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Network Working Group                                         Y. RekhterRequest for Comments: 3107                              Juniper NetworksCategory: Standards Track                                       E. Rosen                                                     Cisco Systems, Inc.                                                                May 2001Carrying Label Information in BGP-4Status 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 Contents1      Specification of Requirements  ..........................22      Overview  ...............................................23      Carrying Label Mapping Information  .....................34      Advertising Multiple Routes to a Destination  ...........45      Capability Advertisement  ...............................46      When the BGP Peers are not Directly Adjacent  ...........57      Security Considerations  ................................58      Acknowledgments  ........................................69      References  .............................................610      Authors' Addresses  .....................................711      Full Copyright Statement  ...............................8Rekhter & Rosen             Standards Track                     [Page 1]

RFC 3107          Carrying Label Information in BGP-4           May 20011. 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 inRFC 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 SAFIRekhter & 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 200110. 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.comRekhter & Rosen             Standards Track                     [Page 7]

RFC 3107          Carrying Label Information in BGP-4           May 200111.  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]