CENG/BILM 362. Midterm Review

Transkript

1 CENG/BILM 362 Midterm Review

2 Ques:on # 1 How many types of delay are present in an end- to- end transfer path? Explain each briefly (Uçtan uca transfer yolunda kaç çeşit gecikme vardır? Herbirini kısaca açıklayınız)

3 Answer Processing Delay(düğüm işlem gecikmesi) : Time to examine the packet s header, decide route of the packet, do error checks Queuing Delay(kuyrukta gecikme) : Time spent in the queue of the node wai:ng to be transmi_ed onto the link Transmission Delay(ile:m gecikmesi): Time required to push all of the packet s bits into the link Propaga:on Delay(yayınım gecikmesi): Time required for a bit to travel from one end to the other

4 Ques:on #2 How many types of access networks are there? (Kaç çeşit giriş ağı vardır?)

5 Answer Home access (Ev ağı): DSL, Cable, FTTH, Dial- up, Satellite Enterprise access(kurumsal ağ) :Ethernet, WiFi Wide- area wireless access(geniş alan kablosuz ağ): 3G, LTE

6 Ques:on #3 List the types of physical media(fiziksel ortam türlerini listeleyiniz)? Answer: Twisted pair Cooper Wire(Çie sarımlı bakır kablo) Coaxial Cable(koaksiyel kablo) Fiber Op:cs Terrestrial Radio (3G, Wifi, microwave) Satellite Radio (uydu)

7 Ques:on #4 This elementary problem begins to explore propaga:on delay and transmission delay, two central concepts in data networking. Consider two hosts, Hosts A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propaga:on speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B. (Host A ve Host B, R bps ile:m oranına sahip bir linkle birbirine bağlıdır ve m metre kadar birbirinden uzaktadır ve yayınım hızı ise s metre/saniyedir. Host A, L bit boyutundaki pake: Host B ye göndermeye çalışmaktadır ) a) Express the propaga:on delay, d prop in terms of m and s. (Buna göre yayınım gecikmesini (d prop ) m ve s cinsinden yazınız) b) Determine the transmission :me of the packet, d trans in terms of L and R(İle:m hızını(d trans ) L ve R cinsinden yazınız). c) Ignoring processing and queuing delays, obtain an expression for the end- to- end delay (işlem ve kuyruk gecikmelerini hesaba katmayarak uçtan- uca toplam gecikmeyi hesaplayınız).

8 Ques:on #4 This elementary problem begins to explore propaga:on delay and transmission delay, two central concepts in data networking. Consider two hosts, Hosts A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propaga:on speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B. d) Suppose Host A begins to transmit the packet at :me t=0. At :me t=d trans, where is the last bit of the packet? (Host A pake: t=0. saniyede göndermeye başlıyor. t= d trans eşit olduğunda pake:n son bi: nerededir) e) Suppose d prop is greater than d trans. At :me t=d trans, where is the first bit of the packet? (d prop, d trans dan büyük ise t= d trans da pake:n ilk bi: nerededir?) f) Suppose d prop is less than d trans. At :me t=d trans, where is the first bit of the packet?((d prop, d trans dan küçük ise t= d trans da pake:n ilk bi: nerededir?) g) Suppose s=2.5*10 8, L=100bits and R=28 kbps. Find the distance m so that d prop equals d trans. (s = 2.5*108, L = 100bit ve R=28kbps ise d prop ve d trans ın birbirine eşit olduğu m değerini bulunuz)

9 Answer

10 Ques:on #5 What are cookies used for in HTTP? Is the cookie stored at the client or the server?(http protokolünde Çerezler ne için kullanılır? Çerez istemci mi yoksa sunucu tararnda mı depolanır?) Answer: Cookies are used to hold state informa:on in HTTP. They are stored both at the client and the server (Durum bilgisi tutmak için kullanılır. Her iki taraea da depolanır)

