Subject: [pim] PIM IPv6 pseudo header length for Register messages.

From: "Ravi Shekhar" <[email-masked]>
Date: Sat, 18 Oct 2003 16:26:37 -0400

Folks,
  Should the length field in the pseudo-header created for calculating
  the checksum of a Register message include the length of the
encapsulated
  data packet, or should it only include PIM Message header and the
Register 
  header part over which the checksum is actually calculated (i.e. 8
bytes)?
  
  Checksum section states that it should be the "length of the PIM
message".

  If that includes the encapsulated data length, it could preclude an
implementation
  from pre-calculating the checksum, as appears to be the intention from
specification
  in Register Packet section where it states that data packet should not
be included
  in checksum calculation. 

  Thanks.
  - Ravi Shekhar.

Checksum
     The checksum is a standard IP checksum, i.e.  the 16-bit one's
     complement of the one's complement sum of the entire PIM message,
     excluding the "Multicast data packet" section of the Register
     message.  For computing the checksum, the checksum field is zeroed.

     For IPv6, the checksum also includes the IPv6 "pseudo-header", as
     specified in RFC 2460, Section 8.1 [5]. This "pseudo-header" is
     prepended to the PIM header for the purposes of calculating the
     checksum.  The "Upper-Layer Packet Length" in the pseudo-header is
     set to the length of the PIM message.  The Next Header value used
     in the pseudo-header is 103.  If the packet's length is not an
     integral number of 16-bit words, the packet is padded with a byte
     of zero before performing the checksum.

....
PIM Version, Type, Reserved, Checksum
     Described in Section 4.10. Note that in order to reduce
     encapsulation overhead, the checksum for Registers is done only on
     first 8 bytes of the packet, including the PIM header and the next
     4 bytes, excluding the data packet portion. For interoperability
     reasons, a message carrying a checksum calculated over the entire
     PIM Register message should also be accepted.

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