semantics for the ECN field that differ from the
default semantics of <xref target="RFC3168"></xref>. A particular PCN
encoding scheme needs to describe how it meets the guidelines of BCP
124 <xref target="RFC4774"></xref> for specifying alternative
semantics for the ECN field. In summary the approach is to:</t>
<t><list style="symbols">
<t>use a DSCP to allow PCN-nodes to distinguish PCN-traffic that
uses the alternative ECN semantics;</t>
<t>define these semantics for use within a controlled region, the
PCN-domain;</t>
<t>take appropriate action if ECN capable, non-PCN traffic arrives
at a PCN-ingress-node with the DSCP used by PCN.</t>
</list>For the baseline encoding <xref target="PCN08-1"></xref>, 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.</t>
<t>Extended encoding schemes may need to take different 'appropriate
action'.</t>
</section>
</section>
<section anchor="detailed-arch" title="Detailed Functional architecture">
<t>This section is intended to provide a systematic summary of the new
functional architecture in the PCN-domain. First it describes functions
needed at the three specific types of PCN-node; these are data plane
functions and are in addition to their normal router functions. Then it
describes further functionality needed for both flow admission control
and flow termination; these are signalling and decision-making
functions, and there are various possibilities for where the functions
are physically located. The section is split into:</t>
<t><list style="numbers">
<t>functions needed at PCN-interior-nodes</t>
<t>functions needed at PCN-ingress-nodes</t>
<t>functions needed at PCN-egress-nodes</t>
<t>other functions needed for flow admission control</t>
<t>other functions needed for flow termination control</t>
</list>Note: Probing is covered in the Appendix.</t>
<t>The section then discusses some other detailed topics:</t>
<t><list style="numbers">
<t>addressing</t>
<t>tunnelling</t>
<t>fault handling</t>
</list></t>
<section title="PCN-interior-node functions">
<t>Each link of the PCN-domain is configured with the following
functionality:</t>
<t><list style="symbols">
<t>Behaviour aggregate classification – determine whether an
incoming packet is a PCN-packet or not.</t>
<t>PCN-meter – measure the ‘amount of
PCN-traffic’. The measurement is made on the overall
PCN-traffic, and not per flow. Algorithms determine whether to
indicate to the PCN-marking functionality that packets should be
PCN-marked.</t>
<t>PCN-mark – as triggered by indications from the PCN-meter
functionality, if necessary PCN-mark packets wth the appropiate
encoding.</t>
<t>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.</t>
</list></t>
<t>The functions are defined in <xref target="PCN08-2"></xref> and the
baseline encoding in <xref target="PCN08-1"></xref> (extended
encodings are to be defined in other documents).</t>
<t><figure>
<preamble></preamble>
<artwork><![CDATA[
+---------+ Result
+->|Threshold|-------+
| | Meter | |
| +---------+ V
+----------+ +- - - - -+ | +------+
| BA | | | | | | Marked
Packet =>|Classifier|==>| Dropper |==?===============>|Marker|==> Packet
Stream | | | | | | | Stream
+----------+ +- - - - -+ | +------+
| +---------+ ^
| | Excess | |
+->| Traffic |-------+
| Meter | Result
+---------+
]]></artwork>
<postamble>Figure 4: Schematic of PCN-interior-node
functionality</postamble>
</figure></t>
</section>
<section title="PCN-ingress-node functions">
<t>Each ingress link of the PCN-domain is configured with the
following functionality:</t>
<t><list style="symbols">
<t>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).</t>
<t>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 <xref
target="RFC1633"></xref> for a microflow and its NSIS
equivalent).</t>
<t>PCN-colour – set the DSCP and ECN fields appropriately
for the PCN-domain, for example as in <xref
target="PCN08-1"></xref>.</t>
<t>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.</t>
</list></t>
<t>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 <xref
target="Briscoe06"></xref> for an example using RSVP). PCN-colouring
allows the rest of the PCN-domain to recognise PCN-packets.</t>
</section>
<t></t>
<section title="PCN-egress-node functions">
<t>Each egress link of the PCN-domain is configured with the following
functionality:</t>
<t><list style="symbols">
<t>Packet classify – determine which PCN-ingress-node a
PCN-packet has come from.</t>
<t>Meter – "measure PCN-traffic" or "monitor PCN-marks".</t>
<t>PCN-colour – for PCN-packets, set the DSCP and ECN fields
to the appropriate values for use outside the PCN-domain.</t>
</list></t>
<t>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-marked packets. For PCN-colouring, <xref target="PCN08-1"></xref>
specifies that the PCN-egress-node re-sets the ECN field to 00; other
encodings may define different behaviour.</t>
</section>
<t></t>
<section title="Admission control functions">
<t>As well as the functions covered above, other specific admission
control functions need to be performed (others might be possible):</t>
<t><list style="symbols">
<t>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", then the decision is based
on whether one (or several) packets is (are) PCN-marked or not (eg
the RSVP PATH message). In either case, the admission decision
also takes account of policy and application layer requirements
<xref target="RFC2753"></xref>.</t>
<t>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.</t>
</list>There are various possibilities for how the functionality
could be distributed (we assume the operator would configure which is
used):</t>
<t><list style="symbols">
<t>The decision is made at the PCN-egress-node and the decision
(admit or block) is signalled to the PCN-ingress-node.</t>
<t>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, but the
PCN-ingress-node is the policy enforcement point <xref
target="RFC2753"></xref>, which polices incoming traffic to ensure
it is part of an admitted PCN-flow.</t>
<t>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.</t>
<t>The decision is made at a centralised node (see Appendix).</t>
</list></t>
</section>
<t>Note: Admission control functionality is not performed by normal
PCN-interior-nodes.</t>
<section title="Flow termination functions">
<t>As well as the functions covered above, other specific termination
control functions need to be performed (others might be possible):</t>
<t><list style="symbols">
<t>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, and to
"monitor PCN-marks" and react to one (or several) PCN-marks.</t>
<t>(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.</t>
<t>(if required) Communicate PCN-feedback-information to the node
that makes the flow termination decision. For example, as in <xref
target="Briscoe06"></xref>, communicate the PCN-egress-node's
measurements to the PCN-ingress-node.</t>
<t>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 <xref target="RFC2753"></xref>.</t>
<t>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.</t>
</list>There are various possibilities for how the functionality
could be distributed, similar to those discussed above in the
Admission control section.</t>
<t>Note: Flow termination functionality is not performed by normal
PCN-interior-nodes.</t>
</section>
<t></t>
<section title="Addressing">
<t>PCN-nodes may need to know the address of other PCN-nodes. Note: in
all cases PCN-interior-nodes don't need to know the address of any
other PCN-nodes (except as normal their next hop neighbours, for
routing purposes).</t>
<t>The PCN-egress-node needs to know the address of the
PCN-ingress-node associated with a flow, at a minimum so that the
PCN-ingress-node can be informed to enforce the admission decision
(and any flow termination decision) through policing. There are
various possibilities for how the PCN-egress-node can do this, ie
associate the received packet to the correct ingress-egress-aggregate.
