---

Programming with Timeout

---

• Recall: the Stop-and-wait protocol
  • The stop-and-wait protocol (click here click here ) transmits a message and must wait for acknowledgement:

    stop-and-wait-send( input-data ) { seqno ++; // use next sequence number msg.seqno = seqno; // label message with the seqno msg.data = input-data // put inpute data inside msg send msg; // send message to receive (use sendto( ) !) ack_recv = false; while ( ! ack_recv ) { wait for acknowledgement; if ( timeout while waiting for ACK ) { send msg (again); // retransmit message } else { if ( ACK.seqno = seqno ) ack_recv = true; } } }

  ---

  ---

  ---

  ---

• Functions that support "Timeout programming"
  • Time out:

    Network programs often use timeout to recover from transmission errors

    ---

  • System calls (= functions) that can be used in timeout programming:

    select( ) (originally defined in the BSD (Berkeley Software Distribution) UNIX) poll( ) (originally from UNIX System V)

    Most UNIX systems nowadays provide both system calls.