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Versions: (draft-napierala-mpls-targeted-mldp) 00 01 02 03 04 RFC 7060

MPLS Working Group                                       Maria Napierala
Internet Draft                                                      AT&T
Intended Status: Standards Track
Expires: March 4, 2014                                     Eric C. Rosen
                                                       IJsbrand Wijnands
                                                     Cisco Systems, Inc.

                                                       September 4, 2013


        Using LDP Multipoint Extensions on Targeted LDP Sessions


                  draft-ietf-mpls-targeted-mldp-04.txt

Abstract

   Label Distribution Protocol (LDP) can be used to set up Point-to-
   Multipoint (P2MP) and Multipoint-to-Multipoint (MP2MP) Label Switched
   Paths.  However, the specification for the Multipoint Extensions to
   LDP presupposes that the two endpoints of an LDP session are directly
   connected.  The LDP base specification allows for the case where the
   two endpoints of an LDP session are not directly connected; such a
   session is known as a "Targeted LDP" session.  This document provides
   the specification for using the LDP Multipoint Extensions over a
   Targeted LDP session.


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
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   http://www.ietf.org/ietf/1id-abstracts.txt.

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



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Copyright and License Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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  ..........................................   3
 2          Targeted mLDP and the Upstream LSR  ....................   3
 2.1        Selecting the Upstream LSR  ............................   3
 2.2        Sending data from U to D  ..............................   4
 3          Applicability of Targeted mLDP  ........................   5
 4          LDP Capabilities  ......................................   5
 5          Targeted mLDP with Unicast Replication  ................   6
 6          Targeted mLDP with Multicast Tunneling  ................   7
 7          IANA Considerations  ...................................   8
 8          Security Considerations  ...............................   8
 9          Acknowledgments  .......................................   8
10          Authors' Addresses  ....................................   9
11          Normative References  ..................................   9
















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1. Introduction

   The Label Distribution Protocol (LDP) extensions for setting up
   Point-to-MultiPoint (P2MP) Label Switched Paths (LSPs) and
   Multipoint-to-Multipoint (MP2MP) LSPs are specified in [mLDP].  This
   set of extensions is generally known as "Multipoint LDP" (mLDP).

   A pair of Label Switched Routers (LSRs) that are the endpoints of an
   LDP session are considered to be "LDP peers".  When a pair of LDP
   peers are "directly connected" (e.g., they are connected by a layer 2
   medium, or are otherwise considered to be neighbors by the network's
   interior routing protocol), the LDP session is said to be a "directly
   connected" LDP session.  When the pair of LDP peers are not directly
   connected, the session between them is said to be a "Targeted" LDP
   session.

   The base specification for mLDP does not explicitly cover the case
   where the LDP multipoint extensions are used over a targeted LDP
   session.  This document provides that specification.

   We will use the term "Multipoint" to mean "either P2MP or MP2MP".

   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 [RFC2119].


2. Targeted mLDP and the Upstream LSR

2.1. Selecting the Upstream LSR

   In mLDP, a multipoint LSP (MP-LSP) has a unique identifier that is an
   ordered pair of the form <root, opaque value>.  The first element of
   the ordered pair is the IP address of the MP-LSP's "root node".  The
   second element of the ordered pair is an identifier that is unique in
   the context of the root node.

   If LSR D is setting up the MP-LSP <R, X>, D must determine the
   "upstream LSR" for <R, X>.  In [mLDP], the upstream LSR for <R, X>,
   U, is defined to be the "next hop" on D's path to R, and "next hop"
   is tacitly assumed to mean "IGP next hop".  It is thus assumed that
   there is a direct LDP session between D and U.  In this
   specification, we extend the notion of "upstream LSR" to cover the
   following cases:







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     - U is the "BGP next hop" on D's path to R, where U and D are not
       necessarily IGP neighbors, and where there is a Targeted LDP
       session between U and D.  In this case, we allow D to select U as
       the "upstream LSR" for <R,X>.

