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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