Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720, February 12, 2019
rfc6179, rfc6706 (if
approved)
Intended status: Standards Track
Expires: August 16, 2019
Asymmetric Extended Route Optimization (AERO)
draft-templin-intarea-6706bis-04.txt
Abstract
This document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). Nodes attached
to AERO links can exchange packets via trusted intermediate routers
that provide forwarding services to reach off-link destinations and
route optimization services for improved performance. AERO provides
an IPv6 link-local address format that supports operation of the IPv6
Neighbor Discovery (ND) protocol and links ND to IP forwarding.
Dynamic link selection, mobility management, quality of service (QoS)
signaling and route optimization are naturally supported through
dynamic neighbor cache updates, while IPv6 Prefix Delegation (PD) is
supported by network services such as the Dynamic Host Configuration
Protocol for IPv6 (DHCPv6). AERO is a widely-applicable tunneling
solution especially well-suited to aviation services, mobile Virtual
Private Networks (VPNs) and other applications as described in this
document.
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 August 16, 2019.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 7
3.1. AERO Link Reference Model . . . . . . . . . . . . . . . . 8
3.2. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 9
3.3. AERO Routing System . . . . . . . . . . . . . . . . . . . 10
3.4. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 12
3.5. AERO Interface Characteristics . . . . . . . . . . . . . 14
3.6. AERO Interface Initialization . . . . . . . . . . . . . . 16
3.6.1. AERO Relay Behavior . . . . . . . . . . . . . . . . . 16
3.6.2. AERO Server Behavior . . . . . . . . . . . . . . . . 17
3.6.3. AERO Client Behavior . . . . . . . . . . . . . . . . 17
3.6.4. AERO Proxy Behavior . . . . . . . . . . . . . . . . . 18
3.7. AERO Interface Neighbor Cache Maintenance . . . . . . . . 18
3.8. AERO Interface Forwarding Algorithm . . . . . . . . . . . 20
3.8.1. Client Forwarding Algorithm . . . . . . . . . . . . . 21
3.8.2. Proxy Forwarding Algorithm . . . . . . . . . . . . . 21
3.8.3. Server Forwarding Algorithm . . . . . . . . . . . . . 21
3.8.4. Relay Forwarding Algorithm . . . . . . . . . . . . . 22
3.8.5. Processing Return Packets . . . . . . . . . . . . . . 23
3.9. AERO Interface Encapsulation and Re-encapsulation . . . . 23
3.10. AERO Interface Decapsulation . . . . . . . . . . . . . . 24
3.11. AERO Interface Data Origin Authentication . . . . . . . . 24
3.12. AERO Interface Packet Size Issues . . . . . . . . . . . . 25
3.13. AERO Interface Error Handling . . . . . . . . . . . . . . 27
3.14. AERO Router Discovery, Prefix Delegation and
Autoconfiguration . . . . . . . . . . . . . . . . . . . . 30
3.14.1. AERO ND/PD Service Model . . . . . . . . . . . . . . 30
3.14.2. AERO Client Behavior . . . . . . . . . . . . . . . . 31
3.14.3. AERO Server Behavior . . . . . . . . . . . . . . . . 33
3.15. AERO Route Optimization . . . . . . . . . . . . . . . . . 35
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3.15.1. Reference Operational Scenario . . . . . . . . . . . 35
3.15.2. Concept of Operations . . . . . . . . . . . . . . . 37
3.15.3. Sending NS Messages . . . . . . . . . . . . . . . . 37
3.15.4. Re-encapsulating and Relaying the NS . . . . . . . . 38
3.15.5. Processing NSs and Sending NAs . . . . . . . . . . . 39
3.15.6. Processing NAs . . . . . . . . . . . . . . . . . . . 40
3.15.7. Server-Based Route Optimization . . . . . . . . . . 40
3.16. Neighbor Unreachability Detection (NUD) . . . . . . . . . 42
3.17. Mobility Management and Quality of Service (QoS) . . . . 43
3.17.1. Forwarding Packets on Behalf of Departed Clients . . 44
3.17.2. Announcing Link-Layer Address and/or QoS Preference
Changes . . . . . . . . . . . . . . . . . . . . . . 44
3.17.3. Bringing New Links Into Service . . . . . . . . . . 44
3.17.4. Removing Existing Links from Service . . . . . . . . 45
3.17.5. Implicit Mobility Management . . . . . . . . . . . . 45
3.17.6. Moving to a New Server . . . . . . . . . . . . . . . 45
3.18. Multicast Considerations . . . . . . . . . . . . . . . . 46
4. The AERO Proxy . . . . . . . . . . . . . . . . . . . . . . . 46
5. Direct Underlying Interfaces . . . . . . . . . . . . . . . . 48
6. Operation on AERO Links with /64 ASPs . . . . . . . . . . . . 48
7. AERO Adaptations for SEcure Neighbor Discovery (SEND) . . . . 49
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 49
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
10. Security Considerations . . . . . . . . . . . . . . . . . . . 50
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 51
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 52
12.1. Normative References . . . . . . . . . . . . . . . . . . 52
12.2. Informative References . . . . . . . . . . . . . . . . . 54
Appendix A. AERO Alternate Encapsulations . . . . . . . . . . . 59
Appendix B. When to Insert an Encapsulation Fragment Header . . 61
Appendix C. Autoconfiguration for Constrained Platforms . . . . 62
Appendix D. Operational Deployment Alternatives . . . . . . . . 63
D.1. Operation on AERO Links Without DHCPv6 Services . . . . . 63
D.2. Operation on Server-less AERO Links . . . . . . . . . . . 63
D.3. Operation on Client-less AERO Links . . . . . . . . . . . 63
D.4. Manually-Configured AERO Tunnels . . . . . . . . . . . . 64
D.5. Encapsulation Avoidance on Relay-Server Dedicated Links . 64
D.6. Encapsulation Protocol Version Considerations . . . . . . 64
D.7. Extending AERO Links Through Security Gateways . . . . . 64
Appendix E. Change Log . . . . . . . . . . . . . . . . . . . . . 66
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 68
1. Introduction
This document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). The AERO link
can be used for tunneling between neighboring nodes over either IPv6
or IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as
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equivalent links for tunneling. Nodes attached to AERO links can
exchange packets via trusted intermediate routers that provide
forwarding services to reach off-link destinations and route
optimization services for improved performance [RFC5522].
AERO provides an IPv6 link-local address format that supports
operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and
links ND to IP forwarding. Dynamic link selection, mobility
management, quality of service (QoS) signaling and route optimization
are naturally supported through dynamic neighbor cache updates, while
IPv6 Prefix Delegation (PD) is supported by network services such as
the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC8415].
A node's AERO interface can be configured over multiple underlying
interfaces. From the standpoint of ND, AERO interface neighbors
therefore may appear to have multiple link-layer addresses (i.e., the
IP addresses assigned to underlying interfaces). Each link-layer
address is subject to change due to mobility and/or QoS fluctuations,
and link-layer address changes are signaled by ND messaging the same
as for any IPv6 link.
AERO is applicable to a wide variety of use cases. For example, it
can be used to coordinate the Virtual Private Network (VPN) links of
mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
connect into a home enterprise network via public access networks
using services such as OpenVPN [OVPN]. AERO is also applicable to
aviation services for both manned and unmanned aircraft where the
aircraft is treated as a mobile node that can connect an Internet of
Things (IoT). Other applicable use cases are also in scope.
The following sections present the AERO specification.
2. Terminology
The terminology in the normative references applies; the following
terms are defined within the scope of this document:
IPv6 Neighbor Discovery (ND)
an IPv6 control message service for coordinating neighbor
relationships between nodes connected to a common link. The ND
service used by AERO is specified in [RFC4861].
IPv6 Prefix Delegation (PD)
a networking service for delegating IPv6 prefixes to nodes on the
link. The nominal PD service is DHCPv6 [RFC8415], however
alternate services (e.g., based on ND messaging) are also in scope
[I-D.templin-v6ops-pdhost][I-D.templin-6man-dhcpv6-ndopt].
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(native) Internetwork
a connected IP network topology over which the AERO link virtual
overlay is configured and native peer-to-peer communications are
supported. Example Internetworks include the global public
Internet, private enterprise networks, aviation networks, etc.
AERO link
a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
configured over an underlying Internetwork. Nodes on the AERO
link appear as single-hop neighbors from the perspective of the
virtual overlay even though they may be separated by many
underlying Internetwork hops. The AERO mechanisms can also
operate over native link types (e.g., Ethernet, WiFi etc.) when
tunneling is not needed.
AERO interface
a node's attachment to an AERO link. Since the addresses assigned
to an AERO interface are managed for uniqueness, AERO interfaces
do not require Duplicate Address Detection (DAD) and therefore set
the administrative variable 'DupAddrDetectTransmits' to zero
[RFC4862].
AERO address
an IPv6 link-local address constructed as specified in
Section 3.4.
AERO node
a node that is connected to an AERO link.
AERO Client ("Client")
a node that requests PDs from one or more AERO Servers. Following
PD, the Client assigns a Client AERO address to the AERO interface
for use in ND exchanges with other AERO nodes. A node that acts
as an AERO Client on one AERO interface can also act as an AERO
Server on a different AERO interface.
AERO Server ("Server")
a node that configures an AERO interface to provide default
forwarding services for AERO Clients. The Server assigns an
administratively-provisioned AERO address to the AERO interface to
support the operation of the ND/PD services. An AERO Server can
also act as an AERO Relay.
AERO Relay ("Relay")
an IP router that can relay IP packets between AERO Servers and/or
forward IP packets between the AERO link and the native
Internetwork. AERO Relays are standard IP routers that do not
require any AERO-specific functions.
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AERO Proxy ("Proxy")
a node that provides proxying services, e.g., when the Client is
located in a secured internal enclave and the Server is located in
the external Internetwork. The AERO Proxy is a conduit between
the secured enclave and the external Internetwork in the same
manner as for common web proxies, and behaves in a similar fashion
as for ND proxies [RFC4389].
ingress tunnel endpoint (ITE)
an AERO interface endpoint that injects encapsulated packets into
an AERO link.
egress tunnel endpoint (ETE)
an AERO interface endpoint that receives encapsulated packets from
an AERO link.
underlying network
the same as defined for Internetwork.
underlying link
a link that connects an AERO node to the underlying network.
underlying interface
an AERO node's interface point of attachment to an underlying
link.
link-layer address
an IP address assigned to an AERO node's underlying interface.
When UDP encapsulation is used, the UDP port number is also
considered as part of the link-layer address. Packets transmitted
over an AERO interface use link-layer addresses as encapsulation
header source and destination addresses. Destination link-layer
addresses can be either "reachable" or "unreachable" based on
dynamically-changing network conditions.
network layer address
the source or destination address of an encapsulated IP packet.
end user network (EUN)
an internal virtual or external edge IP network that an AERO
Client connects to the rest of the network via the AERO interface.
The Client sees each EUN as a "downstream" network and sees the
AERO interface as its point of attachment to the "upstream"
network.
AERO Service Prefix (ASP)
an IP prefix associated with the AERO link and from which more-
specific AERO Client Prefixes (ACPs) are derived.
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AERO Client Prefix (ACP)
an IP prefix derived from an ASP and delegated to a Client, where
the ACP prefix length must be no shorter than the ASP prefix
length.
base AERO address
the lowest-numbered AERO address from the first ACP delegated to
the Client (see Section 3.4).
secured enclave
a private access network (e.g., a corporate enterprise network,
radio access network, cellular service provider network, etc.)
with secured links and perimeters. Link-layer security services
such as IEEE 802.1X and physical-layer security such as campus
wired LANs prevent unauthorized access from within the enclave,
while border network-layer security services such as firewalls and
proxies prevent unauthorized access from the external
Internetwork.
Potential Router List (PRL)
a geographically and/or topologicallly referenced list of IP
addresses of Servers for the AERO link.
Throughout the document, the simple terms "Client", "Server", "Relay"
and "Proxy" refer to "AERO Client", "AERO Server", "AERO Relay" and
"AERO Proxy", respectively. Capitalization is used to distinguish
these terms from DHCPv6 client/server/relay [RFC8415].
The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including
the names of node variables, messages and protocol constants) is used
throughout this document. Also, the term "IP" is used to generically
refer to either Internet Protocol version, i.e., IPv4 [RFC0791] or
IPv6 [RFC8200].
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]. Lower case
uses of these words are not to be interpreted as carrying RFC2119
significance.
3. Asymmetric Extended Route Optimization (AERO)
The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:
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3.1. AERO Link Reference Model
.-(::::::::)
.-(::::::::::::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +--------+-------+ +--------------+
|AERO Server S1| | AERO Relay R1 | |AERO Server S2|
| Nbr: C1, R1 | | Nbr: S1, S2 | | Nbr: C2, R1 |
| default->R1 | |(X1->S1; X2->S2)| | default->R1 |
| X1->C1 | | ASP A1 | | X2->C2 |
+-------+------+ +--------+-------+ +------+-------+
| AERO Link | |
X---+---+-------------------+-+----------------+---+---X
| | |
+-----+--------+ +----------+------+ +--------+-----+
|AERO Client C1| | AERO Proxy P1 | |AERO Client C2|
| Nbr: S1 | |(Proxy Nbr Cache)| | Nbr: S2 |
| default->S1 | +--------+--------+ | default->S2 |
| ACP X1 | | | ACP X2 |
+------+-------+ .--------+------. +-----+--------+
| (- Proxyed Clients -) |
.-. `---------------' .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. +-------+ +-------+ .-(_ IP )-.
(__ EUN )--|Host H1| |Host H2|--(__ EUN )
`-(______)-' +-------+ +-------+ `-(______)-'
Figure 1: AERO Link Reference Model
Figure 1 presents the AERO link reference model. In this model:
o AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as a
default router for its associated Servers (S1 and S2), and
connects the AERO link to the rest of the Internetwork.
o AERO Servers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.
o AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive AERO Client Prefix (ACP) delegations
X1 and X2, and also act as default routers for their associated
physical or internal virtual EUNs. Simple hosts H1 and H2 attach
to the EUNs served by Clients C1 and C2, respectively.
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o AERO Proxy P1 provides proxy services for AERO Clients in secured
enclaves that cannot associate directly with other AERO link
neighbors.
Each node on the AERO link maintains an AERO interface neighbor cache
and an IP forwarding table the same as for any link. Although the
figure shows a limited deployment, in common operational practice
there may be many additional Relays, Servers, Clients and Proxies.
3.2. AERO Node Types
AERO Relays are standard IP routers that provide default forwarding
services for AERO Servers. Each Relay also peers with Servers and
other Relays in a dynamic routing protocol instance to discover the
list of active ACPs (see Section 3.3). Relays forward packets
between neighbors connected to the same AERO link and also forward
packets between the AERO link and the native Internetwork. Relays
present the AERO link to the native Internetwork as a set of one or
more AERO Service Prefixes (ASPs) and serve as a gateway between the
AERO link and the Internetwork. Relays maintain tunnels with
neighboring Servers, and maintain an IP forwarding table entry for
each AERO Client Prefix (ACP).
