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Re: HTTP/2 flow control <draft-ietf-httpbis-http2-17>

From: Bob Briscoe <bob.briscoe@bt.com>
Date: Fri, 20 Mar 2015 18:22:11 +0000
Message-ID: <201503201822.t2KIMA75012552@bagheera.jungle.bt.co.uk>
To: Stuart Douglas <stuart.w.douglas@gmail.com>
CC: <mbelshe@chromium.org>, <fenix@google.com>, <martin.thomson@gmail.com>, <ietf-http-wg@w3.org>
Stuart,

At 00:18 20/03/2015, Stuart Douglas wrote:


>==a) Intermediate buffer control==
>For this, sliding window-based flow control 
>would be appropriate, because the goal is to 
>keep the e2e pipeline full without wasting buffer.
>
>Let me prove HTTP/2 cannot do window flow 
>control. For window flow control, the sender 
>needs to be able to advance both the leading and 
>trailing edges of the window. In the draft:
>* WINDOW_UPDATE frames can only advance the 
>leading edge of a 'window' (and they are constrained to positive values).
>* To advance the trailing edge, window flow 
>control would need a continuous stream of 
>acknowledgements back to the sender (like TCP). 
>The draft does not provide ACKs at the 
>app-layer, and the app-layer cannot monitor ACKs 
>at the transport layer, so the sending app-layer 
>cannot advance the trailing edge of a 'window'.
>
>So the protocol can only support credit-based 
>flow control. It is incapable of supporting window flow control.
>
>
>Next, I don't understand how a receiver can set 
>the credit in 'WINDOW_UPDATE' to a useful value. 
>If the sender needed the receiver to answer the 
>question "How much more can I send than I have 
>seen ACK'd?" that would be easy. But because the 
>protocol is restricted to credit, the sender 
>needs the receiver to answer the much harder 
>open-ended question, "How much more can I send?" 
>So the sender needs the receiver to know how 
>many ACKs the sender has seen, but neither of them know that.
>
>The receiver can try, by taking a guess at the 
>bandwidth-delay product, and adjusting the guess 
>up or down, depending on whether its buffer is 
>growing or shrinking. But this only works if the 
>unknown bandwidth-delay product stays constant.
>
>However, BDP will usually be highly variable, as 
>other streams come and go. So, in the time it 
>takes to get a good estimate of the per-stream 
>BDP, it will probably have changed radically, or 
>the stream will most likely have finished 
>anyway. This is why TCP bases flow control on a 
>window, not credit. By complementing window 
>updates with ACK stream info, a TCP sender has 
>sufficient info to control the flow.
>
>The draft is indeed correct when it says:
>"¬ ¬  this can lead to suboptimal use of available
>¬ ¬  network resources if flow control is enabled without knowledge of the
>¬ ¬  bandwidth-delay product (see [RFC7323]).
>"
>Was this meant to be a veiled criticism of the 
>protocol's own design? A credit-based flow 
>control protocol like that in the draft does not 
>provide sufficient information for either end to 
>estimate the bandwidth-delay product, given it will be varying rapidly.
>
>
> From my point of view as a server/proxy 
> implementor the flow control window mostly 
> represent the amount of data I am prepared to buffer.¬
>
> From the server as a receiver case we are 
> basically acting as an intermediary between the 
> network and an end user application.

Yes.

>In this case the flow control credit basically 
>represents the maximum amount of data that I am 
>prepared to buffer at the HTTP layer. In this 
>case the answer to 'how much more can I send' is 
>simple, it is basically the amount of data I am 
>prepared to buffer. Because I have no idea how 
>quickly (if at all) the user application will 
>consume the data, all I can really do is buffer 
>it and deliver it as the user application requests it.¬
>
>If I set the flow control window to larger than 
>I am prepared to buffer and the application does 
>not consume data quickly enough then I have two 
>options, which are basically stop reading (head 
>of line blocking), or reset the stream, neither 
>of which is particularly good (head of line 
>blocking is particularly problematic if the user 
>application is trying to write a response before 
>reading the request, as window updates will not be processed).¬
>
>If I am only prepared to buffer a small amount 
>of data then my performance is not going to be 
>great no matter what flow control implementation 
>is in use, and I think that this is basically a 
>limitation of a multiplexed protocol (unless you 
>are prepared to accept potential HOL blocking).
>
>This mostly only affects server uploads, as 
>clients will likely have to buffer the whole 
>response anyway so are not constrained by buffer size.¬
>
>All the issues above basically apply to the 
>intermediary/proxy use case as well.¬
>
>Basically I guess what I am getting to is that 
>yes, there are some situations where HTTP2 might 
>perform worse than HTTP1, however I think the 
>underlying problem is intrinsic to any sort of 
>multiplexed protocol, rather than HTTP2's flow control mechanism.

