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Network Working Group B. Davie
Request for Comments: 5129 Cisco Systems, Inc.
Category: Standards Track B. Briscoe
J. Tay
BT Research
January 2008
Explicit Congestion Marking in MPLS
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
RFC 3270 defines how to support the Diffserv architecture in MPLS
networks, including how to encode Diffserv Code Points (DSCPs) in an
MPLS header. DSCPs may be encoded in the EXP field, while other uses
of that field are not precluded. RFC 3270 makes no statement about
how Explicit Congestion Notification (ECN) marking might be encoded
in the MPLS header. This document defines how an operator might
define some of the EXP codepoints for explicit congestion
notification, without precluding other uses.
Davie, et al. Standards Track [Page 1]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Intent . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use of MPLS EXP Field for ECN . . . . . . . . . . . . . . . . 5
3. Per-Domain ECT Checking . . . . . . . . . . . . . . . . . . . 7
4. ECN-Enabled MPLS Domain . . . . . . . . . . . . . . . . . . . 8
4.1. Pushing (Adding) One or More Labels to an IP Packet . . . 8
4.2. Pushing One or More Labels onto an MPLS Labeled Packet . . 8
4.3. Congestion Experienced in an Interior MPLS Node . . . . . 8
4.4. Crossing a Diffserv Domain Boundary . . . . . . . . . . . 8
4.5. Popping an MPLS Label (Not the End of the Stack) . . . . . 9
4.6. Popping the Last MPLS Label in the Stack . . . . . . . . . 9
4.7. Diffserv Tunneling Models . . . . . . . . . . . . . . . . 10
5. ECN-Disabled MPLS Domain . . . . . . . . . . . . . . . . . . . 10
6. The Use of More Codepoints with E-LSPs and L-LSPs . . . . . . 10
7. Relationship to Tunnel Behavior in RFC 3168 . . . . . . . . . 11
8. Deployment Considerations . . . . . . . . . . . . . . . . . . 11
8.1. Marking Non-ECN-Capable Packets . . . . . . . . . . . . . 11
8.2. Non-ECN-Capable Routers in an MPLS Domain . . . . . . . . 12
9. Example Uses . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. RFC 3168-Style ECN . . . . . . . . . . . . . . . . . . . . 13
9.2. ECN Co-Existence with Diffserv E-LSPs . . . . . . . . . . 13
9.3. Congestion-Feedback-Based Traffic Engineering . . . . . . 14
9.4. PCN Flow Admission Control and Flow Termination . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Extension to Pre-Congestion Notification . . . . . . 16
A.1. Label Push onto IP Packet . . . . . . . . . . . . . . . . . 16
A.2. Pushing Additional MPLS Labels . . . . . . . . . . . . . . 16
A.3. Admission Control or Flow Termination Marking Inside
MPLS Domain . . . . . . . . . . . . . . . . . . . . . . . . 17
A.4. Popping an MPLS Label (Not End of Stack) . . . . . . . . . 17
A.5. Popping the Last MPLS Label to Expose IP Header . . . . . . 17
Normative References . . . . . . . . . . . . . . . . . . . . . . . 18
Informative References . . . . . . . . . . . . . . . . . . . . . . 18
Davie, et al. Standards Track [Page 2]
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1. Introduction
1.1. Background
[RFC3168] defines Explicit Congestion Notification (ECN) for IP. The
primary purpose of ECN is to allow congestion to be signalled without
dropping packets.
[RFC3270] defines how to support the Diffserv architecture in MPLS
networks, including how to encode Diffserv Code Points (DSCPs) in an
MPLS header. DSCPs may be encoded in the EXP field, while other uses
of that field are not precluded. RFC 3270 makes no statement about
how Explicit Congestion Notification (ECN) marking might be encoded
in the MPLS header.
This document defines how an operator might define some of the EXP
codepoints for explicit congestion notification, without precluding
other uses. In parallel to the activity defining the addition of ECN
to IP [RFC3168], two proposals were made to add ECN to MPLS
[Floyd][Shayman]. These proposals, however, fell by the wayside.
With ECN for IP now being a proposed standard, and developing
interest in using pre-congestion notification (PCN) for admission
control and flow termination [PCN], there is consequent interest in
being able to support ECN across IP networks consisting of MPLS-
enabled domains. Therefore, it is necessary to specify the protocol
for including ECN in the MPLS shim header and the protocol behavior
of edge MPLS nodes.
We note that in [RFC3168], there are four codepoints used for ECN
marking, which are encoded using two bits of the IP header. The MPLS
EXP field is the logical place to encode ECN codepoints, but with
only 3 bits (8 codepoints) available, and with the same field being
used to convey DSCP information as well, there is a clear incentive
to conserve the number of codepoints consumed for ECN purposes.
