1. Mobile IP & Wireless Transport

2. Announcements Feedback on presentations Presentation schedule
Altered one more time!
Student presentations for this set will happen only on October 19th and October 24th
No review class before exam

3. Puzzle
10 bags with coins. 9 bags have coins of equal weights. 1 bag has defective coins (heavier or lighter by 1 lb)
Using a spring balance, how many weighings do you need to find the defective bag?
Take 1 coin from 1st bag, 2 coins from 2nd bag, …, and 10
coins from the 10th bag. If X is the weight of a good coin,
ideally total weight should be 55X. Assume the observed weight is 55X+/-Y. Yth bag is the defective bag.

4. Outline
What is the problem at the routing layer when Internet hosts move?!
Can the problem be solved?
What is the standard solution? – mobile IP
What are the problems with the solution?
Other approaches?

5. 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 …

6. 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

7. 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

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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

15. 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?

16. 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)

17. Illustration

18. 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

19. 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

20. 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

21. 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)

22. Illustration

23. 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 …

24. Outline TCP over wireless networks
TCP assumptions
Wireless network characteristics
Impact on TCP performance
Approaches to improve TCP performance
Link layer approaches
TCP-aware link layer approaches
Split connection approaches
End-to-end approaches
TCP over satellite and ad-hoc networks

25. TCP Congestion control Window based Slow start LIMD
Congestion detection
Self-clocking
Window limitation

26. TCP (Contd.) Reliability Cumulative ACKs DUPACKs and Timeouts
Timeout calculation
RTTavg + 4 * RTTmdev
Coupling between congestion control and reliability

27. Wireless Networks Wide-area wireless Metropolitan-area wireless
Local-area wireless
Ad-hoc wireless
Satellite wireless

28. Wireless Characteristics
Low bandwidths
10-20Kbps WWANs, 1-10Mbps WLANs, Kbps satellite
Random wireless losses
Upto 10% packet loss rates
Hand-offs
Depending on cell coverage and user mobility (30m/s in an network will result in a hand-off every seconds)

29. Wireless Characteristics (Contd.)
Black-outs
Fading, temporary disconnections etc.
Can last from a few seconds to less than a minute
Large and Varying Delay
WWANs have typical RTTs of 400ms and deviations of up-to a few seconds
Path asymmetry
Reverse path characteristics different from forward path characteristics (e.g. satellite, WWANs)

30. Characteristics (Contd.)
Local-area wireless
Frequent hand-offs when mobile
Ad-hoc wireless
Routers mobile! Frequent disconnections
Network partitions?
Satellite wireless
Large bandwidth delay products (why?)

31. TCP on Wireless – Random Losses
TCP uses losses as indication of congestion
Reduces congestion window by half (LIMD) when it experiences congestion
Even when no congestion, if packets are dropped due to random losses, TCP will cut down its rate (is this right?)

32. Other Losses
TCP will interpret hand-offs related losses also as congestion based losses
Hence, it will reduce the congestion window every time hand-offs related losses occur
Black-outs will further result in TCP experiencing multiple timeouts of increasing granularity

33. Large and Varying Delay
TCP uses RTTavg + 4 * RTTmdev as the retransmission timeout
If there is large variance in delay, mean deviation is high resulting in inflated timeout values
Hence, if there are burst losses resulting in a timeout, the sender would take longer time to detect losses and recover

34. Path Asymmetry
TCP relies on ACK arrivals for congestion window progression
If path asymmetry exists, a TCP connection’s performance will be influenced by the reverse path characteristics also
Indirect effects of path asymmetry (ACK bunching)

35. Low Bandwidths TCP uses window based congestion control
If there is free space in the congestion window, TCP will transmit
Hence, TCP’s output can be bursty
This coupled with the low bandwidths can result in queue build-ups in the network adversely affecting RTT calculations and causing packet drops

36. Large Bandwidth Delay Products
TCP’s header has 16 bits allocated for receiver window advertisement
Maximum of 64KB can be advertised
Consider a satellite link with 1Mbps bandwidth and 1 second RTT (BDP = 1Mb)
But, TCP can only achieve 500Kbps (resulting in only 50% utilization)

37. Slow-start
TCP uses slow-start to ramp up rate to the available capacity
Whenever timeouts occur, TCP uses slow-start
If hand-offs, black-outs, or route re-computations are frequent, TCP will constantly be in slow-start, lowering performance

38. Recap TCP over wireless networks
Several factors contribute to the performance degradation of TCP when used in a wireless environment
Approaches to improve TCP performance …

39. Approaches Reliable link layers TCP-aware link layers
Split connection protocols

40. Reliable Link Layers
Help in recovering from losses that occur on the wireless link
Can potentially hide such losses from the TCP layer
Can be implemented without requiring any changes at all to the sender and the receiver

41. Reliable Link Layer (contd.)
Losses can still occur
Retransmission can interfere with TCP retransmissions worsening the performance
TCP timeouts similar LL timeouts
DUPACKs
Can introduce variations in TCP’s rtt estimation increasing the RTO

42. Snoop Module Resides in the base-station
Caches packets sent from fixed host to mobile host
TCP-aware functionality for retransmissions, and ACK suppression
Improves on the performance of reliable link layers

43. Snoop (Contd.) Two modules:
Snoop_data() – for processing data packets on the forward path (from FH – MH)
Snoop_ACK() – for processing ACK packets on the reverse path (from MH – FH)

44. Snoop_data() 3 scenarios Normal packet in sequence Cache packet
Forward to MH
Timestamp if necessary (once per window) – for RTOs

45. Snoop_data() Scenario 2 Out of sequence (S) but already cached
If highestACK < S
Forward packet
Else
Generate an ACK from the snoop module for the highestACK

46. Snoop_data() Scenario 3
Out of sequence (S) and has not been cached earlier
Either out-of-order or packet that was lost earlier
Heuristically assume retransmission
Packet forwarded, and marked as having been retransmitted due to congestion loss

47. Snoop_ACK() Scenario 1 New ACK Spurious ACK Clean snoop cache
Round-trip estimate updated
Spurious ACK
ACK# less than highestACK#
Discard ACK

48. Snoop_ACK() Duplicate ACK (DUPACK)
If sender retransmitted packet or packet not in cache, forward ACK to sender
Unexpected DUPACK (loss between BS and MH)
Retransmit packet
Keep track of maximum number of relevant DUPACKs possible
Expected DUPACKs
Suppress

49. Mobile host to Fixed host
Cannot use only base-station based mechanisms
Need to change the mobile host
NACKs from the base-station to the mobile-station
Mobile-station retransmits immediately and does not perform window reduction for NACKed losses

50. Split Connection Approaches
Fixed Host
Base Station
Mobile Host
TCP
Wireless aware TP

51. Advantages Wireless aware congestion control and flow control
Wireless link characteristics completely decoupled from the progress of TCP on the wired leg
Better service to mobile applications possible
Potentially simpler protocol stack at mobile
Improved performance

52. Cons Application layer re-linking End-to-end semantics
Software overheads
Hand-off latency

53. Puzzle Two great mathematicians S & P
S knows the sum of two positive integers (> 1) x and y
P knows the product of x and y
S calls P and says “You cannot find the two numbers”
P replies “I know the two numbers”
S responds “I know the two numbers too”
What are the two numbers?!! Why?