option 212 for 6RD

Tore Anderson tore at fud.no
Tue Jan 15 09:04:05 CET 2013

* Mikael Abrahamsson
> On Tue, 15 Jan 2013, Jeroen Massar wrote:
>> Note also that setting your local network to a wrong MTU does not 
>> resolve this. Typically packets from the server will be large
>> while the client only sends small one, and you are only controlling
>> the sending of packets with your local MTU, not what the server is 
>> sending, which will in most cases happily be at 1500 and maybe
>> even jumbo.
> With TCP the client advertised MSS will be derived from the IPv6 MTU,
> so the server will never send 1500 byte packets on TCP.

So the RA LAN MTU trick really only helps TCP, correct? What is gained
from lowering the LAN MTU by means of RA, rather than rewriting the TCP
MSS field as we've "always" done in IPv4?

I'm not buying the argument that having a slightly lower LAN MTU is a
significant problem for the end user, but depending on all the HGWs in
the world to behave according to your wishes is - and the RA LAN MTU
trick must be implemented in the HGW.

TCP MSS clamping, on the other hand, could be done by the 6RD BR, and
yield beneficial results for all the subscribers (regardless of their
HGWs implementing the RA LAN MTU trick or not).

Tore Anderson

More information about the ipv6-ops mailing list