Internet DRAFT - draft-villamizar-mpls-multipath-use
draft-villamizar-mpls-multipath-use
MPLS C. Villamizar, Ed.
Internet-Draft Outer Cape Cod Network
Intended status: Informational Consulting
Expires: July 29, 2013 January 25, 2013
Use of Multipath with MPLS-TP and MPLS
draft-villamizar-mpls-multipath-use-02
Abstract
Many MPLS implementations have supported multipath techniques and
many MPLS deployments have used multipath techniques, particularly in
very high bandwidth applications, such as provider IP/MPLS core
networks. MPLS-TP has strongly discouraged the use of multipath
techniques. Some degradation of MPLS-TP OAM performance cannot be
avoided when operating over many types of multipath implementations.
Using MPLS Entropy label, MPLS LSPs can be carried over multipath
links while also providing a fully MPLS-TP compliant server layer for
MPLS-TP LSPs. This document describes the means of supporting MPLS
as a server layer for MPLS-TP. The use of MPLS-TP LSPs as a server
layer for MPLS LSPs is also discussed.
Status of this Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on July 29, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. MPLS as a Server Layer for MPLS-TP . . . . . . . . . . . . . . 5
3.1. MPLS-TP Forwarding and Server Layer Requirements . . . . . 6
3.2. Methods of Supporting MPLS-TP client LSPs over MPLS . . . 7
4. MPLS-TP as a Server Layer for MPLS . . . . . . . . . . . . . . 10
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Today the requirement to handle large aggregations of traffic, can be
handled by a number of techniques which we will collectively call
multipath. Multipath applied to parallel links between the same set
of nodes includes Ethernet Link Aggregation [IEEE-802.1AX], link
bundling [RFC4201], or other aggregation techniques some of which may
be vendor specific. Multipath applied to diverse paths rather than
parallel links includes Equal Cost MultiPath (ECMP) as applied to
OSPF, ISIS, or BGP, and equal cost LSPs. Some vendors support load
split across equal cost MPLS LSPs where the load is split
proportionally to the reserved bandwidth of the set of LSPs.
RFC 5654 requirement 33 requires the capability to carry a client
MPLS-TP or MPLS layer over a server MPLS-TP or MPLS layer [RFC5654].
This is possible in all cases with one exception. When an MPLS LSP
exceeds the capacity of any single component link it may be carried
by a network using multipath techniques, but may not be carried by a
single MPLS-TP LSP due to the inherent MPLS-TP capacity limitation
imposed by MPLS-TP OAM fate sharing constraints and MPLS-TP LM OAM
packet ordering constraints (see Section 3.1).
The term composite link is more general than terms such as link
aggregation (which is specific to Ethernet) or ECMP (which implies
equal cost paths within a routing protocol). The use of the term
composite link here is consistent with the broad definition in
[ITU-T.G.800]. Multipath is very similar to composite link as
defined by ITU, but specifically excludes inverse multiplexing.
2. Definitions
Multipath
The term multipath includes all techniques in which
1. Traffic can take more than one path from one node to a
destination.
2. Individual packets take one path only. Packets are not
subdivided and reassembled at the receiving end.
3. Packets are not resequenced at the receiving end.
4. The paths may be:
a. parallel links between two nodes, or
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b. may be specific paths across a network to a destination
node, or
c. may be links or paths to an intermediate node used to
reach a common destination.
Link Bundle
Link bundling is a multipath technique specific to MPLS
[RFC4201]. Link bundling supports two modes of operations.
Either an LSP can be placed on one component link of a link
bundle, or an LSP can be load split across all members of the
bundle. There is no signaling defined which allows a per LSP
preference regarding load split, therefore whether to load split
is generally configured per bundle and applied to all LSPs across
the bundle.
Link Aggregation
The term "link aggregation" generally refers to Ethernet Link
Aggregation [IEEE-802.1AX] as defined by the IEEE. Ethernet Link
Aggregation defines a Link Aggregation Control Protocol (LACP)
which coordinates inclusion of LAG members in the LAG.
Link Aggregation Group (LAG)
A group of physical Ethernet interfaces that are treated as a
logical link when using Ethernet Link Aggregation is referred to
as a Link Aggregation Group (LAG).
Equal Cost Multipath (ECMP)
Equal Cost Multipath (ECMP) is a specific form of multipath in
which the costs of the links or paths must be equal in a given
routing protocol. The load may be split equally across all
available links (or available paths), or the load may be split
proportionally to the capacity of each link (or path).
