IP Services

When we talk about IP services, we refer to the applications, protocols, and services that not only use IP themselves, but also provide data to be used by other IP protocols.
Nội dung trích xuất từ tài liệu:
IP ServicesIP ServicesWhen we talk about IP services, we refer to the applications, protocols, and services thatnot only use IP themselves, but also provide data to be used by other IP protocols. TheseIP services typically allow for IP communications between hosts by performing functionssuch as physical address resolution, automatic address assignment, name resolution, andtime synchronization.Address Resolution Protocol (ARP) is an IP service that was developed to map physicaladdresses to logical addresses. This resolution is required because hosts need to be able tophysically address the data frames at Layer 2 to the appropriate destination. In mostcases, the hosts only know each other by IP address. Consequently, ARP functions bybroadcasting on the local subnet for the host that has the IP address that needs to beresolved. Because it is a broadcast, all hosts will process the packet, but only the host thatactually has that IP address will respond with its MAC address, thereby allowing theoriginal source to properly address the packet both logically by IP address and physicallyby MAC address.DHCP exists to provide unconfigured hosts on a network with the appropriate IPaddressing information necessary to enable them to communicate on the network.Because a host that is a DHCP client does not have an IP address to use, DHCP usesbroadcasts between the DHCP client and the DHCP server. When a DHCP client isturned on, it issues a broadcast known as a DHCP Discover packet. The objective of thisbroadcast is to discover whether there is a DHCP server on the network. If there is, theDHCP server responds with a broadcast known as a DHCP Offer packet, because theclient still has no IP address. Because the packet is a broadcast, the client still processes iteven though the client has no configured IP address. If the network has multiple DHCPservers, they all respond with a DHCP Offer, and the client accepts the first offer itreceives. The client then broadcasts a packet known as a DHCP Request packet. Thisserves two purposes. First, it notifies the selected DHCP server that the client has electedto accept the DHCP Offer. Second, it notifies all other DHCP servers that the clientselected another DHCP server, allowing them to place that offered IP address back in thepool of IP addresses they can offer to clients. Finally, the DHCP server responds with afinal broadcast known a DHCP Acknowledgement packet, which notifies the client that itcan use the IP address that it was offered and provides additional configurationinformation such as subnet mask, default gateway, name servers, and so on. At this point,the client configures itself accordingly and can begin engaging in full TCP/IPcommunications.DNS provides for the resolution of host names and fully qualified domain names to IPaddresses. The reason for this is simple. Most humans do not know the IP address of theremote hosts that they are trying to communicate with. They know the name, such aswww.cisco.com. Unfortunately, the network hosts require the IP address of the remotehost to communicate with that host. When an IP host receives a request to communicatewith a remote host by name, the host automatically and transparently contacts a DNSserver to resolve the name to an IP address, which if successful allows the two hosts tocommunicate with each other. A detailed discussion of DNS is beyond the scope of thisbook, but an excellent resource for understanding DNS is DNS and BIND, FourthEdition, by Paul Albitz and Cricket Liu.Network Time Protocol (NTP) is used to synchronize the time across hosts over thenetwork. Time synchronization is critical for applications such as Windowsauthentication, Kerberos, IPsec, as well as event logging. NTP functions over UDP port123 and allows clients to synchronize their time with a master time server, known as astratum 1 or stratum 2 server.