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Versions: (draft-so-yong-rtgwg-cl-framework) 00 01 02 03 04

RTGWG                                                            S. Ning
Internet-Draft                                       Tata Communications
Intended status: Informational                                D. McDysan
Expires: January 16, 2014                                        Verizon
                                                              E. Osborne
                                                                   Cisco
                                                                 L. Yong
                                                              Huawei USA
                                                           C. Villamizar
                                                  Outer Cape Cod Network
                                                              Consulting
                                                           July 15, 2013


                  Advanced Multipath Framework in MPLS
                    draft-ietf-rtgwg-cl-framework-04

Abstract

   This document specifies a framework for support of Advanced Multipath
   in MPLS networks.  As defined in this framework, an Advanced
   Multipath consists of a group of homogenous or non-homogenous links
   that have the same forward adjacency (FA) and can be considered as a
   single TE link or an IP link when advertised into IGP routing.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on January 16, 2014.

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



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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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Architecture Summary . . . . . . . . . . . . . . . . . . .  4
     1.3.  Conventions used in this document  . . . . . . . . . . . .  5
     1.4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.5.  Document Issues  . . . . . . . . . . . . . . . . . . . . .  5
   2.  Advanced Multipath Key Characteristics . . . . . . . . . . . .  7
     2.1.  Flow Identification  . . . . . . . . . . . . . . . . . . .  7
       2.1.1.  Flow Identification Granularity  . . . . . . . . . . .  8
       2.1.2.  Flow Identification Summary  . . . . . . . . . . . . .  9
       2.1.3.  Flow Identification Using Entropy Label  . . . . . . .  9
     2.2.  Advanced Multipath in Control Plane  . . . . . . . . . . . 10
     2.3.  Advanced Multipath in Data Plane . . . . . . . . . . . . . 13
   3.  Architecture Tradeoffs . . . . . . . . . . . . . . . . . . . . 14
     3.1.  Scalability Motivations  . . . . . . . . . . . . . . . . . 14
     3.2.  Reducing Routing Information and Exchange  . . . . . . . . 15
     3.3.  Reducing Signaling Load  . . . . . . . . . . . . . . . . . 15
       3.3.1.  Reducing Signaling Load using LDP MPTP . . . . . . . . 16
       3.3.2.  Reducing Signaling Load using Hierarchy  . . . . . . . 16
       3.3.3.  Using Both LDP MPTP and RSVP-TE Hierarchy  . . . . . . 17
     3.4.  Reducing Forwarding State  . . . . . . . . . . . . . . . . 17
     3.5.  Avoiding Route Oscillation . . . . . . . . . . . . . . . . 17
   4.  New Challenges . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.1.  Control Plane Challenges . . . . . . . . . . . . . . . . . 19
       4.1.1.  Delay and Jitter Sensitive Routing . . . . . . . . . . 19
       4.1.2.  Local Control of Traffic Distribution  . . . . . . . . 20
       4.1.3.  Path Symmetry Requirements . . . . . . . . . . . . . . 20
       4.1.4.  Requirements for Contained LSP . . . . . . . . . . . . 21
       4.1.5.  Retaining Backwards Compatibility  . . . . . . . . . . 21
     4.2.  Data Plane Challenges  . . . . . . . . . . . . . . . . . . 22
       4.2.1.  Very Large LSP . . . . . . . . . . . . . . . . . . . . 22
       4.2.2.  Very Large Microflows  . . . . . . . . . . . . . . . . 23
       4.2.3.  Traffic Ordering Constraints . . . . . . . . . . . . . 23
       4.2.4.  Accounting for IP and LDP Traffic  . . . . . . . . . . 23
       4.2.5.  IP and LDP Limitations . . . . . . . . . . . . . . . . 24
   5.  Existing Mechanisms  . . . . . . . . . . . . . . . . . . . . . 25



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     5.1.  Link Bundling  . . . . . . . . . . . . . . . . . . . . . . 25
     5.2.  Classic Multipath  . . . . . . . . . . . . . . . . . . . . 26
   6.  Mechanisms Proposed in Other Documents . . . . . . . . . . . . 27
     6.1.  Loss and Delay Measurement . . . . . . . . . . . . . . . . 27
     6.2.  Link Bundle Extensions . . . . . . . . . . . . . . . . . . 28
     6.3.  Pseudowire Flow and MPLS Entropy Labels  . . . . . . . . . 28
     6.4.  Multipath Extensions . . . . . . . . . . . . . . . . . . . 29
   7.  Required Protocol Extensions and Mechanisms  . . . . . . . . . 29
     7.1.  Brief Review of Requirements . . . . . . . . . . . . . . . 29
     7.2.  Proposed Document Coverage . . . . . . . . . . . . . . . . 30
       7.2.1.  Component Link Grouping  . . . . . . . . . . . . . . . 31
       7.2.2.  Delay and Jitter Extensions  . . . . . . . . . . . . . 31
       7.2.3.  Path Selection and Admission Control . . . . . . . . . 32
       7.2.4.  Dynamic Multipath Balance  . . . . . . . . . . . . . . 32
       7.2.5.  Frequency of Load Balance  . . . . . . . . . . . . . . 33
       7.2.6.  Inter-Layer Communication  . . . . . . . . . . . . . . 33
       7.2.7.  Packet Ordering Requirements . . . . . . . . . . . . . 33
       7.2.8.  Minimally Disruption Load Balance  . . . . . . . . . . 34
       7.2.9.  Path Symmetry  . . . . . . . . . . . . . . . . . . . . 34
       7.2.10. Performance, Scalability, and Stability  . . . . . . . 35
       7.2.11. IP and LDP Traffic . . . . . . . . . . . . . . . . . . 35
       7.2.12. LDP Extensions . . . . . . . . . . . . . . . . . . . . 35
       7.2.13. Pseudowire Extensions  . . . . . . . . . . . . . . . . 36
       7.2.14. Multi-Domain Advanced Multipath  . . . . . . . . . . . 36
     7.3.  Framework Requirement Coverage by Protocol . . . . . . . . 36
       7.3.1.  OSPF-TE and ISIS-TE Protocol Extensions  . . . . . . . 37
       7.3.2.  PW Protocol Extensions . . . . . . . . . . . . . . . . 37
       7.3.3.  LDP Protocol Extensions  . . . . . . . . . . . . . . . 37
       7.3.4.  RSVP-TE Protocol Extensions  . . . . . . . . . . . . . 37
       7.3.5.  RSVP-TE Path Selection Changes . . . . . . . . . . . . 37
       7.3.6.  RSVP-TE Admission Control and Preemption . . . . . . . 37
       7.3.7.  Flow Identification and Traffic Balance  . . . . . . . 37
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 38
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 39
     11.2. Informative References . . . . . . . . . . . . . . . . . . 39
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42












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1.  Introduction

   Advanced Multipath functional requirements are specified in
   [I-D.ietf-rtgwg-cl-requirement].  Advanced Multipath use cases are
   described in [I-D.ietf-rtgwg-cl-use-cases].  This document specifies
   a framework to meet these requirements.

   This document describes an Advanced Multipath framework in the
   context of MPLS networks using an IGP-TE and RSVP-TE MPLS control
   plane with GMPLS extensions [RFC3209] [RFC3630] [RFC3945] [RFC5305].

   Specific protocol solutions are outside the scope of this document,
   however a framework for the extension of existing protocols is
   provided.  Backwards compatibility is best achieved by extending
   existing protocols where practical rather than inventing new
   protocols.  The focus is on examining where existing protocol
   mechanisms fall short with respect to [I-D.ietf-rtgwg-cl-requirement]
   and on the types of extensions that will be required to accommodate
   functionality that is called for in [I-D.ietf-rtgwg-cl-requirement].

1.1.  Background

   Classic multipath, including Ethernet Link Aggregation has been
   widely used in today's MPLS networks [RFC4385][RFC4928].  Classic
   multipath using non-Ethernet links are often advertised using MPLS
   Link bundling.  A link bundle [RFC4201] bundles a group of
   homogeneous links as a TE link to make IGP-TE information exchange
   and RSVP-TE signaling more scalable.  An Advanced Multipath allows
   bundling non-homogenous links together as a single logical link.

   An Advanced Multipath is a single logical link in MPLS network that
   contains multiple parallel component links between two MPLS LSR.
   Unlike a link bundle [RFC4201], the component links in an Advanced
   Multipath can have different properties such as cost, capacity,
   delay, or jitter.

1.2.  Architecture Summary

   Networks aggregate information, both in the control plane and in the
   data plane, as a means to achieve scalability.  A tradeoff exists
   between the needs of scalability and the needs to identify differing
   path and link characteristics and differing requirements among flows
   contained within further aggregated traffic flows.  These tradeoffs
   are discussed in detail in Section 3.

   Some aspects of Advanced Multipath requirements present challenges
   for which multiple solutions may exist.  In Section 4 various
   challenges and potential approaches are discussed.



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   A subset of the functionality called for in
   [I-D.ietf-rtgwg-cl-requirement] is available through MPLS Link
   Bundling [RFC4201].  Link bundling and other existing standards
   applicable to Advanced Multipath are covered in Section 5.

   The most straightforward means of supporting Advanced Multipath
   requirements is to extend MPLS protocols and protocol semantics and
   in particular to extend link bundling.  Extensions which have already
   been proposed in other documents which are applicable to Advanced
   Multipath are discussed in Section 6.

   A goal of most new protocol work within IETF is to reuse existing
   protocol encapsulations and mechanisms where they meet requirements
   and extend existing mechanisms.  This approach minimizes additional
   complexity while meeting requirements and tends to preserve backwards
   compatibility to the extent it is practical to do so.  These goals
   are considered in proposing a framework for further protocol
   extensions and mechanisms in Section 7.

1.3.  Conventions used in this document

   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].

