![]() The TCP stream then continues as normal after that. If you expand the TCP Options you’ll again see the TCP Fast Open option, this time including the cookie value of 037110cc0de73575 Here’s the resulting SYN+ACK from the server: Here the client sends the Fast Open Cookie Request option asking the server give it a TFO cookie for next time. ![]() If you expand the TCP Options you’ll see that the TCP Fast Open option is included (ALT or OPTION-Click to expand everything). Notice that the TCP Length (the Len: field) is zero, since there’s no data in the initial handshake: Here’s an initial SYN packet from this example capture. Interestingly, TLS 1.3 has a very similar feature for saving a round trip, called “0-RTT” or zero round trip, which we’ll go into more detail about in our series on TLS. The RFC goes into detail on the protocol sequence in section 3. Normally, the client and server would need to do a full TCP handshake every time before sending application data. The next time you connect, you can include data in the SYN packet, and the server can include its response data in the first ACK. TCP Fast Open works by allowing the server to set a special “TFO” cookie to use for your later connections. You know you can make collections just like this with CloudShark! Learn more. Follow along with these sample capturesĪs always, we've gathered up the captures mentioned in this article into this collection over on CloudShark TFO allows data to be carried in the SYN and SYN-ACK packets and consumed by the receiving end during the initial connection handshake, and saves up to one full round-trip time (RTT) compared to the standard TCP, which requires a three-way handshake (3WHS) to complete before data can be exchanged. This speeds things up for endpoints that are going to keep talking to each other in the future and is especially beneficial on high-latency networks where time-to-first-byte is critical.įast Open is defined in RFC 7413 which explains: TCP Fast Open (TFO) is an optional mechanism within TCP that lets endpoints that have established a full TCP connection in the past eliminate a round-trip of the handshake and send data right away. Part 3 of our series exploring TCP examines the TCP “Fast Open” option and what to look for when troubleshooting
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