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+Ideas on TCP unit testing:
+<https://www.snellman.net/blog/archive/2015-07-09-unit-testing-a-tcp-stack/>
+
+Check behaviour around TCP zero-window probing. Is the correct
+behaviour already a consequence of the way `send-outbound` works?
+
+Do something smarter with TCP timers and RTT estimation than the
+nothing that's already being done.
+
+TCP options negotiation.
+ - SACK
+ - Window scaling