•

• 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 waiting to be transmitted onto the link
• Transmission Delay(iletim gecikmesi) : Time required to push all of the packet’s bits into the link
• Propagation Delay(yayınım gecikmesi) : Time required for a bit to travel from one end to the other

•

•
•
•

•
•
– Twisted pair Cooper Wire(Çift sarımlı bakır kablo)
– Coaxial Cable(koaksiyel kablo)
– Fiber Optics
– Terrestrial Radio (3G, Wifi, microwave)
– Satellite Radio (uydu)

• This elementary problem begins to explore propagation 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 propagation 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 iletim oranına sahip bir linkle birbirine bağlıdır ve m metre kadar birbirinden uzaktadır ve yayınım hızı ise s metre/saniyedir. Host A, L bit boyutundaki paketi Host B’ye göndermeye çalışmaktadır )
• a) Express the propagation delay, d gecikmesini (d prop prop in terms of m and s. (Buna göre yayınım
) m ve s cinsinden yazınız)
• b) Determine the transmission time of the packet, d trans hızını(d trans
) L ve R cinsinden yazınız).
in terms of L and R(İletim
• 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 ).

• This elementary problem begins to explore propagation 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 propagation 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 time t=0. At time t=d d trans eşit olduğunda paketin son biti nerededir) trans, where is the last bit of the packet? (Host A paketi t=0. saniyede göndermeye başlıyor. t=
• e) Suppose d prop packet? (d prop
, d is greater than d trans trans
. At time t=d
’dan büyük ise t= d trans trans
, where is the first bit of the
‘da paketin ilk biti nerededir?)
• f) Suppose d packet?((d prop prop
, d is less than d trans trans
. At time t=d
’dan küçük ise t= d trans trans
, where is the first bit of the
‘da paketin ilk biti nerededir?)
• g) Suppose s=2.5*10 8 , L=100bits and R=28 kbps. Find the distance m so that d equals d trans
. (s = 2.5*108, L = 100bit ve R=28kbps ise d prop olduğu m değerini bulunuz) ve d trans prop
’ın birbirine eşit

• 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 tarafında mı depolanır?)
• Answer: Cookies are used to hold state information in HTTP. They are stored both at the client and the server (Durum bilgisi tutmak için kullanılır. Her iki tarafta da depolanır)

• 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 questions, indicating 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/20040804 Netscape/7.2 (ax) <cr><lf>
Accept:ext/xml, application/xml, application/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-8859-1,utf-8;q=0.7,*;q=0.7<cr><lf>Keep-Alive
300<cr><lf> Connection:keepalive<cr><lf><cr><lf>
• a. What is the URL of the document requested by the browser? (Tarayıcı tarafından 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 connection? (Tarayıcı kalıcı mı yoksa kalıcı olmayan bağlantı mı istemektedir?)
• d. What is the IP address of the host on which the browser is running? (Tarayıcının çalıştığı makinenin IP adresi nedir?)
• e. What type of browser initiates this message? Why is the browser type needed in an HTTP request message? (Ne tip bir tarayıcı bu mesajı oluşturmuştur? HTTP mesajında tarayıcı tipine neden ihtiyaç duyulmaktadır?)

• 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.

• With the TCP connection established POP3 progresses through three phases. List them. (TCP bağlantısı oluşturulduktan sonra POP3
3 aşamadan geçer. Bunları listeleyin)
• Answer:
– Authorization(Yetki verme): The user agent sends a username and a password to authenticate the user(kullanıcı, kullanıcı ismi ve parolayı gönderir)
– Transaction(İşlem yapma) : User agent retrieves messages, mark messages for deletion and obtain statistics (email mesajları indirilir, silinmek için işaretlenir, istatistik bilgisi alınır)
– Update(Güncelleme): After the client issues a QUIT ending the session, the mail server deleted messages marked for deletion (QUIT komutundan sonra silinmek için işaretlenen mesajlar sunucu tarafından silinir.)

• Compare the sizes of UDP and TCP headers (TCP ve UDP başlıklarının büyüklüklerini karşılaştırın)
• 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 congestion control and flow control throttle the sender, then how are they different? (Sıkışıklık ve akış kontrolü hangi yönleriyle birbirinden farklıdır?)
• Answer: Flow control handles congestion at the receiver while congestion control handles congestion at the network core
(Sıkışıklık kontrolü ağ merkezinde sıkışıklık meydana gelmesini engellerken, akış kontrolü alıcının bufferlarının dolmasını engeller)

• Consider a reliable data transfer protocol that uses only negative acknowledgements. Suppose the sender sends data only infrequently. Would a NAK-only protocol be preferable to a protocol that uses ACKs? Why? (Sadece negatif 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-toend connection 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?)