Loop Free Alternate Paths
"Loop-free alternate paths" (LFA) are defined in RFC 5714,
Section 5.2 [RFC5714] as follows. "Such a path exists when a
direct neighbor of the router adjacent to the failure has a path
to the destination that can be guaranteed not to traverse the
failure." Further detail can be found in [RFC5286]. LFA as
defined for IPFRR can be used to load balance by relaxing the
equal cost criteria of ECMP, though IPFRR defined LFA for use in
selecting protection paths. When used with IP, proportional
split is generally not used. LFA use in load balancing is
implemented by some vendors though it may be rare or non-existent
in deployments.
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Composite Link
The term Composite Link had been a registered trademark of Avici
Systems, but was abandoned in 2007. The term composite link is
now defined by the ITU in [ITU-T.G.800]. The ITU definition
includes multipath as defined here, plus inverse multiplexing
which is explicitly excluded from the definition of multipath.
Inverse Multiplexing
Inverse multiplexing either transmits whole packets and
resequences the packets at the receiving end or subdivides
packets and reassembles the packets at the receiving end.
Inverse multiplexing requires that all packets be handled by a
common egress packet processing element and is therefore not
useful for very high bandwidth applications.
Component Link
The ITU definition of composite link in [ITU-T.G.800] and the
IETF definition of link bundling in [RFC4201] both refer to an
individual link in the composite link or link bundle as a
component link. The term component link is applicable to all
multipath.
LAG Member
Ethernet Link Aggregation as defined in [IEEE-802.1AX] refers to
an individual link in a LAG as a LAG member. A LAG member is a
component link. An Ethernet LAG is a composite link. IEEE does
not use the terms composite link or component link.
load split
Load split, load balance, or load distribution refers to
subdividing traffic over a set of component links such that load
is fairly evenly distributed over the set of component links and
certain packet ordering requirements are met. Some existing
techniques better acheive these objectives than others.
A small set of requirements are discussed. These requirements make
use of keywords such as MUST and SHOULD as described in [RFC2119].
3. MPLS as a Server Layer for MPLS-TP
An MPLS LSP may be used as a server layer for MPLS-TP LSPs as long as
all MPLS-TP requirements are met. Section 3.1 reviews the basis for
requirements of a server layer that supports MPLS-TP as a client
layer. Key requirements include OAM "fate-sharing" the the
requirement that packets within an MPLS-TP LSP are not reordered,
including both payload and OAM packets. Section 3.2 discusses
implied requirements where MPLS is the server layer for MPLS-TP
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client LSPs, and describes a set of solutions using existing MPLS
mechanisms.
3.1. MPLS-TP Forwarding and Server Layer Requirements
[RFC5960] defines the date plane requirements for MPLS-TP. Two very
relevant paragraphs in "Section 3.1.1 LSP Packet Encapsulation and
Forwarding" are the following.
RFC5960, Section 3.1.1, Paragraph 3
Except for transient packet reordering that may occur, for
example, during fault conditions, packets are delivered in order
on L-LSPs, and on E-LSPs within a specific ordered aggregate.
RFC5960, Section 3.1.1, Paragraph 6
Equal-Cost Multi-Path (ECMP) load-balancing MUST NOT be performed
on an MPLS-TP LSP. MPLS-TP LSPs as defined in this document MAY
operate over a server layer that supports load-balancing, but
this load-balancing MUST operate in such a manner that it is
transparent to MPLS-TP. This does not preclude the future
definition of new MPLS-TP LSP types that have different
requirements regarding the use of ECMP in the server layer.
[RFC5960] paragraph 3 requires that packets within a specific ordered
aggregate be delivered in order. This same requirement is already
specified by Differentiated Services [RFC2475]. [RFC5960] paragraph
6 explicitly allows a server layer to use ECMP provided that it is
transparent to the MPLS-TP client layer.
[RFC6371] adds a requirement for data traffic and OAM traffic "fate-
sharing". The following paragraph in "Section 1 Introduction"
summarizes this requirement.
RFC6371, Section 1, Paragraph 7
OAM packets that instrument a particular direction of a transport
path are subject to the same forwarding treatment (i.e., fate-
share) as the user data packets and in some cases, where
Explicitly TC-encoded-PSC LSPs (E-LSPs) are employed, may be
required to have common per-hop behavior (PHB) Scheduling Class
(PSC) End-to-End (E2E) with the class of traffic monitored. In
case of Label-Only-Inferred-PSC LSP (L-LSP), only one class of
traffic needs to be monitored, and therefore the OAM packets have
common PSC with the monitored traffic class.
