Guide to TCP/IP Fourth Edition Chapter 7: IP Address Autoconfiguration

Guide to TCP/IP Fourth Edition

Chapter 7:IP Address Autoconfiguration

2

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

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

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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


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

5

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


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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


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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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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


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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


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DHCP Lease Types

• Manual address lease– Administrator assigns an IP address manually

• Dynamic address lease– DHCP server assigns addresses for specific periods

of time


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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 [email protected] must be resolved


IPv4 Autoconfiguration

• Two types of IPv4 address autoconfiguration mechanisms– DHCP– Automatic Private IP Addressing (APIPA)


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 self-assign an IPv4 address

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


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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


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DHCP Address Discovery

• Uses four packets– DHCP Discover packet– DHCP Offer packet– DHCP Request packet– DHCP Acknowledgment packet


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DHCP Address Discovery (cont’d.)


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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


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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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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


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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


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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


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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)


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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


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Rebinding Time (T2) (cont’d.)


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Rebinding Time (T2) (cont’d.)


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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


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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


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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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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)


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DHCP Option 53: Message Type (cont’d.)


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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


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Broadcast and Unicast in DHCP (cont’d.)


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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


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Communications with a DHCP Relay Agent (cont’d.)



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


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


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


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


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


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


Node Interface Identifiers (cont’d.)


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


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


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


DHCPv6 (cont’d.)

• DHCPv6 Stateless Message Exchange– Host sends a Router Solicitation– Router replies with a Router Advertisement– 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 Router Solicitation– Router replies with a Router Advertisement– Host sends a Solicit message


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


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


IPv6 Autoconfiguration Process (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


IPv6 Autoconfiguration Process (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


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


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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 non-

consecutive IP addresses


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Setting Up A Simple DHCP Server

• Source of information– D-Link Xtreme N Gigabit Router (model number DIR-

655)




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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



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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


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


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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


Documents

Guide to TCP/IP Fourth Edition Chapter 7: IP Address Autoconfiguration