RFC 4813
OSPF Link-Local Signaling
RFC 4813 🎀 OSPF Link-Local Signaling
okay bestie, this one is literally about letting OSPF routers whisper arbitrary data to each other on a link 🦋 It's an Experimental RFC, published February 2007. The authors are B. Friedman, L. Nguyen, A. Roy, D. Yeung (all at Cisco Systems) and A. Zinin (Alcatel). The title block doesn't list anything it obsoletes or updates, so it's a fresh drop 💅
Status of This Memo 📜
This memo defines an Experimental Protocol for the Internet community. It is not an Internet standard of any kind, and 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 swap info on a link using packets that follow a well-defined format. The problem is that the OSPF packet format isn't flexible enough for apps to send arbitrary data, and sometimes that's exactly what's needed 🙈 This memo describes a vendor-specific, backward-compatible technique for link-local signaling (LLS), which is basically exchanging arbitrary data on a link. Iconic glow-up ✨
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 ......................................91. Introduction 🧚
okay so OSPF [RFC2328] packets are not very flexible, which makes it kinda hard to do opaque data transfer 😭 But it seems really useful to let OSPF routers do it anyway. One example: trading some capabilities on a link. Standard OSPF uses the Options field in Hello and Exchange packets for that, but there aren't that many bits left in there, so it's giving scarcity 🍓
One possible fix would be inventing a brand new packet type. The catch is that extra packets on the network may not be desirable, so this document goes a different route and explains how to exchange data using existing, standard OSPF packet types. Main character energy, no extra packets 💜
2. Proposed Solution 🎀
To do link-local signaling (LLS), OSPF routers tack a special data block onto the end of OSPF packets, or right after the authentication data block when cryptographic authentication is in use. Just like OSPF cryptographic authentication, the LLS block's length is not included in the OSPF packet length, but it is included in the IP packet length. Figure 1 shows 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 BlockThe LLS data block can ride along on two OSPF packet types: type 1 (OSPF Hello) and type 2 (OSPF DBD). Hello packets can go out at any moment, so LLS data in them can be used for dynamic signaling. The tradeoff is that delivery of LLS data in Hello packets is not guaranteed 🙈 Data sent with Database Description (DBD) packets, though, is guaranteed to be delivered as part of the adjacency forming process. Lowkey the most reliable vibe 🫶
This memo does not specify how OSPF routers should interpret the data sent through the LLS mechanism. 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 added to the OSPF Options field (see Figure 2). Its value 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 FieldL-bit 💅
This bit is set only in Hello and DBD packets. It is not set in OSPF Link State Advertisements (LSAs), and it may be used in those for different purposes 💀
2.2. LLS Data Block 🧁
The data block used for link-local signaling is formatted as described below (Figure 3 illustrates it) 👑
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 BlockChecksum 🔢
The Checksum field holds the standard IP checksum of the entire contents of the LLS block.
LLS Length 📏
The 16-bit LLS Data Length field holds the length (in 32-bit words) of the LLS block, header and payload included. 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 that 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 is 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 LLS data block is built out of TLVs. Figure 4 shows the TLV format 🎀
The Type field holds the TLV ID, which is unique for each type of TLV. The Length field holds the length of the Value field (in bytes). The Value field is variable and holds 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 TLVsNote that TLVs are always padded to a 32-bit boundary, but the padding bytes are not included in the TLV Length field. They are 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. Figure 5 shows the EO-TLV format 💖
From the LLS mechanism's point of view, the bits in the Value field don't have any semantics. This field may be used to announce some OSPF capabilities that are link-specific. Other OSPF extensions may also 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-TLVCurrently, [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 🔐
okay so this doc defines a special TLV for cryptographic authentication, aka the CA-TLV 💅. It should be included in the LLS block when cryptographic (MD5) authentication is enabled on the corresponding interface. The message digest of the LLS block should be calculated using the same key that's used for the main OSPF packet. The cryptographic sequence number lives inside 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 built like this, periodt 👑
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 Type value for CA-TLV is 2 🌸
the Length field in the header holds the length of the data portion of the TLV, which covers 4 bytes for the sequence number plus the length of the message digest (MD5) block for the whole LLS block in bytes. for MD5 that's always 16 bytes, so the AuthLen field ends up with a value of 20 ✨
the Sequence Number field holds the cryptographic sequence number that's used to stop simple replay attacks. for the LLS block to be considered authentic, the sequence number in the CA-TLV has to match the sequence number in the OSPF packet 🔥
the AuthData field holds the message digest calculated for the LLS data block 🦋
the CA-TLV can show up in the LLS block only once. also, when it's present, this TLV should be the last one in the LLS block 💜
3. Backward Compatibility ✨
the changes to OSPF packet formats play nicely with standard OSPF 🫶 because routers that aren't LLS-capable just won't look at the extra data after the packet. in other words, they'll ignore the LLS data block completely 🙈
4. Security Considerations 🔒
okay, serious moment 🥹 this function doesn't create any new security issues for the OSPF protocol. the technique gives you the same level of security as OSPF itself, because it lets LLS data be authenticated (see Section 2.4.2 for more details) 💖
5. IANA Considerations 📋
LLS TLV types are maintained by the IANA. any extension to OSPF that needs a new LLS TLV type must be reviewed by a designated expert from the routing area 🧚
following the policies in [RFC2434], LLS type values in the range of 0-32767 get 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, like so:
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 Usethis 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 want to give a shoutout to Russ White for reviewing this document 💅
Authors' Addresses 🏡
the five authors, all from Cisco Systems except Alex Zinin 💜
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.comFull Copyright Statement ©️
Copyright (C) The IETF Trust (2007) 🌷
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📦 where this page comes from
- ietf: rfc4813 2007-03
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