AERO Servers provide default forwarding services for AERO Clients.
Each Server also peers with Relays in a dynamic routing protocol
instance to advertise its list of associated ACPs (see Section 3.3).
Servers facilitate PD exchanges with Clients, where each delegated
prefix becomes an ACP taken from an ASP. Servers forward packets
between AERO interface neighbors, and maintain AERO interface
neighbor cache entries for Relays. They also maintain both neighbor
cache entries and IP forwarding table entries for each of their
associated Clients.
AERO Clients act as requesting routers to receive ACPs through PD
exchanges with AERO Servers over the AERO link. Each Client can
associate with a single Server or with multiple Servers, e.g., for
fault tolerance, load balancing, etc. Each IPv6 Client receives at
least a /64 IPv6 ACP, and may receive even shorter prefixes.
Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
singleton IPv4 address), and may receive even shorter prefixes.
Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.
AERO Proxies provide a transparent conduit for AERO Clients connected
to secured enclaves to associate with AERO link Servers. The Client
sends all of its control plane messages to the Server's link-layer
address and the Proxy intercepts them before they leave the secured
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enclave. The Proxy forwards the Client's control and data plane
messages to and from the Client's current Server(s). The Proxy may
also discover a more direct route toward a target destination via
AERO route optimization, in which case future outbound data packets
would be forwarded via the more direct route. The Proxy function is
specified in Section 4.
AERO Relays, Servers and Proxies are critical infrastructure elements
in fixed (i.e., non-mobile) deployments. AERO Relays and Servers
must use public link-layer addresses that do not change and can be
reached from any correspondent in the underlying Internetwork (i.e.,
in the same fashion as for popular Internet services). AERO Clients
may be mobile, and may not have any public link-layer addresses,
e.g., if they are located behind NATs or Proxies.
3.3. AERO Routing System
The AERO routing system comprises a private instance of the Border
Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays
and Servers and does not interact with either the public Internet BGP
routing system or the native Internetwork routing system. Relays
advertise only a small and unchanging set of ASPs to the native
Internetwork routing system instead of the full dynamically changing
set of ACPs.
In a reference deployment, each AERO Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP instance,
and each Server further uses eBGP to peer with one or more Relays but
does not peer with other Servers. All Relays are members of the same
hub AS using a common ASN, and use iBGP to maintain a consistent view
of all active ACPs currently in service.
Each Server maintains a working set of associated ACPs, and
dynamically announces new ACPs and withdraws departed ACPs in its
eBGP updates to Relays. Clients are expected to remain associated
with their current Servers for extended timeframes, however Servers
SHOULD selectively suppress updates for impatient Clients that
repeatedly associate and disassociate with them in order to dampen
routing churn.
Each Relay configures a black-hole route for each of its ASPs. By
black-holing the ASPs, the Relay will maintain forwarding table
entries only for the ACPs that are currently active, and packets
destined to all other ACPs will correctly incur Destination
Unreachable messages due to the black hole route. Relays do not send
eBGP updates for ACPs to Servers, but instead only originate a
default route. In this way, Servers have only partial topology
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knowledge (i.e., they know only about the ACPs of their directly
associated Clients) and they forward all other packets to Relays
which have full topology knowledge.
Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. As of 2015, the
global public Internet BGP routing system manages more than 500K
routes with linear growth and no signs of router resource exhaustion
[BGP]. More recent network emulation studies have also shown that a
single Relay can accommodate at least 1M dynamically changing BGP
routes even on a lightweight virtual machine, i.e., and without
requiring high-end dedicated router hardware.
Therefore, assuming each Relay can carry 1M or more routes, this
means that at least 1M Clients can be serviced by a single set of
Relays. A means of increasing scaling would be to assign a different
set of Relays for each set of ASPs. In that case, each Server still
peers with one or more Relays, but the Server institutes route
filters so that it only sends BGP updates to the specific set of
Relays that aggregate the ASP. For example, if the ASP for the AERO
link is 2001:db8::/32, a first set of Relays could service the ASP
segment 2001:db8::/40, a second set of Relays could service
2001:db8:0100::/40, a third set could service 2001:db8:0200::/40,
etc.
Assuming up to 1K sets of Relays, the AERO routing system can then
accommodate 1B or more ACPs with no additional overhead for Servers
and Relays (for example, it should be possible to service 1B /64 ACPs
taken from a /34 ASP and even more for shorter prefixes). In this
way, each set of Relays services a specific set of ASPs that they
advertise to the native Internetwork routing system, and each Server
configures ASP-specific routes that list the correct set of Relays as
next hops. This arrangement also allows for natural incremental
deployment, and can support small scale initial deployments followed
by dynamic deployment of additional Clients, Servers and Relays
without disturbing the already-deployed base.
In an alternate routing arrangement, each set of Relays could
advertise an aggregated ASP for the link into the native Internetwork
routing system even though each Relay services only smaller segments
of the ASP. In that case, a Relay upon receiving a packet with a
destination address covered by the ASP segment of another Relay can
simply tunnel the packet to the other Relay. The tradeoff then is
the penalty for Relay-to-Relay tunneling compared with reduced
routing information in the native routing system.
A full discussion of the BGP-based routing system used by AERO is
found in [I-D.ietf-rtgwg-atn-bgp]. The system provides for
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Distributed Mobility Management (DMM) per the distributed mobility
anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring].
3.4. AERO Addresses
A Client's AERO address is an IPv6 link-local address with an
interface identifier based on the Client's delegated ACP. Relay and
Server AERO addresses are assigned from the range fe80::/96 and
include an administratively-provisioned value in the lower 32 bits.
For IPv6, Client AERO addresses begin with the prefix fe80::/64 and
include in the interface identifier (i.e., the lower 64 bits) a
64-bit prefix taken from one of the Client's IPv6 ACPs. For example,
if the AERO Client receives the IPv6 ACP:
2001:db8:1000:2000::/56
it constructs its corresponding AERO addresses as:
fe80::2001:db8:1000:2000
fe80::2001:db8:1000:2001
fe80::2001:db8:1000:2002
... etc. ...
fe80::2001:db8:1000:20ff
For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6
address formed from an IPv4 ACP and with a Prefix Length of 96 plus
the ACP prefix length. For example, for the IPv4 ACP 192.0.2.32/28
the IPv4-mapped IPv6 ACP is:
0:0:0:0:0:FFFF:192.0.2.16/124
The Client then constructs its AERO addresses with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as:
fe80::FFFF:192.0.2.16
fe80::FFFF:192.0.2.17
fe80::FFFF:192.0.2.18
... etc. ...
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fe80:FFFF:192.0.2.31
Relay and Server AERO addresses are allocated from the range
fe80::/96, and MUST be managed for uniqueness by the administrative
authority for the link. For interfaces that assign static IPv4
addresses, the lower 32 bits of the AERO address includes the IPv4
address, e.g., for the IPv4 address 192.0.2.1 the corresponding AERO
address is fe80::192.0.2.1. For other interfaces, the lower 32 bits
of the AERO address includes a unique integer value, e.g., fe80::1,
fe80::2, fe80::3, etc. (Note that the address fe80:: is reserved as
the IPv6 link-local Subnet Router Anycast address [RFC4291], and the
address fe80::ffff:ffff is reserved as the prefix solicitation
address; hence, these values are not available for administrative
assignment.)
When the Server delegates ACPs to the Client, the lowest-numbered
AERO address from the first ACP delegation serves as the "base" AERO
address (for example, for the ACP 2001:db8:1000:2000::/56 the base
AERO address is fe80::2001:db8:1000:2000). The Client then assigns
the base AERO address to the AERO interface and uses it for the
purpose of maintaining the neighbor cache entry. The Server likewise
uses the AERO address as its index into the neighbor cache for this
Client.
If the Client has multiple AERO addresses (i.e., when there are
multiple ACPs and/or ACPs with prefix lengths shorter than /64), the
Client originates ND messages using the base AERO address as the
source address and accepts and responds to ND messages destined to
any of its AERO addresses as equivalent to the base AERO address. In
this way, the Client maintains a single neighbor cache entry that may
be indexed by multiple AERO addresses.
AERO addresses that embed an IPv6 prefix can be statelessly
transformed into an IPv6 Subnet Router Anycast address and vice-
versa. For example, for the AERO address fe80::2001:db8:2000:3000
the corresponding Subnet Router Anycast address is
2001:db8:2000:3000::. In the same way, for the IPv6 Subnet Router
Anycast address 2001:db8:1:2:: the corresponding AERO address is
fe80::2001:db8:1:2. In other words, the low-order 64 bits of an AERO
address can be used as the high-order 64 bits of a Subnet Router
Anycast address, and vice-versa.
AERO links additionally reserve an IPv6 prefix to support
encapsulated forwarding of IPv6 ND messages between Servers on the
link. Although any non-link-local IPv6 prefix could be reserved for
this purpose, a Unique Local Address (ULA) prefix [RFC4389] would be
a good candidate since it is not routable outside of the AERO link.
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For example, if the reserved (ULA) prefix is fd00:db8::/64 the AERO
Server Subnet Router Anycast Address is fd00:db8::.
A full discussion of the AERO addressing service is found in
[I-D.templin-6man-aeroaddr].
3.5. AERO Interface Characteristics
AERO interfaces use encapsulation (see: Section 3.9) to exchange
packets with neighbors attached to the AERO link.
AERO interfaces maintain a neighbor cache for tracking per-neighbor
state the same as for any interface. AERO interfaces use ND messages
including Router Solicitation (RS), Router Advertisement (RA),
Neighbor Solicitation (NS), Neighbor Advertisement (NA) and Redirect
for neighbor cache management.
AERO interface ND messages include one or more Source/Target Link-
Layer Address Options (S/TLLAOs) formatted as shown in Figure 2:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID | UDP Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IP Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: AERO Source/Target Link-Layer Address Option (S/TLLAO)
Format
In this format:
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o Type is set to '1' for SLLAO or '2' for TLLAO.
o Length is set to the constant value '5' (i.e., 5 units of 8
octets).
o Reserved is set to the value '0' on transmission and ignored on
receipt.
o Interface ID is set to a 16-bit integer value corresponding to an
underlying interface of the AERO node. Once the node has assigned
an Interface ID to an underlying interface, the assignment must
remain unchanged until the node fully detaches from the AERO link.
o UDP Port Number and IP Address are set to the addresses used by
the AERO node when it sends encapsulated packets over the
specified underlying interface (or to '0' when the addresses are
left unspecified). When UDP is not used as part of the
encapsulation, UDP Port Number is set to '0'. When the
encapsulation IP address family is IPv4, IP Address is formed as
an IPv4-mapped IPv6 address as specified in Section 3.4.
o P(i) is a set of Preferences that correspond to the 64
Differentiated Service Code Point (DSCP) values [RFC2474]. Each
P(i) is set to the value '0' ("disabled"), '1' ("low"), '2'
("medium") or '3' ("high") to indicate a QoS preference level for
packet forwarding purposes.
AERO interfaces may be configured over multiple underlying interface
connections to underlying links. For example, common mobile handheld
devices have both wireless local area network ("WLAN") and cellular
wireless links. These links are typically used "one at a time" with
low-cost WLAN preferred and highly-available cellular wireless as a
standby. In a more complex example, aircraft frequently have many
wireless data link types (e.g. satellite-based, cellular,
terrestrial, air-to-air directional, etc.) with diverse performance
and cost properties.
A Client's underlying interfaces are classified as follows:
o Native interfaces connect to the open Internetwork, and have a
global IP address that is reachable from any open Internetwork
correspondent.
o NATed interfaces connect to a closed network that is separated
from the open Internetwork by a Network Address Translator (NAT).
The NAT does not participate in any AERO control message
signaling, but the AERO Server can issue control messages on
behalf of the Client.
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o VPNed interfaces use security encapsulation over the Internetwork
to a Virtual Private Network (VPN) gateway that also acts as an
AERO Server. As with NATed links, the AERO Server can issue
control messages on behalf of the Client.
o Proxyed interfaces connect to a closed network that is separated
from the open Internetwork by an AERO Proxy. Unlike NATed and
VPNed interfaces, the AERO Proxy can also issue control messages
on behalf of the Client.
o Direct interfaces connect the Client directly to a neighbor
without crossing any networked paths. An example is a line-of-
sight link between a remote pilot and an unmanned aircraft.
If a Client's multiple underlying interfaces are used "one at a time"
(i.e., all other interfaces are in standby mode while one interface
is active), then ND messages include only a single S/TLLAO with
Interface ID set to a constant value. In that case, the Client would
appear to have a single underlying interface but with a dynamically
changing link-layer address.
If the Client has multiple active underlying interfaces, then from
the perspective of ND it would appear to have multiple link-layer
addresses. In that case, ND messages MAY include multiple S/TLLAOs
-- each with an Interface ID that corresponds to a specific
underlying interface of the AERO node.
When the Client includes an S/TLLAO for an underlying interface for
which it is aware that there is a NAT or Proxy on the path to the
Server, or when a node includes an S/TLLAO solely for the purpose of
announcing new QoS preferences, the node sets both UDP Port Number
and IP Address to 0 to indicate that the addresses are unspecified at
the network layer and must instead be derived from the link-layer
encapsulation headers.
When an ND message includes multiple S/TLLAOs, the first S/TLLAO MUST
correspond to the AERO node's underlying interface used to transmit
the message.
3.6. AERO Interface Initialization
3.6.1. AERO Relay Behavior
When a Relay enables an AERO interface, it first assigns an
administratively-provisioned AERO address fe80::ID to the interface.
Each fe80::ID address MUST be unique among all AERO nodes on the
link. The Relay then engages in a dynamic routing protocol session
with one or more Servers and all other Relays on the link (see:
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Section 3.3), and advertises its assigned ASPs into the native
Internetwork. Each Relay subsequently maintains an IP forwarding
table entry for each active ACP covered by its ASP(s).
3.6.2. AERO Server Behavior
When a Server enables an AERO interface, it assigns an
administratively-provisioned AERO address fe80::ID the same as for
Relays. The Server further configures a service to facilitate ND/PD
exchanges with AERO Clients. The Server maintains neighbor cache
entries for one or more Relays on the link, and manages per-Client
neighbor cache entries and IP forwarding table entries based on
control message exchanges. The Server also engages in a dynamic
routing protocol with its neighboring Relays (see: Section 3.3).
When the Server receives an NS/RS message on the AERO interface it
authenticates the message and returns an NA/RA message. (When the
Server receives an unsolicited NA message, it likewise authenticates
the message and processes it locally.) The Server further provides a
simple link-layer conduit between AERO interface neighbors. In
particular, when a packet sent by a source Client arrives on the
Server's AERO interface and is destined to another AERO node, the
Server forwards the packet from within the AERO interface at the link
layer without ever disturbing the network layer.
