Application Development for Mobile and Ubiquitous Computing

Transcription

1 Department of Computer Science Institute for System Architecture, Chair for Computer Networks Application Development for Mobile and Ubiquitous Computing 2. Mobile Internet Dr. Ing. Thomas Springer Technische Universität Dresden Chair of Computer Networks

2 Lecture Structure Application Development Mobile Business Apps Cross-Platform Development Adaptation and Contextawareness Mobile Web Apps Android ios.net Compact Framework/ Windows Phone 7 Java ME Mobile Middleware Disconnected Operations Mobile Databases Locationbased Services Communication Mechanisms Mobile Internet Enabling Technologies and Challenges Slide 2

3 Lecture Overview Prerequisits IP Addressing Routing Issues of mobility Network Layer DHCP Mobile IP Cellular IP GPRS Tunneling Protocol (GTP) Transportation layer Mobile TCP 3

4 Protocol stack OSI Reference Model TCP/IP Protocol Suite IEEE 802 Standards Application Layer Presentation Layer Application Layer Session Layer Transport Layer Mobile TCP Transport Layer Network DHCP, Layer Mobile IP, Network Cellular Layer IP Link Layer Physical Layer Link Layer Physical Layer Logical Link Control MAC layer Physical Layer 4

5 Mobile Internet Extension of existing infrastructure mobile devices wireless network technologies mobility of users Extension and modification of existing protocols network layer: assignment of addresses o DHCP for dynamic address assignement o Mobile IP/Cellular IP/GTP for mobility support transport layer: optimizations for wireless communication 5

6 Host Mobility Home Network /24 Mobile Host Address: Change of subnet Foreign Network /24 Mobile Host Address: Transport layer connection is broken down and has to be re-established Classless Inter-Domain Routing Subnetmask defines separation of network id and host id Suffix notation for subnetmask /24 Corresponding Host

7 Network Layer Challenge IP address has dual significance o unique id o location pointer Change of subnet/cell implicates changes of IP address o reconfiguration of network required (new IP, new standard gateway, new DNS) o breakage of established connections o mobile device not reachable without further support (DNS does not work with fast changing IP addresses) Dynamic Host Configuration Protocol (DHCP) supports automatic assignement of IP addresses automatic configuration of network (router, subnetmask, DNS) + optional parameters (web server, mail server) automatic configuration of all network parameters 7

8 Dynamic assignment DHCP DISCOVER DHCP Server DHCP Server DHCP DISCOVER DHCP DISCOVER DHCP DISCOVER DHCP Client 8

9 Dynamic assignment DHCP OFFER DHCP Server DHCP Server DHCP DISCOVER DHCP OFFER Network Configuration DHCP OFFER Network Configuration DHCP DISCOVER DHCP Client 9

10 Dynamic assignment DHCP ACK DHCP Server DHCP Server DHCP REQUEST Accept Network Configuration DHCP Client 10

11 Dynamic assignment DHCP ACK DHCP Server DHCP Server DHCP REQUEST DHCP ACK Acknowledge Network Configuration DHCP Client 11

12 DHCP Address Assignment 3 kinds of address assignment manual: o fixed assignment of IP addresses to devices (identified by MAC address) defined by system administrator automatic: o available IP addresses are assigned to mobile devices for unlimited time frame dynamic: o available IP addresses are assigned for limited time frame (Lease) o periodic refreshment of address assignment by mobile device required dynamic address assignment fits best for mobile hosts Dr. Thomas Springer Application Development - 2. Mobile Internet 12

13 Refresh Lease Result is assigned IP Address (Lease) Lease is valid for a defined time frame (T=lease time) Depends on configuration, e.g. several hours or one day For refresh two times defined by server T1 50 % of lease time T2 87,5 % of lease time T1 Client tries to refresh lease Client sends DHCP REQUEST to formerly used DHCP Server if DHCP ACK, then lease time for IP address starts again T2 if no positiv answer for DHCP REQUEST clients sends DHCP DISCOVER via broadcast to all local DHCP Servers to refresh lease 13

14 DHCP state diagram 14

15 Mobile IP Challenge IP address of mobile device changes in each subnetwork connection break in case of subnetwork change services offered by mobile device not reachable because of changing IP address Goal no changes of IP routing Mobile IP Permanent IP address for mobile device infrastructure for packet forwarding and address updates 15

16 Main Principle Two addresses per mobile host (network adapter) Home Address o permanent address of Mobile Host independent of subnet Care-of Address (COA): o temporary address of Mobile Host in visited network o Two kinds: - Foreign-Agent Care-of-Address: - foreign agent is responsible for packet forwarding to Mobile Host - multiple Mobile Host can use same Foreign-Agent COA - Co-located Care-of-Address - address is assigned to Mobile Host in Visiting Network and registered with the Home Agent - No Foreign Agent used - COAs have to be unique Two level registration and forwarding mechanism Home agent forwards datagrams to care-of address 16

