[e2e] Re: [Tsvwg] Re: ECN & PMTU

mbashyam mbashyam at cisco.com
Tue Apr 9 22:53:00 PDT 2002

Arun Prasad wrote:

> Srivathsa wrote:
> > >     Why cant we generalise this stuff.... ie.,  the need here is some way to
> > > communicate between the intermediate router and the endpoint.... Not sure
> > > which of the two ways is advantageous, but cant we use the same method for
> > > both..... ie.,
> > > As for the PMTU discovery, the intermediate router can send an ICMP message
> > > to the endpoint carrying a message "CONGESTION", by receiving this the
> > > endpoint will do all appropriate actions as it does when it receives the packet set
> > > with ECN-ECHO flag (as in present ECN procedure...)
> > >
> > > or vice versa ( ie., adopt the method followed by ECN  for PMTU discovery aslo,
> > > that might be slighly tough, considering the Backward compatibility....)
> > >
> >
> > You cant use a ECN-like-mechanism for PMTU discovery.   As soon as an
> > intermediate router discovers that the packet (with DontFragment bit set
> > ) is too large for one of its outbound interfaces,   it will have to
> > discard the packet.  I.e it cant relay the packet to the
> > destination.      In ECN scenario,  packet can still be relayed further.
> >
>             Ya, I agree that......  But the  method by which PMTU discovery is done
> can be changed to avoid packet drops (if dropping a packet is costly... I think
> yes, because it triggers Retransmissions....). We might rethink about implementing PMTU
> discovery like the ECN like mechanism, with this new proposal..... The idea is
> there should be  someother bit like the DF bit, say IF bit (Inform on Fragmentation) in the
> IP header..... This IF bit will be set by the Transport layers if it is planning to do the PMTU
> discovery.....  The router upon receiving a packet of size greater than the MTU size of the
> Egress interface and the IF bit is set in the IP header... then IP layer there will go ahead
>  fragmenting the packet and sending it to the destination..., but also send an ICMP
> "Message too Long" message to the Sender.....  The router also sets the DF bit, and
> unsets the IF bit in the IP header of the fragment packets (which ensure no more
> fragmentation is done on the packet....)  ..................The sender upon receiving the ICMP
> packet will just update his PMTU... Thus there is no Packet drops... so the procedure of
> PMTU discovery will not  trigger any Retransmissions.... (which is expensive, because
> the transport layer shrinks the Congestion window if it senses that a packe is dropped...)

I dont understand, PMTU discovery requires that TCP go into a retransmission of all its
unacknowledged packets if there is a change in MSS.

Also in ur scheme by forwarding the fragments with DF bit on won't u  be triggering
drops in a downstream router with a smaller next hop MTU than the just announced


> Not sure whether this is already discussed or proposed etc...
> Thanks
> -arun
> >
> > -Srivathsa
> --
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> V.Arun Prasad
> HCL Technologies Ltd.
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