One document matched: draft-templin-aerolink-40.xml
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<rfc category="std" docName="draft-templin-aerolink-40.txt" ipr="trust200902"
obsoletes="rfc5320, rfc5558, rfc5720, rfc6179, rfc6706">
<front>
<title abbrev="AERO">Transmission of IP Packets over AERO Links</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="25" month="September" year="2014"/>
<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 and provisioning are supported by
the Dynamic Host Configuration Protocol for IPv6 (DHCPv6), and node
mobility is naturally supported through dynamic neighbor cache updates.
Although DHCPv6 and IPv6 ND messaging is used in the control plane, both
IPv4 and IPv6 are supported in the data plane.</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 that addresses the requirements outlined in <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 and provisioning are supported by the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) <xref target="RFC3315"/>, and
node mobility is naturally supported through dynamic neighbor cache
updates. Although DHCPv6 and IPv6 ND message signalling is used in the
control plane, both IPv4 and IPv6 can be used in the data plane. 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.</t>
<t hangText="AERO interface"><vspace/>a node's attachment to an AERO
link.</t>
<t hangText="AERO address"><vspace/>an IPv6 link-local address
constructed as specified in Section 3.2 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
applies an AERO address to an AERO interface and receives an IP
prefix via a DHCPv6 Prefix Delegation (PD) exchange with one or more
AERO Servers.</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 applies the IPv6
link-local subnet router anycast address (fe80::) to the AERO
interface and also applies an administratively assigned IPv6
link-local unicast address used for operation of DHCPv6 and the IPv6
ND protocol.</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 applies an administratively
assigned IPv6 link-local unicast address to the AERO interface the
same as for a 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.</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. 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.</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"/>.</t>
</section>
<section title="Asymmetric Extended Route Optimization (AERO)">
<t>The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:</t>
<section 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 | |(H1->S1; H2->S2)| | default->R1 |
| H1->C1 | +--------+-------+ | H2->C2 |
+-------+------+ | +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client C1| |AERO Client C2|
| Nbr: S1 | | Nbr: S2 |
| default->S1 | | default->S2 |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. .-(_ IP )-.
(__ EUN ) (__ EUN )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host H1| | Host H2|
+--------+ +--------+
]]></artwork>
</figure><xref target="chaining-fig"/> above presents the AERO link
reference model. In this model:</t>
<t><list style="symbols">
<t>Relay R1 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>Servers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.</t>
<t>Clients C1 and C2 associate with Servers S1 and S2,
respectively and also act as default routers for their associated
EUNs</t>
<t>Hosts H1 and H2 attach to the EUNs served by Clients C1 and C2,
respectively</t>
</list>In common operational practice, there may be many additional
Relays, Servers and Clients.</t>
</section>
<section anchor="node-types" title="AERO 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
Internetwork. Relays present the AERO link to the native Internetwork
as a set of one or more AERO Service Prefixes (ASPs). Each Relay
advertises the ASPs for the AERO link into the native IP Internetwork
and serves 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).</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 Clients and Relays, as well as between
Clients and other Clients associated with the same Server. AERO
Servers maintain an AERO interface neighbor cache entry for each AERO
Relay. They also maintain both a neighbor cache entry and an IP
forwarding table entry for each of their associated Clients.</t>
<t>AERO Clients act as requesting routers to receive ACPs through
DHCPv6 PD exchanges with AERO Servers over the AERO link and
sub-delegate portions of their ACPs to EUN interfaces. (Each Client
MAY associate with a single Server or with multiple Servers, e.g., for
fault tolerance and/or load balancing.) 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 Clients that act as hosts typically configure a TUN/TAP
interface as a point-to-point linkage between the IP layer and the
AERO interface. The IP layer therefore sees only the TUN/TAP
interface, while the AERO interface provides an intermediate conduit
between the TUN/TAP interface and the underlying interfaces. AERO
Clients that act as hosts assign one or more IP addresses from their
ACPs to the TUN/TAP interface.</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. In that case, the Client
MUST accept the address as equivalent to the base address, but then
use the base address as the source address of any IPv6 ND message
replies. 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 but uses fe80::2001:db8:1000:2000 as the source address of any
IPv6 ND replies.</t>
</section>
<section anchor="interface" title="AERO Interface Characteristics">
<t>AERO interfaces use IP-in-IPv6 encapsulation <xref
target="RFC2473"/> to exchange tunneled packets with AERO neighbors
attached to an underlying IPv6 network, and use IP-in-IPv4
encapsulation <xref target="RFC2003"/><xref target="RFC4213"/> to
exchange tunneled packets with AERO neighbors attached to an
underlying IPv4 network. AERO interfaces can also coordinate secured
tunnel types such as IPsec <xref target="RFC4301"/> or TLS <xref
target="RFC5246"/>. When Network Address Translator (NAT) traversal
and/or filtering middlebox traversal may be necessary, a UDP header is
further inserted immediately above the IP encapsulation header.</t>
<t>AERO interfaces maintain a neighbor cache, and AERO Clients and
Servers use an adaptation of standard unicast IPv6 ND messaging. AERO
interfaces use unicast 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
redirection message types -- the first known as a Predirect message
and the second being the standard Redirect message (see Section 3.12).
