Re: Large Frame Proposal

Reading through I am -1 on the frame layout changes.

I would be fine with un-reserving the first two bits and simply
requiring 64kb frames, but I feel like needing to compare allowed
sizes against SETTINGS that may change dynamically is a more
complicated change.

On Tue, Jul 8, 2014 at 5:37 PM, William Chan (陈智昌)
<willchan@chromium.org> wrote:
> 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 02:13:40 UTC