It is not the intention of this document to mandate a particular
mechanism.<list style="symbols">
<t>The addressing information can be gathered from signalling. For
example, regular processing of an RSVP PATH message, as the
PCN-ingress-node is the previous RSVP hop (PHOP) (<xref
target="Lefaucheur06"></xref>). Or the PCN-ingress-node could
signal its address to the PCN-egress-node.</t>
<t>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).</t>
</list></t>
</section>
<section title="Tunnelling">
<t>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 <xref target="RFC2983"></xref> should be
re-applied in the PCN context. In line with this and the approach of
<xref target="RFC4303"></xref> and <xref target="Briscoe08-2"></xref>,
the following rule is applied if encapsulation is done within the
PCN-domain:</t>
<t><list style="symbols">
<t>any PCN-marking is copied into the outer header</t>
</list>Note: A tunnel will not provide this behaviour if it complies
with <xref target="RFC3168"></xref> tunnelling in either mode, but it
will if it complies with <xref target="RFC4301"></xref> IPSec
tunnelling.</t>
<t>Similarly, in line with the “uniform conceptual model”
of <xref target="RFC2983"></xref>, the “full-functionality
option” of <xref target="RFC3168"></xref>, and <xref
target="RFC4301"></xref>, the following rule is applied if
decapsulation is done within the PCN-domain:</t>
<t><list style="symbols">
<t>if the outer header's marking state is more severe then it is
copied onto the inner header.</t>
</list>Note: the order of increasing severity is: not PCN-marked;
threshold-marked; excess-traffic-marked.</t>
<t>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; even if ECMP is running, all
PCN-packets on a particular ingress-egress-aggregate follow the same
path. (ECMP: Equal Cost Multi-Path, Section 6.4.) But it also has
drawbacks, for example the additional overhead in terms of bandwidth
and processing, and the cost of setting up a mesh of tunnels between
PCN-boundary-nodes (there is an N^2 scaling issue).</t>
<t>Potential issues arise for a “partially PCN-capable
tunnel”, ie where only one tunnel endpoint is in the PCN
domain:</t>
<t><list style="numbers">
<t>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.</t>
<t>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.</t>
</list>In line with the solution for partially capable Diffserv
tunnels in <xref target="RFC2983"></xref>, the following rules are
applied:</t>
<t><list style="symbols">
<t>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.</t>
<t>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.</t>
</list> 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.</t>
<t></t>
<t>Tunnelling constraints were a major factor in the choice of the
baseline encoding. As explained in <xref target="PCN08-1"></xref>,
with current tunnelling endpoints only the 11 codepoint of the ECN
field survives decapsulation, and hence the baseline encoding only
uses the 11 codepoint to indicate PCN-marking. Extended encoding
schemes need to explain their interactions with (or assumptions about)
tunnelling. A lengthy discussion of all the issues associated with
layered encapsulation of congestion notification (for ECN as well as
PCN) is in <xref target="Briscoe08-2"></xref>.</t>
</section>
<section title="Fault handling">
<t>If a PCN-interior-node (or one of its links) fails, then lower
layer protection mechanisms or the regular IP routing protocol will
eventually re-route around it. If the new route can carry all the
admitted traffic, flows will gracefully continue. If instead this
causes early warning of pre-congestion on the new route, then
admission control based on pre-congestion notification will ensure new
flows will not be admitted until enough existing flows have departed.
Re-routing may result in heavy (pre-)congestion, when the flow
termination mechanism will kick in.</t>
<t>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 <xref target="Briscoe08-2"></xref> considers what
happens if RSVP is the QoS signalling protocol.</t>
</section>
</section>
<section title="Operations and Management">
<t>This Section considers operations and management issues, under the
FCAPS headings: the Operations and Management of Faults, Configuration,
Accounting, Performance and Security. Provisioning is discussed with
performance.</t>
<section title="Configuration Operations and Management">
<t>Threshold-metering and -marking and excess-traffic-metering and
-marking are standardised in <xref target="PCN08-2"></xref>. 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.</t>
<t>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.</t>
<t>PCN configuration control variables fall into the following
categories:</t>
<t><list style="symbols">
<t>system options (enabling or disabling behaviours)</t>
<t>parameters (setting levels, addresses etc)</t>
</list>One possibility is that all configurable variables sit within
an SNMP management framework <xref target="RFC3411"></xref>, being
structured within a defined management information base (MIB) on each
node, and being remotely readable and settable via a suitably secure
management protocol (SNMPv3).</t>
<t>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.</t>
<t>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.</t>
<t></t>
<section title="System options">
<t>On PCN-interior-nodes there will be very few system options:</t>
<t><list style="symbols">
<t>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.</t>
<t>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).</t>
</list> PCN-boundary-nodes (ingress and egress) will have more
system options:</t>
<t><list style="symbols">
<t>Which of admission and flow termination are enabled. If any
PCN-interior-node is configured to generate a marking, all
PCN-boundary-nodes must be able to interpret that marking (which
includes understanding, in a PCN-domain that uses only one type
of PCN-marking, whether they are generated by
PCN-interior-nodes' threshold meters or the excess traffic
meters). Therefore all PCN-boundary-nodes must be configured the
same in this respect.</t>
<t>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.</t>
<t>How PCN-markings are translated into admission control and
flow termination decisions (see Section 3.1 and Section
3.2).</t>
</list>PCN-egress-nodes will have further system options:</t>
<t><list style="symbols">
<t>How the mapping should be established between each packet and
its aggregate, eg by MPLS label, by IP packet filter spec; and
how to take account of ECMP.</t>
<t>If an equipment vendor provides a choice, there may be
options to select which smoothing algorithm to use for
measurements.</t>
</list></t>
</section>
<section title="Parameters">
<t>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.</t>
<t>Setting these rates on first deployment of PCN will be very
similar to the traditional process for sizing an admission
controlled network, depending on: the operator's requirements for
minimising flow blocking (grade of service), the expected PCN
traffic load on each link and its statistical characteristics (the
traffic matrix), contingency for re-routing the PCN traffic matrix
in the event of single or multiple failures, and the expected load
from other classes relative to link capacities <xref
target="Menth07"></xref>. But once a domain is in operation, a PCN
design goal is to be able to determine growth in these configured
rates much more simply, by monitoring PCN-marking rates from actual
rather than expected traffic (see Section 5.2 on Performance &
Provisioning).</t>
<t>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.</t>
<t>Threshold-metering requires a threshold token bucket depth to be
configured, excess-traffic-metering needs a value for the MTU
(maximum size of a PCN-packet on the link) and both require setting
a maximum size of their token buckets. It will be preferable for
there to be rules to set defaults for these parameters, but then
allow operators to change them, for instance if average traffic
characteristics change over time.</t>
<t>The PCN-egress-node may allow configuration of the following:</t>
<t><list style="symbols">
<t>how it smooths metering of PCN-markings (eg EWMA
parameters)</t>
</list>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:</t>
<t><list style="symbols">
<t>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;</t>
<t>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;</t>
<t>a parameter to control the trade-off between how quickly
excess flows are terminated, and over-termination.</t>
</list></t>
<t>One particular approach, <xref target="Charny07-2"></xref> would
require a global parameter to be defined on all PCN-nodes, but only
needs one PCN marking rate to be configured on each link. The global
parameter is a scaling factor between admission and termination (the
PCN-traffic rate on a link up to which flows are admitted vs the
rate above which flows are terminated). <xref
target="Charny07-2"></xref> discusses in full the impact of this
particular approach on the operation of PCN.</t>
</section>
</section>
<section title="Performance & Provisioning Operations and Management">
<t>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 <xref
target="RFC3393"></xref>, <xref target="Y.1541"></xref>, loss rate
<xref target="RFC4656"></xref>, mean opinion score <xref
target="P.800"></xref>, 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.</t>
<t>Even better, PCN indicates which resources will probably cause
problems if they are not upgraded soon. This can be achieved by the
management system monitoring the total amount (in bytes) of
PCN-marking generated by each queue over a period. Given possible long
provisioning lead times, pre-congestion volume is the best metric to
reveal whether sufficient persistent demand has occurred to warrant an
upgrade. Because, even before utilisation becomes problematic, the
statistical variability of traffic will cause occasional bursts of
pre-congestion. This 'early warning system' decouples the process of
adding customers from the provisioning process. This should cut the
time to add a customer when compared against admission control
provided over native Diffserv <xref target="RFC2998"></xref>, because
it saves having to verify the capacity planning process before adding
each customer.</t>
<t>Alternatively, before triggering an upgrade, the long term
pre-congestion volume on each link can be used to balance traffic load
across the PCN-domain by adjusting the link weights of the routing
system. When an upgrade to a link’s configured PCN-rates is
required, it may also be necessary to upgrade the physical capacity
available to other classes. But usually there will be sufficient
physical capacity for the upgrade to go ahead as a simple
configuration change. Alternatively, <xref
target="Songhurst06"></xref> 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.</t>
<t>All the above processes require just three extra counters
associated with each PCN queue: threshold-markings,
excess-traffic-markings and drop. Every time a PCN packet is marked or
dropped its size in bytes should be added to the appropriate counter.