     - If the "next hop interface" on D's path to R is an RSVP-TE P2P
       tunnel whose remote endpoint is U, and if there is known to be an
       RSVP-TE P2P tunnel from U to D, and if there is a Targeted LDP
       session between U and D, then we allow D to select U as the
       "upstream LSR" for <R,X>.  This is useful when D and U are part
       of a network area that is fully meshed via RSVP-TE P2P tunnels.

   The particular method used to select an "upstream LSR" is determined
   by the Service Provider (SP), and must be made known a priori (i.e.,
   by provisioning) to all the LSRs involved.

   Other methods than the two specified above MAY be used; however the
   specification of other methods is outside the scope of this document.


2.2. Sending data from U to D

   By using Targeted mLDP, we can construct an MP-LSP <R,X> containing
   an LSR U, where U has one or more downstream LSR neighbors (D1, ...,
   Dn) to which it is not directly connected.  In order for a data
   packet to travel along this MP-LSP, U must have some way of
   transmitting the packet to D1, ..., Dn.  We will cover two methods of
   transmission:

     - Unicast Replication.

       In this method, U creates n copies of the packet, and unicasts
       each copy to exactly one of D1, ..., Dn.

     - Multicast tunneling.

       In this method, U becomes the root node of a multicast tunnel,
       with D1, ..., Dn as leaf nodes.  When a packet traveling along
       the MP-LSP <R,X> arrives at U, U transmits it through the
       multicast tunnel, and as a result it arrives at D1, ..., Dn.

       When this method is used, it may be desirable to carry traffic of
       multiple MP-LSPs through a single multicast tunnel.  We specify
       procedures that allow for the proper demultiplexing of the MP-
       LSPs at the leaf nodes of the multicast tunnel.  We do not assume
       that all the leaf nodes of the tunnel are on all the MP-LSPs
       traveling through the tunnel; thus some of the tunnel leaf nodes
       may need to discard some of the packets received through the



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       tunnel.  For example, suppose MP-LSP <R1,X1> contains node U with
       downstream LSRs D1 and D2, while MP-LSP <R2,X2> contains node U
       with downstream LSRs D2 and D3.  Suppose also that there is a
       multicast tunnel with U as root and with D1, D2, and D3 as leaf
       nodes.  U can aggregate both MP-LSPs in this one tunnel.
       However, D1 will have to discard packets that are traveling on
       <R2,X1>, while D3 will have to discard packets that are traveling
       on <R1,X2>.


3. Applicability of Targeted mLDP

   When LSR D is setting up MP-LSP <R,X>, it MUST NOT use targeted mLDP
   unless D implements a procedure that can select, as the "upstream
   LSR" for <R,X>, an LSR U that is a Targeted mLDP peer of D.  See
   section 2.1.

   Whether D uses Targeted mLDP when this condition holds is determined
   by provisioning, or by other methods that are outside the scope of
   this specification.

   When Targeted mLDP is used, the choice between unicast replication
   and multicast tunneling is determined by provisioning, or by other
   methods that are outside the scope of this specification.  It is
   presupposed that all nodes will have a priori knowledge of whether to
   use unicast replication or to use multicast tunneling.  If the
   latter, it is presupposed that all nodes will have a priori knowledge
   of the type of multicast tunneling to use.


4. LDP Capabilities

   Per [mLDP], any LSR that needs to set up an MP-LSP must support the
   procedures of [LDP-CAP], and in particular must send and receive the
   P2MP Capability and/or the MP2MP Capability.  This specification does
   not define any new capabilities; the advertisement of the P2MP and/or
   MP2MP Capabilities on a Targeted LDP session means that the
   advertising LSR is capable of following the procedures of this
   document.

   Some of the procedures of this document require the use of upstream-
   assigned labels [LDP-UP].  In order to use upstream-assigned labels
   as part of Targeted mLDP, an LSR must advertise the LDP Upstream-
   Assigned Label Capability [LDP-UP] on the Targeted LDP session.







