RTGWG                                                            S. Ning
Internet-Draft                                       Tata Communications
Intended status: Informational                                  A. Malis
Expires: February 13, 2013                                    D. McDysan
                                                                 L. Yong
                                                              Huawei USA
                                                           C. Villamizar
                                                  Outer Cape Cod Network
                                                         August 12, 2012

           Composite Link Use Cases and Design Considerations


   This document provides a set of use cases and design considerations
   for composite links.

   Composite link is a formalization of multipath techniques currently
   in use in IP and MPLS networks and a set of extensions to multipath

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 13, 2013.

Copyright Notice

   Copyright (c) 2012 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

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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions used in this document  . . . . . . . . . . . . . .  3
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Composite Link Foundation Use Cases  . . . . . . . . . . . . .  4
   4.  Delay Sensitive Applications . . . . . . . . . . . . . . . . .  7
   5.  Large Volume of IP and LDP Traffic . . . . . . . . . . . . . .  7
   6.  Composite Link and Packet Ordering . . . . . . . . . . . . . .  8
     6.1.  MPLS-TP in network edges only  . . . . . . . . . . . . . . 10
     6.2.  Composite Link at core LSP ingress/egress  . . . . . . . . 11
     6.3.  MPLS-TP as a MPLS client . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  More Details on Existing Network Operator
                Practices and Protocol Usage  . . . . . . . . . . . . 15
   Appendix B.  Existing Multipath Standards and Techniques . . . . . 17
     B.1.  Common Multpath Load Spliting Techniques . . . . . . . . . 18
     B.2.  Simple and Adaptive Load Balancing Multipath . . . . . . . 19
     B.3.  Traffic Split over Parallel Links  . . . . . . . . . . . . 20
     B.4.  Traffic Split over Multiple Paths  . . . . . . . . . . . . 20
   Appendix C.  Characteristics of Transport in Core Networks . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

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1.  Introduction

   Composite link requirements are specified in
   [I-D.ietf-rtgwg-cl-requirement].  A composite link framework is
   defined in [I-D.so-yong-rtgwg-cl-framework].

   Multipath techniques have been widely used in IP networks for over
   two decades.  The use of MPLS began more than a decade ago.
   Multipath has been widely used in IP/MPLS networks for over a decade
   with very little protocol support dedicated to effective use of

   The state of the art in multipath prior to composite links is
   documented in Appendix B.

   Both Ethernet Link Aggregation [IEEE-802.1AX] and MPLS link bundling
   [RFC4201] have been widely used in today's MPLS networks.  Composite
   link differs in the following caracteristics.

   1.  A composite link allows bundling of non-homogenous links together
       as a single logical link.

   2.  A composite link provides more information in the TE-LSDB and
       supports more explicit control over placement of LSP.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.1.  Terminology

   Terminology defined in [I-D.ietf-rtgwg-cl-requirement] is used in
   this document.

   In addition, the following terms are used:

   classic multipath:
       Classic multipath refers to the most common current practice in
       implementation and deployment of multipath (see Appendix A).  The
       most common current practice makes use of a hash on the MPLS
       label stack and if IPv4 or IPv6 are indicates under the label
       stack, makes use of the IP source and destination addresses
       [RFC4385] [RFC4928].

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   classic link bundling:
       Classic link bundling refers to the use of [RFC4201] where the
       "all ones" component is not used.  Where the "all ones" component
       is used, link bundling behaves as classic multipath does.
       Classic link bundling selects a single component link on which to
       put any given LSP.

   Among the important distinctions between classic multipath or classic
   link bundling and Composite Link are:

   1.  Classic multipath has no provision to retain order among flows
       within a subset of LSP.  Classic link bundling retains order
       among all flows but as a result does a poor job of splitting load
       among components and therefore is rarely (if ever) deployed.
       Composite Link allows per LSP control of load split

   2.  Classic multipath and classic link bundling do not provide a
       means to put some LSP on component links with lower delay.
       Composite Link does.

   3.  Classic multipath will provide a load balance for IP and LDP
       traffic.  Classic link bundling will not.  Neither classic
       multipath or classic link bundling will measure IP and LDP
       traffic and reduce the advertised "Available Bandwidth" as a
       result of that measurement.  Composite Link better supports
       RSVP-TE used with significant traffic levels of native IP and
       native LDP.

   4.  Classic link bundling cannot support an LSP that is greater in
       capacity than any single component link.  Classic multipath and
       Composite Link support this capability but will reorder traffic
       on such an LSP.  Composite Link can retain order of an LSP that
       is carried within an LSP that is greater in capacity than any
       single component link if the contained LSP has such a

   None of these techniques, classic multipath, classic link bundling,
   or Composite Link, will reorder traffic among IP microflows.  None of
   these techniques will reorder traffic among PW, if a PWE3 Control
   Word is used [RFC4385].

3.  Composite Link Foundation Use Cases

   A simple composite link composed entirely of physical links is
   illustrated in Figure 1, where a composite link is configured between
   LSR1 and LSR2.  This composite link has three component links.

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   Individual component links in a composite link may be supported by
   different transport technologies such as wavelength, Ethernet VLAN.
   Even if the transport technology implementing the component links is
   identical, the characteristics (e.g., bandwidth, latency) of the
   component links may differ.

