Path MTU discovery
Setup TCP connection. Use default MSS = Min (local MTU - headers, destination MSS). Send packet with DF bit set.
If ICMP error, reduce segment size and retransmit.
- Old ICMP: “unreachable” error. New: “cant fragment” error. ICMP message may not specify exact MTU value.
Since routes change dynamically, a larger value can be tried again after a time interval (RFC 1191 recommends 10 min, but Solaris uses 30 s).
Large packets not necessarily good. Store-and-forward paradigm => small packets create a pipelining-like effect (fig 24.3 & 24.4)