- From: James M Snell <jasnell@gmail.com>
- Date: Sat, 21 Jul 2012 12:06:14 -0700
- To: ietf-http-wg@w3.org
- Message-ID: <CABP7RbdsCKq3f0in5cMCGCGqYFMW5LBf-47pN1HZ3+uO=4UpXA@mail.gmail.com>
Continuing my review of the SPDY draft... have a few questions relating to SPDY and load balancers / reverse proxy set ups... The intent is not to poke holes but to understand what the SPDY authors had in mind for these scenarios... 1. Imagine two client applications (A and B) accessing an Origin (D) via a Reverse Proxy (C). When a client accesses /index.html on Origin D, the Origin automatically pushes static resources /foo.css, /images/a.jpg and /video/a.mpg to the client. Basic flow looks something like... A RP O | | | | | | |==================>| | | 1)SYN | | |<==================| | | 2)SYN_ACK | | |==================>| | | 3)ACK | | |==================>| | | 4)SYN_STREAM (1) | | | |================>| | | 5) SYN | | |<================| | | 6) SYN_ACK | | |================>| | | 7) ACK | | |================>| | | 8) SYN_STREAM(1)| | |<================|-- | | 9) SYN_STREAM(2)| | | | uni=true | | |<==================| | | | 10) SYN_STREAM(2) | | | | uni=true | | | Content Push | |<================| | | | 11) SYN_REPLY(1)| | |<==================| | | | 12) SYN_REPLY(1) | | | | | | | | |<================| | |<==================| 13) DATA (2,fin)|-- | 14) DATA (2,fin) | | | | | | | | My question is: what does this picture look like if Client's A and B concurrently request /index.html? With HTTP/1.1, static resources can be pushed off to CDN's, stored in caches, distributed around any number of places in order to improve overall performance. Suppose /index.html is cached at the RP. Is the RP expected to also cache the pushed content? Is the RP expected to keep track of the fact that /foo.css, images/a.jpg and /video/a.mpg were pushed before and push those automatically from it's own cache when it returns the cached instance of /index.html? If not, when the caching proxy returns index.html from it's cache, A and B will be forced to issue GETs for the static resources defeating the purpose of pushing those resources in the first place. In theory, we could introduce new Link rels in the same spirit as http://tools.ietf.org/html/draft-nottingham-linked-cache-inv-03 that tell caches when to push cached content... e.g. SYN_STREAM id=2 unidirectional=true Content-Location: http://example.org/images/foo.jpg Content-Type: image/jpeg Cache-Control: public Link: </index.html>; rel="cache-push-with" What does cache validation look like for pushed content? E.g. what happens if the cached /index.html is fresh and served from the cache but the related pushed content also contained in the cache is stale? I'm sure I can come up with many more questions, but it would appear to me that server push in SPDY is, at least currently, fundamentally incompatible with existing intermediate HTTP caches and RP's, which is definitely a major concern. As a side note, however, it does open up the possibility for a new type of proxy that can be configured to automatically push static content on the Origin's behalf... e.g. A SPDY Proxy that talks to a backend HTTP/1.1 server and learns that /images/foo.jpg is always served with /index.html so automatically pushes it to the client. Such services would be beneficial in general, but the apparent incompatibility with existing deployed infrastructure is likely to significantly delay adoption. Unless, of course, I'm missing something fundamental :-) 2. While we on the subject of Reverse Proxies... the SPDY spec currently states: When a SYN_STREAM and HEADERS frame which contains an Associated-To-Stream-ID is received, the client must not issue GET requests for the resource in the pushed stream, and instead wait for the pushed stream to arrive. Question is: Does this restriction apply to intermediaries like Reverse Proxies? For instance, suppose the server is currently pushing a rather large resource to client A and Client B comes along and sends a GET request for that specific resource. Assume that the RP ends up routing both requests to the same backend Origin server. A strict reading of the above requirement means that the RP is required to block Client B's get request until the push to Client A is completed. Further, the spec is not clear if this restriction only applies for requests sent over the same TCP connection. Meaning, a strict reading of this requirement means that even if the RP opens a second connection to the Origin server, it is still forbidden to forward Client B's GET request until Client A's push has been completed. - James
Received on Saturday, 21 July 2012 19:07:03 UTC