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In today's the class,
we review examples of protocols and services.
Network Architecture Design evolution is driven by services and applications.
Examples Include: Internet transfer of individual block of information,
the Internet reliable transfer of a stream of
bytes and in real-time transfer of a voice signal.
Applications built upon communications services.
For example, short message service builds on
internet reliable streams service and a singular telephone text message.
So overall communication process between machines
connected across one or more networks is very complex.
Layering partitions related communication functions into groups that are manageable.
Each layer provides a service to the layer
above and each layer operates according to a protocol.
Let's use examples to show what we mean.
In Web browsing, say a user clicks on www.nytimes.com.
The URL contains internet name of machine but not its Internet address.
Internet needs Internet address to send or receive information to or from a machine.
Browser software uses Domain Name Service
(DNS) to send a query for the internet address of the server.
So the responding address is also cached in the browser for future use.
The Browser software uses HTTP protocol to send request for document.
HTTP server of the website.
Waits for the request by listening to a well-known port number, 80 for HTTP service.
HTTP client sends a request message through an ephemeral port number.
For example, 1127, which is used only for the duration of the connection.
HTTP needs a Transmission Control Protocol called TCP
reliable connection between the HTTP client
and the HTTP server to transfer messages reliably.
After the TCP connection is established,
the client uses HTTP to request a document,
which specifies the message "Get".
The server sends a status response encoded 200,
indicating the client request was successful,
and follows by sending the requested file.
Let's see how protocols and layers come into play.
Protocol's is a set of precise rules that
governs how two or more communicating entities in a layer are to interact.
What kind of message that can be send and receive.
What kind of actions that are to be taken when a certain event occurs.
In short, the purpose of a protocol is to provide a service to the layer above.
For example, HTTP is an application layer protocol.
It will retrieve documents on behalf of a browser application program.
It specifies field in request and responsive messages.
It also specifies actions to be taken upon receipt of certain messages.
HTTP use reliable stream service of TCP.
For illustration, HTTP Get request is sent to the underlying TCP entity,
which got encapsulated into a TCP segment
with a header of source and destination port numbers.
The segment is sent to the server by TCP reliable communication.
The server's TCP deprives
the header and gives the Get request to its up layer HTTP server.
HTTP server process the Get request.
The response is sent to its underlying TCP entity,
which got encapsulated into a TCP segment with a head
of source and destination point numbers.
So the segment is sent back to the client side by TCP reliable communication.
The client side TCP
deprives the header and gives the response to its upper layer HTTP client.
DNS is another application layer protocol.
It use a distributed database that resides in multiple machines in the Internet.
DNS protocol allows queries of different types.
Mainly domain name to IP address or IP address to domain name.
It has a well-known port number 53.
DNS servers are organized in a hierarchy.
Local Name Server resolve frequently used domain names.
If it cannot resolve a query,
it contacts Root Name Servers.
If a Root Name Server cannot resolve a query,
it goes to the last resort, Authoritative Name Servers.
The Authoritative Name Servers are a network of hundreds of servers around the world.
They are configured in the DNS's Root Zone as a team named Authorities.
A National Administration Agency called NTIA,
under United States Department of Commerce,
had exercised the ultimate authority
over the DNS root.There were bad hosts about to control over
Authority by different countries and organizations.
The control is eventually moved to International Assigned Numbers Authority called IANA.
Note that a DNS servers one primary target of cyber attacks.
Xiaobo ZhouProfessor, Interim Dean
