TCP & UDP Operations Notes

Transportation Layer Protocols

• Three main Layer 4 protocols in TCP/IP:
  • TCP (Transmission Control Protocol):
  • Connection-oriented and reliable.
  • UDP (User Datagram Protocol):
  • Connectionless and not reliable.
  • SCTP (Stream Control Transmission Protocol):
  • New protocol designed for reliable transmission over a connectionless network.

TCP Overview

• Reliable host-to-host connection:
  • Controls for initiation, flow control, and termination.
• Various revisions exist:
  • Slow Start, Reno, Tahoe, Vegas, Compound TCP, BIC/CUBIC.

TCP Header Format

• Structure of TCP header:
  • Source Port, Destination Port
  • Sequence number, Acknowledgement number
  • Data offset, Checksum, Options
  • Window, Urgent Pointer

IP Header Overview

• Key Fields:
  • IHL, Version, Type of Service (TOS), Total Length.
  • Identification, Fragment Offset, Time to Live (TTL), Protocol.
  • Source Address, Destination Address.
  • Important Flags: Do Not Fragment (D), More Fragments (M).

Well-Known Port Numbers (IANA)

• Selected protocols and their corresponding ports:
  • UDP:
  • Port 53 for DNS
  • Port 67 for DHCP
  • TCP:
  • Port 80 for HTTP
  • Port 25 for SMTP
  • Port 21 for FTP

TCP Header Fields

• Main fields:
  • Sequence Number: Identifier for transmitted segment.
  • Acknowledgement Number: Next sequence number expected.
  • Data Offset: Where the payload begins.
• Control Flags (1 bit each):
  • URG, ACK, PSH, RST, SYN, FIN.
• Window: Number of bytes that can be accepted.
• Checksum: Validates integrity of the segment.

TCP Connection Establishment

• Steps in Establishing a Connection:
  • 1. SYN: Request to establish a connection.
  • 2. SYN+ACK: Server acknowledges request.
  • 3. ACK: Client acknowledges to complete the process.

Terminating a TCP Connection

• Steps in Termination:
  • 1. FIN: Client requests to close.
  • 2. ACK: Server acknowledges the request.
  • 3. FIN: Server requests to close, and the process concludes with ACK.

TCP State Machine

• States involved in TCP connection management:
  • CLOSED, LISTEN, SYNRCVD, SYNSENT, ESTABLISHED, FINWAIT1, FINWAIT2, CLOSEWAIT, LASTACK, TIME_WAIT.

UDP Overview

• Characteristics:
  • Minimal delivery capabilities, not reliable.
  • Lacks robust features such as error checking and flow control.

UDP Header Format

• Key structure:
  • 16-bit Source Port, Destination Port.
  • 16-bit Length of the segment, 16-bit Checksum.

Review Points

• Understand basic features of both TCP and UDP, especially why TCP is reliable.
• Familiarize with TCP and UDP headers, their functions, and the differences between their operations.
• Know the reasoning behind the existence of both protocols and their comparison in different applications.