14.1 Protocols

The need for protocols

When sending and receiving data, both parties need to agree the protocol being used to ensure successful communication takes place.

Internet protocol suite

The 4 layer structure for internet protocol suite looks like:

• Application layer

• Transport layer

• Network layer

• Data link layer

where sending goes from the application layer to the link layer, and receiving goes from the link layer to the application layer.

Application layer

• Contains all the programs that exchange data, such as web browsers or server software

• It sends files to the transport layer

• Allows applications to access the services used in other layers

• Defines the protocols that any app uses to allow the exchange of data

1. HTTP (Hypertext transfer protocol)
   • Used when fetching an HTML document from a web server
   • Defines the format of the messages sent and received
   • Web browser initiates the web page request and also converts HTML into a format which can be displayed on the user’s screen

2. FTP (File transfer protocol)
   • Used when transferring files from one computer/device to another via the internet or other networks

3. POP3 (Post office protocol)
   • Used when receiving emails from the email server
   • A pull protocol
   • Does not keep the server and client in synchronisation
   • When emails are downloaded by the client, they are then deleted by the server

4. IMAP (Internet message access protocol)
   • Used when receiving emails from the email server
   • A pull protocol
   • Keeps the server and client in synchronisation
   • Only a copy of the email is downloaded with the original remaining on the server

5. SMTP (Simple mail transfer protocol)
   • Used when sending emails
   • A push protocol
   • Client opens a connection to a server and keeps the connection at all times
   • Client uploads a new email to the server

6. BitTorrent protocol
   • Computers share files directly with each other
   • No structure and no controlling mechanism
   • Peers act as both clients and servers
   • Each peer is just one end-system
   • When a peer acts as a server, it is called a “seed”
   • How does a peer find others that have the wanted content?
     • Every content provider provides a content description, which is a file that contains the name of the tracker and a list of the chunks that make up the content
     • The tracker is a server that maintains a list of all the other peers (the “swarm”) actively downloading and uploading the content
   • How do peers replicate content to provide high-speed downloads for everyone?
     • Peers download and upload chunks at the same time, but peers have to exchange lists of chunks and aim to download rare chunks for preference
     • Each time a rare chunk is downloaded it automatically becomes less rare
   • How do peers encourage other peers to provide content rather than just using the protocol to download for themselves?
     • Requires dealing with “leechers” who only download
     • Randomly try other peers but then to only continue to upload to those peers that provide regular downloads
     • If a peer is not downloading or only downloading slowly, the peer will eventually be isolated

Transport layer

• Regulates the network connections

• Where data is broken up into packets which are then sent to the network layer

• Ensures that packets arrive in sequence without errors, by swapping acknowledgements and retransmitting packets if they become lost or corrupted

TCP (Transmission control protocol)

• Responsible for the safe delivery of a message by creating sufficient packets for transmission

• Automatically resends a data packet if it has not received a positive acknowledgement

• Connection-oriented since it establishes an end-to-end connection between two host computers using handshakes

Network layer

• Network layer identifies the intended network and host

• Common protocol is IP

IP (Internet protocol)

• Ensure correct routing of packets of data over the network

• Responsible for protocols when communicating between networks

• Take a packet from the transport layer and add its own header which will include the IP addresses of both sender and recipient

• The IP packet is sent to the data link layer where it gets assembled into frames for transmission

Data link layer

• Identifies and moves traffic across local segments

• Encapsulates IP packets into frames for transmission

• Maps IP addresses to MAC addresses and ensures correct protocols are followed

• Identifies network protocols in the packet header and delivers packets to the network

1. Ethernet protocols
   • Designed for use in a local area network
   • Connects a number of computers or devices together to form a LAN
   • Uses protocols to control the movement of frames between computers or devices and to avoid simultaneous transmission by two or more devices
   • A local protocol, does not communicate with external devices

14.2 Circuit switching and packet switching

Circuit switching

• Method used in the traditional telephone system

• End-systems connected to local exchanges which have a switching function and which are connected via a number of intermediate nodes with a switching function

• For a data transfer to take place, the following has to happen:
  • The sender provides the identity of the intended receiver
  • The system checks whether or not the receiver is ready to accept data
  • If the receiver is available, a sequence of links is established across the network
  • The data is transferred
  • The links are removed

• Pros:
  • Circuit is dedicated to the single transmission only
  • Whole of the bandwidth is available
  • The data transfer rate is faster than with packet switching
  • Packets arrive in the same order as they were sent
  • Data will not get lost since all packets follow the same route
  • Works better than packet switching in real time applications

• Cons:
  • Not very flexible (has to use a single line)
  • Nobody can use the circuit even when it is idle
  • Circuit is always there whether or not it is used
  • If there is a failure, no alternate routing available
  • Dedicated channels require a greater bandwidth
  • Time to establish a link can be long

Packet switching

• Allows data transmission without a circuit being established

• Shortest path available is selected

• Data cannot be sent in a continuous stream, and they arrive the destination in a different order

• Data is packaged in portions inside packets

• A packet consists of a header which contains instructions for delivery plus the data body

• Pros:
  • No need to tie up a communication line
  • Possible to overcome faulty lines by rerouting
  • Easy to expand traffic usage
  • Charges user on the distance instead of the duration
  • High data transmission is possible
  • Uses digital networks which means digital data is transmitted directly to the destination

• Cons:
  • The protocols can be more complex
  • If a packet is lost, the sender must resend the packet
  • Does not work well with real-time data streams
  • The circuit has to share its bandwidth with other packets
  • There is a delay at the destination while packets are reassembled
  • Needs large amounts of RAM to handle the large amounts of data