Re: [boost] [asio] RFC on new reliable UDP library

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). https://en.wikipedia.org/wiki/Micro_Transport_Protocol On Tue, Sep 9, 2014 at 11:59 AM, Niall Douglas wrote:

Dear Boost and ASIO users,

CC: boost@lists.boost.org, asio-users@lists.sourceforge.net

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 (http://www.hyc.io/boost/expected-proposal.pdf) 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 http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/

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