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Questions and Answers of Computer Networking

Give the ASN.1 encoding for the following three integers. Note that ASN.1 integers, like those in XDR, are 32 bits in length.(a) 15.(b) 29,496,729.(c) 58,993,458.
Give the big-endian and little-endian representation for the integers from Exercise 7.Exercise 7Give the ASN.1 encoding for the following three integers. Note that ASN.1 integers, like those in XDR,
Give the big-endian and little-endian representation for the integers from Exercise 8.Exercise 8Give the ASN.1 encoding for the following three integers. Note that ASN.1 integers, like those in XDR,
Give the big-endian and little-endian representations for the integers from the previous problem.Exercise 10Give the big-endian and little-endian representation for the integers from Exercise
XDR is used to encode/decode the header for the SunRPC protocol illustrated by Figure 5.18. The XDR version is determined by the RPCVersion field. What potential difficulty does this present? Would
The presentation-formatting process is sometimes regarded as an autonomous protocol layer, separate from the application. If this is so, why might including data compression in the presentation layer
Suppose you have a machine with a 36-bit word size. Strings are represented as five packed 7-bit characters per word. What presentation issues on this machine have to be addressed for it to exchange
Using the programming language of your choice that supports userdefined automatic type conversions, define a type netint and supply conversions that enable assignments and equality comparisons
Different architectures have different conventions on bit order as well as byte order—whether the least significant bit of a byte, for example, is bit 0 or bit 7. RFC 791 defines the standard
Let p ≤ 1 be the fraction of machines in a network that are bigendian; the remaining 1 − p fraction are little-endian. Suppose we choose two machines at random and send an int from one to the
Describe a representation for XML documents that would be more compact and more efficient to process than XML text.
Experiment with a compression utility (e.g., compress, gzip, or pkzip).What compression ratios are you able to achieve? See if you can generate data files for which you can achieve 5:1 or 10:1
Suppose a file contains the lettersa, b,c, andd. Nominally we require 2 bits per letter to store such a file.(a) Assume the letter a occurs 50% of the time, b occurs 30% of the time, and c and d each
Suppose we have a compression functionc, which takes a bit string s to a compressed string c(s).(a) Show that for any integer N, there must be a string s of length N for which length(c(s)) ≥ N;
Give an algorithm for run length encoding that requires only a single byte to represent nonrepeated symbols.
Write a program to construct a dictionary of all “words,” defined to be runs of consecutive nonwhitespace, in a given text file. We might then compress the file (ignoring the loss of whitespace
The one-dimensional discrete cosine transform is similar to the twodimensional transform, except that we drop the second variable (j or y) and the second cosine factor. We also drop, from the inverse
Compare the size of an all-white image in JPEG format with a “typical” photographic image of the same dimensions. At what stage or stages of the JPEG compression process does the white image
Create a grayscale image consisting of an 8 × 8 grid with a vertical black line in the first column. Compress into JPEG format and decompress. How far off are the resultant bytes at the default
Create an 8 × 8 grayscale image consisting of a 64-character ASCII text string. Use lowercase letters only, with no whitespace or punctuation. Compress into JPEG format and decompress. How
Write a program that implements forward and backward DCT using floating-point arithmetic. Run the program on a sample grayscale image. Since DCT is lossless, the image output by the program should
Express DCT(0, 0) in terms of the average of the pixel(x, y)s.
Think about what functions might reasonably be expected from a video standard: fast-forward, editing capabilities, random access, and so on. (See the paper by Le Gall, “MPEG: A video compression
Suppose you want to implement fast-forward and reverse for MPEG streams. What problems do you run into if you limit your mechanism to displaying I frames only? If you do not, then to display a given
Use mpeg_play to play an MPEG-encoded video. Experiment with options, particularly -nob and -nop, which are used to omit the B and P frames, respectively, from the stream. What are the visible
The mpeg_stat program can be used to display statistics for video streams. Use it to determine, for several streams:(a) number and sequence of I, B, and P frames,(b) average compression rate for the
Suppose we have a video of two white points moving toward each other at a uniform rate against a black background. We encode it via MPEG. In one I frame, the two points are 100 pixels apart; in the
Find or install an encryption utility (e.g., the Unix des command or pgp) on your system. Read its documentation and experiment with it. Measure how fast it is able to encrypt and decrypt data. Are
Diagram cipher block chaining as described in Section 8.2.1. Plaintext block 3. Plaintext block 2 Plaintext block 1 Plaintext block 0 Initialization vector (For block 0 only) Encryption function
Learn about a key escrow, or key surrender, scheme (for example, Clipper). What are the pros and cons of key escrow?
