8.2 Message Transmission Requirements
8.2.1 Persistent Connections and Flow Control
HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
8.2.2 Monitoring Connections for Error Status Messages
An HTTP/1.1 (or later) client sending a message-body SHOULD monitor the network connection for an error status while it is transmitting the request. If the client sees an error status, it SHOULD immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (section 3.6), a zero length chunk and empty trailer MAY be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client MUST close the connection.
8.2.3 Use of the 100 (Continue) Status
The purpose of the 100 (Continue) status (see section 10.1.1) is to allow a client that is sending a request message with a request body to determine if the origin server is willing to accept the request (based on the request headers) before the client sends the request body. In some cases, it might either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking at the body.
Requirements for HTTP/1.1 clients:
- If a client will wait for a 100 (Continue) response before
sending the request body, it MUST send an Expect request-header
field (section 14.20) with the "100-continue" expectation.
- A client MUST NOT send an Expect request-header field (section
14.20) with the "100-continue" expectation if it does not intend
to send a request body.
Because of the presence of older implementations, the protocol allows ambiguous situations in which a client may send "Expect: 100- continue" without receiving either a 417 (Expectation Failed) status or a 100 (Continue) status. Therefore, when a client sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) status, the client SHOULD NOT wait for an indefinite period before sending the request body.
Requirements for HTTP/1.1 origin servers:
- Upon receiving a request which includes an Expect request-header
field with the "100-continue" expectation, an origin server MUST
either respond with 100 (Continue) status and continue to read
from the input stream, or respond with a final status code. The
origin server MUST NOT wait for the request body before sending
the 100 (Continue) response. If it responds with a final status
code, it MAY close the transport connection or it MAY continue
to read and discard the rest of the request. It MUST NOT
perform the requested method if it returns a final status code.
- An origin server SHOULD NOT send a 100 (Continue) response if
the request message does not include an Expect request-header
field with the "100-continue" expectation, and MUST NOT send a
100 (Continue) response if such a request comes from an HTTP/1.0
(or earlier) client. There is an exception to this rule: for
compatibility with RFC 2068, a server MAY send a 100 (Continue)
status in response to an HTTP/1.1 PUT or POST request that does
not include an Expect request-header field with the "100-
continue" expectation. This exception, the purpose of which is
to minimize any client processing delays associated with an
undeclared wait for 100 (Continue) status, applies only to
HTTP/1.1 requests, and not to requests with any other HTTP-
version value.
- An origin server MAY omit a 100 (Continue) response if it has
already received some or all of the request body for the
corresponding request.
- An origin server that sends a 100 (Continue) response MUST
ultimately send a final status code, once the request body is
received and processed, unless it terminates the transport
connection prematurely.
- If an origin server receives a request that does not include an
Expect request-header field with the "100-continue" expectation,
the request includes a request body, and the server responds
with a final status code before reading the entire request body
from the transport connection, then the server SHOULD NOT close
the transport connection until it has read the entire request,
or until the client closes the connection. Otherwise, the client
might not reliably receive the response message. However, this
requirement is not be construed as preventing a server from
defending itself against denial-of-service attacks, or from
badly broken client implementations.
Requirements for HTTP/1.1 proxies:
- If a proxy receives a request that includes an Expect request-
header field with the "100-continue" expectation, and the proxy
either knows that the next-hop server complies with HTTP/1.1 or
higher, or does not know the HTTP version of the next-hop
server, it MUST forward the request, including the Expect header
field.
- If the proxy knows that the version of the next-hop server is
HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
respond with a 417 (Expectation Failed) status.
- Proxies SHOULD maintain a cache recording the HTTP version
numbers received from recently-referenced next-hop servers.
- A proxy MUST NOT forward a 100 (Continue) response if the
request message was received from an HTTP/1.0 (or earlier)
client and did not include an Expect request-header field with
the "100-continue" expectation. This requirement overrides the
general rule for forwarding of 1xx responses (see section 10.1).
8.2.4 Client Behavior if Server Prematurely Closes Connection
If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field with the "100-continue" expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the client sees the connection close before receiving any status from the server, the client SHOULD retry the request. If the client does retry this request, it MAY use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response:
1. Initiate a new connection to the server
2. Transmit the request-headers
3. Initialize a variable R to the estimated round-trip time to the
server (e.g., based on the time it took to establish the
connection), or to a constant value of 5 seconds if the round-
trip time is not available.
4. Compute T = R * (2**N), where N is the number of previous
retries of this request.
5. Wait either for an error response from the server, or for T
seconds (whichever comes first)
6. If no error response is received, after T seconds transmit the
body of the request.
7. If client sees that the connection is closed prematurely,
repeat from step 1 until the request is accepted, an error
response is received, or the user becomes impatient and
terminates the retry process.
If at any point an error status is received, the client
- SHOULD NOT continue and
- SHOULD close the connection if it has not completed sending the
request message.