There I disagree. The problem only applies to h2 
because it sits above TCP, and is not able to see 
the ACK information that TCP sees. If stream flow 
control is implemented in the same layer as both 
connection flow control and segment 
acknowledgement, then I suspect it could be made to work properly.

SCTP, Minion and QUIC have this architecture, so 
implementing stream flow control would not be an 
unsolvable problem (if a use for it were found, 
and if it could be made deadlock-free).


>==b) Control by the ultimate client app==
>For this case, I believe neither window nor 
>credit-based flow control is appropriate:
>* There is no memory management issue at the 
>client end - even if there's a separate HTTP/2 
>layer of memory between TCP and the app, it 
>would be pointless to limit the memory used by 
>HTTP/2, because the data is still going to sit 
>in the same user-space memory (or at least about 
>the same amount of memory) when HTTP/2 passes it over for rendering.
>
>
>Not necessarily, not all clients are browsers, 
>and even with browsers when downloading a file I 
>imagine the data will generally be transferred 
>straight to disk rather staying in memory. In 
>general I agree with you though, and I think 
>most clients will want to set a large window size.
>¬
>* Nonetheless, the receiving client does need to 
>send messages to the sender to supplement stream 
>priorities, by notifying when the state of the 
>receiving application has changed (e.g. if the 
>user's focus switches from one browser tab to another).
>* However, credit-based flow control would be 
>very sluggish for such control, because credit 
>cannot be taken back once it has been given 
>(except HTTP/2 allows 
>SETTINGS_INITIAL_WINDOW_SIZE to be reduced, but 
>that's a drastic measure that hits all streams together).
>
>
>This support is provided by PRIORITY frames,

Yes (as you can see, I suggested priorities 
myself). I was assuming per-stream flow control 
had been proposed for some reason, and having 
knocked down the other reasons I could think of, 
the only one left was for the client to use flow 
control to complement priorities.

You're saying per-stream flow control is not even 
safe for this case, and your reasoning below 
seems sound. So I guess you're implying there is 
very little, if anything, that per-stream flow 
control is good for. Am I putting words in your mouth?

>using flow control for such use cases is very 
>problematic and has a good chance of leading to deadlocks.

Setting aside whether stream flow control is useful,...
... if proper stream flow control were feasible 
(e.g. if it were implemented in the transport 
layer), I believe the protocol could be made 
structurally impossible to deadlock within its 
own layer (aside from any deadlock at the 
app-layer, as you point out later). I've written 
the rules for avoiding flow control deadlock into 
the Inner Space spec (using understanding gained 
from multipath TCP). My ambition is to give a 
proof that it cannot deadlock, even when combined 
with the byzantine behaviours of middleboxes. 
It's a tough ambition, but needed to determine 
any necessary conditions for the proof to hold.

>For example consider a Java Servlet container, 
>new requests will be read from the underlying 
>connection, and then dispatched to a worker 
>thread to generate the page. Once a server has 
>started processing a request there is no way to 
>pause the request in a way that frees up the 
>resources (threads, database connections etc) in 
>use. I think almost every use case more complex 
>than simple file serving has this issue, once a 
>server has started processing a request there is 
>generally no way to suspend it and free the resources.¬

I suspect you are right. The deadlock avoidance I 
was talking about above is a simpler problem (tho 
by no means trivial), because it solely concerns 
flow-through buffers, not endpoint resources.


