Solution for Assignment 01
1. Question 1
1.1. (a)
In circuit switching, the delay mainly consists of the connection setup delay and the propagation delay along the reserved circuit. After the circuit has been established, there is usually little or no queuing delay at intermediate nodes.
In packet switching, the delay consists of processing delay, queuing delay, transmission delay, and propagation delay at each hop. Since packets are forwarded hop by hop, these delays accumulate along the path.
1.2. (b)
With circuit switching, each user must be assigned a dedicated 5 Mbit/s circuit. Therefore, the 100 Mbit/s link can support at most
100 / 5 = 20
users concurrently.
In packet switching, bandwidth is not reserved permanently. Since each user is active only 20% of the time, the network can exploit statistical multiplexing and support many more than 20 users (about 100 users on average, since 5 * 0.2 = 1 Mbit/s average demand per user). It is better because idle users do not waste reserved bandwidth.
2. Question 2
2.1. (a)
Application layer: Provides network services to user applications such as web, email, and name resolution. It defines the protocols used by applications to exchange data.
Transport layer: Provides end-to-end communication between processes. It can offer reliability, flow control, congestion control, and multiplexing/demultiplexing.
Network layer: Responsible for moving packets from the source host to the destination host across multiple networks. It provides logical addressing and routing.
Link layer: Responsible for transferring frames across a single communication link between neighboring nodes. It handles framing, medium access, and error detection on the local link.
2.2. (b)
When H1 sends a packet to H2, the packet follows the path H1 -> S1 -> R1 -> S2 -> H2.
- H1: H1 is the sender, so it encapsulates the data into a transport segment, then into an IP packet, and finally into a link-layer frame.
- S1: As a switch, S1 only examines the link-layer frame header (especially the destination MAC address) and forwards the frame. It does not decapsulate up to the network layer.
- R1: As a router, R1 removes the incoming link-layer header/trailer, extracts the IP packet, examines the destination IP address, and then re-encapsulates the packet into a new link-layer frame for the next link.
- S2: Like S1, S2 only processes the link-layer frame and forwards it based on the MAC address.
- H2: H2 is the final destination, so it fully decapsulates the frame: link layer, network layer, and transport layer, then delivers the data to the application.
3. Question 3
3.1. (a)
we have chosen the following universities and written a script to ping them.
for h in tum.de lmu.de uni-heidelberg.de fu-berlin.de www.ox.ac.uk ens.psl.eu www.elte.hu www.sorbonne-universite.fr www.caltech.edu mit.edu sydney.edu.au unimelb.edu.au; do echo "===== $h =====" ping -c 10 "$h" done
the result is:
===== tum.de ===== PING tum.de (129.187.255.109) 56(84) bytes of data. 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=1 ttl=246 time=22.2 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=2 ttl=246 time=23.9 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=3 ttl=246 time=24.0 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=4 ttl=246 time=23.8 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=5 ttl=246 time=23.9 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=6 ttl=246 time=24.6 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=7 ttl=246 time=25.6 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=8 ttl=246 time=23.9 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=9 ttl=246 time=24.0 ms 64 bytes from tu-muenchen.de (129.187.255.109): icmp_seq=10 ttl=246 time=24.9 ms --- tum.de ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9012ms rtt min/avg/max/mdev = 22.206/24.080/25.568/0.831 ms ===== lmu.de ===== PING lmu.de (141.84.44.56) 56(84) bytes of data. 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=1 ttl=53 time=31.2 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=2 ttl=53 time=31.9 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=3 ttl=53 time=32.3 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=4 ttl=53 time=30.9 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=5 ttl=53 time=32.7 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=6 ttl=53 time=31.8 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=7 ttl=53 time=31.9 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=8 ttl=53 time=31.7 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=9 ttl=53 time=32.0 ms 64 bytes from www.bio.lmu.de (141.84.44.56): icmp_seq=10 ttl=53 time=33.0 ms --- lmu.de ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9014ms rtt min/avg/max/mdev = 30.901/31.939/32.974/0.584 ms ===== uni-heidelberg.de ===== PING uni-heidelberg.de (129.206.13.71) 56(84) bytes of data. 