datagram. The protocol module assembling fragments judges fragments to belong to the same datagram if they have the same source, destination, protocol, and Identifier. Thus, the sender must choose the Identifier to be unique for this source, destination pair and protocol for the time the datagram (or any fragment of it) could be alive in the internet.
It seems then that a sending protocol module needs to keep a table of Identifiers, one entry for each destination it has communicated with in the last maximum packet lifetime for the internet.
However, since the Identifier field allows 65,536 different values, some host may be able to simply use unique identifiers independent of destination.
It is appropriate for some higher level protocols to choose the identifier. For example, TCP protocol modules may retransmit an identical TCP segment, and the probability for correct reception would be enhanced if the retransmission carried the same identifier as the original transmission since fragments of either datagram could be used to construct a correct TCP segment.
Type of Service
The type of service (TOS) is for internet service quality selection. The type of service is specified along the abstract parameters precedence, delay, throughput, and reliability. These abstract parameters are to be mapped into the actual service parameters of the particular networks the datagram traverses.
Precedence. An independent measure of the importance of this datagram.
Delay. Prompt delivery is important for datagrams with this indication.
Throughput. High data rate is important for datagrams with this indication.
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September 1981 Internet Protocol Specification
Reliability. A higher level of effort to ensure delivery is important for datagrams with this indication.
For example, the ARPANET has a priority bit, and a choice between "standard" messages (type 0) and "uncontrolled" messages (type 3), (the choice between single packet and multipacket messages can also be considered a service parameter). The uncontrolled messages tend to be less reliably delivered and suffer less delay. Suppose an internet datagram is to be sent through the ARPANET. Let the internet type of service be given as:
In this example, the mapping of these parameters to those available for the ARPANET would be to set the ARPANET priority bit on since the Internet precedence is in the upper half of its range, to select standard messages since the throughput and reliability requirements are indicated and delay is not. More details are given on service mappings in "Service Mappings" [8].
Time to Live
The time to live is set by the sender to the maximum time the datagram is allowed to be in the internet system. If the datagram
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