Questions and Answers of Computer Network

The network of Fig. 5-37 uses RSVP with multicast trees for hosts 1 and 2 as shown. Suppose that host 3 requests a channel of bandwidth 2 MB/sec for a flow from host 1 and another channel of
The CPU in a router can process 2 million packets/sec. The load offered to it is 1.5 million packets/sec, if a route from source to destination contains 10 routers, how much time is spent being
Consider the user of differentiated services with expedited forwarding. Is there a guarantee that expedited packets experience a shorter delay than regular packets? Why or why not?
Is fragmentation needed in concatenated virtual-circuit internets or only in datagram systems?
Tunneling through a concatenated virtual-circuit subnet is straightforward: the multi protocol router at one end just sets up a virtual circuit to the other end and passes packets through it. Can
Suppose that host A is connected to a router R 1, R 1 is connected to another router, R 2, and R 2 is connected to host B. Suppose that a TCP message that contains 900 bytes of data and 20 bytes of
A router is blasting out IP packets whose total length (data plus header) is 1024 bytes. Assuming that packets live for 10 sec, what is the maximum line speed the router can operate at without danger
An IP datagram using the strict source routing option has to be fragmented. Do you think the option is copied into each fragment, or is it sufficient to just put it in the first fragment? Explain
Suppose that instead of using 16 bits for the network part of a class B address originally, 20 bits had been used. How many class B networks would there have been?
Convert the IP address whose hexadecimal representation is C22F1582 to dotted decimal notation.
A network on the Internet has a subnet mask of 255.255.240.0. What is the maximum number of hosts it can handle?
A large number of consecutive IP address are available starting at 198.16.0.0. Suppose that four organizations, A, B, C, and D, request 4000, 2000, 4000, and 8000 addresses, respectively, and in that
A router has just received the following new IP addresses: 57.6.96.0/21, 57.6.104.0/21, 57.6.112.0/21, and 57.6.120.0/21. If all of them use the same outgoing line, can they be aggregated, If so to
The set of IP addresses from 29.18.0.0 to 19.18.128.255 has been aggregated to 29.18.0.0/17. However, there is a gap of 1024 unassigned addresses from 29.18.60.0 to 29.18.63.255 that are now suddenly
A router has the following (CIDR) entries in its routing table: Address/mask Next hop 135.46.56.0/22 Interface 0 135.46.60.0/22 Interface 1 192.53.40.0/23 Router 1
Many companies have a policy of having two (or more) routers connecting the company to the Internet to provide some redundancy in case one of them goes down. Is this policy still possible with NAT?
You have just explained the ARP protocol to a friend. When you are all done, he says: ''I've got it. ARP provides a service to the network layer, so it is part of the data link layer.'' What do you
ARP and RARP both map addresses from one space to another. In this respect, they are similar. However, their implementations are fundamentally different. In what major way do they differ?
Describe a way to reassemble IP fragments at the destination.
Most IP datagram reassembly algorithms have a timer to avoid having a lost fragment tie up reassembly buffers forever. Suppose that a datagram is fragmented into four fragments. The first three
In both IP and ATM, the checksum covers only the header and not the data. Why do you suppose this design was chosen?
A person who lives in Boston travels to Minneapolis, taking her portable computer with her. To her surprise, the LAN at her destination in Minneapolis is a wireless IP LAN, so she does not have to
IPv6 uses 16-byte addresses. If a block of 1 million addresses is allocated every picosecond, how long will the addresses last?
The Protocol field used in the IPv4 header is not present in the fixed IPv6 header. Why not?
When the IPv6 protocol is introduced, does the ARP protocol have to be changed? If so, are the changes conceptual or technical?
In our example transport primitives of Fig. 6-2, LISTEN is a blocking call. Is this strictly necessary? If not, explain how a non-blocking primitive could be used. What advantage would this have over
In the model underlying Fig. 6-4, it is assumed that packets may be lost by the network layer and thus must be individually acknowledged. Suppose that the network layer is 100 percent reliable and
In both parts of Fig. 6-6, there is a comment that the value of SERVER_PORT must be the same in both client and server. Why is this so important?
Suppose that the clock-driven scheme for generating initial sequence numbers is used with a 15-bit wide clock counter. The clock ticks once every 100 msec, and the maximum packet lifetime is 60 sec.
Why does the maximum packet lifetime, T, have to be large enough to ensure that not only the packet but also its acknowledgements have vanished?
Imagine that a two-way handshake rather than a three-way handshake were used to set up connections. In other words, the third message was not required. Are deadlocks now possible? Give an example or
Imagine a generalized n-army problem, in which the agreement of any two of the blue armies is sufficient for victory. Does a protocol exist that allows blue to win?
