The Transmission Control Protocol (TCP) is specified in [TRANS:2].
A router that implements TCP MUST be compliant, and SHOULD be unconditionally compliant, with the requirements of [INTRO:2], except that:
It should particularly be noted that a TCP implementation in a router must conform to the following requirements of [INTRO:2]:
The general paradigm applied is that if a particular interface is visible outside the router, then all requirements for the interface must be followed. For example, if a router provides a telnet function, then it will be generating traffic, likely to be routed in the external networks. Therefore, it must be able to set the type of service correctly or else the telnet traffic may not get through.