One document matched: draft-templin-aerolink-30.xml
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<rfc category="std" docName="draft-templin-aerolink-30.txt" ipr="trust200902"
obsoletes="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="21" month="July" 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 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 IPv6 ND message signalling is used in the control
plane, either of 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 applied 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 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 delegation.</t>
<t hangText="AERO Server ("Server")"><vspace/>a node that
configures an AERO interface to provide default forwarding 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 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.</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. This is an important distinction to avoid
ambiguity, e.g., an AERO Server also acts as a DHCPv6 relay, an AERO
Relay may also act as a DHCPv6 server, etc.</t>
<t>Throughout the document, it is said that an address is "applied" to
an AERO interface since the address need not always be "assigned" to the
interface in the traditional sense. However, the address must at least
be bound to the interface in some fashion for operation of the IPv6 ND
protocol.</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 ::) | DHCPv6 |
`-(::::::::::::)-' | Server X |
`-(::::::)-' +-----------+
|
+--------------+ +------+-------+ +--------------+
|AERO Server S1| | AERO Relay R | |AERO Server S2|
| (default->R) | |(C->S1; D->S2)| | (default->R) |
| Nbr: A | +-------+------+ | Nbr: B |
+-------+------+ | +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client A | |AERO Client B |
| default->S1 | | default->S2 |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. .-(_ IP )-.
(__ EUN ) (__ EUN )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host C | | Host D |
+--------+ +--------+
]]></artwork>
</figure><xref target="chaining-fig"/> above presents the AERO link
reference model. In this model:</t>
<t><list style="symbols">
<t>DHCPv6 Server X is the prefix delegation authority for the
delegation of ACPs taken from the AERO link's ASPs</t>
<t>Relay R associates with Servers S1 and S2, and connects the
link to the rest of the IP Internetwork</t>
<t>Servers S1 and S2 associate with Relay R and also act as
default routers for their associated Clients A and B</t>
<t>Clients A and B associate with Servers S1 and S2, respectively
and also act as default routers for their associated EUNs</t>
<t>Hosts C and D attach to the EUNs served by Clients A and B,
respectively</t>
</list>In this model, there may be many additional Relays, Servers
and Clients. Each Sever peers with each Relay in a dynamic routing
protocol session to advertise its list of associated Clients. 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. Clients may associate with only a single Server or with
multiple Server, e.g., for fault tolerance and/or load balancing.</t>
</section>
<section anchor="node-types" title="AERO Node Types">
<t>The DHCPv6 server is authoritative for the management of the AERO
link's AERO Service Prefixes (ASPs). The DHCPv6 server is therefore
critical infrastructure for the AERO link, but need not otherwise
participate as an AERO node. AERO Servers communicate with the DHCPv6
server either via the AERO link itself or via a different IPv6
link.</t>
<t>AERO Relays relay packets between nodes connected to the same AERO
link and also forward packets between the AERO link and the native
Internetwork. The relaying process entails re-encapsulation of IP
packets that were received from a first AERO node and are to be
forwarded without modification to a second AERO node. AERO Relays
present the AERO link to the native Internetwork as a set of one or
more ASPs.</t>
<t>AERO Servers provide default routing services to AERO Clients. AERO
Servers configure a DHCPv6 relay function and facilitate Prefix
Delegation (PD) exchanges between AERO Clients and the DHCPv6 server.
Each delegated prefix becomes an AERO Client Prefix (ACP) taken from
an ASP.</t>
<t>AERO Clients act as requesting routers to receive ACPs through
DHCPv6 PD exchanges via AERO Servers over the AERO link. (Each Client
MAY associate with a single Server or with multiple Servers.) Each
IPv6 AERO Client receives at least a /64 IPv6 ACP, and may receive
even shorter prefixes. Similarly, each IPv4 AERO Client receives at
least a /32 IPv4 ACP (i.e., a singleton IPv4 address), and may receive
even shorter prefixes.</t>
<t>AERO Clients that act as routers sub-delegate portions of their
ACPs to links on EUNs. End system applications on AERO Clients that
act as routers bind to EUN interfaces (i.e., and not the AERO
interface).</t>
<t>AERO Clients that act as ordinary hosts assign one or more IP
addresses from their ACPs to the AERO interface but DO NOT assign the
ACP itself to the AERO interface. Instead, the Client assigns the ACP
to a "black hole" route so that unused portions of the prefix are
nullified. End system applications on AERO Clients that act as hosts
bind directly to the AERO 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 applied 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 as 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 a priori
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 3.9).
