Asymmetric Extended Route Optimization (AERO)Boeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftThis document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). Nodes attached to
AERO links can exchange packets via trusted intermediate routers that
provide forwarding services to reach off-link destinations and
redirection services for route optimization. AERO provides an IPv6
link-local address format known as the AERO address that supports
operation of the IPv6 Neighbor Discovery (ND) protocol and links IPv6 ND
to IP forwarding. Admission control, provisioning and mobility are
supported by the Dynamic Host Configuration Protocol for IPv6 (DHCPv6),
and route optimization is naturally supported through dynamic neighbor
cache updates. Although DHCPv6 and IPv6 ND messaging are used in the
control plane, both IPv4 and IPv6 are supported in the data plane. AERO
is a widely-applicable tunneling solution using standard control
messaging exchanges as described in this document.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 .AERO provides an IPv6 link-local address format known as the AERO
address that supports operation of the IPv6 Neighbor Discovery (ND)
protocol and links IPv6 ND to IP forwarding.
Admission control, provisioning and mobility are supported by the
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) , and route optimization is naturally supported
through dynamic neighbor cache updates. Although DHCPv6 and IPv6 ND
messaging are used in the control plane, both IPv4 and IPv6 can be used
in the data plane. AERO is a widely-applicable tunneling solution using
standard control messaging exchanges as described in this document. The
remainder of this document presents the AERO specification.The terminology in the normative references applies; the following
terms are defined within the scope of this document: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.a node's attachment to an AERO
link. Nodes typically have a single AERO interface; support for
multiple AERO interfaces is also possible but out of scope for this
document.an IPv6 link-local address
constructed as specified in and
assigned to a Client's AERO interface.a node that is connected to an AERO
link and that participates in IPv6 ND and DHCPv6 messaging over the
link.a node that
issues DHCPv6 messages using the special IPv6 link-local address
'fe80::ffff:ffff:ffff:ffff' to receive IP Prefix Delegations (PD)
from one or more AERO Servers. Following PD, the Client assigns an
AERO address to the AERO interface which it uses in IPv6 ND
messaging to coordinate with other AERO nodes.a node that
configures an AERO interface to provide default forwarding and
DHCPv6 services for AERO Clients. The Server assigns an
administratively provisioned IPv6 link-local unicast address to
support the operation of DHCPv6 and the IPv6 ND protocol. An AERO
Server can also act as an AERO Relay.a node that
configures an AERO interface to relay IP packets between nodes on
the same AERO link and/or forward IP packets between the AERO link
and the native Internetwork. The Relay assigns an administratively
provisioned IPv6 link-local unicast address to the AERO interface
the same as for a Server. An AERO Relay can also act as an AERO
Server.a
node that performs data plane forwarding services as a companion to
an AERO Server.an AERO
interface endpoint that injects tunneled packets into an AERO
link.an AERO
interface endpoint that receives tunneled packets from an AERO
link.a connected IPv6 or IPv4
network routing region over which the tunnel virtual overlay is
configured. A typical example is an enterprise network.an AERO node's interface
point of attachment to an underlying network.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.the source or
destination address of the encapsulated IP packet.an internal virtual or
external edge IP network that an AERO Client connects to the rest of
the network via the AERO interface.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).a more-specific IP
prefix taken from an ASP and delegated to a Client.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 .The terminology of (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).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 .
Lower case uses of these words are not to be interpreted as carrying
RFC2119 significance.The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links: presents the AERO link
reference model. In this model: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
InternetworkServers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.Clients C1 and C2 associate with Servers S1 and S2,
respectively and also act as default routers for their associated
EUNsHosts H1 and H2 attach to the EUNs served by Clients C1 and C2,
respectivelyEach node maintains a neighbor cache and IP forwarding table.
(For example, AERO Relay R1 in the diagram has neighbor cache entries
for Servers S1 and S2 and IP forwarding table entries for ACPs H1 and
H2.) In common operational practice, there may be many additional
Relays, Servers and Clients. (Although not shown in the figure, AERO
Forwarding Agents may also be provided for data plane forwarding
offload services.)AERO Relays provide default forwarding services to AERO Servers.
Relays forward packets between Servers connected to the same AERO link
and also forward packets between the AERO link and the native IP
Internetwork. Relays present the AERO link to the native Internetwork
as a set of one or more AERO Service Prefixes (ASPs) and serve as a
gateway between the AERO link and the Internetwork. AERO Relays
maintain an AERO interface neighbor cache entry for each AERO Server,
and maintain an IP forwarding table entry for each AERO Client Prefix
(ACP). AERO Relays can also be configured to act as AERO Servers.AERO Servers provide default forwarding services to AERO Clients.
Each Server also peers with each Relay in a dynamic routing protocol
instance to advertise its list of associated ACPs. Servers configure a
DHCPv6 server function to facilitate Prefix Delegation (PD) exchanges
with Clients. Each delegated prefix becomes an ACP taken from an ASP.
