1. TCP over Wireless Networks

2. WTCP Reliable transport protocol for wide-area wireless networks
Alternative to TCP (but completely different unlike the schemes we have seen so far)
End-to-end approach

3. WTCP Approach Rate based transmission control
Inter-packet separation based congestion detection
Distinguishing cause of packet loss
Transmission control computations at receiver
Variable granularity rate adjustment
Quick start

4. WTCP Reliability
SACK algorithm
Probe packets

5. Reliability Selective acknowledgements
SACKs very useful in TCP when random losses occur
Better utilization of the network resources (no unnecessary retransmissions)

6. Reliability (contd.) No retransmit timeouts
Since RTOs cannot be relied upon, WTCP does not use RTOs
Use probe packets to recover from suffix losses
ACK frequency tuned by sender based on factors such as (a) observed loss rate on reverse path, (b) round-trip time, (c ) half-duplex or full-duplex nature of channel

7. WTCP Algorithm Congestion control
3 phases: increase, decrease, and maintain
Chosen based on long-term and short term averages in inter-packet separation
RTT computation (for probing)
Using loss profile
Increase, decrease, maintain phases

8. WTCP Recap E2E approach Completely different from TCP
Uses rate based transmissions
Uses inter-packet separation as congestion indication
Performs loss classification
No RTOs
SACK + probe packets

9. pTCP: A Transport Layer Approach for Achieving Aggregate Bandwidths on Multi-homed Mobile Hosts

10. Introduction A multitude of wireless access technologies
How can these technologies co-exist in providing the best service to a multi-homed mobile user?
Use only the interface with the highest data rate at any given time
Objective
Allow simultaneous use of multiple wireless access technologies (striping)
Design a transport layer protocol that provides reliable and sequenced delivery like in TCP, and achieves effective bandwidth aggregation
existing solutions have been considered using the best…
for example, …
However, the objective of our work is to consider simultaneous use…
For example, 7Mbps
To achieve this objective, we design… --
effective cost-qos tradeoffs
an interesting problem that arises is …
the concept of vertical handoff has been proposed to address this problem, where a mobile user is provided access through the higher data connection when it migrates across coverage areas.
For example, when a user with 2Mbps Wireless WAN access moves within the coverage of 10Mbps Wireless LAN, …
why transport layer
emphasize simultaneous use and bandwidth aggregation
illustrate bandwidth aggregation?

11. Why Not Lower Layers? Link layer striping [Adiseshu ’96, Snoeren ’99]
Stripe according to estimates of link bandwidths
Optimal link layer striping  optimal bandwidth aggregation across multi-hop paths
Not applicable to multi-homed mobile hosts
Network layer striping [Phatak ’02]
IP performs tunneling and striping
TCP’s RTT estimates lose meaning
TCP adversely reacts to packet reordering
bandwidth aggregation at the link layer has been proposed --
But more importantly, TCP’s adversely… causes the problem.
adverse reaction: mention fast retransmit causes window cutdown
Lower layer approaches fail due to the multi-hop nature of paths and TCP’s reliance on FIFO delivery!

12. Why Not Multiple Sockets?
Application layer striping
Open multiple TCP sockets
Each socket is in charge of only one path (pipe)
Perform bandwidth estimation at the sending end and packet re-sequencing at the receiving end
Optimal aggregate bandwidth = sum of maximum achievable bandwidths on the individual pipes
pipe 1
pipe 2
an alternative approach is to use application layer striping by opening multiple …
to achieve bandwidth aggregation, the sending end…, and to provide in-sequence, ..
Note that what we mean by effective bandwidth aggregation is to achieve.. --
smart application coarse level
Problems
Data rate differential
Data rate fluctuations
Blackouts

