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Network Working Group Y. Rekhter Request for Comments: 3107 Juniper Networks Category: Standards Track E. Rosen

                                                   Cisco Systems, Inc.
                                                              May 2001

                Carrying Label Information in BGP-4

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

 This document specifies the way in which the label mapping
 information for a particular route is piggybacked in the same Border
 Gateway Protocol (BGP) Update message that is used to distribute the
 route itself.  When BGP is used to distribute a particular route, it
 can be also be used to distribute a Multiprotocol Label Switching
 (MPLS) label which is mapped to that route.

Table of Contents

  1      Specification of Requirements  ..........................   2
  2      Overview  ...............................................   2
  3      Carrying Label Mapping Information  .....................   3
  4      Advertising Multiple Routes to a Destination  ...........   4
  5      Capability Advertisement  ...............................   4
  6      When the BGP Peers are not Directly Adjacent  ...........   5
  7      Security Considerations  ................................   5
  8      Acknowledgments  ........................................   6
  9      References  .............................................   6
 10      Authors' Addresses  .....................................   7
 11      Full Copyright Statement  ...............................   8

Rekhter & Rosen Standards Track [Page 1] RFC 3107 Carrying Label Information in BGP-4 May 2001

1. Specification of Requirements

 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.

2. Overview

 When BGP is used to distribute a particular route, it can also be
 used to distribute an MPLS label that is mapped to that route [MPLS-
 ARCH].  This document specifies the way in which this is done.  The
 label mapping information for a particular route is piggybacked in
 the same BGP Update message that is used to distribute the route
 itself.

 This can be useful in the following situations:

If two immediately adjacent Label Switched Routers (LSRs) are

also BGP peers, then label distribution can be done without the

       need for any other label distribution protocol.

Suppose one's network consists of two "classes" of LSR:

exterior LSRs, which interface to other networks, and interior

       LSRs, which serve only to carry traffic between exterior LSRs.
       Suppose that the exterior LSRs are BGP speakers.  If the BGP
       speakers distribute MPLS labels to each other along with each
       route they distribute, then as long as the interior routers
       support MPLS, they need not receive any of the BGP routes from
       the BGP speakers.

       If exterior router A needs to send a packet to destination D,
       and A's BGP next hop for D is exterior router B, and B has
       mapped label L to D, then A first pushes L onto the packet's
       label stack.  A then consults its IGP to find the next hop to
       B, call it C.  If C has distributed to A an MPLS label for the
       route to B, A can push this label on the packet's label stack,
       and then send the packet to C.

 If a set of BGP speakers are exchanging routes via a Route Reflector
 [BGP-RR], then by piggybacking the label distribution on the route
 distribution, one is able to use the Route Reflector to distribute
 the labels as well.  This improves scalability quite significantly.
 Note that if the Route Reflector is not in the forwarding path, it
 need not even be capable of forwarding MPLS packets.

 Label distribution can be piggybacked in the BGP Update message by
 using the BGP-4 Multiprotocol Extensions attribute [RFC 2283].  The
 label is encoded into the NLRI field of the attribute, and the SAFI

Rekhter & Rosen Standards Track [Page 2] RFC 3107 Carrying Label Information in BGP-4 May 2001

 ("Subsequent Address Family Identifier") field is used to indicate
 that the NLRI contains a label.  A BGP speaker may not use BGP to
 send labels to a particular BGP peer unless that peer indicates,
 through BGP Capability Advertisement, that it can process Update
 messages with the specified SAFI field.

3. Carrying Label Mapping Information

 Label mapping information is carried as part of the Network Layer
 Reachability Information (NLRI) in the Multiprotocol Extensions
 attributes.  The AFI indicates, as usual, the address family of the
 associated route.  The fact that the NLRI contains a label is
 indicated by using SAFI value 4.

 The Network Layer Reachability information is encoded as one or more
 triples of the form <length, label, prefix>, whose fields are
 described below:

    +---------------------------+
    |   Length (1 octet)        |
    +---------------------------+
    |   Label (3 octets)        |
    +---------------------------+
    .............................
    +---------------------------+
    |   Prefix (variable)       |
    +---------------------------+

 The use and the meaning of these fields are as follows:

    a) Length:

       The Length field indicates the length in bits of the address
       prefix plus the label(s).

    b) Label:

       The Label field carries one or more labels (that corresponds to
       the stack of labels [MPLS-ENCAPS]).  Each label is encoded as 3
       octets, where the high-order 20 bits contain the label value,
       and the low order bit contains "Bottom of Stack" (as defined in
       [MPLS-ENCAPS]).

    c) Prefix:

       The Prefix field contains address prefixes followed by enough
       trailing bits to make the end of the field fall on an octet
       boundary.  Note that the value of trailing bits is irrelevant.

Rekhter & Rosen Standards Track [Page 3] RFC 3107 Carrying Label Information in BGP-4 May 2001

 The label(s) specified for a particular route (and associated with
 its address prefix) must be assigned by the LSR which is identified
 by the value of the Next Hop attribute of the route.

 When a BGP speaker redistributes a route, the label(s) assigned to
 that route must not be changed (except by omission), unless the
 speaker changes the value of the Next Hop attribute of the route.

 A BGP speaker can withdraw a previously advertised route (as well as
 the binding between this route and a label) by either (a) advertising
 a new route (and a label) with the same NLRI as the previously
 advertised route, or (b) listing the NLRI of the previously
 advertised route in the Withdrawn Routes field of an Update message.
 The label information carried (as part of NLRI) in the Withdrawn
 Routes field should be set to 0x800000.  (Of course, terminating the
 BGP session also withdraws all the previously advertised routes.)

