Re: Clarification of dynamic table size change
Patrick McManus <mcmanus@ducksong.com> Thu, 18 August 2016 14:11 UTC
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From: Patrick McManus <mcmanus@ducksong.com>
Date: Thu, 18 Aug 2016 10:06:29 -0400
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To: Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com>
Cc: Bence Béky <bnc@chromium.org>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>, Hervé Ruellan <herve.ruellan@crf.canon.fr>, Cory Benfield <cory@lukasa.co.uk>, Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: Clarification of dynamic table size change
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Tatsuhiro's description matches my understanding. -Patrick On Thu, Aug 18, 2016 at 9:50 AM, Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com> wrote: > Hi, > > On Thu, Aug 18, 2016 at 10:31 PM, Bence Béky <bnc@chromium.org> wrote: > >> Hi all, >> >> I'm sorry to revive this old thread, but there is one more case that I >> would >> like to request clarification for. I was looking at both RFC 7540 and >> 7541, but >> could not find a definitive answer to the following question: What is the >> initial maximum size of the dynamic table if there was a >> SETTINGS_HEADER_TABLE_SIZE value in the initial SETTINGS frame (the one >> part of >> the connection preface)? >> >> For example, suppose that the decoder sends a SETTINGS_HEADER_TABLE with >> value >> 64 * 1024 in the initial SETTINGS frame. Do we think about the HPACK >> context to >> be created after the connection preface is sent, with a maximum dynamic >> table >> size of the current SETTINGS_HEADER_TABLE_SIZE value of 64 kB? Or do we >> think >> about the HPACK context to be created before the connection preface is >> sent, >> with a default maximum dynamic table size of 4 kB? Note that there is no >> synchronization issue even in the former case: if the decoder only evicts >> dynamic table entries above 64 kB from the very beginning, there is no >> harm in >> the encoder not starting to reference entries above 4 kB until it >> processes the >> decoder's initial SETTINGS frame. >> >> Suppose that the encoder does not emit a "dynamic table size update" HPACK >> instruction after this. The consensus on this e-mail thread seems to be >> that >> this is acceptable as long as the encoder means "no change" to the maximum >> dynamic table size. It is, however, important that the encoder and the >> decoder >> are in agreement about the initial maximum dynamic table size, relative >> to which >> the encoder means "no change". For example, if the decoder is under the >> impression that the maximum dynamic table size is 4 kB, while the encoder >> takes >> it to be 64 kB, then the decoder will signal a CONNECTION_ERROR as soon >> as the >> encoder references an entry above 4 kB. If, on the other hand, the >> encoder >> thinks it's 4 kB and never references entries above that, then the >> decoder would >> waste memory if it kept 64 kB worth of entries. >> >> Given that a decoder can send a SETTINGS_HEADER_TABLE_SIZE with a value >> lower >> than the default, and the encoder can start compressing headers before >> receiving >> the initial SETTINGS frame, it seems necessary to me to understand the >> initial >> maximum dynamic table size to be 4 kB, and to require the decoder to >> store this >> much entries until it receives the dynamic table size update HPACK >> instruction >> from the encoder. Otherwise a COMPRESSION_ERROR arises due to the >> synchronization issue even if the peers agree that the initial size is >> the new >> (lower) value. Unless, of course, we want to formulate different >> requirements >> depending on whether the SETTINGS_HEADER_TABLE_SIZE value is greater than >> or >> less than the default. >> >> If I implement a decoder in this spirit, that is, one that sends a >> SETTINGS_HEADER_TABLE_SIZE of 64 kB in the initial SETTINGS frame, but >> does not >> allow more than default memory for the dynamic table until it receives a >> dynamic >> table size update from the encoder, would it be incompatible with >> anybody's >> current implementation? >> >> > According to this thread, I'm under impression that this is OK, and > until you get dynamic table size update, default 4KiB dynamic table limit > still applies. > > As for initial value of dynamic table size, I think it is 4KiB regardless > of SETTINGS. We create HTTP/2 session before doing any parameter > modification, including header table size change. At this moment, table > size if 4KiB, RFC default. After that, decoder send > SETTINGS_HEADER_TABLE_SIZE with whatever value they want. Then after > SETTINGS ACK, and HPACK table size update, dynamic table size is finally > synchronized, and changed to the value encoder sent in HPACK table size > update (as long as it is equal or smaller than decoder sent in SETTINGS). > > Best regards, > Tatsuhiro Tsujikawa > > > > >> Best regards, >> >> Bence Béky >> >> >> On Fri, Oct 23, 2015 at 12:15 PM, Ilari Liusvaara < >> ilariliusvaara@welho.com> wrote: >> >>> On Sat, Oct 24, 2015 at 12:45:49AM +0900, Tatsuhiro Tsujikawa wrote: >>> > Hi, >>> > >>> > On Fri, Oct 23, 2015 at 1:18 AM, Hervé Ruellan < >>> herve.ruellan@crf.canon.fr> >>> > wrote: >>> > >>> > > I agree that the wording is ambiguous here. >>> > > >>> > > However, my reading is the same a Cory's: you don't have to send a >>> dynamic >>> > > table update if the *actual* value is not changed. >>> > > >>> > > >>> > I also found the discussion in this ML indicating you are right. >>> Thank >>> > you for clarification. >>> > I have to ask one more question: what is *actual* value? Is it the >>> table >>> > size both peer agreed before reading SETTINGS, or the value in >>> > SETTINGS_HEADER_TABLE_SIZE decoder sent? >>> > >>> > I think this is a good item to add in FAQ section.. >>> >>> The way negotiation works: >>> - Decoder side sets the upper bound via SETTINGS_HEADER_TABLE_SIZE. >>> - Encoder side sets the actual size via dynamic table updates (inside >>> HPACK bitstream) within limits set by decoder. >>> - If between headers decoder reduces the limit below size signaled by >>> encoder, the encoder must first reduce the table size to the minimum >>> it was between the frames or less (it can then increase it up to >>> current limit). >>> >>> As example of the last point: >>> [4k dynamic table size in use] >>> --> HEADERS >>> <-- SETTINGS(SETTINGS_HEADER_TABLE_SIZE=4k) >>> <-- SETTINGS(SETTINGS_HEADER_TABLE_SIZE=2k) >>> <-- SETTINGS(SETTINGS_HEADER_TABLE_SIZE=4k) >>> <-- SETTINGS(SETTINGS_HEADER_TABLE_SIZE=8k) >>> <-- SETTINGS(SETTINGS_HEADER_TABLE_SIZE=6k) >>> --> HEADERS >>> >>> The second HEADERS must first reduce the dynamic table to at most >>> 2k. It can then increase dynamic table size to up to 6k. >>> >>> >>> -Ilari >>> >>> >> >
- Re: Clarification of dynamic table size change Patrick McManus
- Re: Clarification of dynamic table size change Tatsuhiro Tsujikawa
- Re: Clarification of dynamic table size change Bence Béky
- Clarification of dynamic table size change Tatsuhiro Tsujikawa
- Re: Clarification of dynamic table size change Cory Benfield
- Re: Clarification of dynamic table size change Tatsuhiro Tsujikawa
- Re: Clarification of dynamic table size change Cory Benfield
- Re: Clarification of dynamic table size change Hervé Ruellan
- Re: Clarification of dynamic table size change Tatsuhiro Tsujikawa
- Re: Clarification of dynamic table size change Ilari Liusvaara
- Re: Clarification of dynamic table size change Bence Béky
- Re: Clarification of dynamic table size change Tatsuhiro Tsujikawa