Servers forward packets between AERO interface neighbors only, i.e.,
and not between the AERO link and the native IP Internetwork. AERO
Servers maintain an AERO interface neighbor cache entry for each AERO
Relay. They also maintain both a neighbor cache entry and an IP
forwarding table entry for each of their associated Clients. AERO
Servers can also be configured to act as AERO Relays.AERO Clients act as requesting routers to receive ACPs through
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, load balancing, etc.) Each IPv6 Client receives at
least a /64 IPv6 ACP, and may receive even shorter prefixes.
Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
singleton IPv4 address), and may receive even shorter prefixes. AERO
Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.AERO Clients 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, i.e., and not to the AERO
interface.AERO Forwarding Agents provide data plane forwarding services as
companions to AERO Servers. Note that while Servers are required to
perform both control and data plane operations on their own behalf,
they may optionally enlist the services of special-purpose Forwarding
Agents to offload data plane traffic.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:fe80::[ACP]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:2001:db8:1000:2000::/56it constructs its AERO address as:fe80::2001:db8:1000:2000For IPv4, the AERO address is formed from the lower 64 bits
of an IPv4-mapped IPv6 address that includes
the base prefix taken from the Client's IPv4 ACP. For example, if the
AERO Client receives the IPv4 ACP:192.0.2.32/28it constructs its AERO address as:fe80::FFFF:192.0.2.32The AERO address remains stable as the Client moves between
topological locations, i.e., even if its link-layer addresses
change.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.AERO interfaces use encapsulation (see )
to exchange packets with neighbors attached to the AERO link. AERO
interfaces maintain a neighbor cache, and AERO Clients and Servers use
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 ). 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.AERO interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in :In this format, Link ID is an integer value between 0 and 255
corresponding to an underlying interface of the target node, NDSCPs
encodes an integer value between 1 and 64 indicating the number of
Differentiated Services Code Point (DSCP) octets that follow. Each
DSCP octet is a 6-bit integer DSCP value followed by a 2-bit
Preference ("Prf") value. Each DSCP value encodes an integer between 0
and 63 associated with this Link ID, where the value 0 means "default"
and other values are interpreted as specified in . The 'Prf' qualifier for each DSCP value is set to
the value 0 ("deprecated'), 1 ("low"), 2 ("medium"), or 3 ("high") to
indicate a preference level for packet forwarding purposes. 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 .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.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.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.When an administrative authority first deploys a set of AERO Relays
and Servers that comprise an AERO link, they also assign a unique
domain name for the link, e.g., "linkupnetworks.example.com". Next, if
administrative policy permits Clients within the domain to serve as
correspondent nodes for Internet mobile nodes, the administrative
authority adds a Fully Qualified Domain Name (FQDN) for each of the
AERO link's ASPs to the Domain Name System (DNS) . The FQDN is based on the suffix
"aero.linkupnetworks.net" with a prefix formed from the
wildcard-terminated reverse mapping of the ASP , and resolves to a DNS PTR
resource record. For example, for the ASP '2001:db8:1::/48' within the
domain name "linkupnetworks.example.com", the DNS database
contains:'*.1.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net. PTR
linkupnetworks.example.com'This DNS registration advertises the AERO link's ASPs to
prospective correspondent nodes.When a Relay enables an AERO interface, it first assigns an
administratively provisioned link-local address fe80::ID to the
interface. Each fe80::ID address MUST be unique among all AERO nodes
on the link, and MUST NOT collide with any potential AERO addresses
nor the special addresses fe80:: and fe80::ffff:ffff:ffff:ffff. (The
fe80::ID addresses are typically taken from the available range
fe80::/96, e.g., as fe80::1, fe80::2, fe80::3, etc.) The Relay then
engages in a dynamic routing protocol session with all Servers on
the link (see: ), and advertises its
assigned ASP prefixes into the native IP Internetwork.Each Relay subsequently maintains an IP forwarding table entry
for each Client-Server association, and maintains a neighbor cache
entry for each Server on the link. Relays exchange NS/NA messages
with AERO link neighbors the same as for any AERO node, however they
typically do not perform explicit Neighbor Unreachability Detection
(NUD) (see: ) since the dynamic routing protocol
already provides reachability confirmation.When a Server enables an AERO interface, it assigns an
administratively provisioned link-local address fe80::ID the same as
for Relays. The Server further configures a DHCPv6 server function
to facilitate DHCPv6 PD exchanges with AERO Clients. The Server
maintains a neighbor cache entry for each Relay on the link, and
manages per-Client neighbor cache entries and IP forwarding table
entries based on control message exchanges. Each Server also engages
in a dynamic routing protocol with each Relay on the link (see:
).When the Server receives an NS/RS message from a Client on the
AERO interface it returns an NA/RA message but does not update the
neighbor cache. The Server further provides a simple conduit between
AERO interface neighbors. Therefore, packets enter the Server's AERO
interface from the link layer and are forwarded back out the link
layer without ever leaving the AERO interface and therefore without
ever disturbing the network layer.When a Client enables an AERO interface, it uses the special
address fe80::ffff:ffff:ffff:ffff to obtain an ACP from an AERO
Server via DHCPv6 PD. 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 autoconfiguration 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 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, 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 .)When a Forwarding Agent enables an AERO interface, it assigns the
