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Questions and Answers of Computer Networking

In the source routing example, the address received by B is not reversible and does not help B know how to reach A. Propose a modification to the delivery mechanism that does allow for reversibility.
Propose a mechanism that virtual circuit switches might use so that if one switch loses all its state regarding connections, then a sender of packets along a path through that switch is informed of
Propose a mechanism that might be used by datagram switches so that if one switch loses all or part of its forwarding table, affected senders are informed of the failure.
The virtual circuit mechanism assumes that each link is point-to-point. Extend the forwarding algorithm to work in the case that links are shared-media connections, for example, Ethernet.
Suppose, in Figure 3.2, that a new link has been added, connecting switch 3 port 1 (where G is now) and switch 1 port 0 (where D is now);neither switch is “informed” of this link. Furthermore,
Give an example of a working virtual circuit whose path traverses some link twice. Packets sent along this path should not, however, circulate indefinitely.
Each switch chose the VCI value for the incoming link. Show that it is also possible for each switch to choose the VCI value for the outbound link and that the same VCI values will be chosen by each
Given the extended LAN shown in Figure 3.46, indicate which ports are not selected by the spanning tree algorithm.Figure 3.46) E G A B1 1 B7 B4 D B2 (B3) B6 (B5 F H J B
Given the extended LAN shown in Figure 3.46, assume that bridge B1 suffers catastrophic failure. Indicate which ports are not selected by the spanning tree algorithm after the recovery process and a
Consider the arrangement of learning bridges shown in Figure3.47.Assuming all are initially empty, give the forwarding tables for each of the bridges B1–B4 after the following transmissions:■ A
As in the previous problem, consider the arrangement of learning bridges shown in Figure 3.47. Assuming all are initially empty, give the forwarding tables for each of the bridges B1–B4 after the
Consider hosts X, Y, Z, W and learning bridges B1, B2, B3, with initially empty forwarding tables, as in Figure 3.48.(a) Suppose X sends to Z. Which bridges learn where X is? Does Y’s network
Give the spanning tree generated for the extended LAN shown in Figure 3.49, and discuss how any ties are resolved.Figure 3.49) B2 B1 B3
Suppose two learning bridges B1 and B2 form a loop as shown in Figure 3.50 and do not implement the spanning tree algorithm. Each bridge maintains a single table of address, interface pairs.(a) What
Suppose that M in Figure 3.50 sends to itself (this normally would never happen). State what would happen, assuming:(a) the bridges’ learning algorithm is to install (or update) the new
Consider the extended LAN of Figure 3.10. What happens in the spanning tree algorithm if switch S1 does not participate and(a) simply forwards all spanning tree algorithm messages?(b) drops all
Suppose some repeaters (hubs), rather than bridges, are connected into a loop.(a) What will happen when somebody transmits?(b) Why would the spanning tree mechanism be difficult or impossible to
Suppose a bridge has two of its ports on the same network. How might the bridge detect and correct this?
What percentage of an ATM link’s total bandwidth is consumed by the ATM cell headers? Ignore padding to fill cells.
Cell switching methods (like ATM) essentially always use virtual circuit routing rather than datagram routing. Give a specific argument why this is so.
Suppose a workstation has an I/O bus speed of 800 Mbps and a memory bandwidth of 2 Gbps. Assuming DMA in and out of main memory, how many interfaces to 100-Mbps Ethernet links could a switch based on
Suppose a workstation has an I/O bus speed of 1 Gbps and a memory bandwidth of 2 Gbps. Assuming DMA in and out of main memory, how many interfaces to 100 Mbps Ethernet links could a switch based on
Suppose a switch can forward packets at a rate of 500,000 per second, regardless (within limits) of size. Assuming the workstation parameters described in the previous problem, at what packet size
Suppose that a switch is designed to have both input and output FIFO buffering. As packets arrive on an input port, they are inserted at the tail of the FIFO. The switch then tries to forward the
Suppose a 10-Mbps Ethernet hub (repeater) is replaced by a 10-Mbps switch, in an environment where all traffic is between a single server and N “clients.” Because all traffic must still traverse
What aspect of IP addresses makes it necessary to have one address per network interface rather than just one per host? In light of your answer, why does IP tolerate point-to-point interfaces that
Why does the Offset field in the IP header measure the offset in 8-byte units? (Hint: Recall that the Offset field is 13 bits long.)
