1. Ad-hoc Transport Layer Protocol (ATCP)
EECS 6590

2. Overview
What is TCP?
TCP Challenges in MANETs
ATCP

3. Transport Layer
In the OSI model, the transport layer is responsible for:
Reliable end-to-end connection
End-to-end delivery
Flow control
Congestion control
In-order packet delivery

4. TCP: A Brief Review TCP: Transmission Control Protocol
Specified in 1974 (TCP Tahoe)
Data stream  TCP packets
Reliable end-to-end connection
In-order packet delivery
Flow and congestion control

5. How does TCP work? Establishes an end-to-end connection:
Acknowledgement based packet delivery
Assigns a congestion window Cw:
Initial value of Cw = 1 (packet)
If tx successful, congestion window doubled. Continues until Cmax is reached
After Cw ≥ Cmax, Cw = Cw + 1
If timeout before ACK, TCP assumes congestion

6. How does TCP work? (2) TCP response to congestion is drastic:
A random backoff timer disables all transmissions for duration of timer
Cw is set to 1
Cmax is set to Cmax / 2
Congestion window can become quite small for successive packet losses.
Throughput falls dramatically as a result.
Random backoff timer done to avoid further congestion

7. TCP Congestion Window

8. Why does TCP struggle in MANETs?
Dynamic network topology
Node mobility
Dynamic channel quality
Multi-hop paths
Variable path lengths per node
Longer path = higher failure rate
1) Dynamic network topology means the nodes are mobile, and the structure of the network changes regularly. TCP must account for gaps in transmission where the routing protocol must update the paths before TCP can continue
2) The

9. Why does TCP struggle in MANETs? (2)
Lost packets due to high BER (Bit Error Rate):
BER in wired: 10-8 – 10-10
BER in wireless: 10-3 – 10-5

10. Solutions for TCP in MANETs
Various solutions
Most solutions tackle only a subset of problems
Often fixing one part of TCP may break another part

11. Solution Topology

12. Why focus on TCP based solutions and not new transport protocols?
We want to choose solutions which maintain close connection to TCP
Upper layers in the OSI model affected by choice of transport layer protocol
Modifications may affect interactions with the Internet
Alternative methods only useful for isolated networks

13. TCP Summary Works well in wired networks
Fails in wireless networks due to frequent link breaks:
Node mobility
Packets lost due to lossy channels
Multi-hop paths more prone to failure
Most solutions tackle only a subset of problems

14. ATCP Overview: What is ATCP? Motivation for ATCP ATCP Infrastructure
How ATCP works
Is ATCP Successful?
Performance vs. TCP
ATCP Recap

15. What is ATCP? Overview: Ad-hoc TCP Network layer feedback mechanism
TCP state control
End-to-end semantics
Dependent on routing protocols

16. ATCP in Solution Topology

17. Motivation for ATCP Issues addressed by ATCP:
Packet loss due to high BER or collision
Route changes due to node mobility
Network partitions
Out-of-order packets
Congestion
Congestion window size adjustment

18. ATCP infrastructure
ATCP is a thin layer that is layered between TCP and IP
Sender’s ATCP states:
Normal, Disconnected, Congested, and Loss
TCP
TCP
ATCP IP IP
Sender states: persist state, congestion control state and retransmission state
A thin layer called ATCP is inserted into between TCP and IP, Standard TCP is maintained
On receiving a ICMP “Destination Unreachable” messages, the sender enters the persist state. TCP at the sender is frozen and no packets are sent until a new route found;congestion control is not invoked.

19. How ATCP works – Lossy Channels
Disconnected *
+ ATCP transmits unacknowledged segments from TCP’s buffer, and maintains its own timers for these segments.
+ When a new ACK comes, ATCP forwards it to TCP and returns to normal state.
Normal
New
ACK
RTO about
to expire OR
3 dup ACKs
Loss *
Congested
* TCP sender in persist mode
ATCP retransmits
segments in buffer

20. How ATCP works - Congestion
Disconnected *
+ Congestion: ECN flag set in ACK and data packets
+ ATCP does not interfere.
+ After TCP transmits a new segment, ATCP returns to normal state.
Normal
Receive
ECN
TCP transmits
a new packet
New
ACK
RTO about
to expire OR
3 dup ACKs
Loss *
Congested
* TCP sender in persist mode
ATCP retransmits
segments in buffer

21. How ATCP works – Disconnected
Network generates ICMP message “Destination Unreachable”.
ATCP: disconnected mode
TCP: persist mode
generates probe packets
Receiver responds with a data packet or ACK.
ATCP and TCP return to normal state.

22. How ATCP works – Disconnected
Receive dup ACK
or packet from receiver
Receive “Destination
Unreachable” ICMP
Normal
Receive
ECN
TCP transmits
a new packet
New
ACK
RTO about
to expire OR
3 dup ACKs
Loss *
Congested
* TCP sender in persist mode
ATCP retransmits
segments in buffer

23. ATCP Diagram

24. Is ATCP Successful? Pros: Cons:
Maintenance of end-to-end TCP semantics
Compatibility with traditional TCP
Invisibility to TCP
Cons:
Dependency on the network layer protocol to detect route changes and partitions
Addition of a thin ATCP layer to TCP

25. Performance: Loss Case
BER = 10-5
1 MB File Transfer
20 measurements
90% confidence interval
+ 5 Pentium PCs
+ 32-Kbps channels
+ Simulated hop-by-hop delay

26. ATCP vs. TCP: Congestion Window

27. Performance: BER and Congestion
200ms period every 5s (80 congestions)
1200s to 3000s transfer time (TCP)
460s to 500s transfer time (ATCP)

28. Performance: BER and Partition
Partitioned for 1 min every
5 minutes

29. ATCP vs. TCP: File Transfer Time
1MB file transfer

30. ATCP Summary Introduces a thin layer between IP and TCP
Maintain end-to-end semantics
Does not interfere with TCP functions
Depends on the network layer to detect route changes and partitions

31. References [1] Split-TCP for Mobile Ad Hoc Networks; Kopparty et al.
[2] ATCP: TCP for Mobile Ad Hoc Networks; Jian Liu, Suresh Singh, IEEE Journal, 2001.
[3] A Feedback-Based Scheme for Improving TCP Performance in Ad Hoc Wireless Networks; Kartik Chandran et al.
[4] Ad Hoc Wireless Networks: Architectures and Protocols; C. Siva Ram Murthy and B. S. Manoj
[5] Improving TCP Performance over Wireless Networks; Kenan Xu, Queen’s University 2003