Decreasing the default initial maximum size to 1250

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Stanislav Slusny

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Dec 8, 2022, 10:08:50 AM12/8/22
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Hi all,

QUICHE change 36d9a1f decreased the default initial maximum size of a QUIC packet from 1350 to 1250.
Would it be possible to share the motivation and impact ?

RFC says "QUIC assumes a minimum IP packet size of at least 1280 bytes ... this results in a maximum datagram size of 1232 bytes for IPv6 and 1252 bytes for IPv4."
The figure 12 in article [1] suggests that there is just  a very small difference between 1250 bytes and 1350 bytes in reachability.  Was the unreachability of some clients the main motivation  ?

I assume that Chrome uses Path MTU Discovery, but still, would it be possible to share the performance impact ?

Thanks!

Stanislav Slusny  

[1] The QUIC Transport Protocol: Design and Internet-Scale Deployment

Haoyue Wang

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Dec 8, 2022, 11:21:50 AM12/8/22
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Hi Stanislav,

"Was the unreachability of some clients the main motivation?" Yes. With a smaller default QUIC packet size, there is potentially more QUIC usage. Understandably the downside of this change is that there can be more packets sent for clients already reachable and different applications should tune this trade-off differently. 1250 is selected based on experiments. The numbers from figure 12 in article [1] are still reasonable. Hence the net impact is modest but it opens more headroom for Path MTU Discovery.

Haoyue

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