1.4.  Terminology

   Terminology defined in [I-D.ietf-rtgwg-cl-requirement] is used in
   this document.  The additional terms defined in
   [I-D.ietf-rtgwg-cl-use-cases] are also used.

   The abbreviation IGP-TE is used as a shorthand indicating either
   OSPF-TE [RFC3630] or ISIS-TE [RFC5305].

1.5.  Document Issues

   This subsection exists solely for the purpose of focusing the RTGWG
   meeting and mailing list discussions on areas within this document
   that need attention in order for the document to achieve the level of
   quality necessary to advance the document through the IETF process.
   This subsection will be removed before work group last call.

   The following issues need to be resolved.

   1.  The feasibility of symmetric paths for all flows is questionable.
       The only case where this is practical is where LSP are smaller
       than component links and where classic link bundling (not using
       the all-ones component) is used.  Perhaps the emphasis on this



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       (mis)feature should be reduced in the requirements document.  See
       Section 4.1.3.

   2.  There is a tradeoff between supporting delay optimized routing
       and avoiding oscillation.  This may be sufficiently covered, but
       a careful review by others and comments would be beneficial.

   3.  Any measurement of jitter (delay variation) that is used in route
       decision is likely to cause oscillation.  Trying to optimize a
       path to reduce jitter may be a fools errand.  How do we say this
       in the draft or does the existing text cover it adequately?

   4.  RTGWG needs to consider the possibility of using multi-topology
       IGP extensions in IP and LDP routing where the topologies reflect
       differing requirements (see Section 4.2.5).  This idea is similar
       to TOS routing, which has been discussed for decades but has
       never been deployed.  One possible outcome of discussion would be
       to declare TOS routing out of scope in the requirements document.

   5.  The following referenced drafts have expired:

       A.  [I-D.ospf-cc-stlv]

       B.  [I-D.villamizar-mpls-multipath-extn]

       A replacement for [I-D.ospf-cc-stlv] is expected to be submitted.
       [I-D.villamizar-mpls-multipath-extn] is expected to emerge in a
       simplified form, removing extensions for which existing
       workarounds are considered adequate based on feedback at a prior
       IETF.

   6.  Clarification of what we intend to do with Multi-Domain Advanced
       Multipath is needed in Section 7.2.14.

   7.  The following topics in the requirements document are not
       addressed.  Since they are explicitly mentioned in the
       requirements document some mention of how they are supported is
       needed in this document.

       A.  Migration (incremental deployment) may not be adequately
           covered in Section 4.1.5.  It might also be necessary to say
           more here on performance, scalability, and stability as it
           related to migration.  Comments on this from co-authors or
           the WG?

       B.  We may need a performance section in this document to
           specifically address #DR6 (fast convergence), and #DR7 (fast
           worst case failure convergence).  We do already have



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           scalability discussion and make a recommendation for a
           separate document.  At the very least the performance section
           would have to say "no worse than before, except were there
           was no alternative to make it very slightly worse" (in a bit
           more detail than that).  It might also be helpful to better
           define the nature of the performance criteria implied by #DR6
           and #DR7.

   The above list has been in this document for the better part of a
   year with very little discussion (or none) of the above issues on the
   RTGWG mailing list.


2.  Advanced Multipath Key Characteristics

   [I-D.ietf-rtgwg-cl-requirement] defines external behavior of Advanced
   Multipath.  The overall framework approach involves extending
   existing protocols in a backwards compatible manner and reusing
   ongoing work elsewhere in IETF where applicable, defining new
   protocols or semantics only where necessary.  Given the requirements,
   and this approach of extending MPLS, Advanced Multipath key
   characteristics can be described in greater detail than given
   requirements alone.

2.1.  Flow Identification

   Traffic mapping to component links is a data plane operation.
   Control over how the mapping is done may be directly dictated or
   constrained by the control plane or by the management plane.  When
   unconstrained by the control plane or management plane, distribution
   of traffic is entirely a local matter.  Regardless of constraints or
   lack or constraints, the traffic distribution is required to keep
   packets belonging to individual flows in sequence and meet QoS
   criteria specified per LSP by either signaling or management
   [RFC2475] [RFC3260].

   Key objectives of the traffic distribution are to not overload any
   component link, and to be be able to perform local recovery when a
   subset of component links fails.

   The network operator may have other objectives such as placing a
   bidirectional flow or LSP on the same component link in both
   direction, bounding delay and/or jitter, Advanced Multipath energy
   saving, and etc.  These new requirements are described in
   [I-D.ietf-rtgwg-cl-requirement].

   Examples of means to identify a flow may in principle include:




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   1.  an LSP identified by an MPLS label,

   2.  a pseudowire (PW) [RFC3985] identified by an MPLS PW label,

   3.  a flow or group of flows within a pseudowire (PW) [RFC6391]
       identified by an MPLS flow label,

   4.  a flow or flow group in an LSP [RFC6790] identified by an MPLS
       entropy label,

   5.  all traffic between a pair of IP hosts, identified by an IP
       source and destination pair,

   6.  a specific connection between a pair of IP hosts, identified by
       an IP source and destination pair, protocol, and protocol port
       pair,

   7.  a layer-2 conversation within a pseudowire (PW), where the
       identification is PW payload type specific, such as Ethernet MAC
       addresses and VLAN tags within an Ethernet PW [RFC4448].  This is
       feasible but not practical (see below).

   Although in principle a layer-2 conversation within a pseudowire
   (PW), may be identified by PW payload type specific information, in
   practice this is impractical at LSP midpoints when PW are carried.
   The PW ingress may provide equivalent information in a PW flow label
   [RFC6391].  Therefore, in practice, item #8 above is covered by
   [RFC6391] and may be dropped from the list.

2.1.1.  Flow Identification Granularity

   An LSR must at least be capable of identifying flows based on MPLS
   labels.  Most MPLS LSP do not require that traffic carried by the LSP
   are carried in order.  MPLS-TP is a recent exception.  If it is
   assumed that no LSP require strict packet ordering of the LSP itself
   (only of flows within the LSP), then the entire label stack can be
   used as flow identification.  If some LSP may require strict packet
   ordering but those LSP cannot be distinguished from others, then only
   the top label can be used as a flow identifier.  If only the top
   label is used (for example, as specified by [RFC4201] when the "all-
   ones" component described in [RFC4201] is not used), then there may
   not be adequate flow granularity to accomplish well balanced traffic
   distribution and it will not be possible to carry LSP that are larger
   than any individual component link.

   The number of flows can be extremely large.  This may be the case
   when the entire label stack is used and is always the case when IP
   addresses are used in provider networks carrying Internet traffic.



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   Current practice for native IP load balancing at the time of writing
   were documented in [RFC2991] and [RFC2992].  These practices as
   described, make use of IP addresses.

   The common practices described in [RFC2991] and [RFC2992] were
   extended to include the MPLS label stack and the common practice of
   looking at IP addresses within the MPLS payload.  These extended
   practices require that pseudowires use a PWE3 Control Word and are
   described in [RFC4385] and [RFC4928].  Additional detail on current
   multipath practices can be found in the appendices of
   [I-D.ietf-rtgwg-cl-use-cases].

   Using only the top label supports too coarse a traffic balance.
   Prior to MPLS Entropy Label [RFC6790] using the full label stack was
   also too coarse.  Using the full label stack and IP addresses as flow
   identification provides a sufficiently fine traffic balance, but is
   capable of identifying such a high number of distinct flows, that a
   technique of grouping flows, such as hashing on the flow
   identification criteria, becomes essential to reduce the stored
   state, and is an essential scaling technique.  Other means of
   grouping flows may be possible.

2.1.2.  Flow Identification Summary

   In summary:

   1.  Load balancing using only the MPLS label stack provides too
       coarse a granularity of load balance.

   2.  Tracking every flow is not scalable due to the extremely large
       number of flows in provider networks.

   3.  Existing techniques, IP source and destination hash in
       particular, have proven in over two decades of experience to be
       an excellent way of identifying groups of flows.

   4.  If a better way to identify groups of flows is discovered, then
       that method can be used.

   5.  IP address hashing is not required, but use of this technique is
       strongly encouraged given the technique's long history of
       successful deployment.

2.1.3.  Flow Identification Using Entropy Label

   MPLS Entropy Label [RFC6790] provides a means of making use of the
   entropy from information that would require deeper packet inspection,
   such as inspection of IP addresses, and putting that entropy in the



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   form of a hashed value into the label stack.  Midpoint LSR that
   understand the Entropy Label Indicator can make use of only label
   stack information but still obtain a fine load balance granularity.

2.2.  Advanced Multipath in Control Plane

   An Advanced Multipath is advertised as a single logical interface
   between two connected routers, which forms forwarding adjacency (FA)
   between the routers.  The FA is advertised as a TE-link in a link
   state IGP, using either OSPF-TE or ISIS-TE.  The IGP-TE advertised
   interface parameters for the Advanced Multipath can be preconfigured
   by the network operator or be derived from its component links.
   Advanced Multipath advertisement requirements are specified in
   [I-D.ietf-rtgwg-cl-requirement].

   In IGP-TE, an Advanced Multipath is advertised as a single TE link
   between two connected routers.  This is similar to a link bundle
   [RFC4201].  Link bundle applies to a set of homogenous component
   links.  Advanced Multipath allows homogenous and non-homogenous
   component links.  Due to the similarity, and for backwards
   compatibility, extending link bundling is viewed as both simple and
   as the best approach.

   In order for a route computation engine to calculate a proper path
   for a LSP, it is necessary for Advanced Multipath to advertise the
   summarized available bandwidth as well as the maximum bandwidth that
   can be made available for single flow (or single LSP where no finer
   flow identification is available).  If an Advanced Multipath contains
   some non-homogeneous component links, the Advanced Multipath also
   should advertise the summarized bandwidth and the maximum bandwidth
   for single flow per each homogeneous component link group.

