Impact of transmission errors on TCP performance (Nitin Vaidya)

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Impact of transmission errorson TCP
performance(Nitin Vaidya)
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Tutorial Outline

• Wireless technologies
• TCP basics
• Impact of transmission errors on TCP performance
• Approaches to improve TCP performance
• Classification
• Discussion of selected approaches

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Random Errors

• If number of errors is small, they may be
  corrected by an error correcting code
• Excessive bit errors result in a packet being
  discarded, possibly before it reaches the
  transport layer

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Random Errors May Cause Fast Retransmit
Example assumes delayed ack - every other packet
ackd
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Random Errors May Cause Fast Retransmit
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Example assumes delayed ack - every other packet
ackd
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Random Errors May Cause Fast Retransmit
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dupack
Duplicate acks are not delayed
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Random Errors May Cause Fast Retransmit
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Duplicate acks
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Random Errors May Cause Fast Retransmit
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3 duplicate acks trigger fast retransmit at sender
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Random Errors May Cause Fast Retransmit

• Fast retransmit results in
• retransmission of lost packet
• reduction in congestion window
• Reducing congestion window in response to errors
  is unnecessary
• Reduction in congestion window reduces the
  throughput

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Sometimes Congestion Response May be Appropriate
in Response to Errors

• On a CDMA channel, errors occur due to
  interference from other user, and due to noise
  Karn99pilc
• Interference due to other users is an indication
  of congestion. If such interference causes
  transmission errors, it is appropriate to reduce
  congestion window
• If noise causes errors, it is not appropriate to
  reduce window
• When a channel is in a bad state for a long
  duration, it might be better to let TCP backoff,
  so that it does not unnecessarily attempt
  retransmissions while the channel remains in the
  bad state Padmanabhan99pilc

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Various Schemes

• Link level mechanisms
• Split connection approach
• TCP-Aware link layer
• TCP-Unaware approximation of TCP-aware link layer
• Explicit notification
• Receiver-based discrimination
• Sender-based discrimination
• For a brief overview, see Dawkins99,Montenegro99

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Split Connection Approach
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Split Connection Approach

• End-to-end TCP connection is broken into one
  connection on the wired part of route and one
  over wireless part of the route
• A single TCP connection split into two TCP
  connections
• if wireless link is not last on route, then more
  than two TCP connections may be needed

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Split Connection Approach

• Connection between wireless host MH and fixed
  host FH goes through base station BS
• FH-MH FH-BS BS-MH

FH
MH
BS
Fixed Host
Base Station
Mobile Host
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Split Connection Approach

• Split connection results in independent flow
  control for the two parts
• Flow/error control protocols, packet size,
  time-outs, may be different for each part

FH
MH
BS
Fixed Host
Base Station
Mobile Host
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Split Connection Approach
Per-TCP connection state
TCP connection
TCP connection
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Split Connection ApproachIndirect TCP
Bakre95,Bakre97

• FH - BS connection Standard TCP
• BS - MH connection Standard TCP

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Split Connection ApproachSelective Repeat
Protocol (SRP) Yavatkar94

• FH - BS connection standard TCP
• BS - FH connection selective repeat protocol on
  top of UDP
• Performance better than Indirect-TCP (I-TCP),
  because wireless portion of the connection can be
  tuned to wireless behavior

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Split Connection Approach Other Variations

• Asymmetric transport protocol (Mobile-TCP)
  Haas97icc
• Low overhead protocol at wireless hosts, and
  higher overhead protocol at wired hosts
• smaller headers used on wireless hop (header
  compression)
• simpler flow control - on/off for MH to BS
  transfer
• MH only does error detection, BS does error
  correction too
• No congestion control over wireless hop

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Split Connection Approach Other Variations

• Mobile-End Transport Protocol Wang98infocom
• Terminate the TCP connection at BS
• TCP connection runs only between BS and FH
• BS pretends to be MH (MHs IP functionality moved
  to BS)
• BS guarantees reliable ordered delivery of
  packets to MH
• BS-MH link can use any arbitrary protocol
  optimized for wireless link
• Idea similar to Yavatkar94

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Split Connection Approach Classification

• Hides transmission errors from sender
• Primary responsibility at base station
• If specialized transport protocol used on
  wireless, then wireless host also needs
  modification

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Split Connection Approach Advantages

• BS-MH connection can be optimized independent of
  FH-BS connection
• Different flow / error control on the two
  connections
• Local recovery of errors
• Faster recovery due to relatively shorter RTT on
  wireless link
• Good performance achievable using appropriate
  BS-MH protocol
• Standard TCP on BS-MH performs poorly when
  multiple packet losses occur per window (timeouts
  can occur on the BS-MH connection, stalling
  during the timeout interval)
• Selective acks improve performance for such cases

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Split Connection Approach Disadvantages

• End-to-end semantics violated
• ack may be delivered to sender, before data
  delivered to the receiver
• May not be a problem for applications that do not
  rely on TCP for the end-to-end semantics

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Split Connection Approach Disadvantages

• BS retains hard state
• BS failure can result in loss of data
  (unreliability)
• If BS fails, packet 40 will be lost
• Because it is ackd to sender, the sender does
  not buffer 40

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Split Connection Approach Disadvantages

• BS retains hard state
• Hand-off latency increases due to state transfer
• Data that has been ackd to sender, must be moved
  to new base station

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Split Connection Approach Disadvantages

• Buffer space needed at BS for each TCP connection
• BS buffers tend to get full, when wireless link
  slower (one window worth of data on wired
  connection could be stored at the base station,
  for each split connection)
• Window on BS-MH connection reduced in response to
  errors
• may not be an issue for wireless links with small
  delay-bw product

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Split Connection Approach Disadvantages

• Extra copying of data at BS
• copying from FH-BS socket buffer to BS-MH socket
  buffer
• increases end-to-end latency
• May not be useful if data and acks traverse
  different paths (both do not go through the base
  station)
• Example data on a satellite wireless hop, acks
  on a dial-up channel

data
FH
MH
ack