Efficient use of the EXP field has been a focus of prior documents
[Floyd] [Shayman], and we draw on those efforts in this document as
well.
We also note that [RFC3168] defines default usage of the ECN field,
but it allows for the possibility that some Diffserv Per Hop
Behaviors (PHBs) might include different specifications on how the
ECN field is to be used. This document seeks to preserve that
capability.
Davie, et al. Standards Track [Page 3]
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1.2. Intent
Our intent is to specify how the MPLS shim header [RFC3032] should
denote ECN marking and how MPLS nodes should understand whether the
transport for a packet will be ECN capable. We offer this as a
building block, from which to build different congestion-notification
systems. We do not intend to specify how the resulting congestion
notification is fed back to an upstream node that can mitigate
congestion. For instance, unlike [Shayman], we do not specify edge-
to-edge MPLS domain feedback, but we also do not preclude it.
Nonetheless, we do specify how the egress node of an MPLS domain
should copy congestion notification from the MPLS shim into the
encapsulated IP header if the ECN is to be carried onward towards the
IP receiver; but we do *not* mandate that MPLS congestion
notification must be copied into the IP header for onward
transmission. This document aims to be generic for any use of
congestion notification in MPLS. Support of [RFC3168] is our primary
motivation; some additional potential applications to illustrate the
flexibility of our approach are described in Section 9. In
particular, we aim to support possible future schemes that may use
more than one level of congestion marking.
1.3. Terminology
This document draws freely on the terminology of ECN [RFC3168] and
MPLS [RFC3031]. For ease of reference, we have included some
definitions here, but refer the reader to the references above for
complete specifications of the relevant technologies:
o CE: Congestion Experienced. One of the states with which a packet
may be marked in a network supporting ECN. A packet is marked in
this state by an ECN-capable router to indicate that this router
was experiencing congestion at the time the packet arrived.
o ECT: ECN-capable Transport. One of the ECN states that a packet
may be in when it is sent by an end system. An end system marks a
packet with an ECT codepoint to indicate that the endpoints of the
transport protocol are ECN-capable. A router may not mark a
packet as CE unless the packet was marked ECT when it arrived.
o Not-ECT: Not ECN-capable transport. An end system marks a packet
with this codepoint to indicate that the endpoints of the
transport protocol are not ECN-capable. A congested router cannot
mark such packets as CE, and thus it can only drop them to
indicate congestion.
Davie, et al. Standards Track [Page 4]
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o EXP field. A 3-bit field in the MPLS label header [RFC3032] that
may be used to convey Diffserv information (and is also used in
this document to carry ECN information).
o PHP. Penultimate Hop Popping. An MPLS operation in which the
penultimate Label Switching Router (LSR) on a Label Switched Path
(LSP) removes the top label from the packet before forwarding the
packet to the final LSR on the LSP.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Use of MPLS EXP Field for ECN
We propose that LSRs configured for explicit congestion notification
should use the EXP field in the MPLS shim header. However, [RFC3270]
already defines use of codepoints in the EXP field for differentiated
services. Although it does not preclude other compatible uses of the
EXP field, this clearly seems to limit the space available for ECN,
given the field is only 3 bits (8 codepoints).
[RFC3270] defines two possible approaches for requesting
differentiated service treatment from an LSR:
o In the EXP-Inferred-PSC LSP (E-LSP) approach, different codepoints
of the EXP field in the MPLS shim header are used to indicate the
packet's per hop behavior (PHB).
o In the Label-Only-Inferred-PSC LSP (L-LSP) approach, an MPLS label
is assigned for each PHB scheduling class (PSC, as defined in
[RFC3260], so that an LSR determines both its forwarding and its
scheduling behavior from the label.
If an MPLS domain uses the L-LSP approach, there is likely to be
space in the EXP field for ECN codepoint(s). Where the E-LSP
approach is used, codepoint space in the EXP field is likely to be
scarce. This document focuses on interworking ECN marking with the
E-LSP approach, as it is the tougher problem. Consequently, the same
approach can also be applied with L-LSPs.
We recommend that explicit congestion notification in MPLS should use
codepoints instead of bits in the EXP field. Since not every PHB
will necessarily require an associated ECN codepoint, it would be
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wasteful to assign a dedicated bit for ECN. (There may also be cases
where a given PHB might need more than one ECN-like codepoint; see
Section 9.4 for an example).