   Both LDP [RFC5036] and RSVP-TE [RFC3209] can be used to signal a LSP
   over an Advanced Multipath.  LDP cannot be extended to support
   traffic engineering capabilities [RFC3468].

   When an LSP is signaled using RSVP-TE, the LSP MUST be placed on the
   component link that meets the LSP criteria indicated in the signaling
   message.

   When an LSP is signaled using LDP, the LSP MUST be placed on the
   component link that meets the LSP criteria, if such a component link
   is available.  LDP does not support traffic engineering capabilities,
   imposing restrictions on LDP use of Advanced Multipath.  See
   Section 4.2.5 for further details.

   If the Advanced Multipath solution is based on extensions to IGP-TE
   and RSVP-TE, then in order to meet requirements defined in



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   [I-D.ietf-rtgwg-cl-requirement], the following derived requirements
   MUST be met.

   1.  An Advanced Multipath MAY contain non-homogeneous component
       links.  The route computing engine MAY select one group of
       component links for a LSP.  The The route computing engine MUST
       accommodate service objectives for a given LSP when selecting a
       group of component links for a LSP.

   2.  The routing protocol MUST make a grouping of component links
       available in the TE-LSDB, such that within each group all of the
       component links have similar characteristics (the component links
       are homogeneous within a group).

   3.  The route computation used in RSVP-TE MUST be extended to include
       only the capacity of groups within an Advanced Multipath which
       meet LSP criteria.

   4.  The signaling protocol MUST be able to indicate either the
       criteria, or which groups may be used.

   5.  An Advanced Multipath MUST place each LSP on a component link or
       group which meets or exceeds the LSP criteria.

   Advanced Multipath capacity is aggregated capacity.  LSP capacity MAY
   be larger than individual component link capacity.  Any aggregated
   LSP can determine a bounds on the largest microflow that could be
   carried and this constraint can be handled as follows.

   1.  If no information is available through signaling, management
       plane, or configuration, the largest microflow is bound by one of
       the following:

       A.  the largest single LSP if most traffic is RSVP-TE signaled
           and further aggregated,

       B.  the largest pseudowire if most traffic is carrying pseudowire
           payloads that are aggregated within RSVP-TE LSP,

       C.  or the largest interface or component lisk capacity carrying
           IP or LDP if a large amount of IP or LDP traffic is contained
           within the aggregate.

       If a very large amount of traffic being aggregated is IP or LDP,
       then the largest microflow is bound by the largest component link
       on which IP traffic can arrive.  For example, if an LSR is acting
       as an LER and IP and LDP traffic is arriving on 10 Gb/s edge
       interfaces, then no microflow larger than 10 Gb/s will be present



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       on the RSVP-TE LSP that aggregate traffic across the core, even
       if the core interfaces are 100 Gb/s interfaces.

   2.  The prior conditions provide a bound on the largest microflow
       when no signaling extensions indicate a bounds.  If an LSP is
       aggregating smaller LSP for which the largest expected microflow
       carried by the smaller LSP is signaled, then the largest
       microflow expected in the containing LSP (the aggregate) is the
       maximum of the largest expected microflow for any contained LSP.
       For example, RSVP-TE LSP may be large but aggregate traffic for
       which the source or sink are all 1 Gb/s or smaller interfaces
       (such as in mobile applications in which cell sites backhauls are
       no larger than 1 Gb/s).  If this information is carried in the
       LSP originated at the cell sites, then further aggregates across
       a core may make use of this information.

   3.  The IGP must provide the bounds on the largest microflow that an
       Advanced Multipath can accommodate, which is the maximum capacity
       on a component link that can be made available by moving other
       traffic.  This information is needed by the ingress LER for path
       determination.

   4.  A means to signal an LSP whose capacity is larger than individual
       component link capacity is needed [I-D.ietf-rtgwg-cl-requirement]
       and also signal the largest microflow expected to be contained in
       the LSP.  If a bounds on the largest microflow is not signaled
       there is no means to determine if an LSP which is larger than any
       component link can be subdivided into flows and therefore should
       be accepted by admission control.

   When a bidirectional LSP request is signaled over an Advanced
   Multipath, if the request indicates that the LSP must be placed on
   the same component link, the routers of the Advanced Multipath MUST
   place the LSP traffic in both directions on a same component link.
   This is particularly challenging for aggregated capacity which makes
   use of the label stack for traffic distribution.  The two
   requirements are mutually exclusive for any one LSP.  No one LSP may
   be both larger than any individual component link and require
   symmetrical paths for every flow.  Both requirements can be
   accommodated by the same Advanced Multipath for different LSP, with
   any one LSP requiring no more than one of these two features.

   Individual component link may fail independently.  Upon component
   link failure, an Advanced Multipath MUST support a minimally
   disruptive local repair, preempting any LSP which can no longer be
   supported.  Available capacity in other component links MUST be used
   to carry impacted traffic.  The available bandwidth after failure
   MUST be advertised immediately to avoid looped crankback.



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   When an Advanced Multipath is not able to transport all flows, it
   preempts some flows based upon holding priority and informs the
   control plane of these preempted flows.  To minimize impact on
   traffic, the Advanced Multipath MUST support soft preemption
   [RFC5712].  The network operator SHOULD enable soft preemption.  This
   action ensures the remaining traffic is transported properly.  FR#10
   requires that the traffic be restored.  FR#12 requires that any
   change be minimally disruptive.  These two requirements are
   interpreted to include preemption among the types of changes that
   must be minimally disruptive.

2.3.  Advanced Multipath in Data Plane

   The data plane must identify groups of flows.  Flow identification is
   covered in Section 2.1.  Having identified groups of flows the groups
   must be placed on individual component links.  This step following
   flow group identification is called traffic distribution or traffic
   placement.  The two steps together are known as traffic balancing or
   load balancing.

   Traffic distribution may be determined by or constrained by control
   plane or management plane.  Traffic distribution may be changed due
   to component link status change, subject to constraints imposed by
   either the management plane or control plane.  The distribution
   function is local to the routers in which an Advanced Multipath
   belongs to and its implementation is not specified here.

   When performing traffic placement, an Advanced Multipath does not
   differentiate multicast traffic vs. unicast traffic.

   In order to maintain scalability, existing data plane forwarding
   retains state associated with the top label only.  Using UHP (UHP is
   the absence of the more common PHP), zero of more labels may be POPed
   and packet and byte counters incremented prior to processing what
   becomes the top label after the POP operations are completed.  Flow
   group identification may be a parallel step in the forwarding
   process.  Data plane forwarding makes use of the top label to select
   an Advanced Multipath, or a group of components within an Advanced
   Multipath or for the case where an LSP is pinned (see [RFC4201]), a
   specific component link.  For those LSP for which the LSP selects
   only the Advanced Multipath or a group of components within an
   Advanced Multipath, the load balancing makes use of the set of
   component links selected based on the top label, and makes use of the
   flow group identification to select among that group.

   The simplest traffic placement techniques uses a modulo operation
   after computing a hash.  This techniques has significant
   disadvantages.  The most common traffic placement techniques uses the



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   a flow group identification as an index into a table.  The table
   provides an indirection.  The number of bits of hash is constrained
   to keep table size small.  While this is not the best technique, it
   is the most common.  Better techniques exist but they are outside the
   scope of this document and some are considered proprietary.

   Requirements to limit frequency of load balancing can be adhered to
   by keeping track of when a flow group was last moved and imposing a
   minimum period before that flow group can be moved again.  This is
   straightforward for a table approach.  For other approaches it may be
   less straightforward.


3.  Architecture Tradeoffs

   Scalability and stability are critical considerations in protocol
   design where protocols may be used in a large network such as today's
   service provider networks.  Advanced Multipath is applicable to
   networks which are large enough to require that traffic be split over
   multiple paths.  Scalability is a major consideration for networks
   that reach a capacity large enough to require Advanced Multipath.

   Some of the requirements of Advanced Multipath could potentially have
   a negative impact on scalability.  This section is about
   architectural tradeoffs, many motivated by the need to maintain
   scalability and stability, a need which is reflected in
   [I-D.ietf-rtgwg-cl-requirement], specifically in DR#6 and DR#7.

3.1.  Scalability Motivations

   In the interest of scalability, information is aggregated in
   situations where information about a large amount of network capacity
   or a large amount of network demand provides is adequate to meet
   requirements.  Routing information is aggregated to reduce the amount
   of information exchange related to routing and to simplify route
   computation (see Section 3.2).

   In an MPLS network large routing changes can occur when a single
   fault occurs.  For example, a single fault may impact a very large
   number of LSP traversing a given link.  As new LSP are signaled to
   avoid the fault, resources are consumed elsewhere, and routing
   protocol announcements must flood the resource changes.  If
   protection is in place, there is less urgency to converging quickly.
   If multiple faults occur that are not covered by shared risk groups
   (SRG), then some protection may fail, adding urgency to converging
   quickly even where protection is deployed.

   Reducing the amount of information allows the exchange of information



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   during a large routing change to be accomplished more quickly and
   simplifies route computation.  Simplifying route computation improves
   convergence time after very significant network faults which cannot
   be handled by preprovisioned or precomputed protection mechanisms.
   Aggregating smaller LSP into larger LSP is a means to reduce path
   computation load and reduce RSVP-TE signaling (see Section 3.3).

   Neglecting scaling issues can result in performance issues, such as
   slow convergence.  Neglecting scaling in some cases can result in
   networks which perform so poorly as to become unstable.

3.2.  Reducing Routing Information and Exchange

   Link bundling provides a means of aggregating control plane
   information.  Even where the all-ones component link supported by
   link bundling is not used, the amount of control information is
   reduced by the number of component links in a bundle.

   Fully deaggregating link bundle information would negate this
   benefit.  If there is a need to deaggregate, such as to distinguish
   between groups of links within specified ranges of delay, then no
   more deaggregation than is necessary should be done.