same link-local address(es) as the companion AERO Server. The
Forwarding Agent thereafter provides data plane forwarding services
based solely on the forwarding information assigned to it by the
companion AERO Server.Relays require full topology knowledge of all ACP/Server
associations for the ASPs they service, while individual Servers at a
minimum only need to know the ACPs for their current set of associated
Clients. This is accomplished through the use of an internal instance
of the Border Gateway Protocol (BGP)
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.In a reference BGP arrangement, each AERO Server is configured as
an Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP instance,
and each Server further peers with each Relay but does not peer with
other Servers. Similarly, Relays do not peer with each other, since
they will reliably receive all updates from all Servers and will
therefore have a consistent view of the AERO link ACP delegations.Each Server maintains a working set of associated ACPs, and
dynamically announces new ACPs and withdraws departed ACPs in its BGP
updates to Relays. Clients are expected to remain associated with
their current Servers for extended timeframes, however Servers SHOULD
selectively suppress BGP updates for impatient Clients that repeatedly
associate and disassociate with them in order to dampen routing
churn.Each Relay configures a black-hole route for each ASP associated
with the AERO link. By black-holing the ASPs, the Relay will maintain
active forwarding table entries only for the ACPs that are currently
active, and all other ACPs will correctly result in destination
unreachable failures due to the black hole route.Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. Assuming O(10^6)
as a reasonable maximum number of BGP routes, this means that O(10^6)
Clients can be serviced by a single set of Relays. A means of
increasing scaling would be to assign a different set of Relays for
each set of ASPs. In that case, each Server still peers with each
Relay, but the Server institutes route filters so that each set of
Relays only receives BGP updates for the ACPs they aggregate. For
example, if the ASP for the AERO link is 2001:db8::/32, a first set of
Relays could service the ASP segment 2001:db8::/40, a second set of
Relays could service 2001:db8:0100::/40, a third set could service
2001:db8:0200::/40, etc.Assuming up to O(10^3) sets of Relays, the system can then
accommodate O(10^9) Clients with no additional overhead for Servers
and Relays. In this way, each set of Relays services a specific set of
ASPs that they advertise to the native routing system outside of the
AERO link, and each Server configures ASP-specific routes that list
the correct set of Relays as next hops. This arrangement also allows
for natural incremental deployment, and can support small scale
initial deployments followed by dynamic deployment of additional
Clients, Servers and Relays without disturbing the already-deployed
base.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 .
AERO interface neighbor cache entires are said to be one of
"permanent", "static" or "dynamic".Permanent neighbor cache entries are created through explicit
administrative action; they have no timeout values and remain in place
until explicitly deleted. AERO Relays maintain a permanent neighbor
cache entry for each Server on the link, and AERO Servers maintain a
permanent neighbor cache entry for each Relay. Each entry maintains
the mapping between the neighbor's fe80::ID network-layer address and
corresponding link-layer address.Static neighbor cache entries are created though DHCPv6 PD
exchanges and remain in place for durations bounded by prefix
lifetimes. AERO Servers maintain static neighbor cache entries for the
ACPs of each of their associated Clients, and AERO Clients maintain a
static neighbor cache entry for each of their associated Servers. When
an AERO Server sends a DHCPv6 Reply message response to a Client's
DHCPv6 Solicit/Request, Rebind or Renew message, it creates or updates
a static neighbor cache entry based on the AERO address corresponding
to the Client's ACP 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.Dynamic neighbor cache entries are created or updated based on
receipt of an IPv6 ND message, and are garbage-collected if not used
within a bounded timescale. AERO Clients maintain dynamic neighbor
cache entries for each of their active correspondent Client ACPs with
lifetimes based on IPv6 ND messaging constants. When an AERO Client
receives a valid Predirect message it creates or updates a dynamic
neighbor cache entry for the Predirect target network-layer and
link-layer addresses plus prefix length. The node then sets an
"AcceptTime" variable in the neighbor cache entry to ACCEPT_TIME
seconds and uses this value to determine whether packets received from
the correspondent can be accepted. When an AERO Client receives a
valid Redirect message it creates or updates a dynamic neighbor cache
entry for the Redirect target network-layer and link-layer addresses
plus prefix length. The Client then sets a "ForwardTime" variable in
the neighbor cache entry to FORWARD_TIME seconds and uses this value
to determine whether packets can be sent directly to the
correspondent. The Client also sets a "MaxRetry" variable to MAX_RETRY
to limit the number of keepalives sent when a correspondent may have
gone unreachable.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.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 .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.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 .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.IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system), or
from the link layer (i.e., from the AERO tunnel virtual link). Packets
that enter the AERO interface from the network layer are encapsulated
and admitted into the AERO link, i.e., they are tunnelled to an AERO
interface neighbor. Packets that enter the AERO interface from the
link layer are either re-admitted into the AERO link or delivered to
the network layer where they are subject to either local delivery or
IP forwarding. Since each AERO node may have only partial information
about neighbors on the link, AERO interfaces may forward packets with
link-local destination addresses at a layer below the network layer.
