1 / 47

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

liang
Télécharger la présentation

Special Topics on Wireless Ad-hoc Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related