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1
Answers of Exercise 10
1. 100 byte data is sent using IP across an
Ethernet. Before sent, the data will be first
formed an IP datagram and then the datagram will
be encapsulated into an Ethernet Frame.
Calculate the percentage of headers in sending
the 100 byte data. Assume no optional field
in both IP and frame header. Answer As
Ethernet frame header length is 26 bytes and IP
datagram header is 20 bytes,
total length 2620100146 bytes. Header
percentage46/14631.5. 2. Suppose a file of 20
Kbytes to be sent from host H1 to host H2 across
three networks as shown in the following
figure. How many IP datagrams will be sent from
H1? And how many IP datagrams will be
received by H2? Assume no datagram loss,
duplication and disorder during the
transmissions. Answer As
20k20x1024204804x44641x2624 thus,
number of IP datagram sent from H1 is 5 (four
4464 datagrams and one 2624 datagram)
Further, 44642x15001x1464 and
26241x15001x1124 Therefore, number of IP
datagram received by H2 is 4x3214. Note
that there is no need of sub-fragmentation and
assembly in R2.
Toking Ring MTU4464
Ethernet MTU1500
FDDI MTU4352
H1
R1
R2
H2
2
Answers of Exercise 10
3. Host A sends a message to host B and never
receive reply from B. However, host receives
an ICMP message with a header in hexadecimal
format as the follows 03 01 1A C8 31 00
B7 Give possible reasons that A does not
receive reply from B. Answer In the above
ICMP message, type03 shows destination
unreachable error,
further, the code01 shows host unreachable
error. Therefore, the reasons is the
host has hardware failure, is not connected
to network, or powered down. 4. Explain how
traceroute utility works. Use the utility in a
Windows OS environment to probe the Internet
organization web server. The command is tracert
www.ietf.org . How many routes have been
passed when your packet travel to the web server?
Which one is the slowest? Answer Each IP
datagram has a TTL field which specifies the
number of routers the packet can pass. If
TTL0, a router will not forward the packet,
discard it and send a ICMP message to the
host. The traceroute utility uses the TTL and
ICMP error report mechanism to detect
routers in route. The traceroute application sets
TTL0, 1, 2, , n, and shows routers as
well as time according to corresponding ICMP
messages. Note that the information
displayed is usually different when using
traceroute utility in different sites,
computers and time. When I tested it at 100PM in
June 21, 2000, the number of routers
between my computers to the web server at
www.ietf.org is 21, and the slowest router
is at 206.204.271.97 with name hevacis-01-pos3-0-0
.conxion.net. 5. Summarize main features of IPv6
as compared with IPv4. Answer (1) Large
address size - 128bits (2) Extensible functions
because of extensible header (3) Support for
resource allocation (QoS) - audio and video
applications (4) Support for security.