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 Target Link-Layer
Address Options (TLLAOs) formatted as shown in <xref
target="tllaov6"/>:</t>
<t><figure anchor="tllaov6"
title="AERO Target Link-Layer Address Option (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 | Preference | 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, and
Preference is an integer value between 0 and 255 indicating the node's
preference for this underlying interface (with 255 being the highest
preference, 1 being the lowest, and 0 meaning "link disabled"). 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 the encapsulation IP address family is IPv4, IP Address is formed
as an IPv4-mapped IPv6 address <xref target="RFC4291"/>.</t>
<t>When a Relay enables an AERO interface, it assigns an
administratively assigned link-local address fe80::ID to the
interface. Each fe80::ID address MUST be unique among all Relays and
Servers on the link, and MUST NOT collide with any potential AERO
addresses. The addresses are typically taken from the range fe80::/96,
e.g., as fe80::1, fe80::2, fe80::3, etc. The Relay also maintains an
IP forwarding table entry for each Client-Server association and
maintains a neighbor cache entry for each Server on the link. Relays
do not require the use of IPv6 ND messaging for reachability
determination since Relays and Servers engage in a dynamic routing
protocol over the AERO interface. At a minimum, however, Relays
respond to NS messages by returning an NA.</t>
<t>When a Server enables an AERO interface, it assigns the address
fe80:: to the interface as a link-local Subnet Router Anycast address,
and also assigns an administratively assigned link-local address
fe80::ID the same as for Relays. (The Server then accepts DHCPv6 and
IPv6 ND solicitation messages destined to either the fe80:: or
fe80::ID addresses, but always uses fe80::ID as the source address in
the replies it generates.) 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-Client neighbor cache entries and IP forwarding
table entries based on DHCPv6 exchanges. When the Server receives an
NS/RS message on the AERO interface it returns an NA/RA message but
does not update the neighbor cache. Each Server also engages in a
dynamic routing protocol with all Relays on the link. Finally, the
Server provides a simple conduit between Clients and Relays, or
between Clients and other Clients. 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>
<t>When a Client enables an AERO interface, it invokes DHCPv6 PD to
receive an ACP from an AERO Server. Next, it assigns the corresponding
AERO address to the AERO interface and creates a neighbor cache entry
for the Server, i.e., the PD exchange bootstraps the provisioning of a
unique link-local address. 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. Unsolicited NA messages update the
cached link-layer addresses for correspondent Clients (e.g., following
a link-layer address change due to node mobility) but do not create
new neighbor cache entries. NS/NA messages used for Neighbor
Unreachability Detection (NUD) update timers in existing neighbor
cache entires but do not update link-layer addresses nor create new
neighbor cache entries. Finally, the Client need not maintain any IP
forwarding table entries for its Servers or correspondent Clients.
Instead, it can set a single "route-to-interface" default route in the
IP forwarding table pointing to the AERO interface, and all forwarding
decisions can be made within the AERO interface based on neighbor
cache entries. (On systems in which adding a default route would
violate security policy, the default route could instead be installed
via a "synthesized RA", e.g., as discussed in Section 3.11.2.)</t>
<section anchor="multi"
title="Coordination of Multiple Underlying Interfaces">
<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 a single
link-local address with multiple link-layer addresses. In that case,
Redirect, Predirect and unsolicited NA messages MAY include multiple
TLLAOs -- each with a different Link ID that corresponds to a
specific underlying interface of the Client.</t>
</section>
</section>
<section 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 on the link.</t>
<t>Static neighbor cache entries are created though DHCPv6 PD
exchanges and remain in place for durations bounded by prefix
lifetimes. AERO Servers maintain a static neighbor cache entry for
each of their associated Clients, and AERO Clients maintain a static
neighbor cache for each of their associated Servers. When an AERO
Server sends a DHCPv6 Reply message response to a Client's DHCPv6
Solicit/Request or Renew message, it creates or updates a static
neighbor cache entry based on the Client's AERO address as the
network-layer address, 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 as the length to apply to
the AERO address. When an AERO Client receives a DHCPv6 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 based on receipt of an
IPv6 ND message, and are garbage-collected if not used within a short
timescale. AERO Clients maintain dynamic neighbor cache entries for
each of their active correspondent Clients 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 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 and uses this value
to determine whether packets can be sent directly to the
correspondent. The Client also maintains a "MaxRetry" variable to
limit the number of keepalives sent when a correspondent may have gone
unreachable.</t>
<t>For dynamic neighbor cache entries, when an AERO Client receives a
valid NS message it (re)sets AcceptTime for the neighbor to
ACCEPT_TIME. When an AERO Client receives a valid solicited NA
message, it (re)sets ForwardTime for the neighbor to FORWARD_TIME and
sets MaxRetry to MAX_RETRY. When an AERO Client receives a valid
unsolicited NA message, it updates the correspondent's link-layer
addresses but DOES NOT reset AcceptTime, ForwardTime or MaxRetry.</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 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 has 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 and sub-IP layer
forwarding agents. 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 static 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 any permanent neighbor cache entry 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 neighbor cache
entry for the next-hop Server 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 is 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 or 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 for the next-hop Server 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: Section 3.10).</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>Note that in the above that the link-layer address for
encapsulation may be determined through consulting either the neighbor
cache or the IP forwarding table. IP forwarding is therefore linked to
IPv6 ND via the AERO address.</t>
</section>
<section title="AERO Interface Encapsulation, Re-encapsulation and Decapsulation">
<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>AERO interfaces encapsulate packets per the base tunneling
specifications (e.g., <xref target="RFC2003"/><xref
target="RFC2473"/><xref target="RFC4213"/><xref
target="RFC4301"/><xref target="RFC5246"/>, etc.) except that the
interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class"
and "Congestion Experienced" values in the packet's IP header into the
corresponding fields in the encapsulation IP header. For packets
undergoing re-encapsulation, the AERO interface instead copies the
"TTL/Hop Limit", "Type of Service/Traffic Class" 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>The AERO interface encapsulates the packet per the base tunneling
specification except that it inserts a UDP header following the
encapsulation IP header and before any other encapsulation headers.
