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An Application Layer Gateway for Air Traffic Management Communication by Satellite. Erling Kristiansen European Space Agency Simone Patella, Massimo Mazzoccanti Vitrociset. ATM traffic profile. Short messages The majority of messages are ~20 to a few hundred bytes - PowerPoint PPT Presentation
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ICSSC 2008 San Diego 1
ATM Application Layer Gateway
An Application Layer Gateway for Air Traffic Management Communication by Satellite
Erling KristiansenEuropean Space Agency
Simone Patella, Massimo Mazzoccanti Vitrociset
ICSSC 2008 San Diego 2
ATM Application Layer GatewayATM 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
ICSSC 2008 San Diego 3
ATM Application Layer GatewayATM 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.
ICSSC 2008 San Diego 4
ATM Application Layer GatewayATM 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.
ICSSC 2008 San Diego 5
ATM Application Layer GatewayATM 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/wasn’t congestion one minute ago says nothing about now.
ICSSC 2008 San Diego 6
ATM Application Layer GatewayATM transport layer issues
In summary: 2 problems:
1. Congestion control is ineffective for the traffic pattern
2. Inelastic traffic over an elastic transport protocol
– Two approaches to mitigate this situation were investigated:– Transport relay (“PEP”)
– Application layer gateway (“AGW”)
ICSSC 2008 San Diego 7
ATM Application Layer Gateway
Transport layer relay
More commonly known as
Performance Enhancing Proxy (“PEP”)
ICSSC 2008 San Diego 8
ATM Application Layer GatewayTransport 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.
ICSSC 2008 San Diego 9
ATM Application Layer GatewayTransport relay (PEP)
ICSSC 2008 San Diego 10
ATM Application Layer Gateway
The Application Layer Gateway(“AGW”)
ICSSC 2008 San Diego 11
ATM Application Layer GatewayCongestion 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.
ICSSC 2008 San Diego 12
ATM Application Layer GatewayApplication 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
ICSSC 2008 San Diego 13
ATM Application Layer GatewayApplication gateway (AGW)
ICSSC 2008 San Diego 14
ATM Application Layer GatewayApplication 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
ICSSC 2008 San Diego 15
ATM Application Layer GatewayApplication 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
ICSSC 2008 San Diego 16
ATM Application Layer GatewayApplication 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
ICSSC 2008 San Diego 17
ATM Application Layer GatewayThe AGW test bed
NEWNETWORK
COMPONENTS
Simulated by DummyNet
Ground DLP
Aircraft DLP
TCPLINK 2
TCPLINK 2
Aircraft AGW
Ground AGW
TCPLINK 3
TCPLINK 1
SIMULATOR
FDP
Load Generator
Load Receiver
ICSSC 2008 San Diego 18
ATM Application Layer GatewayTest 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
ICSSC 2008 San Diego 19
ATM Application Layer GatewayTest 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.
ICSSC 2008 San Diego 20
ATM Application Layer GatewayTest bed results
HIGH PRIORITY MSGS With AGW Without AGW
Transmitted Messages 2500 2500
Messages delivered in Time 2500 1200
Average Delay 1369.45 ms 5441.99 ms
MEDIUM PRIORITY MSGS
With AGW Without AGW
Transmitted Messages 5000 5000
Messages delivered in Time 5000 5000
Average Delay 9879.62 ms 5465.45 ms
LOW PRIORITY With AGW Without AGW
Transmitted Messages 2500 2500
Messages delivered in Time 1657 2500
Average Delay 19863.17 ms 5480.58 ms
ICSSC 2008 San Diego 21
ATM Application Layer Gateway
Thank you for your attention