One document matched: draft-templin-aerolink-66.xml
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<rfc category="std" docName="draft-templin-aerolink-66.txt" ipr="trust200902"
obsoletes="rfc5320, rfc5558, rfc5720, rfc6179, rfc6706">
<front>
<title abbrev="AERO">Asymmetric Extended Route Optimization (AERO)</title>
<author fullname="Fred L. Templin" initials="F. L." role="editor"
surname="Templin">
<organization>Boeing Research & Technology</organization>
<address>
<postal>
<street>P.O. Box 3707</street>
<city>Seattle</city>
<region>WA</region>
<code>98124</code>
<country>USA</country>
</postal>
<email>fltemplin@acm.org</email>
</address>
</author>
<date day="2" month="February" year="2016"/>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<abstract>
<t>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
redirection services for route optimization. AERO provides an IPv6
link-local address format known as the AERO address that supports
operation of the IPv6 Neighbor Discovery (ND) protocol and links IPv6 ND
to IP forwarding. Admission control, provisioning and mobility are
supported by the Dynamic Host Configuration Protocol for IPv6 (DHCPv6),
and route optimization is naturally supported through dynamic neighbor
cache updates. Although DHCPv6 and IPv6 ND messaging are used in the
control plane, both IPv4 and IPv6 are supported in the data plane. AERO
is a widely-applicable tunneling solution using standard control
messaging exchanges as described in this document.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>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 to neighboring nodes over either IPv6 or IPv4
networks, i.e., AERO views the IPv6 and IPv4 networks as 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 redirection services for route
optimization <xref target="RFC5522"/>.</t>
<t>AERO provides an IPv6 link-local address format known as the AERO
address that supports operation of the IPv6 Neighbor Discovery (ND)
<xref target="RFC4861"/> protocol and links IPv6 ND to IP forwarding.
Admission control, provisioning and mobility are supported by the
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) <xref
target="RFC3315"/>, and route optimization is naturally supported
through dynamic neighbor cache updates. Although DHCPv6 and IPv6 ND
messaging are used in the control plane, both IPv4 and IPv6 can be used
in the data plane. AERO is a widely-applicable tunneling solution using
standard control messaging exchanges as described in this document. The
remainder of this document presents the AERO specification.</t>
</section>
<section anchor="terminology" title="Terminology">
<t>The terminology in the normative references applies; the following
terms are defined within the scope of this document:</t>
<t><list style="hanging">
<t hangText="AERO link"><vspace/>a Non-Broadcast, Multiple Access
(NBMA) tunnel virtual overlay configured over a node's attached IPv6
and/or IPv4 networks. 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 network hops.
AERO can also operate over native multiple access link types (e.g.,
Ethernet, WiFi etc.) when a tunnel virtual overlay is not
needed.</t>
<t hangText="AERO interface"><vspace/>a node's attachment to an AERO
link. Nodes typically have a single AERO interface; support for
multiple AERO interfaces is also possible but out of scope for this
document. AERO interfaces do not require Duplicate Address Detection
(DAD) and therefore set the administrative variable
DupAddrDetectTransmits to zero <xref target="RFC4862"/>.</t>
<t hangText="AERO address"><vspace/>an IPv6 link-local address
constructed as specified in <xref target="aero-address"/> and
assigned to a Client's AERO interface.</t>
<t hangText="AERO node"><vspace/>a node that is connected to an AERO
link and that participates in IPv6 ND and DHCPv6 messaging over the
link.</t>
<t hangText="AERO Client ("Client")"><vspace/>a node that
issues DHCPv6 messages to receive IP Prefix Delegations (PDs) from
one or more AERO Servers. Following PD, the Client assigns an AERO
address to the AERO interface for use in DHCPv6 and IPv6 ND
exchanges with other AERO nodes.</t>
<t hangText="AERO Server ("Server")"><vspace/>a node that
configures an AERO interface to provide default forwarding and
DHCPv6 services for AERO Clients. The Server assigns an
administratively provisioned IPv6 link-local unicast address to
support the operation of DHCPv6 and the IPv6 ND protocol. An AERO
Server can also act as an AERO Relay.</t>
<t hangText="AERO Relay ("Relay")"><vspace/>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 assigns an administratively
provisioned IPv6 link-local unicast address to the AERO interface
the same as for a Server. An AERO Relay can also act as an AERO
Server.</t>
<t
hangText="AERO Forwarding Agent ("Forwarding Agent")"><vspace/>a
node that performs data plane forwarding services as a companion to
an AERO Server.</t>
<t hangText="ingress tunnel endpoint (ITE)"><vspace/>an AERO
interface endpoint that injects tunneled packets into an AERO
link.</t>
<t hangText="egress tunnel endpoint (ETE)"><vspace/>an AERO
interface endpoint that receives tunneled packets from an AERO
link.</t>
<t hangText="underlying network"><vspace/>a connected IPv6 or IPv4
network routing region over which the tunnel virtual overlay is
configured. A typical example is an enterprise network, but many
other use cases are also in scope.</t>
<t hangText="underlying interface"><vspace/>an AERO node's interface
point of attachment to an underlying network.</t>
<t hangText="link-layer address"><vspace/>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; otherwise, UDP port number is set to the constant value
'0'. Link-layer addresses are used as the encapsulation header
source and destination addresses.</t>
<t hangText="network layer address"><vspace/>the source or
destination address of the encapsulated IP packet.</t>
<t hangText="end user network (EUN)"><vspace/>an internal virtual or
external edge IP network that an AERO Client connects to the rest of
the network via the AERO interface.</t>
<t hangText="AERO Service Prefix (ASP)"><vspace/>an IP prefix
associated with the AERO link and from which AERO Client Prefixes
(ACPs) are derived (for example, the IPv6 ACP 2001:db8:1:2::/64 is
derived from the IPv6 ASP 2001:db8::/32).</t>
<t hangText="AERO Client Prefix (ACP)"><vspace/>a more-specific IP
prefix taken from an ASP and delegated to a Client.</t>
</list>Throughout the document, the simple terms "Client", "Server"
and "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
respectively. Capitalization is used to distinguish these terms from
DHCPv6 client/server/relay <xref target="RFC3315"/>.</t>
<t>The terminology of <xref target="RFC4861"/> (including the names of
node variables and protocol constants) applies to this document. Also
throughout the document, the term "IP" is used to generically refer to
either Internet Protocol version (i.e., IPv4 or IPv6).</t>
<t>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 <xref target="RFC2119"/>.
Lower case uses of these words are not to be interpreted as carrying
RFC2119 significance.</t>
</section>
<section anchor="aerospec"
title="Asymmetric Extended Route Optimization (AERO)">
<t>The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:</t>
<section anchor="aerolink" title="AERO Link Reference Model">
<t><figure anchor="chaining-fig" title="AERO Link Reference Model">
<artwork><![CDATA[ .-(::::::::)
.-(:::: IP ::::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +--------+-------+ +--------------+
|AERO Server S1| | AERO Relay R1 | |AERO Server S2|
| Nbr: C1; R1 | | Nbr: S1; S2 | | Nbr: C2; R1 |
| default->R1 | |(P1->S1; P2->S2)| | default->R1 |
| P1->C1 | | ASP A1 | | P2->C2 |
+-------+------+ +--------+-------+ +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client C1| |AERO Client C2|
| Nbr: S1 | | Nbr: S2 |
| default->S1 | | default->S2 |
| ACP P1 | | ACP P2 |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. .-(_ IP )-.
(__ EUN ) (__ EUN )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host H1| | Host H2|
+--------+ +--------+
]]></artwork>
</figure><xref target="chaining-fig"/> presents the AERO link
reference model. In this model:</t>
<t><list style="symbols">
<t>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 IP Internetwork.</t>
<t>AERO Servers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.</t>
<t>AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive AERO Client Prefix (ACP) delegations P1
and P2, and also act as default routers for their associated
physical or internal virtual EUNs. (Alternatively, clients can act
as multi-addressed hosts without serving any EUNs).</t>
<t>Simple hosts H1 and H2 attach to the EUNs served by Clients C1
and C2, respectively.</t>
</list>Each AERO node maintains an AERO interface neighbor cache and
an IP forwarding table. For example, AERO Relay R1 in the diagram has
neighbor cache entries for Servers S1 and S2 as well as IP forwarding
table entries for the ACPs delegated to Clients C1 and C2. In common
operational practice, there may be many additional Relays, Servers and
Clients. (Although not shown in the figure, AERO Forwarding Agents may
also be provided for data plane forwarding offload services.)</t>
</section>
<section anchor="node-types" title="AERO Link Node Types">
<t>AERO Relays provide default forwarding services to AERO Servers.
Relays forward packets between Servers connected to the same AERO link
and also forward packets between the AERO link and the native IP
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. AERO Relays
maintain an AERO interface neighbor cache entry for each AERO Server,
and maintain an IP forwarding table entry for each AERO Client Prefix
(ACP). AERO Relays can also be configured to act as AERO Servers.</t>
<t>AERO Servers provide default forwarding services to AERO Clients.
Each Server also peers with each Relay in a dynamic routing protocol
instance to advertise its list of associated ACPs. Servers configure a
DHCPv6 server function to facilitate Prefix Delegation (PD) exchanges
with Clients. Each delegated prefix becomes an ACP taken from an ASP.
Servers forward packets between AERO interface neighbors only, i.e.,
and not between the AERO link and the native IP Internetwork. AERO
Servers maintain an AERO interface neighbor cache entry for each AERO
Relay. 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.</t>
<t>AERO Clients act as requesting routers to receive ACPs through
DHCPv6 PD exchanges with AERO Servers over the AERO link. Each Client
MAY 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. AERO
Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.</t>
<t>AERO Forwarding Agents provide data plane forwarding services as
companions to AERO Servers. Note that while Servers are required to
perform both control and data plane operations on their own behalf,
they may optionally enlist the services of special-purpose Forwarding
Agents to offload data plane traffic.</t>
</section>
<section anchor="aero-address" title="AERO Addresses">
<t>An AERO address is an IPv6 link-local address with an embedded ACP
and assigned to a Client's AERO interface. The AERO address is formed
as follows:</t>
<t><list style="empty">
<t>fe80::[ACP]</t>
</list>For IPv6, the AERO address begins with the prefix fe80::/64
and includes in its interface identifier the base prefix taken from
the Client's IPv6 ACP. The base prefix is determined by masking the
ACP with the prefix length. For example, if the AERO Client receives
the IPv6 ACP:</t>
<t><list style="empty">
<t>2001:db8:1000:2000::/56</t>
</list>it constructs its AERO address as:</t>
<t><list style="empty">
<t>fe80::2001:db8:1000:2000</t>
</list>For IPv4, the AERO address is formed from the lower 64 bits
of an IPv4-mapped IPv6 address <xref target="RFC4291"/> that includes
the base prefix taken from the Client's IPv4 ACP. For example, if the
AERO Client receives the IPv4 ACP:</t>
<t><list style="empty">
<t>192.0.2.32/28</t>
</list>it constructs its AERO address as:</t>
<t><list style="empty">
<t>fe80::FFFF:192.0.2.32</t>
</list>The AERO address remains stable as the Client moves between
topological locations, i.e., even if its link-layer addresses
change.</t>
<t>NOTE: In some cases, prospective neighbors may not have advanced
knowledge of the Client's ACP length and may therefore send initial
IPv6 ND messages with an AERO destination address that matches the ACP
but does not correspond to the base prefix. For example, if the Client
receives the IPv6 ACP 2001:db8:1000:2000::/56 then subsequently
receives an IPv6 ND message with destination address
fe80::2001:db8:1000:2001, it accepts the message as though it were
addressed to fe80::2001:db8:1000:2000.</t>
</section>
<section anchor="interface" title="AERO Interface Characteristics">
<t>AERO interfaces use encapsulation (see: <xref
target="aeroencaps"/>) to exchange packets with neighbors attached to
the AERO link. AERO interfaces maintain a neighbor cache, and AERO
nodes use both DHCPv6 PD and IPv6 ND control messaging. AERO Clients
send DHCPv6 Solicit, Rebind, Renew and Release messages to AERO
Servers, which respond with DHCPv6 Reply messages. These messages
result in the creation, modification and deletion of neighbor cache
entries.</t>
<t>AERO interfaces use unicast IPv6 ND Neighbor Solicitation (NS),
Neighbor Advertisement (NA), Router Solicitation (RS) and Router
Advertisement (RA) messages the same as for any IPv6 link. AERO
interfaces use two IPv6 ND redirection message types -- the first
known as a Predirect message and the second being the standard
Redirect message (see <xref target="predirect"/>). AERO links further
use link-local-only addressing; hence, AERO nodes ignore any Prefix
Information Options (PIOs) they may receive in RA messages over an
AERO interface.</t>
<t>AERO interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in <xref
target="tllaov6"/>:</t>
<t><figure anchor="tllaov6"
title="AERO Source/Target Link-Layer Address Option (S/TLLAO) Format">
