[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: 00 01 02 03 04

MPLS Working Group                                              R. Singh
INTERNET-DRAFT                                                   Y. Shen
Intended Status: Proposed Standard                              J. Drake
Expires: April 24, 2014                                 Juniper Networks
                                                        October 21, 2013


                    Entropy label for seamless MPLS
              draft-ravisingh-mpls-el-for-seamless-mpls-01


Abstract

   This document describes how entropy labels can be used for load
   balancing in a seamless MPLS architecture. The definition of the
   control plane and data plane behavior at LSP stitching points; and at
   the ingress of an LSP in a hierarchy of LSPs, as described in this
   document, brings the benefits of entropy labels to seamless MPLS as
   MPLS deployments proliferate in the access and aggregation networks.

   This document updates RFC 6790.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This Internet-Draft will expire on August 22, 2013.


Copyright and License Notice




Singh, et al.            Expires April 24, 2014                 [Page 1]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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
   2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3. Key attributes of the entropy label solution: Summary from
      [EL-RFC]  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4. Problems and Motivation . . . . . . . . . . . . . . . . . . . .  6
     4.1 EL applicability for seamless MPLS . . . . . . . . . . . . .  7
     4.2 EL for LSP stitching . . . . . . . . . . . . . . . . . . . .  7
       4.2.1 Spectrum of EL usage behaviors required to be
             supported for stitched LSPs  . . . . . . . . . . . . . .  8
         4.2.1.1 Entropy label for per-segment LSP  . . . . . . . . .  9
         4.2.1.2 Entropy label for notional-segment-LSP(s)  . . . . .  9
         4.2.1.3 Entropy label for e2e LSP  . . . . . . . . . . . . . 10
     4.3 EL for LSP hierarchy . . . . . . . . . . . . . . . . . . . . 10
       4.3.1 Possibility of unnecessary reduction of max-payload of
             the LSP: . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.3.2 Possibility of EL being non-usable for load-balancing: . 11
   5. EL for LSP stitching/hierarchy  . . . . . . . . . . . . . . . . 13
     5.1 Additional EL abstractions: specific to LSP
         stitching/hierarchy  . . . . . . . . . . . . . . . . . . . . 13
     5.2 New abstractions: EL applicability for LSP stitching . . . . 13
       5.2.1 Signaling  . . . . . . . . . . . . . . . . . . . . . . . 13
         5.2.1.1 Signaling ELC at stitching points (Translation
                 rules) . . . . . . . . . . . . . . . . . . . . . . . 14
       5.2.2 Data plane aspects . . . . . . . . . . . . . . . . . . . 15
         5.2.2.1 Stitching: Differing EL dispositions . . . . . . . . 15
     5.3 New abstractions: EL applicability for LSP hierarchy . . . . 18
       5.3.1 Management plane:  . . . . . . . . . . . . . . . . . . . 18
       5.3.2 Data plane aspects . . . . . . . . . . . . . . . . . . . 18
   6. Use-cases . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     6.1 Carrier of carrier L3VPN . . . . . . . . . . . . . . . . . . 20
     6.2 Inter-AS L3VPN: Option C . . . . . . . . . . . . . . . . . . 21



Singh, et al.            Expires April 24, 2014                 [Page 2]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   7. Security considerations . . . . . . . . . . . . . . . . . . . . 21
   8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 21
   9. IANA considerations . . . . . . . . . . . . . . . . . . . . . . 21
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1  Normative References  . . . . . . . . . . . . . . . . . . . 21
     9.2  Informative References  . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23












































Singh, et al.            Expires April 24, 2014                 [Page 3]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


1  Introduction

   [EL-RFC] specifies a way to implement load-balancing in an MPLS
   network such that sub-flows of an LSP may be identified and sent on
   different paths through the network. This is achieved by using
   entropy labels (ELs) to abstract out the flow-identifying information
   into the entropy label and inserting the entropy label underneath the
   LSP label. The transit LSRs perform the load-balancing hash-
   computation, on the label-stack alone, to effect a good load-
   balancing outcome without a need to parse inner headers.

   The key feature of [EL-RFC] is that it defines the EL in the context
   of a given LSP. [EL-RFC] defines the signaling extensions by which
   entropy label capability might be signaled for LSPs setup by RSVP-TE,
   LDP or [LU-BGP]. While that works well for individual LSPs, there are
   additional issues to consider for the seamless MPLS architecture [S-
   MPLS].

   The currently-under-definition framework for seamless MPLS proposes
   an architecture ([S-MPLS]) that shall enable the setting-up of MPLS
   LSPs from access nodes to access nodes using a medley of signaling
   protocols and statically configured LSPs. There are special EL-
   related considerations that need to be dealt with to make EL more
   suitable for seamless MPLS.

   This document defines additional abstractions and rules for the use
   of entropy-label with LSP stitching/hierarchy to enable the use of
   ELs for the seamless MPLS architecture. This document describes how
   entropy labels may be used when the LSP has been setup by stitching
   LSP segments or by tunneling the LSP over other LSPs. It is
   conceivable that different signaling protocols are in use to create
   an e2e LSP.

