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Computer Networking: A Top-Down Approach, 6th Edition
Solutions to Review Questions and Problems
Version Date: May 2012
This document contains the solutions to review questions and problems for the 5th edition of Computer Networking: A Top-Down Approach by Jim Kurose and Keith Ross. These solutions are being made available to instructors ONLY. Please do NOT copy or distribute this document to others (even other instructors). Please do not post any solutions on a publicly-available Web site. We’ll be happy to provide a copy (up-to-date) of this solution manual ourselves to anyone who asks.
Acknowledgments: Over the years, several students and colleagues have helped us prepare this solutions manual. Special thanks goes to HongGang Zhang, Rakesh Kumar, Prithula Dhungel, and Vijay Annapureddy. Also thanks to all the readers who have made suggestions and corrected errors.
All material © copyright 1996-2012 by J.F. Kurose and K.W. Ross. All rights reserved
Chapter 1 Review Questions
1. There is no difference. Throughout this text, the words “host” and “end system” are used interchangeably. End systems include PCs, workstations, Web servers, mail servers, PDAs, Internet-connected game consoles, etc
2. From Wikipedia: Diplomatic protocol is commonly described as a set of international courtesy rules. These well-established and time-honored rules have made it easier for nations and people to live and work together. Part of protocol has always been the acknowledgment of the hierarchical standing of all present. Protocol rules are based on the principles of civility.
3. Standards are important for protocols so that people can create networking systems and products that interoperate.
4. 1. Dial-up modem over telephone line: home; 2. DSL over telephone line: home or small office; 3. Cable to HFC: home; 4. 100 Mbps switched Ethernet: enterprise; 5. Wifi (802.11): home and enterprise: 6. 3G and 4G: wide-area wireless.
5. HFC bandwidth is shared among the users. On the downstream channel, all packets emanate from a single source, namely, the head end. Thus, there are no collisions in the downstream channel.
6. In most American cities, the current possibilities include: dial-up; DSL; cable modem; fiber-to-the-home.
7. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.
8. Today, Ethernet most commonly runs over twisted-pair copper wire. It also can run over fibers optic links.
9. Dial up modems: up to 56 Kbps, bandwidth is dedicated; ADSL: up to 24 Mbps downstream and 2.5 Mbps upstream, bandwidth is dedicated; HFC, rates up to 42.8 Mbps and upstream rates of up to 30.7 Mbps, bandwidth is shared. FTTH: 2-10Mbps upload; 10-20 Mbps download; bandwidth is not shared.
10. There are two popular wireless Internet access technologies today:
a) Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets to/from an base station (i.e., wireless access point) within a radius of few tens of meters. The base station is typically connected to the wired Internet and thus serves to connect wireless users to the wired network.
b) 3G and 4G wide-area wireless access networks. In these systems, packets are transmitted over the same wireless infrastructure used for cellular telephony, with the base station thus being managed by a telecommunications provider. This provides wireless access to users within a radius of tens of kilometers of the base station.
11. At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host completes transmission and the entire packet is received at the router (no propagation delay). Because the router has the entire packet at time t1, it can begin to transmit the packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes transmission and the entire packet is received at the receiving host (again, no propagation delay). Thus, the end-to-end delay is L/R1 + L/R2.
12. A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth. FDM requires sophisticated analog hardware to shift signal into appropriate frequency bands.
13. a) 2 users can be supported because each user requires half of the link bandwidth.
b) Since each user requires 1Mbps when transmitting, if two or fewer users transmit simultaneously, a maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there will be no queuing delay before the link. Whereas, if three users transmit simultaneously, the bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link. In this case, there will be queuing delay before the link.
c) Probability that a given user is transmitting = 0.2
d) Probability that all three users are transmitting simultaneously =
= (0.2)3 = 0.008. Since the queue grows when all the users are transmitting, the fraction of time during which the queue grows (which is equal to the probability that all three users are transmitting simultaneously) is 0.008.
14. If the two ISPs do not peer with each other, then when they send traffic to each other they have to send the traffic through a provider ISP (intermediary), to which they have to pay for carrying the traffic. By peering with each other directly, the two ISPs can reduce their payments to their provider ISPs. An Internet Exchange Points (IXP) (typically in a standalone building with its own switches) is a meeting point where multiple ISPs can connect and/or peer together. An ISP earns its money by charging each of the the ISPs that connect to the IXP a relatively small fee, which may depend on the amount of traffic sent to or received from the IXP.
15. Google's private network connects together all its data centers, big and small. Traffic between the Google data centers passes over its private network rather than over the public Internet. Many of these data centers are located in, or close to, lower tier ISPs. Therefore, when Google delivers content to a user, it often can bypass higher tier ISPs. What motivates content providers to create these networks? First, the content provider has more control over the user experience, since it has to use few intermediary ISPs. Second, it can save money by sending less traffic into provider
networks. Third, if ISPs decide to charge more money to highly profitable content providers (in countries where net neutrality doesn't apply), the content providers can avoid these extra payments.
16. The delay components are processing delays, transmission delays, propagation delays, and queuing delays. All of these delays are fixed, except for the queuing delays, which are variable.
17. a) 1000 km, 1 Mbps, 100 bytes
b) 100 km, 1 Mbps, 100 bytes
18. 10msec; d/s; no; no
19. a) 500 kbps
b) 64 seconds
c) 100kbps; 320 seconds
20. End system A breaks the large file into chunks. It adds header to each chunk, thereby generating multiple packets from the file. The header in each packet includes the IP address of the destination (end system B). The packet switch uses the destination IP address in the packet to determine the outgoing link. Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on, given the packet’s destination address.
21. The maximum emission rate is 500 packets/sec and the maximum transmission rate is 350 packets/sec. The corresponding traffic intensity is 500/350 =1.43 > 1. Loss will eventually occur for each experiment; but the time when loss first occurs will be different from one experiment to the next due to the randomness in the emission process.
22. Five generic tasks are error control, flow control, segmentation and reassembly, multiplexing, and connection setup. Yes, these tasks can be duplicated at different layers. For example, error control is often provided at more than one layer.
23. The five layers in the Internet protocol stack are – from top to bottom – the application layer, the transport layer, the network layer, the link layer, and the physical layer. The principal responsibilities are outlined in Section 1.5.1.
24. Application-layer message: data which an application wants to send and passed onto the transport layer; transport-layer segment: generated by the transport layer and encapsulates application-layer message with transport layer header; network-layer datagram: encapsulates transport-layer segment with a network-layer header; linklayer frame: encapsulates network-layer datagram with a link-layer header.
25. Routers process network, link and physical layers (layers 1 through 3). (This is a little bit of a white lie, as modern routers sometimes act as firewalls or caching
components, and process Transport layer as well.) Link layer switches process link and physical layers (layers 1 through2). Hosts process all five layers.
26. a) Virus
Requires some form of human interaction to spread. Classic example: E-mail viruses.
b) Worms
No user replication needed. Worm in infected host scans IP addresses and port numbers, looking for vulnerable processes to infect.
27. Creation of a botnet requires an attacker to find vulnerability in some application or system (e.g. exploiting the buffer overflow vulnerability that might exist in an application). After finding the vulnerability, the attacker needs to scan for hosts that are vulnerable. The target is basically to compromise a series of systems by exploiting that particular vulnerability. Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability. An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the nodes in the botnet. Hence, it becomes possible for the attacker to issue a command to all the nodes, that target a single node (for example, all nodes in the botnet might be commanded by the attacker to send a TCP SYN message to the target, which might result in a TCP SYN flood attack at the target).
28. Trudy can pretend to be Bob to Alice (and vice-versa) and partially or completely modify the message(s) being sent from Bob to Alice. For example, she can easily change the phrase “Alice, I owe you $1000” to “Alice, I owe you $10,000”. Furthermore, Trudy can even drop the packets that are being sent by Bob to Alice (and vise-versa), even if the packets from Bob to Alice are encrypted.
Chapter 1 Problems
Problem 1
There is no single right answer to this question. Many protocols would do the trick. Here's a simple answer below:
Messages from ATM machine to Server Msg name purpose
HELO <userid> Let server know that there is a card in the ATM machine
ATM card transmits user ID to Server
PASSWD <passwd> User enters PIN, which is sent to server BALANCE
User requests balance
WITHDRAWL <amount> User asks to withdraw money
BYE user all done
Messages from Server to ATM machine (display)
Msg name purpose
PASSWD
Ask user for PIN (password) OK last requested operation (PASSWD, WITHDRAWL) OK
ERR last requested operation (PASSWD, WITHDRAWL) in ERROR
AMOUNT <amt> sent in response to BALANCE request BYE user done, display welcome screen at ATM
Correct operation:
client server
HELO (userid)
PASSWD <passwd>
> (check if valid userid)
< PASSWD
> (check password)
< OK (password is OK)
BALANCE >
WITHDRAWL <amt>
ATM dispenses $ BYE
< AMOUNT <amt>
> check if enough $ to cover withdrawl
< OK
In situation when there's not enough money:
HELO (userid)
PASSWD <passwd>
BYE
> (check if valid userid)
< PASSWD
> (check password)
< OK (password is OK)
BALANCE >
WITHDRAWL <amt>
< AMOUNT <amt>
> check if enough $ to cover withdrawl
< ERR (not enough funds) error msg displayed no $ given out
BYE
BYE
Problem 2
At time N*(L/R) the first packet has reached the destination, the second packet is stored in the last router, the third packet is stored in the next-to-last router, etc. At time N*(L/R) + L/R, the second packet has reached the destination, the third packet is stored in the last router, etc. Continuing with this logic, we see that at time N*(L/R) + (P-1)*(L/R) = (N+P-1)*(L/R) all packets have reached the destination.
