One of the most widely used routing protocols in IP networks is the Routing Information Protocol(RIP)．RIP is the canonical example of a routing protocol built on the (71) algorithm．Routing protocols in internetworks differ slightly from the idealized graph model．In aninternetwork，the goal of the routers to forward packets to various (72) ．
Routers running RIP send their advertisement about cost every (73) seconds．A router also sends an update message whenever an update from another router causes it to change itsrouting table．
It is possible to use a range of different metrics or costs for the links in a routing protocol．RIP takes the simplest approach，with all link costs being equal (74) Thus it always tries tofind the minimum hop route．Valid distances are 1 through (75) ．This also limits RIP torunning on fairly small networks．
（73）处应填（）。
A．10
B．30
C．60
D．180