   LSP stitching is the process of connecting LSP segments in the data
   plane to form a single e2e data plane LSP.  This is achieved by
   setting up LSP segments through signaling or through management
   action, and then signaling an e2e LSP that "uses" these LSP segments
   as hops in its path. The procedures for LSP stitching are described
   in [STITCHING]. Labeled data traffic flowing over e2e MPLS LSPs, that
   have been setup using multiple different protocols (by stitching
   together segments), would benefit from having the entropy label be
   included in it.

   LSP hierarchy is defined in [MPLS-ARCH] and [GMPLS-HIER]. Usage of
   entropy label in LSP hierarchies has some peculiar practical issues
   that will benefit from some additional flexibility in inserting ELs
   for a specific layer in an LSP hierarchy.




Singh, et al.            Expires April 24, 2014                 [Page 4]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


2. Terminology

   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].


   The following acronyms/terms are used:

   e2e: End to end LSP that has been setup by stitching together LSP
        segments

   ECMP: Equal Cost Multi-Path

   EL: Entropy Label

   ELC: Entropy Label Capability or Entropy Label Capable

   ELI: Entropy Label Indicator

   Intrinsic ELC: Entropy label capability/capable as in [EL-RFC]. In
                  this document, an LSP is considered to be
                  "intrinsically" EL-capable when the:
                  - the ingress of the LSP has the ability to compute
                    and PUSH the EL before PUSHing the ELI before
                    PUSHing the LSP label; and
                  - the egress/PHR of the LSP-segment has the ability
                    to POP the (ELI+EL) at the egress/PHR while
                    POPing-transport-label/ELI-is-top-label
                    respectively.

   LAG: Link Aggregation Group

   LER: Label Edge Router

   LSP: Label Switched Path

   LSR: Label Switching Router

   Notional ingress: Ingress LER for an LSP segment that is inserting
                     the (ELI+EL) on data traffic going over an e2e LSP

   Notional egress: egress LER for an LSP segment that is removing the
                    (ELI+EL) from data traffic going over an e2e LSP

   Notional LSP segment: the portion of the e2e LSP between a notional
                         ingress and a notional egress




Singh, et al.            Expires April 24, 2014                 [Page 5]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   PHP: Penultimate Hop Popping

   PHR: Penultimate Hop Router

   UHP: Ultimate Hop Popping


   NOTE: this document references the (ELI+EL) pair simply as EL when
         the presence of the ELI is of no significance for the issue
         being described. The presence of ELI is mandatory as per
         [EL-RFC] when EL is in use.


3. Key attributes of the entropy label solution: Summary from [EL-RFC]
     - Transport-label-PUSHing router inserts (ELI+EL)
       The (ELI+EL) insertion is done, if at all, by a router that is
       PUSHing the transport LSP's label.

     - Ingress-LER (transport-label-PUSHing-router) inserts (ELI+EL)
       only if the PHR/egress has signaled ability to strip it off.

     - Transport-label-POPing router POPs (ELI+EL) PHR/egress of the
       LSP is responsible for POPing off the (ELI+EL) after it has
       been exposed as the top label on the packet due to POPing the
       transport label. The removal of the (ELI+EL) is done either
       when the ELI is the top label; or when the ELI is next label
       below the top label being POPed.

     - Max-payload size for the LSP gets reduced by 8 bytes after the
       insertion of the (ELI+EL).

4. Problems and Motivation

   [EL-RFC] defines EL signaling/usage suitable for single-segment LSPs.
   However, as MPLS proliferates in the network access leading to the
   setup of e2e LSPs using LSP stitching and hierarchies, there is a
   need to define the EL behavior for LSP stitching and LSP hierarchies.

   [EL-RFC] does not explicitly specify the EL-signaling-interaction
   between stitched LSPs segments. Similarly, peculiarities in the data-
   plane related to LSP stitching need further specification. Likewise,
   the signaling and data-plane peculiarities for using EL over LSP
   hierarchies could be further specified.

   It is desirable to get the benefits of EL even for stitched LSPs.

   Certain aspects peculiar to stitched LSPs need additional handling to
   increase the applicability of [EL-RFC]. [EL-RFC] needs to be extended



Singh, et al.            Expires April 24, 2014                 [Page 6]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   to define the behavior for LSP stitching and LSP hierarchies
   (tunneling) when using EL.

   The sub-sections below list the specific additional requirements for
   making entropy label more deployable when used with LSP stitching,
   and LSP hierarchy.


4.1 EL applicability for seamless MPLS

   The seamless MPLS architecture relies on the use of LSP stitching and
   hierarchy to signal an e2e LSP between access-nodes, such that the
   e2e LSP is going over aggregation/transport/cores nodes.

   The signaling of such e2e LSPs is enabled by using the
   stitching/hierarchy mechanisms that exist, using [LU-BGP]/LDP/RSVP.

