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RFC 4813 - OSPF Link-Local Signaling

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Network Working Group                                        B. Friedman
Request for Comments: 4813                                     L. Nguyen
Category: Experimental                                            A. Roy
                                                                D. Yeung
                                                           Cisco Systems
                                                                A. Zinin
                                                           February 2007

                       OSPF Link-Local Signaling

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   OSPF is a link-state intra-domain routing protocol used in IP
   networks.  OSPF routers exchange information on a link using packets
   that follow a well-defined format.  The format of OSPF packets is not
   flexible enough to enable applications to exchange arbitrary data,
   which may be necessary in certain situations.  This memo describes a
   vendor-specific, backward-compatible technique to perform link-local
   signaling, i.e., exchange arbitrary data on a link.

Table of Contents

   1. Introduction ....................................................2
   2. Proposed Solution ...............................................2
      2.1. Options Field ..............................................3
      2.2. LLS Data Block .............................................4
      2.3. LLS TLVs ...................................................5
      2.4. Predefined TLV .............................................5
           2.4.1. Extended Options TLV ................................5
           2.4.2. Cryptographic Authentication TLV ....................6
   3. Backward Compatibility ..........................................7
   4. Security Considerations .........................................7
   5. IANA Considerations .............................................7
   6. References ......................................................8
      6.1. Normative References .......................................8
      6.2. Informative References .....................................8
   Appendix A.  Acknowledgements ......................................9

1.  Introduction

   Formats of OSPF [RFC2328] packets are not very flexible to provide an
   acceptable mechanism for opaque data transfer.  However, this appears
   to be very useful to allow OSPF routers to do so.  An example where
   such a technique could be used is exchanging some capabilities on a
   link (standard OSPF utilizes the Options field in Hello and Exchange
   packets, but there are not so many bits left in it).

   One potential way of solving this task could be introducing a new
   packet type.  However, that would mean introducing extra packets on
   the network, which may not be desirable, so this document describes
   how to exchange data using existing, standard OSPF packet types.

2.  Proposed Solution

   To perform link-local signaling (LLS), OSPF routers add a special
   data block at the end of OSPF packets or right after the
   authentication data block when cryptographic authentication is used.
   Like with OSPF cryptographic authentication, the length of the LLS-
   block is not included into the length of OSPF packet, but is included
   in the IP packet length.  Figure 1 illustrates how the LLS data block
   is attached.

                         +---------------------+ --
                         | IP Header           | ^
                         | Length = HL+X+Y+Z   | | Header Length
                         |                     | v
                         +---------------------+ --
                         | OSPF Header         | ^
                         | Length = X          | |
                         |.....................| | X
                         |                     | |
                         | OSPF Data           | |
                         |                     | v
                         +---------------------+ --
                         |                     | ^
                         | Authentication Data | | Y
                         |                     | v
                         +---------------------+ --
                         |                     | ^
                         |  LLS Data           | | Z
                         |                     | v
                         +---------------------+ --

                    Figure 1: Attaching LLS Data Block

   The LLS data block may be attached to OSPF packets of two types --
   type 1 (OSPF Hello), and type 2 (OSPF DBD).  The data included in the
   LLS block attached to a Hello packet may be used for dynamic
   signaling, since Hello packets may be sent at any moment in time.
   However, delivery of LLS data in Hello packets is not guaranteed.
   The data sent with Database Description (DBD) packets is guaranteed
   to be delivered as part of the adjacency forming process.

   This memo does not specify how the data transmitted by the LLS
   mechanism should be interpreted by OSPF routers.  The interface
   between the OSPF LLS component and its clients is implementation-

2.1.  Options Field

   A new bit, called L (L stands for LLS), is introduced to the OSPF
   Options field (see Figure 2).  The value of the bit is 0x10.  Routers
   set the L-bit in Hello and DBD packets to indicate that the packet
   contains the LLS data block.

                     | * | O | DC| L |N/P| MC| E | * |

                        Figure 2: The Options Field


      This bit is set only in Hello and DBD packets.  It is not set in
      OSPF Link State Advertisements (LSAs) and may be used in them for
      different purposes.

2.2.  LLS Data Block

   The data block used for link-local signaling is formatted as
   described below (see Figure 3 for illustration).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |            Checksum           |       LLS Data Length         |
    |                                                               |
    |                           LLS TLVs                            |
    .                                                               .
    .                                                               .
    .                                                               .

                  Figure 3: Format of the LLS Data Block


      The Checksum field contains the standard IP checksum of the entire
      contents of the LLS block.

   LLS Length

      The 16-bit LLS Data Length field contains the length (in 32-bit
      words) of the LLS block including the header and payload.
      Implementations should not use the Length field in the IP packet
      header to determine the length of the LLS data block.

   Note that if the OSPF packet is cryptographically authenticated, the
   LLS data block must also be cryptographically authenticated.  In this
   case, the regular LLS checksum is not calculated and the LLS block
   will contain a cryptographic authentication TLV (see Section 2.4.2).

   The rest of the block contains a set of Type/Length/Value (TLV)
   triplets as described in Section 2.3.  All TLVs must be 32-bit
   aligned (with padding if necessary).

2.3.  LLS TLVs

   The contents of the LLS data block is constructed using TLVs.  See
   Figure 4 for the TLV format.