Then the management system can read the counters at any time and
subtract a previous reading to establish the incremental volume of
each type of (pre-)congestion. Readings should be taken frequently, so
that anomalous events (eg re-routes) can be distinguished from regular
fluctuating demand if required.</t>
</section>
<t></t>
<section title="Accounting Operations and Management">
<t>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.</t>
</section>
<t></t>
<section title="Fault Operations and Management">
<t>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.</t>
<t>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).</t>
<t><xref target="detailed-arch"></xref> describes how the PCN
architecture has been designed to ensure admitted flows continue
gracefully after recovering automatically from link or node failures.
The need to record and monitor re-routing events affecting signalling
is unchanged by the addition of PCN to a Diffserv domain. Similarly,
re-routing events within the PCN-domain will be recorded and monitored
just as they would be without PCN.</t>
<t>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.</t>
<t>Finally, of course, many faults are caused by failings in the
management process ('human error'): a wrongly configured address in a
node, a wrong address given in a signalling protocol, a wrongly
configured parameter in a queueing algorithm, a node set into a
different mode from other nodes, and so on. Generally, a clean design
with few configurable options ensures this class of faults can be
traced more easily and prevented more often. Sound management practice
at run-time also helps. For instance: a management system should be
used that constrains configuration changes within system rules (eg
preventing an option setting inconsistent with other nodes);
configuration options should also be recorded in an offline database;
and regular automatic consistency checks between live systems and the
database should be performed. PCN adds nothing specific to this class
of problems.</t>
</section>
<t></t>
<section title="Security Operations and Management">
<t>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 <xref target="RFC4778"></xref>, other
than those below. The correct functions of the system should be
monitored (Section 5.2) in multiple independent ways and correlated to
detect possible security breaches. Persistent (pre-)congestion marking
should raise an alarm (both on the node doing the marking and on the
PCN-egress-node metering it). Similarly, persistently poor external
QoS metrics (such as jitter or mean opinion score) should raise an
alarm. The following are examples of symptoms that may be the result
of innocent faults, rather than attacks, but until diagnosed they
should be logged and trigger a security alarm:</t>
<t><list style="symbols">
<t>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).</t>
<t>PCN-capable packets arriving at a PCN-egress-node with no
associated state for mapping them to a valid
ingress-egress-aggregate.</t>
<t>A PCN-ingress-node receiving feedback signals about the
pre-congestion level on a non-existent aggregate, or that are
inconsistent with other signals (eg unexpected sequence numbers,
inconsistent addressing, conflicting reports of the pre-congestion
level, etc).</t>
<t>Pre-congestion marking arriving at a PCN-egress-node with
(pre-)congestion markings focused on particular flows, rather than
randomly distributed throughout the aggregate.</t>
</list></t>
</section>
<t></t>
</section>
<section title="Applicability of PCN">
<t></t>
<section title="Benefits">
<t>The key benefits of the PCN mechanisms are that they are simple,
scalable, and robust because: <list style="symbols">
<t>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).</t>
<t>Admission control is resilient: with PCN QoS is decoupled from
the routing system. Hence in general admitted flows can survive
capacity, routing or topology changes without additional
signalling. The PCN-admissible-rate on each link can be chosen
small enough that admitted traffic can still be carried after a
rerouting in most failure cases <xref target="Menth07"></xref>.
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 <xref target="Iyer03"></xref>.</t>
<t>The PCN-metering behaviours only operate on the overall
PCN-traffic on the link, not per flow.</t>
<t>The information of these measurements is signalled to the
PCN-egress-nodes by the PCN-marks in the packet headers, ie <xref
target="Style"></xref> "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.</t>
<t>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.</t>
<t>The admitted PCN-load is controlled dynamically. Therefore it
adapts as the traffic matrix changes, and also if the network
topology changes (eg after a link failure). Hence an operator can
be less conservative when deploying network capacity, and less
accurate in their prediction of the PCN-traffic matrix.</t>
<t>The termination mechanism complements admission control. It
allows the network to recover from sudden unexpected surges of
PCN-traffic on some links, thus restoring QoS to the remaining
flows. Such scenarios are expected to be rare but not impossible.
They can be caused by large network failures that redirect lots of
admitted PCN-traffic to other links, or by malfunction of the
measurement-based admission control in the presence of admitted
flows that send for a while with an atypically low rate and then
increase their rates in a correlated way.</t>
<t>Flow termination can also enable an operator to be less
conservative when deploying network capacity. It is an alternative
to running links at low utilisation in order to protect against
link or node failures. This is especially the case with SRLGs
(shared risk link groups, which are links that share a resource,
such as a fibre, whose failure affects all those links <xref
target="RFC4216"></xref>). Fully protecting traffic against a
single SRLG failure requires low utilisation (~10%) of the link
bandwidth on some links before failure <xref
target="Charny08"></xref>.</t>
<t>The PCN-supportable-rate may be set below the maximum rate that
PCN-traffic can be transmitted on a link, in order to trigger
termination of some PCN-flows before loss (or excessive delay) of
PCN-packets occurs, or to keep the maximum PCN-load on a link
below a level configured by the operator.</t>
<t>Provisioning of the network is decoupled from the process of
adding new customers. By contrast, with the Diffserv architecture
<xref target="RFC2475"></xref> operators rely on subscription-time
Service Level Agreements, which statically define the parameters
of the traffic that will be accepted from a customer, and so the
operator has to verify provision is sufficient each time a new
customer is added to check that the Service Level Agreement can be
fulfilled. A PCN-domain doesn't need such traffic
conditioning.</t>
</list></t>
<t></t>
</section>
<section anchor="deployment-scenarios" title="Deployment scenarios">
<t>Operators of networks will want to use the PCN mechanisms in
various arrangements, for instance depending on how they are
performing admission control outside the PCN-domain (users after all
are concerned about QoS end-to-end), what their particular goals and
assumptions are, how many PCN encoding states are available, and so
on.</t>
<t>A PCN-domain may have three encoding states (or pedantically, an
operator may choose to use up three encoding states for PCN): not
PCN-marked, threshold-marked, excess-traffic-marked. Then both PCN
admission control and flow termination can be supported. As
illustrated in Figure 1, admission control accepts new flows until the
PCN-traffic rate on the bottleneck link rises above the
PCN-threshold-rate, whilst if necessary the flow termination mechanism
terminates flows down to the PCN-excess-rate on the bottleneck
link.</t>
<t>On the other hand, a PCN-domain may have two encoding states (as in
<xref target="PCN08-1"></xref>) (or pedantically, an operator may
choose to use up two encoding states for PCN): not PCN-marked,
PCN-marked. Then there are three possibilities, as discussed in the
following paragraphs (see also Section 3.3).</t>
<t>First, an operator could just use PCN's admission control, solving
heavy congestion (caused by re-routing) by 'just waiting' - as
sessions end, PCN-traffic naturally reduces, and meanwhile the
admission control mechanism will prevent admission of new flows that
use the affected links. So the PCN-domain will naturally return to
normal operation, but with reduced capacity. The drawback of this
approach would be that, until sufficient sessions have ended to
relieve the congestion, all PCN-flows as well as lower priority
services will be adversely affected.</t>
<t>Second, an operator could just rely for admission control on
statically provisioned capacity per PCN-ingress-node (regardless of
the PCN-egress-node of a flow), as is typical in the hose model of the
Diffserv architecture <xref target="RFC2475"></xref>. 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.</t>
<t>Third, both admission control and flow termination can be triggered
from the single type of PCN-marking; the main downside is that
admission control is less accurate <xref target="Charny07-2"></xref>.
