Re: Update to draft QUIC DPLPMTUD text i draft-ietf-tsvwg-datagram-plpmtud

[Christian Huitema <[e-mail]>]

On 5/15/2020 8:56 AM, Magnus Westerlund wrote:
> Hi,
>
> So what Gorry says appear correct. And it is a constant but its value could in
> worst case be not only interface but actually path dependent. The reason being
> that if the lower layers adds headers in specific cases. 
>
> Although use of IPv6 extension headers are not problem free there might be cases
> where such a header is added for some paths but not others. Thus, specifying the
> MIN_PLPMTU correctly to be stable acrosss interfaces and paths could be
> problematic. But for QUIC as Gorry says the stake is already in the ground and
> it is in fact reasonable maybe to be more precise to say that a default
> MIN_PLPMTU to use can be 1200 bytes as we don't expect QUIC to work over paths
> that don't support this. 

Quic servers will only accept an Initial packet if it is at least 1200
bytes long. This pretty much enforces a min MTU requirement of 1200
bytes. Quic will not work on a network path does not support that.

> We might get some interesting questions on the padded handshake packets when
> some want to run QUIC as IOT transport over radios such as what LPWAN WG
> discusses where it is very costly to send 1200 bytes packets. The radio blocks
> are more in the range of 32-64 bytes if I remember correctly. 

It is pretty hard to design an end-to-end transport that works well with
a very small end-to-end MTU. There are just too many trade-offs. For
example, the 1200 minimum MTU in Quic was chosen to mitigate DDOS
attacks. That's the kind of trade-off mandated by the requirement to
work end-to-end over the Internet. I assume that IOT devices with
limited networking capability will also have limited connectivity, for
example to a local relay.

>
> So I think this discussion is pointing to that we should actually move Section
> 6.3 from draft-ietf-tsvwg-datagram-plpmtud into draft-ietf-quic-transport so
> that the details can still be polished. 

I am comparing the specification in the paper with my early
implementation of PMTU discovery in Quic (Picoquic). They are pretty
much aligned, except for one important point regarding the sending of
probes. As the draft says, the Picoquic implementation will send probes
to discover the Path MTU, "by selecting step sizes from a table of
common PMTU sizes". But there is an additional step in the Picoquic
algorithm, a decision to probe or not as a trade-off between potential
gains in efficiency and potential costs in probing.

End-to-end probes consume resource. Sending a 1500 bytes probe consumes
about the same resource as sending 1500 bytes of data using the already
discovered packet size. Nodes send such probes because they expect a
gain in efficiency. If they discover that MTU size is 1500 bytes instead
of 1200 bytes, they can send a 1.5 MB file in 1000 packets instead of
1250. There will be 25% fewer packet headers, 25% fewer packet
processing, 25% fewer packet acknowledgements. That seems "worth the
try". But these gains depends on how many bytes the nodes expect to send
during the course of the connection.

Suppose now that instead of 1.5MB of data the node only anticipates
sending 15KB. Successful probe means sending 10 packets of 1500 bytes
instead of 13 packets of 1200 bytes. The typical overhead per Quic
packet would be about 60 bytes, between link header, IPv6 header, UDP
header and Quic header. Sending larger packets will save 180 bytes in
packet overhead, which is less than the 1500 bytes of sending a probe.
The node will achieve better throughput by not bothering with PMTU probes.

Of course, the numbers in the examples above are arbitrary. But there is
a principle there: at a given time, nodes send probes because they
anticipate a gain in efficiency over the reminder of the connection, if
the probe is successful. This gives us a simple test: only send probes
if the anticipated gain is larger than the cost of sending the probe:

    if (anticipated_gain * probability_of_success > cost-of_probe)

        then: send probe

        else: keep current MTU

That inequality informs the decision to send a probe or not. For
example, suppose that discovery so far has assessed that the PMTU is at
least 1400 bytes but lower than 1500. The table of step sizes says that
the most likely value between 1400 and 1500 is 1430. Is it a good idea
to try 1430? Maybe, but the efficiency gain will be just over 2%. In
case of success, the cost of probing will only be amortized after
sending 50 packets. If the probability of success is about 50%, the node
should only send the probe if it anticipates sending at least 100 packets.

The inequality also informs the decision of which probe to send. For
example, assume that the discovery so far has assessed that PMTU is at
least 1200 bytes. Will the node probe for 1500 bytes or 1400 bytes? To
make the decision, the node can assess the potential gains and the
probability of success. 1400 bytes might be a safer bet if the node
anticipates just sending a small amount of data, 1500 would provide a
higher gain if the node anticipates sending a large amount. Trying 8K
might be even more attractive if both peers support that size, as the
lower probability of success is compensated by a much higher gain.

The inequality also informs the decision on whether to repeat a probe.
Suppose that a node tried a 1500 bytes probe and failed. There are two
explanations: either the path MTU is lower than 1500 bytes, or the probe
was lost for other reasons. Applying Bayesian logic, the node can
reevaluate the probability of success of a 1500 bytes probe. It may well
be that sending a 1400 bytes probe will now be more interesting than
trying 1500 bytes again.

It may be that this discussion is too much detail for the plpmtud draft.
I could of course submit a draft discussing the probabilistic approach
to PMTUD. But it still might be a good idea to add a general
consideration in the description of the algorithm, asking implementation
to take into account probability of success and expected gains before
sending a probe.

-- Christian Huitema