[Libwebsockets] Do we have plan to support web socket over http2 client on libwebsockets.

Andy Green andy at warmcat.com
Wed Sep 19 09:29:42 CEST 2018



On 19/09/2018 15:11, zhang guojun wrote:
> Andy, glad to receive you email.
> Thank you for the very detail reply.
> 
> Please see my comments inline with red color.
> 
> Thanks
> Guojun
> 
> 
>> On Sep 18, 2018, at 3:39 PM, Andy Green <andy at warmcat.com 
>> <mailto:andy at warmcat.com>> wrote:
>>
>>
>>
>> On 19/09/2018 00:40, zhang guojun wrote:
>>> Hi Andy,
>>> Thanks for the quickly reply and very detail explanation.
>>> In my case, there is a manage server to manage various network 
>>> device. Right now we are using websocket as the transport tunnel, but 
>>> as we know, we can see head-of-line-blocking issue.
>>
>> I know what that is, but what do you mean by it?
>>
>> Actually you can't properly solve that without QUIC... but since you 
>> talk about a non-h2 ws solution these are presumably individual tcp 
>> connections... one can't block the other.
>>
>> TCP itself can block itself where there is packet loss due to its 
>> requirement to deliver in-order... if that's what you mean h2 won't 
>> solve it since it's also on top of TCP.  QUIC is designed on top of 
>> UDP transport and to the extent its packets are formed from content 
>> from one stream, loss + retry does not block packets containing 
>> content from other streams.
>>
>> You should clarify exactly where the blocking you think makes trouble 
>> is coming from, because h2 won't necessarily solve it.
> 
> [GZ]: in my case, multiple device send message to websocket server at 
> almost same time, but one of device have a real-time application need 
> higher priority and lower latency, if some of the connection have occupy 

Mmm... the network decides about the latency.  Network programming is 
not like designing a system where everything is local, you cannot always 
control latency.  You can do things to reduce it generally but if your 
application cares so much about it and is using tcp / websockets, it's 
likely got problems.

> longer time for response, it will cause the higher priority connection 
> block, since H2 have multiplexing, so it should solve the problem of my 
> case.

No, latency on two h2 streams being muxed is always going to be worse 
than two separate h1 connections.  So if you already see this problem, 
h2 won't fix it.

> My understand is websocket doesn’t have multiplexing, if I understand 
> wrong, please correct me.
> I agree with you, QUIC solve TCP level head-of-line blocking. It should 
> be ultimate solution of all kinds of networks blocking issue.
>>
>>> So I consider the websocket over h2, in this way,  we can reuse some 
>>> of the existing code. We may loss a little bit bandwidth compare to 
>>> raw websocket, but it still can acceptable.
>>> I go through the ws-over-h2 draft, the technology is flawless and you 
>>> already implement server side code. In your perspective, what’s the 
>>> reason the draft haven’t been a RFC?
>>
>> It's simply going through the process, which takes time.  The author 
>> is one of the Great and Good from Mozilla, and the proposal is very 
>> reasonable and limited in scope.
> [GZ]: that’s great, looking forward to see it become RFC soon.
>>
>>> I’m also considering to use H2 directly.
>>
>> By default what flows on DATA is understood to be http.  What happens 
>> without a content-length on both sides and unlimited DATA in both 
>> directions depends on the implementation... this isn't in the h2 spec 
>> and the compliance tools don't test for it.  ws-over-h2 upgrades the 
>> connection so it is logically no longer http protocol on the stream.
> [GZ]: Looks like I have some wrong understanding of ws-over-h2, I though 
> websocket frame is the payload of HTTP2, going through the new protocol 
> again, I get it. After handshake, it just a websocket on the stream. So 

Yes, the protocol on the stream is told to be "websocket" after the 
ws-over-h2 upgrade.  So endpoints that understand that no longer think 
of traffic on the stream in http terms.

> my question is, does ws-over-h2 still inherit the multiplexing of H2? Or 
> you don’t think multiplexing is necessary for ws-over-h2.

It doesn't really 'inherit' anything, the only way for an h2 stream to 
participate on the h2 "bundle" / "master" / "network connection" is as a 
subordinate h2 stream.

So it must follow the rules about h2 tx credit... it can't send anything 
the remote stream endpoint hasn't already told it is willing to receive. 
The ws-over-h2 steam WRITABLE callback is not directly related to the 
"bundle" being WRITABLE, because it does not itself own the network 
connection... it participates in a round-robin sharing of the bundle / 
connection writability with all the streams, be they http or ws protocol 
on them.

In other words, there's a new way ws streams on h2 have to "wait their 
turn".  Hence ws-over-h2 is always going to have same or worse latency 
than ws-over-h1.

