Design and Deployment of a Reliable File Transfer Protocol over Asymmetric Satellite Networks

1
Design and Deployment of a Reliable File
Transfer Protocol over Asymmetric Satellite
Networks

• Anupam Goyal
• Roll no. 01329001
• Guided By
• Dr. Sridhar Iyer

2
Outline of the Talk

• Why Satellite networks for reliable transfer ?
• Reliable Multicast Transfer Strategies
• DEP Satellite Network
• The Problem and the proposed solution
• RTAN
• Deployment Problems
• Field Experiments and results
• Conclusions and Future Work

3
Satellite Networks

• Widely used for audio-video transmissions
• Major disadvantages
• large delay
• higher bit error rates
• limited bandwidth

4
Satellite Networks for Reliable Transfer

• Interesting for reliable data transfer to clients
  dispersed over a large area due to
• Inherent multicast nature
• Long reach

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Reliable Multicast Protocols

• Types based on error recovery responsibility
• Sender-Initiated
• Sender keeps track of all the clients
• Sender Asks for ACKs periodically
• Receiver-Initiated
• Receiver detects packet loss through timers
• Receiver sends NACKs to server
• Tree Based
• ACK Tree of receivers
• Group leaders perform error recovery
• Ring Based
• Token site performs error recovery
• Token passing
• Various implementations MFTP, RMTP, SRM, LBRM,
  MDP etc.

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DEP Satellite Network

• Very Small Aperture Terminal (VSAT) Network
• Topology
• 512 kbps Demand Assigned Multiple Access channel
• 16 kbps Time Division Multiple Access channel

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The DEP Network Setup
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The Requirement

• To send any file reliably over the satellite
  network to all or a particular receiving station.
• Large files of the order of 1 GB videotaped
  lecture
• Small files of the order of KBs tutorials etc.

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The Problem

• The need to have TCP like reliability.
• The need to send files of the order of 1 GB.
• The 16 kbps full duplex reliable channel is too
  small for sending large files in reasonable time.
• The 512 kbps DAMA channel is a multicast channel
  and hence offers no guarantee of reliable data
  delivery.

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The Solution

• Use the 512 kbps half duplex channel for data
  transfer.
• Use the 16 kbps full duplex channel for control
  information exchange with the receivers.
• Design an application level UDP based reliable
  file transfer protocol.

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Logical TDMA point to point channel
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Designed Protocol

• Reliable Transfer over Asymmetric Networks (RTAN)
• Basic Idea of RTAN
• The overall transfer consists of many Transfer
  Cycles
• Each Transfer cycle consists of one Data Cycle
  and many Recovery Cycles
• A fixed number of packets are sent per Data Cycle
  to all the clients.
• All the packets dropped by at least one client
  are remulticast in Recovery Cycles until all the
  clients have received all the packets sent in the
  Data Cycle.

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The Protocol Sequence Graph
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The Server State Transition Graph
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The Client State Transition Graph
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RTAN - The Basic Protocol Server

• Server side processing
• Start ConnectionAcceptorThread. Wait for Client
  connections. Add connecting clients to a global
  Connection List.
• Wait for file transfer request.
• Start KeepAlive thread.
• Start Transfer.
• Transmission setup cycle
• Mark all the clients in the connection list as in
  session.
• Send file Transfer_Start message to all the
  clients on control channel. Information like file
  name, file size etc.
• Wait for ACKs.

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RTAN - The Basic Protocol Server

• Start Transfer Cycle.
• Data Cycle
• Create transfer buffer and index.
• Read N data packets of size M bytes from the
  file.
• Buffer them in the memory.
• Creates a N bit send index, one bit for each
  packet.
• M,N pre-decided.
• Send Data_Start message start packet index, end
  packet index
• Wait for Acks from all clients.
• Multicast N data packets on the DAMA channel.

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RTAN - The Basic Protocol Server

• Recovery Cycle
• Reuest N bit index from the RSs on the control
  channel from all clients.
• Send Send_Index message to all clients.
• Wait for ACKs.
• Perform boolean AND with the server send index.
• Create recovery list from send buffer.
• Remulticast the packets dropped by the clients.
• Again request index from all clients.
• If recovery list empty then transfer cycle ends
• Send Recovery_End message to all clients.
• Wait for ACKs.
• If no more data to send then Transfer ends.
• Send Transfer_End message to all clients.
• Wait for ACKs.

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RTAN - The Basic Protocol Client

• Client Side Processing
• Start the ControlProcessorThread and the
  DataProcessorThread.
• Wait for Server messages.
• If Transfer_Start message, create a session
  object.
• If Data_Start message, create a buffer and N bit
  index.
• If Send_Index message, send index to the server.
• If Recovery_End message, flush buffer to disk.
• If Transfer_End message, delete session object.

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RTAN - The Basic Protocol

• Contents of data packet
• RTAN header
• Actual data bytes
• RTAN header is of 17 bytes
• SessionID (4 bytes)
• FileID (4 bytes)
• Sequence Number (4 bytes)
• FileSize (4 bytes)
• Flags (1 byte)

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RTAN - The Basic Protocol

• Disadvantages
• The protocol is only as fast as the maximum
  faulting RS.
• Limited by the number of RSs that can be
  accommodated on the control channel.

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Deployment of RTAN on DEP Satellite Network

• Server at IIT Bombay
• Client at NCST, Juhu, Mumbai.

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Deployment Problems

• The Router Configuration problem
• Multicast group, multicast port added to router
• The Time To Live problem
• TTL value set to greater than 3
• The Router Table Update problem
• 2-3 seconds of multicast route setup time
• The Link Data rate Synchronization problem
• The server throttled according to the link data
  rate
• The Link Up-Down problem
• Dummy KeepAlive packet were introduced

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Field Experiments

• Achievable data rates
• Granularity of 10 ms
• 24, 30, 37.5, 49.5 and 75 Kilobytes per second
• Error rate of DEP Satellite Network
• Very low 0.5 in good channel conditions
• 2-3 in early morning hours
• Data Packet Size
• DEP Satellite Network MTU 1500 bytes
• Max Application data 1440 bytes
• 17 byte RTAN header

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Experiment 1 Transfer Time vs File Size
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Experiment 2 Transfer Time vs Packets Per
Transfer
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Experiment 3 Transfer Time vs Data Rate
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Experiment 2 Transfer Time vs Packets Per
Transfer for different Data Rates
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Conclusions Drawn from Field Experiments

• Effect of time spent on exchanging control
  information on overall transfer time is greater
  for small files.
• Data rate is the main contributing factor to the
  overall transfer time as against packets per
  transfer.
• Data rates closest to link capacity result in
  maximum throughput.

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Conclusions

• A protocol for reliable transfer over asymmetric
  networks (RTAN) has been proposed.
• Implementation of RTAN for writing a file
  transfer utility for DEP satellite network.
• RTAN Basic implementation works quite well for
  the DEP Satellite Network owing to very low error
  rates.

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Future Work

• To extract a reliable transport layer protocol
  for asymmetric networks
• To optimize the basic protocol implementation
• Version 1.1 Multiple Multicast Per cycle
• Version 1.2 Ignore Unruly Client
• Version 1.3 Regional Division of Clients
• To adapt the protocol to changing DEP network
  topology

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Thank You