Internet DRAFT - draft-templin-aerolink
draft-templin-aerolink
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720, May 10, 2018
rfc6179, rfc6706 (if
approved)
Intended status: Standards Track
Expires: November 11, 2018
Asymmetric Extended Route Optimization (AERO)
draft-templin-aerolink-82.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 November 11, 2018.
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Copyright Notice
Copyright (c) 2018 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
publication of this document. Please review these documents
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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 . . . . . . . . . . . . . . . . 7
3.2. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 9
3.3. AERO Routing System . . . . . . . . . . . . . . . . . . . 10
3.4. AERO Interface Link-local Addresses . . . . . . . . . . . 11
3.5. AERO Interface Characteristics . . . . . . . . . . . . . 13
3.6. AERO Interface Initialization . . . . . . . . . . . . . . 16
3.6.1. AERO Relay Behavior . . . . . . . . . . . . . . . . . 16
3.6.2. AERO Server Behavior . . . . . . . . . . . . . . . . 16
3.6.3. AERO Client Behavior . . . . . . . . . . . . . . . . 17
3.6.4. AERO Proxy Behavior . . . . . . . . . . . . . . . . . 17
3.7. AERO Interface Neighbor Cache Maintenance . . . . . . . . 18
3.8. AERO Interface Forwarding Algorithm . . . . . . . . . . . 19
3.8.1. Client Forwarding Algorithm . . . . . . . . . . . . . 20
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 . . . . . . . . . . . . . . 22
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 Interface 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. Re-encapsulating and Relaying NAs . . . . . . . . . 40
3.15.7. Processing NAs . . . . . . . . . . . . . . . . . . . 41
3.15.8. Server and Proxy Extended Route Optimization . . . . 41
3.16. Neighbor Unreachability Detection (NUD) . . . . . . . . . 43
3.17. Mobility Management and Quality of Service (QoS) . . . . 44
3.17.1. Forwarding Packets on Behalf of Departed Clients . . 44
3.17.2. Announcing Link-Layer Address and QoS Preference
Changes . . . . . . . . . . . . . . . . . . . . . . 44
3.17.3. Bringing New Links Into Service . . . . . . . . . . 45
3.17.4. Removing Existing Links from Service . . . . . . . . 45
3.17.5. Implicit Mobility Management . . . . . . . . . . . . 45
3.17.6. Moving to a New Server . . . . . . . . . . . . . . . 46
3.18. Multicast Considerations . . . . . . . . . . . . . . . . 46
4. The AERO Proxy . . . . . . . . . . . . . . . . . . . . . . . 47
5. Direct Underlying Interfaces . . . . . . . . . . . . . . . . 49
6. Operation on AERO Links with /64 ASPs . . . . . . . . . . . . 50
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 50
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
9. Security Considerations . . . . . . . . . . . . . . . . . . . 51
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 52
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 53
11.1. Normative References . . . . . . . . . . . . . . . . . . 53
11.2. Informative References . . . . . . . . . . . . . . . . . 55
Appendix A. AERO Alternate Encapsulations . . . . . . . . . . . 60
Appendix B. When to Insert an Encapsulation Fragment Header . . 62
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 . . . . . . . . . . . 64
D.3. Operation on Client-less AERO Links . . . . . . . . . . . 64
D.4. Manually-Configured AERO Tunnels . . . . . . . . . . . . 64
D.5. Encapsulation Avoidance on Relay-Server Dedicated Links . 65
D.6. Encapsulation Protocol Version Considerations . . . . . . 65
D.7. Extending AERO Links Through Security Gateways . . . . . 65
Appendix E. Change Log . . . . . . . . . . . . . . . . . . . . . 67
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)
[RFC3315][RFC3633].
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
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 remainder of this document presents 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 [RFC3315] [RFC3633],
however other services (e.g., alternate ND options, network
management, static configuration, etc.) are also possible.
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(native) Internetwork
a connected IPv6 or IPv4 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. All 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 a
tunnel virtual overlay 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 IP PDs from one or more AERO Servers.
Following PD, the Client assigns an 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 IPv6 link-local 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")
a node that configures an AERO interface to relay IP packets
between nodes on the same AERO link and/or forward IP packets
between the AERO link and the native Internetwork. The Relay
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assigns an administratively-provisioned IPv6 link-local address to
the AERO interface the same as for a Server. An AERO Relay can
also act as an AERO Server.
AERO Proxy ("Proxy")
a node that provides proxying services for Clients that cannot
associate directly with Servers, 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
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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.
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 and must be no longer than 64 for IPv6 or 32 for IPv4.
base AERO address
the lowest-numbered AERO address from the first ACP delegated to
the Client (see Section 3.4).
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 [RFC3315].
The terminology of DHCPv6 [RFC3315][RFC3633] 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:
3.1. AERO Link Reference Model
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.-(::::::::)
.-(::::::::::::)-.
