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Unformatted text preview: o somehow detect the missing datagram and retransmit.
ICMP, IGMP, UDP, and TCP all use the same checksum algorithm, although TCP and UDP
include various fields from the IP header, in addition to their own header and data. RFC 1071
[Braden, Borman, and Partridge 1988] contains implementation techniques for computing the
Internet checksum. Since a router often changes only the TTL field (decrementing it by 1), a router
can incrementally update the checksum when it forwards a received datagram, instead of
calculating the checksum over the entire IP header again. RFC 1141 [Mallory and Kullberg 1990]
describes an efficient way to do this.
The standard BSD implementation, however, does not use this incremental update feature when forwarding a
datagram. Every IP datagram contains the source IP address and the destination IP address. These are the 32bit values that we described in Section 1.4.
The final field, the options, is a variable-length list of optional information for the datagram. The
options currently defined are:
q q security and handling restrictions (for military applications, refer to RFC 1108 [Kent 1991]
record route (have each router record its IP address. Section 7.3),
timestamp (have each router record its IP address and time. Section 7.4),
loose source routing (specifying a list of IP addresses that must be traversed by the
datagram. Section 8.5), and
strict source routing (similar to loose source routing but here only the addresses in the list
can be traversed. Section 8.5). These options are rarely used and not all host and routers support all the options.
The options field always ends on a 32-bit boundary. Pad bytes with a value of 0 are added if
necessary. This assures that the IP header is always a multiple of 32 bits (as required for the header
length field). 3.3 IP Routing
Conceptually, IP routing is simple, especially for a host. If the destination is directly connected to
the host (e.g., a point-to-point link) or on a shared network (e.g., Ethernet or token ring), then the
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12