From: John Beckett 

"Geo"  wrote in message
news::
> Doesn't UDP max out at about 1K packet size? I remember dns responses
> will switch to tcp when the response size exceeds a certain size because
> UDP won't support that size.

Interesting point. Actually, UDP just uses an underlying IP datagram to
send its payload, so the limit is due to IP. An IP datagram can carry 64K
bytes (including headers), so a single UDP packet can carry just less than
64K bytes of payload.

However, IP has to fragment any datagram that exceeds the capacity of the
underlying network frame size (e.g. 1500 bytes for Ethernet).

Just guessing, but perhaps DNS servers switch from UDP to TCP at much less
than 64K bytes because the DNS server would have to have a buffer of the
required size to send a large UDP payload. It would be tempting for the DNS
server code to maintain a small number of fixed-size buffers to handle
common queries. A response too large for one of these buffers might be
handled by streaming the data via TCP (and indeed, the DNS client might not
really want to receive, say, 64K bytes of response when all the client
asked for was a simple name resolution).

John

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