AERO links further use link-local-only addressing; hence, AERO nodes
ignore any Prefix Information Options (PIOs) they may receive in RA
messages.</t>
<t>AERO interface Redirect, Predirect and unsolicited NA messages
include 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 (or 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-- --+
| |
+-- 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 0 being highest
preference and 255 being lowest. 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 no UDP
encapsulation is used, UDP Port Number is set to 0. When the
encapsulation IP address family is IPv4, IP Address is formed as an
IPv4-mapped IPv6 address <xref target="RFC4291"/>.</t>
<t>When a Relay enables an AERO interface, it applies an
administratively assigned link-local address fe80::ID to the interface
for communicating with Servers on the link. Each fe80::ID address MUST
be unique among all Relays and Servers on the link, and MUST NOT
collide with any potential AERO addresses, e.g., the addresses could
be assigned 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 applies the address
fe80:: to the interface as a link-local Subnet Router Anycast address,
and also applies an administratively assigned link-local address
fe80::ID to support the operation of the IPv6 ND protocol and to
communicate with Relays on the link. The Server maintains a neighbor
cache entry for each Relay on the link, and also creates per-Client
neighbor cache entries whenever it discovers a new Client. At a
minimum, when the Server receives an NS/RS messages on the AERO
interface it returns an NA/RA message. When the Server receives an
NS/NA, it also update timers in existing neighbor cache entries but
does not create new neighbor cache entries nor update cached
link-layer addresses. Servers also engage 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 prefix
delegation to receive an ACP. Next, it applies the corresponding AERO
address to the AERO interface, i.e., the prefix delegation 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
peer 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 address for the neighbor Client (e.g.,
following a link-layer address change due to node mobility) but do not
create new neighbor cache entries. RA messages as well as 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. Redirect, Predirect
and unsolicited NA messages SHOULD include a Timestamp option (see
Section 5.3 of <xref target="RFC3971"/>) that other AERO nodes can use
to verify the message time of origin. Predirect, NS and RS messages
SHOULD include a Nonce option (see Section 5.3 of <xref
target="RFC3971"/>) that recipients echo back in corresponding
responses. Finally, the Client need not maintain any IP forwarding
table entries for neighboring 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.</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 (e.g., 0).</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. Further details on
coordination of multiple active underlying interfaces are outside
the scope of this specification.</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"/>.
Neighbor cache entries are created and maintained as follows:</t>
<t>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. AERO Clients maintain a
neighbor cache entry for each of their associated Servers, and AERO
Servers maintain a neighbor cache for each of their associated Clients
with a lifetime based on the DHCPv6 lease lifetime. AERO Clients
maintain neighbor cache entries for each of their active correspondent
Clients with lifetimes based on IPv6 ND messaging constants.</t>
<t>When an AERO Server relays a DHCPv6 Reply message to an AERO
Client, it creates or updates a neighbor cache entry for the Client
based on the AERO address corresponding to the Client's ACP as the
network-layer address and with the Client's encapsulation IP address
and UDP port number as the link-layer address. The Server also records
the ACP's lease lifetime and prefix length in the neighbor cache
entry.</t>
<t>When an AERO Client receives a DHCPv6 Reply message from an AERO
Server, it creates or updates a neighbor cache entry for the Server
based on the Reply message link-local source address as the
network-layer address, the lease 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>When an AERO Client receives a valid Predirect message it creates
or updates a neighbor cache entry for the Predirect target
network-layer and link-layer addresses plus prefix length. The node
then sets an "AcceptTime" variable for the neighbor and uses this
value to determine whether packets received from the predirected
neighbor can be accepted.</t>
<t>When an AERO Client receives a valid Redirect message it creates or
updates a neighbor cache entry for the Redirect target network-layer
and link-layer addresses plus prefix length. The node then sets a
"ForwardTime" variable for the neighbor and uses this value to
determine whether packets can be sent directly to the redirected
neighbor. The node also maintains a "Retry" variable to limit the
number of keepalives sent when a neighbor may have gone
unreachable.</t>
<t>When an AERO Client receives a valid NS message corresponding to a
neighbor cache entry for another Client, it (re)sets AcceptTime for
the neighbor to ACCEPT_TIME.</t>
<t>When an AERO Client receives a valid solicited NA message