3.6.3. AERO Client Behavior
When a Client enables an AERO interface, it sends RS messages with
ND/PD parameters over an underlying interface to one or more AERO
Servers, which return RA messages with corresponding PD parameters.
See [I-D.templin-6man-dhcpv6-ndopt] for the types of ND/PD parameters
that can be included in the RS/RA message exchanges.
After the initial ND/PD message exchange, the Client assigns AERO
addresses to the AERO interface based on the delegated prefix(es).
The Client can then register additional underlying interfaces with
the Server by sending a simple RS message (i.e., one with no PD
parameters) over each underlying interface using its base AERO
address as the source network layer address. The Server will update
its neighbor cache entry for the Client and return a simple RA
message.
The Client maintains a neighbor cache entry for each of its Servers
and each of its active correspondent Clients. When the Client
receives ND messages on the AERO interface it updates or creates
neighbor cache entries, including link-layer address and QoS
preferences.
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3.6.4. AERO Proxy Behavior
When a Proxy enables an AERO interface, it maintains per-Client proxy
neighbor cache entries based on control message exchanges. Proxies
forward packets between their associated Clients and each Client's
associated Servers.
When the Proxy receives an RS message from a Client in the secured
enclave, it creates an incomplete proxy neighbor cache entry and
sends a proxyed RS message to a Server selected by the Client while
using its own link-layer address as the source address. When the
Server returns an RA message, the Proxy completes the proxy neighbor
cache entry based on autoconfiguration information in the RA and
sends a proxyed RA to the Client while using its own link-layer
address as the source address. The Client, Server and Proxy will
then have the necessary state for managing the proxy neighbor
association.
3.7. AERO Interface Neighbor Cache Maintenance
Each AERO interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the AERO
link, the same as for any IPv6 interface [RFC4861]. AERO interface
neighbor cache entries are said to be one of "permanent", "static",
"proxy" or "dynamic".
Permanent neighbor cache entries are created through explicit
administrative action; they have no timeout values and remain in
place until explicitly deleted. AERO Relays maintain permanent
neighbor cache entries for their associated Relays and Servers on the
link, and AERO Servers maintain permanent neighbor cache entries for
their associated Relays. Each entry maintains the mapping between
the neighbor's fe80::ID network-layer address and corresponding link-
layer address.
Static neighbor cache entries are created and maintained through ND/
PD exchanges as specified in Section 3.14, and remain in place for
durations bounded by ND/PD lifetimes. AERO Servers maintain static
neighbor cache entries for each of their associated Clients, and AERO
Clients maintain static neighbor cache entries for each of their
associated Servers.
Proxy neighbor cache entries are created and maintained by AERO
Proxies when they process Client/Server ND/PD exchanges, and remain
in place for durations bounded by ND/PD lifetimes. AERO Proxies
maintain proxy neighbor cache entries for each of their associated
Clients.
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Dynamic neighbor cache entries are created or updated based on
receipt of route optimization messages as specified in Section 3.15,
and are garbage-collected when keepalive timers expire. AERO nodes
maintain dynamic neighbor cache entries for each of their active
correspondents with lifetimes based on ND messaging constants.
When a target AERO node receives a valid NS message used for route
optimization, it returns an NA message and also creates or updates a
dynamic neighbor cache entry for the source network-layer and link-
layer addresses. The node then sets a "ReportTime" variable in the
neighbor cache entry to REPORT_TIME seconds. The node resets
ReportTime when it receives a new NS message, and otherwise
decrements ReportTime while no NS messages have been received. It is
RECOMMENDED that REPORT_TIME be set to the default constant value 40
seconds to allow a 10 second window so that the AERO route
optimization procedure can converge before ReportTime decrements
below FORWARD_TIME (see below).
When a source AERO node receives a valid NA message response to its
NS message, it creates or updates a dynamic neighbor cache entry for
the target network-layer and link-layer addresses. The node then
sets a "ForwardTime" variable in the neighbor cache entry to
FORWARD_TIME seconds and uses this value to determine whether packets
can be forwarded directly to the correspondent, i.e., instead of via
a default route. The node resets ForwardTime when it receives a new
NA, and otherwise decrements ForwardTime while no further NA messages
arrive. It is RECOMMENDED that FORWARD_TIME be set to the default
constant value 30 seconds to match the default REACHABLE_TIME value
specified in [RFC4861].
The node also sets a "MaxRetry" variable to MAX_RETRY to limit the
number of keepalives sent when a correspondent may have gone
unreachable. It is RECOMMENDED that MAX_RETRY be set to 3 the same
as described for address resolution in Section 7.3.3 of [RFC4861].
Different values for REPORT_TIME, FORWARD_TIME and MAX_RETRY MAY be
administratively set; however, if different values are chosen, all
nodes on the link MUST consistently configure the same values. Most
importantly, REPORT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO route
optimization procedure to converge.
When there may be a NAT or Proxy between the Client and the Server,
or if the path from the Client to the Server should be tested for
reachability, the Client can send periodic RS messages to the Server
without PD parameters to receive RA replies. The RS/RA messaging
will keep NAT/Proxy state alive and test Server reachability without
disturbing the PD service.
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3.8. AERO Interface Forwarding Algorithm
IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system) or
from the link layer (i.e., from the AERO tunnel virtual link).
Packets that enter the AERO interface from the network layer are
encapsulated and forwarded into the AERO link, i.e., they are
tunneled to an AERO interface neighbor. Packets that enter the AERO
interface from the link layer are either re-admitted into the AERO
link or forwarded to the network layer where they are subject to
either local delivery or IP forwarding. In all cases, the AERO
interface itself MUST NOT decrement the network layer TTL/Hop-count
since its forwarding actions occur below the network layer.
AERO interfaces may have multiple underlying interfaces and/or
neighbor cache entries for neighbors with multiple Interface ID
registrations (see Section 3.5). The AERO node uses each packet's
DSCP value to select an outgoing underlying interface based on the
node's own QoS preferences, and also to select a destination link-
layer address based on the neighbor's underlying interface with the
highest preference. AERO implementations SHOULD allow for QoS
preference values to be modified at runtime through network
management.
AERO nodes MAY include a configuration option that maps transport
layer port numbers to DSCP values, e.g., in case the application is
unable to set the DSCP value in the IP header. In that case, nodes
on the AERO link should maintain a map of port numbers to DSCP
values, e.g., TCP port 22 maps to DSCP value 0x08, TCP port 443 maps
to DSCP value 0x14, UDP port 8060 maps to DSCP value 0x22, etc. As
for QoS preferences, AERO implementations SHOULD allow for the map to
be modified at runtime through network management.
If multiple outgoing interfaces and/or neighbor interfaces have a
preference of "high", the AERO node sends one copy of the packet via
each of the (outgoing / neighbor) interface pairs; otherwise, the
node sends a single copy of the packet via the interface with the
highest preference. AERO nodes keep track of which underlying
interfaces are currently "reachable" or "unreachable", and only use
"reachable" interfaces for forwarding purposes.
The following sections discuss the AERO interface forwarding
algorithms for Clients, Proxies, Servers and Relays. In the
following discussion, a packet's destination address is said to
"match" if it is a non-link-local address with a prefix covered by an
ASP/ACP, or if it is an AERO address that embeds an ACP, or if it is
the same as an administratively-provisioned AERO address.
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3.8.1. Client Forwarding Algorithm
When an IP packet enters a Client's AERO interface from the network
layer the Client searches for a dynamic neighbor cache entry that
matches the destination. If there is a match, the Client uses one or
more "reachable" link-layer addresses in the entry as the link-layer
addresses for encapsulation and admits the packet into the AERO link.
Otherwise, the Client uses the link-layer address in a static
neighbor cache entry for a Server as the encapsulation address
(noting that there may be a Proxy on the path to the real Server).
When an IP packet enters a Client's AERO interface from the link-
layer, if the destination matches one of the Client's ACPs or link-
local addresses the Client decapsulates the packet and delivers it to
the network layer. Otherwise, the Client drops the packet and MAY
return a network-layer ICMP Destination Unreachable message subject
to rate limiting (see: Section 3.13).
3.8.2. Proxy Forwarding Algorithm
When the Proxy receives a packet from a Client within the secured
enclave, the Proxy searches for a dynamic neighbor cache entry that
matches the destination. If there is a match, the Proxy uses one or
more "reachable" link-layer addresses in the entry as the link-layer
addresses for encapsulation and admits the packet into the AERO link.
Otherwise, the Proxy uses the link-layer address for one of the
Client's Servers as the encapsulation address.
When the Proxy receives a packet from an AERO interface neighbor, it
searches for a proxy neighbor cache entry for a Client within the
secured enclave that matches the destination. If there is a match,
the Proxy forwards the packet to the Client. Otherwise, the Proxy
returns the packet to the neighbor, i.e., by reversing the source and
destination link-layer addresses and re-admitting the packet into the
AERO link.
3.8.3. Server Forwarding Algorithm
When an IP packet enters a Server's AERO interface from the network
layer, the Server searches for a static neighbor cache entry for a
Client that matches the destination. If there is a match, the Server
uses one or more link-layer addresses in the entry as the link-layer
addresses for encapsulation and admits the packet into the AERO link.
Otherwise, the Server uses the link-layer address in a permanent
neighbor cache entry for a Relay (selected through longest-prefix
match) as the link-layer address for encapsulation.
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When an IP packet enters a Server's AERO interface from the link
layer, the Server processes the packet according to the network-layer
destination address as follows:
o if the destination matches one of the Server's own addresses the
Server decapsulates the packet and forwards it to the network
layer for local delivery.
o else, if the destination matches a static neighbor cache entry for
a Client the Server first determines whether the neighbor is the
same as the one it received the packet from. If so, the Server
drops the packet silently to avoid looping; otherwise, the Server
uses the neighbor's link-layer address(es) as the destination for
encapsulation and re-admits the packet into the AERO link.
o else, the Server uses the link-layer address in a neighbor cache
entry for a Relay (selected through longest-prefix match) as the
link-layer address for encapsulation.
3.8.4. Relay Forwarding Algorithm
When an IP packet enters a Relay's AERO interface from the network
layer, the Relay searches its IP forwarding table for an ACP entry
that matches the destination the same as for any IP router. If there
is a match, the Relay uses the link-layer address in the
corresponding neighbor cache entry as the link-layer address for
encapsulation and forwards the packet to the AERO neighbor.
Otherwise, the Relay drops the packet and returns a network-layer
ICMP Destination Unreachable message subject to rate limiting (see:
Section 3.13).
When an IP packet enters a Relay's AERO interface from the link-
layer, (i.e., when it receives an encapsulated packet from a Server)
the Relay processes the packet as follows:
o if the destination does not match an ASP, or if the destination
matches one of the Relay's own addresses, the Relay decapsulates
the packet and forwards it to the network layer where it will be
subject to either IP forwarding or local delivery.
o else, if the destination matches an ACP entry in the IP forwarding
table the Relay first determines whether the neighbor is the same
as the one it received the packet from. If so the Relay MUST drop
the packet silently to avoid looping; otherwise, the Relay uses
the neighbor's link-layer address as the destination for
encapsulation and re-admits the packet into the AERO link.
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o else, the Relay drops the packet and returns an ICMP Destination
Unreachable message subject to rate limiting (see: Section 3.13).
3.8.5. Processing Return Packets
When an AERO Server receives a return packet from an AERO Proxy (see
Section 3.8.2), it proceeds according to the AERO link trust basis.
Namely, the return packets have the same trust profile as for link-
layer Destination Unreachable messages. If the Server has sufficient
trust basis to accept link-layer Destination Unreachable messages, it
can then process the return packet by searching for a dynamic
neighbor cache entry that matches the destination. If there is a
match, the Server marks the corresponding link-layer address as
"unreachable", selects the next-highest priority "reachable" link-
layer address in the entry as the link-layer address for
encapsulation then (re)admits the packet into the AERO link. If
there are no "reachable" link-layer addresses, the Server instead
sets ForwardTime in the dynamic neighbor cache entry to 0 (noting
that ReportTime may still be non-zero). Otherwise, the Server SHOULD
drop the packet and treat it as an indication that a path may be
failing, and MAY use Neighbor Unreachability Detection (NUD) (see:
Section 3.13) to test the path for reachability.
When an AERO Relay receives a return packet from an AERO Server, it
searches its routing table for an entry that matches the inner
destination address. If there is a routing table entry that lists a
different Server as the next hop, the Relay forwards the packet to
the different Server; otherwise, the Relay drops the packet.
3.9. AERO Interface Encapsulation and Re-encapsulation
AERO interfaces encapsulate IP packets according to whether they are
entering the AERO interface from the network layer or if they are
being re-admitted into the same AERO link they arrived on. This
latter form of encapsulation is known as "re-encapsulation".
The AERO interface encapsulates packets per the Generic UDP
Encapsulation (GUE) procedures in
[I-D.ietf-intarea-gue][I-D.ietf-intarea-gue-extensions], or through
an alternate encapsulation format (e.g., see: Appendix A, [RFC2784],
[RFC8086], [RFC4301], etc.). For packets entering the AERO interface
from the network layer, the AERO interface copies the "TTL/Hop
Limit", "Type of Service/Traffic Class" [RFC2983], "Flow
Label"[RFC6438] (for IPv6) and "Congestion Experienced" [RFC3168]
values in the packet's IP header into the corresponding fields in the
encapsulation IP header. For packets undergoing re-encapsulation,
the AERO interface instead copies these values from the original
encapsulation IP header into the new encapsulation header, i.e., the
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values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header. (Note
especially that by copying the TTL/Hop Limit between encapsulation
headers the value will eventually decrement to 0 if there is a
(temporary) routing loop.) For IPv4 encapsulation/re-encapsulation,
the AERO interface sets the DF bit as discussed in Section 3.12.
When GUE encapsulation is used, the AERO interface next sets the UDP
source port to a constant value that it will use in each successive
packet it sends, and sets the UDP length field to the length of the
encapsulated packet plus 8 bytes for the UDP header itself plus the
length of the GUE header (or 0 if GUE direct IP encapsulation is
used). For packets sent to a Server or Relay, the AERO interface
sets the UDP destination port to 8060, i.e., the IANA-registered port
number for AERO. For packets sent to a Client, the AERO interface
sets the UDP destination port to the port value stored in the
neighbor cache entry for this Client. The AERO interface then either
includes or omits the UDP checksum according to the GUE
specification.
Clients normally use the IP address of the underlying interface as
the encapsulation source address. If the underlying interface does
not have an IP address, however, the Client uses an IP address taken
from an ACP as the encapsulation source address (assuming the node
has some way of injecting the ACP into the underlying network routing
system). For IPv6 addresses, the Client normally uses the ACP Subnet
Router Anycast address [RFC4291].