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=1 ttl=53 time=18.9 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=2 ttl=53 time=20.6 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=3 ttl=53 time=20.7 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=4 ttl=53 time=21.1 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=5 ttl=53 time=19.5 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=6 ttl=53 time=20.4 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=7 ttl=53 time=19.7 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=8 ttl=53 time=19.6 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=9 ttl=53 time=18.3 ms 64 bytes from lb1-ocp-west.urz.uni-heidelberg.de (129.206.13.71): icmp_seq=10 ttl=53 time=19.9 ms --- uni-heidelberg.de ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9016ms rtt min/avg/max/mdev = 18.259/19.874/21.112/0.824 ms ===== fu-berlin.de ===== PING fu-berlin.de (160.45.170.10) 56(84) bytes of data. 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=1 ttl=48 time=27.0 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=2 ttl=48 time=25.6 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=3 ttl=48 time=26.7 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=4 ttl=48 time=25.6 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=5 ttl=48 time=25.5 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=6 ttl=48 time=25.8 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=7 ttl=48 time=26.8 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=8 ttl=48 time=26.1 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=9 ttl=48 time=26.6 ms 64 bytes from www.fu-berlin.de (160.45.170.10): icmp_seq=10 ttl=48 time=28.8 ms --- fu-berlin.de ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9015ms rtt min/avg/max/mdev = 25.522/26.457/28.774/0.939 ms ===== www.ox.ac.uk ===== PING www.ox.ac.uk.cdn.cloudflare.net (172.66.169.161) 56(84) bytes of data. 64 bytes from 172.66.169.161: icmp_seq=1 ttl=59 time=10.2 ms 64 bytes from 172.66.169.161: icmp_seq=2 ttl=59 time=11.2 ms 64 bytes from 172.66.169.161: icmp_seq=3 ttl=59 time=11.6 ms 64 bytes from 172.66.169.161: icmp_seq=4 ttl=59 time=11.5 ms 64 bytes from 172.66.169.161: icmp_seq=5 ttl=59 time=11.9 ms 64 bytes from 172.66.169.161: icmp_seq=6 ttl=59 time=11.0 ms 64 bytes from 172.66.169.161: icmp_seq=7 ttl=59 time=18.5 ms 64 bytes from 172.66.169.161: icmp_seq=8 ttl=59 time=13.2 ms 64 bytes from 172.66.169.161: icmp_seq=9 ttl=59 time=12.0 ms 64 bytes from 172.66.169.161: icmp_seq=10 ttl=59 time=10.9 ms --- www.ox.ac.uk.cdn.cloudflare.net ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9013ms rtt min/avg/max/mdev = 10.202/12.201/18.507/2.230 ms ===== ens.psl.eu ===== PING ens.psl.eu (129.199.166.212) 56(84) bytes of data. 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=1 ttl=49 time=23.3 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=2 ttl=49 time=25.2 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=3 ttl=49 time=24.2 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=4 ttl=49 time=25.6 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=5 ttl=49 time=25.7 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=6 ttl=49 time=24.6 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=7 ttl=49 time=24.9 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=8 ttl=49 time=26.0 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=9 ttl=49 time=25.0 ms 64 bytes from website-front-45.ens.fr (129.199.166.212): icmp_seq=10 ttl=49 time=26.2 ms --- ens.psl.eu ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9005ms rtt min/avg/max/mdev = 23.285/25.064/26.185/0.838 ms ===== www.elte.hu ===== PING www.elte.hu (157.181.152.2) 56(84) bytes of data. 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=1 ttl=50 time=27.7 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=2 ttl=50 time=29.3 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=3 ttl=50 time=29.3 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=4 ttl=50 time=31.3 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=5 ttl=50 time=28.7 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=6 ttl=50 time=34.5 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=7 ttl=50 time=28.5 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=8 ttl=50 time=40.3 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=9 ttl=50 time=28.5 ms 64 bytes from wwwprod.elte.hu (157.181.152.2): icmp_seq=10 ttl=50 time=36.1 ms --- www.elte.hu ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9015ms rtt min/avg/max/mdev = 27.663/31.414/40.275/3.969 ms ===== www.sorbonne-universite.fr ===== PING www.sorbonne-universite.fr (151.101.194.216) 56(84) bytes of data. 64 bytes from 151.101.194.216: icmp_seq=1 ttl=56 time=12.5 ms 64 bytes from 151.101.194.216: icmp_seq=2 ttl=56 time=14.3 ms 64 bytes from 151.101.194.216: icmp_seq=3 ttl=56 time=14.0 ms 64 bytes from 151.101.194.216: icmp_seq=4 ttl=56 time=14.4 ms 64 bytes from 151.101.194.216: icmp_seq=5 ttl=56 time=13.7 ms 64 bytes from 151.101.194.216: icmp_seq=6 ttl=56 time=13.8 ms 64 bytes from 151.101.194.216: icmp_seq=7 ttl=56 