corresponding to a neighbor cache entry for another Client, it
(re)sets ForwardTime for the neighbor to FORWARD_TIME and sets Retry
to MAX_RETRY. (When an AERO Client receives a valid unsolicited NA
message, it updates the neighbor's link-layer address but DOES NOT
reset ForwardTime or Retries.)</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>
<t>For AERO Client<->Server neighbor cache entries, AcceptTime
and ForwardTime are set based on the DHCPv6 lease lifetime and may be
modified based on the Router Lifetime advertised in the Server's RA
messages.</t>
</section>
<section title="AERO Interface Sending Algorithm">
<t>When an IP packet enters a Client's AERO interface from the network
layer, the Client searches its neighbor cache for an entry with an
AERO address that matches the packet's destination address. If there
is a match, the Client uses the link-layer address in the neighbor
cache entry as the link-layer address for encapsulation then admits
the packet into the tunnel. If there is no match, the Client instead
uses the link-layer address of a neighboring Server as the link-layer
address for encapsulation. (Note that the Client caches the ASPs for
the AERO link and can thus search the neighbor cache only for
destination addresses that are covered by an ASP.)</t>
<t>When an IP packet enters a Server's AERO interface from the link
layer, the Server searches for a neighbor cache match the same as for
a Client. If there is a match, the Server uses the link-layer address
in the neighbor cache entry as the link-layer address for
re-encapsulation. If there is no match, the Server instead uses the
link-layer address of a neighboring Relay as the link-layer address
for encapsulation. Servers also relay Predirect, Redirect and
unsolicited Neighbor Advertisement messages received from a Client and
with an AERO destination address. If the AERO destination address is
the address of a neighbor, the Server changes the link-layer source
address to its own address, changes the link-layer destination address
to the address of the neighbor and forwards the message to the
neighbor. If the AERO destination address is not a neighbor, the
Server instead forwards the message to a Relay. When an AERO Relay
forwards either a data packet or an IPv6 ND message to an AERO Server,
the Server MUST NOT forward the packet back to the same or a different
Relay.</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. When an IP packet enters a Relay's AERO interface from
the link-layer, if the destination is not covered by an ASP the Relay
forwards the packet to another IP link as indicated by the IP
forwarding table. If the destination is covered by an ASP, and there
is a more-specific forwarding table entry that matches the
destination, the Relay uses the link-layer address in the neighbor
cache entry for the next-hop Server as the link-layer address for
encapsulation. If there is no more-specific entry, the Relay instead
drops the packet. Relays also relay Predirect, Redirect and
unsolicited Neighbor Advertisement messages by searching for an IP
forwarding table entry that matches the message's AERO destination
address. If there is a match, the Relay proxies the packet in the same
manner as described for Servers above; otherwise, the Relay drops the
packet. When an AERO Server forwards either a data packet or an IPv6
ND message to an AERO Relay, the Relay MUST NOT forward the packet
back to the same Server.</t>
<t>Note that in the above this tunnel exit determination is often
based on consulting the neighbor cache instead of the IP forwarding
table. IP forwarding is therefore linked to IPv6 ND via the AERO
address.</t>
<t>When an AERO node forwards a packet back out the same AERO
interface the packet arrived on, the node MUST NOT decrement the
network layer TTL/Hop-count.</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 forwarded out the same AERO interface that they arrived on. This
latter form of encapsulation is known as "re-encapsulation".</t>
<t>AERO interfaces encapsulate packets per the specifications in <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
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
header into the corresponding fields in the new encapsulation header
(i.e., the values are transferred between encapsulation headers and
*not* copied from the encapsulated packet's network-layer header).</t>
<t>When AERO UDP encapsulation is used, the AERO interface
encapsulates the packet per the specifications in <xref
target="RFC2003"/><xref target="RFC2473"/><xref target="RFC4213"/>
except that it inserts a UDP header between the encapsulation header
and the packet's IP header. The AERO interface sets the UDP source
port to a constant value that it will use in each successive packet it
sends, sets the UDP checksum field to zero (see: <xref
target="RFC6935"/><xref target="RFC6936"/>) and sets the UDP length
field to the length of the IP 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) when AERO-only encapsulation is used. For packets sent to a
neighboring Client, the AERO interface sets the UDP destination port
to the port value stored in the neighbor cache entry for this
neighbor.</t>
<t>The AERO interface next sets the IP protocol number in the
encapsulation header to the appropriate value for the first protocol
layer within the encapsulation (e.g., IPv4, IPv6, UDP, IPsec, etc.).
