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IETF RFC 5613
OSPF Link-Local Signaling
Last modified on Friday, August 14th, 2009
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Network Working Group A. Zinin
Request for Comments: 5613 Alcatel-Lucent
Obsoletes: 4813 A. Roy
Category: Standards Track L. Nguyen
Cisco Systems
B. Friedman
Google, Inc.
D. Yeung
Cisco Systems
August 2009
OSPF Link-Local Signaling
 Abstract
OSPF is a link-state intra-domain routing protocol. OSPF routers
exchange information on a link using packets that follow a well-
defined fixed format. The format is not flexible enough to enable
new features that need to exchange arbitrary data. This document
describes a backward-compatible technique to perform link-local
signaling, i.e., exchange arbitrary data on a link. This document
replaces the experimental specification published in RFC 4813 to
bring it on the Standards Track.
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) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Zinin, et al. Standards Track PAGE 1 
RFC 5613 OSPF Link-Local Signaling August 2009
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 2
2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 3
2.1. L-Bit in Options Field . . . . . . . . . . . . . . . . . . 4
2.2. LLS Data Block . . . . . . . . . . . . . . . . . . . . . . 4
2.3. LLS TLVs . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Extended Options and Flags TLV . . . . . . . . . . . . . . 5
2.5. Cryptographic Authentication TLV (OSPFv2 ONLY) . . . . . . 6
2.6. Private TLVs . . . . . . . . . . . . . . . . . . . . . . . 7
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4. Compatibility Issues . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Appendix B. Changes from RFC 4813 . . . . . . . . . . . . . . . . 11
1. Introduction
This document describes an extension to OSPFv2 [OSPFV2] and OSPFv3
[OSPFV3] allowing additional information to be exchanged between
routers on the same link. OSPFv2 and OSPFv3 packet formats are fixed
and do not allow for extension. This document proposes appending an
optional data block composed of Type/Length/Value (TLV) triplets to
existing OSPFv2 and OSPFv3 packets to carry this additional
information. Throughout this document, OSPF will be used when the
specification is applicable to both OSPFv2 and OSPFv3. Similarly,
OSPFv2 or OSPFv3 will be used when the text is protocol specific.
One potential way of solving this task could be introducing a new
packet type. However, that would mean introducing extra packets on
the network that may not be desirable and may cause backward
compatibility issues. This document describes how to exchange data
using standard OSPF packet types.
1.1. Requirements Notation
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 [KEY].
Zinin, et al. Standards Track PAGE 2
RFC 5613 OSPF Link-Local Signaling August 2009
2. Proposed Solution
To perform link-local signaling (LLS), OSPF routers add a special
data block to the end of OSPF packets or right after the
authentication data block when cryptographic authentication is used.
The length of the LLS block is not included into the length of the
OSPF packet, but is included in the IPv4/IPv6 packet length. Figure
1 illustrates how the LLS data block is attached.
+---------------------+ -- -- +---------------------+
| IP Header | ^ ^ | IPv6 Header |
| Length = HL+X+Y+Z | | Header Length | | Length = HL+X+Y |
| | v v | |
+---------------------+ -- -- +---------------------+
| OSPF Header | ^ ^ | OSPFv3 Header |
| Length = X | | | | Length = X |
|.....................| | X | X |.....................|
| | | | | |
| OSPFv2 Data | | | | OSPFv3 Data |
| | v v | |
+---------------------+ -- -- +---------------------+
| | ^ ^ | |
| Authentication Data | | Y | Y | LLS Data |
| | v v | |
+---------------------+ -- -- +---------------------+
| | ^
| LLS Data | | Z
| | v
+---------------------+ --
Figure 1: LLS Data Block in OSPFv2 and OSPFv3
The LLS block MAY be attached to OSPF Hello and Database Description
(DD) packets. The LLS block MUST NOT be attached to any other OSPF
packet types on generation and MUST be ignored on reception.
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
time. However, delivery of LLS data in Hello packets is not
guaranteed. The data sent with DD packets is guaranteed to be
delivered as part of the adjacency forming process.
This document does not specify how the data transmitted by the LLS
mechanism should be interpreted by OSPF routers. As routers that do
not understand LLS may receive these packets, changes made due to LLS
block TLV's do not affect the basic routing when interacting with
non-LLS routers.
Zinin, et al. Standards Track PAGE 3
RFC 5613 OSPF Link-Local Signaling August 2009
2.1. L-Bit in Options Field
A new L-bit (L stands for LLS) is introduced into the OSPF Options
field (see Figures 2a and 2b). Routers set the L-bit in Hello and DD
packets to indicate that the packet contains an LLS data block. In
other words, the LLS data block is only examined if the L-bit is set.