This means that AERO nodes act as both IP routers and sub-IP layer
forwarding agents. AERO interface sending considerations for Clients,
Servers and Relays are given below.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.When an IP packet enters a Server's AERO interface from the link
layer, if the destination is covered by an ASP the Server searches for
a neighbor cache entry with an AERO address that matches the packet's
destination address. (The destination address may be either an address
covered by the neighbor's ACP or the AERO address itself.) If there is
a match, the Server uses a link-layer address in the entry as the
link-layer address for encapsulation and re-admits the packet into the
AERO link. If there is no match, the Server instead uses the
link-layer address in a permanent neighbor cache entry for a Relay as
the link-layer address for encapsulation.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 a permanent neighbor
cache entry for a 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 does not match an ASP the Relay removes the
packet from the AERO interface and uses IP forwarding to forward the
packet to the Internetwork. If the destination address is a link-local
address or a non-link-local address that matches an ASP, and there is
a more-specific ACP entry in the IP forwarding table, the Relay uses
the link-layer address in the corresponding neighbor cache entry as
the link-layer address for encapsulation and re-admits the packet into
the AERO link. When an IP packet enters a Relay's AERO interface from
either the network layer or link-layer, and the packet's destination
address matches an ASP but there is no more-specific ACP entry, the
Relay drops the packet and returns an ICMP Destination Unreachable
message (see: ).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.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.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.When an AERO node forwards a data packet to the primary link-layer
address of a Server, it may receive Redirect messages with an SLLAO
that include the link-layer address of an AERO Forwarding Agent. The
AERO node SHOULD record the link-layer address in the neighbor cache
entry for the neighbor and send subsequent data packets via this
address instead of the Server's primary address (see: ).AERO interfaces encapsulate IP packets according to whether they
are entering the AERO interface from the network layer or if they are
being re-admitted into the same AERO link they arrived on. This latter
form of encapsulation is known as "re-encapsulation".The AERO interface encapsulates packets per the Generic UDP
Encapsulation (GUE) encapsulation procedures in . During encapsulation, the
AERO 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. (When IPv6 is
used as the encapsulation protocol, the interface also sets the Flow
Label value in the encapsulation header per .)
For packets undergoing re-encapsulation, the AERO interface instead
copies the "TTL/Hop Limit", "Type of Service/Traffic Class", "Flow
Label" and "Congestion Experienced" values in the original
encapsulation IP header into the corresponding fields in the new
encapsulation IP header, i.e., the values are transferred between
encapsulation headers and *not* copied from the encapsulated packet's
network-layer header.The AERO interface next sets the UDP source port to a constant
value that it will use in each successive packet it sends, and sets
the UDP length field to the length of the encapsulated packet plus 8
bytes for the UDP header itself, plus the length of the GUE header.
For packets sent 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 per the procedures specified in .The AERO interface next sets the IP protocol number in the
encapsulation header to 17 (i.e., the IP protocol number for UDP).
When IPv4 is used as the encapsulation protocol, the AERO interface
sets the DF bit as discussed in .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. Decapsulation is per the
procedures specified in .AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:AERO Relays and Servers accept encapsulated packets with a
link-layer source address that matches a permanent neighbor cache
entry.AERO Servers accept authentic encapsulated DHCPv6 messages from
Clients, and create or update a static neighbor cache entry for
the source based on the specific message type.AERO Servers accept encapsulated packets if there is a 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.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.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.Note that this simple data origin authentication is effective
in environments in which link-layer addresses cannot be spoofed. In
other environments, each AERO message must include a signature that
the recipient can use to authenticate the message origin.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 (i.e., 64KB-ENCAPS), since the
maximum packet size in the base IP specifications is 64KB . While IPv6 jumbograms can
be up to 4GB, they are considered optional for IPv6 nodes and therefore out of scope
for this document.AERO interfaces are considered to have an indefinite MTU , i.e.,
instead of clamping the MTU to a finite size. The interface MTU is
therefore constrained by the minimum of (64KB-ENCAPS) and the path MTU
within the tunnel.IPv6 specifies a minimum link MTU of 1280 bytes . 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 , AERO interfaces also observe a 1280 byte minimum
for IPv4.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 resulting in failure of the IP Path MTU Discovery
(PMTUD) mechanisms .