Some signaling errors can cause entire ranges of bits in a packet to be overwritten by all 0s or all 1s. Suppose all the bits in the packet including the Internet checksum are overwritten. Could a
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
Path MTU is the smallest MTU of any link on the current path (route)between two hosts. Assume we could discover the Path MTU of the path used in the previous exercise and that we use this value as
Suppose an IP packet is fragmented into 10 fragments, each with a 1% (independent) probability of loss. To a reasonable approximation, this means there is a 10% chance of losing the whole packet due
Suppose the fragments of Figure 3.19(b) all pass through another router onto a link with an MTU of 380 bytes, not counting the link header. Show the fragments produced. If the packet were originally
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;
Why do you think IPv4 has fragment reassembly done at the endpoint rather than at the next router? Why do you think IPv6 abandoned fragmentation entirely? Hint: Think about the differences between
Having ARP table entries time out after 10–15 minutes is an attempt at a reasonable compromise. Describe the problems that can occur if the timeout value is too small or too large.
IP currently uses 32-bit addresses. If we could redesign IP to use the 6-byte MAC address instead of the 32-bit address, would we be able to eliminate the need for ARP? Explain why or why not.
Suppose hosts A and B have been assigned the same IP address on the same Ethernet, on which ARP is used. B starts up after A.What will happen to A’s existing connections? Explain how
Suppose an IP implementation adheres literally to the following algorithm on receipt of a packet, P, destined for IP address D:(a) If the IP layer receives a burst of packets destined for D, how
For the network given in Figure 3.51, give global distance-vector tables like those of Tables 3.12 and 3.15 when:(a) each node knows only the distances to its immediate neighbors.(b) each node has
For the network given in Figure 3.52, give global distance-vector tables like those of Tables 3.12 and 3.15 when:(a) each node knows only the distances to its immediate neighbors.(b) each node has
For the network given in Figure 3.51, show how the link-state algorithm builds the routing table for node D.Figure 3.51) D A 8 3 B 2 2 1 E 6 F LL
Use the Unix utility traceroute (Windows tracert) to determine how many hops it is from your host to other hosts in the Internet(e.g., cs.princeton.edu or www.cisco.com). How many routers do you
What will happen if traceroute is used to find the path to an unassigned address? Does it matter if the network portion or only the host portion is unassigned?
A site is shown in Figure 3.53. R1 and R2 are routers; R2 connects to the outside world. Individual LANs are Ethernets. RB is a bridge router; it routes traffic addressed to it and acts as a bridge
Suppose we have the forwarding tables shown in Table 3.18 for nodes A and F in a network where all links have cost 1.Give a diagram of the smallest network consistent with these tables. Table 3.18)
Suppose we have the forwarding tables shown in Table 3.19 for nodes A and F in a network where all links have cost 1.Give a diagram of the smallest network consistent with these tables.Table 3.19)
For the network in Figure 3.51, suppose the forwarding tables are all established as in Exercise 44 and then the C–E link fails. Give:(a) the tables of A, B, D, and F after C and E have reported
Suppose a router has built up the routing table shown in Table 3.20.The router can deliver packets directly over interfaces 0 and 1, or it can forward packets to router R2, R3, or R4. Describe what
Suppose a router has built up the routing table shown in Table 3.21.The router can deliver packets directly over interfaces 0 and 1, or it can forward packets to router R2, R3, or R4. Assume the
Consider the simple network in Figure 3.54, in which A and B exchange distance-vector routing information. All links have cost 1.Suppose the A–E link fails.(a) Give a sequence of routing table
Consider the situation involving the creation of a routing loop in the network of Figure 3.30 when the A–E link goes down. List all sequences of table updates among A, B, and C, pertaining to
Suppose a set of routers all use the split-horizon technique; we consider here under what circumstances it makes a difference if they use poison reverse in addition.(a) Show that poison reverse makes
