its contain all tcp related to mobile ip and tcp rano ,taho ,sack

1. Mobile Transport Layer
Prepared By :
Maulik Patel

2. Outline
 TCP and its Usability : Congestion Avoidance, Flow control
 Slow Start and Retransmission
 TCP Variants : Tahoe, Reno and Vegas
 Indirect TCP
 Snoop TCP and Mobile TCP
 Transaction Oriented TCP and TCP over 2.5G/3G

3. TCP and Its Usability
 Reliable ordered delivery
 TCP has built-in mechanisms to behave in a ‗network friendly‘
manner.
 Guarantees that packet will be delivered in same order in
which they were sent
 Implements congestion avoidance and control
 Reliability achieved by means of retransmissions if necessary

4. Cont…
 End-to-end semantics
Acknowledgements sent to TCP sender confirm delivery
of data received by TCP receiver
Ack for data sent only after data has reached receiver

5. Congestion
 Congestion in a network may occur if the load on the
network—the number of packets sent to the network—is
greater than the capacity of the network—the number of
packets a network can handle.
 Congestion occurs when bandwidth is insufficient and
network data traffic exceeds capacity.

6. Effect of Congestion
 Congestion causes packet loss
 Results in retransmission
 Reduces data throughput

7. Congestion Avoidance
 In the congestion avoidance algorithm, the size of congestion
window increases additively until congestion is detected.
 It maintains a network congestion state after reducing the
initial data load.

9. Flow Control
 Flow control refers to a set of procedures used to restrict the
amount of data that the sender can send before waiting for
acknowledgment.
 limits the rate a sender transfers data to guarantee reliable
delivery.
 The receiver continually hints the sender on how much data can be
received.
 When the receiving host's buffer fills, the next acknowledgment
contains a 0 in the window size, to stop transfer.
 TCP uses an end-to-end flow control protocol to avoid having the
sender send data too fast for the TCP receiver to receive and
process it reliably

10. Slow Start
 Linear additive increase takes too long to ramp up a new TCP
connection from cold start
 Beginning with TCP Tahoe, the slow start mechanism was
added to provide an initial exponential increase in the size of
Congestion Window(cwnd).
 Moreover, slow start is slower than sending a full advertised
window‘s worth of packets all at once.

11. Cont…
 The source starts with cwnd = 1.
 Every time an ACK arrives, cwnd is incremented.
 cwnd is effectively doubled per RTT.
 Two slow start situations:
 At the very beginning of a connection.
 When the connection goes dead waiting for a timeout to
occur (i.e, the advertized window goes to zero!)

12. Slow Start
Add one packet
per ACK
Slow Start
Source Destination

13. Cont…
 However, in the second case the source has more
information. The current value of cwnd can be saved as a
congestion threshold.
 This is also known as the ―slow start threshold‖ ssthresh.

14. Retransmission
 Coarse timeouts remained a problem, and Fast retransmit
was added with TCP Tahoe.
 Since the receiver responds every time a packet arrives, this
implies the sender will see duplicate ACKs.
 Generally, fast retransmit eliminates about half the coarse-
grain timeouts.
 Fast retransmit does not eliminate all the timeouts due to
small window sizes at the source.

15. Fast Retransmission
Based on three
duplicate ACKs
 Receipt of three duplicate ACKs, the TCP Sender
retransmits the lost packet.
Packet 1
Packet 2
Packet 3
Packet 4
Packet 5
Packet 6
Retransmit
packet 3
ACK 1
ACK 2
ACK 2
ACK 2
ACK 6
ACK 2
Sender Receiv er

16. TCP Recap...
 TCP is a reliable protocol
 Ensuring reliability by requiring receiver‘s acknowledgement
 TCP uses cumulative ack. schema
 TCP start timer, maintains for retransmission
 TCP is end-to-end delivery oriented mechanism
 TCP can work directly on wireless but it require some
modification in TCP as it have mobility so high bit error rate
occurs

17. TCP Algorithms:
Four algorithms used commonly in TCP implementations
 Slow Start - Every ack increases the sender‘s window (cwnd)
size by 1
 Congestion Avoidance - Reducing sender‘s window size by
half at experience of loss, and increase the sender‘s window at
the rate of about one packet per RTT
 Fast Retransmit - Don‘t wait for retransmit timer to go off,
retransmit packet if 3 duplicate acks received
 Fast Recovery - Since duplicate ack came through, one packet
has left the wire. Perform congestion avoidance, don‘t jump
down to slow start

18. TCP Variants :
 TCP-Tahoe:
implements the slow start, congestion avoidance,
and fast retransmit algorithms
 TCP-Reno:
implements the slow start, congestion avoidance,
fast retransmit, and fast recovery algorithms
 TCP-Vegas:
Improvement of TCP Reno
New Retransmission mechanism.

