1. 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.

3. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

4. 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
© 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

6. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

7. How DHCP Works (cont'd.) Client In reply
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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

8. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

9. Role of Leases Leases Leases that range from one to three weeks
“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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

10. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

11. DHCP Lease Types Manual address lease Dynamic address lease
Administrator assigns an IP address manually
Dynamic address lease
DHCP server assigns addresses for specific periods of time
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

12. 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:
addresses of the form must be resolved
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

13. IPv4 Autoconfiguration
Two types of IPv4 address autoconfiguration mechanisms
DHCP
Automatic Private IP Addressing (APIPA)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

14. Automatic Private IP Addressing (APIPA)
Dynamic configuration of IPv4 link-local addresses
Address block /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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

15. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

16. DHCP Address Discovery
Uses four packets
DHCP Discover packet
DHCP Offer packet
DHCP Request packet
DHCP Acknowledgment packet
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

17. DHCP Address Discovery (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

18. Discover Packet During DHCP Discovery process Preferred address
Client broadcasts Discover packet that identifies client’s hardware address
Header contains
Source IP address
Preferred address
Typically the last address the client used
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

19. Offer Packet Send by DHCP server to offer IP address to DHCP client
Includes IP address that is offered to the client
Sometimes, answers to the requested options in the DHCP Discover packet
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

20. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

21. Request Packet Once Offer packet is received Typically
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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

22. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

23. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

24. Renewal Time (T1) T1 The Renewal packet Default value for 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)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

25. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

26. Rebinding Time (T2) (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

27. Rebinding Time (T2) (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

28. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

29. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

30. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

31. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

32. 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)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

33. DHCP Option 53: Message Type (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

34. Broadcast and Unicast in DHCP
DHCP clients
Must broadcast service requests until they obtain IP addresses
Use unicast addressing after they obtain an address for a local DHCP server or relay agent
DHCP servers
Examine DHCP packets coming from clients
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

35. Broadcast and Unicast in DHCP (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

36. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

37. Communications with a DHCP Relay Agent (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

38. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

39. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

40. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

41. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

42. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

43. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

44. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

45. Node Interface Identifiers (cont’d.)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

46. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

47. DHCPv6 DHCPv6 DHCPv6 uses two specific multicast addresses:
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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

48. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

49. 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 Solicit message
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

50. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

51. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

52. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

53. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

54. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

55. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

56. Setting Up A Simple DHCP Server
Source of information
D-Link Xtreme N Gigabit Router (model number DIR-655)
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

57. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

58. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

59. Troubleshooting DHCP One good way to troubleshoot DHCP/DHCPv6 Analyzer
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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

60. © 2013 Course Technology/Cengage Learning. All Rights Reserved.

61. Summary DHCP From the administrative side DHCP’s origins
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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

62. Summary (cont'd.) DHCP When a DHCP client starts
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
Supports a variety of message types and options
Only MessageType 53 (DHCP Message) is mandatory for any given DHCP message
© 2013 Course Technology/Cengage Learning. All Rights Reserved.

63. 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
© 2013 Course Technology/Cengage Learning. All Rights Reserved.