Added compression spec

doc/PROTOCOL-HTTP2.md

@@ -38,7 +38,7 @@ Request-Headers are delivered as HTTP2 headers in HEADERS + CONTINUATION frames.
 * **Nanosecond** → "n"
 * **Content-Type** → "content-type" "application/grpc" [("+proto" / "+json" / {_custom_})]
 * **Content-Coding** → "identity" / "gzip" / "deflate" / "snappy" / {_custom_}
-* **Message-Encoding** → "grpc-encoding" Content-Coding
+* <a name="message-encoding"></a>**Message-Encoding** → "grpc-encoding" Content-Coding
 * **Message-Accept-Encoding** → "grpc-accept-encoding" Content-Coding \*("," Content-Coding)
 * **User-Agent** → "user-agent" {_structured user-agent string_}
 * **Message-Type** → "grpc-message-type" {_type name for message schema_}
@@ -83,7 +83,7 @@ binary values' lengths being post-Base64.
 The repeated sequence of **Length-Prefixed-Message** items is delivered in DATA frames
 
 * **Length-Prefixed-Message** → Compressed-Flag Message-Length Message
-* **Compressed-Flag** → 0 / 1   # encoded as 1 byte unsigned integer
+* <a name="compressed-flag"></a>**Compressed-Flag** → 0 / 1   # encoded as 1 byte unsigned integer
 * **Message-Length** → {_length of Message_}  # encoded as 4 byte unsigned integer
 * **Message** → \*{binary octet}
 

doc/compression.md

+## **gRPC Compression**
+
+The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be
+interpreted as described in [RFC 2119](http://www.ietf.org/rfc/rfc2119.txt).
+
+### Intent
+
+Compression is used to reduce the amount of bandwidth used between peers. The
+compression supported by gRPC acts _at the individual message level_, taking
+_message_ [as defined in the wire format
+document](PROTOCOL-HTTP2.md).
+
+The implementation supports different compression algorithms. A _default
+compression level_, to be used in the absence of message-specific settings, MAY
+be specified for during channel creation.
+
+The ability to control compression settings per call and to enable/disable
+compression on a per message basis MAY be used to prevent CRIME/BEAST attacks.
+It also allows for asymmetric compression communication, whereby a response MAY
+be compressed differently, if at all.
+
+### Specification
+
+Compression MAY be configured by the Client Application by calling the
+appropriate API method. There are two scenarios where compression MAY be
+configured:
+
++  At channel creation time, which sets the channel default compression and
+   therefore the compression that SHALL be used in the absence of per-RPC
+   compression configuration.
++  At response time, via:
+   +  For unary RPCs, the {Client,Server}Context instance. 
+   +  For streaming RPCs, the {Client,Server}Writer instance. In this case,
+      configuration is reduced to disabling compression altogether.
+
+### Compression Method Asymmetry Between Peers
+
+A gRPC peer MAY choose to respond using a different compression method to that
+of the request, including not performing any compression, regardless of channel
+and RPC settings (for example, if compression would result in small or negative
+gains).
+
+A compressed message from a client with an algorithm unsupported by a server,
+WILL result in an INVALID\_ARGUMENT error, alongside the receiving peer's
+`grpc-accept-encoding` header specifying the algorithms it accepts. If an
+INTERNAL error is returned from the server despite having used one of the
+algorithms from the `grpc-accept-encoding h`eader, the cause MUST NOT be related
+to compression. Data sent from a server compressed with an algorithm not
+supported by the client will also result in an INTERNAL error.
+
+Note that a peer MAY choose to not disclose all the encodings it supports.
+However, if it receives a message compressed in an undisclosed but supported
+encoding, it MUST include said encoding in the response's `grpc-accept-encoding
+h`eader.
+
+For every message a server is requested to compress using an algorithm it knows
+the client doesn't support (as indicated by the last `grpc-accept-encoding`
+header received from the client), it SHALL send the message uncompressed. 
+
+### Specific Disabling of Compression
+
+If the user (through the previously described mechanisms) requests to disable
+compression the next message MUST be sent uncompressed. This is instrumental in
+preventing BEAST/CRIME attacks. This applies to both the the unary and streaming
+cases.
+
+### Compression Levels and Algorithms
+
+We currently (as of July 2015) support _gzip_ and _deflate_ as algorithms (with
+the possible future addition of snappy). In order to simplify the public API,
+it's intended to abstract the algorithms as _compression levels_ (such as "low",
+"medium", "high") that'd map to concrete algorithms and/or their settings (such
+as "low" mapping to "gzip -3" and "high" mapping to "gzip -9"). However, we
+can't presently (July 2015) implement said compression levels at the client side
+without either a initial negotiation of capabilities or an automatic retry
+mechanism. Therefore, compression levels are only supported at the server side,
+which is aware of the client's capabilities by virtue of the incoming
+Message-Accept-Encoding header.
+
+### Propagation to child RPCs
+
+The inheritance of the compression configuration by child RPCs is left up to the
+implementation. Note that in the absence of changes to the parent channel, its
+configuration will be used.
+
+### Test cases
+
+1. When a compression level is not specified for either the channel or the
+message, the default channel level _none_ is considered: data MUST NOT be
+compressed.
+1. When per-RPC compression configuration isn't present for a message, the
+channel compression configuration MUST be used.
+1. When a compression method (including no compression) is specified for an
+outgoing message, the message MUST be compressed accordingly.
+1. A message compressed in a way not supported by its endpoint MUST fail with
+INVALID\_ARGUMENT status, its associated description indicating the unsupported
+condition as well as the supported ones. The returned `grpc-accept-encoding`
+header MUST NOT contain the compression method (encoding) used.
+1. An ill-constructed message with its [Compressed-Flag
+bit](PROTOCOL-HTTP2.md#compressed-flag)
+set but lacking a
+"[grpc-encoding](PROTOCOL-HTTP2.md#message-encoding)"
+entry different from _identity_ in its metadata MUST fail with INTERNAL status,
+its associated description indicating the invalid Compressed-Flag condition.