Re: Options for QUIC Multipath

Martin Duke <> Mon, 15 February 2021 20:07 UTC

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From: Martin Duke <>
Date: Mon, 15 Feb 2021 12:07:08 -0800
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Subject: Re: Options for QUIC Multipath
Cc: Christian Huitema <>, IETF QUIC WG <>
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Thanks for all of this, Christian.

FWIW, I am deeply concerned about the complexity of the second approach,
and like that the first approach's changes are almost entirely sender-side.
IIUC on the receiver side, the only requirement is enough state to keep
more than 1 path validated and active at once. If the objective is to
provide a way to get MP-QUIC experiments up and running, this is an
important consideration.

I am also quite fearful of undermining the assumption that sequence numbers
increase monotonically; in particular, I'm afraid of messing up the key
update logic.

On Mon, Feb 15, 2021 at 7:48 AM Behcet Sarikaya <>

> On Sun, Feb 14, 2021 at 4:23 PM Christian Huitema <>
> wrote:
>> I authored two drafts proposing two different solutions for Multipath
>> QUIC: QUIC Multipath Negotiation Option (
>>; and,
>> in collaboration with colleagues at Ali Baba, Multipath Extension for QUIC (
>> Apart from
>> some details that could easily be aligned, the main difference is that the
>> “negotiation option” maintains the property of QUIC Transport
>> <> to have a
>> single packet number space for all application packets while the “multipath
>> extension for QUIC” specifies that there will be a specific packet number
>> space for each path. I have now implemented both options in Picoquic. This
>> blog describes what I learned:
>> .
>> To summarize, I believe now that both options work. The simple option
>> requires some additional work for managing acknowledgement, but the
>> multiple number space option adds a lot more complexity (41 new code
>> branches compared to only 6), and will require a lot more testing because
>> it also change the processing of the "single path" scenarios. The multiple
>> number space option also prevents the use of zero-length connection IDs,
>> and thus causes additional overhead in some important deployment scenarios.
>> So, yes, both options work, but the simpler option provides simpler code
>> and also less overhead.
> Great!
> I thought it was a given.
> Thanks for your hard work Christian.
> Behcet
>> In any case, I hope that this exercise will inform our efforts to
>> standardize multipath support in QUIC.
>> -- Christian Huitema