For these reasons, the source AERO interface (i.e., the tunnel
ingress) sends encapsulated packets to the destination AERO interface
(i.e., the tunnel egress) subject to operational expectation that
PMTUD will convey the correct information to the original source in
the event that the packet is too large.When the original source, ingress and egress are all within the
same well-managed administrative domain, the ingress admits a packet
into the tunnel if it is not too large to traverse the tunnel in one
piece. Otherwise, the ingress drops the packet and sends a PTB message
back to the original source. Additionally, the ingress can translate
PTB messages received from a router on the path to the egress and/or
cache the MTU value reported for future reference. These procedures
apply when the path MTU from the ingress to egress is no smaller than
(1280+ENCAPS) bytes. Otherwise, the ingress can either shut down the
tunnel or begin fragmenting packets that are no larger than 1280 bytes
but larger than the path MTU minus ENCAPS using tunnel fragmentation
as described in the following paragraphs. This exactly parallels the
behavior specified in .When the original source, ingress and egress are not all within the
same well-managed administrative domain, the ingress admits all
packets up to 1500 bytes in length even if some fragmentation is
necessary, and admits larger packets without fragmentation in case
they are able to traverse the tunnel in one piece. The use of
fragmentation entails additional considerations as described
below.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 (see: ). 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 through insertion of a
GUE fragment header and
application of tunnel fragmentation as described in Section 3.1.7 of
. Since the GUE 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.The tunnel ingress therefore sends encapsulated packets to the
tunnel egress according to the following algorithm: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, the 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.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 GUE fragment header. Next, the tunnel ingress
fragments 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 IP/UDP/GUE encapsulation headers 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. When
the tunnel egress receives the fragments, it reassembles them into
a whole packet. 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.For IPv4 packets that are larger than 1500 bytes and with the
DF bit set to 0, the tunnel ingress uses ordinary IPv4
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 fragments will be delivered to the final
destination via the tunnel egress.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 . 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) .While sending packets according to the above algorithm, the
tunnel ingress MAY also send 1500 byte or larger probe packets to
determine whether they can reach the tunnel egress without
fragmentation. If the probes succeed, the tunnel ingress can
discontinue fragmentation and (for IPv4) set DF 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.To construct a probe, the tunnel ingress can prepare an NS message
with a Nonce option plus trailing NULL padding octets added to the
probe length 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. (Note that other means
of probing such as sending a self-addressed data packet that will be
looped back if received by the egress are also possible.)When probing is used, it is essential that probes follow the same
paths used to convey actual data packets to avoid path variations due
to Equal Cost MultiPath (ECMP) and Link Aggregation Gateway (LAG)
equipment in the path. In order to ensure that probes follow the same
paths as data packets, the tunnel ingress sets both the Differentiated
Service Code Point (DSCP) and (for IPv6) Flow
Label fields to a constant value during
encapsulation instead of copying these values from the original
packet. In that case, it is RECOMMENDED that these fields be set to
the value 0.Control messages (i.e., IPv6 ND, DHCPv6, etc.) 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:For control messages that are larger than (1280-ENCAPS)
bytes, the tunnel ingress encapsulates the packet and inserts a
GUE fragment header. Next, the tunnel ingress uses the
fragmentation algorithm in 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.Control messages that exceed the 2KB minimum reassembly
size 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 control
messages become more common in the future.The tunnel ingress MAY send large control messages without
fragmentation if there is assurance that large packets can traverse
the tunnel without fragmentation.The tunnel ingress MAY send 1500 byte or larger probe packets as
specified in the previous section to determine a size for which
fragmentation can be avoided.When fragmentation is needed, there must be assurance that
reassembly can be safely conducted without incurring data
corruption. Sources of corruption can include implementation errors,
memory errors and misassociations of fragments from a first datagram
with fragments of another datagram. The first two conditions
(implementation and memory errors) are mitigated by modern systems
and implementations that have demonstrated integrity through decades
of operational practice. The third condition (reassembly
misassociations) must be accounted for by AERO.The AERO fragmentation procedure described in the above
algorithms reuses standard IPv6 fragmentation and reassembly code.
