---
# System prepended metadata

title: Network midterm 2017
tags: [computer network]

---

1. Compare the primary characteristics /differences between circuit and packet switching strategies in the following aspects:
   (A) Connection setup/teardown
   (B) Bandwidth reservation
   (C\) Routing path selection
   (D) Queueing delay
   (E) Channel efficiency

2. Identify and briefly describe the five layers in the Internet protocol stack, outlining their primary responsibilities.

3. Assume Alice, utilizing a Web-based email account (such as Hotmail or Gmail), sends a message to Bob, who accesses his mail using POP3 from his mail server. List the series of application-layer protocols employed to transfer messages from Alice's host computer to Bob's host computer.

4. Host A is transmitting a file of L - 2000 bits to Host B using a packet switching strategy. There exist three links (and two packet switches) between A and B, and these links are uncongested (meaning no queue delays). The propagation speed, s, on each link is 2 * 10^8 m/s, and the packet switch processing delay, t_proc, encountered at each switch is 1 millisecond. The transmission rates for the links are R_1 = 1 Mbps, R_2 = 2 Mbps, and R_3 = 500 Kbps, respectively. The lengths of the links are d_1 = 600 km, d_2 = 200 km, and d_3 = 400 km. Determine the total end-to-end delay, D_total, for sending this file. Please derive an equation in terms of d_i, R_i (where i = 1, 2, 3), t_proc, s, and L to calculate the final answer.

5. Assume you click a web browser to access a web page, but the Web caching proxy server within your host doesn't contain the IP address for the associated URL. Consequently, a DNS lookup is performed to obtain the IP address from DNS, incurring successive RTTs (RTT_1 ... RTT_4). Let RTT_0 represent the RTT between the local host and the server containing the web page. The web page comprises simple HTML text referencing five very small objects. Assuming zero transmission time for the web page and objects, calculate the time elapsed from your initial click on the web browser until you receive all objects in the following scenarios:
   (a) Non-persistent HTTP with parallel connections
   (b) Non-persistent HTTP with no parallel TCP connections
   (c\) Persistent HTTP with parallel connections

6. Considering the transfer of a large file of F bits from Host A to Host B using a packet switching strategy, and assuming two links (and one switch) that are uncongested (without queueing delays). Host A segments the file into segments of S bits each, where F is an integer multiple of S. Each segment has an additional H bits of header, forming a packet of L = H + S bits. Each link has a transmission rate of R bps, and we assume negligible propagation delay. Determine:
   (a) The end-to-end delay in moving the file from Host A to Host B.
   (b) Find the optimal value of S that minimizes the end-to-end delay.
   
   
   
  
  
  ### Unofficial solution 🤐
  
  1. (a) c: need to setup a physical path at begin, and teardown after  connection end. p: no need to setup, each packet have its own route which define by routing.
    (b) c: reserve a const bandwidth for connection, and occupy it while whole connection. p: dynamic distribute bandwidth when packet need to be transfer
    (c\) c: use path pre-defined at setup process. p: every packet can have its own route independently, may dynamic routing by component.
    (d) c: No queueing delay since the route being reserve for connection. p: It may have delay at switch.
    (e) c: low efficiency, since whole bandwidth reserved for connection, evne there are no data transfering. p: High efficiency, since the bandwidth only reserved when data transfering.
1.
     Application layer: provide network service for end-point application. ex: HTTP, SMTP, FTP
     Transport layer: manage data transfer between process and process. ex: TCP, UTP
     Network layer: routing datagrams from source to destination. ex: IP
     Link layer: transfer datagrams from one ineternet endpoint to another. ex: LAN, WLAN
     Physics layer: Transfer data on physical medium(媒介) (bits on the wire)
1. A -> Mail server: SMTP/HTTP
   Mail server -> another Mail server: SMTP
   Mail server -> B: POP3/HTTP
1. $\frac{\sum{d_i}}{s}+2\cdot t_{proc}+\sum{\frac{L}{R_i}}=15ms$ 
1. (a) $\sum_{i=1}^{4}{\text{RTT}_i}+4\text{RTT}_0$
(b) $\sum_{i=1}^{4}{\text{RTT}_i}+12\text{RTT}_0$ 
(c\) $\sum_{i=1}^{4}{\text{RTT}_i}+3\text{RTT}_0$
1. (a)$2\cdot\frac{H+S}{R}+(\frac{F}{S}-1)\frac{H+S}{R}=\frac{H+S}{R}(\frac{F}{S}+1)$
(b) $1\sqrt{FH}$

