Created: 2014-05-19 05:19
Updated: 2014-06-06 06:46



creating a socket: http://www.cs.rutgers.edu/~pxk/417/notes/sockets/udp.html

beej's guide: http://www.beej.us/guide/bgnet/output/html/singlepage/bgnet.html#getaddrinfo


  • TA's link to demo: http://www.ccs-labs.org/teaching/rn/animations/gbn_sr/
  • Main Idea
    • sender allowed to transmit multpile packets without waiting for ackowlegement, but is constrained to having no more than N unacknowledged packets in the pipeline
  • window size N = max range of permissible sequence numbers for transmitted but not yet acknowledged sequence numbers
  • about packet sequence numbers
    • carried in a fixed-length field in packet header
    • if k is the number of bits in the packet sequence number field, then range of number is 0 to 2^k-1
    • with finite range of sequence numbers => use modulo 2^k arithmetic
  • Finite state machine of ACK-based, NAK-free Go-Back-N protocol on page 221 in textbook (ch 3.4.3)
  • 3 types of events that Go-Back-N sender must respond to
    • Invocation from above: ~ call send(), then first check to see if window is full (N unACK-ed packages) ~If window not full, packet created and sent (& variables updated) ~If window full, sender returns data back to upper layer (upper layer would try again later or perhaps would buffer the data or have a sycnhoronization mechanism that allos the upper layer to call send() only when it's not full)
    • Receipt of an ACK: ~ acknowledgement for a packet with sequence number n is a cumulative acknowledgment, i.e. an indication that all packets with a sequence number up to and including n have been correctly received at the receiver
    • Timeout event: ~ timer used to recover from lost data or ACK packets (like in the stop-and-wait protocol) ~ if timeout occurs, sender resends all packets that have been previously sent but not yet acknowleged ~ if ACK is received but there are still additional transmitted but not yet acknowledged packets, timer is restarted ~ if there are no outstanding, unacknowledged packets, the timer is stopped
  • receiver's actions in Go-Back-N
    • if a packet with sequence number n is received correctly in order (right after a packet with sequence number n-1 was delivered to the upper layer), receiver sends an ACK for packet n and delivers the data to the upper layer
    • in all other cases, receiver discards packet and resends an ACK for the most recently received in-order packet (note: since packets are delivered one at a time to upper layer, if packet k has been received & delivered, then all packets with a sequence number lower than k have also been delivered)
  • receivers don't need to buffer out of order packets because, if packet n is lost, both packet n and packet n + 1 will eventually be retransmitted => receiver only needs to maintain sequence number of the next-in-order package => sender must maintain upper and lower bounds of its window and the position of the next sequence number within this window
  • summary of techniques in GBN protocol: sequence numbers, cumulative acknowledgements, checksums, and a timeout/retransmit operation
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