Since the GUE 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, 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 wireless data links commonly used by AERO Clients
support vastly lower data rates, the aggregate data rates between
AERO Servers and Relays may be substantial. However, high speed data
links in the network core are expected to configure larger MTUs,
e.g., 4KB, 8KB or even larger such that unfragmented packets can be
used. Hence, no integrity check is included to cover fragmentation
and reassembly procedures.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 . 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 due to misconfigurations.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, 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.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.When an AERO node admits encapsulated packets into the AERO
interface, it may receive link-layer (L2) or network-layer (L3) error
indications.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.The IP header is followed by an ICMP header that includes an error
Type, Code and Checksum. For ICMPv6 , the
error Types include "Destination Unreachable", "Packet Too Big (PTB)",
"Time Exceeded" and "Parameter Problem". For ICMPv4 , the error Types include "Destination Unreachable",
"Fragmentation Needed" (a Destination Unreachable Code that is
analogous to the ICMPv6 PTB), "Time Exceeded" and "Parameter
Problem".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, 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, specifies that the
packet-in-error includes: "Internet Header + 64 bits of Original Data
Datagram", however Section 4.3.2.3 updates
this specification by stating: "the ICMP datagram SHOULD contain as
much of the original datagram as possible without the length of the
ICMP datagram exceeding 576 bytes".The L2 error message format is shown in :The AERO node rules for processing these L2 error messages
is as follows: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 .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: ).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 ). 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.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.When an AERO Server receives persistent L2 Destination
Unreachable messages in response to tunneled packets that it sends
to one of its static neighbor Clients, the Server SHOULD test the
path to the Client using NUD. If NUD fails, the Server SHOULD
cancel the DHCPv6 PD for the Client's ACP, withdraw its route for
the ACP from the AERO routing system and delete the neighbor cache
entry (see and ).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.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.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.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: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.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.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.(*) 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.Each AERO Server configures a DHCPv6 server function to
facilitate PD requests from Clients. Each Server is provisioned with
a database of ACP-to-Client ID mappings for all Clients enrolled in
the AERO system, as well as any information necessary to
authenticate each Client. The Client database is maintained by a
central administrative authority for the AERO link and securely
distributed to all Servers, e.g., via the Lightweight Directory
Access Protocol (LDAP) or a similar
distributed database service.Therefore, no Server-to-Server DHCPv6 PD delegation state
synchronization is necessary, and Clients can optionally hold
separate delegations for the same ACP from multiple Servers. In this
way, Clients can associate with multiple Servers, and can receive
new delegations from new Servers before deprecating delegations
received from existing Servers.AERO Clients and Servers exchange Client link-layer address
information using an option format similar to the Client Link Layer
Address Option (CLLAO) defined in . Due to
practical limitations of CLLAO, however, AERO interfaces instead use
Vendor-Specific Information Options as described in the following
sections.AERO Clients discover the link-layer addresses of AERO Servers
via static configuration, or through an automated means such as DNS
name resolution. In the absence of other information, the Client
resolves the FQDN "linkupnetworks.[domainname]" where
"linkupnetworks" is a constant text string and "[domainname]" is the
connection-specific DNS suffix for the Client's underlying network
connection (e.g., "example.com"). After discovering the link-layer
addresses, the Client associates with one or more of the
corresponding Servers.To associate with a Server, the Client acts as a requesting
router to request an ACP through a two-message (i.e., Solicit/Reply)
or four-message (i.e., Solicit/Advertise/Request/Reply) DHCPv6 PD
exchange . The
Client's Solicit/Request message includes fe80::ffff:ffff:ffff:ffff
as the IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as
the IPv6 destination address and the link-layer address of the
Server as the link-layer destination address. The Solicit/Request
message also includes a Rapid Commit option, a Client Identifier
option with a DHCP Unique Identifier (DUID) and an Identity
Association for Prefix Delegation (IA_PD) option. If the Client is
pre-provisioned with an ACP associated with the AERO service, it MAY
also include the ACP in the IA_PD to indicate its preference to the
DHCPv6 server.The Client also SHOULD include an AERO Link-registration Request
(ALREQ) option to register one or more links with the Server. The
Server will include an AERO Link-registration Reply (ALREP) option
in the corresponding DHCPv6 Reply message as specified in . (The Client MAY omit the ALREQ option, in
which case the Server will still include an ALREP option in its
Reply with "Link ID" set to 0, "DSCP" set to 0, and "Prf" set to
3.)The format for the ALREQ option is shown in :In the above format, the Client sets 'option-code' to
OPTION_VENDOR_OPTS, sets 'option-len (1)' to the length of the
option following this field, sets 'enterprise-number' to 45282 (see:
"IANA Considerations"), sets opt-code to the value 0
