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Computer Networks A Systems Approach 6th Edition Larry L. Peterson, Bruce S. Davie - Solutions

The text states that additive increase is a necessary condition for a congestion control mechanism to be stable. Outline a specific instability that might arise if all increases were exponential; that is, if TCP continued to use “slow” start after CongestionWindow increased beyond
Discuss the relative advantages and disadvantages of marking a packet (as in the DECbit mechanism) versus dropping a packet (as in RED gateways).
Consider a RED gateway with MaxP = 0.02 and with an average queue length halfway between the two thresholds.(a) Find the drop probability Pcount for count = 1 and count = 50.(b) Calculate the probability that none of the first 50 packets are dropped. Note that this is (1 − P1) ×···× (1 −
Consider a RED gateway with MaxP = p and with an average queue length halfway between the two thresholds.(a) Calculate the probability that none of the first n packets are dropped.(b) Find p such that the probability that none of the first n packets are dropped is α.
Explain the intuition behind setting MaxThreshold = 2 × MinThreshold in RED gateways.
In RED gateways, explain why MaxThreshold is actually less than the actual size of the available buffer pool.
Explain the fundamental conflict between tolerating burstiness and controlling network congestion.
Why do you think that the drop probability P of a RED gateway does not simply increase linearly from P = 0 at MinThresh to P = 1 at MaxThresh?
In TCP Vegas, the calculation of ActualRate is done by dividing the amount of data transmitted in one RTT interval by the length of the RTT.(a) Show that for any TCP, if the window size remains constant, then the amount of data transmitted in one RTT interval is constant once a full window-full is
Suppose a TCP Vegas connection measures the RTT of its first packet and sets BaseRTT to that, but then a network link failure occurs and all subsequent traffic is routed via an alternative path with twice the RTT. How will TCP Vegas respond? What will happen to the value of CongestionWindow? Assume
Consider the following two causes of a 1-second network delay(assume ACKs return instantaneously):■ one intermediate router with a 1-second outbound per-packet bandwidth delay and no competing traffic,■ one intermediate router with a 100-ms outbound per-packet bandwidth delay and with a
Give an argument why the congestion control problem is better managed at the internet level than the ATM level, at least when only part of the internet is ATM. In an exclusively IP-over-ATM network, is congestion better managed at the cell level or at the TCP level? Why?
Consider the taxonomy of Figure 6.24.(a) Give an example of a real-time application that is intolerant/rate adaptive.(b) Explain why you might expect a loss-tolerant application to be at least somewhat rate adaptive.(c) Part (b) notwithstanding, give an example of an application that might be
The transmission schedule (Table 6.3) for a given flow lists for each second the number of packets sent between that time and the following second. The flow must stay within the bounds of a token bucket filter. What bucket depth does the flow need for the following token rates? Assume the bucket is
The transmission schedule (Table 6.4) for a given flow is for each second the number of packets sent between that time and the following second. The flow must stay within the bounds of a token bucket filter. Find the necessary bucket depth D as a function of token rate r.Note that r takes only
Suppose a router has accepted flows with the TSpecs shown in Table 6.5, described in terms of token bucket filters with token rate r packets per second and bucket depth B packets. All flows are in the same direction, and the router can forward one packet every 0.1 second. (a) What is the maximum
Suppose an RSVP router suddenly loses its reservation state but otherwise remains running.(a) What will happen to the existing reserved flows if the router handles reserved and nonreserved flows via a single FIFO queue?(b) What might happen to the existing reserved flows if the router used weighted
Consider the following C code:where num_raises + 1 corresponds to the number of valid entries in array salary_history. Show the on-the-wire representation of employee0 that is generated by XDR. #define MAXSTR 100 struct date I 1: char month[MAXSTR]: int day: int year: struct employee ! }: char
Show the on-the-wire representation of employee1 from the previous problem that is generated by XDR.Exercise 1where num_raises + 1 corresponds to the number of valid entries in array salary_history. Show the on-the-wire representation of employee0 that is generated by XDR. #define MAXSTR 100 struct
For the data structures given in the previous problem, give the XDR routine that encodes/decodes these structures. If you have XDR available to you, run this routine and measure how long it takes to encode and decode an example instance of structure employee.Data from in previous problem where
Using library functions like htonl and Unix’s bcopy or Windows’CopyMemory, implement a routine that generates the same onthe-wire representation of the structures given in Exercise 1 as XDR does. If possible, compare the performance of your “by-hand”encoder/decoder with the corresponding
Use XDR and htonl to encode a 1000-element array of integers. Measure and compare the performance of each. How do these compare to a simple loop that reads and writes a 1000-element array of integers? Perform the experiment on a computer for which the native byte order is the same as the network
Write your own implementation of htonl. Using both your own htonl and (if little-endian hardware is available) the standard library version, run appropriate experiments to determine how much longer it takes to byte-swap integers versus merely copying them.
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) 101.(b) 10,120.(c) 16,909,060.
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, are 32 bits in length.(a) 101.(b) 10,120.(c) 16,909,060.
