North American Network Operators Group
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Types of packet modifications allowed for networks

  • From: Jean-François Mezei
  • Date: Sat May 31 17:59:53 2008 
I would like any pointers to good documents that outline what sort of
packet modifications are allowed (in terms of Internet culture/policies)
by networks.

Notably:

For a transit network (neither sending or destination IPs belong to the
network)

For the sending network (originating IP belongs to that network)

For the destination network (destination IP belongs to that network).


Obviously, every router will change/decrement the TTL (and recalculate
the header checksum) in the IP header. Are there other fields that are
routinely changed at every hop ?

Would it also be correct to state that any network along the way would
have the right to fragment a packet in two or more pieces ? Or would
that only be the destination network needing to fragment a packet to fit
the last mile (PPP dialup or PPPoE ) in cases where MTU negotiations
failed ?

Are there sacred rules documented anywhere about not modifying anything
else in the packets during transit ?  Or has there never been any formal
documentation on this because it was so obvious nobody was allowed to
modify packets in transit ?

(I an in Canada and currently participating in CRTC proceedings about
the Bell throttling issue, and would like to consider the angle where
the CRTC might decides Bell has the right to manage PPPoE packets as if
they were TCPIP packets (by looking inside).

Thanks in advance for any hints/pointers on this issue.