   For example, in supporting the requirement for heterogeneous
   component links, it makes little sense to fully deaggregate link
   bundles when adding support for groups of component links with common
   attributes within a link bundle can maintain most of the benefit of
   aggregation while adequately supporting the requirement to support
   heterogeneous component links.

   Routing information exchange is also reduced by making sensible
   choices regarding the amount of change to link parameters that
   require link readvertisement.  For example, if delay measurements
   include queuing delay, then a much more coarse granularity of delay
   measurement would be called for than if the delay does not include
   queuing and is dominated by geographic delay (speed of light delay).

3.3.  Reducing Signaling Load

   Aggregating traffic into very large hierarchical LSP in the core very
   substantially reduces the number of LSP that need to be signaled and
   the number of path computations any given LSR will be required to
   perform when a network fault occurs.

   In the extreme, applying MPLS to a very large network without
   hierarchy could exceed the 20 bit label space.  For example, in a
   network with 4,000 nodes, with 2,000 on either side of a cutset,
   would have 4,000,000 LSP crossing the cutset.  Even in a degree four



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   cutset, an uneven distribution of LSP across the cutset, or the loss
   of one link would result in a need to exceed the size of the label
   space.  Among provider networks, 4,000 access nodes is not at all
   large.  Hierarchy is an absolute requirement if all access nodes were
   interconnected in such a network.

   In less extreme cases, having each node terminate hundreds of LSP to
   achieve a full mesh creates a very large computational load.
   Computational complexity is a function of the number of nodes (N) and
   links (L) in a topology, and the number of LSP that need to be set
   up.  In the common case where L is proportional to N (relatively
   constant node degree with growth), the time complexity of one CSPF
   computation is order(N log N).  If each node must perform order(N)
   computations when a fault occurs, then the computational load
   increases as order(N^2 log N) as the number of nodes increases (where
   "^" is the power of operator and "N^2" is read "N-squared").  In
   practice at the time of writing, this imposes a limit of a few
   hundred nodes in a full mesh of MPLS LSP before the computational
   load is sufficient to result in unacceptable convergence times.

   Two solutions are applied to reduce the amount of RSVP-TE signaling.
   Both involve subdividing the MPLS domain into a core and a set of
   regions.

3.3.1.  Reducing Signaling Load using LDP MPTP

   LDP can be used for edge-to-edge LSP, using RSVP-TE to carry the LDP
   intra-core traffic and also optionally also using RSVP-TE to carry
   the LDP intra-region traffic within each region.  LDP does not
   support traffic engineering, but does support multipoint-to-point
   (MPTP) LSP, which require less signaling than edge-to-edge RSVP-TE
   point-to-point (PTP) LSP.  A drawback of this approach is the
   inability to use RSVP-TE protection (FRR or GMPLS protection) against
   failure of the border LSR sitting at a core/region boundary.

3.3.2.  Reducing Signaling Load using Hierarchy

   When the number of nodes grows too large, the amount of RSVP-TE
   signaling can be reduced using the MPLS PSC hierarchy [RFC4206].  A
   core within the hierarchy can divide the topology into M regions of
   on average N/M nodes.  Within a region the computational load is
   reduced by more than M^2.  Within the core, the computational load
   generally becomes quite small since M is usually a fairly small
   number (a few tens of regions) and each region is generally attached
   to the core in typically only two or three places on average.

   Using hierarchy improves scaling but has two consequences.  First,
   hierarchy effectively forces the use of platform label space.  When a



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   containing LSP is rerouted, the labels assigned to the contained LSP
   cannot be changed but may arrive on a different interface.  Second,
   hierarchy results in much larger LSP.  These LSP today are larger
   than any single component link and therefore force the use of the
   all-ones component in link bundles.

3.3.3.  Using Both LDP MPTP and RSVP-TE Hierarchy

   It is also possible to use both LDP and RSVP-TE hierarchy.  MPLS
   networks with a very large number of nodes may benefit from the use
   of both LDP and RSVP-TE hierarchy.  The two techniques are certainly
   not mutually exclusive.

3.4.  Reducing Forwarding State

   Both LDP and MPLS hierarchy have the benefit of reducing the amount
   of forwarding state.  Using the example from Section 3.3, and using
   MPLS hierarchy, the worst case generally occurs at borders with the
   core.

   For example, consider a network with approximately 1,000 nodes
   divided into 10 regions.  At the edges, each node requires 1,000 LSP
   to other edge nodes.  The edge nodes also require 100 intra-region
   LSP.  Within the core, if the core has only 3 attachments to each
   region the core LSR have less than 100 intra-core LSP.  At the border
   cutset between the core and a given region, in this example there are
   100 edge nodes with inter-region LSP crossing that cutset, destined
   to 900 other edge nodes.  That yields forwarding state for on the
   order of 90,000 LSP at the border cutset.  These same routers need
   only reroute well under 200 LSP when a multiple fault occurs, as long
   as only links are affected and a border LSR does not go down.

   Interior to the core, the forwarding state is greatly reduced.  If
   inter-region LSP have different characteristics, it makes sense to
   make use of aggregates with different characteristics.  Rather than
   exchange information about every inter-region LSP within the intra-
   core LSP it makes more sense to use multiple intra-core LSP between
   pairs of core nodes, each aggregating sets of inter-region LSP with
   common characteristics or common requirements.

3.5.  Avoiding Route Oscillation

   Networks can become unstable when a feedback loop exists such that
   moving traffic to a link causes a metric such as delay to increase,
   which then causes traffic to move elsewhere.  For example, the
   original ARPANET routing used a delay based cost metric and proved
   prone to route oscillations [DBP].




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   Delay may be used as a constraint in routing for high priority
   traffic, when this high priority traffic makes a minor contribution
   to total load, such that the movement of the high priority traffic
   has a small impact on the delay experienced by other high priority
   traffic.  The safest way to measure delay is to make measurements
   based on traffic which is prioritized such that it is queued ahead of
   the lower priority traffic which will be affected if high priority
   traffic is moved.  The amount of high priority traffic must be
   constrained to consume a fraction of link capacities with the
   remaining capacity available to lower priority traffic.

   Any measurement of jitter (delay variation) that is used in route
   decision is likely to cause oscillation.  Jitter that is caused by
   queuing effects and cannot be measured using a very high priority
   measurement traffic flow.

   It may be possible to find links with constrained queuing delay or
   jitter using a theoretical maximum or a probability based bound on
   queuing delay or jitter at a given priority based on the types and
   amounts of traffic accepted and combining that theoretical limit with
   a measured delay at very high priority.  Using delay or jitter as
   path metrics without creating oscillations is challenging.

   Instability can occur due to poor performance and interaction with
   protocol timers.  In this way a computational scaling problem can
   become a stability problem when a network becomes sufficiently large.


4.  New Challenges

   New technical challenges are posed by [I-D.ietf-rtgwg-cl-requirement]
   in both the control plane and data plane.

   Among the more difficult challenges are the following.

   1.  The requirements related to delay or jitter conflict with
       requirements for scalability and stability (see Section 4.1.1),

   2.  The combination of ingress control over LSP placement and
       retaining an ability to move traffic as demands dictate can pose
       challenges and such requirements can even be conflicting (see
       Section 4.1.2),

   3.  Path symmetry requires extensions and is particularly challenging
       for very large LSP (see Section 4.1.3),

   4.  Accommodating a very wide range of requirements among contained
       LSP can lead to inefficiency if the most stringent requirements



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       are reflected in aggregates, or reduce scalability if a large
       number of aggregates are used to provide a too fine a reflection
       of the requirements in the contained LSP (see Section 4.1.4),

   5.  Backwards compatibility is somewhat limited due to the need to
       accommodate legacy multipath interfaces which provide too little
       information regarding their configured default behavior, and
       legacy LSP which provide too little information regarding their
       LSP requirements (see Section 4.1.5),

   6.  Data plane challenges include those of accommodating very large
       LSP, large microflows, traffic ordering constraints imposed by a
       subset of LSP, and accounting for IP and LDP traffic (see
       Section 4.2).

4.1.  Control Plane Challenges

   Some of the control plane requirements are particularly challenging.
   Handling large flows which aggregate smaller flows must be
   accomplished with minimal impact on scalability.  Potentially
   conflicting are requirements for jitter and requirements for
   stability.  Potentially conflicting are the requirements for ingress
   control of a large number of parameters, and the requirements for
   local control needed to achieve traffic balance across an Advanced
   Multipath.  These challenges and potential solutions are discussed in
   the following sections.

4.1.1.  Delay and Jitter Sensitive Routing

   Delay and jitter sensitive routing are called for in
   [I-D.ietf-rtgwg-cl-requirement] in requirements FR#2, FR#7, FR#8,
   FR#9, FR#15, FR#16, FR#17, FR#18.  Requirement FR#17 is particularly
   problematic, calling for constraints on jitter.

   A tradeoff exists between scaling benefits of aggregating
   information, and potential benefits of using a finer granularity in
   delay reporting.  To maintain the scaling benefit, measured link
   delay for any given Advanced Multipath SHOULD be aggregated into a
   small number of delay ranges.  IGP-TE extensions MUST be provided
   which advertise the available capacities for each of the selected
   ranges.

   For path selection of delay sensitive LSP, the ingress SHOULD bias
   link metrics based on available capacity and select a low cost path
   which meets LSP total path delay criteria.  To communicate the
   requirements of an LSP, the ERO MUST be extended to indicate the per
   link constraints.  To communicate the type of resource used, the RRO
   SHOULD be extended to carry an identification of the group that is



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   used to carry the LSP at each link bundle hop.

4.1.2.  Local Control of Traffic Distribution

   Many requirements in [I-D.ietf-rtgwg-cl-requirement] suggest that a
   node immediately adjacent to a component link should have a high
   degree of control over how traffic is distributed, as long as network
   performance objectives are met.  Particularly relevant are FR#18 and
   FR#19.