+---+---+---+---+---+---+---+---+
| * | O | DC| L |N/P| MC| E | * |
+---+---+---+---+---+---+---+-+-+
Figure 2a: OSPFv2 Options Field
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
| | | | | | | | | | | | | | |L|AF|*|*|DC| R| N|MC| E|V6|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
Figure 2b: OSPFv3 Options Field
The L-bit MUST NOT be set except in Hello and DD packets that contain
an LLS block.
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 LLS Data Block
The Checksum field contains the standard IP checksum for the entire
contents of the LLS block. Before computing the checksum, the
checksum field is set to 0. If the checksum is incorrect, the OSPF
packet MUST be processed, but the LLS block MUST be discarded.
Zinin, et al. Standards Track PAGE 4
RFC 5613 OSPF Link-Local Signaling August 2009
The 16-bit LLS Data Length field contains the length (in 32-bit
words) of the LLS block including the header and payload.
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, but is instead set
to 0.
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 an LLS data block are constructed using TLVs. See
Figure 4 for the TLV format.
The Type field contains the TLV ID, which is unique for each type of
TLV. The Length field contains the length of the Value field (in
bytes). The Value field 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 a 32-bit boundary, but padding
bytes are not included in the TLV Length field (though they are
included in the LLS Data Length field in the LLS block header).
Unrecognized TLV types are ignored.
2.4. Extended Options and Flags TLV
This subsection describes a TLV called the Extended Options and Flags
(EOF) TLV. The format of the EOF-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. Bits MAY be allocated to announce OSPF
link-local capabilities. Bits MAY also be allocated to perform
boolean link-local signaling.
Zinin, et al. Standards Track PAGE 5
RFC 5613 OSPF Link-Local Signaling August 2009
The length of the Value field in the EOF-TLV is 4 bytes.
The value of the Type field in the EOF-TLV is 1. The EOF-TLV MUST
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 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Options and Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Format of the EOF-TLV
Currently, [OOB] and [RESTART] use bits in the Extended Options field
of the EOF-TLV.
The Extended Options and Flags bits are defined in Section 3.
2.5. Cryptographic Authentication TLV (OSPFv2 ONLY)
This document defines a special TLV that is used for cryptographic
authentication (CA-TLV) of the LLS data block. This TLV MUST only be
included in the LLS block when cryptographic authentication is
enabled on the corresponding interface. The message digest of the
LLS block MUST be calculated using the same key and authentication
algorithm as used for the OSPFv2 packet. The cryptographic sequence
number is included in the TLV and MUST be the same as the one in the
OSPFv2 authentication data for the LLS block to be considered
authentic.
The TLV is constructed as shown in 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
Zinin, et al. Standards Track PAGE 6
RFC 5613 OSPF Link-Local Signaling August 2009
The value of the Type field for the CA-TLV is 2.
The Length field in the header contains the length of the data
portion of the TLV including 4 bytes for Sequence Number and the
length of the message digest block for the whole LLS block in bytes.
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 OSPFv2 packet header Authentication field
(which MUST be present). In the event of Sequence Number mismatch or
Authentication failure, the whole LLS block MUST be ignored.
The CA-TLV MUST NOT appear more than once in the LLS block. Also,
when present, this TLV MUST be the last TLV in the LLS block. If it
appears more than once, only the first occurrence is processed and
any others MUST be ignored.
The AuthData field contains the message digest calculated for the LLS
data block up to the CA-TLV AuthData field (i.e., excludes the CA-TLV
AuthData).
The CA-TLV is not applicable to OSPFv3 and it MUST NOT be added to
any OSPFv3 packet. If found on reception, this TLV MUST be ignored.
2.6. Private TLVs
LLS type values in the range of 32768-65536 are reserved for private
use. The first four octets of the Value field MUST be the private
enterprise code [ENTNUM]. This allows multiple vendor private
extensions to coexist in a network.
3. IANA Considerations
This document uses the registry that was originally created in
[RFC 4813]. IANA updated the following registry to point to this
document instead:
o "Open Shortest Path First (OSPF) Link-Local Signalling (LLS) -
Type/Length/Value Identifiers (TLV)"
IANA allocated L-bit in the "OSPFv2 Options Registry" and "OSPFv3
Options Registry" as per Section 2.1.
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.
