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Subject: Re: [boost] [asio] RFC on new reliable UDP library
From: Cory Nelson (phrosty_at_[hidden])
Date: 2014-09-10 14:36:24

I'd suggest looking at BitTorrent's uTP. It's well established and is built
with reliability, perf, and firewall compatibility in mind. It does have
congestion control tuned to be a lower priority than any other traffic, but
I see no reason that couldn't be changed (or made configurable).

On Tue, Sep 9, 2014 at 11:59 AM, Niall Douglas <s_sourceforge_at_[hidden]>

> Dear Boost and ASIO users,
> CC: boost_at_[hidden], asio-users_at_[hidden]
> I am writing to ask for comments on the design of and interest in a
> generic reliable messaging library based on ASIO. Such a library
> could bring to ASIO very considerably improved data transfer
> performance over TCP, plus the ability to ignore NAT firewalls. The
> design imperatives for such a library are proving tricky to reach
> consensus upon, so perhaps the community might be able to help.
> Firstly, what features should such a reliable messaging library have?
> Here are some for you to consider:
> * Should it have wire transport backends with an abstraction from
> wire transports, so a UDT backend could be interchangeable with TCP
> for example with just a recompile needed? Upper level code need
> therefore not consider what format the wire transport takes, just
> that messages are reliably delivered or we are told of the connection
> failure. Some mechanism for dealing with out of order messages, and
> message expiry in case congestion control excessively delays outbound
> messages would also be included.
> You should be aware that congestion control for a reliable messaging
> library means one has no choice but to pace message construction
> (i.e. block/EWOULDBLOCK handing out new message send templates),
> otherwise one ends up buffering more data for sending than
> potentially can ever be delivered. This is because we cannot drop
> reliable messages, nor can we pace writes, so pacing new message
> construction is the only alternative.
> * Should it be zero copy for both receipt and send? If so, we'll have
> to break with the existing ASIO API somewhat - namely that
> MutableBufferSequence will need to now become
> MutableFileOffsetBufferSequence so one can supply scatter/gathers of
> user space buffers and kernel side buffers. We'll also have to
> templatise the buffer management so client code can supply the
> virtualisation (e.g. an iterator) which remaps inbound message
> fragments into apparently contiguous regions. By the time we have
> done these, the API looks quite different.
> * Should it be written using C++ 14 idioms instead of the existing 03
> idioms ASIO uses? If so, basing read and write handlers on the C++ 17
> proposed experimental::expected<T, E>
> ( makes much more sense
> than the fixed ASIO handler prototype void (*handler)(const
> boost::system::error_code& ec, std::size_t bytes_transferred) - it is
> more powerful, more expressive, optionally integrates cleanly with
> no-alloc future-promise and the rest of the C++ 11/14 STL, and lets
> handlers use any mix of error reporting mechanism they choose. It
> also opens the door to a much cleaner and tighter integration of the
> future C++ 17 Networking TS with Concurrency TS.
> Equally, especially if combined with the earlier changes, it goes
> further and further away from the present ASIO design.
> * How concurrent should it be capable of? Unlike TCP where it makes
> not much sense to have concurrent writers, UDP allows enormous
> parallelism, potentially into queue depths of thousands on decent
> NICs, sufficiently so where a strand based design may simply not
> scale. We believe that with some effort a completely lock free design
> which can make full use of the batch datagram send and receive APIs
> on Linux and Windows is possible - we believe that such a design
> would scale close to linearly to CPU cores available. Unfortunately,
> as batch datagram send/receive is not supported by ASIO and on POSIX
> the main dispatcher appears to be locked by a mutex, this would
> require a custom code path and i/o service. If combined with all of
> the above, it starts to look like a rewrite of the core of ASIO which
> seems very big just to implement reliable messaging.
> I guess that is good for now for starting discussion. My thanks for
> your time in advance.
> Niall
> --
> ned Productions Limited Consulting
> _______________________________________________
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Cory Nelson

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