Networking Fundamentals Quiz

Chapter 4: Network Layer

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Chapter Goals

• Understand the principles behind network layer services
• Learn about network layer service models
• Learn about forwarding versus routing
• Understand how a router functions.
• Understand routing, path selection
• Learn about broadcast and multicast instantiation and implementation in the Internet.

Chapter 4: Outline

• Introduction: General overview of the chapter and its topics
• Virtual Circuit and Datagram Networks: Explains the two networking methods and their differences.
• What's Inside a Router: Describes the internal components and functionality of a router.
• Internet Protocol (IP): Explains the Internet Protocol including; datagram formats, IPv4 addressing, ICMP (Internet Control Message Protocol), and IPv6.
• Routing Algorithms: Details on link state, distance vector, and hierarchical routing algorithms.
• Routing in the Internet: Elaborates on different routing protocols found in the Internet, such as RIP, OSPF, BGP
• Broadcast and Multicast Routing: Explanations of the processes related to broadcast and multicast routing.

Network Layer

• Transport Segment: Transports segments from sender to receiver using the network layer.
• Encapsulates Segments: Encapsulates segments into datagrams during sending
• Delivers Segments: Delivers segments to transport layer during receiving
• Network Layer Protocols: Protocols in every host and router
• Examines Header Fields: Router examines header fields in IP datagrams passing through.

Two Key Network Layer Functions

• Forwarding: Moves packets from input to appropriate router output
• Routing: Determines route taken by packets from source to destination. Includes routing algorithms.

Interplay Between Routing and Forwarding

• The routing algorithm determines the end-to-end path through the network.
• The forwarding table specifies local forwarding in the router.
• The value in the incoming packet's header is used to determine output link.

Connection Setup

• 3rd important function in some network architectures (ATM, Frame Relay, X.25)
• End hosts and intervening routers create virtual connections before data flow.

Network Service Model

• Individual Datagrams: Services for handling individual datagrams include guaranteed delivery and guaranteed delivery with less than 40-msec delay.
• Flow of Datagrams: Services for a flow of datagrams include in-order datagram delivery, guaranteed minimum bandwidth, restrictions on bandwidth changes, and inter-packet spacing.

Network Layer Service Models: Table

• Presents a table showing different network architectures, service models, and their guarantees (bandwidth, loss, order, timing and congestion feedback). Explains various service models like best-effort, constant rate, guaranteed rate, etc.

Virtual Circuits

• Source-to-destination path: Similar to telephone circuits, with consistent performance.
• Call setup/teardown: Necessary before data flow.
• VC identifier: Carried in each packet, guiding routing through routers.
• Router state: Maintaining 'state' for each connection.
• Resource allocation: Network resources (e.g., bandwidth, buffers) dedicated to a VC for predictable service.

VC Implementation

• Path from source to destination: Defines the VC path's route.
• VC numbers: Unique number assigned to each link within the path.
• Forwarding tables: Includes entries specific to VCs in routers.
• VC number in packet: Carried rather than destination address.
• Changing VC numbers: VC number changes along the path; each link may use new VC number coming from a forwarding table.

VC Forwarding Table

• Shows an example forwarding table used in northwest router. Contains incoming interface, incoming VC numbers, outgoing interface and outgoing VC numbers. VC routers keep state information.

Virtual Circuits: Signaling Protocols

• Used in ATM, Frame Relay, X.25 for setup, maintenance, and teardown of VC.
• Today's Internet does not use these.

Datagram Networks

• No call setup: No setup required at network layer for end-to-end connections.
• No state: Routers don't maintain state about connections.
• Destination addresses: Packets forwarded based on destination host addresses.

Datagram Forwarding Table

• Shows a local forwarding table using destination host addresses. An aggregate table includes ranges of addresses

Longest Prefix Matching

• Used for choosing the longest prefix that matches the destination address in forwarding table, which is preferred when there is a range of addresses

Datagram or VC Network: Why?

• Internet (Datagram) is designed for general-purpose data exchange between computers, is adaptable, and has many types of links and characteristics. The uniform service may be hard to fulfill. It has smart end-systems, and recovery error is adaptable and achievable. Inside the network is simple, but complexity is on the edge.
• ATM (VC) is based on telephony and has strict time requirements. It supports guaranteed services, with dumb end-systems (e.g., telephones). The complexity is centrally inside the network.

