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Re: [partly OT]: in-between ISPs - solved



Ok, I'll try to be more detailed here, but the document i wrote should be
clear enough.
1) having an MTU smaller than 1500 is not a problem. It is perfectly ok
and the ip stack has the mechanism to deal with it.

2) the problem is caused by black holing the server: somewhere in the path
the icmp packets telling him that the packets need fragmentation are
droped. The droping router may be either in Barak or Netvision.
3) hosts do not "negotiate" MSS . Each one tells the other about his MSS.
For reasons outlined in the conclusions part of my doc I recommand to
reduce the MSS advertised by hosts in the masqueraded segment (this is
acomplished by reducing the ethernet MaxMTU ).
4) note that this blach holing of server do not hapen for normal dial up
hosts since the MTU of the dial up adapter (the ppp interface) is 1500 ,
the same as the Ethernet of the servers, and thus the path MTU is 1500.

Send the doc to your ISP's , maybe they finaly fix their network
configuration.

http://damyen.technion.ac.il/~dani/adsl-mtu.txt

Dani

On 7 Aug 2001, Oleg Goldshmidt wrote:

> Dani Arbel <darbel@techunix.technion.ac.il> writes:
>
> > All the "odities" are caused by misunderstanding the root of the problem:
> > it is in the specific server, and the ISP network. The effect is
> > blackholing the server. Now whatever service this server is giving you,
> > that will be the service you feel the connection freeze. The problem is in
> > the server side, not the client. We can bypass this by changing the MTU of
> > the client to the actual path MTU. This causes the client to advertise,
> > during the tcp connection setup, MSS that fits the path MTU , and then the
> > server never becomes black hole.
>
> Let me understand this: you are saying that the root of the problem is
> barak (whose ADSL service we are using here). They do not allow
> packets of more that 1452 (or whatever) bytes through. To overcome the
> problem, the server (e.g. the Netvision POP3 server, or theregister
> web server) and the client (my computer) need to negotiate an MTU less
> than the Barak's limit. Since the server can't be expected to choose
> an MTU that will satisfied all the quirks of every connection, I have
> to choose an MTU that will satisfy Barak.
>
> Is my understanding correct?
>
> --
> Oleg Goldshmidt | ogoldshmidt@NOSPAM.computer.org
> "I'd rather write programs to write programs than write programs."
>


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