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Re: Large Frame Proposal

From: (wrong string) 陈智昌 <willchan@chromium.org>
Date: Tue, 8 Jul 2014 17:36:27 -0700
Message-ID: <CAA4WUYgv4mM24gZOweFHrEc0rNtmYBXNY53gAFj8S6dB+pbMHA@mail.gmail.com>
To: Mike Belshe <mike@belshe.com>
Cc: Greg Wilkins <gregw@intalio.com>, HTTP Working Group <ietf-http-wg@w3.org>
On Tue, Jul 8, 2014 at 3:09 PM, Mike Belshe <mike@belshe.com> wrote:

> Thanks, Greg.
>
> I believe your proposal is preferable to what we have today in H2.
>
>
> On Mon, Jul 7, 2014 at 8:14 PM, Greg Wilkins <gregw@intalio.com> wrote:
>
>>
>> Mike,
>>
>> I think the settings are needed with the ability to send larger frame
>> sizes.   If I understand the feedback from SPDY correctly, the larger frame
>> sizes there were some endpoints that were lazy with their frame sizes and
>> sent overly large ones that hurt multiplexing.
>>
>
> OK - thanks for clarifying.   I don't recall any such learning from SPDY.
>  It certainly wasn't learning while I was at Google.
>

Yes, Patrick identified these issues after you stopped working on SPDY.


>
> BTW, some history:
> * SPDY originally used large frames too - 24bits - and let you use it as
> you wished.  Some people worried that these large frame sizes would be
> abused someday and felt compelled to shrink them.  But, as far as I know,
> those arguments were only theoretical.  The fact is that you're really only
> going to hurt yourself; and 'you' is the server.  Proxies are a different
> story.
>
> * SPDY's final field sizes were not arbitrary - they were an evolution
> based on lessons of what is valuable and what works.  Initially all headers
> were limited to 16k-ish.  This was because we didn't "like" large headers.
>  But, when put to practice there are edge cases which needed to be dealt
> with, and some apps out there, wisely or not, use ridiculously sized
> headers.  It just happens.  Increasing the size was always preferable
> because:
>    a) we learned that the constraint itself wasn't very valuable and
> didn't make anything materially better.
>    b) there were edge cases where the constraint just didn't work, and the
> solutions were worse than the initial problem.
>
>
>
>>
>> The intention of the proposal is to allow large frame sizes IF NEEDED,
>> but to use the settings to constrain implementations to the 16KB that has
>> been selected as a reasonable one-size-fits-all for todays traffic.   The
>> settings can then be adjusted without needing to rev the spec or deploy
>> extensions as experience is gained, traffic changes, networks change
>> etc.     It may well be that they are adjusted down initially as much as
>> they are adjusted up.
>>
>
>> So I don't see it as a compromise - it is simply moving the limit that
>> must exist from being a fixed capability of the framing layer to be an
>> explicitly managed parameter of the protocol.
>>
>>
> Ok. I don't see any real need for these either.  They may introduce new
> oddities if someone goes crazy with them.  I'd lean toward not adding these
> if possible.
>
> Mike
>
>
>
>
>
>
>> cheers
>>
>>
>>
>>
>>
>> On 8 July 2014 10:07, Mike Belshe <mike@belshe.com> wrote:
>>
>>>
>>>
>>>
>>> On Mon, Jul 7, 2014 at 12:50 AM, Greg Wilkins <gregw@intalio.com> wrote:
>>>
>>>>
>>>> https://github.com/http2/http2-spec/pull/548
>>>>
>>>>  Background
>>>>
>>>> The HTTP2 protocol has a requirement to be able to transport large
>>>> headers, that exceed the payload size of a single frame at the current 16KB
>>>> maximum size.
>>>>
>>>> To address this requirement, the current draft (13) includes the
>>>> CONTINUATION frames, 0 or more of which may be sent after a HEADERS or
>>>> PUSH_PROMISE frame to contain the large headers. There has been significant
>>>> criticism of the CONTINUATION design, including:
>>>>
>>>>    -
>>>>
>>>>    The total length of a HEADERS+CONTINUATION* sequence is not known
>>>>    until the last frame in the sequence is processed. A receiver that wishes
>>>>    to reject streams headers larger than a specific limit may have to process
>>>>    many frames and hold the results in memory before it discovers the header
>>>>    is too large.
>>>>    -
>>>>
>>>>    The size of header that an endpoint is prepared to receive is not
>>>>    known in advance. The only way a sender can know if a header too large is
>>>>    by attempting to send it and receiving an error in response. Error handling
>>>>    of headers may be difficult for an endpoint to handle efficiently and can
>>>>    result in the closure of the entire connection.
