15-441 Computer Networking

1. 15-441 Computer Networking Mobility

2. Many Mobility Scenario’s • Mobile hosts connecting to networked infrastructure (focus of this lecture) • Cell phones connecting to cell towers • Laptops connecting to access points • Ad hoc networks: a set of mobile nodes communicating with each other with no additional networked infrastructure • Extra challenges: routing and possible mobility • Sensor networks (ad hoc +) • More resource constrained mobile nodes • Vehicular networks • Extreme mobility

3. Routing to Mobile Nodes • Obvious solution: have mobile nodes advertise route to mobile address/32 • Should work!!! • Why is this bad? • Consider forwarding tables on backbone routers • Would have an entry for each mobile host • Not very scalable • What are some possible solutions?

4. How to Handle Mobile Nodes?(Addressing) • Dynamic Host Configuration (DHCP) • Host gets new IP address in new locations • Problems • Host does not have constant name/address  how do others contact host • What happens to active transport connections?

5. How to Handle Mobile Nodes?(Naming) • Naming • Use DHCP and update name-address mapping whenever host changes address • Fixes contact problem but not broken transport connections

6. How to Handle Mobile Nodes? (Transport) • TCP currently uses 4 tuple to describe connection • <Src Addr, Src port, Dst addr, Dst port> • Modify TCP to allow peer’s address to be changed during connection • Security issues • Can someone easily hijack connection? • Difficult deployment  both ends must support mobility

7. How to Handle Mobile Nodes?(Link Layer) • Link layer mobility • Learning bridges can handle mobility  this is how it is handled at CMU • Encapsulated PPP (PPTP)  Have mobile host act like he is connected to original LAN • Works for IP AND other network protocols

8. How to Handle Mobile Nodes?(Routing) • Allow mobile node to keep same address and name • How do we deliver IP packets when the endpoint moves? • Can’t just have nodes advertise route to their address • What about packets from the mobile host? • Routing not a problem • What source address on packet?  this can cause problems • Key design considerations • Scale • Incremental deployment

9. Basic Solution to Mobile Routing • Same as other problems in computer science • Add a level of indirection • Keep some part of the network informed about current location • Need technique to route packets through this location (interception) • Need to forward packets from this location to mobile host (delivery)

10. Interception • Somewhere along normal forwarding path • At source • Any router along path • Router to home network • Machine on home network (masquerading as mobile host) • Clever tricks to force packet to particular destination • “Mobile subnet” – assign mobiles a special address range and have special node advertise route

11. Delivery • Need to get packet to mobile’s current location • Tunnels • Tunnel endpoint = current location • Tunnel contents = original packets • Source routing • Loose source route through mobile current location

12. Mobile IP (RFC 2290) • Interception • Typically home agent – a host on home network • Delivery • Typically IP-in-IP tunneling • Endpoint – either temporary mobile address or foreign agent • Terminology • Mobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA) • Care-of-address, home address

13. Lecture 21: 2006-11-16 Mobile IP (MH at Home) Packet Correspondent Host (CH) Internet Visiting Location Home Mobile Host (MH)

14. Mobile IP (MH Moving) Packet Correspondent Host (CH) Internet Visiting Location Home Home Agent (HA) Mobile Host (MH) I am here

15. Mobile IP (MH Away – FA) Packet Correspondent Host (CH) Mobile Host (MH) Internet Visiting Location Home Encapsulated Home Agent (HA) Foreign Agent (FA)

16. Mobile IP (MH Away - Collocated) Packet Correspondent Host (CH) Internet Visiting Location Home Encapsulated Home Agent (HA) Mobile Host (MH)

17. Other Mobile IP Issues • Route optimality • Resulting paths can be sub-optimal • Can be improved with route optimization • Unsolicited binding cache update to sender • Authentication • Registration messages • Binding cache updates • Must send updates across network • Handoffs can be slow • Problems with basic solution • Triangle routing • Reverse path check for security

18. State of Mobile Networks • Getting a new name and address via DHCP is the most commonly used solution for laptops, as long as • it is a client machine (what is the difference between a server and client machine?) • Single consistent TCP connection during mobility is not important • Cell network is the commonly used solution to support continuous mobility • Overview slides attached. For information only. Not required for exam. • Mobile IP not as widely used • Why? • Will cover ad hoc networks, sensor networks, vehicular networks in two weeks • Check out Professor Srini Seshan’s 15-446 class next semester

19. GSM Mobility Management (not required for exam) Originals by: Rashmi Nigalye, Mouloud Rahmani, Aruna Vegesana, Garima Mittal, Fall 2001 Prof. M. Veeraraghavan, Polytechnic University, New York • GSM architecture overview • Network layout • Protocols • Addresses & identifiers • Location management • Call delivery + location update • Handover management

20. GSM network layout PLMN: Public Land Mobile Network MSC: Mobile Switching Center BTS: Base Transceiver Station BSC: Base Station Controller GSM Network (PLMN) MSC region MSC region Location area Location area BSC BSC MSC region BTS BTS

21. GSM network layout PSTN ISDN OMC BSC MSC GMSC E Abis BSC A B,C BTS HLR EIR BTS VLR AUC Um BTS

22. GSM MAP protocol • GSM MAP similar to IS41 MAP • MAP uses Transactions Capabilities Part (TCAP) of the SS7 stack • MAP functions: • Updating of location information in VLRs • Storing routing information in HLRs • Updating and supplementing user profiles in HLRs • Handoff of connections between MSCs

23. What is a location area (LA)? • A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell • One extreme is to page every cell in the network for each call - a waste of radio bandwidth • Other extreme is to have a mobile send location updates at the cell level. Paging cut to 1 cell, but large number of location updating messages. • Hence, in GSM, cells are grouped into Location Areas – updates sent only when LA is changed; paging message sent to all cells in last known LA

24. MCC MNC MSIN MCC: Country Code MNC: Mobile Network Code MSIN: Mobile Subscriber Identification Number Addresses and Identifiers • International Mobile Station Equipment Identity (IMEI) • It is similar to a serial number. It is allocated by equipment manufacturer, registered by network, and stored in EIR • International Mobile Subscriber Identity (IMSI) When subscribing for service with a network, subscriber receives (IMSI) and stores it in the SIM (Subscriber Identity Module) card. The HLR can be identified by a VLR/MSC from the IMSI.

