From: Volkov, Dmitry (IDS Canada) (dmitry_volkov@ca.ml.com)
Date: Sun Jun 29 2003 - 18:44:57 GMT-3
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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