RFC 3358 (rfc3358) - Page 2 of 4

Optional Checksums in Intermediate System to Intermediate System (ISIS)

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