Ground-Based LISP for the Aeronautical Telecommunications Network
draft-haindl-ground-lisp-atn-00

Abstract

This document describes the use of the LISP architecture and protocols to address the requirements of the worldwide Aeronautical Telecommunications Network with Internet Protocol Services, as articulated by the International Civil Aviation Organization.

The ground-based LISP overlay provides mobility and multi-homing services to the IPv6 networks hosted on commercial aircrafts, to support Air Traffic Management communications with Air Traffic Controllers and Air Operation Controllers. The proposed architecture doesn't require support for LISP protocol in the airborne routers, and can be easily deployed over existing ground infrastructures.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on March 26, 2018.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Definition of Terms
• 3. Design Overview
• 4. Basic Protocol Operation
• 4.1. Endsystem Registration
• 4.2. Ground to Airborne Traffic Flow
• 4.3. Airborne to Ground Traffic Flow
• 4.4. Default forwarding path
• 4.5. Traffic symmetry
• 5. Multi-Homing and Mobility
• 6. Convergence
• 6.1. Use of RLOC-probing
• 6.2. Use of Solicit-Map-Request
• 6.3. Use of LISP pub-sub
• 7. Security Considerations
• 8. IANA Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

This document describes the use of the LISP [RFC6830] architecture and protocols to address the requirements of the worldwide Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS), as articulated by the International Civil Aviation Organization (ICAO).

ICAO is proposing to replace the existing aeronautical communication services with an IPv6 based infrastructure that supports Air Traffic Management (ATM) between commercial aircrafts, Air Traffic Controllers (ATC) and Air Operation Controllers (AOC).

This document describes how a LISP overlay can be used to offer mobility and multi-homing services to the IPv6 networks hosted on commercial aircrafts without requiring LISP support in the airborne routers. Use of the LISP protocol is limited to the ground-based routers, hence the name "ground-based LISP". The material for this document is derived from [GBL].

2. Definition of Terms

[RFC6830].

• AOC: Airline Operational Control
• ATN/IPS: Aeronautical Telecommunications Network with Internet Protocol Services
• AC-R: Access Ground Router
• A/G-R: Air/Ground Router
• G/G-R: Ground/Ground Router
• A-R: Airborne Router
• A-E: Airborne Endsystem
• ATS-E: ATS Endsystem

For definitions of other terms, notably Map-Register, Map-Request, Map-Reply, Routing Locator (RLOC), Solicit-Map-Request (SMR), Ingress Tunnel Router (ITR), Egress Tunnel Router (ETR), xTR (ITR or ETR), Map-Server (MS), and Map-Resolver (MR) please consult the LISP specification

3. Design Overview

In the ATN/IPS architecture the airborne endsystems hosted on an aircraft are part of an IPV6 network connected to the ground network by one or more Airborne Routers (A-R). A-Rs have multiple radio interfaces that connects them via various radios infrastructures (e.g. SATCOM, LDACS, AeroMACS) to a given radio region, also known as subnetwork, on the ground. Typically an A-R has a corresponding ground based Access Router (AC-R) that terminates the radio protocol with the A-R and provides access services to the ground based portion of the radio network infrastructure. Each radio region is interconnected with the ATN/IPS ground network via an Air-to-Ground router (AG-R).

Similarly, the Air Traffic Controllers and Air Operation Controllers Endsystems (ATS-E and AOC-E) are part of IPv6 networks reachable via one or more Ground-to-Ground Routers (G/G-Rs).

The ATN/IPS ground network infrastructure is the internetworking region located between the A/G routers and the G/G routers.

In the ground-based LISP architecture a LISP overlay is laid over the ATN/IPS internetworking region (that is in the LISP RLOC space) and provides connectivity between endsystems (that are in the LISP EID space) hosted in the aircrafts and in the AOC/ATS regions. The A/G-Rs and the G/G-Rs assume the role of LISP xTRs supported by a LISP mapping system infrastructure.