The AERO interface 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. For packets sent via 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
also sets the UDP checksum field to zero (see: <xref
target="RFC6935"/><xref target="RFC6936"/>) unless an integrity check
is required (see: Section 3.9.2).</t>
<t>The AERO interface next sets the IP protocol number in the
encapsulation header to 17 (i.e., the IP protocol number for UDP).
When IPv6 is used as the encapsulation protocol, the interface then
sets the flow label value in the encapsulation header the same as
described in <xref target="RFC6438"/>. When IPv4 is used as the
encapsulation protocol, the AERO interface sets the DF bit as
discussed in Section 3.9.</t>
<t>AERO interfaces decapsulate packets destined either to the node
itself or to a destination reached via an interface other than the
AERO interface the packet was received on. When the AERO interface
receives a UDP packet, it examines the first octet of the encapsulated
packet. The packet is accepted if the most significant four bits of
the first octet encode the value '0110' (i.e., the version number
value for IPv6) or the value '0100' (i.e., the version number value
for IPv4). Otherwise, the packet is accepted if the first octet
encodes a valid IP protocol number per the IANA "protocol-numbers"
registry that matches a supported encapsulation type. Otherwise, the
packet is discarded.</t>
<t>Further decapsulation then proceeds according to the appropriate
base tunneling specification.</t>
</section>
<section 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 Relays and Servers 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, and
create or update a static neighbor cache entry for the source
based on the specific message type.</t>
<t>AERO Servers accept encapsulated packets if there is a static
neighbor cache entry with an AERO address that matches the
packet's network-layer source address and with a link-layer
address that matches the packet's link-layer source address.</t>
<t>AERO Clients accept encapsulated packets if there is a static
neighbor cache entry with a link-layer source address that matches
the packet's link-layer source address.</t>
<t>AERO Clients and Servers accept encapsulated packets if there
is a dynamic neighbor cache entry with an AERO address that
matches the packet's network-layer source address, with a
link-layer address that matches the packet's link-layer source
address, and with a non-zero AcceptTime.</t>
</list>Note that this simple data origin authentication only applies
to environments in which link-layer addresses cannot be spoofed.
Additional security mitigations may be necessary in other
environments.</t>
</section>
<section title="AERO Interface MTU and Fragmentation">
<t>The AERO interface is the node's point of attachment to the AERO
link. AERO links over IP networks have a maximum link MTU of 64KB
minus the encapsulation overhead (termed here "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, they are considered optional for IPv6 nodes <xref
target="RFC2675"/><xref target="RFC6434"/>).</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. Additionally, 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 lost in
the network <xref target="RFC2923"/> resulting in failure of the IP
MTU discovery mechanisms <xref target="RFC1191"/><xref
target="RFC1981"/>.</t>
<t>For these reasons, AERO interfaces MUST admit packets up to 1500
bytes in length even if some fragmentation is necessary. AERO
interfaces MAY admit even larger packets as long as they can be
accommodated without fragmentation.</t>
<t>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: Section 3.9.2). 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 it is performed within the AERO interface (i.e., instead of at
the encapsulating IP layer) through the insertion of an IPv6 Fragment
Header <xref target="RFC2460"/>. Since the Fragment Header reduces the
room available for packet data, but the original source has no way to
control its insertion, the Fragment Header length MUST be included in
the ENCAPS length even for packets in which the header does not
appear.</t>
<t>The source AERO interface (i.e., the tunnel ingress) therefore
sends encapsulated packets to the destination AERO interface (i.e.,
the tunnel egress) according to the following algorithm:</t>
<t><list style="symbols">
<t>For IP packets that are no larger than (1280-ENCAPS) bytes, the
tunnel ingress encapsulates the packet and admits it into the
tunnel without fragmentation. For IPv4 AERO links, tunnel ingress
sets the Don't Fragment (DF) bit to 0 so that these packets will
be delivered to the tunnel 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 tunnel ingress encapsulates the packet
and inserts a Fragment Header above the UDP/IP encapsulation
headers. Next, the tunnel ingress uses the fragmentation algorithm
in <xref target="RFC2460"/> to break the packet into two
non-overlapping fragments where the first fragment (including
ENCAPS) is no larger than 1024 bytes and the second is no larger
than the first. Each fragment consists of identical UDP/IP
encapsulation headers, followed by the Fragment Header followed by
the fragment of the encapsulated packet itself. The tunnel ingress
then admits both 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 tunnel egress.</t>
<t>For IPv4 packets that are larger than 1500 bytes and with the
DF bit set to 0, the tunnel ingress uses ordinary IP fragmentation
