Optional IS-IS checksums: Optional Checksums in Intermediate System to Intermediate System (ISIS)
Adding integrity checks to IS-IS routing protocol packets.
Przygienda, T., "Optional Checksums in Intermediate System to Intermediate System (ISIS)", RFC 3358, DOI 10.17487/RFC3358, August 2002, <https://www.rfc-editor.org/info/rfc3358>.
@misc{rfc3358,
series = {Request for Comments},
number = 3358,
howpublished = {RFC 3358},
publisher = {RFC Editor},
doi = {10.17487/RFC3358},
url = {https://www.rfc-editor.org/info/rfc3358},
author = {T. Przygienda},
title = {{Optional Checksums in Intermediate System to Intermediate System (ISIS)}},
pagetotal = 4,
year = 2002,
month = aug,
} RFC text
Contents
Network Working Group T. Przygienda
Request for Comments: 3358 Xebeo
Category: Informational August 2002
Optional Checksums in
Intermediate System to Intermediate System (ISIS)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes an optional extension to the Intermediate
System to Intermediate System (ISIS) protocol, used today by several
Internet Service Proviers (ISPs) for routing within their clouds.
ISIS is an interior gateway routing protocol developed originally by
OSI and used with IP extensions as Interior Gateway Protocol (IGP).
ISIS originally does not provide Complete Sequence Numbers Protocol
Data (CSNP) and Partial Sequence Numbers Protocol Data Unit (PSNP)
checksums, relying on the underlying layers to verify the integrity
of information provided. Experience with the protocol shows that
this precondition does not always hold and scenarios can be imagined
that impact protocol functionality. This document introduces a new
optional Type, Length and Value (TLV) providing checksums.
1. Introduction
ISIS [ISO90, Cal90a, Cal90b] CSNPs and PSNPs and IIHs can be
corrupted in case of faulty implementations of L2 hardware or lack of
checksuming on a specific network technology. As a particularly ugly
case, corruption of length and/or TLV length fields may lead to the
generation of extensive numbers of "empty" LSPs in the receiving
node. Since we cannot rely on authentication as a checksum
mechanism, this document proposes an optional TLV to add checksums to
the elements.
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 [Bra97].
2. TLV Description
This optional TLV MAY BE included in all CSNP, PSNP and IIH packets
and an implementation that implements optional checksums MUST accept
PDUs if they do NOT contain the optional checksum. Implementations
that receive an optional checksum TLV and support it MUST discard the
PDU if the checksum is incorrect. An implementation that does NOT
implement optional checksums MUST accept a PDU that contains the
checksum TLV. An implementation that supports optional checksums and
receives it within any other PDU than CSNP, PSNP or IIH MUST discard
the PDU. Such an implementation MUST discard the PDU as well if more
than one optional checksum TLVs are included within it.
Additionally, any implementation supporting optional checksums MUST
accept PDUs with an optional checksum with the value 0 and consider
such a checksum as correct.
3. Checksum Computation
The checksum is a fletcher checksum computed according to [ISO98],
Annex C over the complete PDU. To compute the correct checksum, an
implementation MUST add the optional checksum TLV to the PDU with the
initial checksum value of 0 and compute the checksum over such a PDU.
4. Interaction with TLVs using PDU Data to Compute Signatures
The implementation MUST either omit the optional checksum on an
interface or send a 0 checksum value if it includes in the PDU
signatures that provide equivalent or stronger functionality, such as
HMAC or MD5. Otherwise an implementation that handles such
signatures but does not handle the optional checksums, may fail to
compute the MD5 signature on the packet. Such a failure would be
caused by the fact that MD5 is computed with the checksum value set
to 0 and only as a final step is the checksum value being filled in.
5. TLV Format
[Prz01] lists the according value of the TLV type and discusses
issues surrounding the assignment of new TLV codepoints.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type =12 | TLV Length =2 | Checksum (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6. Acknowledgments
Tony Li mentioned the original problem. Mike Shand provided
comments. Somehow related problems with purging on LSP checksum
errors have been observed by others before. Nischal Sheth spelled
out the issues of interaction between MD5 and the optional checksums.
7. Security Considerations
ISIS security applies to the work presented. No specific security
issues as to the new element are known.
References
[Bra97] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[Cal90a] Callon, R., "OSI ISIS Intradomain Routing Protocol", RFC
1142, February 1990.
[Cal90b] Callon, R., "Use of OSI ISIS for Routing in TCP/IP and Dual
Environments", RFC 1195, December 1990.
[ISO90] ISO. Information Technology - Telecommunications and
Information Exchange between Systems - Intermediate System
to Intermediate System Routing Exchange Protocol for Use in
Conjunction with the Protocol for Providing the
Connectionless-Mode Network Service. ISO, 1990.
[ISO98] ISO. Information Technology - Protocol for Providing the
Connectionless-Mode Network Service: Protocol
Specification. ISO, 1998.
[Prz01] Przygienda, T., "Reserved Type, Length and Value (TLV)
Codepoints in Intermediate System to Intermediate System",
RFC 3359, August 2002.
Author's Address
Tony Przygienda
Xebeo
One Cragwood Road
South Plainfield, NJ 07080
Phone: (908) 222 4225
Email: prz@xebeo.com
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Optional Checksums in ISIS
IS-IS (Intermediate System to Intermediate System) is an interior gateway routing protocol used by ISPs to determine how to forward packets within their networks. It is a link-state protocol, meaning routers exchange information about the network topology with each other. ISIS originally did not include checksums in its PDUs (protocol data units); instead it relied on the underlying data-link layer to verify data integrity. In practice, however, some network technologies lack checksums, and faulty hardware implementations can introduce corruption that goes undetected.
RFC 3358 extends ISIS with an optional checksum field that protects CSNP (Complete Sequence Number PDU), PSNP (Partial Sequence Number PDU), and IIH (ISIS Hello) packets against corruption.
ISIS is a protocol routers use to tell each other about the network. It trusted lower layers to catch errors, but sometimes errors slipped through, so this RFC adds a checksum to catch them.
§2The Checksum TLV and Compatibility
The optional checksum is carried in a TLV field with type number 12 and length 2, containing the computed checksum. An implementation that supports optional checksums MAY include the TLV in packets, but MUST accept packets without it. Implementations that receive a checksum TLV MUST discard the packet if the checksum is incorrect, but implementations without checksum support MUST still accept the packet and ignore the TLV. This backward compatibility ensures that mixed networks of old and new routers work correctly. Additionally, if a packet includes signatures (like HMAC or MD5) for authentication, the checksum should either be omitted or sent as zero to avoid interference with signature computation.
The checksum is optional, so old routers that do not know about it can still read the packet. New routers check the seal and throw away smudged packets.
§3Checksum Computation
The checksum is a Fletcher checksum computed according to ISO 8859 over the complete PDU. To compute it correctly, the implementation adds the optional checksum TLV to the packet with an initial checksum value of zero, then computes the checksum over the modified packet. This approach ensures that the checksum includes itself in the computation. By special convention, an implementation MAY send a checksum value of zero, which is treated as correct. This flexibility allows implementations to signal that they did not compute a checksum while still being compatible with systems that require it.
The checksum math works by going through the packet once and computing a number. If the packet gets corrupted, that number will be wrong, and routers know to discard it.