Problem 3
a) A circuit-switched network would be well suited to the application, because the application involves long sessions with predictable smooth bandwidth requirements. Since the transmission rate is known and not bursty, bandwidth can be reserved for each application session without significant waste. In addition, the overhead costs of setting up and tearing down connections are amortized over the lengthy duration of a typical application session.
b) In the worst case, all the applications simultaneously transmit over one or more network links. However, since each link has sufficient bandwidth to handle the sum of all of the applications' data rates, no congestion (very little queuing) will occur. Given such generous link capacities, the network does not need congestion control mechanisms.
Problem 4
a) Between the switch in the upper left and the switch in the upper right we can have 4 connections. Similarly we can have four connections between each of the 3 other pairs of adjacent switches. Thus, this network can support up to 16 connections.
b) We can 4 connections passing through the switch in the upper-right-hand corner and another 4 connections passing through the switch in the lower-left-hand corner, giving a total of 8 connections.
c) Yes. For the connections between A and C, we route two connections through B and two connections through D. For the connections between B and D, we route two connections through A and two connections through C. In this manner, there are at most 4 connections passing through any link.
Problem 5
Tollbooths are 75 km apart, and the cars propagate at 100km/hr. A tollbooth services a car at a rate of one car every 12 seconds.
a) There are ten cars. It takes 120 seconds, or 2 minutes, for the first tollbooth to service the 10 cars. Each of these cars has a propagation delay of 45 minutes (travel 75 km) before arriving at the second tollbooth. Thus, all the cars are lined up before the second tollbooth after 47 minutes. The whole process repeats itself for traveling between the second and third tollbooths. It also takes 2 minutes for the third tollbooth to service the 10 cars. Thus the total delay is 96 minutes.
b) Delay between tollbooths is 8*12 seconds plus 45 minutes, i.e., 46 minutes and 36 seconds. The total delay is twice this amount plus 8*12 seconds, i.e., 94 minutes and 48 seconds.
Problem 6
a) s m d prop / seconds.
b) R L d
seconds.
c)
seconds.
d) The bit is just leaving Host A.
e) The first bit is in the link and has not reached Host B.
f) The first bit has reached Host B.
g) Want
Problem 7
Consider the first bit in a packet. Before this bit can be transmitted, all of the bits in the packet must be generated. This requires
The
to transmit the packet
Propagation delay = 10 msec.
The delay until decoding is
7msec + 224 sec + 10msec = 17.224msec
A similar analysis shows that all bits experience a delay of 17.224 msec.
Problem 8
a) 20 users can be supported b) 1 0 p .
c)
d)
We use the central limit theorem to approximate this probability. Let j X be independent random variables such that
Problem 9 a) 10,000
Problem 10
The first end system requires L/R1 to transmit the packet onto the first link; the packet propagates over the first link in d1/s1; the packet switch adds a processing delay of dproc; after receiving the entire packet, the packet switch connecting the first and the second
link requires L/R2 to transmit the packet onto the second link; the packet propagates over the second link in d2/s2. Similarly, we can find the delay caused by the second switch and the third link: L/R3, dproc, and d3/s3. Adding these five delays gives dend-end = L/R1 + L/R2 + L/R3
To answer the second question, we simply plug the values into the equation to get 6 + 6 + 6 + 20+16 + 4 + 3 + 3 = 64 msec.
Problem 11
Because bits are immediately transmitted, the packet switch does not introduce any delay; in particular, it does not introduce a transmission delay. Thus, dend-end = L/R + d1/s1 + d2/s2+ d3/s3
For the values in Problem 10, we get 6 + 20 + 16 + 4 = 46 msec.
Problem 12
The arriving packet must first wait for the link to transmit 4.5 *1,500 bytes = 6,750 bytes or 54,000 bits. Since these bits are transmitted at 2 Mbps, the queuing delay is 27 msec. Generally, the queuing delay is (nL + (L - x))/R.
Problem 13
a) The queuing delay is 0 for the first transmitted packet, L/R for the second transmitted packet, and generally, (n-1)L/R for the n th transmitted packet. Thus, the average delay for the N packets is:
(L/R + 2L/R + ....... + (N-1)L/R)/N = L/(RN) * (1 + 2 + ..... + (N-1)) = L/(RN) * N(N-1)/2 = LN(N-1)/(2RN) = (N-1)L/(2R)
Note that here we used the well-known fact:
1 + 2 + ....... + N = N(N+1)/2
b) It takes R LN / seconds to transmit the N packets. Thus, the buffer is empty when a each batch of N packets arrive. Thus, the average delay of a packet across all batches is the average delay within one batch, i.e., (N-1)L/2R.
Problem 14
a) The transmission delay is R L / . The total delay is I R
R L I R IL 1 / ) (1
b) Let R L x /
Total delay = ax x 1
For x=0, the total delay =0; as we increase x, total delay increases, approaching infinity as x approaches 1/a.
Problem 15
Total delay
Problem 16
The total number of packets in the system includes those in the buffer and the packet that is being transmitted. So, N=10+1.
Because d a N , so (10+1)=a*(queuing delay + transmission delay). That is, 11=a*(0.01+1/100)=a*(0.01+0.01). Thus, a=550 packets/sec.
Problem 17
a) There are Q nodes (the source host and the 1 Q routers). Let q proc d denote the processing delay at the q th node. Let q R be the transmission rate of the q th link and let q q trans R L d / . Let q prop d be the propagation delay across the q th link. Then
b) Let q queue d denote the average queuing delay at node q . Then
Problem 18
On linux you can use the command traceroute www.targethost.com and in the Windows command prompt you can use tracert www.targethost.com
In either case, you will get three delay measurements. For those three measurements you can calculate the mean and standard deviation. Repeat the experiment at different times of the day and comment on any changes.
Here is an example solution:
Traceroutes between San Diego Super Computer Center and www.poly.edu
a) The average (mean) of the round-trip delays at each of the three hours is 71.18 ms, 71.38 ms and 71.55 ms, respectively. The standard deviations are 0.075 ms, 0.21 ms, 0.05 ms, respectively.
b) In this example, the traceroutes have 12 routers in the path at each of the three hours. No, the paths didn’t change during any of the hours.
c) Traceroute packets passed through four ISP networks from source to destination. Yes, in this experiment the largest delays occurred at peering interfaces between adjacent ISPs.
Traceroutes from www.stella-net.net (France) to www.poly.edu (USA).
d) The average round-trip delays at each of the three hours are 87.09 ms, 86.35 ms and 86.48 ms, respectively. The standard deviations are 0.53 ms, 0.18 ms, 0.23 ms, respectively. In this example, there are 11 routers in the path at each of the three hours. No, the paths didn’t change during any of the hours. Traceroute packets passed three ISP networks from source to destination. Yes, in this experiment the largest delays occurred at peering interfaces between adjacent ISPs.
Problem 19
An example solution:
Traceroutes from two different cities in France to New York City in United States
a) In these traceroutes from two different cities in France to the same destination host in United States, seven links are in common including the transatlantic link.
b) In this example of traceroutes from one city in France and from another city in Germany to the same host in United States, three links are in common including the transatlantic link
Traceroutes to two different cities in China from same host in United States
c) Five links are common in the two traceroutes. The two traceroutes diverge before reaching China
Problem 20
Throughput = min{Rs, Rc, R/M}
Problem 21
If only use one path, the max throughput is given by:
If use all paths, the max throughput is given by
Problem 22
Probability of successfully receiving a packet is: ps= (1-p)N .
The number of transmissions needed to be performed until the packet is successfully received by the client is a geometric random variable with success probability ps. Thus, the average number of transmissions needed is given by: 1/ps Then, the average number of re-transmissions needed is given by: 1/ps -1.