   The rules of section 5 provide a general-purpose way for the use of
   ELs across e2e LSPs by defining:

   - the rules of ELC propagation at stitching points;

   - the data-plane guidelines for the stitching point LSR; and

   - the data-plane guidelines for LSP hierarchies for inserting
     (ELI+EL) at ingress LER of an LSP in an LSP hierarchy.

4.2 EL for LSP stitching

   A stitched e2e LSP might be stitched from greater than 2 segment LSPs
   (along the length of the e2e LSP), with 2 LSPs forming the stitch at
   each stitching point.

   An LSP segment is considered to be intrinsically EL capable when it
   supports the attributes summarized in section 3.

   Some of the segment LSPs in the e2e LSP may intrinsically support EL
   and some may not. So, the e2e LSP may not intrinsically support EL
   from end to end. However, to obtain the benefits of EL for stitched
   LSPs, it is important that an EL should be present on the data
   packets traversing as many segments of the e2e LSP as is possible
   within data plane abilities of the routers on the way.

   In using EL with LSP stitching, the aims are BOTH of the following:
        a. Get EL benefits wherever possible: on all segments where
           possible. Just because a given segment does not support EL
           is not a reason to deny EL benefits to other segments of the
           e2e LSP.



Singh, et al.            Expires April 24, 2014                 [Page 7]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


        b. Not run into data-plane problems where an
           intermediate-segment whose ingress LER can not look deeper
           to remove EL when the subsequent segment does not support EL.

   - Independent setup of LSP segments:

   LSP stitching typically relies on LSP segments that have been
   independently setup. In an e2e LSP (made of stitched segments), it is
   unlikely that all of the stitching points (i.e., segment LSP end
   points) as well as the ultimate ingress and ultimate egress support
   EL as defined in section 3.

   However, there would be individual LSP segments that would completely
   satisfy the requirements of section 2 (i.e. are intrinsically EL
   capable). This document describes how EL may be used for (portions
   of) the e2e LSP while still working within the framework for [EL-
   RFC].


        S---A---B---C---D

   In the above topology, for an e2e LSP from S to D, the segments AB
   and CD could be intrinsically EL capable while the segments SA, & BC
   may not be. For data traffic going over the LSP from S to D, using EL
   on the segments AB and CD would be beneficial for load-balancing over
   LAGs/ECMP.

   - Dealing with different protocols being used to setup the segments
     of the e2e LSP.

4.2.1 Spectrum of EL usage behaviors required to be supported for
   stitched LSPs

   To cater for an incremental deployment of intrinsically-ELC routers
   in a network, the multiple different modes for EL use with LSP
   stitching need to be to be supported.

   The spectrum of supported behaviors are listed below by referencing
   the following diagram.

            S1          S2         S3         S4

      A-----------B-----------C-----------D-----------E

   LSP segments S1, S2, S3, S4 are between LERs A/B/C/D/E. There may or
   may not be other routers between the per-segment ingress<->egress
   LERs.




Singh, et al.            Expires April 24, 2014                 [Page 8]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   Transport LSP signaling protocol: could be any: LDP/RSVP/([LU-BGP]
   tunneled over RSVP/LDP).

4.2.1.1 Entropy label for per-segment LSP

   Each of the segments will have their independent EL capability based
   on BOTH the:

        - Per-segment ingress having the ability to insert the EL.

        - Per-segment egress (or PH router) having the ability to strip
           the EL.

   This is very similar to [EL-RFC] with the additional data-plane rule
   of section 5.2.2.1 "A. Rationalizing EL for the outgoing LSP
   segment:".

   Reasoning for why per-segment EL may be attractive for certain use
   scenarios:

   Opportunity to get benefits on those segments where EL benefits are
   available. Even though the e2e LSP may not support ELC, this allows
   the EL benefits on those segments that are EL-capable.



4.2.1.2 Entropy label for notional-segment-LSP(s)

   In the case of stitched LSPs, it is useful to:

        - Insert EL at first per-segment ingress LER (per-segment ingress
          LER closest to the e2e ingress LER) that has the ability to
          insert EL.
        - Carry the EL on the data packets as far along the stitched LSP as
          the last per-segment egress LER that ability to strip the EL
          on a series of contiguous EL-supporting segments.

   The above is enabled by the rules of section "5.2.1.1 Signaling ELC
   at stitching points (Translation rules)".

   The benefit of using EL with notional-segment LSPs:

   An operator might be able to use EL for the MPLS traffic on its path
   to a stitching point even though the stitching-point router (or its
   PHR) itself may not have the data-plane capabilities required as in
   [EL-RFC].

   Additionally, even if the stitching-point (or its PHR) do have the



Singh, et al.            Expires April 24, 2014                 [Page 9]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   data-plane capabilities of [EL-RFC], it is just more efficient to
   forward the data packets without having to strip the EL and then
   reinsert the EL when the downstream segment is also intrinsically
   ELC.