Hold down is another distance-vector loop avoidance technique, whereby hosts ignore updates for a period of time until link failure news has had a chance to propagate. Consider the networks in Figure
Consider the network in Figure 3.56, using link-state routing. Suppose the B–F link fails, and the following then occur in sequence:(a) Node H is added to the right side with a connection to G.(b)
Give the steps as in Table 3.16 in the forward search algorithm as it builds the routing database for node A in the network shown in Figure 3.57.Table 3.16)Figure 3.57) Table 3.16 Steps for
Give the steps as in Table 3.16 in the forward search algorithm as it builds the routing database for node A in the network shown in Figure 3.58.Table 3.16)Figure 3.58) Table 3.16 Steps for
Suppose that nodes in the network shown in Figure 3.59 participate in link-state routing, and C receives contradictory LSPs: one from A arrives claiming the A–B link is down, but one from B arrives
Suppose IP routers learned about IP networks and subnets the way Ethernet learning bridges learn about hosts: by noting the appearance of new ones and the interface by which they arrive. Compare this
IP hosts that are not designated routers are required to drop packets misaddressed to them, even if they would otherwise be able to forward them correctly. In the absence of this requirement, what
Read the man page or other documentation for the Unix/Windows utility netstat. Use netstat to display the current IP routing table on your host. Explain the purpose of each entry. What is the
An organization has been assigned the prefix 200.1.1/24 (a class C)and wants to form subnets for four departments, with hosts as follows:There are 145 hosts in all.(a) Give a possible arrangement of
Suppose hosts A and B are on an Ethernet LAN with IP network address 200.0.0/24. It is desired to attach a host C to the network via a direct connection to B (see Figure 3.60). Explain how to do this
An alternative method for connecting host C in Exercise 67 is to use proxy ARP and routing: B agrees to route traffic to and from C, and also answers ARP queries for C received over the Ethernet.(a)
Suppose two subnets share the same physical LAN; hosts on each subnet will see the other subnet’s broadcast packets.(a) How will DHCP fare if two servers, one for each subnet, coexist on the shared
Table 3.22 is a routing table using CIDR. Address bytes are in hexadecimal. The notation “/12” in C4.50.0.0/12 denotes a netmask with 12 leading 1 bits, that is, FF.F0.0.0. Note that the last
Table 3.23 is a routing table using CIDR. Address bytes are in hexadecimal. The notation “/12” in C4.50.0.0/12 denotes a netmask with 12 leading 1 bits, that is, FF.F0.0.0. State to what next hop
An ISP that has authority to assign addresses from a /16 prefix(an old class B address) is working with a new company to allocate it a portion of address space based on CIDR. The new company needs IP
Consider the network shown in Figure 4.29, in which horizontal lines represent transit providers and numbered vertical lines are interprovider links.(a) How many routes to P could provider Q’s BGP
Give an example of an arrangement of routers grouped into autonomous systems so that the path with the fewest hops from a point A to another point B crosses the same AS twice. Explain what BGP would
Let A be the number of autonomous systems on the Internet and let D (for diameter) be the maximum AS path length.(a) Give a connectivity model for which D is of order logA and another for which D is
Propose a plausible addressing plan for IPv6 that runs out of bits.Specifically, provide a diagram such as Figure 4.11, perhaps with additional ID fields, that adds up to more than 128 bits, together
Suppose P, Q, and R are network service providers, with respective CIDR address allocations C1.0.0.0/8, C2.0.0.0/8, and C3.0.0.0/8. Each provider’s customers initially receive address allocations
In the previous problem, assume each provider connects to both others. Suppose customer PA switches to provider Q and customer QB switches to provider R. Use the CIDR longest match rule to give
Suppose most of the Internet used some form of geographical addressing but that a large international organization has a single IP network address and routes its internal traffic over its own
The telephone system uses geographical addressing. Why do you think this was not adopted as a matter of course by the Internet?