time=13.9 ms 64 bytes from 151.101.194.216: icmp_seq=8 ttl=56 time=13.5 ms 64 bytes from 151.101.194.216: icmp_seq=9 ttl=56 time=13.8 ms 64 bytes from 151.101.194.216: icmp_seq=10 ttl=56 time=13.7 ms --- www.sorbonne-universite.fr ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9015ms rtt min/avg/max/mdev = 12.480/13.743/14.397/0.499 ms ===== www.caltech.edu ===== PING www.caltech.edu.cdn.cloudflare.net (172.64.151.221) 56(84) bytes of data. 64 bytes from 172.64.151.221: icmp_seq=1 ttl=59 time=9.49 ms 64 bytes from 172.64.151.221: icmp_seq=2 ttl=59 time=11.0 ms 64 bytes from 172.64.151.221: icmp_seq=3 ttl=59 time=11.7 ms 64 bytes from 172.64.151.221: icmp_seq=4 ttl=59 time=11.2 ms 64 bytes from 172.64.151.221: icmp_seq=5 ttl=59 time=11.8 ms 64 bytes from 172.64.151.221: icmp_seq=6 ttl=59 time=11.8 ms 64 bytes from 172.64.151.221: icmp_seq=7 ttl=59 time=12.1 ms 64 bytes from 172.64.151.221: icmp_seq=8 ttl=59 time=11.1 ms 64 bytes from 172.64.151.221: icmp_seq=9 ttl=59 time=10.9 ms 64 bytes from 172.64.151.221: icmp_seq=10 ttl=59 time=11.3 ms --- www.caltech.edu.cdn.cloudflare.net ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9014ms rtt min/avg/max/mdev = 9.493/11.237/12.095/0.696 ms ===== mit.edu ===== PING mit.edu (104.102.37.33) 56(84) bytes of data. 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=1 ttl=53 time=13.2 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=2 ttl=53 time=14.4 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=3 ttl=53 time=15.7 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=4 ttl=53 time=15.9 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=5 ttl=53 time=18.2 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=6 ttl=53 time=16.2 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=7 ttl=53 time=15.1 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=8 ttl=53 time=15.0 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=9 ttl=53 time=14.4 ms 64 bytes from a104-102-37-33.deploy.static.akamaitechnologies.com (104.102.37.33): icmp_seq=10 ttl=53 time=14.6 ms --- mit.edu ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9014ms rtt min/avg/max/mdev = 13.182/15.252/18.238/1.295 ms ===== sydney.edu.au ===== PING sydney.edu.au (20.248.131.216) 56(84) bytes of data. 64 bytes from 20.248.131.216: icmp_seq=1 ttl=108 time=309 ms 64 bytes from 20.248.131.216: icmp_seq=2 ttl=108 time=327 ms 64 bytes from 20.248.131.216: icmp_seq=3 ttl=108 time=351 ms 64 bytes from 20.248.131.216: icmp_seq=4 ttl=108 time=269 ms 64 bytes from 20.248.131.216: icmp_seq=5 ttl=108 time=293 ms 64 bytes from 20.248.131.216: icmp_seq=6 ttl=108 time=316 ms 64 bytes from 20.248.131.216: icmp_seq=7 ttl=108 time=339 ms 64 bytes from 20.248.131.216: icmp_seq=8 ttl=108 time=361 ms 64 bytes from 20.248.131.216: icmp_seq=9 ttl=108 time=283 ms 64 bytes from 20.248.131.216: icmp_seq=10 ttl=108 time=305 ms --- sydney.edu.au ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9009ms rtt min/avg/max/mdev = 269.366/315.306/360.849/28.020 ms ===== unimelb.edu.au ===== PING unimelb.edu.au (43.245.41.62) 56(84) bytes of data. 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=1 ttl=59 time=9.29 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=2 ttl=59 time=11.3 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=3 ttl=59 time=11.4 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=4 ttl=59 time=11.2 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=5 ttl=59 time=11.8 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=6 ttl=59 time=11.5 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=7 ttl=59 time=12.0 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=8 ttl=59 time=21.4 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=9 ttl=59 time=12.7 ms 64 bytes from 62.41.245.43.in-addr.arpa (43.245.41.62): icmp_seq=10 ttl=59 time=11.5 ms --- unimelb.edu.au ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9011ms rtt min/avg/max/mdev = 9.285/12.414/21.372/3.099 ms
The measured average RTTs are:
- TUM (`tum.de`): 24.080 ms
- LMU Munich (`lmu.de`): 31.939 ms
- Heidelberg University (`uni-heidelberg.de`): 19.874 ms
- Free University of Berlin (`fu-berlin.de`): 26.457 ms
- University of Oxford (`www.ox.ac.uk`): 12.201 ms
- École normale supérieure (`ens.psl.eu`): 25.064 ms
- Eötvös Loránd University (`www.elte.hu`): 31.414 ms
- Sorbonne University (`www.sorbonne-universite.fr`): 13.743 ms
- California Institute of Technology (`www.caltech.edu`): 11.237 ms
- Massachusetts Institute of Technology (`mit.edu`): 15.252 ms
- University of Sydney (`sydney.edu.au`): 315.306 ms
- University of Melbourne (`unimelb.edu.au`): 12.414 ms
3.2. (b)
Yes. There are targets with a smaller RTT even though they are geographically farther away. For example, Caltech (11.237 ms), MIT (15.252 ms), and the University of Melbourne (12.414 ms) all have smaller RTTs than several German universities such as TUM (24.080 ms), Heidelberg (19.874 ms), and FU Berlin (26.457 ms), even though they are much farther away.
