Go-back-N Protocol

In this project, a Go-Back-N network protocol is implemented in C using the socket library.

The protocol consists of the following three stages: (a) connection setup, (b) data transmission, and (c) connection teardown.

A general description of the protocol

Connection Setup

First, the connection is set up with a 3-way handshake between the sender and receiver as shown in the diagram below.

It is implemented on the sender in gbn_connect() and on the receiver in gbn_accept().

Data Transmission

Once the connection is established, the data is sent in DATA packets by the sender in gbn_send(). Each DATA packet is acknowleged by the receiver by sending an ACK packet in gnb_recv().

The design is illustrated in the diagram below.

Connection Teardown

One the data transmission is completed, the connection is torn down by both sender and receiver by sending FIN packets in gbn_close().

The design is illustrated in the diagram below.

The tricky parts of the implementation

The most tricky part of the implementation is figuring out the logic for the the gbn_sender() and gbn_receiver(). We tackled this by relying on the Finite State Machine (FSM) as below:

Known Issues

There are timeout issues when sending and receiving ACK during data transmission using gun_send() and gbn_recv(). Despite hours spent, we were unable to resolve this issue. We tested that 3-way hand shake works successfully. Then we tested with sendto() function and it works as well. However, after changing with maybe_sendto() function the timeout issue occurs. We thought it possible occurs from gnn_receive().

Output from sender:

$ ./sender 127.0.0.1 9999 README.md
FUNCTION: gbn_socket()... Create socket.... socket_descriptor: 4
FUNCTION: gbn_connect()  4...
STATE: SYN_SENT
SUCCESS: Sent SYN.
SUCCESS: Received SYNACK...
type: 1	seqnum:73checksum(received)65206checksum(calculated)65206
SUCCESS: Received valid SYN_ACK!
FUNCTION: gbn_send() 4...
STATE: ESTABLISHED
INFO: DATA length of 429 packets left to be sent...
SUCCESS: Sent DATA packet (73)...
type: 2	73seqnum: 46584	checksum(received): 46584	checksum(calculated): 
SUCCESS: Received ACK packet.
SUCCESS: Received valid SYNACK packet.
SUCCESS: Sent SYNACK.
ERROR: Unable to receive ACK!
ERROR: Timeout when receiving ACK.
STATE: ESTABLISHED
INFO: DATA length of 429 packets left to be sent...
SUCCESS: Sent DATA packet (73)...
type: 2	73seqnum: 46584	checksum(received): 46584	checksum(calculated): 
SUCCESS: Received ACK packet.
ERROR: Unable to receive ACK!
ERROR: Timeout when receiving ACK.
STATE: ESTABLISHED
INFO: DATA length of 429 packets left to be sent...
SUCCESS: Sent DATA packet (73)...
type: 2	73seqnum: 46584	checksum(received): 46584	checksum(calculated): 
SUCCESS: Received ACK packet.
ERROR: Unable to receive ACK!
ERROR: Timeout when receiving ACK.
STATE: ESTABLISHED
INFO: DATA length of 429 packets left to be sent...
SUCCESS: Sent DATA packet (73)...
type: 2	73seqnum: 46584	checksum(received): 46584	checksum(calculated): 
SUCCESS: Received ACK packet.
ERROR: Unable to receive ACK!
ERROR: Timeout when receiving ACK.
STATE: ESTABLISHED
INFO: DATA length of 429 packets left to be sent...
SUCCESS: Sent DATA packet (73)...
type: 2	73seqnum: 46584	checksum(received): 46584	checksum(calculated): 
SUCCESS: Received ACK packet.
ERROR: Unable to receive ACK!
ERROR: Timeout when receiving ACK.
STATE: ESTABLISHED
INFO: DATA length of 429 packets left to be sent...
ERROR: Max attempts are reached.
STATE: CLOSED
gbn_send: Success

Process finished with exit code 255

Ouput from receiver:

$ ./receiver 9999 output.txt
FUNCTION: gbn_socket()... Create socket.... socket_descriptor: 4
FUNCTION: gbn_bind() 4...
FUNCTION: gbn_listen() 4...
FUNCTION: gbn_accept() 4...
STATE: CLOSED
SUCCESS: Received SYN
SUCCESS: Received a valid SYN packet
STATE: SYN_RCVD
SUCCESS: Sent SYNACK.
SUCCESS: Accepted a valid ACK packet.
STATE: ESTABLISHED.
FUNCTION: gbn_accept returns 4.
FUNCTION: gbn_recv()
INFO: keep reading data until no more new data to be received.
SUCCESS: Received a packet.
SUCCESS: Receiving a valid DATA packet
SUCCESS: DATA packet has the correct sequence number.
SUCCESS: Sent duplicate ACK packet.
FUNCTION: gbn_recv()
INFO: keep reading data until no more new data to be received.
SUCCESS: Received a packet.
INFO: keep reading data until no more new data to be received.
ERROR: Unable to receive a packet.
INFO: keep reading data until no more new data to be received.
SUCCESS: Received a packet.
SUCCESS: Receiving a valid DATA packet
INFO: DATA packet has the incorrect sequence number.
SUCCESS: Sent duplicate ACK packet.
INFO: keep reading data until no more new data to be received.
SUCCESS: Received a packet.
SUCCESS: Receiving a valid DATA packet
INFO: DATA packet has the incorrect sequence number.
SUCCESS: Sent duplicate ACK packet.
INFO: keep reading data until no more new data to be received.
SUCCESS: Received a packet.
SUCCESS: Receiving a valid DATA packet
INFO: DATA packet has the incorrect sequence number.
SUCCESS: Sent duplicate ACK packet.
INFO: keep reading data until no more new data to be received.
SUCCESS: Received a packet.
SUCCESS: Receiving a valid DATA packet
INFO: DATA packet has the incorrect sequence number.
SUCCESS: Sent duplicate ACK packet.
INFO: keep reading data until no more new data to be received.

Reference

• The TCP/IP Guide: http://www.tcpipguide.com/free/index.htm

• Reference Implementations
  • https://github.com/JordanMontgomery/go-back-n
  • https://github.com/JasonHettmansperger/Go-Back-N-UDP/blob/master/udpSender.c
• State machine:
  • Open: http://www.tcpipguide.com/free/t_TCPConnectionEstablishmentProcessTheThreeWayHandsh-3.htm
  • Terminate: http://www.tcpipguide.com/free/t_TCPConnectionEstablishmentProcessTheThreeWayHandsh-4.htm