This possibility is illustrated in Figure 3.</t>
<t></t>
<t>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 also discussed elsewhere, such as in <xref
target="Menth08-3"></xref>.</t>
<t></t>
<t>The flow admission and termination decisions need to be enforced
through per flow policing by the PCN-ingress-nodes. If there are
several PCN-domains on the end-to-end path, then each needs to police
at its PCN-ingress-nodes. One exception is if the operator runs both
the access network (not a PCN-domain) and the core network (a
PCN-domain); per flow policing could be devolved to the access network
and not done at the PCN-ingress-node. Note: to aid readability, the
rest of this draft assumes that policing is done by the
PCN-ingress-nodes.</t>
<t></t>
<t>PCN admission control has to fit with the overall approach to
admission control. For instance <xref target="Briscoe06"></xref>
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 <xref
target="RFC2998"></xref>. It would also be possible for the RSVP
signalling to be originated and/or terminated by proxies, with
application-layer signalling between the end user and the proxy (eg
SIP signalling with a home hub). A similar example would use NSIS
signalling instead of RSVP. (NSIS: Next Steps in Signalling, <xref
target="RFC3726"></xref>.)</t>
<t></t>
<t>It is possible that a user wants its inelastic traffic to use the
PCN mechanisms but also react to ECN marking outside the PCN-domain
<xref target="Sarker08"></xref>. 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 <xref target="Moncaster08"></xref>. Tunnelling is discussed
further in Section 4.7.</t>
<t></t>
<t>Some further possible deployment models are outlined in the
Appendix.</t>
</section>
<section title="Assumptions and constraints on scope">
<t>The scope is restricted by the following assumptions:</t>
<t><list style="numbers">
<t>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</t>
<t>all flows handled by these mechanisms are inelastic and
constrained to a known peak rate through policing or shaping</t>
<t>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.</t>
<t>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.</t>
</list></t>
<section title="Assumption 1: Trust and support of PCN - controlled environment">
<t>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 this assumption: <list style="symbols">
<t>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.</t>
<t>Similarly, a PCN-boundary-node has to trust that all the
PCN-nodes mark PCN-traffic consistently. A node not performing
PCN-marking wouldn't be able to alert when it suffered
pre-congestion, which potentially would lead to too many
PCN-flows being admitted (or too few being terminated). Worse, a
rogue node could perform various attacks, as discussed in the
Security Considerations section.</t>
</list></t>
<t>One way of assuring the above two points is that the entire
PCN-domain is run by a single operator. Another possibility is that
there are several operators that trust each other in their handling
of PCN-traffic.</t>
<t>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.</t>
<t></t>
</section>
<section title="Assumption 2: Real-time applications">
<t>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 <xref
target="RFC4594"></xref>. For example, voice and video requiring low
delay, jitter and packet loss, the Controlled Load Service, <xref
target="RFC2211"></xref>, and the Telephony service class, <xref
target="RFC4594"></xref>. This assumption is to help focus the
effort where it looks like PCN would be most useful, ie the sorts of
applications where per flow QoS is a known requirement. In other
words we focus on PCN providing a benefit to inelastic traffic (PCN
may or may not provide a benefit to other types of traffic).</t>
<t>As a consequence, it is assumed that PCN-metering and PCN-marking
is being applied to traffic scheduled with the expedited forwarding
per-hop behaviour, <xref target="RFC3246"></xref>, or a per-hop
behaviour with similar characteristics.</t>
</section>
<section title="Assumption 3: Many flows and additional load">
<t>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, however
the actual network performance must be acceptable during the call
some time later. One issue is that if there are only a few variable
rate flows, then the aggregate traffic level may vary a lot, perhaps
enough to cause some packets to get dropped. If there are many flows
then the aggregate traffic level should be statistically smoothed.
How many flows is enough depends on a number of factors such as the
variation in each flow's rate, the total rate of PCN-traffic, and
the size of the "safety margin" between the traffic level at which
we start admission-marking and at which packets are dropped or
significantly delayed.</t>
<t>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 assumption on
aggregation at the ingress-egress-aggregate level.</t>
</section>
<section title="Assumption 4: Emergency use out of scope">
<t>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 of this document.</t>
</section>
<t></t>
</section>
<section title="Challenges">
<t>Prior work on PCN and similar mechanisms has thrown up a number of
considerations about PCN's design goals (things PCN should be good at)
and some issues that have been hard to solve in a fully satisfactory
manner. Taken as a whole it represents a list of trade-offs (it is
unlikely that they can all be 100% achieved) and perhaps as evaluation
criteria to help an operator (or the IETF) decide between options.</t>
<t>The following are open issues. They are mainly taken from <xref
target="Briscoe06"></xref>, which also describes some possible
solutions. Note that some may be considered unimportant in general or
in specific deployment scenarios or by some operators.</t>
<t>NOTE: Potential solutions are out of scope for this document.</t>
<t><list style="symbols">
<t>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:<list style="numbers">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list></t>
<t>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 <xref target="Hancock02"></xref>.</t>
<t>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.</t>
<t>Scenarios with only one tunnel endpoint in the PCN domain may
make it harder for the PCN-egress-node to gather from the
signalling messages (eg RSVP, NSIS) the identity of the
PCN-ingress-node.</t>
<t>Bi-Directional Sessions: Many applications have bi-directional
sessions - hence there are two microflows that should be admitted
(or terminated) as a pair - for instance a bi-directional voice
call only makes sense if microflows in both directions are
admitted. However, the PCN mechanisms concern admission and
termination of a single flow, and coordination of the decision for
both flows is a matter for the signalling protocol and out of
scope of PCN. One possible example would use SIP pre-conditions.
However, there are others.</t>
<t>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.</t>
<t>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.</t>
<t>Flash crowds and Speed of Reaction: PCN is a measurement-based
mechanism and so there is an inherent delay between packet marking
by PCN-interior-nodes and any admission control reaction at
PCN-boundary-nodes. For example, potentially if a big burst of
admission requests occurs in a very short space of time (eg
prompted by a televote), they could all get admitted before enough
PCN-marks are seen to block new flows. In other words, any
additional load offered within the reaction time of the mechanism
must not move the PCN-domain directly from a no congestion state
to overload. This 'vulnerability period' may have an impact at the
signalling level, for instance QoS requests should be rate limited
to bound the number of requests able to arrive within the
vulnerability period.</t>
<t>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.</t>
</list></t>
</section>
</section>
<!-- -->
<!-- ================================================================ -->
<!-- ================================================================ -->
<section title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section>
<section title="Security considerations">
<t>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.</t>
<t><list style="symbols">
<t>Because the PCN-boundary-nodes are flow-aware, they are trusted
to use that awareness correctly. The degree of trust required
depends on the kinds of decisions they have to make and the kinds of
information they need to make them. There is nothing specific to
PCN.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>The PCN-boundary-nodes should be able to deal with DoS attacks
and state exhaustion attacks based on fast changes in per flow
signalling.</t>
<t>The signalling between the PCN-boundary-nodes must be protected
from attacks. For example the recipient needs to validate that the
message is indeed from the node that claims to have sent it.
Possible measures include digest authentication and protection
against replay and man-in-the-middle attacks. For the specific
protocol RSVP, hop-by-hop authentication is in <xref
target="RFC2747"></xref>, and <xref target="Behringer07"></xref> may
also be useful.</t>
</list></t>
<t>Operational security advice is given in Section 5.5.</t>
</section>
<!-- -->
<!-- ================================================================ -->
<section title="Conclusions">
<t>The document describes a general architecture for flow admission and
termination based on pre-congestion information in order to protect the
quality of service of established inelastic flows within a single
Diffserv domain. The main topic is the functional architecture. It also
mentions other topics like the assumptions and open issues.</t>
</section>
<!-- ================================================================ -->
<section title="Acknowledgements">
<t></t>
<t>This document is a revised version of an earlier individual draft
authored by: P. Eardley, J. Babiarz, K. Chan, A. Charny, R. Geib, G.
Karagiannis, M. Menth, T. Tsou. They are therefore contributors to this
document.</t>
<t>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, David Ward, Lars Westberg, Magnus
Westerlund, Delei Yu. Thanks to Bob Briscoe who extensively revised the
Operations and Management section.</t>
<t>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, A.