   The composite link in Figure 1 may carry LSP traffic flows and
   control plane packets.  Control plane packets may appear as IP
   packets or may be carried within a generic associated channel (G-Ach)
   [RFC5586].  A LSP may be established over the link by either RSVP-TE
   [RFC3209] or LDP [RFC5036] signaling protocols.  All component links
   in a composite link are summarized in the same forwarding adjacency
   LSP (FA-LSP) routing advertisement [RFC3945].  The composite link is
   summarized as one TE-Link advertised into the IGP by the composite
   link end points.  This information is used in path computation when a
   full MPLS control plane is in use.  The individual component links or
   groups of component links may optionally be advertised into the IGP
   as sub-TLV of the composite link advertisement to indicate capacity
   available with various characteristics, such as a delay range.

               Management Plane
           Configuration and Measurement <------------+
                      ^                               |
                      |                               |
              +-------+-+                           +-+-------+
              |       | |                           | |       |
         CP Packets   V |                           | V     CP Packets
              |  V    | |     Component Link 1      | |    ^  |
              |  |    |=|===========================|=|    |  |
              |  +----| |     Component Link 2      | |----+  |
              |       |=|===========================|=|       |
    Aggregated LSPs   | |                           | |       |
             ~|~~~~~~>| |     Component Link 3      | |~~~~>~~|~~
              |       |=|===========================|=|       |
              |       | |                           | |       |
              | LSR1    |                           |    LSR2 |
              +---------+                           +---------+
                      !                               !
                      !                               !
                      !<------ Composite Link ------->!

    Figure 1: a composite link constructed with multiple physical links
                              between two LSR

   [I-D.ietf-rtgwg-cl-requirement] specifies that component links may
   themselves be composite links.  Figure 2 shows three three forms of
   component links which may be deployed in a network.

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    +-------+                 1. Physical Link             +-------+
    |     |-|----------------------------------------------|-|     |
    |     | |                                              | |     |
    |     | |     +------+                     +------+    | |     |
    |     | |     | MPLS |    2. Logical Link  | MPLS |    | |     |
    |     |.|.... |......|.....................|......|....|.|     |
    |     | |-----| LSR3 |---------------------| LSR4 |----| |     |
    |     | |     +------+                     +------+    | |     |
    |     | |                                              | |     |
    |     | |                                              | |     |
    |     | |     +------+                     +------+    | |     |
    |     | |     |GMPLS |    3. Logical Link  |GMPLS |    | |     |
    |     |.|. ...|......|.....................|......|....|.|     |
    |     | |-----| LSR5 |---------------------| LSR6 |----| |     |
    |       |     +------+                     +------+    |       |
    | LSR1  |                                              |  LSR2 |
    +-------+                                              +-------+
          |<------------- Composite Link ------------------->|

          Figure 2: Illustration of Various Component Link Types

   The three forms of component link shown in Figure 2 are:

   1.  The first component link is configured with direct physical

   2.  The second component link is a TE tunnel that traverses LSR3 and
       LSR4, where LSR3 and LSR4 are the nodes supporting MPLS, but
       supporting few or no GMPLS extensions.

   3.  The third component link is formed by lower layer network that
       has GMPLS enabled.  In this case, LSR5 and LSR6 are not the nodes
       controlled by the MPLS but provide the connectivity for the
       component link.

   A composite link forms one logical link between connected LSR and is
   used to carry aggregated traffic [I-D.ietf-rtgwg-cl-requirement].
   Composite link relies on its component links to carry the traffic
   over the composite link.  The endpoints of the composite link maps
   incoming traffic into component links.

   For example, LSR1 in Figure 1 distributes the set of traffic flows
   including control plane packets among the set of component links.
   LSR2 in Figure 1 receives the packets from its component links and
   sends them to MPLS forwarding engine with no attempt to reorder
   packets arriving on different component links.  The traffic in the
   opposite direction, from LSR2 to LSR1, is distributed across the set
   of component links by the LSR2.

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   These three forms of component link are only example.  Many other
   examples are possible.  A component link may itself be a composite
   link.  A segment of an LSP (single hop for that LSP) may be a
   composite link.

4.  Delay Sensitive Applications

   Most applications benefit from lower delay.  Some types of
   applications are far more sensitive than others.  For example, real
   time bidirectional applications such as voice communication or two
   way video conferencing are far more sensitive to delay than
   unidirectional streaming audio or video.  Non-interactive bulk
   transfer is almost insensitive to delay if a large enough TCP window
   is used.

   Some applications are sensitive to delay but unwilling to pay extra
   to insure lower delay.  For example, many SIP end users are willing
   to accept the delay offerred to best effort services as long as call
   quality is good most of the time.

   Other applications are sensitive to delay and willing to pay extra to
   insure lower delay.  For example, financial trading applications are
   extremely sensitive to delay and with a lot at stake are willing to
   go to great lengths to reduce delay.

   Among the requirements of Composite Link are requirements to
   advertise capacity available within configured ranges of delay within
   a given composite link and the support the ability to place an LSP
   only on component links that meeting that LSP's delay requirements.

