6.2 mtu now limits size of incomming packet

[Robert Watson]

On Fri, 13 Jul 2007, Mike Karels wrote:

>> The ability to receive packets larger than mtu was not accidental. This 
>> should be fixed, if it is, as is suggested, a deliberate change.
>
> I'd be happy to see the change undone as well.  I (well, our test group) 
> found this change in a similar way, and it didn't agree with our previous 
> usage.

A related change that should probably be discussed if we want to think more 
about asymmetry in maximum transmission unit is this one:

   ----------------------------
   revision 1.98
   date: 2006/06/26 17:54:53;  author: andre;  state: Exp;  lines: +2 -0
   In syncache_respond() do not reply with a MSS that is larger than what
   the peer announced to us but make it at least tcp_minmss in size.

   Sponsored by:   TCP/IP Optimization Fundraise 2005
   ----------------------------

In this change, we cap the advertised MSS in SYN/ACK to the received 
advertised MSS, which presumably avoids an extra PMTU round trip if jumbograms 
are enabled on the receiving endpoint.  However, it also prevents use of 
larger packet sizes if asymmetric MTU is supported.  I think I suggested after 
this was committed that we at least add an administrative twiddle to 
enable/disable this mode of operation, but don't see one in there currently. 
Does the Secure Computing scenario use TCP in this way, and is the potential 
win in avoiding a PMTU round-trip worth disallowing asymmetric MSS at the TCP 
layer?

Robert N M Watson
Computer Laboratory
University of Cambridge