Problem 23
Let’s call the first packet A and call the second packet B.
a) If the bottleneck link is the first link, then packet B is queued at the first link waiting for the transmission of packet A. So the packet inter-arrival time at the destination is simply L/Rs.
b) If the second link is the bottleneck link and both packets are sent back to back, it must be true that the second packet arrives at the input queue of the second link before the second link finishes the transmission of the first packet. That is,
L/Rs + L/Rs + dprop < L/Rs + dprop + L/Rc
The left hand side of the above inequality represents the time needed by the second packet to arrive at the input queue of the second link (the second link has not started transmitting the second packet yet). The right hand side represents the time needed by the first packet to finish its transmission onto the second link.
If we send the second packet T seconds later, we will ensure that there is no queuing delay for the second packet at the second link if we have:
L/Rs + L/Rs + dprop + T >= L/Rs + dprop + L/Rc
Thus, the minimum value of T is L/Rc L/Rs .
Problem 24
40 terabytes = 40 * 1012 * 8 bits. So, if using the dedicated link, it will take 40 * 1012 * 8 / (100 *106 ) =3200000 seconds = 37 days. But with FedEx overnight delivery, you can guarantee the data arrives in one day, and it should cost less than $100.
Problem 25
a) 160,000 bits
b) 160,000 bits
c) The bandwidth-delay product of a link is the maximum number of bits that can be in the link
d) the width of a bit = length of link / bandwidth-delay product, so 1 bit is 125 meters long, which is longer than a football field
e) s/R
Problem 26 s/R=20000km, then R=s/20000km= 2.5*108/(2*107)= 12.5 bps
Problem 27
a) 80,000,000 bits
b) 800,000 bits, this is because that the maximum number of bits that will be in the link at any given time = min(bandwidth delay product, packet size) = 800,000 bits.
c) .25 meters
Problem 28
a) ttrans + tprop = 400 msec + 80 msec = 480 msec
b) 20 * (ttrans + 2 tprop) = 20*(20 msec + 80 msec) = 2 sec.
c) Breaking up a file takes longer to transmit because each data packet and its corresponding acknowledgement packet add their own propagation delays.
Problem 29
Recall geostationary satellite is 36,000 kilometers away from earth surface.
a) 150 msec
b) 1,500,000 bits
c) 600,000,000 bits
Problem 30
Let’s suppose the passenger and his/her bags correspond to the data unit arriving to the top of the protocol stack. When the passenger checks in, his/her bags are checked, and a tag is attached to the bags and ticket. This is additional information added in the Baggage layer if Figure 1.20 that allows the Baggage layer to implement the service or separating the passengers and baggage on the sending side, and then reuniting them (hopefully!) on the destination side. When a passenger then passes through security and additional stamp is often added to his/her ticket, indicating that the passenger has passed through a security check. This information is used to ensure (e.g., by later checks for the security information) secure transfer of people.
Problem 31
a) Time to send message from source host to first packet switch = 4sec sec 10 2 10 8 6 6
With store-and-forward switching, the total time to move message from source host to destination host = 12sec 3 4sec hops
b) Time to send 1st packet from source host to first packet switch = .
. Time at which 2nd packet is received at the first switch = time at which 1st packet is received at the second switch = sec 10 sec 5 2 m m
c) Time at which 1st packet is received at the destination host = sec 15 3 sec 5 m hops m . After this, every 5msec one packet will be received; thus time at which last (800th) packet is received = 01sec 4 sec 799*5 sec 15 m m . It can be seen that delay in using message segmentation is significantly less (almost 1/3rd).
d)
e)
i. Without message segmentation, if bit errors are not tolerated, if there is a single bit error, the whole message has to be retransmitted (rather than a single packet).
ii. Without message segmentation, huge packets (containing HD videos, for example) are sent into the network. Routers have to accommodate these huge packets. Smaller packets have to queue behind enormous packets and suffer unfair delays.
i. Packets have to be put in sequence at the destination.
ii. Message segmentation results in many smaller packets. Since header size is usually the same for all packets regardless of their size, with message segmentation the total amount of header bytes is more.
Problem 32
Yes, the delays in the applet correspond to the delays in the Problem 31.The propagation delays affect the overall end-to-end delays both for packet switching and message switching equally.
Problem 33
There are F/S packets. Each packet is S=80 bits. Time at which the last packet is received at the first router is S F R S 80 sec. At this time, the first F/S-2 packets are at the destination, and the F/S-1 packet is at the second router. The last packet must then be transmitted by the first router and the second router, with each transmission taking R S 80 sec Thus delay in sending the whole file is 2) ( 80 S F R S delay To calculate the value of S which leads to the minimum delay, F S delay dS d 40 0
Problem 34
The circuit-switched telephone networks and the Internet are connected together at "gateways". When a Skype user (connected to the Internet) calls an ordinary telephone, a circuit is established between a gateway and the telephone user over the circuit switched network. The skype user's voice is sent in packets over the Internet to the gateway. At the gateway, the voice signal is reconstructed and then sent over the circuit. In the other direction, the voice signal is sent over the circuit switched network to the gateway. The gateway packetizes the voice signal and sends the voice packets to the Skype user.
Chapter 2 Review Questions
1. The Web: HTTP; file transfer: FTP; remote login: Telnet; e-mail: SMTP; BitTorrent file sharing: BitTorrent protocol
2. Network architecture refers to the organization of the communication process into layers (e.g., the five-layer Internet architecture). Application architecture, on the other hand, is designed by an application developer and dictates the broad structure of the application (e.g., client-server or P2P).
3. The process which initiates the communication is the client; the process that waits to be contacted is the server.
4. No. In a P2P file-sharing application, the peer that is receiving a file is typically the client and the peer that is sending the file is typically the server.
5. The IP address of the destination host and the port number of the socket in the destination process
6. You would use UDP. With UDP, the transaction can be completed in one roundtrip time (RTT) - the client sends the transaction request into a UDP socket, and the server sends the reply back to the client's UDP socket. With TCP, a minimum of two RTTs are needed - one to set-up the TCP connection, and another for the client to send the request, and for the server to send back the reply.
7. One such example is remote word processing, for example, with Google docs. However, because Google docs runs over the Internet (using TCP), timing guarantees are not provided.
8. a) Reliable data transfer
TCP provides a reliable byte-stream between client and server but UDP does not.
b) A guarantee that a certain value for throughput will be maintained
Neither
c) A guarantee that data will be delivered within a specified amount of time
Neither
d) Confidentiality (via encryption)
Neither
9. SSL operates at the application layer. The SSL socket takes unencrypted data from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, she has to include the SSL code in the application.
10. A protocol uses handshaking if the two communicating entities first exchange control packets before sending data to each other. SMTP uses handshaking at the application layer whereas HTTP does not.
11. The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not.
12. When the user first visits the site, the server creates a unique identification number, creates an entry in its back-end database, and returns this identification number as a cookie number. This cookie number is stored on the user’s host and is managed by the browser. During each subsequent visit (and purchase), the browser sends the cookie number back to the site. Thus the site knows when this user (more precisely, this browser) is visiting the site.
13. Web caching can bring the desired content “closer” to the user, possibly to the same LAN to which the user’s host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links.
14. Telnet is not available in Windows 7 by default. to make it available, go to Control Panel, Programs and Features, Turn Windows Features On or Off, Check Telnet client. To start Telnet, in Windows command prompt, issue the following command > telnet webserverver 80
where "webserver" is some webserver. After issuing the command, you have established a TCP connection between your client telnet program and the web server. Then type in an HTTP GET message. An example is given below:
Since the index.html page in this web server was not modified since Fri, 18 May 2007 09:23:34 GMT, and the above commands were issued on Sat, 19 May 2007, the server returned "304 Not Modified". Note that the first 4 lines are the GET message and header lines inputed by the user, and the next 4 lines (starting from HTTP/1.1 304 Not Modified) is the response from the web server.
15. FTP uses two parallel TCP connections, one connection for sending control information (such as a request to transfer a file) and another connection for actually transferring the file. Because the control information is not sent over the same connection that the file is sent over, FTP sends control information out of band.
16. The message is first sent from Alice’s host to her mail server over HTTP. Alice’s mail server then sends the message to Bob’s mail server over SMTP. Bob then transfers the message from his mail server to his host over POP3.
17.
