Is Path MTU Discovery(PMTUD) really efficient?

Question

As i know, PMTUD uses ICMPv6 to find minimum MTU value in the network path. I understood why IPv6 doesn't allow fragmentation (by reading this article), but PMTUD doesn't look like good manner.

here are my thoughts.

1. it has to find the network path from src to dst to get min MTU value. i think this is additional delay.
2. when path is changed due to inter-router failure or something else, it has to repeat 1. process which increases delay again.

am i thinking wrong? any comments is my pleasure. thanks.

Answer 1

Efficient in comparison to what else?

The main point for PMTUD is to get rid of in-flight fragmentation which is costly on the routers. Routers need to be lean and efficient to keep up with the traffic demands.

1. The network path is "found" anyway by the first data or handshake packets. PMTUD requires just one round trip for each hop where MTU is lower than the currently discovered value. Note that this is per connection as opposed to overhead per packet with fragmentation.
2. A later reduction of MTU again just requires one round trip. An increase in MTU is probed and detected on a regular basis using the data stream.

Answer 2

It's efficient when you are transferring bulk data using a connection orientated transport protocol on a network that reliably supplies packet too big notification.

Unfortunately that doesn't always match up with the real world.

1. In the real world packet too big notifications often simply do not arrive leading to connections that transmit small amounts of data ok but then mysteriously hang.
2. For a connectionless protocol by the time the server gets the "packet too big" notification it's too late. The packet no longer exists so it can't just be sent out again. Instead the sever must cache the packet too big, then the client must time out and re-send the request.
3. For small requests the extra latency of sending a packet, getting the packet too big error back may make up a significant percentage of total request time.