Chapter 1

esponse, it too considers itself bound in a session: the next response/request datagram it
sends will be sequenced number one.
The listening partner sends the first sequenced packet. For the listening partner, receiving
response n (from the initiating partner) acknowledges its request n. For the initiating partner,
receiving response n + 1 (from the listening partner) acknowledges its request n. The
initiate-request and initiate-response datagrams may be resent using the usual RGCP
resend facility.
To terminate a session normally, the terminating partner sends a 'terminate' request. After
sending a terminate request, the terminating partner is in the blank state: it does not expect
to receive a response. The partner receiving a terminate request must therefore terminate
without sending a response. A terminate request has a zero sequence number because it
can be sent at any time, without waiting for the terminating partner's turn in the conversation.
A terminate request cannot be resent.
Although a terminate request has a zero sequence number, it is uniquely identified with a
particular RGCP session because it specifies GSDP port numbers that are unique to that
session.
Abnormal termination may occur through either partner in a session – or during session
setup – simply ceasing to communicate. The other partner must realize by other means that
this has happened, abandon their current session, and decide what further action to take.
Standard opcodes
RGCP defines three request opcodes:
� 0×ff: initiate – also used to indicate initiate-response
� 0×fe: terminate
� 0×00: should not be used.
Other opcodes may be defined by specific RGCP implementations.
Responses are uniquely associated with their corresponding requests, so a response
opcode system is not strictly necessary. RGCP protocol designers may, however, find it
convenient and/or safer to use the same opcodes for responses as they use for requests.
States and transitions
As perceived by either endpoint, an RGCP session may have the following states:
State Meaning
Blank No GSDP connection.
Initiating An initiate-request has been sent, but no response has been received.
Listening Ready to receive an initiate-request, but none has yet been received.
Responding The session has been bound, a request is being handled, and the
partner must compose a response. This state lasts only for the duration
of a single synchronous function call.
Requesting The session has been bound, and it is this partner's turn to compose
and send a request.
Waiting The session has been bound, this party has sent a response/request,
and is now waiting for its response and another request. Incoming