Ground-Based LISP for the
Aeronautical Telecommunications NetworkFrequentisbernhard.haindl@frequentis.comFrequentismanfred.lindner@frequentis.comCisco Systemsrrahman@cisco.comCisco Systemsmportole@cisco.comCisco Systemsvmoreno@cisco.comCisco Systemsfmaino@cisco.comCisco Systemsbvenkata@cisco.com
Internet
LISP Working GroupLISP; deploymentThis 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.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 .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
(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 .AOC: Airline Operational ControlATN/IPS: Aeronautical Telecommunications Network with Internet
Protocol ServicesAC-R: Access Ground RouterA/G-R: Air/Ground RouterG/G-R: Ground/Ground RouterA-R: Airborne RouterA-E: Airborne EndsystemATS-E: ATS EndsystemFor 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 .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.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. Each G/G-R is basically an xTR
which has direct connections only to the terrestrial regions, i.e. no direct
connection to the radio regions.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 .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, but with
the RLOC of the A/G-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 aircraftairborne routers can assume that any ground system is
reachable via any A/G router. Static routing policies can be used
on boardno need for routing/mobility protocols on board.
Routing/mobility is managed on the ground ATN/IPS networkon-board outgoing link selection can be done with simple
static policyseamless support for aircraft mobility and multi-homing with
minimal traffic overhead on the A/G datalinkminimize complexity of ground deploymentground-based LISP can be easily deployed over existing
ATN/IPS ground infrastructureit is based on COTS solutionscan ease IPv4 to IPv6 transition issues provides the reference topology for a
description of the basic operation. A more detailed description of the
basic protocol operation is described in .The following are the steps via which airborne endsystem prefixes
are registered with the LISP mapping system: 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.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.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, but with the RLOC
of the A/G-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 .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): The default route in the ATS region takes the traffic to xTR3
which is also a Ground/Ground Router (G/G-R).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.xTR3 encapsulates the traffic to xTR2 using the RLOC
information in the Map-Reply message.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.AC-R2 sends the traffic to the Airborne Router of the aircraft
and the A-R sends it to the endsystem.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): 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.The default route on AC-R2 points to xTR2 (also an A/G-R), so
the traffic is sent from AC-R2 to xTR2.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.xTR2 encapsulates the traffic to xTR3 using the RLOC
information in the Map-Reply message.xTR3 decapsulates the traffic coming from xTR2, and forwards it
to ATS-E2.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 , 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 .
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. It is only the RTR which needs to subscribe
to all the aircraft EIDs, the XTRs (i.e. the A/G-Rs and G/G-Rs) do not need
to subscribe.The requirements for traffic symmetry are still TBD.Multi-homing support builds on the procedures described in : The Airborne Router (A-R) on an aircraft attaches to multiple
radio regions. As an example, and referring to , the A-R attaches to the LDACS and SATCOM
regions, via AC-R2 and AC-R1 respectively.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.Both xTR1 and xTR2 register the IPv6 EID prefixes with the LISP
mapping system using merge semantic, as specified in section 4.6 of
. 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.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. Both xTRs are notified because they have
both set the merge-request and want-map-notify bits in their respective
Map-Register message.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: The A-R sends a radio advertisement to AC-R1 indicating a better
preference for the SATCOM link.This leads to AC-R1 lowering its quality parameter (e.g. IGP metric)
for the IPv6 EID prefixes.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. 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.The procedure for mobility is derived from .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.RLOC-probing is described in section 6.3.2 of . 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.Solicit-Map-Request is used as described in : xTR3 is still sending traffic to xTR2 since its map-cache has
not been updated yet.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.As specified in ,
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.For LISP control-plane message security, please refer to
. This addresses the control-plane threats
that target EID-to-RLOC mappings, including manipulations of Map-Request and
Map-Reply messages, and malicious ETR EID prefix overclaiming.No IANA considerations.The authors would like to thank Dino Farinacci for his review of the document.Ground Based LISP for Multilink Operation,
https://www.icao.int/safety/acp/ACPWGF/CP WG-I 19/WP06
Ground_Based_LISP 2016-01-14.pdf