From: sohu (xieji@sohu.com)
Date: Mon Jun 30 2003 - 00:46:00 GMT-3
Hello
I know that the host shall negotiate with the peer the MSS value by TCP SYN option.when the host receives icmp 3 4,the tcp conncetion shall automaticlly adjust its MSS to the value the ICMP gave.Because only tcp protocol shall reponse to ICMP 3 4 ,I have a little confusion about how UDP or IP to adjust theire sending MTU ?
For example:
h1(MTU 1500)---r1(MTU1400)---r2--(MTU1500)host2
if I issue ping -s 1500 to the destination to h2 with DF bit =1.When the packet reaches r1,the r1 shall drop this packet becasue the MTU value is 1400 and the packet DF=1 set.The R1 shall send icmp 3 4 with recommend mtu=1400 to the source of the packet which is h1.If the h1 can not adjust the sending MTU,so the ping can not succeed any time.Wheather this statement is true ??
----- Original Message -----
From: "Volkov, Dmitry (IDS Canada)" <dmitry_volkov@ca.ml.com>
To: "'Howard C. Berkowitz'" <hcb@gettcomm.com>; <ccielab@groupstudy.com>
Cc: <security@groupstudy.com>
Sent: Monday, June 30, 2003 6:58 AM
Subject: RE: reaction on ICMP 3 4
> I was blind new next Hop MTU is inside ICMP 3 4:
>
> 7 and 8 th bytes of ICMP header.
>
> To support the Path MTU Discovery
> technique specified in this memo, the router MUST include the MTU of
> that next-hop network in the low-order 16 bits of the ICMP header
> field that is labelled "unused" in the ICMP specification [7]. The
> high-order 16 bits remain unused, and MUST be set to zero.
> http://www.ietf.org/rfc/rfc1191.txt
>
> http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00
> 800d6979.shtml#second
>
> Thanks, Howard.
>
> 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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