Why IP & TCP Precedence Query


Jim Stevens (Stevens@A.ISI.EDU)
Thu 7 Jan 88 19:58:20-EST


Michael Padlipsky wanted to know why I asked about IP & TCP
Precedence.

Well, I am trying (for quite a while now actually) to complete an RFC
on a new protocol called Transaction Transport Protocol (TTP). TTP
differs from TCP and UDP by providing reliable message delivery
instead of reliable byte stream or unreliable packet delivery.
Anyway, one of the optional TTP services is precedence and
preemption. Note that the optional TTP service is more than just a
field reservation since the draft TTP RFC actually describes the
processing which is performed at the TTP level based upon the TOS.

I propose to get the priority level out of the precedence field
within the IP Type Of Service (TOS) field. In fact, my TTP packet
format includes the IP TOS within the TTP pseudo header of
information which is obtained from the IP layer and is TTP
checksummed (similar to UDP & TCP pseudo headers except that the zero
octet is replaced by the IP TOS). My question to the TCP-IP
distribution list was prompted when a reviewer questioned whether the
IP TOS field was available from all IP implementations.

Since no one has said otherwise, I presume that the IP TOS is
available in all IP implementations.

As pointed out by John Lekashman <lekash@orville.nas.nasa.gov>,
authentication is required for fair TOS processing by IP to prevent
people from setting their priority to the highest possible and thus
hogging the network. Also as pointed out by Dan Lynch
<LYNCH@A.ISI.EDU>, the IP TOS processing would have to be implemented
by everyone if the TOS parameters are ever to have any use.

I feel that precedence makes more sense at the Transport Level since
the authentication issues are simpler than at the Internet Level
and also since fewer people would have to implement precedence for it
to make sense.

Jim

P.S I have been asked if I could forward any responses I got to the
TCP-IP distribution list. Well so far all of the responses that I
have received were also CC'd to the list.

P.P.S I feel that there is a flaw to TCP's use of precedence and
security. The TCP Military Standard (MIL-STD-1778) specifies the
processing to be performed based upon the IP precedence field and
security options. (I believe that the TCP RFC also specifies the use
of the IP TOS and Security options but do not have a copy of the RFC
with me to check right now). The use of the IP fields is fine and
dandy since it is a standard and since it saves space in the TCP
header. Unfortunately, TCP does not checksum the IP precedence field
or the security options. Thus these fields could have been changed
or corrupted en route between the 2 TCP implementations and this
change would not be detected by TCP.

You could say that the IP header checksum would detect this sort of
error. Well if we must depend upon the IP header checksum why does
TCP have a checksummed pseudo header at all? I am sure that the
reason we have the TCP pseudo header is to detect if IP corrupted the
addresses and protocol number. I am also sure that the reason why
the TCP pseudo header does not include the precedence and security
fields is just an oversight.

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