>>>>    -
>>>>
>>>>    The END_STREAM flag is not present on the CONTINUATION frame, thus
>>>>    it is possible for a stream to send CONTINUATION frames after a HEADERS
>>>>    frame that has the END_STREAM flag set. This is confusing and increases the
>>>>    complexity of the state machine required to process streams. It is highly
>>>>    desirable that a set END_STREAM flag truly indicates the last non control
>>>>    frame of a stream.
>>>>    -
>>>>
>>>>    There is a significant discontinuity in the code path required to
>>>>    process headers. Headers up to an indeterminate size (roughly 20-something
>>>>    KB) can be handled in a single frame. Headers that exceed this size must be
>>>>    handled in multiple frames of different types with different frame flags
>>>>    and stream control logic. Because the vast majority of headers sent
>>>>    (>99.99%) are below this indeterminate size, implementations will have a
>>>>    code path that is seldom executed and probably insufficiently tested. This
>>>>    invites poor and/or partial and/or incorrect implementation.
>>>>    -
>>>>
>>>>    Because of the HPACK compression algorithm, a sequence of
>>>>    HEADERS+CONTINUATIONS frames may not be interleaved with any other frame.
>>>>    This effectively makes the sequence a single large frame. Because of the
>>>>    simplicity of description and implementation it is proposed that it would
>>>>    be far simpler to meet the requirement of large headers by supporting large
>>>>    frames.
>>>>
>>>> This proposal has been prepared as it is possible to meet the
>>>> requirements of CONTINUATIONS without the complications and criticisms
>>>> above.
>>>>
>>>> This proposal addresses the issue of sending/receiving large HTTP
>>>> headers without giving endpoints and intermediaries unlimited resource
>>>> commitments nor unknown limits
>>>>
>>>> Additional Frame Size Issues Addressed
>>>>
>>>> The current draft (13) has maximum frame size of 16KB, which is an
>>>> arbitrary value that has been selected on the basis of experience to
>>>> provide a reasonable compromise between the efficiency of transmitting data
>>>> vs the quality of service for multiplexed channels.
>>>>
>>>> Whilst this educated guess may be near optimal for today's networks and
>>>> traffic, it is entirely possible that some current and/or future networks
>>>> may require a different value to achieve an optimal balance. There have
>>>> already been proposals [1] put to the WG to reduce the frame size to
>>>> optimise multiplexing , as well as discussion that high capacity, low
>>>> latency networks can achieve satisfactory multiplexing quality of service
>>>> with large frame sizes.
>>>>
>>>> This proposal addresses the issue that a fixed frame size does not
>>>> allow tuning multiplexing performance based on current/future experience.
>>>>
>>>> It has also been noted that 16KB is near the middle of the peak of the
>>>> current HTTP Object size histogram [2], so that a small change in the frame
>>>> size may have a significant impact on the number of HTTP messages that can
>>>> be sent in a single frame, without significant impacts on QoS. The HTTP
>>>> Object size histogram has changed significantly over time and is expected
>>>> to continue to do so.
>>>>
>>>> This proposal addresses the issue of tuning the frame size based on
>>>> experience of actual payload sizes.
>>>>
>>>> There have also been issues raised that a 16KB frame size does not
>>>> allow efficient data transfer [3] even when the end points are aware that
>>>> only a single stream is likely to be required for the imminent future, or
>>>> that a particular stream is of high priority.
>>>>
>>>> This proposal addresses the issue of tuning the frame size for
>>>> transport efficiency for specific streams in specific situations.
>>>>
>>>> Large Frame Header Proposal
>>>>
>>>> This proposal is to alter the core http2 protocol to address the issues
>>>> identified above by supporting a variable length maximum frame size
>>>> controlled by peer limits.
>>>>
>>>> This proposal increases the length field in the frame header to 31
>>>> bits, to match the maximum flow control window size. However,
>>>> implementations will not be able to use the full frame size without
>>>> explicit consent from peers using newly defined SETTINGS or an optional
>>>> WINDOW_UPDATE field.
>>>> Frame Size Settings
>>>>
>>>> Two settings parameters are proposed: SETTINGS_HEADER_FRAME_SIZE for
>>>> the maximum header size and SETTINGS_FRAME_SIZE for all other frames.
>>>>
>>>> The SETTINGS_HEADER_FRAME_SIZE parameter supports the current behaviour
>>>> where large headers can be sent without changing the frame size allowed for
>>>> other frame types. ie A large header size limit can be set without
>>>> affecting the multiplexing efficiency of DATA frames.
