Unit 9 - Network Layer

Question 1

What is forwarding?

Answer 1

Moving packets from router’s input link to output link. It is an action local to routers

Question 2

What is routing?

Answer 2

Determines the route taken by packets from source to destination. It is a global action

Question 3

What is a forwarding table?

Answer 3

Inside the router, a forwarding table stores mappings from datagram header values to output links. The next route is determined by finding an entry in the table who’s header value matches the packet header value

Question 4

Name 2 services for individual datagrams

Answer 4

1. Guaranteed delivery

2. Guaranteed delivery with less than 40ms delay

Question 5

Name 3 services for a flow of datagrams

Answer 5

1. In-order datagram delivery
2. Guaranteed minimal bandwidth
3. Guaranteed maximal inter-packet timespacing

Question 6

What does ATM stand for?

Answer 6

Asynchronous Transfer Mode

Question 7

What does the Internet Network Architecture provide a guarantee on?

Answer 7

Nothing - not bandwidth, loss, order or timing

Question 8

What is the difference between a datagram network and a virtual circuit network? Give an example of what architecture uses each

Answer 8

A datagram network (e.g. Internet) provides connectionless services, virtual-circuit network (e.g. ATM - telephony) is connection-oriented.

Question 9

How is performance guaranteed in virtual circuit networks?

Answer 9

1. There are setups and teardowns of each call before data can flow
2. Every routers on source - destination path maintains a state for each passing connection
3. Link and router resources (e.g. bandwidth, buffers) may be allocated/dedicated
4. Each packet carries virtual-circuit (VC) identifier

Question 10

What 3 components make up a VC?

Answer 10

1. Path from source to destination
2. Series of VC numbers, one for each link along the path
3. Entries in forwarding tables in routers along path

Question 11

What happens to a forwarding table in each router when a VC network is torn down?

Answer 11

The routers erase the entries in their forwarding tables

Question 12

What is longest prefix matching?

Answer 12

When looking for forwarding table entry for given destination address, use the longest address prefix that matches destination address

Question 13

Given two destination address ranges and link interfaces:
1) 0001 1000 - interface 1 &
2) 0001 1*** - interface 0
Which interface do the following addresses belong in:
a) 0001 1110
b) 0001 1000

Answer 13

a) 2

b) 1