For each PHB that uses ECN marking, we assume one EXP codepoint will
be defined as not congestion marked (Not-CM), and at least one other
codepoint will be defined as congestion marked (CM). Therefore, each
PHB that uses ECN marking will consume at least two EXP codepoints,
but PHBs that do not use ECN marking will only consume one.
Further, we wish to use minimal space in the MPLS shim header to tell
interior LSRs whether each packet will be received by an ECN-capable
transport (ECT). Nonetheless, we must ensure that an endpoint that
would not understand an ECN mark will not receive one, otherwise it
will not be able to respond to congestion as it should. In the past,
three solutions to this problem have been proposed:
o One possible approach is for congested LSRs to mark the ECN field
in the underlying IP header at the bottom of the label stack.
Although many commercial LSRs routinely access the IP header for
other reasons (equal cost multi-path - ECMP), there are numerous
drawbacks to attempting to find an IP header beneath an MPLS label
stack. Notably, there is the challenge of detecting the absence
of an IP header when non-IP packets are carried on an LSP.
Therefore, we will not consider this approach further.
o In the scheme suggested by [Floyd], ECT and CE are overloaded into
one bit, so that a 0 means ECT while a 1 might either mean Not-ECT
or it might mean CE. A packet that has been marked as having
experienced congestion upstream, and then is picked out for
marking at a second congested LSR, will be dropped by the second
LSR since it cannot determine whether the packet has previously
experienced congestion or if ECN is not supported by the
transport.
While such an approach seemed potentially palatable, we do not
recommend it here for the following reasons. In some cases, we
wish to be able to use ECN marking long before actual congestion
(e.g., pre-congestion notification). In these circumstances,
marking rates at each LSR might be non-negligible most of the
time, so the chances of a previously marked packet encountering an
LSR that wants to mark it again will also be non-negligible. In
the case where CE and not-ECT are indistinguishable to core
routers, such a scenario could lead to unacceptable drop rates.
If the typical marking rate at every router or LSR is p, and the
typical diameter of the network of LSRs is d, then the probability
that a marked packet will be chosen for marking more than once is
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1-[Pr(never marked) + Pr(marked at exactly one hop)] = 1- [(1-p)^d
+ dp(1-p)^(d-1)]. For instance, with 6 LSRs in a row, each
marking ECN with 1% probability, the chances of a packet that is
already marked being chosen for marking a second time is 0.15%.
The bit-overloading scheme would therefore introduce a drop rate
of 0.15% unnecessarily. Given that most modern core networks are
sized to introduce near-zero packet drop, it may be unacceptable
to drop over one in a thousand packets unnecessarily.
o A third possible approach was suggested by [Shayman]. In this
scheme, interior LSRs assume that the endpoints are ECN-capable,
but this assumption is checked when the final label is popped. If
an interior LSR has marked ECN in the EXP field of the shim
header, but the IP header says the endpoints are not ECN-capable,
the edge router (or penultimate router, if using penultimate hop
popping) drops the packet. We recommend this scheme, which we
call `per-domain ECT checking', and define it more precisely in
the following section. Its chief drawback is that it can cause
packets to be forwarded after encountering congestion only to be
dropped at the egress of the MPLS domain. The rationale for this
decision is given in Section 8.1.
3. Per-Domain ECT Checking
For the purposes of this discussion, we define the egress nodes of an
MPLS domain as the nodes that pop the last MPLS label from the label
stack, exposing the IP (or, potentially non-IP) header. Note that
such a node may be the ultimate or penultimate hop of an LSP,
depending on whether penultimate hop popping (PHP) is employed.
In the per-domain ECT checking approach, the egress nodes take
responsibility for checking whether the transport is ECN-capable.
This document does not specify how these nodes should pass on
congestion notification because different approaches are likely in
different scenarios. However, if congestion notification in the MPLS
header is copied into the IP header, the procedure MUST conform to
the specification given here.
If congestion notification is passed to the transport without first
passing it onward in the IP header, the approach used must take
similar care to check that the transport is ECN-capable before
passing it ECN markings. Specifically, if the transport for a
particular congestion marked MPLS packet is found not to be ECN-
capable, the packet MUST be dropped at this egress node.
In the per-domain ECT checking approach, only the egress nodes check
whether an IP packet is destined for an ECN-capable transport.
Therefore, any single LSR within an MPLS domain MUST NOT be
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configured to enable ECN marking unless all the egress LSRs
surrounding it are already configured to handle ECN marking.