[RFC6371] does not prohibit multilink techniques in "Section 4.6
Fate-Sharing Considerations for Multilink", where multilink is
defined as Ethernet Link Aggregation and the use of Link Bundling for
MPLS, but does declare that such a network would be only partially
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MPLS-TP compliant. The characteristic that is to be avoided is
contained in the following sentence in this section.
RFC6371, Section 4.6, Paragraph 1, last sentence
These techniques frequently share the characteristic that an LSP
may be spread over a set of component links and therefore be
reordered, but no flow within the LSP is reordered (except when
very infrequent and minimally disruptive load rebalancing
occurs).
A declaration that implies that Link Bundling for MPLS yields a
partially MPLS-TP compliant network, is perhaps overstated since only
the Link Bundling all-ones component link has this characteristic.
[RFC6374] defines a direct Loss Measurement (LM) where LM OAM packets
cannot be reordered with respect to payload packets. This will
require that payload packets themselves not be reordered. The
following paragraph in "Section 2.9.4 Equal Cost Multipath" gives the
reason for this restriction.
RFC6374, Section 2.9.4, Paragraph 2
The effects of ECMP on loss measurement will depend on the LM
mode. In the case of direct LM, the measurement will account for
any packets lost between the sender and the receiver, regardless
of how many paths exist between them. However, the presence of
ECMP increases the likelihood of misordering both of LM messages
relative to data packets and of the LM messages themselves. Such
misorderings tend to create unmeasurable intervals and thus
degrade the accuracy of loss measurement. The effects of ECMP
are similar for inferred LM, with the additional caveat that,
unless the test packets are specially constructed so as to probe
all available paths, the loss characteristics of one or more of
the alternate paths cannot be accounted for.
3.2. Methods of Supporting MPLS-TP client LSPs over MPLS
Supporting MPLS-TP LSPs over a fully MPLS-TP conformant MPLS LSP
server layer where the MPLS LSPs are making use of multipath,
requires special treatment of the MPLS-TP LSPs such that those LSPs
meet MPLS-TP forwarding requirements (see Section 3.1). This implies
the following brief set of requirements.
MP#1 It MUST be possible for a midpoint MPLS-TP LSR which is serving
as ingress to a server layer MPLS LSP to identify MPLS-TP LSPs,
so that MPLS-TP forwarding requirements can be applied, or to
otherwise accommodate the MPLS-TP forwarding requirements.
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MP#2 It SHOULD be possible to completely exclude MPLS-TP LSPs from
the multipath hash and load split. If the selected component
link no longer meets requirements, an LSP is considered down
which may trigger protection and/or may require that the
ingress LSR select a new path and signal a new LSP.
MP#3 It SHOULD be possible to insure that MPLS-TP LSPs will not be
moved to another component link as a result of a composite link
load rebalancing operation. If the selected component link no
longer meets requirements, another component link may be
selected, however a change in path should not occur solely for
load balancing.
MP#4 Where an RSVP-TE control plane is used, it MUST be possible for
an ingress LSR which is setting up an MPLS-TP or an MPLS LSP to
determine at path selection time whether a link or Forwarding
Adjacency (FA, see [RFC4206]) within the topology can support
the MPLS-TP requirements of the LSP.
The reason for requirement MP#1 may not be obvious. A MPLS-TP LSP
may be aggregated along with other client LSP by a midpoint LSR into
a very large MPLS server layer LSP, as would be the case in a core
node to core node MPLS LSP between major cities. In this case the
ingress of the MPLS LSP cannot through any existing signaling
mechanism determine which client LSP contained within it as MPLS-TP
or not MPLS-TP. Multipath load splitting can be avoided for MPLS-TP
LSP if at the MPLS server layer LSP ingress LSR an Entropy Label
Indicator (ELI) and Entropy Label (EL) are added to the label stack
[RFC6790]. For those client LSP that are MPLS-TP LSP, a single EL
value must be chosen. For those client LSP that are MPLS LSP, per
packet entropy below the top label must, for practical reasons, be
used to determine the entropy label value. Requirement MP#1 simply
states that there must be a means to make this decision.
There is currently no signaling mechanism defined to support
requirement MP#1, though that does not preclude a new extension being
defined later. In the absense of a signaling extension, MPLS-TP can
be identified through some form of configuration, such as
configuration which provides an MPLS-TP compatible server layer to
all LSP arriving on a specific interface or originating from a
specific set of ingress LSR.