to break the unencapsulated packet into a minimum number of
non-overlapping fragments where the first fragment is no larger
than 1024-ENCAPS and all other fragments are no larger than the
first fragment. The tunnel ingress then encapsulates each fragment
(and for IPv4 sets the DF bit to 0) then admits them into the
tunnel. These encapsulated fragments will be delivered to the
final destination via the tunnel egress.</t>
<t>For all other IP packets, if the packet is too large to enter
the underlying interface following encapsulation, the tunnel
ingress drops the packet and returns a network-layer (L3) PTB
message to the original source with MTU set to the larger of 1500
bytes or the underlying interface MTU minus ENCAPS. Otherwise, the
tunnel ingress encapsulates the packet and admits it into the
tunnel without fragmentation (and for IPv4 sets the DF bit to 1)
and translates any link-layer (L2) PTB messages it may receive
from the network into corresponding L3 PTB messages to send to the
original source as specified in Section 3.10. Since both L2 and L3
PTB messages may be either 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>While sending packets according to the above algorithm, the
tunnel ingress MAY also send 1500 byte probe packets to determine
whether they can reach the tunnel egress without fragmentation. If the
probes succeed, the tunnel ingress can begin sending packets that are
no larger than 1500 bytes without fragmentation (and for IPv4 with DF
set to 1). Since the path MTU within the tunnel may fluctuate due to
routing changes, the tunnel ingress SHOULD continue to send additional
probes subject to rate limiting and SHOULD process any L2 PTB messages
as an indication that the path MTU may have decreased. If the path MTU
within the tunnel becomes insufficient, the source MUST resume
fragmentation.</t>
<t>To construct a probe, the tunnel ingress prepares an NS message
with a Nonce option plus trailing NULL padding octets added to a
length of 1500 bytes without including the length of the padding in
the IPv6 Payload Length field, but with the length included in the
encapsulating IP header. The tunnel ingress then encapsulates the
padded NS message in the encapsulation headers (and for IPv4 sets DF
to 1) then sends the message to the tunnel egress. If the tunnel
egress returns a solicited NA message with a matching Nonce option,
the tunnel ingress deems the probe successful.</t>
<t>When the tunnel egress receives the fragments of a fragmented
packet, it reassembles them into a whole packet per the reassembly
algorithm in <xref target="RFC2460"/> then discards the Fragment
Header. The tunnel egress therefore MUST be capable of reassembling
packets up to 1500+ENCAPS bytes in length; hence, it is RECOMMENDED
that the tunnel egress be capable of reassembling at least 2KB.</t>
<section title="Accommodating Large IPv6 ND and DHCPv6 Messages">
<t>IPv6 ND and DHCPv6 messages MUST be accommodated even if some
fragmentation is necessary. These packets are therefore accommodated
through a modification of the second rule in the above algorithm as
follows:</t>
<t><list style="symbols">
<t>For IPv6 ND and DHCPv6 messages that are larger than
(1280-ENCAPS) bytes, the tunnel ingress encapsulates the packet
and inserts a Fragment Header above the UDP/IP encapsulation
headers. Next, the tunnel ingress uses the fragmentation
algorithm in <xref target="RFC2460"/> to break 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 tunnel
ingress then encapsulates each fragment (and for IPv4 sets the
DF bit to 0) then admits them into the tunnel.</t>
</list>IPv6 ND and DHCPv6 messages that exceed the minimum
reassembly size listed above rarely occur in the modern era, however
the tunnel egress SHOULD be able to reassemble them if they do. This
means that the tunnel egress SHOULD include a configuration knob
allowing the operator to set a larger reassembly buffer size if
large IPv6ND and DHCPv6 messages become more common in the
future.</t>
<t>The tunnel ingress can send large IPv6 ND and DHCPv6 messages
without fragmentation if there is assurance that large packets can
traverse the tunnel without fragmentation. The tunnel ingress MAY
send probe packets of 1500 bytes or larger as specified above to
determine a size for which fragmentation can be avoided.</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 misassociation 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 AERO fragmentation procedure described in the above
algorithms uses the IPv6 Fragment Header and reuses standard IPv6
fragmentation and reassembly code. Since the Fragment Header
includes 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 tunnel 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 typical
wireless data links 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. Hence, no integrity check is included to cover the AERO
fragmentation and reassembly procedures.</t>
<t>When the tunnel 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 include
"poorly managed" IPv4 link MTUs.</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 tunnel 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 many 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 tunnel ingress MAY omit an integrity check
for the IPv4 fragmentation and reassembly procedures; otherwise, the
tunnel ingress SHOULD include an integrity check. When an upper
layer encapsulation (e.g., IPsec) already includes an integrity
check, the tunnel ingress need not include an additional check.
