Apache HttpClient 4.5: Why does the second package wait for the ack of the first package?

Question

I am using apache http client in version 4.5.13 within my java application to send a post request. I used following line of code to set up the http client.

        SocketConfig socketConfig = SocketConfig.custom()
            .setSoKeepAlive(true)
            .setTcpNoDelay(true)
            .build();

        ConnectionConfig connectionConfig = ConnectionConfig.custom()
            .setMalformedInputAction(CodingErrorAction.IGNORE)
            .setUnmappableInputAction(CodingErrorAction.IGNORE)
            .setCharset(Consts.UTF_8)
            .setMessageConstraints(messageConstraints)
            .build();

        RequestConfig defaultRequestConfig = RequestConfig.custom()
            .setCookieSpec(CookieSpecs.DEFAULT)
            .setExpectContinueEnabled(true)
            .setTargetPreferredAuthSchemes(Arrays.asList(AuthSchemes.NTLM, AuthSchemes.DIGEST))
            .setContentCompressionEnabled(true)
            .build();

        BasicHttpClientConnectionManager connectionManager = new BasicHttpClientConnectionManager();
        connectionManager.setSocketConfig(socketConfig);
        connectionManager.setConnectionConfig(connectionConfig);

        CloseableHttpClient httpClient = HttpClients.custom()
            .setConnectionManager(connectionManager)
            .setDefaultRequestConfig(defaultRequestConfig)
            .build();

And I am sending the data via

CloseableHttpResponse response = httpClient.execute(postRequest);

The issue I am experiencing is that when I look how the messages are send (using tshark) I can see that the application data is split in two messages. The first one leaves my system around 0.5ms after the httpClient.execute(postRequest), but the second part is send around 10ms-20ms after the first one. It looks like the second part is waiting to receive the ack for the first part of the message. I tried to change a a lot of configurations (buffer sizes, TcpNoDelay, different TLS ...) but cannot figure out what is causing this behavior.

I also tried http.net client to send post requests. With this client the message was also split in two messages but they where both send right after each other (with around 0.3ms delay).

I am pretty new to network so I would appreciate a helpful answer and apologize upfront if I did not explain it very well (I do not know all the specific wordings).

Thanks

Answer 1

Try disabling expect-continue handshake.