12 DHCP and Cellular IP

Suchit Purohit

epgp books
  • Introduction
  • Need of Dynamic Configuration
  • DHCP message format
  • Address acquisition state
  • Cellular IP
  • Network of CIP
  • Summary

Introduction

 

Internet service provider (ISP) provides a block of Internet Protocol (IP) addresses to the organization. There are many ways to do address assignment in the network of any organization. IP address assignment is done in the organization by network administrator. Each computer connected with the TCP/IP network required to get an IP address of the network. Also, other information such as address of router, address of name server and address mask for the network are required for communication within a network.

 

There was a one general protocol Bootstrap Protocol (BOOTP) which allows a host machine to obtain all the information automatically during the startup process. When a computer is booting on the network, the operating system send BOOTP messages to the network to get an IP address. BOOTP was implemented using User Datagram Protocol (UDP). It was initially used for UNIX disk-less workstation to get the location of network to assign IP address. Some network card manufacturers have embedded this protocol in the Basic Input Output System (BIOS) of the network interface card which allow direct network booting.

 

Later on the standardized network protocols Dynamic Host Configuration Protocol (DHCP) was developed which provide functionality of leasing of an IP address. DHCP is used to assign dynamic IP addresses to the host of network.

 

Need of Dynamic configuration

 

In the current era of Internet most of the people are using laptops or mobile phone for wireless internet connections. ISPs have a continually changing set of customers, and portable laptop computers with wireless connections make it possible to move a computer from one place to another place very easily and quickly. Whenever any new computer is connected to the network it sends the request to the server for an IP address. The server selects one of the addresses from the range of IP addresses available and assigned the address to the computer based on the list provided by ISP.

 

DHCP allows three types of address assignment manual configuration, automatic configuration and dynamic configuration.

  • In manual IP configuration technique an administrator or manager configures a specific address for each and every computer connected to the network.
  • While in automatic configuration an administrator or manager allows a DHCP server to assign a fixed address when a computer is attached to the network first time.
  • In dynamic configuration method an IP address is assigned to the machine for a short period of time e. based on the loan period a server will assign an IP address. Dynamic IP addressing is most powerful aspects of DHCP. When a client contacts a DHCP server, the client sends an identifier, usually the client’s hardware address. The server uses the client’s identifier and the network to which the client has connected to determine how to assign the client and IP address.

DHCP message format

 

DHCP client and server both are using the same message format which is shown in the figure

 

1.1. Each and every field has their own usage and requirements. Table 1.1 describes the value which is available in the message format.

 

Figure 1.1 Format of DHCP message [Reference: Internetworking with TCP/IP Principles, Protocols and Architecture, 5th Edition Douglas E. Comer]

Field Name Description
OP Specifies which operation is performed (1) Request or (2) Reply
HTYPE Hardware Type (1 for Ethernet)
HLEN Hardware Length (6 for Ethernet)
HOPS Initially Zero (0), Server will increment Hops count value.
TRANSACTION ID Unique integer ID used to match reply with request.
SECONDS Specifies the number of seconds before the client computer started.
FLAGS It controls the server response via unicast or broadcast.
CLIENT IP ADDRES Client’s IP address if it is known by client.
YOUR IP ADDRESS Server return the client’s IP address if previous field contain ZERO
SERVER IP ADDRESS Client specifies the server IP address if server is specific.
ROUTER IP ADDRESS IP address of router if it is known by client machine.
CLIENT HARDWARE ADDRESS MAC address of client machine specify by client
SERVER HOST NAME Name of the specific server from where client want response.
BOOT FILE NAME Client specifies the name of booting file for small diskless machine.
OPTIONS Uses Type-Length-Value style encoding.

