Introduction. Binding Protocol Addresses. Address Resolution. Address Resolution Protocol (ARP) ARP (2) Support Protocols and Technologies

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TCP/IP

Support Protocols and Technologies

Gail Hopkins

TCP/IP

Introduction

Address Resolution Protocol (ARP)

For Binding protocol addresses to hardware addresses

Internet Control Message Protocol (ICMP)

Error reporting mechanism

Dynamic Host Configuration Protocol (DHCP)

For allocating IP addresses during bootstrapping

Network Address Translation (NAT)

For allowing multiple computers at a site to share a single, globally valid IP address

Part 4 – Internetworking Using TCP/IP

Binding Protocol Addresses

An Internet packet passes through a series

of routers

 Each hop takes it over a particular network, either to a specific computer on that network or to the next router

 In either case, the sending router has to map between the protocol (IP) address and a hardware address

 This called address resolution

Address Resolution

• Address resolution is local to a network

From Comer, 2009

Address Resolution Protocol

(ARP)

TCP/IP defines the Address Resolution

Protocol (ARP) which defines the format of resolution requests and responses

This technique is usually combined with

local caching of hardware addresses

ARP (2)

Consider scenario:

Computer A wants to resolve address of Computer B

A sends broadcast message: “I’m looking for the MAC address of the computer that has IP address B”

Broadcast message only travels across one network

When C receives a copy of the request it sends one message back “I’m the computer with IP address B and my MAC address is M”

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ARP (3)

Request message reaches all computers on a network...

....Reply doesn’t From Comer, 2009

ARP Message Format

Most implementations of ARP are used to

bind IP addresses to Ethernet addresses But the protocol is designed to be used to bind

arbitrary high-level addresses to arbitrary hardware addresses

Designers have included

A fixed-size field at the beginning of an ARP message to specify the size of hardware addresses being used

An address length field to specify size of protocol addresses

ARP Encapsulation

Frame type field specifies that data contains an ARP message

Same type used for ARP requests and responses... ...receiver must therefore examine OPERATION field to determine if a request or response From Comer, 2009

ARP Caching and Message

Processing

Sending an ARP request for each datagram is

inefficient

Also, most computer communication involves

a sequence of packets

ARP software therefore caches responses

ARP maintains a small table of bindings in

memory

An entry is replaced when a response arrives

Oldest entry removed when table runs out of space or after entry hasn’t been updated for a long time

The Conceptual Address

Boundary

From Comer, 2009

Internet Control Message

Protocol (ICMP)

IP offers a best-effort communication

service

But it does still try to avoid errors and report them when they occur

E.g. Use of header checksum to detect transmission errors

Also, TIME TO LIVE field used to prevent a datagram circulating forever

If checksum error – datagram is discarded immediately

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TCP/IP

ICMP (2)

For problems that are less severe than

checksum errors:

 ICMP used to report errors back to sender

ICMP and IP co-dependent

 IP depends on ICMP to report errors

 ICMP uses IP to carry error messages

TCP/IP

Key ICMP Messages

Number Type Purpose

0 Echo Reply Used by the ping program

3 Dest. Unreachable Datagram could not be delivered

5 Redirect Host must change a route

8 Echo Used by the ping program

11 Time Exceeded TTL expired or fragments timed out

12 Parameter Problem IP header is incorrect

30 Traceroute Used by the traceroute

program

From Comer, 2009

ICMP Message Format and

Encapsulation

From Comer, 2009

Protocol Software, Parameters

and Configuration

How does protocol software in a host or

router begin operation? Router:

Manager must specify initial values, e.g. IP address of each network connection, protocol software to run, initial values for a forwarding table

Host configuration: Done during Bootstrapping

Dynamic Host Configuration Protocol

(DHCP)

Special DHCP server that assigns IP

addresses to hosts

Newly booted machine broadcasts a DHCP

Discover packet

DHCP server sends back Reply with an IP

address – configured to provide either: Permanent IP addresses

Manually assigned by administrator

Automatic IP address from a pool of addresses to be allocated on demand

Leased for a finite period of time

DHCP Operation with Relays

DHCP server does not need to be on the same

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Network Address Translation

(NAT)

Allows multiple computers at a site to share

a single, globally valid IP address  Transparent communication

A host at a site appears to have a normal Internet connection

A host in the Internet always appears to receive communication from a single computer rather than from one of many computers at a site

 NAT provides this illusion

NAT (2)

Hosts at a site using NAT run conventional

TCP/IP software and applications

NAT is placed on the connection between

the Internet and the site

NAT is often embedded in a device

e.g. Wi-Fi wireless access point or an Internet router

NAT Conceptual Architecture

From Comer, 2009

Basic NAT Translation

NAT Translation Table From Comer, 2009

Transport-Layer NAT (NAPT)

Basic version of NAT doesn’t handle:

 When two hosts at a site attempt to communicate with the same remote server

 When two or more applications on a given host at a site attempt simultaneous communication with different destinations on the Internet

NAPT allows a site to have arbitrary numbers

of applications running on arbitrary hosts, all communicating simultaneously with arbitrary destinations on the Internet

NAPT (2)

NAPT uses protocol port numbers to

distinguish among services:

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TCP/IP

Summary

Address resolution (ARP) and assignment

(DHCP)

ICMP error reporting mechanism

Network Address Translation

Reading:

 Chapter 23, Computer Networks and Internets, Comer, 5th_{Edition, 2009}