[1Day-1CS] Transport Layer

Transport Layer

Provides communication services that connect senders and receivers. Provides connection-oriented data stream support, reliability, and flow control Serves as a data delivery intermediary between the application and Internet layers

• TCP: Guarantees order, uses connection-oriented protocol, acknowledges receipt (virtual line packet exchange)

• UDP: ordering X, acknowledgments X, simple data delivery (datagram packet exchange)

Virtual line packet exchange method

• Each packet contains a virtual line identifier

• Virtual line is released when all packets have been sent, packets arrive in order of transmission

How datagram packets are exchanged

• Packets travel independently, choose the best path

• Can be sent on different paths and in different order

How TCP establishes and tears down connections

connection: 3-way handshake

1. client sends connection request flag with its ISN
2. server sends back server's ISN and client's ISN + 1 with authorization number
3. client replies back with server's ISN + 1 with authorization number

Disconnecting: 4-way handshake

1. client sends FIN set segment and enters FIN_WAIT
2. the server that received the data sends an ACK acknowledgment segment to the client and enters CLOSE_WAIT
3. sends FIN segment after some time has passed since ACK was sent
4. client receives ACK, enters TIME_WAIT and resends ACK back to server
5. the receiving server sends a CLOSE

Reasons for using TIME_WAIT: Possibility of delayed packets, confirmation of disconnection