Network Layer Functions and Protocols Quiz

Chapter 4: Network Layer

• The slides are freely available to faculty, students, and readers.
• The PowerPoint slides include animations and can be modified.
• Use of the slides should mention their source.
• Any posting of slides on a website must include a note about adapting the slides and copyright.
• Authors of the slides are Jim Kurose and Keith Ross.

Chapter Goals

• Understand principles behind network layer services.
• Understand network layer service models.
• Understand forwarding versus routing.
• Understand how routers operate.
• Understand routing (path selection).
• Understand broadcast, multicast, and instantiation, implementation in the internet.

Chapter Outline

• Introduction
• Virtual circuit and datagram networks
• What's inside a router
• IP: Internet Protocol
  • Datagram format
  • IPv4 addressing
  • ICMP
  • IPv6
• Routing algorithms
  • Link state
  • Distance vector
  • Hierarchical routing
• Routing in the internet
  • RIP
  • OSPF
  • BGP
• Broadcast and multicast routing
• Network layer protocols and functions

Network Layer

• Transports segments from sending to receiving host
• Encapsulates segments into datagrams
• Delivers segments to the transport layer on receiving side
• Network layer protocols are in every host and router.
• Routers examine header fields in all IP datagrams passing through them

Two Key Network Layer Functions

• Forwarding: Moves packets from a router's input to appropriate output port.
• Routing: Determines route taken by packets from source to destination.
• Routing algorithms are used to determine routes.

Interplay between Routing and Forwarding

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

Connection Setup

• A third important function in network architectures
• Establish virtual connections before datagrams flow.
• Routers get involved in connection setup in some architectures like ATM, frame relay or X.25.

Network Service Model

• Q: What service model for transporting datagrams from sender to receiver?
• Individual datagrams: guaranteed delivery with less than 40 msec delay.
• Flow of datagrams: in-order datagram delivery, minimum bandwidth guaranteed to a flow of datagrams, constraints on changes in inter packet spacing.

Network Layer Service Models: Table

• Attributes: network architecture, service model, bandwidth, loss, order, timing, congestion feedback
• Examples of architectures and service models are shown (including Internet, ATM, CBR, VBR, ABR, and UBR).

Virtual Circuits

• The source-to-destination path behaves like a telephone circuit.
• Performance is consistent and network resources are allocated in advance
• Includes call setup & teardown ,VC identifier, and router state maintenance
• Link-level and router resources can be allocated to virtual circuits.

Virtual Circuits Implementation

• Consists of path from source to destination
• Contains VC numbers for each link along path
• Creates entries in forwarding tables in routers.
• Packets carry VC number rather than destination address.
• VC number can change on each link based on forwarding table.

Virtual Circuits: Signaling Protocols

• Used to set up, maintain, and tear down VCs.
• Used in ATM, frame relay and X.25 but are not used in the modern Internet.

Datagram Networks

• No call setup; no state about end-to-end connections in routers.
• No network-level concept of connection.
• Packets are forwarded using destination host address
• Forwarding tables only look at destination address rather than maintaining connections.

Datagram Forwarding Table

• 4 billion IP addresses but rather than each one, lists ranges (aggregate table entries) for simplicity.
• Routers use destination address in arriving packets to look up next hop using a forwarding table.
• Longest prefix matching is used in the tables, and it finds the longest match for destination address within the table.

Datagram or VC Networks: Why Choose One?

• Internet (datagram): elastic service, many link types, different network characteristics, smart end systems (computers).
• ATM (VC): evolved from telephony, strict timing and reliability requirements, need for guaranteed service, dumb end systems (telephones).

