Protocol Layers & Encapsulation

Question 1

Network intercommunication was the main design goal of the Internet

Answer 1

Question 2

TCP/IP was designed to enable network intercommunication

Answer 2

Question 3

Abstraction is the solution!

Question 4

Abstraction leads to layering

Answer 4

Question 5

The Internet Protocol Stack (TCP/IP)

Answer 5

1. Packet Switching: the
   fundamental Internet principle!
2. Routing: necessary to
   interconnect distinct networks
3. Bit Transmission: enables data
   communication

Question 6

Benefits of Protocol Layering

Answer 6

• Structure
  – Does not specify an implementation
  – Instead, tells us how to organize functionality
• Modularity
  – Eases maintenance, updating of system
  – Allows identification, relationship of complex system’s pieces
• Flexibility
  – Reuse of code across the network
• Extensibility
  – Module implementations may change

Question 7

Layer Features

Answer 7

• Service
  – What does this layer do?
• Interface
  – How do you access this layer?
• Protocol
  – How is this layer implemented?

Question 8

Communication Service

Answer 8

• Specification of
  – Functionality a layer offers
  – How this functionality is provided
  – Interaction between layers (service user & service provider)
• Service definitions abstract from how the service is actually implemented
  – Service interface hides complexity
  – Service is implemented through protocols

Question 9

Communication Protocol

Answer 9

• Specification of
  – How and when data is transmitted and received
  – Allowed messages and expected/required replies
  – Ordering and timing of transmissions
  – Exact format of transmitted data
• Protocols often make assumptions that demand lower-level protocols
  – We talk of communicating peers using the same protocol

Question 10

Internet Protocol Stack: Application Layer

Answer 10

• Service
  – Whatever we want :)
• Interface
  – Whatever you want :D
• Protocol
  – Whatever you want ;)
• Examples:
  – File Transfer (FTP)
  – World Wide Web (HTTP)
  – Email (SMTP)
  – Directory Services (DNS)

Question 11

Internet Protocol Stack: Transport Layer

Answer 11

• Service
  – Multiplexing/demultiplexing
  – Congestion control
  – Reliable, in-order delivery
• Interface
  – Send messages to a destination
• Protocol
  – Port numbers
  – Reliability/error correction
  – Flow-control information
• Examples:
  – Transmission Control Protocol (TCP)
  – User Datagram Protocol (UDP)

Question 12

Internet Protocol Stack: Network Layer

Answer 12

• Service
  – Deliver packets across the network
  – Handle fragmentation/reassembly
  – Packet scheduling
  – Buffer management
• Interface
  – Send a packet to a specific destination
• Protocol
  – Define globally unique addresses
  – Maintain routing tables
• Example: Internet Protocol (IP), IPv6

Question 13

Internet Protocol Stack: Data Link Layer

Answer 13

• Service
  – Data framing: boundaries between packets
  – Media access control (MAC)
  – Per-hop reliability and flow-control
• Interface
  – Send one packet between two hosts connected to the same media
• Protocol
  – Physical addressing (e.g. MAC address)
• Examples:
  – Ethernet
  – Wifi
  – DOCSIS

Question 14

Internet Protocol Stack: Physical Layer

Answer 14

• Service
  – Move information between two systems connected by a physical link
• Interface
  – Specifies how to send one bit
• Protocol
  – Encoding scheme for one bit
  – Voltage levels
  – Timing of signals
• Examples:
  – coaxial cable
  – fiber optics
  – radio frequency transmitters
  – Twisted pair
  – Infrared (IR)

Question 15

The ISO/OSI model

Answer 15

• Presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine specific conventions
• Session: synchronization, checkpointing, recovery of data exchange
• Internet model “misses” these layers!
  – These services, if needed, must be implemented in application
  – Needed?

Question 16

Data flow in the TCP/IP model

Answer 16

Question 17

Services, Layering and Encapsulation (Application)

Answer 17

1. Application exchanges messages to
   implement some application service using
   services of transport layer

Question 18

Services, Layering and Encapsulation (Transport)

Answer 18

Question 19

Services, Layering and Encapsulation (Network)

Answer 19

Question 20

Services, Layering and Encapsulation (Link)

Answer 20

Question 21

Services, Layering and Encapsulation Flow

Answer 21

Question 22

Encapsulation: an end-end view

Answer 22

Question 23

Encapsulation

Answer 23

Question 24

Encapsulation in practice

Answer 24

Question 25

The Internet Hourglass

Answer 25

Question 26

Why is the internet core is “dumb” by design

Answer 26

• Minimal functionality in Layer below the Application layer
• The End-to-End Principle:
  “ Don’t implement anything in the network that can be implemented correctly by the hosts
• Generality, Net Neutrality, Low cost and complexity
• Tradeoffs?

Question 27

Layering sometimes can be harmful

Answer 27

• Lower performance due to data and processing overhead added by protocol headers
• The same functionality may be duplicated by different layers (e.g. error detection)
• Lack of transparency in lower layers hinders optimization

Question 28

Reality Check

Answer 28

• Layering regularly violated:
  
  • Firewalls
  • Transparent proxies
  • NATs
• Conflicting Interests:
  
  • Architectural purity
  • Commercial necessity