Network Working Group B. Davie
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track B. Briscoe
Expires: August 24, 2007 J. Tay
BT Research
February 20, 2007
Explicit Congestion Marking in MPLS
draft-ietf-tsvwg-ecn-mpls-00.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 24, 2007.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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. RFC3270 makes no statement about
how Explicit Congestion Notification (ECN) marking might be encoded
in the MPLS header. This draft defines how an operator might define
Davie, et al. Expires August 24, 2007 [Page 1]
Internet-Draft ECN for MPLS February 2007
some of the EXP codepoints for explicit congestion notification,
without precluding other uses.
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].
Davie, et al. Expires August 24, 2007 [Page 2]
Internet-Draft ECN for MPLS February 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Change History . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Intent . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Use of MPLS EXP Field for ECN . . . . . . . . . . . . . . . . 6
3. Per-domain ECT checking . . . . . . . . . . . . . . . . . . . 8
4. ECN-enabled MPLS domain . . . . . . . . . . . . . . . . . . . 9
4.1. Pushing (adding) one or more labels to an IP packet . . . 9
4.2. Pushing one or more labels onto an MPLS labelled packet . 9
4.3. Congestion experienced in an interior MPLS node . . . . . 9
4.4. Crossing a Diffserv Domain Boundary . . . . . . . . . . . 10
4.5. Popping an MPLS label (not the end of the stack) . . . . . 10
4.6. Popping the last MPLS label in the stack . . . . . . . . . 10
4.7. Diffserv Tunneling Models . . . . . . . . . . . . . . . . 11
4.8. Extension to Pre-Congestion Notification . . . . . . . . . 11
4.8.1. Label Push onto IP packet . . . . . . . . . . . . . . 12
4.8.2. Pushing Additional MPLS Labels . . . . . . . . . . . . 12
4.8.3. Admission Control or Pre-emption Marking inside
MPLS domain . . . . . . . . . . . . . . . . . . . . . 12
4.8.4. Popping an MPLS Label (not end of stack) . . . . . . . 12
4.8.5. Popping the last MPLS Label to expose IP header . . . 12
5. ECN-disabled MPLS domain . . . . . . . . . . . . . . . . . . . 13
6. The use of more codepoints with E-LSPs and L-LSPs . . . . . . 13
7. Relationship to tunnel behavior in RFC 3168 . . . . . . . . . 14
7.1. Alternative approach to support ECN in an MPLS domain . . 14
8. Example Uses . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. RFC3168-style ECN . . . . . . . . . . . . . . . . . . . . 15
8.2. ECN Co-existence with Diffserv E-LSPs . . . . . . . . . . 15
8.3. Congestion-feedback-based Traffic Engineering . . . . . . 16
8.4. PCN flow admission control and flow pre-emption . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17
9.1. Marking non-ECN Capable Packets . . . . . . . . . . . . . 17
9.2. Non-ECN capable routers in an MPLS Domain . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . . 19
13.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
Davie, et al. Expires August 24, 2007 [Page 3]
Internet-Draft ECN for MPLS February 2007
1. Introduction
1.1. Change History
[Note to RFC Editor: This section to be removed before publication]
This version (draft-ietf-tsvwg-ecn-mpls-00.txt) differs from the last
(draft-davie-mpls-ecn-01.txt) only in title, date, and updated
references.
Changes from draft-davie-ecn-mpls-00 to draft-davie-ecn-mpls-01:
o Corrected the description of ECN-MPLS marking proposed in
[Shayman], which closely corresponds to that proposed in this
document.
o Pre-congestion notification (PCN) marking is now described in a
way that does not require normative references to PCN
specifications. PCN discussion now serves only to illustrate how
the ECN marking concepts can be extended to cover more complex
scenarios, with PCN being an example.
o Added specification of behavior when MPLS encapsulated packets
cross from an ECN-enabled domain to a domain that is not ECN-
enabled.
o Clarified that copying MPLS ECN or PCN marking into exposed IP
header on egress is not mandatory
o Fixed typos and nits
1.2. Background
[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. RFC3270 makes no statement about
how Explicit Congestion Notification (ECN) marking might be encoded
in the MPLS header. This draft 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 pre-emption
[I-D.briscoe-tsvwg-cl-architecture], there is consequent interest in
being able to support ECN across IP networks consisting of MPLS-
Davie, et al. Expires August 24, 2007 [Page 4]
Internet-Draft ECN for MPLS February 2007
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 drafts
[Floyd] [Shayman] and we draw on those efforts in this draft as well.
1.3. 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
underlying 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
draft aims to be generic for any use of congestion notification in
MPLS. PCN or traffic engineering are merely two of many motivating
applications (see Section 8.)