11 Ques:on #6 Consider the following string of ASCII characters that were captured by Wireshark when the browser sent an HTTP GET message (i.e., this is the actual content of an HTTP GET message). The characters <cr><lf> are carriage return and line- feed characters (that is, the italized character string <cr> in the text below represents the single carriage- return character that was contained at that point in the HTTP header). Answer the following ques:ons, indica:ng where in the HTTP GET message below you find the answer. (Aşağıdaki HTTP GET bağlığına bakarak soruları cevaplayın) GET /cs453/index.html HTTP/1.1<cr><lf>Host: gaia.cs.umass.edu<cr><lf>user- Agent: Mozilla/5.0 (Windows;U; Windows NT 5.1; en- US; rv:1.7.2) Gecko/ Netscape/7.2 (ax) <cr><lf> Accept:ext/xml, applicauon/xml, applicauon/xhtml+xml, text/html;q=0.9, text/ plain;q=0.8,image/png,*/*;q=0.5<cr><lf>accept- Language: enus, en;q=0.5<cr><lf>accept- Encoding: zip,deflate<cr><lf>accept- Charset: ISO ,u`- 8;q=0.7,*;q=0.7<cr><lf>Keep- Alive 300<cr><lf> ConnecUon:keepalive<cr><lf><cr><lf> a. What is the URL of the document requested by the browser? (Tarayıcı tararndan istenen dokümanın URL i nedir?) b. What version of HTTP is the browser running? (Tarayıcı hangi versiyon HTTP kullanmaktadır?) c. Does the browser request a non- persistent or a persistent connec:on? (Tarayıcı kalıcı mı yoksa kalıcı olmayan bağlanu mı istemektedir?) d. What is the IP address of the host on which the browser is running? (Tarayıcının çalışuğı makinenin IP adresi nedir?) e. What type of browser ini:ates this message? Why is the browser type needed in an HTTP request message? (Ne :p bir tarayıcı bu mesajı oluşturmuştur? HTTP mesajında tarayıcı :pine neden ih:yaç duyulmaktadır?)

12 Answer a. The document request was h_p://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 reques:ng a persistent connec:on, as indicated by the Connec:on: keep- alive. d. This is a trick ques:on. This informa:on 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 informa:on from the IP datagrams (that carried the TCP segment that carried the HTTP GET request) to answer this ques:on. e. Mozilla/5.0. The browser type informa:on is needed by the server to send different versions of the same object to different types of browsers.

13 Ques:on #7 With the TCP connec:on established POP3 progresses through three phases. List them. (TCP bağlanusı oluşturulduktan sonra POP3 3 aşamadan geçer. Bunları listeleyin) Answer: Authoriza:on(Yetki verme): The user agent sends a username and a password to authen:cate the user(kullanıcı, kullanıcı ismi ve parolayı gönderir) Transac:on(İşlem yapma) : User agent retrieves messages, mark messages for dele:on and obtain sta:s:cs ( mesajları indirilir, silinmek için işaretlenir, ista:s:k bilgisi alınır) Update(Güncelleme): Aeer the client issues a QUIT ending the session, the mail server deleted messages marked for dele:on (QUIT komutundan sonra silinmek için işaretlenen mesajlar sunucu tararndan silinir.)

14 Ques:on #8 You enter a web page address to your browser. The webpage is an html page with 9 references to different JPEG objects. The IP address of the web page is not cached in your local host. You need a DNS look up. If we assume that 10 DNS servers are visited before your host receives the DNS address- IP conversion. Visi:ng 4 of these servers takes 5 ms RTT each. Visi:ng the rest of it takes 3ms RTT each. Considering the RTT between the client host and the server of the webpage is 6ms, and the transmission :me of the webpage and referenced objects are negligible, If your browser uses non- persistent HTTP with no parallel connec:ons, If your browser uses persistent HTTP How much :me elapses from when the client clicks on the link un:l the client receives all of the objects? Show your calcula:ons.

15 Ques:on #8 Browserınıza bir web adresi girdiğinizi ve bu web sayfasının 9 tane JPEG nesnesine referans yapuğını varsayın. Web sayfasının IP adresi yerel cache de bulunmamaktadır ve IP adresini alabilmek için 10 ayrı DNS sunucusu dolaşmanız gerekmektedir. Bunlardan 4 ü ile aranızdaki her bir RTT 5ms, geri kalanlar ile ise 3ms dir. Eğer web sunucusuyla aranızdaki RTT 6ms ise web adresinizi girdiğiniz andan i:baren web sayfası nesnelerle beraber browserınıza gelene kadar Kalıcı olmayan HTTP kullanılıyorsa ne kadar zaman geçer?, Kalıcı HTTP kullanılıyorsa ne kadar zaman geçer? Nesnelerin ile:m hızını hesaba katmayın ve hesaplamalarınızı gösterin.

16 Answer DNS: 4 x 5ms + 6x 3ms = 38ms HTTP: 10x 2RTT = 20 x6 ms = 120ms ms = 158ms DNS: 4 x 5ms + 6x 3ms = 38ms HTTP: 2RTT = 2x 6ms =12ms = 50ms

17 Ques:on #9 QuesUon 6 P2P (20 points): a 10Gbit file has to be distributed to N peers and the server has an upload rate of 40Mbps while each pair has a download rate of 4Mbps and an upload rate of u. Prepare a graphic that presents the file distribu:on :me for N = 5, 25 and 125 peers; u = 150Kbps, 300Kbps and 1Mbps for both client- server and P2P model. (You may use excel to do the calcula:ons)

18 Ques:on #9 10Gbitlik bir dosya N tane peer a dağıulacakur. Sunucunun upload kapasitesi 40Mbps ve her bir peer in download kapasitesi 4Mbps, upload kapasitesi ise u kadardır. N = 5, 25 ve 125; u = 150Kbps, 300Kbps ve 1Mbps olması durumunda hem istemci- sunucu hem de P2P mimarisinde dağıum süresini gösteren bir tablo hazırlayınız.