(:: 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.
o AERO Proxy P1 provides proxy services for AERO Clients in secured
enclaves that cannot associate directly with other AERO link
neighbors.
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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 provide default forwarding services to 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 AERO interface neighbor cache entries for Servers,
and maintain an IP forwarding table entry for each AERO Client Prefix
(ACP). AERO Relays can also be configured to act as AERO Servers.
AERO Servers provide default forwarding services to 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 Servers can also be configured to act as
AERO Relays.
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 conduit for AERO Clients connected to secured
enclaves to associate with AERO link Servers. The Proxy can either
be explicit or transparent. In the explicit case, the Client sends
all of its control plane messages addressed to the Server to the
link-layer address of the Proxy. In the transparent case, 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. In both cases, 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.
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
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. At the time of
this writing, the global public Internet BGP routing system manages
more than 500K routes with linear growth and no signs of router
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resource exhaustion [BGP]. 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.
Note that 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.templin-atn-bgp].
3.4. AERO Interface Link-local Addresses
AERO interface link-local address types include administratively-
provisioned addresses and AERO addresses.
Administratively-provisioned addresses are allocated from the range
fe80::/96 and assigned to a Server or Relay's AERO interface.
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Administratively-provisioned addresses MUST be managed for uniqueness
by the administrative authority for the AERO link. The address
fe80:: is reserved as the IPv6 link-local subnet router anycast
address, and the address fe80::ffff:ffff is reserved as the "prefix-
solicitation" address used by Clients to bootstrap AERO address
autoconfiguration. These reserved addresses are therefore not
available for general assignment.
An AERO address is an IPv6 link-local address with an embedded prefix
based on an ACP and associated with a Client's AERO interface. AERO
addresses remain stable as the Client moves between topological
locations, i.e., even if its link-layer addresses change.
For IPv6, 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, AERO addresses are based on an IPv4-mapped IPv6 address
[RFC4291] 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
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... etc. ...
fe80:FFFF:192.0.2.31
When the Server delegates ACPs to the Client, both the Server and
Client use the lowest-numbered AERO address from the first ACP
delegation 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 short prefix lengths), 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.
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 Neighbor Solicitation (NS), Neighbor Advertisement (NA),
Router Solicitation (RS), Router Advertisement (RA) and Redirect for
neighbor cache management. AERO interfaces use RS/RA messages with
an embedded PD message (e.g., see: [I-D.templin-6man-dhcpv6-ndopt]).
AERO interfaces include routing information in ND messages to support
route optimization.
AERO interface ND messages include one or more Source/Target Link-
Layer Address Options (S/TLLAOs) formatted as shown in Figure 2:
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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 |X| 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:
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 X (proXy) is set to '1' in an S/TLLAO if the address corresponds
to a Proxy; otherwise, X is set to '0'.
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. The value 255 is reserved
for Server-based route optimization (see: Section 3.15.8).
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
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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 64 Preference values 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 NAT'ed 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 AERO control messages on
behalf of the Client.
o VPN'ed interfaces use security encapsulation over the Internetwork
to a Virtual Private Network (VPN) gateway that also acts as an
AERO Server. As with NAT'ed links, the AERO Server can issue
control messages on behalf of the Client.
o Proxy'ed interfaces connect to a closed network that is separated
from the open Internetwork by an AERO Proxy. Unlike NAT'ed and
VPN'ed interfaces, the AERO Proxy (rather than the Server) can
issue control message on behalf of the Client.
o Direct interfaces connect the Client directly to a peer 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
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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.
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 link-local 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: 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), and maintains neighbor cache
entries for all Servers on the link. Relays exchange NS/NA messages
with AERO link neighbors the same as for any AERO node. However,
Neighbor Unreachability Detection (NUD) (see: Section 3.16) is
optional since the dynamic routing protocol already provides
reachability confirmation.
3.6.2. AERO Server Behavior
When a Server enables an AERO interface, it assigns an
administratively-provisioned link-local address fe80::ID the same as
for Relays. The Server further configures a service to facilitate PD
exchanges with AERO Clients. The Server maintains neighbor cache
entries for one or more Relays on the link, and manages per-Client
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neighbor cache entries and IP forwarding table entries based on
control message exchanges. Each Server also engages in a dynamic
routing protocol with their neighboring Relays (see: Section 3.3).
When the Server receives an NS/RS message from a Client on the AERO
interface it authenticates the message and returns an NA/RA message.
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 driver 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 PD
"Solicit" options over an underlying interface using the prefix-
solicitation address as the source network layer address and all-
routers [RFC4861] as the destination network layer address to obtain
ACPs from one or more AERO Servers. Each Server processes the
message and returns an RA message with a PD "Reply" option with the
Server's link-layer address as the source and the base AERO address
as the destination network layer addresses. In this way, the ND/PD
control messages securely perform all autoconfiguration operations in
a single request/response exchange.