>So in your example if the user switches tabs and 
>you stop sending window updates for streams in 
>use by the old browser tab, while sending new 
>requests for the new tab it is possible the 
>server will be in a situation where it is not 
>prepared to allocate resources for the new 
>requests until the existing ones are complete, 
>however these will never complete as the browser 
>has stopped sending window updates.¬
>
>You could argue that a server should limit the 
>max streams value to the number of streams that 
>it is prepared to allocate resources for, 
>however this greatly limits the utility of the 
>priority mechanism, as it means that servers 
>will always handle requests on a first come 
>first served basis. If we set the maximum 
>streams value to a higher amount than what we 
>are prepared to allocate resources for and queue 
>requests then when a request finishes we can 
>pick the highest priority request from the queue 
>to allocate resources to (not to mention there 
>is no round trip delay because the request is queued).
>
>Basically IMHO flow control should not be used 
>to control priority, that is what PRIORITY 
>frames are for, and in general servers can only 
>ever do priority on a best effort approach 
>anyway. If you try and use flow control to 
>enforce a strict priority mechanism you run a very real risk of deadlocks.

That sounds like a reasonable rule, at least for 
now until we gain better understanding.

This relates to the part of my review about h2 
entering uncharted theoretical territory, by 
allowing implementers free rein on what they do 
with the protocol elements, with no guidance or 
constraints. One end might write code that 
interprets flow control messages with some 
priority-related semantics, while the 
implementation at the other end did not intend that.

>¬
>
>==Flow control problem summary==
>
>With only a credit signal in the protocol, a 
>receiver is going to have to allow generous 
>credit in the WINDOW_UPDATEs so as not to hurt 
>performance. But then, the receiver will not be 
>able to quickly close down one stream (e.g. when 
>the user's focus changes), because it cannot 
>claw back the generous credit it gave, it can only stop giving out more.
>
>IOW: Between a rock and a hard place,... but 
>don't tell them where the rock is.
>
>
>For the reasons I outlined above I don't think this is actually a problem.

Certainly, if flow control is not used to close 
down a stream, then not being able to close down 
a stream won't be a problem. That does leave the 
problem of what per-stream flow control /can/ be 
used for, if it can't be used to slow down streams!

The only believeable use-case I've seen so far is 
blocking a stream's progress when it is first created.

>Also in terms of clawing back credit I thought 
>that in general it was a bad idea? The TCP RFC 
>explicitly states that "shrinking the window" is 
>strongly discouraged, although I must admit I am 
>not fully aware of the reasoning.

 From memory, the reasoning was simply that TCP 
can't unsend what it has already sent. So 
shrinking the window leaves the protocol in an 
indeterminate state where more buffer might be needed anyway.


