CS 326E Computer Networks - Packets, Routing, and Performance

Reasons for Layering

• Purpose of Layers:

  • Simplifies networking through modularization.

  • Different layers handle different aspects of communication.

• Benefits:

  • Easier to troubleshoot and isolate network issues.

  • Enhances flexibility and compatibility between different systems.

• Downsides:

  • Complexity may lead to inefficiencies or overhead due to multiple layers.

Networking Layers Breakdown

• Layers:

  • Link (e.g., Ethernet, Wi-Fi): Responsible for bit-wise delivery between physically adjacent nodes.

  • Network (e.g., IP): Handles data transfer between host devices across multiple networks.

  • Transport (e.g., TCP, UDP): Manages communication between processes (end-to-end communication).

  • Application (e.g., HTTP, FTP): Focuses on application-specific data transfer needs.

• Header Information:

  • Example: TCP header includes source and destination port numbers and IP addresses.

Packet Forwarding vs Routing

• Switches: Forward packets within the same network.

• Routers: Forward packets across different networks.

• Forwarding:

  • Local Action: Deciding the appropriate output link based on destination address in the packet's header.

  • Utilizes a local forwarding table.

• Routing:

  • Global Action: Calculates the best paths for packets to travel from source to destination, based on routing algorithms.

Packet Delay Components

• Types of Delays:

  1. Nodal Processing (d_proc): Time taken for routers/switches to process packets.

  2. Queueing Delay (d_queue): Delay while waiting in queues for transmission, impacted by network congestion.

  3. Transmission Delay (d_trans): The time required to push all bits of a packet onto the output link.

  4. Propagation Delay (d_prop): Time for a packet to travel through the physical medium.

  Formula:
  d{nodal} = d{proc} + d{queue} + d{trans} + d_{prop}

• Transmission Delay:
  d_{trans} = \frac{L}{R}

  • L = packet length (bits)

  • R = link transmission rate (bps)

• Propagation Delay:
  d_{prop} = \frac{d}{s}

  • d = distance of physical link

  • s = propagation speed (e.g., ~2x10^8 m/s in optical fibers)

Circuit Switching vs Packet Switching

• Circuit Switching:

  • All dedicated resources are reserved for a call between source and destination.

  • Used in traditional telephony; guarantees performance but resources can be idle.

• Packet Switching:

  • Resources are shared among multiple users, with probabilities of congestion and delay.

  • Enables more users to share a network.

Packet Loss

• Packet loss occurs when a queue reaches its capacity, and incoming packets are dropped.

• Possible causes for packet loss include:

  • Buffer overflow in a router.

  • Network congestion leading to dropped packets.

Queueing Delay Analysis

• Variables:

  • a: Average packet arrival rate.

  • L: Packet length (bits).

  • R: Transmission rate (bps).

  • Traffic intensity: \frac{La}{R}

• High traffic intensity leads to increased queueing delay and potential infinite delays.

Throughput

• Definition: The rate at which bits are sent from the original sender to the final receiver.

• Factors:

  • Instantaneous vs average throughput over time.

  • Bottleneck situations when end-to-end throughput is constrained by the slowest link in the path.

Other Notable Points

• Questionable reliability of shared networks under high load versus reserved circuit networks.

• The concept of resource sharing in packet networks leads to potential congestion but allows for better overall utilization of resources.

Key Questions for Discussion

• Differences between forwarding and routing.

• Store-and-forward necessity and its implications.

• Packet loss occurrences and their consequences for data integrity and transmission efficiency.