Otherwise, the tunnel ingress calculates the UDP checksum over the
encapsulated packet and writes the value into the UDP encapsulation
header, i.e., instead of writing the value 0. The tunnel egress will
then verify the UDP checksum and discard the packet if the checksum
is incorrect.</t>
</section>
</section>
<section 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: Section 3.9.2).</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 Section 3.14). 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 lease for the Client's ACP, withdraw its
route for the ACP from the AERO routing system and delete the
neighbor cache entry (see Sections 3.11 and 3.12).</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 routing system is likely in a temporary transitional state
that will soon re-converge.</t>
<t>When an AERO node receives an L2 PTB message, it translates the
message into an L3 PTB message if possible (*) and forwards the
message toward the original source as described below.</t>
</list>To translate an L2 PTB message to an L3 PTB message, the AERO
node first caches the MTU field value of the L2 ICMP header. The node
next 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. In the process, the node
writes the maximum of 1500 bytes and (L2 MTU - ENCAPS) into the MTU
field of the L3 ICMP header.</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>
<t>(*) 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. It can therefore be said that 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>
</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 pre-configured
with an identical list of ACP-to-Client ID mappings for all Clients
enrolled in the AERO system, as well as any information necessary to
authenticate Clients. The configuration information is maintained by
a central administrative authority for the AERO link and securely
propagated to all Servers whenever a new Client is enrolled or an
existing Client is withdrawn.</t>
<t>With these identical configurations, each Server can function
independently of all other Servers, including the maintenance of
active leases. Therefore, no Server-to-Server DHCPv6 state
synchronization is necessary, and Clients can optionally hold
separate leases for the same ACP from multiple Servers.</t>
<t>In this way, Clients can easily associate with multiple Servers,
and can receive new leases from new Servers before deprecating
leases held through old Servers. This enables a graceful
"make-before-break" capability.</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 Fully-Qualified Domain Name (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 an ACP through a DHCPv6 PD exchange<xref
target="RFC3315"/><xref target="RFC3633"/> in which the Client's
Solicit/Request messages use the IPv6 "unspecified" address (i.e.,
"::") 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 Client also includes a Client Identifier
option with a DHCP Unique Identifier (DUID) plus any necessary
authentication options to identify itself to the DHCPv6 server, and
includes a Client Link Layer Address Option (CLLAO) <xref
target="RFC6939"/> with the format shown in <xref
target="cllao"/>:</t>
<t><figure anchor="cllao"
title="AERO Client Link-Layer Address Option (CLLAO) 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_CLIENT_LINKLAYER_ADDR | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link-layer type (16 bits) | Link ID | Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>The Client sets the CLLAO 'option-length' field to 4 and
sets the 'link-layer type' field to TBD1 (see: IANA Considerations),
then includes appropriate Link ID and Preference values for the
underlying interface over which the Solicit/Request will be issued
(note that these are the same values that would be included in a
TLLAO as shown in <xref target="tllaov6"/>). If the Client is
pre-provisioned with an ACP associated with the AERO service, it MAY
also include the ACP in the Solicit/Request message Identity
Association (IA) option to indicate its preferred ACP to the DHCPv6
server. The Client then sends the encapsulated DHCPv6 request via
the underlying interface.</t>
<t>When the Client receives its ACP and the set of ASPs via a DHCPv6
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 records the lifetime for the ACP in the neighbor
cache entry and marks the neighbor cache entry as "default", i.e.,
the Client considers the Server as a default router. If the Reply
message contains a Vendor-Specific Information Option (see: Section
3.11.3) the Client also caches each ASP in the option.</t>
<t>The Client then applies the AERO address to the AERO interface
and sub-delegates the ACP to nodes and links within its attached
EUNs (the AERO address thereafter remains stable as the Client
moves). The Client also assigns a default IP route to the AERO
interface as a route-to-interface, i.e., with no explicit next-hop.
The next hop will then be determined after a packet has been
submitted to the AERO interface by inspecting the neighbor cache
(see above).</t>
<t>On some platforms (e.g., popular cell phone operating systems),
the act of assigning a default IPv6 route to the AERO interface may
not be permitted from a user application due to security policy.
Typically, those platforms include a TUN/TAP interface 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 fe80::</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"/>. Note that in
this method, the Client appears as a mobility proxy for applications
that bind to the (point-to-point) TUN/TAP interface. The arrangement
can be likened to a Proxy AERO scenario in which the mobile node and
Client are located within the same physical platform (see Section
3.20 for further details on Proxy AERO).</t>
<t>The Client subsequently renews its ACP delegation through each of
its Servers by performing DHCPv6 Renew/Reply exchanges with its AERO
address as the IPv6 source address,
'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address,
the link-layer address of a Server as the link-layer destination
address and the same Client identifier, authentication options and
CLLAO option as was used in the initial PD request. Note that if the
Client does not issue a DHCPv6 Renew before the Server has
terminated the lease (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. If the Client sends synthesized RA and/or DHCPv4 messages
(see above), it also sends a new synthesized message when issuing a
DHCPv6 Renew or when re-initiating the DHCPv6 PD procedure.</t>
<t>Since the Client's AERO address is configured from the unique ACP
delegation it receives, there is no need for Duplicate Address
Detection (DAD) on AERO links. Other nodes maliciously attempting to
hijack an authorized Client's AERO address will be denied access to
the network by the DHCPv6 server due to an unacceptable link-layer
address and/or security parameters (see: Security
Considerations).</t>
<t>AERO Clients ignore the IP address and UDP port number in any
S/TLLAO options in ND messages they receive directly from another
AERO Client, but examine the Link ID and Preference values to match
the message with the correct link-layer address information.</t>
<t>When a source Client forwards a packet to a prospective
destination Client (i.e., one for which the packet's destination
address is covered by an ASP), the source Client initiates an AERO
route optimization procedure as specified in <xref
target="predirect"/>.</t>
</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 DHCPv6 PD
Solicit/Request message, it first authenticates the message. If
authentication succeeds, the Server determines the correct ACP to
delegate to the Client by matching the Client's DUID within an
online directory service (e.g., LDAP). The Server then delegates the
ACP and creates a static neighbor cache entry for the Client's AERO
address with lifetime set to no more than the lease lifetime and the
Client's link-layer address as the link-layer address for the Link
ID specified in the CLLAO option. The Server then creates an IP
forwarding table entry so that the AERO routing system will
propagate the ACP to all Relays (see: <xref target="scaling"/>).
