Re: ISCSI: Urgent Flag requirement violates TCP.

[Matt Wakeley <matt_wakeley@agilent.com>]

Douglas Otis wrote:

> There is little assurance of pointer accuracy as
> most machine's pointers do not resolve to the byte

The "urgent pointer" is not a pointer to memory.  It is a 16 bit integer that is
added to the 32 bit tcp sequence number (also an integer) to indicate the
position in the byte stream where the urgent data is.  Last time I checked, most
all machines are able to do integer arithmetic.


> and TCP never intended a urgent pointer to be used as a record mark.

Sure it did. From the glossary of rfc793:

urgent pointer - A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer which indicates the data
octet associated with the sending user's urgent call.

Did you get that? "indicates the data octet associated with the sending user's
urgent call" sure sounds like a marker to me.

Also note rfc1122, section 4.2.2.4 Urgent Pointer:

... the Urgent mechanism may be used for any application...

> This urgent pointer simply
> defines a change in state where the actual pointer is not directly presented
> to the application but used to expedite processing.

You're right.  The "pointer" is not presented to the application.  But the data
byte the urgent pointer points to is presented to the application.

Matt Wakeley
Agilent Technologies