When GUE encapsulation is not available, encapsulation between
Servers and Relays can use standard mechanisms such as Generic
Routing Encapsulation (GRE) [RFC2784], GRE-in-UDP [RFC8086] and IPSec
[RFC4301] so that Relays can be standard IP routers with no AERO-
specific mechanisms.
3.10. AERO Interface Decapsulation
AERO interfaces decapsulate packets destined either to the AERO node
itself or to a destination reached via an interface other than the
AERO interface the packet was received on. Decapsulation is per the
procedures specified for the appropriate encapsulation format.
3.11. AERO Interface Data Origin Authentication
AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:
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o AERO Relays and Servers accept encapsulated packets with a link-
layer source address that matches a permanent neighbor cache
entry.
o AERO Servers accept authentic encapsulated ND messages from
Clients (either directly or via a Proxy), and create or update a
static neighbor cache entry for the Client based on the specific
message type.
o AERO Clients and Servers accept encapsulated packets if there is a
static neighbor cache entry with a link-layer address that matches
the packet's link-layer source address.
o AERO Proxies accept encapsulated packets if there is a proxy
neighbor cache entry that matches the packet's network-layer
address.
Each packet should include a signature that the recipient can use to
authenticate the message origin, e.g., as for common VPN systems such
as OpenVPN [OVPN]. In some environments, however, it may be
sufficient to require signatures only for ND control plane messages
(see: Section 10) and omit signatures for data plane messages.
3.12. AERO Interface Packet Size Issues
The AERO interface is the node's attachment to the AERO link. The
AERO interface acts as a tunnel ingress when it sends a packet to an
AERO link neighbor and as a tunnel egress when it receives a packet
from an AERO link neighbor. AERO interfaces observe the packet
sizing considerations for tunnels discussed in
[I-D.ietf-intarea-tunnels] and as specified below.
The Internet Protocol expects that IP packets will either be
delivered to the destination or a suitable Packet Too Big (PTB)
message returned to support the process known as IP Path MTU
Discovery (PMTUD) [RFC1191][RFC1981]. However, PTB messages may be
crafted for malicious purposes such as denial of service, or lost in
the network [RFC2923]. This can be especially problematic for
tunnels, where a condition known as a PMTUD "black hole" can result.
For these reasons, AERO interfaces employ operational procedures that
avoid interactions with PMTUD, including the use of fragmentation
when necessary.
AERO interfaces observe two different types of fragmentation. Source
fragmentation occurs when the AERO interface (acting as a tunnel
ingress) fragments the encapsulated packet into multiple fragments
before admitting each fragment into the tunnel. Network
fragmentation occurs when an encapsulated packet admitted into the
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tunnel by the ingress is fragmented by an IPv4 router on the path to
the egress. Note that a packet that incurs source fragmentation may
also incur network fragmentation.
IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
bytes [RFC8200]. Although IPv4 specifies a smaller minimum link MTU
of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
for IPv4 even if encapsulated packets may incur network
fragmentation.
IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes
[RFC8200], while the minimum MRU for IPv4 is only 576 bytes [RFC1122]
(note that common IPv6 over IPv4 tunnels already assume a larger MRU
than the IPv4 minimum).
AERO interfaces therefore configure an MTU that MUST NOT be smaller
than 1280 bytes, MUST NOT be larger than the minimum MRU among all
nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also
configure a Maximum Segment Unit (MSU) as the maximum-sized
encapsulated packet that the ingress can inject into the tunnel
without source fragmentation. The MSU value MUST NOT be larger than
(MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
operational assurance that a larger size can traverse the link along
all paths.
All AERO nodes MUST configure the same MTU/MSU values for reasons
cited in [RFC3819][RFC4861]; in particular, multicast support
requires a common MTU value among all nodes on the link. All AERO
nodes MUST configure an MRU large enough to reassemble packets up to
(MTU+ENCAPS) bytes in length; nodes that cannot configure a large-
enough MRU MUST NOT enable an AERO interface.
The network layer proceeds as follow when it presents an IP packet to
the AERO interface. For each IPv4 packet that is larger than the
AERO interface MTU and with the DF bit set to 0, the network layer
uses IPv4 fragmentation to break the packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MTU and the remaining fragments are no larger than the first.
For all other IP packets, if the packet is larger than the AERO
interface MTU, the network layer drops the packet and returns a PTB
message to the original source. Otherwise, the network layer admits
each IP packet or fragment into the AERO interface.
For each IP packet admitted into the AERO interface, the interface
(acting as a tunnel ingress) encapsulates the packet. If the
encapsulated packet is larger than the AERO interface MSU the ingress
source-fragments the encapsulated packet into a minimum number of
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non-overlapping fragments where the first fragment is no larger than
the MSU and the remaining fragments are no larger than the first.
The ingress then admits each encapsulated packet or fragment into the
tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
header in case any network fragmentation is necessary. The
encapsulated packets will be delivered to the egress, which
reassembles them into a whole packet if necessary.
Several factors must be considered when fragmentation is needed. For
AERO links over IPv4, the IP ID field is only 16 bits in length,
meaning that fragmentation at high data rates could result in data
corruption due to reassembly misassociations [RFC6864][RFC4963]. For
AERO links over both IPv4 and IPv6, studies have also shown that IP
fragments are dropped unconditionally over some network paths [I-
D.taylor-v6ops-fragdrop]. In environments where IP fragmentation
issues could result in operational problems, the ingress SHOULD
employ intermediate-layer source fragmentation (see: [RFC2764] and
[I-D.ietf-intarea-gue-extensions]) before appending the outer
encapsulation headers to each fragment. Since the encapsulation
fragment header reduces the room available for packet data, but the
original source has no way to control its insertion, the ingress MUST
include the fragment header length in the ENCAPS length even for
packets in which the header is absent.
3.13. AERO Interface Error Handling
When an AERO node admits encapsulated packets into the AERO
interface, it may receive link-layer or network-layer error
indications.
A link-layer error indication is an ICMP error message generated by a
router in the underlying network on the path to the neighbor or by
the neighbor itself. The message includes an IP header with the
address of the node that generated the error as the source address
and with the link-layer address of the AERO node as the destination
address.
The IP header is followed by an ICMP header that includes an error
Type, Code and Checksum. Valid type values include "Destination
Unreachable", "Time Exceeded" and "Parameter Problem"
[RFC0792][RFC4443]. (AERO interfaces ignore all link-layer IPv4
"Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
only emit packets that are guaranteed to be no larger than the IP
minimum link MTU as discussed in Section 3.12.)
The ICMP header is followed by the leading portion of the packet that
generated the error, also known as the "packet-in-error". For
ICMPv6, [RFC4443] specifies that the packet-in-error includes: "As
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much of invoking packet as possible without the ICMPv6 packet
exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For
ICMPv4, [RFC0792] specifies that the packet-in-error includes:
"Internet Header + 64 bits of Original Data Datagram", however
[RFC1812] Section 4.3.2.3 updates this specification by stating: "the
ICMP datagram SHOULD contain as much of the original datagram as
possible without the length of the ICMP datagram exceeding 576
bytes".
The link-layer error message format is shown in Figure 3 (where, "L2"
and "L3" refer to link-layer and network-layer, respectively):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| L2 IP Header of |
| error message |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2 ICMP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ ~ P
| IP and other encapsulation | a
| headers of original L3 packet | c
~ ~ k
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e
~ ~ t
| IP header of |
| original L3 packet | i
~ ~ n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~ e
| Upper layer headers and | r
| leading portion of body | r
| of the original L3 packet | o
~ ~ r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
Figure 3: AERO Interface Link-Layer Error Message Format
The AERO node rules for processing these link-layer error messages
are as follows:
o When an AERO node receives a link-layer Parameter Problem message,
it processes the message the same as described as for ordinary
ICMP errors in the normative references [RFC0792][RFC4443].
o When an AERO node receives persistent link-layer Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
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awaiting reassembly have been processed. In that case, the node
SHOULD begin including integrity checks and/or institute rate
limits for subsequent packets.
o When an AERO node receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its dynamic neighbor correspondents, the node
SHOULD process the message as an indication that a path may be
failing, and MAY initiate NUD over that path. If it receives
Destination Unreachable messages on many or all paths, the node
SHOULD set ForwardTime for the corresponding dynamic neighbor
cache entry to 0 and allow future packets destined to the
correspondent to flow through a default route.
o When an AERO Client receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its static neighbor Servers, the Client SHOULD
mark the path as unusable and use another path. If it receives
Destination Unreachable messages on many or all paths, the Client
SHOULD associate with a new Server and release its association
with the old Server as specified in Section 3.17.6.
o When an AERO Server receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its static neighbor Clients, the Server SHOULD
mark the underlying path as unusable and use another underlying
path. If it receives Destination Unreachable messages on multiple
paths, the Server should take no further actions unless it
receives an explicit ND/PD release message or if the PD lifetime
expires. In that case, the Server MUST release the Client's
delegated ACP, withdraw the ACP from the AERO routing system and
delete the neighbor cache entry.
o When an AERO Relay or Server receives link-layer Destination
Unreachable messages in response to an encapsulated packet that it
sends to one of its permanent neighbors, it treats the messages as
an indication that the path to the neighbor may be failing.
However, the dynamic routing protocol should soon reconverge and
correct the temporary outage.
When an AERO Relay receives a packet for which the network-layer
destination address is covered by an ASP, if there is no more-
specific routing information for the destination the Relay drops the
packet and returns a network-layer Destination Unreachable message
subject to rate limiting. The Relay writes the network-layer source
address of the original packet as the destination address and uses
one of its non link-local addresses as the source address of the
message.
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When an AERO node receives an encapsulated packet for which the
reassembly buffer it too small, it drops the packet and returns a
network-layer Packet Too Big (PTB) message. The node first writes
the MRU value into the PTB message MTU field, writes the network-
layer source address of the original packet as the destination
address and writes one of its non link-local addresses as the source
address.
3.14. AERO Router Discovery, Prefix Delegation and Autoconfiguration
AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated as discussed in the following Sections.
3.14.1. AERO ND/PD Service Model
Each AERO Server configures a PD service to facilitate Client
requests. Each Server is provisioned with a database of ACP-to-
Client ID mappings for all Clients enrolled in the AERO system, as
well as any information necessary to authenticate each Client. The
Client database is maintained by a central administrative authority
for the AERO link and securely distributed to all Servers, e.g., via
the Lightweight Directory Access Protocol (LDAP) [RFC4511], via
static configuration, etc. Therefore, no Server-to-Server PD state
synchronization is necessary, and Clients can optionally hold
separate PDs for the same ACPs from multiple Servers. In this way,
Clients can associate with multiple Servers, and can receive new PDs
from new Servers before releasing PDs received from existing Servers.
This provides the Client with a natural fault-tolerance and/or load
balancing profile.
AERO Clients and Servers use ND messages to maintain neighbor cache
entries. AERO Servers configure their AERO interfaces as advertising
interfaces, and therefore send unicast RA messages with configuration
information in response to a Client's RS message. Thereafter,
Clients send additional RS messages to the Server's unicast address
to refresh prefix and/or router lifetimes.
AERO Clients and Servers include PD parameters in the RS/RA messages
they exchange (see: [I-D.templin-6man-dhcpv6-ndopt]). The unified
ND/PD messages are exchanged between Client and Server according to
the prefix management schedule required by the PD service.
On Some AERO links, PD arrangements may be through some out-of-band
service such as network management, static configuration, etc. In
those cases, AERO nodes can use simple RS/RA message exchanges with
no explicit PD options. Instead, the RS/RA messages use AERO
addresses as a means of representing the delegated prefixes, e.g., if
a message includes a source address of "fe80::2001:db8:1:2" then the
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recipient can infer that the sender holds the prefix delegation
"2001:db8:1:2::/N" (where 'N' is the prefix length common to all ACPs
for the link).
The following sections specify the Client and Server behavior.
3.14.2. AERO Client Behavior
AERO Clients discover the link-layer addresses of AERO Servers in the
Potential Router List (PRL) via static configuration (e.g., from a
flat-file map of Server addresses and locations), or through an
automated means such as Domain Name System (DNS) name resolution
[RFC1035]. In the absence of other information, the Client resolves
the DNS Fully-Qualified Domain Name (FQDN)
"linkupnetworks.[domainname]" where "linkupnetworks" is a constant
text string and "[domainname]" is a DNS suffix for the Client's
underlying interface (e.g., "example.com"). After discovering the
link-layer addresses, the Client associates with one or more of the
corresponding Servers.
To associate with a Server, the Client acts as a requesting router to
request ACPs through an ND/PD message exchange. The Client sends an
RS message with PD parameters and with all-routers multicast as the
IPv6 destination address, the address of the Client's underlying
interface as the link-layer source address and the link-layer address
of the Server as the link-layer destination address. If the Client
already knows its own AERO address, it uses the AERO address as the
IPv6 source address; otherwise, it uses the prefix-solicitation
address as the source address. If the Client's underlying interface
connects to a subnetwork that supports ACP injection, the Client can
use the ACP's Subnet Router Anycast address as the link-layer source
address.
The Client next includes one or more SLLAOs in the RS message
formatted as described in Section 3.5 to register its link-layer
address(es) with the Server. The first SLLAO MUST correspond to the
underlying interface over which the Client will send the RS message.
The Client MAY include additional SLLAOs specific to other underlying
interfaces, but if so it sets their UDP Port Number and IP Address
fields to 0. The Client can instead register additional link-layer
addresses with the Server by sending additional RS messages including
SLLAOs via other underlying interfaces after the initial RS/RA
exchange.
The Client then sends the RS message to the AERO Server and waits for
an RA message reply (see Section 3.14.3) while retrying MAX_RETRY
times until an RA is received. If the Client receives no RAs, or if
it receives an RA with Router Lifetime set to 0 and/or with no ACP PD
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parameters, the Client SHOULD discontinue autoconfiguration attempts
through this Server and try another Server. Otherwise, the Client
processes the ACPs found in the RA message.
Next, the Client creates a static neighbor cache entry with the
Server's link-local address as the network-layer address and the
Server's encapsulation source address as the link-layer address. The
Client then autoconfigures AERO addresses for each of the delegated
ACPs and assigns them to the AERO interface.
The Client next examines the P bit in the RA message flags field
[RFC5175]. If the P bit value was 1, the Client infers that there is
a NAT or Proxy on the path to the Server via the interface over which
it sent the RS message. In that case, the Client sets UDP Port
Number and IP Address to 0 in the S/TLLAOs of any subsequent ND
messages it sends to the Server over that link.
The Client also caches any ASPs included in Route Information Options
(RIOs) [RFC4191] as ASPs to associate with the AERO link, and assigns
the MTU/MSU values in the MTU options to its AERO interface while
configuring an appropriate MRU. This configuration information
applies to the AERO link as a whole, and all AERO nodes will receive
the same values.