4.2.1.3 Entropy label for e2e LSP

   This correspond to having the notional-LSP and the e2e LSP being the
   same.

   This is covered  by the rules of section 5.2.1.1 "Signaling ELC at
   stitching points (Translation rules):" with the additional
   requirement that the data-plane be exactly the same as [EL-RFC],
   i.e.
        the (ELI+EL) insertion is done by a label PUSHing router,
        the (ELI+EL) POP is done by the PHR/egress for the e2e LSP.

4.3 EL for LSP hierarchy

   For the purpose of highlighting the problem to be addressed and the
   resultant requirements to be met, the following diagram is presented
   as an example.

   Let there be an LSP hierarchy with the ingress for the different
   levels of LSP hierarchy being at different routers, such that each
   LSP in the hierarchy is intrinsically EL capable. The individual LSPs
   in the hierarchy could be a single-segment LSP or a stitched e2e LSP.


                S1                  D1
                  \    ---------    /
                   A===B===C===D===E
                  /    ---------    \
                S2                  D2

   In the above topology, let there be the following LSPs:

        L1: B->D
        L2: A->E, tunneled through LSP L1
        L3: S1->D1, tunneled through LSP L2
        L4: S2->D2, tunneled through LSP L2

   All of the LSPs above are assumed to be intrinsically EL capable.

4.3.1 Possibility of unnecessary reduction of max-payload of the LSP:

   Even though the aim of using EL is to get better load-balancing
   support, in some cases the insertion of (ELI+EL) may unnecessarily



Singh, et al.            Expires April 24, 2014                [Page 10]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   reduce the effective payload of an LSP.

   In above diagram, as per [EL-RFC] for a data packet on LSP L3, the
   insertion of (ELI+EL) for each of the 3 LSPs: L1, L2 and L3 is not
   explicitly prohibited. As a result it is possible that the packet on
   LSP L3, might end up with 3 (ELI+EL)s (one for each LSP level in the
   hierarchy) thus reducing the effective payload of the LSP L3.
   Likewise for L4. The presence of the (ELI+EL) for the outer LSPs L1
   and L2 is not strictly useful for load-balancing the data traffic on
   the LSPs L3 and L4.

   The solution for this issue is presented in section 5.3.2: it relies
   on inserting the (ELI+EL) in the context of only 1 LSP in a
   hierarchy.

   This issues results in the following requirement for EL usage in the
   presence of LSP hierarchies:
   - Desirability of having a single (ELI+EL) on data packets over an
     LSP hierarchy: The LSP for which the (ELI+EL) is inserted, is
     preferably the innermost intrinsically EL-capable LSP, as the
     notion of a user-flow is more specifically indicated by fields
     deeper inside the packet headers. Having an EL be present deeper
     in the packet provides load-balancing benefits of EL for the
     traversal of the packet across a longer stretch of the network.

   If there is to be only 1 (ELI+EL) in the label stack, it imposes an
   additional data plane requirement on the ingress LER as described in
   section 5.3.2.

4.3.2 Possibility of EL being non-usable for load-balancing: Even though
   the aim of using EL is to get better load-balancing, in some cases
   the insertion of (ELI+EL) may actually offer no load-balancing
   benefits at all. Whether the presence of an EL offers load-balancing
   benefits on a given transit router, depends on:

    - whether the transit router has a LAG or an ECMP as an outgoing
      interface for the LSP, AND
    - whether the forwarding ASICs of the transit routers have the
      ability to include the EL (appearing at a specific position in
      the label stack) in the hash computation, either by:
        +   looking up the ELI and then picking the EL, or
        +   computing the hash on the maximum number of labels that it can
             pick from the label-stack for hash-computation which
             happens to also include the EL.

   When the EL on a packet is outside the portion-of-the-label-stack
   that the ASIC of a transit router can use for hash computation, the
   forwarding hardware may include only the top few labels or the bottom



Singh, et al.            Expires April 24, 2014                [Page 11]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   few labels in the hash computation. This may prevent the inclusion of
   EL for hash-computation.

   In the above diagram, for a data packet going over LSP L3 let the
   issue of section 4.3.1 have been resolved by the router S1 inserting
   the (ELI+EL) underneath the label for LSP L3 and none of the other
   routers inserting the (ELI+EL). When this data packet arrives at
   router C, its label stack looks thus:

         Label-LSP1 | Label-LSP2 | Label-LSP3 | ELI | EL
         Top-label              ->                    Bottom label


   Let's say that the router C is able to include only the top 4 labels
   in a label stack for the hash-computation due to the ability of its
   forwarding ASICs.

   So, the router C is not able to get the benefit of the presence of
   the EL in the data packet. If the only ECMP/LAG in this network is
   the link between C&D, then the presence of the EL serves no purpose
   for the above network example and it ends up reducing the payload
   capacity of the LSPs L3 and L4 by 8 bytes.

   This example could be further generalized in the case of seamless
   MPLS, where there may be deeper LSP hierarchies.

