EE 4272Spring, 2003 Protocols & Architecture A Protocol Architecture is the layered structure of hardware & software that supports the exchange of data between systems At each layer, one or more common protocols are implemented in communication systems. Each protocol provides a set of rules for the exchanges of data between systems Key Functions of a protocol: encapsulation, segmentation & reassembly, connection control, ordered delivery, flow control, error control, addressing, and multiplexing Chapter 2

EE 4272Spring, 2003 The OSI Protocol Model

EE 4272Spring, 2003 Functions of Protocols Segmentation and reassembly Encapsulation Connection control Ordered delivery Flow control Error control Addressing Multiplexing Transmission services

EE 4272Spring, 2003 Segmentation (Fragmentation) Data blocks are of bounded size at each layer Application layer messages may be large Network packets may be smaller Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP)  ATM blocks (cells) are 53 octets long  Ethernet blocks (frames) are up to 1526 octets long Why Fragment? Advantages: 1).More efficient error control; 2) More equitable access to network facilities; 3) Shorter delays; 4). Smaller buffers needed Disadvantages: 1). Overheads; 2). Increased interrupts at receiver; 3). More processing time

EE 4272Spring, 2003 Encapsulation Addition of control information to data  Address information  Error-detecting code  Protocol control: data + control information -> PDU

EE 4272Spring, 2003 Connection Control Connection Establishment Data transfer Connection termination

EE 4272Spring, 2003 Ordered Delivery & Flow Control Ordered Delivery  PDUs may traverse different paths through network  PDUs may arrive out of order  Sequentially number PDUs to allow for ordering Flow Control  Done by receiving entity  Limit amount or rate of data  Stop and wait  Credit systems: Sliding window  Needed at application as well as network layers

EE 4272Spring, 2003 Error Control Guard against loss or damage Error detection  Sender inserts error detecting bits  Receiver checks these bits  If OK, acknowledge  If error, discard packet Retransmission  If no acknowledge in given time, re-transmit Performed at various levels

EE 4272Spring, 2003 Addressing Addressing level Addressing scope  Globally unique : Global address identifies unique system  Locally Unique: Multiple simultaneous applications Addressing mode  Unicast address: Sent to one machine or person  Broadcast: Sent to all machines or users  Multicast: Sent to some machines or a group of users

EE 4272Spring, 2003 Addressing level Level in architecture at which entity is named Unique address for each end system (computer) and router Network level address  IP or internet address (TCP/IP)  Network service access point or NSAP (OSI) Process within the system  Port number (TCP/IP)  Service access point or SAP (OSI)

EE 4272Spring, 2003 Address Concepts – in TCP/IP Architecture

EE 4272Spring, 2003 Multiplexing Supporting multiple connections on one machine Mapping of multiple connections at one level to a single connection at another: e.g.  Carrying a number of connections on one fiber optic cable  Aggregating or bonding low speed lines to gain bandwidth B B C C A A B C A B C A MUX (a) (b) Trunk group

EE 4272Spring, 2003 Transmission Services Some common service example: Different Priorities for Different messages Quality of Service Guarantee  Minimum acceptable throughput  Maximum acceptable delay Security: Access Restrictions

EE 4272Spring, 2003 The OSI Protocol Model

EE 4272Spring, 2003 OSI - 7 Layers Model A layer model Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers

EE 4272Spring, 2003 OSI Layers – Layer Provide fundamental network functionalities/services Physical: Covers the physical interface between devices & the rules by which bits are passed from one to another Data Link  Provide a reliable link by Error detection and control  Higher layers may assume error free transmission Network: routing/switching; establish/maintain/terminate connections Transport: Exchange of data between end-to-end systems  Error free; In sequence; No losses; No duplicates; QoS

EE 4272Spring, 2003 OSI Layers – Layer Provide finer network service (more software orientated) Session: Control of dialogues between applications in end system  Dialogue discipline: full-duplex or half-duplex  Recovery: provide checkpoint mechanism. Presentation: Defines data formats and coding. e.g.,  Data compression  Encryption Application: provide means for application program to access OSI environment  management functions; ; FTP

EE 4272Spring, 2003 Use of a Relay

EE 4272Spring, 2003 TCP/IP Protocol Architecture Application Layer  Communication between processes or applications End to end or transport layer (TCP/UDP/…)  End to end transfer of data  May include reliability mechanism (TCP)  Hides detail of underlying network Internet Layer (IP): Routing of data Network Access Layer  Logical interface between end system and network Physical Layer  Transmission medium  Signal rate and encoding

EE 4272Spring, 2003 PDUs in TCP/IP Architecture

EE 4272Spring, 2003 Some Protocols in TCP/IP Suite

EE 4272Spring, 2003 Further Reading Reference Stallings chapter 2 Comer,D. Internetworking with TCP/IP volume I Comer,D. and Stevens,D. Internetworking with TCP/IP volume II and volume III, Prentice Hall Halsall, F> Data Communications, Computer Networks and Open Systems, Addison Wesley RFCs