Bhargava.</t>
</section>
<!-- ================================================================ -->
<section title="Comments Solicited (to be removed by RFC Editor)">
<t>Comments and questions are encouraged and very welcome. They can be
addressed to the IETF PCN working group mailing list
<pcn@ietf.org>.</t>
<t></t>
</section>
<section title="Changes (to be removed by RFC Editor)">
<section title="Changes from -10 to -11">
<t>Changes to deal with IESG comments from routing area review:</t>
<t><list style="symbols">
<t>Small clarifications to Introduction</t>
<t>the term "marking" now only used to refer only to setting the
codepoint (not as a shorthand for 'metering and setting the
codepoint')</t>
<t>Added Figure 4 (Schematic of PCN-interior-node functionality)
(from <xref target="PCN08-2"></xref></t>
<t>Appendix A brought back into the main body.</t>
<t>Other minor clarifications</t>
</list></t>
</section>
<section title="Changes from -09 to -10">
<t>Changes to deal with IESG comments:</t>
<t><list style="symbols">
<t>New introduction to provide gentler introduction for the PCN
novice: quick summary of PCN’s applicability; quick example
of how it all hangs together in one end-to-end qos scenario; quick
summary of PCN “documentation”</t>
<t>OAM changed to Operations and Management</t>
<t>Processed some of the minor suggestions in the Gen-ART Review
by Francis Dupont</t>
<t>Two wording tweaks in Sections 3.2 & 3.4 (as agreed on
mailing list)</t>
<t>Updated boilerplate. this draft may include material pre- Nov
10 2008 blah.</t>
</list></t>
</section>
<section title="Changes from -08 to -09">
<t>Small changes to deal with WG Chair comments:</t>
<t><list style="symbols">
<t>tweak language in various places to make it more RFC-like and
less that of a scholarly work, for instance from "we propose" to
"this document describes"</t>
<t>tweak language in various places to make it a stand alone
architecture document rather than a discussion of the PCN WG. Now
only mentions WG at start of Annex.</t>
<t>References: IDs are no longer referenced to by the draft
name</t>
<t>References: removed some of less important references to
IDs</t>
</list></t>
</section>
<section title="Changes from -07 to -08">
<t>Small changes from second WG last call:</t>
<t><list style="symbols">
<t>Section 2: added definition for PCN-admissible-rate and
PCN-supportable-rate. Small changes to use these terms as follows:
Section 3, bullets 2 & 9; S6.1 para 1; S6.2 para1; S6.3 bullet
3; added to Figs 1 & 2.</t>
<t>added the phrase "(others might be possible") before the list
of approaches in Section 6.3, 7.4 & 7.5.</t>
<t>added references to RFC2753 (A framework for policy-based
admission control) in S7.4 & S7.5.</t>
<t>throughout, updated references now that marking behaviour &
baseline encoding are WG drafts.</t>
<t>a few typos corrected</t>
</list></t>
</section>
<section title="Changes from -06 to -07">
<t>References re-formatted to pass ID nits. No other changes.</t>
</section>
<section title="Changes from -05 to -06">
<t>Minor clarifications throughout, the least insignificant are as
follows:</t>
<t><list style="symbols">
<t>Section 1: added to the list of encoding states in an
'extended' scheme: "or perhaps further encoding states as
suggested in draft-westberg-pcn-load-control"</t>
<t>Section 2: added definition for PCN-colouring (to clarify that
the term is used consistently differently from 'PCN-marking')</t>
<t>Section 6.1 and 6.2: added "(others might be possible)" before
the list of high level approaches for making flow admission
(termination) decisions.</t>
<t>Section 6.2: corrected a significant typo in 2nd bullet (more
-> less)</t>
<t>Section 6.3: corrected a couple of significant typos in Figure
2</t>
<t>Section 6.5 (PCN-traffic) re-written for clarity. Non
PCN-traffic contributing to PCN meters is now given as an example
(there may be cases where don't need to meter it).</t>
<t>Section 7.7: added to the text about encapsulation being done
within the PCN-domain: "Note: A tunnel will not provide this
behaviour if it complies with [RFC3168] tunnelling in either mode,
but it will if it complies with [RFC4301] IPSec tunnelling."</t>
<t>Section 7.7: added mention of [RFC4301] to the text about
decapsulation being done within the PCN-domain.</t>
<t>Section 8: deleted the text about design goals, since this is
already covered adequately earlier eg in S3.</t>
<t>Section 11: replaced the last sentence of bullet 1 by "There is
nothing specific to PCN."</t>
<t>Appendix: added to open issues: possibility of automatically
and periodically probing.</t>
<t>References: Split out Normative references (RFC2474 &
RFC3246).</t>
</list></t>
</section>
<section title="Changes from -04 to -05">
<t>Minor nits removed as follows:</t>
<t><list style="symbols">
<t>Further minor changes to reflect that baseline encoding is
consensus, standards track document, whilst there can be
(experimental track) encoding extensions</t>
<t>Traffic conditioning updated to reflect discussions in Dublin,
mainly that PCN-interior-nodes don't police PCN-traffic (so
deleted bullet in S7.1) and that it is not advised to have non
PCN-traffic that shares the same capacity (on a link) as
PCN-traffic (so added bullet in S6.5)</t>
<t>Probing moved into Appendix A and deleted the 'third viewpoint'
(admission control based on the marking of a single packet like an
RSVP PATH message) - since this isn't really probing, and in any
case is already mentioned in S6.1.</t>
<t>Minor changes to S9 Operations and management - mainly to
reflect that consensus on marking behaviour has simplified things
so eg there are fewer parameters to configure.</t>
<t>A few terminology-related errors expunged, and two pictures
added to help.</t>
<t>Re-phrased the claim about the natural decision point in
S7.4</t>
<t>Clarified that extended encoding schemes need to explain their
interactions with (or assumptions about) tunnelling (S7.7) and how
they meet the guidelines of BCP124 (S6.6)</t>
<t>Corrected the third bullet in S6.2 (to reflect consensus about
PCN-marking)</t>
</list></t>
</section>
<section title="Changes from -03 to -04">
<t><list style="symbols">
<t>Minor changes throughout to reflect the consensus call about
PCN-marking (as reflected in <xref target="PCN08-2"></xref>).</t>
<t>Minor changes throughout to reflect the current decisions about
encoding (as reflected in <xref target="PCN08-1"></xref> and <xref
target="Moncaster08"></xref>).</t>
<t>Introduction: re-structured to create new sections on Benefits,
Deployment scenarios and Assumptions.</t>
<t>Introduction: Added pointers to other PCN documents.</t>
<t>Terminology: changed PCN-lower-rate to PCN-threshold-rate and
PCN-upper-rate to PCN-excess-rate; excess-rate-marking to
excess-traffic-marking.</t>
<t>Benefits: added bullet about SRLGs.</t>
<t>Deployment scenarios: new section combining material from
various places within the document.</t>
<t>S6 (high level functional architecture): re-structured and
edited to improve clarity, and reflect the latest PCN-marking and
encoding drafts.</t>
<t>S6.4: added claim that the most natural place to make an
admission decision is a PCN-egress-node.</t>
<t>S6.5: updated the bullet about non-PCN-traffic that uses the
same DSCP as PCN-traffic.</t>
<t>S6.6: added a section about backwards compatibility with
respect to <xref target="RFC4774"></xref>.</t>
<t>Appendix A: added bullet about end-to-end PCN.</t>
<t>Probing: moved to Appendix B.</t>
<t>Other minor clarifications, typos etc.</t>
</list></t>
</section>
<section title="Changes from -02 to -03">
<t><list style="symbols">
<t>Abstract: Clarified by removing the term 'aggregated'.
