If the network is unreliable, it’s better to keep the window small. The TCP window is the maximum number of bytes that can be sent before the ACK must be received. Receiving the ACK packet means the device can flush that old data out of the buffer. Each device maintains a buffer of all the data just in case it needs to send it again. If everything isn’t received, some or all of the data needs to be retransmitted. Then device B validates all the packet-level checksums and sends an ACK packet that says everything was received correctly. Device A sends a bunch of data to device B. What’s a TCP window?Ī key reliability feature of TCP is the acknowledgement (ACK) packet. One of them has to do with a feature called TCP windowing. So it shouldn’t come as a surprise that the bulletproof reliability mechanisms designed into the protocol sometimes cause problems. Today’s broadband networks are many orders of magnitude faster, as well as vastly more reliable. These features were invented when WAN bandwidth of 56Kbps was fast and packet drop rates of 1% were not uncommon. The Transmission Control Protocol (TCP) has built-in mechanisms for reliability that include validating a checksum on every packet, as well as detection and retransmission of dropped or out-of-order packets. So one of the main considerations in early protocol design was reliability. It was invented in an era when networks were very slow and packet loss was high. The TCP/IP protocol sometimes shows its age.

Are you seeing poor network performance but with link utilization that’s well below 100%? You might have an issue with your TCP window size.

Tcp retransmission wireshark causes