9781133019862_PPT_ch07

Guide to TCP/IP

Fourth Edition

Chapter 7:

IP Address Autoconfiguration

Objectives

• Explain the basic services that DHCP/DHCPv6 offers to its clients and explain its background

• Explain the specifics of IP/IPv6 address management using DHCP/DHCPv6

• Explain the DHCP Discovery, renewal, and release processes

• Explain the basic DHCP/DHCPv6 packet structure and types of DHCP/DHCPv6 messages in use

2

© 2013 Course Technology/Cengage Learning. All Rights Reserved.

Objective (cont'd.)

• Describe broadcast and unicast addressing for

IPv4 as well as multicast addressing for IPv6

• Describe relay agent communications for both IPv4 and IPv6

• Discuss Microsoft DHCP scopes and classes IPv4 and differences in IPv6 scope configuration

• Use DHCP/DHCPv6 troubleshooting utilities

3


Understanding Autoaddressing

• DHCP

– Considered the most common form of address autoconfiguration

• Automatic Private IP Addressing (APIPA)

– Introduced by Microsoft a few years after DHCP

– Requires no addressing servers

• DHCPv6

– A more robust and controllable system for address autoconfiguration


4

Introducing Dynamic Host

Configuration Protocol

• DHCP

– Service that provides a way for client computer to request IP address assignment

– Delivers necessary configuration information to clients

– Traces its origins back to an earlier protocol named

BOOTP

– Servers can manage one or more ranges of IP addresses

5


How DHCP Works

• When TCP/IP is configured on client computer

– The Obtain an IP address automatically option is the only necessary set-up element

• Next time workstation attempts to access network

– It broadcasts a DHCP address request to the network because it has no IP address

• All DHCP servers present on same broadcast domain

– Receive request and send back message that indicates a willingness to grant an address lease

6


How DHCP Works (cont'd.)

• Client

– Accepts address lease offer and sends packet to server that extended offer

• In reply

– Server proffers an IP address for a specific period of time that the client uses thereafter

• When half the lease period expires

– Client attempts to renew the lease

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8

Role of Leases

• Leases

– “Loans” of an address for a specific amount of time

• Leases that range from one to three weeks

– Typical on networks in which machines seldom move and the workforce is stable

• Lease periods

– Average between one and three days for networks on which roving workers come and go regularly

• Lease periods of four to eight hours

– Common on ISP networks


9

DHCP Software Elements

• DHCP client

– Enabled at client machine when you select the

Obtain an IP address automatically option

• DHCP server

– Manages address pools and configuration data

• DHCP relay agent

– Intercepts address requests on local cable segment

– Repackages requests as a unicast to one or more

DHCP servers

10


DHCP Lease Types

• Manual address lease

– Administrator assigns an IP address manually

• Dynamic address lease

– DHCP server assigns addresses for specific periods of time

11


More About DHCP Leases

• How DHCP integrates with DNS

– Server addresses are advertised using DNS

– All address updates must be entered manually

– Client addresses usually come into play only when:

• E-mail addresses of the form user@domain.name must be resolved

12


IPv4 Autoconfiguration

• Two types of IPv4 address autoconfiguration mechanisms

– DHCP

– Automatic Private IP Addressing (APIPA)


13

Automatic Private IP Addressing

(APIPA)

• Dynamic configuration of IPv4 link-local addresses

• Address block 169.254.0.0/16

– Reserved for this use

• Used by interfaces as a failover mechanism to selfassign an IPv4 address

• APIPA address will not allow routed communications to hosts on other networks

14


Understanding IP Address

Management with DHCP

• DHCP Discovery

– Occurs when DHCP client broadcasts a request for an IP address

– Relies on an initial DHCP broadcast

• DHCP servers

– PING an address before offering it to the client

• Windows clustering

– Allows two or more servers to be managed as a single system

15


DHCP Address Discovery

• Uses four packets

– DHCP Discover packet

– DHCP Offer packet

– DHCP Request packet

– DHCP Acknowledgment packet


16

DHCP Address Discovery (cont’d.)


