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

Roberto Peon <> Fri, 20 March 2015 17:34 UTC

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Date: Fri, 20 Mar 2015 10:29:41 -0700
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From: Roberto Peon <>
To: Bob Briscoe <>
Cc: Jason Greene <>, Patrick McManus <>, HTTP Working Group <>
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Subject: Re: HTTP/2 flow control <draft-ietf-httpbis-http2-17>
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The appetite for complexity was very low here, and so what we have is the
least complex not-obviously-broken functionality that seems likely to
prevent HOL blocking.

On Fri, Mar 20, 2015 at 9:50 AM, Bob Briscoe <> wrote:

>  Jason,
> At 16:00 19/03/2015, Jason Greene wrote:
> I think thats a good argument for why it’s not suitable for
> rate-limiting relative to a variable bandwidth product of a single
> connection (which i took as your use-case a).
> I believe there is no need for an intermediate node to do flow control for
> individual streams. It does need to control the whole envelope within which
> all the streams are flowing through the proxy's app-layer buffer memory
> (e.g. due to a thick incoming pipe feeding a thin outgoing pipe). The best
> mechanism for controlling the app-layer buffer consumption of the aggregate
> connection is for the intermediate node to control the TCP receive window
> of the incoming stream.
> That doesn't preclude the intermediate node passing on any per-stream flow
> control messages emanating from the ultimate receiver so that the ultimate
> sender controls each stream's rate, which will alter the balance between
> streams within the overall envelope at the proxy.
> I explained all this in my review.
>  However, memory limits are typically constant, and an HTTP/2 server can
> be configured to limit the capacity of the server based on the combination
> of the “window†size, the number of streams allowed, and the number of
> connections allowed. Without a hard data credit, the server would have to
> either HOL block, prevent valid large requests from processing, or run out
> of resources and start dropping requests.
> I think you've moved away from the intermediate node case here? Your
> suggestions are valid. They are the crude controls available because more
> fine-grained per-stream control seems unattainable in h2, even tho I think
> the impression has been given that this requirement had been satisfied.
> Altho per-stream flow control seems unattainable, there is still stream
> priority.
> Bob
>  On Mar 19, 2015, at 10:38 AM, Bob Briscoe <> wrote:
> Jason,
> Yes, I agree this is what we /want/ per-stream flow control to do. My
> review explained why the flow control in h2 won't be able to do this.
> Specifically:
>  ==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.
> Bob
> At 15:08 19/03/2015, Jason Greene wrote:
> Hi Bob,
> I agree with you that HTTP/2 flow control is not useful for your a)
> use-case, intermediate buffer control. It is, however, necessary for the b)
> use case, memory management of an endpoint.
> An HTTP server is often divided into a core set of components which manage
> work scheduling and HTTP protocol aspects, as well as a set of
> independantly developed  “application” components, that  are often, but
> not necessarily,  provided by the end-user of the server. There is
> typically an abstraction between application code and server code, where
> the application is only ever aware of its stream. The stream may be
> provided by a single HTTP/1.1 TCP socket, or it might be multiplexed over
> HTTP/2 or SPDY, or some other protocol.  In all of these cases the
> abstraction appears the same to the application, as it need not care. The
> application code typically isnÂ’t required (nor able) to read everything
> immediately; instead, it is  allowed to read as it processes the data, and
> this ultimately requires the server to buffer the data in the interim
> (since other streams should not be blocked). HTTP/2 flow control allows the
> server to limit the maximum memory usage of this buffering, which would
> otherwise be significant since HTTP data can be of arbitrary length.
> -Jason
> On Mar 19, 2015, at 6:54 AM, Bob Briscoe <> wrote:
> Patrick,
> Thank you for taking the time to read my review carefully. I've been away
> from mail a few days, which should have allowed time for anything
> substantive to come from the people with SPDY experience.
> We only had the Rob/Greg exchange which merely talked about what some
> theoretical thing called flow control might be useful for, rather than
> really addressing my concern that the credit-based protocol provided in h2
> is unlikely to be able to provide control if it is needed. Nonetheless,
> Greg did summarise well the limited usefulness of h2's mechanism.