Following autoconfiguration, the Client sub-delegates the ACPs to its
attached EUNs and/or the Client's own internal virtual interfaces as
described in [I-D.templin-v6ops-pdhost] to support the Client's
downstream attached "Internet of Things (IoT)". The Client
subsequently maintains its ACP delegations through each of its
Servers by sending RS messages with PD parameters to receive
corresponding RA messages.
After the Client registers its Interface IDs and their associated
UDP/IP addresses and 'P(i)' values, it may wish to change one or more
Interface ID registrations, e.g., if an underlying interface changes
address or becomes unavailable, if QoS preferences change, etc. To
do so, the Client prepares an unsolicited NA message to send over any
available underlying interface. The target address of the NA message
is set to the Client's link-local address, and the destination
address is set to all-nodes multicast. The NA MUST include a TLLAO
specific to the selected available underlying interface as the first
TLLAO and MAY include any additional TLLAOs specific to other
underlying interfaces. The Client includes fresh 'P(i)' values in
each TLLAO to update the Server's neighbor cache entry. If the
Client wishes to update 'P(i)' values without updating the link-layer
address, it sets the UDP Port Number and IP Address fields to 0. If
the Client wishes to disable the interface, it sets all 'P(i)' values
to '0' ("disabled").
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If the Client wishes to discontinue use of a Server it issues an RS
message with PD parameters that will cause the Server to release the
Client. When the Server processes the message, it releases the ACP,
deletes its neighbor cache entry for the Client, withdraws the IP
route from the routing system and returns an RA reply containing any
necessary PD parameters.
3.14.3. AERO Server Behavior
AERO Servers act as IPv6 routers and support a PD service for
Clients. AERO Servers arrange to add their encapsulation layer IP
addresses (i.e., their link-layer addresses) to a static map of
Server addresses for the link and/or the DNS resource records for the
FQDN "linkupnetworks.[domainname]" before entering service. The list
of Server addresses should be geographically and/or topologically
referenced, and forms the Potential Router List (PRL) for the AERO
link.
When an AERO Server receives a prospective Client's RS message with
PD parameters on its AERO interface, and the Server is too busy, it
SHOULD return an immediate RA reply with no ACPs and with Router
Lifetime set to 0. Otherwise, the Server authenticates the RS
message and processes the PD parameters. The Server first determines
the correct ACPs to delegate to the Client by searching the Client
database. When the Server delegates the ACPs, it also creates an IP
forwarding table entry for each ACP so that the AERO BGP-based
routing system will propagate the ACPs to the Relays that aggregate
the corresponding ASP (see: Section 3.3).
Next, the Server prepares an RA message that includes the delegated
ACPs and any other PD parameters. The Server then returns the RA
message using its link-local address as the network-layer source
address, the network-layer source address of the RS message as the
network-layer destination address, the Server's link-layer address as
the source link-layer address, and the source link-layer address of
the RS message as the destination link-layer address. The Server
next sets the P flag in the RA message flags field [RFC5175] to 1 if
the source link-layer address in the RS message was different than
the address in the first SLLAO to indicate that there is a NAT or
Proxy on the path; otherwise it sets P to 0. The Server then
includes one or more RIOs that encode the ASPs for the AERO link.
The Server also includes two MTU options - the first MTU option
includes the MTU for the link and the second MTU option includes the
MSU for the link (see Section 3.12). The Server finally sends the RA
message to the Client.
The Server next creates a static neighbor cache entry for the Client
using the base AERO address as the network-layer address and with
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lifetime set to no more than the smallest PD lifetime. Next, the
Server updates the neighbor cache entry link-layer address(es) by
recording the information in each SLLAO option indexed by the
Interface ID and including the UDP port number, IP address and P(i)
values. For the first SLLAO in the list, however, the Server records
the actual encapsulation source UDP and IP addresses instead of those
that appear in the SLLAO in case there was a NAT or Proxy in the
path.
After the initial RS/RA exchange, the AERO Server maintains the
neighbor cache entry for the Client until the PD lifetimes expire.
If the Client issues additional RS messages with PD renewal
parameters, the Server extends the PD lifetimes. If the Client
issues an RS with PD release parameters, or if the Client does not
issue a renewal before the lifetime expires, the Server deletes the
neighbor cache entry for the Client and withdraws the IP routes from
the AERO routing system. The Server processes these and any other
Client PD messages, and returns an RA reply. The Server may also
issue an unsolicited RA message with PD reconfigure parameters to
inform the Client that it needs to renegotiate its PDs.
3.14.3.1. Lightweight DHCPv6 Relay Agent (LDRA)
When DHCPv6 is used as the ND/PD service back end, AERO Clients and
Servers are always on the same link (i.e., the AERO link) from the
perspective of DHCPv6. However, in some implementations the DHCPv6
server and ND function may be located in separate modules. In that
case, the Server's AERO interface module can act as a Lightweight
DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from
the DHCPv6 server module.
When the LDRA receives an authentic RS message, it extracts the PD
message parameters and uses them to fabricate an IPv6/UDP/DHCPv6
message. It sets the IPv6 source address to the source address of
the RS message, sets the IPv6 destination address to
'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values
that will be understood by the DHCPv6 server.
The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message
header and includes an 'Interface-Id' option that includes enough
information to allow the LDRA to forward the resulting Reply message
back to the Client (e.g., the Client's link-layer addresses, a
security association identifier, etc.). The LDRA also wraps the
information in all of the SLLAO options from the RS message into the
Interface-Id option, then forwards the message to the DHCPv6 server.
When the DHCPv6 server prepares a Reply message, it wraps the message
in a 'Relay-Reply' message and echoes the Interface-Id option. The
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DHCPv6 server then delivers the Relay-Reply message to the LDRA,
which discards the Relay-Reply wrapper and IPv6/UDP headers, then
uses the DHCPv6 message to fabricate an RA response to the Client.
The Server uses the information in the Interface-Id option to prepare
the RA message and to cache the link-layer addresses taken from the
SLLAOs echoed in the Interface-Id option.
3.15. AERO Route Optimization
When a source Client forwards packets to a prospective correspondent
Client within the same AERO link domain (i.e., one for which the
packet's destination address is covered by an ASP), the source Client
MAY initiate an AERO link route optimization procedure. The
procedure is based on an exchange of IPv6 ND messages using a chain
of AERO Servers and Relays as a trust basis.
Although the Client is responsible for initiating route optimization,
the Server is the policy enforcement point that determines whether
route optimization is permitted. For example, on some AERO links
route optimization would allow traffic to circumvent critical
network-based traffic inspection points. In those cases, the Server
can simply discard any route optimization messages instead of
forwarding them.
The following sections specify the AERO route optimization procedure.
3.15.1. Reference Operational Scenario
Figure 4 depicts the AERO route optimization reference operational
scenario, using IPv6 addressing as the example (while not shown, a
corresponding example for IPv4 addressing can be easily constructed).
The figure shows an AERO Relay ('R1'), two AERO Servers ('S1', 'S2'),
two AERO Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1',
'H2'):
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+--------------+ +--------------+ +--------------+
| Server S1 | | Relay R1 | | Server S2 |
+--------------+ +--------------+ +--------------+
fe80::2 fe80::1 fe80::3
L2(S1) L2(R1) L2(S2)
| | |
X-----+-----+------------------+-----------------+----+----X
| AERO Link |
L2(C1) L2(C2)
fe80::2001:db8:0:0 fe80::2001:db8:1:0
+--------------+ +--------------+
|AERO Client C1| |AERO Client C2|
+--------------+ +--------------+
2001:DB8:0::/48 2001:DB8:1::/48
| |
.-. .-.
,-( _)-. 2001:db8:0::1 2001:db8:1::1 ,-( _)-.
.-(_ IP )-. +-------+ +-------+ .-(_ IP )-.
(__ EUN )--|Host H1| |Host H2|--(__ EUN )
`-(______)-' +-------+ +-------+ `-(______)-'
Figure 4: AERO Reference Operational Scenario
In Figure 4, Relay ('R1') assigns the administratively-provisioned
AERO address fe80::1 to its AERO interface with link-layer address
L2(R1), Server ('S1') assigns the address fe80::2 with link-layer
address L2(S1), and Server ('S2') assigns the address fe80::3 with
link-layer address L2(S2). Servers ('S1') and ('S2') next arrange to
add their link-layer addresses to a published list of valid Servers
for the AERO link.
AERO Client ('C1') receives the ACP 2001:db8:0::/48 in an ND/PD
exchange via AERO Server ('S1') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(C1). Client ('C1') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::2 and link-
layer address L2(S1), then sub-delegates the ACP to its attached
EUNs. IPv6 host ('H1') connects to the EUN, and configures the
address 2001:db8:0::1.
AERO Client ('C2') receives the ACP 2001:db8:1::/48 in an ND/PD
exchange via AERO Server ('S2') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C2). Client ('C2') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::3 and link-
layer address L2(S2), then sub-delegates the ACP to its attached
EUNs. IPv6 host ('H2') connects to the EUN, and configures the
address 2001:db8:1::1.
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3.15.2. Concept of Operations
Again, with reference to Figure 4, when source host ('H1') sends a
packet to destination host ('H2'), the packet is first forwarded over
the source host's attached EUN to Client ('C1'). Client ('C1') then
forwards the packet via its AERO interface to Server ('S1') and also
sends an NS message toward Client ('C2') via Server ('S1').
Server ('S1') then forwards both the packet and the NS message out
the same AERO interface toward Client ('C2') via Relay ('R1'). When
Relay ('R1') receives the packet and NS message, it consults its
forwarding table to discover Server ('S2') as the next hop toward
Client ('C2'). Relay ('R1') then forwards both the packet and the NS
message to Server ('S2'), which then forwards them to Client ('C2').
After Client ('C2') receives the NS message, it process the message
and creates or updates a dynamic neighbor cache entry for Client
('C1'), then sends the NA response to the link-layer address of
Client ('C1').
After Client ('C1') receives the NA message, it processes the message
and creates or updates a dynamic neighbor cache entry for Client
('C2'). Thereafter, forwarding of packets from Client ('C1') to
Client ('C2') without involving any intermediate nodes is enabled.
The mechanisms that support this exchange are specified in the
following sections.
3.15.3. Sending NS Messages
When a Client forwards a packet with a source address from one of its
ACPs toward a destination address covered by an ASP (i.e., toward
another AERO Client connected to the same AERO link), the source
Client MAY send an NS message forward toward the destination Client
via the Server.
In the reference operational scenario, when Client ('C1') forwards a
packet toward Client ('C2'), it MAY also send an NS message forward
toward Client ('C2'), subject to rate limiting (see Section 8.2 of
[RFC4861]). Client ('C1') prepares the NS message as follows:
o the link-layer source address is set to 'L2(C1)' (i.e., the link-
layer address of Client ('C1')).
o the link-layer destination address is set to 'L2(S1)' (i.e., the
link-layer address of Server ('S1')).
o the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the base AERO address of Client ('C1')).
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o the network-layer destination address is set to the AERO address
corresponding to the destination address of Client ('C2').
o the Type is set to 135.
o the Target Address is set to the destination address of the packet
that triggered route optimization.
o the message includes SLLAOs set to appropriate values for the
Client ('C1')'s underlying interfaces The first SLLAO serves as
the "Report-To" address for the Client, which is the address to
which the target will announce mobility events and/or other
dynamic updates.
o the message includes one or more RIOs that include Client ('C1')'s
ACPs [I-D.templin-6man-rio-redirect].
o the message SHOULD include a Timestamp option and a Nonce option.
Note that the act of sending NS messages is cited as "MAY", since
Client ('C1') may have advanced knowledge that the direct path to
Client ('C2') would be unusable or otherwise undesirable. If the
direct path later becomes unusable after the initial route
optimization, Client ('C1') simply allows packets to again flow
through Server ('S1').
3.15.4. Re-encapsulating and Relaying the NS
When Server ('S1') receives an NS message from Client ('C1'), it
first verifies that the SLLAOs in the NS are a proper subset of the
link-layer addresses in Client ('C1')'s neighbor cache entry. If the
Client's SLLAOs are not acceptable, Server ('S1') discards the
message.
Server ('S1') then examines the network-layer destination address of
the NS to determine the next hop toward Client ('C2') by searching
for the AERO address in the neighbor cache. Since Client ('C2') is
not one of its neighbors, Server ('S1') then inserts an additional
layer of encapsulation between the outer IP header and the NS
message. This mid-layer IP header uses the AERO Server Subnet Router
Anycast address as the source address and the Subnet Router Anycast
address corresponding to Client ("C2")'s AERO address as the
destination address (in this case, C2's Subnet Router Anycast address
is 2001:db8:1:0::). The Server then forwards this double-
encapsulated NS message to Relay ('R1') by changing the link-layer
source address of the message to 'L2(S1)' and changing the link-layer
destination address to 'L2(R1)'. Server ('S1') finally forwards the
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message to Relay ('R1') without decrementing the network-layer TTL/
Hop Limit field.
When Relay ('R1') receives the double-encapsulated NS message from
Server ('S1') it discards the outer IP header and determines that
Server ('S2') is the next hop toward Client ('C2') by consulting its
standard IP forwarding table for the Client Subnet Router Anycast
destination address. Relay ('R1') then encapsulates and forwards the
message to Server ('S2') the same as for any IP router.
When Server ('S2') receives the double-encapsulated NS message from
Relay ('R1') it removes the mid-layer IP header and determines that
Client ('C2') is a neighbor on a native underlying interface by
consulting its neighbor cache for Client ('C2')'s AERO address.
Server ('S2') then re-encapsulates the NS while changing the link-
layer source address to 'L2(S2)' and changing the link-layer
destination address to 'L2(C2)'. Server ('S2') then forwards the
message to Client ('C2').
3.15.5. Processing NSs and Sending NAs
When Client ('C2') receives the NS message, it accepts the NS only if
the message has a link-layer source address of one of its Servers
(e.g., L2(S2)). Client ('C2') further accepts the message only if it
is willing to serve as a route optimization target.
In the reference operational scenario, when Client ('C2') receives a
valid NS message, it either creates or updates a dynamic neighbor
cache entry that stores the source address of the message as the
network-layer address of Client ('C1') and stores the link-layer
addresses found in the SLLAOs as the link-layer addresses of Client
('C1'). Client ('C2') then sets ReportTime for the neighbor cache
entry to REPORT_TIME.
After processing the message, Client ('C2') prepares an NA message
response as follows:
o the link-layer source address is set to 'L2(C2)' (i.e., the link-
layer address of Client ('C2')).
o the link-layer destination address is set to 'L2(C1)' (i.e., the
link-layer address of Client ('C1')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the base AERO address of Client ('C2')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the base AERO address of Client ('C1')).
