I have two programs, one is a C-language program using libcurl (Linux)
to send a document to a perl-based receiver (also Linux), but the
conversation I see between the two machines is very strange:

1. 18:05:34.974033: receives "PUT / HTTP/1.1" header from libcurl PUT
server; the packet (in tcpdump) shows that it has Accept: */* and
Content-Length: 1095, then ends with "Expect: 100-continue"

2. 18:05:34.974049: client sends ACK

3. 18:05:34.976442: client sends an HTTP response which contains
"(HTTP/1.1 100 Continue"

4. 18:05:35.065209: client receives ACK from libcurl server

5. 18:05:36.065523: client receives the HTTP body from libcurl

Forgive my noviceness, but there's two things I don't understand here,
and one of them is bogging down my transmissions:

- Why doesn't the HTTP PUT message already contain the body? In the C
code I set up the curl call as one atomic item containing headers and
the body expecting this to be one transmission; why does the client
need to send an HTTP 100-continue response to the original request?
Is this discussed in RFC 2616? (I must have missed it) or is this
perhaps an ill-configured PUT request?

- I can live with 100-Continue handshakes, but here is the really
bothersome part: Notice how the delay between the 100 Continue request
and the start of the response with the body is very nearly exactly one
second! I see this on every transfer. Why one second? that seems a
long time for a pause when the typical HTTP transmission is only a few
K in size. Again, is there perhaps some misconfig of the libcurl that
is causing this?

Here's the sequence of curl library calls:

  curl = curl_easy_init();
  if (!curl) return NULL;

  curl_easy_setopt(curl, CURLOPT_DNS_CACHE_TIMEOUT, 150);
  curl_easy_setopt(curl, CURLOPT_TIMEOUT, RESTIMEOUT);
  curl_easy_setopt(curl, CURLOPT_MAXCONNECTS, 1);
  curl_easy_setopt(curl, CURLOPT_FRESH_CONNECT, TRUE);

  curl_easy_setopt(curl, CURLOPT_READDATA, mem_buffer);
  curl_easy_setopt(curl, CURLOPT_URL, target);
  curl_easy_setopt(curl, CURLOPT_UPLOAD, TRUE) ;

  curl_easy_setopt(curl, CURLOPT_FORBID_REUSE, TRUE);
        sprintf( string, "Content-Type: text/xml; charset=\"%s\"", charset);
        slist = curl_slist_append(slist, string );
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, slist);
  curl_easy_setopt(curl, CURLOPT_INFILESIZE, file_size);
  res = curl_easy_perform(curl);

Why is curl_easy_perform(curl) introducing that one-second pause? Can
it be avoided?
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Received on 2008-09-21