This can happen for several reasons. First, Internet routing is not based on geographical shortest distance, so packets may take a more efficient path to a far-away target than to a geographically closer one. Second, RTT includes not only propagation delay but also queuing and processing delays, which depend on congestion and router load. Third, some university domains are served by CDNs or edge servers (for example Cloudflare or Akamai), so the ping reply may come from a nearby edge node rather than from the university’s actual campus network.
4. Question 4 — Sources of Packet Delay
Given: \[ v = 10^8 \ \text{m/s}, \qquad R = 2 \times 10^6 \ \text{bit/s} \]
\[ L_1 = 1 \ \text{km} = 1000 \ \text{m}, \qquad L_3 = 300 \ \text{km} = 300000 \ \text{m} \]
Processing delay is assumed to be \(0\), and the reply packet has the same size as the outgoing packet.
4.1. (a)
A sends a packet of \(250\) bytes to C and measures an RTT of \(20\) ms.
Packet size: \[ 250 \ \text{bytes} = 250 \times 8 = 2000 \ \text{bits} \]
Propagation delay on \(L_1\): \[ d_{\text{prop},1} = \frac{1000}{10^8} = 10^{-5} \ \text{s} = 0.01 \ \text{ms} \]
Propagation delay on \(L_3\): \[ d_{\text{prop},3} = \frac{300000}{10^8} = 3 \times 10^{-3} \ \text{s} = 3 \ \text{ms} \]
One-way propagation delay: \[ d_{\text{prop, one-way}} = 0.01 + 3 = 3.01 \ \text{ms} \]
Round-trip propagation delay: \[ d_{\text{prop, RTT}} = 2 \times 3.01 = 6.02 \ \text{ms} \]
Transmission delay per link: \[ d_{\text{trans}} = \frac{2000}{2 \times 10^6} = 10^{-3} \ \text{s} = 1 \ \text{ms} \]
In one RTT, the packet is transmitted over \(4\) links total: \[ d_{\text{trans, RTT}} = 4 \times 1 = 4 \ \text{ms} \]
Thus, without queueing: \[ RTT_{\text{no queue}} = 6.02 + 4 = 10.02 \ \text{ms} \]
Measured RTT: \[ RTT = 20 \ \text{ms} \]
So the total queueing delay is: \[ d_{\text{queue}} = 20 - 10.02 = 9.98 \ \text{ms} \]
Therefore: \[ \boxed{d_{\text{queue}} \approx 10 \ \text{ms}} \]
4.2. (b)
C sends a packet to A and measures an RTT of \(65\) ms. Assume empty queues.
Let the packet size be \(x\) bits.
