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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.

Friedman, et al. Experimental [Page 1] RFC 4813 OSPF Link-Local Signaling February 2007

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.

Friedman, et al. Experimental [Page 2] RFC 4813 OSPF Link-Local Signaling February 2007

                       +---------------------+ --
                       | 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.

Friedman, et al. Experimental [Page 3] RFC 4813 OSPF Link-Local Signaling February 2007

                   | * | 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).

Friedman, et al. Experimental [Page 4] RFC 4813 OSPF Link-Local Signaling February 2007

 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.

Friedman, et al. Experimental [Page 5] RFC 4813 OSPF Link-Local Signaling February 2007

   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

Friedman, et al. Experimental [Page 6] RFC 4813 OSPF Link-Local Signaling February 2007

 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

Friedman, et al. Experimental [Page 7] RFC 4813 OSPF Link-Local Signaling February 2007

 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.

Friedman, et al. Experimental [Page 8] RFC 4813 OSPF Link-Local Signaling 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
 Liem Nguyen
 Cisco Systems
 225 West Tasman Drive
 San Jose, CA  95134
 Abhay Roy
 Cisco Systems
 225 West Tasman Drive
 San Jose, CA  95134
 Derek Yeung
 Cisco Systems
 225 West Tasman Drive
 San Jose, CA  95134
 Alex Zinin
 Sunnyvale, CA

Friedman, et al. Experimental [Page 9] RFC 4813 OSPF Link-Local Signaling February 2007

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 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Friedman, et al. Experimental [Page 10]

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