1. Layering & protocol stacks Johan Lukkien
Computer Networks 2002/2003
Layering & protocol stacks
Johan Lukkien
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

2. Architectural pattern: layers
“The Layers architectural pattern helps to structure applications”
that can be decomposed into groups of subtasks
in which each group of subtask is at a particular level of abstraction”
from “Pattern-oriented software architecture”, Buschmann et.al
Layering supports
stable interfaces
re-use
mastering complexity: separation of concerns
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

3. Layering Packets impose additional structure onto signals...
...Several packets can form again a higher level structure...
Network software is organized in layers of protocols, services and interfaces
separation of concerns
separation of what is provided (the service) from how it is accessed (the interface) and internally realized (the protocol)
different layers have different concerns
ease of design
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

4. Layering in communication
provided
service
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

5. Protocol stacks 14-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

6. Protocol stacks Level k message is payload of level k-1
add headers and trailers of level k-1
Binding: exact description of
A: mapping one protocol onto another
messages, communication behavior
B: mapping abstract functions onto a service interface
connecting layers at the interface
Tunneling: level k-1 tunnels level k
transport without interpretation of payload
term ‘tunneling’ usually reserved for a stack in which a service occurs twice
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

7. OSI reference model Open Systems Interconnection
Abstract architecture of communicating systems
Names and identifies relevant concepts
services, protocols and interfaces
service data units (xSDU) and protocol data units (xPDU), x is the layer
Introduces layer semantics
issues solved by a layer; layer services
Description of “how it should be done”
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

8. The OSI model 14-Jan-19 PSDU SSDU TSDU NSDU DSDU PhSDU
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

9. TCP/IP model 14-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

10. Remarks TCP/IP ‘model’ OSI model OSI protocol stack
named after the dominating protocols
model constructed after the fact
host-to-network
not really a layer; just get IP packets across
OSI model
well-discussed, before designing protocols
OSI protocol stack
not widely used
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

11. Some stacks and the OSI model
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

12. Model in the book... 14-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

13. Communication modes Sender and receiver
One to one
One to many
un-addressed: broadcast
collection of destinations: multicast
Many to one
messages appear at receiver in undetermined order
Fragmentation & synchronization
send and receive of message combined
fixed units (packets, frames, cells)
streaming
synchronization on availability of data and buffers
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

14. Communication modes Connections Message existence Timing
Connection oriented:
Setup connection, communicate, destroy connection
control: reliability, flow, congestion
Connection less:
Single shot: just send a message/packet
Message existence
Persistent: subsystem will deliver messages
Transient: message life depends on life of sender, receiver, subsystem
Timing
Time-independent
Time-dependent (real-time)
soft real-time, hard real-time
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

15. Issues, to be resolved by the layers
Error correction
Flow control
Addressing
Multiplexing
Naming
Congestion control
Mobility
Routing
Fragmentation
Security
....
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

16. Layer semantics trade-off’s
Important design decision: the choice in which level to solve a particular issue
too high: inefficient
error correction at the application layer
broadcasting at the transport layer
too low: issue may re-occur
multiplexing at the link layer
connection orientation below the network layer
Transparency: hiding lower layer details
good for portability, generality etc.
but must be considered carefully
e.g. mobility, location, multi-casting
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

17. Layer generality & re-usability
Layer k cooperates with many different types layers k-1
moderate required service
pitfall: layer becomes
complex and large
Layer k provides useful services to many different types k+1
powerful provided service
must be possible to indicate several next layers
pitfall: layer becomes complex and large
Hence, work towards standardization as quickly as possible
Service required by layer k
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

18. Definition: service
Service: an overall functionality (semantics) of an object.
Service quality: non-functional properties of a service (e.g. speed, reliability, ...).
Service interface: actions (“primitives”) and responses that make the service available; these responses can be autonomous (“events”). In addition, a specification that
describes their effect on state variables and parameters, as well as their results;
describes rules as how and in what sequence to call them;
describes the functional and non-functional properties of sequences of calls.
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

19. Service examples (cnt’d)
Typically, the communication modes
Quality dimensions
reliable, acknowledged, guaranteed bandwidth, low latency
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

20. Example: primitives for connection oriented service
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

21. Service examples (cnt’d)
Frame relay:
connection oriented
no error or flow control
typically, for connecting LANs
X.25:
connection oriented, typically across phone lines
still supported in e.g. GPRS
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

22. Definition: protocol
Protocol: A formal set of rules that dictates how information exchange as well as interaction between objects (can be devices, execution threads, etc.) should take place.
The rules specify
the format of the messages exchanged;
a number of different protocol states and what messages are allowed to be sent in each state; these states determine, among others, the order of the messages.
timing constraints and other non-functional properties, if any.
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

23. Exercises Exercises 11, 12, 13, 19, 20 on p82
Discuss the transmission of a letter through the postal system. Describe service, interface and protocol.
Discuss the trade-offs of providing reliable versus datagram service at the data link layer when the transport layer must provide reliable connection oriented service. Take into account
the probability of packet loss per link
the number of hops
the delay needed to judge when a retransmission is needed
the resulting throughput at the transport layer
14-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking