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Gen-art last call review of draft-ietf-httpbis-http2-16

2015-01-19 04:37:52
I am the assigned Gen-ART reviewer for this draft. For background on
Gen-ART, please see the FAQ at

<http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.

Please resolve these comments along with any other Last Call comments
you may receive.

Document: draft-ietf-httpbis-http2-16.txt
Reviewer: Elwyn Davies
Review Date:  2015-01-18
IETF LC End Date: 2015-01-14
IESG Telechat date: 2015-01-22

Summary:
Almost ready. A well written document with just a few nits really. I am slightly surprised (having not been following httpbis in detail) that HTTP/2 is so tightly wedded to TCP - this is doubtless pragmatic but it adds to the ossification of the Internet and makes me slightly suspicious that it is an attempt to really confirm HTTP/2 as the waist of the neo-Internet. Can't we ever use any other transport? A couple of minor issues: (1) I am not sure that I see the (security) value of the padding mechanisms specified, but I am not an expert so I will defer to the security experts, and (2) the extension method for SETTINGS seems to be flawed. Apologies for the slightly delayed review.

Major Issues:
s3, para 1: Just checking: HTTP/2 is defined to run over TCP connections only. I take it that this is something that the WG has formally decided upon. Is there something that stops HTTP/2 running over any other reliable byte stream protocol with in-order delivery? Could there be a more general statement?

Minor Issues:
s4.3, next to last para; s5, bullet #1:
(Just a bit more than a nit)
s4.3 says:
Header blocks MUST be
    transmitted as a contiguous sequence of frames, with no interleaved
    frames of any other type or from any other stream.
s5 says:
    o  A single HTTP/2 connection can contain multiple concurrently open
       streams, with either endpoint interleaving frames from multiple
       streams.
If s4.3 is correct (which multiple repetitions elsewhere indicate that it is) then the bullet in s5 needs to reflect the constraint in s4.3 as they are currently inconsistent.

I am not quite sure whether the last para of s4.3 implies that the client must wait until it has the whole header block before trying to decompress or whether decompression
might happen as fragments are received - please clarify this in the text.

s6.1 and s6.2: I am dubious about the value of the padding story in DATA and HEADERS frames, even given the caveats in s10.7. An attacker can use the header to deduce the padding section. It seems a bit odd to say that you can add arbitrary padding and then insist it is all zeroes. However, I am not an expert in this area and will defer to security experts.

s6.5.3: If an endpoint sends a SETTINGS value that the receiving endpoint doesn't understand (for an extension, say), the receiver will ignore it but still send an ACK. This leaves the sending endpoint unaware that the other endpoint didn't understand it. I suspect that this makes the extension mechanism for settings effectively useless. I note that s6.5 says that implementations MUST support the values defined in the draft so that the ACK mechanism works fine for the base system, but any extension of SETTINGS cannot be used to limit the behaviour of the receiving endpoint because the sender can't rely on the receiver actioning the limitation; the only useful things might be to expand a limit that the sender would otherwise have to conform to. It would be possible to alter the spec so that the receiver can send back any values it didn't understand with a frame with ACK set - not exactly negotiation but just rejection.

Editorial/Nits:
Abstract:
allowing multiple
    concurrent messages on the same connection.
Technically the *messages* are not on the connection concurrently. How about: allowing a server to simultaneously process multiple requests submitted via a single
    connection with arbitrary ordering of responses.

s2, last para:
allowing many requests to be compressed into one TCP packet.
At this point in the document, nothing has been said about TCP.
Better:
allowing many requests to be compressed into one transport connection packet.

s2.2: I think definitions or references or deprecations for message/message body/message payload/message headers/payload data and entity/entity headers/entity body are needed. With HTTP/2 the distinction between message body and entity body is no longer needed I think. It appears that message body is not currently used but message payload, payload data and entity body are. I think that it might be useful to stick to message payload in the body of the draft and add a note in terminology to indicate that message payload covers both message body and entity body in HTTP/1.x and stress that there is only one encoding.

s3.2.1, para 4:
OLD:
production for "token68"
NEW:
production for "token68", allowing all the characters in a base64url string,

s3.4, para 3: s/sever/server/

s3.5, paras 2-5: It would be worth explaining that what is being done here is to send what would be a method request for the PRI method which will not be recognized by well-implemented HTTP/1.x servers. The PRI method is then reserved for HTTP/1.1 so that it can't be accidentally implemented later (see Section 11.6).

s4.1, Figure 1 (and other figures): The frame header layout doesn't match the usual conventions for protocol component specifications where each 32 bit 'word' is filled out. I'm not sure how strictly we would want to adhere to this convention... consult an AD.