We call a domain surrounded by ECN-capable egress LSRs an ECN-enabled
MPLS domain. This term only implies that all the egress LSRs are
ECN-enabled; some interior LSRs may not be ECN-enabled. For
instance, it would be possible to use some legacy LSRs incapable of
supporting ECN in the interior of an MPLS domain as long as all the
egress LSRs were ECN-capable. Note that if PHP is used, the
"penultimate hop" routers that perform the pop operation do need to
be ECN-enabled since they are acting in this context as egress LSRs.
4. ECN-Enabled MPLS Domain
In the following subsections, we describe various operations
affecting the ECN marking of a packet that may be performed at MPLS-
edge and core LSRs.
4.1. Pushing (Adding) One or More Labels to an IP Packet
On encapsulating an IP packet with an MPLS label stack, the ECN field
must be translated from the IP packet into the MPLS EXP field. The
Not-CM (not congestion marked) state is set in the MPLS EXP field if
the ECN status of the IP packet is Not-ECT or ECT(1) or ECT(0). The
CM state is set if the ECN status of the IP packet is CE. If more
than one label is pushed at one time, the same value should be placed
in the EXP value of all label stack entries.
4.2. Pushing One or More Labels onto an MPLS Labeled Packet
The EXP field is copied directly from the topmost label before the
push to the newly added outer label. If more than one label is being
pushed, the same EXP value is copied to all label-stack entries.
4.3. Congestion Experienced in an Interior MPLS Node
If the EXP codepoint of the packet maps to a PHB that uses ECN
marking, and the marking algorithm requires the packet to be marked,
the CM state is set (irrespective of whether it is already in the CM
state).
If the buffer is full, a packet is dropped.
4.4. Crossing a Diffserv Domain Boundary
If an MPLS-encapsulated packet crosses a Diffserv domain boundary, it
may be the case that the two domains use different encodings of the
same PHB in the EXP field. In such cases, the EXP field must be
Davie, et al. Standards Track [Page 8]
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rewritten at the domain boundary. If the PHB is one that supports
ECN, then the appropriate ECN marking should also be preserved when
the EXP field is mapped at the boundary.
If an MPLS-encapsulated packet that is in the CM state crosses from a
domain that is ECN-enabled (as defined in Section 3) to a domain that
is not ECN-enabled, then it is necessary to perform the egress
checking procedures at the egress LSR of the ECN-enabled domain.
This means that if the encapsulated packet is not ECN-capable, the
packet MUST be dropped. Note that this implies the egress LSR must
be able to look beneath the MPLS header without popping the label
stack.
The related issue of Diffserv tunnel models is discussed in
Section 4.7.
4.5. Popping an MPLS Label (Not the End of the Stack)
When a packet has more than one MPLS label in the stack and the top
label is popped, another MPLS label is exposed. In this case, the
ECN information should be transferred from the outer EXP field to the
inner MPLS label in the following manner. If the inner EXP field is
Not-CM, the inner EXP field is set to the same CM or Not-CM state as
the outer EXP field. If the inner EXP field is CM, it remains
unchanged whatever the outer EXP field. Note that an inner value of
CM and an outer value of not-CM should be considered anomalous, and
SHOULD be logged in some way by the LSR.
4.6. Popping the Last MPLS Label in the Stack
When the last MPLS label is popped from the packet, its payload is
exposed. If that packet is not IP, and does not have any capability
equivalent to ECT, it is assumed Not-ECT, and it is treated as such.
That means that if the EXP value of the MPLS header is CM, the packet
MUST be dropped.
Assuming an IP packet was exposed, we have to examine whether or not
that packet is ECT. A Not-ECT packet MUST be dropped if the EXP
field is CM.
For the remainder of this section, we describe the behavior that is
required if the ECN information is to be transferred from the MPLS
header into the exposed IP header for onward transmission. As noted
in Section 1.2, such behavior is not mandated by this document, but
may be selected by an operator.
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If the inner IP packet is Not-ECT, its ECN field remains unchanged if
the EXP field is Not-CM. If the ECN field of the inner packet is set
to ECT(0), ECT(1), or CE, the ECN field remains unchanged if the EXP
field is set to Not-CM. The ECN field is set to CE if the EXP field
is CM. Note that an inner value of CE and an outer value of not-CM
should be considered anomalous, and SHOULD be logged in some way by
the LSR.
4.7. Diffserv Tunneling Models
[RFC3270] describes three tunneling models for Diffserv support
across MPLS Domains, referred to as the uniform, short pipe, and pipe
models. The differences between these models lie in whether the
Diffserv treatment that applies to a packet while it travels along a
particular LSP is carried to the ingress of the last hop, to the
egress of the last hop, or beyond the last hop. Depending on which
mode is preferred by an operator, the EXP value or DSCP value of an
exposed header following a label pop may or may not be dependent on
the EXP value of the label that is removed by the pop operation. We
believe that, in the case of ECN marking, the use of these models
should only apply to the encoding of the Diffserv PHB in the EXP
value, and that the choice of codepoint for ECN should always be made
based on the procedures described above, independent of the tunneling
model.
