Avaya Configuring IP Services Bedienungsanleitung Seite 14

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Configuring IP Services
1-2
IP routers process each datagram individually. The datagram header provides
the router with the destination IP address, as well as other routing
information. Routers select a transmission path based on the IP address of the
destination network, not of the destination host.
IP Datagrams
An IP datagram is the unit of data exchanged between IP modules. In addition to
data, a datagram includes a header with fields that provide the following
information used by IP routers:
Type of Service
This field indicates the quality of service the datagram requires. The IP router
inspects the Type of Service field to obtain information about the datagram’s
precedence and expected delay characteristics.
Time to Live
This field determines the datagram’s lifetime in the Internet system. Each time
an IP router processes the datagram header, it decrements the value in the
Time to Live field by at least one hop. When the value reaches zero, the IP
router discards the datagram, unless it is destined for the router itself, thus
preventing undeliverable datagrams from looping endlessly through the
network, consuming Internet resources.
Options
This field may or may not be present in a datagram; therefore, IP datagrams
vary in length. There are three classes of options:
Security, which specifies security level and distribution restrictions
Timestamps, which is a 32-bit value measured in milliseconds since
midnight universal time, or any other value if the high-order bit is set to 1
Special Routing, which specifies host-discovered paths to other hosts, or a
specific path for the datagram to take
Header Checksum
This field contains a value that the IP router calculates each time it processes a
datagram’s IP header. The algorithm used to calculate the checksum value is a
16-bit ones complement addition of the 16-bit words contained only within
the IP header. The IP router discards datagrams received with an incorrect IP
header checksum.
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