   The Type field contains the TLV ID that is unique for each type of
   TLVs.  The Length field contains the length of the Value field (in
   bytes) that is variable and contains arbitrary data.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |            Type               |           Length              |
    |                                                               |
    .                                                               .
    .                             Value                             .
    .                                                               .

                       Figure 4: Format of LLS TLVs

   Note that TLVs are always padded to 32-bit boundary, but padding
   bytes are not included in the TLV Length field (though it is included
   in the LLS Data Length field of the LLS block header).

2.4.  Predefined TLV

2.4.1.  Extended Options TLV

   This subsection describes a TLV called Extended Options (EO) TLV.
   The format of EO-TLV is shown in Figure 5.

   Bits in the Value field do not have any semantics from the point of
   view of the LLS mechanism.  This field may be used to announce some
   OSPF capabilities that are link-specific.  Also, other OSPF
   extensions may allocate bits in the bit vector to perform boolean
   link-local signaling.

   The length of the Value field in EO-TLV is 4 bytes.

   The value of the Type field in EO-TLV is 1.

   EO-TLV should only appear once in the LLS data block.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |             1                 |           Length              |
    |                       Extended Options                        |

                        Figure 5: Format of EO-TLV

   Currently, [RFC4811] and [RFC4812] use bits in the Extended Options
   field of the EO-TLV.  The Extended Options bits are also defined in
   Section 5.

2.4.2.  Cryptographic Authentication TLV

   This document defines a special TLV that is used for cryptographic
   authentication (CA-TLV) of the LLS data block.  This TLV should be
   included in the LLS block when the cryptographic (MD5) authentication
   is enabled on the corresponding interface.  The message digest of the
   LLS block should be calculated using the same key as that used for
   the main OSPF packet.  The cryptographic sequence number is included
   in the TLV and must be the same as the one in the main OSPF packet
   for the LLS block to be considered authentic.

   The TLV is constructed as shown Figure 6.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |              2                |         AuthLen               |
    |                         Sequence Number                       |
    |                                                               |
    .                                                               .
    .                           AuthData                            .
    .                                                               .

           Figure 6: Format of Cryptographic Authentication TLV

   The value of the Type field for CA-TLV is 2.

   The Length field in the header contains the length of the data
   portion of the TLV that includes 4 bytes for the sequence number and
   the length of the message digest (MD5) block for the whole LLS block

   in bytes (this will always be 16 bytes for MD5).  So the AuthLen
   field will have value of 20.

   The Sequence Number field contains the cryptographic sequence number
   that is used to prevent simple replay attacks.  For the LLS block to
   be considered authentic, the sequence number in the CA-TLV must match
   the sequence number in the OSPF packet.

   The AuthData field contains the message digest calculated for the LLS
   data block.

   The CA-TLV may appear in the LLS block only once.  Also, when
   present, this TLV should be the last in the LLS block.

3.  Backward Compatibility

   The modifications to OSPF packet formats are compatible with standard
   OSPF because LLS-incapable routers will not consider the extra data
   after the packet; i.e., the LLS data block will be ignored by routers
   that do not support the LLS extension.

4.  Security Considerations

   The function described in this document does not create any new
   security issues for the OSPF protocol.  The described technique
   provides the same level of security as the OSPF protocol by allowing
   LLS data to be authenticated (see Section 2.4.2 for more details).

5.  IANA Considerations

   LLS TLV types are maintained by the IANA.  Extensions to OSPF that
   require a new LLS TLV type must be reviewed by a designated expert
   from the routing area.

   Following the policies outlined in [RFC2434], LLS type values in the
   range of 0-32767 are allocated through an IETF consensus action, and
   LLS type values in the range of 32768-65536 are reserved for private
   and experimental use.

   This document assigns LLS types 1 and 2, as follows:

        LLS Type    Name                                      Reference
            0       Reserved
            1       Extended Options                          [RFC4813]
            2       Cryptographic Authentication              [RFC4813]
            3-32767 Reserved for assignment by the IANA
        32768-65535 Private Use

   This document also assigns the following bits for the Extended
   Options bits field in the EO-TLV outlined in Section 2.4.1:

        Extended Options Bit      Name                        Reference
          0x00000001              LSDB Resynchronization (LR) [RFC4811]
          0x00000002              Restart Signal (RS-bit)     [RFC4812]

   Other Extended Options bits will be allocated through an IETF
   consensus action.

6.  References

6.1.  Normative References

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

6.2.  Informative References

   [RFC4811]  Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band Link
              State Database (LSDB) Resynchronization", RFC 4811,
              February 2007.

   [RFC4812]  Nguyen, L., Roy, A., and A. Zinin, "OSPF Restart
              Signaling", RFC 4812, February 2007.

Appendix A.  Acknowledgments

   The authors would like to acknowledge Russ White for his review of
   this document.

Authors' Addresses

   Barry Friedman
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA  95134
   EMail: friedman@cisco.com

   Liem Nguyen
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA  95134
   EMail: lhnguyen@cisco.com

   Abhay Roy
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA  95134
   EMail: akr@cisco.com

   Derek Yeung
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA  95134
   EMail: myeung@cisco.com

   Alex Zinin
   Sunnyvale, CA
   EMail: zinin@psg.com

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