Transport Layer: Services, Protocols, and Reliability Mechanisms

Transport Layer Overview

The Transport Layer (Layer 4 of the OSI model) sits between the Network Layer (Layer 3) and the Application Layer (Layer 5). Its primary responsibilities are:

1. End‑to‑end communication – ensuring that data sent by an application on one host arrives intact at the application on the destination host.
2. Segmentation and reassembly – breaking large application messages into manageable segments and reassembling them at the receiver.
3. Multiplexing – allowing multiple applications to share a single network connection through port numbers.
4. Reliability, flow control, and congestion control – guaranteeing correct delivery, preventing sender overload, and avoiding network congestion.

The Transport Layer is defined by the Internet Engineering Task Force (IETF) in RFC 793 (TCP) and RFC 768 (UDP).

Key Protocols

TCP – Transmission Control Protocol

TCP is a stateful, byte‑stream protocol that guarantees reliable, ordered delivery. Its header fields include:

sequenceDiagram
    participant Client
    participant Server
    Client->>Server: SYN (Seq=x)
    Server->>Client: SYN-ACK (Seq=y, Ack=x+1)
    Client->>Server: ACK (Ack=y+1)
    Note right of Server: Connection Established
    Client->>Server: Data (Seq=z)
    Server->>Client: ACK (Ack=z+1)
    Client->>Server: FIN
    Server->>Client: ACK
    Server->>Client: FIN
    Client->>Server: ACK
    Note right of Server: Connection Terminated

TCP Three-Way Handshake and Connection Termination

• Source/Destination Port – 16‑bit identifiers for application processes.
• Sequence Number – byte offset of the first byte in the segment.
• Acknowledgement Number – next expected byte from the peer.
• Data Offset – header length.
• Flags – SYN, ACK, FIN, RST, PSH, URG.
• Window Size – number of bytes the receiver can accept.
• Checksum – error detection.

TCP Handshake (Three‑Way)

1. SYN – Client → Server: Seq = x
2. SYN‑ACK – Server → Client: Seq = y, Ack = x+1
3. ACK – Client → Server: Ack = y+1

After the handshake, data transfer begins.

Example: Segmentation and Reassembly

Suppose an application sends a 1,200‑byte message. The TCP Maximum Segment Size (MSS) is 500 bytes.

The receiver acknowledges each segment. If segment 2 is lost, the sender retransmits it after a timeout or upon receiving a duplicate ACK.

Flow Control – Sliding Window

TCP uses a receiver window to inform the sender of available buffer space. The sender may transmit up to Window Size bytes without waiting for ACKs.

graph LR
    A[Sender Buffer] -->|Window=1000| B[Network]
    B --> C[Receiver Buffer]
    C -->|ACK| A
    style A fill:#f9f,stroke:#333
    style C fill:#bbf,stroke:#333
    style B fill:#f99,stroke:#333
    note right of A: Sender transmits up to window size
    note right of C: Receiver advertises available space

TCP Sliding Window Flow Control

Worked Example

• Receiver window = 1,000 bytes.
• Sender transmits 1,000 bytes (segments 1–2).
• Receiver acknowledges 1,000.
• Sender can now send next 1,000 bytes.

If the receiver’s buffer becomes full, it advertises a window of 0, forcing the sender to pause.

Congestion Control

TCP employs several algorithms:

1. Slow Start – starts with cwnd = 1 segment; doubles each RTT until reaching ssthresh.
2. Congestion Avoidance – increases cwnd linearly after ssthresh.
3. Fast Retransmit – retransmits a segment after receiving three duplicate ACKs.
4. Fast Recovery – reduces cwnd to ssthresh and enters congestion avoidance.

These mechanisms adapt the sending rate to network capacity, preventing congestion collapse.

UDP – User Datagram Protocol

UDP is a lightweight, connectionless protocol. Its header contains only source/destination ports, length, and checksum. It offers no sequencing, flow control, or congestion control. UDP is suitable for real‑time applications where occasional packet loss is acceptable.

Transport Layer Services

ARQ Protocols – Error Control

Stop‑and‑Wait ARQ

• Sender transmits one frame and waits for ACK.
• If ACK not received within timeout, retransmit.
• Simple but low throughput.

graph TD
    A[Send Frame 1] -->|Timeout?| B{No ACK?}
    B -->|No| C[Proceed]
    B -->|Yes| D[Retransmit Frame 1]
    D --> A
    C --> E[Send Frame 2]
    style A fill:#f9f,stroke:#333
    style D fill:#f99,stroke:#333
    style E fill:#bbf,stroke:#333
    note right of B: Wait for ACK or timeout

Stop-and-Wait ARQ Process Flow

Worked Example
Assume a 1 kB frame, transmission time = 10 ms, RTT = 50 ms.
Throughput = Frame Size / (Transmission Time + RTT)
= 1 kB / (10 ms + 50 ms) = 16.7 kB/s.

Sliding‑Window ARQ (TCP)

• Sender can transmit multiple frames up to the window size.
• Receives cumulative ACKs.
• Improves throughput by keeping the pipeline full.

Illustration

Sender:  [S1] [S2] [S3] [S4] [S5]
          |  |  |  |  |  
Receiver ACKs:  |  |  |  |  |

If S3 is lost, the receiver acknowledges up to S2. The sender retransmits S3 and continues with S4, S5.

Distance Vector Algorithm – Brief Mention

Although primarily a routing protocol at the Network Layer, the Distance Vector algorithm’s principles (e.g., Bellman‑Ford) influence Transport Layer congestion control by estimating path costs (RTT, loss). Understanding its operation helps in analyzing TCP’s RTT estimation and timeout calculations.

Practical Applications

Advantages & Disadvantages

Exam Tip

• Define each Transport Layer service clearly.
• Compare TCP and UDP using a table.
• Explain the TCP three‑way handshake and state transitions.
• Illustrate a Stop‑and‑Wait or Sliding‑Window ARQ with a simple diagram.
• Calculate throughput for Stop‑and‑Wait given frame size, transmission time, and RTT.
• Discuss congestion control phases (slow start, congestion avoidance).
• Answer “Why is DNS required?” by highlighting name resolution and the role of UDP.

Focus on concise, structured answers, use tables and diagrams where appropriate, and practice tracing TCP segment exchanges to reinforce understanding.