After the initial ND/PD message exchange, the Client can register
additional underlying interfaces with the Server by sending an RS
message over each underlying interface using its base AERO address as
the source network layer address and without including a PD option.
The Server will update its neighbor cache entry for the Client and
return an 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.
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 the Clients'
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
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forwards the 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 forwards
the 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 proxyed 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 Servers on the link, and AERO Servers
maintain permanent neighbor cache entries for 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 by gleaning information from 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, and include pointers to the Client's current set
of Servers.
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 with an AERO
source address, 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 an "AcceptTime" variable in the
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neighbor cache entry to ACCEPT_TIME seconds and uses this value to
determine whether packets received from the correspondent can be
accepted. The node resets AcceptTime when it receives a new ND
message, and otherwise decrements AcceptTime while no ND messages
have been received. It is RECOMMENDED that ACCEPT_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 AcceptTime
decrements below FORWARD_TIME (see below).
When a source AERO node receives a valid NA message with an AERO
source address that matches 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 have been received. 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 ACCEPT_TIME, FORWARD_TIME and MAX_RETRY MAY be
administratively set, if necessary, to better match the AERO link's
performance characteristics; however, if different values are chosen,
all nodes on the link MUST consistently configure the same values.
Most importantly, ACCEPT_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 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 a PD
option to receive RA replies. The RS/RA messaging will keep NAT
state alive and test Server reachability without disturbing the PD
service.
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
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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. 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 link-local address.
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).
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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.
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.
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.
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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 and otherwise searches for a neighbor
cache entry that matches the destination (e.g., for administratively-
provisioned link-local addresses). 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
into the AERO link. Otherwise, the Relay drops the packet and (for
non-link-local addresses) 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, 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, or if the destination matches the link-local address in a
permanent neighbor cache entry, 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.
o else, the Relay drops the packet and (for non-link-local
addresses) returns an ICMP Destination Unreachable message subject
to rate limiting (see: Section 3.13).
3.8.5. Processing Return Packets
When an AERO node receives a return packet such as generated by 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
node has sufficient trust basis to accept link-layer Destination
Unreachable messages, it can then process the return packet as
described in the following paragraph. Otherwise, the node SHOULD
drop the packet and treat it as an indication that a path may be
failing, and MAY use NUD to test the path for reachability.
If the node has sufficient trust basis to accept return packets, it
searches for a dynamic neighbor cache entry that matches the
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destination. If there is a match, the neighbor 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
neighbor instead sets FowardTime in the dynamic neighbor cache entry
to 0. If the source address corresponds to one of the neighbor's own
addresses, the neighbor also forwards the packet to the corresponding
Server; otherwise, it 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 (see: Appendix A). 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 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.
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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].
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:
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, 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 Clients and Servers accept encapsulated packets if there is a
dynamic neighbor cache entry with an AERO address that matches the
packet's network-layer source address, with a link-layer address
that matches the packet's link-layer source address, and with a
non-zero AcceptTime.
o AERO Proxies accept encapsulated packets if there is a proxy
neighbor cache entry that matches the packet's network-layer
destination address (i.e., the address of the Client) and link-
layer source address (i.e., the address of one of the Client's
Servers). When the proxy is configured to accept packets
originating from any address in the open Internetwork however
(e.g., from another Proxy), it omits the source address check.
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Note that this simple data origin authentication is effective in
environments in which link-layer addresses cannot be spoofed. In
other environments, each AERO message must 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 environments where
end systems use end-to-end security, however, it may be sufficient to
require signatures only for ND and ICMP control plane messages 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
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).
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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
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
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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
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):
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| 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
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
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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
math 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 send a PD "Release" message
to 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 path as unusable and use another path. If it receives
Destination Unreachable messages on multiple paths, the Server
should take no further actions unless it receives a 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 first writes the network-layer
source address of the original packet as the destination address of
the message and determines the next hop to the destination. If the
next hop is reached via the AERO interface, the Relay uses the IPv6
address "::" or the IPv4 address "0.0.0.0" as the source address of
the message, then encapsulates the message and forwards it to the
next hop within the AERO interface. Otherwise, the Relay uses one of
its non link-local addresses as the source address of the message and
forwards it via a link outside the AERO interface.
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 of the message and determines the next hop to the
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destination. If the next hop is reached via the AERO interface, the
node uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the
source address of the message, then encapsulates the message and
forwards it to the next hop within the AERO interface. Otherwise,
the node uses one of its non link-local addresses as the source
address of the message and forwards it via a link outside the AERO
interface.
When an AERO node receives any network-layer error message via the
AERO interface, it examines the network-layer destination address.
If the next hop toward the destination is via the AERO interface, the
node re-encapsulates and forwards the message to the next hop within
the AERO interface. Otherwise, if the network-layer source address
is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node
writes one of its non link-local addresses as the source address,
recalculates the IP and/or ICMP checksums then forwards the message
via a link outside the AERO interface.