Suppose Alice uses the Needham–Schroeder authentication protocol to initiate a session with Bob. Further suppose that an adversary is able to eavesdrop on the authentication messages and, long
One mechanism for resisting “replay” attacks in password authentication is to use one-time passwords: a list of passwords is prepared, and once password[N] has been accepted, the server
Suppose a user employs one-time passwords as above (or, for that matter, reusable passwords) but that the password is transmitted“sufficiently slowly.”(a) Show that an eavesdropper can gain
The Diffie–Hellman key exchange protocol is vulnerable to a “manin-the-middle” attack as shown in Figure 8.7. Outline how Diffie–Hellman can be extended to protect against this
Suppose we have a very short secret s (e.g., a single bit or even a Social Security number), and we wish to send someone else a message m now that will not reveal s but that can be used later to
Suppose two people want to play poker over the network. To “deal”the cards, they need a mechanism for fairly choosing a random number x between them; each party stands to lose if the other party
Estimate the probabilities of finding two messages with the same MD5 checksum, given total numbers of messages of 263, 264, and 265. This is the birthday problem and again the probability that the (k
Suppose we wanted to encrypt a Telnet session with, say, 3DES. Telnet sends lots of 1-byte messages, while 3DES encrypts in blocks of 8 bytes at a time. Explain how 3DES might be used securely in
Consider the following simple UDP protocol (based loosely on TFTP, Request for Comments 1350) for downloading files:■ Client sends a file request.■ Server replies with first data packet.■
Using the browser of your choice, find out what certification authorities for HTTPS your browser is configured by default to trust. Do you trust these agencies? Find out what happens when you disable
Use an OpenPGP implementation such as GnuPG to do the following. Note that no email is involved—you are working exclusively with files on a single machine.(a) Generate a public–private key
PuTTY (pronounced “putty”) is a popular free SSH client—an application that implements the client side of SSH connections—for Unix and Windows. Its documentation is accessible on the Web.(a)
Suppose you want your firewall to block all incoming Telnet connections but to allow outbound Telnet connections. One approach would be to block all inbound packets to the designated Telnet
Suppose a firewall is configured to allow outbound TCP connections but inbound connections only to specified ports. The FTP protocol now presents a problem: when an inside client contacts an outside
Suppose filtering routers are arranged as in Figure 8.22; the primary firewall is R1. Explain how to configure R1 and R2 so that outsiders can Telnet to net 2 but not to hosts on net 1. To avoid
Why might an Internet service provider want to block certain outbound traffic?
It is said that IPsec may not work with Network Address Translation (NAT) (RFC 1631). However, whether IPsec will work with NAT depends on which mode of IPsec and NAT we use. Suppose we use true NAT,
ARP and DNS both depend on caches; ARP cache entry lifetimes are typically 10 minutes, while DNS cache is on the order of days. Justify this difference. What undesirable consequences might there be
IPv6 simplifies ARP out of existence by allowing hardware addresses to be part of the IPv6 address. How does this complicate the job of DNS? How does this affect the problem of finding your local DNS
DNS servers also allow reverse lookup; given an IP address 128.112.169.4, it is reversed into a text string 4.169.112.128.in-addr.arpa and looked up using DNS PTR records (which form a hierarchy of
What is the relationship between a domain name (e.g., cs.princeton.edu) and an IP subnet number (e.g., 192.12.69.0)? Do all hosts on the subnet have to be identified by the same name server? What
Suppose a host elects to use a name server not within its organization for address resolution. When would this result in no more total traffic, for queries not found in any DNS cache, than with a
Figure 9.17 shows the hierarchy of name servers. How would you represent this hierarchy if one name server served multiple zones? In that setting, how does the name server hierarchy relate to the
Use the whois utility/service to find out who is in charge of your site, at least as far as the InterNIC is concerned. Look up your site both by DNS name and by IP network number; for the latter, you
Many smaller organizations have their websites maintained by a third party. How could you use whois to find if this is the case, and, if so, the identity of the third party?
One feature of the existing DNS .com hierarchy is that it is extremely“wide.”(a) Propose a more hierarchical reorganization of the .com hierarchy. What objections might you foresee to your
Suppose, in the other direction, we abandon any pretense at all of DNS hierarchy and simply move all the .com entries to the root name server: www.cisco.com would become www.cisco, or perhaps just
What DNS cache issues are involved in changing the IP address of, say, a Web server host name? How might these be minimized?
Take a suitable DNS-lookup utility (e.g., dig) and disable the recursive lookup feature (e.g., with +norecursive), so that when your utility sends a query to a DNS server and that server is unable to
Discuss how you might rewrite SMTP or HTTP to make use of a hypothetical general-purpose request/reply protocol. Could an appropriate analog of persistent connections be moved from the application
Most Telnet clients can be used to connect to port 25, the SMTP port, instead of to the Telnet port. Using such a tool, connect to an SMTP server and send yourself (or someone else, with permission)
What features might be used by (or added to) SMTP and/or a mail daemon such as sendmail to provide some resistance to email forgeries as in the previous exercise?Exercise 14Most Telnet clients can be
Find out how SMTP hosts deal with unknown commands from the other side and how in particular this mechanism allows for the evolution of the protocol (e.g., to “extended SMTP”). You can either
As presented in the text, SMTP involves the exchange of several small messages. In most cases, the server responses do not affect what the client sends subsequently. The client might thus implement
Find out what other features DNS MX records provide in addition to supplying an alias for a mail server; the latter could, after all, be provided by a DNS CNAME record. MX records are provided to
One of the central problems faced by a protocol such as MIME is the vast number of data formats available. Consult the MIME RFC to find out how MIME deals with new or system-specific image and text
MIME supports multiple representations of the same content using the multipart/alternative syntax; for example, text could be sent as text/plain, text/richtext, and application/postscript. Why do you
Consult the MIME RFC to find out how base64 encoding handles binary data of a length not evenly divisible by three bytes.
