Module 4 - Application Layer & HTTP
List the steps required to retrieve a single HTML file from an HTTP server.
1. Client initiates TCP connection to the server 2. Client sends HTTP request message to server 3. Server processes request, retrieves object, encapsulates object in HTTP response message, and sends message to client 4. Server tells TCP to close the connection 5. Client receives response message and analyzes it 6. First four steps are repeated to get other objects referenced in the file received
What status code is returned for a redirect?
301 Moved Permanently
In the client-server architecture, which side makes the first contact?
Client
Which HTTP request is sent to a web proxy server?
GET requests
What line in a HTTP request presents a stored cookie to a server?
In a header called "Cookie:"
What line in a HTTP response provided a cookie to a client to store?
In a header called "Set-cookie:"
What is a socket?
Interface into which an application process can both send/receive messages to/from another application process.
What is an application-level protocol?
It defines how an application's processes, running on different end-systems, pass messages to each other.
Is SMTP secure?
It is not secure nor encrypted.
If a redirect is indicated, where does the client find the new URI to request?
It is specified in the "Location:" header of the response message?
Is HTTP a stateful or stateless protocol? What does this mean?
It is stateless, and it means that the server maintains no information about past browser requests.
What does the "Last-Modified:" header option indicate?
It means that the object being requested was last modified on a specific date.
What is the purpose of a cookie?
It remembers information about a user as they use the Internet.
What does a web proxy server do if the requested object is not found in the cache?
It returns a "200 OK" status code in the response message, and it fetches and returns the requested object.
What does a web proxy server do if the requested object is found in the cache?
It returns a "304 Not Modified" status code in the response message, and it does not return the requested object.
What does the "304 Not Modified" code status mean?
It tells the cache that it can go ahead and forward its cached copy of the object to the requested browser.
Does FTP use out-of-band or in-band control?
It uses out-of-band control due to it using a separate connection for the control information, and another connection for sending data.
How are images and other binary attachments included in email messages if everything in SMTP must be 7-bit ASCII?
It uses the Multipart Type format where it is broken up into multiple sections separated by the "parts separator".
What information is typically sent in an HTTP request message?
Method, path, version, fields of information, and an entity body.
What are three email access protocols?
POP, IMAP, and HTTP
What is the difference between passive and active FTP?
Passive: client and server agree on a new server port for data transfer, and then client contacts server on that new data port for transfer Active: client and server have contact, and the server uses the data port to contact client and transfer data
What is the default TCP port used for SSH?
Port 22
What is the default port used for Apple Wide Area Connectivity Service?
Port 4488
What is the default port used for HTTP?
Port 80
What are the two types of HTTP messages, and who sends each type?
Requests are sent by the client, and responses are from the server.
What protocol is used between mail servers?
SMTP
What protocol is used to send mail from a client to a mail server?
SMTP
In the client-server architecture, which side is "always-on"?
Server
What is the difference between a session and persistent cookie?
Session cookies are deleted when a browser closes. Persistent cookies are not deleted when a browser closes.
Which transport protocol does email use and why?
TCP because it is reliable.
What two elements are needed to address a particular communicating process on a particular end-system (that is, how are sockets addressed)?
The IP address and port number of the end-system.
What is the main difference between GET and POST HTTP methods, especially in the context of web forms?
The POST method will contain the requested information in the entity body, unlike GET where it puts it in the URL itself.
What does a status code of 200 mean?
The request succeeded
What does a status code of 404 mean?
The requested document could not be found.
How can a web proxy server tell if an object that it does have in its cache is up-to-date?
The server sends a response message with the requested object to the cache, and if the "Last-Modified:" header is the same as it was before, the cache is up-to-date; otherwise, it needs to fetch it again to update it.
If UDP does not offer reliable transport, why do any applications use it?
They can sacrifice some loss of packets, but it requires a minimum rate to be effective.
What information is typically included in an HTTP response message?
Version, code, phrase, fields of information, and an entity body.