13. Data Rate Differential
Head of line blocking will result in the slower pipes stalling the faster pipes
Example
Two pipes with mismatched bandwidths
Unaware application
Write until blocked
Sequence Number (Unaware Application) 1 2 3 4 5 6 35 36 37 38 39 40
Time (sec)
Sequence Number (x1000)
Pipe 1 (2500kbps)
Pipe 2 (500kbps)
Throughput vs. Bandwidth Ratio 1 2 3 4 5 6 7 8 9 10
Bandwidth Ratio (to 500kbps)
Throughput (Mbps)
Ideal
Smart Application
Unaware Application
mention ideal here
snapshot , a more generic scenario …
describe the problem
put up ideal with unaware application
smart application stripes based on 5:1
Unaware application does not work. Need some kind of bandwidth estimation
Smart application
Coarse-grained bandwidth estimation
The aggregate bandwidth is limited by the slower pipe in the unaware application

14. Data Rate Fluctuations
Data rate fluctuations cause disproportional striping
Data Rate Fluctuations
1.5
1.7
1.9
2.1
2.3
2.5
2.7
2.9
3.1
3.3
3.5 10 9 8 7 6 5 4 3 2 1
Fluctuation Period (sec)
Throughput (Mbps)
Ideal
Smart Application
Unaware Application 1 2 3 4 5 6 20 40 60 80
100
Bandwidth (Mbps)
Time (sec)
Bandwidth Fluctuations (5Mbps Pipe)
Example
Two pipes with random data rate fluctuations every t seconds
Smart application
Performs bandwidth estimation every T seconds
put up smart application after the fluctuation curve
1. general description
2. describe the link fluctuations (1 to 5Mbps, 200Kbps to 1Mbps)
3. smart application performs coarse grain for every T seconds
Need aggressive bandwidth estimation, which will cause overhead
Packets will be unnecessarily held up in each socket
Smart application’s performance is limited by disproportional striping

15. Blackouts
Undelivered packet in the blackout pipe will cause head of line blocking
Smart Application during Blackouts 5 10 15 20 25 30 40 50 60 70 80 90
100
Time (sec)
Sequence Number (x1000)
Pipe 1 (does not experience blackout)
Pipe 2 (experiences blackout)
Example
One of the two pipes experiences blackout
The other pipe does not experience any blackout
stalled due to pipe 1 blackout
blackout is a phenomenon where the data rate goes to zero --
1. one generic statement
2. scenario
true even for multiple pipes
blackout
Blackouts on one pipe will stall the entire aggregate connection

16. Design Elements Decoupling of functionalities
Per-pipe (TCP-v) behavior vs. aggregate connection (pTCP) behavior
TCP-v: virtual packets; pTCP: data packets
TCP-v: congestion control; pTCP: reliability
TCP-v: how much to send; pTCP: what to send
The fundamental design element of the pTCP protocol is the decoupling of functionalities…
in terms of key functionalities, TCP-v handles…
Note that although we present pTCP as a wrapper around TCP-v, it can be used in tandem with any single-pipe transport layer protocol
TCP-v congestion control, pTCP reliability
Allow plug-in of different congestion control schemes for different pipes

17. Design Elements Congestion window based striping Dynamic reassignment
A packet is given to a TCP-v only when its congestion window has space
Reuse TCP’s congestion control for bandwidth estimation
Dynamic reassignment
Reassign packets that fall out of TCP-v’s window
Avoid packets being held up by one pipe
-> differential [say here that pTCP can use any congestion control; also mention that this is fine-grained bandwidth-based striping]
-> fluctuations [mention that this addresses the problem caused by fluctuations (disproportional striping)]
-> blackouts
binding mention
Redundant striping
Redundantly stripe the first packet after timeouts
Allow the concerned pipe to probe while not potentially stalling the rest

18. pTCP Architecture Application pTCP TCP-v IP data packet virtual packet
control
read
write
ptcp-recv
ip-output
Application IP
pTCP
TCP-v
open/close
established/closed
receive
send
shrunk
resume
recv buffer
send buffer
bindings
active
pipes
virtual recv buffer
virtual send buffer
briefly mention open/close/established/closed only
can link back to the design?

