Simone Patella, Massimo Mazzoccanti 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 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

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.

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.

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/wasn’t congestion one minute ago says nothing about now.

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”)

More commonly known as Performance Enhancing Proxy (“PEP”) Transport layer relay More commonly known as Performance Enhancing Proxy (“PEP”)

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.

Transport relay (PEP)

The Application Layer Gateway (“AGW”)

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.

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

Application gateway (AGW)

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

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

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

The AGW test bed

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

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.

Test bed results HIGH PRIORITY MSGS With AGW Without AGW Transmitted Messages 2500 Messages delivered in Time 1200 Average Delay 1369.45 ms 5441.99 ms MEDIUM PRIORITY MSGS With AGW Without AGW Transmitted Messages 5000 Messages delivered in Time Average Delay 9879.62 ms 5465.45 ms LOW PRIORITY With AGW Without AGW Transmitted Messages 2500 Messages delivered in Time 1657 Average Delay 19863.17 ms 5480.58 ms

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