On 11 Mar 2004, Goha wrote:

> To all the Cisco Gurus out there.....
>
I am no guru but I'll try to answer as best as I can.

>
> I am studing for my CCNA 640-801 exam and I have a few questions on
> TCP. Please keep in mind that I am still a newbie to this and keep the
> examples simple. Thank You.
>
>
> 1. According to the Sybex 640-801 Deluxe book the reserved field in
> TCP is always set to zero. Why? What is the actual purpose of this
> field? The defenithin to this was only a one liner in the entire book.
> I could not find a reference to this in the Cisco Press books. Please
> also supply an example.
>
By convention, the reserved field is set to zero. The field will be used
in the future. In fact, the two bits before the TCP flags (0,0,U,A,P,R,F,S)
are now being used for Explicit Congestion Notification (ECN). I would
start with "Internetworking with TCP/IP - Volume One" by Comer. Also,
read RFC3540 if you want to know more about ECN

> 2. What is the Urgent Pointer used for? I could not find a reference
> to this in the Cisco Press books. Please also supply an example.
>
This flag indicates that "urgent" data should take precedence over other
data. An example of this is pressing Control C to abort an FTP download.

Doan