1# gRPC over HTTP2
2
3## Introduction
4This document serves as a detailed description for an implementation of gRPC carried over <a href="https://tools.ietf.org/html/rfc7540">HTTP2 framing</a>. It assumes familiarity with the HTTP2 specification.
5
6## Protocol
7Production rules are using <a href="http://tools.ietf.org/html/rfc5234">ABNF syntax</a>.
8
9### Outline
10
11The following is the general sequence of message atoms in a GRPC request & response message stream
12
13* Request → Request-Headers \*Length-Prefixed-Message EOS
14* Response → (Response-Headers \*Length-Prefixed-Message Trailers) / Trailers-Only
15
16
17### Requests
18
19* Request → Request-Headers \*Length-Prefixed-Message EOS
20
21Request-Headers are delivered as HTTP2 headers in HEADERS + CONTINUATION frames.
22
23* **Request-Headers** → Call-Definition \*Custom-Metadata
24* **Call-Definition** → Method Scheme Path TE [Authority] [Timeout] Content-Type [Message-Type] [Message-Encoding] [Message-Accept-Encoding] [User-Agent]
25* **Method** →  ":method POST"
26* **Scheme** → ":scheme "  ("http" / "https")
27* **Path** → ":path" "/" Service-Name "/" {_method name_}  # But see note below.
28* **Service-Name** → {_IDL-specific service name_}
29* **Authority** → ":authority" {_virtual host name of authority_}
30* **TE** → "te" "trailers"  # Used to detect incompatible proxies
31* **Timeout** → "grpc-timeout" TimeoutValue TimeoutUnit
32* **TimeoutValue** → {_positive integer as ASCII string of at most 8 digits_}
33* **TimeoutUnit** → Hour / Minute / Second / Millisecond / Microsecond / Nanosecond
34* **Hour** → "H"
35* **Minute** → "M"
36* **Second** → "S"
37* **Millisecond** → "m"
38* **Microsecond** → "u"
39* **Nanosecond** → "n"
40* **Content-Type** → "content-type" "application/grpc" [("+proto" / "+json" / {_custom_})]
41* **Content-Coding** → "identity" / "gzip" / "deflate" / "snappy" / {_custom_}
42* <a name="message-encoding"></a>**Message-Encoding** → "grpc-encoding" Content-Coding
43* **Message-Accept-Encoding** → "grpc-accept-encoding" Content-Coding \*("," Content-Coding)
44* **User-Agent** → "user-agent" {_structured user-agent string_}
45* **Message-Type** → "grpc-message-type" {_type name for message schema_}
46* **Custom-Metadata** → Binary-Header / ASCII-Header
47* **Binary-Header** → {Header-Name "-bin" } {_base64 encoded value_}
48* **ASCII-Header** → Header-Name ASCII-Value
49* **Header-Name** → 1\*( %x30-39 / %x61-7A / "\_" / "-" / ".") ; 0-9 a-z \_ - .
50* **ASCII-Value** → 1\*( %x20-%x7E ) ; space and printable ASCII
51
52
53HTTP2 requires that reserved headers, ones starting with ":" appear before all other headers. Additionally implementations should send **Timeout** immediately after the reserved headers and they should send the **Call-Definition** headers before sending **Custom-Metadata**.
54
55Some gRPC implementations may allow the **Path** format shown above
56to be overridden, but this functionality is strongly discouraged.
57gRPC does not go out of its way to break users that are using this kind
58of override, but we do not actively support it, and some functionality
59(e.g., service config support) will not work when the path is not of
60the form shown above.
61
62If **Timeout** is omitted a server should assume an infinite timeout. Client implementations are free to send a default minimum timeout based on their deployment requirements.
63
64**Custom-Metadata** is an arbitrary set of key-value pairs defined by the application layer. Header names starting with "grpc-" but not listed here are reserved for future GRPC use and should not be used by applications as **Custom-Metadata**.
65
66Note that HTTP2 does not allow arbitrary octet sequences for header values so binary header values must be encoded using Base64 as per https://tools.ietf.org/html/rfc4648#section-4. Implementations MUST accept padded and un-padded values and should emit un-padded values. Applications define binary headers by having their names end with "-bin". Runtime libraries use this suffix to detect binary headers and properly apply base64 encoding & decoding as headers are sent and received.
67
68**Custom-Metadata** header order is not guaranteed to be preserved except for
69values with duplicate header names. Duplicate header names may have their values
70joined with "," as the delimiter and be considered semantically equivalent.
71Implementations must split **Binary-Header**s on "," before decoding the
72Base64-encoded values.
73
74**ASCII-Value** should not have leading or trailing whitespace. If it contains
75leading or trailing whitespace, it may be stripped. The **ASCII-Value**
76character range defined is more strict than HTTP. Implementations must not error
77due to receiving an invalid **ASCII-Value** that's a valid **field-value** in
78HTTP, but the precise behavior is not strictly defined: they may throw the value
79away or accept the value. If accepted, care must be taken to make sure that the
80application is permitted to echo the value back as metadata. For example, if the
81metadata is provided to the application as a list in a request, the application
82should not trigger an error by providing that same list as the metadata in the
83response.
