Lect 6 Mobility.pdf

University of Science & Technology Faculty of Computer Science & Information Technology Department of information for communication and technology Mobile communications & wireless technology_ it 709 4year/sem 7 Tawffeeg Mohammed Tawfeeg...

University of Science & Technology Faculty of Computer Science & Information Technology Department of information for communication and technology Mobile communications & wireless technology_ it 709 4year/sem 7 Tawffeeg Mohammed Tawfeeg 1 Mobility 2 What is mobility?  spectrum of mobility, from the network perspective: no mobility high mobility mobile wireless user, mobile user, mobile user, passing using same access connecting/ through multiple point disconnecting from access point while network using maintaining ongoing DHCP. connections (like cell phone) Wireless, Mobile Networks 3 Mobility: vocabulary home network: permanent home agent: entity that will “home” of mobile perform mobility functions on (e.g., 128.119.40/24) behalf of mobile, when mobile is remote wide area network permanent address: address in home network, can always be used to reach mobile e.g., 128.119.40.186 Wireless, Mobile Networks 4 Mobility: more vocabulary permanent address: remains visited network: network in constant (e.g., 128.119.40.186) which mobile currently resides (e.g., 79.129.13/24) care-of-address: address in visited network. (e.g., 79,129.13.2) wide area network foreign agent: entity in visited network that performs mobility correspondent: wants functions on behalf of to communicate with mobile. mobile 5 How do you contact a mobile friend: Consider friend frequently I wonder where changing addresses, how do you Alice moved to? find her?  search all phone books?  call her parents?  expect her to let you know where he/she is? 6 Multiple BSSs In order to increase the physical range of a wireless LAN, companies and universities will often deploy multiple BSSs within the same IP subnet. BBS 1 BBS 2 7  Mobility among the BSSs This naturally raises the issue of mobility among the BSSs—how do wireless stations seamlessly move from one BSS to another while maintaining ongoing TCP sessions? BBS 1 H1 BBS 2 8 Mobility within same IP subnet. The Previous Figure shows two interconnected BSSs with a host, H1, moving from BSS1 to BSS2. Because in this example the interconnection device that connects the two BSSs is not a router, all of the stations in the two BSSs, including the APs, belong to the same IP subnet. 9 Mobility within same IP subnet. Thus, when H1 moves from BSS1 to BSS2, it may keep its IP address and all of its ongoing TCP connections. If the interconnection device were a router, then H1 would have to obtain a new IP address in the subnet in which it was moving. This address change would disrupt (and eventually terminate) any on-going TCP connections at H1. 10 H1 moving from BSS1 to BSS2 As H1wanders away from AP1, H1 detects a weakening signal from AP1 and starts to scan for a stronger signal. H1 receives beacon frames from AP2 (which in many corporate and university settings will have the same SSID as AP1). H1 then disassociates with AP1 and associates with AP2, while keeping its IP address and maintaining its ongoing TCP sessions. 11 H1 moving from BSS1 to BSS2 Cont. This addresses the handoff problem from the host and AP viewpoint. But what about the switch in the Figure? How does it know that the host has moved from one AP to another? 12  Self-Learning As you know, switches are “self-learning” and automatically build their forwarding tables. This self-learning feature nicely handles occasional moves (for example, when an employee gets transferred from one department to another); 13  Highly mobile users However, switches were not designed to support highly mobile users who want to maintain TCP connections while moving between BSSs. To appreciate the problem here, recall that before the move, the switch has an entry in its forwarding table that pairs H1’s MAC address with the outgoing switch inter- face through which H1 can be reached. 14  Highly mobile users Cont. If H1 is initially in BSS1, then a datagram destined to H1 will be directed to H1 via AP1. Once H1 associates with BSS2, however, its frames should be directed to AP2. 15  A Solution One solution is for AP2 to send a broadcast Ethernet frame with H1’s source address to the switch just after the new association. When the switch receives the frame, it updates its forwarding table, allowing H1 to be reached via AP2. 16 How a mobile user can maintain ongoing connections while moving between networks?. 17 How a mobile user can maintain ongoing connections while moving between networks? Addressing We noted above that in order for user mobility to be transparent to network applications, it is desirable for a mobile node to keep its address as it moves from one network to another. When a mobile node is resident in a foreign network, all traffic addressed to the node’s permanent address now needs to be routed to the foreign network. 18 Mobility: approaches How can this be done? One role of the foreign agent is to create a so-called care-of address (COA) for the mobile node, with the network portion of the COA matching that of the foreign network. There are thus two addresses associated with a mobile node, its permanent address and it’s COA, sometimes known as a foreign address. In the example in Figure in previous slides, the permanent address of the mobile node is 128.119.40.186. When visiting network 79.129.13/24, the mobile node has a COA of 79.129.13.2 A second role of the foreign agent is to inform the home agent that the mobile node is resident in its (the foreign agent’s) network and has the given COA. 19 Mobility Management & Mobile Routing 20 Mobility: approaches  let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.  routing tables indicate where each mobile located  no changes to end-systems  let end-systems handle it:  indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote  direct routing: correspondent gets foreign 21 address of mobile, sends directly to mobile  Mobility: approaches  let routing handle it: routers advertise permanent address of mobile-nodes-in-residenncoet via usual routing table exchange.  routing tables to s c a al b l e in millions d i c aof t e where each mobile  located no changes to enmdo-bsiyel sstems  let end-systems handle it:  indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote  direct routing: correspondent gets foreign address of mobile, sends directly to mobile Wireless, Mobile Networks 22  Mobility: registration visited network home network 1 wid2e area network mobile contacts foreign agent contacts home foreign agent on agent home: “this mobile is entering visited resident in my network” network end result:  foreign agent knows about mobile  home agent knows location of mobile 23  Mobility via indirect routing foreign agent receives packets, home agent intercepts forwards to mobile packets, forwards to visited foreign agent networ home k network 3 wide area networ2k 1 correspondent 4 addresses packets mobile replies using home address of directly to mobile correspondent 24  Indirect Routing: comments mobile uses two addresses:  permanent address: used by correspondent (hence mobile location is transparent to correspondent)  care-of-address: used by home agent to forward datagrams to mobile foreign agent functions may be done by mobile itself triangle routing: correspondent-home- network- mobile  inefficient when correspondent, 25 Indirect routing: moving between networks suppose mobile user moves to another network  registers with new foreign agent  new foreign agent registers with home agent  home agent update care-of-address for mobile  packets continue to be forwarded to mobile (but with new care-of-address) mobility, changing foreign networks transparent: on going connections can be maintained! 26  Mobility via direct routing foreign agent receives packets, correspondent forwards forwards to mobile to foreign agent v i s home i network t 3 e 2 d 1 n 4 mobile repliese correspondent t directly to requests, receives w correspondent foreign address o of mobile r k 27 Mobility via direct routing: comments overcome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent  what if mobile changes visited network? 3 1 2 4 6-28 28  Accommodating mobility with direct routing  anchor foreign agent: FA in first visited network  data always routed first to anchor FA  when mobile moves: new FA arranges to have data forwarded from old FA (chaining) foreign net visited at anchor foreign session start agent wide area 2 network 1 4 3 5 new correspondent foreign new foreign agent networ correspondent agent k 29 Thanks for attentions These slides are adapted from Computer Networking: A Top Down Approach Jim Kurose, Keith Ross Addison-Wesley March 2012 30