("OPTION_ALREQ") and sets 'option-len (2)' to the length of the
remainder of the option. The Client includes appropriate 'Link ID,
'DSCP' and 'Prf' values for the underlying interface over which the
DHCPv6 PD Solicit/Request message will be issued the same as
specified for an S/TLLAO . The Client MAY
include multiple (DSCP, Prf) values with this Link ID, with the
number of values indicated by option-len (2). The Server will
register each value with the Link ID in the Client's neighbor cache
entry. The Client finally includes any necessary authentication
options to identify itself to the DHCPv6 server, and sends the
encapsulated DHCPv6 PD Solicit/Request message via the underlying
interface corresponding to Link ID. (Note that this implies that the
Client must perform additional Rebind/Reply DHCPv6 exchanges with
the server following the initial PD exchange using different
underlying interfaces and their corresponding Link IDs if it wishes
to register additional link-layer addresses and their associated
DSCPs.)When the Client receives its ACP 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
considers the link-layer address of the Server as the primary
default encapsulation address for forwarding packets for which no
more-specific forwarding information is available. The Client
further caches any ASPs included in the ALREP option as ASPs to
apply to the AERO link.Next, the Client autoconfigures an AERO address from the
delegated ACP, assigns the AERO address to the AERO interface and
sub-delegates the ACP to its attached EUNs and/or the Client's own
internal virtual interfaces. 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 Client can then determine the correct next
hops for packets submitted to the AERO interface by inspecting the
neighbor cache.The Client subsequently renews its ACP delegation through each of
its Servers by performing DHCPv6 Renew/Reply exchanges with the
link-layer address of a Server as the link-layer destination address
and the same options that were used in the initial PD request. Note
that if the Client does not issue a DHCPv6 Renew before the
delegation expires (e.g., if the Client has been out of touch with
the Server for a considerable amount of time) it must re-initiate
the DHCPv6 PD procedure.Since the Client's AERO address is obtained 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).When a Client attempts to perform a DHCPv6 PD exchange with a
Server that is too busy to service the request, the Client may
receive a "NoPrefixAvail" status code in the Server's Reply per
. In that case, the Client SHOULD
discontinue DHCPv6 PD attempts through this Server and try another
Server.On some platforms (e.g., popular cell phone operating systems),
the act of assigning a default IPv6 route and/or assigning an
address to an interface may not be permitted from a user
application due to security policy. Typically, those platforms
include a TUN/TAP interface 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 and is prepared as
follows:the IPv6 source address is the Client's AERO addressthe IPv6 destination address is all-nodes multicastthe Router Lifetime is set to a time that is no longer than
the ACP DHCPv6 lifetimethe message does not include a Source Link Layer Address
Option (SLLAO)the message includes a Prefix Information Option (PIO) with
a /64 prefix taken from the ACP as the prefix for
autoconfigurationThe 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 .In the initial DHCPv6 PD message exchanges, AERO Clients use
the special IPv6 source address 'fe80::ffff:ffff:ffff:ffff' since
their AERO addresses are not yet configured. After AERO address
autoconfiguration, however, AERO Clients can either continue to
use 'fe80::ffff:ffff:ffff:ffff' as the source address for further
DHCPv6 messaging or begin using their AERO address as the source
address.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.When an AERO Server receives a prospective Client's DHCPv6 PD
Solicit/Request on its AERO interface, and the Server is too busy to
service the message, it returns a Reply with status code
"NoPrefixAvail" per . Otherwise, the Server
authenticates the message.If authentication succeeds, the Server determines the correct ACP
to delegate to the Client by searching the Client database. In
environments where spoofing is not considered a threat, the Server
MAY use the Client's DUID as the identification value. Otherwise,
the Server SHOULD use a signed certificate provided by the
Client.When the Server delegates the ACP, it also creates an IP
forwarding table entry so that the AERO routing system will
propagate the ACP to all Relays that aggregate the corresponding ASP
(see: ). Next, the Server prepares a DHCPv6
Reply message to send to the Client while using fe80::ID as the IPv6
source address, the link-local address taken from the Client's
Solicit/Request as the IPv6 destination address, the Server's
link-layer address as the source link-layer address, and the
Client's link-layer address as the destination link-layer address.
The server also includes an IA_PD option with the delegated ACP.
Since the Client may experience a fault that prevents it from
issuing a DHCPv6 Release before departing from the network, Servers
should set a short prefix lifetime (e.g., 40 seconds) so that stale
prefix delegation state can be flushed out of the network.The Server also includes an ALREP option that includes the UDP
Port Number and IP Address values it observed when it received the
ALREQ in the Client's original DHCPv6 message (if present) followed
by the ASP(s) for the AERO link. The ALREP option is formatted as
shown in :In the ALREP, the Server sets 'option-code' to
OPTION_VENDOR_OPTS, sets 'option-length (1)' to the length of the
option, sets 'enterprise-number' to 45282 (see: "IANA
Considerations"), sets opt-code to OPTION_ALREP (1), and sets
'option-len (2)' to the length of the remainder of the option. Next,
the Server sets 'Link ID' to the same value that appeared in the
ALREQ, sets Reserved to 0 and sets 'UDP Port Number' and 'IP
address' to the Client's link-layer address. The Server next
includes one or more ASP with the IP prefix as it would appear in
the interface identifier portion of the corresponding AERO address
(see: ), except that the low-order 8