Received: from 65.54.246.203 (EHLO bay0-omc3-s3.bay0.hotmail.com) (65.54.246.203) by mta419.mail.mud.yahoo.com with SMTP; Sat, 19 May 2007 16:53:51 -0700
Received: from hotmail.com ([65.55.135.106]) by bay0-omc3-s3.bay0.hotmail.com with Microsoft SMTPSVC(6.0.3790.2668); Sat, 19 May 2007 16:52:420700
Received: from mail pickup service by hotmail.com with Microsoft SMTPSVC; Sat, 19 May 2007 16:52:41 -0700
Message-ID: <BAY130-F26D9E35BF59E0D18A819AFB9310@phx.gbl>
Received: from 65.55.135.123 by by130fd.bay130.hotmail.msn.com with HTTP; Sat, 19 May 2007 23:52:36 GMT
From: "prithula dhungel" <prithuladhungel@hotmail.com>
To: prithula@yahoo.com
Bcc:
Subject: Test mail
Date: Sat, 19 May 2007 23:52:36 +0000
Mime-Version:1.0
Content-Type: Text/html; format=flowed
Return-Path: prithuladhungel@hotmail.com
Figure: A sample mail message header
Received: This header field indicates the sequence in which the SMTP servers send and receive the mail message including the respective timestamps. In this example there are 4 “Received:” header lines. This means the mail message passed through 5 different SMTP servers before being delivered to the receiver’s mail box. The last (forth) “Received:” header indicates the mail message flow from the SMTP server of the sender to the second SMTP server in the chain of servers. The sender’s SMTP server is at address 65.55.135.123 and the second SMTP server in the chain is by130fd.bay130.hotmail.msn.com.
The third “Received:” header indicates the mail message flow from the second SMTP server in the chain to the third server, and so on.
Finally, the first “Received:” header indicates the flow of the mail messages from the forth SMTP server to the last SMTP server (i.e. the receiver’s mail server) in the chain.
Message-id: The message has been given this number BAY130F26D9E35BF59E0D18A819AFB9310@phx.gbl (by bay0-omc3s3.bay0.hotmail.com. Message-id is a unique string assigned by the mail system when the message is first created.
From: This indicates the email address of the sender of the mail. In the given example, the sender is “prithuladhungel@hotmail.com”
To: This field indicates the email address of the receiver of the mail. In the example, the receiver is “prithula@yahoo.com”
Subject: This gives the subject of the mail (if any specified by the sender). In the example, the subject specified by the sender is “Test mail”
Date: The date and time when the mail was sent by the sender. In the example, the sender sent the mail on 19th May 2007, at time 23:52:36 GMT.
Mime-version: MIME version used for the mail. In the example, it is 1.0.
Content-type: The type of content in the body of the mail message. In the example, it is “text/html”.
Return-Path: This specifies the email address to which the mail will be sent if the receiver of this mail wants to reply to the sender. This is also used by the sender’s mail server for bouncing back undeliverable mail messages of mailer-daemon error messages. In the example, the return path is “prithuladhungel@hotmail.com”.
18. With download and delete, after a user retrieves its messages from a POP server, the messages are deleted. This poses a problem for the nomadic user, who may want to access the messages from many different machines (office PC, home PC, etc.). In the download and keep configuration, messages are not deleted after the user retrieves the messages. This can also be inconvenient, as each time the user retrieves the stored messages from a new machine, all of non-deleted messages will be transferred to the new machine (including very old messages).
19. Yes an organization’s mail server and Web server can have the same alias for a host name. The MX record is used to map the mail server’s host name to its IP address.
20. You should be able to see the sender's IP address for a user with an .edu email address. But you will not be able to see the sender's IP address if the user uses a gmail account.
21. It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur even if Alice provides chunks to Bob throughout a 30-second interval.
22. Recall that in BitTorrent, a peer picks a random peer and optimistically unchokes the peer for a short period of time. Therefore, Alice will eventually be optimistically unchoked by one of her neighbors, during which time she will receive chunks from that neighbor.
23. The overlay network in a P2P file sharing system consists of the nodes participating in the file sharing system and the logical links between the nodes. There is a logical link (an “edge” in graph theory terms) from node A to node B if there is a semipermanent TCP connection between A and B. An overlay network does not include routers.
24. Mesh DHT: The advantage is in order to a route a message to the peer (with ID) that is closest to the key, only one hop is required; the disadvantage is that each peer must track all other peers in the DHT. Circular DHT: the advantage is that each peer needs to track only a few other peers; the disadvantage is that O(N) hops are needed to route a message to the peer that is closest to the key.
25.
a) File Distribution
b) Instant Messaging
c) Video Streaming
d) Distributed Computing
26. With the UDP server, there is no welcoming socket, and all data from different clients enters the server through this one socket. With the TCP server, there is a welcoming socket, and each time a client initiates a connection to the server, a new socket is created. Thus, to support n simultaneous connections, the server would need n+1 sockets.
27. For the TCP application, as soon as the client is executed, it attempts to initiate a TCP connection with the server. If the TCP server is not running, then the client will fail to make a connection. For the UDP application, the client does not initiate connections (or attempt to communicate with the UDP server) immediately upon execution
Chapter 2 Problems
Problem 1
a) F
b) T
c) F
d) F
e) F
Problem 2
Access control commands: USER, PASS, ACT, CWD, CDUP, SMNT, REIN, QUIT.
Transfer parameter commands: PORT, PASV, TYPE STRU, MODE.
Service commands: RETR, STOR, STOU, APPE, ALLO, REST, RNFR, RNTO, ABOR, DELE, RMD, MRD, PWD, LIST, NLST, SITE, SYST, STAT, HELP, NOOP
Problem 3
Application layer protocols: DNS and HTTP
Transport layer protocols: UDP for DNS; TCP for HTTP
Problem 4
a) The document request was http://gaia.cs.umass.edu/cs453/index.html. The Host : field indicates the server's name and /cs453/index.html indicates the file name.
b) The browser is running HTTP version 1.1, as indicated just before the first <cr><lf> pair.
c) The browser is requesting a persistent connection, as indicated by the Connection: keep-alive.
d) This is a trick question. This information is not contained in an HTTP message anywhere. So there is no way to tell this from looking at the exchange of HTTP messages alone. One would need information from the IP datagrams (that carried the TCP segment that carried the HTTP GET request) to answer this question.
e) Mozilla/5.0. The browser type information is needed by the server to send different versions of the same object to different types of browsers.
Problem 5
a) The status code of 200 and the phrase OK indicate that the server was able to locate the document successfully. The reply was provided on Tuesday, 07 Mar 2008 12:39:45 Greenwich Mean Time.
b) The document index.html was last modified on Saturday 10 Dec 2005 18:27:46 GMT.
c) There are 3874 bytes in the document being returned.
d) The first five bytes of the returned document are : <!doc. The server agreed to a persistent connection, as indicated by the Connection: Keep-Alive field
Problem 6
a) Persistent connections are discussed in section 8 of RFC 2616 (the real goal of this question was to get you to retrieve and read an RFC). Sections 8.1.2 and 8.1.2.1 of the RFC indicate that either the client or the server can indicate to the other that it is going to close the persistent connection. It does so by including the connection-token "close" in the Connection-header field of the http request/reply.
b) HTTP does not provide any encryption services.
c) (From RFC 2616) “Clients that use persistent connections should limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy.”
d) Yes. (From RFC 2616) “A client might have started to send a new request at the same time that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress.”
Problem 7
The total amount of time to get the IP address is
Once the IP address is known, ORTT elapses to set up the TCP connection and another ORTT elapses to request and receive the small object. The total response time is
Problem 8
a)
b)
c)
Problem 9
a) The time to transmit an object of size L over a link or rate R is L/R. The average time is the average size of the object divided by R:
= (850,000 bits)/(15,000,000 bits/sec) = .0567 sec
The traffic intensity on the link is given by =(16 requests/sec)(.0567 sec/request) = 0.907. Thus, the average access delay is (.0567 sec)/(1 - .907) .6 seconds. The total average response time is therefore .6 sec + 3 sec = 3.6 sec.
b) The traffic intensity on the access link is reduced by 60% since the 60% of the requests are satisfied within the institutional network. Thus the average access delay is (.0567 sec)/[1 – (.4)(.907)] = .089 seconds. The response time is approximately zero if the request is satisfied by the cache (which happens with probability .6); the average response time is .089 sec + 3 sec = 3.089 sec for cache misses (which happens 40% of the time). So the average response time is (.6)(0 sec) + (.4)(3.089 sec) = 1.24 seconds. Thus the average response time is reduced from 3.6 sec to 1.24 sec.
Problem 10
Note that each downloaded object can be completely put into one data packet. Let Tp denote the one-way propagation delay between the client and the server.
First consider parallel downloads using non-persistent connections. Parallel downloads would allow 10 connections to share the 150 bits/sec bandwidth, giving each just 15 bits/sec. Thus, the total time needed to receive all objects is given by:
(200/150+Tp + 200/150 +Tp + 200/150+Tp + 100,000/150+ Tp ) + (200/(150/10)+Tp + 200/(150/10) +Tp + 200/(150/10)+Tp + 100,000/(150/10)+ Tp ) = 7377 + 8*Tp (seconds)
Now consider a persistent HTTP connection. The total time needed is given by:
(200/150+Tp + 200/150 +Tp + 200/150+Tp + 100,000/150+ Tp ) + 10*(200/150+Tp + 100,000/150+ Tp ) =7351 + 24*Tp (seconds)
Assuming the speed of light is 300*106 m/sec, then Tp=10/(300*106)=0.03 microsec. Tp is therefore negligible compared with transmission delay.
