IETF Working Group

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IETF Working Group
CSCI 344 Spring 1998
Presentation
Beth Johnson TCP over Satellite (tcpsat)
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TCP over Satellite

• ( tcpsat)

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General Description

• GOAL - To create an informational RFC
  describing the issues affecting TCP
  throughput over satellite links.
• ISSUES
• - domains for each issue
• - network topology
• - satellite orbit
• LEO- ( Low Earth Orbit )
• MEO-( Medium Earth Orbit )
• GSO-( Geostationary Orbit )
• - link rates
• - fixing protocol
• - fixing implementation

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Scope of working group

• Transport layer issues affecting TCP over
  Satellite
• Existing TCP options
• Compliant implementation with known improved
  performance over satellite links
• Recommendations of well understood protocol
  changes
• Identification of protocol changes that are
  potentially promising

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URL and Mailing List
URLs http//www.ieft.org/html.charters/tcpsat-ch
arter.html http//tcpsat.lerc.nasa.gov/tcpsat/ M
ailing List General Discussiontcp-over-satellit
e_at_listserv.trw.com To Subscribe
majordomo_at_listserve.trw.com In Body subscribe
tcp-over-satellite
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Enhancing TCP over Satellite Channels using
Standard Mechanisms

• Satellite channel characteristics have an effect
  on the way transport protocols (ex. TCP) behave.
• When protocols such as TCP perform poorly,
  channel utilization is low.
• Improving TCP in the satellite environment

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Satellite Characteristics

• Delay in the delivery of a message over a
  satellite link due to finite speed of light and
  the altitude of communications satellites
• Many communications satellites are located at GSO
  at an altitude of approximately 36,000 km.
• -orbit period is the same as Earths
• -ground station can always see
• -one round-trip (RTT) would be about 558 ms
• Other orbits of communications satellites at LEO
  and MEO
• -use constellations of satellites for
  constant coverage
• -propagation delay from several milliseconds
  to 80 ms

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Fundamental Characteristics

• Noise The strength of a radio signal falls in
  proportion to the square of the distance
  traveled.
• Bandwidth The radio spectrum is a limited
  natural resource.
• - bandwidth available to satellite
  systems is limited

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Advantages of Satellite

• Natural broadcast capability
• Multicast applications
• Reach geographically remote areas
• Reach mobile users

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Disadvantages of Satellites

• Long feedback loop
• Large delay bandwidth product (DBP)
• - defines the amount of data a protocol
  should have in flight
• Transmission errors
• Asymmetric use
• Variable round trip times
• Intermittent connectivity ( in non-GSO)

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Two non-TCP Mechanisms

• Path MTU Discovery
• - used to determine the maximum packet size
  a connection can use on a given network path
  without being subjected to IP packet
  fragmentation
• - disadvantage is that it may cause a long
  pause before TCP is able to start sending data
• Forward Error Correction (FEC)
• - should be used to bring the performance
  of the link to at least fiber quality

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Standard TCP Mechanisms

• Congestion Control algorithms
• - slow start - initializing the congestion
  window to one segment waits for
  corresponding acknowledgement (ACK)
• - congestion avoidance
• - fast retransmit
• - fast recovery

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Recommendations for TCP over Satellite

• Path-MTU Discovery
• FEC
• Fast Retransmit
• Fast Recovery
• TCP Large Window
• - window scaling
• - PAWS - RTTM
• TCP SACKS

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Requirements for TCP over Satellite

• Slow Start
• Congestion Avoidance

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Ongoing TCP Research Related to Satellites

• Outlines mechanisms that may help the TCP better
  utilize the bandwidth provided by long-delay
  satellite environments

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Satellite Architectures

• Asymmetric satellite networks
• Satellite link as last hop
• Hybrid satellite networks
• Point-to-point satellite networks
• Point-to-multipoint satellite networks
• Multiple satellite hops

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Connection Setup

• TCP uses a three-way handshake to setup a
  connection between two hosts
• T/TCP bypasses the three-way handshake sends data
  and connection setup information
• T/TCP requires changes in the TCP stacks of both
  the sender and receiver

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Slow Start

• used to gradually increase the size of TCPs
  sliding window
• Larger Initial Window
• - initial window be more than a single
  segment
• - triggers more ACKs opening window more
  rapidly
• - requires changes to the sender TCP stack
• Byte Counting
• - window increases based on the number of
  previously unacknowledged bytes ACKed,
  rather than on the number of ACKs
  received
• - leads to slightly larger line-rate bursts
  of segments

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Slow Start cont.

• -requires changes to the senders TCP stack
• Terminating Slow Start
• - use the packet-pai algorithm to determine
  a more appropriate value for sstresh
• - requires changes to the senders TCP stack

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Spoofing

• break a TCP connection between a client and a
  server into two parts
• - client and its gateway router over
  satellite/wireless link
• - gateway router and server over
  Internet/wired link
• gateway breaks incoming TCP connections in two by
  acting on clients behalf
• allows the server to complete the transfer
  without delays of the satellite
• allows the gateway to use more appropriate
  version of TCP over the satellite hop
• requires modifications to the gateway routers

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Multiple Data Connections

• Start transmission uses an effective window of N
  segments rather than a single segment
• Transfer increases the window by Nsegments per
  RTT rather than one segment
• Larger over all window size
• Overall window decrease in the face of dropped
  segments is reduced when using N parallel
  connections
• The use of multiple parallel connections in a
  shared network, such as the Internet may lead to
  congestive collapse

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ACK Spacing

• Bursts can be spread over time by making a
  gateway separate ACKs by at least two segments
  between ACKs
• Allow the sender to transmit at the correct rate
  and thus avoid dropped segments
• Implemented at the router

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TCP Header Compression

• Replaces the data in the TCP and IP headers that
  remains constant, changes slowly, or changes in a
  predictable manner
• The sender first sends a full TCP header
  including in it a connection number that the
  sender will use to reference the connection
• Receiver stores the full header and uses it as a
  template, filling in some fields from the limited
  information contained in later compressed headers
• Requires changes at both the sender and receiving
  ends

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42nd IETF Meeting

• Los Angles, CA
• March 30 - April 3
• TCP over Satellite meets
• Tuesday, March 31
• 900 - 1115