1. Definitions
Protocol (1) An agreement between the communicating parties on how communication is to proceed.
(2) A set of rules that governs how two or more communicating entities in a layer are to interact.
Protocol Stack A list of the protocols used by a certain system, one protocol per layer.
Peers The entities comprising the corresponding layers on different machines.
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

2. Why Layers?
Layering simplifies design, implementation, and testing by partitioning overall communications process into parts
Protocol in each layer can be designed separately from those in other layers
Protocol makes “calls” for services from layer below
Layering provides flexibility for modifying and evolving protocols and services without having to change layers below
Monolithic non-layered architectures are costly, inflexible, and soon obsolete
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

3. Protocol Layers (1)
Protocol layering is the main structuring method used to divide up network functionality.
Each protocol instance talks virtually to its peer
Each layer communicates only by using the one below
Lower layer services are accessed by an interface
At bottom, messages are carried by the medium
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

4. Protocol Layers (2)
Example: the philosopher-translator-secretary architecture Each protocol at different layers serves a different purpose
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

5. Protocol Layers (3)
Each lower layer adds its own header (with control inform- ation) to the message to transmit and removes it on receive Layers may also split and join messages, etc.
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

6. Example: HTTP HTTP is an application layer protocol
Retrieves documents on behalf of a browser application program
HTTP specifies fields in request messages and response messages
Request types; Response codes
Content type, options, cookies, …
HTTP specifies actions to be taken upon receipt of certain messages
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

7. HTTP Protocol HTTP Client HTTP Server
HTTP assumes messages can be exchanged directly between HTTP client and HTTP server
In fact, HTTP client and server are processes running in two different machines across the Internet
HTTP uses the reliable stream transfer service provided by TCP
GET
HTTP
Client
HTTP
Server
Response
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

8. Example: TCP TCP is a transport layer protocol
Provides reliable byte stream service between two processes in two computers across the Internet
Sequence numbers keep track of the bytes that have been transmitted and received
Error detection and retransmission used to recover from transmission errors and losses
TCP is connection-oriented: the sender and receiver must first establish an association and set initial sequence numbers before data is transferred
Connection ID is specified uniquely by
(send port #, send IP address, receive port #, receiver IP address)
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

9. HTTP using the TCP service
server
HTTP
client
Response
GET
Port 80
Port 1127
TCP
80, 1127
GET
1127, 80
bytes
Response
TCP
GET
Response
TCP
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

10. Design Issues for the Layers
Each layer solves a particular problem but must include mechanisms to address a set of recurring design issues
Issue
Example mechanisms at different layers
Reliability despite failures
Codes for error detection/correction (§3.2, 3.3)
Routing around failures (§5.2)
Network growth and evolution
Addressing (§5.6) and naming (§7.1)
Protocol layering (§1.3)
Allocation of resources like bandwidth
Multiple access (§4.2)
Congestion control (§5.3, 6.3)
Security against various threats
Confidentiality of messages (§8.2, 8.6)
Authentication of communicating parties (§8.7)
The point is that there are some issues that are not wholly the responsibility of any one layer, and they crop up again and again in the text. For example, reliability is often considered a key function of the transport layer (i.e., making transport reliable) yet reliability mechanisms also appear in other layers (error codes in the link layer, routing around failures in the network layer, and replication at the application layer).
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

11. Connection-Oriented vs. Connectionless
Service provided by a layer may be kinds of either:
Connection-oriented, must be set up for ongoing use (and torn down after use), e.g., phone call
Connectionless, messages are handled separately, e.g., postal delivery
TCP provides a reliable bytestream service at the Transport layer, IP provides unreliable datagram service at the Network layer.
More examples: RTP (used to carry VoIP data) provides unreliable connection service; (WiFi) provides acknowledged datagram service; Ethernet provides unreliable datagram service.
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

12. Service Primitives (1)
A service is provided to the layer above as primitives Hypothetical example of service primitives that may provide a reliable byte stream (connection-oriented) service:
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

13. Service Primitives (2)
Hypothetical example of how these primitives may be used for a client-server interaction
Client
Server
LISTEN (0)
ACCEPT
RECEIVE
SEND (4)
DISCONNECT (6)
CONNECT (1)
SEND
DISCONNECT (5)
Connect request
Accept response
Request for data
Reply
Disconnect
(2)
(3)
The primitives are called at the client and server by the higher layer using the service. The layer implements the primitives by sending messages using the services of the lower layer; these messages are assumed to be reliable for simplicity and the lower layer service is not otherwise described.
This is similar to the way that simple Web browsers and Web servers work today.
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