17

Discover Packet

• During DHCP Discovery process

– Client broadcasts Discover packet that identifies client’s hardware address

– Header contains

• Source IP address 0.0.0.0

• Preferred address

– Typically the last address the client used


18

Offer Packet

• Send by DHCP server to offer IP address to DHCP client

• Offer packet

– Includes IP address that is offered to the client

– Sometimes, answers to the requested options in the

DHCP Discover packet

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20

Request Packet

• Once Offer packet is received

– Client can either accept offer by issuing a DHCP

Request packet, or

– Reject offer by sending a DHCP Decline packet

• Typically

– Client sends a Decline only if it receives more than one Offer

21


Acknowledgment Packet

• Sent from server to client to indicate the completion of the four-packet DHCP Discovery process

• Response

– Contains answers to any configuration options requested by client in the previous Request packet

22


Address Renewal Process

• When DHCP client receives an address from a

DHCP server

– Client also receives a lease time and notes the time that the address was received

• Lease time

– Defines how long the client can keep the address

• In middle of the lease period

– Client starts a renewal process

23


Renewal Time (T1)

• T1

– Defined as the time that the client tries to renew its network address

• The Renewal packet

– Is unicast directly to the DHCP server

• Default value for T1

– 0.5 * duration_of_lease (i.e., lease time)

24


Rebinding Time (T2)

• Time that client begins to:

– Broadcast a renewal request for an extended lease time from another DHCP server

• DHCP specification, RFC 2131, defines default value for T2 as

– 0.875 * duration_of_lease

25


Rebinding Time (T2) (cont’d.)


26

Rebinding Time (T2) (cont’d.)


27

DHCP Address Release Process

• Client should release its address by

– Sending a DHCP Release packet to the server

• DHCP Release packet

– Sent over UDP

– DHCP server does not send any acknowledgment

• If client does not send the DHCP Release packet:

– DHCP server automatically releases the address at the lease expiration time

28


DHCP Packet Structures

• Fields

– Operation Code (OPCODE) or Message Type Field

– Hardware Type Field

– Hardware Length Field

– Hops Field

– Transaction ID Number Field

– Seconds Since Boot or Seconds Elapsed Field

– Flags Field

29


DHCP Packet Structures (cont'd.)

• Fields

– Client IP Address Field

– Your IP Address Field

– Server IP Address Field

– Gateway or Relay Agent IP Address Field

– Client Hardware Address Field

– Server Host Name Field

– Boot File Field

– DHCP Option field


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31

DHCP Option 53: Message Type

• Only DHCP option required in all DHCP packets

• Indicates general purpose of any DHCP message

• DHCP boot sequence uses these message types:

– DHCP Message Type 1: Discover (client to server)

– DHCP Message Type 2: Offer (server to client)

– DHCP Message Type 3: Request (client to server)

– DHCP Message Type 5: ACK (server to client)

32


DHCP Option 53: Message Type

(cont’d.)


33

Broadcast and Unicast in DHCP

• DHCP clients

– Must broadcast service requests until they obtain IP addresses

• DHCP clients

– Use unicast addressing after they obtain an address for a local DHCP server or relay agent

• DHCP servers

– Examine DHCP packets coming from clients

34


Broadcast and Unicast in DHCP

(cont’d.)


35

Communications with a DHCP Relay

Agent

• DHCP boot up process

– Relies heavily on broadcasts, but most routers do not forward broadcasts

• Relay agent function

– Typically loaded on a router connected to the segment containing DHCP clients

• Relay agent device

– Configured with the address of the DHCP server

– Can communicate using unicast packets directly with that server

36


Communications with a DHCP Relay

Agent

(cont’d.)


37


38

IPv6 Autoconfiguration

• Two basic approaches to IPv6 autoconfiguration

• Stateless autoconfiguration

– Simply presents required router configuration information to all comers

• Stateful autoconfiguration

– DHCPv6 server must maintain awareness of the status or state of its pool of available addresses


39

Types of IPv6 Autoconfiguration

• Stateless address autoconfiguration

– ND protocol allows routers to be configured to present the minimum information a host needs when joining a network link

• Stateful address autoconfiguration

– DHCPv6 is much like DHCPv4 under IPv4

– Both rely on dedicated servers to hold databases of information about hosts and their IP and other configuration parameters

40


Types of IPv6 Autoconfiguration

(cont’d.)

