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Special Topics on Wireless Ad-hoc Networks

Special Topics on Wireless Ad-hoc Networks. Lecture 9: Wireless Internet, Mobile IP. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Covered topics. How to build a global wireless network? Some considerations Mobility Routing Transport layer

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Special Topics on Wireless Ad-hoc Networks

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  1. Special Topicson Wireless Ad-hoc Networks Lecture 9: Wireless Internet, Mobile IP University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Computer Network

  2. Covered topics • How to build a global wireless network? • Some considerations • Mobility • Routing • Transport layer • References • Chapter 4 of the book • Alex C. Snoeren and Hari Balakrishnan, “An End-to-End Approach to Host Mobility” Computer Network

  3. Outline • Mobility consideration • Mobile IP • Multicast approach • End to End approach Computer Network

  4. Motivation • Connectivity everywhere • Overlapping, heterogeneous networks • Small, portable devices • Maintaining ongoing connections as the user moves • Why maintain connectivity? • Avoid restarting applications/networks • Avoid losing “distributed state” Computer Network

  5. Problems? • The IP address associated with a mobile host is network dependent! • When user connects to another network, IP address needs to change • Packets belonging to ongoing connections somehow need to be delivered to the mobile host • Solutions?

  6. Build in the network • The traditional approach: support in the network • Intelligence and expense is in the network • End-points are cheap (handsets) • Allows for supporting infrastructure • Requires agreements/trust amongst multiple vendors • Examples: • A link/physical level (many wireless networks) • At routing level (Columbia, VIP) • Doesn’t work when switching between technologies and often not between vendors • In Internet would require modifying lots of routers Computer Network

  7. Build in end points • The Internet approach: end-to-end • Intelligence (and expense) is in the end-points • Network is cheap (relatively) and as fast as possible • Implies self-support for many activities • Less work/trust required amongst multiple vendors • End-to-end support at transport/naming/application levels • May be ideal in future, but requires extensive changes • Not currently backwards compatible • TRIAD may be interesting approach Computer Network

  8. Problems in Wireless • Mobility: Nodes move from a network to another. How to keep connectivity • Broken connectivity and high error rate in the link: Degrades badly the performance. • TCP timeout • How to deal with Mobility?: Any solution should satisfy • Compatibility • Scalability • Transparency Computer Network

  9. Modify all applications? application Modify TCP, UDP, etc.? transport Modify IP end-points? routing Modify all device drivers? link How does this work across network technologies? physical E2E in routing level • Use end-to-end support at routing level • Makes problem transparent at layers above and below • Current Internet standard: Mobile IP (RFC 2002) TCP/IP network stack: Computer Network

  10. IP address problem • Internet hosts/interfaces are identified by IP address • Domain name service translates host name to IP address • IP address identifies host/interface and locates its network • Mixes naming and location • Moving to another network requires different network address • But this would change the host’s identity • How can we still reach that host? Computer Network

  11. ether Mobile host radio Domains versus interfaces • Switching domains & switching interfaces are the same problem at the routing level Network interfaces: Administrative domains: Stanford.edu 171.64.14.X 171.64.X.X Berkeley.edu 42.13.0.X 128.32.X.X Computer Network

  12. Intuitive Solution • Take up the analogy of you moving from one apartment to another • What do you do? • Leave a forwarding address with your old post-office! • The old post-office forwards mails to your new post-office, which then forwards them to you

  13. HA MH MH SH FA Reverse path? • Same as in the post-office analogy • Packets originating from the mobile host go directly to the static corresponding host … • Hence the name • triangular routing

  14. CH MH Foreign network Home network HA FA Basic Mobile IP MH = mobile host CH = correspondent host HA = home agent FA = foreign agent (We’ll see later that FA is not necessary or even desirable) • MH registers new “care-of address” (FA) with HA • HA tunnels packets to FA • FA decapsulates packets and delivers them to MH Computer Network

  15. Packet Tunneling Packet from CH to MH Source address = address of CH Destination address = home IP address of MH Payload Home agent intercepts above packet and tunnels it Source address = address of HA Destination address = care-of address of MH Source address = address of CH Destination address = home IP address of MH Original payload Computer Network