Thus, we see that persistent HTTP is not significantly faster (less than 1 percent) than the non-persistent case with parallel download.
Problem 11
a) Yes, because Bob has more connections, he can get a larger share of the link bandwidth.
b) Yes, Bob still needs to perform parallel downloads; otherwise he will get less bandwidth than the other four users.
Problem 12
Server.py
fromsocketimport*
serverPort=12000
serverSocket=socket(AF_INET,SOCK_STREAM) serverSocket.bind(('',serverPort)) serverSocket.listen(1) connectionSocket,addr=serverSocket.accept() while1:
sentence=connectionSocket.recv(1024) print 'From Server:', sentence, '\n' serverSocket.close()
Problem 13
The MAIL FROM: in SMTP is a message from the SMTP client that identifies the sender of the mail message to the SMTP server. The From: on the mail message itself is NOT an SMTP message, but rather is just a line in the body of the mail message.
Another random document with no related content on Scribd:
houseclean her sentiments. A man never likes to throw anything away, you know, according to the tradition. He doesn’t like to throw away sex. He’s used it badly, spotted it up and all that—in his lucid moments he’ll even admit it. But none the less it’s very often the one thing which can excite his tenderness and reverence and when he sees us invade the home, as he says, it isn’t that he’s afraid the dishes won’t get washed. He says that, but what he is afraid of is that we’ll find the secret places of his sentiment and ravish them. I’m awfully sorry for some of the men. They’re going through a lot just now. They seem to feel so left out, under all their loud jocosity and foolish talk—you know,” she ended a little weakly.
“I know. I’ve been sorry for Gage myself. Terribly sorry for him. But I don’t see how one can make concessions. What I’m afraid of is that in his bitterness he’ll break Helen down. And she might give in but she’d never forgive him now.”
“You’ve done some speedy work, haven’t you? Smashed up homes and everything. What happened to your own pet Sinn Feiner?”
“He’s lecturing somewhere or other.”
“Is that all off?”
“It never was on.”
“You couldn’t get absorbed in his enthusiasm because you’ve got one of your own, haven’t you?”
“I have yours.”
Mrs. Thompson patted her affectionately on the arm. Her contacts were all warm and intimate. With men and women alike, she seemed to get inside their minds and look out on the world as they saw it.
“You’re a dear girl,” she told her, “but you must remember that humanity is a bigger thing even than feminism, Margaret. Be a little more tender towards the poor men. After all, they can’t all be transported.”
All afternoon the crowds swelled. In the evening the great hotel diningrooms were filled with people who represented almost everything—power, wealth, notoriety, ambition. Headquarters were established. Newspaper men idled to and fro, joking, prophesying, gossiping. Underneath the fatuousness of much of the pretense that this was a great popular meeting, most of the people knew that the rules were already laid down and things would take their course—must take their course. And yet there was a certain amount of
fair speculation as to whether in some way the great leaders might not be outwitted after all—whether some new element might not show sudden strength, whether the unorganized, half formulated hopes and ideals of millions of ordinary people might this time put themselves across and lead instead of follow. The Convention, like the world, was attuned to surprises, revolts, inexplicable overturnings.
Helen was more than excited. A little less than two days ago she had felt that she could not go on with this—that the personal agony had to be fought out first. To her amazement she found that Margaret had been right. Helen had always agreed with her that women were really not dependent on emotion but that had been because Margaret’s contention seemed reasonable and to take the other side hardly worthy. An inner feeling had persisted that after all Margaret was unmarried and didn’t—couldn’t know the strength of the emotional pull. But now she found herself breaking through personal emotional wreckage to impersonal interest—or if not impersonal interest at least interest in which sex played no part. She had at first kept up the signs of control because she must. Now she no longer needed the signs. She thought of Gage almost dispassionately. Now and then the tragedy of the ruined feeling between them shook her violently. But she could see their situation spread out before her. She could see it in relation to the children, to other work. She could see where things must be stopped in that they did not improve. And most of the time she hardly thought of Gage at all. She was responding to all the excitement and interest and admiration around her. She felt part of a great organization about to act in ways which would affect the world. The great sensibility of her woman’s imagination, undulled by much experience in the direction of things beyond her contact, was played upon by a vision of great power which she might help direct.
For the first time she understood what Margaret meant by the freedom of women and why she was not content with the formal letting down of the gates. She understood what some of the others meant when they talked of the easy contemporary victories as obscuring the real things which women needed.
The best of them did not talk the lingo of the “new day.” They knew that any day was not new long enough to get used to its title. They talked of adjustment of contemporary circumstances to an evolution as old as that of civilization, as old as that of man—merged with male development and yet
distinct from it again. They avoided catch words and the flattery which was sprinkled so thickly, avoided it not pedantically but with humorous knowledge of its purpose.
They dined with Mrs. Thompson and three other women and held a kind of informal court afterwards in one of the parlors of the hotel. Every notable man who came into the hotel seemed to want a word with Mrs. Thompson. She had a way with them that they all liked, a kind of keen camaraderie, especially effective in a woman who, like Mrs. Thompson, could never be accused of trying out any arts of sex attraction. They liked her company and her brisk tongue and there was added interest in finding such company in a woman. Helen met the Senators—the men whose names the party conjured with and handled them as she handled most people—skillfully.
It was a little after nine when Helen saw Gage at the end of the room. She had been talking with a gray haired, affable Senator who was telling her what a beneficent influence the women were to have on the Convention and she was excited, amused and sparkling. She was wearing a dinner dress of black lace that Gage had always liked and as she caught his eyes on her that was the first thought which flashed through her mind. It was followed by a quick appraisal of Gage himself. He was looking a little untidy and showing clearly the signs of recent strain and worry.
He did not make his way to her at once. He stopped to talk casually to some men whom he knew. Helen thought suddenly that Gage was not a big man politically. He did not have nearly as much prestige as Mrs. Thompson of course, not nearly as much strength as she herself might have—. She saw Margaret introducing him to Mrs. Thompson. His manners were bad. He had none of the easy pleasant way with which the other men had come up to her. He must be making an extremely bad impression. It humiliated her somewhat. They were still too close for her not to feel that.
She joined the group where were Gage and Mrs. Thompson and Margaret.
“Have you had a good day, Gage? I called up the house and the children are fine. Not a trace of their colds, Esther said.”
He nodded gravely. Their eyes met, denying any intimacies of exchange, coldly, a little cruelly.
“I hear your wife is to make the prize seconding speech to-morrow, Mr. Flandon,” said Harriet Thompson, bending towards him.
“I have no doubt of it,” said Gage.
“Do your views agree?” asked the older woman lightly.
“Very seldom,” answered Gage. He had not made it light. It was like the flick of a whip.
Margaret interposed.
“Gage doesn’t believe in women’s progress, Harriet—don’t get him started, please.”
“I wish he would get started. There are plenty of times when I think we’re all talking balderdash and it would be a relief to hear some one give testimony against us. What is the matter with women, Mr. Flandon?”
Gage’s tired, half-haunted eyes looked at her as if he suspected mockery but he found none.
“According to most belief, there is nothing the matter with them. They are supremely successful. They’ve got what they wanted. If they don’t like the taste of their little mess of pottage as they eat it, it will be unfortunate.”
“You don’t think they will like it?”
“I may be mistaken. It may suit their taste.”
“I’m afraid it won’t but it’s the best food we’re able to provide so far. Perhaps we’ve overpaid for it.”
“You have.”
He stopped, abruptly conscious of being drawn into discussion in public. Margaret and Helen had been listening to the brief dialogue, and he stiffened to the sense of their presence.
“I can’t stay, Helen—I’ve an appointment. Is there anything I can do?”
She walked with him to the end of the room, impelled by a desire to preserve what she could of appearances but more by an unexpected pain at having him leave her. She did not want him to stay—she was clear about that, but she hated to have him go away in that lonely fashion. The gentleness that welled up only lasted for a moment. He was ugly still. She could tell by the set of his lips. It brought back her terribly painful memories.
“Good night, Gage.”
That was all. Towards morning Gage went to bed. He had been drinking other people’s whisky and he was ill enough to suspect it had not been good
stuff.
II
He did not go to the great Auditorium until the next afternoon. It took some time to get himself into shape. The heat had begun, heat which settled thickly on the city for three days and played its own part in making possible agreements and compromises. By noon the smart look, the brisk look had gone from everything and everybody and the sticky battle with the weather had begun. Gage had met some men from his own part of the country and they entered the great hall where the banners hung limp from the ceiling and the delegates were already coming back to their places after the noon recess. Gage did not look for his wife but after a while he saw her—as usual looking the mistress of herself. His head was hot and thick and he hated her for the fine mastery of her health and beauty. He wanted to see her in tears —prostrate—and because he knew his desire was ugly he slipped down in his own self-respect, which already was becoming such a frail reed to cling to.
