Network Architecture Layered Architectures Network Protocols Reference Models Network Standards Network Devices 13/10/10 04-Architecture

Network Architecture Definition of Network Architecture A highly structured framework within which networks can be analysed, designed and implemented, incorporating a defined set of layers and protocols 13/10/10 04-Architecture

Layered Architectures Each layer is functionally independent Each layer has a defined interface to the previous and preceding layer Each layer builds on the previous layer Virtual communication takes place between layers at the same level Layered architectures are often called protocol stacks 13/10/10 04-Architecture

Layered Architectures Advantages They divide up complex operations into more manageable groups Changes to one layer can be carried out without having to change all the others Standard interfaces can be defined for each layer (allows protocols to be mixed and matched) 13/10/10 04-Architecture

Interfaces An interface exists between two adjacent layers. It defines which services and primitives the lower layer offers to the upper layer 13/10/10 04-Architecture

Services and Primitives A layer offers a service to the layer immediately above it A service is formally specified by a set of operations called primitives which can be thought of as procedure calls with parameters E.g. The IP network service has two primitives: send to transmit a packet deliver to receive a packet 13/10/10 04-Architecture

Service Access Points The layer n SAP is the place where the layer n+1 entity can access the layer n services offered (Network layer SAPs are often referred to as NSAPs) Each SAP has a unique address called the SAP address (Network layer addressed are often referred to as NSAP addresses) 13/10/10 04-Architecture

Protocols A protocol is a set of mutually agreed rules that allows two peer layer entities to communicate successfully Protocol Characteristics Symmetric/Asymmetric Standard/Proprietary Connection-oriented / Connectionless 13/10/10 04-Architecture

Symmetric and Asymmetric Protocols Communication between equal peer entities where each entity can invoke the same services Asymmetric One entity has access to services that the other does not have. E.g. Client/server 13/10/10 04-Architecture

Standard and Proprietary Protocols A standard protocol is a protocol that is controlled by a recognised standards body A proprietary protocol is a protocol that is not controlled by a recognised standards body and is usually under the control of a commercial organisation Customers today are usually wary of proprietary protocols as their use tends to lock them into suppliers 13/10/10 04-Architecture

Connection-oriented / Connectionless Protocols Connection-oriented protocols support three phases: Connection Establishment Data Transfer Connection Termination In most cases data is received in the same order that it was transmitted Connectionless Protocols only support one phase (data transfer) All packets (called datagrams) are routed independently Packets are often received in an order different to that in which they were transmitted 13/10/10 04-Architecture

Service and Protocol Data Units The SDU is the data unit passed down from layer n+1 to layer n, then passed across the network and up to the peer layer n+1 entity The PDU is the data passed between peer layer n entities The layer n PDU consist of the layer n+1 SDU preceded by the layer n header Some layer n PDUs do not carry any layer n+1 SDUs, but are used to carry control information between layer n entities in the layer n header 13/10/10 04-Architecture

Service and Protocol Data Units Layer n+1 Entity Layer n Entity Layer n+1 SDU Layer n PDU The layer n PDU consists of the layer n+1 SDU preceded by a layer n protocol header. Protocol headers can include address information, control information and error detecting codes 13/10/10 04-Architecture

Generic Protocol Functions Encapsulation Segmentation Connection Control Ordered Delivery Flow Control Error Control Addressing Multiplexing Priority Grade of Service Security Data Compression Data Encoding Data Encryption 13/10/10 04-Architecture

Encapsulation When a layer n+1 SDU is transmitted in a layer n PDU, the layer n+1 SDU is said to be encapsulated in the layer n PDU Encapsulation really means that a layer n protocol header is added to the layer n+1 SDU prior to transmission Decapsulation is the reverse process where the layer n protocol header is removed before the SDU is passed to layer n+1 13/10/10 04-Architecture

Encapsulation Example Application Protocol Data Unit (message) E.g. Email Addresses Application Header Transport Protocol Data Unit (segment) E.g. Port Numbers Transport Header E.g. Network Addresses Network Protocol Data Unit (packet) Network Header 13/10/10 04-Architecture

Segmentation Different layers often support different maximum PDU sizes Sometimes SDUs needs to be segmented (or fragmented) into smaller PDUs by a lower layer protocol and re-assembled back into the original size SDU before being passed back to the upper layer protocol 13/10/10 04-Architecture

Segmentation Layer n+1 Entity Layer n Entity Layer n+1 SDU Layer n PDU 1 Layer n PDU 2 Layer n does not support a large enough SDU size to transmit the SDU as one PDU. It therefore segments it and reassembles it at the distant end, before passing the original SDU back to Layer n+1 13/10/10 04-Architecture

Connection Control Connection Control is required for Connection-oriented protocols which must have primitives to establish, terminate and reset connections and use special PDUs to establish and close connections 13/10/10 04-Architecture

Ordered Delivery Protocols need mechanisms to ensure that data can be reordered back into the order that it was originally transmitted This is usually achieved by including a sequence number in the protocol header 13/10/10 04-Architecture

Flow Control and Error Control Flow control is performed by the receiving entity to limit the rate at which data is sent by the transmitting entity to avoid data loss. Error Control guards against the the loss of or damage to data or control information Both these functions are often implemented by using sequence numbers in protocol headers and requiring that the receiver acknowledges receipt of a PDU. The receiver can slow down the transmitter by slowing the rate at which it issues acknowledgements. Sequence numbers can also be used to detect and report the non-delivery or corruption of PDUs 13/10/10 04-Architecture

Addressing All entities that transmit or receive data must have a unique unambiguous address Most protocols headers have a source and destination address field which identify the peer layer entities involved in the communication 13/10/10 04-Architecture

Multiplexing A protocol can carry SDUs between several different pairs of communicating entities to make more efficient use of lower level services Protocols support multiplexing by using fields in the protocol header (often addresses) to uniquely identify which PDUs belong to which communication, so that they can be demultiplexed at the receiving end and passed to the correct layer n+1 entity 13/10/10 04-Architecture

Priority Certain messages such as control messages may need to get through with minimum, delay Protocol headers often contain a priority field so that some messages can be given differential treatment 13/10/10 04-Architecture

Grade of Service Certain classes of data may require a minimum throughput or a maximum delay Protocol headers often contain a field to specify the grade of service required 13/10/10 04-Architecture

Security Security mechanisms restricting access may be invoked in protocols (E.g. Closed User Groups and authentication fields) Protocol headers often contain fields that are used to identify and authenticate users and can thus control access 13/10/10 04-Architecture

Data Compression Bandwidth is frequently a scarce resource and some data transmissions are often inefficient and can be compressed before being transmitted (E.g. MPEG 4 for video) Protocols sometimes support the compression and decompression of data so that the quantity of data transmitted is significantly reduced 13/10/10 04-Architecture

Data Encoding In order to be understood at the receiving end, it is essential that the receiving entity knows how to interpret the data being sent This is achieved through using a common protocol to encode the data (E.g. ASCII, HTML etc.) 13/10/10 04-Architecture

Data Encryption To keep data (and control information) secure, it is often necessary to use encryption to scramble the data so that other parties cannot understand it Some protocols support the encryption and decryption of data (and control information) 13/10/10 04-Architecture