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http2: send HEADER & DATA together if possible #11420

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This PR is based on the conversation here.

When sending an HTTP/2 POST request, curl will always defer request body to a different TLS record (if SSL/TLS is enabled) and TCP packet. This problem is previously addressed in #6363.

Belows are network packets captured by Wireshark by running SSLKEYLOGFILE=/tmp/sslkeylog.log src/curl -k "" -d "moo":


We can combine the TCP packet by disabling TCP_NODELAY. Belows is a TCP packet sent by running SSLKEYLOGFILE=/tmp/sslkeylog.log src/curl -k "" -d "moo" --no-tcp-nodelay:


As can be seen from the image above, curl still generates 2 TLS records for HEADER and DATA frames, respectively (ignore the first TLS record from MAGIC frame).

Therefore, this PR provides an optimization that allows request body to be sent together with request headers in a single TLS record and TCP packet if the body size is small enough. This is achieved by attaching request body to header data so that h2_submit() can write the body directly after it has created a stream.

Belows is a TCP packet sent by running SSLKEYLOGFILE=/tmp/sslkeylog.log src/curl -k "" -d "moo" with this change:


@github-actions github-actions bot added the HTTP label Jul 11, 2023
@bagder bagder requested a review from icing July 11, 2023 08:35
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LGTM! Nicely done.

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@bagder bagder added the HTTP/2 label Jul 11, 2023
@bagder bagder closed this in c9ec851 Jul 11, 2023
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bagder commented Jul 11, 2023


@peppapighs peppapighs deleted the http2-coalesce-frames branch July 12, 2023 00:35
bch pushed a commit to bch/curl that referenced this pull request Jul 19, 2023
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3 participants