Consider the problem of recovering from host crashes (i.e., Fig. 6-18). If the interval between writing and sending an acknowledgement, or vice versa, can be made relatively small, what are the two
Are deadlocks possible with the transport entity described in the text (Fig. 6-20)?
Out of curiosity, the implementer of the transport entity of Fig. 6-20 has decided to put counters inside the sleep procedure to collect statistics about the conn array. Among these are the number of
What happens when the user of the transport entity given in Fig. 6-20 sends a zero-length message? Discuss the significance of your answer.
For each event that can potentially occur in the transport entity of Fig. 6-20, tell whether it is legal when the user is sleeping in sending state.
Discuss the advantages and disadvantages of credits versus sliding window protocols.
Why does UDP exist? Would it not have been enough to just let user processes send raw IP packets?
Consider a simple application-level protocol built on top of UDP that allows a client to retrieve a file from a remote server residing at a well-known address. The client first sends a request with
A client sends a 128-byte request to a server located 100 km away over a 1-gigabit optical fiber. What is the efficiency of the line during the remote procedure call?
Consider the situation of the previous problem again. Compute the minimum possible response time both for the given 1-Gbps line and for a 1-Mbps line. What conclusion can you draw?
Both UDP and TCP use port numbers to identify the destination entity when delivering a message. Give two reasons for why these protocols invented a new abstract ID (port numbers); instead of using
What is the total size of the minimum TCP MTU, including TCP and IP overhead but not including data link layer overhead?
Datagram fragmentation and reassembly are handled by IP and are invisible to TCP. Does this mean that TCP does not have to worry about data arriving in the wrong order?
RTP is used to transmit CD-quality audio, which makes a pair of 16-bit samples 44,100 times/sec, one sample for each of the stereo channels, how many packets per second must RTP transmit?
Would it be possible to place the RTP code in the operating system kernel, along with the UDP code? Explain your answer.
A process on host 1 has been assigned port p, and a process on host 2 has been assigned port q. Is it possible for there to be two or more TCP connections between these two ports at the same time?
In Fig. 6-29 we saw that in addition to the 32-bit Acknowledgement field, there is an ACK bit in the fourth word. Does this really add anything? Why or why not?
The maximum payload of a TCP segment is 65,495 bytes. Why was such a strange number chosen?
Describe two ways to get into the SYN RCVD state of Fig. 6-33.
Give a potential disadvantage when Nagle's algorithm is used on a badly-congested network.
Consider the effect of using slow start on a line with a 10-msec round-trip time and no congestion. The receive window is 24 KB and the maximum segment size is 2 KB. How long does it take before the
Suppose that the TCP congestion window is set to 18 KB and a timeout occurs. How big will the window be if the next four transmission bursts are all successful? Assume that the maximum segment size
If the TCP round-trip time, RTT, is currently 30 msec and the following acknowledgements come in after 26, 32, and 24 msec, respectively, what is the new RTT estimate using the Jacobson algorithm?
A TCP machine is sending full windows of 65,535 bytes over a 1-Gbps channel that has a 10-msec one-way delay. What is the maximum throughput achievable? What is the line efficiency?
What is the fastest line speed at which a host can blast out 1500-byte TCP payloads with a 120-sec maximum packet lifetime without having the sequence numbers wrap around? Take TCP, IP, and Ethernet
In a network that has a maximum TPDU size of 128 bytes, a maximum TPDU lifetime of 30 sec, and an 8-bit sequence number, what is the maximum data rate per connection?
Suppose that you are measuring the time to receive a TPDU. When an interrupt occurs, you read out the system clock in milliseconds. When the TPDU is fully processed, you read out the clock again. You
A CPU executes instructions at the rate of 1000 MIPS. Data can be copied 64 bits at a time, with each word copied costing 10 instructions. If a coming packet has to be copied four times, can this
To get around the problem of sequence numbers wrapping around while old packets still exist, one could use 64-bit sequence numbers. However, theoretically, an optical fiber can run at 75 Tbps. What
Give one advantage of RPC on UDP over transactional TCP. Give one advantage of T/TCP over RPC.
In Fig. 6-40(a), we see that it takes 9 packets to complete the RPC. Are there any circumstances in which it takes exactly 10 packets?