   The requirements to allow local control are potentially in conflict
   with requirement FR#21 which gives full control of component link
   select to the LSP ingress.  While supporting this capability is
   mandatory, use of this feature is optional per LSP.

   A given network deployment will have to consider this set of
   conflicting requirements and make appropriate use of local control of
   traffic placement and ingress control of traffic placement to best
   meet network requirements.

4.1.3.  Path Symmetry Requirements

   Requirement FR#21 in [I-D.ietf-rtgwg-cl-requirement] includes a
   provision to bind both directions of a bidirectional LSP to the same
   component.  This is easily achieved if the LSP is directly signaled
   across an Advanced Multipath.  This is not as easily achieved if a
   set of LSP with this requirement are signaled over a large
   hierarchical LSP which is in turn carried over an Advanced Multipath.
   The basis for load distribution in such as case is the label stack.
   The labels in either direction are completely independent.

   This could be accommodated if the ingress, egress, and all midpoints
   of the hierarchical LSP make use of an entropy label in the
   distribution, and the ingress use a fixed value per contained LSP in
   the entropy label.  A solution for this problem may add complexity
   with very little benefit.  There is little or no true benefit of
   using symmetrical paths rather than component links of identical
   characteristics.

   Traffic symmetry and large LSP capacity are a second pair of
   conflicting requirements.  Any given LSP can meet one of these two
   requirements but not both.  A given network deployment will have to
   make appropriate use of each of these features to best meet network
   requirements.







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4.1.4.  Requirements for Contained LSP

   [I-D.ietf-rtgwg-cl-requirement] calls for new LSP constraints.  These
   constraints include frequency of load balancing rearrangement, delay
   and jitter, packet ordering constraints, and path symmetry.

   When LSP are contained within hierarchical LSP, there is no signaling
   available at midpoint LSR which identifies the contained LSP let
   alone providing the set of requirements unique to each contained LSP.
   Defining extensions to provide this information would severely impact
   scalability and defeat the purpose of aggregating control information
   and forwarding information into hierarchical LSP.  For the same
   scalability reasons, not aggregating at all is not a viable option
   for large networks where scalability and stability problems may occur
   as a result.

   As pointed out in Section 4.1.3, the benefits of supporting symmetric
   paths among LSP contained within hierarchical LSP may not be
   sufficient to justify the complexity of supporting this capability.

   A scalable solution which accommodates multiple sets of LSP between
   given pairs of LSR is to provide multiple hierarchical LSP for each
   given pair of LSR, each hierarchical LSP aggregating LSP with common
   requirements and a common pair of endpoints.  This is a network
   design technique available to the network operator rather than a
   protocol extension.  This technique can accommodate multiple sets of
   delay and jitter parameters, multiple sets of frequency of load
   balancing parameters, multiple sets of packet ordering constraints,
   etc.

4.1.5.  Retaining Backwards Compatibility

   Backwards compatibility and support for incremental deployment
   requires considering the impact of legacy LSR in the role of LSP
   ingress, and considering the impact of legacy LSR advertising
   ordinary links, advertising Ethernet LAG as ordinary links, and
   advertising link bundles.

   Legacy LSR in the role of LSP ingress cannot signal requirements
   which are not supported by their control plane software.  The
   additional capabilities supported by other LSR has no impact on these
   LSR.  These LSR however, being unaware of extensions, may try to make
   use of scarce resources which support specific requirements such as
   low delay.  To a limited extent it may be possible for a network
   operator to avoid this issue using existing mechanisms such as link
   administrative attributes and attribute affinities [RFC3209].

   Legacy LSR advertising ordinary links will not advertise attributes



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   needed by some LSP.  For example, there is no way to determine the
   delay or jitter characteristics of such a link.  Legacy LSR
   advertising Ethernet LAG pose additional problems.  There is no way
   to determine that packet ordering constraints would be violated for
   LSP with strict packet ordering constraints, or that frequency of
   load balancing rearrangement constraints might be violated.

   Legacy LSR advertising link bundles have no way to advertise the
   configured default behavior of the link bundle.  Some link bundles
   may be configured to place each LSP on a single component link and
   therefore may not be able to accommodate an LSP which requires
   bandwidth in excess of the size of a component link.  Some link
   bundles may be configured to spread all LSP over the all-ones
   component.  For LSR using the all-ones component link, there is no
   documented procedure for correctly setting the "Maximum LSP
   Bandwidth".  There is currently no way to indicate the largest
   microflow that could be supported by a link bundle using the all-ones
   component link.

   Having received the RRO, it is possible for an ingress to look for
   the all-ones component to identify such link bundles after having
   signaled at least one LSP.  Whether any LSR collects this information
   on legacy LSR and makes use of it to set defaults, is an
   implementation choice.

4.2.  Data Plane Challenges

   Flow identification is briefly discussed in Section 2.1.  Traffic
   distribution is briefly discussed in Section 2.3.  This section
   discusses issues specific to particular requirements specified in
   [I-D.ietf-rtgwg-cl-requirement].

4.2.1.  Very Large LSP

   Very large LSP may exceed the capacity of any single component of an
   Advanced Multipath.  In some cases contained LSP may exceed the
   capacity of any single component.  These LSP may make use of the
   equivalent of the all-ones component of a link bundle, or may use a
   subset of components which meet the LSP requirements.

   Very large LSP can be accommodated as long as they can be subdivided
   (see Section 4.2.2).  A very large LSP cannot have a requirement for
   symmetric paths unless complex protocol extensions are proposed (see
   Section 2.2 and Section 4.1.3).







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4.2.2.  Very Large Microflows

   Within a very large LSP there may be very large microflows.  A very
   large microflow is one which cannot be further subdivided and
   contributes a very large amount of capacity.  Flows which cannot be
   subdivided must be no larger that the capacity of any single
   component link.

   Current signaling provides no way to specify the largest microflow
   that a can be supported on a given link bundle in routing
   advertisements.  Extensions which address this are discussed in
   Section 6.4.  Absent extensions of this type, traffic containing
   microflows that are too large for a given Advanced Multipath may be
   present.  There is no data plane solution for this problem that would
   not require reordering traffic at the Advanced Multipath egress.

   Some techniques are susceptible to statistical collisions where an
   algorithm to distribute traffic is unable to disambiguate traffic
   among two or more very large microflow where their sum is in excess
   of the capacity of any single component.  Hash based algorithms which
   use too small a hash space are particularly susceptible and require a
   change in hash seed in the event that this were to occur.  A change
   in hash seed is highly disruptive, causing traffic reordering among
   all traffic flows over which the hash function is applied.

4.2.3.  Traffic Ordering Constraints

   Some LSP have strict traffic ordering constraints.  Most notable
   among these are MPLS-TP LSP.  In the absence of aggregation into
   hierarchical LSP, those LSP with strict traffic ordering constraints
   can be placed on individual component links if there is a means of
   identifying which LSP have such a constraint.  If LSP with strict
   traffic ordering constraints are aggregated in hierarchical LSP, the
   hierarchical LSP capacity may exceed the capacity of any single
   component link.  In such a case the load balancing may be constrained
   through the use of an entropy label [RFC6790].  This and related
   issues are discussed further in Section 6.4.

4.2.4.  Accounting for IP and LDP Traffic

   Networks which carry RSVP-TE signaled MPLS traffic generally carry
   low volumes of native IP traffic, often only carrying control traffic
   as native IP.  There is no architectural guarantee of this, it is
   just how network operators have made use of the protocols.

   [I-D.ietf-rtgwg-cl-requirement] requires that native IP and native
   LDP be accommodated (DR#2 and DR#3).  In some networks, a subset of
   services may be carried as native IP or carried as native LDP.  Today



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   this may be accommodated by the network operator estimating the
   contribution of IP and LDP and configuring a lower set of available
   bandwidth figures on the RSVP-TE advertisements.

   The only improvement that Advanced Multipath can offer is that of
   measuring the IP and LDP traffic levels and automatically reducing
   the available bandwidth figures on the RSVP-TE advertisements.  The
   measurements would have to be filtered.  This is similar to a feature
   in existing LSR, commonly known as "autobandwidth" with a key
   difference.  In the "autobandwidth" feature, the bandwidth request of
   an RSVP-TE signaled LSP is adjusted in response to traffic
   measurements.  In this case the IP or LDP traffic measurements are
   used to reduce the link bandwidth directly, without first
   encapsulating in an RSVP-TE LSP.

   This may be a subtle and perhaps even a meaningless distinction if
   Advanced Multipath is used to form a Sub-Path Maintenance Element
   (SPME).  A SPME is in practice essentially an unsignaled single hop
   LSP with PHP enabled [RFC5921].  An Advanced Multipath SPME looks
   very much like classic multipath, where there is no signaling, only
   management plane configuration creating the multipath entity (of
   which Ethernet Link Aggregation is a subset).

4.2.5.  IP and LDP Limitations

   IP does not offer traffic engineering.  LDP cannot be extended to
   offer traffic engineering [RFC3468].  Therefore there is no traffic
   engineered fallback to an alternate path for IP and LDP traffic if
   resources are not adequate for the IP and/or LDP traffic alone on a
   given link in the primary path.  The only option for IP and LDP would
   be to declare the link down.  Declaring a link down due to resource
   exhaustion would reduce traffic to zero and eliminate the resource
   exhaustion.  This would cause oscillations and is therefore not a
   viable solution.

   Congestion caused by IP or LDP traffic loads is a pathologic case
   that can occur if IP and/or LDP are carried natively and there is a
   high volume of IP or LDP traffic.  This situation can be avoided by
   carrying IP and LDP within RSVP-TE LSP.

