Optional Checksums in Intermediate System to Intermediate System (ISIS)
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 (C) The Internet Society (2002). All Rights Reserved.
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.
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 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].
Przygienda Informational [Page 1]
RFC 3358 SNP Checksums in ISIS August 2002
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) |
Przygienda Informational [Page 2]
RFC 3358 SNP Checksums in ISIS August 2002
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
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