PMTU Discovery

[Guus Sliepen]

On Wed, Jan 05, 2011 at 11:23:16AM +0100, Daniel Schall wrote:

> while improving the PMTU Discovery algorithm, I found the following behavior
> in the method "send_udppacket":
> 
> 1)      The code checks, whether the data size is smaller than the MTU, thus
> if it fits into a single UDP packet.
> 
> If not, you send the packet via TCP.
> 
> 2)      The data is compressed, changing its size. (Usually, making it
> smaller, but that's not always true.)
> 
> 3)      The compressed data is sent.
> 
> My question is: What use is it, to check the data size prior to compression?

The PMTU discovery process already takes compression into account, and the MTU
it sets is always small enough so that even an incompressible packet that will
expand somewhat can still be safely sent via UDP.

Furthermore, ICMP packets will have been sent and PMTU clamping will be used so
that the sender will usually not send packets that are larger than the PMTU
tinc found. The fallback to TCP is only used when the PMTU discovery has not
yet finished, and when someone still sends a packet that is too large and
cannot be handled otherwise (for example, non-IP protocols or some routing
protocols that add their own footer to IP packets).

> I suggest to implement a check, if the compressed data really is smaller
> than the uncompressed one and decide, what to send over the wire.

Although that could be done I don't think it is worth the effort for the few
packets that might benefit from it.

-- 
Met vriendelijke groet / with kind regards,
     Guus Sliepen <guus at tinc-vpn.org>
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