Transport Layer (Wireless TCP)

David (Bong Jun) Choi

Department of Computer ScienceThe State University of New York Korea

CSE534: Fundamentals of Computer Networks

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TCP Basics Reliable Transfer Segment retransmission if necessary

End-to-End Semantics Flow Control Congestion Control Slow Start, Congestion Avoidance, Fast Recovery

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slow start

congestion avoidance

fast recovery

TCP Basics Acknowledgements

(ACK) Cumulative Selective

Sequence Number Receive Window

(rwnd) @ receiver sender

Congestion Window (cwnd) @ sender

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32 bitssource port # dest port #

applicationdata

(variable length)

sequence number

acknowledgement number

Urg data pointerchecksum

FSRPAUheadlen

notused

options (variable length)

receive window

Impact of Wireless and Mobility

Negative Effects of the Wireless Channel Fading (Multipath, Doppler) Shadowing Hidden Terminal High Bit Error Rates (BER)

Handover (H/O) due to Mobility H/O: Changes in the Point of Attachment

(PoA) Frequent and Lengthy Disconnections

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Impact of Wireless and Mobility on Higher Layer Protocols

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Internet

Q: Packet loss from (1) congestion or from (2) poor wireless channel condition (and/or from disconnection due to mobility)?

congestion?

dup ACK 14 received / timeout

poor channel?

receiver

senderPacket 15

not received

AP1

AP2

Impact of Wireless and Mobility on Higher Layer Protocols

Link Layer (L2) H/O – move within same subnet Increase in RTT IP is not aware (i.e., no IP address change within same subnet) May not effect TCP much

Network Layer (L3) H/O (e.g., Mobile IP) – move to different subnet Route update needed IP address change (neighbor discovery protocol needed) Slow: packet loss during handover

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Internet

a) L2 handover

b) L3 handover

Subnet 1

Subnet 2

AP1

AP2

AP3

Impact of Wireless and Mobility on Higher Layer Protocols Traditional TCP is okay?Reacts to losses with slow start although

there may be no congestionNeeds a new point of attachment (POA)

using the same serviceHas long delay due to handovers

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Wireless TCP Protocols General Approaches

a. Mask wireless packet loss from the TCP sender• Indirect TCP (I-TCP)• Snooping TCP

b. Notify cases of packet loss to the TCP sender • Mobile TCP (M-TCP)

Additional Approaches Support mobility Depending on application

Apply Changes to Host Node: TCP sender/receiver Intermediate Node: typically at the “last mile” (ex.

Base station, WLAN AP).

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I-TCP

Indirect TCP “Split” a TCP connection at the “last mile” Wireless TCP, Standard TCP WLAN/AP (Foreign Agent) acts as proxy for both

directions Re-transmission on wireless link handled locally

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Internet

a) Wireless TCP b) Standard TCP

WLAN/AP

A. Bakre and B. R. Badrinath, “I-TCP: Indirect TCP for Mobile Hosts,” In Proc. IEEE ICDCS, May 1995.A. Bakre and B. R. Badrinath, “Implementation and Performance Evaluation of Indirect TCP,” IEEE Trans. on Computers, March 1997.

I-TCP

Handover Step (4): Migration of buffered packets and

system state (packet sequence, ACKs, ports, etc.) necessaryMobile IP support

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InternetAP1

(1)

(1)

(3)

(3)

(4)

(2)

I-TCP Advantages Transparent

• Hosts in the fixed part do not notice the wireless part• No change to the TCP protocol of the host connected to

the wired Internet• No change to the Internet

Simple• Separate wireless TCP only for one hop between the

foreign agent and the wireless host• Local fast retransmission of packets possible between

the foreign agent and the wireless host Flexibility

• Easy to use different (customized) protocols for wired and wireless networks (i.e., cellular networks)

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I-TCP

DisadvantagesNo End-to-End Semantics

• ACK received by a sender does not necessarily mean that a receiver has received a packet

• Crash at the foreign agent? Larger Buffer Space at FAHigher Latency Possible

• Buffering data within WLAN/APSecurity Issue

• Foreign agent must be a trusted entity

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Snooping TCP

Snooping TCP Foreign agent “snoops” traffic and

buffers packets for fast-retransmission to the mobile host

One TCP connection

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InternetSnooping ACKs

Buffering of data

End-to-End TCP Connection

WLAN/APLocal

Retransmission

H. Balakrishnan, S. Seshan, R. H. Katz, "Improving Reliable Transport and Handoff Performance in Cellular Wireless Networks," ACM Wireless Networks, vol. 1, no. 4, Dec. 1995.