  16. Foreign network #1 Foreign network #2 CH MH MH FA #1 FA #2 HA host moves again Home network • MH registers new address (FA #2) with HA & FA #1 • HA tunnels packets to FA #2, which delivers them to MH • Packets in flight can be forwarded from FA #1 to FA #2 Computer Network

  17. CH MH Foreign network Home network HA FA Basic Mobile IP (cont) Mobile hosts also send packets • Mobile host uses its home IP address as source address • Lower latency • Still transparent to correspondent host • No obvious need to encapsulate packet to CH • This is called a “triangle route” Computer Network

  18. Mobile IP (RFC 2002) • Leaves Internet routing fabric unchanged • Does assume “foreign Agent ” exist everywhere • Simple • Correspondent hosts don’t need to know about mobility • Works both for changing domains and network interfaces Computer Network

  19. CH MH HA Problems with ingress filtering Home network Foreign network • Mobile host uses its home IP address as source address • Security-conscious boundary routers will drop this packet Computer Network

  20. Home network Foreign network CH MH HA Solution: bi-directional tunnel • Provide choice of “safe” route through home agent both ways • This is the slowest but most conservative option • At the other extreme… Computer Network

  21. Discovering the care-of address • Discovery process built on top of an existing standard protocol: router advertisement (RFC 1256) • Router advertisements extended to carry available care-of addresses called: agent advertisements • Foreign agents (and home agents) send agent advertisements periodically • A mobile host can choose not to wait for an advertisement, and issue a solicitation message

  22. Agent advertisements • Foreign agents send advertisements to advertise available care-of addresses • Home agents send advertisements to make themselves known • Mobile hosts can issue agent solicitations to actively seek information • If mobile host has not heard from a foreign agent its current care-of address belongs to, it seeks for another care-of address

  23. Registering the Care-of Address • Once mobile host receives care-of address, it registers it with the home agent • A registration request is first sent to the home agent (through the foreign agent) • Home agent then approves the request and sends a registration reply back to the mobile host • Security?

  24. Registration Authentication • Mobile IP requires the home agent and mobile host to share a security association • MD5 with 128-bit keys to create digital signatures for registration requests to be used (registration message & header used for creating signature) • Any problems? – replay attacks • Solved by using an unique message identifier (timestamp or pseudorandom number)

  25. Foreign Agent Security? • No foreign agent authentication required • Foreign agent can potentially discard data once registration happens • However, the problem is same as in unauthenticated route advertisements (RFC 1256) in the wireline context

  26. Home agent discovery • If the mobile host is unable to communicate with the home agent, a home agent discovery message is used • The message is sent as a broadcast to the home agents in the home network

  27. Problems with Foreign Agents • Assumption of support from foreign networks • A foreign agent exists in all networks you visit? • The foreign agent is robust and up and running? • The foreign agent is trustworthy? • Correctness in security-conscious networks • “triangle route” has problems • MH under its own control can eliminate this problem • Other undesirable features • Some performance improvements are harder with FAs • We want end-to-end solution that allows flexibility Computer Network

  28. CH MH Foreign network Home network HA Solution • Mobile host is responsible for itself (With help from infrastructure • in its home network) • Mobile host decapsulates packets • Mobile host sends its own packets • “Co-located” FA on MH • MH must acquire its own IP address in foreign network • This address is its new “care-of” address • Mobile IP spec allows for this option Computer Network

  29. Obtaining a foreign IP address • Can we expect to obtain an IP address? • DHCP becoming more common • Dynamic IP address binding like some dial-up services • Your friend can reserve an IP address for you • Various other tricks • More support for dynamic IP address binding in IPv6 • This assumes less than getting others to run a FA Computer Network

  30. Design implications • New issues: the mobile host now has two roles: • Home role • Local role • More complex mobile host • Loss of in-flight packets? (This can happen anyway.) • Can visit networks without a foreign agent • Can join local multicast groups, etc. • More control over packet routing = more flexibility Computer Network