   It is also not possible to route LDP traffic differently for
   different FEC.  LDP traffic engineering is specifically disallowed by
   [RFC3468].  It may be possible to support multi-topology IGP
   extensions to accommodate more than one set of criteria.  If so, the
   additional IGP could be bound to the forwarding criteria, and the LDP
   FEC bound to a specific IGP instance, inheriting the forwarding
   criteria.  Alternately, one IGP instance can be used and the LDP SPF
   can make use of the constraints, such as delay and jitter, for a



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   given LDP FEC.


5.  Existing Mechanisms

   In MPLS the one mechanism which supports explicit signaling of
   multiple parallel links is Link Bundling [RFC4201].  The set of
   techniques known as "classis multipath" support no explicit
   signaling, except in two cases.  In Ethernet Link Aggregation the
   Link Aggregation Control Protocol (LACP) coordinates the addition or
   removal of members from an Ethernet Link Aggregation Group (LAG).
   The use of the "all-ones" component of a link bundle indicates use of
   classis multipath, however the ability to determine if a link bundle
   makes use of classis multipath is not yet supported.

5.1.  Link Bundling

   Link bundling supports advertisement of a set of homogenous links as
   a single route advertisement.  Link bundling supports placement of an
   LSP on any single component link, or supports placement of an LSP on
   the all-ones component link.  Not all link bundling implementations
   support the all-ones component link.  There is no way for an ingress
   LSR to tell which potential midpoint LSR support this feature and use
   it by default and which do not.  Based on [RFC4201] it is unclear how
   to advertise a link bundle for which the all-ones component link is
   available and used by default.  Common practice is to violate the
   specification and set the Maximum LSP Bandwidth to the Available
   Bandwidth.  There is no means to determine the largest microflow that
   could be supported by a link bundle that is using the all-ones
   component link.

   [RFC6107] extends the procedures for hierarchical LSP but also
   extends link bundles.  An LSP can be explicitly signaled to indicate
   that it is an LSP to be used as a component of a link bundle.  Prior
   to that the common practice was to simply not advertise the component
   link LSP into the IGP, since only the ingress and egress of the link
   bundle needed to be aware of their existence, which they would be
   aware of due to the RSVP-TE signaling used in setting up the
   component LSP.

   While link bundling can be the basis for Advanced Multipath, a
   significant number of small extension needs to be added.

   1.  To support link bundles of heterogeneous links, a means of
       advertising the capacity available within a group of homogeneous
       links needs to be provided.





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   2.  Attributes need to be defined to support the following parameters
       for the link bundle or for a group of homogeneous links.

       A.  delay range

       B.  jitter (delay variation) range

       C.  group metric

       D.  all-ones component capable

       E.  capable of dynamically balancing load

       F.  largest supportable microflow

       G.  support for entropy label

   3.  For each of the prior extended attributes, the constraint based
       routing path selection needs to be extended to reflect new
       constraints based on the extended attributes.

   4.  For each of the prior extended attributes, LSP admission control
       needs to be extended to reflect new constraints based on the
       extended attributes.

   5.  Dynamic load balance must be provided for flows within a given
       set of links with common attributes such that Performance
       Objectives are not violated including frequency of load balance
       adjustment for any given flow.

5.2.  Classic Multipath

   Classic multipath is described in [I-D.ietf-rtgwg-cl-use-cases].

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

   Classic multipath provides a highly scalable means of load balancing.
   Dynamic multipath has proven value in assuring an even loading on
   component link and an ability to adapt to change in offered load that
   occurs over periods of hundreds of milliseconds or more.  Classic
   multipath scalability is due to the ability to effectively work with
   an extremely large number of flows (IP host pairs) using relatively
   little resources (a data structure accessed using a hash result as a
   key or using ranges of hash results).



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   Classic multipath meets a small subset of Advanced Multipath
   requirements.  Due to scalability of the approach, classic multipath
   seems to be an excellent candidate for extension to meet the full set
   of Advanced Multipath forwarding requirements.

   Additional detail can be found in [I-D.ietf-rtgwg-cl-use-cases].


6.  Mechanisms Proposed in Other Documents

   A number of documents which at the time of writing are works in
   progress address parts of the requirements of Advanced Multipath, or
   assist in making some of the goals achievable.

6.1.  Loss and Delay Measurement

   Procedures for measuring loss and delay are provided in [RFC6374].
   These are OAM based measurements.  This work could be the basis of
   delay measurements and delay variation measurement used for metrics
   called for in [I-D.ietf-rtgwg-cl-requirement].

   Currently there are three documents that address delay and delay
   variation metrics.

   draft-ietf-ospf-te-metric-extensions
       [I-D.ietf-ospf-te-metric-extensions] provides a set of OSPF-TE
       extension to support delay, jitter, and loss.  Stability is not
       adequately addressed and some minor issues remain.

   I-D.previdi-isis-te-metric-extensions
       [I-D.previdi-isis-te-metric-extensions] provides the set of
       extensions for ISIS that [I-D.ietf-ospf-te-metric-extensions]
       provides for OSPF.  This draft mirrors
       [I-D.ietf-ospf-te-metric-extensions] sometimes lagging for a
       brief period when the OSPF version is updated.

   I-D.atlas-mpls-te-express-path
       [I-D.atlas-mpls-te-express-path] provides information on the use
       of OSPF and ISIS extensions defined in
       [I-D.ietf-ospf-te-metric-extensions] and
       [I-D.previdi-isis-te-metric-extensions] and a modified CSPF path
       selection to meet LSP performance criteria such as minimal delay
       paths or bounded delay paths.

   Delay variance, loss, residual bandwidth, and available bandwidth
   extensions are particular prone to network instability.  The question
   as to whether queuing delay and delay variation should be considered,
   and if so for which diffserv Per-Hop Service Class (PSC) is not



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   adequately addressed in the current versions of these drafts.  These
   drafts are actively being discussed and updated and remaining issues
   are expected to be resolved.

6.2.  Link Bundle Extensions

   A set of extension are needed to indicate a group of component links
   in the ERO or RRO, where the group is given an interface
   identification like the bundle itself.  The extensions could also be
   further extended to support specification of the all-ones component
   link in the ERO or RRO.

   [I-D.ospf-cc-stlv] provides a baseline draft for extending link
   bundling to advertise components.  A new component TLV (C-TLV) is
   proposed, which must reference an Advanced Multipath Link TLV.
   [I-D.ospf-cc-stlv] is intended for the OSPF WG and submitted for the
   "Experimental" track.  The 00 version expired in February 2012.  A
   replacement is expected that will be submitted for consideration on
   the standards track.

6.3.  Pseudowire Flow and MPLS Entropy Labels

   Two documents provide a means to add entropy for the purpose of
   improving load balance.  MPLS encapsulation can bury information that
   is needed to identify microflows.  These two documents allow a
   pseudowire ingress and LSP ingress respectively to add a label solely
   for the purpose of providing a finer granularity of microflow groups.

   [RFC6391] allows pseudowires which carry a large volume of traffic,
   where microflows can be identified to be load balanced across
   multiple members of an Ethernet LAG or an MPLS link bundle.  This is
   accomplished by adding a flow label below the pseudowire label in the
   MPLS label stack.  For this to be effective the link bundle load
   balance must make use of the label stack up to and including this
   flow label.

   [RFC6790] provides a means for a LER to put an additional label known
   as an entropy label on the MPLS label stack.  Only the LER can add
   the entropy label.  The LER of a PSC LSP would have to add a entropy
   label for contained LSPs for which it is a midpoint LSR.

   Core LSR acting as LER for aggregated LSP can add entropy labels
   based on deep packet inspection and place an entropy label indicator
   (ELI) and entropy label (EL) just below the label being acted on.
   This would be helpful in situations where the label stack depth to
   which load distribution can operate is limited by implementation or
   is limited for other reasons such as carrying both MPLS-TP and MPLS
   with entropy labels within the same hierarchical LSP.



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6.4.  Multipath Extensions

   The multipath extensions drafts address the issue of accommodating
   LSP which have strict packet ordering constraints in a network
   containing multipath.  MPLS-TP has become the one important instance
   of LSP with strict packet ordering constraints and has driven this
   work.

   [I-D.ietf-mpls-multipath-use] proposed to use MPLS Entropy Label
   [RFC6790] to allow MPLS-TP to be carried within MPLS LSP that make
   use of multipath.  Limitations of this approach in the absence of
   protocol extensions is discussed.

   [I-D.villamizar-mpls-multipath-extn] provides protocol extensions
   needed to overcome the limitations in the absence of protocol
   extensions is discussed in [I-D.ietf-mpls-multipath-use].


7.  Required Protocol Extensions and Mechanisms

   Prior sections have reviewed key characteristics, architecture
   tradeoffs, new challenges, existing mechanisms, and relevant
   mechanisms proposed in existing new documents.

   This section first summarizes and groups requirements specified in
   [I-D.ietf-rtgwg-cl-requirement] (see Section 7.1).  A set of
   documents coverage groupings are proposed with existing works-in-
   progress noted where applicable (see Section 7.2).  The set of
   extensions are then grouped by protocol affected as a convenience to
   implementors (see (see Section 7.3).

