Chapter 12

12.1	Does broadcasting increase the amount of network traffic?
A:	Broadcasting by itself does not add to network traffic, but it adds extra host processing. Broadcasting can lead to additional network traffic if the receiving hosts incorrectly respond with errors such as ICMP port unreachables. Also, routers normally don't forward broadcast packets, whereas bridges normally do, so broadcasts on a bridged network can travel much farther than they would on a routed network.
12.2	Consider 50 hosts on an Ethernet: 20 running TCP/IP and 30 running some other protocol suite. How are broadcasts from one protocol suite handled by hosts running the other protocol suite?
A:	Every host receives a copy of every broadcast. The interface layer receives the frame, and passes it to the device driver. If the type field is for some other protocol, it is the device driver that discards the frame.
12.3	You login to a Unix system that you've never used before and want to find the subnet-directed broadcast address for all attached interfaces that support broadcasting. How can you do this?
A:	First execute `netstat -r` to see the routing table. This shows the names of all the interfaces. Then execute `ifconfig` (Section 3.8) for each interface: the flags tell if the interface supports broadcasting, and if so the broadcast address is also output.
12.4	If we `ping` the broadcast address with a large packet size , as in sun % ping 140.252.13.63 1472 PING 140.252.13.63: 1472 data bytes 1480 bytes from sun (140.252.13.33): icmp_seq=0. time=6. ms 1480 bytes from svr4 (140.252.13.34): icmp_seq=0. time=84. ms 1480 bytes from bsdi (140.252.13.35): icmp_seq=0. time=128. ms it works, but increasing the packet size by 1 byte gives us the following error: sun % ping 140.252.13.63 1473 PING 140.252.13.63: 1473 data bytes sendto: Message too long What's going on?
A:	Berkeley-derived implementations do not allow a broadcast datagram to be fragmented. When we specified the length of 1472 bytes, the resulting IP datagram was exactly 1500 bytes, the Ethernet MTU. Refusing to allow a broadcast datagram to be fragmented is a policy decision ”there is no technical reason (other than a desire to reduce the number of broadcast packets).
12.5	Redo Exercise 10.6 assuming the eight RIP messages are multicast instead of broadcast (assume RIP Version 2 is being used). What changes?
A:	Depending on the multicasting support in the various Ethernet interface cards in the 100 hosts, the multicast datagram can be ignored by the interface card, or discarded by the device driver.