>>> BTW, does libwebsockets support SERVER PUSH and POST?
>>
>> There's no lws api to use PUSH_PROMISE.  I proposed on httpbis that 
>> ws-over-h2 support PUSH_PROMISE, because this would allow what was 
>> originally a "GET index.html" to have sent the first data on a ws link 
>> index.html will want to open before the client has finished receiving 
>> index.html... instead of a RTT setting up the ws link much later it 
>> could have delivered the first data before the browser realized it 
>> wanted the ws link: instant ws data as soon as the JS opened the ws 
>> without any network activity.  But it was told it complicated the spec 
>> too much.
>>
>> For me I don't have a need to implement PUSH_PROMISE without that. 
>> PUSH_PROMISE on http traffic has the internal contradiction the server 
>> doesn't know what the client has in its private cache.  So if it 
>> starts setting up and partially sending CSS, JS, IMG or whatever, 
>> after the first time if the private cache policy is reasonable, that 
>> is just complete waste and the client will ignore the streams every 
>> time since it has them in private cache already.  So PUSH_PROMISE is 
>> kind of useless AFAIK.
> [GZ]: hope I understand right. it may useless on B/S mode, but it still 
> useful in C/S mode. In our case(C/S mode), server push some commands to 
> clients whenever it want, clients need to apply that command and 
> response the status to server. There no any duplicate data pushed to 
> client. So as a end-to-end transport, PUSH_PROMISE + POST provide a 
> bi-direction transmit capability.

That is not what h2 "server push" / PUSH_PROMISE does.  Section 8.2 is 
literally called "server push" in the RFC

https://tools.ietf.org/html/rfc7540#section-8.2

You seem to be wrongly thinking it's some kind of long poll mechanism 
but it's completely unrelated.  It's the server trying to get ahead of 
things by inferring that if it is sending you index.html, you will soon 
want mysite.css, mysite.js and starting the streams for that speculatively.

You can open an XHR back to the server and do something like that though.

But if your basic problem is a tight latency requirement, and it's not 
that your existing implementation is just poor, you need to go back and 
study exactly where this latency is coming from.

-Andy

>> Lws h2 supports POST (including multipart / file upload), CGI 
>> (translating headers from the h1-only CGI) and proxying h2 <-> h1, 
>> including on unix domain sockets.
>>
>> -Andy
>>
>>> Thanks
>>> Guojun
>>>> On Sep 14, 2018, at 5:57 PM, Andy Green <andy at warmcat.com 
>>>> <mailto:andy at warmcat.com>> wrote:
>>>>
>>>>
>>>>
>>>> On 15/09/2018 02:10, zhang guojun wrote:
>>>>> Dear libwebsockets developers.
>>>>> I’m glad to see libwebsocket support websocket over http2 server, 
>>>>> do we have any plan to support web socket over HTTP2 client?
>>>>
>>>> Yeah, but not immediately.  H2 client is working (including multi 
>>>> client stream coalescing into one h2 connection) for HTTP GET 
>>>> already, just not the ws-over-h2.
>>>>
>>>> - I don't personally need it atm.  There's a really big benefit 
>>>> implementing server side as lws already has, if your client is a 
>>>> browser with it implemented.  Once the TLS + h2 is up for fetching 
>>>> html / css / js, an additional ws connection can be established 
>>>> inside the h2 connection, and start sending data, in just one RTT. 
>>>>  It only benefits clients that are opening multiple streams on the 
>>>> server... if the client just opens the one ws connection, it still 
>>>> has to do the TLS and start the h2 connection from scratch, so 
>>>> there's no advantage for that case.
>>>>
>>>> - It's not a small subproject... it interacts with both h1, h2 and 
>>>> ws roles / parsers and h2 needs all stream tx initiated from the h2 
>>>> round-robin scheduler.
>>>>
>>>> - Aside from the Chrome browser developers who contacted me while we 
>>>> mutually used each other's implementation for testing, IIRC you're 
>>>> the first person to acknowledge the existence of even the server work.
>>>>
>>>> - Although the draft RFC is small and unlikely to change, it's not 
>>>> offical yet (it seems just a matter of time though)
>>>>
>>>> - Implementation status outside of Chrome (it's in Canary 67+ if you 
>>>> enable special flags) is opaque.  I asked twice on httpbis and just 
>>>> got ignored.  I guess they won't enable it by default until the RFC 
>>>> is formally accepted.  If you know the server has it and your 
>>>> non-browser client has it, you don't care about browser status though.
>>>>
>>>> Of course if someone wants to pay my consultancy rate for the couple 
>>>> of weeks it would take I could get religion about it.
>>>>
>>>> -Andy
>>>>
>>>>
>>>>> Thanks
>>>>> Guojun
>>>>> _______________________________________________
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>>>>> <mailto:Libwebsockets at ml.libwebsockets.org>
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> 


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