   The Composite Link requirements to accommodate delay sensitive
   applications are analogous to diffserv requirements to accomodate
   applications requiring higher quality of service on the same
   infrastructure as applications with less demanding requirements.  The
   ability to share capacity with less demanding applications, with best
   effort applications being the least demanding, can greatly reduce the
   cost of delivering service to the more demanding applications.

5.  Large Volume of IP and LDP Traffic

   IP and LDP do not support traffic engineering.  Both make use of a
   shortest (lowest routing metric) path, with an option to use equal
   cost multipath (ECMP).  Note that though ECMP is prohibited in LDP
   specifications, it is widely implemented.  Where implemented for LDP,
   ECMP is generally disabled by default for standards compliance, but
   often enabled in LDP deployments.

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   Without traffic engineering capability, there must be sufficient
   capacity to accomodate the IP and LDP traffic.  If not, persistent
   queuing delay and loss will occur.  Unlike RSVP-TE, a subset of
   traffic cannot be routed using constraint based routing to avoid a
   congested portion of an infrastructure.

   In existing networks which accomodate IP and/or LDP with RSVP-TE,
   either the IP and LDP can be carried over RSVP-TE, or where the
   traffic contribution of IP and LDP is small, IP and LDP can be
   carried native and the effect on RSVP-TE can be ignored.  Ignoring
   the traffic contribution of IP is certainly valid on high capacity
   networks where native IP is used primarily for control and network
   management and customer IP is carried within RSVP-TE.

   Where it is desireable to carry native IP and/or LDP and IP and/or
   LDP traffic volumes are not negligible, RSVP-TE needs improvement.
   The enhancement offerred by Composite Link is an ability to measure
   the IP and LDP, filter the measurements, and reduce the capacity
   available to RSVP-TE to avoid congestion.  The treatment given to the
   IP or LDP traffic is similar to the treatment when using the "auto-
   bandwidth" feature in some RSVP-TE implementations on that same
   traffic, and giving a higher priority (numerically lower setup
   priority and holding priority value) to the "auto-bandwidth" LSP.
   The difference is that the measurement is made at each hop and the
   reduction in advertised bandwidth is made more directly.

6.  Composite Link and Packet Ordering

   A strong motivation for Composite Link is the need to provide LSP
   capacity in IP backbones that exceeds the capacity of single
   wavelengths provided by transport equipment and exceeds the practical
   capacity limits acheivable through inverse multiplexing.  Appendix C
   describes characteristics and limitations of transport systems today.
   Section 2 defines the terms "classic multipath" and "classic link
   bundling" used in this section.

   For purpose of discussion, consider two very large cities, city A and
   city Z. For example, in the US high traffic cities might be New York
   and Los Angeles and in Europe high traffic cities might be London and
   Amsterdam.  Two other high volume cities, city B and city Y may share
   common provider core network infrastructure.  Using the same
   examples, the city B and Y may Washington DC and San Francisco or
   Paris and Stockholm.  In the US, the common infrastructure may span
   Denver, Chicago, Detroit, and Cleveland.  Other major traffic
   contributors on either US coast include Boston, northern Virginia on
   the east coast, and Seattle, and San Diego on the west coast.  The
   capacity of IP/MPLS links within the shared infrastructure, for

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   example city to city links in the Denver, Chicago, Detroit, and
   Cleveland path in the US example, have capacities for most of the
   2000s decade that greatly exceeded single circuits available in
   transport networks.

   For a case with four large traffic sources on either side of the
   shared infrastructure, up to sixteen core city to core city traffic
   flows in excess of transport circuit capacity may be accomodated on
   the shared infrastructure.

   Today the most common IP/MPLS core network design makes use of very
   large links which consist of many smaller component links, but use
   classic multipath techniques rather than classic link bundling or
   Composite Link.  A component link typically corresponds to the
   largest circuit that the transport system is capable of providing (or
   the largest cost effective circuit).  IP source and destination
   address hashing is used to distribute flows across the set of
   component links as described in Appendix B.3.

   Classic multipath can handle large LSP up to the total capacity of
   the multipath (within limits, see Appendix B.2).  A disadvantage of
   classic multipath is the reordering among traffic within a given core
   city to core city LSP.  While there is no reordering within any
   microflow and therefore no customer visible issue, MPLS-TP cannot be
   used across an infrastructure where classic multipath is in use,
   except within pseudowires.

   These capacity issues force the use of classic multipath today.
   Classic multipath excludes a direct use of MPLS-TP.  The desire for
   OAM, offerred by MPLS-TP, is in conflict with the use of classic
   multipath.  There are a number of alternatives that satisfy both
   requirements.  Some alternatives are described below.

   MPLS-TP in network edges only

       A simple approach which requires no change to the core is to
       disallow MPLS-TP across the core unless carried within a
       pseudowire (PW).  MPLS-TP may be used within edge domains where
       classic multipath is not used.  PW may be signaled end to end
       using single segment PW (SS-PW), or stitched across domains using
       multisegment PW (MS-PW).  The PW and anything carried within the
       PW may use OAM as long as fat-PW [RFC6391] load splitting is not
       used by the PW.

