RFC 3358 (rfc3358) - Page 2 of 4
Optional Checksums in Intermediate System to Intermediate System (ISIS)
Alternative Format: Original Text Document
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