Re: Pseudo-Headers & Checksumming


Thomas Narten (narten@purdue.edu)
Mon, 11 Jan 88 10:05:42 EST


>VMTP has no need for pseudo headers a la TCP because the "entity identifiers",
>the transport-level endpoints, are (inter)network level independent as is the
>rest of the packet.

One other important implication of the independence between the VMTP
and IP layer concerns ICMP errors. The ICMP spec requires that the
first 64 bits (8 bytes) of the transport level header be returned
along with the IP header. In protocols like TCP/UDP, the 8 byte
source/destination pair is actually part of the IP header, and by
careful arrangement of the transport header fields, source/destination
ports reside at the beginning of the packet. Because of the pseudo
header, TCP/UDP actually get back 12 bytes of useful transport header
information. Errors such as src quench and port unreachables can then
matched up with the protocol control block that originates the
offending datagram.

In newer protocols like VMTP, I wonder if 8 bytes of transport header
is sufficient. According the packet format given in the 86 SIGCOMM
paper, entity identifiers are 32 bits long, hence the
source/destination identifiers would use up all 64 bits of data. This
leaves no room for other possibly important information like the
forwarded entity identifier. Without the forwarded entity identifier
field, ICMP error processing would appear to be much more difficult if
not impossible in some cases.

For instance, consider a request to read a file. The request would
first go to the directory server, which might forward the request to a
second server which answers the query directly. If the second server
is unreachable, where would ICMP errors go? Most likely, they go back
to the first server (not the originating client). The server would not
be able to match the source entity indentifier with any of its own,
and the ICMP error would likely be ignored. Meanwhile, the client
originating the request retransmits a few times and finally times out.

Is ICMP useful to newer protocols, and if not, can the Internet
operate effectively without it?

Thomas



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