                                              ,------,
                                ,---------.   : A-E1 :
                              ,'           `./'------'
                             (    AIRCRAFT   )
                              `.  +-----+  ,'\ ,------,
                                `-| A-R |-'   \: A-E2 :
                                  +-----+      '------'
                                //       \\
                               //         \\
                        +---+--+           +-+--+--+
                  .--.-.| AC-R1|'.-.      .| AC-R2 |.-.
                 (      +---+--+    )    ( +-+--+--+   )
                (                __.    (              '.
              ..'SATCOM Region  )      .' LDACS Region  )
             (              .'-'      (              .'-'
              '  +-------+  )          '  +-------+   )
               '-| A/G-R |-'            '-| A/G-R |-''
                 |       |                |       |
                 |  xTR1 |                | xTR2  |
                 +-------+                +-------+
                       \                      /
                        \    .--..--. .--. ../
                         \  (    '           '.--.
                         .-.'  Internetworking   ''   '-------'
                        (          region          )--: MS/MR :
                         (     (RLOC SPACE)    '-''   '-------'
                          ._.'--'._.'.-._.'.-._)
                            /                   \
                        +---+---+               +-+--+--+
                    -.-.| G/G-R |'.            .| G/G-R |.
                   (    |       |  )          ( |       | )
                  (     |  xTR3 |   )        (  | xTR4  |  )
                 (      +---+---+    )      (   +-+--+--+   )
                (                _._.      (               '.
              ..'  ATS Region   )         .'   AOC Region  )
             (              .'-'          (             .'-'
               '--'._.'.    )\            '--'._.'.    )\
                /       '--'  \            /       '--'  \
            '--------'    '--------'   '--------'   '--------'
            : ATS-E1 :    : ATS-E2 :   : AOC-E1 :   : AOC-E2 :
            '--------'    '--------'   '--------'   '--------'

         

Figure 1: ATN/IPS and ground-based LISP overlay

Endsystems in the AOC/ATS regions are mapped in the LISP overlay by the G/G-Rs, that are responsible for the registration of the AOC/ATS endsystems to the LISP mapping system.

Aircrafts will attach to a specific radio region, via the radio interfaces of the A-Rs. How the radio attachment works is specific to each particular radio infrastructure, and out of the scope of this document, see [GBL].

Typically at the end of the attachment phase, the access router (AC-R) corresponding to the A-R, will announce the reachability of the EID prefixes corresponding to the attached aircraft (the announcement is specific to each particular radio infrastructure, and is out of the scope of this document). A/G-Rs in that particular radio region are responsible to detect those announcements, and, since they act as xTRs, register to the LISP mapping systems the corresponding IPv6 EID prefixes on behalf of the A-R.

The EID prefixes registered by the A/G-Rs are then reachable by any of the AOC/ATS Endsystems that are part of the ground based LISP overlay.

The LISP infrastructure is used to support seamless aircraft mobility from one radio network to another, as well as multi-homing attachment of an aircraft to multiple radio networks with use of LISP weight and priorities to load balance traffic directed toward the aircraft.

The rest of this document provides further details on how ground-based LISP is used to address the requirements of the ATN/IPS use cases. The main design goals are:

• minimize added complexity on the aircraft
  • airborne routers can assume that any ground system is reachable via any A/G router. Static routing policies can be used on board
  • no need for routing/mobility protocols on board. Routing/mobility is managed on the ground ATN/IPS network
  • on-board outgoing link selection can be done with simple static policy
• seamless support for aircraft mobility and multi-homing with minimal traffic overhead on the A/G datalink
• minimize complexity of ground deployment
  • ground-based LISP can be easily deployed over existing ATN/IPS ground infrastructure
  • it is based on COTS solutions
  • can ease IPv4 to IPv6 transition issues

4. Basic Protocol Operation

Figure 1 provides the reference topology for a description of the basic operation. A more detailed description of the basic protocol operation is described in [GBL].

4.1. Endsystem Registration

The following are the steps via which airborne endsystem prefixes are registered with the LISP mapping system: [RFC6830].

1. Each Airborne Endsystem (A-E) is assigned an IPv6 address that is the endsystem EID. Each EID includes a Network-ID prefix that comprises (1) an ICAO ID which uniquely identifies the aircraft, and possibly (2) an aircraft network identifier. Airborne devices are grouped in one (and possibly several) IPv6 EID prefixes. As an example an IPv6 EID prefix could be used for all ATC applications located in a safety critical domain of the aircraft network, another IPv6 EID prefix could be used for AOC applications located in a less safety critical domain.
2. After the Airborne Router (A-R) on an aircraft attaches to one radio region, the corresponding Access Router (AC-R) learns the IPv6 EID prefixes belonging to the aircraft. The AC-R also announces reachability of these prefixes in the radio region (subnetwork) e.g. by using an IGP protocol like OSPF. The attachment to a radio includes a preference parameter and a quality parameter, these parameters are used e.g. to calculate the IGP reachability advertisement metric.
3. The Air/Ground Router (A/G-R) in the subnetwork receives the radio region announcements which contain reachablity information for the IPv6 EID prefixes corresponding to the Airborne Endsystems. Since each A/G-R is also an xTR, the A/G-R registers the IPv6 EID prefixes with the LISP MS/MR on behalf of the A-R. The included quality parameter (e.g. IGP metric) is converted to a LISP priority, so that a lower quality metric results in a lower LISP priority value.

