>> >> OK, does PMTUD lower the outgoing packet size on my system due to the >> >> hotel router's lower MTU or does the hotel router itself fragment my >> >> 1500 byte packets in order to send them out? Just curious. >> > >> > If you are sending out packets with the DF bit set no fragmentation will >> > take place - the packet is dropped and an appropriate message is >> > returned to sender. Otherwise the router will fragment them and send >> > them on to the recipient address. >> >> Shouldn't PMTUD change my MTU based on the hotel router's lower MTU? > > Yes, it should. At the start of the connection the sender sends DF in the > header to find out what is the MRU that the network nodes will support. Then > sends packets of the appropriate size so that they get through with no > fragmentation. This is the optimal scenario. > > Now, imagine another scenario where some router/firewall/server does not send > back the correct ICMP packet with its required MRU, or even worse it sends > back a 1500 (full ethernet) size with DF set, or also drops fragments ... This > reminds me of MSN IM which was a particularly bad implementation back when. > > The sender may eventually try a smaller packet, after initially increasing the > time it waits for a response, and you could well get something through 30 > seconds later, or even give up and time out.
Here's my layout: laptop+shorewall (MTU:1500) -> hotel router (MTU:?) -> internet -> Westell modem/router (MTU:1492) -> desktop+shorewall (MTU:1500) Shouldn't PMTUD change the desktop's MTU to 1492? Is the fact that it doesn't due to a flaw in the Westell's operation? Should I manually change the desktop's MTU to 1492 along with that of other systems on its LAN? > If you are using Shorewall at your remote server I would expect it to behave > properly and return the correct ICMP packet when it receives a DF. However, I > am not familiar with the Shorewall properties and settings, so if you suspect > this as the cause of your problem it would be better if you look into it > properly. So I'm sure I understand, this doesn't apply if ICMP is dropped at the Westell? - Grant