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 3.7.</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 AERO UDP encapsulation
is used (i.e., when a UDP header with destination port 8060 is
present) the interface examines the first octet of the encapsulated
packet. 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), the packet is
accepted and the encapsulating UDP header is discarded; otherwise, the
packet is discarded.</t>
<t>Further decapsulation then proceeds according to the appropriate
tunnel type <xref target="RFC2003"/><xref target="RFC2473"/><xref
target="RFC4213"/><xref target="RFC4301"/><xref target="RFC5246"/>
(etc.).</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. In particular,
AERO Clients accept encapsulated packets with a link-layer source
address belonging to one of their current AERO Servers, and AERO
Clients and Servers accept encapsulated packets with a link-layer
source address that is correct for the network-layer source
address.</t>
<t>The AERO node considers the link-layer source address correct for
the network-layer source address if there is an AERO interface
neighbor cache entry with an AERO address that matches the packet's
network-layer source address prefix, with a link-layer address that
matches the packet's link-layer source address, and AcceptTime is
non-zero.</t>
<t>An AERO Server also accepts packets with a link-layer source
address that matches one of its associated Relays, and an AERO Relay
accepts packets with a source address that matches one of its
associated Servers.</t>
<t>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 Considerations">
<t>The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
encapsulation overhead for IPv4 as the link-layer <xref
target="RFC0791"/> and 4GB minus the encapsulation overhead for IPv6
as the link layer <xref target="RFC2675"/>. This is the most that IPv4
and IPv6 (respectively) can convey within the constraints of protocol
constants, but actual sizes available for tunneling will frequently be
much smaller.</t>
<t>The base tunneling specifications for IPv4 and IPv6 typically set a
static MTU on the tunnel interface to 1500 bytes minus the
encapsulation overhead or smaller still if the tunnel is likely to
incur additional encapsulations on the path. This can result in path
MTU related black holes when packets that are too large to be
accommodated over the AERO link are dropped, but the resulting ICMP
Packet Too Big (PTB) messages are lost on the return path. As a
result, AERO nodes use the following MTU mitigations to accommodate
larger packets.</t>
<t>AERO nodes set their AERO interface MTU to the larger of the
underlying interface MTU minus the encapsulation overhead, and 1500
bytes. (If there are multiple underlying interfaces, the node sets the
AERO interface MTU according to the largest underlying interface MTU,
or 64KB /4G minus the encapsulation overhead if the largest MTU cannot
be determined.) AERO nodes optionally cache other per-neighbor MTU
values in the underlying IP path MTU discovery cache initialized to
the underlying interface MTU.</t>
<t>AERO nodes admit packets that are no larger than 1280 bytes minus
the encapsulation overhead (*) as well as packets that are larger than
1500 bytes into the tunnel without fragmentation, i.e., as long as
they are no larger than the AERO interface MTU before encapsulation
and also no larger than the cached per-neighbor MTU following
encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit
to 0 for packets no larger than 1280 bytes minus the encapsulation
overhead (*) and sets the DF bit to 1 for packets larger than 1500
bytes. If a large packet is lost in the path, the node may optionally
cache the MTU reported in the resulting PTB message or may ignore the
message, e.g., if there is a possibility that the message is
spurious.</t>
<t>For packets destined to an AERO node that are larger than 1280
bytes minus the encapsulation overhead (*) but no larger than 1500
bytes, the node uses IP fragmentation to fragment the encapsulated
packet into two pieces (where the first fragment contains 1024 bytes
of the original IP packet) then admits the fragments into the tunnel.
If the link-layer protocol is IPv4, the node admits each fragment into
the tunnel with DF set to 0 and subject to rate limiting to avoid
reassembly errors <xref target="RFC4963"/><xref target="RFC6864"/>.
For both IPv4 and IPv6, the node also sends a 1500 byte probe message
(**) to the neighbor, subject to rate limiting.</t>
<t>To construct a probe, the node prepares an NS message with a Nonce
option plus trailing padding octets added to a length of 1500 bytes
without including the length of the padding in the IPv6 Payload Length
field. The node then encapsulates the NS in the encapsulation headers
(while including the length of the padding in the encapsulation header
length fields), sets DF to 1 (for IPv4) and sends the padded NS
message to the neighbor. If the neighbor returns an NA message with a
correct Nonce value, the node may then send whole packets within this
size range and (for IPv4) relax the rate limiting requirement. (Note
that the trailing padding SHOULD NOT be included within the Nonce
option itself but rather as padding beyond the last option in the NS
message; otherwise, the (large) Nonce option would be echoed back in
the solicited NA message and may be lost at a link with a small MTU
along the reverse path.)</t>
<t>AERO nodes MUST be capable of reassembling packets up to 1500 bytes
plus the encapsulation overhead length. It is therefore RECOMMENDED
that AERO nodes be capable of reassembling at least 2KB.</t>
<t>(*) Note that if it is known without probing that the minimum Path
MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500)
then MINMTU can be used instead of 1280 in the fragmentation threshold
considerations listed above.</t>
<t>(**) It is RECOMMENDED that no probes smaller than 1500 bytes be
used for MTU probing purposes, since smaller probes may be fragmented
if there is a nested tunnel somewhere on the path to the neighbor.