Router Architecture Overview

• Routing protocols (e.g., RIP, OSPF, BGP) run within the router, to compute and update forwarding tables and push it to input ports.
• Routing processor (control plane)
• High-speed switching fabric
• Forwarding data plane (hardware)

Input Port Functions

• Physical layer: bit-level reception
• Link layer (receive): receives frame and extracts the network-layer packet
• Lookup: using forwarding table
• Forwarding: Based on lookup
• Queuing: to handle delay if packets come too quickly

Switching Fabrics

• Transfer packets from input buffer to appropriate output buffer.
• Switching rate: Transfer rate of packets from inputs to outputs
• Three types of fabrics: memory, bus, crossbar
• Memory/CPU based switching
• Bus switching
• Interconnection Network switching (faster than bus)

Switching via Memory

• Packets are copied to system's memory.
• Speed is limited by memory bandwidth.
• 2 bus crossings need to occur for each datagram

Switching via Bus

• Datagrams from input port memory to output port memory via a shared bus.
• Switching rate limited by bus bandwidth.
• 32 Gbps bus sufficient speed for access, enterprise routers

Switching via Interconnection Network

• Allows overcoming bus limitations
• Utilizes technologies like banyan networks, crossbars
• It fragments datagram into smaller cells and switches cells through the fabric
• Cisco 12,000 and above devices have this kind of architecture

Output Ports

• Buffer packets from the switching fabric.
• Requires buffering to handle differences between the rate at which data flows from the fabric and the output ports
• Queuing: if arriving rate exceeds output line speed.
• Scheduling: how datagrams from the queue are processed.

Input Port Queuing

• Fabric slower; queueing at input queues could occur
• Head-of-the-line blocking
• Output Port Contention: only one packet is transferred at a time.

Buffering

• The recent recommendation is that the buffering amount at the output ports is proportional to the link capacity and the RTT (round trip time).

ICMP

• Internet Control Message Protocol
• Used by hosts & routers to communicate network-level information.
• ICMP messages (types and codes)
  • Error reporting
  • Echo request, reply

IPv6 Motivation

• Initial motivation: 32-bit address space exhaustion.
• Additional motivation: header format helps processing/forwarding, header changes accommodate QoS.

IPv6 Datagram Format

• Fixed-length header (40 bytes)
• No fragmentation allowed from the beginning.
• Priority, flow label, next-header, hop limit, source, destination addresses in the header.

Other Changes from IPv4

• Removed checksums
• Options outside the header
• ICMPv6 with Packet Too Big messages
• Multicast group management functions

Transition from IPv4 to IPv6

• Tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers.
• Allows interoperability while IPv6 network deployment is still ongoing.

Subnets

• Subnets are isolated network islands.
• Interfaces with same subnet portion of the IP address can communicate directly without requiring a router.

Hierarchical Addressing: Route Aggregration

• Efficient advertisement of routing information through ISP blocks and subnets with hierarchical addressing and aggregation
• Route aggregation to advertisement from ISP to ISP and within Organizations reduces routing load and traffic.

Hierarchical Addressing: More Specific Routes

• More specific routes for organizations, given by Internet service providers (ISPs).
• Allows ISP-level route advertisements
• Examples are shown in diagrams

IP Addressing: Last Word...

• ICANN (Internet Corporation for Assigned Names and Numbers)
• Allocates IP addresses, manages DNS, assigns domain names, and address resolution.

DHCP Overview

• DHCP dynamically assigns the IP address to a client on a network
• Protocols and messages involved in the process
  • discover
  • offer
  • request
  • ACK.
• Useful for mobile users and when it dynamically assigns the IP address based on the network usage.

DHCP Client-Server Scenarios

• arriving client needs the IP in a network
• DHCP server gives an IP to the client.

DHCP: More than IP Addresses

• DHCP can return addresses of first-hop router, DNS server, and network mask to client.

IP Addresses: How to Get One?

• Hard-coded in a file by system administrator
• DHCP(Dynamic Host Configuration Protocol)

NAT (Network Address Translation)

• Motivation: Local networks use just one IP address for outside world visibility.
• Implementation: Router needs NAT translation table to remember source IP & port to NAT IP & port mapping for outgoing datagrams and the opposite for incoming ones.

NAT Traversal Problem

• Solutions: Static configuration to forward incoming connections through NAT.