   Composite Link at core LSP ingress/egress

       The interior of the core network may use classic link bundling,
       with the limitation that no LSP can exceed the capacity of a

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       single circuit.  Larger non-MPLS-TP LSP can be configured using
       multiple ingress to egress component MPLS-TP LSP.  This can be
       accomplished using existing IP source and destination address
       hashing configured at LSP ingress and egress, or using Composite
       Link configured at ingress and egress.  Each component LSP, if
       constrained to be no larger than the capacity of a single
       circuit. can make use of MPLS-TP and offer OAM for all top level
       LSP across the core.

   MPLS-TP as a MPLS client

       A third approach involves modifying the behavior of LSR in the
       interior of the network core, such that MPLS-TP can be used on a
       subset of LSP, where the capacity of any one LSP within that
       MPLS-TP subset of LSP is not larger than the capacity of a single
       circuit.  This requirement is accommodated through a combination
       of signaling to indicate LSP for which traffic splitting needs to
       be constrained, the ability to constrain the depth of the label
       stack over which traffic splitting can be applied on a per LSP
       basis, and the ability to constrain the use of IP addresses below
       the label stack for traffic splitting also on a per LSP basis.

   The above list of alternatives allow packet ordering within an LSP to
   be maintained in some circumstances and allow very large LSP
   capacities.  Each of these alternatives are discussed further in the
   following subsections.

6.1.  MPLS-TP in network edges only

   Classic MPLS link bundling is defined in [RFC4201] and has existed
   since early in the 2000s decade.  Classic MPLS link bundling place
   any given LSP entirely on a single component link.  Classic MPLS link
   bundling is not in widespread use as the means to accomodate large
   link capacities in core networks due to the simplicity and better
   multiplexing gain, and therefore lower network cost of classic

   If MPLS-TP OAM capability in the IP/MPLS network core LSP is not
   required, then there is no need to change existing network designs
   which use classic multipath and both label stack and IP source and
   destination address based hashing as a basis for load splitting.

   If MPLS-TP is needed for a subset of LSP, then those LSP can be
   carried within pseudowires.  The pseudowires adds a thin layer of
   encapsulation and therefore a small overhead.  If only a subset of
   LSP need MPLS-TP OAM, then some LSP must make use of the pseudowires
   and other LSP avoid them.  A straihtforward way to accomplish this is
   with administrative attributes [RFC3209].

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6.2.  Composite Link at core LSP ingress/egress

   Composite Link can be configured only for large LSP that are made of
   smaller MPLS-TP component LSP.  This approach is capable of
   supporting MPLS-TP OAM over the entire set of component link LSP and
   therefore the entire set of top level LSP traversing the core.

   There are two primary disadvantage of this approach.  One is the
   number of top level LSP traversing the core can be dramatically
   increased.  The other disadvantage is the loss of multiplexing gain
   that results from use of classic link bundling within the interior of
   the core network.

   If component LSP use MPLS-TP, then no component LSP can exceed the
   capacity of a single circuit.  For a given composite LSP there can
   either be a number of equal capacity component LSP or some number of
   full capacity component links plus one LSP carrying the excess.  For
   example, a 350 Gb/s composite LSP over a 100 Gb/s infrastructure may
   use five 70 Gb/s component LSP or three 100 Gb/s LSP plus one 50 Gb/s
   LSP.  Classic MPLS link bundling is needed to support MPLS-TP and
   suffers from a bin packing problem even if LSP traffic is completely
   predictable, which it never is in practice.

   The common means of setting composite link bandwidth parameters uses
   long term statistical measures.  For example, many providers base
   their LSP bandwidth parameters on the 95th percentile of carried
   traffic as measured over a one week period.  It is common to add
   10-30% to the 95th percentile value measured over the prior week and
   adjust bandwidth parameters of LSP weekly.  It is also possible to
   measure traffic flow at the LSR and adjust bandwidth parameters
   somewhat more dynamically.  This is less common in deployments and
   where deployed, make use of filtering to track very long term trends
   in traffic levels.  In either case, short term variation of traffic
   levels relative to signaled LSP capacity are common.  Allowing a
   large overallocation of LSP bandwidth parameters (ie: adding 30% or
   more) avoids overutilization of any given LSP, but increases unused
   network capacity and increases network cost.  Allowing a small
   overallocation of LSP bandwidth parameters (ie: 10-20% or less)
   results in both underutilization and overutilization but
   statistically results in a total utilization within the core that is
   under capacity most or all of the time.

   The classic multipath solution accomodates the situation in which
   some composite LSP are underutilizing their signaled capacity and
   others are overutilizing their capacity with the need for far less
   unused network capacity to accomodate variation in actual traffic
   levels.  If the actual traffic levels of LSP can be described by a
   probability distribution, the variation of the sum of LSP is less

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   than the variation of any given LSP for all but a constant traffic
   level (where the variation of the sum and the components are both

   There are two situations which can motivate the use of this approach.
   This design is favored if the provider values MPLS-TP OAM across the
   core more than efficiency (or is unaware of the efficiency issue).
   This design can also make sense if transport equipment or very low
   cost core LSR are available which support only classic link bundling
   and regardless of loss of multiplexing gain, are more cost effective
   at carrying transit traffic than using equipment which supports IP
   source and destination address hashing.