Finally, the Server sends a DHCPv6 Reply message to the Client while
using fe80::ID as the IPv6 source address, the Client's AERO address
as the IPv6 destination address, and the Client's link-layer address
as the destination link-layer address. The Server also includes a
Server Unicast option with server-address set to fe80::ID so that
all future Client/Server transactions will be link-local-only
unicast over the AERO link.</t>
<t>When the Server sends the DHCPv6 Reply message, it also includes
a DHCPv6 Vendor-Specific Information Option with 'enterprise-number'
set to "TBD2" (see: IANA Considerations). The option is formatted as
shown in<xref target="RFC3315"/> and with the AERO
enterprise-specific format shown in <xref target="vendor-specific">
</xref>:</t>
<t><figure anchor="vendor-specific"
title="AERO Vendor-Specific Information Option">
<artwork><![CDATA[ 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number ("TBD2") |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ASP (1) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ASP (2) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ASP (3) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. (etc.) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>Per <xref target="vendor-specific"/>, the option includes
one or more ASP. The ASP field contains the IP prefix as it would
appear in the interface identifier portion of the corresponding AERO
address (see: Section 3.3). For IPv6, valid values for the Prefix
Length field are 0 through 64; for IPv4, valid values are 0 through
32.</t>
<t>After the initial DHCPv6 PD exchange, the AERO Server maintains
the neighbor cache entry for the Client as long as the lease
lifetime remains current. If the Client issues a Renew/Reply
exchange, the Server extends the lifetime. If the Client issues a
Release/Reply exchange, or if the Client does not issue a
Renew/Reply within the lease lifetime, the Server deletes the
neighbor cache entry for the Client and withdraws the IP route from
the AERO routing system.</t>
</section>
</section>
<section anchor="scaling" title="AERO Relay/Server Routing System">
<t>Relays require full topology information of all Client/Server
associations, while individual Servers only require partial topology
information, i.e., they only need to know the ACPs associated with
their current set of associated Clients. This is accomplished through
the use of an internal instance of the Border Gateway Protocol (BGP)
<xref target="RFC4271"/> coordinated between Servers and Relays. This
internal BGP instance does not interact with the public Internet BGP
instance; therefore, the AERO link is presented to the IP Internetwork
as a small set of ASPs as opposed to the full set of individual
ACPs.</t>
<t>In a reference BGP arrangement, each AERO Server is configured as
an Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) (possibly using a private AS Number (ASN) <xref
target="RFC1930"/>), and each Server further peers with each Relay but
does not peer with other Servers. Similarly, Relays need not peer with
each other, since they will 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 Clients, and
dynamically announces new ACPs and withdraws departed ACPs in its BGP
updates to Relays (this is typically accomplished via a "redistribute
static" routing directive). Relays do not send BGP updates to Servers,
however, such that the BGP route reporting is unidirectional from the
Servers to the Relays.</t>
<t>The Relays therefore discover the full topology of the AERO link in
terms of the working set of ACPs associated with each Server, while
the Servers only discover the ACPs of their associated Clients. Since
Clients are expected to remain associated with their current set of
Servers for extended timeframes, the amount of BGP control messaging
between Servers and Relays should be minimal. However, BGP peers
SHOULD dampen any route oscillations caused by impatient Clients that
repeatedly associate and disassociate with Servers.</t>
</section>
<section anchor="predirect" title="AERO Redirection">
<section anchor="avoidance-fig" title="Reference Operational Scenario">
<t><xref target="no-onlink-prefix-fig"/> depicts the AERO
redirection 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') applies
the address fe80::1 to its AERO interface with link-layer address
L2(R1), Server ('S1') applies the address fe80::2 with link-layer
address L2(S1),and Server ('S2') applies 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 applies 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 applies 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 ('H1') connects to the EUN, and configures
the address 2001:db8:1::1.</t>
</section>
<section 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
ICMPv6 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
Preference 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 ICMPv6 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 ICMPv6
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
Preference 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 AERO 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 ICMPv6 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 Predirect; 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 Predirect to determine the next hop toward Client ('C2') by
searching for the AERO address in the neighbor cache. Since Client
('C2') is not a neighbor, Server ('S2') re-encapsulates the
Predirect 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 Predirect 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 Predirect 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 Predirect via Server
('S1').</t>
<t>When Server ('S1') receives the Predirect message from Relay
('R1') it determines that Client ('C1') is a neighbor by consulting
its neighbor cache. Server ('S1') then re-encapsulates the Predirect
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 ICMPv6 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: Section 3.13.6), except that it writes its
own link-layer address in the TLLAO option. The Server must then
maintain a neighbor cache entry for the redirected source
Client.</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
Section 3.10) or proactively to refresh 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 proactively
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 a target Client receives an NS message from a source Client,
it resets AcceptTime to ACCEPT_TIME if a neighbor cache entry exists;
otherwise, it discards the NS message. If ForwardTime is non-zero, the
target Client then sends a solicited NA message to the link-layer
address of the source Client; otherwise, it sends the solicited NA
message to the link-layer address of one of its Servers.</t>
<t>When a source Client receives a solicited NA message from a target
Client, it resets ForwardTime to FORWARD_TIME if a neighbor cache