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5. Targeted mLDP with Unicast Replication

   When unicast replication is used, the mLDP procedures are exactly the
   same as described in [mLDP], with the following exception.  If LSR D
   is setting up MP-LSP <R,X>, its "upstream LSR" is selected according
   to the procedures of section 2.1, and is not necessarily the "IGP
   next hop" on D's path to R.

   Suppose that LSRs D1 and D2 are both setting up the P2MP MP-LSP
   <R,X>, and that LSR U is the upstream LSR on each of their paths to
   R.  D1 and D2 each binds a label to <R,X>, and each uses a label
   mapping message to inform U of the label binding.  Suppose D1 has
   assigned label L1 to <R,X> and D2 has assigned label L2 to <R,X>.
   (Note that L1 and L2 could have the same value or different values;
   D1 and D2 do not coordinate their label assignments.)  When U has a
   packet to transmit on the MP-LSP <R,X>, it makes a copy of the
   packet, pushes on label L1, and unicasts the resulting packet to D1.
   It also makes a second copy of the packet, pushes on label L2, and
   then unicasts the resulting packet to D2.

   This procedure also works when the MP-LSP <R,X> is a MP2MP LSP.
   Suppose that in addition to labels L1 and L2 described above, U has
   assigned label L3 for <R,X> traffic received from D1, and label L4
   for <R,X> traffic received from D2.  When U processes a packet with
   label L3 at the top of its label stack, it knows the packet is from
   D1, so U sends a unicast copy of the packet to D2, after swapping L3
   for L2.  U does not send a copy back to D1.

   Note that all labels used in this procedure are downstream-assigned
   labels.

   The method of unicast is a local matter, outside the scope of this
   specification.  The only requirement is that D1 will receive the copy
   of the packet carrying label L1, and that D1 will process the packet
   by looking up label L1.  (And similarly, D2 must receive the copy of
   the packet carrying label L2, and must process the packet by looking
   up label L2.)

   Note that if the method of unicast is MPLS, U will need to push
   another label on each copy of the packet before transmitting it.
   This label needs to ensure that delivery of the packet to the
   appropriate LSR, D1 or D2.  Use of penultimate-hop popping for that
   label is perfectly legitimate.








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6. Targeted mLDP with Multicast Tunneling

   Suppose that LSRs D1 and D2 are both setting up MP-LSP <R,X>, and
   that LSR U is the upstream LSR on each of their paths to R.  Since
   multicast tunneling is being used, when U has a packet to send on
   this MP-LSP, it does not necessarily send two copies, one to D1 and
   one to D2.  It may send only one copy of the packet, which will get
   replicated somewhere downstream in the multicast tunnel.  Therefore,
   the label that gets bound to the MP-LSP must be an upstream-assigned
   label, assigned by U.  This requires a change from the procedures of
   [mLDP].  D1 and D2 do not send label mapping messages to U; instead
   they send label request messages to U, following the procedures of
   Section 4 of [LDP-UP], asking U to assign a label to the MP-LSP
   <R,X>.  U responds with a label mapping message containing an
   upstream-assigned label, L (using the procedures specified in [LDP-
   UP]).  As part of the same label mapping message, U also sends an
   Interface TLV (as specified in [LDP-UP]) identifying the multicast
   tunnel in which data on the MP-LSP will be carried.  When U transmits
   a packet on this tunnel, it first pushes on the upstream-assigned
   label L, and then pushes on the label that corresponds to the
   multicast tunnel.

   If the numerical value L of the upstream-assigned label is the value
   3, defined in [LDP] and [RFC3032] as "Implicit NULL", then the
   specified multicast tunnel will carry only the specified MP-LSP.
   That is, aggregation of multiple MP-LSPs into a single multicast
   tunnel is not being done.  In this case, no upstream-assigned label
   is pushed onto a packet that is transmitted through the multicast
   tunnel.

   Various types of multicast tunnel may be used.  The choice of tunnel
   type is determined by provisioning, or by some other method that is
   outside the scope of this document.  [LDP-UP] specifies encodings
   allowing U to identify an mLDP MP-LSP, and RSVP-TE P2MP LSP, as well
   as other types of multicast tunnel.