<artwork><![CDATA[ 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 = 2 | Length = 3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID | NDSCPs | DSCP #1 |Prf| DSCP #2 |Prf|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DSCP #3 |Prf| DSCP #4 |Prf| ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Port Number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| IP Address |
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>In this format, Link ID is an integer value between 0 and 255
corresponding to an underlying interface of the target node, NDSCPs
encodes an integer value between 0 and 64 indicating the number of
Differentiated Services Code Point (DSCP) octets that follow. Each
DSCP octet is a 6-bit integer DSCP value followed by a 2-bit
Preference ("Prf") value. Each DSCP value encodes an integer between 0
and 63 associated with this Link ID, where the value 0 means "default"
and other values are interpreted as specified in <xref
target="RFC2474"/>. The 'Prf' qualifier for each DSCP value is set to
the value 0 ("deprecated'), 1 ("low"), 2 ("medium"), or 3 ("high") to
indicate a preference level for packet forwarding purposes. When a
particular DSCP value is not specified, its preference level is set to
"medium" by default.</t>
<t>UDP Port Number and IP Address are set to the addresses used by the
target node when it sends encapsulated packets over the underlying
interface. When UDP is not used as part of the encapsulation, UDP Port
Number is set to the value '0'. When the encapsulation IP address
family is IPv4, IP Address is formed as an IPv4-mapped IPv6 address
<xref target="RFC4291"/>.</t>
<t>AERO interfaces may be configured over multiple underlying
interfaces. 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, terrestrial, air-to-air directional,
etc.) with diverse performance and cost properties.</t>
<t>If a Client's multiple underlying interfaces are used "one at a
time" (i.e., all other interfaces are in standby mode while one
interface is active), then Redirect, Predirect and unsolicited NA
messages include only a single TLLAO with Link ID set to a constant
value.</t>
<t>If the Client has multiple active underlying interfaces, then from
the perspective of IPv6 ND it would appear to have multiple link-layer
addresses. In that case, Redirect and Predirect messages MAY include
multiple TLLAOs -- each with a Link ID that corresponds to a specific
underlying interface of the Client.</t>
</section>
<section anchor="aerolinkinit" title="AERO Link Registration">
<t>When an administrative authority first deploys a set of AERO Relays
and Servers that comprise an AERO link, they also assign a unique
domain name for the link, e.g., "linkupnetworks.example.com". Next, if
administrative policy permits Clients within the domain to serve as
correspondent nodes for Internet mobile nodes, the administrative
authority adds a Fully Qualified Domain Name (FQDN) for each of the
AERO link's ASPs to the Domain Name System (DNS) <xref
target="RFC1035"/>. The FQDN is based on the suffix
"aero.linkupnetworks.net" with a prefix formed from the
wildcard-terminated reverse mapping of the ASP <xref
target="RFC3596"/><xref target="RFC4592"/>, and resolves to a DNS PTR
resource record. For example, for the ASP '2001:db8:1::/48' within the
domain name "linkupnetworks.example.com", the DNS database
contains:</t>
<t>'*.1.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net. PTR
linkupnetworks.example.com'</t>
<t>This DNS registration advertises the AERO link's ASPs to
prospective correspondent nodes.</t>
</section>
<section anchor="aeroinit" title="AERO Interface Initialization">
<section anchor="rinit" title="AERO Relay Behavior">
<t>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, and MUST NOT collide with any potential AERO addresses
nor the special addresses fe80:: and fe80::ffff:ffff:ffff:ffff. (The
fe80::ID addresses are typically taken from the available range
fe80::/96, e.g., as fe80::1, fe80::2, fe80::3, etc.) The Relay then
engages in a dynamic routing protocol session with all Servers on
the link (see: <xref target="scaling"/>), and advertises its
assigned ASP prefixes into the native IP Internetwork.</t>
<t>Each Relay subsequently maintains an IP forwarding table entry
for each ACP covered by its ASP(s), and maintains a neighbor cache
entry for each Server on the link. Relays exchange NS/NA messages
with AERO link neighbors the same as for any AERO node, however they
typically do not perform explicit Neighbor Unreachability Detection
(NUD) (see: <xref target="nud"/>) since the dynamic routing protocol
already provides reachability confirmation.</t>
</section>
<section anchor="sinit" title="AERO Server Behavior">
<t>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 DHCPv6 server function
to facilitate DHCPv6 PD exchanges with AERO Clients. The Server
maintains a neighbor cache entry for each Relay on the link, and
manages per-ACP neighbor cache entries and IP forwarding table
entries based on control message exchanges. Each Server also engages
in a dynamic routing protocol with each Relay on the link (see:
<xref target="scaling"/>).</t>
<t>When the Server receives an NS/RS message from a Client on the
AERO interface it returns an NA/RA message. The Server further
provides a simple link-layer conduit between AERO interface
neighbors. Therefore, packets enter the Server's AERO interface from
the link layer and are forwarded back out the link layer without
ever leaving the AERO interface and therefore without ever
disturbing the network layer.</t>
</section>
<section anchor="cinit" title="AERO Client Behavior">
<t>When a Client enables an AERO interface, it uses the special
address fe80::ffff:ffff:ffff:ffff to obtain one or more ACPs from an
AERO Server via DHCPv6 PD. Next, it assigns the corresponding AERO
address(es) to the AERO interface and creates a neighbor cache entry
for the Server, i.e., the DHCPv6 PD exchange bootstraps
autoconfiguration of unique link-local address(es). The Client
maintains a neighbor cache entry for each of its Servers and each of
its active correspondent Clients. When the Client receives
Redirect/Predirect messages on the AERO interface it updates or
creates neighbor cache entries, including link-layer address
information.</t>
</section>
<section anchor="feinit" title="AERO Forwarding Agent Behavior">
<t>When a Forwarding Agent enables an AERO interface, it assigns the
same link-local address(es) as the companion AERO Server. The
Forwarding Agent thereafter provides data plane forwarding services
based solely on the forwarding information assigned to it by the
companion AERO Server.</t>
</section>
</section>
<section anchor="scaling" title="AERO Routing System">
<t>The AERO routing system is based on a private instance of the
Border Gateway Protocol (BGP) <xref target="RFC4271"/> that is
coordinated between Relays and Servers and does not interact with
either the public Internet BGP routing system or the native IP
Internetwork interior routing system. Relays advertise only a small
and unchanging set of ASPs to the native routing system instead of the
full dynamically changing set of ACPs.</t>
<t>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 peers with each Relay but does not peer with
other Servers. Similarly, Relays do not peer with each other, since
they will reliably receive all updates from all Servers and will
therefore have a consistent view of the AERO link ACP delegations.</t>
<t>Each Server maintains a working set of associated ACPs, and
dynamically announces new ACPs and withdraws departed ACPs in its BGP
updates to Relays. Clients are expected to remain associated with
their current Servers for extended timeframes, however Servers SHOULD
selectively suppress BGP updates for impatient Clients that repeatedly
associate and disassociate with them in order to dampen routing
churn.</t>
<t>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 all other
ACPs will correctly result in destination unreachable failures due to
the black hole route.</t>
<t>Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. Assuming O(10^6)
as a reasonable maximum number of BGP routes, this means that O(10^6)
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 each
Relay, but the Server institutes route filters so that each set of
Relays only receives BGP updates for the ASPs they aggregate. 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.</t>
<t>Assuming up to O(10^3) sets of Relays, the AERO routing system can
then accommodate O(10^9) ACPs with no additional overhead for Servers
and Relays (for example, it should be possible to service 4 billion
/64 ACPs taken from a /32 ASP and even more for shorter ASPs). In this
way, each set of Relays services a specific set of ASPs that they
advertise to the native 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.</t>
<t>Note that in an alternate routing arrangement each set of Relays
could advertise the aggregated ASP for the link into the native
routing system even though each Relay services only a segment 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 correct Relay. The tradeoff then is the penalty for
Relay-to-Relay tunneling compared with reduced routing information in
the native routing system.</t>
</section>
<section anchor="aeroncache"
title="AERO Interface Neighbor Cache Maintenace">
<t>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 <xref target="RFC4861"/>.
AERO interface neighbor cache entires are said to be one of
"permanent", "static" or "dynamic".</t>
<t>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 a permanent neighbor
cache entry for each Server on the link, and AERO Servers maintain a
permanent neighbor cache entry for each Relay. Each entry maintains
the mapping between the neighbor's fe80::ID network-layer address and
corresponding link-layer address.</t>
<t>Static neighbor cache entries are created through DHCPv6 PD
exchanges and remain in place for durations bounded by prefix
lifetimes. AERO Servers maintain static neighbor cache entries for the
ACPs of each of their associated Clients, and AERO Clients maintain a
static neighbor cache entry for each of their associated Servers. When
an AERO Server sends a Reply message response to a Client's Solicit,
Rebind or Renew message, it creates or updates a static neighbor cache
entry based on the Client's DHCP Unique Identifier (DUID) as the
Client identifier, the AERO address(es) corresponding to the Client's
ACP(s) as the network-layer address(es), the prefix lifetime as the
neighbor cache entry lifetime, the Client's encapsulation IP address
and UDP port number as the link-layer address and the prefix length(s)
as the length to apply to the AERO address(es). When an AERO Client
receives a Reply message from a Server, it creates or updates a static
neighbor cache entry based on the Reply message link-local source
address as the network-layer address, the prefix lifetime as the
neighbor cache entry lifetime, and the encapsulation IP source address
and UDP source port number as the link-layer address.</t>
<t>Dynamic neighbor cache entries are created or updated based on
receipt of a Predirect/Redirect message, and are garbage-collected if
not used within a bounded timescale. AERO Clients maintain dynamic
neighbor cache entries for each of their active correspondent Client
ACPs with lifetimes based on IPv6 ND messaging constants. When an AERO
Client receives a valid Predirect message it creates or updates a
dynamic neighbor cache entry for the Predirect target network-layer
and link-layer addresses plus prefix length. The node then sets an
"AcceptTime" variable in the neighbor cache entry to ACCEPT_TIME
seconds and uses this value to determine whether packets received from
the correspondent can be accepted. When an AERO Client receives a
valid Redirect message it creates or updates a dynamic neighbor cache
entry for the Redirect target network-layer and link-layer addresses
plus prefix length. The Client then sets a "ForwardTime" variable in
the neighbor cache entry to FORWARD_TIME seconds and uses this value
to determine whether packets can be sent directly to the
correspondent. The Client also sets a "MaxRetry" variable to MAX_RETRY
to limit the number of keepalives sent when a correspondent may have
gone unreachable.</t>
<t>It is RECOMMENDED that FORWARD_TIME be set to the default constant
value 30 seconds to match the default REACHABLE_TIME value specified
for IPv6 ND <xref target="RFC4861"/>.</t>
<t>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
redirection procedure can converge before AcceptTime decrements below
FORWARD_TIME.</t>
<t>It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
for IPv6 ND address resolution in Section 7.3.3 of <xref
target="RFC4861"/>.</t>
<t>Different values for FORWARD_TIME, ACCEPT_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 redirection
procedure to converge.</t>
</section>
<section anchor="aeroalg" title="AERO Interface Sending Algorithm">
<t>IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system), or
from the link layer (i.e., from the AERO tunnel virtual link). Packets
that enter the AERO interface from the network layer are encapsulated
and admitted into the AERO link, i.e., they are tunnelled to an AERO
interface neighbor. Packets that enter the AERO interface from the
link layer are either re-admitted into the AERO link or delivered to
the network layer where they are subject to either local delivery or
IP forwarding. Since each AERO node may have only partial information
about neighbors on the link, AERO interfaces may forward packets with
link-local destination addresses at a layer below the network layer.
This means that AERO nodes act as both IP routers/hosts and sub-IP
layer forwarding nodes. AERO interface sending considerations for
Clients, Servers and Relays are given below.</t>
<t>When an IP packet enters a Client's AERO interface from the network
layer, if the destination is covered by an ASP the Client searches for
a dynamic neighbor cache entry with a non-zero ForwardTime and an AERO
address that matches the packet's destination address. (The
destination address may be either an address covered by the neighbor's
ACP or the (link-local) AERO address itself.) If there is a match, the
Client uses a link-layer address in the entry as the link-layer
address for encapsulation then admits the packet into the AERO link.