1.4. 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 which 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 end-points of
the transport protocol are ECN-capable. A router may not mark a
Davie, et al. Expires August 24, 2007 [Page 5]
Internet-Draft ECN for MPLS February 2007
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 end-points of the
transport protocol are not ECN-capable. A congested router cannot
mark such packets as CE, and thus can only drop them to indicate
congestion.
o EXP field. A 3 bit field in the MPLS label header [RFC3032] which
may be used to convey Diffserv information (and used in this draft
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.
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, RFC 3270
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).
RFC 3270 defines two possible approaches for requesting
differentiated service treatment from an LSR.
o In the 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 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, then codepoint space in the EXP field is likely to
be scarce. This draft 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
Davie, et al. Expires August 24, 2007 [Page 6]
Internet-Draft ECN for MPLS February 2007
will need an associated ECN codepoint and in some applications a
given PHB might need two ECN codepoints (see, for
example,[I-D.briscoe-tsvwg-cl-architecture]) it would be wasteful to
assign a dedicated bit for ECN.
For each PHB that uses ECN marking, we assume one EXP codepoint will
be defined meaning not congestion marked (Not-CM), and at least one
other codepoint will be defined meaning 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 end-point 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 (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
Davie, et al. Expires August 24, 2007 [Page 7]
Internet-Draft ECN for MPLS February 2007
that a marked packet will be chosen for marking more than once is
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 9.1.
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 draft 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
Davie, et al. Expires August 24, 2007 [Page 8]
Internet-Draft ECN for MPLS February 2007
whether an IP packet is destined for an ECN-capable transport.
Therefore, any single LSR within an MPLS domain MUST NOT be
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 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 which 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 labelled 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.
Davie, et al. Expires August 24, 2007 [Page 9]
Internet-Draft ECN for MPLS February 2007
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
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 treated as such. That
means that if the EXP value of the MPLS header was CM, the packet
MUST be dropped.
Assuming an IP packet was exposed, we have to examine whether that
packet is ECT or not. 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
Davie, et al. Expires August 24, 2007 [Page 10]
Internet-Draft ECN for MPLS February 2007
in Section 1.3, such behavior is not mandated by this document, but
may be selected by an operator.
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 last hop of the LSP and 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.
4.8. Extension to Pre-Congestion Notification
This section describes how the preceding mechanisms can be extended
to support PCN [I-D.briscoe-tsvwg-cl-architecture]. Our intent here
is to show that the mechanisms are readily extended to more complex
scenarios than ECN, but this section may be safely ignored if one is
interested only in supporting ECN.
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), pre-emption marked (PM) and not marked (NM)
states. (See Section 8.4 for further discussion of PCN and the
possibility of using fewer codepoints.)
o A packet can go from NM to AM, from NM to PM, or from AM to PM,
but no other transition is possible.
o Whereas ECN-capable packets are identified by the ECT value in the
IP header, PCN-capability is determined by the PHB of the packet.
Davie, et al. Expires August 24, 2007 [Page 11]
Internet-Draft ECN for MPLS February 2007
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), pre-emption marked
(PM) and not marked (NM) states. The procedures described 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.
4.8.1. 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
PM, set the EXP value of all entries in the label stack to PM. For
any other marking of the IP header, set the EXP value of all entries
in the label stack to NM.
4.8.2. Pushing Additional MPLS Labels
The procedures of Section 4.2 apply.
4.8.3. Admission Control or Pre-emption Marking inside MPLS domain
The EXP value can be set to AM or PM according to the same procedures
as described in [I-D.briscoe-tsvwg-cl-phb]. For the purposes of this
document, it does not matter exactly what algorithms are used to
decide when to set AM or PM; all that matters is that if a router
would have marked AM (or PM) in the IP header, it should set the EXP
value in the MPLS header to the AM (or PM) codepoint.
4.8.4. Popping an MPLS Label (not end of stack)
When popping an MPLS Label exposes another MPLS label, the AM or PM
marking should be transferred to the exposed EXP field in the
following manner: 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. If the inner EXP value is AM, it should be unchanged if the
popped EXP value was AM, and it should be set to PM if the popped EXP
value was PM. If the popped EXP value was NM, this should be logged
in some way and the inner EXP value should be unchanged. If the
inner EXP value is PM, it should be unchanged whatever the popped EXP
value was, but any EXP value other than PM should be logged.
4.8.5. 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:
Davie, et al. Expires August 24, 2007 [Page 12]
Internet-Draft ECN for MPLS February 2007
o the popping LSR is NOT the egress router of the PCN region, in
which case AM or PM 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
[I-D.briscoe-tsvwg-cl-architecture] 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: if the
inner IP header value is neither AM nor PM, 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. 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 PM if the popped EXP value was PM. If the popped EXP value
was NM, this should be logged in some way and the inner IP header
value should be unchanged. If the IP header value is PM, it should
be unchanged whatever the popped EXP value was, but any EXP value
other than PM should be logged.