19 Answer

20 Ques:on #10 Compare the sizes of UDP and TCP headers (TCP ve UDP başlıklarının büyüklüklerini karşılaşurın) Answer: 20 bytes of overhead for TCP and 8 bytes of overhead for UDP What is receiver window in TCP header used for? (TCP başlığındaki alıcı penceresi ne için kullanılır) Answer : Flow Control (Akış kontrolü) Both conges:on control and flow control thro_le the sender, then how are they different? (Sıkışıklık ve akış kontrolü hangi yönleriyle birbirinden farklıdır?) Answer: Flow control handles conges:on at the receiver while conges:on control handles conges:on at the network core (Sıkışıklık kontrolü ağ merkezinde sıkışıklık meydana gelmesini engellerken, akış kontrolü alıcının bufferlarının dolmasını engeller)

21 Ques:on # 11 Consider a reliable data transfer protocol that uses only nega:ve acknowledgements. Suppose the sender sends data only infrequently. Would a NAK- only protocol be preferable to a protocol that uses ACKs? Why? (Sadece nega:f ACK kullanan bir güvenilir transfer protokolü düşünün. Gönderici sık olmayan aralıklarla paket gönderyorsa sadece- NAK kullanan bir protokol sadece ACK kullanan bir protokole tercih edilebilir mi? Neden?) Now suppose the sender has a lot of data to send and the end- to- end connec:on experiences few losses. In this second case, would a NAK- only protocol be preferable to a protocol that uses ACKs? Why?(Eğer gönderici sık olarak paket gönderiyor ve kayıp çok az yaşanıyorsa, sadece NAK kullanan bir protokol sadece ACK kullanan bir protokole tercih edilebilir mi?)

22 Answer In a NAK only protocol, the loss of packet x is only detected by the receiver when packet x+1 is received. That is, the receivers receives x- 1 and then x+1, only when x+1 is received does the receiver realize that x was missed. If there is a long delay between the transmission of x and the transmission of x+1, then it will be a long :me un:l x can be recovered, under a NAK only protocol. (NAK tercih edilmez çünkü kaybolan x pake: ancak x+1. paket yerine ulaşuğında farkedilir) On the other hand, if data is being sent oeen, then recovery under a NAK- only scheme could happen quickly. Moreover, if errors are infrequent, then NAKs are only occasionally sent (when needed), and ACK are never sent a significant reduc:on in feedback in the NAK- only case over the ACK- only case. (Sık paket gönderilen bir durumda kayıplar çabuk farkedilir ve kayıp ta çok az yaşanıyorsa az sayıda NAK gönderilerek gönderilen geri dönüşüm paketlerinin sayısında büyük bir azalma olur.)

23 ) Ques:on # 12

24 Ques:on # 12 5 kez ölçüm sonucunda SampleRTT değerleri şu şekildedir: 106ms, 120ms, 140ms, 90ms,115ms. Es:matedRTT değerini her ölçümden sonra hesaplayınız (α = ve ölçümlerden önceki Es:mated RTT değeri 100ms dir DevRTT değerini her bir ölçümden sonra hesaplayınız. Ölçümlerden öndeki DevRTT değeri 5ms dir ve β=0.25 En son :meoutinterval ı hesaplayın

25 Answer Es:matedRTT = x.samplertt + (1- x)es:matedrtt DevRTT = y. SampleRTT Es:matedRTT + (1- y) DevRTT TimeOutInterval = Es:matedRTT + 4. DevRTT

26 Answer

27 Answer

28 Answer

29 Ques:on #13 Compare GBN, SR, and TCP (no delayed ACK). Assume the :meout values for all three protocols are sufficiently long such that 5 consecu:ve data segments and their corresponding ACKs can be received (if not lost on the channel) by the receiving host(host B) and the sending host (Host A) respec:vely. Suppose A sends 5 data segments to B and the 2 nd segment(sent from A) is lost. In the end all 5 segments have been correctly received by B. (GBN, SR ve TCP (gecikmeli ACK kullanılmamaktadır) protokollerini karşılaşurın. Timeout aralığının 5 paket gönderilip ACKlerinin alınmasından daha fazla bir süre oluğunu varsayalım. A, B ye 5 segment göndermekte ve 2. segment kaybolmaktadır. Sonunda bütün segmentler yerine ulaşıp ACKler alınmaktadır.) a) How many segments has A sent in total and how many ACKs has B sent in total? What are their sequence numbers? Answer for the three protocols and show the :me diagram of the protocol opera:on. (Toplamda A kaç segment göndermiş:r ve B kaç tane ACK göndermiş:r. Segmentlerin ACKlerin sıra numaraları nedir? Zaman diagramını çizin) B) If the :meout values for all three protocols are much longer than 5 RTT, then which protocol successfully delivers all five data segments in shortest :me interval. (Eğer :meout aralığı 5RTT den daha büyük ise hangi protokol segmentleri daha hızlı gönderir, neden?)