Router Architecture Overview

• Key Functions: Routing algorithms (RIP, OSPF, BGP) and forwarding datagrams.
• Routing Processor: Managing routing algorithms.
• High-Speed Switching Fabric: Fast packet transfer between input and output ports.
• Input Ports: Packet processing (including bit-level reception, data link layer, see chapter 5)
• Output Ports: Datagram buffering and scheduling, and potential queueing
• Routing, Management control plane: Software managing protocol and routing functionality
• Forwarding Data Plane: Hardware responsible for forwarding and data transfer

Input Port Functions

• Line Termination: Physical layer bit-level reception. Data Link Layer tasks like Ethernet (e.g., Ethernet see chapter 5)
• Link Layer (receive): Processing of received frames in the link layer.
• Lookup-Forwarding-Queueing: Finding target and output interface, and handling packet queues.
• Decentralized Switching: Finding destination and selecting output port using forwarding tables stored in memory

Switching Fabrics

• Transfer Packets: Moving packets to desired output port.
• Switching Rate: Rate of data transfer between input and output ports, often measured in multiples of input output line rates
• Three Categories: Different types of switching fabrics like memory-based, bus-based, and interconnection network-based.

Switching via Memory

• First Generation Routers: Traditional routers with switching under CPU direct control.
• Packet Copy: Copying data packets into system memory
• Speed Limited: Limited by memory bandwidth.

Switching via Bus

• Shared Bus: Data transfer via a shared bus.
• Contention: Switching speed is limited by bus bandwidth's contention
• 32Gbps bus: Sufficient for access and enterprise routers. Rapid and high quality traffic forwarding is possible.

Switching via Interconnection Network

• Bus Bandwidth Limiting: Overcomes limitations of bus bandwidth
• Interconnection Network: Developed to connect processors and other devices in multiprocessor.
• Advanced Design: Fragmenting datagrams into cells and switching cells through the fabric.
• 60Gbps Switching: Cisco 12000 switches with high speed throughput.

Output Ports

• Buffering: Necessary as packets may arrive faster than the fabric can transfer, leading to buffering.
• Scheduling: Strategies for allocating out-going ports and processing queueing.
• Queuing: Potential delay and loss due to a queue overflow

How Much Buffering?

• Rule of thumb from RFC 3439: buffer size equals the product of typical round-trip time (e.g., 250 msec) and link capacity.

Input Port Queuing

• Fabric Slower: Queueing may occur if fabric is slower than input ports combined
• HOL blocking: This is caused because of queued datagram delays and loss in front of queue which prevents other queued datagrams from moving forward.
• Output Port Contention: Contention for output port, leading to delay and loss when arrival rate exceeds switch output line speed
• Buffer Overflow: Queueing delay and loss due to input buffer overflow.

Chapter 4 Outline

• A repeat of previous topics, but possibly with different sub-sections or slightly different wording from before

IP Addresses: Introduction

• IP Addresses are 32-bit identifiers for hosts and routers.
• Interfaces are connections between hosts/routers and physical links.
• Routers and hosts can have multiple interfaces.
• Each interface is assigned a unique IP address.

IP Addressing: CIDR

• Variable-Length Subnet Masks: Subnets can have variable-length, giving flexibility to create networks.
• CIDR Format: Addresses have the format "a.b.c.d/x" where x is the bit size of the subnet addressing

IP Addresses: How to Get One?

• Hard-coded: Configuration files to determine address allocation
• Windows: System admin configures manually from control panel
• UNIX: Configuration typically from /etc/rc.config
• DHCP (Dynamic Host Configuration Protocol): for dynamically getting IP address

DHCP: Dynamic Host Configuration Protocol

• Allows Dynamic Allocation: Hosts automatically obtain address.
• Reusing Addresses: Reuse addresses when devices disconnect.
• Mobile User Support: Supports mobile users who dynamically join networks.

DHCP Client-Server Scenario (Steps)

• DHCP Discover: Client broadcasts to locate a DHCP server.
• Offer: DHCP server responds with an available IP address.
• Request / Select: Client requests the offered address.
• Ack: Server acknowledges the successful configuration.

DHCP: More Than IP Addresses

• DHCP can provide more than just IP addresses.
• It can return the first-hop router address
• DNS server Address
• Network mask for different host parts.

DHCP: Example

• DHCP Server: Returns IP address, first-hop router, and DNS server details
• Encapsulation: Addresses are encapsulated in UDP and IP, and then encapsulated in an Ethernet frame.

