Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!samsung!uunet!auspex!guy From: guy@auspex.auspex.com (Guy Harris) Newsgroups: comp.protocols.tcp-ip Subject: Re: SLIP documents Message-ID: <5848@auspex.auspex.com> Date: 11 Feb 91 00:42:02 GMT References: <4320@ns-mx.uiowa.edu> <5827@auspex.auspex.com> Organization: Auspex Systems, Santa Clara Lines: 39 >This sounds like you are willing to compare apples to oranges. IP >differs not at all whether it originates on a coaxial Ethernet, or a >slow speed serial line. Yes, I know IP doesn't differ, but how reliably the link level checks the link-level packets in which IP packets are wrapped *DOES* differ - Ethernet has a checksum, and SLIP doesn't, as I said. >UDP without checksums is only mentioning that you are willing to disallow >checksums at the transport level. This option exists under both Ethernet >and SLIP connections. If you disable UDP checksumming, the transmission of your UDP datagrams will be checked by checksum when the transmission goes over Ethernet, but not when it goes over SLIP, so if the transmission includes a SLIP connection, that connection won't have any checksumming other than the IP checksum you mention, which does *NOT* checksum the entire packet, just the IP header. If the transmission includes only Ethernet connections, it will be checksummed on each one of those hops. (It won't necessarily be checksummed as it's moved internally to any of the intermediate machines, of course....) >The network level (IP) still has a checksum that resides at byte 12. Which only checksums IP headers; it doesn't check the entire datagram. >I believe that V. Jacobsen (RFC 1144) states that the COMPRESSED_IP checksum >will be accomplished at byte 5. He also states that: This compression is specific to TCP/IP datagrams./2/ The author investigated compressing UDP/IP datagrams but found that they were too infrequent to be worth the bother and either there was insufficient datagram-to-datagram coherence for good compression (e.g., name server queries) or the higher level protocol headers overwhelmed the cost of the UDP/IP header (e.g., Sun's RPC/NFS). so it's irrelevant to the discussion of UDP anyway.