Probe sizes larger than 1500 bytes MAY be used, but may be unnecessary
since original sources are expected to implement <xref
target="RFC4821"/> when sending large packets.</t>
</section>
<section anchor="aeropd"
title="AERO Router Discovery, Prefix Delegation and Address Configuration">
<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 "[domainname]" is the
connection-specific DNS suffix for the Client's underlying network
connection. 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 DHCPv6 PD <xref
target="RFC3315"/><xref target="RFC3633"/><xref target="RFC6355"/>
using 'fe80::ffff:ffff:ffff:ffff' as the IPv6 source address,
'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address
and the link-layer address of the Server as the link-layer
destination address. The Client includes a DHCPv6 Unique Identifier
(DUID) in the Client Identifier option of its DHCPv6 messages (as
well as a DHCPv6 authentication option if necessary) to identify
itself to the DHCPv6 server. The Client also includes a DHCPv6
Client Link Layer Address Option (CLLAO) <xref target="RFC6939"/>
with the link-layer address format shown in <xref target="tllaov6"/>
with Link ID followed by Preference followed by the values 0 for
both the UDP Port Number and IP Address.The Client sets the CLLAO
'option-length' field to 22 (2 plus the length of the link-layer
address) and sets the 'link-layer type' field to TBD1 (see: IANA
Considerations). If the Client is pre-provisioned with an ACP
associated with the AERO service, it MAY also include the ACP in its
DHCPv6 PD Request to indicate its preferred ACP to the DHCPv6
server. The Client then sends the encapsulated DHCPv6 request via an
underlying interface.</t>
<t>When the Client receives its ACP and the set of ASPs via a Reply
from the DHCPv6 server, i.e., via an AERO Server acting as a DHCPv6
relay, it creates a neighbor cache entry with the Server's
link-local address (i.e., fe80::ID) as the network-layer address and
the Server's encapsulation address as the link-layer address.</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.</t>
<t>The Client subsequently renews its ACP delegation 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 DUID and authentication
information. If the Client wishes to associate with multiple
Servers, it can perform DHCPv6 Renew/Reply exchanges via each of the
Servers.</t>
<t>The Client then sends an RS message to each of its associated
Servers to receive an RA message with a Router Lifetime and any
other link-specific parameters. The Client uses the Router Lifetime
to set the lifetime for the neighbor cache entry for this Server.
The Client further ignores any RS messages it might receive, since
only Servers may process RS messages.</t>
<t>The Client then sends periodic RS messages to obtain new RA
messages, and further initiates a new DHCPv6 Renew/Reply exchange
before the Router Lifetime expires. The Client can also forward IP
packets destined to networks beyond its local EUNs via a Server as a
default router.</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 relay 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. Here, "linkupnetworks" is a constant text string, and
"[domainname]" is the connection-specific DNS suffix for this
underlying network connection.</t>
<t>When an AERO Server relays a prospective Client's DHCPv6 PD
messages to the DHCPv6 server, it wraps each message in a
"Relay-forward" message per <xref target="RFC3315"/> and includes a
DHCPv6 Interface Identifier option that encodes a value that
identifies the AERO link to the DHCPv6 server. Without creating
internal state, the Server then modifies the Client's link-layer
address in the CLLAO <xref target="RFC6939"/> by writing the
client's UDP Port number and IP adddress in the corresponding fields
of the option. The Server finally includes a DHCPv6 Echo Request
Option (ERO) <xref target="RFC4994"/> that encodes the option code
for the CLLAO in a 'requested-option-code-n' field, then relays the
message to the DHCPv6 server. The CLLAO information will therefore
subsequently be echoed back in the DHCPv6 server's "Relay-reply"
message.</t>
<t>When the DHCPv6 server issues the ASPs and ACP prefix delegation
in a "Relay-reply" message via the AERO Server (acting as a DHCPv6
relay), the Server obtains the Client's link-layer address from the
echoed CLLAO option and also obtains the Client's delegated ACP and
lease lifetime from the message. The Server then creates a neighbor
cache entry for the Client's AERO address with the Client's
link-layer address as the link-layer address and with lifetime set
to no more than the lease lifetime. The Server finally injects the
ACP as an IP route into the inter-Server/Relay routing system (see:
<xref target="scaling"/>) then relays the DHCPv6 message to the
Client while using fe80::ID as the IPv6 source address, the
link-local address found in the "peer address" field of the
Relay-reply message as the IPv6 destination address, and the
Client's link-layer address as the destination link-layer
address.</t>
<t>Servers respond to NS/RS messages from Clients on their AERO
interfaces by returning an NA/RA message. When the Server returns an
RA message, it sets Router Lifetime to the neighbor cache entry
lifetime but does not include any Prefix Information Options (PIOs)
since the AERO link is link-local-only. The server decrements the
neighbor cache entry lifetime according to the system clock.</t>
<t>Servers ignore any RA messages they may receive from a Client,
but they MAY examine RA messages received from other Servers for
consistency verification purposes.</t>
</section>
<section title="DHCPv6 Server Behavior">