entry exists; otherwise, it discards the NA message.</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 title="Mobility Management">
<section 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 performs an immediate DHCPv6 Rebind/Reply
exchange via each of its Servers using the new link-layer address as
the source and with a CLLAO that includes the correct Link ID and
Preference values. If authentication succeeds, the Server then
update its neighbor cache and sends a DHCPv6 Reply. Note that if the
Client does not issue a DHCPv6 Rebind before the Server has
terminated the lease (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 unsolicited NA messages to each of its
correspondent Client neighbors using the same procedures as
specified in Section 7.2.6 of <xref target="RFC4861"/>, except that
it sends the messages as unicast to each neighbor via a Server
instead of multicast. In this process, the Client should send no
more than MAX_NEIGHBOR_ADVERTISEMENT messages separated by no less
than RETRANS_TIMER seconds to each neighbor.</t>
<t>With reference to <xref target="no-onlink-prefix-fig"/>, when
Client ('C2') needs to change its link-layer address it sends
unicast unsolicited NA messages to Client ('C1') via Server ('S2')
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 136.</t>
<t>the Code is set to 0.</t>
<t>the Solicited flag is set to 0.</t>
<t>the Override flag is set to 1.</t>
<t>the Target Address is set to fe80::2001:db8:1:0 (i.e., the
AERO address of Client ('C2')).</t>
<t>the message includes one or more TLLAOs with Link ID and
Preference 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.</t>
</list>When Server ('S1') receives the NA message, it relays the
message in the same way as described for relaying Redirect messages
in Section 3.13.7. In particular, Server ('S1') copies the correct
UDP port numbers and IP addresses into the TLLAOs, changes the
link-layer source address to its own address, changes the link-layer
destination address to the address of Relay ('R1'), then forwards
the NA message via the relaying chain the same as for a
Redirect.</t>
<t>When Client ('C1') receives the NA message, it accepts the
message only if it already has a neighbor cache entry for Client
('C2') then updates the link-layer addresses for Client ('C2') based
on the addresses in the TLLAOs. However, Client ('C1') MUST NOT
update ForwardTime since Client ('C2') will not have updated
AcceptTime.</t>
<t>Note that these unsolicited NA messages are unacknowledged;
hence, Client ('C2') has no way of knowing whether Client ('C1') has
received them. If the messages are somehow lost, however, Client
('C1') will soon learn of the mobility event via the NUD procedures
specified in Section 3.14.</t>
</section>
<section 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 performs an
immediate Rebind/Reply exchange via each of its Servers using the
new link-layer address as the source address and with a CLLAO that
includes the new Link ID and Preference values. If authentication
succeeds, the Server then updates its neighbor cache and sends a
DHCPv6 Reply. The Client MAY then send unsolicited NA messages to
each of its correspondent Clients to inform them of the new
link-layer address as described in Section 3.15.1.</t>
</section>
<section 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
performs an immediate Rebind/Reply exchange via each of its Servers
over any available link with a CLLAO that includes the deprecated
Link ID and a Preference value of 0. If authentication succeeds, the
Server then updates its neighbor cache and sends a DHCPv6 Reply. The
Client SHOULD then send unsolicited NA messages to each of its
correspondent Clients to inform them of the deprecated link-layer
address as described in Section 3.15.1.</t>
</section>
<section title="Moving to a New Server">
<t>When a Client associates with a new Server, it performs the
Client procedures specified in Section 3.12.</t>
<t>When a Client disassociates with an existing Server, it sends a
DHCPv6 Release message to the unicast link-local network layer
address of the old Server. The Client SHOULD send the message via a
new Server (i.e., by setting the link-layer destination address to
the address of the new Server) in case the old Server is unreachable
at the link layer, e.g., if the old Server is in a different network
partition. The new Server will forward the message to a Relay, which
will in turn forward the message to the old Server.</t>
<t>When the old Server receives the DHCPv6 Release, it first
authenticates the message. If authentication succeeds, the old
Server withdraws the IP route from the AERO routing system and
deletes the neighbor cache entry for the Client. (The old Server MAY
impose a small delay before deleting the neighbor cache entry so
that any packets already in the system can still be delivered to the
Client.) The old Server then returns a DHCPv6 Reply message via a
Relay. 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 the Server's Reply to the Client's Release message may be lost,
e.g., if the AERO routing system has not yet converged. Since the
Client is responsible for reliability, however, it will retry until
it gets an indication that the Release was successful.)</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 due to
routing protocol dampening. 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="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 the Server.</t>
</section>
<section title="Multicast Considerations">
<t>When the underlying network does not support multicast, AERO nodes
map IPv6 link-scoped multicast addresses (including
'All_DHCP_Relay_Agents_and_Servers') to the link-layer address of a
Server.</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 direct
multicast mapping for IPv6 underlying networks. (In the latter case,
"direct multicast mapping" means that if the IPv6 multicast
destination address of the encapsulated packet is "M", then the IPv6
multicast destination address of the encapsulating header is also
"M".)</t>
</section>
<section 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 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 title="Proxy AERO">
<t>Proxy Mobile IPv6 (PMIPv6) <xref target="RFC5213"/><xref
target="RFC5844"/> presents a localized mobility management scheme for
use within an access network domain. It is typically used in cellular
wireless service provider networks, and allows mobile nodes to receive
and retain a stable IP address without needing to implement any
special mobility protocols. In the PMIPv6 architecture, access network
devices known as Mobility Access Gateways (MAGs) provide mobile nodes
with an access link abstraction and receive prefixes for the mobile
nodes from a Local Mobility Anchor (LMA).</t>
<t>The AERO Client (acting as a MAG) can similarly provide proxy
services for mobile nodes that do not participate in AERO messaging.