Ground based endsystems are part of ground subnetworks where the Ground/Ground Router (G/G-R) is an xTR. Each G/G-R therefore registers the prefixes corresponding to the AOC endsystems and ATS endsystems with the LISP mapping system, as specified in

4.2. Ground to Airborne Traffic Flow

Here is an example of how traffic flows from the ground to the airborne endsystems, when ATS endsystem 1 (ATS-E1) has traffic destined to airborne endsystem 1 (A-E1):

1. The default route in the ATS region takes the traffic to xTR3 which is also a Ground/Ground Router (G/G-R).
2. xTR3 sends a Map-Request message for the address of A-E1 to the LISP mapping system. xTR2 sends a Map-Reply to xTR3 with RLOC set to its address which is reachable from xTR3 via the internetworking region.
3. xTR3 encapsulates the traffic to xTR2 using the RLOC information in the Map-Reply message.
4. xTR2 decapsulates the traffic coming from xTR3. The destination address of the inner packet belongs to A-E1 which has been advertised by the AC-R in the same region. The traffic is therefore forwarded to AC-R2.
5. AC-R2 sends the traffic to the Airborne Router of the aircraft and the A-R sends it to the endsystem.

4.3. Airborne to Ground Traffic Flow

Here is an example of how traffic flows from the airborne endsystems to the ground when airborne endsystem 2 (A-E2) has traffic destined to ATS endsystem 2 (ATS-E2):

1. The default route in the aircraft points to the Airborne Router (A-R). The latter forwards the traffic over the radio link to AC-R2.
2. The default route on AC-R2 points to xTR2 (also an A/G-R), so the traffic is sent from AC-R2 to xTR2.
3. xTR2 sends a Map-Request message for the address of ATS-E2 to the LISP mapping system. xTR3 sends a Map-Reply to xTR2 with RLOC set to its address which is reachable from xTR2 via the internetworking region.
4. xTR2 encapsulates the traffic to xTR3 using the RLOC information in the Map-Reply message.
5. xTR3 decapsulates the traffic coming from xTR2, and forwards it to ATS-E2.

4.4. Default forwarding path

When an xTR is waiting for a Map-Reply for an EID, the xTR does not know how to forward the packets destined to that EID. This means that the first packets for ground-to-air traffic would get dropped until the Map-Reply is received and a map-cache entry is created. However if a device acting as RTR, see [I-D.ermagan-lisp-nat-traversal], has mappings for all EIDs, the xTR could use the RTR as default path for packets which have to be encapsulated. How the RTR gets all the mappings is outside the scope of this document but one example is the use of LISP pub-sub as specified in [I-D.rodrigueznatal-lisp-pubsub]. Note that the RTR does not have to be a new device, the device which has the MS/MR role can also act as RTR.

4.5. Traffic symmetry

There is a requirement for air-to-ground traffic and ground-to-air traffic to follow the same path. As described in Section 4.3, the path for air-to-ground traffic is controlled by the A-R: the A-R decides which radio link to use. The path for ground-to-air traffic is governed by the quality metrics in the radio advertisement from the A-R, this is described in Section 4.2. This means that the responsibility for enforcing traffic symmetry lies on the A-R.

5. Multi-Homing and Mobility

Multi-homing support builds on the procedures described in Section 4:.

1. The Airborne Router (A-R) on an aircraft attaches to multiple radio regions. As an example, and referring to Figure 1, the A-R attaches to the LDACS and SATCOM regions, via AC-R2 and AC-R1 respectively.
2. Through the preference parameter sent to each region, the A-R has control over which path (i.e. radio region) ground to air traffic flows. For example, A-R would indicate preference of the LDACS region by choosing a better preference value for the LDACS region compared to the preference value sent to the SATCOM region.
3. Both xTR1 and xTR2 register the IPv6 EID prefixes with the LISP mapping system using merge semantic, as specified in section 4.6 of [I-D.ietf-lisp-rfc6833bis]. Since the priority used in the LISP registrations is derived from the preference and quality parameters, xTR2 would use a lower priority value than xTR1. In this way the LISP mapping system will favour xTR2 (A/G-R for the LDACS region) over xTR1 (A/G-R for the SATCOM region), as specified by the preference and quality parameters.
4. Upon registration the LISP MS/MR will send Map-Notify messages to both xTR1 and xTR2, to inform that they have reachability to the aircraft's IPv6 EID prefixes.
5. Upstream and downstream traffic flows on the same path, i.e. both use the LDACS region.

