Transport Layer

Transport Layer - responsible for the end-to-end communication between processes - unlike the network layer, which is responsible for the end-to-end communication between hosts;
Provides logical communication between processes running on different hosts;
Sender - breaks the message into segments and passes them to the network layer;
Receiver - reassembles the segments into the message and passes it to the application layer;
The two most common transport layer protocols are TCP and UDP.

Multiplexing and Demultiplexing

Multiplexing - the process of combining multiple data streams into a single data stream;
- At the sender, handles data from multiple sockets, adds header information to each segment, and passes the segments to the network layer;
Demultiplexing - the process of splitting a single data stream into multiple data streams;
- At the receiver, the transport layer uses the header information to deliver the segments to the correct socket;
- Connectionless demultiplexing - the transport layer uses the destination port number to deliver the segments to the correct socket - used by UDP;
- Connection-oriented demultiplexing - the transport layer uses the connection identifier to deliver the segments to the correct socket - used by TCP - source and destination port numbers and IP addresses are used to identify the connection.

User Datagram Protocol (UDP) - a connectionless transport layer protocol that provides unreliable and unsequenced data delivery;
Best-effort delivery - UDP does not guarantee that the data will be delivered, nor does it guarantee the order of delivery;~~
Used at streaming applications, DNS and SNMP;
Pros: simple, low overhead, no connection establishment, no connection state, no congestion control (blast), small header;
UDP Header:
- Source Port - 16 bits - identifies the sending port;
- Destination Port - 16 bits - identifies the receiving port;
- Length - 16 bits - length of the UDP header and data;
- Checksum - 16 bits - used for error-checking the header and data;
  - Sender calculates the checksum and includes it in the header - calculated using a sum of the header and data;
  - Receiver calculates the checksum and compares it with the one in the header - if they match, no errors are detected.

Reliable Data Transfer - the process of ensuring that data is delivered error-free, in-order, and without loss;
RDT 1.0 - consider the network error-free and no packet loss;
- Sender sends the data and receiver reads it;
RDT 2.0 - underlying channel with bit errors;
- Checksum used to detect errors;
- Acknowledgments (ACKs) used to confirm the data was received and was error-free;
- Negative Acknowledgments (NAKs) used to request retransmission of the data;
- Stop-and-Wait - sender sends one packet and waits for the acknowledgment before sending the next packet;
- Sequence Numbers - used to order the packets, and detect duplicates;
RDT 2.1 - NAKs are not used;
- Instead, the receiver sends an ACK for the last correctly received packet;
RDT 3.0 - underlying channel with bit errors and packet loss;
- Timeouts - used to detect packet loss;

Pipelined Protocols - allow the sender to have multiple packets in transit at the same time, without waiting for an acknowledgment for each packet;
Go-Back-N - sender can have up to N unacknowledged packets in transit;
- Sender - sends up to N packets without waiting for an acknowledgment;
- Receiver - sends cumulative ACK for the last correctly received packet;
- Sender - retransmits all unacknowledged packets after a timeout;
Selective Repeat - sender can have up to N unacknowledged packets in transit;
- Sender - sends up to N packets without waiting for an acknowledgment;
- Receiver - sends ACK for each correctly received packet;
- Sender - retransmits only the unacknowledged packets after a timeout.

Transmission Control Protocol (TCP) - a connection-oriented transport layer protocol that provides reliable and sequenced data delivery;
Connection-oriented - connection establishment and termination;
Reliable - error detection, retransmission, acknowledgments, flow control, and congestion control;
Full-duplex - data can be sent in both directions at the same time;
Point-to-point - data is sent from one sender to one receiver;
The structure of the TCP header is more complex than the UDP header:
- Source Port - 16 bits - identifies the sending port;
- Destination Port - 16 bits - identifies the receiving port;
- Sequence Number - 32 bits - used to order the packets;
- Acknowledgment Number - 32 bits - used to acknowledge the received packets;
- Data Offset - 4 bits - length of the TCP header;
- Reserved - 6 bits - reserved for future use;
- Flags - 6 bits - used to control the connection - SYN, ACK, FIN, RST, PSH, URG;
- Window Size - 16 bits - used for flow control;
- Checksum - 16 bits - used for error-checking the header and data;
- Urgent Pointer - 16 bits - used to indicate the urgent data;
- Options - variable length - used for additional features.

Used to establish a connection between the sender and receiver;
Better than a two-way handshake because it prevents old or duplicate connections;
- Sender sends a SYN packet to the receiver;
- Receiver sends a SYN-ACK packet to the sender;
- Sender sends an ACK packet to the receiver;
To terminate the connection, a four-way handshake is used:
- Sender sends a FIN packet to the receiver;
- Receiver sends an ACK packet to the sender;
- Receiver sends a FIN packet to the sender;
- Sender sends an ACK packet to the receiver.