   A transit router that has the ability to hash on an EL (based on its
   depth in the label stack) does not have multiple paths; while another
   router that has multiple paths and the ability hash on the EL
   (appearing at a specific depth in the label stack) is unable to do so
   as the EL appears outside the depth of the label stack that may be
   included in the hash. In neither of the foregoing cases is the
   presence of an EL helpful.

   This translates into a requirement for EL: Flexibility in choice of
   LSP tunnel for which EL is inserted:

   There is a need to have a way by which to include an EL underneath a
   specific label in a label-hierarchy based on it serving the most
   useful purpose (i.e. taking into consideration location of multiple-
   forwarding-paths and stack-depth-concerns).

   [EL-RFC] has no way of influencing the insertion of (ELI+EL) at a
   certain LSP level in the stack. Thus, there is a need for a mechanism
   by which one of the many intrinsically-EL-capable LSPs in an LSP
   hierarchy could be picked for inserting the (ELI+EL).





Singh, et al.            Expires April 24, 2014                [Page 12]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


5. EL for LSP stitching/hierarchy
5.1 Additional EL abstractions: specific to LSP stitching/hierarchy

   Given the previous sections, following additional abstractions need
   to be defined to make EL more useful for LSP stitching and hierarchy.

5.2 New abstractions: EL applicability for LSP stitching
5.2.1 Signaling

   New abstractions need to be defined to handle the differences in the
   use of ELs for stitched-LSPs as compared to their use for single-
   segment LSPs.

   The differences are:

    - Notion of ingress for EL insertion:
      (ELI+EL) insertion might not necessarily be done by a
      label-PUSHing router. A stitching point where the label is being
      swapped might do the (ELI+EL) insertion, and serves as a
      "notional ingress".

    - Notion of egress for EL:
      "Notional-egress" might not be the segment egress for the segment
      of the notional-ingress.
      Even though certain stitching-points (segment LERs) might not
      support POPing (ELI+EL), it may be acceptable to let the (ELI+EL)
      continue to be on the packet since the egress of a subsequent
      segment has the capability to POP the (ELI+EL) (which may not
      necessarily be along with POPing the transport label). A
      "notional-ingress and notional-egress" pair might actually be the
      segment-ingress and segment-egress for different LSP segments that
      are part of the same e2e LSP.

   The portion of the stitched e2e LSP, between a notional-ingress and a
   notional-egress is referred to as the "notional-LSP-segment" in this
   document.

   As a packet traverses an e2e LSP, it may have an (ELI+EL) imposed on
   it and then removed at different routers.

   It is desirable for there to not be more than one instance of an
   (ELI+EL) to appear on a packet at any given time. However, the
   insertion followed by removal of an (ELI+EL) may happen more than
   once as the packet traverses the e2e LSP.  Each router doing the
   (ELI+EL) insertion is the notional-ingress and each router doing the
   (ELI+EL) removal is the notional-egress (or notion EL-stripping-PH-
   router).




Singh, et al.            Expires April 24, 2014                [Page 13]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   Thus, there may be more than 1 "notional ingress" for EL insertion,
   and there may be more than 1 "notional egress" for EL removal.

   For each notional "ingress ingress", there will be a "notional
   egress" with the "notional ingress"es and "notional egress"es
   alternating along the path of the e2e LSP when there are more than 1
   notional ingress and egress for an e2e LSP.

   In the simplest case, this boils down to the case of there being just
   one notional ingress and one notional egress; and the notional
   ingress coincides with the e2e ingress, and the notional-egress
   coincides with the e2e egress. That is the case that [EL-RFC]
   addresses.

   Separation of control/data-plane implies that certain routers

    - Might be running software that supports signaling ELC and
      understanding an egress' ELC.
    - However, might not have the capability to insert (ELI+EL).

   Such routers should not be allowed to play a spoil-sport in
   preventing EL benefits for traffic traversing over them via stitched
   LSPs. In other words, such routers can not act as notional-ingress or
   notional-egress. However, the presence of such per-segment
   ingress/egress routers on the path of a notional segment-LSP should
   not prevent the notional segment-LSP from benefiting from the use of
   EL.

5.2.1.1 Signaling ELC at stitching points (Translation rules)

   The rules for propagating ELC, at stitching points, from a downstream
   segment LSP to an upstream segment LSP are listed in this section.

   There is benefit in propagating ELC across stitching points is to not
   have to re-compute the EL at different segment ingress for those
   segments that are EL capable, including when the LSP segments have
   been setup using different protocols.

   Additionally, in certain cases it should be possible to get benefits
   of (ELI+EL) on LSP segments that are not "intrinsically EL capable",
   where the lack of "intrinsic EL capability" is due to:
    - The per-segment ingress does not support EL insertion.
    - The per-segment PHR/egress does not support EL POPing.

   However, such a stitching point might support ELC signaling.

   At a stitching point, when 2 LSP segments: L1 (incoming LSP) and L2
   (the outgoing LSP) are being stitched, the following rules should be



Singh, et al.            Expires April 24, 2014                [Page 14]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   followed by stitching point in signaling its ELC.

