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and untested,hung from his chest,like tools in a toolshed．The felt on his wading boots was as white as SnOW．A handerafted net swung on his back．The rod had never gotten wet,much less caught a fish,and the line was shiny from lack ofUSe．The reel was on backward．Typical beginner”,I thought,Rich and,like many rich people in my experienee,probably demanding． I reached out to shake his hand．A firm grabbed back．His wife,aⅡ1 attractive wol'Ilan brimming with confidenee,took a photo of us,then waved goodbye with an arlTl heavily weighted with turquoise bracelets．First I turned his reel round．He smiled and shrugged．Then We began his casting lesson on the lawn behind the main lodge． T0 my surprise,he was one of those rare people who connect with a fly rod almost immediately．It just looked right from the beginning,and he was charmed by the way the line seemed to magically flow above his silhouette on the lawn．I could just stand here all day and east,”he said,smiling．We did not have to trave]far to the water,since a perfectly sweet little creek ran along the last nine kilometres of the rutted dirt road I had tmvelled t}lat morning．The warmth of the sun raised the water temperature enough to awaken the rainbow and cutthroat trout that slept,and the caddis flies were dancing their erratic dance,here and there,over the water．ven in hip waders we were overdressed for the an．kle—deep creek,but we stepped in,waded out to the middle and faced upstream．My client cast,and I pointed to the place the fly should land．Oh,hey!Look at that,’’he said when the first fish struck．He WaS truly awed．

Then I corked the bottle and earried it down to the river．I threw the bottle as far out as I could．It splashed,bobbed up,and floated.1 watehed it go out of sight．I kept thinking about my$eere[projeet．Maybe my botne was on t}le way to Hawaii．Maybe it was on the way to France．Maybe it was Olq the way to China．Maybe 1 would write letters to the person who found it,and we would become friends．1 would go visit the person where he or she lived．I could see myself in Rio de Janeiro,dancing in the streets．I could see myseff in India,riding Oil an elephant．I could see myself in Africa．taming wild lions．A week went by．1 wondered how long I’d have to wait before I heard from the person who got my bottle．It might be months．Maybe my bottle would go to the North Pole and be found stuek in the iee by Eskimo hunters．Then I realized it might lie in the iee for years before it was found.Somebody might phone or write me,and 1 would evl~rl have forgotten about my bottle．I decided I should write a note to myself and hide it in my desk．where 1 would find it when I grew up．SO I could remind myself about the bottle then．Dear old Julian 1 wrote．Remember the bottle you throw in the river?And then I put down the day and the year that I threw it in．I had just finished hiding this message in the back of my desk when the phone rang．It was Gloria．“Julian.I have some news!”she said．

establishing itself in less than a decade as a genuine rival to the dollar as a world currency．But that SUCCESS disguises two failings．The first is that some countries have adapted a lot better to the discipline of the euro than others．Germany and the Netherlands have cut labour costs and introduced enough reforms to make cheir economies more competitive．France,Spain and especially Italy have done less—and are suffering more．from both the euro’s rise and the global slowdown10．trimftnm]vt．to reduce the amount or number of sth1 1．1iquidity_IItkwldlt~]n．1iquid assets；cash 1 2．crunch[’krnntI,]n．a severe shortage of money or credit deteriorate[dl,t18rlareff]vi．to become worse pick up：increase bite[balt]vi．(of a policy or situation)take effect with unpleasant consequences( delusion[dl-lu：3n]n．an idea or belief that is not ture barrel[-baerN]vi．to drive or move in a way that is so fast as to almost be out of control bust r bASt]m a period of economic difficulty or demeltdown[tmeltldaon]n．a sudden and complete failure of a company,organization,or system(20 topple[’t3p1]vt．to cause to fall in such a way 21 weather-webof．to manage a difficult experience without being seriously harmed sell[sel_vt．to persuade someone to do．have．or use Something 23．faltering[’f3：lt(3)r10]n．becoming less effective and Successful fallout f·f3：laot 1 n．the unpleasant e仟ects of something that has happened squeal f‘skwi：1]vi.to complain or protest about Something head for the might Euro—zolze economies face external woes‘and internal tensions )At the World Economic Forum in Davos in January 2001,the mood was sombre.．The dotcom bubble had burst spectacularly,the Nasdaq stockmarket had crashed,and the American economy was tipping~into recession．

