Re: [Tsvwg] ECN & PMTU

[Jacob Heitz <[e-mail]>]

One reason: sending extra packets in the face
of congestion will worsen the congestion.
Congestion collapse could be a very serious possible
consequence of sending extra packets.
Notice the ECN does not go towards the sender,
but in the other direction. I think the intention
is to have the upper layer (transport) in the
receiver withhold acknowledgements to cause the
sender to back off. I'm making this up, so please
correct me if I'm wrong.

PMTU discovery does not face this problem. It's ok to send
an extra packet back to the sender in this case.

Arun Prasad wrote:
> 
> Hi all,
>     Some doubts on the procedure followed for PMTU (Path mtu) discovery and
> ECN method.....
>     The doubt is not related to the procedure as such, but the relation between the
> two procedures stated above......
> Tcp and Sctp uses ICMP message to handle the PMTU discovery procedure....
> ie., the intermediate router will send an ICMP messge of "Message too Long"
> to the end point sending the data packet of size greater than the MTU size of some
> intermediate path to the destination.....
> 
> For ECN, the intermediate router should set the appropriate bits in the IP header
> (if it supports ECN ....) whenever there is congestion and risk of packet drops
> is high....... The endpoints then correct their Congestion window accordingly and
> help in reducing the Congestion in the intermediate router.....
> 
> Why, we adopt two different methods to carry out similar (in the sense, both are
> related to the Intermediate routers....)?????
>     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....)
> 
>     What we can achieve by this is the simplicity  in the router implementation,
> which doesnt need to do different procedure for different features of the Transport
> layer..... and by maintaining this uniformity, the future extentions which demands a
> similar requirement can use the same way......
> 
> I could have missed some points....
> 
> Any comments on this????
> 
> Thanks
> -arun
> --
> ****************************************************************
> V.Arun Prasad
> HCL Technologies Ltd.
> 51, Jawaharlal Nehru Road,
> Ekkattuthangal,
> Guindy Industrial Estate,
> Chennai - 600097.
> 
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-- 
// Jacob Heitz. Tel:[phone]