Bob


>Stuart
>¬
>
>==Towards a solution?==
>
>I think 'type-a' flow control (for intermediate 
>buffer control) does not need to be at 
>stream-granularity. Indeed, I suspect a proxy 
>could control its app-layer buffering by 
>controlling the receive window of the incoming 
>TCP connection. Has anyone assessed whether this would be sufficient?
>
>I can understand the need for 'type-b' 
>per-stream flow control (by the ultimate client 
>endpoint). Perhaps it would be useful for the 
>receiver to emit a new 'PAUSE_HINT' frame on a 
>stream? Or perhaps updating per-stream PRIORITY 
>would be sufficient? Either would minimise the 
>response time to a half round trip. Whereas 
>credit flow-control will be much more sluggish 
>(see 'Flow control problem summary').
>
>Either approach would correctly propagate e2e. 
>An intermediate node would naturally tend to 
>prioritise incoming streams that fed into 
>prioritised outgoing streams, so priority 
>updates would tend to propagate from the 
>ultimate receiver, through intermediate nodes, up to the ultimate sender.
>
>==Flow control coverage==
>
>The draft exempts all TCP payload bytes from 
>flow control except HTTP/2 data frames. No 
>rationale is given for this decision. The draft 
>says it's important to manage per-stream memory, 
>then it exempts all the frame types except data, 
>even tho each byte of a non-data frame consumes 
>no less memory than a byte of a data frame.
>
>What message does this put out? "Flow control is 
>not important for one type of bytes with 
>unlimited total size, but flow control is so 
>important that it has to be mandatory for the other type of bytes."
>
>It is certainly critical that WINDOW_UPDATE 
>messages are not covered by flow control, 
>otherwise there would be a real risk of 
>deadlock. It might be that there are 
>dependencies on other frame types that would 
>lead to a dependency loop and deadlock. It would 
>be good to know what the rationale behind these rules was.
>
>
>I think a lot of people had similar concerns. 
>There is a discussion about it here:¬ 
><https://lists.w3.org/Archives/Public/ietf-http-wg/2014AprJun/0647.html>https://lists.w3.org/Archives/Public/ietf-http-wg/2014AprJun/0647.html
>¬
>
>==Theory?==
>
>I am concerned that HTTP/2 flow control may have 
>entered new theoretical territory, without 
>suitable proof of safety. The only reassurance 
>we have is one implementation of a flow control 
>algorithm (SPDY), and the anecdotal non-evidence 
>that no-one using SPDY has noticed a deadlock 
>yet (however, is anyone monitoring for deadlocks?).
>
>Whereas SPDY has been an existence proof that an 
>approach like http/2 'works', so far all the 
>flow control algos have been pretty much 
>identical (I think that's true?). I am concerned 
>that the draft takes the InterWeb into uncharted 
>waters, because it allows unconstrained 
>diversity in flow control algos, which is an untested degree of freedom.
>
>The only constraints the draft sets are:
>* per-stream flow control is mandatory
>* the only protocol message for flow control 
>algos to use is the WINDOW_UPDATE credit message, which cannot be negative
>* no constraints on flow control algorithms.
>* and all this must work within the outer flow control constraints of TCP.
>
>Some algos might use priority messages to make 
>flow control assumptions. While other algos 
>might associate PRI and WINDOW_UPDATE with 
>different meanings. What confidence do we have 
>that everyone's optimisation algorithms will 
>interoperate? Do we know there will not be 
>certain types of application where deadlock is likely?
>
>"¬ ¬  When using flow
>¬ ¬  control, the receiver MUST read from the TCP receive buffer in a
>¬ ¬  timely fashion.¬  Failure to do so could lead to a deadlock when
>¬ ¬  critical frames, such as WINDOW_UPDATE, are not read and acted upon.
>"
>I've been convinced (offlist) that deadlock will 
>not occur as long as the app consumes data 
>'greedily' from TCP. That has since been 
>articulated in the above normative text. But how 
>sure can we be that every implementer's 
>different interpretations of 'timely' will still prevent deadlock?
>
>Until a good autotuning algorithm for TCP 
>receive window management was developed, good 
>window management code was nearly non-existent. 
>Managing hundreds of interdependent stream 
>buffers is a much harder problem. But 
>implementers are being allowed to just 'Go forth 
>and innovate'. This might work if everyone 
>copies available open source algo(s). But they 
>might not, and they don't have to.
>
>This all seems like 'flying by the seat of the pants'.
>
>==Mandatory Flow Control? ==
>
>"¬ ¬ ¬ ¬ ¬  3. [...] A sender
>¬ ¬ ¬ ¬ ¬ ¬  MUST respect flow control limits imposed by a receiver."
>This ought to be a 'SHOULD' because it is 
>contradicted later - if settings change.
>
>"¬ ¬  6.¬  Flow control cannot be disabled."
>Also effectively contradicted half a page later:
>"¬ ¬  Deployments that do not require this capability can advertise a flow
>¬ ¬  control window of the maximum size (2^31-1), and by maintaining this
>¬ ¬  window by sending a WINDOW_UPDATE frame when any data is received.
