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On Sat, 17 Dec 2011, Martijn Grooten wrote:
On Fri, Dec 16, 2011 at 6:54 PM, Gordon Henderson wrote:So it is working OK now? If so, I'd leave it alone :)It is. I'm still a little confused about why though: one of the default options for the MTU in yast is 1492 which, it explicitly says, is for PPPoE broadband.
In ye olden days, the maximum amount of data you could put down a bit of co-ax Ethernet was 1500 bytes and that's been carried on through the years right up to Gb Ethernet - with the addition now of "jumbo frames" (9000 bytes)
When you're encapsulating one protocol inside another there is almost always overhead - and the overhead of encapsulating PPP inside Ethernet is 8 bytes - hence the MTU of 1500-8 = 1492. (And incidentally, to get 1500 bytes of payload over Ethernet, there is an additional 18 bytes of headers containing things like the source and destination Ethernet MAC addresses, VLAN tag, checksums and so on)
All sorts of complications arise from changing the MTU - not normally on your own system but on remote systems - if they didn't know that the MTU at your end was 1492 then they might send packets of 1500 bytes which then leads to fragmentation - it might send a packet of 1492 bytes then a packe of 8 bytes - doubling the packet overhead and reducing throughput. What's worse is that some systems simply don't support fragmentation, so you'll see things like ping work, but anything transmitting larger chunks of data might fail.
(And I think some protocolls refuse to work the fragmentation too - e.g. IPsec, but I'd need to double check on that)
However, there is a part of TCP called Path MTU discovery and that has the effect of propogating your MTU to the far end (and vice versa) Even so, some sites still break it if they filter out ICMP, so you can't win.
See here http://en.wikipedia.org/wiki/Path_MTU_Discovery for more details. But if it's working, don't fiddle :) Gordon -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/listfaq