The Internet Protocol - IPv4

UNIVERSITY OF AARHUS

Department of Computer Science Network Protocols and Internetworking03/09/2007 - 1

The Internet Protocol - IPv4

Lars M. KristensenDepartment of Computer ScienceUniversity of AarhusDenmark



The Internetworking Model

Abstraction (TCP/IP software)

Network Interface layer

Internet Protocol layer

Application Protocol layer

Basic Network Level Services:Connectionless Packet Delivery Service (IP/UDP) (loss, duplication, and reordering).Reliable Stream Transport Service (IP/TCP).

Transparent interconnection of heterogeneous physical networks into a single uniform and virtual network



Internet Addressing



Internet AddressingMain goals:

Universal: hosts must be globally addressable.Routing/forwarding must be efficient.

Basic idea:Each host is assigned a 32-bit internet address.Addresses consists of a network identifier (prefix) and a host identifier (suffix).

Dotted decimal notation:Addresses by convention divided into 8-bit blocks and written in decimal notation separated by dots (.):Example:

10000000000010100000001000011110

10000000 00001010 00000010 00011110

128.10.2.30



Original Classful Internet Addressing Scheme

Physical networks will have different number of hosts attached:



Special AddressesNetwork address:

All bits for host identifier equal to 0.Example (class B network): 192.255.0.0

Directed broadcast (all hosts on a specified network):All bits for host identifier equal to 1.Usually implemented via hardware broadcast (if supported).Example (class B Network) : 192.254.255.255

Limited broadcast (all host on local network):All address bits set to 1: 255.255.255.255

Startup source address:All address bits set to 0: 0.0.0.0Use for identification when host has not yet an IP address.

Loopback address:Network prefix 127.0.0.0For testing and interprocess communication purposes on a machine.



Example

IP addresses identifies network connections (interfaces).

Questions:Network address?Network class?Limited broadcast?Directed broadcast?



DiscussionUnique network prefix for each physical network quickly exhausts the address space.

Subnetting alleviates this.

Static classful hierarchy leads to poor utilisation of the address space.

Classless addressing alleviates this.

IP adresss of a host must be changed if moved to another physical network.

Mobile IP developed to support mobile hosts.

IP addresses identifies network connections (interfaces).



Address Resolution



The ProblemCommunication on the physical networks uses physical network addresses (e.g., Ethernet):

Communication in an IP network uses IP addresses.

Address Resolution Problem: Mapping from IP addresses to physical addresses.



The Address Resolution Protocol

Request-reply protocol:ARP request (broadcast)

ARP reply (unicast)



ARP Protocol

ARP cache:Contains recently acquired IP-to-physical address bindings.Each cache entry has an associated timeout (~ 20 minutes).Reduces the number of broadcasts and delay.

Further refinements:Sender IP address typically contained in the ARP request.Gratuitous ARP request broadcast to inform of a new physical address.Early revalidation: refresh cache entry before timeout to avoid jitter (delays).



ARP Protocol Format

HARDWARE TYPE: type of physical network (1 for Ethernet).PROTOCOL TYPE: higher-level protocol (0800 for IP).HLEN: length of hardware addresses.PLEN: length of higher-level protocol addresses.OPERATION: ARP request(1), ARP response (2).



ARP Operation

ARP request (broadcast)

ARP reply (unicast)

Question:Sender HA/IP? Target HA/IP?



Example

ARP messages for transmission of an IP datagram from host Lancelot to host Arthur via router Taliesyn?



IPv4: Connectionless Datagram Delivery



The Internet ProtocolService provided:

Unreliable: loss, duplication, delay, out-of-order delivery.Connectionless: each datagram (packet) treated independently.

Protocol components:Packet format: basic units of data transfer in an TCP/IP internet.Rules for forwarding of packets across multiple physical networks. Rules for packet processing and error signalling.



The IPv4 Datagram

IP header is at least 20 octets.Maximum datagram size is 65,535 octets.



FragmentationMaximum transfer units (MTU) differs between physical networks:

Multiple frames may be required to transmit the IP datagram.



FragmentationOriginal IP datagram

IP datagram 1

IP datagram 2

IP datagram 3

IDENTIFICATION field used to group fragments belonging to the same original IP datagram.FRAGMENT OFFSET specifies offset in original datagram.FLAGS used to tag the last fragment.

Reassembly is only done at the final destination.



IP Datagram Options



Forwarding IP Datagrams



IP Forwarding BasicsDirect delivery (by host or router):

IP destination of the datagram is on the same physical network.The network prefix can be used to determine whether direct delivery is possible.

Indirect delivery (by host):IP destination of the datagram is not on the same physical network.IP datagram is passed on to a router which forwards the datagramtowards the destination network.

Forwarding based upon the contents of routing tables.Content of routing tables computed by routing algorithms.Routing tables contains pairs consisting of a network prefix and a next-hop router IP address.



Routing Example

Size of routing tables independent of the number of hosts.Routing table may also contain a default route.Routing table may also contain host routes.



Forwarding Algorithm



Forwarding and Addressing

ComputerComputer

10.0.0.4 40.0.0.8

10.0.0.440.0.0.8



ICMP: Error and Control Messages



Internet Control Message Protocol (ICMP)

Protocol for control and error reporting among hosts and routers.ICMP messages encapsulated in IP datagrams:



ICMP Message Types

Examples of use:Testing reachability (ping).Reporting an unreachable destination.Tracing a route (traceroute).



SummaryInternet Addressing:

Division into network prefix and host suffix.IP addresses identifies network connections (interfaces).

Address Resolution:ARP Protocol for mapping IP addresses to physical addresses.

IP Protocol:Unreliable connectionless delivery of datagrams.

IP Datagram Forwarding:Forwarding based on routing tables computed by routing protocols.Routing table entries are pairs (network prefix, next-hop IP address).

Error and Control Messages:ICMP Protocol for control, error reporting, and diagnostics.



Introduction to project 1

www.daimi.au.dk/NPaI/project1.html

Documents

The Internet Protocol - IPv4