[SATLUG] OT: question about TCP headers

redpill toddwbucy at grandecom.net
Wed Jan 27 08:15:41 CST 2010


On Tue, 2010-01-26 at 22:27 -0600, Jon Mark Allen wrote: 
> On Tue, Jan 26, 2010 at 22:12, redpill <toddwbucy at grandecom.net> wrote:
> > background:
> > I am currently taking Kolars TCP/IP class at SAC and this question
> > relates to a non-graded lab.
> >
> > references:
> > class text - TCP/IP Illustrated Volume 1: the protocols cira 1994
> > web reference png image illustrating the TCP header breakdown -
> > http://freebie.fatpipe.org/~mjb/Drawings/TCP-Header.png
> >
> > question:
> > the text notes that there are 6 reserved bits between the header length
> > and the flags (page 225).  The image noted above has shown 4 bits
> > reserved between the length and the flags.  2 additional flags are added
> > on the png file that are not listed in the book: C 0x80 Reduced (CWR)
> > and E 0x40 ECN Echo (ECE).  Which one is correct?  I was shown this file
> > by a classmate and I need to confirm its accuracy.
> >
> > Challenge:
> > you will win my vote for uber geek of the year if you can give me the
> > RFC's that may clarify this issue.
> >
> >
> > Thanks
> > Todd
> >
> 
> RFC 793 is the original RFC for TCP.  Viewing [1] shows at the top
> that RFC has been "updated" by RFCs 1122 [2] and 3168 [3].  The
> abstract of RFC3168 states:
> 
> "This memo specifies the incorporation of ECN (Explicit Congestion
> Notification) to TCP and IP, including ECN's use of two bits in the IP
> header."
> 
> Given that 3168 was issue in September of 2001, that would explain why
> it isn't mentioned in the book.
> 
> [1] http://tools.ietf.org/html/rfc793
> [2] http://tools.ietf.org/html/rfc1122
> [3] http://tools.ietf.org/html/rfc3168
> 
> -- 
> JM
> 
> /* If you haven't found something strange during the day, it hasn't
> been much of a day.
> -- John A. Wheeler */

thanks for the info.  I was originally referring to to the faqs.org's
copy of the RFC 793, http://www.faqs.org/rfcs/rfc793.html, which doesn't
refer to the other updates.  So not only did you give me the info that I
needed but you pointed me to a better source for my RFC's.  You win my
vote for Uber Geek of the year:)  The year is still very young so keep
showing off that brain of yours.

thanks
Todd



More information about the SATLUG mailing list