Lecture 1: Protocols and services

Internet Protocol Stack (IPS) Layers

Application
Transport
Network
Link
Physical

Internet Protocol Stack

Application: Mail (SMTP/IMAP), WWW (HTTP/HTTPS)
Transport: TCP, UDP
Network: IP
Link & Physical: Ethernet, Wifi, Bluetooth, 3G

Application Layer

Network applications and their application-layer protocols e.g., web (HTTP), email (SMTP, IMAP), files (FTP)

Distributed over multiple end systems e.g., client and server.

Transport Layer

End-to-end (e2e) transport of application layer messages.

TCP: Transmission Control Protocol
1) connection-oriented service
2) guaranteed delivery
3) flow control (sender/receiver speed matching)
4) congestion control (throttle transmission rate when the network is congested)

UDP: User Datagram Protocol
1) connectionless service
2) no reliability
3) best effort

Network Layer

Moves datagrams from one host to another.
Input: address, transport layer packet (segment).
IP (Internet Protocol)
Routing protocols (determine the routes between source and destination)

Link Layer

Moves network layer datagrams from one node to the next node along the route.

Different (optional) functionality:
1) reliable delivery over one link
2) avoid collision on a shared medium
3) fair sharing of a medium

Ethernet, WiFi, Cable access, etc.
Every link along a route may run a different link layer protocol.

Physical Layer

Moves individual bits within a link layer frame from one node to the next.

IPS Protocol Data Units (PDU)s

Application: Message
Transport: Segment
Network: Datagram
Link: Frame
Physical: -

Message PDU purpose & content

Raw application content:
1) app headers (request/response metadata)
2) app payload

Format:
[ App Data ]

Segment PDU purpose & content

Segment (TCP segment or UDP datagram):
Purpose: provide reliable, connection-oriented, ordered, and error-checked process-to-process delivery. Wraps message PDU.
Header Content:
1) Source & destination port: identifies the specific software programs communicating on each machine.
2) Sequence & acknowledgement numbers (TCP only): Tracks byte order so lost data can be retransmitted and packets reassembled in order.
3) Control flags (TCP only): Manages connection state (SYN, ACK, FIN, RST).
4) Window size (TCP only): Controls flow so sender does not overpower reciever.
5) Checksum: Validates data integrity for segment

Format:
[ Transport Header | App Data ]

Datagram / IP Packet PDU purpose & content

Purpose: Handles logical addressing, path determination, and cross-network routing from source host to destination host. Wraps Segment PDU.
Header Content:
1) Source IP address: logical global address of originating device
2) Destination IP address: logical global address of target device
3) Time to live (TTL): a counter decreased by 1 at each router to destroy looping packets.
4) Protocol: declares what Transport protocol is inside (6 = TCP, 17 = UDP)
5) Header checksum: verifies IP header integrity
Wraps

Format:
[ IP Header | Transport Header | App Data ]

Frame PDU purpose & content

Purpose: Handles point-to-point data transfer between two directly connected physical devices on the same local network. Wraps Datagram PDU.
Header Content:
1) Source MAC address: physical hardware address of the current sending network interface card (NIC)
2) Destination MAC address: physical hardware address of the immediate next hop (e.g., local Wi-Fi router or switch port)
3) EtherType: declares what Network protocol is inside (e.g., 0x0800 for IPv4)
Trailer content:
1) Frame check sequence (FCS / CRC): a mathematical check added to the end of the frame so the receiver can detect bit errors caused by physical noise over the cable or radio signals.

Service Multiplexing

At the software layers, multiplexing allows multiple applications running on the same host to share a single network link and IP address simultaneusly.

Instead of needing a separate network cable for your browser, email, and music app, all application traffic is funneled through the same IP layer (multiplexing at the sender) and then routed back to the correct app (demultiplexing at the receiver).

Channel Multiplexing (Physical/Link layer)

At the bottom of the stack, multiplexing allows multiple communications to share a single physical wire, cable, or radio spectrum without interfering with each other.
FDM (Frequence Division Multiplexing: The total available bandwith is divided into separate frequency bands. Multiple signals travel simultaneously, but each uses a different frequence range.
TDM (Time Division Multiplexing): Devices take turns transmitting using the entire frequency band for brief, dedicated time slots (like round-robin scheduling).

Also check out WDM, PDM, SDM, OAM and CDM.

Demultiplexing

Service Demultiplexing:
Inspects protocol headers as data moves up the sofware stack to deliver payloads to the correct application process to the destination machine.

Channel Demultiplexing:
Separates physical signals sharing the same medium back into independent data channels.