30 Answer A) GoBackN: A sends 9 segments in total. They are ini:ally sent segments 1, 2, 3, 4, 5 and later re- sent segments 2, 3, 4, and 5. B sends 8 ACKs. They are 4 ACKS with sequence number 1, and 4 ACKS with sequence numbers 2, 3, 4, and 5. Selec:ve Repeat: A sends 6 segments in total. They are ini:ally sent segments 1, 2, 3, 4, 5 and later re- sent segments 2. B sends 5 ACKs. They are 4 ACKS with sequence number 1, 3, 4, 5. And there is one ACK with sequence number 2.

31 Answer TCP: A sends 6 segments in total. They are ini:ally sent segments 1, 2, 3, 4, 5 and later re- sent segments 2. B sends 5 ACKs. They are 4 ACKS with sequence number 2. There is one ACK with sequence numbers 6. Note that TCP always send an ACK with expected sequence number. B)TCP. This is because TCP uses fast retransmit without wai:ng un:l :me out.

32 Ques:on #14 35 TCP CongesUon Control cwnd Cwnd transmission round

33 Ques:on #14 Iden:fy the intervals SS and CA are working? (SS ve CA in çalışuğı aralıkları belir:n) Answer: SS- > 1-5, CA- > 5-24, What is the value of ssthresh at round 21 (21.turda ssthresh değeri nedir?) Answer: 18/2 Which round packet 80 is sent? (80.paket hangi turda gönderilir) Answer: = 85 at round 8 At which rounds conges:on is detected and what is the cause of conges:on for each? (Hangi turlarda sıkışıklık tespit edilir ve sıkışıklığın sebebi nedir?) Answer: 11 (3DUPACK), 16(3DUPACK), 24 (:meout), 30 (3DUPACK)

34 Ques:on 15 Consider that only a single TCP connec:on uses one 10Mbps link which does not buffer any data. Suppose that this link is the only congested link between the sending and receiving hosts. Assume that the TCP sender has a huge file to send and the receiver s buffer is much larger than the conges:on window. Each TCP segment is 1500 bytes and propaga:on delay is 150 msec and this tcp connec:on is always in conges:on avoidance phase. (Tek bir TCP bağlanfsı 10Mbps lik bir linki kullanmaktadır. TCP bağlanfsı büyük bir dosyayı göndermekte ve TCP alıcı bufferları sıkışıklık penceresinden büyüktür. Her segment 1500 byte ve aradaki gecikme 150msdir. Bu bağlanf hep CA fazında çalışmaktadır.) What is the maximum window size in segments that this TCP connec:on can achieve?(bu bağlanfnın ulaşabileceği en büyük pencere büyüklüğü nedir?) What is the average window size and average throughput? (Ortalama pencere büyüklüğü ve ortalama throughput nedir?) How long would it take for this TCP connec:on to reach its maximum window size again aeer recovering from a packet loss?(bağlanfnın paket kaybından sonra en yüksek pencere büyüklüğüne ulaşması ne kadar vakit alır?)

35 Answer Let W denote the max window size measured in segments. Then, W*MSS/ RTT = 10Mbps, as packets will be dropped if the maximum sending rate exceeds link capacity. Thus, we have W*1500*8/0.15=10*10^6, then W is about 125 segments. (W maximum pencere büyüklüğü olarak verilirse ve segment cinsinden ise W*MSS/RTT link gönderim hızını geçmemeli yoksa paket kaybına sebep olabilir. Öyleyse W*1500*8/0.15=10*10^6 dan W yaklaşık 125 segment olur.) As conges:on window size varies from W/2 to W, then the average window size is 0.75W=94 (ceiling of 93.75) segments. Average throughput is 94*1500*8/0.15 =7.52Mbps.(Pencere büyüklüğü W ve W/2 arasında değişuği için ortalama büyüklük 0.75W=94 segmensr) 94/2 *0.15 = 7.05 seconds, as the number of RTTs (that this TCP connec:ons needs in order to increase its window size from W/2 to W) is given by W/2. Recall the window size increases by one in each RTT. (Her paket kaybında pencere yarılandığından ve daha sonra her RTT de 1 artğında dolayı ortalama pencere büyüklüğüne ulaşmak için 94/2*0.15= 7.05 saniye gereklidir.)