Re: Large Frame Proposal

Hi Greg et al,

Thanks for the proposal! Some non-anticipated feedback:

>    -
>    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.
This is orthogonal to CONTINUATION (and could also be addressed by
pre-pending the encoding length within the HPACK encoding, as has been
suggested). The problem with doing so is that it precludes the possibility
of streaming HPACK.

>    -
>    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.
It's not a requirement that an endpoint close the connection after deciding
that headers on a particular stream are too large. It does require that the
endpoint be willing to incrementally process header block fragments as they
arrive, to avoid unbounded buffering. With that, it's fully possible for an
endpoint to tightly bound it's memory commitment in the face of large
headers on a stream, without tearing down the connection.

Realistically as a service operator, even if given
SETTINGS_HEADER_FRAME_SIZE as proposed here, I'll still want to maintain
two thresholds of "large headers": A server-side threshold which sends an
error and tears down the stream, and a larger (potentially much larger)
client-side threshold. I want this because I otherwise have no
visibility into the impact that my selection has upon clients who wish to
communicate with me.

>    -
>    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.
> Yes, this is a concern, but is easily solved (and is why Chromium is
emitting smaller continuation frames to ensure good interop coverage within


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.
> 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
> 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
>    - Greg Wilkins
>    - Jason Greene
>    - Keith Morgan
>    - Poul-Henning Kamp
> [1]
> [2]
> [3]
> --
> Greg Wilkins <>
> HTTP, SPDY, Websocket server and client that
> scales
>  advice and support for jetty and cometd.

Received on Monday, 7 July 2014 18:24:25 UTC