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Application layer is the layer the user actually interfaces with.

Application layer provides :

Presentation to the user. 
The interface to the functionality.
Security Encryption/decryption.
Login/log-out - if appropriate.
Requesting and monitoring the connection between client and remote service
and reporting back to user.
Reporting success, progress, and/or failure of various tasks. 
Filezilla, Coreftp, IE, Firefox, Putty, ssh, etc.



Transport (host-to-host) or TCP layer establishes the connection between client 
and service.  And maintains the connection appropriate to the particular 
protocol.


Transport layer provides :

Actual negotiation of a virtual connection between client and service.
Maintaining the connection if appropriate.
Traffic congestion control.
Port number assignments. Ports are used to identify access/connection to a 
  service or application task on a node.
  ssh 22, HTTP/HTTP 80/443, ftp 20/21, etc.
Reassembly of multi-packet messages.
Elimination of duplicated packets.
Implementation of the various TCP protocols.

  https://en.wikipedia.org/wiki/List_of_IP_protocol_numbers

  TCP - Transmission Control Protocol  (most common?)
    Multi-packet transmission with connection negotiation, sequencing, and 
    error correction. Called connection based.

  UDP - User Datagram Protocol
    single packet requests/response with hope no guarantee of response. 
    Classified  connection-less.

  ICMP - Internet Control Message Protocol 
    Use to send error messages or queries between nodes.    
 
  IGP/EGP/BGP - Internet/External/Border Gateway Protocols
    Used by gateway systems that are portal between different networks to
    exchange routing information. 

  There are around 140 defined varieties of packets, some obsolete,


TCP protocol provided as shared library/drivers on most systems and 
handled by the OS or system level daemons.


Internet (IP) layer provides the mechanism to move packets of information 
across inter-networked networks (Internet) irrelevant of technology of 
individual networks.

Internet layer provides :

Independent of 'mechanics' of actual delivery, Ethernet, token ring, etc.
Identification protocol of source and destination nodes. IP numbers.
Packet routing across hops.
Delivery of individual packets to destination.
Protocol Ids - different ways information being transmitted can be packaged.
Protocols implemented in TCP layer but flagged in IP layer.


Link (data-link) layer establishes connections between nodes along the
route between any two communication end points.

Link layer provides :

Routing at a node to next hop on path.
Optimization of route choice to next hop.
Success of packet delivery to next node.
Conversion of logical packet bits to electronic (bit) signals.
Encoding of timing within electronic bit stream.


Printed circuits have essentially merged the physical and data-link levels
of ISO-OSI into a single functional unit.

Lectures