25. CC NDC SN Addresses and Identifiers • Mobile Subscriber ISDN (MSISDN) • The “real telephone number”: assigned to the SIM • The SIM can have several MSISDN numbers for selection of different services like voice, data, fax NDC: National Destination Code (NDC identifies operator); SN: Subscriber Number; CC: Country Code; Digits following NDC identifies the HLR

26. CC NDC SN Addresses and Identifiers • Mobile Station Roaming Number (MSRN) • It is temporary location dependent ISDN number • It is assigned by local VLR to each MS in its area.

27. Addresses and identifiers • Temporary Mobile Subscriber Identity (TMSI) • It is an alias of the IMSI and is used in its place for privacy. • It is used to avoid sending IMSI on the radio path. • It is an temporary identity that is allocated to an MS by the VLR at inter-VLR registration, and can be changed by the VLR • TMSI is stored in MS SIM card and in VLR.

28. TMSI, IMSI, MSRN and MSISDN • Unlike MSISDN, IMSI is not known to the GSM user. The CC of MSISDN translates to an MCC of IMSI as follows, e.g, Denmark CC: 45 MCC: 238 • TMSI is used instead of IMSI during location update to protect privacy. As user moves, TMSI is used to send location update. Thus a third party snooping on the wireless link cannot track a user as he/she moves. • MSRN is the routing number that identifies the current location of the called MS. • MSRN is temporary network identity assigned to a mobile subscriber. • MSRN identifies the serving MSC/VLR. • MSRN is used for call delivery (calls incoming to an MS). • MSISDN is the dialed number to reach a GSM user

29. CC MNC LAC Addresses and Identifiers • Location Area ID (LAI) • CC: Country Code, MNC:Mobile Network Code, LAC: Location Area Code • LAI is broadcast regularly by Base Station on BCCH • Each cell is identified uniquely as belonging to an LA by its LAI

30. Location management • Set of procedures to: • track a mobile user • find the mobile user to deliver it calls • Current location of MS maintained by 2-level hierarchical strategy with HLRs and VLRs.

31. Ways to obtain MSRN • Obtaining at location update – MSRN for the MS is assigned at the time of each location update, and is stored in the HLR. This way the HLR is in a position to immediately supply the routing info (MSRN) needed to switch a call through to the local MSC. • Obtaining on a per call basis – This case requires that the HLR has at least an identification for the currently responsible VLR. When routing info is requested from the HLR, it first has to obtain the MSRN from the VLR. This MSRN is assigned on a per call basis, i.e. each call involves a new MSRN assignment

32.   IMSI MSISDN MSRN MSRN   MSISDN MSRN GMSC Routing information: case when MSRN is selected per call by VLR/MSC • If MSRN is allocated to each subscriber visiting at an MSC, then the number of MSRNs required is large. If instead, an MSRN is allocated only when a call is to be established, then the number of MSRNs is roughly equal to number of circuits at MSC – a much smaller number – hence MSRNs typically allocated per call by VLR/MSC MSISDNIMSI, VLR number HLR MSC/VLR

33. Call routing to a mobile station: case when HLR returns MSRN 1 MSISDN GMSC ISDN LA 1 1 4 MSRN 2 MSISDN 3 MSRN BSC MSC MSC HLR BTS 7 TMSI 5 MSRN 7 TMSI LA 2 BSC EIR BTS VLR 8 TMSI 7 TMSI AUC 6 TMSI BTS MS

34. Messages exchanged: call delivery 5 GMSC 1 4 PSTN HLR 2 VLR 3 6 Target MSC Target MSC VLR HLR GMSC Originating Switch 1. ISUP IAM 2. MAP_SEND_ROUTING_INFO 3. MAP_PROVIDE_ROAMING_NUMBER 4. MAP_PROVIDE_ROAMING_NUMBER_ack 5. MAP_SEND_ROUTING_INFO_ack 6. ISUP IAM

35. Find operation in GSM • ISDN switch recognizes from the MSISDN that the call subscriber is a mobile subscriber. Therefore, forward the call to the GMSC of the home PLMN (Public Land Mobile Network) • GMSC requests the current routing address (MSRN) from the HLR using MAP • By way of MSRN the call is forwarded to the local MSC • Local MSC determines the TMSI of the MS (by querying VLR) and initiates the paging procedure in the relevant LA • After MS responds to the page the connection can be switched through.

36. Abbreviations • ISC: International switching center • OMC: Operations and maintenance center • GMSC: Gateway switching center • MSC: Mobile switching center • VLR: Visitor location register • HLR: Home Location register • EIR: Equipment Identification register • AUC: Authentication center • BSC: Base station controller • BTS: Base transceiver station • MS: Mobile subscriber • TMSI: Temporary Mobile Subscriber Identity • IMSI: International Mobile Subscriber Identity