<t>The DHCPv6 server observes both the base DHCPv6 specification
<xref target="RFC3315"/> and the DHCPv6 PD specification <xref
target="RFC3633"/>. The DHCPv6 server further MUST honor the DHCPv6
Echo Request Option (ERO) and Client Link-Layer Address Option
(CLLAO) as discussed in Section 3.10.1.</t>
<t>The DHCPv6 server 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 Prefix Length field must contain a value
between 0 - 64 and the ASP field contains the leading 64 bits of the
ASP. When the Client receives an AERO Vendor-Specific Information
Option it accepts the option and caches each ASP that observes the
format specified above. If the Client cannot parse the ASPs, it
ignores the option.</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. 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 ('R'), two AERO Servers ('S1', 'S2'), two AERO Clients ('A',
'B') and two ordinary IPv6 hosts ('C', 'D'):</t>
<figure anchor="no-onlink-prefix-fig"
title="AERO Reference Operational Scenario">
<artwork><![CDATA[ +--------------+ +--------------+ +--------------+
| Server S1 | | Relay R | | Server S2 |
| Nbr: A | |(C->S1; D->S2)| | Nbr: B |
+--------------+ +--------------+ +--------------+
fe80::2 fe80::1 fe80::3
L2(S1) L2(R) L2(S2)
| | |
X-----+-----+------------------+-----------------+----+----X
| AERO Link |
L2(A) L2(B)
fe80::2001:db8:0:0 fe80::2001:db8:1:0
+--------------+ +--------------+
| AERO Client A| | AERO Client B|
| (default->S1)| | (default->S2)|
+--------------+ +--------------+
2001:DB8:0::/48 2001:DB8:1::/48
| |
.-. .-.
,-( _)-. 2001:db8:0::1 2001:db8:1::1 ,-( _)-.
.-(_ IP )-. +---------+ +---------+ .-(_ IP )-.
(__ EUN )--| Host C | | Host D |--(__ EUN )
`-(______)-' +---------+ +---------+ `-(______)-'
]]></artwork>
</figure>
<t>In <xref target="no-onlink-prefix-fig"/>, Relay ('R') applies the
address fe80::1 to its AERO interface with link-layer address L2(R),
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 ('A') receives the ACP 2001:db8:0::/48 in a DHCPv6 PD
exchange via AERO Server ('S1') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(A). Client ('A') 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 ('C') connects to the EUN, and configures
the address 2001:db8:0::1.</t>
<t>AERO Client ('B') receives the ACP 2001:db8:1::/48 in a DHCPv6 PD
exchange via AERO Server ('S2') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(B). Client ('B') 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 ('D') 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 ('C') sends a packet to destination host ('D'), the
packet is first forwarded over the source host's attached EUN to
Client ('A'). Client ('A') then forwards the packet via its AERO
interface to Server ('S1') and also sends a Predirect message toward
Client ('B') via Server ('S1'). Server ('S1') then re-encapsulates
and forwards both the packet and the Predirect message out the same
AERO interface toward Client ('B') via Relay ('R').</t>
<t>When Relay ('R') receives the packet and Predirect message, it
consults its forwarding table to discover Server ('S2') as the next
hop toward Client ('B'). Relay ('R') then forwards both messages to
Server ('S2'), which then forwards them to Client ('B').</t>
<t>After Client ('B') receives the Predirect message, it process the
message and returns a Redirect message toward Client ('A') via
Server ('S2'). During the process, Client ('B') also creates or
updates a neighbor cache entry for Client ('A').</t>
<t>When Server ('S2') receives the Redirect message, it
re-encapsulates the message and forwards it on to Relay ('R'), which
forwards the message on to Server ('S1') which forwards the message
on to Client ('A'). After Client ('A') receives the Redirect
message, it processes the message and creates or updates a neighbor
cache entry for Client ('C').</t>
<t>Following the above Predirect/Redirect message exchange,
forwarding of packets from Client ('A') to Client ('B') 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 ('A') forwards
a packet toward Client ('B'), it MAY also send a Predirect message
forward toward Client ('B'), subject to rate limiting (see Section
8.2 of <xref target="RFC4861"/>). Client ('A') prepares the
Predirect message as follows:</t>
<t><list style="symbols">
<t>the link-layer source address is set to 'L2(A)' (i.e., the
link-layer address of Client ('A')).</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 ('A')).</t>
<t>the network-layer destination address is set to
fe80::2001:db8:1:0 (i.e., the AERO address of Client ('B')).</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
applied to the Target Address.</t>
<t>the Target Address is set to fe80::2001:db8:0:0 (i.e., the
AERO address of Client ('A')).</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 a TLLAO with Link ID and Preference set
to appropriate values for Client ('A')'s underlying interface,
and with UDP Port Number and IP Address set to 0'.</t>
<t>the message SHOULD include a Timestamp option.</t>
<t>the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated 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 ('A') may have advanced knowledge that the
direct path to Client ('B') would be unusable. If the direct path
later becomes unusable after the initial route optimization, Client
('A') 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
('A'), it first verifies that the requested redirection is
authorized. If the redirection is not permitted, 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 ('A') 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 number and IP Address for Client ('A') into the (previously
empty) TLLAO.</t>
<t>Server ('S1') then examines the network-layer destination address