Alternately, the need for requirement MP#1 can be eliminated if evey
MPLS-TP LSP can be created by the MPLS-TP ingress makes use of an
Entropy Label Indicator (ELI) and Entropy Label (EL) below the
MPLS-TP label [RFC6790]. This would require that all MPLS-TP LSR in
a deployment support Entropy Label, which may render it impractical
in many deployments.
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Some hardware which exists today can support requirement MP#2.
Signaling in the absense of MPLS Entropy Label can make use of link
bundling with the path pinned to a specific component for MPLS-TP LSP
and link bundling using the all-ones component for MPLS LSP. This
prevents MPLS-TP LSP from being carried within MPLS LSP but does
allow the co-existance of MPLS-TP and very large MPLS LSP.
MPLS-TP LSPs can be carried as client LSPs within an MPLS server LSP
if an Entropy Label Indicator (ELI) and Entropy Label (EL) is added
after the server layer LSP label(s) in the label stack, just above
the MPLS-TP LSP label entry [RFC6790]. The value of EL can be
randomly selected at the client MPLS-TP LSP setup time and the same
EL value used for all packets of that MPLS-TP LSP. This allows
MPLS-TP LSP to be carried as client LSP within MPLS LSP and satisfies
MPLS-TP forwarding requirements but requires that MPLS LSR be able to
identify MPLS-TP LSP (requirement MP#1).
MPLS-TP traffic can be protected from an degraded performance due to
an imperfect load split if the MPLS-TP traffic is given queuing
priority (using strict priority and policing or shaping at ingress or
locally or weighted queuing locally). This can be accomplished using
the Traffic Class field and Diffserv treatment of traffic
[RFC5462][RFC2475]. In the event of congestion due to load
imbalance, other traffic will suffer as long as there is a minority
of MPLS-TP traffic.
If MPLS-TP LSP are carried within MPLS LSP and ELI and EL are used,
requirement MP#3 is satisfied only for uncongested links where load
balancing is not required, or if MPLS-TP LSP use TC and Diffserv and
the load rebalancing implementation rebalances only the less
preferred traffic. Load rebalance is generally needed only when
congestion occurs, therefore restricting MPLS-TP to be carried only
over MPLS LSP that are known to traverse only links which are
expected to be uncongested can satisfy requirement MP#3.
An MPLS-TP LSP can be pinned to a Link Bundle component link if the
behavior of requirement MP#2 is preferred. An MPLS-TP LSP can be
assigned to a Link Bundle but not pinned if the behavior of
requirement MP#3 is preferred. In both of these cases, the MPLS-TP
LSP must be the top level LSP, except as noted above.
If MPLS-TP LSP can be moved among component links, then the Link
Bundle all-ones component link can be used or server layer MPLS LSPs
can be used with no restrictions on the server layer MPLS use of
multipath except that Entropy Label must be supported along the
entire path. An Entropy Label must be used to insure that all of the
MPLS-TP payload and OAM traffic are carried on the same component,
except during very infrequent transitions due to load balancing.
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An MPLS-TP LSP may not traverse multipath links on the path where
MPLS-TP forwarding requirements cannot be met. Such links include
any using pre-RFC6790 Ethernet Link Aggregation, pre-RFC6790 Link
Bundling using the all-ones component link, or other form of
multipath not supporting termination of the entropy search at the EL
label as called for in [RFC6790]. An MPLS-TP LSP must not traverse a
server layer MPLS LSP which traverses any form of multipath not
supporting termination of the entropy search at the EL label. For
this to occur, the MPLS-TP ingress LSR must be aware of these links.
This is the reason for requirement MP#4.
Requirement MP#4 can be supported using administrative attributes.
Administrative attributes are defined in [RFC3209]. Some
configuration is required to support this.
4. MPLS-TP as a Server Layer for MPLS
Carrying MPLS LSP which are larger than a component link over a
MPLS-TP server layer requires that the large MPLS client layer LSP be
accommodated by multiple MPLS-TP server layer LSPs. MPLS multipath
can be used in the client layer MPLS.
Creating multiple MPLS-TP server layer LSP places a greater Incoming
Label Map (ILM) scaling burden on the LSR. High bandwidth MPLS cores
with a smaller amount of nodes have the greatest tendency to require
LSP in excess of component links, therefore the reduction in number
of nodes offsets the impact of increasing the number of server layer
LSP in parallel. Today, only in cases where deployed LSR ILM are
small would this be an issue.
