Page 175 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes 11.3.1 The Example Service Primitives

The abstract service primitives also called as system calls are connection-oriented such as LISTEN,
CONNECT, SEND, RECEIVE and DISCONNECT. They are listed below along with their functions:
LISTEN: Broadcast willingness to accept connections and provide queue size.

ACCEPT: Block the caller unless a communication attempt arrives.
CONNECT: Actively try to establish a connection.
SEND: Send data over the connection.

RECIEVE: Receive data from the connection.
CLOSE: Release the connection.
In the client server architecture, a machine (client) requests to another machine (server) to create
a connection for providing some service. The services running on the server run on ports. The
ports are application identifiers. The client machine should know the address of the server
machine for getting the desired services from this port and to connect to the server machine.
However, the server machine should not know the address or the port of the client machine at
the time of connection initiation. The first packet transmitted by the client machine as a request
to the server machine contains details about the client which are further used by the server to
send any information. Client machine acts as the active device which makes the first move to
establish the connection whereas the server machine passively waits for such requests from
some client.
11.3.2 The Example Transport Entity

The transport layer uses the network layer primitives to send and receive TPDUs. The transport
entity resides in:

the host operating system kernel,
a separate user process,
a package of library routines running within the user’s address space, or
a co-processor chip or network board plugged into the host’s backplane.
The interface to the network layer is given as below:

to_net(int cid, int q, int m, pkt_type pt, unsigned char *p, int bytes);
from_net(int *cid, int *q, int *m, pkt_type *pt, unsigned char *p, int *bytes);
The network layer packets that are used are given below:

CALL REQUEST: Sent to establish a connection
CALL ACCEPTED: Response to CALL REQUEST
CLEAR REQUEST: Sent to release a connection
CLEAR CONFIRMATION: Response to CLEAR REQUEST
DATA: Used to transport data

CREDIT: Control packet for managing the window
When information is passed as procedure parameters rather than the actual outgoing or incoming
packet itself, the transport layer is shielded from the details of the network layer protocol. The

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