Module 3 application layer:

The Presentation layer has three primary functions:


• Coding and conversion of Application layer data to ensure that data from the 
source device
 can be interpreted by the appropriate application on the destination device.
• Compression of the data in a manner that can be decompressed by the destination device.
• Encryption of the data for transmission and the decryption of data upon receipt by the destination.

common protocols in tcp/ip:

• Domain Name Service Protocol (DNS) is used to resolve Internet names to IP addresses.
• Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web.
• Simple Mail Transfer Protocol (SMTP)
 is used for the transfer of mail messages and attachments.
• Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices.
• File Transfer Protocol (FTP) is used for interactive file transfer between systems.


Exploration module 4: OSI transport layer.
Transport layer is responsible for end-to-end transport of data and integrity of such data.

Transport layer:
Enables multiple applications to communicate over the network at the same time on a single device 
Ensures that, if required, all the data is received reliably and in order by the correct application
Employs error handling mechanisms
transport layer prepares data for transmition over the network.
It also segments data and handles which application receives which data.
Transport layer adds headers to allow segmenting of data and the reassembly of these segments into streams.
Applications do not need to know specifics of the network the data is travelling on at the transport layer.
TCP/IP uses port numbers to differentiate between conversations.
data is segmented to prevent monopolization of the network by various applications.
Tcp provides 

Connection-oriented conversations
Reliable delivery
Ordered data reconstruction
Flow control

Because data can arrive via different paths and at different times tcp needs to reorder segments.

TCP is end to end reliable, handles error correction and packet retransmition. UDP is fire and forget, low overhead, no acknowledgement.
pieces of data in UDP are called datagrams.
tcp header length 20 bytes.
udp header 8 bytes.

discuss source and destination ports.

combination of ip address and port number is known as a socket.

Well known server ports 0-1023
registered ports 1024-49151
49152-65536 dynamic ports assigned to applications on an discuss netstat show usage.
 as needs basis.

Discuss netstat and explain usage.

Tcp uses sequence numbers and ack numbers.

tcp must create a session between 2 hosts for communication to function.
sequence number, last byte that host has sent.
ack number: next byte host expects to receive.

only one application can listen on a particular port on a server.

3-way handshake, syn ack syn.
client sends sequence number, server sends ack number 1 higher than sequence number.
note this happens in both directions.

sequence number is randomly chosen at connection establishment to prevent guessing sequence numbers.
Sequence number is incremented for each byte of data sent.
Segments are held in a buffer until all segments in correct order then data is handed up to application.
This buffer should be as big as the window size for the session.
sequence number is number of bytes plus 1. ack should be same as sequence number or the next byte expected to be received.

also selective acks can occur.
if segment is not acked it is resent with an exponential backoff timer.

selective ack means that segments can be acked then only missing segments need to be retransmitted.

window size can grow and shrink depending on network conditions.


5: Network layer.
Network PDUs are called packets.
Packets have source and destination addresses.
packets are routed at the network layer and devices called routers do this.
each router a packet passes through is called a hop.

IPV4 does the following
• Connectionless - No connection is established before sending data packets.
• Best Effort (unreliable) - No overhead is used to guarantee packet delivery.
• Media Independent - Operates independently of the medium carrying the data.
Largest packet that media can take sets mtu.

discuss ttl
prevents routing loops
TOS is important also.
sets priority of packets
ip has header checksum.