Re: NAT Traversal

[Henry Spencer <henry@spsystems.net>]

On Thu, 28 Feb 2002, Tero Kivinen wrote:
> > - What "local policy" is being reffered to here? 
> 
> Implementations own local policy. If the implementation knows for sure
> that it's TCP/UDP stack does not care about checksums, it can simply
> zero them.

There is no such thing as a (standard conforming) TCP/UDP stack which does
not care about checksums.  Checking checksums is mandatory; discarding
packets with bad checksums is mandatory.  A UDP implementation may
optionally choose not to include a checksum with a packet it generates (by
setting the checksum field to zero), but it must still check checksums on
incoming packets and discard bad ones.  A TCP implementation does not even
have that little bit of flexibility; the TCP checksum is not optional. 
See RFC 1122. 

                                                          Henry Spencer
                                                       henry@spsystems.net