http2/http2-spec — pull request #548 by gregw (2014-07-07 07:46:37+00:00)
Title: Large Frame Proposal

Background
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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
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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
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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.

Frame Size Updates
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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
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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
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This proposal was prepare by:
* Amos Jeffries
* Greg Wilkins
* Jason Greene
* Keith Morgan
* Poul-Henning Kamp

[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