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Re: tcpip stops working after some time



On Wednesday 2008 December 24 16:16:26 owens@netptc.net wrote:
> AFAIK the MTU is set by the initiator of the IP connection and any
> intermediate router can't change it.
> Larry

Not exactly.  The MTU is really a link-level property that's immutable.  
However, path MTU discovery is not required for IPv4, so the MTU is generally 
just set to a reasonable value at the endpoint.  However, any intermediate 
(or end-point) node can fragment any packet.  Intermediate routers tend to do 
that anytime the packet size exceeds the link MTU that they choose to route 
down.  The alternative is to down the packet and, IIRC, send back and ICMP 
message that indicates the packet exceeds the MTU (which, in IPv4 is 
generally ignored or lost).

In IPv6, path MTU discovery is required and fragmentation is only allowed at 
the end points.  Intermediate routers are required to drop (or re-route) 
packets that exceed the MTU, and send a MTU notification (over ICMPv6) back.  
The end-point is required to use this packet to adjust it's path MTU to allow 
packets to be send via the smaller link.

IPv6 is not implemented widely yet, and there are certainly issues that may 
continue to prevent it from being so.  We need (or could certainly use) the 
extra addresses, but there's a lot more to that involved in IPv6 -- AAAA 
records in DNS, mandated IPSec (I think), aforementioned constraints on 
intermediate nodes, etc.
-- 
Boyd Stephen Smith Jr.                     ,= ,-_-. =. 
bss@iguanasuicide.net                     ((_/)o o(\_))
ICQ: 514984 YM/AIM: DaTwinkDaddy           `-'(. .)`-' 
http://iguanasuicide.net/                      \_/     

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