>>>>
>>>> The SETTINGS_FRAME_SIZE applies to all other frames including DATA
>>>> frames and any other frame that may be defined by an extension. The use of
>>>> this parameter is intended to tune/optimise the connection for the general
>>>> case of multiple streams over the specific connection.
>>>>
>>>
>>> Greg -
>>>
>>> Overall, great writeup.  Thank you.
>>>
>>> Clarifying question:  are you bundling these settings parameters because
>>> you think we need them?  Or simply as a compromise?  If you had to choose
>>> between a larger frame size and no settings change vs the current frame
>>> size, which would you find simpler?
>>>
>>> Thanks,
>>> Mike
>>>
>>>
>>>
>>>
>>>
>>>
>>>>  Frame Size Updates
>>>>
>>>> To handle the issue of efficiently sending large data when an end point
>>>> is prepared to risk multiplexing efficiency, this proposal allows a Max
>>>> Frame Size to be applied to a specific stream as an optional field in a
>>>> WINDOW_UPDATE frame.
>>>>
>>>> By including a variable frame size in the flow control mechanism this
>>>> proposal allows the decision to increase the frame size to be deferred
>>>> until more knowledge about the specific situation are known and limited to
>>>> the stream that will benefit from the increased size.
>>>>
>>>> Consider the example of a server that has commenced sending a large
>>>> content to a client. The server may initially send 4 x 16KB frames to
>>>> consume the default stream flow control window, at which time it must wait
>>>> for the client to send a WINDOW_UPDATE frame before continuing. When
>>>> generating the WINDOW_UPDATE frame, the client may have knowledge of:
>>>>
>>>>    - The content-length header - so it knows that the amount of data
>>>>    expected is large or is perhaps just slightly larger than a single frame.
>>>>    - The content-type header - so it knows if the content has high
>>>>    priority in rendering the current page, or if the content is likely to
>>>>    include references to other resources which may need to be multiplexed.
>>>>    - How many other streams are currently open and/or reserved - so it
>>>>    knows if multiplexing is actually required.
>>>>    - How many other requests are pending - so it knows if new
>>>>    multiplexed stream could soon be opened.
>>>>    - An approximate rough measure of the network latency and
>>>>    throughput - This can be derived from the timing of the receipt of the
>>>>    first few data frames and used to estimate the impact on QoS of any change
>>>>    to the maximum frame size.
>>>>
>>>> The client can use this knowledge to make an informed decision as to
>>>> the benefit of changing the allowed max frame size against any risk to
>>>> multiplexing QoS. It can make several choices:
>>>>
>>>>    -
>>>>
>>>>    No change. It can just not adjust the frame size, either because it
>>>>    is too hard to consider or that there are too many other streams, or that
>>>>    the content is video that needs to be received slowly. In any of these
>>>>    cases the max frame size can be left unchanged and the protocol continues
>>>>    as it currently does.
>>>>    -
>>>>
>>>>    Large frame. If the stream is the only expected stream or of
>>>>    sufficiently high priortiy, then the window and frame size can be set to
>>>>    allow as much of the remaining data as possible to be sent in a single
>>>>    frame.
>>>>    -
>>>>
>>>>    Medium frame. The client can momentarily trade some QoS (for an
>>>>    estimated duration) by increasing the frame size to something >16KB and <
>>>>    content-length
>>>>    -
>>>>
>>>>    Sufficient frame. If the remaining content is only a small
>>>>    increment over the current SETTING_FRAME_SIZE, the Max Frame Size can be
>>>>    increased to receive the remaining content in a single frame without any
>>>>    significant QoS impact.
>>>>
>>>> Minimal Compliance
>>>>
>>>> A minimally compliant implementation MUST handle the SETTING_FRAME_SIZE
>>>> and SETTINGS_HEADER_SIZE and ensure that no frame sent exceeds the
>>>> applicable limit. However no implementation is required to send frames at
>>>> or near these limits when set above the default 16KB.
>>>>
>>>> There is no requirement for an implementation to send or to handle the
>>>> Max Frame Size in a WINDOW_UPDATE and it is allowable for it to be ignored
>>>> if received.
>>>> Anticipated Feedback
>>>>
>>>> *It is too late in the process to change the framing layer and to do so
>>>> after so much discussion is an implicit fail of the WG*
>>>>
>>>> To not consider issues and proposal brought to the WG would be a fail
>>>> of the process. This proposal is based on all the hard work to date done by
>>>> the WG and contributors to identify issues and test solutions.
>>>>
>>>> *These issues can be handled in extensions.*
>>>>
>>>> Optimising data transfers for large content could possibly be done in
>>>> an extension, however:
>>>>
>>>>    - It is not yet clear if extensions will be a viable way to enhance
>>>>    the http2 protocol. There are significant hurdle to overcome to deploy
>>>>    extensions.
>>>>    - Many of the issues are aimed at complexity and tuning of the core
>>>>    protocol, and these cannot be addressed in an extension.
>>>>    - It is asymmetric to support large headers with one mechanism and
>>>>    large data with another.
>>>>
>>>>
>>>>
>>>> *The proposed header costs 2 extra bytes per frame*
>>>>
>>>> There is a small data cost to adopt this proposal, however this is
>>>> mitigated as:
>>>>
>>>>    - The proposal may be able to reduce the number of frames needed
>>>>    for some content, thus saving 8 bytes. Whilst not likely to be a 25% frame
>>>>    saving required to break even, it will still reduce cost to below 2 bytes.
>>>>    - There are options to have variable length headers or optional
>>>>    extended headers that will preserve the semantics of this proposal and keep
>>>>    an 8 byte header for small frames. If the 2 byte cost is considered
>>>>    prohibitive, then these alterations can be considered.
>>>>
>>>>
>>>>
>>>> *The header is 10 bytes long and not 32bit word aligned.*
>>>>
>>>> Frames sent after arbitrary data will not be word aligned anyway. If
>>>> alignment is important, then padding could changed to be part of the base
>>>> frame format, 2 header bytes used for a padding length (giving an aligned
>>>> 12 byte header) and all frames padded to a word boundary.
>>>>
>>>>  *31 bits is also an arbitrary length*
>>>>
>>>> It is true that a 31 bit large frame length is also an arbitrary limit
>>>> to the size of a frame. However, it is believed that 31 bits is
>>>> sufficiently large to efficiently handle almost all conceivable present and
>>>> future use cases. It would be possible to implement an unlimited size
>>>> length field, but this would also need changes to the flow control
>>>> mechanism, which currently also has a 31 bit maximum size.
>>>>
>>>>  *It does not support unlimited response headers*
>>>>
>>>> A SETTINGS_HEADER_FRAME_SIZE of 2^31-1 is effectively unlimited for all
>>>> foreseeable response headers.
>>>>
>>>>
>>>>  *This was tried with SPDY and rejected*
>>>>
>>>> SPDY did not have the settings to allow peers to set limits on the max
>>>> frame size. This proposal will not change to default behaviour of http2
>>>> with regards to frame size.
>>>>
>>>>
>>>>  *Intermediaries will destroy multiplexing by setting frame size to
>>>> 2^31-1*
>>>>
>>>> Large frames require the participation of both sender and receiver. A
>>>> receiver may advise that it is willing to accept large frames, but a sender
>>>> is under no obligation to send them. Thus intermediaries nor any end point
>>>> can unilaterally change multiplexing QoS.
>>>>
>>>>
>>>>  *Cannot be hardware accelerated.*
>>>>
>>>> Hardware acceleration is not part of the WG brief to support, nor is it
>>>> clear that this proposal is any less suitable than others for hardware
>>>> acceleration.
>>>> Contributors
>>>>
>>>> This proposal was prepare by:
>>>>
>>>>    - Amos Jeffries squid3@treenet.co.nz
>>>>    - Greg Wilkins gregw@intalio.com
>>>>    - Jason Greene jgreene@redhat.com
>>>>    - Keith Morgan K.Morgan@iaea.org
>>>>    - Poul-Henning Kamp phk@phk.freebsd.dk
>>>>
>>>> [1]
>>>> http://lists.w3.org/Archives/Public/ietf-http-wg/2013AprJun/0926.html
>>>> [2] http://httparchive.org/interesting.php
>>>> [3]
>>>> http://lists.w3.org/Archives/Public/ietf-http-wg/2014AprJun/1664.html
>>>>
>>>>
>>>> --
>>>> Greg Wilkins <gregw@intalio.com>
>>>> http://eclipse.org/jetty HTTP, SPDY, Websocket server and client that
>>>> scales
>>>> http://www.webtide.com  advice and support for jetty and cometd.
>>>>
>>>
>>>
>>
>>
>> --
>> Greg Wilkins <gregw@intalio.com>
>> http://eclipse.org/jetty HTTP, SPDY, Websocket server and client that
>> scales
>> http://www.webtide.com  advice and support for jetty and cometd.
>>
>
>
Received on Wednesday, 9 July 2014 00:36:56 UTC

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