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.