17 Mobile IP 3 Parts 1. Agent Discovery discover network type (home network, visiting network) and agent (home agent or foreign agent) 2 ways for discovery Agent Advertisment o Agent periodically sends broadcasts to all hosts in subnet o Mobile Host can discover Agent using the broadcast message Agent Solicitation o if advertisment period to long, Mobile Host sends request for Agent Advertisment via broadcast to the subnet o no Advertisment message Mobile Host assumes Home Network o Otherwise get IP address via DHCP as Care-of-Address o CoA registered to Home Agent 2. Registration 3. Forwarding 17

18 2. Agent Registration Home Network /24 2. Register mobile host 1. Registration Request CoA: HA: MH: lifetime: 5000 ID: 501 Internet Foreign Network /24 Mobile Host Permanent Address: Care-of address: permanent address COA mobility binding list Home Agent Address: Authentification of Mobile Host Home Mobile Foreign/ Foreign - Home Limited validity of CoA, periodic reregistration of Mobile Host required Corresponding Host Foreign Agent Address: Registration Reply HA: MH: lifetime: 5000 ID:

19 3. Forwarding Foreign Agent CoA Home Network /24 4. Foreign Agents extracts encapsulated packet and forwards it to Mobile Host Foreign Network /24 HAddr 2. Resolve COA COA Home Agent Address: Home Agent encapsulates Packet and forwards it to Foreign Host (Tunneling) S = , D= Internet S = , D= user data S = , D= Foreign Agent Address: Mobile Host Permanent Address: Current Address: Mobile Host sends answer to source address 1. Packet send to Home Agent of Mobile Host S = , D= Corresponding Host

20 3. Forwarding Co-located CoA Home Network /24 4. Mobile Host extracts encapsulated packet Foreign Network /24 S = , D= Resolve COA 3. Home Agent encapsulates packet and forwards it to Mobile Host (Tunneling) S = , D= Internet S = , D= user data Mobile Host Permanent Address: Current Address: HAddr COA Home Agent Address: Foreign Agent Address: Mobile Host sends answer to source address 1. Packet send to Home Agent of Mobile Host S = , D= Corresponding Host

21 Mobile IP - Issues Triangle Routing Mobile and corresponding host far away from home network (maybe in same subnet) o Permanent address refers to home agent o Home agent forwards datagrams to mobile host Solution: mechanism in IPv6 Micromobility in cellular networks Frequent updates of (far away) home agent required due to frequent cell/coa changes Solution: Cellular IP o Hierarchical updates: Cellular IP for managing cell changes in location area, Mobile IP updated only for changes of location area 21

22 Mobile IP IPv6 Mobile IP is developed on top of IPv4 for Mobile IP additional hosts necessary o One Home Agent per Home Network o One Foreign Agent in each Visiting Network (if Foreign- Agent CoA is used) Mobile IP in IPv6 integrated into IP protocol o Co-Located CoAs used only - no Foreign Agents required o Functionality of Home Agent directly integrated into IPv6 router no separate Home Agents required Mobile Hosts can inform corresponding hosts about new CoA enabling direct communication between MH and CH (to avoid triangle routing issues) 22

23 Cellular IP Frequent handoffs in cellular networks cause: High traffic for location update messages to HA Increased handoff latency Goals of Cellular IP: Full compatibility with IP Minimized message number and load for location management Minimized handoff overhead Support of active and passive hosts Minimized complexity for mobile hosts Access Network A Cellular IP Internet with Mobile IP Mobile IP location update Hierarchical system Mobile IP supports macromobility Gateway Gateway Access Network B Cellular IP Cellular IP handles mobility within cellular access network Mobile IP for macro mobility (changes of access network/provider) 23

24 Cellular IP - Routing Hop-by-hop routing base stations forward packets to neighbours only Base stations maintain mappings Mobile host id (IP adress) -> outgoing port Created by monitoring packets send by mobile hosts Deleted based on timeouts Two mapping caches maintained Routing cache (for active hosts) o Monitoring of traffic or periodic route-update packets o Timeout in order of packet time scale Paging cache (for passive and active hosts) o Monitoring of periodic paging-update packets o Timeout in order of host mobility time scale 24

25 Cellular IP Routing Example X: from G X: from F,G X: from C X: from F Internet with Mobile IP Gateway A C E G cell change B paging/routing update or traffic from MH traffic to MH D F X Passive MHs periodically send paging updates with low frequency and long timeout When data packets to be routed to passive MH Gateway queues packets to MH Gateway sends paging packet -> routed based on paging cache If no entry in paging cache -> paging packet send to all outports MH responds with route-update packet which updates routing cache Further packets to MH can be routed based on routing cache 25