• 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 time until x can be recovered, under a NAK only protocol. (NAK tercih edilmez çünkü kaybolan x paketi ancak x+1. paket yerine ulaştığında farkedilir)
• On the other hand, if data is being sent often, 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 reduction 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.)

• )

• 5 kez ölçüm sonucunda SampleRTT değerleri şu şekildedir: 106ms, 120ms, 140ms, 90ms,115ms.
EstimatedRTT değerini her ölçümden sonra hesaplayınız (α = 0.125 ve ölçümlerden önceki
Estimated RTT değeri 100ms’dir
• DevRTT değerini her bir ölçümden sonra hesaplayınız. Ölçümlerden öndeki DevRTT değeri
5ms’dir ve β=0.25
• En son timeoutInterval’ı hesaplayın


EstimatedRTT
 xSampleRTT

(1
 x ) EstimatedRTT
DevRTT
 y SampleRTT

EstimatedRTT

(1
 y ) DevRTT


4 *

TCP Congestion Control
25
20
15
10
35
30
5
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 transmission round
Cwnd

• Identify the intervals SS and CA are working? (SS ve CA’in çalıştığı aralıkları belirtin)
• Answer: SS-> 1-5, 25-30 CA-> 5-24, 31-36
• 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: 1+2+4+8+16+17+18+19 = 85 at round 8
• At which rounds congestion is detected and what is the cause of congestion for each? (Hangi turlarda sıkışıklık tespit edilir ve sıkışıklığın sebebi nedir?)
• Answer: 11 (3DUPACK), 16(3DUPACK), 24 (timeout), 30 (3DUPACK)

• Compare GBN and SR. Assume that the timeout values for all three protocols are sufficiently long such that 5 consecutive data segments and their ACKs can be received if not lost in the channel by the receiving host and the sending host respectively. Suppose
Host A sends 5 data segments to Host B and the ACK for the 2 nd segment is lost. In the end, all 5 data segments have been received by Host B. How many segments has Host A sent in total and how many ACKs has Host B sent in total? What are their sequence numbers ? (Go-back-N, Selective Repeat protokollerini karşılaştırınız. Üç protocol için de time out süresinin 5 tane veri segmentinin gönderilip ACKlerinin alınmasına imkan tanıyacak kadar uzun olduğunu varsayın(Eğer bağlantı üzerinde kaybolmazlarsa). Host A Host B ye 5 veri segmenti gönderir fakat 2 nolu segmentin ACK paketi kaybolur.A ve B toplamda kaç segment ve ACK göndermiştir ve sıra numaraları nedir?)

•
– 5 segments sent by A: 1,2,3,4,5
– 5 ACKs sent by B: 1,2,3,4,5
•
– 6 segments sent by A: 1,2,3,4,5,2
– 6ACKs sent by B: 1,2,3,4,5,2

• Consider that only a single TCP connection 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 congestion window. Each TCP segment is 1500 bytes and propagation delay is 150 msec and this tcp connection is always in congestion avoidance phase. (Tek bir TCP bağlantısı 10Mbps lik bir linki kullanmaktadır. TCP bağlantısı büyük bir dosyayı göndermekte ve TCP alıcı bufferları sıkışıklık penceresinden büyüktür. Her segment 1500 byte ve aradaki gecikme 150msdir. Bu bağlantı hep CA fazında çalışmaktadır.)
– What is the maximum window size in segments that this TCP connection can achieve?(Bu bağlantının 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 connection to reach its maximum window size again after recovering from a packet loss?(Bağlantının paket kaybından
sonra en yüksek pencere büyüklüğüne ulaşması ne kadar vakit alır?)

• 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 congestion 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
değiştiği için ortalama büyüklük 0.75W=94 segmenttir)
• 94/2 *0.15 = 7.05 seconds, as the number of RTTs (that this TCP connections 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 arttığında dolayı ortalama pencere büyüklüğüne ulaşmak için 94/2*0.15=
7.05 saniye gereklidir.)