This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ne of ASCII
characters. We can test this service using the telnet command:
sun % telnet bsdi daytime
Trying 184.108.40.206 ...
Connected to bsdi.
Escape character is "^]"
Wed Feb 3 16:38:33 1993
Connection closed by
foreign host. first three lines output are from the Telnet
here's the daytime service output
this is also from the Telnet client The time server, on the other hand, returns a 32-bit binary value with the number of
seconds since midnight January 1, 1900, UTC. While this provides the date, the time
value is in units of a second. (The rdate command that we mentioned earlier uses
the TCP time service.)
2. Serious timekeepers use the Network Time Protocol (NTP) described in RFC 1305
[Mills 1992]. This protocol uses sophisticated techniques to maintain the clocks for a
group of systems on a LAN or WAN to within millisecond accuracy. Anyone
interested in precise timekeeping on computers should read this RFC.
3. The Open Software Foundation's (OSF) Distributed Computing Environment (DCE)
defines a Distributed Time Service (DTS) that also provides clock synchronization
between computers. [Rosenberg, Kenney, and Fisher 1992] provide additional
details on this service. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (9 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol 4. Berkeley Unix systems provide the daemon timed(8) to synchronize the clocks of
systems on a local area network. Unlike NTP and DTS, timed does not work across
wide area networks. 6.5 ICMP Port Unreachable Error
The last two sections looked at ICMP query messages-the address mask and time-stamp
queries and replies. We'll now examine an ICMP error message, the port unreachable
message, a subcode of the ICMP destination unreachable message, to see the additional
information returned in an ICMP error message. We'll watch this using UDP (Chapter 11).
One rule of UDP is that if it receives a UDP datagram and the destination port does not
correspond to a port that some process has in use, UDP responds with an ICMP port
View Full Document
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