An Application Layer Gateway for Air Traffic Management Communication by Satellite

1
An Application Layer Gateway for Air Traffic
Management Communication by Satellite

• Erling Kristiansen
• European Space Agency
• Simone Patella, Massimo Mazzoccanti
• Vitrociset

2
ATM traffic profile

• Short messages
• The majority of messages are 20 to a few hundred
  bytes
• Some longer messages (a few KB)
• Irregular, infrequent message interval
• Inter-message interval seconds to minutes,
  depending on flight phase
• Many different types of messages, each with its
  own pattern

3
ATM transport layer issues

• ATM traffic is inelastic
• Traffic is generated by events
• (Time-triggered messages are also considered
  events)
• ATN TP4 reliable transport was designed for
  elastic traffic (by the way, so was TCP)
• Speed of transmission is driven by the transport
  protocol
• Source is capable of slowing down if the
  transport tells it to
• Reliable transport insists on delivering all
  data, and delivering in sequence.

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ATM transport layer issues

• There is a fundamental incompatibility between
  inelastic sources and elastic transport
• As long as traffic volume is well below network
  capacity, and no significant volume of
  retransmissions take place, all is well
• But if even mild congestion is encountered, all
  traffic is delayed.
• Significant congestion, even for a short time,
  may cause very large delays to all traffic.
  Timeouts may expire, causing unnecessary
  retransmissions, thus increasing congestion
  further.

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ATM transport layer issues

• Congestion control
• ATM traffic to/from any given aircraft is very
  thin
• Infrequent, mostly short messages
• TP4 and TCP congestion control was designed for
  large file transfers
• Feed-back from receiver to sender via ACKs and
  ACK timing
• TP4/TCP congestion control does not work well
  with thin, intermittentt raffic
• Knowing that there was/wasnt congestion one
  minute ago says nothing about now.

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ATM transport layer issues

• In summary 2 problems
• Congestion control is ineffective for the traffic
  pattern
• Inelastic traffic over an elastic transport
  protocol
• Two approaches to mitigate this situation were
  investigated
• Transport relay (PEP)
• Application layer gateway (AGW)

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Transport layer relay

• More commonly known as
• Performance Enhancing Proxy (PEP)

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Transport relay (PEP)

• The PEP is a transport layer proxy
• Breaks the e2e transport into 3 parts
• Ingress network
• Satellite link
• Egress network
• Solves problem 1 the inadequacy of congestion
  control for the traffic profile
• Does not solve problem 2 The incompatibility
  between inelastic traffic and elastic transport.

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Transport relay (PEP)
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The Application Layer Gateway(AGW)
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Congestion will happen

• Unless you have an extremely high
  over-provisioning of bandwidth, you have to
  assume that
• Congestion will happen
• And it will happen when you least want it In an
  unusual operational situation such as massive
  flight re-routing due to bad weather or an
  incident
• You can reduce the incidence rate as much as you
  can afford by providing more bandwidth, but you
  cannot reduce it to zero.
• The only thing you can do when congestion happens
  is to discard messages.
• Randomly or intelligently.
• With e2e reliable transport, there is no way the
  network can discard traffic. Only the sending
  application can.

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Application gateway (AGW)

• The AGW is an application layer message proxy
• The AGW intercepts messages
• Transports the message to the peer AGW at the
  other end of the satellite link
• The peer AGW delivers the message to the
  destination
• The AGW can re-order and discard traffic
  selectively

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Application gateway (AGW)
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Application gateway (AGW)

• AGW functionality
• The AGW builds a queue of messages to be sent
  over the satellite link
• The AGW attempts to build a schedule for
  transmission that meets the CoS/QoS requirements
  for all messages
• If such a schedule cannot be built, congestion is
  present
• In case of congestion, the AGW will discard
  messages according to set rules

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Application gateway (AGW)

• AGW rules may consider such elements as
• Priority
• Time-to-live
• Context
• AGW rules might include such features as
• Try to deliver all within time-to-live (deadline
  scheduling), even if it sometimes means low
  priority goes before high
• High priority before low if both meet deadline
• If a message supersedes another one (e.g. new
  position vs. old position), new goes before old

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Application gateway (AGW)

• Solves both problem 1 and 2
• Drawbacks
• AGW needs to know message formats
• Must be updated if new messages are introduced or
  formats changed
• For some rules, AGW needs to know message context
• Incompatible with end-to-end encryption
• Extra benefits
• May serve as interface between heterogeneous
  technologies
• E.g. ATN in the aircraft, TCP/IP on the ground
• Future proof for future network technologies
• Effectively decouples ground, satellite link,
  on-board network

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The AGW test bed
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Test cases

• 4 types of test were carried out
• Very light load.
• The objective is to verify that the AGW
  interferes only minimally with traffic when no
  congestion is present
• Very heavy load.
• The objective is to verify that the AGW performs
  as designed under heavy congestion. This test is
  not representative of any foreseen operational
  situation
• Operational heavy load situation.
• The traffic load in somewhat below congestion
  most of the time, with short periods of
  congestion. The objective is to show that the AGW
  can improve overall performance significantly
  under light congestion.
• Demonstration in a realistic ATC environment

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Test cases

• The tests were carried out with a mix of 3 types
  of messages.
• CPDLC (Controller-Pilot Data Link Communication).
  These are high-priority, urgent messages
• FLIPCY (Flight Plan Consistency). These were
  considered of medium priority and urgency.
• ADS-C (Automatic Dependent Surveillance
  Contract) reports. These are regular position
  reports. Because the reports are repeated at
  rather short, regular intervals, we considered
  these of low priority.

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Test bed results
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Thank you for your attention