4. Advertising Multiple Routes to a Destination

 A BGP speaker may maintain (and advertise to its peers) more than one
 route to a given destination, as long as each such route has its own
 label(s).

 The encoding described above allows a single BGP Update message to
 carry multiple routes, each with its own label(s).

 In the case where a BGP speaker advertises multiple routes to a
 destination, if a route is withdrawn, and a label(s) is specified at
 the time of withdrawal, only the corresponding route with the
 corresponding label is withdrawn.  If a route is withdrawn, and no
 label is specified at the time of withdrawal, then only the
 corresponding unlabeled route is withdrawn; the labeled routes are
 left in place.

5. Capability Advertisement

 A BGP speaker that uses Multiprotocol Extensions to carry label
 mapping information should use the Capabilities Optional Parameter,
 as defined in [BGP-CAP], to inform its peers about this capability.
 The MP_EXT Capability Code, as defined in [BGP-MP], is used to
 advertise the (AFI, SAFI) pairs available on a particular connection.

 A BGP speaker should not advertise this capability to another BGP
 speaker unless there is a Label Switched Path (LSP) between the two
 speakers.

Rekhter & Rosen Standards Track [Page 4] RFC 3107 Carrying Label Information in BGP-4 May 2001

 A BGP speaker that is capable of handling multiple routes to a
 destination (as described above) should use the Capabilities Optional
 Parameter, as defined in [BGP-CAP], to inform its peers about this
 capability.  The value of this capability is 4.

6. When the BGP Peers are not Directly Adjacent

 Consider the following LSR topology: A--B--C--D.  Suppose that D
 distributes a label L to A.  In this topology, A cannot simply push L
 onto a packet's label stack, and then send the resulting packet to B.
 D must be the only LSR that sees L at the top of the stack.  Before A
 sends the packet to B, it must push on another label, which was
 distributed by B.  B must replace this label with yet another label,
 which was distributed by C.  In other words, there must be an LSP
 between A and D.  If there is no such LSP, A cannot make use of label
 L.  This is true any time labels are distributed between non-adjacent
 LSRs, whether that distribution is done by BGP or by some other
 method.

 This document does NOT specify any procedure for ensuring in real
 time that label distribution between non-adjacent LSRs is done only
 when the appropriate MPLS infrastructure exists in the network or
 networks connecting the two LSRs.  Ensuring that the proper
 infrastructure exists is an issue for network management and
 operation.

7. Security Considerations

 When an LSR A is directly connected to an LSR B via a point-to-point
 interface, then when A receives packets over that interface, it knows
 that they come from B.  This makes it easy for A to discard any
 packets from B whose top labels are not among the labels that A
 distributed to B.  That is, A can easily ensure that B only uses
 those labels which it is entitled to use.  This technique can be used
 to prevent "label spoofing", i.e., the situation in which an LSR
 imposes a label which has not been properly distributed to it.

 The procedures discussed in this document would commonly be used when
 the label distribution peers are separated not merely by a point-to-
 point link, but by an MPLS network.  This means that when an LSR A
 processes a labeled packet, it really has no way to determine which
 other LSR B pushed on the top label.  Hence it cannot tell whether
 the label is one which B is entitled to use.  In fact, when Route
 Reflectors are in use, A may not even know the set of LSRs which
 receive its label mappings.  So the previous paragraph's technique
 for preventing label spoofing does not apply.

Rekhter & Rosen Standards Track [Page 5] RFC 3107 Carrying Label Information in BGP-4 May 2001

 It is possible though to use other techniques to avoid label spoofing
 problems.  If, for example, one never accepts labeled packets from
 the network's "external" interfaces, and all the BGP-distributed
 labels are advertised via IBGP, then there is no way for an untrusted
 router to put a labeled packet into the network.  One can generally
 assume that one's IBGP peers (or the IBGP peers of one's Route
 Reflector) will not attempt label spoofing, since they are all under
 the control of a single administration.

 This condition can actually be weakened significantly.  One doesn't
 need to refuse to accept all labeled packets from external
 interfaces.  One just needs to make sure that any labeled packet
 received on an external interface has a top label which was actually
 distributed out that interface.

 Then a label spoofing problem would only exist if there are both
 trusted and untrusted systems out the same interface.  One way to
 avoid this problem is simply to avoid this situation.

8. Acknowledgments

 Thanks to Ravi Chandra, Enke Chen, Srihari Ramachandra, Eric Gray and
 Liam Casey for their comments.

9. References

 [BGP-4]       Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
               (BGP-4)", RFC 1771, March 1995.

 [BGP-CAP]     Chandra, R. and J. Scudder, "Capabilities Advertisement
               with BGP-4", RFC 2842, May 2000.

 [BGP-MP]      Bates, T., Rekhter, Y, Chandra, R. and D. Katz,
               "Multiprotocol Extensions for BGP-4", RFC 2858, June
               2000.

 [BGP-RR]      Bates, T. and R. Chandra, "BGP Route Reflection: An
               alternative to full mesh IBGP", RFC 1966, June 1996.

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

 [MPLS-ENCAPS] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
               Farinacci, D., Li, T. and A. Conta, "MPLS Label Stack
               Encoding", RFC 3032, January 2001.

Rekhter & Rosen Standards Track [Page 6] RFC 3107 Carrying Label Information in BGP-4 May 2001

10. Authors' Addresses

 Yakov Rekhter
 Juniper Networks
 1194 N. Mathilda Avenue
 Sunnyvale, CA 94089

 EMail: yakov@juniper.net

 Eric Rosen
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA 01824

 EMail: erosen@cisco.com

Rekhter & Rosen Standards Track [Page 7] RFC 3107 Carrying Label Information in BGP-4 May 2001

11. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.

 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.

 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Rekhter & Rosen Standards Track [Page 8]