If there is no match, the Client instead uses the link-layer address
of a neighboring Server as the link-layer address for
encapsulation.</t>
<t>When an IP packet enters a Server's AERO interface from the link
layer, if the destination is covered by an ASP the Server searches for
a neighbor cache entry with an AERO address that matches the packet's
destination address. (The destination address may be either an address
covered by the neighbor's ACP or the AERO address itself.) If there is
a match, the Server uses a link-layer address in the entry as the
link-layer address for encapsulation and re-admits the packet into the
AERO link. If there is no match, the Server instead 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.</t>
<t>When an IP packet enters a Relay's AERO interface from the network
layer, the Relay searches its IP forwarding table for an entry that is
covered by an ASP and also matches the destination. 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
admits the packet into the AERO link. When an IP packet enters a
Relay's AERO interface from the link-layer, if the destination is not
a link-local address and does not match an ASP the Relay removes the
packet from the AERO interface and uses IP forwarding to forward the
packet to the Internetwork. If the destination address is a link-local
address or a non-link-local address that matches an ASP, and there is
a more-specific ACP entry in the IP forwarding table, the Relay uses
the link-layer address in the corresponding neighbor cache entry as
the link-layer address for encapsulation and re-admits the packet into
the AERO link. When an IP packet enters a Relay's AERO interface from
either the network layer or link-layer, and the packet's destination
address matches an ASP but there is no more-specific ACP entry, the
Relay drops the packet and returns an ICMP Destination Unreachable
message (see: <xref target="aeroerr"/>).</t>
<t>When an AERO Server receives a packet from a Relay via the AERO
interface, the Server MUST NOT forward the packet back to the same or
a different Relay.</t>
<t>When an AERO Relay receives a packet from a Server via the AERO
interface, the Relay MUST NOT forward the packet back to the same
Server.</t>
<t>When an AERO node re-admits a packet into the AERO link without
involving the network layer, the node MUST NOT decrement the network
layer TTL/Hop-count.</t>
<t>When an AERO node forwards a data packet to the primary link-layer
address of a Server, it may receive Redirect messages with an SLLAO
that include the link-layer address of an AERO Forwarding Agent. The
AERO node SHOULD record the link-layer address in the neighbor cache
entry for the neighbor and send subsequent data packets via this
address instead of the Server's primary address (see: <xref
target="aeropd-agent"/>).</t>
</section>
<section anchor="aeroencaps"
title="AERO Interface Encapsulation and Re-encapsulation">
<t>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".</t>
<t>The AERO interface encapsulates packets per the Generic UDP
Encapsulation (GUE) encapsulation procedures in <xref
target="I-D.ietf-nvo3-gue"/><xref
target="I-D.herbert-gue-fragmentation"/>, or through an alternate
minimal encapsulation format (see: <xref target="minimal"/>). For
packets entering the AERO link from the IP layer, the AERO interface
copies the "TTL/Hop Limit", "Type of Service/Traffic Class" <xref
target="RFC2983"/>, "Flow Label"<xref target="RFC6438"> </xref>.(for
IPv6) and "Congestion Experienced" <xref target="RFC3168"/> values in
the packet's IP header into the corresponding fields in the
encapsulation IP header. For packets undergoing re-encapsulation
within the AERO link, the AERO interface instead copies the "TTL/Hop
Limit", "Type of Service/Traffic Class", "Flow Label" and "Congestion
Experienced" values in the original encapsulation IP header into the
corresponding fields in the new encapsulation IP header, i.e., the
values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header.</t>
<t>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. For packets sent to a
Server, the AERO interface sets the UDP destination port to 8060,
i.e., the IANA-registered port number for AERO. For packets sent to a
correspondent Client, the AERO interface sets the UDP destination port
to the port value stored in the neighbor cache entry for this
correspondent. The AERO interface then either includes or omits the
UDP checksum according to the GUE specification.</t>
<t>For IPv4 encapsulation, the AERO interface sets the DF bit as
discussed in <xref target="aeromtu"/>.</t>
</section>
<section anchor="aerodecaps" title="AERO Interface Decapsulation">
<t>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.</t>
</section>
<section anchor="aeroauth"
title="AERO Interface Data Origin Authentication">
<t>AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:</t>
<t><list style="symbols">
<t>AERO Servers and Relays accept encapsulated packets with a
link-layer source address that matches a permanent neighbor cache
entry.</t>
<t>AERO Servers accept authentic encapsulated DHCPv6 messages from
Clients, and create or update a static neighbor cache entry for
the Client based on the specific DHCPv6 message type.</t>
<t>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.</t>
<t>AERO Clients, Servers and Relays 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.</t>
</list>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.</t>
</section>
<section anchor="aeromtu" title="AERO Interface MTU and Fragmentation">
<t>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 links over IP networks have a maximum
link MTU of 64KB minus the encapsulation overhead (i.e., 64KB-ENCAPS),
since the maximum packet size in the base IP specifications is 64KB
<xref target="RFC0791"/><xref target="RFC2460"/>. While IPv6
jumbograms can be up to 4GB <xref target="RFC2675"/>, they are
considered optional for IPv6 nodes <xref target="RFC6434"/> and
therefore out of scope for this document.</t>
<t>The AERO interface is considered to have an indefinite MTU , i.e.,
instead of clamping the MTU to a fixed size. The MTU for each AERO
interface neighbor is therefore constrained by the minimum of 64KB,
the MTU of the underlying interface used for tunneling, and the path
MTU within the tunnel (minus ENCAPS in each case).</t>
<t>IPv6 specifies a minimum link MTU of 1280 bytes <xref
target="RFC2460"/>. This is the minimum packet size the AERO interface
MUST admit without returning an ICMP Packet Too Big (PTB) message.
Although IPv4 specifies a smaller minimum link MTU of 68 bytes <xref
target="RFC0791"/>, AERO interfaces also observe a 1280 byte minimum
for IPv4 even if some fragmentation is needed.</t>
<t>The vast majority of links in the Internet configure an MTU of at
least 1500 bytes. Original source hosts have therefore become
conditioned to expect that IP packets up to 1500 bytes in length will
either be delivered to the final destination or a suitable PTB message
returned. However, PTB messages may be crafted for malicious purposes
such as denial of service, or lost in the network <xref
target="RFC2923"/> resulting in failure of the IP Path MTU Discovery
(PMTUD) mechanisms <xref target="RFC1191"/><xref target="RFC1981"/>.
For these reasons, the tunnel ingress sends encapsulated packets to
the tunnel egress subject to whether standard PMTUD can be leveraged
within the specific deployment model. The two cases for consideration
are as follows:</t>
<section anchor="samedomain"
title="All Elements in Same Administrative Domain">
<t>When the original source, ingress and egress are all within the
same well-managed administrative domain, the ingress admits a packet
into the tunnel if it is no larger than the current path MTU
estimate for this egress (initially set to the MTU of the underlying
link to be used for tunneling minus ENCAPS). Otherwise, the ingress
drops the packet and sends a network layer (L3) PTB message back to
the original source. Additionally, the ingress SHOULD cache the MTU
value in any link-layer (L2) PTB messages it receives from a router
on the path to the egress as a new path MTU estimate. Thereafter,
the ingress SHOULD periodically reset the path MTU estimate to the
MTU of the underlying link minus ENCAPS to detect path MTU
increases.</t>
<t>These procedures apply when the path MTU for this egress is no
smaller than (1280+ENCAPS) bytes; otherwise, the ingress can either
declare the egress unreachable or commence fragmentation in a manner
that parallels the standard behavior specified in <xref
target="RFC2473"/>. In that case, the ingress encapsulates all
packets that are no larger than 1280 bytes while using encapsulation
layer fragmentation if necessary as specified in <xref
target="diffdomain"/>. (For IPv4 packets with DF=0 that are larger
than 1280 bytes, the ingress instead uses IPv4 fragmentation before
encapsulation.)</t>
</section>
<section anchor="diffdomain"
title="Not All Elements in Same Administrative Domain">
<t>When the original source, ingress and egress are not all within
the same well-managed administrative domain, the ingress admits all
packets up to 1500 bytes in length even if some fragmentation is
needed, and admits larger packets without fragmentation in case they
are able to traverse the tunnel in one piece.</t>
<t>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 <xref
target="RFC6864"/><xref target="RFC4963"/> (see: <xref
target="integrity"/>). 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].
For these reasons, when fragmentation is needed the ingress inserts
an encapsulation layer fragment header and applies tunnel
fragmentation in the manner suggested in Section 3.1.7 of <xref
target="RFC2764"/> instead of IP fragmentation. Since the 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.</t>
<t>The ingress therefore sends encapsulated packets to the egress
according to the following algorithm:</t>
<t><list style="symbols">
<t>For IP packets that are no larger than (1280-ENCAPS) bytes,
the ingress encapsulates the packet and admits it into the
tunnel without fragmentation. For IPv4 AERO links, the ingress
sets the Don't Fragment (DF) bit to 0 so that these packets will
be delivered to the egress even if there is a restricting link
in the path, i.e., unless lost due to congestion or routing
errors.</t>
<t>For IP packets that are larger than (1280-ENCAPS) bytes but
no larger than 1500 bytes, the ingress encapsulates the packet
and inserts an encapsulation layer fragment header. Next, the
ingress fragments the packet into a minimum number of
non-overlapping fragments where the first fragment (including
ENCAPS) is no larger than 1024 bytes and the remaining fragments
are no larger than the first. Each fragment consists of
corresponding encapsulation headers followed by the fragment of
the encapsulated packet itself. The ingress then admits the
fragments into the tunnel, and for IPv4 sets the DF bit to 0 in
the IP encapsulation header. These fragmented encapsulated
packets will be delivered to the egress, which reassembles them
into a whole packet. The egress therefore MUST be capable of
reassembling packets up to (1500+ENCAPS) bytes in length; hence,
it is RECOMMENDED that the egress be capable of reassembling at
least 2KB.</t>
<t>For IPv4 packets that are larger than 1500 bytes and with the
DF bit set to 0, the ingress uses ordinary IPv4 fragmentation to
break the unencapsulated packet into a minimum number of
non-overlapping fragments where the first fragment (including
ENCAPS) is no larger than 1024 bytes and the remaining fragments
are no larger than the first. The ingress then encapsulates each
fragment (and for IPv4 sets the DF bit to 0) then admits them
into the tunnel. These fragments will be delivered to the final
destination via the egress.</t>
<t>For all other IP packets, if the packet is larger than the
current path MTU estimate for this egress, the ingress drops the
packet and returns an L3 PTB message to the original source with
MTU set to the larger of 1500 bytes or the current path MTU
estimate. Otherwise, the ingress encapsulates the packet and
admits it into the tunnel without fragmentation (and for IPv4
sets the DF bit to 1). Since PTB messages may either be lost or
contain insufficient information, however, it is RECOMMENDED
that original sources that send unfragmentable IP packets larger
than 1500 bytes use Packetization Layer Path MTU Discovery
(PLPMTUD) <xref target="RFC4821"/>.</t>
</list>A first exception to these procedures occurs when the
ingress and egress are both within the same well-managed
administrative domain. In that case, the ingress MAY initially admit
all packets into the tunnel without fragmentation. If the ingress
subsequently receives an L2 PTB message reporting a size smaller
than (1500+ENCAPS) it can commence fragmentation per the above
algorithm.</t>
<t>A second exception occurs when the original source and ingress
are both within the same well-managed administrative domain. In that
case, if the underlying interface used by the ingress for tunneling
configures an MTU smaller than (1500+HLEN) bytes, the ingress MAY
drop packets that are larger than 1280 bytes and larger than the
underlying interface MTU following encapsulation, and return an L3
PTB message to the original source.</t>
</section>
<section anchor="aerobignd"
title="Accommodating Large Control Messages">
<t>The tunnel ingress MUST accommodate control messages (i.e., IPv6
ND, DHCPv6, etc.) even if the path MTU is insufficient to deliver
the message without fragmentation. For control messages that are
larger than the known or assumed minimum path MTU, the ingress
encapsulates the packet and inserts an encapsulation layer fragment
header. Next, the ingress breaks the packet into a minimum number of
non-overlapping fragments where the first fragment (including
ENCAPS) is no larger than 1024 bytes and the remaining fragments are
no larger than the first. The ingress then encapsulates each
fragment (and for IPv4 sets the DF bit to 0) then admits them into
the tunnel.</t>
<t>Control messages that exceed the 2KB minimum reassembly size
rarely occur in current operational practices, however the egress
SHOULD be able to reassemble them if they appear in future
applications. This means that the egress SHOULD include a
configuration knob allowing the operator to set a larger reassembly
buffer size if large control messages become more common in the
future.</t>
<t>The ingress MAY send large control messages without fragmentation
if there is assurance that large packets can traverse the tunnel
without fragmentation.</t>
</section>
<section anchor="integrity" title="Integrity">
<t>When fragmentation is needed, there must be assurance that
reassembly can be safely conducted without incurring data
corruption. Sources of corruption can include implementation errors,
memory errors and misassociations of fragments from a first datagram
with fragments of another datagram. The first two conditions
(implementation and memory errors) are mitigated by modern systems
and implementations that have demonstrated integrity through decades
of operational practice. The third condition (reassembly
misassociations) must be accounted for by AERO.</t>
<t>The fragmentation procedure described in the above algorithms can
reuse standard IPv6 fragmentation and reassembly code. Since
encapsulation layer fragment headers include a 32-bit ID field,
there would need to be 2^32 packets alive in the network before a
second packet with a duplicate ID enters the system with the
(remote) possibility for a reassembly misassociation. For 1280 byte
packets, and for a maximum network lifetime value of 60 seconds
<xref target="RFC2460"/>, this means that the ingress would need to
produce ~(7 *10^12) bits/sec in order for a duplication event to be
possible. This exceeds the bandwidth of data link technologies of
the modern era, but not necessarily so going forward into the
future. Although wireless data links commonly used by AERO Clients
support vastly lower data rates, the aggregate data rates between
AERO Servers and Relays may be substantial. However, high speed data
links in the network core are expected to configure larger MTUs
(e.g., 4KB, 8KB or even larger) such that unfragmented packets can
be used. Hence, no integrity check is included to cover
fragmentation and reassembly procedures.</t>
<t>When the ingress sends an IPv4-encapsulated packet with the DF
bit set to 0 in the above algorithms, there is a chance that the
packet may be fragmented by an IPv4 router somewhere within the
tunnel. Since the largest such packet is only 1280 bytes, however,
it is very likely that the packet will traverse the tunnel without
incurring a restricting link. Even when a link within the tunnel
configures an MTU smaller than 1280 bytes, it is very likely that it
