The terms internal port and inside port are used interchangeably. They both refer to the original source port before NAT is performed.
The terms external port and outside port are used interchangeably. They both refer to the translated source port after NAT is performed.
The PCP PORT_SET option is defined in RFC 7753, Port Control Protocol (PCP) Extension for Port-Set Allocation, and is used by applications that require a consecutive block of port forwards. The reasons to provide a block of ports in a single request as opposed to multiple requests for single port in a MAP request are described in Section 2 of RFC 7753.
A PCP PORT_SET option indicates to the PCP server (SR OS) that a client needs a block of sequential port forwards. The number of requested port forwards must be greater than one (otherwise, a plain MAP opcode can be used). The ports in the block start at the Internal Port (in MAP opcode) and map to the same number of ports on the outside (or the external side), starting from either the Suggested External Port (in MAP opcode) or from an arbitrary external port. The returned number of ports may be fewer than the requested number, but the number cannot be larger. The allocated port set cannot fall outside of the range defined by the Internal Port (as requested in MAP opcode) plus the PORT_SET Size. Before a port set is assigned to a client, the SR OS always checks if any of the internal ports in the MAP request carrying the PORT_SET option has already been allocated.
The following figure shows an example of PORT_SET option format.
In the MAP request, the needed number of ports is specified in the PORT_SET Size field.
The First Internal Port is set to the same value as the Internal Port field in the MAP opcode.
In the MAP response, the PORT_SET Size represents the number of allocated ports. The First Internal Port represents the first internal port in the port set, which may be different than the Internal Port field in the MAP opcode (see example in Section 6.3 in RFC 7753, overlapping requests). The first external port is the value returned in Assigned External Port field in the MAP opcode.