A Connection Oriented communication protocol that ensures no data loss in communication.
Packet Structure
- Source port: 16-bits, represents sending port
- Dest port: 16-bits, represents receiving port
- Sequence Number: 32-bits, used for keeping data in sequence
- If SYN flag set (1), then this is initial sequence number
- If SYN flag unset (0), this is accumulated sequence number
- Acknowledgement Number: 32-bits:
- If ACK flag is set, this number is the next Sequence Number the sender expects
- Data Offset: 4-bits, specifies size of TCP header in 32-bit Words
- Reversed: 4-bits, never used
- CWR: 1-bit, used to indicate that congestion window has been reduced
- ECE: 1-bit
- If SYN is set, TCP is ECN capable
- if SYN is unset, signifies congestion or incoming congestion
- URG: 1-bit indicates that urgent pointer is significant
- ACK: 1-bit, indicates that acknowledgement fields are significant
- PSH: 1-bit, asks to push buffered data to receiving application
- RST: 1-bit, resets the connection
- SYN: 1-bit, used to synchronize sequence numbers
- FIN: 1-bit, used to signal this is the last packet from the sender
- Window: 16-bits, the size of the receive window that the sender is willing to recieve
- Checksum: 16-bits a Checksum
- Urgent Pointer: 16-bits: if set, indicates the offset from sequence number is the last urgent data byte
Protocol
- SEQ: used to track bytes sent
- ACK: used to track bytes received
Three Way Handshake
- Gets the initial sequence number (ISN)
Communication
- Alice sends packet with SEQ=ISN+1
- Alice is able to continue sending until she reaches the window size specified by Bob
- Bob after Delayed Acknowledgement. replies with acknowledgement: ACK=SEQ+data size+1
- If the ACK does not match what alice should receive, receiver starts TCP retransmission
Retransmission
- If we dont recieve an ACK in 200ms, we resend the message from cache
- If the server recieves two of the same message, then we know we had a failure in ACK, so we resend the same ACK from cache
Additional Features
Flow Control
Ability to limit the data sent