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, NOT 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
RFC 3270 gives different options with E-LSPs and L-LSPs and some of
those could potentially provide ample EXP codepoints for ECN/PCN.
However, deploying L-LSPs vs E-LSPs has many implications such as
platform support and operational complexity. The above ECN/PCN 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/PCN
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/PCN codepoints, even if
this comes with some compromise on ECN/PCN optimality. See Section 8
for discussion of some possible deployment scenarios.
Davie, et al. Expires August 24, 2007 [Page 13]
Internet-Draft ECN for MPLS February 2007
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 reason for this different design choice is to give
interior routers and LSRs more information about upstream marking in
multi-bottleneck cases. For instance, the flow pre-emption marking
mechanism proposed for PCN works by only considering packets for
marking that have not already been marked upstream. Unless existing
pre-emption marking is copied from the inner to the outer header at
tunnel ingress, the mechanism doesn't pre-empt enough traffic in
cases where anomalous events hit multiple MPLS 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.
So, rather than define different encapsulation methods for ECN and
PCN, Section 4 defines a common approach for both.
7.1. Alternative approach to support ECN in an MPLS domain
It is possible to define an approach for MPLS support of ECN that
more closely resembles that of the full functionality mode of
[RFC3168]. This approach would differ from that described in
Section 4 in the following ways:
o when pushing one or more MPLS labels onto an IP packet, the not-CM
state is set in the EXP field of all label stack entries
o when pushing one or more MPLS labels onto an MPLS packet, the
not-CM state is set in the EXP field of all newly added label
stack entries
o when popping an MPLS label and the exposed header is MPLS (i.e.
this is not the end of stack), the EXP field of the MPLS packet
should be set to CM if the popped label's EXP value was CM and
left unchanged otherwise
Davie, et al. Expires August 24, 2007 [Page 14]
Internet-Draft ECN for MPLS February 2007
o when popping an MPLS label and the exposed header is IP, the IP
ECN field should be set to CE if the EXP value was CM and if the
IP header indicated that the packet was ECN capable. If the IP
header indicated not-ECT and the EXP value was CM, the packet MUST
be dropped. If the EXP value was not-CM, the ECN field in the IP
header is unchanged.
The advantages of this scheme over that described in Section 4 are
greater similarity to [RFC3168], and the ability to determine, at the
end of an LSP, that congestion either did or did not occur along that
LSP (since the initial state is always not-CM at the start of an
LSP).
A disadvantage of this approach is that exceptions to this rule are
necessary in cases where the marking process on LSRs needs to depend
on whether a packet has already suffered upstream marking. The
currently proposed pre-emption marking in PCN is an example where
such an exception would be necessary (see the discussion at the start
of Section 7).
8. Example Uses
8.1. RFC3168-style ECN
[RFC3168] proposes the use of ECN in TCP and introduces the use of
ECN-Echo and 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.
8.2. 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 single PHB, this can be
accomplished by simply allocated a second codepoint to the PHB for
the "CM" state of that PHB and retaining the old codepoint for the
Davie, et al. Expires August 24, 2007 [Page 15]
Internet-Draft ECN for MPLS February 2007
"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 described in [I-D.chan-tsvwg-diffserv-class-aggr].
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
[I-D.chan-tsvwg-diffserv-class-aggr]. 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.
8.3. Congestion-feedback-based Traffic Engineering
Shayman's traffic engineering [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 RSVP
TUNNEL CONGESTION message which was sent to the ingress LSR and
ignored by transit LSRs.
8.4. PCN flow admission control and flow pre-emption
[I-D.briscoe-tsvwg-cl-architecture] 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
pre-empt 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 signalling used to initiate each flow in order to
inform the admission control decision for the whole region at once.
As an example, a minor extension to RSVP signalling has been proposed
[I-D.lefaucheur-rsvp-ecn] to carry this message, but a similar
approach has also been proposed that uses NSIS signalling
[I-D.ietf-nsis-rmd].
If it is possible for LSRs to signify congestion in MPLS, PCN marking
could be used for admission control and flow pre-emption 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. Section 4.8 describes the mechanisms by which
the necessary markings for PCN could be carried in the MPLS header.
As an illustrative example of how the EXP field might be used in this
Davie, et al. Expires August 24, 2007 [Page 16]
Internet-Draft ECN for MPLS February 2007
case, consider the example of an operator who is using the aggregated
service classes described in [I-D.chan-tsvwg-diffserv-class-aggr].
He may choose to support PCN only for the Real Time Treatment
Aggregate, using the EXP codepoint 100 for the not-marked (NM) state,
101 for the Admission Marked (AM) state, and 111 for the Pre-emption
Marked (PM) state. All other codepoints could be the same as in
[I-D.chan-tsvwg-diffserv-class-aggr]. 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.
