Snowflake broker protocol 0. Scope and Preliminaries The Snowflake broker is used to hand out Snowflake proxies to clients using the Snowflake pluggable transport. There are some similarities to the function of the broker and how BridgeDB hands out Tor bridges. This document specifies how the Snowflake broker interacts with other parts of the Tor ecosystem, starting with the metrics CollecTor module and to be expanded upon later. 1. Metrics Reporting (version 1.1) Metrics data from the Snowflake broker can be retrieved by sending an HTTP GET request to https://[Snowflake broker URL]/metrics and consists of the following items: "snowflake-stats-end" YYYY-MM-DD HH:MM:SS (NSEC s) NL [At start, exactly once.] YYYY-MM-DD HH:MM:SS defines the end of the included measurement interval of length NSEC seconds (86400 seconds by default). "snowflake-ips" [CC=NUM,CC=NUM,...,CC=NUM] NL [At most once.] List of mappings from two-letter country codes to the number of unique IP addresses of Snowflake proxies that have polled. Each country code only appears once. "snowflake-ips-total" NUM NL [At most once.] A count of the total number of unique IP addresses of Snowflake proxies that have polled. "snowflake-ips-standalone" NUM NL [At most once.] A count of the total number of unique IP addresses of snowflake proxies of type "standalone" that have polled. "snowflake-ips-badge" NUM NL [At most once.] A count of the total number of unique IP addresses of snowflake proxies of type "badge" that have polled. "snowflake-ips-webext" NUM NL [At most once.] A count of the total number of unique IP addresses of snowflake proxies of type "webext" that have polled. "snowflake-idle-count" NUM NL [At most once.] A count of the number of times a proxy has polled but received no client offer, rounded up to the nearest multiple of 8. "client-denied-count" NUM NL [At most once.] A count of the number of times a client has requested a proxy from the broker but no proxies were available, rounded up to the nearest multiple of 8. "client-restricted-denied-count" NUM NL [At most once.] A count of the number of times a client with a restricted or unknown NAT type has requested a proxy from the broker but no proxies were available, rounded up to the nearest multiple of 8. "client-unrestricted-denied-count" NUM NL [At most once.] A count of the number of times a client with an unrestricted NAT type has requested a proxy from the broker but no proxies were available, rounded up to the nearest multiple of 8. "client-snowflake-match-count" NUM NL [At most once.] A count of the number of times a client successfully received a proxy from the broker, rounded up to the nearest multiple of 8. "client-http-count" NUM NL [At most once.] A count of the number of times a client has requested a proxy using the HTTP rendezvous method from the broker, rounded up to the nearest multiple of 8. "client-http-ips" [CC=NUM,CC=NUM,...,CC=NUM] NL [At most once.] List of mappings from two-letter country codes to the number of times a client has requested a proxy using the HTTP rendezvous method, rounded up to the nearest multiple of 8. Each country code only appears once. "client-ampcache-count" NUM NL [At most once.] A count of the number of times a client has requested a proxy using the ampcache rendezvous method from the broker, rounded up to the nearest multiple of 8. "client-ampcache-ips" [CC=NUM,CC=NUM,...,CC=NUM] NL [At most once.] List of mappings from two-letter country codes to the number of times a client has requested a proxy using the ampcache rendezvous method, rounded up to the nearest multiple of 8. Each country code only appears once. "client-sqs-count" NUM NL [At most once.] A count of the number of times a client has requested a proxy using the sqs rendezvous method from the broker, rounded up to the nearest multiple of 8. "client-sqs-ips" [CC=NUM,CC=NUM,...,CC=NUM] NL [At most once.] List of mappings from two-letter country codes to the number of times a client has requested a proxy using the sqs rendezvous method, rounded up to the nearest multiple of 8. Each country code only appears once. "snowflake-ips-nat-restricted" NUM NL [At most once.] A count of the total number of unique IP addresses of snowflake proxies that have a restricted NAT type. "snowflake-ips-nat-unrestricted" NUM NL [At most once.] A count of the total number of unique IP addresses of snowflake proxies that have an unrestricted NAT type. "snowflake-ips-nat-unknown" NUM NL [At most once.] A count of the total number of unique IP addresses of snowflake proxies that have an unknown NAT type. "snowflake-proxy-poll-with-relay-url-count" NUM NL [At most once.] A count of snowflake proxy polls with relay url extension present. This means this proxy understands relay url, and is sending its allowed prefix. "snowflake-proxy-poll-without-relay-url-count" NUM NL [At most once.] A count of snowflake proxy polls with relay url extension absent. This means this proxy is not yet updated. "snowflake-proxy-rejected-for-relay-url-count" NUM NL [At most once.] A count of snowflake proxy polls with relay url extension rejected based on broker's relay url extension policy. This means an incompatible allowed relay pattern is included in the proxy poll message. 2. Broker messaging specification and endpoints The broker facilitates the connection of snowflake clients and snowflake proxies 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: ``` POST /client HTTP [offer SDP] ``` If the broker is behind a domain-fronted connection, this request is accompanied with the necessary HOST information. If the client is matched up with a proxy, they receive a 200 OK response with the proxy's answer SDP in the request body: ``` HTTP 200 OK [answer SDP] ``` 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 
elements. The 
 elements are then surrounded by AMP boilerplate. To
decode, search the HTML for 
 elements, concatenate their contents
and join on whitespace, discard the "0" prefix, and base64 decode.

2.2 Proxy interactions with the broker

Proxies poll the broker with a proxy poll request to `/proxy`:

```
POST /proxy HTTP

{
  Sid: [generated session id of proxy],
  Version: 1.3,
  Type: ["badge"|"webext"|"standalone"|"mobile"],
  NAT: ["unknown"|"restricted"|"unrestricted"],
  Clients: [number of current clients, rounded down to multiples of 8],
  AcceptedRelayPattern: [a pattern representing accepted set of relay domains]
}
```

If the request is well-formed, they receive a 200 OK response.

If a client is matched:
```
HTTP 200 OK

{
  Status: "client match",
  {
    type: offer,
    sdp: [WebRTC SDP]
  },
  RelayURL: [the WebSocket URL proxy should connect to relay Snowflake traffic]
}
```

If a client is not matched:
```
HTTP 200 OK

{
    Status: "no match"
}
```

If the request is malformed:
```
HTTP 400 BadRequest
```

If they are matched with a client, they provide their SDP answer with a POST
request to `/answer`:
```
POST /answer HTTP

{
  Sid: [generated session id of proxy],
  Version: 1.3,
  Answer:
  {
    type: answer,
    sdp: [WebRTC SDP]
  }
}
```

If the request is well-formed, they receive a 200 OK response.

If the client retrieved the answer:
```
HTTP 200 OK

{
  Status: "success"
}
```

If the client left:
```
HTTP 200 OK

{
  Status: "client gone"
}

3) If the request is malformed:
HTTP 400 BadRequest
```