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Thread: Good old URG...

  1. #1
    Senior Member
    Join Date
    Apr 2003

    Good old URG...

    Does anyone know of any current application of the URG flag? I mean besides from pen-testing and such, I mean legitamate network traffic using the URG flag. I was thinking of asking the same thing about the PSH flag... but I remembered it's uses in telnet, and I capped itunes radio, and it uses the PSH flag.

    I searched this site alot, and google quite a bit, and found no mention of a current use.

    We all [should] know the RFC's point of view (they came up with it they'd better have a viewpoint):

    From RFC 793

    The Communication of Urgent Information

    The objective of the TCP urgent mechanism is to allow the sending user
    to stimulate the receiving user to accept some urgent data and to
    permit the receiving TCP to indicate to the receiving user when all
    the currently known urgent data has been received by the user.

    This mechanism permits a point in the data stream to be designated as
    the end of urgent information. Whenever this point is in advance of
    the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
    must tell the user to go into "urgent mode"; when the receive sequence
    number catches up to the urgent pointer, the TCP must tell user to go
    into "normal mode". If the urgent pointer is updated while the user
    is in "urgent mode", the update will be invisible to the user.

    The method employs a urgent field which is carried in all segments
    transmitted. The URG control flag indicates that the urgent field is
    meaningful and must be added to the segment sequence number to yield
    the urgent pointer. The absence of this flag indicates that there is
    no urgent data outstanding.

    To send an urgent indication the user must also send at least one data
    octet. If the sending user also indicates a push, timely delivery of
    the urgent information to the destination process is enhanced
    Is there anything that needs that? I mean, is anything slow enough these days to need to cut mid packet and send the data on? I know it may have had it's uses... yada yadda yadda... Is anything like that in IPv6? is tcp changing in IPv6? Why am I too lazy too look that up myself? because I've already spent too long researching the stupid URG pointer

    This has to have been discussed plenty of times before somewhere... unless of course there's a common use for the URG pointer and I'm just ignorant

    I'd be happy to get links to previous discussions, as I can't find any of consequence.

    Otherwise can we discuss what the point is?


  2. #2
    Senior Member
    Join Date
    Mar 2004

    the best I could come up with is this link:


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