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Re: [RRG] Perplexing PMTUD and packet length observations

On 11 aug 2008, at 18:55, Robin Whittle wrote:


I expected that most hosts would send packets with DF=1, using RFC
1191 PMTUD, since it has been around since 1990 . . .
Most hosts do. It's enabled by default on Win, Mac, Linux and FreeBSD. Probably more. 


and that it
would be considered antisocial to send any longish packets into the
Net with DF=0


Why?


saying the Network should do extra work if the
packets are too long for it.
Well, it's better than setting DF and ignoring the ICMP too bigs, like some people do (see recent thread on NANOG). 


In fact, some servers - including at least some Google servers, send
only DF=0 packets, and therefore never do any PMTUD.  They won't be
tempted by the client having a higher MSS than their own 1430, but
as long as the client's MSS is this or above, the server frequently
sends 1470 byte packets with DF=0.


Hm, didn't know that, but yes, that's what I'm seeing too.


 1 - I guess Google figure 1470 is a packet size which generally
     works in the core.


Or they use a 30 byte encapsulation somewhere.


 2 - I guess they figure that if the client advertises an MSS and
     when 40 (IP and TCP headers) is added to that figure if the
     result is larger than any MTU limit between the client and
     Google's servers, then it is the client's problem.
??? Why would advertising a large MSS be a problem? You send what the other advertises he/she can handle and obviously _they_ will be sending you what they can handle. 

Should try with 1500 byte DF=1 packets just for kicks, though.


If lots of hosts are sending long packets with DF=0, we need to cope
with then in any map-encap scheme.
Like I've been saying for a long time, reducing the user-visible MTU of the internet is not an acceptable approach. 


Another perplexing discovery is that my web server in Texas does
ordinary TCP SYN handshakes with MSS values such as 1460 from both
sides and then quite merrily cranks up to packets of 7260 bytes and
longer.
Hm, could it be that this is happening between two hosts on the same subnet? I gather some systems ignore the MSS if the traffic is deemed "local".
BTW: if you haven't seen it before, have a look at this before it expires two weeks from now: 

http://www.ietf.org/internet-drafts/draft-van-beijnum-multi-mtu-02.txt

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