In Sec. 6.6.5, we calculated that a gigabit line dumps 80,000 packets/sec on the host, giving it only 6250 instructions to process it and leaving half the CPU time for applications, this calculation
For a 1-Gbps network operating over 4000 km, the delay is the limiting factor, not the bandwidth. Consider a MAN with the average source and destination 20 km apart. At what data rate does the
Calculate the bandwidth-delay product for the following networks: (1) T1 (1.5 Mbps), (2) Ethernet (10 Mbps), (3) T3 (45 Mbps), and (4) STS-3 (155 Mbps). Assume an RTT of 100 msec. Recall that a TCP
What is the bandwidth-delay product for a 50-Mbps channel on a geostationary satellite? If the packets are all 1500 bytes (including overhead), how big should the window be in packets?
Many business computers have three distinct and worldwide unique identifiers. What are they?
According to the information given in Fig. 7-3, is little-sister.cs.vu.nl on a class A, B, or C network?
In Fig. 7-3, there is no period after rowboat? Why not?
Make a guess about what the smiley :-X (sometimes written as :-#) might mean.
DNS uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it solved?
In addition to being subject to loss, UDP packets have a maximum length, potentially as low as 576 bytes. What happens when a DNS name to be looked up exceeds this length? Can it be sent in two
Can a machine with a single DNS name have multiple IP addresses? How could this occur?
Can a computer have two DNS names that fall in different top-level domains? If so, give a plausible example. If not, explain why not.
The number of companies with a Web site has grown explosively in recent years. As a result, thousands of companies are registered in the com domain, causing a heavy load on the top-level server for
Some e-mail systems support a header field Content Return:. It specifies whether the body of a message is to be returned in the event of non-delivery. Does this field belong to the envelope or to the
Electronic mail systems need directories so people's e-mail addresses can be looked up. To build such directories, names should be broken up into standard components (e.g., first name, last name) to
A person's e-mail address is his or her login name @ the name of a DNS domain with an MX record. Login names can be first names, last names, initials, and all kinds of other names. Suppose that a
A binary file is 3072 bytes long. How long will it be if encoded using base64 encoding, with a CR+LF pair inserted after every 80 bytes sent and at the end?
Consider the quoted-printable MIME encoding scheme. Mention a problem not discussed in the text and proposes a solution.
Name five MIME types not listed in the book. You can check your browser or the Internet for information.
Suppose that you want to send an MP3 file to a friend, but your friend's ISP limits the amount of incoming mail to 1 MB and the MP3 file is 4 MB. Is there a way to handle this situation by using RFC
Suppose that someone sets up a vacation daemon and then sends a message just before logging out. Unfortunately, the recipient has been on vacation for a week and also has a vacation daemon in place.
In any standard, such as RFC 822, a precise grammar of what is allowed is needed so that different implementations can interwork. Even simple items have to be defined carefully. The SMTP headers
Is the vacation daemon part of the user agent or the message transfer agent? Of course, it is set up using the user agent, but does the user agent actually send the replies? Explain your answer.
POP3 allows users to fetch and download e-mail from a remote mailbox. Does this mean that the internal format of mailboxes has to be standardized so any POP3 program on the client side can read the
From an ISP's point of view, POP3 and IMAP differ in an important way. POP3 users generally empty their mailboxes every day. IMAP users keep their mail on the server indefinitely. Imagine that you
Does Web mail use POP3, IMAP, or neither? If one of these, why was that one chosen? If neither, which one is it closer to in spirit?
When Web pages are sent out, they are prefixed by MIME headers. Why?
When are external viewers needed? How does a browser know which one to use?
Is it possible that when a user clicks on a link with Netscape, a particular helper is started, but clicking on the same link in Internet Explorer causes a completely different helper to be started,
A multithreaded Web server is organized as shown in Fig. 7-21. It takes 500 µsec to accept a request and check the cache. Half the time the file is found in the cache and returned immediately. The
The standard http URL assumes that the Web server is listening on port 80. However, it is possible for a Web server to listen to some other port. Devise a reasonable syntax for a URL accessing a file
Although it was not mentioned in the text, an alternative form for a URL is to use the IP address instead of its DNS name. An example of using an IP address is 192.31.231.66/index.html. How does the
Imagine that someone in the CS Department at Stanford has just written a new program that he wants to distribute by FTP. He puts the program in the FTP directory ftp/pub/freebies/newprog.c. What is
In Fig. 7-25, www.aportal.com keeps track of user preferences in a cookie. A disadvantage of this scheme is that cookies are limited to 4 KB, so if the preferences are extensive, for example, many
Sloth Bank wants to make on-line banking easy for its lazy customers, so after a customer signs up and is authenticated by a password, the bank returns a cookie containing a customer ID number. In
In Fig. 7-26, the ALT parameter is set in the tag. Under what conditions does the browser use it, and how?
How do you make an image clickable in HTML? Give an example.

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