[Snort-users] TCP reserved flags: which is it?
cpw at ...440...
Mon Jul 22 09:19:03 EDT 2002
Everybody is wrong, cause they are refered to in the RFC as
bit 9* and bit 8! But, that's in relation to the 32 bit word which
which is word 3 of the tcp header (start counting at 0 of course).
0 ! * 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| OFF=10| | | | |W|E|U|A|P|R|S|F| Window = 5840 |
data | reserved | flags |
* ECN-Echo flag
! Congestion Window Reduced flag
So, if we go with the flow, bit W (congestion _W_indow reduced (ECN)**
and bit E (ecn _E_cho sent (ECN))** are the first two bits in the newly(1999)
defined (6->8) bit tcp flags field. Consequently, they should be numbered
bit 0 and bit 1 of the tcp flags field. Ah, but what happens to all the
old documentation that might refer to the Urgent bit as bit 0 or bit 10.
or when the flags fields expands further into the reserved space?
** See print-tcp.c in tcpdump source from tcpdump.org.
On Sun, Jul 21, 2002 at 10:59:42PM -0700, John Sage wrote:
> Actually, if you had read my initial post, the *real* question was
> why snort reported the flags as 12****S* while ACID reports the flags
> as flags=21****S*
It was one of the once over lightly reads.
> Notice the "1" and the "2" are reversed between the two.
> I know *what* the flags mean; I'm just trying to understand why snort
> and ACID seem to be reporting them differently...
> (That, and I was kinda funnin' with Erek, but he doesn't seem to have
> noticed :-)
> - John
> "Obviously, we do not want to leave zombies around."
> PGP key http://www.finchhaven.com/pages/gpg_pubkey.html
> Fingerprint FE 97 0C 57 08 43 F3 EB 49 A1 0C D0 8E 0C D0 BE C8 38 CC B5
> On Sun, Jul 21, 2002 at 12:14:27PM -0600, Phil Wood wrote:
> > On Sat, Jul 20, 2002 at 10:10:00PM -0700, John Sage wrote:
> > > On Wed, Jul 17, 2002 at 11:38:31PM -0700, John Sage wrote:
> > > > Received some tcp:25 packets with the reserved flag bits set.
> > >
> > > <snip>
> > >
> > > What about my question?
> > >
> > > Guys?
> > Take a look at rfc2481 and rfc2914.txt. Those bits are being used
> > for explicit congestion control. Of course it only works if both ends
> > and intervening routers are participating. Here is a snippit from rfc 2481:
> > 6.1. TCP
> > The following sections describe in detail the proposed use of ECN in
> > TCP. This proposal is described in essentially the same form in
> > [Floyd94]. We assume that the source TCP uses the standard congestion
> > control algorithms of Slow-start, Fast Retransmit and Fast Recovery
> > [RFC 2001].
> > This proposal specifies two new flags in the Reserved field of the
> > TCP header. The TCP mechanism for negotiating ECN-Capability uses
> > the ECN-Echo flag in the TCP header. (This was called the ECN Notify
> > flag in some earlier documents.) Bit 9 in the Reserved field of the
> > TCP header is designated as the ECN-Echo flag. The location of the
> > 6-bit Reserved field in the TCP header is shown in Figure 3 of RFC
> > 793 [RFC793].
> 8< snip >8
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Phil Wood, cpw at ...440...
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