5. ECN-Disabled MPLS Domain
If ECN is not enabled on all the egress LSRs of a domain, ECN MUST
NOT be enabled on any LSRs throughout the domain. If congestion is
experienced on any LSR in an ECN-disabled MPLS domain, packets MUST
be dropped; they MUST NOT be marked. The exact algorithm for
deciding when to drop packets during congestion (e.g., tail-drop,
RED, etc.) is a local matter for the operator of the domain.
6. The Use of More Codepoints with E-LSPs and L-LSPs
[RFC3270] gives different options with E-LSPs and L-LSPs, and some of
those could potentially provide ample EXP codepoints for ECN.
However, deploying L-LSPs vs. E-LSPs has many implications, such as
platform support and operational complexity. The above ECN MPLS
solution should provide some flexibility. If the operator has
deployed one L-LSP per PHB scheduling class, then EXP space will be a
non-issue, and it could be used to achieve more sophisticated ECN
behavior if required. If the operator wants to stick to E-LSPs and
uses a handful of EXP codepoints for Diffserv, it may be desirable to
operate with a minimum number of extra ECN codepoints, even if this
comes with some compromise on ECN optimality. See Section 9 for
discussion of some possible deployment scenarios.
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We note that in a network where L-LSPs are used, ECN marking SHOULD
NOT cause packets from the same microflow, but with different ECN
markings, to be sent on different LSPs. As discussed in [RFC3270],
packets of a single microflow should always travel on the same LSP to
avoid possible misordering. Thus, ECN marking of packets on L-LSPs
SHOULD only affect the EXP value of the packets.
7. Relationship to Tunnel Behavior in RFC 3168
[RFC3168] defines two modes of encapsulating ECN-marked IP packets
inside additional IP headers when tunnels are used. The two modes
are the "full functionality" and "limited functionality" modes. In
the full functionality mode, the ECT information from the inner
header is copied to the outer header at the tunnel ingress, but the
CE information is not. In the limited functionality mode, neither
ECT nor CE information is copied to the outer header, and thus ECN
cannot be applied to the encapsulated packet.
The behavior that is specified in Section 4 of this document
resembles the "full functionality" mode in the sense that it conveys
some information from inner to outer header, and in the sense that it
enables full ECN support along the MPLS LSP (which is analogous to an
IP tunnel in this context). However it differs in one respect, which
is that the CE information is conveyed from the inner header to the
outer header. Our original reason for this different design choice
was to give interior routers and LSRs more information about upstream
marking in multi-bottleneck cases. For instance, the flow
termination marking mechanism proposed for PCN works by only
considering packets for marking that have not already been marked
upstream. Unless existing flow termination marking is copied from
the inner to the outer header at tunnel ingress, the mechanism
doesn't terminate enough traffic in cases where anomalous events hit
multiple domains at once. [RFC3168] does not give any reasons
against conveying CE information from the inner header to the outer
in the "full functionality" mode. Furthermore, [RFC4301] specifies
that the ECN marking should be copied from inner header to outer
header in IPSEC tunnels, consistent with the approach defined here.
[BRISCOE-ECN] discusses this issue in more detail. In summary, the
approach described in Section 4 appears to be both a sound technical
choice and consistent with the current state of thinking in the IETF.
8. Deployment Considerations
8.1. Marking Non-ECN-Capable Packets
What are the consequences of marking a packet that is not ECN-
capable? Even if it will be dropped before leaving the domain,
doesn't this consume resources unnecessarily?
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The problem only arises if there is congestion downstream of an
earlier congested queue in the same MPLS domain. Congested LSRs
downstream might forward packets already marked, even though they
will be dropped later when the inner IP header is found to be Not-ECT
on decapsulation. Such packets might cause the downstream LSRs to
mark (or drop) other packets that they would otherwise not have had
to.
We expect congestion will typically be rare in MPLS networks, but it
might not be. The extra unnecessary load at downstream LSRs will not
be more than the fraction of marked packets from upstream LSRs, even
in the worst case where no transports are ECN-capable. Therefore,
the amount of unnecessary marking (or drop) on an LSR will not be
more than the product of its local marking rate and the marking rate
due to upstream LSRs within the same domain -- typically the product
of two small (often zero) probabilities.
