Computer Networks 20022003 Layering protocol stacks Johan Lukkien

Computer Networks 2002/2003 Layering & protocol stacks Johan Lukkien 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 1

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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 2

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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 3

Layering in communication provided service 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 4

Protocol stacks 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 5

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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 6

OSI reference model • Open Systems Interconnection • Abstract architecture of communicating systems • Names and identifies relevant concepts – services, protocols and interfaces – service data units (x. SDU) and protocol data units (x. PDU), x is the layer • Introduces layer semantics – issues solved by a layer; layer services • Description of “how it should be done” 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 7

The OSI model PSDU SSDU TSDU NSDU DSDU Ph. SDU 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 8

TCP/IP model 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 9

Remarks • TCP/IP ‘model’ – 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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 10

Some stacks and the OSI model 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 11

Model in the book. . . 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 12

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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 13

Communication modes • Connections – 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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 14

Issues, to be resolved by the layers • • • Error correction Flow control Addressing Multiplexing Naming Congestion control Mobility Routing Fragmentation Security. . 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 15

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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 16

Layer generality & re-usability • Layer k cooperates with many different types layers k-1 – moderate required service • pitfall: layer becomes complex and large Service required by layer k • 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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 17

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. 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 18

Service examples (cnt’d) • Typically, the communication modes • Quality dimensions – reliable, acknowledged, guaranteed bandwidth, low latency 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 19

Example: primitives for connection oriented service 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 20

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 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 21

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. 12/30/2021 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 22

Exercises • Exercises 11, 12, 13, 19, 20 on p 82 • 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 – – 12/30/2021 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 Johan J. Lukkien, j. j. lukkien@tue. nl TU/e Computer Science, System Architecture and Networking 23