From: John Beckett 

"Geo"  wrote in message
news::
> Ok but when you talk about buffers, routers use buffers too, isn't that the
> reason for checking MTU?

It is conventional (not compulsory) for the start of a TCP session to test
the end-to-end MTU. The sending and receiving systems will then never send
a frame that requires fragmentation.

With UDP (or raw IP), there is no such initial testing. Say my client sends
a DNS query to resolve www.cisco.com. My client will send a SINGLE packet
to its DNS server. That DNS server will send a SINGLE packet to some other
DNS server until the query is resolved.

The reply will again be a single packet. There are no "test the
MTU" packets or other options. Each system simply sends a UDP packet
which it hopes will not exceed the size of the recipient's buffer.

Re routers: A router doesn't have to receive a full UDP or IP datagram. A
router only has to have a buffer that can handle the max size of an IP
fragment. That size is the maximum frame size of the attached interface
(e.g. 1500 bytes if using Ethereal).

Suppose that all networks are Ethernet. My computer could send a single UDP
message to your computer. Say the message is 4000 bytes. My IP layer will
send three fragments (roughly 1500, 1500 and 1000 bytes). Each router
simply receives and forwards each fragment, so the router needs a 1500 byte
buffer only. Your computer has to have a buffer of 4000 bytes if it wants
to receive the whole message.

John

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