Follow-up clarifications later in draft: S1: expanded
PCN-egress-nodes bullet to mention case where the
PCN-feedback-information is about one (or a few) PCN-marks, rather
than aggregated information; S3 clarified PCN-meter; S5 minor
changes; conclusion.</t>
<t>S1: added a paragraph about how the PCN-domain looks to the
outside world (essentially it looks like a Diffserv domain).</t>
<t>S2: tweaked the PCN-traffic terminology bullet: changed PCN
traffic classes to PCN behaviour aggregates, to be more in line
with traditional Diffserv jargon (-> follow-up changes later in
draft); included a definition of PCN-flows (and corrected a couple
of 'PCN microflows' to 'PCN-flows' later in draft)</t>
<t>S3.5: added possibility of downgrading to best effort, where
PCN-packets arrive at PCN-ingress-node already ECN marked (CE or
ECN nonce)</t>
<t>S4: added note about whether talk about PCN operating on an
interface or on a link. In S8.1 (OAM) mentioned that PCN
functionality needs to be configured consistently on either the
ingress or the egress interface of PCN-nodes in a PCN-domain.</t>
<t>S5.2: clarified that signalling protocol installs flow filter
spec at PCN-ingress-node (& updates after possible
re-route)</t>
<t>S5.6: addressing: clarified</t>
<t>S5.7: added tunnelling issue of N^2 scaling if you set up a
mesh of tunnels between PCN-boundary-nodes</t>
<t>S7.3: Clarified the "third viewpoint" of probing (always
probe).</t>
<t>S8.1: clarified that SNMP is only an example; added note that
an operator may be able to not run PCN on some PCN-interior-nodes,
if it knows that these links will never become (pre-)congested;
added note that it may be possible to have different
PCN-boundary-node behaviours for different
ingress-egress-aggregates within the same PCN-domain.</t>
<t>Appendix: Created an Appendix about "Possible work items beyond
the scope of the current PCN WG Charter". Material moved from near
start of S3 and elsewhere throughout draft. Moved text about
centralised decision node to Appendix.</t>
<t>Other minor clarifications.</t>
</list></t>
</section>
<section title="Changes from -01 to -02">
<t><list style="symbols">
<t>S1: Benefits: provisioning bullet extended to stress that PCN
does not use RFC2475-style traffic conditioning.</t>
<t>S1: Deployment models: mentioned, as variant of PCN-domain
extending to end nodes, that may extend to LAN edge switch.</t>
<t>S3.1: Trust Assumption: added note about not needing
PCN-marking capability if known that an interface cannot become
pre-congested.</t>
<t>S4: now divided into sub-sections</t>
<t>S4.1: Admission control: added second proposed method for how
to decide to block new flows (PCN-egress-node receives one (or
several) PCN-marked packets).</t>
<t>S5: Probing sub-section removed. Material now in new S7.</t>
<t>S5.6: Addressing: clarified how PCN-ingress-node can discover
address of PCN-egress-node</t>
<t>S5.6: Addressing: centralised node case, added that
PCN-ingress-node may need to know address of PCN-egress-node</t>
<t>S5.8: Tunnelling: added case of "partially PCN-capable tunnel"
and degraded bullet on this in S6 (Open Issues)</t>
<t>S7: Probing: new section. Much more comprehensive than old
S5.5.</t>
<t>S8: Operations and Management: substantially revised.</t>
<t>other minor changes not affecting semantics</t>
</list></t>
</section>
<section title="Changes from -00 to -01">
<t>In addition to clarifications and nit squashing, the main changes
are:</t>
<t><list style="symbols">
<t>S1: Benefits: added one about provisioning (and contrast with
Diffserv SLAs)</t>
<t>S1: Benefits: clarified that the objective is also to stop
PCN-packets being significantly delayed (previously only mentioned
not dropping packets)</t>
<t>S1: Deployment models: added one where policing is done at
ingress of access network and not at ingress of PCN-domain (assume
trust between networks)</t>
<t>S1: Deployment models: corrected MPLS-TE to MPLS</t>
<t>S2: Terminology: adjusted definition of PCN-domain</t>
<t>S3.5: Other assumptions: corrected, so that two assumptions
(PCN-nodes not performing ECN and PCN-ingress-node discarding
arriving CE packet) only apply if the PCN WG decides to encode
PCN-marking in the ECN-field.</t>
<t>S4 & S5: changed PCN-marking algorithm to marking
behaviour</t>
<t>S4: clarified that PCN-interior-node functionality applies for
each outgoing interface, and added clarification: "The
functionality is also done by PCN-ingress-nodes for their outgoing
interfaces (ie those 'inside' the PCN-domain)."</t>
<t>S4 (near end): altered to say that a PCN-node "should" dedicate
some capacity to lower priority traffic so that it isn't starved
(was "may")</t>
<t>S5: clarified to say that PCN functionality is done on an
'interface' (rather than on a 'link')</t>
<t>S5.2: deleted erroneous mention of service level agreement</t>
<t>S5.5: Probing: re-written, especially to distinguish probing to
test the ingress-egress-aggregate from probing to test a
particular ECMP path.</t>
<t>S5.7: Addressing: added mention of probing; added that in the
case where traffic is always tunnelled across the PCN-domain, add
a note that he PCN-ingress-node needs to know the address of the
PCN-egress-node.</t>
<t>S5.8: Tunnelling: re-written, especially to provide a clearer
description of copying on tunnel entry/exit, by adding explanation
(keeping tunnel encaps/decaps and PCN-marking orthogonal),
deleting one bullet ("if the inner header's marking state is more
sever then it is preserved" - shouldn't happen), and better
referencing of other IETF documents.</t>
<t>S6: Open issues: stressed that "NOTE: Potential solutions are
out of scope for this document" and edited a couple of sentences
that were close to solution space.</t>
<t>S6: Open issues: added one about scenarios with only one tunnel
endpoint in the PCN domain .</t>
<t>S6: Open issues: ECMP: added under-admission as another
potential risk</t>
<t>S6: Open issues: added one about "Silent at start"</t>
<t>S10: Conclusions: a small conclusions section added</t>
</list></t>
</section>
<t></t>
</section>
</middle>
<back>
<!-- ================================================================ -->
<references title="Normative References">
<?rfc include="reference.RFC.2474"?>
<?rfc include="reference.RFC.3246"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.1633" ?>
<?rfc include="reference.RFC.2205" ?>
<?rfc include="reference.RFC.2211" ?>
<?rfc include="reference.RFC.2475" ?>
<?rfc include="reference.RFC.2747" ?>
<?rfc include="reference.RFC.2753" ?>
<?rfc include="reference.RFC.2983" ?>
<?rfc include="reference.RFC.2998" ?>
<?rfc include="reference.RFC.3168" ?>
<?rfc include="reference.RFC.3270" ?>
<?rfc include="reference.RFC.3393" ?>
<?rfc include="reference.RFC.3411" ?>
<?rfc include="reference.RFC.3726" ?>
<?rfc include="reference.RFC.4216" ?>
<?rfc include="reference.RFC.4301" ?>
<?rfc include="reference.RFC.4303" ?>
<?rfc include="reference.RFC.4594" ?>
<?rfc include="reference.RFC.4656" ?>
<?rfc include="reference.RFC.4774" ?>
<?rfc include="reference.RFC.4778" ?>
<?rfc include="reference.RFC.5129" ?>
<?rfc include="reference.RFC.5462" ?>
<reference anchor="P.800">
<front>
<title>Methods for subjective determination of transmission
quality</title>
<date month="August" year="1996" />
</front>
<seriesInfo name="ITU-T Recommendation" value="P.800" />
</reference>
<reference anchor="Y.1541">
<front>
<title>Network Performance Objectives for IP-based Services</title>
<date month="February" year="2006" />
</front>
<seriesInfo name="ITU-T Recommendation" value="Y.1541" />
</reference>
<reference anchor="PCN08-1">
<front>
<!--http://tools.ietf.org/html/draft-ietf-pcn-baseline-encoding-->
<title>Baseline Encoding and Transport of Pre-Congestion Information
(work in progress)</title>
<author fullname="T. Moncaster" surname=""></author>
<author fullname="B. Briscoe" surname=""></author>
<author fullname="M. Menth" surname=""></author>
<date month="Oct" year="2008" />
</front>
</reference>
<reference anchor="PCN08-2">
<front>
<!--http://tools.ietf.org/html/draft-ietf-pcn-marking-behaviour-->
<title>Metering and marking behaviour of PCN-nodes (work in
progress)</title>
<author fullname="P. Eardley (editor)" surname=""></author>
<date month="Oct" year="2008" />
</front>
</reference>
<reference anchor="PWE3-08">
<front>
<!--http://tools.ietf.org/html/draft-ietf-pwe3-congestion-frmwk-->
<title>Pseudowire Congestion Control Framework (work in
progress)</title>
<author fullname="S. Bryant" surname=""></author>
<author fullname="B. Davie" surname=""></author>
<author fullname="L. Martini" surname=""></author>
<author fullname="E. Rosen" surname=""></author>
<date month="May" year="2008" />
</front>
</reference>
<reference anchor="Babiarz06">
<front>
<!--http://www.watersprings.org/pub/id/draft-babiarz-pcn-sip-cap-00.txt-->
