From: Volkov, Dmitry (IDS Canada) (dmitry_volkov@ca.ml.com)
Date: Sun Jun 29 2003 - 18:51:57 GMT-3
Sorry i forgot to add.
Yes W2K PC is enabled for PMTUD. During TCP handshake they tell each other
MSS 1460
However I believe PMTUD doesn't work for UDP
Pls, correct me if I'm wrong
When I send UDP with IP MTU 1488 (1460 payload + 20 IP + 8 UDP)
after receiving ICMP 3 4 it lowers down to 1420.
Dmitry
> -----Original Message-----
> From: Volkov, Dmitry (IDS Canada) [mailto:dmitry_volkov@ca.ml.com]
> Sent: Sunday, June 29, 2003 5:45 PM
> To: 'Howard C. Berkowitz'; ccielab@groupstudy.com
> Cc: 'security@groupstudy.com'
> Subject: RE: reaction on ICMP 3 4
>
>
> HostA(W2K)----R1---(S0)R2(Tu0)---ethernet----(Tu0)R3---HostB(W2K)
> R2#
> int tu0
> ip mtu 1420
>
> Host A generates traffic (TCP or UDP) towards to Host B
> R2(S0) sends ICMP 3 4 back and drops 1500 bytes IP packet,
> Host A retransmits 1420 bytes IP packet
>
>
> Dmitry
>
> > -----Original Message-----
> > From: Howard C. Berkowitz [mailto:hcb@gettcomm.com]
> > Sent: Sunday, June 29, 2003 5:34 PM
> > To: ccielab@groupstudy.com
> > Cc: 'security@groupstudy.com'
> > Subject: Re: reaction on ICMP 3 4
> >
> >
> > At 5:09 PM -0400 6/29/03, Volkov, Dmitry (IDS Canada) wrote:
> > >How TCP/IP stack reacts on receiving ICMP type 3 code 4
> > Fragmentation needed
> > >but DF set ?
> > >I mean how many bytes will be sent next time after receiving ICMP
> > >unreachable.
> > >I lowered IP mtu to 1420 and router sent ICMP and host
> > started send 1420 !!
> > >I sniffed ICMP packed and I didn't see anything inside ICMP
> > indicating
> > >allowable MTU.
> > >How source knows what size frame to retransmit ?
> > >
> >
> > More information is needed to answer this. Is the host actively
> > participating in MTU autodiscovery, or is it just setting
> DF? There
> > are valid reasons for the latter. For example, the old IBM RSRB
> > method of Fast Sequenced Transport sets DF, and then steals the
> > fragmentation fields in the header for IBM information.
> >
> > In any case, this is going to be a host implementation matter.
> >
> >
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