ccna 200-301 changes

ccna 200-301 changes

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The MTU of the path between two hosts is not necessarily a constant. It depends on the route chosen at the time. The routing is not necessarily symmetrical ( the route from A to B may be different from the route from B to A ), so the path MTU may not be consistent in both directions. Problems brought by RARP include the use of link-layer broadcasts, which prevents most routers from forwarding RARP requests and returns little information: only the system's IP address. In Chapter 16 , we will see that BOOTP will return more information when booting a diskless system: the IP address and the name of the boot host. The identification field uniquely identifies each datagram sent by the host. It is usually incremented by one each time a message is sent . In 11.5 discussed in section when it is re-fragmentation and reassembly in detail. Similarly, when discussing fragmentation, we will analyze the flag field and the slice offset field. RFC 791 [Postel 1981a] believes that the identification field should be selected by the upper layers that let IP send datagrams. Assuming there are two consecutive IP datagrams, one of which is generated by TCP and the other by UDP , then they may have the same identification field. Although this works as usual (by the reorganization algorithm), in most systems derived from Berkeley ,ccna 200-301 changes, the IP layer adds 1 to the value of a kernel variable for each IP datagram sent , regardless of the IP Which layer the data came from. The initial value of the kernel variable is set according to the time when the system boots. The TTL ( time-to-live ) time-to-live field sets the maximum number of routers that a datagram can pass through. It specifies the lifetime of the datagram. The initial value of TTL is set by the source host (usually 3 2 or 6 4 ). Once it passes a router that processes it, its value is subtracted from 1.. When the value of this field is 0 , the datagram is discarded and an ICMP message is sent to notify the source host. We will return to this field in Chapter 8 when we discuss the Traceroute program.

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