Internet Quality-of-Service (QoS)

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Internet Quality-of-Service (QoS)

• Henning Schulzrinne
• Columbia University
• Fall 2003

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Quality of Service

• Motivation
• Service availability
• Elementary queueing theory
• Traffic characterization control
• Integrated services (RSVP, NSIS)
• Differentiated services (DiffServ)

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What is quality of service?

• Many applications are sensitive to the effects of
  delay ( jitter) and packet loss
• may have floor below which utility drops to
  zero
• The existing Internet architecture provides a
  best effort service.
• All traffic is treated equally (generally, FIFO
  queuing)
• No mechanism for distinguishing between delay
  sensitive and best effort traffic
• Original IP architecture (IPv4) has TOS
  (type-of-service byte) in packet header
• RFC 795 defined multiple axes (delay,
  throughput, reliability)
• rarely used outside some (rumor) military
  networks

utility ()
bandwidth
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Motivation

• QoS ? service availability
• not good enough if all but 2 minutes of my phone
  call sound perfect
• Support mission-critical applications that cant
  tolerate disruption
• VoIP
• VPNs (LAN emulation)
• high-availability computing
• Charge more for business applications vs.
  consumer applications

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Service availability

• Users do not care about QoS
• at least not about packet loss, jitter, delay
• rather, its service availability ? how likely is
  it that I can place a call and not get
  interrupted?
• availability MTBF / (MTBF MTTR)
• MTBF mean time between failures
• MTTR mean time to repair
• availability successful calls / first call
  attempts
• equipment availability 99.999 (5 nines) ? 5
  minutes/year
• ATT (2003)
• Sprint IP frame relay SLA 99.5

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Availability PSTN metrics

• PSTN metrics (Worldbank study)
• fault rate
• should be less than 0.2 per main line
• fault clearance ( MTTR)
• next business day
• call completion rate
• during network busy hour
• varies from about 60 - 75
• dial tone delay

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Example PSTN statistics
Source Worldbank
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Measurement setup
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Measurement setup

• Active measurements
• call duration 3 or 7 minutes
• UDP packets
• 36 bytes alternating with 72 bytes (FEC)
• 40 ms spacing
• September 10 to December 6, 2002
• 13,500 call hours

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Call success probability

• 62,027 calls succeeded, 292 failed ? 99.53
  availability
• roughly constant across I2, I2, commercial ISPs

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Overall network loss

• PSTN once connected, call usually of good
  quality
• exception mobile phones
• compute periods of time below loss threshold
• 5 causes degradation for many codecs
• others acceptable till 20

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Network outages

• sustained packet losses
• arbitrarily defined at 8 packets
• far beyond any recoverable loss (FEC,
  interpolation)
• 23 outages
• make up significant part of 0.25 unavailability
• symmetric A?B ?? B?A?
• spatially correlated A?B ? ? A?X?
• not correlated across networks (e.g., I2 and
  commercial)

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Network outages
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Network outages
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Outage-induced call abortion probability

• Long interruption ? user likely to abandon call
• from E.855 survey Pholding e-t/17.26 (t in
  seconds)
• ? half the users will abandon call after 12s
• 2,566 have at least one outage
• 946 of 2,566 expected to be dropped ? 1.53 of
  all calls

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Conclusions from measurement

• Availability in space is (mostly) solved ?
  availability in time restricts usability for new
  applications
• initial investigation into service availability
  for VoIP
• need to define metrics for, say, web access
• unify packet loss and no Internet dial tone
• far less than 5 nines
• working on identifying fault sources and
  locations
• looking for additional measurement sites

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Whats next?

• Existing SLAs are mostly useless
• too many exceptions
• wrong time scales month vs. minutes
• no guarantees for interconnects
• Existing measurements similarly dubious
• Limited ability to learn from mistakes
• what are the primary causes of service
  unavailability?
• what can I do to protect myself multi-homing
  via same fiber? diverse access mechanisms?
• Consumers of services have no good ways to
  compare service availability
• only some very large customers may get access to
  carrier-internal data
• Thus, market failure
• Need published metrics
• similar to switch availability reporting

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What's hard to scale (and not)

• Signaling does not have be hard
• one message, on a reliable peering channel or IP
  router alert option
• NSIS effort in the IETF?
• YESSIR RTCP-based signaling
• 700 MHz Celeron processor
• 10,000 flow setups/second ? 300,000 softstate
  flows
• If scaling matters, sink-tree based reservation
  (BGRP)

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Diversity is good

• Unlike routing, no need for single signaling
  protocol
• multicast is much harder
• dumb end devices
• edge "pop-up" ? only show up in edge nodes

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AAA

• Signaling can easily be done in ASIC (no harder
  than IP), but
• need cryptographic verification of request
• need interface to Authentication, Authorization,
  Accounting (AAA)
• cross-domain authentication ? hard, but 3G
  networks will do it anyway
• easier if both sides ask their own access router
• see also iPass for dial-up, OSP (open settlement
  protocol)

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AAA example
reserves for both directions
Internet
AR1
AR2
source
destination
signs request
Cell phone model both sides pay
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Reservation scaling

• Example every long-distance call in the US uses
  VoIP with per-flow resource reservation
• 2000 567.4 billion minutes _at_ 10 minutes each ?
  1,800 calls/second
• single mySQL server can sustain 5002,000
  queriesupdates/second

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Business models don't work

• Most of the time, "tin" service is no worse than
  "platinum" service
• can't impress others with platinum AmEx card
• no frequent flyer bonuses
• ? everybody switches only when the network is in
  bad shape

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Resource control reservation
Application
Tspec
Y/N
Reservation Protocol
Admission Control
Routing Protocols DBs
Traffic Control DB
Packet Scheduler
Classifier route selection
Data
USC EE-S 555
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RED (Random Early Detection)

• TCP synchronization effect ? during overload,
  many connections lose packets and go into
  slowstart
• RED start dropping based on average queue
  occupancy (vs. instantaneous queue occupancy)
• Parameter setting critical and non-trivial
• See also RFC 2309

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ECN (Explicit Congestion Notification)

• Extension of RED mark instead of drop
• RFC 2481 (A Proposal to add Explicit Congestion
  Notification (ECN) to IP)
• IP TOS6 bit indicates congestion ECN
• IP TOS7 bit indicates support for mechanism
• Needs cooperation of TCP (or similar protocols)
• TCP should act almost as if packet was dropped
• ½ congestion window
• but dont do slow-start

ECT1 ECN0
ECT1 ECN1
TCP ACK ECN echo
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Next steps in signaling (NSIS)

• RSVP not widely used for resource reservation
• but is used for MPLS path setup
• design heavily biased by multicast needs
• marginal and after-the-fact security
• limited support for IP mobility
• Thus, IETF NSIS working group developing new
  framework for general state management protocol
• resource reservation
• NAT and firewall control
• traffic and QoS measurement
• MPLS and lambda path setup
• Split into two components
• NSLP services
• NTLP transport

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NSIS

• On-path vs. off-path
• off-path ? bandwidth brokers
• Discovery of next NTLP or NSLP hop
• use router alert option

QoS
NAT/FW
measure
NTLP
SCTP
UDP
TCP
SCTP