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Document /amp/client in broker-spec.txt.

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David Fifield 2021-07-26 10:23:12 -06:00
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doc/broker-spec.txt
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@ -107,6 +107,11 @@ through the exchange of WebRTC SDP information with its endpoints.
2.1. Client interactions with the broker
The broker offers multiple ways for clients to exchange registration
messages.
2.1.1. HTTPS POST
Clients interact with the broker by making a POST request to `/client` with the
offer SDP in the request body:
```
@ -130,6 +135,38 @@ If no proxies were available, they receive a 503 status code:
HTTP 503 Service Unavailable
```
2.1.2. AMP
The broker's /amp/client endpoint receives client poll messages encoded
into the URL path, and sends client poll responses encoded as HTML that
conforms to the requirements of AMP (Accelerated Mobile Pages). This
endpoint is intended to be accessed through an AMP cache, using the
-ampcache option of snowflake-client.
The client encodes its poll message into a GET request as follows:
```
GET /amp/client/0[0 or more bytes]/[base64 of client poll message]
```
The components of the path are as follows:
* "/amp/client/", the root of the endpoint.
* "0", a format version number, which controls the interpretation of the
rest of the path. Only the first byte matters as a version indicator
(not the whole first path component).
* Any number of slash or non-slash bytes. These may be used as padding
or to prevent cache collisions in the AMP cache.
* A final slash.
* base64 encoding of the client poll message, using the URL-safe
alphabet (which does not include slash).
The broker returns a client poll response message in the HTTP response.
The message is encoded using AMP armor, an AMP-compatible HTML encoding.
The data stream is notionally a "0" byte (a format version indicator)
followed by the base64 encoding of the message (using the standard
alphabet, with "=" padding). This stream is broken into
whitespace-separated chunks, which are then bundled into HTML <pre>
elements. The <pre> elements are then surrounded by AMP boilerplate. To
decode, search the HTML for <pre> elements, concatenate their contents
and join on whitespace, discard the "0" prefix, and base64 decode.
2.2 Proxy interactions with the broker