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,
notably:
* 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
approach
* 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
known.
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 <[email protected]> on behalf of Christian Huitema
<[email protected]>
Date: Sunday, 14. February 2021 at 23:23
To: IETF QUIC WG <[email protected]>
Subject: Options for QUIC Multipath
I authored two drafts proposing two different solutions for Multipath QUIC: QUIC
Multipath Negotiation Option
(https://datatracker.ietf.org/doc/draft-huitema-quic-mpath-option/); and, in
collaboration with colleagues at Ali Baba, Multipath Extension for QUIC
(https://datatracker.ietf.org/doc/draft-liu-multipath-quic/). Apart from some details
that could easily be aligned, the main difference is that the “negotiation option”
maintains the property of QUIC
Transport<https://datatracker.ietf.org/doc/draft-ietf-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:
https://huitema.wordpress.com/2021/02/14/how-many-packet-number-spaces-for-quic-multipath/.
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
