AppleTalk DSP (ADSP)
AppleTalk Data Stream Protocol (ADSP)
One of the significant new features of AppleTalk included as part of system
full-duplex data stream connection between two nodes in an AppleTalk
internet. ( ADSP was available as an extension under System 6. ) Like the
AppleTalk Session Protocol, Printer Access Protocol, and AppleTalk
Transaction Protocol, ADSP uses the Datagram Delivery Protocol (DDP) to
send its data over the internet. Therefore, even though ADSP appears to its
clients to handle data as a stream of bytes, the data is actually transmitted and
received by DDP in packets. ADSP takes advantage of this fact by including
control and status information in the DDP packet header. You can use
takes care of the implementation of ADSP for you.
Every ADSP connection is between two sockets in an AppleTalk internet. Each socket can maintain concurrent ADSP connections with several other sockets,
but there can be only one ADSP connection between any two sockets at one
time. When a pair of sockets establishes an ADSP connection, each socket
client initializes and maintains a certain amount of control and state
information that it uses for synchronizing communication with the other
socket client and for error checking.
The combination of a socket and the ADSP information maintained by the
socket client is referred to as a connection end. When two connection ends
establish communication, the connection is considered an open connection.
When both connection ends terminate the link and dispose of the connection
information each maintains, the connection is considered a closed
connection. If one connection end is established but the other connection end
is unreachable or has disposed of its connection information, the connection is
considered a half-open connection. No communication can occur over a
half-open or closed connection. To prevent a half-open connection from tying
up resources, ADSP automatically closes any half-open connection that cannot
reestablish communication within 2 minutes.
Using ADSP
You can use ADSP to implement a data stream connection between any two
sockets on an internet. (Note that although there can be only one ADSP
connection between any two sockets, a single socket can maintain connections
with several other sockets.)
maintain, and close a connection between two sockets on an internet.
connection listener-that is, a connection end that waits passively to
receive a connection request and then passes the connection request on to its
client, the connection server. Finally,
(referred to as a user routine) that ADSP calls when your connection end
receives an unsolicited connection event.
When you establish an ADSP connection end, you must allocate a
nonrelocatable block of memory for, and provide a pointer to, a connection
control block (CCB) data structure, which is used by ADSP to store state information about the connection end. You may read the fields in the CCB to obtain information about the connection end, but you are not allowed to write to
any of the fields except one, the userFlags field. The CCB requires 242 bytes and is defined by the TRCCB data type.