19. TCP Tahoe
 Two major congestion control schema use
Slow start and congestion avoidance
Increase cwnd size
Fast Retransmission
Detect congestion
 Problem
Take a complete timeout interval to detect a packet loss

20. TCP Tahoe‘s Fast Retransmit
1. Sender
receives 3
dupACKS.
2. Sender infers
that the
segment is
lost.
3. Sender re-
sends the
segment
immediately!
4. Sender
returns to
slow-start.
cwnd = 1
cwnd = 2
cwnd = 4
3 duplicate
ACKs
fast-retransmit
of segment 4
S R

21. TCP Tahoe’s Graph

22. (TCP Reno)
Fast Retransmit & Fast Recovery
 After receiving 3 dupACKS:
1. Retransmit the lost segment.
2. Set ssthresh = flight size/2.
3. Set cwnd = ssthresh, and ndupacks = 3.
 If dupACK arrives:
 ++ ndupacks
 Transmit new segment, if allowed.
 If new ACK arrives:
 ndupacks = 0
 Exit fast recovery.
 If RTO expires:
 ndupacks = 0
 Perform slow-start - ( ssthresh = flight size/2, cwnd = 1 )

23. TCP Reno vs TCP Tahoe

24. TCP Vegas
 Developed by Brakmo, O'Malley & Peterson
[SIGCOMM,1994]
 TCP Vegas emphasis on packet delay, not packet loss.
 Sender-side protocol
Inter-operates with other TCP variants
 Depends heavily on accurate calculation of the Base RTT
value.
If it is too small then throughput of the connection will be
less than the bandwidth available while if the value is too
large then it will overrun the connection.

25. TCP Vegas
 Adjust window size even before packet loss
 Adjust window based on difference between actual
and expected throughput
 Advantage
Detects losses faster than TCP Reno and also
recover from multiple drops more efficiently.
 Disadvantage
On heavily traffic it react same as TCP Reno.
Vegas are not stabilize if buffer are too small.
Fairness issues of Vegas still unresolved

26. TCP Vegas new
retransmission mechanism
 Keeps track of when each segment was sent and it also
calculates an estimate of the RTT.
 Whenever a duplicate acknowledgement is received it checks
to see if the (current time-segment transmission time)> RTT
estimate
 So no need to wait for 3 dupack, send retransmite packet

27. Indirect TCP
 Split a TCP connection at the foreign agent into 2 TCP
connections
 hosts in the fixed part of the network do not notice the
characteristics of the wireless part
 no changes to the TCP protocol for hosts connected to
the wired Internet, millions of computers use (variants
of) this protocol
 optimized TCP protocol for mobile hosts

28. Cont…

29. Cont…
 The access point acts as proxy in both directions.
 AP acknowledges to both the sender and receiver.
 Re-transmission on wireless links is handled locally.
 During handover, the buffered packets, as well as the system
state (packet sequence number, acknowledgements, ports,
etc), must migrate the new agent.

30. I-TCP Socket and State
Migration

31. Advantages of I-TCP
 No changes in the fixed network necessary, no changes for
the hosts (TCP protocol) necessary, all current optimizations
to TCP still work
 Simple to control, mobile TCP is used only for one hop
between, e.g., a foreign agent and mobile host
 transmission errors on the wireless link do not propagate
into the fixed network
 therefore, a very fast retransmission of packets is possible,
the short delay on the mobile hop is known

32. Cont…
 It is always dangerous to introduce new mechanisms in a
huge network without knowing exactly how they behave.
 New optimizations can be tested at the last hop, without
jeopardizing the stability of the Internet.
 It is easy to use different protocols for wired and wireless
networks.

33. Disadvantages of I-TCP
 Loss of end-to-end semantics
 an acknowledgement to a sender no longer means that a
receiver really has received a packet ---foreign agents
might crash.
 Higher latency possible
 due to buffering of data within the foreign agent and
forwarding to a new foreign agent
 Security issue
 The foreign agent must be a trusted entity.

34. Snoop TCP
 Indirect TCP
 2 TCP sessions.
 Snoop TCP
 One TCP session.
 The access point snoops into the traffic and buffers
packets for fast re-transmission.