does so due to limited performance characteristics <xref
target="RFC3819"/>. This means that the tunnel would not be able to
convey fragmented IPv4-encapsulated packets fast enough to produce
reassembly misassociations, as discussed above. However, AERO must
also account for the possibility of tunnel paths that traverse a
high-speed IPv4 link with a degenerate MTU.</t>
<t>Since the IPv4 header includes only a 16-bit ID field, there
would only need to be 2^16 packets alive in the network before a
second packet with a duplicate ID enters the system. For 1280 byte
packets, and for a maximum network lifetime value of 120
seconds<xref target="RFC0791"/>, this means that the ingress would
only need to produce ~(5 *10^6) bits/sec in order for a duplication
event to be possible - a value that is well within range for modern
wired and wireless data link technologies.</t>
<t>Therefore, if there is strong operational assurance that no IPv4
links capable of supporting data rates of 5Mbps or more configure an
MTU smaller than 1280 the ingress MAY omit an integrity check for
the IPv4 fragmentation and reassembly procedures; otherwise, the
ingress SHOULD include an integrity check. When an upper layer
encapsulation (e.g., IPsec) already includes an integrity check, the
ingress need not include an additional check. Otherwise, the ingress
calculates the encapsulation layer checksum over the encapsulated
packet and writes the value into the encapsulation layer checksum
header. The egress will then verify the checksum and discard the
packet if the checksum is incorrect.</t>
</section>
</section>
<section anchor="aeroerr" title="AERO Interface Error Handling">
<t>When an AERO node admits encapsulated packets into the AERO
interface, it may receive link-layer (L2) or network-layer (L3) error
indications.</t>
<t>An L2 error indication is an ICMP error message generated by a
router 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.</t>
<t>The IP header is followed by an ICMP header that includes an error
Type, Code and Checksum. For ICMPv6 <xref target="RFC4443"/>, the
error Types include "Destination Unreachable", "Packet Too Big (PTB)",
"Time Exceeded" and "Parameter Problem". For ICMPv4 <xref
target="RFC0792"/>, the error Types include "Destination Unreachable",
"Fragmentation Needed" (a Destination Unreachable Code that is
analogous to the ICMPv6 PTB), "Time Exceeded" and "Parameter
Problem".</t>
<t>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, <xref target="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, <xref target="RFC0792"/> specifies that the
packet-in-error includes: "Internet Header + 64 bits of Original Data
Datagram", however <xref target="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".</t>
<t>The L2 error message format is shown in <xref
target="icmp2err"/>:</t>
<t><figure anchor="icmp2err"
title="AERO Interface L2 Error Message Format">
<artwork><![CDATA[ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
]]></artwork>
</figure>The AERO node rules for processing these L2 error messages
is as follows:</t>
<t><list style="symbols">
<t>When an AERO node receives an L2 Parameter Problem message, it
processes the message the same as described as for ordinary ICMP
errors in the normative references <xref target="RFC0792"/><xref
target="RFC4443"/>.</t>
<t>When an AERO node receives persistent L2 IPv4 Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
have been processed. In that case, the node SHOULD begin including
IPv4 integrity checks (see: <xref target="integrity"/>).</t>
<t>When an AERO Client receives persistent L2 Destination
Unreachable messages in response to tunneled packets that it sends
to one of its dynamic neighbor correspondents, the Client SHOULD
test the path to the correspondent using Neighbor Unreachability
Detection (NUD) (see <xref target="nud"/>). If NUD fails, the
Client SHOULD set ForwardTime for the corresponding dynamic
neighbor cache entry to 0 and allow future packets destined to the
correspondent to flow through a Server.</t>
<t>When an AERO Client receives persistent L2 Destination
Unreachable messages in response to tunneled packets that it sends
to one of its static neighbor Servers, the Client SHOULD test the
path to the Server using NUD. If NUD fails, the Client SHOULD
delete the neighbor cache entry and attempt to associate with a
new Server.</t>
<t>When an AERO Server receives persistent L2 Destination
Unreachable messages in response to tunneled packets that it sends
to one of its static neighbor Clients, the Server SHOULD test the
path to the Client using NUD. If NUD fails, the Server SHOULD
cancel the DHCPv6 PD for the Client's ACP, withdraw its route for
the ACP from the AERO routing system and delete the neighbor cache
entry (see <xref target="nud"/> and <xref target="aeromob"/>).</t>
<t>When an AERO Relay or Server receives an L2 Destination
Unreachable message in response to a tunneled packet that it sends
to one of its permanent neighbors, it discards the message since
the AERO routing system is likely in a temporary transitional
state that will soon re-converge. In case of a prolonged outage,
however, the AERO routing system will compensate for Relays or
Servers that have fallen silent.</t>
<t>When an AERO node receives an L2 PTB message, it caches the MTU
field value of the L2 ICMP header then translates the message into
an L3 PTB message if possible and forwards the message toward the
original source as described below. Note that in some instances
the packet-in-error field of an L2 PTB message may not include
enough information for translation to an L3 PTB message. In that
case, the AERO interface simply discards the L2 PTB message since
translation of L2 PTB messages to L3 PTB messages can provide a
useful optimization when possible, but is not critical for sources
that correctly use PLPMTUD.</t>
</list>To translate an L2 PTB message to an L3 PTB message, the AERO
node discards the L2 IP and ICMP headers, and also discards the
encapsulation headers of the original L3 packet. Next the node
encapsulates the included segment of the original L3 packet in an L3
IP and ICMP header, and sets the ICMP header Type and Code values to
appropriate values for the L3 IP protocol. When the AERO node, AERO
link neighbor and original source are all within the same
administrative domain, the node writes the maximum of 1280 bytes and
(L2 MTU - ENCAPS) into the MTU field of the L3 ICMP header. Otherwise,
the node translates L2 PTB messages for which (L2 MTU - ENCAPS) is no
less than 1500 bytes and discards all other L2 PTBs.</t>
<t>The node next writes the IP source address of the original L3
packet as the destination address of the L3 PTB message and determines
the next hop to the 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 IP source address of the L3 PTB message.
Otherwise, the node uses one of its non link-local addresses as the
source address of the L3 PTB message. The node finally calculates the
ICMP checksum over the L3 PTB message and writes the Checksum in the
corresponding field of the L3 ICMP header. The L3 PTB message
therefore is formatted as follows:</t>
<t><figure anchor="icmp3err"
title="AERO Interface L3 Error Message Format">
<artwork><![CDATA[ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| L3 IP Header of |
| error message |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L3 ICMP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ ~ p
| IP header of | k
| original L3 packet | t
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i
~ ~ n
| Upper layer headers and |
| leading portion of body | e
| of the original L3 packet | r
~ ~ r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
]]></artwork>
</figure>After the node has prepared the L3 PTB message, it either
forwards the message via a link outside of the AERO interface without
encapsulation, or encapsulates and forwards the message to the next
hop via the AERO interface.</t>
<t>When an AERO Relay receives an L3 packet for which the 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
an L3 Destination Unreachable message. The Relay first writes the IP
source address of the original L3 packet as the destination address of
the L3 Destination Unreachable 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 IP source address of the L3 Destination Unreachable message and
forwards the message 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 L3 Destination Unreachable message and forwards
the message via a link outside the AERO interface.</t>
<t>When an AERO node receives any L3 error message via the AERO
interface, it examines the destination address in the L3 IP header of
the message. If the next hop toward the destination address of the
error message is via the AERO interface, the node re-encapsulates and
forwards the message to the next hop within the AERO interface.
Otherwise, if the source address in the L3 IP header of the message 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 of the L3
message and recalculates the IP and/or ICMP checksums. The node
finally forwards the message via a link outside of the AERO
interface.</t>
</section>
<section anchor="aeropd"
title="AERO Router Discovery, Prefix Delegation and Address Configuration">
<section anchor="aeropd-dhcp" title="AERO DHCPv6 Service Model">
<t>Each AERO Server configures a DHCPv6 server function to
facilitate PD requests from Clients. 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) <xref target="RFC4511"/> or a similar
distributed database service.</t>
<t>Therefore, no Server-to-Server DHCPv6 PD delegation state
synchronization is necessary, and Clients can optionally hold
separate delegations for the same ACPs from multiple Servers. In
this way, Clients can associate with multiple Servers, and can
receive new delegations from new Servers before deprecating
delegations received from existing Servers. This provides the Client
with a natural fault-tolerance and/or load balancing profile.</t>
<t>AERO Clients and Servers exchange Client link-layer address
information using an option format similar to the Client Link Layer
Address Option (CLLAO) defined in <xref target="RFC6939"/>. Due to
practical limitations of CLLAO, however, AERO interfaces instead use
Vendor-Specific Information Options as described in the following
sections.</t>
</section>
<section anchor="aeropd-client" title="AERO Client Behavior">
<t>AERO Clients discover the link-layer addresses of AERO Servers
via static configuration, or through an automated means such as DNS
name resolution. In the absence of other information, the Client
resolves the FQDN "linkupnetworks.[domainname]" where
"linkupnetworks" is a constant text string and "[domainname]" is the
connection-specific DNS suffix for the Client's underlying network
connection (e.g., "example.com"). After discovering the link-layer
addresses, the Client associates with one or more of the
corresponding Servers.</t>
<t>To associate with a Server, the Client acts as a requesting
router to request ACPs through a two-message (i.e., Solicit/Reply)
DHCPv6 PD exchange <xref target="RFC3315"/><xref target="RFC3633"/>.
The Client's Solicit message includes fe80::ffff:ffff:ffff:ffff as
the IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the
IPv6 destination address and the link-layer address of the Server as
the link-layer destination address. The Solicit message also
includes a Client Identifier option with a DUID and an Identity
Association for Prefix Delegation (IA_PD) option. If the Client is
pre-provisioned with ACPs associated with the AERO service, it MAY
also include the ACPs in the IA_PD to indicate its preferences to
the DHCPv6 server.</t>
<t>The Client also SHOULD include an AERO Link-registration Request
(ALREQ) option to register one or more links with the Server. The
Server will include an AERO Link-registration Reply (ALREP) option
in the corresponding Reply message as specified in <xref
target="aeropd-server"/>. (The Client MAY omit the ALREQ option, in
which case the Server will still include an ALREP option in its
Reply with "Link ID" set to 0.)</t>
<t>The format for the ALREQ option is shown in <xref
target="alireq"/>:<figure anchor="alireq"
title="AERO Link-registration Request (ALREQ) Option">
<artwork><![CDATA[ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VENDOR_OPTS | option-len (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number = 45282 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| opt-code = OPTION_ALREQ (0) | option-len (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID | DSCP #1 |Prf| DSCP #2 |Prf| ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
]]></artwork>
</figure></t>
<t>In the above format, the Client sets 'option-code' to
OPTION_VENDOR_OPTS, sets 'option-len (1)' to the length of the
option following this field, sets 'enterprise-number' to 45282 (see:
"IANA Considerations"), sets opt-code to the value 0
("OPTION_ALREQ") and sets 'option-len (2)' to the length of the
remainder of the option. The Client includes appropriate 'Link ID,
'DSCP' and 'Prf' values for the underlying interface over which the
Solicit message will be issued the same as specified for an S/TLLAO
<xref target="interface"/>. The Server will register each value with
the Link ID in the Client's neighbor cache entry. The Client finally
includes any necessary authentication options to identify itself to
the DHCPv6 server, and sends the encapsulated Solicit message via
the underlying interface corresponding to Link ID. (Note that this
implies that the Client must send additional Rebind messages to the
server following the initial PD exchange using different underlying
interfaces and their corresponding Link IDs if it wishes to register
additional link-layer addresses and their associated DSCPs.)</t>
<t>When the Client receives its ACP via a Reply from the AERO
Server, it creates a static neighbor cache entry with the Server's
link-local address as the network-layer address and the Server's
encapsulation address as the link-layer address. The Client then
considers the link-layer address of the Server as the primary
default encapsulation address for forwarding packets for which no
more-specific forwarding information is available. The Client
further caches any ASPs included in the ALREP option as ASPs to
apply to the AERO link.</t>
<t>Next, the Client autoconfigures an AERO address for each of the
delegated ACPs, assigns the address(es) to the AERO interface and
sub-delegates the ACPs to its attached EUNs and/or the Client's own
internal virtual interfaces. Alternatively, the Client can configure
as many addresses as it wants from /64 prefixes taken from the ACPs
and assign them to either an internal virtual interface ("weak
end-system") or to the AERO interface itself ("strong end-system")
<xref target="RFC1122"/> while black-holing the remaining portions
of the /64s. Finally, the Client assigns one or more default IP
routes to the AERO interface with the link-local address of a Server
as the next hop.</t>
<t>After AERO address autoconfiguration, the Client SHOULD begin
using the AERO address as the source address for further DHCPv6
messaging. The Client subsequently renews its ACP delegations
through each of its Servers by sending Renew messages with the
link-layer address of a Server as the link-layer destination address
and the same options that were used in the initial PD request. Note
that if the Client does not issue a Renew before the delegations
expire (e.g., if the Client has been out of touch with the Server
for a considerable amount of time) it must re-initiate the DHCPv6 PD
procedure.</t>
<t>Since the addresses assigned to the Client's AERO interface are
obtained from the unique ACP delegations it receives, there is no
need for DAD on AERO links. Other nodes maliciously attempting to
hijack addresses from an authorized Client's ACPs will be denied
access to the network by the Server due to an unacceptable
link-layer address and/or security parameters (see: Security
Considerations).</t>
<t>When a Client attempts to perform a DHCPv6 PD exchange with a
Server that is too busy to service the request, the Client may
receive either a "NoPrefixAvail" status code in the Server's Reply
per <xref target="RFC3633"/> or no reply at all. In that case, the
Client SHOULD discontinue DHCPv6 PD attempts through this Server and
try another Server.</t>
<section title="Autoconfiguration for Constrained Platforms">
<t>On some platforms (e.g., popular cell phone operating systems),
the act of assigning a default IPv6 route and/or assigning an
address to an interface may not be permitted from a user
application due to security policy. Typically, those platforms
include a TUN/TAP interface <xref target="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 <xref