6.3.  MPLS-TP as a MPLS client

   Accomodating MPLS-TP as a MPLS client requires a small change to
   forwarding behavior and is therefore most applicable to major network
   overbuilds or new deployments.  The change to forwarding is an
   ability to limit the depth of MPLS labels used in hashing on the
   label stack on a per LSP basis.  Some existing hardware, particularly
   microprogrammed hardware, may be able to accomodate this forwarding
   change.  Providing support in new hardware is not difficult, a much
   smaller change than, for example, changes required to disable PHP in
   an environment where LSP hierarchy is used.

   The advantage of this approach is an ability to accommodate MPLS-TP
   as a client LSP but retain the high multiplexing gain and therefore
   efficency and low network cost of a pure MPLS deployment.  The
   disadvantage is the need for a small change in forwarding.

7.  Security Considerations

   This document is a use cases document.  Existing protocols are
   referenced such as MPLS.  Existing techniques such as MPLS link
   bundling and multipath techniques are referenced.  These protocols
   and techniques are documented elsewhere and contain security
   considerations which are unchanged by this document.

   This document also describes use cases for Composite Link, which is a
   work-in-progress.  Composite Link requirements are defined in
   [I-D.ietf-rtgwg-cl-requirement].  [I-D.so-yong-rtgwg-cl-framework]
   defines a framework for Composite Link.  Composite Link bears many
   similarities to MPLS link bundling and multipath techniques used with
   MPLS.  Aditional security considerations, if any, beyond those
   already identified for MPLS, MPLS link bundling and multipath
   techniques, will be documented in the framework document if specific
   to the overall framework of Composite Link, or in protocol extensions

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   if specific to a given protocol extension defined later to support
   Composite Link.

8.  Acknowledgments

   Authors would like to thank [ no one so far ] for their reviews and
   great suggestions.

   In the interest of full disclosure of affiliation and in the interest
   of acknowledging sponsorship, past affiliations of authors are noted.
   Much of the work done by Ning So occurred while Ning was at Verizon.
   Much of the work done by Curtis Villamizar occurred while at
   Infinera.  Infinera continues to sponsor this work on a consulting

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.

9.2.  Informative References

              Villamizar, C., McDysan, D., Ning, S., Malis, A., and L.
              Yong, "Requirements for MPLS Over a Composite Link",
              draft-ietf-rtgwg-cl-requirement-04 (work in progress),
              March 2011.

              So, N., Malis, A., McDysan, D., Yong, L., Villamizar, C.,
              and T. Li, "Composite Link Framework in Multi Protocol
              Label Switching (MPLS)",
              draft-so-yong-rtgwg-cl-framework-04 (work in progress),
              June 2011.

              IEEE Standards Association, "IEEE Std 802.1AX-2008 IEEE
              Standard for Local and Metropolitan Area Networks - Link
              Aggregation", 2006, <http://standards.ieee.org/getieee802/

              ITU-T, "Spectral grids for WDM applications: CWDM
              wavelength grid", 2003,

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              ITU-T, "Unified functional architecture of transport
              networks", 2007,

              ITU-T, "Internet protocol data communication service - IP
              packet transfer and availability performance parameters",
              2007, <http://www.itu.int/rec/T-REC-Y.1540/en>.

              ITU-T, "Network performance objectives for IP-based
              services", 2006, <http://www.itu.int/rec/T-REC-Y.1541/en>.

   [RFC1717]  Sklower, K., Lloyd, B., McGregor, G., and D. Carr, "The
              PPP Multilink Protocol (MP)", RFC 1717, November 1994.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC2615]  Malis, A. and W. Simpson, "PPP over SONET/SDH", RFC 2615,
              June 1999.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991, November 2000.

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, November 2000.

   [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.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, April 2002.

   [RFC3809]  Nagarajan, A., "Generic Requirements for Provider
              Provisioned Virtual Private Networks (PPVPN)", RFC 3809,
              June 2004.

   [RFC3945]  Mannie, E., "Generalized Multi-Protocol Label Switching
              (GMPLS) Architecture", RFC 3945, October 2004.

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   [RFC4201]  Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
              in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, June 2007.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

   [RFC6391]  Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan,
              J., and S. Amante, "Flow-Aware Transport of Pseudowires
              over an MPLS Packet Switched Network", RFC 6391,
              November 2011.

Appendix A.  More Details on Existing Network Operator Practices and
             Protocol Usage

   Often, network operators have a contractual Service Level Agreement
   (SLA) with customers for services that are comprised of numerical
   values for performance measures, principally availability, latency,
   delay variation.  Additionally, network operators may have Service
   Level Sepcification (SLS) that is for internal use by the operator.
   See [ITU-T.Y.1540], [ITU-T.Y.1541], RFC3809, Section 4.9 [RFC3809]
   for examples of the form of such SLA and SLS specifications.  In this
   document we use the term Network Performance Objective (NPO) as
   defined in section 5 of [ITU-T.Y.1541] since the SLA and SLS measures
   have network operator and service specific implications.  Note that
   the numerical NPO values of Y.1540 and Y.1541 span multiple networks
   and may be looser than network operator SLA or SLS objectives.
   Applications and acceptable user experience have an important
   relationship to these performance parameters.