>¬ ¬  This effectively disables flow control for that receiver."
>
>And contradicted in the definition of half closed (remote):
>"¬  half closed (remote):
>¬ ¬ ¬ ¬ ¬  [...] an endpoint is no longer
>¬ ¬ ¬ ¬ ¬  obligated to maintain a receiver flow control window.
>"
>
>And contradicted in 
><https://tools.ietf.org/html/draft-ietf-httpbis-http2-17#section-8.3>8.3. 
>The CONNECT Method, which says:
>"¬  Frame types other than DATA
>¬ ¬  or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
>¬ ¬  MUST NOT be sent on a connected stream, and MUST be treated as a
>¬ ¬  stream error (Section 5.4.2) if received.
>"
>
>Why is flow control so important that it's 
>mandatory, but so unimportant that you MUST NOT do it when using TLS e2e?
>
>Going back to the earlier quote about using the 
>max window size, it seems perverse for the spec 
>to require endpoints to go through the motions 
>of flow control, even if they arrange for it to 
>affect nothing, but to still require 
>implementation complexity and bandwidth waste 
>with a load of redundant WINDOW_UPDATE frames.
>
>HTTP is used on a wide range of devices, down to 
>the very small and challenged. HTTP/2 might be 
>desirable in such cases, because of the improved 
>efficiency (e.g. header compression), but in 
>many cases the stream model may not be complex 
>enough to need stream flow control.
>
>So why not make flow control optional on the 
>receiving side, but mandatory to implement on 
>the sending side? Then an implementation could 
>have no machinery for tuning window sizes, but 
>it would respond correctly to those set by the 
>other end, which requires much simpler code.
>
>If a receiving implemention chose not to do 
>stream flow control, it could still control flow 
>at the connection (stream 0) level, or at least at the TCP level.
>
>
>==Inefficiency?==
>
><https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-5.2>5.2. 
>Flow Control
>"Flow control is used for both individual
>¬ ¬  streams and for the connection as a whole."
>Does this means that every WINDOW_UPDATE on a 
>stream has to be accompanied by another 
>WINDOW_UPDATE frame on stream zero? If so, this 
>seems like 100% message redundancy. Surely I must¬  have misunderstood.
>
>==Flow Control Requirements===
>
>I'm not convinced that clear understanding of 
>flow control requirements has driven flow control design decisions.
>
>The draft states various needs for flow-control 
>without giving me a feel of confidence that it 
>has separated out the different cases, and 
>chosen a protocol suitable for each. I tried to 
>go back to the early draft on flow control 
>requirements < 
>http://tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01  
> >, and I was not impressed.
>
>I have quoted below the various sentences in the 
>draft that state what flow control is believed 
>to be for. Below that, I have attempted to 
>crystalize out the different concepts, each of 
>which I have tagged within the quotes.
>
>* 
><https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-2>2. 
>HTTP/2 Protocol Overview says
>¬  "Flow control and prioritization ensure that 
>it is possible to efficiently use multiplexed streams. [Y]
>¬ ¬  Flow control (Section 5.2) helps to ensure 
>that only data that can be used by a receiver is transmitted. [X]"
>* 
><https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-5.2>5.2. 
>Flow Control says:
>¬  "Using streams for multiplexing introduces 
>contention over use of the TCP connection [X], 
>resulting in blocked streams [Z]. A flow control 
>scheme ensures that streams on the same 
>connection do not destructively interfere with each other [Z]."
>* 
><https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-5.2.2>5.2.2. 
>Appropriate Use of Flow Control
>"¬  Flow control is defined to protect endpoints that are operating under
>¬ ¬  resource constraints.¬  For example, a proxy needs to share memory
>¬ ¬  between many connections, and also might have a slow upstream
>¬ ¬  connection and a fast downstream one [Y].¬  Flow control addresses cases
>¬ ¬  where the receiver is unable to process data on one stream, yet wants
>¬ ¬  to continue to process other streams in the same connection [X]."
>"¬  Deployments with constrained resources (for example, memory) can
>¬ ¬  employ flow control to limit the amount of memory a peer can consume. [Y]
>
>Each requirement has been tagged as follows:
>[X] Notification of the receiver's changing utility for each stream
>[Y] Prioritisation of streams due to contention 
>over the streaming capacity available to the whole connection.
>[Z] Ensuring one stream is not blocked by another.
>
>[Z] might be a variant of [Y], but [Z] sounds 
>more binary, whereas [Y] sounds more like 
>optimisation across a continuous spectrum.
>
>
>Regards
>
>
>
>Bob
>
>________________________________________________________________
>Bob Briscoe,¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 
>¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬  BT

________________________________________________________________
Bob Briscoe,                                                  BT 
Received on Friday, 20 March 2015 18:22:51 UTC

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