TCP Handshake, SYN and SYNACK

Onload User Guide (UG1586)

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1.2 English

During the TCP connection establishment three-way handshake, Onload negotiates the MSS, Window Scale, SACK permitted, ECN, PAWS and RTTM timestamps.

For TCP connections Onload will negotiate an appropriate MSS for the MTU configured on the interface. However, when using jumbo frames, Onload will currently negotiate an MSS value up to a maximum of 2048 bytes minus the number of bytes required for packet headers. This is due to the fact that the size of buffers passed to the Solarflare network interface card is 2048 bytes and the Onload stack cannot currently handle fragmented packets on its TCP receive path.

TCP options advertised during the handshake can be selected using the EF_TCP_SYN_OPTS environment variable. Refer to Parameter Reference for details of environment variables.