19. pTCP Architecture Application pTCP TCP-v IP data packet virtual packet
control
read
write
ptcp-recv
ip-output
Application IP
pTCP
TCP-v
open/close
established/closed
recv buffer
send buffer
bindings
active
pipes
receive
send
virtual recv buffer
virtual send buffer
resume
mention application reads at the end
briefly mention open/close/established/closed only
can link back to the design?
shrunk

20. pTCP Protocol Congestion control Flow control Reliability
Performed by each TCP-v on a per-pipe basis
Any congestion control mechanism can be used
Flow control
Managed by pTCP through window advertisement
No flow control by TCP-v
Reliability
pTCP delegates reliability to each TCP-v unless the packet has to be reassigned
congestion control: Any congestion control mechanism can be used (current prototype and results using TCP’s CC)
flow control: No flow control by TCP-v
which component handles it
how does it do it
Connection management
pTCP manages the aggregation connection while TCP-v manages the individual pipes

21. Simulation Environment
Topology
Pipe 1: BW = 500kbps ~ 5Mbps, RTT = 100ms
Pipe 2: BW = 2Mbps, RTT = 400ms
Pipe 3: BW = 500kbps, RTT = 400ms
Approaches
Ideal
pTCP
Smart application
Unaware application
3 types of pipes representative of different wireless evironments
and we consider 4 different types of approaches
why such values
simulation does not matter

22. Data Rate Differential
Throughput vs. Bandwidth Ratio 1 2 3 4 5 6 7 8 9 10
Bandwidth Ratio (to 500kbps)
Throughput (Mbps)
Ideal
pTCP
Smart Application
Unaware Application

23. Data Rate Fluctuations
Throughput vs. Number of Pipes (with Fluctuations) 2 4 6 8 10 12 14 16 3 5 7 9
Number of Pipes
Throughput (Mbps)
Ideal
pTCP
Smart Application
Unaware Application
take the same scenario as the “different number of pipes”, and we now introduce data rate fluctuations

24. Blackouts (pTCP) pTCP during Blackouts 10 20 30 40 50 60 70 80 90 10010 20 30 40 50 60 70 80 90
100
Time (sec)
Sequence Number (x1000)
Pipe 1 (does not experience blackout)
Pipe 2 (experiences blackout)
continues progression
blackout

25. Multiple Congestion Control Schemes
Use TCP-ELN for lossy link (Pipe 2)
Multiple Congestion Control Schemes 1 2 3 4 5 6 7
1.00E-05
1.00E-04
1.00E-03
1.00E-02
Packet Drop Probability (Pipe 2)
Throughput (Mbps)
Ideal: TCP (Pipe 1) + ELN (Pipe 2)
pTCP: TCP (Pipe 1) + ELN (Pipe 2)
Smart Application: TCP (Pipe 1) + ELN (Pipe 2)
Smart Application: TCP (Pipe 1) + TCP (Pipe 2)
mention bw for each pipe
ideal first
link back to decoupling of functionalities

26. Related Work Link layer Network layer Transport layer
Scalable striping [Adiseshu ’96]
Adaptive inverse multiplexing [Snoeren ’99]
Network layer
Multi-IP links streaming [Phatak ’02]
Transport layer
SCTP [Stewart ’00]
RMTP
no animation?
RMTP [Magalhaes ’01]
Application layer
PSockets [Sivakumar ’00]

27. Recap
Bandwidth aggregation can be used to provide better service to multi-homed mobile hosts
A transport layer approach is more effective than other layers in the target environment
pTCP achieves effective bandwidth aggregation despite the data rate mismatch, fluctuations, and blackouts of different pipes
mention 3 characteristics first
then target environment
For more information:

28. Puzzle You have a chain consisting of 63 inter-linked gold links
You have to stay at a motel where the charge per day is 1 gold link
You do not trust the motel manager and neither does he trust you
What is the minimum number of links you need to break in order to stay for 63 days?
Clue: The manager will not sell the links till you check out