Chapter 6

6.1	At the end of Section 6.2 we listed five special conditions under which an ICMP error message is not sent. What would happen if these five conditions weren't followed and we sent a broadcast UDP datagram to an unlikely port on the local cable?
A:	If there were one hundred hosts on the local cable, each could try to send an ICMP port unreachable at about the same time. Many of these transmissions could lead to collisions (if an Ethernet is being used), which can render the network useless for a second or two.
6.2	Read the Host Requirements RFC [Braden 1989a] to see if the generation of an ICMP port unreachable is a "must", "should", or "may". What section and page is this found on?
A:	It is a "should."
6.3	Read RFC 1349 [Almquist 1992] to see how the IP type-of-service field (Figure 3.2) should be set by ICMP.
A:	An ICMP error is always sent with a TOS of 0, as we indicated in Figure 3.2. An ICMP query request can be sent with any TOS, and the corresponding reply should be sent with the same TOS.
6.4	If your system provides the `netstat` command, use it to see what types of ICMP messages are received and sent.
A:	`netstat -s` is the common way to see the per-protocol statistics. On a SunOS 4.1.1 host ( `gemini` ) that has received 48 million IP datagrams, the ICMP statistics are: Output histogram: echo reply: 1757 destination unreachable: 700 time stamp reply: 1 Input histogram: echo reply: 211 destination unreachable: 3071 source quench: 249 routing redirect: 2789 echo: 1757 #10: 21 time exceeded: 56 time stamp: 1 The 21 input messages of type 10 are router solicitations that SunOS 4.1.1 doesn't support. SNMP can also be used (Figure 25.26) and some systems, such as Solaris 2.2, generate `netstat -s` output that uses SNMP variable names .