Re: Options for QUIC Multipath

Christian Huitema <> Sat, 10 April 2021 09:11 UTC

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To: Mirja Kuehlewind <>, IETF QUIC WG <>
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From: Christian Huitema <>
Subject: Re: Options for QUIC Multipath
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Date: Sat, 10 Apr 2021 02:09:18 -0700
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On 4/9/2021 6:36 AM, Mirja Kuehlewind wrote:

> Hi Christian, hi all,
> reviving this thread. First of all thanks for implementing both options 
and writing the blog post!
> After re-reading you post and reviewing the drafts, however, I have to say that I’m more convinced now that multiple packet number spaces might be the better approach forward. I actually always thought of the packet number as a per-path property where the wire image for each path should be as much independent as possible. So that seem architecturally more clean to me.
There are pros and cons with the "path property" approach. The main 
issue is that the definition of a path is not as obvious as it appears 
at first sight. We may think of a path as defined by the pair of sender 
and receiver addresses and ports, but this gets unclear if NATs are 
present. We end up with either a narrower definition using connection 
identifiers as in draft-liu, or avoiding the problem altogether by 
making packet numbers global. There are issues with both approaches, 

* Support for 0-length connection identifiers in draft-liu

* Need to maintain a path object with wider scope than the connection 
identifier in draft-liu, e.g. remembering congestion control, PMTU and 
RTT through a connection-ID renewal

* Size of acknowledgement frames if using a global number approach

The good news is that we are making progress on these issues, using a 
bit more sophistication. Ideas include:

* Identifying paths through using the corresponding source ID if the 
DCID is zero length. This enables control per path such as "don't use 
this path anymore" even if the peer uses zero-length DCID.

* If a peer uses zero length CID, use global path numbers when sending 
to that peer -- essentially a fusion of draft-liu and global numbers 

* Manage the size of ACK frames by limiting the number of times a given 
ACK range is repeated in ACKs. This is a very simple implementation 
recommendation that mitigates the growth of ACK frames when global 
numbers are used, including in the zero-length CID case.

> However, I’m really not convinced by your argument about implementation complexity below. First of all when we talk about implementation 
complexity, we should not only consider lines of code or number of tests or something like that but I think it is more important to assess the potential for implementation error. That is harder to assess but I think having a clean design and reduce the number of interdependencies is a factor.
> Further, implementation complexity should never be considered as a the sole metric. You actually convinced me in your blog post that what you call efficiency might be even more important because there are two aspects here: number of bits on the wire (for ACK frames that might have a lot of 
wholes) and amount of bits in local memory.

My personal concern is the impact on code quality. If we end up with 
code branches that are rarely used, these branches will not be as well 
tested as the "main path", and  bugs in these branches may surface 
later. The inverse correlation between complexity and security is well 

> With this conclusion I see draft-liu-multipath-quic as a really good starting point for future work (however, that so far my personal assessment). In both cases I support the approach to design a multipath extension that minimizes the changes needed from the base protocol. So reusing the connection ID and connection ID update mechanism is I think definitely the 
right approach to take.

I am certainly willing to use draft-liu as a starting point. I would not 
be a co-author of that draft if I did not believe that.

> I also think that any mechanism for address/path negotiation do not need to be part of the initial extension. In the most common scenario the client might just open a second path without further negotiation or coordination with the server when the interface/IP address of that new path come 
available. However, even if any negotiation is needed, this can be done on the application layer or added by another extension later on.

We agree.

> For draft-liu-multipath-quic I would even recommend to even move the part about scheduling and QoS support into the separate draft. I think QoS signal can definitely be a separate extension because that might even be useful without multiple paths (e.g. as input for congestion control). And 
for scheduling, I recommend to just specify some per-stream scheduling as 
the default behavior for now, but leave more complex schemes for future work (or research; scheduling doesn’t need standardization as it can be changed sender-side only).

My co-authors have been doing an excellent work investigating scheduling 
issues in a multipath environment. The question that we want to answer 
is essentially, when is a multipath setup worse than a single path 
configuration, and how can we mitigate that? The main answer is that 
multipath does degrade performance if packets are scheduled on a lossy 
path and later need to be repeated on another path, creating additional 
delays. I would rather wait the next draft release for explaining 
mitigations, because it is a bit long for email. But describing such 
mitigations absolutely belongs in the multipath draft.

> So as soon as we could converge on the packet number question, I think we have a good starting to move on!
> Again thanks for your work and for the drafts!
> Mirja

You are welcome.

-- Christian Huitema

> From: QUIC <> on behalf of Christian Huitema <>
> Date: Sunday, 14. February 2021 at 23:23
> Subject: Options for QUIC Multipath
> 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.
> In any case, I hope that this exercise will inform our efforts to standardize multipath support in QUIC.
> -- Christian Huitema