Objectives:

Chapter 5: Network/Internet Layer

How Networks are connected Network/Internet Layer Routed Protocols Routing Protocols Autonomous Systems (AS) Border/Boundary and hierarchy routers

Internetworking

• How Networks Differ

• How Networks Can Be Connected

• Concatenated Virtual Circuits

• Connectionless Internetworking

• Tunneling

• Internetwork Routing

Connecting Networks

A collection of interconnected networks.

How Networks Differ

Some of the many ways networks can differ.

5-43

How Networks Can Be Connected

(a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers.

Concatenated Virtual Circuits

Internetworking using concatenated virtual circuits.

Connectionless Internetworking

A connectionless internet.

Tunneling

Tunneling a packet from Paris to London.

Internetwork Routing

(a) An internetwork. (b) A graph of the internetwork.

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Routed Protocols Versus Routing Protocols

Routing protocols determine the path that routed protocols follow to their destinations.

The Network Layer in the Internet(routed protocols)

• Internet Protocol (IP) • Network Address Translation Protocol (NAT)• Internet Control Message Protocol (ICMP)• Address Resolution Protocol (ARP)• Dynamic Host Configuration Protocol (DHCP)• Internet Control Protocols

Internet Protocol (IP)

Network layer (Packet) fields

IP Protocol Options

Some of the IP options.

5-54

NAT – Network Address Translation

Placement and operation of a NAT table.

network address translation (NAT) is the process of modifying network address information by hiding an entire address space, usually consisting of private network addresses, behind a single IP address in another public address space. This mechanism is implemented in a routing device that uses stateful translation tables to map the "hidden" addresses into a single address and then rewrites the outgoing IP packets on exit so that they appear to originate from the router.

Internet Control Message Protocol

The principal ICMP message types.

The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is mainly used by networked computers' operating systems to send error messages—indicating, for instance, that a requested service is not available or that a host or router could not be reached.ICMP relies on IP to perform its tasks, and it is an integral part of IP protocol. It is usually not used directly by user network applications, with some notable exceptions being the ping tool and traceroute.

ARP– The Address Resolution Protocol

Three interconnected /24 networks: two Ethernets and an FDDI ring.

the Address Resolution Protocol (ARP) is the method for finding a host's link layer (hardware) address when only its Internet Layer (IP) is known.

ARP is primarily used to translate IP addresses to Ethernet MAC addresses. It is also used for IP over other LAN technologies, such as Token Ring,FDDI, or IEEE 802.11, and for IP over ATM.

Dynamic Host Configuration Protocol

DHCP Operation

Dynamic Host Configuration Protocol (DHCP) is a network application protocol used by devices (DHCP clients) to obtain configuration information for operation in an Internet Protocol network. This protocol reduces system administration workload, allowing devices to be added to the network with little or no manual intervention. Dynamic Host Configuration Protocol automates network parameter assignment to network devices from one or multiple DHCP servers.

Routing Protocols

• Shortest Path Routing• Distance Vector Routing: RIP, IGRP• Link State Routing • Interior Gateway Routing Protocol: OSPF• Exterior Gateway Routing Protocol: BGP• Hierarchical Routing

Link State Routing

Each router must do the following:

1. Discover its neighbors, learn their network address.

2. Measure the delay or cost to each of its neighbors.

3. Construct a packet telling all it has just learned.

4. Send this packet to all other routers.

5. Compute the shortest path to every other router.

Building Link State Packets

(a) A subnet. (b) The link state packets for this subnet.

Open Shortest Path First (OSPF)

The five types of OSPF messages.

5-66

OSPF – The Interior Gateway Routing Protocol

The relation between Autonomous Systems (AS), backbones, and areas in OSPF.

BGP – The Exterior Gateway Routing Protocol

(a) A set of BGP routers. (b) Information sent to F.

Internet inter-AS routing: BGP

a) BGP (Border Gateway Protocol): the de facto standardb) BGP provides each AS a means to:

1. Obtain subnet reachability information from neighboring ASs.

2. Propagate the reachability information to all routers internal to the AS.

3. Determine “good” routes to subnets based on reachability information and policy.

c) Allows a subnet to advertise its existence to rest of the Internet:

Boundary/border routers Hierarchical

Hierarchical Routing

a) Two-level hierarchy: local area, backbone.

– Link-state advertisements only in area – each nodes has detailed area topology; only

know direction (shortest path) to nets in other areas.

b) Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

c) Backbone routers: run OSPF routing limited to backbone.

d) Boundary routers: connect to other Autonomous Systems.

Why different Intra- and Inter-AS routing ?

Policy: a) Inter-AS: admin wants control over how its traffic routed, who routes

through its net.

b) Intra-AS: single admin, so no policy decisions needed

Scale:a) hierarchical routing saves table size, reduced update traffic

Performance:

a) Intra-AS: can focus on performance

b) Inter-AS: policy may dominate over performance