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. The RS/RA
messaging is conducted in the same fashion as specified in [RFC5214].
AERO Clients and Servers include PD messages as options in the RS/RA
messages they exchange (see: [I-D.templin-6man-dhcpv6-ndopt]).
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Client-initiated PD options are included in RS messages, and Server-
initiated PD options are included in RA messages. The unified ND/PD
messages are exchanged between Client and Server according to the
prefix management schedule determined by the PD service. The unified
messages can be protected using SEcure Neighbor Discovery (SEND)
[RFC3971].
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
recipient can infer that the sender holds the prefix delegation
"2001:db8:1:2::/64".
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 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-Qualfied 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 a combined ND/PD message exchange. The Client
includes a PD "Solicit" message as an ND option in an RS message with
the prefix-solicitation address as the IPv6 source address, 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's underlying interface does not have an IP
address, the Client can use the ACP Subnet Router Anycast address as
the link-layer source address.)
The Client next includes a "Client Identifier" and an "IA_PD" (i.e.,
prefix request) code in the PD "Solicit" message. If the Client is
pre-provisioned with ACPs associated with the AERO service, it MAY
also include the ACPs in the "IA_PD" option to indicate its
preferences to the Server. The Client finally includes any
additional PD codes (e.g., "Rapid Commit").
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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 MUST have assurance that there will be no
NATs or Proxies on the paths to the Server via those interfaces.
(Otherwise, the Client can register additional link-layer addresses
with the Server by sending subsequent NS/RS messages via different
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 no RA is received, or if it
receives an RA with Router Lifetime set to 0 and/or a "Reply" with no
ACPs, the Client SHOULD discontinue autoconfiguration attempts
through this Server and try another Server. Otherwise, the Client
processes the ACPs in the embedded "Reply" 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 assumes 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]. The Client subsequently
maintains its ACP delegations through each of its Servers by sending
RS "Renew", "Rebind", and/or "Release" messages. The Server will in
turn send RA "Reply" 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
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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 source and target address of the
NA message are set to the Client's AERO 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").
If the Client wishes to discontinue use of a Server it issues an RS
"Release" message. 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".
3.14.3. AERO Server Behavior
AERO Servers act as IPv6 routers and support a PD service on their
AERO links. 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.
When an AERO Server receives a prospective Client's RS "Solicit"
message on its AERO interface, and the Server is too busy, it SHOULD
return an immediate RA "Reply" message with no ACPs and with Router
Lifetime set to 0. Otherwise, the Server authenticates the RS
message and processes the embedded "Solicit" option. 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 "Reply" message that includes the
delegated ACPs. For IPv4 ACPs, the ACP is in IPv4-mapped IPv6
address format and with prefix length set as specified in
Section 3.4. The Server then prepares an RA "Reply" message using
its link-local address (i.e., fe80::ID) as the network-layer source
address, the Client's base AERO address from the first ACP 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
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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
"Reply" 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
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 an RS "Renew", the Server extends the PD
lifetimes. If the Client issues an RS "Release", or if the Client
does not issue a "Renew" 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 "Reconfigure" message 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 PD service, 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 driver 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 option and wraps it in IPv6/UDP headers. 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
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the UDP fields to values that will be understood by the DHCPv6
server.
The LDRA then wraps the message in a 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
DHCPv6 server then delivers the Relay-Reply message to the LDRA,
which discards the Relay-Reply wrapper and IPv6/UDP headers, then
delivers the DHCPv6 message to be wrapped into 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 Interface 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 on behalf of
any of its native underlying interfaces. 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 link route optimization
procedure.
3.15.1. Reference Operational Scenario
Figure 4 depicts the AERO link 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
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Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary
IPv6 hosts ('H1', 'H2'):
+--------------+ +--------------+ +--------------+
| 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
link-local 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-
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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.
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 re-encapsulates and 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
Server ('S2'). When Server ('S2') receives the NA message it re-
encapsulates the message and forwards it on to Relay ('R1'), which
re-encapsulates and forwards the message on to Server ('S1') which
re-encapsulates and forwards the message on to 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:
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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')).
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 one or more SLLAOs set to appropriate values
for Client ('C1')'s native underlying interfaces.
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. Otherwise, Server ('S1') verifies that Client ('C1') is
authorized to use the ACPs encoded in the RIOs of the NS and discards
the NS if verification fails.
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') re-encapsulates the NS and
relays it via Relay ('R1') by changing the link-layer source address
of the message to 'L2(S1)' and changing the link-layer destination
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address to 'L2(R1)'. Server ('S1') finally forwards the re-
encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R1') receives the NS message from Server ('S1') it
determines that Server ('S2') is the next hop toward Client ('C2') by
consulting its forwarding table. Relay ('R1') then re-encapsulates
the NS while changing the link-layer source address to 'L2(R1)' and
changing the link-layer destination address to 'L2(S2)'. Relay
('R1') then relays the NS via Server ('S2').