<title>SIP Controlled Admission and Preemption (work in
progress)</title>
<author fullname="J. Babiarz" surname=""></author>
<date month="Oct" year="2006" />
</front>
</reference>
<reference anchor="Behringer07">
<front>
<!--http://tools.ietf.org/html/draft-ietf-tsvwg-rsvp-security-groupkeying-->
<title>Applicability of Keying Methods for RSVP Security (work in
progress)</title>
<author fullname="M. Behringer" surname=""></author>
<author fullname="F. Faucheur" surname=""></author>
<date month="Nov" year="2007" />
</front>
</reference>
<reference anchor="Briscoe06">
<front>
<!--http://www.watersprings.org/pub/id/draft-briscoe-tsvwg-cl-architecture-04.txt-->
<title>An edge-to-edge Deployment Model for Pre-Congestion
Notification: Admission Control over a Diffserv Region (work in
progress)</title>
<author fullname="B. Briscoe et al" surname=""></author>
<date month="October" year="2006" />
</front>
</reference>
<reference anchor="Briscoe08-1">
<front>
<!--http://tools.ietf.org/html/draft-briscoe-re-pcn-border-cheat-->
<title>Emulating Border Flow Policing using Re-PCN on Bulk Data
(work in progress)</title>
<author fullname="B. Briscoe" surname=""></author>
<date month="Sept" year="2008" />
</front>
</reference>
<reference anchor="Briscoe08-2">
<front>
<!--http://tools.ietf.org/html/draft-ietf-tsvwg-ecn-tunnel-->
<title>Tunnelling of Congestion Notification (work in
progress)</title>
<author fullname="B. Briscoe" surname=""></author>
<date month="July" year="2008" />
</front>
</reference>
<reference anchor="Charny07-1">
<front>
<!--http://tools.ietf.org/html/draft-charny-pcn-comparison-->
<title>Comparison of Proposed PCN Approaches (work in
progress)</title>
<author fullname="A. Charny" surname=""></author>
<date month="November" year="2007" />
</front>
</reference>
<reference anchor="Charny07-2">
<front>
<!--http://tools.ietf.org/id/draft-charny-pcn-single-marking-03.txt-->
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<title>Pre-Congestion Notification Using Single Marking for
Admission and Termination (work in progress)</title>
<author fullname="A. Charny" surname=""></author>
<author fullname="X. Zhang" surname=""></author>
<author fullname="F. Faucheur" surname=""></author>
<author fullname="V. Liatsos" surname=""></author>
<date month="November" year="2007" />
</front>
</reference>
<reference anchor="Charny07-3"
target="http://www1.ietf.org/mail-archive/web/pcn/current/msg00871.html">
<front>
<title>Email to PCN WG mailing list</title>
<author fullname="A Charny"></author>
<date day="6" month="November" year="2007" />
</front>
</reference>
<reference anchor="Charny08"
target="http://www1.ietf.org/mail-archive/web/pcn/current/msg01359.html">
<front>
<title>Email to PCN WG mailing list</title>
<author fullname="A Charny"></author>
<date day="20" month="March" year="2008" />
</front>
</reference>
<reference anchor="Eardley07"
target="http://www1.ietf.org/mail-archive/web/pcn/current/msg00831.html">
<front>
<title>Email to PCN WG mailing list</title>
<author fullname="P Eardley"></author>
<author fullname="B Strulo"></author>
<date day="30" month="October" year="2007" />
</front>
</reference>
<reference anchor="Hancock02"
target="http://www-nrc.nokia.com/sua/nsis/interim/nsis-framework-outline.ppt">
<front>
<title>Slide 14 of 'NSIS: An Outline Framework for QoS
Signalling'</title>
<author fullname="Robert Hancock" surname=""></author>
<author fullname="Eleanor Hepworth" surname=""></author>
<date month="May" year="2002" />
</front>
</reference>
<reference anchor="Iyer03"
target="http://www.ieee-infocom.org/2003/papers/10_04.pdf">
<front>
<title>An approach to alleviate link overload as observed on an IP
backbone</title>
<author fullname="Sundar Iyer" surname=""></author>
<author fullname="Supratik Bhattacharyya"></author>
<author fullname="Nina Taft"></author>
<author fullname="Christophe Diot"></author>
<date month="" year="2003" />
</front>
<seriesInfo name="IEEE INFOCOM" value="" />
</reference>
<reference anchor="Lefaucheur06">
<front>
<!--http://tools.ietf.org/html/draft-lefaucheur-rsvp-ecn-->
<title>RSVP Extensions for Admission Control over Diffserv using
Pre-congestion Notification (PCN) (work in progress)</title>
<author fullname="F. Faucheur" surname=""></author>
<date month="June" year="2006" />
</front>
</reference>
<reference anchor="Menth07"
target="http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/Menth07-PCN-Config.pdf">
<front>
<title>PCN-Based Resilient Network Admission Control: The Impact of
a Single Bit"</title>
<author fullname="M. Menth" surname=""></author>
<author fullname="M. Hartmann"></author>
<date month="" year="2007" />
</front>
<seriesInfo name="Technical Report" value="" />
</reference>
<reference anchor="Menth08-1">
<front>
<!--http://tools.ietf.org/html/draft-menth-pcn-emft-->
<title>Edge-Assisted Marked Flow Termination (work in
progress)</title>
<author fullname="M. Menth" surname=""></author>
<author fullname="F. Lehrieder" surname=""></author>
<author fullname="P. Eardley" surname=""></author>
<author fullname="A. Charny" surname=""></author>
<author fullname="J. Babiarz" surname=""></author>
<date month="February" year="2008" />
</front>
</reference>
<reference anchor="Menth08-2">
<front>
<!--http://tools.ietf.org/html/draft-menth-pcn-psdm-encoding-->
<title>PCN Encoding for Packet-Specific Dual Marking (PSDM) (work in
progress)</title>
<author fullname="M. Menth" surname=""></author>
<author fullname="J. Babiarz" surname=""></author>
<author fullname="T. Moncaster" surname=""></author>
<author fullname="B. Briscoe" surname=""></author>
<date month="July" year="2008" />
</front>
</reference>
<reference anchor="Menth08-3"
target="http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/Menth08-PCN-Comparison.pdf">
<front>
<title>PCN-Based Admission Control and Flow Termination</title>
<author fullname="Michael Menth" surname=""></author>
<date month="" year="2008" />
</front>
</reference>
<reference anchor="Moncaster08">
<front>
<!--http://tools.ietf.org/id/draft-moncaster-pcn-3-state-encoding-->
<title>A three state extended PCN encoding scheme (work in
progress)</title>
<author fullname="T. Moncaster" surname=""></author>
<author fullname="B. Briscoe" surname=""></author>
<author fullname="M. Menth" surname=""></author>
<date month="June" year="2008" />
</front>
</reference>
<reference anchor="Sarker08">
<front>
<!--http://tools.ietf.org/id/draft-sarker-pcn-ecn-pcn-usecases-->
<title>Usecases and Benefits of end to end ECN support in PCN
Domains (work in progress)</title>
<author fullname="Z.Sarker" surname=""></author>
<author fullname="I. Johansson" surname=""></author>
<date month="November" year="2008" />
</front>
</reference>
<reference anchor="Songhurst06"
target="http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/ipe2eqos/gqs/papers/GQS_shared_tr.pdf">
<front>
<title>Guaranteed QoS Synthesis for Admission Control with Shared
Capacity</title>
<author fullname="David J. Songhurst" surname=""></author>
<author fullname="Philip Eardley"></author>
<author fullname="Bob Briscoe"></author>
<author fullname="Carla Di Cairano Gilfedder"></author>
<author fullname="June Tay"></author>
<date month="Feburary" year="2006" />
</front>
<seriesInfo name="BT Technical Report" value="TR-CXR9-2006-001" />
</reference>
<reference anchor="Style" target="http://www.guardian.co.uk/styleguide/">
<front>
<title>Guardian Style</title>
<author fullname="David Marsh (editor)" surname=""></author>
<date month="" year="2007" />
</front>
<seriesInfo name="Note: "
value="This document uses the abbreviations 'ie' and 'eg' (not 'i.e.' and 'e.g.'), as in many style guides, eg" />
</reference>
<reference anchor="Tsou08">
<front>
<!--http://tools.ietf.org/id/draft-tsou-pcn-racf-applic-->
<title>Applicability Statement for the Use of Pre-Congestion
Notification in a Resource-Controlled Network (work in
progress)</title>
<author fullname="T. Tsou" surname=""></author>
<author fullname="F. Huang" surname=""></author>
<author fullname="T. Taylor" surname=""></author>
<date month="November" year="2008" />
</front>
</reference>
<reference anchor="Westberg08">
<front>
<!--http://tools.ietf.org/id/draft-westberg-pcn-load-control-->
<title>LC-PCN: The Load Control PCN Solution (work in
progress)</title>
<author fullname="L. Westberg" surname=""></author>
<author fullname="A. Bhargava" surname=""></author>
<author fullname="A. Bader" surname=""></author>
<author fullname="G. Karagiannis" surname=""></author>
<author fullname="H. Mekkes" surname=""></author>
<date month="November" year="2008" />
</front>
</reference>
</references>
<section title="Possible future work items">
<t>This section mentions some topics that are outside the PCN WG's
current charter, but which have been mentioned as areas of interest.
