What TCP port to use with SeaCat


SeaCat requires to specify one TCP port that is eventually used for client-gateway communication. Clients connect to this port to establish TLS channel that is used to exchange requests and related responses. SPDY-based communication protocol is used for traffic in this channel.

SeaCat protocol stack
SeaCat: Protocol stack

TCP ports reachability

Google performed investigation during WebSocket implementation (late 2009) that showed surprising facts about success rate of client-to-server connections:

  • HTTP port 80: 67%
  • Custom TCP port (61985): 86%
  • HTTPS port 443: 95%

The reason for low HTTP score is that the Internet is today full of proxies and firewalls that are configured to be transparent to HTTP traffic. However, non-HTTP traffic doesn't successfully pass them. Detailed reasoning can be found here: SPDY Essentials presentation from Google at approx. 18th minute of the video.

443 or custom port

SeaCat communication protocol is up to Session layer compatible with HTTPS protocol stack, HTTPS client that connects to SeaCat gateway is politely rejected with no harm on both sides. SeaCat traffic is indistinguishable from HTTPS traffic for intermediates on the network path, and,therefore, it shares the same success rate of connections. For these reasons TCP port 443 is recommended choice for SeaCat.

If for whatever reason you cannot use this port, use any TCP port above 1024 (non-reserved ports). You will likely got little bit worst connection success rate but still useful for practical deployments in common network scenarios.

Need help?

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About the Author

Ales Teska

TeskaLabs’ founder and CEO, Ales Teska, is a driven innovator who proactively builds things and comes up with solutions to solve practical IT problems.


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