   A. Segment-egress:

      1. A segment-egress signals ELC for an LSP-segment L1 when:
         a. The segment-egress is intrinsically ELC, or
         b. When it is not intrinsically-ELC, however segment-egress for
            LSP-segment L2 (downstream of L1)- for which this
            stitching-point is segment-ingress - is signaling ELC.
            [This handles the case: Support the signaling even though it
             may not support the data-plane behavior.]

      2. A segment-egress MUST NOT signal ELC if BOTH of the following
         are true:
         a. It is also segment-ingress for another LSP-segment whose
            segment-egress is not signaling ELC.
         b. This router does not have the ability to remove an (ELI+EL)
            inserted by the segment-ingress for the LSP-segment for
            which this router is the segment-egress.


   B. Segment-ingress:

      The following is relevant only for RSVP as defined in [EL-RFC].
      When this router acting as segment-egress for an LSP L1 (that is
      stitched to downstream LSP L2) is signaling ELC for L1, then this
      router must signal ELC in its Path messages using the mechanism
      defined in [EL-RFC].

      This is relevant only in the context of bidirectional LSPs.

5.2.2 Data plane aspects
5.2.2.1 Stitching: Differing EL dispositions

   At a stitching point, when 2 LSP segments: L1 (incoming LSP) and L2
   (the outgoing LSP) are being stitched, the following rules should be
   followed by the stitching point in its data-plane behavior.

   A. Rationalizing EL for the outgoing LSP segment:
      When the LSP segments L1 and L2 differ in their ELC, the stitching
      point router needs to take the following data-plane actions
      depending on its role for the e2e LSP:

        a. Notional egress behavior:
           When L1 intrinsically supports ELC and L2 does not, then
           the stitching-point router must remove the (ELI+EL), if
           present under top label, from the received data packets
           before effectively SWAPing the top label. In other words,



Singh, et al.            Expires April 24, 2014                [Page 15]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


           in the presence of the ELI, the operations performed
           should be:

            POP(IncomingLabel), POP(ELI+EL), PUSH(OutgoingLabel)
                 or alternately:
            POP, POP, SWAP(OutgoingLabel)

           Translation rule "A 2" of section 5.2.1.1 would have ensured that
            the above is doable at the stitching point.

        b. Notional ingress behavior:
           When L1 does not intrinsically support ELC and L2 does, then the
           stitching point router must POP the incoming label, insert
           (ELI+EL) before PUSHing the label for the LSP segment L2.

           The label operations performed would be:
            POP(IncomingLabel), PUSH(EL), PUSH(ELI), PUSH(OutgoingLabel),
                        or
            SWAP(EL), PUSH(ELI), PUSH(OutgoingLabel)

        c. Implicit notional ingress behavior:
           When L1 is intrinsically ELC and so is L2, the arriving data
           traffic should already have (ELI+EL) on it.

           However, it is possible that due to local configuration on the
           notional-ingress, (ELI+EL) is not being inserted. In that
           case, traffic arriving on L1 will not have (ELI+EL) on it.

           In that case, this stitching-point is the "implicit notional
           ingress" and it should insert (ELI+EL) just as if it were a
           "notional ingress".


   B. Preventing multiple (ELI+EL) pairs underneath a given forwarding
      label in the stack:

      A segment-ingress that is intrinsically-EL-capable should have
      the ability to inspect received data packets to check whether an
      (ELI+EL) already exists on the data packet underneath the top
      label.

      Not causing multiple ELs on a data packet:
            When both the LSP segments L1 and L2 support ELC, the
            stitching point router SHOULD insert an (ELI+EL) only if the
            incoming packet does not contain an (ELI+EL) underneath the
            top label. In that case, the label operations are as below:
                 POP(IncomingLabel), PUSH(ELI+EL), PUSH(OutgoingLabel)




Singh, et al.            Expires April 24, 2014                [Page 16]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


      If the incoming packet already contains an (ELI+EL) underneath the
      top label, an additional (ELI+EL) MUST NOT be inserted on the
      packet underneath the top label that is being effectively SWAPed.

      This prevents a segment ingress from inserting an (ELI+EL) when
      the notional ingress has already inserted an (ELI+EL).


   C. Rationalizing EL insertion (at stitching-point) for LSP hierarchy:

      A stitching point router that is intrinsically-EL-capable should
      have the ability to inspect received data packets to check whether
      an (ELI+EL) already exists, underneath any label in the
      label-stack.

      If the router has such a ability, then this router MUST NOT insert
      an (ELI+EL) as in "A a" above.

      This helps to prevent multiple (ELI+EL)s on the packet inserted
      (at a stitching point) in the context of different transport
      labels in a label hierarchy.


   D. Notional ingress role change at a router:

      This role can change due to local configuration on the router or
      due to segment egress starting/stopping to signal ELC possibly due
      to a configuration change at the segment egress or due to a
      configuration change at this router.
      When this router becomes a notional ingress, it reacts to the
      change as in "A b" above.