They might be work items for: the PCN WG after a future re-chartering;
some other IETF WG; another standards body; an operator-specific usage
that is not standardised.</t>
<t>NOTE: it should be crystal clear that this section discusses
possibilities only.</t>
<t>The first set of possibilities relate to the restrictions described
in Section 6.3:</t>
<t><list style="symbols">
<t>a single PCN-domain encompasses several autonomous systems that
do not trust each other, perhaps by using a mechanism like re-PCN,
<xref target="Briscoe08-1"></xref>.</t>
<t>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).</t>
<t>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.</t>
<t>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.</t>
<t>the applicability of PCN mechanisms for emergency use (911, GETS,
WPS, MLPP, etc.)</t>
</list>Other possibilities include:</t>
<t><list style="symbols">
<t>Probing. This is discussed in Section A.1 below.</t>
<t>The PCN-domain extends to the end users. The scenario is
described in <xref target="Babiarz06"></xref>. 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.</t>
<t>indicating pre-congestion through signalling messages rather than
in-band (in the form of PCN-marked packets)</t>
<t>the decision-making functionality is at a centralised node rather
than at the PCN-boundary-nodes. This requires that the
PCN-egress-node signals PCN-feedback-information to the centralised
node, and that the centralised node signals to the PCN-ingress-node
the decision about admission (or termination). It may need the
centralised node and the PCN-boundary-nodes to be configured with
each other's addresses. The centralised case is described further in
<xref target="Tsou08"></xref>.</t>
<t>Signalling extensions for specific protocols (eg RSVP, NSIS). For
example: the details of how the signalling protocol installs the
flowspec at the PCN-ingress-node for an admitted PCN-flow; and how
the signalling protocol carries the PCN-feedback-information.
Perhaps also for other functions such as: coping with failure of a
PCN-boundary-node (<xref target="Briscoe06"></xref> considers what
happens if RSVP is the QoS signalling protocol); establishing a
tunnel across the PCN-domain if it is necessary to carry ECN marks
transparently.</t>
<t>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.</t>
<t>PCN for Pseudowire: PCN may be used as a congestion avoidance
mechanism for edge to edge pseudowire emulations <xref
target="PWE3-08"></xref>.</t>
<t>PCN for MPLS: <xref target="RFC3270"></xref> defines how to
support the Diffserv architecture in MPLS networks (Multi-protocol
label switching). <xref target="RFC5129"></xref> 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 <xref
target="RFC5462"></xref> re-names the Class of Service (CoS)
field).</t>
<t>PCN for Ethernet: Similarly, it may be possible to extend PCN
into Ethernet networks, where PCN-marking is done in the Ethernet
header. NOTE: Specific consideration of this extension is outside
the IETF's remit.</t>
</list></t>
<section title="Probing">
<t></t>
<section title="Introduction">
<t>Probing is a potential mechanism to assist admission control.</t>
<t>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.</t>
<t>One approach is to be “optimistic” and simply admit
the new flow. However it’s possible to envisage a scenario
where the traffic levels on other ingress-egress-aggregates are
already so high that they’re blocking new PCN-flows, and
admitting a new flow onto this 'empty' ingress-egress-aggregate adds
extra traffic onto a link that is already pre-congested –
which may ‘tip the balance’ so that PCN’s flow
termination mechanism is activated or some packets are dropped. This
risk could be lessened by configuring on each link sufficient
‘safety margin’ above the PCN-threshold-rate.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Probing functions">
<t>The probing functions are:</t>
<t><list style="symbols">
<t>Make decision that probing is needed. As described above,
this is when the ingress-egress-aggregate (or the ECMP path -
Section 6.4) carries no PCN-traffic. An alternative is always to
probe, ie probe before admitting every PCN-flow.</t>
<t>(if required) Communicate the request that probing is needed
– the PCN-egress-node signals to the PCN-ingress-node that
probing is needed</t>
<t>(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.</t>
<t>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.</t>
<t>Consume probe packets - the PCN-egress-node consumes probe
packets to ensure that they don't travel beyond the
PCN-domain.</t>
</list></t>
</section>
<section title="Discussion of rationale for probing, its downsides and open issues">
<t>It is an unresolved question whether probing is really needed,
but two viewpoints have been put forward as to why it is useful. The
first is perhaps the most obvious: there is no PCN-traffic on the
ingress-egress-aggregate. The second assumes that multipath routing
ECMP is running in the PCN-domain. We now consider each in turn.</t>
<t>The first viewpoint assumes the following:</t>
<t><list style="symbols">
<t>There is no PCN-traffic on the ingress-egress-aggregate (so a
normal admission decision cannot be made).</t>
<t>Simply admitting the new flow has a significant risk of
leading to overload: packets dropped or flows terminated.</t>
</list>On the former bullet, <xref target="Eardley07"></xref>
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.</t>
<t>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.</t>
<t>The downsides of probing for this viewpoint are:</t>
<t><list style="symbols">
<t>Probing adds delay to the admission control process.</t>
<t>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.</t>
</list></t>
<t></t>
<t>The second viewpoint applies in the case where there is multipath
routing (ECMP) in the PCN-domain. Note that ECMP is often used on
core networks. There are two possibilities:</t>
<t>(1) If admission control is based on measurements of the
ingress-egress-aggregate, then the viewpoint that probing is useful
assumes:</t>
<t><list style="symbols">
<t>there’s a significant chance that the traffic is
unevenly balanced across the ECMP paths, and hence there’s
a significant risk of admitting a flow that should be blocked
(because it follows an ECMP path that is pre-congested) or
blocking a flow that should be admitted.</t>
<t>Note: <xref target="Charny07-3"></xref> 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.</t>
</list></t>
<t>(2) If admission control is based on measurements of
pre-congestion on specific ECMP paths, then the viewpoint that
probing is useful assumes:</t>
<t><list style="symbols">
<t>There is no PCN-traffic on the ECMP path on which to base an
admission decision.</t>
<t>Simply admitting the new flow has a significant risk of
leading to overload.</t>
<t>The PCN-egress-node can match a packet to an ECMP path.</t>
<t>Note: This is similar to the first viewpoint and so similarly
could occur in a flash crowd if a new flow starts more-or-less
simultaneously on many of the empty ECMP paths. Because there
are several (sometimes many) ECMP paths between each pair of
PCN-boundary-nodes, it’s presumably more likely that an
ECMP path is ‘empty’ than an
ingress-egress-aggregate is. To constrain the number of ECMP
paths, a few tunnels could be set-up between each pair of
PCN-boundary-nodes. Tunnelling also solves the issue in the
bullet immediately above (which is otherwise hard because an
ECMP routing decision is made independently on each node).</t>
</list></t>
<t>The downsides of probing for this viewpoint are:</t>
<t><list style="symbols">
<t>Probing adds delay to the admission control process.</t>
<t>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.</t>
<t>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).</t>
</list></t>
</section>
<t>The open issues associated with this viewpoint include:</t>
<t><list style="symbols">
<t>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?</t>
<t>What difficulty does the delay (whilst probing is done), and
possible packet drops, cause applications?</t>
<t>Can the delay be alleviated by automatically and periodically
probing on the ingress-egress-aggregate? Or does this add too much
overhead?</t>
<t>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.</t>
<t>(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).</t>
</list></t>
</section>
<t></t>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-22 23:51:51 |