All that day he did not go near her. He watched her furtively sometimes while he was in the auditorium but most of the time he spent with other men in hotel rooms which grew hotter in spite of the efforts of electric fans and all the time the whisky which he drank made his brain hot and seething with misconceptions and desires and hatreds.
By the afternoon of the second day it had settled down to an endurance meeting.
Watching the restless, heated crowd, going through the same old formalities, Gage wondered whether Helen was aware now what kind of game this was she had chosen to sit in—whether the farce of it was clear. He did not wonder clearly. It was a kind of vindictive spite which pricked a muddled brain.
He had not intend to be there when she spoke but in the end he stayed. He heard the round commonplace phrases of the man who was nominating the candidate she was working for—a good man, as Gage admitted—better caliber than most, but without a ghost of a show for nomination. He listened with irritation to the outburst of applause. Then he saw his wife before the great crowd. It seemed quite unreal.
He had not guessed her voice would carry like that. He had not known she would show up like that. She came like a breath of cool air into that heated place. In her blue linen gown and white feather hat she looked cool, fresh, immaculate. When she spoke they listened to her and for a few minutes Gage caught himself listening eagerly. She was talking well. No nonsense. It was to the point. Just then he heard a man behind him.
“Some looker, isn’t she? That’s the kind of dames we ought to have in politics all right.”
Blind rage swept over Gage again. He wanted to turn on the man and fight. But he did nothing of the sort, being held by a thousand inhibitions. Instead he watched his wife and as she talked he seemed to see her offering her beauty to the crowd, seemed to see in every man’s face—as they watched her—amusement, desire, lust.
He heard the burst of applause when she finished, applause with real enthusiasm and at every hand clap he felt fury rising. Getting up, he found his way to the door.
If Helen had expected a tribute from him, piled on the many she received that night, she was mistaken. Men, women, newspapers all congratulated her on having put some real fire into the speeches. Her speech, printed and flashed all over the country was given its own share of praise. It was clear, forceful, new in its outlook. The women of the country had chosen a good spokesman, said the papers. But from Gage there was only a note at the hotel, saying briefly that he had thought it best to return to St. Pierre—the convention was the usual farce.
Helen twisted his note in her hands.
“So he couldn’t stay away from Freda Thorstad even that long,” she thought. “Well,—”
CHAPTER XVI
MR. SABLE STARTS SOMETHING
WHAT Mr. Sable had counted on was that Gage, once away for the Convention, with his wife travelling with him, would stay away for a week or so. In any case, he had taken matters rather summarily into his own hands in the case of Freda Thorstad. The presence of the girl in the office was to him like an open scandal. He knew that she had to get out of the office and the time to get her out was while Gage was away. Of course Flandon might raise trouble when he came back but there would be no scene with the girl and he could put it flatly and finally that amours had to be conducted outside of the office if at all. He was extremely correct and secure in his own position and he felt he was most delicate.
So he was at the office a little earlier than usual the morning after Gage left for the Convention. Freda was at her desk sorting the first batch of mail. She looked very neat and capable, in her white blouse open a little at the throat, her thick golden red hair pulled back smoothly from her forehead, and her head industriously bent over her letters.
“Will you come in my office for a moment, Miss Thorstad?”
Freda followed him obediently. He closed the door and, taking his arm chair, left her standing a little dubiously before him.
“Miss Thorstad,” he said, “I think—er—that it will be best for you to sever your connection with this office.”
His tone, wholly disapproving, weighted with meaning, told his reasons with almost comic flatness.
Freda’s brow contracted and she looked sharply at him. Then she laughed. A brazen laugh, he would have said. Truly a laugh with no more fear or care or apprehension in it than the laugh of any child who comes upon something ridiculous.
Mr. Sable frowned. She was a hussy, he thought. Might try to bulldoze him a little—he became increasingly stern.
“I have no desire to go into our reasons for this but I think it will be best for you to simply leave at once. You may find the work too heavy for you. I
am sure you understand that no office of this kind could take the situation differently.”
“Wouldn’t it be better for me to wait until Mr. Flandon’s return?” she asked.
He had feared that.
“Surely you understand that your presence here is embarrassing to Mr. Flandon,” he said sharply.
If she had guessed what he suspected she might have contended. But all that he said struck her as true. She evidently was being gossiped about and if it did make it embarrassing for Mr. Flandon—Perhaps that was why he had been so over courteous, to conceal a deep embarrassment.
“Very well, Mr. Sable,”—she straightened her shoulders a little—“I shall not go on with my work here.”
“Exactly.” With victory so easily accomplished, Mr. Sable became different, adept at smoothing things over. “Of course when a young lady cannot typewrite, an office like this has hardly the right kind of work—”
“I know that. I told Mr. Flandon that at the start.”
“Mr. Flandon being absent, I will give you a check for this week’s work.”
“I’ve done only one day’s work, Mr. Sable. It is only”—she calculated —“two and a half dollars.”
She took his check for that—he did not dare press the point—and left his office. Mr. Sable smoothed his little white mustache, straightened his papers with the air of having done a good day’s work already, and pressed the buzzer for his own secretary.
It was only half past nine o’clock. Freda got her hat and coat from the tiny dressing-room. Her desk was in order and there was no use in fussing over it. She wanted to get out into the clean air. The pompous little lawyer’s insinuations while they did not strike deep enough to insult her, made her feel soiled and dirty.
Cele followed her into the dressing-room.
“Where you going?”
“Going out to look for a job.”
“He let you out?”
“That’s the substance of his message.”
“Well—I call that—” Cele stopped, a veil of thought coming over her eyes. “Look here,” she went on, “if there’s anything I can do—” She stressed the last word violently, as if the need for action pressed upon her.
“Not a thing.”
“You aren’t going back to—what-you-may-call-it? The place you came from?”
“Mohawk? No—mother’s coming here in a day or two. I’ll wait, and look for a job.”
“A job,” said Cele, reflectively, “don’t worry too much, will you? Say— I’ll be around to see you to-night. I think it’s rotten.”
Freda went out, wondering if the slanderous tongues had found even Cele’s ear.
But she did not linger on thoughts of her dismissal. She was sorry to leave Mr. Flandon so, but after all he knew and understood and the whole business was so temporary anyway. Gregory should be back any day now— and they would go away and never think of such ugliness any more. It was like her that no thought of personal justification, of setting people straight on the gossip, ever entered her head. She wanted to shake them off—that was all. She wanted to get away into light and clearness and cleanness with Gregory. And it seemed to her that merely being with Gregory would make an atmosphere like that.
She had received no letter from him in five or six days now and she missed one sadly. She needed that written touch of vigor and sweetness which set her days aflame with happiness. Especially now, with the knowledge that she was probably to bear his child. The lack of an address so she might send him that delightful information was distressing. She could have reached him through his lecture bureau but she had a dread of the letter going astray if it were not sent directly to him. Not a word or thought of resentment did she allow to penetrate her love. She kept herself free from that. It was harder to keep fear away.
She was strolling along, passing through the shopping district, now and then stopping to look idly at something in the window when she heard herself greeted. Looking up she saw Ted Smillie. He was quietly affable and there seemed no escape from speaking to him.
“How are you, Freda?” he asked calmly.
She resented his use of her name though he had come to using it before their disastrous evening.
“Quite well,” she said, and looked at him evenly, waiting for him to pass her.
He did not pass. He lingered, showing in his face the return of that avid attraction which he had felt so strongly before. She was thinner than she had been when he had seen her last and the shadows under her eyes made her face more delicate—more interesting!
“I wanted to see you again and luck’s come my way. You know that I did call on you the next day at the Brownleys’ and found you’d gone. I’m afraid I acted like an awful fool that night. Didn’t I?”
“Worse than that.”
“But it truly wasn’t my fault. I had been drinking. I know I can’t stand the stuff. And you made me quite lose my head.”
She reflected that of course it hadn’t been his fault as much as Barbara’s. And not knowing or dreaming that he was the agency which had violated the privacy of those two days at the Roadside Inn, she did not persist in great resentment. She disliked him of course but she was very idle and ready for distraction.
He went on talking, eagerly.
“It’s been on my mind ever since. I hated to let you think I was like that. Look here, Freda, I’ve got a free afternoon. Come in and have a cool drink somewhere with me—won’t you?”
“I don’t think so,” said Freda.
“Please.”
“I might hurt your reputation,” she said, with a scornful little laugh. “I understand I’m causing a lot of talk among your friends.”
“They always talk about every one—especially if a girl has the courage not to be conventional—”
She did not trust him in the least. Nor did she like him. It was sheer ennui which made her consent. She needed company.