Suppose a site A is multihomed, in that it has two Internet connections from two different providers, P and Q. Provider-based addressing as in Exercise 5 is used, and A takes its address assignment
Suppose a network N within a larger organization A acquires its own direct connection to an Internet service provider, in addition to an existing connection via A. Let R1 be the router connecting N
How do routers determine that an incoming IP packet is to be multicast?
Suppose a multicast group is intended to be private to a particular routing domain. Can an IP multicast address be assigned to the group without consulting with other domains with no risk of
Under what conditions could a nonrouter host on an Ethernet receive a IP multicast packet for a multicast group it has not joined?
Consider the example internet shown in Figure 4.30, in which sources D and E send packets to multicast group G, whose members are shaded in gray. Show the shortest path multicast trees for each
Consider the example internet shown in Figure 4.31, in which sources S1 and S2 send packets to multicast group G, whose members are shaded in gray. Show the shortest-path multicast trees for each
Suppose host A is sending to a multicast group; the recipients are leaf nodes of a tree rooted at A with depth N and with each nonleaf node having k children; there are thus kN recipients.(a) How
The existing Internet depends in many respects on participants being good “network citizens”—cooperating above and beyond adherence to standard protocols.(a) In the PIM-SM scheme, who
(a) Draw an example internetwork where the BIDIR-PIM route from a source’s router to a group member’s router is longer than the PIM-SM source-specific route.(b) Draw an example where they are the
Determine whether or not the following IPv6 address notations are correct:(a) ::0F53:6382:AB00:67DB:BB27:7332.(b) 7803:42F2:::88EC:D4BA:B75D:11CD.(c) ::4BA8:95CC::DB97:4EAB.(d) 74DC::02BA.(e)
MPLS labels are usually 20 bits long. Explain why this provides enough labels when MPLS is used for destination-based forwarding.
MPLS has sometimes been claimed to improve router performance.Explain why this might be true, and suggest reasons why in practice this may not be the case.
Assume that it takes 32 bits to carry each MPLS label that is added to a packet when the “shim” header of Figure 4.20(b) is used.(a) How many additional bytes are needed to tunnel a packet using
RFC 791 describes the Internet Protocol and includes two options for source routing. Describe three disadvantages of using IP source route options compared to using MPLS for explicit routing.
If a UDP datagram is sent from host A, port P to host B, port Q, but at host B there is no process listening to port Q, then B is to send back an ICMP Port Unreachable message to A. Like all ICMP
Consider a simple UDP-based protocol for requesting files (based somewhat loosely on the Trivial File Transport Protocol [TFTP]). The client sends an initial file request, and the server answers (if
Design a simple UDP-based protocol for retrieving files from a server. No authentication is to be provided. Stop-and-wait transmission of the data may be used. Your protocol should address the
This chapter explains three sequences of state transitions during TCP connection teardown. There is a fourth possible sequence, which traverses an additional arc from FIN_WAIT_1 to TIME_WAIT and
When closing a TCP connection, why is the two-segment-lifetime timeout not necessary on the transition from LAST_ACK to CLOSED?
A sender on a TCP connection that receives a 0 advertised window periodically probes the receiver to discover when the window becomes nonzero. Why would the receiver need an extra timer if it were
Read the man page (or Windows equivalent) for the Unix/Windows utility netstat. Use netstat to see the state of the local TCP connections. Find out how long closing connections spend in TIME_WAIT.
The sequence number field in the TCP header is 32 bits long, which is big enough to cover over 4 billion bytes of data. Even if this many bytes were never transferred over a single connection, why
You are hired to design a reliable byte-stream protocol that uses a sliding window (like TCP). This protocol will run over a 100-Mbps network. The RTT of the network is 100 ms, and the maximum
You are hired to design a reliable byte-stream protocol that uses a sliding window (like TCP). This protocol will run over a 1-Gbps network. The RTT of the network is 140 ms, and the maximum segment

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