14. Relationship of Services to Protocols
Recap:
A layer provides a service to the one above [vertical]
A layer talks to its peer using a protocol [horizontal]
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

15. OSI Reference Model
A principled, international standard, seven layer model to connect different systems
– Provides functions needed by users
– Converts different representations
– Manages task dialogs
– Provides end-to-end delivery
– Sends packets over multiple links
– Sends frames of information
– Sends bits as signals
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

16. 7-Layer OSI Reference Model
Application
Application
End-to-End Protocols
Application
Layer
Application
Layer
Presentation
Layer
Presentation
Layer
Session
Layer
Session
Layer
Transport
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Communicating End Systems
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

17. Physical Layer Transfers bits across link
Definition & specification of the physical aspects of a communications link
Mechanical: cable, plugs, pins...
Electrical/optical: modulation, signal strength, voltage levels, bit times, …
functional/procedural: how to activate, maintain, and deactivate physical links…
Ethernet, DSL, cable modem, telephone modems…
Twisted-pair cable, coaxial cable optical fiber, radio, infrared, …

18. Data Link Layer Transfers frames across direct connections
Groups bits into frames
Detection of bit errors; Retransmission of frames
Activation, maintenance, & deactivation of data link connections
Medium access control for local area networks
Flow control
frames
Data Link
Layer
Data Link
Layer
bits
Physical
Layer
Physical
Layer

19. Network Layer
Transfers packets across multiple links and/or multiple networks
Addressing must scale to large networks
Nodes jointly execute routing algorithm to determine paths across the network
Forwarding transfers packet across a node
Congestion control to deal with traffic surges
Connection setup, maintenance, and teardown when connection-based

20. Transport Layer
Transfers data (segments or datagrams) end-to-end from process in a machine to process in another machine
Reliable stream transfer or quick-and-simple single-block transfer
Port numbers enable multiplexing
Message segmentation and reassembly
Connection setup, maintenance, and release
Transport
Layer
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Communication Network

21. Application & Upper Layers
Application Layer: Provides services that are frequently required by applications: DNS, web acess, file transfer, …
Presentation Layer: machine-independent representation of data…
Session Layer: dialog management, recovery from errors, …
Application
Layer
Transport
Application
Layer
Presentation
Session
Transport
Incorporated into Application Layer

22. Headers & Trailers
Each protocol uses a header that carries addresses, sequence numbers, flag bits, length indicators, etc…
CRC check bits may be appended for error detection
Application
Application
APP DATA
Application
Layer
Application
Layer AH
APP DATA
Transport
Layer
Transport
Layer TH AH
APP DATA
Network
Layer
Network
Layer NH TH AH
APP DATA
Data Link
Layer
Data Link
Layer DH NH TH AH
APP DATA
CRC
Physical
Layer
Physical
Layer
bits

23. TCP/IP Reference Model
A four layer model derived from experimentation; omits some OSI layers and uses the IP as the network layer.
IP is the “narrow waist” of the Internet
Network
NIC
The comment about the narrow waist refers to the fact that the network layer of the Internet is IP (Internet Protocol) such that the network layer is called the “Internet” layer. The significance is that all Internet devices speak IP, which provides a point of interoperability that enables innovation both above (new applications and transports) and below (new link technologies).
Protocols are shown in their respective layers
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

24. Model Used in this Book
It is based on the TCP/IP model but we call out the physical layer and look beyond Internet protocols.
Port Numbers
IP Address
NIC
MAC Address
(eg. Ethernet Address)
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

25. TCP/IP Protocol Suite Distributed applications User
HTTP
SMTP
DNS
RTP
Distributed applications
Reliable stream service
User
datagram service
TCP
UDP
Best-effort
connectionless packet transfer IP
(ICMP, ARP)
Network
interface 1
Network
interface 2
Network
interface 3
Diverse network technologies

26. Critique of OSI & TCP/IP
Very influential model with clear concepts
Models, protocols and adoption all bogged down by politics and complexity
TCP/IP:
Very successful protocols that worked well and thrived
Weak model derived after the fact from protocols
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011