Lecture #4: Network Architecture - Network Software prof. Boyanov

1. Lecture #4: Network Architecture - Network Softwareprof. Boyanov C o n t e n t s • Characteristics and Structure of the Network Software • Layered SW Design • Connectivity Services • Service Primitives 2 8 12 18

2. Network Software: Characteristics • Based on structural programming approach • Network Layers: hierarchy of SW modules providing communication services to the next upper layer • Transparency of the layered structure: independence of layer n of the implementation of the lower layers

3. Network Software: Structure 1/9 • Layered structure: • Protocol - rules and convention of data exchange between layer n of host1 and layer n of host2 • Peers - entities that locally implement the functionality of a given layer • Interface - the set of primitive operations and services that lower layer provide to the upper one • Physical media - the signal carrier that is used by the 1st layer for transmission

4. Network Software • Considerations: • Virtual exchange between the equilevel layers of two hosts according the protocols • Physical exchange between the neighbor layers of one host according to the interface • Portability of the layers: based on clear simple interfaces and well defined set of functions of each layer

5. Network Architecture • Network architecture - the set of layers and protocols; ignores the interfaces as the interfaces of the hosts in a network may differ • Protocol stack - the list of protocol hierarchy in the network; matches the layered structure • Analogies to the network protocol stack 1/10

6. Network Architecture • Example network architecture: • 5-layer protocol stack • Layer 5: Application process generates message M and deposit it to Layer 4 • Layer 4: Extends M with the header H4 containing control information (ordering, size, time, etc.) and deposits H4M to Layer 3 • Layer 3: Brakes H4M into smaller fixed size packets (e.g. H4M1 and M2); extends them with its header H3; selects an outgoing line for transmission and passes the packets to Layer 2 • Layer 2: Adds its header and trailer to each packet and deposit them to Layer 1 for physical transmission 1/11

7. Network Architecture • Receiving of the message M at the destination machine consists in: • moving of its packets upward the layers, • stripping of control header and trailers, • merging the packets in a message and • interpreting the message by the application • Lower layers have hardware implementation • Medium layer[s] have firmware implementation • High layers have software implementation

8. Layers Design • Layer’s design issues: • identification mechanism for senders/receivers - process ID, machine ID, net ID, etc.; • data transfer mode - simplex, half-duplex and full-duplex • support of multiple logical channels with priority scale • Application of error-detecting and error-correcting codes and mechanisms for feed-back • ordering protocols for the packets in messages • buffering between fast and slow processes

9. Interfaces and Services • Terminology • Entities: the active elements of each layer - either software or hardware • Peer Entities: same layer entities of different machines • Service Provider: Layer n entity that serves requests of Layer n+1 entities • Service User: Layer n entity that requests service from Layer n-1 entities • Class of services: Functional set of services in a layer that differs in quality (e.g. fast/slow, expensive/cheap, reliable/unreliable ...)

10. Interfaces and Services • Terminology (Abbreviations): • SAP (Service Access Point) - the unique address of for access to the services of Layer n from the Layer n+1 (Analogs: phone #, street address...) • IDU (Interface Data Unit): fixed data format for exchange between two layers; it consists of SDU and ICI • SDU (Service Data Unit): information passed through the network to the peer entity • ICI (Interface Control Information): control information assisting the lower layer entity to process the request (e.g. SDU’s length) …. 1/12

11. Interfaces and Services • PDU (Protocol Data Unit): fragment of SDU (e.g. packet) that is processed by the lower Layer N. PDU contains also control information in header. Header identifies sequence number of PDU, type of the data (control/information) etc.

12. Connection-Oriented and Connectionless Services • Connection-oriented service: establishes the connection from point to point; caries the exchange, preserving the order of the bitstream and releases the connection. Analogy to telephone system. • Connectionless service: each message is provided with full destination address and it is routed through the system independently to rest of message stream. • QoS (quality of service) - reliability to losing data 13

13. Quality of Services (QoS) • Implementation of reliability: based on acknowledgment by the receiver - acknowledge receipt for each message • Acknowledge receipts produce • communication overhead and • delays • Application: file transfer

14. Reliable Connection-Oriented Service • 2 methods: • Message Stream - preserves message boundaries. Example: stream of pages for phototype printing • Byte Stream - brakes the message sequence into stream of ordered bytes: Example exchange between a terminal and a remote system 1/13

15. Unreliable Connection-Oriented Service • Application for systems where delays are unacceptable, e.g. real-time systems for • voice communication • on-line image transmission

16. Unreliable Connectionless Service • Application - all functions where: • real-time, interactive or on-line features are not essential but • the cost of communications has to be minimized and also • reliability is not of crucial importance • Example: standard e-mail services • Implementation: datagrams - not acknowledged connectionless service

17. Reliable Connectionless Service • Application: non-interactive short messages exchange with guaranteed reliability • Example: banking, military, remote queries in data bases • Implementation: acknowledged datagrams • Variation: Request-Replay services for one-cycle interaction. Mostly in remote data-base access and another client-server applications

18. Service Primitives • Set of operations - primitives - forms the access language to a service. Primitives: • request some elementary service action; • inform the service process for some event in the peer entity • 4 classes of service primitives • Parameters of the primitives are usually • initiating and target entities (peers); • type of the requested service; • connection parameters (e.g. message size, type of coding etc.). 1/14

19. Confirmed and Unconfirmed Services • Confirmed Services: the exchange of primitives between the peer entities follows the pattern: • request • indication • response • confirm • Unconfirmed Services: the exchange of primitives between the peer entities follows the pattern: • request • indication Application: basically for • connection oriented services • reliable services Application: basically for • connectionless services • unreliable services

20. Service Primitives Exchange (Example) • CONNECT.request (request for connection to be established) • CONNECT.indication (signal the called party) • CONNECT.responce (callee accepts/rejects the connection) • CONNECT.confirm (Caller notified for acceptance) • DATA.request (request data to be sent) • DATA.indication (receive data request) • DATA.request (grant data to caller) • DATA.indication (caller accepts the data) • DISCONNECT.request (Caller requests release of the connection) • DISCONNECT.indication (request for connection to be established) 1/15

21. Service and Protocols If the protocols are not distinguished from the services than any change in the protocol is visible to the user and limits the portability of the netware.