1. Wireless Scheduling

2. Puzzle Monty Hall Problem
You are a contestant on a game show. In front of you are three closed doors. The game show host informs you that behind one of these doors is the motor car of your dreams, but behind the other two doors lies a peanut (which you're allergic to anyway!).
The quiz-master asks you to select a door. After you have selected, he then opens one of the other two doors that does not contain the car. He does this every week to build up the suspense for the watching millions. He asks if you would like to open the door you originally selected and take home that prize, or switch to the remaining door and go home with that prize. 
Is it in your best interests to switch, or to remain with your original selection ?

3. Instantiations Channel state dependent packet scheduling (CSDPS)
Idealized wireless fair queuing (IWFQ)
Wireless packet scheduling (WPS)
Channel-condition independent fair queuing (CIFQ)
CBQ-CSDPS
Server based fairness approach (SBFA)
Wireless fair service (WFS)

4. SBFA Any error free service model can be used
SBFA reserves a fraction of the channel bandwidth statically for compensation by specifying a virtual compensation flow
When a flow is unable to use a slot, it queues a slot-request to the compensation flow
Scheduler serves compensation flow just as other flows
When the compensation flow gets a slot, it turns the slot over to the flow represented by the head-of-line slot-request

5. SBFA (Contd.) Scheduled to Tx F1 Cannot transmit because of error
Slot queued
into compensation flow
Cannot transmit
because of error
Compensation Flow of weight w
Slot scheduled for
Tx and handed over to F1

6. SBFA (Contd.) No concept of a leading flow
All bounds supported by SBFA are only with respect to the remaining fraction of the channel bandwidth
Performance of SBFA is sensitive to the statically reserved fraction
No short-term fairness
Long-term fairness dependent upon the reserved fraction

7. Wireless Fair Service
Uses an enhanced version of WFQ in order to support delay-bandwidth decoupling
Lag of a flow incremented only if there is a flow that can use the slot
Both lead and lag are bounded by per-flow parameters
A leading flow with a lead of L and a lead bound of Lmax relinquishes a fraction L/Lmax of the slots allocated to it by the error-free service
This results in an exponential reduction in the number of slots relinquished

8. WFS (Contd.) Service degradation is graceful for leading flows
In-sync flows are not affected
Tightest short-term fairness among all algorithms discussed
Compensation for lagging flows can take up more time than other algorithms

9. Recap Wireless Fair Scheduling Why wireline algorithms cannot be used
Key components of a a wireless fair scheduling algorithm
Different approaches for wireless fair scheduling

10. Mobile IP

11. Internet hosts & Mobility
Wireless networking – allows Internet users to become mobile
As users move, they have to be handed over from one coverage area to another (since the coverage areas of access points are finite) …
Ongoing connections need to be maintained as the user moves …

12. Problems? What are the 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

13. Problems (Contd.)? What are the options?
Make IP address host specific instead of network specific – obvious pitfalls?
Change IP address of host and start using the new IP address in the subsequent packets belonging to the connections

14. 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

15. Mobile IP Basics Same as the post-office analogy
Two other entities – home agent (old post-office), foreign agent (new post-office)
Mobile host registers with home agent the new location
Home agent captures packets meant for mobile host, and forwards it to the foreign agent, which then delivers it to the mobile host

16. 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 MH HA SH FA MH

17. Mobile IP Entities Mobile host Corresponding host Home address
Care-of address
Home agent
Foreign agent

18. Mobile IP in detail … Combination of 3 separable mechanisms:
Discovering the care-of address
Registering the care-of address
Tunneling to the care-of address

19. 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

20. 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

21. 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?

