CCNA Learning Journey

Day 3 : TCP/IP Model

Protocols and Standards:

• A protocol is a set of rules that defines how data should be communicated between devices over a network.
• “Language” computer uses to communicate

history

• Vint Cerf and Bob Kahn (working at DARPA) began developing TCP (Transmission Control Program) in 1974.
• Later divided into two protocols still used today:
  • Transmission Control Protocol (TCP)
  • Internet Protocol (IP)

Who defines standards?

• IEEE (Institute of Electrical and Electronics Engineers)
  • Develops many of the technologies used on local area networks:
    • Ethernet (802.3)
    • Wi-Fi (802.11)
• IETF (Internet Engineering Task Force)
  • Open community that defines protocols used on the Internet:
    • TCP, IP, UDP, HTTP, DNS, etc.
  • Publishes standards in documents called RFCs (Requests for Comments)
• Layered models
  • Networks do different jobs to move data from one device to other like:
    • Physical transmission of signals, local delivery on a LAN, routing traffic between networks, end-to-end conversations, applications, etc.
  • A model lets us group related jobs into layers.
    • Each layer has a specific role
    • Each layer uses the services of the layer below and provides services to the layer above
  • Protocols live(mostly) at one layer.
    • e.g. IP, TCP, HTTP, etc.
    • Together they form a stack of protocols that work as a team (the network stack)

From RFC 791, “Internet Protocol” (1981)

TCP/IP Model

Application layer (layer 5)

• Protocols for communication between application processes.
• Defines how application processes format, send and interpret data
• Prototols at this layer define message formats and rules for specific tasks, such as:
  • browsing web pages (HTTP/HTTPS)
  • Transferring files (FTP, TFTP)
  • Sending/receiving email (SMTP, POP3, IMAP)
• Network infrastructure devices (routers, switches) don’t care about Application-layer details.
  • They just move messages across the network.
  • Only the communicating hosts interpret the data.

Transport layer (layer 4)

• Provides end-to-end communication between application processes using port numbers .
  • also called process-to-process or service-to-service
• Runs mainly on the communicating hosts.
• Protocols at this layer include:
  • UDP (User Datagram Protocol): simple and efficient
  • TCP (Transmission Control Protocol): more robust features beyond basic message addressing

Internet layer (layer 3)

• Provides ene-to-end communication between hosts, using IP addresses and routers.
• Protocols at this layer include:
  • IP (IPv4, IPv6)
  • ICMP (Internet Control Message Protocol)

Local Network layer (layer 2)

• Provides hop-to-hop delivery of messages within a local network using MAC addresses and switches.
• A hop is one step along the path between two devices:
  • from one router or host, to the next router or host in the path.
  • switches don’t count: a switch just extends the local network, allowing multiple devices to connect.
• Uses MAC (Media Access Control) addresses to identify interfaces.
• Protocols at this layer include:
  • Ethernet (IEEE 802.3)
  • Wi-Fi (IEEE 802.11)

Physical layer (layer 1)

• Sends bits as electrical, optical, or radio signals over the physical medium.
• defines things like cables, connectors, signal levels and link speeds.
• e.g. copper UTP cables, fiber-optic cables, Wi-Fi radios and antenna, network interface cards(NICs),

Encapsulation & Decapsulation

• Application layer prepares the data to be sent over the network.
• As the messages moves down the stack, each layer encapsulates the data with a header including the information needed for that layer.
  • source and destination addresses (port numbers, IP addresses, MAC addresses), etc.
  • layer 2 also adds a trailer that the receiving device uses to check for transmission errors.
• Physical layer transmits the bits as signals over the physical medium.
  • the L2 header is transmitted first, and the L2 trailer is transmitted last

• Receiving device receives the message as a stream of bits at Layer 1.
• The device examines the information in the Layer 2 header and trailer, and then removes them (decapsulation).
  • The decapsulation process continues up the stack: Layer 3 removes the L3 header, then Layer 4 removes the L4 header, and then the data is delivered to the Application layer
• The application processes the data and, if needed, generates a response that goes back down the stack.

Protocol Data Units

At every stage of Encapsulation/Decapsulation process, there is a name given to the message:

• The combination of data and L4 header is called a segment(TCP) or datagram(UDP).
  • TCP creates segments, UDP creates datagrams.

• The combination of segment/datagram and L3 header is called a packet.

• The combination of a packet and L2 header/trailer is called a frame.
  • the is the exact thing sent over the wire.

Alternate name to describe message at each stage: Protocol Data Unit (PDU)

• A segment or datagram’s payload is the application data.
• A packet’s payload is a segment or datagram.
• A frame’s payload is a packet.

Adjacent-layer interaction

• Each layer provides a service to the layer above it, and is serviced by the layer below it (adjacent-layer interaction).
  • e.g. Layer 4 provides a service to layer 5 by delivering data to the correct application using port numbers.

Same-layer interaction

• Each layer communicates with the same layer on other devices(same-layer interaction).
  • e.g. Application layer on one host sends data to application layer on the other host.

Separation of layers

• Each layer has its own job and provides a specific service to the layers above.
  • The layers are modular.
• As long as each layer keeps its “contract” with the other layers, we can improve or replace protocols at different layers without redesigning everything.
• That flexibility is one of the main benefits of a layered model.

The OSI Model

• In late 1970s and 1980s, International Organization of Standardization(ISO) designed a 7 layer Open Systems Interconnection (OSI) model and a matching protocol suite.
• OSI protocols ended up being late and complex, and never gained the same deployment as TCP/IP.

7 Layers of OSI

Acronym:

Application : A > All
Presentation : P > People
Session : S > Seem
Transport : T > To
Network : N > Need
Data Link : D > Data
Physical : P > Processing

• 7.Application Layer
  • Closest to the user. Provides network services directrly to software like browser, email client
  • Protocols: HTTPS, DNS, FTP, SMTP, SSH etc.
• 6.Presentation Layer
  • Acts as a translator. translates the data in different format and handles encryption, compression, character encoding
  • Protocols: SSL/TLS (Encryption), JPEG,MPEG(Media format)
• 5.Session Layer
  • Manages “dialogues” (sessions) between communicating hosts. It opens, maintains and terminates the communication sessions between local and remote applications.
  • Protocols: NetBIOS, RPC(Remote Procedure Call), PPTP
• 4.Transport Layer
  • Responsible for end-to-end communication and error recovery. Breaks large pieces of data into smaller segments.
  • Protocols: TCP, UDP
• 3.Network Layer
  • The “routing” layer. It determines best physical path for the data to take based on network conditions and IP addresses. Provides logical addressing (IP addresses)
  • Provides connectivity between end hosts on different networks (i.e. outside LAN)
  • Protocols: IP(IPv4/IPv6), ICMP(Ping), ARP(Address Resolution Protocol), IGMP, OSPF/BGP(Routing Protocols)
• 2.Data Link Layer
  • Handles data trasfers between two directly connected nodes.
  • Convert raw bits into structured frames
  • Error correction from the physical layer
  • manages MAC addresses
  • Protocols: Ethernet(802.2), Wi-Fi(802.11), PPP (Point to Point Protocol)
• 1.Physical Layer
  • The actual hardware level. It defines physical characteristics of the medium to transfer data, like, voltage levels, max transmission distance, physical connectors, cable specifications.
  • Digital bits are converted to electrical (for wired connections) or radio (for wireless connection) signals
  • Hardware/Standards: RJ45 connectors, Fiber Optics, Coaxial Cables
• Network engineers dont usually work with top 3 layers. Application developers work with the top layers to connect their applications over networks.