When Server ('S2') receives the NS message from Relay ('R1') it
determines that Client ('C2') is a neighbor by consulting its
neighbor cache. 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') , stores the link-layer
addresses found in the SLLAOs as the link-layer addresses of Client
('C1'), and stores the ACPs encoded in the RIOs of the NS as the ACPs
for Client ('C1'). Client ('C2') then sets AcceptTime for the
neighbor cache entry to ACCEPT_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(S2)' (i.e., the
link-layer address of Server ('S2')).
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 is
included).
Client ('C2') then sends the NA message to Server ('S2').
3.15.6. Re-encapsulating and Relaying NAs
When Server ('S2') receives an NA message from Client ('C2'), it
first verifies that the TLLAOs in the NA are a proper subset of the
Interface IDs in Client ('C2')'s neighbor cache entry. If the
Client's TLLAOs are not acceptable, Server ('S2') discards the
message. Otherwise, Server ('S2') verifies that Client ('C2') is
authorized to use the ACPs encoded in the RIOs of the NA message. If
validation fails, Server ('S2') discards the NA.
Server ('S2') then examines the network-layer destination address of
the NA to determine the next hop toward Client ('C1') by searching
for the AERO address in the neighbor cache. Since Client ('C1') is
not a neighbor, Server ('S2') re-encapsulates the NA and relays it
via Relay ('R1') by changing the link-layer source address of the
message to 'L2(S2)' and changing the link-layer destination address
to 'L2(R1)'. Server ('S2') finally forwards the re-encapsulated
message to Relay ('R1') without decrementing the network-layer TTL/
Hop Limit field.
When Relay ('R1') receives the NA message from Server ('S2') it
determines that Server ('S1') is the next hop toward Client ('C1') by
consulting its forwarding table. Relay ('R1') then re-encapsulates
the NA while changing the link-layer source address to 'L2(R1)' and
changing the link-layer destination address to 'L2(S1)'. Relay
('R1') then relays the NA via Server ('S1').
When Server ('S1') receives the NA message from Relay ('R1') it
determines that Client ('C1') is a neighbor by consulting its
neighbor cache. Server ('S1') then re-encapsulates the NA while
changing the link-layer source address to 'L2(S1)' and changing the
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link-layer destination address to 'L2(C1)'. Server ('S1') then
forwards the message to Client ('C1').
3.15.7. Processing NAs
When Client ('C1') receives the NA message, it first verifies the
Nonce value matches the value that it included in its NS message (if
any). If the Nonce values match, 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 AcceptTime value. Thereafter, Client ('C1') may forward
ordinary network-layer data packets directly to Client ('C2') without
involving any intermediate nodes, and Client ('C2') can verify that
the packets came from an acceptable source. (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.)
3.15.8. Server and Proxy Extended Route Optimization
Route optimization may be initiated by the source Client by sending
NS messages with SLLAOs corresponding to its native underlying
interfaces. Route optimization for the source Client's other
interfaces may be initiated by Servers and/or Proxies. Each node
initiates route optimization by sending NS messages with SLLAOs only
for those underlying interfaces they are authoritative for. Each
node MUST consistently use the same Interface ID values to denote the
same interfaces. The Interface IDs are established and maintained by
the source Client's RS/RA exchanges.
The target Client's Server serves as a route optimization target if
some or all of the target Client's underlying interfaces connect via
NATs, Proxies and/or VPNs. In that case, when the source sends an NS
message the target Server both forwards the NS toward a native
underlying interface of the target Client (if any) and prepares an NA
response the same as if it were the target Client (see:
Section 3.15.5). (This means that the source may receive two
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separate NA messages - one from the target Server and one from the
target Client. The source must accept the union of the information
from both messages.)
For non-native underlying interfaces, the target Server includes a
first TLLAO option in the NA with Interface ID set to 255 and
includes any additional TLLAOs corresponding to the Client's NATed,
Proxyed and/or VPNed underlying interfaces. The Server writes its
own link-layer address in TLLAOs corresponding to NATed and VPNed
underlying interfaces, and writes the link-layer address of the Proxy
in TLLAOs corresponding to Proxyed underlying interfaces (while also
setting the X flag). The Interface ID and QoS Preference values in
the TLLAOs are those supplied by the Client during the initial RS/RA
exchange and updated by any ensuing unsolicited NA messages. The
target Server must then maintain a dynamic neighbor cache entry for
the Client, but MUST NOT send BGP updates for Clients discovered
through dynamic route optimization.
Thereafter, if the target Client moves to a new Server, 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. The source then allows
future packets destined to the target Client to again flow through
its own Server (or Relay). Note however that the old Server retains
the neighbor cache entry with its associated AcceptTime since there
may be many packets in flight. AcceptTime will then eventually
decrement to 0 once the correspondent node processes and acts on the
unsolicited NAs.