s4.1: Would performance be helped by aligning the length and stream identifiers on
32 bit boundaries?

s4.1, Flags: For the avoidance of doubt it would be good to label the flag bits with the numbers that are used later in the document.

s4.1, Stream Identifier: s5.1 and s5.1.1 say the identifiers are integers - this should be reflected here. Also consistency between s4.1 and s5.1.1 should have the reserved id as either 0 or 0x0 (OK, this is really picky!)

s5, bullets #2 and #5:
    o  Streams can be established and used unilaterally or shared by
       either the client or server.
    o  Streams are identified by an integer.  Stream identifiers are
       assigned to streams by the endpoint initiating the stream.
[Is there a risk of a race condition in which one end allocates a stream id and sends using it and the other end allocates the same id for a different purpose while the first frame is in transit? Maybe clients should always use odd numbered streams and servers even numbered .. or is this a use for the reserved bit to be used by servers?]

OK.. I correctly identified the possibility of race conditions.. Please add a pointer to s5.1.1 which tells you to do what I just proposed. Another pointer in s4.1 definition of stream id would also help.

s5.1:
OLD:
   half closed (local):
      A stream that is in the "half closed (local)" state cannot be used
      for sending frames.  Only WINDOW_UPDATE, PRIORITY and RST_STREAM
      frames can be sent in this state.
NEW:
   half closed (local):
      A stream that is in the "half closed (local)" state cannot be used
      for sending frames with the exceptions of  WINDOW_UPDATE,
      PRIORITY and RST_STREAM frames.
Also: Would it be worth saying that any type of frame can be received in this state?

s5.1:
    half closed (remote):
       A stream that is "half closed (remote)" is no longer being used by
       the peer to send frames.  In this state, an endpoint is no longer
       obligated to maintain a receiver flow control window if it
       performs flow control.
Needs a pointer to the section where the flow control window is discussed.

s5.1, next to last para:
    In the absence of more specific guidance elsewhere in this document,
    implementations SHOULD treat the receipt of a frame that is not
    expressly permitted in the description of a state as a connection
    error (Section 5.4.1  
<http://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-5.4.1>) of type 
PROTOCOL_ERROR.
There are not many other sorts of frames, so I think it would help (possibly even essential, since there doesn't seem to be anywhere else this is stated) to explicitly specify what sorts of frames can be expected to be received in the 'active' states (Open, Half closed(local) at least)

s5.1, next to last para: s/Frame of unknown/Frames of unknown/

s5.1, next to last para; s5.5, para 3, s6.2, END_HEADERS, para 2:
s5.1 says:
OLD:
Frame of unknown types are ignored.

s5.5 and s6.2 qualify this by saying that frames of unknown types inside HEADER frame sequences have to be treated as connection errors. The two sections need to be synced up. Maybe:
NEW:
Frames of unknown types are ignored except when they occur in HEADER frame sequences when they have to be treated as a connection error of type PROTOCOL_ERROR (see Sections 5.5 and 6.2).

s5.1.1, last para/s3.4, last para, s9.1, para 3: Having to open a new connection when stream ids run out interacts with the point made in s3.4 that there is no guarantee that the new connection will support HTTP/2. It might be worth pointing this out in s5.1.1 and s9.1 - it means the client has to be able to switch back to HTTP/1.1 in the very rare case that this happens. Thought: Would it be useful for a server to have a SETTINGS flag that guarantees that it would do HTTP/2 on all subsequent connections?

s5.3.1, para 1: To be absolutely explicit: s/on another stream/on exactly one other stream/

s6.2, END_STREAM flag, 2nd para: s/A HEADERS frame carries/A HEADERS frame can carry/

s6.2;s6.6: The wording on padding should be synchronized with that in s6.1 or a note added to s6.2 to refer readers to the padding comments in s6.1 similar to that in s6.6. Synching 6.2 and s6.6 would be sensible.

s6.3, Weight: Should be explicitly said to be an (unsigned) integer.

s6.5.2, SETTINGS_HEADER_TABLE_SIZE: Should have a reference to [COMPRESSION].

s8.1.3: For clarity, the convention "+ END_HEADERS"/"- END_HEADERS" should be explained.

s8.1.3, last example: It might be worth noting that there could be significant time between the two responses being sent.

s9.1.1, para 1: s/to an origin servers/to an origin server/

s10.3, para 2: s/translater verbatim/translated verbatim/

s10.5.1, para 1:
OLD:
    For this
   an other reasons, such as ensuring cache correctness, means that an
   endpoint might need to buffer the entire header block.
NEW:
This ordering and other reasons, such as ensuring cache correctness, mean that an
   endpoint might need to buffer the entire header block.

s10.5.1, para 2: s/This setting specific/This setting is specific/