• Combination of stateful and stateless address autoconfiguration

– Referenced as DHCPv6 stateless

– Routers on the local link can be configured to provide pointers to DHCPv6 servers

– Router provides the network prefix, and DHCPv6 server provides the DNS server information


41

Functional States of an IPv6

Autoconfigured Address

• Tentative addresses

– Occur as a node initializes an interface on an IPv6 network segment or link

• Valid addresses

– Usable based on the Valid Lifetime field in the Prefix

Information option of an RA or the Valid Lifetime field in the DHCPv6 IA Address option

• Preferred addresses

– Usable based on the Preferred Lifetime field in the

Prefix Information option of an RA or the Preferred

Lifetime field in the DHCPv6 IA Address option


42

Functional States of an IPv6

Autoconfigured Address (cont’d.)

• Deprecated addresses

– Allow nodes to continue to function while they renew the lease on their addresses

• Invalid addresses

– Cannot be used as either the source or destination address when the valid lifetime expires

43


Node Interface Identifiers

• Node interface identifiers (IDs)

– Used to ensure that the IPv6 address is unique among all other IPv6 addresses

– Generally 64 bits long

• Three most common sources

– Modified EUI-64 format

– Random number generator to create a 64-bit number

– Cryptographically Generated Addresses (CGA) process


44

Node Interface Identifiers (cont’d.)


45

Node Interface Identifiers (cont’d.)

• RFC 4941

– Node using SLAAC will compute an additional IPv6 address known as the “temporary” address

• Temporary address

– Assigned “preferred” status

– Used for all outbound communications from the node

46


DHCPv6

• DHCPv6

– Defined in RFC 3315

– Uses UDP ports 546 and 547

• DHCPv6 uses two specific multicast addresses:

– FF02::1:2

– FF05::1:3

• DHCP Unique Identifier (DUID) methods

– DUID-LLT

– DUID-EN

– DUID-LL


47

DHCPv6 (cont’d.)

• Numerous DHCPv6 message types

– Occur between nodes, servers, and relay agents

• DHCPv6 Stateful Message Exchange

– Host sends a Router Solicitation

– Router replies with a Router Advertisement

– Host sends a Solicit message

– DHCPv6 server replies with an Advertise message

– Host sends a Request message

– DHCPv6 server sends the host a Reply message with an IPv6 address


48

DHCPv6 (cont’d.)

• DHCPv6 Stateless Message Exchange



– Host sends an Information-Request message

– DHCPv6 server sends the host a Reply message with the other available configuration options

• DHCPv6 Relay Message Exchange



– Host sends a Solicit message


49

DHCPv6 (cont’d.)

• DHCPv6 Relay Message Exchange (cont’d.)

– Router relay-forwards Solicit message to server

– DHCPv6 server relay-replies router with an

Advertise message

– Router replies with Advertise message to the host

– Host sends a Request message

– Router relay-forwards Request message to server

– DHCPv6 server relay-replies router with a Reply message with IPv6 address and other options

– Router replies with the Reply message to the host


50

IPv6 Autoconfiguration Process

• Basic steps

– Node creates an interface ID

– FE80:: is prepended to the interface ID

– Node sends NS message with its link-local address as the destination

– If an NA is not received, then the address is considered unique

– Host sends an RS to the all-routers multicast address FF02::2

– If an RA is not received, the host starts the stateful autoconfiguration process


51


(cont’d.)

• Basic steps (cont’d.)

– If an RA is received, host examines the RA message looking for variables and flags

– If L flag is set to “on,” host adds the network prefix to its prefix cache

– If A flag is set to “on,” then two IPv6 addresses are created

– Host sends an NS message with its global unicast address as the destination

– If an NA is not received, the address is considered unique

52



(cont’d.)

• Basic steps (cont’d.)