Round-trip propagation delay is the same as before: \[ d_{\text{prop, RTT}} = 6.02 \ \text{ms} \]
Transmission delay per link: \[ \frac{x}{2 \times 10^6} \ \text{s} \]
There are \(4\) transmissions in one RTT, so total transmission delay is: \[ 4 \cdot \frac{x}{2 \times 10^6} \ \text{s} \]
Convert to milliseconds: \[ 4 \cdot \frac{x}{2 \times 10^6} \cdot 1000 = \frac{x}{500} \ \text{ms} \]
Thus: \[ 65 = 6.02 + \frac{x}{500} \]
\[ \frac{x}{500} = 65 - 6.02 = 58.98 \]
\[ x = 58.98 \times 500 = 29490 \ \text{bits} \]
Convert to bytes: \[ \frac{29490}{8} = 3686.25 \ \text{bytes} \]
Therefore: \[ \boxed{\text{packet size} \approx 3686 \ \text{bytes}} \]
4.3. (c)
B sends a packet of \(1000\) bytes to C and measures an RTT of \(32\) ms. Assume queueing delay \(=0\).
Let the length of \(L_2\) be \(x\) meters.
Packet size: \[ 1000 \ \text{bytes} = 8000 \ \text{bits} \]
Transmission delay per link: \[ d_{\text{trans}} = \frac{8000}{2 \times 10^6} = 4 \times 10^{-3} \ \text{s} = 4 \ \text{ms} \]
Total transmission delay in one RTT: \[ d_{\text{trans, RTT}} = 4 \times 4 = 16 \ \text{ms} \]
Propagation delay on \(L_2\): \[ d_{\text{prop},2} = \frac{x}{10^8} \ \text{s} \]
In milliseconds: \[ d_{\text{prop},2} = \frac{x}{10^8} \cdot 1000 = \frac{x}{10^5} \ \text{ms} \]
Propagation delay on \(L_3\): \[ d_{\text{prop},3} = 3 \ \text{ms} \]
One-way propagation delay: \[ d_{\text{prop, one-way}} = \frac{x}{10^5} + 3 \]
Round-trip propagation delay: \[ d_{\text{prop, RTT}} = 2 \left( \frac{x}{10^5} + 3 \right) \]
Total RTT: \[ 32 = 16 + 2 \left( \frac{x}{10^5} + 3 \right) \]
\[ 32 = 16 + \frac{2x}{10^5} + 6 \]
\[ 32 = 22 + \frac{2x}{10^5} \]
\[ 10 = \frac{2x}{10^5} \]
\[ x = \frac{10 \cdot 10^5}{2} = 500000 \ \text{m} \]
\[ x = 500 \ \text{km} \]
Therefore, the theoretical upper bound is: \[ \boxed{L_2 \le 500 \ \text{km}} \]
5. Question 5
5.1. (a) Possible consequences of packet sniffing
Packet sniffing means passively observing and capturing network traffic. Therefore, the correct consequences are those that follow from eavesdropping on packets, not from actively modifying traffic.
5.1.1. Statement 1
“Attackers can modify packets in transit and inject malicious data.”
This is false in the context of pure packet sniffing. Modifying packets or injecting malicious data requires an active attack, such as man-in-the-middle behavior, and is not a direct consequence of merely sniffing traffic.
5.1.2. Statement 2
“Attackers can analyze network traffic patterns to infer sensitive information.”
This is true. Even if payload contents are encrypted, traffic analysis can still reveal communication patterns, timing, endpoints, and other metadata that may leak sensitive information.
5.1.3. Statement 3
“Attackers can forge source IP addresses to evade detection.”
This is false. Forging source IP addresses is IP spoofing, which is different from packet sniffing. It is not a consequence of passively observing traffic.
5.1.4. Statement 4
“Sensitive information such as login credentials can be stolen if transmitted in plaintext.”
This is true. If credentials or other secrets are sent without encryption, a sniffer can capture and read them directly.
Therefore, the correct choices are: \[ \boxed{\text{2 and 4}} \]
5.2. (b) Descriptions of the Internet from the service perspective
The service perspective focuses on what the Internet provides to applications and users, rather than how it is structurally built.
5.2.1. Statement 1
“A platform for building network applications.”
This is true. From the service perspective, the Internet provides infrastructure and communication services that enable applications such as web browsing, email, streaming, and messaging.
5.2.2. Statement 2
“A network of networks.”
This is false for the service perspective. This describes the Internet from a structural or architectural perspective, not from the viewpoint of the services it offers.
5.2.3. Statement 3
“A collection of billions of computing devices and packet switches interconnected by links.”
This is false for the service perspective. This is again a description of the Internet’s physical and structural composition.
5.2.4. Statement 4
“A place we browse information, entertainment and to communicate with people.”
This is true. Although informal, it describes the Internet from the user/service point of view: a medium that provides access to communication, information, and online services.
Therefore, the correct choices are: \[ \boxed{\text{1 and 4}} \]