  31. Problem: performance • Example: short-lived communication • When accessing a web server, why pay for mobility? • Do without location-transparency • Unlikely to move during transfer; can reload page • Works when CH keeps no state about MH Computer Network

  32. CH MH Foreign network Home network HA Solution: yet more flexibility • Use current care-of address and send packet directly • This is regular IP! • More generally: • MH should have flexibility to adapt to circumstances • A range of options: from slow-but-safe to regular IP • Should be an end-to-end packet delivery decision (no FA) Computer Network

  33. Forwarding options • Allow MH to choose from among all forwarding options • Options: • Encapsulate packet or not? • Use home address or care-of address as source address? • Tunnel packet through home agent or send directly? • Choice determined by: • Performance • Desire for transparent mobility • Mobile-awareness of correspondent host • Security concerns of networks traversed • Equivalent choices for CH sending packets to MH Computer Network

  34. Mobility 4x4 Computer Network

  35. Which to use? • With bidirectional tunneling • Probe destination using triangle route • If it works, switch to that option • With triangle route • If packets aren’t getting through after some number of tries Computer Network

  36. Mobile IP issues on local network • Host visiting local network with foreign agent • No real presence on local network • Host visiting local network with its own IP address • Has a role on local network • Reverse name lookups through special name? • Or do you change the DNS entry? • Its IP address / HW address gets into local hosts’ ARP caches • Which IP address should go into cache? • How do you update caches if host moves again? Computer Network

  37. Local ARP cache problem • ARP caches store (IP address, HW address) pairs • MH host visits foreign network • Wants to talk directly back and forth to local hosts • If it wants to maintain connectivity with them after moving • Use home IP address • Other hosts address MH by HW address on local link • But if MH moves again, ARP cache entries are wrong • If it doesn’t care • Use local IP address • If MH moves, ARP cache is wrong, but nobody cares Computer Network

  38. Multicast-based Architecture • Very different from the mobile-IP model • Based on the IP-multicast approach • Leverages the similarities in the two problems (multicast and mobility) • Minor modifications to IP-multicast required

  39. Multicast • Multicast: group membership, packets sent to a multicast address have to be delivered to all members of the group • Members of a multicast group can be located “anywhere” • IP-multicast infrastructure is overlayed on the Internet (construction of infrastructure a separate problem by itself – DVMRP, CBT, etc.) • Forwarding of data happens on the overlayed infrastructure, and routing is group specific

  40. Tunnels Multicast (Illustration)

  41. Tunnels Multicast & Mobility CH Use IP-multicasting to support mobility!

  42. End-to-End Approach • Internet infrastructure does not change (like in mobile IP) • Changes required at both the sender and receiver • Does connection migration when mobile-host moves

  43. E2E Approach (Contd.) • Hostname used as the invariant to identify mobile host • Mobile host uses DNS updates to change hostname to IP address mapping • No consistency problem as DNS entries can be made un-cacheable • If client is mobile, DNS-support not used

  44. E2E Approach (Contd.) • When a mobile-host undergoes a handoff, it re-issues a SYN (with a MIGRATE option identifying the previous connection) • A unique token exchanged during initial connection set-up used to identify connection • The receiver of the SYN changes its state to represent the new address of the mobile-host • Connection proceeds as a regular TCP connection from thereon • Trade-offs?

  45. TCP-level mobility support • Use dynamic DNS for initial name lookup • If name changes during a connect, use TCP migrate option • If name changes between DNS lookup and TCP connection, then do another DNS lookup Computer Network

  46. TCP-level advantages and disadvantages • No tunneling • No need to modify IP layer • Possibly more input from applications • Requires secure dynamic DNS • Scalability issue not entirely dismissable • What if both endpoints are mobile? • Need to modify multiple transport layers • More transport-level changes required than IP-level additions • Security issues more severe (1st paragraph of Section 5 is false) • Requires application-level changes for DNS retries Computer Network

  47. Overall TCP-level questions • Are IP address changes a routing responsibility or an application responsibility? • Is this really end-to-end? • With dynamic DNS requirements, application-level changes, and TCP changes, why not just do DNS retry every time a connection fails? Computer Network

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