[SATLUG] OT: question about TCP headers

Jon Mark Allen jm at allensonthe.net
Tue Jan 26 22:27:02 CST 2010

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

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


/* If you haven't found something strange during the day, it hasn't
been much of a day.
-- John A. Wheeler */

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