target="RFC4861"/> and is prepared as follows:</t>
<t><list style="symbols">
<t>the IPv6 source address is the Client's AERO address</t>
<t>the IPv6 destination address is all-nodes multicast</t>
<t>the Router Lifetime is set to a time that is no longer than
the ACP DHCPv6 lifetime</t>
<t>the message does not include a Source Link Layer Address
Option (SLLAO)</t>
<t>the message includes a Prefix Information Option (PIO) with
a /64 prefix taken from the ACP as the prefix for
autoconfiguration</t>
</list>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 <xref
target="RFC2131"/>.</t>
</section>
</section>
<section anchor="aeropd-server" title="AERO Server Behavior">
<t>AERO Servers configure a DHCPv6 server function on their AERO
links. AERO Servers arrange to add their encapsulation layer IP
addresses (i.e., their link-layer addresses) to the DNS resource
records for the FQDN "linkupnetworks.[domainname]" before entering
service.</t>
<t>When an AERO Server receives a prospective Client's Solicit on
its AERO interface, and the Server is too busy to service the
message, it SHOULD return a Reply with status code "NoPrefixAvail"
per <xref target="RFC3633"/>. Otherwise, the Server authenticates
the message. If authentication succeeds, the Server determines the
correct ACPs to delegate to the Client by searching the Client
database.</t>
<t>When the Server delegates the ACPs, it also creates IP forwarding
table entries so that the AERO routing system will propagate the
ACPs to all Relays that aggregate the corresponding ASP (see: <xref
target="scaling"/>). Next, the Server prepares a Reply message to
send to the Client while using fe80::ID as the IPv6 source address,
the link-local address taken from the Client's Solicit as the IPv6
destination address, the Server's link-layer address as the source
link-layer address, and the Client's link-layer address as the
destination link-layer address. The server also includes IA_PD
options with the delegated ACPs. Since the Client may experience a
fault that prevents it from issuing a Release before departing from
the network, Servers should set a short prefix lifetime (e.g., 40
seconds) so that stale prefix delegation state can be flushed out of
the network.</t>
<t>The Server also includes an ALREP option that includes the UDP
Port Number and IP Address values it observed when it received the
ALREQ in the Client's original DHCPv6 message (if present) followed
by the ASP(s) for the AERO link. The ALREP option is formatted as
shown in <xref target="alrep"/>:</t>
<t><figure anchor="alrep"
title="AERO Link-registration Reply (ALREP) Option">
<artwork><![CDATA[ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VENDOR_OPTS | option-len (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number = 45282 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| opt-code = OPTION_ALREP (1) | option-len (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID | Reserved | UDP Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IP Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ AERO Service Prefix (ASP) #1 +-+-+-+-+-+-+-+-+
| | Prefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ AERO Service Prefix (ASP) #2 +-+-+-+-+-+-+-+-+
| | Prefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ ~
]]></artwork>
</figure>In the ALREP, the Server sets 'option-code' to
OPTION_VENDOR_OPTS, sets 'option-length (1)' to the length of the
option, sets 'enterprise-number' to 45282 (see: "IANA
Considerations"), sets opt-code to OPTION_ALREP (1), and sets
'option-len (2)' to the length of the remainder of the option. Next,
the Server sets 'Link ID' to the same value that appeared in the
ALREQ, sets Reserved to 0 and sets 'UDP Port Number' and 'IP
address' to the Client's link-layer address. The Server next
includes one or more ASP with the IP prefix as it would appear in
the interface identifier portion of the corresponding AERO address
(see: <xref target="aero-address"/>), except that the low-order 8
bits of the ASP field encode the prefix length instead of the
low-order 8 bits of the prefix. The longest prefix that can
therefore appear as an ASP is /56 for IPv6 or /24 for IPv4. (Note
that if the Client did not include an ALREQ option in its DHCPv6
message, the Server MUST still include an ALREP option in the
corresponding reply with 'Link ID' set to 0.)</t>
<t>When the Server admits the Reply message into the AERO interface,
it creates a static neighbor cache entry for the Client based on the
DUID and AERO addresses with lifetime set to no more than the
delegation lifetimes and the Client's link-layer address as the
link-layer address for the Link ID specified in the ALREQ. The
Server then uses the Client link-layer address information in the
ALREQ option as the link-layer address for encapsulation based on
the (DSCP, Prf) information.</t>
<t>After the initial DHCPv6 PD exchange, the AERO Server maintains
the neighbor cache entry for the Client until the delegation
lifetimes expire. If the Client issues a Renew, the Server extends
the lifetimes. If the Client issues a 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.</t>
<section title="Lightweight DHCPv6 Relay Agent (LDRA)">
<t>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 AERO interface driver 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)<xref target="RFC6221"> </xref> to relay DHCPv6
messages to and from the DHCPv6 server module.</t>
<t>When the LDRA receives a DHCPv6 message from a client, it
prepares an ALREP option the same as described above then wraps
the option in a Relay-Supplied DHCP Option option (RSOO) <xref
target="RFC6422"/>. The LDRA then incorporates the option into the
Relay-Forward message and forwards the message to the DHCPv6
server.</t>
<t>When the DHCPv6 server receives the Relay-Forward message, it
caches the ALREP option and authenticates the encapsulated DHCPv6
message. The DHCPv6 server subsequently ignores the ALREQ option
itself, since the relay has already included the ALREP option.</t>
<t>When the DHCPv6 server prepares a Reply message, it then
includes the ALREP option in the body of the message along with
any other options, then wraps the message in a Relay-Reply
message. The DHCPv6 server then delivers the Relay-Reply message
to the LDRA, which discards the Relay-Reply wrapper and delivers
the DHCPv6 message to the Client.</t>
</section>
</section>
<section anchor="aeropd-link-dereg"
title="Deleting Link Registrations">
<t>After an AERO Client registers its Link IDs and their associated
(DSCP,Prf) values with the AERO Server, the Client may wish to
delete one or more Link registrations, e.g., if an underlying link
becomes unavailable. To do so, the Client prepares a Rebind message
that includes an AERO Link-registration Delete (ALDEL) option and
sends the Rebind message to the Server over any available underlying
link. The ALDEL option is formatted as shown in <xref
target="aldel"/>:</t>
<t><figure anchor="aldel"
title="AERO Link-registration Delete (ALDEL) Option">
<artwork><![CDATA[ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VENDOR_OPTS | option-len (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number = 45282 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| opt-code = OPTION_ALDEL (2) | option-len (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID #1 | Link ID #2 | Link ID #3 | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
]]></artwork>
</figure>In the ALDEL, the Client sets 'option-code' to
OPTION_VENDOR_OPTS, sets 'option-length (1)' to the length of the
option, sets 'enterprise-number' to 45282 (see: "IANA
Considerations"), sets optcode to OPTION_ALDEL (2), and sets
'option-len (2)' to the length of the remainder of the option. Next,
the Server includes each 'Link ID' value that it wishes to
delete.</t>
<t>If the Client wishes to discontinue use of a Server and thereby
delete all of its Link ID associations, it must issue a Release to
delete the entire neighbor cache entry, i.e., instead of issuing a
Rebind with one or more ALDEL options.</t>
</section>
</section>
<section anchor="aeropd-agent" title="AERO Forwarding Agent Behavior">
<t>AERO Servers MAY associate with one or more companion AERO
Forwarding Agents as platforms for offloading high-speed data plane
traffic. When an AERO Server receives a Client's
Solicit/Renew/Rebind/Release message, it services the message then
forwards the corresponding Reply message to the Forwarding Agent. When
the Forwarding Agent receives the Reply message, it creates, updates
or deletes a neighbor cache entry with the Client's AERO address and
link-layer information included in the Reply message. The Forwarding
Agent then forwards the Reply message back to the AERO Server, which
forwards the message to the Client. In this way, Forwarding Agent
state is managed in conjunction with Server state, with the Client
responsible for reliability.</t>
<t>When an AERO Server receives a data packet on an AERO interface
with a network layer destination address for which it has distributed
forwarding information to a Forwarding Agent, the Server returns a
Redirect message to the source neighbor (subject to rate limiting)
then forwards the data packet as usual. The Redirect message includes
a TLLAO with the link-layer address of the Forwarding Engine.</t>
<t>When the source neighbor receives the Redirect message, it SHOULD
record the link-layer address in the TLLAO as the encapsulation
addresses to use for sending subsequent data packets. However, the
source MUST continue to use the primary link-layer address of the
Server as the encapsulation address for sending control messages.</t>
</section>
<section anchor="predirect" title="AERO Intradomain Route Optimization">
<t>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 intra-domain AERO route optimization
procedure. It is important to note that this procedure is initiated by
the Client; if the procedure were initiated by the Server, the Server
would have no way of knowing whether the Client was actually able to
contact the correspondent over the route-optimized path.</t>
<t>The procedure is based on an exchange of IPv6 ND messages using a
chain of AERO Servers and Relays as a trust basis. This procedure is
in contrast to the Return Routability procedure required for route
optimization to a correspondent Client located in the Internet as
described in <xref target="aerointernet"/>. The following sections
specify the AERO intradomain route optimization procedure.</t>
<section anchor="avoidance-fig" title="Reference Operational Scenario">
<t><xref target="no-onlink-prefix-fig"/> depicts the AERO
intradomain route optimization reference operational scenario, using
IPv6 addressing as the example (while not shown, a corresponding
example for IPv4 addressing can be easily constructed). The figure
shows an AERO Relay ('R1'), two AERO Servers ('S1', 'S2'), two AERO
Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1', 'H2'):</t>
<figure anchor="no-onlink-prefix-fig"
title="AERO Reference Operational Scenario">
<artwork><![CDATA[ +--------------+ +--------------+ +--------------+
| Server S1 | | Relay R1 | | Server S2 |
+--------------+ +--------------+ +--------------+
fe80::2 fe80::1 fe80::3
L2(S1) L2(R1) L2(S2)
| | |
X-----+-----+------------------+-----------------+----+----X
| AERO Link |
L2(A) L2(B)
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 )
`-(______)-' +---------+ +---------+ `-(______)-'
]]></artwork>
</figure>
<t>In <xref target="no-onlink-prefix-fig"/>, Relay ('R1') assigns
the 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.</t>
<t>AERO Client ('C1') receives the ACP 2001:db8:0::/48 in a DHCPv6
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.</t>
<t>AERO Client ('C2') receives the ACP 2001:db8:1::/48 in a DHCPv6
PD exchange via AERO Server ('S2') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C2). Client ('C2') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::3 and
link-layer address L2(S2), then sub-delegates the ACP to its
attached EUNs. IPv6 host ('H2') connects to the EUN, and configures
the address 2001:db8:1::1.</t>
</section>
<section anchor="conops" title="Concept of Operations">
<t>Again, with reference to <xref target="no-onlink-prefix-fig"/>,
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 a Predirect message toward
Client ('C2') via Server ('S1'). Server ('S1') then re-encapsulates
and forwards both the packet and the Predirect message out the same
AERO interface toward Client ('C2') via Relay ('R1').</t>
<t>When Relay ('R1') receives the packet and Predirect 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 Predirect message to Server ('S2'), which then forwards them
to Client ('C2').</t>
<t>After Client ('C2') receives the Predirect message, it process
the message and returns a Redirect message toward Client ('C1') via
Server ('S2'). During the process, Client ('C2') also creates or
updates a dynamic neighbor cache entry for Client ('C1').</t>
<t>When Server ('S2') receives the Redirect message, it
re-encapsulates the message and forwards it on to Relay ('R1'),
which forwards the message on to Server ('S1') which forwards the
message on to Client ('C1'). After Client ('C1') receives the
Redirect message, it processes the message and creates or updates a
dynamic neighbor cache entry for Client ('C2').</t>
<t>Following the above Predirect/Redirect message exchange,
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.</t>
</section>
<section anchor="rmsg" title="Message Format">
<t>AERO Redirect/Predirect messages use the same format as for IPv6
ND Redirect messages depicted in Section 4.5 of <xref
target="RFC4861"/>, but also include a new "Prefix Length" field
taken from the low-order 8 bits of the Redirect message Reserved
field. For IPv6, valid values for the Prefix Length field are 0
through 64; for IPv4, valid values are 0 through 32. The
Redirect/Predirect messages are formatted as shown in <xref
target="aero-redirect"/>:</t>
<figure anchor="aero-redirect"
title="AERO Redirect/Predirect Message Format">
<artwork><![CDATA[ 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 (=137) | Code (=0/1) | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
]]></artwork>
</figure>
<t/>
</section>
<section anchor="sending_pre" title="Sending Predirects">
<t>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 a Predirect message forward toward the
destination Client via the Server.</t>
<t>In the reference operational scenario, when Client ('C1')
forwards a packet toward Client ('C2'), it MAY also send a Predirect
message forward toward Client ('C2'), subject to rate limiting (see
Section 8.2 of <xref target="RFC4861"/>). Client ('C1') prepares the
Predirect message as follows:</t>
<t><list style="symbols">
<t>the link-layer source address is set to 'L2(C1)' (i.e., the
link-layer address of Client ('C1')).</t>
<t>the link-layer destination address is set to 'L2(S1)' (i.e.,
the link-layer address of Server ('S1')).</t>
<t>the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('C1')).</t>
<t>the network-layer destination address is set to
fe80::2001:db8:1:0 (i.e., the AERO address of Client
('C2')).</t>
<t>the Type is set to 137.</t>
<t>the Code is set to 1 to indicate "Predirect".</t>
<t>the Prefix Length is set to the length of the prefix to be
assigned to the Target Address.</t>
<t>the Target Address is set to fe80::2001:db8:0:0 (i.e., the
AERO address of Client ('C1')).</t>
<t>the Destination Address is set to the source address of the
originating packet that triggered the Predirection event. (If
the originating packet is an IPv4 packet, the address is
constructed in IPv4-compatible IPv6 address format).</t>
<t>the message includes one or more TLLAOs with Link ID and
DSCPs set to appropriate values for Client ('C1')'s underlying
interfaces, and with UDP Port Number and IP Address set to
0'.</t>
<t>the message SHOULD include a Timestamp option and a Nonce
option.</t>
<t>the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated if necessary to ensure
that at least the network-layer header is included but the size
of the message does not exceed 1280 bytes.</t>
</list></t>
<t>Note that the act of sending Predirect 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').</t>
</section>
<section anchor="relaying_pre"
title="Re-encapsulating and Relaying Predirects">
<t>When Server ('S1') receives a Predirect message from Client
('C1'), it first verifies that the TLLAOs in the Predirect are a
proper subset of the Link IDs in Client ('C1')'s neighbor cache
entry. If the Client's TLLAOs are not acceptable, Server ('S1')
discards the message. Otherwise, Server ('S1') validates the message
according to the Redirect message validation rules in Section 8.1 of
<xref target="RFC4861"/>, except that the Predirect has Code=1.
