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Re: [partly OT]: in-between ISPs - solved
On Tue, Aug 07, 2001, Dani Arbel wrote about "Re: [partly OT]: in-between ISPs - solved":
> 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.
> Dani
But if the problem was caused only by the server, every users of that server
would have the same problem he has - and he didn't say that actually happened -
he actually said he had used the same server from his home dialup without any
problems.
So this has a problem of the server and client together - both of them have
non-RFC complaint setups: here's my guess as to what happen:
Say the server sends him a packet with a too-large MTU (because his client
uses that as the TCP MSS) *and* with a Do-not-fragment bit set (as most TCP
stacks now do). Something in the ADSL system (near the client) will send
back an ICMP "unreachable - fragmentation needed" message. Now, if the server
has a firewall that refuses letting that ICMP message in (this is contrary
to the RFC, by the way), the server will never know what happen - and will
never be able to send that packet! If this is indeed the problem, there are
two possible solutions:
1) allow ICMP unreachable messages through the server's firewall, and
2) reduce the MTU on the client, so that the server never gets an incorrect
MSS in the first place.
Obviously, having no control over the server means that only solution #2 is
a viable one for him.
--
Nadav Har'El | Tuesday, Aug 7 2001, 18 Av 5761
nyh@math.technion.ac.il |-----------------------------------------
Phone: +972-53-245868, ICQ 13349191 |The 3 stages of sex: Tri-weekly, try
http://nadav.harel.org.il |weekly, try weakly.
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