Larger Site Networks Part2

2 Ethernet Virtual LANs Hubs versus Switches –Hubs broadcast bits out all ports –Switches usually send a frame out a one port More fundamentally –In unicasting, a message is only intended to go to one machine, as when a client sends a message to a server –Switches assume unicasting; it is the basis for sending a frame out a single port

3 Ethernet Virtual LANs Broadcasting –Sometimes, station needs to send a frame to all other stations; this is broadcasting –For example, servers send a frame to advertise their presence with a broadcast message every minute or so

4 Ethernet Virtual LANs Broadcasting with Ethernet Switches –Broadcaster sets the destination MAC address to all ones (48 ones) –When switch sees this address, it broadcasts frame out all stations –All stations read frames with this address Broadcast Frame Ethernet Switch

5 Ethernet Virtual LANs Broadcasting is a Problem in Large Switched Networks –Server broadcasts go to all stations, creating a great deal of network traffic –Create congestion Broadcast Frame

6 Ethernet Virtual LANs In multicasting, messages are only intended to go to some stations –For instance, from a server only to the client PCs it serves –If Ethernet switches can implement multicasting, traffic overload would be avoided Multicast Frame

7 Ethernet Virtual LANs Ethernet switches do implement multicasting –A server and the clients it serves are treated as a single virtual LAN (VLAN) –Can only communicate among themselves, as if they were on their own LAN Frame Marketing VLAN Server Marketing VLAN Client

8 Ethernet Virtual LANs VLAN Benefits –VLANs reduce traffic on the switched network –Other benefits VLANs give ease of management because if a user changes organizational membership, VLAN membership is easily changed centrally

9 Ethernet Virtual LANs VLAN Problems –Communication across VLANs, when necessary, can be difficult –Often, routers must be used to connect members of different VLANs VLAN 1 (Subnet 1) VLAN 2 (Subnet 2) Router

10 Ethernet Virtual LANs VLAN Problems –For cross-VLAN communication, routers actually connect multiple switches Ethernet Switch

11 Bad Switch Organization One Server for All Clients –All traffic goes to and from server –Bottlenecks: no simultaneous conversations –No major benefits compared to hub Bottleneck Ethernet Switch

12 Bad Switch Organization Multiple Servers for Clients –Allows simultaneous conversations –Brings switching’s main benefit Ethernet Switch

Congestion, Latency, and Remedies Peak Loads Congestion and Latency Overprovisioning Capacity Priority Quality of Service Traffic Shaping

14 The Peak Load Problem Capacity Sufficient Most of the Time –Otherwise, get bigger switches and trunk lines! Brief Traffic Peaks can Exceed Capacity –Frames will be delayed in queues or even lost if queue gets full Capacity Traffic Peak

15 Overprovisioning Overprovisioning: Install More Capacity than Will be Needed Nearly All of the Time –Wasteful of capacity –Still, usually the cheapest solution today because of its simplicity Overprovisioned Capacity Traffic Peak

16 Priority Assign Priorities to Frames –High priority for time-sensitive applications (voice) –Low priority for time-insensitive applications ( ) –In traffic peaks, high-priority frames still get through –Low-priority applications do not care about a brief delay for their frames High-Priority Frame Goes Low-Priority Frame Waits Briefly

17 Priority Standardizing Priority –802 Tag Fields are standardizing priority for Ethernet and other 802 LAN technologies –Priority is also being standardized by the IETF for IPv4 and IPv6 (Diffserv for differentiated services) –802 and IETF are harmonizing efforts for end-to-end priority Once Widely Available, Priority Should Replace Overprovisioned Capacity as the Least Expensive Solution to Peak Load Problems High-Priority Frame Goes Low-Priority Frame Waits Briefly

18 Full Quality of Service (QoS) Priority Makes no Quantitative Promises of Maximum Latency, etc. Quality of Service (QoS) Makes Quantitative Promises for such things Different Frames may have Different QoS Guarantees; Some may have no guarantee High Guarantee Low or No Guarantee

19 Full QoS is Expensive For high guarantees, capacity must be reserved at each switch and trunk line along the way Capacity is wasted if not used Like circuit switching but at data link layer –(Circuit switching is at physical layer) High Guarantee Reserved Capacity Low or No Guarantee

20 Full QoS is Not a Cure-All Traffic with no guarantees will not benefit It may not get through at all Often, voice traffic is given strong guarantees while data traffic is given low or no guarantees High Guarantee Reserved Capacity Low or No Guarantee

21 Traffic Shaping Overprovisioning, Priority, and QoS are Ways to Cope with Brief Congestion Traffic Shaping Prevents recognizes that congestion is beginning, acts to stop it Switch Tells Some Sources to Slow or Stop if Congestion is Beginning Called Shaping Because Governs the “Shape” of the Traffic –Mix of traffic of various kinds is governed by policies about traffic Source A Source B Network Slow or Stop Continue