When the target Client (or Proxy) sends unsolicited NA messages to
the target Server to update link-layer address and/or QoS
preferences, the target Server repeats the messages to any of its
dynamic neighbors while using its own link-layer and link-local
addresses as the source addresses. In this way, the target Server
acts as a link-scoped multicast repeater on behalf of the target
Client (or Proxy).
(Note that instead of serving as the route optimization target for
Proxy interfaces, the target Server could instead forward the
source's NS messages and allow the Proxies to return NA messages,
i.e., the same as for Clients on native interfaces. That would mean
that the source could receive multiple NA messages from multiple
Proxies and, if some or all NA messages are lost, the source would
not be able to determine the full picture of the Client's Proxy
affiliations. If this alternate architecture is deemed appropriate
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in some use cases, then the AERO Proxies could be employed to serve
as route optimization targets instead of depending on the Servers to
do so.)
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.
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.
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 AcceptTime to ACCEPT_TIME and updates its cached link-layer
addresses (if necessary). When the source node receives a solicited
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 a solicited 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 has either failed or has become intermittent.
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 AcceptTime for a dynamic neighbor cache
entry expires, the target node discards any subsequent packets
received directly from the source node. When both ForwardTime and
AcceptTime for a dynamic neighbor cache entry expire, the node
deletes the neighbor cache entry.
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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 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 service for
all Clients. AERO Clients coordinate with their regional Servers via
RS/RA exchanges to maintain the DMM profile, and the AERO routing
system tracks the current AERO Client/Server peering relationships.
Mobility management for AERO interfaces is accommodated by sending
unsolicited NA messages the same as for announcing link-layer address
changes for any interface that implements IPv6 ND [RFC4861]. 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.
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 and also returns an unsolicited NA message to the
sender with no TLLAOs. The packets will be delivered to the target
Client's new location, and the sender will realize that it needs to
deprecate its routing information that associated the target with
this Server.
3.17.2. Announcing Link-Layer Address and 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.
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.
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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.
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.
If the Client needs to send the unsolicited NAs 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.
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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.
When a Client disassociates with an existing Server, it sends an RS
"Release" message via a new Server with its base AERO address as the
network-layer source address and the (administratively-provisioned)
link-local address of the old Server as the network-layer destination
address. The new Server then caches the Client's AERO address and
"Release" message parameters (e.g., "transaction ID") and writes its
own administratively-provisioned link-local address as the network-
layer source address. The new Server then forwards the message to a
Relay, which forwards the message to the old Server.
When the old Server receives the "Release", it releases the Client's
ACP prefix delegations and routes. The old Server then deletes the
Client's neighbor cache entry so that any in-flight packets will be
forwarded via a Relay to the new Server, which will forward them to
the Client. The old Server finally returns a "Reply" message via a
Relay to the new Server, which will decapsulate the "Reply" message
and forward it as an RA "Reply" to the Client.
When the new Server forwards the "Reply" message, the Client can
delete both the default route and the neighbor cache entry for the
old Server. (Note that since messages may be lost in the network the
Client SHOULD retry until it gets an RA "Reply" indicating that the
RS "Release" was successful. If the Client does not receive a
"Reply" after MaxRetry attempts, the old Server may have failed and
the Client should discontinue its "Release" attempts.)
Finally, 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 little harm to the network. Examples of
when a Client might wish to change to a different Server include a
Server that has gone unreachable, topological movements of
significant distance, 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.
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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 deployments, AERO Clients may be located in secured enclaves
(e.g., a corporate enterprise network, a radio access network, etc.)
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 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 address along with any
identifying information including PD "transaction IDs", "Client
Identifiers", etc. and/or ND Nonce values. The Proxy then re-
encapsulates the message and forwards it to the Server indicated
by the destination link-layer address in the packet while
substituting its own external address as the source link-layer
address.
o when the Proxy receives an RA message from the Server, it matches
the message with the (INCOMPLETE) proxy neighbor cache entry. The
Proxy then caches the route information in the message 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 message and
forwards it to the Client. At the same time, the Proxy sends an
unsolicited NA message including a TLLAO with the X flag set back
to the Server to assert that it is indeed a Proxy as opposed to an
ordinary NAT. (In environments where spoofing is a threat, the
Proxy signs the NA using SEND.)
After the initial RS/RA handshake, the Proxy can send unsolicited NA
messages to the Client's Server(s) to update Server neighbor cache
entries on behalf of the Client. (For example, the Proxy can send 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
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new QoS preferences for that link). The Proxy also forwards any
unsolicited NA messages originating from the Client to the Client's
Server(s) (e.g. if the Client needs to announce new QoS preferences
on its own behalf), and forwards any data packets originating from
the Client to the Client's primary Server.