This is why we decided to use the per-domain ECT checking approach --
because the most likely effect would be a very slightly increased
marking rate, which would result in very slightly higher drop only
for non-ECN-capable transports. We chose not to use the [Floyd]
alternative, which introduced a low but persistent level of
unnecessary packet drop for all time, even for ECN-capable
transports. Although that scheme did not carry traffic to the edge
of the MPLS domain only to be dropped on decapsulation, we felt our
minor inefficiency was a small price to pay; and it would get smaller
still if ECN deployment widened.
A partial solution would be to preferentially drop packets arriving
at a congested router that were already marked. There is no solution
to the problem of marking a packet when congestion is caused by
another packet that should have been dropped. However, the chance of
such an occurrence is very low, and the consequences are not
significant. It merely causes an application to very occasionally
slow down its rate when it did not have to.
8.2. Non-ECN-Capable Routers in an MPLS Domain
What if an MPLS domain wants to use ECN, but not all legacy routers
are able to support it?
If the legacy router(s) are used in the interior, this is not a
problem. They will simply have to drop the packets if they are
congested, rather than mark them, which is the standard behavior for
IP routers that are not ECN-enabled.
If the legacy router were used as an egress router, it would not be
able to check the ECN-capability of the transport correctly. An
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operator in this position would not be able to use this solution and
therefore MUST NOT enable ECN unless all egress routers are ECN-
capable.
9. Example Uses
9.1. RFC 3168-Style ECN
[RFC3168] proposes the use of ECN in TCP, and it introduces the use
of ECN-Echo and Congestion Window Reduced (CWR) flags in the TCP
header for initialization. The TCP sender responds accordingly (such
as not increasing the congestion window) when it receives an ECN-Echo
(ECE) ACK packet (that is, an ACK packet with ECN-Echo flag set in
the TCP header), then the sender knows that congestion was
encountered in the network on the path from the sender to the
receiver.
It would be possible to enable ECN in an MPLS domain for Diffserv
PHBs like AF and best efforts that are expected to be used by TCP and
similar transports (e.g., DCCP [RFC4340]). Then, end-to-end
congestion control in transports capable of understanding ECN would
be able to respond to approaching congestion on LSRs without having
to rely on packet discard to signal congestion.
9.2. ECN Co-Existence with Diffserv E-LSPs
Many operators today have deployed Diffserv using the E-LSP approach
of [RFC3270]. In many cases, the number of PHBs used is less than 8,
and hence there remain available codepoints in the EXP space. If an
operator wished to support ECN for a single PHB, this could be
accomplished by simply allocating a second codepoint to the PHB for
the CM state of that PHB and retaining the old codepoint for the
not-CM state. An operator with only four deployed PHBs could, of
course, enable ECN marking on all those PHBs. It is easy to imagine
cases where some PHBs might benefit more from ECN than others -- for
example, an operator might use ECN on a premium data service but not
on a PHB used for best-effort Internet traffic.
As an illustrative example of how the EXP field might be used in this
case, consider the example of an operator who is using the aggregated
service classes proposed in [TSVWG]. He may choose to support ECN
only for the Assured Elastic Treatment Aggregate, using the EXP
codepoint 010 for the not-CM state and 011 for the CM state. All
other codepoints could be the same as in [TSVWG]. Of course, any
other combination of EXP values can be used according to the specific
set of PHBs and marking conventions used within that operator's
network.
Davie, et al. Standards Track [Page 13]
RFC 5129 ECN for MPLS January 2008
9.3. Congestion-Feedback-Based Traffic Engineering
Shayman's traffic engineering [Shayman] presents another example
application of ECN feedback in an MPLS domain. Shayman proposed the
use of ECN by an egress LSR feeding back congestion to an ingress LSR
to mitigate congestion by employing dynamic traffic engineering
techniques, such as shifting flows to an alternate path. It proposed
a new Resource Reservation Protocol (RSVP) message, which was sent by
the egress LSR to the ingress LSR (and ignored by transit LSRs) to
indicate congestion along the path. Thus, rather than providing the
same style of congestion notification to endpoints as defined in
[RFC3168], [Shayman] limits its scope to the MPLS domain only. This
application of ECN in an MPLS domain could make use of the ECN
encoding in the MPLS header that is defined in this document.
9.4. PCN Flow Admission Control and Flow Termination
[PCN] proposes using pre-congestion notification (PCN) on routers
within an edge-to-edge Diffserv region to control admission of new
flows to the region and, if necessary, to terminate existing flows in
response to disasters and other anomalous routing events. In this
approach, the current level of PCN marking is picked up by the
signaling used to initiate each flow in order to inform the admission
control decision for the whole region at once. For example,
extensions to RSVP [LEFAUCHEUR] and Next Steps in Signaling (NSIS)
[NSIS], [ARUMAITHURAI] have been proposed.