The proxy Client presents an access link abstraction to mobile nodes,
and performs DHCPv6 PD exchanges over the AERO interface with an AERO
Server (acting as an LMA) to receive a prefix for address provisioning
of the mobile node.</t>
<t>When a mobile node comes onto an access link presented by a proxy
Client, the Client authenticates the node and obtains a unique
identifier that it can use as the DUID in its DHCPv6 PD messages to
the Server. When the Server delegates a prefix, the Client creates a
new AERO address for the mobile node and assigns the delegated prefix
to the mobile node's access link. The Client then generates address
autoconfiguration messages (e.g., IPv6 RA, DHCPv6, DHCPv4, etc.) over
the access link and configures itself as a default router for the
mobile node. Since the Client may serve many such mobile nodes
simultaneously, it may configure multiple AERO addresses, i.e., one
for each mobile node.</t>
<t>When two mobile nodes are associated with the same proxy Client,
the Client can forward traffic between the mobiles without involving
the Server since it configures the AERO addresses of each mobile and
therefore also has the necessary routing information. When two mobiles
are associated with different Clients, the first mobile node's Client
can initiate standard AERO route optimization using the mobile's AERO
address as the source for route optimization messaging. This may
result in a route optimization where the first mobile node's Client
discovers a direct path to the second mobile node's Client.</t>
<t>When a mobile node moves to a new proxy Client, the old proxy
Client issues a DHCPv6 Release message and sends unsolicited NA
messages to any of the mobile node's correspondents the same as
specified for announcing link-layer address changes in Section 3.15.1.
However, since the old Client has no way of knowing where the mobile
has moved to, it sets the Code field in the NA message to 1. When the
correspondent receives such an NA message, it deletes the neighbor
cache entry for the departed mobile node and again allows packets to
flow through its Server.</t>
<t>In addition to the use of DHCPv6 PD 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-to-Server 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. All other considerations are the same as specified in
<xref target="RFC5213"/><xref target="RFC5844"/>.</t>
</section>
<section 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 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 Internet address of
2001:db8:2000::1. While Client ('C1') may not initially know whether
('C2') is in fact an AERO Client, it can 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 AERO 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 presumed IPv6 subnet router anycast
address for ('C2')s ACP)</t>
<t>The encapsulating IPv6 header is followed by a UDP header with
source and destination port set to 8060</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 presumed AERO address for
('C2'))</t>
<t>The encapsulated AERO 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
AERO 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 presumed IPv6 subnet router anycast
address for ('C2')s ACP)</t>
<t>The encapsulating IPv6 header is followed by a UDP header with
source and destination port set to 8060</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 presumed AERO address for
('C2'))</t>
<t>The encapsulated AERO Predirect message includes all of the
securing information that would occur in a MIPv6 "Care-of Test
Init" message (format TBD)</t>
</list>If ('C2') is indeed an AERO Client, it will receive both
Predirect messages through its home network. ('C2') 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, AERO 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 (which may result in a new route
optimization event).</t>
</section>
</section>
<section anchor="implement" title="Implementation Status">
<t>An application-layer implementation is in progress.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>IANA is instructed to assign a new 2-octet Hardware Type number
"TBD1" for AERO in the "arp-parameters" registry per Section 2 of <xref
target="RFC5494"/>. The number is assigned from the 2-octet Unassigned
range with Hardware Type "AERO" and with this document as the
reference.</t>
<t>IANA is instructed to assign a 4-octet Enterprise Number "TBD2" for
AERO in the "enterprise-numbers" registry per <xref
target="RFC3315"/>.</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.</t>
<t>AERO 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. AERO
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 a NAT function.)</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>
</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, Sri Gundavelli, Brian
Haberman, Joel Halpern, Sascha Hlusiak, Lee Howard, Andre Kostur, Ted
Lemon, Joe Touch and Bernie Volz. Members of the IESG also provided
valuable input during their review process that greatly improved the
document. Special thanks go to Stewart Bryant, Joel Halpern and Brian
Haberman for their shepherding guidance.</t>
<t>This work has further been encouraged and supported by Boeing
colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie,
Balaguruna Chidambaram, Claudiu Danilov, Wen Fang, Anthony Gregory, Jeff
Holland, Ed King, Gen MacLean, Kent Shuey, Brian Skeen, Mike Slane,
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>
</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.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.I-D.ietf-dhc-sedhcpv6"?>
</references>
</back>
</rfc>
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