They went to the tea room of a hotel, a cool place, furnished with abundant white willow and great palms. Freda had not been in such a place
before and she, as ever, was esthetically responsive to the oasis of comfort and coolness it made in the sweltering city. Ted ordered for her—a tall glass of cool Russian tea with mint leaves and thin lettuce edged sandwiches. His solicitude for her comfort dulled the edge of whatever resentment she had towards him—she had never bothered to preserve much.
“And what are you doing? Did I hear you were working—like all modern women?”
“Working I was—like all women who need the money,” she answered, “but I’m not working now.”
“You’re not going back to Mohawk?”
She remembered part of his proposition that she need not go back to Mohawk, made some weeks ago, glancing at him guardedly, thinking with a certain amount of interest that this was the very young man who had made suggestions which should have barred him permanently from her presence. Here she was, taking his iced tea. Things were queer. She didn’t even feel particularly angry at him. There wasn’t any use pretending false rigors.
“I don’t know,” she said, as she had said before.
“I’m glad you left the Brownleys’ anyhow. Are you living with other friends?”
“No—I’ve a room by myself.”
Obviously he liked that and the visible signs of his liking amused her.
“Can I come to see you once in a while?”
“Oh, no, indeed. I shouldn’t in the least like you as a caller.”
He was undisturbed.
“I’ll have to make you change your mind somehow.”
She shrugged her shoulders slightly, delicately, in negation.
“Do you know how much I’ve thought about you since that night?” he asked, bending a little closer to her.
“How much?”
“All the time.”
She pushed her glass away from her.
“Don’t be silly. I’m not a half-wit, Ted. I think you ought to be ashamed of yourself. You know quite well you’re engaged to Barbara Brownley. Her mother told Miss Duffield that.”
His face darkened.
“Well,” he said, “I thought you were above a lot of silly conventions.”
She smiled serenely. “I am.”
“Then—just because a man is engaged to a woman is no reason why he should never speak to any one else.”
“No—that would put a pretty high penalty on such things, wouldn’t it?”
“People are getting over their old fashioned notions about such things. Men and women aren’t simpletons as they used to be, you know. We’re regarding these things in a modern way. More like the French,” he said largely.
“The French—oh, yes,” said Freda, gravely, “you mean having wives and mistresses too. I’ve often wondered if the French couldn’t sue us for libel for the things provincial Americans think about them.”
He flushed. “Are you making fun of me?”
“Gracious, no.” But he knew from her laugh that she was. “Why should I make fun of you?”
She was enjoying herself. She felt so secure, so strong. It was fun to bait this temperish young man, make him scuttle about for phrases which had no effect on her at all.
“Anyway you know how I feel.” He pushed aside the glasses and plates between them and bent himself over the tiny table towards her. She sat back in her chair promptly.
“You know how I feel,” he persisted, “I never cared for a girl as I’ve cared for you.”
“Then,” said Freda, with an air of great simplicity, “why not ask me to marry you?”
He threw out his hands in a theatrical gesture of despondency.
“I’m tied hand and foot. This marriage of mine has been cooked up by our families. It’s all arranged for us.”
“I know,” said Freda wisely, “as in France.”
He glanced sharply at her.
“But,” said Freda, “the modern way is not to let your parents put that sort of thing over. Truly. One simply says, ‘Mother—I will wed the girl of my choice.’ ”
“You are making fun of me.”
“Well—who wouldn’t?” Freda collapsed into a laugh. “Here I sit, listening to you make the funniest clandestine love in the world. You feel you’ve got to do it—to uphold your reputation as a—Frenchman! And if you slipped into a serious situation you’d be aghast. You don’t care a thing about me and you know it.”
“Ah, don’t I?” He looked for a moment as if he did.
“You probably care a little about corrupting me. Now look here, Ted, please stop talking such nonsense. You can’t shock me and it’s pretty hard to insult me—I am a little ashamed of not being more insulted—but you probably could make me very angry by persisting in trying to involve me in petty vice. In the first place I don’t like it. In the second place, if I ever went in for vice, it would be on a larger scale than you could dream of. I haven’t the slightest intention of—being French! You’d better go along and make love to Bob Brownley. She’ll bring some excitement into your life, I think. The reason I’m not more angry with you is that you were, indirectly, the cause of the greatest bit of luck that ever happened to me.”
“What?”
“I wouldn’t dream of telling you. But I’m awfully obliged for the tea— truly. It set me up. Shall we go?”
He was not so easy to repulse. He got up and pulled his chair around to her side of the table.
“Freda,” he tried to take her hand, “if I gave up Bob would you let me see you?”
“I wouldn’t if you gave up the world.”
She rose a little impatiently, feeling that this was going too far, and started for the door of the dining-room.
“At least you’ll tell me where you live?” he pressed her. “Let me go home with you now.”
“Don’t you have to work in the middle of the afternoon?”
“Nobody’s working much. It’s too hot.”
“Then go play with Barbara. I’ve other things to do.”
Possibly it was the heat and the sense of effort which got nowhere that made Ted’s face intense and angry. He saw her about to slip away again.
“You can’t go like this, Freda, I’ve only just found you.”
“You’d better let me,” answered Freda, “because I see Maud Dubonnet looking at you and she knows you and obviously isn’t intending to speak to me though I lunched at her house, so you see it will be hot with you when Barbara hears this.”
Against his will Ted looked up and saw Maud truly, with two other girls and three young men coming into the room. They had to pass Ted and Freda as they stood there, discussing. Maud Dubonnet was the only one of the group whom Freda knew. The others all evidently knew Ted and glanced at him with some interest. Maud did not look at Freda. She held her head stiffly as she passed, then said something to the others which made them turn with an attempt at casualness to look at the man and girl. Ted delayed no longer. He followed Freda out.
“You see it doesn’t pay to do things Barbara won’t like. This will get back to her before to-morrow and she won’t be pleasant.”
Ted’s mouth set in an rather ugly line.
“I’ll manage Bob all right.” He looked at Freda. Her face under the plain white hat she wore was mocking, insubordinate, fascinating. “But I want to see you again. To-night?”
“Nonsense. Good-by, Ted. Be good and make your peace with Bob.”
She turned and went in the opposite direction from the one in which they had started, going into the first big department store and retiring to the ladies’ waiting-room where she wrote a letter to her father, and mailed it. Then, having made sure she was rid of Ted she went home. The afternoon dragged along. She read and thought and on an impulse went out again to go to the railway station and get some time tables. She wanted to see just how far Gregory had been away from her when he last wrote.
Each time the postman approached the house there was a leap of her heart. Four times a day he came and each time he brought fresh hope. She would play tricks on herself as she went down to look in the mail box she shared with the people who rented the apartment in which she stayed. Each time she put her hand in the box she hesitated before she looked, then looked quickly as if to catch fate before it tricked her. But it would be an advertisement of a corset firm for Mrs. Miller, an envelope with unmistakable savor of a bill about it, a postcard, a white Louisine envelope
with a woman’s handwriting on it. How she hated all the flatness of the Miller mail.
Each envelope she took in her hands seemed to be mischievously metamorphosed into one of these stupid envelopes which represented such dull contact with the outside world. Nothing to do but to go upstairs and read all over again the old messages of love from him—to wear her wedding ring in the privacy of her room—to make endless computations on the presumable date of her child’s birth—to read with unfailing zest and yet slight nausea the rather mawkish pages of “What Every Mother Should Know” which she had shamefacedly but defiantly bought at a book shop, feeling the necessity for some practical knowledge of marriage.
The next day, breaking through her apprehension and her waiting, cutting across her vague fears, came the letter. It lay between an announcement of the opening of a new hair dressing parlor and Mr. Miller’s water and light bill. How could she hope that the other white envelope would be anything more interesting? Then she turned it over and the address stared at her blackly. It was addressed to Mrs. Gregory Macmillan, in an unfamiliar hand, postmarked at the town from which Gregory had last written. Gregory always addressed her letters to Freda Thorstad to avoid any explanations to the Millers. A quick faint fear came over her. She almost crushed the letter as she flew up the stairs and with her back against her door faced the envelope again.
Then steeling her mind and her heart, presenting only outer senses to what blow it might contain, she opened it. The written words made their sense clear, like some amazingly vital story that thrilled every nerve.
“M M . M :
I obtained your address from your husband and I am writing you to tell you that he is extremely ill. We have done our best for him and he has a nurse with him constantly but I feel that you should come to him if it is at all possible. I do not know what responsibilities of family may hold you but I think it my duty to inform you that your husband is very sick. He lectured here on the fourteenth and the next morning the proprietor of this hotel called me to attend him. I found him in the first stages of typhoid and had him removed to a hospital here which is comfortable and where we have given him every attention. At a time like this his family should be with him. I regret that I must be the agent of such distressing news.
Faithfully yours, L. D. M , M.D.”
She read it through twice carefully. The thing struck her as quite unreal, although she had speculated on the possibility of his illness.
Then her mind, working reasonably, went on. She thought of trains and money. She had fifty—no forty-nine dollars and seventy-five cents. It wasn’t enough, she knew.
