This document specifies a TCP option for service numbers. The current SYN destination port is used both to indicate the desired service and as a connection demultiplexing field. This option separates those two functions, retaining the current destination port solely for demultiplexing and indicating the service separately in a service number option (SNO). By decoupling these two functions, SNO allows a larger number of concurrent connections for a single service, as might be useful between fixed addresses of proxies.