The 3-way Handshake¶

TCP, UDP, and the 3-Way Handshake¶

1. Placement in the OSI Model¶

• OSI Layer: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) both operate at Layer 4 of the OSI model, the Transport Layer.
• Purpose of Layer 4: Responsible for end-to-end communication control, reliability, flow control, multiplexing (via ports), and segmentation of data.

2. TCP (Transmission Control Protocol)¶

2.1 Overview¶

TCP is a connection-oriented, reliable transport protocol that establishes a virtual circuit (session) between two endpoints before transferring data.

2.2 Key Technical Aspects¶

1. Connection-Oriented
   • Initiates a handshake (the 3-Way Handshake) to establish parameters and maintain state.
   • Negotiates sequence numbers, window sizes, and optional features (e.g., MSS, window scaling).
2. Reliable Data Transfer
   • Uses sequence numbers to ensure ordered delivery.
   • Employs ACK (acknowledgment) packets and retransmissions for lost or corrupted segments.
3. Flow Control
   • Utilizes a sliding window to prevent a sender from overwhelming a receiver.
   • Dynamically adjusts the send window based on the receiver's buffer.
4. Congestion Control
   • Implements mechanisms such as Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery to detect and respond to network congestion.
5. Header Structure
   • Includes Source Port, Destination Port, Sequence Number, Acknowledgment Number, Data Offset, flags (SYN, ACK, FIN), Window Size, Checksum, Urgent Pointer, and optional fields.

2.3 Classification of TCP¶

• Protocol: Functions as a Transport Layer protocol.
• Ports: Uses port numbers (1–65535) for multiplexing services on a single host.
• Services: Underpins many application-layer services (HTTP, SSH, SMTP, etc.).
• Function: Provides reliable, ordered, and error-checked delivery of a byte stream.
• Technology: Often discussed alongside IP as part of the TCP/IP technology stack.

3. UDP (User Datagram Protocol)¶

3.1 Overview¶

UDP is a connectionless, best-effort transport protocol that does not guarantee ordered delivery, reliability, or flow control.

3.2 Key Technical Aspects¶

1. Connectionless Communication
   • No handshake required; data (datagrams) are sent immediately.
   • Minimal overhead with no need for maintaining connection state.
2. Best-Effort Delivery
   • Basic checksum is included, but no sequence numbers or retransmissions.
   • Packets may arrive out of order, be duplicated, or be lost without detection at the transport layer.
3. Header Structure
   • Contains Source Port, Destination Port, Length, Checksum, and the data payload.

3.3 Classification of UDP¶

• Protocol: Serves as a Transport Layer protocol (Layer 4).
• Ports: Like TCP, UDP uses port numbers (1–65535) for distinguishing services.
• Services: Suitable for applications where low latency is more important than reliability (DNS, SNMP, streaming).
• Function: Provides a simple, fast transport mechanism without overhead for error correction or sequencing.
• Technology: Part of the broader UDP/IP suite.

4. The TCP 3-Way Handshake¶

4.1 Purpose¶

The 3-Way Handshake establishes a connection-oriented session in TCP, synchronizing sequence numbers and negotiating parameters for reliable data exchange.

4.2 Steps¶

1. SYN
   • Client → Server: Sends a TCP segment with the SYN flag set and an Initial Sequence Number (ISN_C).
   • Signals the start of the connection request.
2. SYN-ACK
   • Server → Client: Responds with SYN and ACK flags set.
   • Includes the server's Initial Sequence Number (ISN_S) and acknowledges the client's SYN (ACK = ISN_C + 1).
3. ACK
   • Client → Server: Sends a final ACK, acknowledging the server's SYN (ACK = ISN_S + 1).
   • Completes connection setup; both sides move to the ESTABLISHED state.

4.3 Significance¶

• Sequence Number Synchronization: Ensures proper ordering and reliability.
• Negotiation of Options: Exchanges settings like Maximum Segment Size (MSS), window scaling, and more.
• State Machine: Each endpoint manages states (e.g., SYN_SENT, SYN_RECEIVED, ESTABLISHED) until the handshake completes.

4.4 Classification of the 3-Way Handshake¶

• Function: An internal mechanism within TCP for establishing a reliable connection.
• Not a Separate Protocol: Part of the TCP specification, not an autonomous service or technology.

5. Classification Table¶

6. Summary¶

TCP and UDP are both Transport Layer protocols.

• TCP provides a reliable, connection-oriented data stream with error checking, flow control, and congestion control.
• UDP offers a simpler, connectionless approach with less overhead, ideal for time-sensitive or high-volume traffic.

The 3-Way Handshake is the process that TCP uses to establish a session, synchronizing sequence numbers and negotiating connection options.

• It is classified as a function (or mechanism) within TCP rather than a protocol of its own.

Ports (1–65535) are used by TCP and UDP to differentiate multiple services running on the same host, acting as a function at the Transport Layer.

In a broader context, services (such as HTTP, DNS, or SMTP) run on top of these protocols (TCP or UDP) at the Application Layer (Layer 7). The entire environment—often called the TCP/IP or UDP/IP stack—is considered a technology suite that forms the backbone of modern computer networking.

Backlinks¶

The following pages link to this page:

• General Networking

Comments