– If M flag is set to “on,” start the stateful autoconfiguration process

– If O flag is set to “on,” start the stateful autoconfiguration process


53

Autoconfiguration in Microsoft

Windows Operating Systems

• Basic guidelines:

– Link-local address will be generated using a RNG

– If A flag is set to “on” in an RA, the global unicast address will be generated

– If A flag is set to “on” in an RA, a temporary global unicast address will be generated

– OS sends the RS to find any on-link routers

– If no RAs are received, OS starts stateful address process

– Manually configuring an IPv6 address does not disable IPv6 address autoconfiguration

54


Microsoft Windows Server 2008 DHCP

Scopes

• Address scopes

– Define a set of addresses that a DHCP server can assign to clients

• Superscopes

– A collection of scopes that contain sets of nonconsecutive IP addresses

55


Setting Up A Simple DHCP Server

• Source of information

– D-Link Xtreme N Gigabit Router (model number

DIR-655)


56


57


58

Troubleshooting DHCP

• One good way to troubleshoot DHCP/DHCPv6

– Use a protocol analyzer, such as Wireshark

• Analyzer

– Can display the sequence of messages that occurs on a network

• Commands to release and/or renew an IP address

– ipconfig /release

– ipconfig /renew

– ipconfig /release6

– ipconfig /renew6


59


60

Summary

• DHCP

– Provides a way for computers to obtain usable, unique IP addresses and necessary TCP/IP configurations

• From the administrative side

– DHCP makes it easy to define and manage pools of

IP addresses

• DHCP’s origins

– Lie in an earlier TCP/IP Application layer protocol, called BOOTP

61


Summary (cont'd.)

• DHCP

– Supports manual or dynamic address allocation

• When a DHCP client starts

– It begins the process of DHCP Discovery, during which the client receives an IP address and lease

• DHCP

– Supports a variety of message types and options

– Only MessageType 53 (DHCP Message) is mandatory for any given DHCP message


62

Summary (cont'd.)

• Because DHCP can ferry a surprisingly large range of configuration information

– The protocol makes use of several message options

• A protocol analyzer is especially effective when diagnosing DHCP/DHCPv6 difficulties

• Clients supporting IPv6 have new operations for address autoconfiguration

• DHCPv6 operates much like DHCP for IPv4

• DHCPv6 is a completely new service, compared to

DHCP’s origins


63

9781133019862_PPT_ch07

Chapter 7:

IP Address Autoconfiguration

Objectives

Objective (cont'd.)

Understanding Autoaddressing

Introducing Dynamic Host

Configuration Protocol

How DHCP Works

How DHCP Works (cont'd.)

Role of Leases

DHCP Software Elements

DHCP Lease Types

More About DHCP Leases

IPv4 Autoconfiguration

Automatic Private IP Addressing

(APIPA)

Understanding IP Address

Management with DHCP

DHCP Address Discovery

DHCP Address Discovery (cont’d.)

Discover Packet

Offer Packet

Request Packet

Acknowledgment Packet

Address Renewal Process

Renewal Time (T1)

Rebinding Time (T2)

Rebinding Time (T2) (cont’d.)

Rebinding Time (T2) (cont’d.)

DHCP Address Release Process

DHCP Packet Structures

DHCP Packet Structures (cont'd.)

DHCP Option 53: Message Type

DHCP Option 53: Message Type

(cont’d.)

Broadcast and Unicast in DHCP

Broadcast and Unicast in DHCP

(cont’d.)

Communications with a DHCP Relay

Agent

Communications with a DHCP Relay

Agent

IPv6 Autoconfiguration

Types of IPv6 Autoconfiguration

Types of IPv6 Autoconfiguration

(cont’d.)

Functional States of an IPv6

Autoconfigured Address

Functional States of an IPv6

Autoconfigured Address (cont’d.)

Node Interface Identifiers

Node Interface Identifiers (cont’d.)

Node Interface Identifiers (cont’d.)

DHCPv6

DHCPv6 (cont’d.)

DHCPv6 (cont’d.)

DHCPv6 (cont’d.)

IPv6 Autoconfiguration Process

IPv6 Autoconfiguration Process

(cont’d.)

IPv6 Autoconfiguration Process

(cont’d.)

Autoconfiguration in Microsoft

Windows Operating Systems

Microsoft Windows Server 2008 DHCP

Scopes

Setting Up A Simple DHCP Server

Troubleshooting DHCP

Summary

Summary (cont'd.)

Summary (cont'd.)

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