[e2e] UDP length field

Steve Deering deering at cisco.com
Wed Apr 18 11:29:42 PDT 2001

At 10:59 AM -0700 4/11/01, Joe Touch wrote:
>	1. the UDP length is used in the pseudoheader (actually twice)
>	   for UDP checksum calculation, and should NOT change E2E
>		- the IP length includes the IP header and its 
>		  options, which MAY change E2E

It was my understanding that the length of IPv4 options is *not* allowed
to change en-route.  At least, none of the currently defined options do.
Regardless, given that TCP is required to determine the TCP payload
length from IP-layer info only, why can UDP not do the same?

>	2. as an additional check when UDP is used without a checksum
>	   (See Stevens/Wright, TCPIP Illustrated, v2, p771).

This sounds revisionist.  If it was recognized that "something extra"
was needed in the case where the UDP checksum was disabled, why did
that something extra not cover the *non*-redundant but vulnerable
UDP header fields, i.e., the UDP port numbers?

>	3. in the case where IP options add trailers
>		in this case, there is insufficient information in the IP_LEN
>		and IP_HLEN fields to determine where the UDP data ends

Again, if UDP needed a length field to deal with that, why didn't TCP?


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