In comp.dcom.sys.cisco Goha <ciscofaq@hamawy.com> wrote:

> 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.

It's just a 6-bit placeholder for future extensions. None are in use now.

> 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.

The Urgent pointer is used in combination with the URG TCP option; when
an application has, err, "Urgent" data that ought to be passed to the other
end of the connection regardless of the current state of the receving-end
application. As usual Stevens has a clear example (TCP/IP Illustrated,
volume 1, P293)

The two most commonly used applications are Telnet and Rlogin, when the
interactive user types the interrupt key [...] If the server process
enters urgent mode, the server TCP immediately sends the urgent pointer
and the URG flag, even though it can't send any data.

If you're really interested in learning about this kind of esoterica, and not
just in passing your test, you owe it to yourself to pick up this book,
RWS is the master.

ISBN 0-201-63346-9

--
Eric Sorenson - Systems / Network Administrator, MIS - Transmeta Corporation

In comp.dcom.sys.cisco Eric Sorenson <eric@transmeta.com> wrote:

> It's just a 6-bit placeholder for future extensions. None are in use now.

This was wrong; as another poster pointed out they're down to 3 reserved and
3 in use for ECN-related options, RFC3168 and RFC3540.

--
Eric Sorenson - Systems / Network Administrator, MIS - Transmeta Corporation

Eric,

If you really are an employee of the Transmeta Corporation, please say a
hearty "Hello!" to Linus Torvalds for all of us open-source folks out here.

Cheers,

SC


"Eric Sorenson" <eric@transmeta.com> wrote in message
news:1079124741.662110@palladium.transmeta.com...
> In comp.dcom.sys.cisco Eric Sorenson <eric@transmeta.com> wrote:
>
> > It's just a 6-bit placeholder for future extensions. None are in use
now.
>
> This was wrong; as another poster pointed out they're down to 3 reserved
and
> 3 in use for ECN-related options, RFC3168 and RFC3540.
>
> --
> Eric Sorenson - Systems / Network Administrator, MIS - Transmeta
Corporation