Table 1.1 DHCP message field and description

DHCP Address Acquisition State

 

During the dynamic IP address assignment process client machine can be in one of six states. The transitions of states are shown in figure 1.2. First of all a client machine is booted and it enters into the INITIALIZE state. From this state a client is discovering about the server from where it can get an IP address. So, client will broadcast the message using DHCPDISCOVER message type. Now all the servers available on the local network will receive the message and only one or more server will give reply depending on the availability of IP addresses. Here, server will send DHCPOFFER message to the client machine. If all the servers are busy than client will not get any response. Client will be in the SELECT state so all the offers of servers can be view by client and if it finds feasibility than send request to server using DHCPREQUEST message type. Here, a client will be in the REQUEST state for the waiting of a response from the server. Server will give DHCPACK message for confirming the request and client will be in a BOUND state for the given period of time.

 

After 50% of lease period expire client can send request to the server for extending the lease period. Depending on the availability of IP addresses and pre-request for an IP address of any client at the server side server will give DHCPACK or DHCPNACK message. If client receive DHCPACK that indicate server has extended the time period of lease else DHCPNACK indicate server has not extended the lease period. Whenever a lease period is extended after 50% the client will be in the RENEW state.

Figure 1.2 DHCP address acquisition state diagram [Reference: Internetworking with TCP/IP Principles, Protocols and Architecture, 5th Edition Douglas E. Comer]

 

After completion of 87.5% of lease period the client can send DHCPREQUEST for extending the lease period. Again the server will check the availability of IP addresses and pre-request for an IP address of any client at the server side than server will give DHCPACK or DHCPNACK message. If DHCPACK message comes than client will be in REBIND state else for DHCPNACK message client machine has to use an IP address for the limited period only.

 

After completing of 100% lease period client will be in INITIALIZE state again. In between if client want to release the lease period or cancel the lease than he/she will send DHCPRELEASE message to the server so, the state of BOUND will be change into INITIALIZE state.

 

So, after the first timer expires, the client can move into renew state. If a second timer expires before renewal state than client can move into rebind state. If the final timer expires before a lease has been renewed, the client stops using the IP address and returns to the initial state to obtain a new IP address.

 

Cellular IP

 

IP technology is improving day by day and significant innovations are happening in the world for Internet Protocol such as Voice over Internet Protocol (VOIP), Mobile IP, and Cellular IP.

 

VOIP is used for sending communication and multimedia sessions over Internet network. Mobile IP allows any mobile device user to move from one place to another by maintaining their permanent IP address. It is also suited for client/server mechanism where security is must. Cellular IP (CIP) is a standard provided by Internet Engineering Task Force (IETF). It was first proposed in the year 2000.

 

CIP is used to work with wireless network where internet work communication is possible. CIP can accommodate large no of users by maintaining distributed Paging and Routing caches. There is no need for new packet formats, address allocation or even for encapsulations.

 

Four fundamental design principles of the cellular internet protocol are:

  1. location information is stored in distributed data bases,
  2. location information referring to a mobile host is created and updated by regular IP datagram originated by the said mobile host
  3. Location information is stored as soft state
  4. Location management for idle mobile hosts is separated from location management of hosts that are actively transmitting or receiving data

Network of Cellular IP

 

A client machine wants wireless interface connection which is changing frequently. There is a mechanism which indicates the delivery of packet from one machine to another machine with high portability. There are two mechanisms for handover in cellular IP hard handover and semi-soft handover. During the handover mechanism quality of signal matters so to avoid the degradation of service semi-soft handoff technique is used. Cellular IP can interwork with Mobile IP to support wide area mobility.

 

Figure 1.3 Cellular IP and Mobile IP Network

 

Summary

  • The dynamic host configuration protocol is always beneficial to those networks where number of IP addresses is limited and host machines are scale in and scale out.
  • DHCP allows a server to assign an IP addresses automatically or dynamically.
  • To use DHCP, a host machine becomes client in the network. Client starts three timers after obtaining an IP address from the server.
  • DHCP can be used in multiple subnets.
  • Cellular  IP   can   work   with   all   Mobile   IP   networks   that   support   wireless communication.
  • CIP  can  accommodate  large  no  of  users  by  maintaining  distributed  Paging  and Routing caches.
you can view video on DHCP and Cellular IP

Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.