84
85Servers may limit the size of **Request-Headers**, with a default of 8 KiB
86suggested.  Implementations are encouraged to compute total header size like
87HTTP/2's `SETTINGS_MAX_HEADER_LIST_SIZE`: the sum of all header fields, for each
88field the sum of the uncompressed field name and value lengths plus 32, with
89binary values' lengths being post-Base64.
90
91The repeated sequence of **Length-Prefixed-Message** items is delivered in DATA frames
92
93* **Length-Prefixed-Message** → Compressed-Flag Message-Length Message
94* <a name="compressed-flag"></a>**Compressed-Flag** → 0 / 1   # encoded as 1 byte unsigned integer
95* **Message-Length** → {_length of Message_}  # encoded as 4 byte unsigned integer
96* **Message** → \*{binary octet}
97
98A **Compressed-Flag** value of 1 indicates that the binary octet sequence of **Message** is compressed using the mechanism declared by the **Message-Encoding** header. A value of 0 indicates that no encoding of **Message** bytes has occurred. Compression contexts are NOT maintained over message boundaries, implementations must create a new context for each message in the stream. If the **Message-Encoding** header is omitted then the **Compressed-Flag** must be 0.
99
100For requests, **EOS** (end-of-stream) is indicated by the presence of the END_STREAM flag on the last received DATA frame. In scenarios where the **Request** stream needs to be closed but no data remains to be sent implementations MUST send an empty DATA frame with this flag set.
101
102### Responses
103
104* **Response** → (Response-Headers \*Length-Prefixed-Message Trailers) / Trailers-Only
105* **Response-Headers** → HTTP-Status [Message-Encoding] [Message-Accept-Encoding] Content-Type \*Custom-Metadata
106* **Trailers-Only** → HTTP-Status Content-Type Trailers
107* **Trailers** → Status [Status-Message] \*Custom-Metadata
108* **HTTP-Status** → ":status 200"
109* **Status** → "grpc-status" 1\*DIGIT ; 0-9
110* **Status-Message** → "grpc-message" Percent-Encoded
111* **Percent-Encoded** → 1\*(Percent-Byte-Unencoded / Percent-Byte-Encoded)
112* **Percent-Byte-Unencoded** → 1\*( %x20-%x24 / %x26-%x7E ) ; space and VCHAR, except %
113* **Percent-Byte-Encoded** → "%" 2HEXDIGIT ; 0-9 A-F
114
115**Response-Headers** & **Trailers-Only** are each delivered in a single HTTP2 HEADERS frame block. Most responses are expected to have both headers and trailers but **Trailers-Only** is permitted for calls that produce an immediate error. Status must be sent in **Trailers** even if the status code is OK.
116
117For responses end-of-stream is indicated by the presence of the END_STREAM flag on the last received HEADERS frame that carries **Trailers**.
118
119Implementations should expect broken deployments to send non-200 HTTP status codes in responses as well as a variety of non-GRPC content-types and to omit **Status** & **Status-Message**. Implementations must synthesize a **Status** & **Status-Message** to propagate to the application layer when this occurs.
120
121Clients may limit the size of **Response-Headers**, **Trailers**, and
122**Trailers-Only**, with a default of 8 KiB each suggested.
123
124The value portion of **Status** is a decimal-encoded integer as an ASCII string,
125without any leading zeros.
126
127The value portion of **Status-Message** is conceptually a Unicode string
128description of the error, physically encoded as UTF-8 followed by
129percent-encoding. Percent-encoding is specified in [RFC 3986
130§2.1](https://tools.ietf.org/html/rfc3986#section-2.1), although the form used
131here has different restricted characters. When decoding invalid values,
132implementations MUST NOT error or throw away the message. At worst, the
133implementation can abort decoding the status message altogether such that the
134user would received the raw percent-encoded form. Alternatively, the
135implementation can decode valid portions while leaving broken %-encodings as-is
136or replacing them with a replacement character (e.g., '?' or the Unicode
137replacement character).
138
139#### Example
140
141Sample unary-call showing HTTP2 framing sequence
142
143**Request**
144
145```
146HEADERS (flags = END_HEADERS)
147:method = POST
148:scheme = http
149:path = /google.pubsub.v2.PublisherService/CreateTopic
150:authority = pubsub.googleapis.com
151grpc-timeout = 1S
152content-type = application/grpc+proto
153grpc-encoding = gzip
154authorization = Bearer y235.wef315yfh138vh31hv93hv8h3v
155
156DATA (flags = END_STREAM)
157<Length-Prefixed Message>
158```
159**Response**
160```
161HEADERS (flags = END_HEADERS)
162:status = 200
163grpc-encoding = gzip
164content-type = application/grpc+proto
165
166DATA
167<Length-Prefixed Message>
168
169HEADERS (flags = END_STREAM, END_HEADERS)
170grpc-status = 0 # OK
171trace-proto-bin = jher831yy13JHy3hc
172```
173
174#### User Agents
175
176While the protocol does not require a user-agent to function it is recommended that clients provide a structured user-agent string that provides a basic description of the calling library, version & platform to facilitate issue diagnosis in heterogeneous environments. The following structure is recommended to library developers
177```
178User-Agent → "grpc-" Language ?("-" Variant) "/" Version ?( " ("  *(AdditionalProperty ";") ")" )
179```
180E.g.