26 Mobility Support in 2.5/3/4G Hierarchical routing based on GPRS Tunneling Protocol (GTP) GTP-C - GTP control o For managing paths, tunnels and mobility o Supports packet forwarding during handovers GTP-U - GTP user data tunneling o Simple IP encapsulation protocol based on UDP/IP o Tunnels established between (e)nodeb and Gateway(GGSN/P-GW) o Particular protocol user over wireless link GTP for transfer of charging data Application IP Packet data protocol context Application IP PDCP PDCP GTP-U GTP-U GTP-U GTP-U RLC RLC UDP/IP UDP/IP UDP/IP UDP/IP MAC MAC L2 L2 L2 L2 L1 L1 L1 L1 L1 L1 UE enodeb S-GW P-GW 26

27 OSI Reference Model TCP/IP Protocol Suite IEEE 802 Standards Application Layer Presentation Layer Application Layer Session Layer Transport Layer Wireless TCP Transport Layer Network Layer Network Layer Link Layer Physical Layer Link Layer Physical Layer Logical Link Control MAC layer Physical Layer 27

28 Transport Layer Challenge TCP is optimized for wired networks o Assumption - Packet loss is mostly caused by congestions o Congestion Control and Avoidance - drastic reduction of packet rate and slow start - larger timeout intervals for retransmissions Assumption is not valid for wireless networks o much higher packet error rates o packet losses during handovers and disconnections o bundled packet losses o Higher delay/round-trip times o necessary reaction: - fast error recovery - no reduction of packet rate Result in case of un-adapted TCP for wireless networks: o data rates far below possible data rate o slow error recovery (high response times) 28

29 TCP Overview Reliable transport protocol sequence numbers per bytes in packets cummulative acknowledgements based on sequence number retransmission of lost packets sliding window protocol for flow control 0 file segment 1 Host A Host B segment 2 RTO segment 1 SEQ=318 ACK= segment 1 SEQ=318 segment RTO segment 1 SEQ=1318 ACK= segment 10 RTO segment 2 SEQ=1318 segment 3 SEQ=2318 X ACK=

30 Algorithms for Congestion Control 4 Algorithms: Slow Start, Congestion Avoidance, Fast Retransmit, Fast Recovery Packet loss detected Retransmission Timeout (Slow Start) o sender window reduced to one packet o Retransmission Timer doubled (RRT estimation, min. 1s) o slow start (exponential grow of sender window size) 3 duplicated ACK (Fast Retransmit) o TCP assumes loss of next unack packet o retransmit packet before Retransmission Timeout o slidely decrease of sender window (to half of the size) o no change of Retransmission Timer 30

31 Slow Start and Fast Recovery Retransmission timeout is doubled after each timeout next time we wait the double time before error is handled 32

32 TCP Performance in microcellular wireless networks transferring 4 Mbytes of data between an MH and an SH MHs connect to 2 Mbit/s WLAN (IEEE ) 4.3 BSD-Tahoe version of TCP from the University of California at Berkeley, Mobile IP software from Columbia University 33

33 Two general Approaches for Wireless TCP No change of TCP Wireless TCP Change of TCP Hiding Packet Losses No Endto-End End-to- End Differentiated Handling of Packet Losses Split Connection Link-Layer Extended Receiver Information Explicit Loss Notification Explicit Congestion Notification TCP-Unaware TCP-Aware 34

34 SplitTCP Approach Mobile Support Router SplitTCP Agent SplitTCP 1 Agent 2 + Separate flow control and error handling to improve performance of connection parts + Hiding of wireless packet losses from wired connection + Optimized protocol can be used between Mobile Host and Base Station + No protocol changes required Mobile Support Router Handover SplitTCP Agent Mobile Support Router Internet 35 No end-to-end semantics for connections Protocol stack has to be processed twice Component required at base station to maintain connections Additional overhead during handover - connection state has to be transfered between Base Stations

35 Change of Link Layer (TCP-Unware) Extension of error correction at link layer Transparent for transport layer (TCP) Hiding packet losses from TCP Two Approaches TCP Unaware - independent from transport layer TCP Aware - Applying information from transport layer TCP Unaware - Component at Base Station performs: Forward error correction (FEC) Automatic Repeat Request (ARQ) Packet retransmission at link layer Disadvantages Interfering mechanisms for error handling at link and transport layer E.g. uncoordinated timeout intervals, duplicated retransmissions Result: performance reduction 36

