1. Internetworking Fundamentals (Lecture #2) Andres Rengifo Copyright 2008

2. Network Services and Architectures What is a network service? –It provides a complete end to end communication solution to users of a system for their own benefit. –Network is expected to operate at all times –It also allows network administrators to have some control over the data flow. –Communication services are provided to the entire network community using one common infrastructure. –Standards are required for uniformity and interoperability –Components can be built with a given specification where the end system does not have to be one specific brand or model.

3. Network Services and Architectures (Cont.) The most important service used to replicate a network service was and still is the telephone system. –How can the administrator charge back the client for use of the network has been a bit difficult to achieve. Data networks are still looking for ways to charge back according to data usage by individual users. –Telephone service is established to charge absolutely every second of usage by any client accessing the voice network.

4. Network Services and Architectures (Cont.) Definitions –Actions executed on network resources are services. –These actions followed in order are scripts. –These scripts are set of steps that are followed to perform some type of action. A switch uses a script or process to move packets very fast between its ports.

5. Network Services and Architectures (Cont.) What is a protocol? –Protocols are a set of rules that when followed in a specific order provide some type of output. It allows two entities to communicate creating a language. –Protocols can be direct or indirect. Direct protocols are point to point Indirect protocols use a proxy for communication –Standard or non-standard Standards protocols should be able to interact with any device regardless of brand or make as it is defined by the ISO (International Organization for Standardization)

6. What should be the standard protocol functionalities? Encapsulation Segmentation and Re-assembly Connection Control Flow control Error Control

7. What should be the standard protocol functionalities? (Cont.) Encapsulation –Data from upper layers of the architecture is packaged or encapsulated with specific Protocol Data Units (PDUs) as the information traverses the layered architecture down towards the physical medium. –It includes control bits which are usually enclosed in the header portion. Each layer of the architecture uses these control bits in order to determine how to deliver that data.

8. What should be the standard protocol functionalities? (Cont.) Segmentation and Re-assembly –Data is normally very large when produced by a source. –It has to be segmented and put into segments which contain portions of the original communication flow. –These are very specific in size and make the transfer more efficient. –As these segments are created, they get an ID which identify them as a portion of a specific communication flow. Re-assembly –This process takes back all the segments sent across the network and using the ID portion of the segment, are put back together to provide the complete data flow originally sent by the source to the destination.

9. What should be the standard protocol functionalities? (Cont.) Connection Control –Two processes Connection Oriented –Three way handshaking is a mechanism used where two entities exchange some preliminary information in order to agree on how to connect such as window size, what protocol to use, sequencing, buffer size, etc. –It is usually used for long term connections using the TCP protocol as the transport. It is a very reliable and guaranteed service.

10. What should be the standard protocol functionalities? (Cont.) Connectionless –No handshaking available and it is best effort. –Used for short term connections but since there is no exchange of parameters between the two entities, there is a possibility of congestion and packet loss.

11. What should be the standard protocol functionalities? (Cont.) Flow Control –It is a mechanism used by two entities to prevent congestion and packet loss. Two entities will agree ahead of time during the handshaking to exchange data at a given rate. If the rate exceeds the agreement, then one entity will tell the other to slow down and throttle back until the other is ready to process data.

12. What should be the standard protocol functionalities? (Cont.) Error Control –There should be error detection and error correction mechanisms integrated under a protocol functionality in order to make sure that all the data that was sent by a sender is truly the data that was received at the receiver’s end. –Cyclic Redundancy Check or CRC algorithms check that every one sent is a one received and every zero sent is a zero received.

13. Classes of Communications Services End to end communications can be categorized into two: –Synchronous There is a notion of a clock. Every 1 and 0 in the stream follows each other separated by very specific delay times. “Ticking like a clock” –Asynchronous All the bits are grouped into packets and there is no notion of a clock. Each packet arrives at the destination at different times.

14. Switching Switching is the idea of how to switch each bit across a network. –The introduction of packet switching is important as the information is grouped into packets and sent across a network using the store and forward mechanism. Each packet arrives to a packet switched node, it gets stored, and then a route determination is implemented to find out where that packet needs to be sent. –Are these bits switched/routed across the network over a permanent path from beginning to end? –Are these bits switched/routed across the network over a path only present during the actual communication? –Or are these bits switched/routed across the network independently of each other? This is referred to respectively as: –Circuit Switching –Virtual Circuit Packet Switching –Datagram Packet Switching

15. Switching (Cont.) Circuit Switching –The telephone system is a topology based on circuit switching. –Switches and links create a mesh-like topology. –Usually the path followed between two telephones use the same route. –path stored in memory, which can be loosely referred to as a “routing” table.

16. Switching (Cont.) Virtual Circuit Packet Switching –A virtual circuit is set up for the duration of the transmission. –Packets of a specific communication flow will follow one another and are labeled with a virtual circuit number, which designates the path. –Each node stores these routing decisions in a routing table that indicates the path to be followed by a packet from the virtual circuit number. –Packets of one specific communication flow will arrive at the destination in order as if they were following a dedicated path. –Since packet switching is more efficient when store and forward is used, it gives each intermediate node a chance to introduce error control mechanisms to make sure that the packet is not corrupted.

17. Switching (Cont.) Datagram Packet Switching –In datagram packet switching, each packet is independently routed across the network. –There is no circuit set up required and each packet can follow any given path. –Packets dynamically re-route if there are issues. –Datagram packet switching is ideal for short-term transmissions. –Packets out of sequence are put back in order by means of sliding window algorithms.