181
182```
183grpc-java/1.2.3
184grpc-ruby/1.2.3
185grpc-ruby-jruby/1.3.4
186grpc-java-android/0.9.1 (gingerbread/1.2.4; nexus5; tmobile)
187```
188
189#### Idempotency and Retries
190
191Unless explicitly defined to be, gRPC Calls are not assumed to be idempotent.  Specifically:
192
193* Calls that cannot be proven to have started will not be retried.
194* There is no mechanism for duplicate suppression as it is not necessary.
195* Calls that are marked as idempotent may be sent multiple times.
196
197
198#### HTTP2 Transport Mapping
199
200##### Stream Identification
201All GRPC calls need to specify an internal ID. We will use HTTP2 stream-ids as call identifiers in this scheme. NOTE: These ids are contextual to an open HTTP2 session and will not be unique within a given process that is handling more than one HTTP2 session nor can they be used as GUIDs.
202
203##### Data Frames
204DATA frame boundaries have no relation to **Length-Prefixed-Message** boundaries and implementations should make no assumptions about their alignment.
205
206##### Errors
207
208When an application or runtime error occurs during an RPC a **Status** and **Status-Message** are delivered in **Trailers**.
209
210In some cases it is possible that the framing of the message stream has become corrupt and the RPC runtime will choose to use an **RST_STREAM** frame to indicate this state to its peer. RPC runtime implementations should interpret RST_STREAM as immediate full-closure of the stream and should propagate an error up to the calling application layer.
211
212The following mapping from RST_STREAM error codes to GRPC error codes is applied.
213
214HTTP2 Code|GRPC Code
215----------|-----------
216NO_ERROR(0)|INTERNAL - An explicit GRPC status of OK should have been sent but this might be used to aggressively lameduck in some scenarios.
217PROTOCOL_ERROR(1)|INTERNAL
218INTERNAL_ERROR(2)|INTERNAL
219FLOW_CONTROL_ERROR(3)|INTERNAL
220SETTINGS_TIMEOUT(4)|INTERNAL
221STREAM_CLOSED|No mapping as there is no open stream to propagate to. Implementations should log.
222FRAME_SIZE_ERROR|INTERNAL
223REFUSED_STREAM|UNAVAILABLE - Indicates that no processing occurred and the request can be retried, possibly elsewhere.
224CANCEL(8)|Mapped to call cancellation when sent by a client.Mapped to CANCELLED when sent by a server. Note that servers should only use this mechanism when they need to cancel a call but the payload byte sequence is incomplete.
225COMPRESSION_ERROR|INTERNAL
226CONNECT_ERROR|INTERNAL
227ENHANCE_YOUR_CALM|RESOURCE_EXHAUSTED ...with additional error detail provided by runtime to indicate that the exhausted resource is bandwidth.
228INADEQUATE_SECURITY| PERMISSION_DENIED … with additional detail indicating that permission was denied as protocol is not secure enough for call.
229
230
231##### Security
232
233The HTTP2 specification mandates the use of TLS 1.2 or higher when TLS is used with HTTP2. It also places some additional constraints on the allowed ciphers in deployments to avoid known-problems as well as requiring SNI support. It is also expected that HTTP2 will be used in conjunction with proprietary transport security mechanisms about which the specification can make no meaningful recommendations.
234
235##### Connection Management
236
237###### GOAWAY Frame
238Sent by servers to clients to indicate that they will no longer accept any new streams on the associated connections. This frame includes the id of the last successfully accepted stream by the server. Clients should consider any stream initiated after the last successfully accepted stream as UNAVAILABLE and retry the call elsewhere. Clients are free to continue working with the already accepted streams until they complete or the connection is terminated.
239
240Servers should send GOAWAY before terminating a connection to reliably inform clients which work has been accepted by the server and is being executed.
241
242###### PING Frame
243Both clients and servers can send a PING frame that the peer must respond to by precisely echoing what they received. This is used to assert that the connection is still live as well as providing a means to estimate end-to-end latency. If a server initiated PING does not receive a response within the deadline expected by the runtime all outstanding calls on the server will be closed with a CANCELLED status. An expired client initiated PING will cause all calls to be closed with an UNAVAILABLE status. Note that the frequency of PINGs is highly dependent on the network environment, implementations are free to adjust PING frequency based on network and application requirements.
244
245###### Connection failure
246If a detectable connection failure occurs on the client all calls will be closed with an UNAVAILABLE status. For servers open calls will be closed with a CANCELLED status.
247
248
249### Appendix A - GRPC for Protobuf
250
251The service interfaces declared by protobuf are easily mapped onto GRPC by
252code generation extensions to protoc. The following defines the mapping
253to be used.
254
255* **Service-Name** → ?( {_proto package name_} "." ) {_service name_}
256* **Message-Type** → {_fully qualified proto message name_}
257* **Content-Type** → "application/grpc+proto"
258
259
260