Snooping TCP

Local RetransmissionData transfer TO the mobile host

1) FA buffers data until ACK received from MH2) FA detects packet loss duplicated ACKs or

timeout3) Fast retransmission from buffered data

Data transfer FROM the mobile host1) FA detects packet loss on the wireless link via

sequence numbers2) FA sets NACK to the MH3) MH retransmits data to the receiver

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Snooping TCP Advantages End-to-End Semantic Preserved

• No changes to MH• Extension of TCP within the FA

Easy Handover• Do not need careful handover of the system state as in I-

TCP (Can improve performance if available)• Standard TCP solution if no enhancements

Disadvantages Snooping TCP does not isolate the wireless link as

good as I-TCP Snooping encrypted TCP header (IPSec)? Asymmetric routes?

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Fast Retransmit / Fast Recovery

Forced Fast Retransmit1) MH connect with a new FA2) MH sends dup ACK (on purpose)3) Dup ACK:

• Forces receiver to enter fast retransmit mode• Forces sender not to go into slow start after

new link connection

R. Caceres and L. Iftode, "Improving the Performance of Reliable Transport Protocols in Mobile Computing Environments," IEEE Journal on Selected Areas in Communications, vol. 13, no. 5, June 1995. 16

Fast Retransmit / Fast Recovery

Advantages Throughput enhancement with little

modification to the standard TCP• No slow start after handover

DisadvantagesOnly handles loss due to handoverNot transparent

• Mix of IP (host) and TCP (session)Cwnd still reduces upon handover (due

to dup ACKs)

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Mobile TCP

Mobile TCP “Splits” TCP connections (as in I-TCP)

• Wireless TCP, Standard TCP• Supervisory Host (SH) to detect disconnection

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Internet

a) Wireless TCP b) Standard TCP

WLAN/AP

Kevin Brown and Suresh Singh, "M-TCP: TCP for Mobile Cellular Networks," ACM Computer Communication Review, Oct. 1999.

Mobile TCP

Supervisory Host (SH) No buffering No local retransmissionMonitors all packets

• (AP withholds the ACK for the last byte) If disconnection detected

1) AP advertises rwnd size to 0: Sender automatically goes into persistent mode

2) Window advertisement received: exit persistent mode

3) RTO and cwnd are same as before the persistent mode

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Mobile TCP Advantages End-to-End Semantics Preserved Preserves cwnd upon handover Easy handover

• No buffering Disadvantages Not Transparent

• Packet loss at the wireless link propagates back to sender

Not good for heavy traffic Level of congestion on the new route is

unknown

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Other Wireless TCP Protocols Time-out Freezing Freeze timeout of TCP upon disconnection until

reconnected Depends on MAC layer information Needs accurate prediction of disconnections

Selective Retransmission TCP ACKs are often cumulative NACK is more complex and requires larger buffer

at the receiver TCP-Peach Satellite Networks

And many more…

K. Leung, V. O. K. Li, "TCP in Wireless Networks: Issues, Approaches, and Challenges," IEEE Communications Surveys and Tutorials, vol. 8, no. 4, pp. 64-79, 2006. 21

Summary Need extra knowledge of the wireless/mobility

conditions to detect loss not due to congestion Congestive loss, random loss, burst loss, packet re-

ordering Open Research Issues Support for End-to-End Semantics?

• Modifying anything on FA contradicts E2E principle Supporting wireless sender and receiver

simultaneously?• Often either sender or receiver, but not both

Integrated solution for all types of wireless problems? Optimization through theoretical basis

Mobile Ad-Hoc Networks WTCP + Mobility + Multihop Link

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