Server ('S1') also verifies that Client ('C1') is authorized to use
the Prefix Length in the Predirect when applied to the AERO address
in the network-layer source address by searching for the AERO
address in the neighbor cache. If validation fails, Server ('S1')
discards the Predirect; otherwise, it copies the correct UDP Port
numbers and IP Addresses for Client ('C1')'s links into the
(previously empty) TLLAOs.</t>
<t>Server ('S1') then examines the network-layer destination address
of the Predirect 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 Predirect 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 address to 'L2(R1)'. Server ('S1')
finally forwards the re-encapsulated message to Relay ('R1') without
decrementing the network-layer TTL/Hop Limit field.</t>
<t>When Relay ('R1') receives the Predirect 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 Predirect 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 Predirect via Server
('S2').</t>
<t>When Server ('S2') receives the Predirect message from Relay
('R1') it determines that Client ('C2') is a neighbor by consulting
its neighbor cache. Server ('S2') then re-encapsulates the Predirect
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').</t>
</section>
<section anchor="processing_pre"
title="Processing Predirects and Sending Redirects">
<t>When Client ('C2') receives the Predirect message, it accepts the
Predirect 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 redirection target.
Next, Client ('C2') validates the message according to the Redirect
message validation rules in Section 8.1 of <xref target="RFC4861"/>,
except that it accepts the message even though Code=1 and even
though the network-layer source address is not that of it's current
first-hop router.</t>
<t>In the reference operational scenario, when Client ('C2')
receives a valid Predirect message, it either creates or updates a
dynamic neighbor cache entry that stores the Target Address of the
message as the network-layer address of Client ('C1') , stores the
link-layer addresses found in the TLLAOs as the link-layer addresses
of Client ('C1') and stores the Prefix Length as the length to be
applied to the network-layer address for forwarding purposes. Client
('C2') then sets AcceptTime for the neighbor cache entry to
ACCEPT_TIME.</t>
<t>After processing the message, Client ('C2') prepares a Redirect
message response as follows:</t>
<t><list style="symbols">
<t>the link-layer source address is set to 'L2(C2)' (i.e., the
link-layer address of Client ('C2')).</t>
<t>the link-layer destination address is set to 'L2(S2)' (i.e.,
the link-layer address of Server ('S2')).</t>
<t>the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C2')).</t>
<t>the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the AERO address of Client
('C1')).</t>
<t>the Type is set to 137.</t>
<t>the Code is set to 0 to indicate "Redirect".</t>
<t>the Prefix Length is set to the length of the prefix to be
applied to the Target Address.</t>
<t>the Target Address is set to fe80::2001:db8:1:0 (i.e., the
AERO address of Client ('C2')).</t>
<t>the Destination Address is set to the destination address of
the originating packet that triggered the Redirection event. (If
the originating packet is an IPv4 packet, the address is
constructed in IPv4-compatible IPv6 address format).</t>
<t>the message includes one or more TLLAOs with Link ID and
DSCPs set to appropriate values for Client ('C2')'s underlying
interfaces, and with UDP Port Number and IP Address set to
'0'.</t>
<t>the message SHOULD include a Timestamp option and MUST echo
the Nonce option received in the Predirect (i.e., if a Nonce
option is included).</t>
<t>the message includes as much of the RHO copied from the
corresponding Predirect message as possible such that at least
the network-layer header is included but the size of the message
does not exceed 1280 bytes.</t>
</list></t>
<t>After Client ('C2') prepares the Redirect message, it sends the
message to Server ('S2').</t>
</section>
<section anchor="relaying_re"
title="Re-encapsulating and Relaying Redirects">
<t>When Server ('S2') receives a Redirect message from Client
('C2'), it first verifies that the TLLAOs in the Redirect are a
proper subset of the Link IDs in Client ('C2')'s neighbor cache
entry. If the Client's TLLAOs are not acceptable, Server ('S2')
discards the message. Otherwise, Server ('S2') validates the message
according to the Redirect message validation rules in Section 8.1 of
<xref target="RFC4861"/>. Server ('S2') also verifies that Client
('C2') is authorized to use the Prefix Length in the Redirect when
applied to the AERO address in the network-layer source address by
searching for the AERO address in the neighbor cache. If validation
fails, Server ('S2') discards the Redirect; otherwise, it copies the
correct UDP Port numbers and IP Addresses for Client ('C2')'s links
into the (previously empty) TLLAOs.</t>
<t>Server ('S2') then examines the network-layer destination address
of the Redirect 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 Redirect
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.</t>
<t>When Relay ('R1') receives the Redirect 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 Redirect 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 Redirect via Server
('S1').</t>
<t>When Server ('S1') receives the Redirect message from Relay
('R1') it determines that Client ('C1') is a neighbor by consulting
its neighbor cache. Server ('S1') then re-encapsulates the Redirect
while changing the link-layer source address to 'L2(S1)' and
changing the link-layer destination address to 'L2(C1)'. Server
('S1') then forwards the message to Client ('C1').</t>
</section>
<section anchor="processing_re" title="Processing Redirects">
<t>When Client ('C1') receives the Redirect message, it accepts the
message only if it has a link-layer source address of one of its
Servers (e.g., ''L2(S1)'). Next, Client ('C1') validates the message
according to the Redirect message validation rules in Section 8.1 of
<xref target="RFC4861"/>, except that it accepts the message even
though the network-layer source address is not that of it's current
first-hop router. Following validation, Client ('C1') then processes
the message as follows.</t>
<t>In the reference operational scenario, when Client ('C1')
receives the Redirect message, it either creates or updates a
dynamic neighbor cache entry that stores the Target 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 Prefix Length as the length to be
applied to the network-layer address for forwarding purposes. Client
('C1') then sets ForwardTime for the neighbor cache entry to
FORWARD_TIME.</t>
<t>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 Predirect/Redirect message exchange is required in the
reverse direction; hence, the mechanism is asymmetric.)</t>
</section>
<section anchor="server_re" title="Server-Oriented Redirection">
<t>In some environments, the Server nearest the target Client may
need to serve as the redirection target, e.g., if direct
Client-to-Client communications are not possible. In that case, the
Server prepares the Redirect message the same as if it were the
destination Client (see: <xref target="processing_pre"/>), except
that it writes its own link-layer address in the TLLAO option. The
Server must then maintain a dynamic neighbor cache entry for the
redirected source Client.</t>
</section>
<section title="Route Optimization Policy">
<t>Although the Client is responsible for initiating route
optimization through the transmission of Predirect messages, 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 interception points. In those cases, the
Server can deny route optimization requests by simply discarding any
Predirect messages instead of forwarding them.</t>
</section>
<section title="Route Optimization and Multiple ACPs">
<t>Clients that receive multiple non-contiguous ACP delegations must
perform route optimization for each of the individual ACPs based on
demand of traffic with source addresses taken from those prefixes.
For example, if Client C1 has already performed route optimization
for destination ACP X on behalf of its source ACP Y, it must also
perform route optimization for X on behalf of its source ACP Z. As a
result, source route optimization state cannot be shared between
non-contiguous ACPs and must be managed separately.</t>
</section>
</section>
<section anchor="nud" title="Neighbor Unreachability Detection (NUD)">
<t>AERO nodes perform Neighbor Unreachability Detection (NUD) by
sending unicast NS messages to elicit solicited NA messages from
neighbors the same as described in <xref target="RFC4861"/>. NUD is
performed either reactively in response to persistent L2 errors (see
<xref target="aeroerr"/>) or proactively to test existing neighbor
cache entries.</t>
<t>When an AERO node sends an NS/NA message, it MUST use its
link-local address as the IPv6 source address and the link-local
address of the neighbor as the IPv6 destination address. When an AERO
node receives an NS message or a solicited NA message, it accepts the
message if it has a neighbor cache entry for the neighbor; otherwise,
it ignores the message.</t>
<t>When a source Client is redirected to a target Client it SHOULD
proactively test the direct path by sending an initial NS message to
elicit a solicited NA response. While testing the path, the source
Client can optionally continue sending packets via the Server,
maintain a small queue of packets until target reachability is
confirmed, or (optimistically) allow packets to flow directly to the
target. The source Client SHOULD thereafter continue to test the
direct path to the target Client (see Section 7.3 of <xref
target="RFC4861"/>) periodically in order to keep dynamic neighbor
cache entries alive.</t>
<t>In particular, while the source Client is actively sending packets
to the target Client it SHOULD also send NS messages separated by
RETRANS_TIMER milliseconds in order to receive solicited NA messages.
If the source Client is unable to elicit a solicited NA response from
the target Client after MAX_RETRY attempts, it SHOULD set ForwardTime
to 0 and resume sending packets via one of its Servers. Otherwise, the
source Client considers the path usable and SHOULD thereafter process
any link-layer errors as a hint that the direct path to the target
Client has either failed or has become intermittent.</t>
<t>When ForwardTime for a dynamic neighbor cache entry expires, the
source Client resumes sending any subsequent packets via a Server and
may (eventually) attempt to re-initiate the AERO redirection process.