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o the Type is set to 136.
o the Target Address is set to the Target Address field in the NS
message.
o the message includes one or more TLLAOs set to appropriate values
for Client ('C2')'s native underlying interfaces.
o the message includes one or more RIOs that include Client ('C2')'s
ACPs [I-D.templin-6man-rio-redirect].
o the message SHOULD include a Timestamp option and MUST echo the
Nonce option received in the NS (i.e., if a Nonce option was
present).
Client ('C2') then sends the NA message to Client ('C1').
3.15.6. Processing NAs
When Client ('C1') receives the NA message, it first verifies that
the NA matches the original NS message. Client ('C1') then processes
the message as follows.
In the reference operational scenario, when Client ('C1') receives
the NA message, it either creates or updates a dynamic neighbor cache
entry that stores the source address of the message as the network-
layer address of Client ('C2'), stores the link-layer addresses found
in the TLLAOs as the link-layer addresses of Client ('C2') and stores
the ACPs encoded in the RIOs of the NA as the ACPs for Client ('C2').
Client ('C1') then sets ForwardTime for the neighbor cache entry to
FORWARD_TIME.
Now, Client ('C1') has a neighbor cache entry with a valid
ForwardTime value, while Client ('C2') has a neighbor cache entry
with a valid ReportTime value. Thereafter, Client ('C1') may forward
ordinary network-layer data packets directly to Client ('C2') without
involving any intermediate nodes, and Client ('C2') can dynamically
report any changes in link-layer information by sending unsolicited
NA messages. (In order for Client ('C2') to forward packets to
Client ('C1'), a corresponding NS/NA message exchange is required in
the reverse direction; hence, the mechanism is asymmetric.)
3.15.7. Server-Based Route Optimization
The source Client itself may initiate route optimization if it has
only native interfaces. If the source Client has Direct, NATed,
Proxyed or VPNed interfaces, route optimization must instead be
initiated by the source Server. The source Server MUST include an
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SLLAO with a "Report-To" address in the route optimization NS
messages it sends. The "Report-To" address must be one of the source
Server's globally routable IP addresses.
In the same way, the target Client may serve as a route optimization
target if it has only native interfaces. If some or all of the
target Client's underlying interfaces are Direct, NATed, Proxyed or
VPNed the target Server must instead serve as the route optimization
target. In that case, when the source sends an NS message the target
Server prepares an NA response the same as if it were the target
Client (see: Section 3.15.5) and does not forward the NS.
When the target Server sends an NA response to a route optimization
NS, it includes a Timestamp option, any necessary security options,
and TLLAOs corresponding to the target Client's underlying
interfaces. The target Server writes the link-layer address of the
Client in TLLAOs corresponding to native underlying interfaces,
writes the link-layer address of the Proxy in TLLAOs corresponding to
Proxyed underlying interfaces and writes its own link-layer address
in TLLAOs corresponding to other interfaces. The Interface ID and
QoS Preference values in the TLLAOs are those supplied by the target
Client during ND exchanges with the target Server. The target Server
then establishes a dynamic neighbor cache entry for the source with
ReportTime set to REPORT_TIME seconds and with a "Report-To" address
set to the address of the source.
When the source Server receives the NA response, it creates or
updates a dynamic neighbor cache entry for the target with
ForwardTime set to FORWARD_TIME seconds and with the information
provided in the TLLAOs as the link-layer addresses and preference
values for the target. The source Server then translates the
solicited NA message into an unsolicited NA message by changing the
source address to its own link-local address, changing the
destination address to all-nodes multicast, recalculating checksums
and any security options, and including the Timestamp option as it
appeared in the original solicited NA. The source Server then
retains this message for subsequent on-demand transmission to any
source neighbors that send packets to the target within the current
ForwardTime window.
While ForwardTime is greater than 0, the source Server sends
unsolicited NA messages (subject to rate limiting) in response to
data packets from source Clients or Proxies that are destined to the
target Client. The unsolicited NA messages update source Client and
Proxy dynamic neighbor cache entries with ForwardTime set to
FORWARD_TIME minus the difference between the current time and the NA
Timestamp. Subsequent packets from the source destined to the target
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Client then travel via the route-optimized path instead of through
the dogleg path through Servers and Relays.
Following route optimization, when the target Client (or Proxy) sends
unsolicited NA messages to the target Server to update link-layer
addresses and/or QoS preferences, the target Server translates the
messages the same as described above and repeats them to any of its
neighbors with non-zero ReportTime. The source Server in turn
translates the messages and repeats them to any of their source
Clients or Proxys to which they recently sent NAs.
If the target Client moves to a new Server, the old Server sends
immediate unsolicited NA messages with no TLLAOs to any of its
dynamic neighbors with non-zero ReportTime, and retains the dynamic
neighbor cache entry until ReportTime expires. While ReportTime is
non-zero, the old Server sends unsolicited NA messages with no TLLAOs
(subject to rate limiting) back to the source in response to data
packets received from a correspondent node while forwarding the
packets themselves to a Relay. The Relay will then either forward
the packets to the new Server if the target Client has moved, or drop
the packets if the target Client is no longer in the network. When
the source receives the unsolicited NAs with no TLLAOs, it allows
future packets destined to the target Client to again flow through
its own Server (or Relay).
3.16. Neighbor Unreachability Detection (NUD)
AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
NS messages to elicit solicited NA messages from neighbors the same
as described in [RFC4861]. NUD is performed either reactively in
response to persistent link-layer errors (see Section 3.13) or
proactively to update neighbor cache entry timers and/or link-layer
address information. (NS messages may include SLLAOs and NA messages
may include TLLAOs in order to update link-layer address
information.)
When an AERO node sends an NS/NA message, it uses one of its link-
local addresses as the IPv6 source address and a link-local address
of the neighbor as the IPv6 destination address. When route
optimization directs a source AERO node to a target AERO node, the
source node SHOULD proactively test the direct path by sending an
initial NS message to elicit a solicited NA response. While testing
the path, the source node can optionally continue sending packets via
its default router, maintain a small queue of packets until target
reachability is confirmed, or (optimistically) allow packets to flow
directly to the target.
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While data packets are still flowing, the source node thereafter
periodically tests the direct path to the target node (see
Section 7.3 of [RFC4861]) in order to keep dynamic neighbor cache
entries alive. When the target node receives a valid NS message, it
resets ReportTime to REPORT_TIME and updates its cached link-layer
addresses (if necessary). When the source node receives a
corresponding NA message, it resets ForwardTime to FORWARD_TIME and
updates its cached link-layer addresses (if necessary). If the
source node is unable to elicit an NA response from the target node
after MaxRetry attempts, it SHOULD set ForwardTime to 0. Otherwise,
the source node considers the path usable and SHOULD thereafter
process any link-layer errors as an indication that the direct path
to the target node may be failing.
When ForwardTime for a dynamic neighbor cache entry expires, the
source node resumes sending any subsequent packets via a Server (or
Relay) and may (eventually) attempt to re-initiate the AERO route
optimization process. When ReportTime for a dynamic neighbor cache
entry expires, the target node ceases to send dynamic mobility and
QoS updates to the source node. When both ForwardTime and ReportTime
for a dynamic neighbor cache entry expire, the node deletes the
neighbor cache entry.
Note that an AERO node may have multiple underlying interface paths
toward the target neighbor. In that case, the node SHOULD perform
NUD over each underlying interface individually and only consider the
neighbor unreachable if NUD fails over multiple underlying interface
paths.
3.17. Mobility Management and Quality of Service (QoS)
AERO is an example of a Distributed Mobility Management (DMM)
service. Each AERO Server is responsible for only a subset of the
Clients on the AERO link, as opposed to a Centralized Mobility
Management (CMM) service where there is a single network mobility
service for all Clients. AERO Clients coordinate with their
associated AERO Servers via RS/RA exchanges to maintain the DMM
profile, and the AERO routing system tracks the current AERO Client/
Server peering relationships.
AERO interfaces accommodate mobility management by sending
unsolicited NA messages the same as for announcing link-layer address
changes for any interface that implements IPv6 ND [RFC4861]. (In
environments where reliability is a concern, AERO interfaces can send
immediate NS messages to receive solicited NA messages, i.e., they
can skip the unreliable unsolicited NA messaging step and move
directly to a reliable NS/NA exchange.)
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When a node sends an unsolicited NA message, it sets the IPv6 source
to its own link-local address, sets the IPv6 destination address to
all-nodes multicast, sets the link-layer source address to its own
address and sets the link-layer destination address to either a
multicast address or the unicast link-layer address of a neighbor.
In the latter case, if the unsolicited NA message must be received by
multiple neighbors, the node sends multiple copies of the NA using a
different unicast link-layer destination address for each neighbor.
Mobility management considerations are specified in the following
sections.
3.17.1. Forwarding Packets on Behalf of Departed Clients
When a Server receives packets with destination addresses that do not
match one of its static neighbor cache Clients, it forwards the
packets to a Relay which delivers them to the target Client's current
location. If the source is not one of its static neighbor Clients,
the Server also returns an unsolicited NA message to the sender with
no TLLAOs - the sender will then realize that it needs to delete its
neighbor cache entry that associated the target with this Server.
3.17.2. Announcing Link-Layer Address and/or QoS Preference Changes
When a Client needs to change its link-layer addresses, e.g., due to
a mobility event, it sends unsolicited NAs to its neighbors using the
new link-layer address as the source address and with TLLAOs that
include the new Client UDP Port Number, IP Address and P(i) values.
(For neighbors that are Servers, the Client can instead initiate an
RS/RA exchange.) If the Client sends the NA solely for the purpose
of updating QoS preferences without updating the link-layer address,
the Client sets the UDP Port Number and IP Address to 0.
The Client MAY send up to MaxRetry unsolicited NA messages in
parallel with sending actual data packets in case one or more NAs are
lost. If all NAs are lost, the neighbor will eventually invoke NUD
by sending NS messages that include SLLAOs.
3.17.3. Bringing New Links Into Service
When a Client needs to bring new underlying interfaces into service
(e.g., when it activates a new data link), it sends unsolicited NAs
to its neighbors using the new link-layer address as the source
address and with TLLAOs that include the new Client link-layer
information. (For neighbors that are Servers, the Client can instead
initiate an RS/RA exchange.)
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3.17.4. Removing Existing Links from Service
When a Client needs to remove existing underlying interfaces from
service (e.g., when it de-activates an existing data link), it sends
unsolicited NAs to its neighbors with TLLAOs with all P(i) values set
to 0. (For neighbors that are Servers, the Client can instead
initiate an RS/RA exchange.)
If the Client needs to send ND messages over an underlying interface
other than the one being removed from service, it MUST include a
current TLLAO for the sending interface as the first TLLAO and
include TLLAOs for any underlying interface being removed from
service as additional TLLAOs.
3.17.5. Implicit Mobility Management
AERO interface neighbors MAY provide a configuration option that
allows them to perform implicit mobility management in which no ND
messaging is used. In that case, the Client only transmits packets
over a single interface at a time, and the neighbor always observes
packets arriving from the Client from the same link-layer source
address.
If the Client's underlying interface address changes (either due to a
readdressing of the original interface or switching to a new
interface) the neighbor immediately updates the neighbor cache entry
for the Client and begins accepting and sending packets to the
Client's new link-layer address. This implicit mobility method
applies to use cases such as cellphones with both WiFi and Cellular
interfaces where only one of the interfaces is active at a given
time, and the Client automatically switches over to the backup
interface if the primary interface fails.
3.17.6. Moving to a New Server
When a Client associates with a new Server, it performs the Client
procedures specified in Section 3.14.2. The Client then sends RS
messages with PD release parameters to the old Server to release
itself from that Server's domain. If the Client does not receive an
RA reply after MaxRetry attempts, the old Server may have failed and
the Client should discontinue its release attempts.
Clients SHOULD NOT move rapidly between Servers in order to avoid
causing excessive oscillations in the AERO routing system. Such
oscillations could result in intermittent reachability for the Client
itself, while causing no harm to the network. Examples of when a
Client might wish to change to a different Server include a Server
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that has gone unreachable, topological movements of significant
distance, movement to a new geographic region, etc.
3.18. Multicast Considerations
When the underlying network does not support multicast, AERO Clients
map link-scoped multicast addresses to the link-layer address of a
Server, which acts as a multicast forwarding agent. The AERO Client
also serves as an IGMP/MLD Proxy for its EUNs and/or hosted
applications per [RFC4605] while using the link-layer address of the
Server as the link-layer address for all multicast packets.
When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in [RFC2529] for IPv4
underlying networks and use a TBD site-scoped multicast mapping for
IPv6 underlying networks. In that case, border routers must ensure
that the encapsulated site-scoped multicast packets do not leak
outside of the site spanned by the AERO link.
4. The AERO Proxy
In some environments, AERO Clients may be located in secured
subnetwork enclaves that do not allow direct communications from the
Client to a Server in the outside Internetwork. In that case, the
secured enclave can employ an AERO Proxy.
The AERO Proxy is located at the secured enclave perimeter and
listens for encapsulated RS messages originating from or RA messages
destined to AERO Clients located within the enclave. The Proxy acts
on these control messages as follows:
o when the Proxy receives an RS message from a Client within the
secured enclave, it first authenticates the message then creates a
proxy neighbor cache entry for the Client in the INCOMPLETE State
and caches the Client and Server link-layer addresses along with
any identifying information including Transaction IDs, Client
Identifiers, Nonce values, etc. The Proxy then re-encapsulates
the RS message using its own external address as the source link-
layer address and forwards the message to the Server.
o when the Server receives the RS message, it authenticates the
message then creates a static neighbor cache entry for the Client
with the Proxy's address as the link-layer address. The Server
then sends an RA message back to the Proxy.
o when the Proxy receives the RA message, it matches the message
with the RS that created the (INCOMPLETE) proxy neighbor cache
entry. The Proxy then caches the route information in the message
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as a mapping from the Client's ACPs to the Client's address within
the secured enclave, and sets the neighbor cache entry state to
REACHABLE. The Proxy then re-encapsulates the RA message using
its own internal address as the source link-layer address and
forwards the message to the Client.
After the initial RS/RA exchange, the Proxy forwards data packets
between the Client and Server with the Server acting as the Client's
default router. The Proxy can send ND messages to the Client's
Server(s) to update Server neighbor cache entries on behalf of the
Client. (For example, the Proxy can send unsolicited NA messages
with a TLLAO with UDP Port Number and IP Address set to 0 and with
valid P(i) values to update the Server(s) with the Client's new QoS
preferences for the path that traverses the Proxy). The Proxy also
forwards any control and data messages originating from the Client to
the Client's primary Server.
