Protocol DHCP

Protocol ICMP

The protocol of interconnection of control messages ICMP (Internet Control Message Protocol) is a part of stack TCP/IP. Protocol ICMP used for testing Of Availability of network nodes and represents by itself Echo protocol (there must be an answer for the send request). ICMP works with 2 types of messages: Echo-request and echo answer. Computer or router sends an Echo-request to the network, in which the IP address of the node, which availability needed to be checked.

Computer (router), which receives echo-request, forms and sends echo-answer and return the answer to the node – the sender of the request. There could be some data in the request (check sum), which must be returned in an answer. Because of echo-request and echo-answer are transmitted through the network inside IP-packets (see Fig. 7.2.9), so their success delivery means normal functioning of all transport system of internet.

Figure 7.2.9 - Encapsulation of ICMP-message in IP-diagram

For sending of echo-request and receiving echo-answers utility (program) ping is used.

The main purpose of DHCP is dynamic assignment of IP-addresses. However, apart from the dynamic, DHCP can maintain and easier ways manual and automatic static assignment.

In automatic static way DHCP-server assigns the IP-address to the client from the pool (array) available IP-addresses without administrator intervention. The boundaries of the pool (array) of the assigned address by the administrator during configuration of DHCP-server. Between the customer ID and theassigned his IP-address, as well as for anual assignment, there is a match.It is installed at the moment of the initial assignment from the serverDHCP IP-address of the client. For all future requests (for IP-address receiving), the server returns to the client the same IP-address.

In the dynamic allocation of addresses DHCP-server gives addressto the client for a limited time, which gives the opportunity to re-use that IP-address receiving to its computers. Dynamic address assignment enables build IP-based network, the number of nodes in which far exceeded the number available to the admin of IP-addresses.

DHCP provides safe and easy way to configure TCP / IP network, ensuring that no conflicts of addresses by centralizing management of their distribution. The administrator manages the process of destination addresses with parameter «lease duration» (lease duration), which determines how long computer can use the assigned IP-address before you once againrequest it from the DHCP server for rent.

Protocol DHCP uses a client-server model. During system startup DHCP - client is in the state of «init» and sends broadcast message discover (find) to search in the network of DHCP-server. DHCP-server receiving this message, replies to it with the message offer (proposal), which contains the IP-address and configuration information. When DHCP - client receives an offer from the DHCP-server it goes into a «request» state and sends a message to request to DHCP-server. DHCP-server sends Message DHCP-acknowledgment (confirmation), which contains the same IP-address which has already been sent earlier in the research stage, as well assetting of rent for the this address. In addition, DHCP-server sends settings of network configuration. Once the customer receives a confirmation, he goes into a state «connection» being in which he can participate in the work of the network TCP / IP.By the end of the rent term of IP-address computer tries to update the parameters of rent from DHCP- server, and if the IP-address can not be selected again, the computeris given another another IP-address. Figure 7.2.10 shows the format of the DHCP-packet.

Figure 7.2.10 – Format of DHCP packet

Using DHCP, except their advantages has several disadvantages. First, there is the problem of matching of information address database in services of DHCP and DNS (Domain Name System). DNS is used to convertcharacter names in the IP-address, if IP-address will dynamically change by the DHCP server, so these changes also need to dynamically contribute to the databaseof DNS. Server's although the protocol is the dynamic interaction between the DNS and DHCP is implemented by some companies (so-called Service Dynamic DNS), a standard for it has not yet been adopted.

Secondly, the instability of the IP-addresses complicates network management. Management system based on the protocol SNMP, developed with the expectation on staticity of IP-addresses.

Thirdly, centralized address assignment procedure reduces the reliability of the system: the DHCP-server refusal, all of his clients are not able to obtain IP-address and other configuration information. The consequences of such refusal can be reduced through the use of multiple servers in a networkDHCP, each of which has its own pool of IP-addresses.

For working with DHCP protocol in the operating system «Windows", use the command ipconfig, which is used to display of all current settings of TCP / IP networkand update The DHCP and DNS parameters. When you call the ipconfig command, you can use a number of parameters such as:

- / all - complete with draw all of TCP / IP configuration for all adapters.Without this setting the ipconfig command displays only the IP-address, subnet mask and main gateway for each adapter.

- / renew - update of the DHCP configuration for all adapters. This option is available only on computers with adapters that are configured to
automatically obtain the IP-addresses.

Дата добавления: 2014-12-27