It might also be possible to deploy a similar solution using PCN
marking over MPLS for just admission control alone, or just flow pre-
emption alone, particularly if codepoint space was at a premium in
the MPLS EXP field. However, the feasibility of deploying one
without the other would require further study.
9. Deployment Considerations
9.1. Marking non-ECN Capable Packets
What is 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?
The problem only arises if there is congestion downstream of an
earlier congested node. It might be that marked packets are carried
through this second congested router when, within the underlying IP
header they are not ECN capable, so they will be dropped later. Such
packets might cause other packets to be marked (or dropped) that
would not otherwise have been.
We decided to use the per-domain ECT checking approach because it
would become optimal as ECN deployment became prevalent. The
situation where traffic is carried beyond a congested LSR only to be
dropped later should become less prevalent as more transports use
ECN. This is why we chose not to use the [Floyd] alternative which
introduced a low but persistent level of unnecessary packet drop for
all time. Although that scheme did not carry droppable traffic to
the edge of the MPLS domain, we felt this was a small price to pay,
and it was anyway only of concern until ECN had become more widely
deployed.
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
Davie, et al. Expires August 24, 2007 [Page 17]
Internet-Draft ECN for MPLS February 2007
significant. It merely causes an application to very occasionally
slow down its rate when it did not have to.
9.2. 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
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.
10. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
11. Security Considerations
We believe no new vulnerabilities are introduced by this draft.
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.
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
draft. The nonce does not need to be present in the MPLS shim
header. 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
Davie, et al. Expires August 24, 2007 [Page 18]
Internet-Draft ECN for MPLS February 2007
all that is necessary for the sender to detect a misbehaving
receiver.
An alternative proposal currently in progress in the IETF
[I-D.briscoe-tsvwg-re-ecn-tcp] allows the network to prevent
misbehavior by senders or receivers or other routers. Like the ECN
nonce, it works correctly without requiring any specific support from
the proposal in this draft. It uses a bit in the IP header (the RE
bit) which is set by the sender and never changed along the path-it
is only read by certain policing elements in the network. There is
no need for a copy of this bit in the MPLS shim, as policing nodes
can examine the IP header if they need to, particularly given they
are intended to only be necessary at domain borders where MPLS
headers are often removed.
12. 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 Joe Babiarz, Ben Niven-Jenkins, Phil Eardley,
and Ruediger Geib for their comments on the draft.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[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.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
Davie, et al. Expires August 24, 2007 [Page 19]
Internet-Draft ECN for MPLS February 2007
[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.
13.2. Informative References
[Floyd] "A Proposal to Incorporate ECN in MPLS", 1999.
Work in progress. http://www.icir.org/floyd/papers/
draft-ietf-mpls-ecn-00.txt
[I-D.briscoe-tsvwg-cl-architecture]
Briscoe, B., "An edge-to-edge Deployment Model for Pre-
Congestion Notification: Admission Control over a
DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04
(work in progress), October 2006.
[I-D.briscoe-tsvwg-cl-phb]
Briscoe, B., "Pre-Congestion Notification marking",
draft-briscoe-tsvwg-cl-phb-03 (work in progress),
October 2006.
[I-D.briscoe-tsvwg-re-ecn-border-cheat]
Briscoe, B., "Emulating Border Flow Policing using Re-ECN
on Bulk Data", draft-briscoe-tsvwg-re-ecn-border-cheat-01
(work in progress), June 2006.
[I-D.briscoe-tsvwg-re-ecn-tcp]
Briscoe, B., "Re-ECN: Adding Accountability for Causing
Congestion to TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-03
(work in progress), October 2006.
[I-D.chan-tsvwg-diffserv-class-aggr]
Chan, K., "Aggregation of DiffServ Service Classes",
draft-chan-tsvwg-diffserv-class-aggr-03 (work in
progress), January 2006.
[I-D.ietf-nsis-rmd]
Bader, A., "RMD-QOSM - The Resource Management in Diffserv
QOS Model", draft-ietf-nsis-rmd-08 (work in progress),
October 2006.
[I-D.lefaucheur-rsvp-ecn]
Faucheur, F., "RSVP Extensions for Admission Control over
Diffserv using Pre-congestion Notification (PCN)",
draft-lefaucheur-rsvp-ecn-01 (work in progress),
June 2006.
Davie, et al. Expires August 24, 2007 [Page 20]
Internet-Draft ECN for MPLS February 2007
[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] "Using ECN to Signal Congestion Within an MPLS Domain",
2000.
Work in progress. http://www.ee.umd.edu/~shayman/papers.d/
draft-shayman-mpls-ecn-00.txt
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. Expires August 24, 2007 [Page 21]
Internet-Draft ECN for MPLS February 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Davie, et al. Expires August 24, 2007 [Page 22]