If LSRs are able to mark packets to signify congestion in MPLS, PCN
marking could be used for admission control and flow termination
across a Diffserv region, irrespective of whether it contained pure
IP routers, MPLS LSRs, or both. Indeed, the solution could be
somewhat more efficient to implement if aggregates could identify
themselves by their MPLS label. Appendix A describes the mechanisms
by which the necessary markings for PCN could be carried in the MPLS
header.
10. Security Considerations
We believe no new vulnerabilities are introduced by this document.
We have considered whether malicious sources might be able to exploit
the fact that interior LSRs will mark packets that are Not-ECT,
relying on their egress LSR to drop them. Although this might allow
sources to engineer a situation where more traffic is carried across
an MPLS domain than should be, we figured that even if we hadn't
introduced this feature, these sources would have been able to
prevent these LSRs dropping this traffic anyway, simply by setting
ECT in the first place.
Davie, et al. Standards Track [Page 14]
RFC 5129 ECN for MPLS January 2008
An ECN sender can use the ECN nonce [RFC3540] to detect a misbehaving
receiver. The ECN nonce works correctly across an MPLS domain
without requiring any specific support from the proposal in this
document. The nonce does not need to be present in the MPLS shim
header to detect a misbehaving receiver. As long as the nonce is
present in the IP header when the ECN information is copied from the
last MPLS shim header, it will be overwritten if congestion has been
experienced by an LSR. This is all that is necessary for the sender
to detect a misbehaving receiver. If there were a need for an ECN
nonce in the MPLS shim header (e.g., to detect if one LSR were
erasing the markings of an upstream LSR in the same domain), we
believe this proposal does not preclude the later addition of an ECN
nonce capability for specific DSCPs, just as it does not preclude any
other use of the EXP codepoints.
11. Acknowledgments
Thanks to K.K. Ramakrishnan and Sally Floyd for getting us thinking
about this in the first place and for providing advice on tunneling
of ECN packets, and to Sally Floyd, Joe Babiarz, Ben Niven-Jenkins,
Phil Eardley, Ruediger Geib, and Magnus Westerlund for their comments
on the document.
Davie, et al. Standards Track [Page 15]
RFC 5129 ECN for MPLS January 2008
Appendix A. Extension to Pre-Congestion Notification
This appendix describes how the mechanisms described in the body of
the document can be extended to support PCN [PCN]. Our intent here
is to show that the mechanisms are readily extended to more complex
scenarios than ECN, particularly in the case where more codepoints
are needed, but this appendix may be safely ignored if one is
interested only in supporting ECN. Note that the PCN standards are
still very much under development at the time of writing; hence, the
precise details contained in this appendix may be subject to change,
and we stress that this appendix is for illustrative purposes only.
The relevant aspects of PCN for the purposes of this discussion are:
o PCN uses 3 states rather than 2 for ECN -- these are referred to
as admission marked (AM), termination marked (TM), and not marked
(NM) states. (See Section 9.4 for further discussion of PCN and
the possibility of using fewer codepoints).
o A packet can go from NM to AM, from NM to TM, or from AM to TM,
but no other transition is possible.
o The determination of whether a packet is subject to PCN is based
on the PHB of the packet.
Thus, to support PCN fully in an MPLS domain for a particular PHB, a
total of 3 codepoints need to be allocated for that PHB. These 3
codepoints represent the admission marked (AM), termination marked
(TM), and not marked (NM) states. The procedures described in
Section 4 above need to be slightly modified to support this
scenario. The following procedures are invoked when the topmost DSCP
or EXP value indicates a PHB that supports PCN.
A.1. Label Push onto IP Packet
If the IP packet header indicates AM, set the EXP value of all
entries in the label stack to AM. If the IP packet header indicates
TM, set the EXP value of all entries in the label stack to TM. For
any other marking of the IP header, set the EXP value of all entries
in the label stack to NM.
A.2. Pushing Additional MPLS Labels
The procedures of Section 4.2 apply.
Davie, et al. Standards Track [Page 16]
RFC 5129 ECN for MPLS January 2008
A.3. Admission Control or Flow Termination Marking Inside MPLS Domain
The EXP value can be set to AM or TM according to the same procedures
as described in [BRISCOE-CL]. For the purposes of this document, it
does not matter exactly which algorithms are used to decide when to
set AM or TM; all that matters is that if a router would have marked
AM (or TM) in the IP header, it should set the EXP value in the MPLS
header to the AM (or TM) codepoint.