   Consider latency as an example.  In some cases, minimizing latency
   relates directly to the best customer experience (e.g., in TCP closer
   is faster).  In other cases, user experience is relatively
   insensitive to latency, up to a specific limit at which point user

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   perception of quality degrades significantly (e.g., interactive human
   voice and multimedia conferencing).  A number of NPOs have. a bound
   on point-point latency, and as long as this bound is met, the NPO is
   met -- decreasing the latency is not necessary.  In some NPOs, if the
   specified latency is not met, the user considers the service as
   unavailable.  An unprotected LSP can be manually provisioned on a set
   of to meet this type of NPO, but this lowers availability since an
   alternate route that meets the latency NPO cannot be determined.

   Historically, when an IP/MPLS network was operated over a lower layer
   circuit switched network (e.g., SONET rings), a change in latency
   caused by the lower layer network (e.g., due to a maintenance action
   or failure) this was not known to the MPLS network.  This resulted in
   latency affecting end user experience, sometimes violating NPOs or
   resulting in user complaints.

   A response to this problem was to provision IP/MPLS networks over
   unprotected circuits and set the metric and/or TE-metric proportional
   to latency.  This resulted in traffic being directed over the least
   latency path, even if this was not needed to meet an NPO or meet user
   experience objectives.  This results in reduced flexibility and
   increased cost for network operators.  Using lower layer networks to
   provide restoration and grooming is expected to be more efficient,
   but the inability to communicate performance parameters, in
   particular latency, from the lower layer network to the higher layer
   network is an important problem to be solved before this can be done.

   Latency NPOs for point-to-point services are often tied closely to
   geographic locations, while latency for multipoint services may be
   based upon a worst case within a region.

   Section 7 of [ITU-T.Y.1540] defines availability for an IP service in
   terms of loss exceeding a threshold for a period on the order of 5
   minutes.  However, the timeframes for restoration (i.e., as
   implemented by pre-determined protection, convergence of routing
   protocols and/or signaling) for services range from on the order of
   100 ms or less (e.g., for VPWS to emulate classical SDH/SONET
   protection switching), to several minutes (e.g., to allow BGP to
   reconverge for L3VPN) and may differ among the set of customers
   within a single service.

   The presence of only three Traffic Class (TC) bits (previously known
   as EXP bits) in the MPLS shim header is limiting when a network
   operator needs to support QoS classes for multiple services (e.g.,
   L2VPN VPWS, VPLS, L3VPN and Internet), each of which has a set of QoS
   classes that need to be supported.  In some cases one bit is used to
   indicate conformance to some ingress traffic classification, leaving
   only two bits for indicating the service QoS classes.  The approach

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   that has been taken is to aggregate these QoS classes into similar
   sets on LER-LSR and LSR-LSR links.

   Labeled LSPs and use of link layer encapsulation have been
   standardized in order to provide a means to meet these needs.

   The IP DSCP cannot be used for flow identification since RFC 4301
   Section 5.5 [RFC4301] requires Diffserv transparency, and in general
   network operators do not rely on the DSCP of Internet packets.  In
   addition, the use of IP DSCP for flow identification is incompatible
   with Assured Forwarding services [RFC2597] or any other service which
   may use more than one DSCP code point to carry traffic for a given

   A label is pushed onto Internet packets when they are carried along
   with L2/L3VPN packets on the same link or lower layer network
   provides a mean to distinguish between the QoS class for these

   Operating an MPLS-TE network involves a different paradigm from
   operating an IGP metric-based LDP signaled MPLS network.  The
   multipoint-to-point LDP signaled MPLS LSPs occur automatically, and
   balancing across parallel links occurs if the IGP metrics are set
   "equally" (with equality a locally definable relation).

   Traffic is typically comprised of a few large (some very large) flows
   and many small flows.  In some cases, separate LSPs are established
   for very large flows.  This can occur even if the IP header
   information is inspected by a LSR, for example an IPsec tunnel that
   carries a large amount of traffic.  An important example of large
   flows is that of a L2/L3 VPN customer who has an access line
   bandwdith comparable to a client-client composite link bandwidth --
   there could be flows that are on the order of the access line

Appendix B.  Existing Multipath Standards and Techniques

   Today the requirement to handle large aggregations of traffic, much
   larger than a single component link, 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
   even BGP, and equal cost LSP, as described in Appendix B.4.  Various
   mutilpath techniques have strengths and weaknesses.

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   the term Composite Link is more general than terms such as Link
   Aggregation which is generally considered to be specific to Ethernet
   and its use here is consistent with the broad definition in
   [ITU-T.G.800].  The term multipath excludes inverse multiplexing and
   refers to techniques which only solve the problem of large
   aggregations of traffic, without addressing the other requirements
   outlined in this document, particularly those described in Section 4
   and Section 5.

B.1.  Common Multpath Load Spliting Techniques

   Identical load balancing techniqes are used for multipath both over
   parallel links and over diverse paths.

   Large aggregates of IP traffic do not provide explicit signaling to
   indicate the expected traffic loads.  Large aggregates of MPLS
   traffic are carried in MPLS tunnels supported by MPLS LSP.  LSP which
   are signaled using RSVP-TE extensions do provide explicit signaling
   which includes the expected traffic load for the aggregate.  LSP
   which are signaled using LDP do not provide an expected traffic load.