35. Cont…

36. Cont…
 Transparent extension of TCP within the foreign agent
 changes of TCP only within the foreign agent
 buffering of packets sent to the mobile host
 lost packets on the wireless link (both directions!) will be
retransmitted immediately by the mobile host or foreign
agent, respectively (so called ―local‖ retransmission)
 the foreign agent therefore ―snoops‖ the packet flow and
recognizes acknowledgements in both directions, it also
filters ACKs

37. Cont…
 Data transfer to the mobile host
 FA buffers data until it receives ACK of the MH, FA detects
packet loss via duplicated ACKs or time-out
 Fast retransmission possible, transparent for the fixed
network
 FA detects packet loss on the wireless link via sequence
numbers, FA answers directly with a NACK to the MH
 MH can now retransmit data with only a very short delay

38. Snoop TCP: Packet Delivery

39. Cont…
 Advantages
 End-to-end semantics is preserved.
 Handover is easy. I-TCP requires a careful handover of the
system state. Here it falls back to the standard solution if
no enhancements.
 Problems
 snooping TCP does not isolate the wireless link as good as
I-TCP
 snooping might be useless depending on encryption
schemes

40. Mobile TCP
 What if the mobile node is disconnected?
 I-TCP
 more packets are buffered at AP.
 Snooping TCP
 no more snooping
 Missing acknowledgement, TCP goes to slow-start.
 Mobile TCP
 Improve efficiency.
 Special handling of lengthy and/or frequent
disconnections.

41. Cont…

42. Cont…
 M-TCP splits as I-TCP does
 unmodified TCP fixed network to supervisory host (SH)
 optimized TCP SH to MH
 Supervisory host
 no caching, no local retransmission
 monitors all packets, if disconnection detected
 set sender window size to 0
 sender automatically goes into persistent mode
old or new SH reopen the window

43. Cont…
 Advantages:
 End-to-end semantics.
 When mobile host is disconnected, it avoids useless
retransmissions and slow-start.
 No buffering, handover is easy.
 Disadvantages:
 Packet loss at the wireless link propagates back to
sender.
 Not a good idea for heavy traffic.

44. Transaction Oriented TCP
 TCP phases
connection setup, data transmission, connection release
using 3-way-handshake needs 3 packets for setup and
release, respectively
thus, even short messages need a minimum of 7 packets!
 Transaction oriented TCP
RFC1644, T-TCP, describes a TCP version to avoid this
overhead
connection setup, data transfer and connection release
can be combined
thus, only 2 or 3 packets are needed

45. Cont…
 Advantage
efficiency
 Disadvantage
requires changed TCP
mobility not longer transparent

46. TCP over 2.5/3G
Wireless Networks
 Fine tuning today‘s TCP
 Learn to live with
 Data rates: 64 kbit/s up, 115-384 kbit/s down; asymmetry: 3-6, but also up
to 1000 (broadcast systems), periodic allocation/release of channels
 High latency, high jitter, packet loss
 Suggestions
 Large (initial) sending windows, large maximum transfer unit, selective
acknowledgement, explicit congestion notification, time stamp, no header
compression
 Widespread use
 i-mode running over FOMA

47. Performance
Enhancing Proxies
 RFC 3135
Transport layer
Local retransmissions and acknowledgements
Additionally on the application layer
Content filtering, compression, picture downscaling
E.g., Internet/WAP gateways
Web service gateways?
Mobile system
PEP
Comm. partner
wireless
Internet

48. Cont…
Big problem: breaks end-to-end semantics
Disables use of IP security
Choose between PEP and security!
 RFC 3150 (slow links)
Recommends header compression, no timestamp
 RFC 3155 (links with errors)
States that explicit congestion notification cannot be used

49. Summary
 TCP is a complex protocol
Minimal support from underlying protocols
Indirect observation of network environment
Large number of competing flows from different hosts
Congestion avoidance is still a research issue

50. Cont…
 TCP does not perform well in a wireless environment where
packets are usually lost due to bit errors, not congestion
 Schemes have been proposed to address TCP performance
problems
Link-level recovery
Split protocols
End-to-end protocols

51. References
 [1]IEEE:TCP-FIT: An improved TCP congestion control algorithm and its
performance
 [2]IEEE: Fast TCP flow control with differentiated services
 [3]Afanasyev, A.; N. Tilley, P. Reiher, and L. Kleinrock (2010). "Host-to-host
congestion control for TCP―
 [4] Jacobson, Van; Karels, MJ (1988). ―Congestion avoidance and control‖
 [5]http://www.scribd.com/doc/63483753/Indirect-TCP-Snooping-TCP-Mobile-
TCP-Mobile-Transport-Layer#download
 [6]Jian Ma, "A simple Fast TCP flow control in IP network or IP/ATM subnet – a
significant improvement over IP", November, 1997
 [7]A Comparative Analysis of TCP Tahoe, Reno, New-Reno, SACK and Vegas
http://inst.eecs.berkeley.edu/~ee122/fa05/projects/Project2/SACKRENEVEGAS.p
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52. Thank You