      When this router stops being a notional ingress, this router
      stops inserting the (ELI+EL) underneath the top label that this
      router is
        SWAPing(if this router is stitching point), or
        PUSHING (if this router is e2e ingress).

   E. Notional egress role change at a router:

      This role can change due to local configuration on the router or
      due the egress of a downstream stitched LSP segment starting to
      signal ELC.

      When this router becomes a notional egress, it reacts to the
      change as in "A a" above.

      When this router stops being a notional egress, this router stops



Singh, et al.            Expires April 24, 2014                [Page 17]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


      performing the label operation described in "A a" above. Instead
      this router now starts to simply SWAP the top label.


5.3 New abstractions: EL applicability for LSP hierarchy
5.3.1 Management plane:

   Moving the (ELI+EL) underneath a different LSP's transport label:
   There are 2 ways to handle the issue of section 4.3.2:
     - Configuration at the ingress LER: a configuration option should
       exist by which an operator can disable the insertion of (ELI+EL)
       on a per-LSP basis. The specific level in the LSP hierarchy for
       which to enable this configuration is based on operator knowledge
       based on:
         * Knowledge of transit routers that need EL benefits : those
           routers that have a multi-path (LAG or LSP ECMP) as egress
           interface.
         * The label hashing abilities of such routers: information
           about the specific number of labels in the label-stack that
           can be used in the hash computation; and any constraints
           about the position of the labels that can be used for
           computation when the label stack is larger than a certain
           ASIC-specific threshold.

     - Configuration-based rewrite of the label stack at the ingress LER
       of an intrinsically-EL-capable LSP:

       An operator will know the forwarding characteristics (with
       regards to the number of labels that can be included in the hash
       computation) of the transit routers across the path of the e2e
       LSP that is part of an LSP hierarchy.

       By making such a configuration, the operator can ensure that the
       EL will appear in the label stack such that all transit routers
       shall be able to include the as part of the hash computation.

       The configuration would cause the label stack of the outgoing
       packet to have its extant (ELI+EL) removed, and an (ELI+EL)
       inserted just underneath the label of the LSP for which this
       ingress LER is setup to insert (ELI+EL).



5.3.2 Data plane aspects

   Preventing insertion of multiple (ELI+EL)s:
   At an ingress LER, the router SHOULD not insert an (ELI+EL) for an
   LSP if the packet already contains an ELI.



Singh, et al.            Expires April 24, 2014                [Page 18]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   This ensures that for a data packet on a hierarchy of LSPs, there
   will be only 1 instance of an (ELI+EL). This helps to prevent the
   issue of section 4.3.1.

   This also ensures that when multiple LSPs in an LSP hierarchy are
   intrinsically-EL-capable, the (ELI+EL) will be inserted just
   underneath the transport label of the innermost LSP in the hierarchy.
   However, based on section 5.3.1 there is a way by which to modify the
   level in the LSP hierarchy for which an (ELI+EL) is inserted.


   A more specific case of this is already covered in section "5.2.2.1
   C. Rationalizing EL insertion (at stitching-point) for LSP
   hierarchy:".


6. Use-cases

   In this document, the definition of LSP-stitching is broadened to
   refer to not just [STITCHING] but also to those cases where  a label
   advertised by one label distribution protocol being:
     - removed at one router (by PH POP) followed by a label PUSH for a
       label distributed by another protocol at the router downstream
       of the PH router of the previous protocol's LSP. Such a router
       which is PUSHing a label for a subsequent protocol is also
       referred to as a  stitching-point router in this document.

     - SWAPed at a router for a label distributed by another protocol
       is also referred to as a stitching-point in this document.

   The list of use-cases of this draft stems from the following:
   A. Not having to insert/remove (ELI+EL) multiple times along an e2e
   labeled path, due to the EL capability not getting signaled e2e. In
   other words, not having to remove (ELI+EL) at a stitching point only
   to re-insert it.

      The lack of e2e EL capability signaling could be either due to
   administrative controls (as in [STITCHING]); or due to a label
   advertised by one label distribution protocol being removed at one
   router (which also causes the removal of (ELI+EL)) and a label
   distributed by a subsequent router being PUSHed along with (ELI+EL)
   at that router.

   B. On an e2e labeled transport-LSP path, it may be possible to get
   the load-balancing benefits of EL on (segment of) the e2e LSP even
   though not every stitching point router (as defined above) may
   intrinsically support EL for the LSP terminating at it.