Zinin, et al. Standards Track PAGE 7
RFC 5613 OSPF Link-Local Signaling August 2009
The criteria for allocating LLS TLVs are:
o LLS should not be used for information that would be better suited
to be advertised in a link-local link state advertisement (LSA).
o LLS should be confined to signaling between direct neighbors.
o Discretion should be used in the volume of information signaled
using LLS due to the obvious MTU and performance implications.
Following the policies outlined in [IANA], LLS type values in the
range of 0-32767 are allocated through an IETF Review and LLS type
values in the range of 32768-65535 are reserved for private use.
This document assigns the following LLS TLV types in OSPFv2/OSPFv3.
TLV Type Name Reference
0 Reserved
1 Extended Options and Flags [RFC 5613]
2 Cryptographic Authentication+ [RFC 5613]
3-32767 Reserved for assignment by the IANA
32768-65535 Private Use
+ Cryptographic Authentication TLV is only defined for OSPFv2
IANA renamed the sub-registry from "LLS Type 1 Extended Options" to
"LLS Type 1 Extended Options and Flags".
This document also assigns the following bits in the EOF-TLV outlined
in Section 2.5:
Bit Name Reference
0x00000001 LSDB Resynchronization (LR) [RFC 4811]
0x00000002 Restart Signal (RS-bit) [RFC 4812]
Future allocation of Extended Options and Flags bits MUST be reviewed
by a Designated Expert from the routing area.
Zinin, et al. Standards Track PAGE 8
RFC 5613 OSPF Link-Local Signaling August 2009
4. Compatibility Issues
The modifications to OSPF packet formats are compatible with standard
OSPF since OSPF routers not supporting LLS will ignore the LLS data
block after the OSPF packet or cryptographic message digest. As of
this writing, there are implementations deployed with [RFC 4813]-
compliant software. Routers not implementing [RFC 4813] ignore the
LLS data at the end of the OSPF packet.
Careful consideration should be given to carrying additional LLS
data, as it may affect the OSPF adjacency bring-up time due to
additional propagation delay and/or processing time.
5. Security Considerations
Security considerations inherited from OSPFv2 are described in
[OSPFV2]. This technique provides the same level of security as the
basic OSPFv2 protocol by allowing LLS data to be authenticated using
the same cryptographic authentication that OSPFv2 uses (see
Section 2.5 for more details).
Security considerations inherited from OSPFv3 are described in
[OSPFV3] and [OSPFV3AUTH]. OSPFv3 utilizes IPsec for authentication
and encryption. With IPsec, the AH (Authentication Header), ESP
(Encapsulating Security Payload), or both are applied to the entire
OSPFv3 payload including the LLS block.
6. References
6.1. Normative References
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
[KEY] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[OSPFV2] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
April 1998.
[OSPFV3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[OSPFV3AUTH] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, June 2006.
Zinin, et al. Standards Track PAGE 9
RFC 5613 OSPF Link-Local Signaling August 2009
6.2. Informative References
[ENTNUM] IANA, "PRIVATE ENTERPRISE NUMBERS",
http://www.iana.org.
[OOB] Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band
Link State Database (LSDB) Resynchronization",
RFC 4811, March 2007.
[RESTART] Nguyen, L., Roy, A., and A. Zinin, "OSPF Restart
Signaling", RFC 4812, March 2007.
[RFC 4813] Friedman, B., Nguyen, L., Roy, A., Yeung, D., and A.
Zinin, "OSPF Link-Local Signaling", RFC 4813,
March 2007.
Zinin, et al. Standards Track PAGE 10
RFC 5613 OSPF Link-Local Signaling August 2009
Appendix A. Acknowledgements
The authors would like to acknowledge Russ White, Acee Lindem, and
Manral Vishwas for their review of this document.
Appendix B. Changes from RFC 4813
This section describes the substantive change from [RFC 4813].
o Added OSPFv3 support
o Private TLVs MUST use private enterprise code
o Clarified requirement levels at several places
o Changed from Experimental to Standards Track
Zinin, et al. Standards Track PAGE 11
RFC 5613 OSPF Link-Local Signaling August 2009
Authors' Addresses
Alex Zinin
Alcatel-Lucent
Singapore
EMail: alex.zinin@alcatel-lucent.com
Abhay Roy
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: akr@cisco.com
Liem Nguyen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: lhnguyen@cisco.com
Barry Friedman
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
EMail: barryf@google.com
Derek Yeung
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: myeung@cisco.com
Zinin, et al. Standards Track PAGE 12
OSPF Link-Local Signaling
RFC TOTAL SIZE: 23092 bytes
PUBLICATION DATE: Friday, August 14th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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