Ch1. Intro to TCP/IP Networking

• the ability to post messages to social media sites, make phone calls, search for information on the Internet, listen to music, and download countless apps to their phones—without caring about how it works or how their favorite device connects to the network.
  

  

TCP/IP Networking Model

Application layer

• the application layer provides an interface between software running on a computer and the network itself.

• HTTP

  

  • Mechanisms

    
    
    1. Bob sends a message with an HTTP header
    2. The response from web server Larry. The message begins with an HTTP header, with a return code (200), which means something as simple as “OK” returned in the header.
    3. HTTP transfers the data by sending multiple messages, each with a part of the file, rather than wasting space by sending repeated HTTP headers.

Transport layer

• Transport layer protocols provide services to the application layer protocols.
• TCP Error Recovery Basics

  • To recover from errors, TCP uses the concept of acknowledgments.

    

    • The TCP header shows a sequence number(SEQ)with each message. In this example, the network has a problem, the network fails to deliver the TCP message (called a segment) with sequence number 2.
    • When Bob receives messages with sequence numbers 1 and 3, but does not receive a message with sequence number 2, Bob realizes that message 2 was lost.
    • That realization by Bob’s TCP logic causes Bob to send a TCP segment back to Larry, asking Larry to send message 2 again.

Network Layer

• comparing IP’s addressing and routing with another commonly known system that uses addressing and routing: the postal service.

• IP

  

  • Routers are networking devices that connect the parts of the TCP/IP network together for the purpose of routing (forwarding) IP packets to the correct destination.

  • IP protocol provides a service of forwarding IP packets from one device to another.

    

    1. Larry’s IP process chooses to send the packet to some router—a nearby router on the same LAN—with the expectation that the router will know how to forward the packet.
    2. R1 looks at the destination address (2.2.2.2), compares that address to its known IP routes, and chooses to forward the packet to Router R2.
    3. R2’s IP process will compare the packet’s destination IP address (2.2.2.2) to R2’s known IP routes and make a choice to forward the packet to the right, on to Bob.

Data-Link and Physical Layers

• define the protocols and hardware required to deliver data across some physical network.
• The TCP/IP physical and data-link layers include two distinct functions, respectively: functions related to the physical transmission of the data, plus the protocols and rules that control the use of the physical media.

1. Larry encapsulates the IP packet between an Ethernet header and Ethernet trailer, creating an Ethernet frame.
2. Larry physically transmits the bits of this Ethernet frame, using electricity flowing over the Ethernet cabling.
3. Router R1 physically receives the electrical signal over a cable and re-creates the same bits by interpreting the meaning of the electrical signals.
4. Router R1 de-encapsulates the IP packet from the Ethernet frame by removing and discarding the Ethernet header and trailer.

Data encapsulation Terminology

• The term encapsulation refers to the process of putting headers (and sometimes trailers) around some data.

  

Names of TCP/IP Messages

• Each term refers to a different layer: segment for the transport layer, packet for the network layer, and frame for the link layer.

  

OSI

• Just as for TCP/IP, the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined.

  

• OSI Data Encapsulation

  • OSI uses the term protocol data unit (PDU). A PDU represents the bits that include the headers and trailers for that layer, as well as the encapsulated data.

    

• it defines services that applications need.

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