Computer Networks Chapter 03

Course Information

• Course Title: Computer Networks
• Course Code: CENG305
• University: Izmir Katip Celebi University
• Semester: Fall 2024-2025
• Chapter: 03
• Instructor: H. Burak Akyol, Ph.D.
• Textbook: Computer Networking: A Top-Down Approach by Jim Kurose and Keith Ross

Transport Layer: Overview

• Goal: Understand principles behind transport layer services.
• Services Include:
  • Multiplexing
  • Demultiplexing
  • Reliable data transfer
  • Flow control
  • Congestion control
• Protocols:
  • Learn about Internet transport layer protocols.
  • UDP: Connectionless transport
  • TCP: Connection-oriented reliable transport
  • TCP congestion control

Transport Layer: Roadmap

• Transport-layer services
• Multiplexing and demultiplexing
• Connectionless transport: UDP
• Principles of reliable data transfer
• Connection-oriented transport: TCP
• Principles of congestion control
• TCP congestion control

Transport Services and Protocols

• Provide logical communication between application processes running on different hosts.
• Transport protocols actions in end systems:
  • Sender: Breaks application messages into segments, passes to network layer.
  • Receiver: Reassembles segments into messages, passes to application layer.
• Two transport protocols available to Internet applications:
  • TCP
  • UDP

Transport vs. Network Layer Services and Protocols

• Transport layer: Communication between processes. Relies on and enhances network layer services.
• Network layer: Communication between hosts.

Transport Layer Actions

• Sender:
  • Receives an application-layer message
  • Determines segment header fields values
  • Creates segment
  • Passes segment to IP layer.
• Receiver:
  • Receives segment from IP layer
  • Checks header values
  • Extracts application-layer message
  • Demultiplexes message up to application via socket

Two Principal Internet Transport Protocols

• TCP: Transmission Control Protocol
  • Reliable, in-order delivery
  • Congestion control
  • Flow control
  • Connection setup
• UDP: User Datagram Protocol
  • Unreliable, unordered delivery
  • No-frills extension of "best-effort" IP
  • Services not available: delay guarantees, bandwidth guarantees

Chapter 3: Roadmap

• Transport-layer services
• Multiplexing and demultiplexing
• Connectionless transport: UDP
• Principles of reliable data transfer
• Connection-oriented transport: TCP
• Principles of congestion control
• TCP congestion control

Multiplexing/Demultiplexing

• Multiplexing (as sender): Handles data from multiple sockets, adds transport header (later used for demultiplexing)
• Demultiplexing (as receiver): Uses header info to deliver received segments to the correct socket

UDP: User Datagram Protocol

• "No frills," "bare bones"
• Internet transport protocol; "best effort" service; UDP segments may be: lost, delivered out-of-order to app.
• Connectionless
• No handshaking between sender and receiver
• Each UDP segment handled independently from others

UDP: Use Cases

• Streaming multimedia apps (loss tolerant, rate sensitive)
• DNS
• SNMP (Simple Network Management Protocol)
• HTTP/3 (if needed reliability and congestion control are added at application layer)

UDP: User Datagram Protocol [RFC 768]

• Protocol intended to provide a datagram mode of communication with minimal protocol mechanism. Delivery and duplicate protection are not guaranteed.
• This protocol assumes the use of the Internet protocol (IP)
• Format includes source port, destination port, length, and checksum fields.

UDP: Transport Layer Actions

• Sender Actions:
  • Passes an application-layer message to the transport layer.
  • Determines UDP segment header values.
  • Creates UDP segment.
  • Passes segment to IP layer.
• Receiver Actions:
  • Receives segment from IP layer.
  • Checks UDP checksum header value
  • Extracts application-layer message
  • Demultiplexes message up to application via socket

UDP Segment Header

• Includes source port, destination port, length, and checksum.

UDP Checksum

• Goal: Detect errors (e.g., flipped bits) in transmitted segments.
• Sender: Treats UDP segment contents (including header and IP addresses) as a sequence of 16-bit integers; computes checksum (one's complement sum) and puts it in UDP checksum field.
• Receiver: Computes checksum of received segment; checks if computed checksum equals checksum field value.

Internet Checksum

• Goal: detect errors.
• Sender: treats contents of UDP segment, computes the checksum, and places the checksum field into the segment.
• Receiver: computes the checksum, checks if the computed checksum equals checksum field value.

Principles of Reliable Data Transfer

• Abstraction: Sending and receiving processes send data over a "reliable channel."
• Complexity: Depends on characteristics of the unreliable channel.

Reliable Data Transfer (RDT) Protocol: Interfaces

• RDT functions: rdt_send(data), deliver_data(data)
• Bi-directional communication over an unreliable channel

Reliable Data Transfer: Getting Started

• Incrementally develop sender and receiver sides of reliable data transfer protocol (RDT)

RDT 1.0: Reliable Transfer over a Reliable Channel

• Underlying channel is perfectly reliable; no bit errors or packet loss.

RDT 2.0: Channel with Bit Errors

• Underlying channel may flip bits in packets.
• Receiver uses checksums to detect bit errors.

RDT 2.0: FSM Specifications

• Defining states and transitions for sender and receiver FSMs (Finite State Machines)

RDT 2.0: Operations with no errors

• Sender sends data, receiver acknowledges (ACKs) successful reception

RDT 2.0: Corrupted Packet Scenarios

• Sender resends data when a corrupted packet is detected
• Receiver discards packets that are duplicates.

RDT 2.0 has a fatal flaw

• Sender does not know what happened at receiver if the ACK or NAK packet is corrupted
• Possible duplicate packets

RDT 2.1: Handling Garbled ACK/NAKs

• Sender and receiver design include error checking and retransmission mechanisms.
• Improved error handling by incorporating sequence numbers.

RDT 2.2: NAK-Free Protocol

• Achieves similar functionality to RDT 2.1 but uses only ACKs for confirmation.
• Receiver must explicitly include the sequence number of the packet being acknowledged.

RDT 2.2: Sender and Receiver Fragments

• Sender and receiver implement fragments.

RDT 3.0: Channels with Errors and Loss

• Underlying channel may also lose packets
• Use checksums, sequence numbers, and retransmissions

RDT 3.0: sender waits "reasonable" amount of time for ACK

• if ACKs are not received, retransmit

RDT 3.0 Sender

• includes a timer mechanism to interrupt operation after a reasonable amount of time

RDT 3.0 in Action

• Diagrammatic illustrations of possible scenarios (no loss, packet loss, ACK loss, etc.)

RDT 3.0 Stop-and-Wait Operation

• Performance analysis
• Utilization calculations

RDT 3.0 Pipelined Protocols Operation

• Illustrates pipelined protocols which allow multiple unacknowledged packets to be sent and received.

Pipelining: Increased Utilization

• Pipelining dramatically increases the utilization level compared to stop-and-wait protocols.