   MPLS LSP may contain other MPLS LSP arranged hierarchically.  When an
   MPLS LSR serves as a midpoint LSR in an LSP carrying other LSP as
   payload, there is no signaling associated with these inner LSP.
   Therefore even when using RSVP-TE signaling there may be insufficient
   information provided by signaling to adequately distribute load based
   solely on signaling.

   Generally a set of label stack entries that is unique across the
   ordered set of label numbers in the label stack can safely be assumed
   to contain a group of flows.  The reordering of traffic can therefore
   be considered to be acceptable unless reordering occurs within
   traffic containing a common unique set of label stack entries.
   Existing load splitting techniques take advantage of this property in
   addition to looking beyond the bottom of the label stack and
   determining if the payload is IPv4 or IPv6 to load balance traffic

   MPLS-TP OAM violates the assumption that it is safe to reorder
   traffic within an LSP.  If MPLS-TP OAM is to be accommodated, then
   existing multipth techniques must be modified.  Such modifications
   are outside the scope of this document.

   For example,a large aggregate of IP traffic may be subdivided into a
   large number of groups of flows using a hash on the IP source and
   destination addresses.  This is as described in [RFC2475] and
   clarified in [RFC3260].  For MPLS traffic carrying IP, a similar hash
   can be performed on the set of labels in the label stack.  These

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   techniques are both examples of means to subdivide traffic into
   groups of flows for the purpose of load balancing traffic across
   aggregated link capacity.  The means of identifying a set of flows
   should not be confused with the definition of a flow.

   Discussion of whether a hash based approach provides a sufficiently
   even load balance using any particular hashing algorithm or method of
   distributing traffic across a set of component links is outside of
   the scope of this document.

   The current load balancing techniques are referenced in [RFC4385] and
   [RFC4928].  The use of three hash based approaches are described in
   [RFC2991] and [RFC2992].  A mechanism to identify flows within PW is
   described in [RFC6391].  The use of hash based approaches is
   mentioned as an example of an existing set of techniques to
   distribute traffic over a set of component links.  Other techniques
   are not precluded.

B.2.  Simple and Adaptive Load Balancing Multipath

   Simple multipath generally relies on the mathematical probability
   that given a very large number of small microflows, these microflows
   will tend to be distributed evenly across a hash space.  Early very
   simple multipath implementations assumed that all component links are
   of equal capacity and perform a modulo operation across the hashed
   value.  An alternate simple multipath technique uses a table
   generally with a power of two size, and distributes the table entries
   proportionally among component links according to the capacity of
   each component link.

   Simple load balancing works well if there are a very large number of
   small microflows (i.e., microflow rate is much less than component
   link capacity).  However, the case where there are even a few large
   microflows is not handled well by simple load balancing.

   An adaptive load balancing multipath technique is one where the
   traffic bound to each component link is measured and the load split
   is adjusted accordingly.  As long as the adjustment is done within a
   single network element, then no protocol extensions are required and
   there are no interoperability issues.

   Note that if the load balancing algorithm and/or its parameters is
   adjusted, then packets in some flows may be briefly delivered out of
   sequence, however in practice such adjustments can be made very

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B.3.  Traffic Split over Parallel Links

   The load spliting techniques defined in Appendix B.1 and Appendix B.2
   are both used in splitting traffic over parallel links between the
   same pair of nodes.  The best known technique, though far from being
   the first, is Ethernet Link Aggregation [IEEE-802.1AX].  This same
   technique had been applied much earlier using OSPF or ISIS Equal Cost
   MultiPath (ECMP) over parallel links between the same nodes.
   Multilink PPP [RFC1717] uses a technique that provides inverse
   multiplexing, however a number of vendors had provided proprietary
   extensions to PPP over SONET/SDH [RFC2615] that predated Ethernet
   Link Aggregation but are no longer used.

   Link bundling [RFC4201] provides yet another means of handling
   parallel LSP.  RFC4201 explicitly allow a special value of all ones
   to indicate a split across all members of the bundle.  This "all
   ones" component link is signaled in the MPLS RESV to indicate that
   the link bundle is making use of classic multipath techniques.

B.4.  Traffic Split over Multiple Paths

   OSPF or ISIS Equal Cost MultiPath (ECMP) is a well known form of
   traffic split over multiple paths that may traverse intermediate
   nodes.  ECMP is often incorrectly equated to only this case, and
   multipath over multiple diverse paths is often incorrectly equated to

   Many implementations are able to create more than one LSP between a
   pair of nodes, where these LSP are routed diversely to better make
   use of available capacity.  The load on these LSP can be distributed
   proportionally to the reserved bandwidth of the LSP.  These multiple
   LSP may be advertised as a single PSC FA and any LSP making use of
   the FA may be split over these multiple LSP.

   Link bundling [RFC4201] component links may themselves be LSP.  When
   this technique is used, any LSP which specifies the link bundle may
   be split across the multiple paths of the LSP that comprise the

Appendix C.  Characteristics of Transport in Core Networks

   The characteristics of primary interest are the capacity of a single
   circuit and the use of wave division multiplexing (WDM) to provide a
   large number of parallel circuits.