At the same time, for data packets originating from a Client within
the enclave with destination addresses that match an ASP, the Proxy
can initiate route optimization by sending an NS message via the
Server to solicit an NA message from a target node on the path to the
destination Client the same as discussed in Section 3.15. The target
must deliver the NA message directly to the Proxy, i.e., instead of
relaying through the backward chain of Relays and Servers, since the
backward chain could deliver the NA to a different Proxy besides the
one that produced the NS. For this reason, the Proxy prepares an NS
message as specified in Section 3.15.3, but with its own link-layer
address as the link-layer source address and with a single SLLAO
containing its link-layer address and with the X flag set to indicate
that direct delivery is required.
When the target receives the NS message, it creates a dynamic
neighbor cache entry in the ACCEPT state and returns an NA message
directly to the Proxy. When the target is a Client, it includes
TLLAOs in the NA message with link-layer addresses corresponding to
its native underling interfaces. When the target is a Server, it
includes a first TLLAO in the NA message with Interface ID set to 255
and with its own link-layer address information, and also includes
additional TLLAOs corresponding to the destination Client's Proxyed,
NATed or VPNed underlying interfaces. (For NATed or VPNed underlying
interfaces the server writes its own link-layer address in the TLLAO,
and for Proxyed interfaces it writes the link-layer address of the
Proxy.) When the source Proxy receives the NA message, it creates a
dynamic neighbor cache entry in the FORWARD state that associates the
TLLAOs of the NA message as the next-hop toward the routes advertised
in the NA RIOs.
When a source Proxy sends route optimization NS messages toward the
target, it can include RIOs to assert specific routes, and the target
will only accept packets from the source Proxy with matching source
addresses. If the source Proxy wishes to assert a "wildcard" route,
it includes an RIO in the NS message with Prefix and Prefix Length
set to 0. In that case, the target will either accept or ignore the
NS based on its configured trust policy. If the target accepts the
NS, it will accept all packets originating from the source Proxy
regardless of their source address.
After the initial NS/NA exchange, the target may need to update the
neighbor cache entries for any source Proxies for which it holds a
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dynamic neighbor cache entry in the ACCEPT state. The target
therefore sends unsolicited NA messages to announce any link layer
changes. As a result:
o the source Proxy may receive unsolicited NA messages with TLLAOs
with new UDP Port Number, IP Address and/or QoS preferences from
the target. In that case, the Proxy updates its neighbor cache
entry and forwards future outbound packets based on the new link
layer information.
o the source Proxy may receive reflected packets destined to the
link-layer address of a departed Client. In that case, the Proxy
proceeds as discussed in Section 3.8.5.
o the source Proxy may receive link-layer Destination Unreachable
messages in response to data packets it sends to one of the target
link-layer addresses. In that case, the Proxy processes the link-
layer error messages as an indication that the path may be failing
and proceeds as discussed in Section 3.13.
After the NS/NA exchange, while data packets are still flowing the
source Proxy sends additional NS messages to the target using the
address in the target's first TLLAO as the destination. The NS
message will update the target's AcceptTime timer, and the resulting
NA reply will update the source Proxy's ForwardTime timer in their
respective neighbor cache entries.
If at some later time the target Client departs from its secured
enclave, the Proxy sends unsolicited NAs to the Client's Servers to
announce the departure.
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.
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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.
7. 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-
1.0.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-
3.0.3a.tgz.
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8. IANA Considerations
The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.
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.
9. Security Considerations
AERO link security considerations are the same as for standard IPv6
Neighbor Discovery [RFC4861] except that AERO improves on some
aspects. In particular, AERO uses a trust basis between Clients and
Servers, where the Clients only engage in the AERO mechanism when it
is facilitated by a trusted Server.
NS and NA messages SHOULD include a Timestamp option (see Section 5.3
of [RFC3971]) that other AERO nodes can use to verify the message
time of origin. NS and RS messages SHOULD include a Nonce option
(see Section 5.3 of [RFC3971]) that recipients echo back in
corresponding responses. In cases where spoofing cannot be mitigated
through other means, however, all AERO IPv6 ND messages should employ
SEND [RFC3971], which also protects the PD information embedded in
RS/RA message options.
AERO links must be protected against link-layer address spoofing
attacks in which an attacker on the link pretends to be a trusted
neighbor. Links that provide link-layer securing mechanisms (e.g.,
IEEE 802.1X WLANs) and links that provide physical security (e.g.,
enterprise network wired LANs) provide a first line of defense,
however AERO nodes SHOULD also use securing services such as SEND for
Client authentication and network admission control. Following
authenticated Client admission and prefix delegation procedures, AERO
nodes MUST ensure that the source of data packets corresponds to the
node to which the prefixes were delegated.
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.)