When AcceptTime for a dynamic neighbor cache entry expires, the target
Client discards any subsequent packets received directly from the
source Client. When both ForwardTime and AcceptTime for a dynamic
neighbor cache entry expire, the Client deletes the neighbor cache
entry.</t>
</section>
<section anchor="aeromob" title="Mobility Management">
<section anchor="llchange"
title="Announcing Link-Layer Address Changes">
<t>When a Client needs to change its link-layer address, e.g., due
to a mobility event, it issues an immediate Rebind to each of its
Servers using the new link-layer address as the source address and
with an ALREQ that includes the correct Link ID and DSCP values. If
authentication succeeds, the Server then updates its neighbor cache
and sends a Reply. Note that if the Client does not issue a Rebind
before the prefix delegation lifetime expires (e.g., if the Client
has been out of touch with the Server for a considerable amount of
time), the Server's Reply will report NoBinding and the Client must
re-initiate the DHCPv6 PD procedure.</t>
<t>Next, the Client sends Predirect messages to each of its
correspondent Client neighbors using the same procedures as
specified in <xref target="sending_pre"/>. The Client sends the
Predirect messages via a Server the same as if it was performing the
initial route optimization procedure with the correspondent. The
Predirect message will update the correspondent' link layer address
mapping for the Client.</t>
</section>
<section anchor="newlink" title="Bringing New Links Into Service">
<t>When a Client needs to bring a new underlying interface into
service (e.g., when it activates a new data link), it issues an
immediate Rebind to each of its Servers using the new link-layer
address as the source address and with an ALREQ that includes the
new Link ID and DSCP values. If authentication succeeds, the Server
then updates its neighbor cache and sends a Reply. The Client MAY
then send Predirect messages to each of its correspondent Clients to
inform them of the new link-layer address as described in <xref
target="llchange"/>.</t>
</section>
<section anchor="rmlink" title="Removing Existing Links from Service">
<t>When a Client needs to remove an existing underlying interface
from service (e.g., when it de-activates an existing data link), it
issues an immediate Rebind to each of its Servers over any available
link with an ALDEL that includes the deprecated Link ID. If
authentication succeeds, the Server then updates its neighbor cache
and sends a Reply. The Client SHOULD then send Predirect messages to
each of its correspondent Clients to inform them of the deprecated
link-layer address as described in <xref target="llchange"/>.</t>
</section>
<section anchor="newsrv" title="Moving to a New Server">
<t>When a Client associates with a new Server, it performs the
Client procedures specified in <xref target="aeropd-client"/>.</t>
<t>When a Client disassociates with an existing Server, it sends a
Release message via a new Server to the unicast link-local network
layer address of the old Server. The new Server then writes its own
link-layer address in the Release message IP source address and
forwards the message to the old Server.</t>
<t>When the old Server receives the Release, it first authenticates
the message. Next, it resets the Client's neighbor cache entry
lifetime to 3 seconds, rewrites the link-layer address in the
neighbor cache entry to the address of the new Server, then returns
a Reply message to the Client via the old Server. When the lifetime
expires, the old Server withdraws the IP route from the AERO routing
system and deletes the neighbor cache entry for the Client. The
Client can then use the Reply message to verify that the termination
signal has been processed, and can delete both the default route and
the neighbor cache entry for the old Server. (Note that since
Release/Reply messages may be lost in the network the Client MUST
retry until it gets a Reply indicating that the Release was
successful. If the Client does not receive a Reply after MAX_RETRY
attempts, the old Server may have failed and the Client should
discontinue its Release attempts.)</t>
<t>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.</t>
</section>
</section>
<section anchor="proxy" title="Proxy AERO">
<t>Proxy Mobile IPv6 (PMIPv6) <xref target="RFC5213"/><xref
target="RFC5844"/><xref target="RFC5949"/> presents a localized
mobility management scheme for use within an access network domain. It
is typically used in WiFi and cellular wireless access networks, and
allows Mobile Nodes (MNs) to receive and retain an IP address that
remains stable within the access network domain without needing to
implement any special mobility protocols. In the PMIPv6 architecture,
access network devices known as Mobility Access Gateways (MAGs)
provide MNs with an access link abstraction and receive prefixes for
the MNs from a Local Mobility Anchor (LMA).</t>
<t>In a proxy AERO domain, a proxy AERO Client (acting as a MAG) can
similarly provide proxy services for MNs that do not participate in
AERO messaging. The proxy Client presents an access link abstraction
to MNs, and performs DHCPv6 PD exchanges over the AERO interface with
an AERO Server (acting as an LMA) to receive ACPs for address
provisioning of new MNs that come onto an access link. This scheme
assumes that proxy Clients act as fixed (non-mobile) infrastructure
elements under the same administrative trust basis as for Relays and
Servers.</t>
<t>When an MN comes onto an access link within a proxy AERO domain for
the first time, the proxy Client authenticates the MN and obtains a
unique identifier that it can use as a DHCPv6 DUID then sends a
Solicit message to its Server. When the Server delegates an ACP and
returns a Reply, the proxy Client creates an AERO address for the MN
and assigns the ACP to the MN's access link. The proxy Client then
configures itself as a default router for the MN and provides address
autoconfiguration services (e.g., SLAAC, DHCPv6, DHCPv4, etc.) for
provisioning MN addresses from the ACP over the access link. Since the
proxy Client may serve many such MNs simultaneously, it may receive
multiple ACP prefix delegations and configure multiple AERO addresses,
i.e., one for each MN.</t>
<t>When two MNs are associated with the same proxy Client, the Client
can forward traffic between the MNs without involving a Server since
it configures the AERO addresses of both MNs and therefore also has
the necessary routing information. When two MNs are associated with
different proxy Clients, the source MN's Client can initiate standard
AERO intradomain route optimization to discover a direct path to the
target MN's Client through the exchange of Predirect/Redirect
messages.</t>
<t>When an MN in a proxy AERO domain leaves an access link provided by
an old proxy Client, the MN issues an access link-specific "leave"
message that informs the old Client of the link-layer address of a new
Client on the planned new access link. This is known as a "predictive
handover". When an MN comes onto an access link provided by a new
proxy Client, the MN issues an access link-specific "join" message
that informs the new Client of the link-layer address of the old
Client on the actual old access link. This is known as a "reactive
handover".</t>
<t>Upon receiving a predictive handover indication, the old proxy
Client sends a Solicit message directly to the new Client and queues
any arriving data packets addressed to the departed MN. The Solicit
message includes the MN's ID as the DUID, the ACP in an IA_PD option,
the old Client's address as the link-layer source address and the new
Client's address as the link-layer destination address. When the new
Client receives the Solicit message, it changes the link-layer source
address to its own address, changes the link-layer destination address
to the address of its Server, and forwards the message to the Server.
At the same time, the new Client creates access link state for the ACP
in anticipation of the MN's arrival (while queuing any data packets
until the MN arrives), creates a neighbor cache entry for the old
Client with AcceptTime set to ACCEPT_TIME, then sends a Redirect
message back to the old Client. When the old Client receives the
Redirect message, it creates a neighbor cache entry for the new Client
with ForwardTime set to FORWARD_TIME, then forwards any queued data
packets to the new Client. At the same time, the old Client sends a
Release message to its Server. Finally, the old Client sends
unsolicited Redirect messages to any of the ACP's correspondents with
a TLLAO containing the link-layer address of the new Client.</t>
<t>Upon receiving a reactive handover indication, the new proxy Client
creates access link state for the MN's ACP, sends a Solicit message to
its Server, and sends a Release message directly to the old Client.
The Release message includes the MN's ID as the DUID, the ACP in an
IA_PD option, the new Client's address as the link-layer source
address and the old Client's address as the link-layer destination
address. When the old Client receives the Release message, it changes
the link-layer source address to its own address, changes the
link-layer destination address to the address of its Server, and
forwards the message to the Server. At the same time, the old Client
sends a Predirect message back to the new Client and queues any
arriving data packets addressed to the departed MN. When the new
Client receives the Predirect, it creates a neighbor cache entry for
the old Client with AcceptTime set to ACCEPT_TIME, then sends a
Redirect message back to the old Client. When the old Client receives
the Redirect message, it creates a neighbor cache entry for the new
Client with ForwardTime set to FORWARD_TIME, then forwards any queued
data packets to the new Client. Finally, the old Client sends
unsolicited Redirect messages to correspondents the same as for the
predictive case.</t>
<t>When a Server processes a Solicit message, it creates a neighbor
cache entry for this ACP if none currently exists. If a neighbor cache
entry already exists, however, the Server changes the link-layer
address to the address of the new proxy Client (this satisfies the
case of both the old Client and new Client using the same Server).</t>
<t>When a Server processes a Release message, it resets the neighbor
cache entry lifetime for this ACP to 3 seconds if the cached
link-layer address matches the old proxy Client's address. Otherwise,
the Server ignores the Release message (this satisfies the case of
both the old Client and new Client using the same Server).</t>
<t>When a correspondent Client receives an unsolicited Redirect
message, it changes the link-layer address for the ACP's neighbor
cache entry to the address of the new proxy Client.</t>
<t>From an architectural perspective, in addition to the use of DHCPv6
PD and IPv6 ND signaling the AERO approach differs from PMIPv6 in its
use of the NBMA virtual link model instead of point-to-point tunnels.
This provides a more agile interface for Client/Server and
Client/Client coordinations, and also facilitates simple route
optimization. The AERO routing system is also arranged in such a
fashion that Clients get the same service from any Server they happen
to associate with. This provides a natural fault tolerance and load
balancing capability such as desired for distributed mobility
management.</t>
</section>
<section anchor="securitygw"
title="Extending AERO Links Through Security Gateways">
<t>When an enterprise mobile device 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
device to the security gateway. During this process, the mobile device
supplies the security gateway with its public Internet address as the
link-layer address for the VPN. The mobile device then acts as an AERO
Client to negotiate with the security gateway to obtain its ACP.</t>
<t>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 device (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 devices 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 devices 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.</t>
<t>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 device 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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section anchor="aerointernet"
title="Extending IPv6 AERO Links to the Internet">
<t>When an IPv6 host ('H1') with an address from an ACP owned by AERO
Client ('C1') sends packets to a correspondent IPv6 host ('H2'), the
packets eventually arrive at the IPv6 router that owns ('H2')s prefix.
This IPv6 router may or may not be an AERO Client ('C2') either within
the same home network as ('C1') or in a different home network.</t>
<t>If Client ('C1') is currently located outside the boundaries of its
home network, it will connect back into the home network via a
security gateway acting as an AERO Server. The packets sent by ('H1')
via ('C1') will then be forwarded through the security gateway then
through the home network and finally to ('C2') where they will be
delivered to ('H2'). This could lead to sub-optimal performance when
('C2') could instead be reached via a more direct route without
involving the security gateway.</t>
<t>Consider the case when host ('H1') has the IPv6 address
2001:db8:1::1, and Client ('C1') has the ACP 2001:db8:1::/64 with
underlying IPv6 Internet address of 2001:db8:1000::1. Also, host
('H2') has the IPv6 address 2001:db8:2::1, and Client ('C2') has the
ACP 2001:db8:2::/64 with underlying IPv6 address of 2001:db8:2000::1.
Client ('C1') can determine whether 'C2' is indeed also an AERO Client
willing to serve as a route optimization correspondent by resolving
the AAAA records for the DNS FQDN that matches ('H2')s prefix,
i.e.:<vspace
blankLines="1"/>'0.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net'</t>
<t>If ('C2') is indeed a candidate correspondent, the FQDN lookup will
return a PTR resource record that contains the domain name for the
AERO link that manages ('C2')s ASP. Client ('C1') can then attempt
route optimization using an approach similar to the Return Routability
procedure specified for Mobile IPv6 (MIPv6) <xref target="RFC6275"/>.
In order to support this process, both Clients MUST intercept and
decapsulate packets that have a subnet router anycast address
corresponding to any of the /64 prefixes covered by their respective
ACPs.</t>
<t>To initiate the process, Client ('C1') creates a specially-crafted
encapsulated Predirect message that will be routed through its home
network then through ('C2')s home network and finally to ('C2')
itself. Client ('C1') prepares the initial message in the exchange as
follows:</t>
<t><list style="symbols">
<t>The encapsulating IPv6 header source address is set to
2001:db8:1:: (i.e., the IPv6 subnet router anycast address for
('C1')s ACP)</t>
<t>The encapsulating IPv6 header destination address is set to
2001:db8:2:: (i.e., the IPv6 subnet router anycast address for
('C2')s ACP)</t>
<t>The encapsulating IPv6 header is followed by any additional
encapsulation headers</t>
<t>The encapsulated IPv6 header source address is set to
fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))</t>
<t>The encapsulated IPv6 header destination address is set to
fe80::2001:db8:2:0 (i.e., the AERO address for ('C2'))</t>
<t>The encapsulated Predirect message includes all of the securing
information that would occur in a MIPv6 "Home Test Init" message
(format TBD)</t>
</list>Client ('C1') then further encapsulates the message in the
encapsulating headers necessary to convey the packet to the security
gateway (e.g., through IPsec encapsulation) so that the message now
appears "double-encapsulated". ('C1') then sends the message to the
security gateway, which re-encapsulates and forwards it over the home
network from where it will eventually reach ('C2').</t>
<t>At the same time, ('C1') creates and sends a second encapsulated
Predirect message that will be routed through the IPv6 Internet
without involving the security gateway. Client ('C1') prepares the
message as follows:</t>
<t><list style="symbols">
<t>The encapsulating IPv6 header source address is set to
2001:db8:1000:1 (i.e., the Internet IPv6 address of ('C1'))</t>
<t>The encapsulating IPv6 header destination address is set to
2001:db8:2:: (i.e., the IPv6 subnet router anycast address for
('C2')s ACP)</t>
<t>The encapsulating IPv6 header is followed by any additional
encapsulation headers</t>
<t>The encapsulated IPv6 header source address is set to
fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))</t>
<t>The encapsulated IPv6 header destination address is set to
fe80::2001:db8:2:0 (i.e., the AERO address for ('C2'))</t>
<t>The encapsulated Predirect message includes all of the securing
information that would occur in a MIPv6 "Care-of Test Init"
message (format TBD)</t>
</list>('C2') will receive both Predirect messages through its home
network then return a corresponding Redirect for each of the Predirect
messages with the source and destination addresses in the inner and
outer headers reversed. The first message includes all of the securing
information that would occur in a MIPv6 "Home Test" message, while the
second message includes all of the securing information that would
occur in a MIPv6 "Care-of Test" message (formats TBD).</t>
<t>When ('C1') receives the Redirect messages, it performs the
necessary security procedures per the MIPv6 specification. It then
prepares an encapsulated NS message that includes the same source and
destination addresses as for the "Care-of Test Init" Predirect
message, and includes all of the securing information that would occur
in a MIPv6 "Binding Update" message (format TBD) and sends the message
to ('C2').</t>
<t>When ('C2') receives the NS message, if the securing information is
correct it creates or updates a neighbor cache entry for ('C1') with
fe80::2001:db8:1:0 as the network-layer address, 2001:db8:1000::1 as
the link-layer address and with AcceptTime set to ACCEPT_TIME. ('C2')
then sends an encapsulated NA message back to ('C1') that includes the
same source and destination addresses as for the "Care-of Test"
Redirect message, and includes all of the securing information that
would occur in a MIPv6 "Binding Acknowledgement" message (format TBD)
and sends the message to ('C1').</t>
<t>When ('C1') receives the NA message, it creates or updates a
neighbor cache entry for ('C2') with fe80::2001:db8:2:0 as the
network-layer address and 2001:db8:2:: as the link-layer address and
with ForwardTime set to FORWARD_TIME, thus completing the route
optimization in the forward direction.</t>
<t>('C1') subsequently forwards encapsulated packets with outer source
address 2001:db8:1000::1, with outer destination address 2001:db8:2::,
with inner source address taken from the 2001:db8:1::, and with inner
destination address taken from 2001:db8:2:: due to the fact that it
has a securely-established neighbor cache entry with non-zero
ForwardTime. ('C2') subsequently accepts any such encapsulated packets
due to the fact that it has a securely-established neighbor cache
entry with non-zero AcceptTime.</t>
<t>In order to keep neighbor cache entries alive, ('C1') periodically
sends additional NS messages to ('C2') and receives any NA responses.