IP Addresses: How to Get One?

• ISP Block: ISPs are allocated blocks of IP addresses.
• Organization Allocation: Smaller organizations get allocated portions from their ISP's block (variable length).
• Example IP allocation schemes using CIDR notation.

Hierarchical Addressing: Route Aggregation

• Efficient Routing Advertisement: Hierarchical structure to summarize routing tables. It reduces overall processing time as less data needs to be used for routing.
• Fly-by-night ISP: Organizations can receive routing details from their provider.
• ISPs-R-Us: ISPs can receive routing details from larger providers.

Hierarchical Addressing: More Specific Routes

• Specific Routes to Organizations Routers maintain more specific routes for higher level organizations, to optimize the traffic flow through the network.

IP Addressing: The Last Word

• ICANN: Manages the allocation and management of IP addresses and DNS.

NAT: Network Address Translation

• Local Network IP Hiding: Local networks use a single public IP for all devices from the outside world.
• Adapting to change: Change of ISP won't affect the devices in the local network.
• Security Benefit: Devices inside the local network are hidden, for security reasons from outside.

NAT: Network Address Translation (Implementation)

• Outgoing Datagrams: NAT router replaces outgoing datagrams’ source IP address and port number with its NAT IP address and a new port.
• Incoming Datagrams: NAT router replaces the destination fields of incoming datagram with corresponding source IP address and port-number stored in translation table from NAT router

NAT: Network Address Translation

• Motivation: Local network uses only one IP address in contrast to outside world usage which uses many IPs.
• Avoiding Address Exhaustion: ISP's provide only one IP address for the local network.
• Simplified Network Configurations: Local network does not need to be aware of outside world devices or address allocations.
• ISP Change :Local network can easily change ISPs without impact to the external visibility/addresses of local devices
• Security: Security feature because it hides the local devices inside the network from public view.

NAT Traversal Problem

• Client-needs external IP: Client wishes to connect to a server outside local range and needs to use its external IP (i.e., 138.76.29.7) as the destination address.
• Internal IP used: Server's IP address is internal to the local network (e.g., 10.0.0.1), so it can't be used as the destination.
• NAT Router Needed: A NAT router is required as a intermediary because the local device does not have a globally routable IP address.

ICMP: Internet Control Message Protocol

• Error reporting: Communicates errors in the network layer.
• Unreachable host: Reports errors when a host is unreachable
• Echo request/reply: Used by ping to check for host reachability.
• ICMP Messages in IP Datagrams: Carried as a part of IP datagrams.
• Example protocol messages are given for Type and Code descriptions

IPv6: Motivation

• 32-bit address exhaustion: IPv4 was running out of addresses, so a new protocol was created.
• Improved Header: This allows quicker processing, and forwarder performance and QoS is facilitated.

IPv6 Datagram Format

• Fixed-length Header: The 40-byte header is fixed for consistent processing.
• No fragmentation: Fragmentation is not permitted to avoid any potential overhead/issues.

IPv6 Datagram Format

• Priority: Identifying priorities among datagrams in flows.
• Flow Label: Identifying datagrams in a flow. Concept not well defined
• Next Header: Identifying the upper-layer protocol associated with data. It's useful for quickly determining the contents of a packet and forwarding it to the related service (app).

Other Changes from IPv4

• Checksum: Removed to reduce processing time

Transition from IPv4 to IPv6

• Tunneling: IPv6 datagrams are relayed over IPv4 networks utilizing IPv4 headers, which carry the payload of an IPv6 datagram to efficiently be processed by intermediary routers.

Tunneling

• Logical view: A simplified perspective of the network operation by the end user; where the intermediary layer, IPv4, remains invisible to the end user.
• Physical view: Perspective of the network underlying infrastructure, showing the actual route of traffic across routers.
• It's useful to show how an IPv6-only network might use IPv4 routers for intermediary routing when transitioning.

IPv6 Adoption

• Adoption rate of IPv6 is still slow; although it was necessary, it's still an on-going process for transition.
• This may be due to application-level changes. Organizations may need to update or retool their applications so that the traffic flow can transition from IPv4 to IPv6.

Test your knowledge on networking fundamentals with this quiz covering key concepts like forwarding, routing, and datagram protocols. Understand the mechanics behind packet travel from source to destination and the significance of longest prefix matching. Perfect for students in computer networking courses.