7.1.  Brief Review of Requirements

   The following list provides a categorization of requirements
   specified in [I-D.ietf-rtgwg-cl-requirement] along with a short
   phrase indication what topic the requirement covers.

   routing information aggregation
       FR#1 (routing summarization), FR#20 (Advanced Multipath may be a
       component of another Advanced Multipath)

   restoration speed
       FR#2 (restoration speed meeting performance objectives), FR#12
       (minimally disruptive load rebalance), DR#6 (fast convergence),
       DR#7 (fast worst case failure convergence)






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   load distribution, stability, minimal disruption
       FR#3 (automatic load distribution), FR#5 (must not oscillate),
       FR#11 (dynamic placement of flows), FR#12 (minimally disruptive
       load rebalance), FR#13 (bounded rearrangement frequency), FR#18
       (flow placement must satisfy performance objectives), FR#19 (flow
       identification finer than per top level LSP), MR#6 (operator
       initiated flow rebalance)

   backward compatibility and migration
       FR#4 (smooth incremental deployment), FR#6 (management and
       diagnostics must continue to function), DR#1 (extend existing
       protocols), DR#2 (extend LDP, no LDP TE)

   delay and delay variation
       FR#7 (expose lower layer measured delay), FR#8 (precision of
       latency reporting), FR#9 (limit latency on per LSP basis), FR#15
       (minimum delay path), FR#16 (bounded delay path), FR#17 (bounded
       jitter path)

   admission control, preemption, traffic engineering
       FR#10 (admission control, preemption), FR#14 (packet ordering),
       FR#21 (ingress specification of path), FR#22 (path symmetry),
       DR#3 (IP and LDP traffic), MR#3 (management specification of
       path)

   single vs multiple domain
       DR#4 (IGP extensions allowed within single domain), DR#5 (IGP
       extensions disallowed in multiple domain case)

   general network management
       MR#1 (polling, configuration, and notification), MR#2 (activation
       and de-activation)

   path determination, connectivity verification
       MR#4 (path trace), MR#5 (connectivity verification)

   The above list is not intended as a substitute for
   [I-D.ietf-rtgwg-cl-requirement], but rather as a concise grouping and
   reminder or requirements to serve as a means of more easily
   determining requirements coverage of a set of protocol documents.

7.2.  Proposed Document Coverage

   The primary areas where additional protocol extensions and mechanisms
   are required include the topics described in the following
   subsections.

   There are candidate documents for a subset of the topics below.  This



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   grouping of topics does not require that each topic be addressed by a
   separate document.  In some cases, a document may cover multiple
   topics, or a specific topic may be addressed as applicable in
   multiple documents.

7.2.1.  Component Link Grouping

   An extension to link bundling is needed to specify a group of
   components with common attributes.  This can be a TLV defined within
   the link bundle that carries the same encapsulations as the link
   bundle.  Two interface indices would be needed for each group.

   a.  An index is needed that if included in an ERO would indicate the
       need to place the LSP on any one component within the group.

   b.  A second index is needed that if included in an ERO would
       indicate the need to balance flows within the LSP across all
       components of the group.  This is equivalent to the "all-ones"
       component for the entire bundle.

   [I-D.ospf-cc-stlv] can be extended to include multipath treatment
   capabilities.  An ISIS solution is also needed.  An extension of
   RSVP-TE signaling is needed to indicate multipath treatment
   preferences.

   If a component group is allowed to support all of the parameters of a
   link bundle, then a group TE metric would be accommodated.  This can
   be supported with the component TLV (C-TLV) defined in
   [I-D.ospf-cc-stlv].

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "routing information aggregation" set of
   requirements.  The "restoration speed", "backward compatibility and
   migration", and "general network management" requirements must also
   be considered.

7.2.2.  Delay and Jitter Extensions

   A extension is needed in the IGP-TE advertisement to support delay
   and delay variation for links, link bundles, and forwarding
   adjacencies.  Whatever mechanism is described must take precautions
   that insure that route oscillations cannot occur.  The following set
   of drafts address this.

   1.  [I-D.ietf-ospf-te-metric-extensions]

   2.  [I-D.previdi-isis-te-metric-extensions]




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   3.  [I-D.atlas-mpls-te-express-path]

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "delay and delay variation" set of
   requirements.  The "restoration speed", "backward compatibility and
   migration", and "general network management" requirements must also
   be considered.

7.2.3.  Path Selection and Admission Control

   Path selection and admission control changes must be documented in
   each document that proposes a protocol extension that advertises a
   new capability or parameter that must be supported by changes in path
   selection and admission control.

   It would also be helpful to have an informational document which
   covers path selection and admission control issues in detail and
   briefly summarizes and references the set of documents which propose
   extensions.  This document could be advanced in parallel with the
   protocol extensions.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 are the "load distribution, stability, minimal
   disruption" and "admission control, preemption, traffic engineering"
   sets of requirements.  The "restoration speed" and "path
   determination, connectivity verification" requirements must also be
   considered.  The "backward compatibility and migration", and "general
   network management" requirements must also be considered.

7.2.4.  Dynamic Multipath Balance

   FR#11 explicitly calls for dynamic placement of flows.  Load
   balancing similar to existing dynamic multipath would satisfy this
   requirement.  In implementations where flow identification uses a
   coarse granularity, the adjustments would have to be equally coarse,
   in the worst case moving entire LSP.  The impact of flow
   identification granularity and potential dynamic multipath approaches
   may need to be documented in greater detail than provided here.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 are the "restoration speed" and the "load
   distribution, stability, minimal disruption" sets of requirements.
   The "path determination, connectivity verification" requirements must
   also be considered.  The "backward compatibility and migration", and
   "general network management" requirements must also be considered.






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7.2.5.  Frequency of Load Balance

   IGP-TE and RSVP-TE extensions are needed to support frequency of load
   balancing rearrangement called for in FR#13, and FR#15-FR#17.
   Constraints are not defined in RSVP-TE, but could be modeled after
   administrative attribute affinities in RFC3209 and elsewhere.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "load distribution, stability, minimal
   disruption" set of requirements.  The "path determination,
   connectivity verification" must also be considered.  The "backward
   compatibility and migration" and "general network management"
   requirements must also be considered.

7.2.6.  Inter-Layer Communication

   Lower layer to upper layer communication called for in FR#7 and
   FR#20.  Specific parameters, specifically delay and delay variation,
   need to be addressed.  Passing information from a lower non-MPLS
   layer to an MPLS layer needs to be addressed, though this may largely
   be generic advice encouraging a coupling of MPLS to lower layer
   management plane or control plane interfaces.  This topic can be
   addressed in each document proposing a protocol extension, where
   applicable.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "restoration speed" set of requirements.
   The "backward compatibility and migration" and "general network
   management" requirements must also be considered.

7.2.7.  Packet Ordering Requirements

   A document is needed to define extensions supporting various packet
   ordering requirements, ranging from requirements to preserve
   microflow ordering only, to requirements to preserve full LSP
   ordering (as in MPLS-TP).  This is covered by
   [I-D.ietf-mpls-multipath-use] and
   [I-D.villamizar-mpls-multipath-extn].

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 are the "admission control, preemption, traffic
   engineering" and the "path determination, connectivity verification"
   sets of requirements.  The "backward compatibility and migration" and
   "general network management" requirements must also be considered.







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7.2.8.  Minimally Disruption Load Balance

   The behavior of hash methods used in classic multipath needs to be
   described in terms of FR#12 which calls for minimally disruptive load
   adjustments.  For example, reseeding the hash violates FR#12.  Using
   modulo operations is significantly disruptive if a link comes or goes
   down, as pointed out in [RFC2992].  In addition, backwards
   compatibility with older hardware needs to be accommodated.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "load distribution, stability, minimal
   disruption" set of requirements.

7.2.9.  Path Symmetry

   Protocol extensions are needed to support dynamic load balance as
   called for to meet FR#22 (path symmetry) and to meet FR#11 (dynamic
   placement of flows).

   Currently path symmetry can only be supported in link bundling if the
   path is pinned.  When a flow is moved both ingress and egress must
   make the move as close to simultaneously as possible to satisfy FR#22
   and FR#12 (minimally disruptive load rebalance).  There is currently
   no protocol to coordinate this move.

   If a group of flows are identified using a hash, then the hash must
   be identical on the pair of LSR at the endpoint, using the same hash
   seed and with one side swapping source and destination.  If the label
   stack is used, then either the entire label stack must be a special
   case flow identification, since the set of labels in either direction
   are not correlated, or the two LSR must conspire to use the same flow
   identifier.  For example, using a common entropy label value, and
   using only the entropy label in the flow identification would satisfy
   the forwarding requirement.  There is no protocol to indicate special
   treatment of a label stack within a hierarchical LSP.  Adding such a
   extension may add significant complexity and ultimately may prove
   unscalable.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 are the "load distribution, stability, minimal
   disruption" and the "admission control, preemption, traffic
   engineering" sets of requirements.  The "backward compatibility and
   migration" and "general network management" requirements must also be
   considered.  Path symmetry simplifies support for the "path
   determination, connectivity verification" set of requirements, but
   with significant complexity added elsewhere.





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7.2.10.  Performance, Scalability, and Stability

   A separate document providing analysis of performance, scalability,
   and stability impacts of changes may be needed.  The topic of traffic
   adjustment oscillation must also be covered.  If sufficient coverage
   is provided in each document covering a protocol extension, a
   separate document would not be needed.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "restoration speed" set of requirements.
   This is not a simple topic and not a topic that is well served by
   scattering it over multiple documents, therefore it may be best to
   put this in a separate document and put citations in documents called
   for in Section 7.2.1, Section 7.2.2, Section 7.2.3, Section 7.2.9,
   Section 7.2.11, Section 7.2.12, Section 7.2.13, and Section 7.2.14.
   Citation may also be helpful in Section 7.2.4, and Section 7.2.5.

7.2.11.  IP and LDP Traffic

   A document is needed to define the use of measurements of native IP
   and native LDP traffic levels which are then used to reduce link
   advertised bandwidth amounts.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 are the "load distribution, stability, minimal
   disruption" and the "admission control, preemption, traffic
   engineering" set of requirements.  The "path determination,
   connectivity verification" must also be considered.  The "backward
   compatibility and migration" and "general network management"
   requirements must also be considered.

7.2.12.  LDP Extensions

   Extending LDP is called for in DR#2.  LDP can be extended to couple
   FEC admission control to local resource availability without
   providing LDP traffic engineering capability.  Other LDP extensions
   such as signaling a bound on microflow size and LDP LSP requirements
   would provide useful information without providing LDP traffic
   engineering capability.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "admission control, preemption, traffic
   engineering" set of requirements.  The "backward compatibility and
   migration" and "general network management" requirements must also be
   considered.






