1[DRAFT] 2 3Introduction: 4 5This document is a draft proposal for Shard Brotli dictionaries in the fetch spec 6(https://fetch.spec.whatwg.org/). 7 8The goal is to add support for custom dictionaries for Brotli. A dictionary is used 9to improve compression. A client can download a dictionary from a server and then 10use it to decompress resources compressed with this dictionary. 11 12This document specifies how the client and server negotiate the dictionary over HTTP. 13A high level overview is as follows: The server adds an HTTP header to the response 14with a URL of the dictionary. The browser downloads the dictionary from the URL and 15then caches it so it can be reused. The server also adds a checksum to an HTTP header 16which the client uses to verify the dictionary. Caching, CORS, and other existing 17mechanisms are used. A dictionary can be a pre-made static dictionary, but does not 18have to be, for example a previous page loaded from this server, or an old version 19of a page, can be used as well. 20 21Below are changes and additions to add Shared Brotli dictionaries to the fetch spec 22at https://fetch.spec.whatwg.org/: 23 24Additions to `4.5. HTTP-network-or-cache fetch` 25 26Add to point `15. Modify httpRequest’s header list per HTTP.`: 27 28 If the recursive-sbr flag is enabled, `Accept-Encoding` may not contain `sbr` 29 [NOTE-BOX] When sbr can be used, it is possible to add a header Available-Dict 30 with the URL and hash code of a cached resource. The server may then use it as 31 shared dictionary. 32 33Additions to `4.6. HTTP-network fetch` 34 35Add after point `10. Run these steps, but abort if the ongoing fetch is terminated`: 36 37 11. Let codings be the result of extracting header list values given 38 `Content-Encoding` and response’s header list. 39 12. If codings contains `sbr` 40 1. If the header list does not contain `Sbr-Dict`, return a network error 41 2. Let dictionaryId be the result of extracting header list values given 42 `Sbr-Dict` and response’s header list. 43 44To point `12. Run these substeps in parallel:`, add new first sub-point: 45 46 1. If codings contains `sbr`, run these subsubsteps: 47 1. Let dictionaryResponse be the result of performing a 48 Shared-Brotli-dictionary fetch given dictionaryId and request. 49 2. If dictionaryResponse is a network error, return a network error. 50 51Change point `12.4. Set bytes to the result of handling content codings given codings and bytes.` to: 52 53 4. Set bytes to the result of handling content codings given codings, bytes 54 and, if codings contains `sbr`, also dictionaryResponse's body. 55 [NOTE-BOX] If the dictionary is still being fetched, which happens in 56 parallel, enqueue bytes in a compressed buffer and handle content coding 57 once the dictionary is fetched 58 59Additions to `2.2.4. Bodies` 60 61 62Change last section `To handle content codings ...` to: 63 64 To handle content codings given codings, bytes and optionally a dictionary, run these substeps: 65 1. If codings are not supported, return bytes. 66 2. If the codings has `sbr`, run these subsubsteps: 67 a. Return the result of decoding bytes and dictionary with the Shared 68 Brotli decoder. 69 [Shared Brotli Spec] [IANA Brotli](https://www.iana.org/assignments/http-parameters/http-parameters.xhtml) 70 3. Else: 71 a. Return the result of decoding bytes with the given codings, as 72 explained in HTTP. [HTTP] [HTTP-SEMANTICS] [HTTP-COND] [HTTP-CACHING] 73 [HTTP-AUTH] 74 75New section `4.10. Shared-Brotli-dictionary fetch` 76 77 To perform a Shared-Brotli-dictionary fetch using dictionaryId, and parentRequest, perform these steps: 78 79 1. Let dictionaryURL be the URL extracted from dictionaryId 80 2. Let dictionaryHash be the hash id extracted from dictionaryId 81 3. Let dictionaryRequest be a new request whose method is `GET`, url is 82 dictionaryURL, mode is "cors", and client is parentRequest's client. 83 4. Let dictionaryResponse be the result of performing an 84 [HTTP-network-or-cache](https://fetch.spec.whatwg.org/#concept-http-network-or-cache-fetch) 85 fetch using dictionaryRequest with the recursive-sbr flag set to true. 86 [NOTE-BOX] For compression benefits, the dictionary should be reused to 87 decode multiple different responses. We rely on caching to achieve this. 88 It is suggested for servers to not add any "no-cache" or short "max-age" 89 Cache-Control directives, and it is suggested for the client to effectively 90 support caching it. 91 [NOTE-BOX] Since the same dictionary can be identified by a hash code, a 92 browser can avoid fetching a dictionary if it already has one with the same 93 hashed cached from a different source URL. 94 [NOTE-BOX] It is suggested that a server does not reuse the same URL 95 to host an updated or different dictionary. Instead the same dictionary URL 96 should contain a dictionary with the same content and same hash. 97 5. If dictionaryResponse is a network error, return a network error. 98 6. If dictionaryResponse's status is not an ok status, return a network error. 99 7. Let tokens be the result of 100 [parsing metadata](https://w3c.github.io/webappsec-subresource-integrity/#parse-metadata) 101 given dictionaryHash. 102 [Subresource Integrity](https://w3c.github.io/webappsec-subresource-integrity/) 103 8. If tokens is no metadata or the length of tokens is not 1, return a network 104 error 105 9. Let algorithm be the alg component of tokens[0]. If alg is 'hw3', set 106 algorithm to 256-bit HighwayHash 107 10. Let digest be the val component of tokens[1]. 108 11. Let hashValue be the result of base64 decoding digest 109 [base64](https://tools.ietf.org/html/rfc4648) 110 12. If hashValue is not a valid base64 encoding, return a network error 111 [NOTE-BOX] All of the supported hashing algorithms are cryptographically 112 secure. 113 13. Compute the hash code of dictionaryResponse's body using algorithm and 114 compare this checksum for equality with hashValue. If the computed 115 checksum does not match hashValue, return a network error. 116 14. Return dictionaryResponse. 117