Voice Over IP RFC 3714

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Voice Over IP RFC 3714

• Presented By

Jaya Ravindran
1st August 2005
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Voice Over IP RFC 3714
Overview

• Introduction
• End to end congestion control for best effort
  voice traffic In the internet
• Concerns
• Fairness
• User Quality
• Congestion Collapse
• Current Efforts
• Recommendations

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Voice Over IP RFC 3714
Motivation Why ?
Callee
Caller
Best Effort
Qos Employed
Qos Employed
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Voice Over IP RFC 3714
Which is the bottleneck ?
Access Links
Access Links
Last Hop
Host
Bottleneck
Avoid congestion collapse along the entire path
including the access links.
Result ! - Bandwidth wasted Possible Measures

Terminate ? Suspend ?
On Persistent high packet drop rate
As VOIP requires a Minimum sending rate
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Voice Over IP RFC 3714
Persistent packet drop rate A problem

• Congestion collapse Important In VoIP
• Congestion collapse Occur mostly for flows that
  traverse multiple congested links.
• For a VoIP network Both ends of the VoIP call
  may be on a congested broadband connection, such
  as DSL, Backbone or Transoceanic link.
• User Quality

Drop occurs randomly, so nobody gets benefited
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Rule of thump

• When packet loss rate gt 20
• Audio quality of VoIP is degraded beyond
  usefulness.
• TCP can compete leading to jitter. (1500 packet
  size)
• Better to terminate.

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Voice Over IP RFC 3714
Fairness

• Consider
• Multiple VoIP sources.
• Multiple TCP sources.
• Bottom line VoIP crowds out TCP

Possible Solutions

• Call Rejection Does not exist for best effort
• End to end congestion control

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Voice Over IP RFC 3714
Current efforts in the IETF

• An upgrade of RTP specification
• TFRC
• DCCP
• Adaptive Rate Audio Codecs
• Differentiated Services Related Topics

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Voice Over IP RFC 3714
RTP RFC 3551

• If best-effort service is being used, RTP
  receivers SHOULD monitor packet loss to ensure
  that the packet loss rate is within acceptable
  parameters. Packet loss is considered acceptable
  if a TCP flow across the same network path and
  experiencing the same network conditions would
  achieve an average throughput, measured on a
  reasonable timescale, that is not less than the
  RTP flow is achieving.
• In essence, this requirement states that it is
  not acceptable to deploy an application (using
  RTP or any other transport protocol) on the
  best-effort Internet which consumes bandwidth
  arbitrarily and does not compete fairly with TCP
  within an order of magnitude. (Timescale
  Throughput)

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Voice Over IP RFC 3714
TFRC RFC 3267 - Equation based congestion
control

• TFRC has a much lower variation of throughput
  over time compared with TCP.
• Suitable for applications such as telephony or
  streaming media where a relatively smooth sending
  rate is of importance.
• TFRC works for fixed packet size and vary their
  sending rate in packets per second.
• Some audio application
• - Need fixed interval of time between packets.
• - Can vary their packet size instead of packet
  rate.
• TFRC-PS (TFRC Packet Size)
• - Meets the above requirement.( Fixed
  PPS,Variable
• Payload Size)
• - Hence, can be effective for congestion
  control.
• - Can be used in a transport protocol such as
  RTP.

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Voice Over IP RFC 3714
DCCP RFC 3267 - Datagram Congestion Control
Protocol

• For unreliable Flows.
• Applications can specify
• TCP like
• TFRC like services.
• Congestion Control Identifiers
• CCID2 TCP like
• CCID3 TFRC like
• CCID4 TFRC-PS - future

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Voice Over IP RFC 3714
Adaptive Rate Audio Codec

• Fixed codecs not helpful.
• Low/Adaptive rate preferable.

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Voice Over IP RFC 3714
Adaptive Rate Audio Codecs

• Varies its encoded frame size. (Between 11 32)
• Each frame size is 20 ms of speech.
• RTP payload format allows multiple AMR frames
  into the same packet.
• Reduces transmission rate during silence
  periods.
• Has employed FCE and frame interleaving.
• Along with TFRC-PS, can provide effective
  congestion control mechanism.
• Highly experimented.

• Improves the scalability of VoIP or TCP sharing a
  congested link.

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Voice Over IP RFC 3714
Assessing Minimum Acceptable sending Rate

• For an AMR application with minimum rate M
• Is M acceptable ?
• Is the limitation in PPS or BPS ?
• For a persistent packet drop D when should the
  application terminate ?
• Consider TCP having the same drop as VoIP
• Sending rate of VoIP gtgt TCP, terminate.
• Reasons
• Avoiding Congestion Collapse.
• Fairness for TCP like traffic.
• Minimum service not achieved.

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Voice Over IP RFC 3714
Recommendations

• Fixed rate codec Ability to specify minimum bit
  throughput.
• Adaptive variable bit rate codec, supporting VoIP
  on shared internet.
• ECN to detect congestion.
• Ensure receiving application is capable of
  getting this information from lower layer.

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Voice Over IP RFC 3714
Thank You
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