Connecting Devices

1. Connecting Devices and Multi-Homed Machines

2. Layer 1 (Physical) Devices • Repeater: • Extends distances by repeating a signal • Any slight variations in the carrier wave for individual bits is corrected when the carrier wave is reproduced • Hub: • As above, but re-creates the carrier wave on multiple ports • Hubs do not decide whether or not to copy data, they do it (necessary or unnecessary) • All ports are part of the same collision domain • With a hub as the centre of the star, any 2 hosts can have frame collisions

3. Layer 2 (Data Link) Devices • Layer 2 switch: • Serves the same purpose as a hub • Can dynamically determine if a frame should be placed on a port (and which one) • The data link layer (e.g. MAC) address is used to make this determination • A table of MAC addresses and corresponding ports is built using incoming frames • Each LAN segment (port) becomes its own collision domain • Only 2 hosts on the same LAN segment can have frame collisions • Layer 2 bridge • As above, but the connected networks do not necessarily have to be of the same type

4. Layer 2/3 Devices • Broadband (Cable/DSL) or Wireless Router • Primarily operates as a layer 2 switch / access point • However, these routers often have additional features (some of which are in layer 3): • Dynamic host configuration (DHCP) – IP address assignment for network hosts, for example • DNS capability (local or distributed) – Provides naming of hosts inside the network • IP masquerading – The router can use one IP given by a broadband provider, but allow all of its hosts to use different IPs inside the network • Layer 3 Switch • Uses layer 3 routing to determine a path for packets • Once a path is found, subsequent packets are switched • This switching typically occurs on layer 2 • These devices will be discussed in more detail later

5. Layer 3 (Network) Devices • Layer 3 Bridge • A bridge that uses network layer addresses (e.g. IP) in its forwarding database, instead of data link layer addresses (e.g. MAC) • This type of bridge more readily allows different types of network to be joined, since they need not share an address type • Cannot handle multiple paths effectively/efficiently: a host is either on a given port or it is not • Router • Determines routes for each packet using network layer addresses (e.g. IP) • Can connect any type of network together • Is capable of determining preferred paths where multiple paths exist

6. Routers

7. What is a Router Made of? • A router has many of the same components as your computer: • CPU • Memory • I/O Interfaces (mostly network interfaces) • Operating System

8. Routers Through History • Gateways: • A computer with installed software to forward packets • These are obsolete, but were common in the early days of ARPANet • Routers: • A computer with specialized hardware and operating system designed for forwarding packets • Switching Routers: • A computer with specialized hardware (switching fabric) that allows packets to be forwarded directly in hardware • The specialized hardware is, in many respects, similar to that of a switch (e.g. ATM switch)

9. Router Hardware • Input buffers (one for each network interface): • Used to store incoming packets before they are processed • Routing processor: • This is often software running on a CPU which: • Maintains and exchanges routing data with other routers • Controls the switching fabric to forward packets • With high-end routers, each network interface may have a local routing processor (for forwarding) so that each can forward the packets in its own input buffer independently • Switching fabric: • A network of connections between network interfaces (and their input and output buffers) • Output buffers (one for each network interface): • Used to store outgoing packets after they are processed, but before the network is available for transmission

10. Routers: Network Interfaces • Often, routers have modularized network interfaces • One can add/remove/replace network interfaces as needs change • Some routers can accept network interface modules of different types (e.g. Ethernet, Token Ring) • Each network interface would have its own: • Input buffer • Output buffer • Routing processor (in high-end routers)

11. Routers: Input Buffers • The incoming packets of a network interface are placed in input buffers • These are banks of very high speed memory for packet queuing prior to processing • The packet is stored here until the routing processor is available • The network interface may have a routing processor, which would: • … have a copy of the forwarding table (to prevent concurrent access) • … lookup the destination address in this forwarding table, to determine the correct output port • … configure the switching fabric to forward the packet to the correct output buffer • Low-end routers would share one routing processor

12. Routers: Routing Processors • Routing processors have two functions: • Maintain and exchange routing data with other routers in the network • Often this involves computing the forwarding table from data received by other routers • Use the forwarding table data to configure the switching fabric to forward the packet to the correct output port

13. Routers: Routing Processors • A routing processor is software which executes on a CPU: • Off-the-shelf CPU • These are very inexpensive • However, the performance of these CPUs is low since they are not optimized for the types of operations a router typically needs to perform • Application-Specific Integrated Circuit (ASIC) • These are expensive to design (time and money) • They are optimized for typical routing operations • High-end routers use these to achieve higher performance levels

14. Routers: Switching Fabric • Switching fabric’s job is to move packets from the input buffer into the correct output buffer • The routing processor determines the correct output port, using the forwarding table

15. Routers: Switching Fabric • Switching fabric comes in 3 major types: • In-memory switching fabric: • The packets are input into the routing processor’s memory, and output into the correct output buffer • Bus-based switching fabric: • The packets move along a shared bus (similar to a network bus) to the correct output buffer • Crossbar switching fabric: • The packets move along a grid of redundant buses • If any bus fails, alternate paths exist so that forwarding can continue

16. Routers: Output Buffers • The switching fabric gets the packet to the right output port • However, that port’s network may not be immediately available • The packets are stored in the output buffer until the network is available

17. Router Performance • Several methods to improve router performance have been discussed: • Use application-specific integrated circuits • Optimized for routing operations • Include much routing functionality otherwise executed as software (in memory) • Many routing functions can execute in parallel, adding new functionality without decreasing throughput • Use efficient switching fabric • Bus or crossbar-based switching fabrics reduce the need for in-memory processing

18. MPLS Multi-Protocol Label Switching

19. MPLS • MPLS is another way to improve router performance • Label switching tries to leverage some of the performance of virtual circuit switched networks (e.g. ATM) • Packets are assigned a label upon entering an MPLS network • This label is used (instead of the IP address) for making forwarding decisions

20. MPLS Labels • An MPLS label is an arbitrary value • This value is typically a numeric identifier • However, labels could also be the frequency (i.e. colour) of light used in multi-mode optical fibre • The label can change from one label-switching router (LSR) to the next • The label must only be unique for the sending and receiving router • IP addresses, in contrast, are usually unique across the network • A value could even be chosen to help the routing processor choose the correct output port

21. MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS: Simplified E-Mail Server User Web Server

22. MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS: Simplified E-Mail Server User • Here, the label is shown as colour • Notice the simplicity of the router’s job: • Red: Up • Blue: Right Web Server

23. MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS: Simplified E-Mail Server User • Notice that two labels can be directed down the same link Web Server

24. MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS LSR MPLS: Label Values 31 31 E-Mail Server User 7 15 7 47 • Notice that label values are not globally unique • Each pair of routers agrees on a label Web Server

25. MPLS Packets • MPLS adds a small pre-header to the start of any IPv4 (or IPv6, IPX, etc.) packet • In other words, between the data link and network headers Label 20 bits The label value 3 bits Class of Service The QoS class of the packet attached (e.g. discardable?) Stack 1 bit Is there a stack of labels? Hop Limit 8 bits The hop limit, copied from/to the IP header

26. MPLS and ATM • LSRs can be ATM-enabled • An LSR can forward a packet (as cells) through an ATM network • This can be for any number of hops through the ATM network • In this situation the source and destination ATM switches must be LSRs • Other switches in between can be normal ATM switches, however • The source LSR will use AAL segmentation to send the cells on the ATM network using a VPI/VCI for the destination LSR • The destination LSR will extract the packet and continue transmission using MPLS