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13534 Views 5 Replies Latest reply: Oct 10, 2013 4:53 PM by Aravindhan RSS

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use of urgent pointer in tcp header

Jan 11, 2010 12:24 AM

stuckInActive 13 posts since
Jun 29, 2009

can anyone help me in understanding the use of 'urgent pointer' in tcp header.

i gone several docs but could not get good understanding of it.

please provide some of the practical scenarios where and how it is used

thanks in advance

  • Conwyn 9,657 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Jan 11, 2010 12:34 AM (in response to stuckInActive)
    Re: use of urgent pointer in tcp header

    HI Krish

     

    The Urgent Pointer is used when some information has to reach the server ASAP. When the TCP/IP stack at the other end sees a packet using the Urgent Pointer, it is duty bound to stop all it's doing and immediately send this packet to the relevant server. Since the packet is plucked out of the processing queue and acted upon immediately, it is known as an Out Of Band (OOB) packet and the data is called Out Of Band (OOB) data. The Urgent Pointer is usually used in Telnet, where an immediate response (e.g. the echoing of characters) is desirable.

     

    http://www.vijaymukhi.com/vmis/tcp.htm

     

    Regards Conwyn

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  • Paul Stewart  -  CCIE Security 7,570 posts since
    Jul 18, 2008
    Currently Being Moderated
    2. Jan 11, 2010 1:51 AM (in response to stuckInActive)
    Re: use of urgent pointer in tcp header

    When an IP stack receives a TCP Segment, its purpose is to hand data or commands up the OSI model.  In other words, TCP data has session, presentation, and applications commands and/or data.  So the IP stack separates this data from the TCP and IP information (headers) and passes this to the process that is associated with the TCP port.  In many instances this is not done on every segment for performance reasons.  Some times a few segments can queue up and be handed to their associated application or process at one time.  This reduces resource load.  However with some applications, the data needs to reach the associated process immediately.  This is when the urg field should be set.

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  • Paul Stewart  -  CCIE Security 7,570 posts since
    Jul 18, 2008
    Currently Being Moderated
    4. Jan 12, 2010 2:16 AM (in response to stuckInActive)
    Re: use of urgent pointer in tcp header

    Yes.  It is an instruction to the TCP stack to hand the data to the application without further delay.

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  • Aravindhan 1 posts since
    Jun 14, 2011
    Currently Being Moderated
    5. Oct 10, 2013 4:53 PM (in response to stuckInActive)
    Re: use of urgent pointer in tcp header

    Hi,

     

    The urgent pointer points to the last byte of the urgent data.

     

    I couldn't understand what's the idea behind this. What could this be used for? I think that begining of the urgent data might be more useful as it would help TCP know where exactly the urgent data begins and ends.

     

    Please help me out here.

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