At some time after data packets have been flowing from the Client to
the Server, the Proxy may receive unsolicited NA messages from the
Server with TLLAOs corresponding to a target Client. The Proxy
establishes a dynamic neighbor cache entry for the target with
ForwardTime set to FORWARD_TIME and allows future data packets
destined to the target to flow directly according to the link-layer
address information instead of through the Server. The Proxy may at
some later point receive additional NA messages with TLLAOs, and if
so resets ForwardTime and updates its cached link-layer address
information. If the Proxy receives no further NA messages, or if it
receives NA messages with no TLLAOs, it deletes the dynamic neighbor
cache entry.
In some subnetworks that employ a Proxy, the Client's ACP can be
injected into the underlying network routing system. In that case,
the Client can send data messages without encapsulation so that the
native underlying network routing system transports the
unencapsulated packets to the Proxy. This can be very beneficial,
e.g., if the Client connects to the network via low-end data links
such as some aviation wireless links. In that case, however, the
Client's control messages are still sent encapsulated so as to supply
the Proxy with the address of the Server and to transport IPv6 ND
messages without decrementing the hop-count. In summary, the
interface becomes one where control messages are encapsulated while
data messages are either unencapsulated or encapsulated according to
the specific use case. This encapsulation avoidance represents a
form of "header compression", meaning that the MTU should be sized
based on the size of full encapsulated messages even if most messages
are sent unencapsulated.
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5. Direct Underlying Interfaces
When a Client's AERO interface is configured over a Direct underlying
interface, the neighbor at the other end of the Direct link can
receive packets without any encapsulation. In that case, the Client
sends packets over the Direct link according to the QoS preferences
associated with its underling interfaces. If the Direct underlying
interface has the highest QoS preference, then the Client's IP
packets are transmitted directly to the peer without going through an
underlying network. If other underlying interfaces have higher QoS
preferences, then the Client's IP packets are transmitted via a
different underlying interface, which may result in the inclusion of
AERO Proxies, Servers and Relays in the communications path. Direct
underlying interfaces must be tested periodically for reachability,
e.g., via NUD, via periodic unsolicited NAs, etc.
6. Operation on AERO Links with /64 ASPs
IPv6 AERO links typically have ASPs that cover many candidate ACPs of
length /64 or shorter. However, in some cases it may be desirable to
use AERO over links that have only a /64 ASP. This can be
accommodated by treating all Clients on the AERO link as simple hosts
that receive /128 prefix delegations.
In that case, the Client sends an RS message to the Server the same
as for ordinary AERO links. The Server responds with an RA message
that includes one or more /128 prefixes (i.e., singleton addresses)
that include the /64 ASP prefix along with an interface identifier
portion to be assigned to the Client. The Client and Server then
configure their AERO addresses based on the interface identifier
portions of the /128s (i.e., the lower 64 bits) and not based on the
/64 prefix (i.e., the upper 64 bits).
For example, if the ASP for the host-only IPv6 AERO link is
2001:db8:1000:2000::/64, each Client will receive one or more /128
IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
2001:db8:1000:2000::2/128, etc. When the Client receives the prefix
delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to
the AERO interface, and assigns the global IPv6 addresses (i.e., the
/128s) to either the AERO interface or an internal virtual interface
such as a loopback. In this arrangement, the Client conducts route
optimization in the same sense as discussed in Section 3.15.
This specification has applicability for nodes that act as a Client
on an "upstream" AERO link, but also act as a Server on "downstream"
AERO links. More specifically, if the node acts as a Client to
receive a /64 prefix from the upstream AERO link it can then act as a
Server to provision /128s to Clients on downstream AERO links.
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7. AERO Adaptations for SEcure Neighbor Discovery (SEND)
SEcure Neighbor Discovery (SEND) [RFC3971] and Cryptographically
Generated Addresses (CGAs) [RFC3972] were designed to secure IPv6 ND
messaging in environments where symmetric network and/or transport-
layer security services are impractical (see: Section 10). AERO
nodes that use SEND/CGA employ the following adaptations.
When a source AERO node prepares a SEND-protected ND message, it uses
a link-local CGA as the IPv6 source address and writes the prefix
embedded in its AERO address (i.e., instead of fe80::/64) in the CGA
parameters Subnet Prefix field. When the neighbor receives the ND
message, it first verifies the message checksum and SEND/CGA
parameters while using the link-local prefix fe80::/64 (i.e., instead
of the value in the Subnet Prefix field) to match against the IPv6
source address of the ND message.
The neighbor then derives the AERO address of the source by using the
value in the Subnet Prefix field as the interface identifier of an
AERO address. For example, if the Subnet Prefix field contains
2001:db8:1:2, the neighbor constructs the AERO address as
fe80::2001:db8:1:2. The neighbor then caches the AERO address in the
neighbor cache entry it creates for the source, and uses the AERO
address as the IPv6 destination address of any ND message replies.
8. Implementation Status
An AERO implementation based on OpenVPN (https://openvpn.net/) was
announced on the v6ops mailing list on January 10, 2018. The latest
version is available at: http://linkupnetworks.net/aero/AERO-OpenVPN-
2.0.tgz.
An initial public release of the AERO proof-of-concept source code
was announced on the intarea mailing list on August 21, 2015. The
latest version is available at: http://linkupnetworks.net/aero/aero-
4.0.0.tgz.
A survey of public domain and commercial SEND implementations is
available at https://www.ietf.org/mail-archive/web/its/current/
msg02758.html.
9. IANA Considerations
The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.
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The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO [RFC6706]. This document obsoletes
[RFC6706] and claims the UDP port number "8060" for all future use.
No further IANA actions are required.
10. Security Considerations
AERO link security considerations include considerations for both the
data plane and the control plane.
Data plane security considerations are the same as for ordinary
Internet communications. Application endpoints in AERO Clients and
their EUNs SHOULD use application-layer security services such as
TLS/SSL [RFC5246], DTLS [RFC6347] and SSH [RFC4251] to assure the
same level of protection as for critical secured Internet services
such as online banking. AERO Clients that require VPN access to
enterprise networks SHOULD use symmetric network and/or transport
layer security services such as TLS/SSL, DTLS, IPsec [RFC4301], etc.
Control plane security considerations are the same as for standard
IPv6 Neighbor Discovery [RFC4861], except that the PRL also provides
AERO Clients with a list of trusted Servers. As fixed infrastructure
elements, AERO Proxys and Servers SHOULD pre-configure security
associations for one another (e.g., using pre-placed keys) and use
symmetric network and/or transport layer security services such as
IPsec, TLS/SSL or DTLS to secure ND messages. AERO Clients that
connect to secured enclaves need not apply security to their ND
messages, since the messages will be intercepted by an enclave
perimeter Proxy. AERO Clients located outside of secured enclaves
SHOULD use symmetric network and/or transport layer security to
secure their ND exchanges with Servers, but when there are many
prospective neighbors with dynamically changing connectivity an
asymmetric security service such as SEND may be needed (see:
Section 7).
AERO Servers and Relays present targets for traffic amplification
Denial of Service (DoS) attacks. This concern is no different than
for widely-deployed VPN security gateways in the Internet, where
attackers could send spoofed packets to the gateways at high data
rates. This can be mitigated by connecting Relays and Servers over
dedicated links with no connections to the Internet and/or when
connections to the Internet are only permitted through well-managed
firewalls. Traffic amplification DoS attacks can also target an AERO
Client's low data rate links. This is a concern not only for Clients
located on the open Internet but also for Clients in secured
enclaves. AERO Servers and Proxys can institute rate limits that
Templin Expires August 16, 2019 [Page 50]
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protect Clients from receiving packet floods that could DoS low data
rate links.
AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on their EUNs to gain access to a protected
network, i.e., AERO Clients that act as routers MUST NOT provide
routing services for unauthorized nodes. (This concern is no
different than for ordinary hosts that receive an IP address
delegation but then "share" the address with other nodes via some
form of Internet connection sharing such as tethering.)
Although public domain and commercial SEND implementations exist,
concerns regarding the strength of the cryptographic hash algorithm
have been documented [RFC6273] [RFC4982].
The PRL MUST be well-managed and secured from unauthorized tampering,
even though the list includes only public information.
Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.
11. Acknowledgements
Discussions in the IETF, aviation standards communities and private
exchanges helped shape some of the concepts in this work.
Individuals who contributed insights include Mikael Abrahamsson, Mark
Andrews, Fred Baker, Bob Braden, Stewart Bryant, Brian Carpenter,
Wojciech Dec, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli,
Brian Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha
Hlusiak, Lee Howard, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy
Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru
Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz,
Ryuji Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt. Members
of the IESG also provided valuable input during their review process
that greatly improved the document. Special thanks go to Stewart
Bryant, Joel Halpern and Brian Haberman for their shepherding
guidance during the publication of the AERO first edition.
This work has further been encouraged and supported by Boeing
colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam
Brodie, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed
King, Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg
Saccone, Kent Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan
Williams, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
BR&T and BIT mobile networking teams. Kyle Bae, Wayne Benson and
Eric Yeh are especially acknowledged for implementing the AERO
functions as extensions to the public domain OpenVPN distribution.
Templin Expires August 16, 2019 [Page 51]
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Earlier works on NBMA tunneling approaches are found in
[RFC2529][RFC5214][RFC5569].
Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:
o The Internet Routing Overlay Network (IRON)
[RFC6179][I-D.templin-ironbis]
o Virtual Enterprise Traversal (VET)
[RFC5558][I-D.templin-intarea-vet]
o The Subnetwork Encapsulation and Adaptation Layer (SEAL)
[RFC5320][I-D.templin-intarea-seal]
o AERO, First Edition [RFC6706]
Note that these works cite numerous earlier efforts that are not also
cited here due to space limitations. The authors of those earlier
works are acknowledged for their insights.
This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.
This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.
This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.
This work is aligned with the Boeing Research and Technology (BR&T)
autonomous systems networking program.
12. References
12.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
DOI 10.17487/RFC3971, March 2005,
<https://www.rfc-editor.org/info/rfc3971>.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<https://www.rfc-editor.org/info/rfc3972>.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <https://www.rfc-editor.org/info/rfc4191>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC5175] Haberman, B., Ed. and R. Hinden, "IPv6 Router
Advertisement Flags Option", RFC 5175,
DOI 10.17487/RFC5175, March 2008,
<https://www.rfc-editor.org/info/rfc5175>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
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[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
12.2. Informative References
[BGP] Huston, G., "BGP in 2015, http://potaroo.net", January
2016.
[I-D.ietf-dmm-distributed-mobility-anchoring]
Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos,
"Distributed Mobility Anchoring", draft-ietf-dmm-
distributed-mobility-anchoring-12 (work in progress),
January 2019.
[I-D.ietf-intarea-gue]
Herbert, T., Yong, L., and O. Zia, "Generic UDP
Encapsulation", draft-ietf-intarea-gue-06 (work in
progress), August 2018.
[I-D.ietf-intarea-gue-extensions]
Herbert, T., Yong, L., and F. Templin, "Extensions for
Generic UDP Encapsulation", draft-ietf-intarea-gue-
extensions-05 (work in progress), August 2018.
[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-09 (work in
progress), July 2018.
[I-D.ietf-rtgwg-atn-bgp]
Templin, F., Saccone, G., Dawra, G., Lindem, A., and V.
Moreno, "A Simple BGP-based Mobile Routing System for the
Aeronautical Telecommunications Network", draft-ietf-
rtgwg-atn-bgp-01 (work in progress), January 2019.
[I-D.templin-6man-aeroaddr]
Templin, F., "The AERO Address", draft-templin-6man-
aeroaddr-04 (work in progress), December 2018.
[I-D.templin-6man-dhcpv6-ndopt]
Templin, F., "A Unified Stateful/Stateless Configuration
Service for IPv6", draft-templin-6man-dhcpv6-ndopt-07
(work in progress), December 2018.
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[I-D.templin-6man-rio-redirect]
Templin, F. and j. woodyatt, "Route Information Options in
IPv6 Neighbor Discovery", draft-templin-6man-rio-
redirect-07 (work in progress), December 2018.
[I-D.templin-intarea-grefrag]
Templin, F., "GRE Tunnel Level Fragmentation", draft-
templin-intarea-grefrag-04 (work in progress), July 2016.
[I-D.templin-intarea-seal]
Templin, F., "The Subnetwork Encapsulation and Adaptation
Layer (SEAL)", draft-templin-intarea-seal-68 (work in
progress), January 2014.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)", draft-
templin-intarea-vet-40 (work in progress), May 2013.
[I-D.templin-ironbis]
Templin, F., "The Interior Routing Overlay Network
(IRON)", draft-templin-ironbis-16 (work in progress),
March 2014.
[I-D.templin-v6ops-pdhost]
Templin, F., "IPv6 Prefix Delegation and Multi-Addressing
Models", draft-templin-v6ops-pdhost-23 (work in progress),
December 2018.
[OVPN] OpenVPN, O., "http://openvpn.net", October 2016.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.
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[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <https://www.rfc-editor.org/info/rfc1981>.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
DOI 10.17487/RFC2003, October 1996,
<https://www.rfc-editor.org/info/rfc2003>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <https://www.rfc-editor.org/info/rfc2473>.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529,
DOI 10.17487/RFC2529, March 1999,
<https://www.rfc-editor.org/info/rfc2529>.
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
<https://www.rfc-editor.org/info/rfc2764>.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<https://www.rfc-editor.org/info/rfc2784>.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, DOI 10.17487/RFC2890, September 2000,
<https://www.rfc-editor.org/info/rfc2890>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<https://www.rfc-editor.org/info/rfc2983>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
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[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004,
<https://www.rfc-editor.org/info/rfc3819>.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213,
DOI 10.17487/RFC4213, October 2005,
<https://www.rfc-editor.org/info/rfc4213>.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
January 2006, <https://www.rfc-editor.org/info/rfc4251>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
2006, <https://www.rfc-editor.org/info/rfc4389>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511,
DOI 10.17487/RFC4511, June 2006,
<https://www.rfc-editor.org/info/rfc4511>.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
August 2006, <https://www.rfc-editor.org/info/rfc4605>.
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[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.
[RFC4982] Bagnulo, M. and J. Arkko, "Support for Multiple Hash
Algorithms in Cryptographically Generated Addresses
(CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007,
<https://www.rfc-editor.org/info/rfc4982>.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
DOI 10.17487/RFC5214, March 2008,
<https://www.rfc-editor.org/info/rfc5214>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320,
February 2010, <https://www.rfc-editor.org/info/rfc5320>.
[RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
Route Optimization Requirements for Operational Use in
Aeronautics and Space Exploration Mobile Networks",
RFC 5522, DOI 10.17487/RFC5522, October 2009,
<https://www.rfc-editor.org/info/rfc5522>.
[RFC5558] Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
RFC 5558, DOI 10.17487/RFC5558, February 2010,
<https://www.rfc-editor.org/info/rfc5558>.
[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569,
January 2010, <https://www.rfc-editor.org/info/rfc5569>.