In practice,hundreds or even thousands of nodes can directly communicate over a broadcast channel． Because all nodes are capable of transmitting frames．more than two nodes can transmit frames at the same time．When this happens．all of the nodes receive multi．ple frames at the same time；that iS,the transmitted frames collide at all of the receivers．Typically,when there iS a collision,none of the receiving nodes can make any sense of any of the frames that were transmitted；in a sense,the signals of the colliding frames become inextricably tangled together．Thus．a11 the frames involved in the collision are lost．and the broadcast channelis wasted during the collision interval．Clearly,if many nodes want to transmit frames frequently．many transmissions will result in collisions,and much of the bandwidth of the broadcast channel will be wasted．In order to ensure that the broadcast channel performs useful work when multiple nodes are active．it is necessary to somehow coordinate the transmissions of the active nodes．This coordination iob is the responsibility of the multiple access protoc01．Over the past 30 years．thousands of Pale)ers and hundreds of PhD dissertations have been written on multiple access protocols；a comprehensive survev of this body of work is 『Ronl 1990]．Furthermore,active research in multiple access protocols continues due to the continued emergence of new types of links．particularly new wireless links．0ver the years,dozens of multiple access protocols have been implemented in a variety of link—layer technologies．Nevertheless,we can classify iust about any multi,pie access protocol as belonging to one of three categories：channel partitioning protocols,random access protocols．and taking-turns protocols．We’11 cover these categories of multiple

hops away from router A．Router D．upon receiving this advertisement,merges the advertisement(Figure 4．37)with the old routing table (Figure 4．36)．In particular,router D learns that there is now a path through router A to subnet Z that iS shorter than the path through router B．Thus,router D updates its routing table to account for the shorter shortest path,as shown in Figure 4_38．How IS it,you might ask,that the shortest path to subnet z has become shorter?Possibly．the decentralized distance—vector algorithm iS still in the process of converging(see Section 4．5．2),or perhaps new links and／or routers were added tO the AS．Thus changing the shortest paths in the AS．Let’S next consider a few of the implementation aspects of RIR Recall that RIP routers exchange advertisements approximately every 30 seconds．If a router does Figure Advertisement from router Figure Routing table in router D after receiving advertisement trom router A not hear from its neighbor at least once every 1 80 seconds．that neighbor iS consid．ered to be no longer reachable；that iS,either the neighbor has died or the connecting link has gone down．W1len this happens．RIP modifies the local routing table and then propagates this information by sending advertisements to its neighboring routers(the ones that are still reachable)．A router can alSO request information about its neighbor’S cost to a given destination using RIP’S request message．Routers send RIP request and response messages to each other over UDP using port number 520．The UDP segment iS carried between routers in a standard IP datagram．The fact that RIP uses a uanspoa—layer protocol(UDP)

Once again．fast processing of IP packets was a central concern．Recall from our discussion of IPv4 in Section 4．4．1 that since the IPv4 header contains a TTL field fsimilar to the hop limit field in 1Pv6)．the|Pv4 header checksum needed to be recomputed at every router．As with fragmentation and reassembly,this too was a costly operation in IPv4 4,4.THE INTERNET PROTOCOL(IP) ,389.Options．An options field is no longer a part of the standard IP header．However, it has not gone away．Instead,the options field is oue of the possible next headers pointed to from within the IPv6 header．That is,just as TCP or UDP protocol headers can be the next header within an IP packet,so too can an options field． The removal of the options field results in a fixed—length,40一byte IP header．Recall from our discussion in Section 4．4．3 that the ICMP protocol is used bv IPnodes to report error conditions and provide 1imited information(for example．The echo reply to a ping message)to an end system．A new version of ICMP has been defined for IPv6 in RFC 4443．In addition to reorganizing the existing ICMP type IPv6 functionality．These include the“Packet Too Big”type．and an“unrecognized IPv6 options”error code．In addition.ICMPv6 subsumes the functionality of the Internet Group Management Protocol fIGMP)that we’11 study in Section 4．7．IGMP which is used to manage a host’s ioining and leaving of multicast groups,was previously a separate protoc01 from ICMP in IPv4．Trar~sitioml~g flT('~lgll{Pv4 lo IPv6 Now that we have seen the technical details of IPv6．1et us consider a very practical matter：How will the public Internet．which is based on IPv4．

Even the presence of congestion in the network must be inferred by the end systems based only on observed network behavior ffor example．packet lOSS and delay)．We will see in Section 3．7 that TCP must necessarily take this end to—end approach toward congestion contr01．since the IP layer provides no feedback to the end systems regarding network congestion．TCP segment loss(as indicated by a timeout or a triple duplicate acknowledgment)is taken as an indication of net work congestion and TCP decreases its window size accordingly．We will also see a more recent proposal for TCP congestion control that uses increasing round—trip delay values as indicators of increased network congestion．Network—assisted congestion contr01．With network—assisted congestion contr01 network—layer components(that 1s,routers)provide explicit feedback to the sender regarding the congestion state in the network．This feedback may be as simple as a single bit indicating congestion at a link．This approach was taken in the early IBM SNA[Schwartz 1982 and DEC DECnet Jain 1989；Ramakrishnan 1990 architectures,was recently proposed for TCP／IP networks Floyd TCP 1994；RFC 31681,and is used in ATM available bit—rate fABR、congestion control as well,as discussed below．More sophisticated network feedback is also pos sible．For example,one form of ATM ABR congestion control that we will study shortly allows a router to inform the sender explicitly of the transmission rate it (the router)can support on an outgoing 1ink．The XCP protocol[Katabi 20021 provldes router—computed feedback to each source,carried in the packet header．regarding how that source should increase or decrease its transmission rate．For network—assisted congestion control,congestion information is typically fed back from the network to the sender in one of two ways,