Singh, et al.            Expires April 24, 2014                [Page 19]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


6.1 Carrier of carrier L3VPN
                                                             -----
                               =======                      /  A1
  ----                         |      |                 ____CE1   |
 /   \       --------         /       \        ------  /    |     |
|  A2 CE2-   / AS2   \       |         |       /      PE1   \    /
\    /  \   /           \_   /           \_  _/ Carrier|     ----
  ----    -PE2           |  |   Carrier's |  |   cust. |
            |            | |   carrier   |   |         |
            |       ASBR1|--ASBR2     ASBR3--|ASBR4    |    ----
            | Carrier    |  |             |  |         |   /     \
            | cust.      /  |     AS1     |  |    AS2  |  CE5  A5 |
            \           /    \            |  |       PE4__/\     /
          ---PE3-------/      |           |  \        /    -----
         /  |                 \           /   --------
  ----  /   |  ----            |          |
 /    \     | /    \           ============
|  A3 CE3   -CE4 A4|
\     /      \     /
  ----         ----
                                         A<n> = Customer site n
                                         CE = Customer Edge Device
                                         PE = Provider Edge Router

   In the above figure, the "carrier's carrier" is providing L3VPN
   service to a carrier customer (carrier cust.) is itself an L3VPN
   provider.
   Let the sites A<n> be the sites of the same L3VPN.
   In order to provide L3VPN service to the sites A<n>, there is
   effectively an e2e LSP between each pair of PEs. For PEs in the same
   carrier customer site, the e2e LSP is an RSVP or LDP LSP. eg. Between
   PE2 and PE3.
   For PEs that are across the carrier-customer's core, there is an e2e
   LSP created by advertising a BGP label for the remote PE's loopback
   address. The BGP label advertised from ASBR2 to ASBR1 rides-on top of
   the RSVP or LDP label in the carrier's-carrier core.

   eg. For having an e2e LSP from PE1 to PE2, a BGP label is advertised
   for PE1's loopback into the carrier customer's site on the left. This
   label could be dealt with by ASBR1 in two ways:
         a. Advertising it into LDP in the carrier customer's site (on
   the left), or
         b. By advertising it over an iBGP session to PE2.

   In the former case (LDP advertising a FEC for PE1), this document
   makes possible for ASBR1 to not have to remove the EL (inserted by
   PE2) and let it be removed by a either a stitching point (ASBR2 or
   ASBR3 or ASBR4) or the egress PE1. This is facilitated by the



Singh, et al.            Expires April 24, 2014                [Page 20]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   translation rules of section 5.2.1.1. The same also facilitates
   traffic with EL to be carried over stitching points such that the EL
   is eventually removed by the last-EL-capable stitching point or the
   EL capable e2e egress.

   Each carrier (carrier's carrier; and carrier-customer) will have LAGs
   and LDP ECMP paths in its network.

6.2 Inter-AS L3VPN: Option C

   Option C is conceptually similar to CoC L3VPN from a point of view of
   setting up the e2e LSP. Therefore, similar EL use-cases also exist
   for Option C.

   This applies for both L3VPN and also BGP-VPLS.


7. Security considerations

   Security considerations as listed in section 9 of [EL-RFC] apply.



8. Acknowledgments

   Many thanks to Adrian Farrel for his inputs on the stitching
   scenarios, and suggesting editorial improvements.

   Thanks to the EL team (Sudharsana Venkataraman, Nitin Singh, Ramji
   Vijayaraghavan, Jie Yan, Abhishek Tripathi) for discussions on some
   of these topics.



9. IANA considerations

   None.



9  References

9.1  Normative References

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




Singh, et al.            Expires April 24, 2014                [Page 21]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


   [GMPLS-HIER] Kompella, K., Y. Rekhter, "Label Switched Paths (LSP)
                Hierarchy with Generalized Multi-Protocol Label
                Switching (GMPLS)", RFC-4206, October 2005.

   [MPLS-ARCH]  Rosen, E.,  Viswanathan, A., R. Callon, "Multiprotocol
                Label Switching Architecture", RFC-3031, January 2001.

   [S-MPLS]     Leymann, N., Decraene, B., Filsfils, C.,
                Konstantynowicz, M., Steinberg, D., "Seamless MPLS
                Architecture", draft-leymann-mpls-seamless-mpls,
                October 2012.

   [STITCHING]  Ayyangar, A., Kompella, K., Vasseur, JP., A. Farrel,
                "Label Switched Path Stitching with Generalized
                Multiprotocol Label Switching Traffic Engineering
                (GMPLS TE)", RFC 5150, February 2008.


9.2  Informative References

   [ISSUE-DEEP] K. Kompella, "Deep Label Stacks",
                http://tools.ietf.org/agenda/84/slides/slides-84-mpls-
                15.pdf, August 2012

   [LU-BGP]     Rekhter, Y., E. Rosen, "Carrying Label Information in
                BGP-4", RFC-3107, May 2001.

























Singh, et al.            Expires April 24, 2014                [Page 22]


INTERNET DRAFT      Entropy label for seamless MPLS     October 21, 2013


Authors' Addresses


   Ravi Singh
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   EMail: ravis@juniper.net

   Yimin Shen
   Juniper Networks
   10 Technology Park Drive
   Westford, MA 01886
   US

   EMail: yshen@juniper.net

   John Drake
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   EMail: jdrake@juniper.net

























Singh, et al.            Expires April 24, 2014                [Page 23]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/