   Procedures for tunneling MP2MP LSPs through P2MP or MP2MP LSPs are
   outside the scope of this document.

   If the multicast tunnel is an mLDP MP-LSP or an RSVP-TE P2MP LSP,
   when U transmits a packet on the MP-LSP <R,X>, the upstream-assigned
   label L will be the second label in the label stack.  Penultimate-hop
   popping MUST NOT be done, because the top label provides the context
   in which the second label is to be interpreted.  See [RFC5331].

   When LSR U uses these procedures to inform LSR D that a particular
   MP-LSP is being carried in a particular multicast tunnel, U and D
   MUST take appropriate steps to ensure that packets U sends into this



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   tunnel will be received by D. The exact steps to take depend on the
   tunnel type.  As long as U is D's upstream LSR for any MP-LSP that
   has been assigned to this tunnel, D must remain joined to the tunnel.

   Note that U MAY assign the same multicast tunnel for multiple
   different MP-LSPs.  However, U MUST assign a distinct upstream-
   assigned label to each MP-LSP.  This allows the packets traveling
   through the tunnel to be demultiplexed into the proper MP-LSPs.

   If U has an MP-LSP <R1,X1> with downstream LSRs D1 and D2, and an MP-
   LSP <R2,X2> with downstream LSRs D2 and D3, U may assign both MP-LSPs
   to the same multicast tunnel.  In this case, D3 will receive packets
   traveling on <R1,X1>.  However, the upstream-assigned label carried
   by those packets will not be recognized by D3, hence D3 will discard
   those packets.  Similarly, D1 will discard the <R2,X2> packets.

   This document does not specify any rules for deciding whether to
   aggregate two or more MP-LSPs into a single multicast tunnel.  Such
   rules are outside the scope of this document.

   Except for the procedures explicitly detailed in this document, the
   procedures of [mLDP] and [LDP-UP] apply unchanged.



7. IANA Considerations

   This document has no considerations for IANA.



8. Security Considerations

   This document raises no new security considerations beyond those
   discussed in [LDP], [LDP-UP], and [RFC5331].


9. Acknowledgments

   The authors wish to thank Lizhong Jin and Lizhen Bin for their
   comments.










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10. Authors' Addresses

   Maria Napierala
   AT&T Labs
   200 Laurel Avenue, Middletown, NJ 07748
   USA
   E-mail: mnapierala@att.com



   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA, 01719
   USA
   E-mail: erosen@cisco.com




   IJsbrand Wijnands
   Cisco Systems, Inc.
   De kleetlaan 6a Diegem 1831
   Belgium
   E-mail: ice@cisco.com



11. Normative References

   [LDP] Loa Andersson, Ina Minei, Bob Thomas, editors, "LDP
   Specification", RFC 5036, October 2007

   [LDP-CAP] Bob Thomas, Kamran Raza, Shivani Aggarwal, Rahul Aggarwal,
   Jean-Louis Le Roux, "LDP Capabilities", RFC 5561, July 2009

   [mLDP] IJsbrand Wijnands, Ina Minei, Kireeti Kompella, Bob Thomas,
   "Label Distribution Protocol Extensions for Point-to-Multipoint and
   Multipoint-to-Multipoint Label Switched Paths", RFC 6388, November
   2011

   [LDP-UP] Rahul Aggarwal, Jean-Louis Le Roux, "MPLS Upstream Label
   Assignment for LDP", RFC 6389, November 2011

   [RFC2119] "Key words for use in RFCs to Indicate Requirement
   Levels.", Bradner, March 1997

   [RFC3032] Eric Rosen, et. al., "MPLS Label Stack Encoding", RFC 3032,



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   January 2001

   [RFC5331] Rahul Aggarwal, Yakov Rekhter, Eric Rosen, "MPLS Upstream
   Label Assignment and Context-Specific Label Space", RFC 5331, August
   2009














































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