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AERO Clients, Servers and Relays on the open Internet are susceptible
to the same attack profiles as for any Internet nodes. For this
reason, IP security SHOULD be used when AERO is employed over
unmanaged/unsecured links using securing mechanisms such as IPsec
[RFC4301], IKE [RFC5996] and/or TLS [RFC5246]. In some environments,
however, the use of end-to-end security from Clients to correspondent
nodes (i.e., other Clients and/or Internet nodes) could obviate the
need for IP security between AERO Clients, Servers and Relays.
AERO Servers and Relays present targets for traffic amplification 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 can institute rate limits that protect Clients from receiving
packet floods that could DoS low data rate links.
Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.
10. 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, Sri Gundavelli, Brian
Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak,
Lee Howard, Andre Kostur, Ted Lemon, Andy Malis, Satoru Matsushima,
Tomek Mrugalski, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa,
Joe Touch, Bernie Volz, Ryuji Wakikawa, 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, Ed King, Gene
MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian
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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. Wayne Benson, Kyle Bae and Eric Yeh are especially
acknowledged for implementing the AERO functions as extensions to the
public domain OpenVPN distribution.
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.
11. References
11.1. 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>.
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[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>.
[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>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <https://www.rfc-editor.org/info/rfc3315>.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<https://www.rfc-editor.org/info/rfc3633>.
[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>.
[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>.
[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>.
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[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>.
11.2. Informative References
[BGP] Huston, G., "BGP in 2015, http://potaroo.net", January
2016.
[I-D.ietf-intarea-gue]
Herbert, T., Yong, L., and O. Zia, "Generic UDP
Encapsulation", draft-ietf-intarea-gue-05 (work in
progress), December 2017.
[I-D.ietf-intarea-gue-extensions]
Herbert, T., Yong, L., and F. Templin, "Extensions for
Generic UDP Encapsulation", draft-ietf-intarea-gue-
extensions-04 (work in progress), March 2018.
[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-08 (work in
progress), January 2018.
[I-D.templin-6man-dhcpv6-ndopt]
Templin, F., "IPv6 Neighbor Discovery Extensions for
Prefix Delegation", draft-templin-6man-dhcpv6-ndopt-04
(work in progress), March 2018.
[I-D.templin-6man-rio-redirect]
Templin, F. and j. woodyatt, "Route Information Options in
IPv6 Neighbor Discovery", draft-templin-6man-rio-
redirect-06 (work in progress), May 2018.
[I-D.templin-atn-bgp]
Templin, F., Saccone, G., Dawra, G., and A. Lindem, "A
Simple BGP-based Mobile Routing System for the
Aeronautical Telecommunications Network", draft-templin-
atn-bgp-06 (work in progress), March 2018.
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[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 Models", draft-
templin-v6ops-pdhost-19 (work in progress), March 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>.
[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>.
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[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[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>.
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[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>.
[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
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:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
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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
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
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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
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.
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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 DHCPv6 PD exchange had
occurred. If the mobile node subsequently withdraws the ACP from the
access 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 link-local address and exchange NS/NA
messages the same as for dynamically-established tunnels.
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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
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 DHCPv6 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.
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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
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.
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Appendix E. Change Log
Changes from -81 to -82:
o Make DHCPv6 the default (but not exclusive) PD service
o Support operation with no PD services nor ND Route Information
Options
o Updates to AERO Proxy function
Changes from -80 to -81:
o Updates to Server and Proxy Extended Route Optimization
o Updates to AERO Proxy section
o Cleanups and clarifications
Changes from -79 to -80:
o Substantial updates to AERO Proxy function
o Removed 'V' bit from SLLAO and replaced with 'X' bit
o Added concept of Direct, Proxyed, NATed, VPNed and Native
underlying interfaces
o Adjusted route optimization text according to underrlying
interface types
Changes from -78 to -79:
o Neighbors now set UDP Port Number and IP Address in S/TLLAOs to 0
if the node is behind a NAT or otherwise does not wish to update
its link-layer address for this underlying interface
o Introduced "proxy" as a new neighbor cache entry type
o updated GUE references
o multipath considerations for error message handling and NUD
Changes from -77 to -78:
o Added "V" bit to SLLAO flags field for NS messages. V=1 indicates
that the NA response must go through the reverse chain of Servers
and Relays
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o Now including DHCPv6 PD messages as IPv6 ND message options
o Clarified the use of the "P" bit in the RA flags field
o Use of SEND to protect the combined DHCPv6/IPv6ND messages
o Proxy now treats a Client's Servers as the default routers (i.e.,
instead of using a Relay as the default).
Changes from -76 to -77:
o Now using IPv6 ND NS/NA messaging for route optimization (no
longer using Predirect/Redirect)
o Now using combined IPv6 ND/DHCPv6 messaging so autoconfiguration
can be conducted in a single message exchange
o Introduced the AERO Proxy construct. Critical for applications
such as ATN/IPS
Changes from -75 to -76:
o Bumped version number ahead of expiration deadline
Changes from -74 to -75:
o Bumped version number ahead of expiration deadline
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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