> I should probably have provided a summary of my (long) review, which I try
> to do below.
> I agree that there could be some use for the h2 credit-based protocol at
> the very start of a stream (in both the C->S and S->C PUSH cases you
> mentioned). And it might possibly be useful for the cases you mention about
> unsolicited data, which sound like they occur at the start too. Altho
> without knowing more about these DoS cases I'm not sure; RST_STREAM might
> have been sufficient.
> However, once a stream has been allowed to open up it's rate, your
> observation that we mostly see very large windows is what I predicted. It
> demonstrates that the credit-based protocol does not have sufficient
> information to be useful to regulate flow. It is effectively being treated
> like the human appendix - something that no longer serves any purpose but
> you have to continually put in effort to keep it healthy otherwise it could
> stop the rest of the system from working.
> For this reason, I questioned why flow control has been made mandatory.
> And I suggested instead that the credit-based flow control in h2 could be
> i) mandatory for a data sender to respond to incoming WINDOW_UPDATEs (and
> therefore a data receiver can gracefully protect itself from DoS by
> discarding data that exceeds the credit it has previously made available)
> ii) optional for a data receiver to emit WINDOW_UPDATEs (i.e. does not
> even have to implement this part of the code).
> Bob
> At 21:12 10/03/2015, Patrick McManus wrote:
> Hi Bob - I think your comments are appreciated. Its just one of those
> things where people have dispersed to other things and aren't necessarily
> in a place to revisit all the ground work again at this stage for a new go
> round. It was in large part the operational feedback and needs of the
> team, who has a lot of experience operating spdy at scale,
> that created the flow control provisions. hopefully those folks will chime
> in more authoritatively than my musings below:
> I'm sure there is quite a bit to learn here - indeed poorly configured use
> of the window_update mechanism has been (predictably) a source of
> unintended bottlenecks during both spdy and h2 trials. The spec does try
> and highlight that there can be dragons here and implementations that don't
> need the features it can bring should provide essentially infinite credits
> to steer clear of them.
> During the various trials I've seen h2 per stream flow control deployed
> successfully for a couple of use cases - both of them essentially deal with
> unsolicited data.
> The primary one is essentially a more flexible version of h1's
> 100-continue. When a client presents a large message body (e.g. a file
> upload) a multiplexing server needs a way of saying "these are how many
> buffers I've got available while I figure out where I'm going to sink this
> incoming data (perhaps to another server I need to connect to)". Presenting
> this on a per-stream basis allows the server to limit one stream while
> another (with a distinct sink) can proceed independently.  IMO this value
> should represent resources available and should be independent of BDP. This
> is why in practice you see clients with extremely large stream windows -
> most circumstances just want the data to flow at line rate (as you
> describe) and aren't trying to invoke flow control. The firefox default
> window is 256MB per stream - that's not going to slow down the sender nor
> require frequent window_update generation.
> The other use case is when the server pushes resources at a client without
> them being requested, which is a new feature of h2. This is conceptually
> similar to the server receiving a POST - suddenly there is a large amount
> of inbound data that the implementation might not have the resources to
> store completely. We can't just let TCP flow control take care of the
> situation because the TCP session is being multiplexed between multiple
> streams that need to be serviced. In this case the client accepts "some" of
> the stream based on policy and resource availability and can leave the
> stream in limbo until an event comes along that tells it to resume the
> transfer by issuing credits or reject it via cancel.
> hth a least a bit.
> On Tue, Mar 10, 2015 at 4:31 PM, Bob Briscoe <> wrote:
> HTTP/2 folks,
> I know extensibility had already been discussed and put to bed, so the WG
> is entitled to rule out opening healed wounds.
> But have points like those I've made about flow control been raised
> before? Please argue. I may be wrong. Discussion can go on in parallel to
> the RFC publication process, even tho the process doesn't /require/ you to
> talk to me.
> If I'm right, then implementers are being mandated to write complex flow
> control code, when it might have little bearing on the performance benefits
> measured for http/2.
> Even if I'm right, and the WG goes ahead anyway, /I/ will understand. My
> review came in after your deadline.
