Chapter 4 The Network Layer - School of Engineering & Applied ...
Chapter 4 The Network Layer
Segmentation & Reassembly:
1. MTU: Maximum Transmission Unit, the longest IP datagram that can be supported by the
underlying physical network.
2. The three fields relevant to segmentation and reassembly: Identification number; Flags;
Offset
3. Example: Consider sending a 2400 byte datagram into a link that has an MTU of 700 bytes.
Suppose the original datagram is stamped with the identification number 422. How many
fragments are generated? What are the values in the various fields in the IP datagrams
generated related to fragmentation?
The datagram is fragmented into 4 fragments.
Length
Identi
First fragment:
700
422
Second:
700
422
Third:
700
422
Forth:
360
422
Flags
1
1
1
1
Offset
0
85
170
255
IPv4 Addressing: CIDR ¨C Classless Interdomain Routing
1. Consider a datagram network using 32-bit host addresses. Suppose a router has four links,
numbered 0 through 3, and packets are to be forwarded to the link interfaces as follows:
Destination Address Range
Link Interface
11100000 00000000 00000000 00000000
through
11100000 00111111 11111111 11111111
0
11100000 01000000 00000000 00000000
Through
11100000 01000000 11111111 11111111
1
11100000 01000001 00000000 00000000
Through
11100001 011111111 11111111 11111111
2
Otherwise
3
a. Provide a forwarding table that has four entries, uses longest prefix matching, and
forwards packets to the correct link interfaces
b. Describe how your forwarding table determines the appropriate link interface for
datagrams with destination addresses:
11001000 10010001 01010001 01010101
11100001 01000000 11000011 00111100
11100001 10000000 00010001 01110111
2. Consider a datagram network using 8-bit host addresses. Suppose a router uses longest
prefix matching and has the following forwarding table:
Prefix Match
00
010
011
10
11
Interface
0
1
2
2
3
For each of the four interfaces, give the associated range of destination host addresses and
the number of addresses in the range.
3. Consider a datagram network using 8-bit host addresses. Suppose a router uses longest
prefix matching and has the following forwarding table:
Prefix Match
1
10
111
otherwise
Interface
0
1
2
3
For each of the four interfaces, give the associated range of destination host addresses and
the number of addresses in the range.
4. Suppose a TCP message that contains 2048 bytes of data and 20 bytes of TCP header is
passed to IP for delivery across two networks of the Internet (i.e. from the source host to a
router to the destination host). The first network has an MTU of 1024B, and the second has
an MTU of 512 bytes. Give the sizes and offsets of the sequence of fragments delivered to
the network layer at the destination host, Assume all IP headers are 20 bytes.
5. What is the maximum bandwidth at which an IP host can send 576-byte packets without
having the Ident field wrap around within 60 seconds? Suppose IP¡¯s maximum segment
lifetime (MSL) is 60 seconds, that is, delayed packets can arrive up to 60 seconds late but no
later. What might happen if this bandwidth were exceeded?
[The Ident field is 16 bits, so we can send 576 ¡Á 216 bytes per 60 seconds, or about 5Mbps. If we send more
than this, then fragments of one packet could conceivably have the same Ident value as fragments of another
packet.]
6. Suppose a router has built up the routing table shown below.
SubnetNumber
SubnetMask
NextHop
128.96.39.0
255.255.255.128
Interface 0
128.96.39.128
255.255.255.128
Interface 1
128.96.40.0
255.255.255.128
R2
192.4.153.0
255.255.255.192
R3
R4
Describe what the router does with a packet addressed to each of the following destinations:
(a)
(b)
(c)
(d)
(e)
128.96.39.10
128.96.40.12
128.96.40.151
192.4.153.17
192.4.153.90
? Interface 0
? R2
? R4
? R3
? R4
7. The following table is a routing table using CIDR. Address bytes are in hexadecimal.
Net/MaskLength
NextHop
C4.50.0.0/12
A
C4.5E.10.0/20
B
C4.60.0.0/12
C
C4.68.0.0/14
D
80.0.0.0/1
E
40.0.0.0/2
F
00.0.0.0/2
G
State to what next hop the following will be delivered:
(a) C4.5E.13.87
?B
(b) C4.5E.22.09
?A
(c) C3.41.80.02
?E
(d) 5E.43.91.12
?F
(e) C4.6D.31.2E
?C
(f) C4.6B.31.2E
?D
8. Consider the network shown below.
(a) How many routes to P could provider Q¡¯s BGP speakers receive?
(b) Suppose Q and P adopt the policy that outbound traffic is routed to the closest link to the
destination¡¯s provider, thus minimizing their own cost. What paths will traffic from host
A to host B and from host B to host A take?
(c) What could Q do to have the B to A traffic use the closer link 1?
(d) What could Q do to have the B to A traffic pass through R?
(a) Q will receive three routes to P, along links 1, 2, and 3.
(b) A?¡úB traffic will take link 1. B?¡úA traffic will take link 2. Note that this strategy minimizes cost to the
source of the traffic.
(c) To have B?¡úA traffic take link 1, Q could simply be configured to prefer link 1 in all cases. The only
general solution, though, is for Q to accept into its routing tables some of the internal structure of P, so that Q
for example knows where A is relative to links 1 and 2.
(d) If Q were configured to prefer AS paths through R, or to avoid AS paths involving links 1 and 2, then Q
might route to P via R.
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