1. A network-layer packet is a datagram. A router forwards a packet based on the packet’s IP (layer 3) address. A link-layer switch forwards a packet based on the packet’s MAC (layer 2) address.
2. Datagram-based network layer: forwarding; routing. Additional function of VC-based network layer: call setup.
3. Forwarding is about moving a packet from a router’s input link to the appropriate output link. Routing is about determining the end-to-routes between sources and destinations.
4. Yes, both use forwarding tables. For descriptions of the tables, see Section 4.2.
5. Single packet: guaranteed delivery; guaranteed delivery with guaranteed maximum delay. Flow of packets: in-order packet delivery; guaranteed minimal bandwidth;
guaranteed maximum jitter. None of these services is provided by the Internet’s network layer. ATM’s CBR service provides both guaranteed delivery and timing.
ABR does not provide any of these services.
6. Interactive live multimedia applications, such as IP telephony and video conference, could benefit from ATM CBR’s service, which maintains timing.
7. With the shadow copy, the forwarding decision is made locally, at each input port, without invoking the centralized routing processor. Such decentralized forwarding avoids creating a forwarding processing bottleneck at a single point within the router.
8. Switching via memory; switching via a bus; switching via an interconnection network
9. Packet loss occurs if queue size at the input port grows large because of slow switching fabric speed and thus exhausting router’s buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the input line speed, where n is the number of input ports.
10. Packet loss can occur if the queue size at the output port grows large because of slow outgoing line-speed.
11. HOL blocking – a queued packet in an input queue must wait for transfer through the fabric because it is blocked by another packet at the head of the line. It occurs at the input port.
12. Yes. They have one address for each interface.
13. 11011111 00000001 00000011 00011100
14. Students will get different correct answers for this question.
15. 8 interfaces; 3 forwarding tables 16. 50% overhead
17. The 8-bit protocol field in the IP datagram contains information about which transport layer protocol the destination host should pass the segment to.
18. Typically the wireless router includes a DHCP server. DHCP is used to assign IP addresses to the 5 PCs and to the router interface. Yes, the wireless router also uses NAT as it obtains only one IP address from the ISP.
19. See Section 4.4.4
20. Yes, because the entire IPv6 datagram (including header fields) is encapsulated in an IPv4 datagram
21. Link state algorithms: Computes the least-cost path between source and destination using complete, global knowledge about the network. Distance-vector routing: The calculation of the least-cost path is carried out in an iterative, distributed manner.
A node only knows the neighbor to which it should forward a packet in order to reach given destination along the least-cost path, and the cost of that path from itself to the destination.
22. Routers are aggregated into autonomous systems (ASs). Within an AS, all routers run the same intra-AS routing protocol. Special gateway routers in the various ASs run the inter-autonomous system routing protocol that determines the routing paths among the ASs. The problem of scale is solved since an intra-AS router need only know about routers within its AS and the gateway router(s) in its AS.
23. No. Each AS has administrative autonomy for routing within an AS.
24. No. The advertisement tells D that it can get to z in 11 hops by way of A.
However, D can already get to z by way of B in 7 hops. Therefore, there is no need to modify the entry for z in the table. If, on the other hand, the advertisement said that A were only 4 hops away from z by way of C, then D would indeed modify its forwarding table.
25. With OSPF, a router periodically broadcasts routing information to all other routers in the AS, not just to its neighboring routers. This routing information sent by a router has one entry for each of the router’s neighbors; the entry gives the distance from the router to the neighbor. A RIP advertisement sent by a router
contains information about all the networks in the AS, although this information is only sent to its neighboring routers.
26. “sequence of ASs on the routes”
27. See “Principles in Practice” on page 401
28. ISP C can use the BGP Multi-Exit Descriptor to suggest to ISP B that the preferred route to ISP D is through the east coast peering point. For example, the east coast BGP router in ISP C can advertise a route to D with an MED value of 5. The west coast router in ISP C can advertise a route to D with an MED value of 10. Since a lower value is preferred, ISP B knows that ISP C wants to receive traffic on the east coast. In practice, a router can ignore the MED value, and so ISP B can still use hot potato routing to pass traffic to ISP C destined to ISP D via the west coast peering point.
29. A subnet is a portion of a larger network; a subnet does not contain a router; its boundaries are defined by the router and host interfaces. A prefix is the network portion of a CDIRized address; it is written in the form a.b.c.d/x ; A prefix covers one or more subnets. When a router advertises a prefix across a BGP session, it includes with the prefix a number of BGP attributes. In BGP jargon, a prefix along with its attributes is a BGP route (or simply a route).
30. Routers use the AS-PATH attribute to detect and prevent looping advertisements;
they also use it in choosing among multiple paths to the same prefix. The NEXT-HOP attribute indicates the IP address of the first router along an advertised path (outside of the AS receiving the advertisement) to a given prefix. When configuring its forwarding table, a router uses the NEXT-HOP attribute.
31. A tier-1 ISP B may not to carry transit traffic between two other tier-1 ISPs, say A and C, with which B has peering agreements. To implement this policy, ISP B would not advertise to A routes that pass through C; and would not advertise to C routes that pass through A.
32. N-way unicast has a number of drawbacks, including:
• Efficiency: multiple copies of the same packet are sent over the same link for potentially many links; source must generate multiple copies of same packet
• Addressing: the source must discover the address of all the recipients
33. a) uncontrolled flooding: T; controlled flooding: T; spanning-tree: F b) uncontrolled flooding: T; controlled flooding: F; spanning-tree: F 34. False
35. IGMP is a protocol run only between the host and its first-hop multicast router.
IGMP allows a host to specify (to the first-hop multicast router) the multicast group it wants to join. It is then up to the multicast router to work with other multicast routers (i.e., run a multicast routing protocol) to ensure that the data for the host-joined multicast group is routed to the appropriate last-hop router and from there to the host.
36. In a group-shared tree, all senders send their multicast traffic using the same routing tree. With source-based tree, the multicast datagrams from a given source are routed over s specific routing tree constructed for that source; thus each source may have a different source-based tree and a router may have to keep track of several source-based trees for a given multicast group.