why TCP congestion control behaves difterently after a tlmeout event than after the receipt of a triple duplicate ACK．Specifically,why does a TCP sender behave conservatively after a timeout event．Reducing its congestion window to 1 MSS,whereas after receiving a triple duplicate ACK it only cuts its congestion window by half?It is interesting that an early version of TCP,known as TCP Tahoe,Hnconditionally cuts its congestion window to 1 MSS and enters the slow—start phase after either type of lOSS event．The newer version of TCE TCP Reno,cancels the slow—start phase after a triple duplicate ACK．The Dhilosophy behind canceling slow start in this case is that even though a packet has been lost,the arrival of three duplicate ACKs indicates that some segments fspecifi．cally,three additional segments beyond the 10st segment)have been received at the sender．Tbus,unlike the case of a timeout,the network is showing itself to be capable of delivering at Ieast some segments,even if other segments are being lost to This canceling of the slow—start phase after a triple duplicate ACK is called fast recovery． Figure 3．53 illustrates the evolution of TCP’s congestion window for both Reno and Tahoe．In this figure,the threshold is initially equal to 8 MSS．For the first eight transmission rounds,Tahoe and Reno take identical actions．The congestion windo of transmission．The congestion window then climbs linearly until a triple duplicate ACK occurs,just after transmission round 8．Note that the congestion window is 12．MSS when this loss event occurs．The threshold is then set to 0．5．CongWin：6．MSS．Under TCP Reno,the congestion window is set to CongWin：6．MSS and then grows linearly．

suppose that B has nothing to send to A．Now consider what happens．As the application process at B empties the buffer,TCP does not send new segments with new RcvWindow values to Host A：indeed．TCP sends a segment to Host A only if it has data to send or if it has an acknowledgment to send．Therefore,Host A is never informed that some space has opened uD in Host B’s receive buffer--Host A is blocked and can transmit no more data!To solve this problem,the TCP specification requires Host A to continue to send segments with one data byte when B’s receive window is zero．These segments will be acknowledged by the receiver．Eventually the buffer will begin to empty and the acknowl．．edgments will contain a nonzero RcvWindow value．The online site at http：Hwww．awl．corn／kurose—ross for this book provides an interactive Java applet that illustrates the operation of the TCP receive window．Having described TCP’s flow—control service,we briefly mention here that UDP does not provide flow contr01．To understand the issue,consider sending a series of UDP segments from a process on HOst A to a process on HOst B．For a typical UDP implementation．UDP will append the segments in a finite—sized buffer that‘‘precedes’the corresponding socket(that 1s,the door to the process)．The process reads one entire segment at a time from the buffeL If the process does not read the segments fast enough from the bu the buffer will overflow and segments will get dropped．3.5.6 TCP Connection Management In this subsection we take a closer look at how a TCP connection is established and torn down．

Pa rt 6 American Poetry Since 1945：The Anti—T radition years of the war,as Kerouac’S father lay dying of cancer,he and Ginsberg shared a need for’a new vision of the world that would later be perceived as the diffuse ideology of“the Beat generation”．The United States seemed as the war ended to be developing a huge and passive middle class—later called “the silent majority”,seemed to lose its sense of purpose and cohesiveness,and in the context of Mc．Carthyite witch—hunts and acute industrial conflicts seemed to pit individuals against each other in a nervous quest for livelihood and security．Post—war America seemed to many to be mechanical and inhuman,smug in its opportunism and weak spiritually．It was the state of America that Allen Ginsberg reported on from San Francisco in his endlessly moving poem“Howl”which begins,saw the best minds of my generation destroyed by madness,starving hysterical naked,dragging themselves through the negro streets at dawn looking for an angry J；x,angelheaded hipsters burning for the ancient heavenly connection to the starry dynamo in the machinery of night The poem has often been described as a rhapsodic elegy for the bohemia of the 1 940 s which voices the growing protest against and an anguished denunciation of the loveless“Moloch”：What sphinx of cement and aluminum bashed open their skulls and ate up their brains and imaginations 9．Moloch!Solitude!Filth! liness!Ashcan and unobtainable dollars Children screaming under the stairways!Boys sobbing in armies!Old men weeping in the parks! Moloch!Moloch!Nightmare of Moloch!Moloch the loveless!Mental Moloch!Moloch the heavyjudger ofmen!．．．Moloch whose love is endless oil and stone!Moloch whose soul is electricity and banks!Moloch whose poverty is the specter of genius!Moloch whose fate is a cloud of sexless hydrogen! Moloch of genius!Moloch whose fate is a cloud of sexless hydrogen!