RFC 4813 EXPERIMENTAL ROUTING · OSPF · SIGNALING
OSPF Link-Local Signaling
Attach extra data to OSPF packets for link-local exchange of capabilities.
RFC EDITOR
Friedman, B., Nguyen, L., Roy, A., Yeung, D., and A. Zinin, "OSPF Link-Local Signaling", RFC 4813, DOI 10.17487/RFC4813, March 2007, <https://www.rfc-editor.org/info/rfc4813>.
BIBTEX
@misc{rfc4813,
series = {Request for Comments},
number = 4813,
howpublished = {RFC 4813},
publisher = {RFC Editor},
doi = {10.17487/RFC4813},
url = {https://www.rfc-editor.org/info/rfc4813},
author = {B. Friedman and L. Nguyen and A. Roy and D. Yeung and A. Zinin},
title = {{OSPF Link-Local Signaling}},
pagetotal = 10,
year = 2007,
month = mar,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Network Working Group B. Friedman
Request for Comments: 4813 L. Nguyen
Category: Experimental A. Roy
D. Yeung
Cisco Systems
A. Zinin
Alcatel
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).
Abstract
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-
specific.
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
L-bit
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
Checksum
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
USA
EMail: friedman@cisco.com
Liem Nguyen
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: lhnguyen@cisco.com
Abhay Roy
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: akr@cisco.com
Derek Yeung
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: myeung@cisco.com
Alex Zinin
Alcatel
Sunnyvale, CA
USA
EMail: zinin@psg.com
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AI ELI5
On this page
PROBLEMStandard OSPF packet formats have limited room for routers to exchange information about capabilities or operational state. The existing Options field in OSPF messages has few available bits left.
MECHANISMLLS adds an optional data block at the end of OSPF Hello and Database Description packets. This block contains type-length-value (TLV) fields that vendors and extensions can populate with arbitrary data without breaking standard OSPF.
WHY IT MATTERSRouters can now exchange vendor-specific information and signal capabilities on a link. The extension is backward-compatible because non-LLS routers simply ignore the extra data.
§1The Problem of Inflexible OSPF Packet Formats
OSPF is a link-state routing protocol where routers exchange information to build a consistent map of the network. The OSPF packet format is well-defined and rigid. Hello packets and Database Description (DBD) packets include an Options field, but this field has limited space and most bits are already assigned for standard OSPF features.
As OSPF evolves, implementers and vendors sometimes need to exchange additional information on a link without creating new OSPF packet types or modifying core message structures. RFC 4813 solves this by introducing a Link-Local Signaling (LLS) extension that adds an optional data block at the end of existing OSPF packets.
OSPF messages follow a strict format with no room for notes. This RFC lets you add a note at the end that new routers read and old ones ignore.
§2LLS Data Block Structure and Format
The LLS data block is appended after the standard OSPF message and any authentication data, but is included in the IP packet length. The block begins with a checksum field and an LLS Data Length field (in 32-bit words), followed by a series of TLV triplets.
Each TLV contains a Type field identifying the data kind, a Length field for the value size in bytes, and a variable-length Value field containing the actual data. All TLVs are padded to 32-bit boundaries, though padding bytes are not counted in the TLV length field. This structure allows vendors to define custom TLVs for their own purposes.
The data block starts with a checksum and length, then has labeled sections (TLVs) where each section says what it is and how long it is.
§3The L-bit and Predefined TLVs
To signal that an OSPF message carries LLS data, a new L-bit is defined in the Options field with value 0x10. Routers set this bit in Hello and DBD packets when they include the LLS data block. Routers that do not support LLS simply ignore the bit and the extra data.
The RFC defines two standard TLV types: the Extended Options (EO) TLV for announcing link-specific capabilities using a 32-bit flags field, and the Cryptographic Authentication (CA) TLV for signing the LLS block when OSPF cryptographic authentication is enabled. The CA-TLV includes a sequence number matching the OSPF packet's sequence number to prevent replay attacks.
The L-bit is a flag saying "I have extra data attached." Standard TLVs exist for extra options and for cryptographic signatures.
§4Backward Compatibility and Extensibility
LLS is completely backward-compatible. Routers that support LLS set the L-bit and routers that do not support it simply ignore the flag and the extra data block. There is no requirement for all routers in a network to support LLS; they can coexist peacefully on the same link.
The RFC reserves LLS type values 0-32767 for IETF-consensus allocation and values 32768-65535 for private and experimental use. This allows vendors to define proprietary TLVs and experimenters to test new LLS extensions without worrying about collisions. Future OSPF extensions can allocate their own TLV types as needed.
Old routers ignore the extra data. New routers can add their own TLVs in a reserved space. Everyone gets along.
§5References
[1]RFC 2328Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.April 1998[2]RFC 2434Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.October 1998[3]RFC 4811Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band Link State Database (LSDB) Resynchronization", RFC 4811, February 2007.March 2007[4]RFC 4812Nguyen, L., Roy, A., and A. Zinin, "OSPF Restart Signaling", RFC 4812, February 2007.March 2007