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, are 32 bits in length.(a) 15.(b) 29,496,729.(c) 58,993,458.
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 8.Exercise 8Give the ASN.1 encoding for the following three integers. Note that ASN.1 integers, like those
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 it be possible for a new version of XDR to switch to little-endian integer format?Figure 5.18) 0 XID
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 be a bad idea?
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 integer and string data with the rest of the world?
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 between ints and netints. Can a generalization of this approach solve the problem of network argument
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 network bit order. Why is bit order then not relevant to presentation formatting?
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 other. Give the average number of byte-order conversions needed for both big-endian network byte order
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 compression ratios.
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 occur 10% of the time. Give an encoding of each letter as a bit string that provides optimal
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; that is, no effective compression is done.(b) Compress some already compressed files (try compressing
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 information)by representing each word as an index in the dictionary. Retrieve the file rfc791.txt
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 DCT only, the leading 1/√2N coefficient. Implement this and its inverse for N = 8 (a spreadsheet
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 become smaller than the photographic image?For the next three exercises, the utilities cjpeg and djpeg
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 quality setting? How would you describe the inaccuracies introduced, visually? What quality setting is
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 recognizable is the result as text? Why might adding whitespace make things worse? With the quality setting
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 match the input. Now modify your program so that it zeroes some of the higher-frequency components and
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 standard for multimedia applications,” given in this chapter’s recommended reading list, for
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 frame in the fast-forward sequence, what is the largest number of frames in the original sequence
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 effects of omitting these frames?
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 entire video,(c) average compression rate for each type of frame.
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 next I frame, they have merged. The final point of merger happens to lie at the center of a 16 × 16
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 these two rates the same? Try to compare these timing results using different key sizes; for example,
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 Blocks of ciphertext
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 after the session has completed, discover the (unencrypted) session key. How could the adversary deceive
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 decrements N and prompts for password[N − 1] next time. At N = 0, a new list is needed. Outline a mechanism
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 access to the remote server with a relatively modest number of guesses. Hint: The eavesdropper starts
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 possibility.Figure 8.7) Alice g mod p gf mod p Mallory g mod p go mod p Bob
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 verify that we did know s. Explain why m = MD5(s) or m = E(s) with RSA encryption would not be secure
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 can unfairly influence the choice of x. Describe such a mechanism. Hint:You may assume that if
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 + 1)th message has a different checksum from each of the preceding k is 1 − k/2128. However, the
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 this setting.
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.■ Client sends ACK, and the two proceed using stop-and-wait.Suppose client and server possess keys KC and
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 trust of some or all of these certification authorities.
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 pair.(b) Use your public key to encrypt a file, as if for secure storage, and then use your private key to
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) How does PuTTY handle authentication of a server that it has not previously connected to?(b) How are
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 port(23).(a) We might want to block inbound packets to other ports as well, but what inbound TCP connections
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 server, the outbound TCP control connection can be opened normally, but the TCP data connection
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 “leapfrogging”break-ins to net 1, also disallow Telnet connections from net 2 to net 1.Figure 8.22)
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, where only IP addresses are translated (without port translation). Will IPsec and NAT work in each
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 in having too long a DNS cache entry lifetime?
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 server?
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 domains analogous to that for the address domain hierarchy).Suppose you want to authenticate the
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 about reverse lookup, as in the previous exercise?Exercise 3DNS servers also allow reverse lookup;
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 local name server? When might this result in a better DNS cache hit rate and possibly less total
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 zone hierarchy? How do you deal with the fact that each zone may have multiple name servers?Figure 9.17
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 may have to try an alternative whois server (e.g., whois -h whois.arin.net. . .). Try princeton.edu
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 proposal’s adoption?(b) What might be some of the consequences of having most DNS domain names contain
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 cisco. How would this affect root name server traffic in general?How would this affect such traffic
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 fully answer the request from its own records, the server sends back the next DNS server in the
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 layer into such a transport protocol? What other application tasks might be moved into this protocol?
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) some forged email. Then examine the headers for evidence the message is not genuine.
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 used to connect to port 25, the SMTP port, instead of to the Telnet port. Using such a tool,
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 read the RFC or contact an SMTP server as in Exercise 14 and test its responses to nonexistent commands.
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 command pipelining: sending multiple commands in a single message.(a) For what SMTP commands does the
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 support email; would an analogous WEB record be of use in supporting HTTP?
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 formats.
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 think plaintext is supposed to be the first format, even though implementations might find it
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 this. How might a general-purpose request/reply protocol address this?
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 protocol or part of an implementation, or is technically even permitted by the protocol.
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 Exercise 14, to connect to port 80 of an HTTP server and find out what happens if you leave the host name
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 an HTTP server deal with noncooperative or poorly implemented clients that do not close from their
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 be relayed.(a) Explain why email servers often no longer permit such relaying from arbitrary
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 doing this.(b) Discuss developing a mechanism within DNS for doing this.Compare the two. Can either
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 group, using something like snmpwalk nodename public system Also try the above with 1 in place of
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 the group again to show the change. Try to do this in such a way that you can be sure your actions

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