[RFC5720] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720,
DOI 10.17487/RFC5720, February 2010,
<https://www.rfc-editor.org/info/rfc5720>.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, DOI 10.17487/RFC5996, September 2010,
<https://www.rfc-editor.org/info/rfc5996>.
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[RFC6179] Templin, F., Ed., "The Internet Routing Overlay Network
(IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
<https://www.rfc-editor.org/info/rfc6179>.
[RFC6221] Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
DOI 10.17487/RFC6221, May 2011,
<https://www.rfc-editor.org/info/rfc6221>.
[RFC6273] Kukec, A., Krishnan, S., and S. Jiang, "The Secure
Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273,
DOI 10.17487/RFC6273, June 2011,
<https://www.rfc-editor.org/info/rfc6273>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6422] Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
RFC 6422, DOI 10.17487/RFC6422, December 2011,
<https://www.rfc-editor.org/info/rfc6422>.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
<https://www.rfc-editor.org/info/rfc6438>.
[RFC6706] Templin, F., Ed., "Asymmetric Extended Route Optimization
(AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
<https://www.rfc-editor.org/info/rfc6706>.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013,
<https://www.rfc-editor.org/info/rfc6864>.
[RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
March 2017, <https://www.rfc-editor.org/info/rfc8086>.
[TUNTAP] Wikipedia, W., "http://en.wikipedia.org/wiki/TUN/TAP",
October 2014.
Appendix A. AERO Alternate Encapsulations
When GUE encapsulation is not needed, AERO can use common
encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic
Routing Encapsulation (GRE) [RFC2784][RFC2890] and others. The
encapsulation is therefore only differentiated from non-AERO tunnels
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through the application of AERO control messaging and not through,
e.g., a well-known UDP port number.
As for GUE encapsulation, alternate AERO encapsulation formats may
require encapsulation layer fragmentation. For simple IP-in-IP
encapsulation, an IPv6 fragment header is inserted directly between
the inner and outer IP headers when needed, i.e., even if the outer
header is IPv4. The IPv6 Fragment Header is identified to the outer
IP layer by its IP protocol number, and the Next Header field in the
IPv6 Fragment Header identifies the inner IP header version. For GRE
encapsulation, a GRE fragment header is inserted within the GRE
header [I-D.templin-intarea-grefrag].
Figure 5 shows the AERO IP-in-IP encapsulation format before any
fragmentation is applied:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IPv4 Header | | Outer IPv6 Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPv6 Frag Header (optional)| |IPv6 Frag Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header | | Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
~ ~ ~ ~
~ Inner Packet Body ~ ~ Inner Packet Body ~
~ ~ ~ ~
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Minimal Encapsulation in IPv4 Minimal Encapsulation in IPv6
Figure 5: Minimal Encapsulation Format using IP-in-IP
Figure 6 shows the AERO GRE encapsulation format before any
fragmentation is applied:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Header |
| (with checksum, key, etc..) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Fragment Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Inner Packet Body ~
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Minimal Encapsulation Using GRE
Alternate encapsulation may be preferred in environments where GUE
encapsulation would add unnecessary overhead. For example, certain
low-bandwidth wireless data links may benefit from a reduced
encapsulation overhead.
GUE encapsulation can traverse network paths that are inaccessible to
non-UDP encapsulations, e.g., for crossing Network Address
Translators (NATs). More and more, network middleboxes are also
being configured to discard packets that include anything other than
a well-known IP protocol such as UDP and TCP. It may therefore be
necessary to determine the potential for middlebox filtering before
enabling alternate encapsulation in a given environment.
In addition to IP-in-IP, GRE and GUE, AERO can also use security
encapsulations such as IPsec and SSL/TLS. In that case, AERO control
messaging and route determination occur before security encapsulation
is applied for outgoing packets and after security decapsulation is
applied for incoming packets.
AERO is especially well suited for use with VPN system encapsulations
such as OpenVPN [OVPN].
Appendix B. When to Insert an Encapsulation Fragment Header
An encapsulation fragment header is inserted when the AERO tunnel
ingress needs to apply fragmentation to accommodate packets that must
be delivered without loss due to a size restriction. Fragmentation
is performed on the inner packet while encapsulating each inner
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packet fragment in outer IP and encapsulation layer headers that
differ only in the fragment header fields.
The fragment header can also be inserted in order to include a
coherent Identification value with each packet, e.g., to aid in
Duplicate Packet Detection (DPD). In this way, network nodes can
cache the Identification values of recently-seen packets and use the
cached values to determine whether a newly-arrived packet is in fact
a duplicate. The Identification value within each packet could
further provide a rough indicator of packet reordering, e.g., in
cases when the tunnel egress wishes to discard packets that are
grossly out of order.
In some use cases, there may be operational assurance that no
fragmentation of any kind will be necessary, or that only occasional
large control messages will require fragmentation. In that case, the
encapsulation fragment header can be omitted and ordinary
fragmentation of the outer IP protocol version can be applied when
necessary.
Appendix C. Autoconfiguration for Constrained Platforms
On some platforms (e.g., popular cell phone operating systems), the
act of assigning a default IPv6 route and/or assigning an address to
an interface may not be permitted from a user application due to
security policy. Typically, those platforms include a TUN/TAP
interface [TUNTAP] that acts as a point-to-point conduit between user
applications and the AERO interface. In that case, the Client can
instead generate a "synthesized RA" message. The message conforms to
[RFC4861] and is prepared as follows:
o the IPv6 source address is the Client's AERO address
o the IPv6 destination address is all-nodes multicast
o the Router Lifetime is set to a time that is no longer than the
ACP DHCPv6 lifetime
o the message does not include a Source Link Layer Address Option
(SLLAO)
o the message includes a Prefix Information Option (PIO) with a /64
prefix taken from the ACP as the prefix for autoconfiguration
The Client then sends the synthesized RA message via the TUN/TAP
interface, where the operating system kernel will interpret it as
though it were generated by an actual router. The operating system
will then install a default route and use StateLess Address
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AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP
interface. Methods for similarly installing an IPv4 default route
and IPv4 address on the TUN/TAP interface are based on synthesized
DHCPv4 messages [RFC2131].
Appendix D. Operational Deployment Alternatives
AERO can be used in many different variations based on the specific
use case. The following sections discuss variations that adhere to
the AERO principles while allowing selective application of AERO
components.
D.1. Operation on AERO Links Without DHCPv6 Services
When Servers on the AERO link do not provide DHCPv6 services,
operation can still be accommodated through administrative
configuration of ACPs on AERO Clients. In that case, administrative
configurations of AERO interface neighbor cache entries on both the
Server and Client are also necessary. However, this may interfere
with the ability for Clients to dynamically change to new Servers,
and can expose the AERO link to misconfigurations unless the
administrative configurations are carefully coordinated.
D.2. Operation on Server-less AERO Links
In some AERO link scenarios, there may be no Servers on the link and/
or no need for Clients to use a Server as an intermediary trust
anchor. In that case, each Client acts as a Server unto itself to
establish neighbor cache entries by performing direct Client-to-
Client IPv6 ND message exchanges, and some other form of trust basis
must be applied so that each Client can verify that the prospective
neighbor is authorized to use its claimed ACP.
When there is no Server on the link, Clients must arrange to receive
ACPs and publish them via a secure alternate PD authority through
some means outside the scope of this document.
D.3. Operation on Client-less AERO Links
In some environments, the AERO service may be useful for mobile nodes
that do not implement the AERO Client function and do not perform
encapsulation. For example, if the mobile node has a way of
injecting its ACP into the access subnetwork routing system an AERO
Server connected to the same access network can accept the ACP prefix
injection as an indication that a new mobile node has come onto the
subnetwork. The Server can then inject the ACP into the BGP routing
system the same as if an AERO Client/Server PD exchange had occurred.
If the mobile node subsequently withdraws the ACP from the access
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network routing system, the Server can then withdraw the ACP from the
BGP routing system.
In this arrangement, AERO Servers and Relays are used in exactly the
same ways as for environments where DHCPv6 Client/Server exchanges
are supported. However, the access subnetwork routing systems must
be capable of accommodating rapid ACP injections and withdrawals from
mobile nodes with the understanding that the information must be
propagated to all routers in the system. Operational experience has
shown that this kind of routing system "churn" can lead to overall
instability and routing system inconsistency.
D.4. Manually-Configured AERO Tunnels
In addition to the dynamic neighbor discovery procedures for AERO
link neighbors described above, AERO encapsulation can be applied to
manually-configured tunnels. In that case, the tunnel endpoints use
an administratively-provisioned AERO address and exchange NS/NA
messages the same as for dynamically-established tunnels.
D.5. Encapsulation Avoidance on Relay-Server Dedicated Links
In some environments, AERO Servers and Relays may be connected by
dedicated point-to-point links, e.g., high speed fiberoptic leased
lines. In that case, the Servers and Relays can participate in the
AERO link the same as specified above but can avoid encapsulation
over the dedicated links. In that case, however, the links would be
dedicated for AERO and could not be multiplexed for both AERO and
non-AERO communications.
D.6. Encapsulation Protocol Version Considerations
A source Client may connect only to an IPvX underlying network, while
the target Client connects only to an IPvY underlying network. In
that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any route
optimization messages and continue to send packets via their Servers.
D.7. Extending AERO Links Through Security Gateways
When an enterprise mobile node moves from a campus LAN connection to
a public Internet link, it must re-enter the enterprise via a
security gateway that has both a physical interface connection to the
Internet and a physical interface connection to the enterprise
internetwork. This most often entails the establishment of a Virtual
Private Network (VPN) link over the public Internet from the mobile
node to the security gateway. During this process, the mobile node
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supplies the security gateway with its public Internet address as the
link-layer address for the VPN. The mobile node then acts as an AERO
Client to negotiate with the security gateway to obtain its ACP.
In order to satisfy this need, the security gateway also operates as
an AERO Server with support for AERO Client proxying. In particular,
when a mobile node (i.e., the Client) connects via the security
gateway (i.e., the Server), the Server provides the Client with an
ACP in a PD exchange the same as if it were attached to an enterprise
campus access link. The Server then replaces the Client's link-layer
source address with the Server's enterprise-facing link-layer address
in all AERO messages the Client sends toward neighbors on the AERO
link. The AERO messages are then delivered to other nodes on the
AERO link as if they were originated by the security gateway instead
of by the AERO Client. In the reverse direction, the AERO messages
sourced by nodes within the enterprise network can be forwarded to
the security gateway, which then replaces the link-layer destination
address with the Client's link-layer address and replaces the link-
layer source address with its own (Internet-facing) link-layer
address.
After receiving the ACP, the Client can send IP packets that use an
address taken from the ACP as the network layer source address, the
Client's link-layer address as the link-layer source address, and the
Server's Internet-facing link-layer address as the link-layer
destination address. The Server will then rewrite the link-layer
source address with the Server's own enterprise-facing link-layer
address and rewrite the link-layer destination address with the
target AERO node's link-layer address, and the packets will enter the
enterprise network as though they were sourced from a node located
within the enterprise. In the reverse direction, when a packet
sourced by a node within the enterprise network uses a destination
address from the Client's ACP, the packet will be delivered to the
security gateway which then rewrites the link-layer destination
address to the Client's link-layer address and rewrites the link-
layer source address to the Server's Internet-facing link-layer
address. The Server then delivers the packet across the VPN to the
AERO Client. In this way, the AERO virtual link is essentially
extended *through* the security gateway to the point at which the VPN
link and AERO link are effectively grafted together by the link-layer
address rewriting performed by the security gateway. All AERO
messaging services (including route optimization and mobility
signaling) are therefore extended to the Client.
In order to support this virtual link grafting, the security gateway
(acting as an AERO Server) must keep static neighbor cache entries
for all of its associated Clients located on the public Internet.
The neighbor cache entry is keyed by the AERO Client's AERO address
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the same as if the Client were located within the enterprise
internetwork. The neighbor cache is then managed in all ways as
though the Client were an ordinary AERO Client. This includes the
AERO IPv6 ND messaging signaling for Route Optimization and Neighbor
Unreachability Detection.
Note that the main difference between a security gateway acting as an
AERO Server and an enterprise-internal AERO Server is that the
security gateway has at least one enterprise-internal physical
interface and at least one public Internet physical interface.
Conversely, the enterprise-internal AERO Server has only enterprise-
internal physical interfaces. For this reason security gateway
proxying is needed to ensure that the public Internet link-layer
addressing space is kept separate from the enterprise-internal link-
layer addressing space. This is afforded through a natural extension
of the security association caching already performed for each VPN
client by the security gateway.
Appendix E. Change Log
<< RFC Editor - remove prior to publication >>
Changes from draft-templin-intarea-6706bis-03 to draft-templin-
intrea-6706bis-04:
o Added definitions for Potential Router List (PRL) and secure
enclave
o Included text on mapping transport layer port numbers to network
layer DSCP values
o Added reference to DTLS and DMM Distributed Mobility Anchoring
working group document
o Reworked Security Considerations
o Updated references.
Changes from draft-templin-intarea-6706bis-02 to draft-templin-
intrea-6706bis-03:
o Added new section on SEND.
o Clarifications on "AERO Address" section.
o Updated references and added new reference for RFC8086.
o Security considerations updates.
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o General text clarifications and cleanup.
Changes from draft-templin-intarea-6706bis-01 to draft-templin-
intrea-6706bis-02:
o Note on encapsulation avoidance in Section 4.
Changes from draft-templin-intarea-6706bis-00 to draft-templin-
intrea-6706bis-01:
o Remove DHCPv6 Server Release procedures that leveraged the old way
Relays used to "route" between Server link-local addresses
o Remove all text relating to Relays needing to do any AERO-specific
operations
o Proxy sends RS and receives RA from Server using SEND. Use CGAs
as source addresses, and destination address of RA reply is to the
AERO address corresponding to the Client's ACP.
o Proxy uses SEND to protect RS and authenticate RA (Client does not
use SEND, but rather relies on subnetwork security. When the
Proxy receives an RS from the Client, it creates a new RS using
its own addresses as the source and uses SEND with CGAs to send a
new RS to the Server.
o Emphasize distributed mobility management
o AERO address-based RS injection of ACP into underlying routing
system.
Changes from draft-templin-aerolink-82 to draft-templin-intarea-
6706bis-00:
o Document use of NUD (NS/NA) for reliable link-layer address
updates as an alternative to unreliable unsolicited NA.
Consistent with Section 7.2.6 of RFC4861.
o Server adds additional layer of encapsulation between outer and
inner headers of NS/NA messages for transmission through Relays
that act as vanilla IPv6 routers. The messages include the AERO
Server Subnet Router Anycast address as the source and the Subnet
Router Anycast address corresponding to the Client's ACP as the
destination.
o Clients use Subnet Router Anycast address as the encapsulation
source address when the access network does not provide a
topologically-fixed address.
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Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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