A.4. Popping an MPLS Label (Not End of Stack)
When popping an MPLS Label exposes another MPLS label, the AM or TM
marking should be transferred to the exposed EXP field in the
following manner:
o If the inner EXP value is NM, then it should be set to the same
marking state as the EXP value of the popped label stack entry.
o If the inner EXP value is AM, it should be unchanged if the popped
EXP value was AM, and it should be set to TM if the popped EXP
value was TM. If the popped EXP value was NM, this should be
logged in some way, and the inner EXP value should be unchanged.
o If the inner EXP value is TM, it should be unchanged whatever the
popped EXP value was, but any EXP value other than TM should be
logged.
A.5. Popping the Last MPLS Label to Expose IP Header
When popping the last MPLS Label exposes the IP header, there are two
cases to consider:
o the popping LSR is *not* the egress router of the PCN region, in
which case AM or TM marking should be transferred to the exposed
IP header field; or
o the popping LSR *is* the egress router of the PCN region.
In the latter case, the behavior of the egress LSR is defined in
[PCN] and is beyond the scope of this document. In the former case,
the marking should be transferred from the popped MPLS header to the
exposed IP header as follows:
o If the inner IP header value is neither AM nor TM, and the EXP
value was NM, then the IP header should be unchanged. For any
other EXP value, the IP header should be set to the same marking
state as the EXP value of the popped label stack entry.
Davie, et al. Standards Track [Page 17]
RFC 5129 ECN for MPLS January 2008
o If the inner IP header value is AM, it should be unchanged if the
popped EXP value was AM, and it should be set to TM if the popped
EXP value was TM. If the popped EXP value was NM, this should be
logged in some way and the inner IP header value should be
unchanged.
o If the IP header value is TM, it should be unchanged whatever the
popped EXP value was, but any EXP value other than TM should be
logged.
Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture",
RFC 3031, January 2001.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label
Stack Encoding", RFC 3032, January 2001.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J.
Heinanen, "Multi-Protocol Label Switching (MPLS)
Support of Differentiated Services", RFC 3270,
May 2002.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
Informative References
[ARUMAITHURAI] Arumaithurai, M., "NSIS PCN-QoSM: A Quality of
Service Model for Pre-Congestion Notification (PCN)",
Work in Progress, September 2007.
[BRISCOE-CL] Briscoe, B., "Pre-Congestion Notification Marking",
Work in Progress, October 2006.
[BRISCOE-ECN] Briscoe, B., "Layered Encapsulation of Congestion
Notification", Work in Progress, July 2007.
Davie, et al. Standards Track [Page 18]
RFC 5129 ECN for MPLS January 2008
[Floyd] Ramakrishnan, K., Floyd, S., and B. Davie, "A
Proposal to Incorporate ECN in MPLS", Work in
Progress, June 1999.
[LEFAUCHEUR] Faucheur, F., Charny, A., Briscoe, B., Eardley, P.,
Barbiaz, J., and K. Chan, "RSVP Extensions for
Admission Control over Diffserv using Pre-congestion
Notification (PCN)", Work in Progress, June 2006.
[NSIS] Bader, A., Westberg, L., Karagiannis, G., Cornelia,
C., and T. Phelan, "RMD-QOSM - The Resource
Management in Diffserv QOS Model", Work in Progress,
November 2007.
[PCN] Eardley, P., "Pre-Congestion Notification
Architecture", Work in Progress, November 2007.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust
Explicit Congestion Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
March 2006.
[Shayman] Shayman, M. and R. Jaeger, "Using ECN to Signal
Congestion Within an MPLS Domain", Work in Progress,
November 2000.
[TSVWG] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
DiffServ Service Classes", Work in Progress,
November 2007.
Davie, et al. Standards Track [Page 19]
RFC 5129 ECN for MPLS January 2008
Authors' Addresses
Bruce Davie
Cisco Systems, Inc.
1414 Mass. Ave.
Boxborough, MA 01719
USA
EMail: bsd@cisco.com
Bob Briscoe
BT Research
B54/77, Sirius House
Adastral Park
Martlesham Heath
Ipswich
Suffolk IP5 3RE
United Kingdom
EMail: bob.briscoe@bt.com
June Tay
BT Research
B54/77, Sirius House
Adastral Park
Martlesham Heath
Ipswich
Suffolk IP5 3RE
United Kingdom
EMail: june.tay@bt.com
Davie, et al. Standards Track [Page 20]
RFC 5129 ECN for MPLS January 2008
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Davie, et al. Standards Track [Page 21]