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7.2.13.  Pseudowire Extensions

   Pseudowire (PW) extensions such as signaling a bound on microflow
   size and signaling requirements specific to PW would provide useful
   information.  This information can be carried in the PW LDP signaling
   [RFC3985] and the the PW requirements could then be used in a
   containing LSP.

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 is the "admission control, preemption, traffic
   engineering" set of requirements.  The "backward compatibility and
   migration" and "general network management" requirements must also be
   considered.

7.2.14.  Multi-Domain Advanced Multipath

   DR#5 calls for Advanced Multipath to span multiple network
   topologies.  Component LSP may already span multiple network
   topologies, though most often in practice these are LDP signaled.
   Component LSP which are RSVP-TE signaled may also span multiple
   network topologies using at least three existing methods (per domain
   [RFC5152], BRPC [RFC5441], PCE [RFC4655]).  When such component links
   are combined in an Advanced Multipath, the Advanced Multipath spans
   multiple network topologies.  It is not clear in which document this
   needs to be described or whether this description in the framework is
   sufficient.  The authors and/or the WG may need to discuss this.
   DR#5 mandates that IGP-TE extension cannot be used.  This would
   disallow the use of [RFC5316] or [RFC5392] in conjunction with
   [RFC5151].

   The primary focus of this document, among the sets of requirements
   listed in Section 7.1 are "single vs multiple domain" and "admission
   control, preemption, traffic engineering".  The "routing information
   aggregation" and "load distribution, stability, minimal disruption"
   requirements need attention due to their use of the IGP in single
   domain Advanced Multipath.  Other requirements such as "delay and
   delay variation", can more easily be accommodated by carrying metrics
   within BGP.  The "path determination, connectivity verification"
   requirements need attention due to requirements to restrict
   disclosure of topology information across domains in multi-domain
   deployments.  The "backward compatibility and migration" and "general
   network management" requirements must also be considered.

7.3.  Framework Requirement Coverage by Protocol

   As an aid to implementors, this section summarizes requirement
   coverage listed in Section 7.2 by protocol or LSR functionality
   affected.



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   Some documentation may be purely informational, proposing no changes
   and proposing usage at most.  This includes Section 7.2.3,
   Section 7.2.8, Section 7.2.10, and Section 7.2.14.

   Section 7.2.9 may require a new protocol.

7.3.1.  OSPF-TE and ISIS-TE Protocol Extensions

   Many of the changes listed in Section 7.2 require IGP-TE changes,
   though most are small extensions to provide additional information.
   This set includes Section 7.2.1, Section 7.2.2, Section 7.2.5,
   Section 7.2.6, and Section 7.2.7.  An adjustment to existing
   advertised parameters is suggested in Section 7.2.11.

7.3.2.  PW Protocol Extensions

   The only suggestion of pseudowire (PW) extensions is in
   Section 7.2.13.

7.3.3.  LDP Protocol Extensions

   Potential LDP extensions are described in Section 7.2.12.

7.3.4.  RSVP-TE Protocol Extensions

   RSVP-TE protocol extensions are called for in Section 7.2.1,
   Section 7.2.5, Section 7.2.7, and Section 7.2.9.

7.3.5.  RSVP-TE Path Selection Changes

   Section 7.2.3 calls for path selection to be addressed in individual
   documents that require change.  These changes would include those
   proposed in Section 7.2.1, Section 7.2.2, Section 7.2.5, and
   Section 7.2.7.

7.3.6.  RSVP-TE Admission Control and Preemption

   When a change is needed to path selection, a corresponding change is
   needed in admission control.  The same set of sections applies:
   Section 7.2.1, Section 7.2.2, Section 7.2.5, and Section 7.2.7.  Some
   resource changes such as a link delay change might trigger
   preemption.  The rules of preemption remain unchanged, still based on
   holding priority.

7.3.7.  Flow Identification and Traffic Balance

   The following describe either the state of the art in flow
   identification and traffic balance or propose changes: Section 7.2.4,



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   Section 7.2.5, Section 7.2.7, and Section 7.2.8.


8.  IANA Considerations

   This is a framework document and therefore does not specify protocol
   extensions.  This memo includes no request to IANA.


9.  Security Considerations

   The security considerations for MPLS/GMPLS and for MPLS-TP are
   documented in [RFC5920] and [RFC6941].

   The types protocol extensions proposed in this framework document
   provide additional information about links, forwarding adjacencies,
   and LSP requirements.  The protocol semantics changes described in
   this framework document propose additional LSP constraints applied at
   path computation time and at LSP admission at midpoints LSR.  The
   additional information and constraints provide no additional security
   considerations beyond the security considerations already documented
   in [RFC5920] and [RFC6941].


10.  Acknowledgments

   Authors would like to thank Adrian Farrel, Fred Jounay, Yuji Kamite
   for his extensive comments and suggestions regarding early versions
   of this document, Ron Bonica, Nabil Bitar, Eric Gray, Lou Berger, and
   Kireeti Kompella for their reviews of early versions and great
   suggestions.

   Authors would like to thank Iftekhar Hussain for review and
   suggestions regarding recent versions of this document.

   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
   basis.


11.  References







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11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              September 2003.

   [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.

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

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

   [RFC5712]  Meyer, M. and JP. Vasseur, "MPLS Traffic Engineering Soft
              Preemption", RFC 5712, January 2010.

   [RFC6107]  Shiomoto, K. and A. Farrel, "Procedures for Dynamically
              Signaled Hierarchical Label Switched Paths", RFC 6107,
              February 2011.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374, September 2011.

   [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.

11.2.  Informative References

   [DBP]      Bertsekas, D., "Dynamic Behavior of Shortest Path Routing
              Algorithms for Communication Networks", IEEE Trans. Auto.
              Control 1982.

   [I-D.atlas-mpls-te-express-path]



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              Atlas, A., Drake, J., Giacalone, S., Ward, D., Previdi,
              S., and C. Filsfils, "Performance-based Path Selection for
              Explicitly Routed LSPs",
              draft-atlas-mpls-te-express-path-02 (work in progress),
              February 2013.

   [I-D.ietf-mpls-multipath-use]
              Villamizar, C., "Use of Multipath with MPLS-TP and MPLS",
              draft-ietf-mpls-multipath-use-00 (work in progress),
              February 2013.

   [I-D.ietf-ospf-te-metric-extensions]
              Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", draft-ietf-ospf-te-metric-extensions-04 (work
              in progress), June 2013.

   [I-D.ietf-rtgwg-cl-requirement]
              Villamizar, C., McDysan, D., Ning, S., Malis, A., and L.
              Yong, "Requirements for Advanced Multipath in MPLS
              Networks", draft-ietf-rtgwg-cl-requirement-11 (work in
              progress), July 2013.

   [I-D.ietf-rtgwg-cl-use-cases]
              Ning, S., Malis, A., McDysan, D., Yong, L., and C.
              Villamizar, "Advannced Multipath Use Cases and Design
              Considerations", draft-ietf-rtgwg-cl-use-cases-04 (work in
              progress), July 2013.

   [I-D.ospf-cc-stlv]
              Osborne, E., "Component and Composite Link Membership in
              OSPF", draft-ospf-cc-stlv-00 (work in progress),
              August 2011.

   [I-D.previdi-isis-te-metric-extensions]
              Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
              A., and C. Filsfils, "IS-IS Traffic Engineering (TE)
              Metric Extensions",
              draft-previdi-isis-te-metric-extensions-03 (work in
              progress), February 2013.

   [I-D.villamizar-mpls-multipath-extn]
              Villamizar, C., "Multipath Extensions for MPLS Traffic
              Engineering", draft-villamizar-mpls-multipath-extn-00
              (work in progress), November 2012.

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



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              Services", RFC 2475, December 1998.

   [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.

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

   [RFC3468]  Andersson, L. and G. Swallow, "The Multiprotocol Label
              Switching (MPLS) Working Group decision on MPLS signaling
              protocols", RFC 3468, February 2003.

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

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 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.

   [RFC4448]  Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, April 2006.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655, August 2006.

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

   [RFC5151]  Farrel, A., Ayyangar, A., and JP. Vasseur, "Inter-Domain
              MPLS and GMPLS Traffic Engineering -- Resource Reservation
              Protocol-Traffic Engineering (RSVP-TE) Extensions",
              RFC 5151, February 2008.

   [RFC5152]  Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain
              Path Computation Method for Establishing Inter-Domain
              Traffic Engineering (TE) Label Switched Paths (LSPs)",
              RFC 5152, February 2008.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS



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              Traffic Engineering", RFC 5316, December 2008.

   [RFC5392]  Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5392, January 2009.

   [RFC5441]  Vasseur, JP., Zhang, R., Bitar, N., and JL. Le Roux, "A
              Backward-Recursive PCE-Based Computation (BRPC) Procedure
              to Compute Shortest Constrained Inter-Domain Traffic
              Engineering Label Switched Paths", RFC 5441, April 2009.

   [RFC5920]  Fang, L., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.

   [RFC5921]  Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
              Berger, "A Framework for MPLS in Transport Networks",
              RFC 5921, July 2010.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, November 2012.

   [RFC6941]  Fang, L., Niven-Jenkins, B., Mansfield, S., and R.
              Graveman, "MPLS Transport Profile (MPLS-TP) Security
              Framework", RFC 6941, April 2013.


Authors' Addresses

   So Ning
   Tata Communications

   Email: ning.so@tatacommunications.com


   Dave McDysan
   Verizon
   22001 Loudoun County PKWY
   Ashburn, VA  20147
   USA

   Email: dave.mcdysan@verizon.com









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   Eric Osborne
   Cisco

   Email: eosborne@cisco.com


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

   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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