If ('C1') moves to a different point of attachment after the initial
route optimization, it sends a new secured NS message to ('C2') as
above to update ('C2')s neighbor cache.</t>
<t>If ('C2') has packets to send to ('C1'), it performs a
corresponding route optimization in the opposite direction following
the same procedures described above. In the process, the
already-established unidirectional neighbor cache entries within
('C1') and ('C2') are updated to include the now-bidirectional
information. In particular, the AcceptTime and ForwardTime variables
for both neighbor cache entries are updated to non-zero values, and
the link-layer address for ('C1')s neighbor cache entry for ('C2') is
reset to 2001:db8:2000::1.</t>
<t>Note that two AERO Clients can use full security protocol messaging
instead of Return Routability, e.g., if strong authentication and/or
confidentiality are desired. In that case, security protocol key
exchanges such as specified for MOBIKE <xref target="RFC4555"/> would
be used to establish security associations and neighbor cache entries
between the AERO clients. Thereafter, NS/NA messaging can be used to
maintain neighbor cache entries, test reachability, and to announce
mobility events. If reachability testing fails, e.g., if both Clients
move at roughly the same time, the Clients can tear down the security
association and neighbor cache entries and again allow packets to flow
through their home network.</t>
</section>
<section anchor="version"
title="Encapsulation Protocol Version Considerations">
<t>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 redirection
messages and continue to send packets via their Servers.</t>
</section>
<section anchor="mcast" title="Multicast Considerations">
<t>When the underlying network does not support multicast, AERO
Clients map link-scoped multicast addresses to the link-layer address
of a Server. The AERO Client also serves as an IGMP/MLD Proxy for its
EUNs and/or hosted applications per <xref target="RFC4605"/> while
using the link-layer address of the Server as the link-layer address
for all multicast packets.</t>
<t>When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in <xref
target="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.</t>
</section>
<section anchor="nodhcp"
title="Operation on AERO Links Without DHCPv6 Services">
<t>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.</t>
</section>
<section anchor="serverless" title="Operation on Server-less AERO Links">
<t>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.</t>
<t>When there is no Server on the link, Clients must arrange to
receive ACPs and publish them via a secure alternate prefix delegation
authority through some means outside the scope of this document.</t>
</section>
<section anchor="static-tunnel" title="Manually-Configured AERO Tunnels">
<t>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-assigned link-local address and exchange NS/NA
messages the same as for dynamically-established tunnels.</t>
</section>
<section anchor="intra-route" title="Intradomain Routing">
<t>After a tunnel neighbor relationship has been established,
neighbors can use a traditional dynamic routing protocol over the
tunnel to exchange routing information without having to inject the
routes into the AERO routing system.</t>
</section>
</section>
<section anchor="implement" title="Implementation Status">
<t>User-level and kernel-level AERO implementations have been developed
and are undergoing internal testing within Boeing.</t>
<t>An initial public release of the AERO source code was announced on
the intarea mailing list on August 21, 2015, and a pointer to the code
is available in the list archives.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.</t>
<t>The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO <xref target="RFC6706"/>. This document
obsoletes <xref target="RFC6706"/> and claims the UDP port number "8060"
for all future use.</t>
<t>No further IANA actions are required.</t>
</section>
<section anchor="secure" title="Security Considerations">
<t>AERO link security considerations are the same as for standard IPv6
Neighbor Discovery <xref target="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 trust anchor. Unless there is some other means of
authenticating the Client's identity (e.g., link-layer security), AERO
nodes SHOULD also use DHCPv6 securing services (e.g., DHCPv6
authentication, Secure DHCPv6 <xref target="I-D.ietf-dhc-sedhcpv6"/>,
etc.) for Client authentication and network admission control. In
particular, Clients SHOULD include authenticating information on each
Solicit/Rebind/Release message they send, but omit authenticating
information on Renew messages. Renew messages are exempt due to the fact
that the Renew must already be checked for having a correct link-layer
address and does not update any link-layer addresses. Therefore, asking
the Server to also authenticate the Renew message would be unnecessary
and could result in excessive processing overhead.</t>
<t>Redirect, Predirect and unsolicited NA messages SHOULD include a
Timestamp option (see Section 5.3 of <xref target="RFC3971"/>) that
other AERO nodes can use to verify the message time of origin.
Predirect, NS and RS messages SHOULD include a Nonce option (see Section
5.3 of <xref target="RFC3971"/>) that recipients echo back in
corresponding responses.</t>
<t>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 that is often
sufficient. In other instances, additional securing mechanisms such as
Secure Neighbor Discovery (SeND) <xref target="RFC3971"/>, IPsec <xref
target="RFC4301"/> or TLS <xref target="RFC5246"/> may be necessary.</t>
<t>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 unauthorized nodes via some form of Internet connection
sharing.)</t>
<t>On some AERO links, establishment and maintenance of a direct path
between neighbors requires secured coordination such as through the
Internet Key Exchange (IKEv2) protocol <xref target="RFC5996"/> to
establish a security association.</t>
<t>An AERO Client's link-layer address could be rewritten by a
link-layer switching element on the path from the Client to the Server
and not detected by the DHCPv6 security mechanism. However, such a
condition would only be a matter of concern on unmanaged/unsecured links
where the link-layer switching elements themselves present a
man-in-the-middle attack threat. For this reason, IP security MUST be
used when AERO is employed over unmanaged/unsecured links.</t>
</section>
<section anchor="ack" title="Acknowledgements">
<t>Discussions both on IETF lists and in private exchanges helped shape
some of the concepts in this work. Individuals who contributed insights
include Mikael Abrahamsson, Mark Andrews, Fred Baker, Stewart Bryant,
Brian Carpenter, Wojciech Dec, Ralph Droms, Adrian Farrel, Sri
Gundavelli, Brian Haberman, Joel Halpern, Tom Herbert, Sascha Hlusiak,
Lee Howard, Andre Kostur, Ted Lemon, Andy Malis, Satoru Matsushima,
Tomek Mrugalski, Alexandru Petrescu, Behcet Saikaya, Joe Touch, Bernie
Volz, Ryuji Wakikawa and Lloyd Wood. Members of the IESG also provided
valuable input during their review process that greatly improved the
document. Discussions on the v6ops list in the December 2015 through
January 2016 timeframe further helped clarify AERO multi-addressing
capabilities. Special thanks go to Stewart Bryant, Joel Halpern and
Brian Haberman for their shepherding guidance during the publication of
the AERO first edition.</t>
<t>This work has further been encouraged and supported by Boeing
colleagues including Dave Bernhardt, Cam Brodie, Balaguruna Chidambaram,
Irene Chin, Bruce Cornish, Claudiu Danilov, Wen Fang, Anthony Gregory,
Jeff Holland, Ed King, Gen MacLean, Rob Muszkiewicz, Sean O'Sullivan,
Kent Shuey, Brian Skeen, Mike Slane, Brendan Williams, Julie Wulff,
Yueli Yang, and other members of the BR&T and BIT mobile networking
teams.</t>
<t>Earlier works on NBMA tunneling approaches are found in <xref
target="RFC2529"/><xref target="RFC5214"/><xref target="RFC5569"/>.</t>
<t>Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:</t>
<t><list style="symbols">
<t>The Internet Routing Overlay Network (IRON) <xref
target="RFC6179"/><xref target="I-D.templin-ironbis"/></t>
<t>Virtual Enterprise Traversal (VET) <xref target="RFC5558"/><xref
target="I-D.templin-intarea-vet"/></t>
<t>The Subnetwork Encapsulation and Adaptation Layer (SEAL) <xref
target="RFC5320"/><xref target="I-D.templin-intarea-seal"/></t>
<t>AERO, First Edition <xref target="RFC6706"/></t>
</list>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.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.0768"?>
<?rfc include="reference.RFC.0791"?>
<?rfc include="reference.RFC.0792"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.2003"?>
<?rfc include="reference.RFC.2460"?>
<?rfc include="reference.RFC.2473"?>
<?rfc include="reference.RFC.2474"?>
<?rfc include="reference.RFC.4213"?>
<?rfc include="reference.RFC.4861"?>
<?rfc include="reference.RFC.4862"?>
<?rfc include="reference.RFC.6434"?>
<?rfc include="reference.RFC.3633"?>
<?rfc include="reference.RFC.3315"?>
<?rfc include="reference.RFC.3971"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.2675"?>
<?rfc include="reference.RFC.1930"?>
<?rfc include="reference.RFC.4271"?>
<?rfc include="reference.RFC.2529"?>
<?rfc include="reference.RFC.5214"?>
<?rfc include="reference.RFC.4301"?>
<?rfc include="reference.RFC.5569"?>
<?rfc include="reference.RFC.6204"?>
<?rfc include="reference.RFC.6980"?>
<?rfc include="reference.RFC.0879"?>
<?rfc include="reference.RFC.4821"?>
<?rfc include="reference.RFC.6691"?>
<?rfc include="reference.RFC.6935"?>
<?rfc include="reference.RFC.6936"?>
<?rfc include="reference.RFC.6438"?>
<?rfc include="reference.RFC.6706"?>
<?rfc include="reference.RFC.4963"?>
<?rfc include="reference.RFC.6864"?>
<?rfc include="reference.RFC.6146"?>
<?rfc include="reference.RFC.7078"?>
<?rfc include="reference.RFC.5996"?>
<?rfc include="reference.RFC.6939"?>
<?rfc include="reference.RFC.5522"?>
<?rfc include="reference.RFC.4291"?>
<?rfc include="reference.RFC.4994"?>
<?rfc include="reference.RFC.5494"?>
<?rfc include="reference.RFC.5246"?>
<?rfc include="reference.RFC.6355"?>
<?rfc include="reference.RFC.2923"?>
<?rfc include="reference.RFC.3819"?>
<?rfc include="reference.RFC.4443"?>
<?rfc include="reference.RFC.1812"?>
<?rfc include="reference.RFC.2131"?>
<?rfc include="reference.RFC.5213"?>
<?rfc include="reference.RFC.5844"?>
<?rfc include="reference.RFC.6275"?>
<?rfc include="reference.RFC.4555"?>
<?rfc include="reference.RFC.1191"?>
<?rfc include="reference.RFC.1981"?>
<?rfc include="reference.RFC.1035"?>
<?rfc include="reference.RFC.4592"?>
<?rfc include="reference.RFC.3596"?>
<?rfc include="reference.RFC.6179"?>
<?rfc include="reference.RFC.5320"?>
<?rfc include="reference.RFC.5558"?>
<?rfc include="reference.RFC.5720"?>
<?rfc include="reference.RFC.2784"?>
<?rfc include="reference.RFC.2890"?>
<?rfc include="reference.RFC.5949"?>
<?rfc include="reference.RFC.6241"?>
<?rfc include="reference.RFC.2983"?>
<?rfc include="reference.RFC.3168"?>
<?rfc include="reference.RFC.2764"?>
<?rfc include="reference.RFC.6221"?>
<?rfc include="reference.RFC.6422"?>
<?rfc include="reference.RFC.4511"?>
<?rfc include="reference.RFC.4605"?>
<?rfc include="reference.RFC.1122"?>
<reference anchor="TUNTAP">
<front>
<title>http://en.wikipedia.org/wiki/TUN/TAP</title>
<author fullname="Wikipedia" initials="W" surname="Wikipedia">
<organization/>
</author>
<date month="October" year="2014"/>
</front>
</reference>
<?rfc include="reference.I-D.ietf-dhc-sedhcpv6"?>
<?rfc include="reference.I-D.templin-ironbis"?>
<?rfc include="reference.I-D.templin-intarea-seal"?>
<?rfc include="reference.I-D.templin-intarea-vet"?>
<?rfc include="reference.I-D.herbert-gue-fragmentation"?>
<?rfc include="reference.I-D.ietf-nvo3-gue"?>
<?rfc include="reference.I-D.templin-intarea-grefrag"?>
<?rfc include="reference.I-D.vandevelde-idr-remote-next-hop"?>
</references>
<section anchor="minimal" title="AERO Minimal Encapsulation">
<t>When GUE encapsulation is not needed, AERO can use common minimal
encapsulations such as IP-in-IP <xref target="RFC2003"/><xref
target="RFC2473"/><xref target="RFC4213"/>, Generic Routing
Encapsulation (GRE) <xref target="RFC2784"/><xref target="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.</t>
<t>As for GUE encapsulation, AERO minimal 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 <xref
target="I-D.templin-intarea-grefrag"/>.</t>
<t><xref target="encaps"/> shows the AERO IP-in-IP minimal encapsulation
format before any fragmentation is applied:</t>
<figure anchor="encaps"
title="Minimal Encapsulation Format using IP-in-IP">
<artwork><![CDATA[
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
]]></artwork>
</figure>
<t><xref target="gre-encaps"/> shows the AERO GRE minimal encapsulation
format before any fragmentation is applied:</t>
<t><figure anchor="gre-encaps" title="Minimal Encapsulation Using GRE">
<artwork><![CDATA[
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Header |
| (with checksum, key, etc..) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Fragment Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Inner Packet Body ~
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>Minimal 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.</t>
<t>GUE encapsulation can traverse network paths that are inaccessible to
minimal encapsulation, 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 minimal
encapsulation in a given environment.</t>
</section>
<section anchor="whentoinsert"
title="When to Insert an Encapsulation Fragment Header">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 09:47:27 |