of the Predirect to determine the next hop toward Client ('B') by
searching for the AERO address in the neighbor cache. Since Client
('B') is not one of its neighbors, Server ('S1') re-encapsulates the
Predirect and relays it via Relay ('R') by changing the link-layer
source address of the message to 'L2(S1)' and changing the
link-layer destination address to 'L2(R)'. Server ('S1') finally
forwards the re-encapsulated message to Relay ('R') without
decrementing the network-layer TTL/Hop Limit field.</t>
<t>When Relay ('R') receives the Predirect message from Server
('S1') it determines that Server ('S2') is the next hop toward
Client ('B') by consulting its forwarding table. Relay ('R') then
re-encapsulates the Predirect while changing the link-layer source
address to 'L2(R)' and changing the link-layer destination address
to 'L2(S2)'. Relay ('R') then relays the Predirect via Server
('S2').</t>
<t>When Server ('S2') receives the Predirect message from Relay
('R') it determines that Client ('B') 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(B)'. Server
('S2') then forwards the message to Client ('B').</t>
</section>
<section anchor="processing_pre"
title="Processing Predirects and Sending Redirects">
<t>When Client ('B') 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 ('B') further accepts the
message only if it is willing to serve as a redirection target.
Next, Client ('B') 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 ('B') receives
a valid Predirect message, it either creates or updates a neighbor
cache entry that stores the Target Address of the message as the
network-layer address of Client ('A') , stores the link-layer
address found in the TLLAO as the link-layer address(es) of Client
('A') and stores the Prefix Length as the length to be applied to
the network-layer address for forwarding purposes. Client ('B') then
sets AcceptTime for the neighbor cache entry to ACCEPT_TIME.</t>
<t>After processing the message, Client ('B') prepares a Redirect
message response as follows:</t>
<t><list style="symbols">
<t>the link-layer source address is set to 'L2(B)' (i.e., the
link-layer address of Client ('B')).</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 ('B')).</t>
<t>the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the AERO address of Client ('A')).</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 ('B')).</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 a TLLAO with Link ID and Preference set
to appropriate values for Client ('B')'s underlying interface,
and with UDP Port Number and IP Address set to '0'.</t>
<t>the message SHOULD include a Timestamp option.</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 ('B') 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 ('B'),
it first verifies that the requested redirection is authorized. If
the redirection is not permitted, 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 ('B') 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 number and IP Address for Client ('B') into the
(previously empty) TLLAO.</t>
<t>Server ('S2') then examines the network-layer destination address
of the Predirect to determine the next hop toward Client ('A') by
searching for the AERO address in the neighbor cache. Since Client
('A') is not one of its neighbors, Server ('S2') re-encapsulates the
Predirect and relays it via Relay ('R') by changing the link-layer
source address of the message to 'L2(S2)' and changing the
link-layer destination address to 'L2(R)'. Server ('S2') finally
forwards the re-encapsulated message to Relay ('R') without
decrementing the network-layer TTL/Hop Limit field.</t>
<t>When Relay ('R') receives the Predirect message from Server
('S2') it determines that Server ('S1') is the next hop toward
Client ('A') by consulting its forwarding table. Relay ('R') then
re-encapsulates the Predirect while changing the link-layer source
address to 'L2(R)' and changing the link-layer destination address
to 'L2(S1)'. Relay ('R') then relays the Predirect via Server
('S1').</t>
<t>When Server ('S1') receives the Predirect message from Relay
('R') it determines that Client ('A') 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(A)'. Server
('S1') then forwards the message to Client ('A').</t>
</section>
<section anchor="processing_re" title="Processing Redirects">
<t>When Client ('A') 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 ('A') 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 ('A') then
processes the message as follows.</t>
<t>In the reference operational scenario, when Client ('A') receives
the Redirect message, it either creates or updates a neighbor cache
entry that stores the Target Address of the message as the
network-layer address of Client ('B'), stores the link-layer address
found in the TLLAO as the link-layer address of Client ('B') and
stores the Prefix Length as the length to be applied to the
network-layer address for forwarding purposes. Client ('A') then
sets ForwardTime for the neighbor cache entry to FORWARD_TIME.</t>
<t>Now, Client ('A') has a neighbor cache entry with a valid
ForwardTime value, while Client ('B') has a neighbor cache entry
with a valid AcceptTime value. Thereafter, Client ('A') may forward
ordinary network-layer data packets directly to Client ("B") without
involving any intermediate nodes, and Client ('B') can verify that
the packets came from an acceptable source. (In order for Client
('B') to forward packets to Client ('A'), 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 destination 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 3.9.6), 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 NUD by sending unicast NS messages to elicit