She had the time tables which she had obtained the day before. She studied them, her body held tautly, her face calm, showing a control which had come unconsciously. She could leave at three fifteen that afternoon. She’d have to change trains at midnight. She’d get there at noon next day. There was money. She must have money. No one to get it from unless she wired her father—better not—Miss Duffield away—Mr. Flandon away— Cele had none. She thought even of Ted Smillie. Better not—she’d pawn something. That was what people did when they needed money.
Like most girls she had a small collection of semi-precious jewelry— nothing of great value. She opened the little mock-ivory box on her dressing table and considered the contents carefully. Then she closed it, put on her hat and stuck the box in her pocket.
Pawning was unlike any experience she had ever had and not as exciting as books had led her to believe. She felt no shame—only a vague hostility to the pawnbroker. She hated his having so obviously the best of the transaction. He was scornful in her array of articles to sell—the gold bracelet that had cost her father thirty-five dollars—the little one carat
diamond ring that had been her mother’s—the opal ring—the seal ring—the little silver locket.
In the end he gave her thirty-three dollars, and with the money in her hands she immediately got his point of view. She had exchanged a lot of things which meant little to her for the boundless power of thirty-three dollars which added to forty-nine made eighty-two.
She bought a ticket to Fairmount, Montana. It cost her twenty-eight dollars and sixty-four cents. She put it in her purse and went home, a splendid sense of action stirring her.
It took her a very short time to pack her bag. There remained two hours before the train. She spent it sitting in her room and letting the knowledge of what she was doing penetrate her mind. It occurred to her that she should let some one know where she was going but in the face of Gregory’s illness it seemed even less possible to confide the news of her marriage. That was to have been a glorious revelation to a few people. She could not turn it into tragedy, so she decided to tell no one.
To her father she wrote a letter.
Yet even to him she could not tell the facts. It seemed now when circumstances seemed to imperil her secret that she clutched it even more tightly to herself. She could not bear the thought of comment breaking in like a destructive barrage on the secret glory and beauty she cherished. In her absorption she did not think much of consequences or possible worry for any one. Only as she told Mrs. Miller that she wanted to pay up for her week and that she had been called out of town, did it occur to her from a comment of Mrs. Miller’s, that her mother was coming in a day or two. The complication puzzled her—then she overrode it boldly. It was one of the things that had to be. So she wrote a note for her mother and entrusted it to Mrs. Miller. It was only a few lines to tell her that she had left the city —“I’m sorry that I can’t be more definite about my plans. There’s nothing to worry you, mother. It’s quite all right and I’m not doing anything I shouldn’t. So please don’t worry about me. Only trust me, won’t you? I know you will.” She sent her love and for all her assurances on paper that she knew her mother would trust her she sealed it with a dubious look.
In the letter to her father she was, if not more informative, at least more expansive.
It was incoherent, reassuring, happy, sad—the kind of letter that carries with it great fear to the one who reads it and who sees how delicate the balance is on which the future is being weighed.
Freda mailed the note to her father, left the one for her mother with Mrs. Miller and went to the station. Almost before she knew it the long train, with a jerk of its loose hung body had gathered itself together and moved out of the yards through the scattering, blackened railroad district. She watched the little houses and let her mind sink into a blur of remembrance and anticipation. She was going to Gregory. No more waiting for letters, no more dreams. Whatever it was that was to come, it was reality—something to feel and to do—not to wait for.
She slipped her wedding ring on her finger and displayed her hand a little absurdly on the edge of the window casing. Marvelous symbol—that ring, she thought. Too bad that people had come to regard it so disdainfully. How ill it must have been treated to have sunk into such disrepute. Across the aisle of the day coach she saw a like ring on the hand of a woman. It was a fleshy hand and the coarse pink skin pushed itself up on either side of the encircling band but Freda felt kinship and friendliness. With this unknown woman, with the unintelligent face she almost felt it possible to converse intimately—as if she might cross to her and say, “I am Mrs. Gregory Macmillan. Are you going far?”
When the shadows fell thick across the prairie and a white-coated waiter, a shade more important in his manner that he had been in passing through the Pullmans, had intrigued a fair number of the day-coach passengers into the diner, Freda rose a little stiffly. Her chin had red marks on it where she had cupped it in her hand for so long and there were streaks of coal dust under her eyes. She made a perfunctory and inadequate attempt to look presentable and faced a new adventure. She had never eaten on a train in her life.
The warm bustle and luxury of the place stirred her senses and brought her out of her lonely rhapsody into an appreciation of what went on around her. The adventure spirit came to the surface. Freda Thorstad—sitting at a tidy table in a dining-car, on the way to her husband. There was no disloyalty to Gregory’s illness that she could not resist the enchantment of shining dishes which looked like silver and warm and savory smells and smiling, interesting travelers, with above and around it all the clatter and
rush of the train, moving on to a hundred destinations, a hundred tragedies and comedies and romances. Her thoughts at least admitted no staleness as a possibility.
The meal stood out in her memory. She never forgot it. Tentatively she ordered, tea, biscuits and lamb chops. When the lamb chops came they lay on a platter with little sprigs of parsley offering them up and made her very hungry. They looked delightful but inadequate. She ate hungrily, for these last few days she had had little food.
Four—five—dragging hours. She bought a magazine with a flaring girl on its cover and read avidly, her mind sinking into its soporific fiction with weariness, getting respite from her own sharp and vivid thoughts.
The conductor came to tell her that this was her station. He lifted her heavy bag for her and carried it to the foot of the steps of the coach.
Then came the excitement of the swoop and pause of the Flyer. Freda was bundled aboard, hat awry, nervously watching for her bag, taken from her hands by some one, porter or conductor. The curtains swung from all the berths. The porter’s voice was low and lazy. He showed her to lower six —a little cubby hole with curtains drawn aside, revealing the delightful neatness of the berth. Freda knew even less of sleeping on trains than she did of eating there. Awkwardly she managed to undress and crept in between the thick white sheets. In the darkness she lay awake, wondering. Wondering at the rush and sound and the mysteries shrouded behind green swinging curtains. When the train shrieked a signal or stopped lurchingly at some station she pushed up the curtain beside her and, propped on her pillows, lay looking into the night tasting the full delight of inexperience. At last she fell asleep and dreamed of Gregory. It was a frightening dream. He did not know who she was, did not remember her. Towards dawn she pushed her way out of it and woke up to see the rain falling lightly over the even country and to realize that she was begrimed with the coal dust and sticky with heat.
At noon she reached Fairmount and stood in the station looking about her for information. The excitement of the last lap and approaching climax of her journey overcame her fatigue and her eyes were brilliant. She decided to take a taxi to the hospital and chose at random one from a row of disheveled looking “For Hire” machines waiting for the daily debouch of passengers from the Flyer. She climbed in with her bag and closed the
shaky door, and the driver started his motor. Freda’s heart was racing. The cab could not go fast enough—nor slow enough. It seemed to her as if she could not bear what might be waiting of joy or sorrow, as if emotion was welling up so strong that it would burst its bounds and overcome her. Through the dusty cab windows she saw Fairmount—ill developed wooden houses with unhealthy looking trees giving little shade—a business district of twelve or fifteen squares with all the machinery of business being conducted as it was at this hour in hundreds of other First National Banks and Gilt Edge Stores and Greek Restaurants and brick office buildings. The cab whisked through it rapidly and came to a section of broader streets where more impressive looking houses of brick or stone appeared at leisurely intervals. A little park with a dusty looking playground adjoining it. A row of apartments and there on the corner where a “Silent Zone” sign, awry and disregarded by a group of boys playing in the street made a vain appeal, was St. Agatha’s Hospital.
Inside the little entry was an office with three or four glass windows, behind which she looked for an informant. A slim, weary looking nun came at last, looking at her from behind steel rimmed glasses without curiosity.
“Macmillan—yes,” she said, “you’re—?”
“I’m his wife.”
The nun accepted the fact simply and as if she yielded Freda certain rights and privileges. Freda felt frightened. She wanted to go into Gregory’s room, kneel down by his bed and tell him to get well. She could see it wasn’t going to be as direct as that.
A buzzing, muffled bell, sounded by the nun, had summoned a nurse, who came into the office thumping heavily on her flat rubber-heeled shoes. She was commissioned to take Freda to the last room in corridor “A”—the “typhoid case.” Freda left her bag in the office and followed the nurse, as she clumped indifferently along. The presence of the nurse bothered her. She wanted to get rid of her—tell her she would go on alone but she did not dare. In corridor “A” the nurse gave her a chair.
“I’ll find his nurse and see if you can see him now.”
“I’m his wife,” said Freda.
The nurse nodded.
“I’ll get the nurse first. She wouldn’t like me to bring any one in without calling her first, you see.” She smiled a little as she explained this