> However, bear in mind that the Webosphere might not be so forgiving. If h2
> goes ahead when potential problems have been identified, it could get a bad
> reputation simply due to the uncertainty, just when you want more people to
> take it up and try it out. Given you've put in a few person-years of
> effort, I would have thought you would not want to risk a reputation flop.
> I'm trying to help - I just can't go any faster.
> Bob
> At 14:43 06/03/2015, Bob Briscoe wrote:
> HTTP/2 folks,
> As I said, consider this as a late review from a clueful but fresh pair of
> eyes.
> My main concerns with the draft are:
> * extensibility (previous posting)
> * flow control (this posting - apologies for the length - I've tried to
> explain properly)
> * numerous open issues left dangling (see subsequent postings)
> The term 'window' as used throughout is incorrect and highly confusing,
> in:
> * 'flow control window' (44 occurrences),
> * 'initial window size' (5),
> * or just 'window size' (8)
> The HTTP/2 WINDOW_UPDATE mechanism constrains HTTP/2 to use only
> credit-based flow control, not window-based. At one point, it actually says
> it is credit-based (in flow control principle #2 <
> >
> ), but otherwise it incorrectly uses the term window.
> This is not just an issue of terminology. The more I re-read the flow
> control sections the more I became convinced that this terminology is not
> just /confusing/, rather it's evidence of /confusion/. It raises the
> questions
> * "Is HTTP/2 capable of the flow control it says it's capable of?"
> * "What type of flow-control protocol ought HTTP/2 to be capable of?"
> * "Can the WINDOW_UPDATE frame support the flow-control that HTTP/2
> needs?"
> To address these questions, it may help if I separate the two different
> cases HTTP/2 flow control attempts to cover (my own separation, not from
> the draft):
> a) Intermediate buffer control
> Here, a stream's flow enters /and/ leaves a buffer (e.g. at the app-layer
> of an intermediate node).
> b) Flow control by the ultimate client app.
> Here flow never releases memory (at least not during the life of the
> connection). The flow is solely consuming more and more memory (e.g. data
> being rendered into a client app's memory).
> ==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  Â
> 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.
> ==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.
> * 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).
> ==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.
> ==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.
> ==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: "Â Â  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. Â Â  This effectively disables flow control for
> that receiver." And contradicted in the definition of half closed
> (remote): "Â  half closed (remote): Â Â Â Â Â  [...] an endpoint is
> no longer       obligated to maintain a receiver flow control
> window. " And contradicted in 8.3. The CONNECT Method
> <>,
> which says: "Â  Frame types other than DATA Â Â  or stream management
> 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?==
>  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 <
> >, 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. * 2. HTTP/2 Protocol Overview
> <> says Â
> "Flow control and prioritization ensure that it is possible to efficiently
> use multiplexed streams. [Y] Â Â  Flow control (Section 5.2) helps to
> ensure that only data that can be used by a receiver is transmitted. [X]" *
> 5.2. Flow Control
> <>
> says: Â  "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]." * 5.2.2. Appropriate Use of Flow Control
> <> "Â
> Flow control is defined to protect endpoints that are operating under Â
>   resource constraints.  For example, a proxy needs to share memory Â
>   between many connections, and also might have a slow upstream  Â
> connection and a fast downstream one [Y].  Flow control addresses cases Â
> Â  where the receiver is unable to process data on one stream, yet wants Â
> Â  to continue to process other streams in the same connection [X]." "Â
> Deployments with constrained resources (for example, memory) can  Â
> 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,                     Â
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
> Â Â  BT
> ________________________________________________________________ Bob
> Briscoe,                     Â
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
> Â Â  BT
> ________________________________________________________________
> Bob Briscoe,                                                  BT
> --
> Jason T. Greene
> WildFly Lead / JBoss EAP Platform Architect
> JBoss, a division of Red Hat
> ________________________________________________________________
> Bob Briscoe,                                                  BT
> --
> Jason T. Greene
> WildFly Lead / JBoss EAP Platform Architect
> JBoss, a division of Red Hat
>  ________________________________________________________________
> Bob Briscoe,                                                  BT