solicited NA messages from neighbors the same as described in <xref
target="RFC4861"/>. When an AERO node sends an NS/NA message, it MUST
use its AERO 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
test the direct path by sending an initial NS message to elicit a
solicited NA response. While testing the path, the source Client can
optionally continue sending packets via the Server, maintain a small
queue of packets until target reachability is confirmed, or
(optimistically) allow packets to flow directly to the target. The
source Client SHOULD thereafter continue to test the direct path to
the target Client (see Section 7.3 of <xref target="RFC4861"/>)
periodically in order to keep 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 the Server which may or may not
result in a new redirection event. 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.</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 neighbor cache entry expires, the source
Client resumes sending any subsequent packets via the Server and may
(eventually) attempt to re-initiate the AERO redirection process. When
AcceptTime for a neighbor cache entry expires, the target Client
discards any subsequent packets received directly from the source
Client. When both ForwardTime and AcceptTime for a 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 Renew/Reply via
each of its Servers using the new link-layer address as the source.
The DHCPv6 Server will re-authenticate the Client and (assuming
authentication succeeds) the DHCPv6 Renew/Reply exchange will update
each Server's neighbor cache.</t>
<t>Next, the Client sends unsolicited NA messages to each of its
active 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"/>, Client
('B') sends unicast unsolicited NA messages to Client ('A') via
Server ('S2') as follows:</t>
<t><list style="symbols">
<t>the link-layer source address is set to 'L2(B)' (i.e., the
link-layer address of Client ('B')).</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 ('B')).</t>
<t>the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the AERO address of Client ('A')).</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 ('B')).</t>
<t>the message includes a TLLAO with Link ID and Preference set
to appropriate values for Client ('B')'s underlying interface,
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 3.12.7. In particular, Server ('S1') copies the correct
UDP port number and IP address into the TLLAO, changes the
link-layer source address to its own address, changes the link-layer
destination address to the address of Relay ('R'), then forwards the
NA message via the relaying chain the same as for a Redirect.</t>
<t>When Client ('A') receives the NA message, it accepts the message
only if it already has a neighbor cache entry for Client ('B') then
updates the link-layer address for Client ('B') based on the address
in the TLLAO. However, Client ('A') MUST NOT update ForwardTime
since Client ('B') will not have updated AcceptTime.</t>
<t>Note that these unsolicited NA messages are unacknowledged;
hence, Client ('B') has no way of knowing whether Client ('A') has
received them. If the messages are somehow lost, however, Client
('A') will soon learn of the mobility event via the NUD procedures
specified in Section 3.13.</t>
</section>
<section title="Moving to a New Server">
<t>When a Client associates with a new Server, it issues a new
DHCPv6 Renew message via the new Server as the DHCPv6 relay. The new
Server then relays the message to the DHCPv6 server and processes
the resulting exchange. After the Client receives the resulting
DHCPv6 Reply message, it sends an RS message to the new Server to
receive a new RA message.</t>
<t>When a Client disassociates with an existing Server, it sends a
"terminating RS" message to the old Server. The terminating RS
message is prepared exactly the same as for an ordinary RS message,
except that the Code field contains the value '1'. When the old
Server receives the terminating RS message, it withdraws the IP
route from the routing system and deletes the neighbor cache entry
for the Client. The old Server then returns an RA message with
default router lifetime set to 0 which the Client can use to verify
that the termination signal has been processed. The client then
deletes both the default route and the neighbor cache entry for the
old Server. The old Server SHOULD 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.</t>
<t>Clients SHOULD NOT move rapidly between Servers in order to avoid
causing unpredictable oscillations in the Server/Relay 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 may 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 the AERO link does 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
Predirect/Redirect 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>
<section anchor="implement" title="Implementation Status">
<t>An application-layer implementation is in progress.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>The 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>The IANA is further 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. AERO also uses DHCPv6 authentication for
Client authentication and network admission control.</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 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, Fred Baker, Stewart Bryant, Brian Carpenter,
Wojciech Dec, Brian Haberman, Joel Halpern, Sascha Hlusiak, Lee Howard,
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, 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"?>
</references>
</back>
</rfc>
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