The most significant disadvantage of MPLS-TP as a Server Layer for
MPLS is that the use MPLS-TP server layer LSP reduces the efficiency
of carrying the MPLS client layer. The service which provides by far
the largest offered load in provider networks is Internet, for which
the LSP capacity reservations are predictions of expected load. Many
of these MPLS LSP may be smaller than component link capacity. Using
MPLS-TP as a server layer results in bin packing problems for these
smaller LSP. For those LSP that are larger than component link
capacity, their capacity are not increments of convenient capacity
increments such as 10Gb/s. Using MPLS-TP as an underlying server
layer greatly reduces the ability of the client layer MPLS LSP to
share capacity. For example, when one MPLS LSP is underutilizing its
predicted capacity, the fixed allocation of MPLS-TP to component
links may not allow another LSP to exceed its predicted capacity.
Using MPLS-TP as a server layer may result in less efficient use of
resources and may result in a less cost effective network.
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No additional requirements beyond MPLS-TP as it is now currently
defined are required to support MPLS-TP as a Server Layer for MPLS.
It is therefore viable but has some undesirable characteristics
discussed above.
5. Acknowledgements
Carlos Pignataro, Dave Allan, and Mach Chen provided valuable
comments and suggestions. Carlos suggested that MPLS-TP requirements
in RFC 5960 be explicitly referenced or quoted. An email
conversation with Dave led to the inclusion of references and quotes
from RFC 6371 and RFC 6374. Mach made suggestions to improve clarity
of the document.
6. Implementation Status
Note: this section is temporary and supports the experiment called
for in draft-sheffer-running-code.
This is an informational document which describes usage of MPLS and
MPLS-TP. No new protocol extensions or forwarding behavior are
specified. Ethernet Link Aggregation and MPLS Link Bundling are
widely implemented and deployed.
Entropy Label is not yet widely implemented and deployed, but both
implementation and deployment are expected soon. At least a few
existing high end commodity packet processing chips are capable of
supporting Entropy Label. It would be helpful if a few LSR suppliers
would state their intentions to support RFC 6790 on the mpls mailing
list.
Dynamic multipath (multipath load split adjustment in response to
observed load) is referred to but not a requirement of the usage
recommendations made in this document. Dynamic multipath has been
implemented and deployed, however (afaik) the only core LSR vendor
supporting dynamic multipath is no longer in the router business
(Avici Systems). At least a few existing high end commodity packet
processing chips are capable of supporting dynamic multipath.
7. IANA Considerations
This memo includes no request to IANA.
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8. Security Considerations
This document specifies requirements with discussion of framework for
solutions using existing MPLS and MPLS-TP mechanisms. The
requirements and framework are related to the coexistence of MPLS/
GMPLS (without MPLS-TP) when used over a packet network, MPLS-TP, and
multipath. The combination of MPLS, MPLS-TP, and multipath does not
introduce any new security threats. The security considerations for
MPLS/GMPLS and for MPLS-TP are documented in [RFC5920] and
[I-D.ietf-mpls-tp-security-framework].
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5654] Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
and S. Ueno, "Requirements of an MPLS Transport Profile",
RFC 5654, September 2009.
[RFC5960] Frost, D., Bryant, S., and M. Bocci, "MPLS Transport
Profile Data Plane Architecture", RFC 5960, August 2010.
[RFC6371] Busi, I. and D. Allan, "Operations, Administration, and
Maintenance Framework for MPLS-Based Transport Networks",
RFC 6371, September 2011.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374, September 2011.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012.
9.2. Informative References
[I-D.ietf-mpls-tp-security-framework]
Fang, L., Niven-Jenkins, B., Mansfield, S., and R.
Graveman, "MPLS-TP Security Framework",
draft-ietf-mpls-tp-security-framework-05 (work in
progress), October 2012.
[IEEE-802.1AX]
IEEE Standards Association, "IEEE Std 802.1AX-2008 IEEE
Standard for Local and Metropolitan Area Networks - Link
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Aggregation", 2006, <http://standards.ieee.org/getieee802/
download/802.1AX-2008.pdf>.
[ITU-T.G.800]
ITU-T, "Unified functional architecture of transport
networks", 2007, <http://www.itu.int/rec/T-REC-G/
recommendation.asp?parent=T-REC-G.800>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, January 2010.
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
Author's Address
Curtis Villamizar (editor)
Outer Cape Cod Network Consulting
Email: curtis@occnc.com
Villamizar Expires July 29, 2013 [Page 13]