22. 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)

23. Illustration

24. 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

25. 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

26. Tunneling to the Care-of address
When home agent receives packets addressed to mobile host, it forwards packets to the care-of address
How does it forward it? - encapsulation
The default encapsulation mechanism that must be supported by all mobility agents using mobile IP is IP-within-IP (RFC 2003)
Using IP-within-IP, home agent inserts a new IP header in front of the IP header of any datagram

27. Tunneling (contd.) Destination address set to the care-of address
Source address set to the home agent’s address
Tunnel header uses 4 for higher protocol id – this ensures that IP after stripping out the first header, processes the packet again
Tunnel header of 55 used if IP minimal encapsulation used (RFC 2004)

28. Illustration

29. Recap Host mobility and Internet addresses Post-office analogy
Home agent, foreign agent, care-of address, home address
Registration and Tunneling
IPv6 and Mobility support …

30. Mobile IP Problems? Triangular routing overhead
What is the worst case scenario?
Registration latency and associated problems
Ingress filtering and consequences
Infrastructure required for mobile IP support?
Firewalls

31. Mobile IP Optimizations
Route optimization
Smooth hand-offs

32. Route Optimizations
Enable direct notification of the corresponding host
Direct tunneling from the corresponding host to the mobile host
Binding cache maintained at corresponding host
Management of cache not stipulated (e.g. least used entry replacement)

33. Route optimizations (contd.)
4 types of messages
Binding update
Binding request
Binding warning
Binding acknowledge

34. Binding Update
When a home agent receives a packet to be tunneled to a mobile host, it sends a binding update message to the corresponding host
When a home agent receives a binding request message, it replies with a binding update message
Also used in the the smooth-handoffs optimization

35. Binding Update (Contd.)
Corresponding host caches binding and uses it for tunneling subsequent packets
Lifetime of binding?
Corresponding host that perceives a near-expiry can choose to ask for a binding confirmation using the binding request message
Home agent can choose to ask for an acknowledgement to which a corresponding host has to reply with a binding ack message

36. Binding warning
When a foreign agent receives a tunneled message, but sees no visitor entry for the mobile host, it generates a binding warning message to the appropriate home agent
When a home agent receives a warning, it issues an update message to the corresponding host
What if the foreign agent does not have the home agent address (why?) ?

37. Illustration Home Agent BU BW BR BA Foreign Agent Corresponding Host
Mobile Host

38. Smooth Hand-offs
When a mobile host moves from one foreign agent to another …
Packets in flight to the old FA are lost and are expected to be recovered through higher layer protocols (e.g. TCP)
How can these packets be saved?

39. Smooth Hand-offs Make previous FA forward packets to the new FA
Send binding updates to the old FA through the new FA
Such forwarding will be done for a pre-specified amount of time (registration lifetime)
Update can also help old FA free any reserved resources immediately
Why better?

40. Mobile IP in IPv6
Route optimization and smooth hand-offs used in IPv6 mobility
Binding updates easier since IPv6 supports destination caches at sources
IPv6 security inherently stronger than in IPv4. Hence, no explicit security mechanisms needed for mobile IP
Source routing to be used instead of encapsulation (why?)

41. Outline Multicast-based architecture Fast handoffs MosquitoNet
End-to-end approach

42. 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

43. 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

44. Multicast (Illustration)
Tunnels

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

46. MSM-IP Architecture MSM-IP: Mobility support using Multicasting in IP
Addressing: mobile host has multicast address
Tunneling architecture: same as IP multicast (sparse mode algorithm required)
Join and prune mechanisms: hand-offs made more efficient
Resource reservation (RSVP) easier

47. Problems? ARP replies TCP support IGMP registration
ICMP message delivery
Multicast address space
IP-multicast maturity

48. Fast Handoffs
Reduce the latency in resuming operations when a hand-off occurs
Use hierarchical foreign agents
Example: domain foreign agents and subnet foreign agents
Mobility within a domain kept transparent from the home agent by appropriate interactions between domain foreign agent and subnet foreign agents

49. Fast Handoffs (Illustration)
Internetwork FA
Subnet A
Subnet B FA FA

50. MosquitoNet One of the first test-bed implementations of Mobile IP
Introduced the notion of co-located foreign agents
Improves deployability of the mobile-IP approach to support host mobility
Trade-offs?

51. 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

52. 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

53. 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?

54. Recap Wireless Fair Scheduling Why wireline algorithms cannot be used
Key components of a a wireless fair scheduling algorithm
Different approaches for wireless fair scheduling

55. Puzzle Man in a boat floating in a swimming pool 
He has a large solid iron ball
If he drops the ball into the water, what happens to the level of water in the swimming pool? (increases, decreases, stays the same?)