In HTTP version 1.0, a server marked the end of a transfer by closing the connection. Explain why, in terms of the TCP layer, this was a problem for servers. Find out how HTTP version 1.1 avoids
Find out how to configure an HTTP server so as to eliminate the 404 not found message and have a default (and hopefully friendlier) message returned instead. Decide if such a feature is part of the
Why does the HTTP GET command GET http://www.cs.princeton.edu/index.html HTTP/1.1 contain the name of the server being contacted? Would the server not already know its name? Use Telnet, as in
When an HTTP server initiates a close() at its end of a connection, it must then wait in TCP state FIN_WAIT_2 for the client to close the other end. What mechanism within the TCP protocol could help
The POP3 Post Office Protocol only allows a client to retrieve email, using a password for authentication. Traditionally, to send email, a client would simply send it to its server and expect that it
Suppose a very large website wants a mechanism by which clients access whichever of multiple HTTP servers is “closest” by some suitable measure.(a) Discuss developing a mechanism within HTTP for
Find out if there is available to you an SNMP node that will answer queries you send it. If so, locate some SNMP utilities (e.g., the ucdsnmp suite) and try the following:(a) Fetch the entire system
Using the SNMP device and utilities of the previous exercise, fetch the tcp group (numerically group 6) or some other group. Then do something to make some of the group’s counters change, and fetch
What information provided by SNMP might be useful to someone planning the IP spoofing attack? What other SNMP information might be considered sensitive?
Application protocols such as FTP and SMTP were designed from scratch, and they seem to work reasonably well. What is it about Business-to-Business and Enterprise Application Integration protocols
Choose a Web Service with equivalent REST and SOAP interfaces, such as those offered by Amazon.com. Compare how equivalent operations are implemented in the two styles.
Get the WSDL for some SOAP-style Web Service and choose an operation. In the messages that implement that operation, identify the fields.
Suppose some receivers in a large conference can receive data at a significantly higher bandwidth than others. What sorts of things might be implemented to address this? Hint: Consider both the
How might you encode audio (or video) data in two packets so that if one packet is lost, then the resolution is simply reduced to what would be expected with half the bandwidth? Explain why this is
Explain the relationship between Uniform Resource Locators (URLs)and Uniform Resource Identifiers (URIs). Given an example of a URI that is not a URL.
What problem would a DNS-based redirection mechanism encounter if it wants to select an appropriate server based on current load information?
Consider the following simplified BitTorrent scenario. There is a swarm of 2n peers and, during the time in question, no peers join or leave the swarm. It takes a peer 1 unit of time to upload or
The purpose of this problem is to give you a better understanding as to the mechanisms of RSA. Write a function that receives as its parameters primes p and q, calculates public and private RSA keys
Write a function that accepts a stream of ASCII characters and encrypts this input using a substitution cipher with the Cipher Block Chaining mode. The block size should be 8 bytes. The program
Write a procedure that computes the SHA-1 hash of a block of data. The procedure should have two parameters: a pointer to the input buffer and a pointer to a 20-byte output buffer. To see the exact
Write a program that encrypts its input by XORing it with a key stream. Find or write as good a random number generator as you can to generate the key stream. The program should act as a filter,
Search the Internet for some court case involving copyright versus fair use and write a 1-page report summarizing your findings.
Rewrite the server of Fig. 6-6 as a true Web server using the GET command for HTTP 1.1. It should also accept the Host message. The server should maintain a cache of files recently fetched from the
Numerous people have registered DNS names that consist of www.word.com, where word is a common word. For each of the following categories, list five such Web sites and briefly summarize what it is
Some cybersquatters have registered domain names that are misspellings of common corporate sites, for example, www.microsfot.com. Make a list of at least five such domains.
Design and implement a chat system that allows multiple groups of users to chat. A chat coordinator resides at a well-known network address, uses UDP for communication with chat clients, sets up chat
One common function that all network protocols need is to manipulate messages. Recall that protocols manipulate messages by adding/striping headers. Some protocols may break a single message into
The file server of Fig. 6-6 is far from perfect and could use a few improvements. Make the following modifications.(a) Give the client a third argument that specifies a byte range.(b) Add a client
Use the trace route (UNIX) or tracert (Windows) programs to trace the route from your computer to various universities on other continents. Make a list of transoceanic links you have discovered. Some
Write a function to do forwarding in an IP router. The procedure has one parameter, an IP address. It also has access to a global table consisting of an array of triples. Each triple contains three
Write a program that simulates a computer network using discrete time. The first packet on each router queue makes one hop per time interval. Each router has only a finite number of buffers. If a
Write a program to simulate routing using flooding. Each packet should contain a counter that is decremented on each hop. When the counter gets to zero, the packet is discarded. Time is discrete,

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