Network Working Group R. Fielding Request for Comments: 2616 ...

- Proxies SHOULD maintain a cache recording the HTTP versionnumbers received from recently-referenced next-hop servers.- A proxy MUST NOT forward a 100 (Continue) response if therequest message was received from an HTTP/1.0 (or earlier)client and did not include an Expect request-header field withthe "100-continue" expectation. This requirement overrides thegeneral rule for forwarding of 1xx responses (see section 10.1).8.2.4 Client Behavior if Server Prematurely Closes ConnectionIf 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 directlyconnected to an HTTP/1.1 origin server, and if the client sees theconnection close before receiving any status from the server, theclient SHOULD retry the request. If the client does retry thisrequest, it MAY use the following "binary exponential backoff"algorithm to be assured of obtaining a reliable response:1. Initiate a new connection to the server2. Transmit the request-headers3. Initialize a variable R to the estimated round-trip time to theserver (e.g., based on the time it took to establish theconnection), or to a constant value of 5 seconds if the roundtriptime is not available.4. Compute T = R * (2**N), where N is the number of previousretries of this request.5. Wait either for an error response from the server, or for Tseconds (whichever comes first)6. If no error response is received, after T seconds transmit thebody of the request.7. If client sees that the connection is closed prematurely,repeat from step 1 until the request is accepted, an errorresponse is received, or the user becomes impatient andterminates the retry process.Fielding, et al. Standards Track [Page 50]RFC 2616 HTTP/1.1 June 1999