36 Snoop Approach (TCP-Aware) Mobile Support Router Mobile Support Router Snoop Agent Handover Snoop Agent Snoop Agent Mobile Support Router Internet + End-to-end semantics + Only transient state information at Base Station no additional overhead during handover + Hinders interfering error correction at link and transport layer + no changes of TCP necessary 37 transparency of link and transport layer broken (link layer depends on transport layer) no support for encrypted TCP packet headers (packet header/sequence numbers not available without decryption) Snoop Agent Transparently intercepts routing code without changing the packet flow Control of packets and acknowledgements Caching of unacknowledged packets from Fixed to Mobile Host at Base Station Detection of packet losses and retransmission before TCP retransmision Support of packet flow from Mobile Host to Fixed Host using negative ACK

37 Differentiated Handling of Packet Losses Extensions to TCP protocol Exchange of information about data transmission between communication partners Extended Receiver Information E.g. selective Acknowledgements (ACK) Array of Sequence Numbers Unchanged TCP SACK Explicit Loss Notification (ELN) Attach information about router state to each packet (Queue length) Detection of concestions possible ELN-Bit of TCP ACK set if loss detected (router not congested) Retransmission if ELN set, else congestion avoidance Explicit Congestion Notification (ECN) ECN-bit of TCP packets set by router in case of packet drop caused by congestion 38

38 TCP over 2.5G and 3G (RFC 3481) Profile for optimizing TCP over 25.G/3G wireless networks Set of configuration options commonly found in modern TCP stacks Appropriate window size (sender and receiver)/window scale option (sender and receiver) Increase default buffer settings (16 KByte) to allow large enought windows Window size > 64 KB Ensure sending enougth data to trigger fast retransmit Increase initial window (sender) From one to up to four segments especially efficient for transmission of few data segments Avoids dropping of sender window to one segment during slow start (faster recovery, more efficient for transmission of few segments) 39

39 TCP over 2.5G and 3G (cont.) Limited Transmit (sender) Sending new data segment in response to each of the first two duplicate ACK Increases possibility of triggering fast retransmit Especially efficient for congestion windows to small to allow triggering of 3 duplicated ACK IP MTU larger than default Mainly applicable to 3G Allows faster growing of sender window Selective Acknowledgements (sender and receiver) Allows improved recovery from multiple segment losses in a single window Explicit Congestion Notification (sender, receiver and intermediate routers) Allows TCP receivers to inform sender about congestions 40

40 Summary Explicit support at the network and transport layer required for wireless connections and mobility support DHCP enables automated reconfiguration of network settings (IP address, gateway, DNS, ) Mobile IP supports mobility of mobile stations o Mobile devices remain reachable after network changes Hierarchical routing more appropriate o Principle of Cellular IP adopted in cellular networks (GTP used) Mobile TCP improves performance of TCP o Split-Connection o Snoop o Differentiated Handling of Packet Losses o TCP over 2.5G and 3G 42

41 References Kurose, J.F., Ross, K. W.: Computer Networking A Top-Down Approach. Pearson International, ISBN , Fourth Edition, 2008 Eds.: Sesia, S., Toufik, I., Baker, M.: LTE The UMTS Long Term Evolution From Theory to Practice, Whiley, 2009 Droms, R.: Dynamic Host Configuration Protocol. RFC 2131, IETF, 1997 Perkins, C.: IP Mobility Support for IPv4. RFC 3344, IETF, 2002 Johnson, D.; Perkins, C. & Arkko, J.: Mobility Support in IPv6. RFC 3775, IETF, 2004 Bakre, A., Badrinath, B. R.: I-TCP: Indirect TCP for Mobile Hosts. In: Proc. 15th International Conf. on Distributed Computing Systems (ICDCS), Balakrishnan, H., Seshan, S., Amir, E., Katz, R.H.: Improving TCP/IP Performance over Wireless Networks. In: Proceedings of Mobicom, November`95 In Proc. 1st ACM Int l Conf. on Mobile Computing and Networking (Mobicom95),

42 References Mathis, M.; Mahdavi, J.; Floyd, S. & Romanow, A.: TCP Selective Acknowledgement Options. RFC 2018, IETF, 1996 Balakrishnan, H., Katz, R.: Explicit Loss Notification and Wireless Web Performance. In: Proceedings of IEEE GLOBECOM, Balakrishnan, H., Padmanabhan, V.N., Seshan, S., Katz, R.H.: A Comparison of Mechanisms for Improving TCP Performance over Wireless Links. IEEE/ACM Transactions on Networking, Vol. 5, No. 6, S , Inamura, H.; Montenegro, G.; Ludwig, R.; Gurtov, A. & Khafizov, F. TCP over Second (2.5G) and Third (3G) Generation Wireless Networks, RFC 3481, IETF, 2003 Valko, A.: Cellular IP: A New Approach to Internet Host Mobility, SIGCOMM Comput. Commun. Rev., Vol. 29, No. 1,