   Wave division multiplexing (WDM) supports multiple independent
   channels (independent ignoring crosstalk noise) at slightly different

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   wavelengths of light, multiplexed onto a single fiber.  Typical in
   the early 2000s was 40 wavelengths of 10 Gb/s capacity per
   wavelength.  These wavelengths are in the C-band range, which is
   about 1530-1565 nm, though some work has been done using the L-band
   1565-1625 nm.

   The C-band has been carved up using a 100 GHz spacing from 191.7 THz
   to 196.1 THz by [ITU-T.G.694.2].  This yields 44 channels.  If the
   outermost channels are not used, due to poorer transmission
   characteristics, then typcially 40 are used.  For practical reasons,
   a 50 GhZ or 25 GHz spacing is used by more recent equipment,
   yielding. 80 or 160 channels in practice.

   The early optical modulation techniques used within a single channel
   yielded 2.5Gb/s and 10 Gb/s capacity per channel.  As modulation
   techniques have improved 40 Gb/s and 100 Gb/s per channel have been

   The 40 channels of 10 Gb/s common in the mid 2000s yields a total of
   400 Gb/s.  Tighter spacing and better modulations are yielding up to
   8 Tb/s or more in more recent systems.

   Over the optical is an electrical encoding.  In the 1990s this was
   typically Synchronous Optical Networking (SONET) or Synchronous
   Digital Hierarchy (SDH), with a maximum defined circuit capacity of
   40 Gb/s (OC-768), though the 10 Gb/s OC-192 is more common.  More
   recently the low level electrical encoding has been Optical Transport
   Network (OTN) defined by ITU-T.  OTN currently defines circuit
   capacities up to a nominal 100 Gb/s (ODU4).  Both SONET/SDH and OTN
   make use of time division multiplexing (TDM) where the a higher
   capacity circuit such as a 100 Gb/s ODU4 in OTN may be subdivided
   into lower fixed capacity circuits such as ten 10 Gb/s ODU2.

   In the 1990s, all IP and later IP/MPLS networks either used a
   fraction of maximum circuit capacity, or at most the full circuit
   capacity toward the end of the decade, when full circuit capacity was
   2.5 Gb/s or 10 Gb/s.  Beyond 2000, the TDM circuit multiplexing
   capability of SONET/SDH or OTN was rarely used.

   Early in the 2000s both transport equipment and core LSR offerred 40
   Gb/s SONET OC-768.  However 10 Gb/s transport equipment was
   predominantly deployed throughout the decade, partially because LSR
   10GbE ports were far more cost effective than either OC-192 or OC-768
   and became practical in the second half of the decade.

   Entering the 2010 decade, LSR 40GbE and 100GbE are expected to become
   widely available and cost effective.  Slightly preceeding this
   transport equipment making use of 40 Gb/s and 100 Gb/s modulations

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   are becoming available.  This transport equipment is capable or
   carrying 40 Gb/s ODU3 and 100 Gb/s ODU4 circuits.

   Early in the 2000s decade IP/MPLS core networks were making use of
   single 10 Gb/s circuits.  Capacity grew quickly in the first half of
   the decade but more IP/MPLS core networks had only a small number of
   IP/MPLS links requiring 4-8 parallel 10 Gb/s circuits.  However, the
   use of multipath was necessary, was deemed the simplest and most cost
   effective alternative, and became thoroughly entrenched.  By the end
   of the 2000s decade nearly all major IP/MPLS core service provider
   networks and a few content provider networks had IP/MPLS links which
   exceeded 100 Gb/s, long before 40GbE was available and 40 Gb/s
   transport in widespread use.

   It is less clear when IP/MPLS LSP exceeded 10 Gb/s, 40 Gb/s, and 100
   Gb/s.  By 2010, many service providers have LSP in excess of 100
   Gb/s, but few are willing to disclose how many LSP have reached this

   At the time of writing 40GbE and 100GbE LSR products are being
   evaluated by service providers and contect providers and are in use
   in network trials.  The cost of components required to deliver 100
   GbE products remains high making these products less cost effective.
   This is expected to change within years.

   The important point is that IP/MPLS core network links have long ago
   exceeded 100 Gb/s and a small number of IP/MPLS LSP exceed 100 Gb/s.
   By the time 100 Gb/s circuits are widely deployed, IP/MPLS core
   network links are likely to exceed 1 Tb/s and many IP/MPLS LSP
   capacities are likely to exceed 100 Gb/s.  Therefore multipath
   techniques are likely here to stay.

Authors' Addresses

   So Ning
   Tata Communications

   Email: ning.so@tatacommunications.com

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   Andrew Malis
   117 West St.
   Waltham, MA  02451

   Phone: +1 781-466-2362
   Email: andrew.g.malis@verizon.com

   Dave McDysan
   22001 Loudoun County PKWY
   Ashburn, VA  20147

   Email: dave.mcdysan@verizon.com

   Lucy Yong
   Huawei USA
   5340 Legacy Dr.
   Plano, TX  75025

   Phone: +1 469-277-5837
   Email: lucy.yong@huawei.com

   Curtis Villamizar
   Outer Cape Cod Network Consulting

   Email: curtis@occnc.com

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