1. Network Architectures Week 3 Part 2

2. Comparing The Internet & OSI

3. Internet “layers” Application – layer 5Message Transport – layer 4Segment Network – layer 3Datagram Data Link – layer 2Frame Physical – layer 1Bit PDU (Protocol Data Unit) Layers as per Kurose & Rose PDU

4. Application – layer 5 Similar to OSI application layer Examples of application services E-mail – SMTP Simple Mail Transfer Protocol FTP – File Transfer Protocol Telnet – Remote login protocol POP3 Post Office Protocol 3 HTTP – Hyper Text Transfer Protocol The Web is not synonymous with the Internet – it is an application service.

5. Transport – layer 4 Has two primary protocols UDP – a connectionless “unreliable” datagram service. Does not provide any re- transmission or congestion control TCP – provides a connection oriented reliable data service with end-to-end error detection and control We will cover both of these in more detail later

6. Network – layer 3 This is the IP layer Transfers packets from source to destination via a connectionless datagram service Uses IP addresses as a road map to locate a host within the Internet Relies on routers to pass on the datagram via the correct link

7. Data Link – layer 2 Moves data in frames from one node to the next Protocol may be different on each link

8. Physical – layer 1 Moves bit from one node to the next Protocols relate to the different media on each link It is worth noting that the Data link and Physical layers are not Specifically Internet layers – e.g. PPP, Frame Relay & ATM

9. Addressing within the Internet We have talked about applications passing messages through the network We know it is more accurate to say that Processes on each host pass messages Thus for two processes to communicate we need: Source & destination host addresses specified by the IP address A processes on the hosts specified by a port number. Common processes have specific post numbers HTTP is port 80 SMTP is port 25

10. Domain Name Service - DNS We prefer to use host names such as “usyd.edu.au” But the Internet uses a hierarchical IP address In IPv4 this is a 32 bit address shown as a four byte address eg 193.92.216.9, where each byte has a range 0-255 i.e. 9=00001001 In IPv6 this will become a 128 bit address, providing around 3 devices per square metre of the globe Network Address Translation (NAT) where many numbers internal to an organisation are not externally visible has reduced the move to 128 bit addresses

11. DNS functions DNS uses three levels of server to translate the domain name into IP address –see Kurose for details Hosts may have alias – DNS will find the real name Load distribution – a domain may have several replicated sites – DNS will provide the different IP addresses to each successive request. Application level protocol Runs over UDP using port 53

12. Defining a Network

13. A Network within the Internet A group of hosts isolated on one side of a router The path between two routers

14. Network IP addresses Was defined in terms of A, B, & C classes each with different numbers of devices This was an inefficient method and was replaced in 1993 with IETF’s Classless Interdomain Routing (CIDR) In this structure the network can be any number of bits. It is expressed in the form a.b.c.d/x where x is the number of bits defining the network and 32-x is the number of devices

15. Allocating Host addresses Manually by the system administrator – usually used for servers Dynamically at boot time following the Dynamic Host Configuration Protocol (DHCP). This is normal for largish organisations for workstations

16. Address Management Domain names and IP Network addresses are managed under the auspices of ICANN (The Internet Corporation for Assigned Numbers & Names) Three registries: US, Europe, Asia Pacific They resolve Domain name disputes and maintain the DNS root servers

17. IPv6 is not universally accepted

18. Transport layer Provides logical communication between two processes on different host. It provides two protocols: UDP – User Datagram Service TCP – Transmission Control Protocol

19. Transport layer The transport layer is implemented on each end host only Application Transport Network Link Physical Network Link Physical Network Link Physical Application Transport Network Link Physical

20. UDP At the Transport layer it does very little. Passes the message straight to the IP packet service It is connectionless No overhead in handshaking No connection state in end systems It has a smaller packet overhead No congestion control Trades off risk of data loss against higher throughput Used in multimedia – telephony, video Some apps. provide own error control

21. TCP – Reliable transmission service To provide a reliable data transmission service it needs: An error detection mechanism. This is based on a checksum calculation A receiver feedback mechanism. The receiver sends an ACK(nowledge) when an error free packet is received, and sends a NAK when the packet has errors. A retransmission service is a packet is damaged or not received

22. Three problems How can you send a NAK if the packet never arrives? But if the second packet is not sent on its way until an ACK is received back, won’t it be slow? The “stop & wait” problem How do you prevent congestion in the network?

23. Packets that do not arrive Packets are given a sequence number and this number is returned on the ACK or NAK. Thus the sending host knows which packets have not been acknowledged and must be presumed lost – it is an implied NAK It is possible for packets to circulate forever and this problem is prevented by giving the packet a life expressed in numbers of links. This number is decremented at each router and the packet discarded when the count is zero The last problem is duplicate packets. The sequence number resolves this one too.

24. Stop & wait problem An example in Kurose shows an effective transfer rate of 267kbps on a 1gbps line The answer is pipelining or a sliding window approach. This allows multiple packets to be sent without waiting for the acknowledgements. It can best be shown in a diagram

25. Sliding Window Protocol

26. End-to-End flow control It is relatively easily to send data too quickly, such that the receiver or a router cannot handle the flow. TCP provides congestion control Host B establishes a connection buffer Host B tells A how much room is available in the connection buffer on each segment it sends back Host A then has to ensure that the amount of data in the pipeline, i.e. data that has not been acknowledged does not exceed the connection buffer It does attempt to dynamically increase flow rate and chokes backs as congestion increases This is a simplistic explanation – see Kurose page 246

27. Network layer The network layer is implemented on each host and router Application Transport Network Link Physical Network Link Physical Network Link Physical Application Transport Network Link Physical

28. IP Provides a connectionless datagram service Source to destination Relies on routers to route the datagram through the network Routers use the IP addresses together with their internal routing tables to direct datagram down the appropriate link

29. IP Datagram header IP protocol i.e. 4 & Transport protocol Header length & header checksum Datagram length – theoretically up to 64k but rarely > 1,500 and often limited to 576 Message fragmentation information Time to live Source & destination IP addresses Timing & routing parameters