Ethereal-dev: Re: [Ethereal-dev] where is the field of Urgent Pointer in TCP

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From: Jason House <jhouse@xxxxxxxxx>
Date: Wed, 23 Jul 2003 09:59:59 -0400


From: Guy Harris <guy@xxxxxxxxxxxx>
On Tuesday, July 22, 2003, at 6:00 PM, li wrote:
when decode packet to TreeView, after show TCP checksum, where is the last two bytes of Urgent Pointer;
Nowhere, unless the URG bit is set in the TCP header. We only put it into the protocol tree if URG is set. (If URG isn't set, the packet isn't setting the urgent pointer, and the urgent pointer field's value is ignored.)
If my understanding is correct, it isn't that the urgent pointer "isn't set", but is actually set to 0. If this is correct, should TCP possibly display non-zero urgent pointers if the URG bit is not set? Am I mistaken about all TCP implementations setting the urgent pointer to 0? Especially with an invalid CRC, I imagine that such functionality might be useful.