With mobility, the aircraft could want to switch traffic from one radio link to another. For example while transiting from an area covered by LDACS to an area covered by SATCOM, the aircraft could desire to switch all traffic from LDACS to SATCOM. For air-to-ground traffic, the A-R has complete control over which radio link to use, and will simply select the SATCOM outgoing interface. For ground-to-air traffic:

1. The A-R sends a radio advertisement to AC-R1 indicating a better preference for the SATCOM link.
2. This leads to AC-R1 lowering its quality parameter (e.g. IGP metric) for the IPv6 EID prefixes.
3. Upon receiving the better preference value, xTR1 registers the IPv6 EID prefixes with the MS/MR, using a lower priority value than what xTR2 had used. Both xTR1 and xTR2 receives Map-Notify messages signaling to xTR2 that xTR1 is now the preferred path toward the aircraft.
4. xTR3 has a map-cache which still points to xTR2, therefore xTR3 still sends traffic via xTR2. xTR2 sends Solicit-Map-Request (SMR) to xTR3 who queries the LISP mapping system again. This results in updating the map-cache on xTR3 which now points to XTR1 so ground-to-air traffic now flows on the SATCOM radio link.

6. Convergence

When traffic is flowing on a radio link and that link goes down, the network has to converge rapidly on the other link available for that aircraft.

For air-to-ground traffic, once the A-R detects the failure it can switch immediatly to the other radio link.

For ground-to-air traffic, when a radio link fails, the corresponding AC-R sends a reachability update that the IPv6 EID prefixes are not reachable anymore. This leads to the A/G-R (also an xTR) in that region to unregister the IPv6 EID prefixes with the MS/MR. This indicates that the xTR in question has no reachability to the EID prefixes. The notification of the failure should reach all relevant xTRs as soon as possible. For example, if the LDACS radio link fails, xTR3 and xTR4 need to learn about the failure so that they stop sending traffic via xTR2 and use xTR1 instead.

In the sub-sections below, we the use of RLOC-probing, Solicit-Map-Request, and LISP pub-sub as alternative mechanisms for link failure notification.

6.1. Use of RLOC-probing

RLOC-probing is described in section 6.3.2 of [RFC6830].

At regular intervals xTR3 sends Map-Request to xTR2 for the aircraft's EID prefixes. When xTR3 detects via RLOC-probing that it can not use xTR2 anymore, it sends a Map-Request for the aircraft's EID prefixes. The corresponding Map-Reply indicates that xTR1 should now be used. The map-cache on xTR3 is updated and air-to-ground traffic now goes through xTR1 to use the SATCOM radio link to the aircraft.

The disadvantage of RLOC-probing is that fast detection becomes more difficult when the number of EID prefixes is large.

6.2. Use of Solicit-Map-Request

Solicit-Map-Request is used as described in Section 5:

1. xTR3 is still sending traffic to xTR2 since its map-cache has not been updated yet.
2. Upon detecting that the link is down, and receiving data plane traffic from the ground network, xTR2 sends an SMR to xTR3 that sends a Map-Request to update its map-cache. The corresponding Map-Reply indicates that xTR1 should now be used.

The disadvantage of this approach is that the traffic is delayed pending control-plane resolution. This method also depends on data traffic being continuous, in many cases data traffic may be sporadic, leading to very slow convergence.

6.3. Use of LISP pub-sub

As specified in [I-D.rodrigueznatal-lisp-pubsub], ITRs can subscribe to changes in the LISP mapping system. So if all ITRs subscribe to the EID prefixes for which they have traffic, the ITRs will be notified when there is mapping change.

In the example where the LDACS radio link fails, when xTR2 unregisters the EID prefixes with the MS/MR, xTR3 would be notified via LISP pub-sub (assuming xTR3 has a map-cache entry for these EID prefixes).

This mechanism provides the fastest convergence at the cost of more state in the LISP mapping system.

7. Security Considerations

For LISP control-plane message security, please refer to [I-D.ietf-lisp-sec].

8. IANA Considerations

No IANA considerations.

9. Acknowledgements

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses