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Re: Large Frame Proposal

From: 陈智昌 <willchan@chromium.org>
Date: Tue, 8 Jul 2014 17:37:56 -0700
Message-ID: <CAA4WUYjY2c4RZLaDbXkA3NZXAYRWC45qt2GcFx0ypH2o4T=NqQ@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 5:36 PM, William Chan (陈智昌) <willchan@chromium.org>
wrote:

> 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.
>

Clarification: Patrick identified actual abuses live in the wild. Not
theoretical. We indeed talked about the theoretical issue back when you
were still at Google.


>
>
>>
>> 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 00:38:25 UTC

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