Tutorial 2 Flashcards
True or false?
A user requests a web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages.
False
True or false?
Two distinct web pages (for example, www.dccomics.com/movies and www.dccomics.com/comics) can be sent over the same persistent connection.
True
true or false?
With nonpersistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.
False
True or false?
The “Date” header in the HTTP response message indicates when the object in the response was last modified.
False
True or false?
HTTP response messages never have an empty message body
False
What reasons would compel a CDN (content delivery network) to install web caches in its network?
- Smaller delay since it reduces response time for requests
- low cost solution for increased performance
- reduced traffic on the access link
A server contains a Type NS record for the domain that includes a specific hostname. Is the server authoritative for the hostname? Why?
No it is not authoritative for the hostname.
Since it stores the name of the authoritative name server for the domain, it itself is not the authoritative name server.
What is the application-layer protocol for “The Web”
HTTP
What is the application-layer protocol for “File transfer”
FTP (file transfer protocol)
What is the application-layer protocol for “Remote login”
Telnet
What is the application-layer protocol for “E-mail”?
SMTP (simple mail transfer protocol)
What is the application-layer protocol for “BitTorrent file sharing”
BitTorrent protocol
For communication session between a pair between a pair of processes, which process is the client and which is the server?
Client process: process that initiates the communication
Server process: process that waits to be contacted
For a P2P file-sharing application, do you agree with the description of clients and servers communicating in a normal communication session? Why or why not?
No.
In P2P, the peer that is receiving a file is typically the client and the peer sending the file is typically the server.
Briefly describe the DNS servers hierarchy
3 classes of DNS servers:
1.) Root DNS Servers: top of hierarchy, provide IP addresses to the TLD servers
2.) Top-level domain (TLD) DNS servers are below root DNS servers. Provide IP addresses for authoritative DNS servers
3.) Authoritative DNS servers are at the bottom, they contain publicly accessible DNS records
When a program is running within a host, two processes within that same host can
communicate using inter-process communication. This is not so when processes are
communicating from different hosts. They instead exchange messages. How can they identify
one another, what information is needed?
The two things needed to address a process are:
- IP address of destination host
- Port number of the socket in the destination process
When you use WhatsApp to video-call a friend, why can’t you term the application as elastic?
Elastic applications are able to adapt their rate to maximise their throughput, when they
encounter resource constraints. They can tolerate delays but are usually strict about data loss.
This is the opposite for other kinds of apps, which are delay-sensitive but loss-tolerant, like Whatsapp.
In this case, because the video call needs a minimum amount of throughput to be effective
(from a certain few kbps-Mbps), you cannot group it among the elastic apps, as those would
make use of whatever throughput they get.
Suppose you wanted to do a transaction from a remote client to a server as fast as possible.
Would you use UDP or TCP? Why?
Since UDP allows you to bypass some restrictions underpinning the TCP service, you would use it as it is faster.
No handshakes in UDP
With UDP, the transaction can be completed in roundtrip time (RTT): client send to UDP socket and server sends the reply back to the client’s UDP socket.
With TCP, a minimum of two RTTs are needed: one to set up TCP connection and another for the client to send the request and for the server to send back the reply.
List the four broad classes of services that a transport protocol can provide. For each of the
service classes, indicate if either UDP or TCP (or both) provides such a service.
1.) Reliable data transfer: TCP provides a reliable byte-stream between client and server,
but UDP does not.
2.) Minimum throughput guarantee: Neither
3.) Low delay or timing guarantee: Neither
4.) Security like confidentiality (via encryption): Neither
Say you develop an e-commerce app and you want to be able to track your customers. More
precisely, you want to be able to maintain a shopping cart and to keep a purchase record for
each of them, so that you may use it for various things such as recommending relevant
shopping items to them. Describe how this can be done with cookies. Be as descriptive as
you’d like.
Be sure to include: cookie files, cookie herder lines in the response and request, unique ID, database entry
For example:
When a prospective customer or user first visits the site, the server creates a unique identification number and creates an entry in its back-end database.
It then returns this identification number as a cookie number in an HTTP response with a “set-cookie: id” header
line.
This cookie number is stored on the user’s host, in a cookie file, and is managed by the browser.
During each subsequent visit (and purchase), the browser sends the cookie number back to the site in an HTTP request message with a “cookie: id” header line. Thus, the site knows when this user (more precisely, this browser) is visiting the site, and can attach their
record to them.
Suppose Alice, with a Web-based e-mail account (such as Hotmail or Gmail), sends a message
to Bob, who accesses his mail from his mail server using POP3.
Explain the stepwise procedure of how the message gets from Alice’s host to Bob’s host. Be
sure to list the series of application-layer protocols that are used to move the message
between the two hosts.
1.) Alice composes the email
2.) she sends the message from her host to her mail sever over HTTP. Here it’s placed in the message queue.
3.) client side of SMTP opens a TCP connection with Bob’s mail sever at port 25
4.)Alice’s mail server then sends the message to Bob’s mail server over SMTP. SMTP begins the handshaking before it proceeds to send the message. In this handshake, client will indicate the address of the sender and the recipient. If there were more messages, they would be sent over this same connection. Once done the SMTP client tells TCP to close the connection.
5.) Bob’s mail server places the message in Bob’s mailbox
6.) Bob then transfers the message from his mail server to his host over POP3. Bob can now invoke his user agent to read the message.