bits of the ASP field encode the prefix length instead of the
low-order 8 bits of the prefix. The longest prefix that can
therefore appear as an ASP is /56 for IPv6 or /24 for IPv4. (Note
that if the Client did not include an ALREQ option in its DHCPv6
message, the Server MUST still include an ALREP option in the
corresponding reply with 'Link ID' set to 0.)When the Server admits the DHCPv6 Reply message into the AERO
interface, it creates a static neighbor cache entry for the Client's
AERO address with lifetime set to no more than the delegation
lifetime and the Client's link-layer address as the link-layer
address for the Link ID specified in the ALREQ. The Server then uses
the Client link-layer address information in the ALREQ option as the
link-layer address for encapsulation based on the (DSCP, Prf)
information.After the initial DHCPv6 PD exchange, the AERO Server maintains
the neighbor cache entry for the Client until the delegation
lifetime expires. If the Client issues a Renew/Reply exchange, the
Server extends the lifetime. If the Client issues a Release/Reply,
or if the Client does not issue a Renew/Reply before the lifetime
expires, the Server deletes the neighbor cache entry for the Client
and withdraws the IP route from the AERO routing system.AERO Clients and Servers are always on the same link (i.e., the
AERO link) from the perspective of DHCPv6. However, in some
implementations the DHCPv6 server and AERO interface driver may be
located in separate modules. In that case, the Server's AERO
interface driver module acts as a Lightweight DHCPv6 Relay Agent
(LDRA) to relay DHCPv6 messages to
and from the DHCPv6 server module.When the LDRA receives a DHCPv6 message from a client, it
prepares an ALREP option the same as described above then wraps
the option in a Relay-Supplied DHCP Option option (RSOO) . The LDRA then incorporates the option into the
Relay-Forward message and forwards the message to the DHCPv6
server.When the DHCPv6 server receives the Relay-Forward message, it
caches the ALREP option and authenticates the encapsulated DHCPv6
message. The DHCPv6 server subsequently ignores the ALREQ option
itself, since the relay has already included the ALREP option.When the DHCPv6 server prepares a Reply message, it then
includes the ALREP option in the body of the message along with
any other options, then wraps the message in a Relay-Reply
message. The DHCPv6 server then delivers the Relay-Reply message
to the LDRA, which discards the Relay-Reply wrapper and delivers
the DHCPv6 message to the Client.After an AERO Client registers its Link IDs and their associated
(DSCP,Prf) values with the AERO Server, the Client may wish to
delete one or more Link registrations, e.g., if an underlying link
becomes unavailable. To do so, the Client prepares a DHCPv6 Rebind
message that includes an AERO Link-registration Delete (ALDEL)
option and sends the Rebind message to the Server over any available
underlying link. The ALDEL option is formatted as shown in :In the ALDEL, the Client sets 'option-code' to
OPTION_VENDOR_OPTS, sets 'option-length (1)' to the length of the
option, sets 'enterprise-number' to 45282 (see: "IANA
Considerations"), sets optcode to OPTION_ALDEL (2), and sets
'option-len (2)' to the length of the remainder of the option. Next,
the Server includes each 'Link ID' value that it wishes to
delete.If the Client wishes to discontinue use of a Server and thereby
delete all of its Link ID associations, it must use a DHCPv6
Release/Reply exchange to delete the entire neighbor cache entry,
i.e., instead of using a DHCPv6 Rebind/Reply exchange with one or
more ALDEL options.AERO Servers MAY associate with one or more companion AERO
Forwarding Agents as platforms for offloading high-speed data plane
traffic. When an AERO Server receives a Client's DHCPv6
Solicit/Request/Renew/Rebind/Release message, it services the message
then forwards the corresponding Reply message to the Forwarding Agent.
When the Forwarding Agent receives the Reply message, it creates,
updates or deletes a neighbor cache entry with the Client's AERO
address and link-layer information included in the Reply message. The
Forwarding Agent then forwards the Reply message back to the AERO
Server, which forwards the message to the Client. In this way,
Forwarding Agent state is managed in conjunction with Server state,
with the Client responsible for reliability. If the Client
subsequently disappears without issuing a Release, the Server is
responsible for purging stale state by sending synthesized Reply
messages to the Forwarding Agent.When an AERO Server receives a data packet on an AERO interface
with a network layer destination address for which it has distributed
forwarding information to a Forwarding Agent, the Server returns a
Redirect message to the source neighbor (subject to rate limiting)
then forwards the data packet as usual. The Redirect message includes
a TLLAO with the link-layer address of the Forwarding Engine.When the source neighbor receives the Redirect message, it SHOULD
record the link-layer address in the TLLAO as the encapsulation
addresses to use for sending subsequent data packets. However, the
source MUST continue to use the primary link-layer address of the
Server as the encapsulation address for sending control messages.When a source Client forwards packets to a prospective
correspondent Client within the same AERO link domain (i.e., one for
which the packet's destination address is covered by an ASP), the
source Client initiates an intra-domain AERO route optimization
procedure. It is important to note that this procedure is initiated by
the Client; if the procedure were initiated by the Server, the Server
would have no way of knowing whether the Client was actually able to
contact the correspondent over the route-optimized path.The procedure is based on an exchange of IPv6 ND messages using a
chain of AERO Servers and Relays as a trust basis. This procedure is
in contrast to the Return Routability procedure required for route
optimization to a correspondent Client located in the Internet as
described in . The following sections
specify the AERO intradomain route optimization procedure. depicts the AERO
intradomain route optimization reference operational scenario, using
IPv6 addressing as the example (while not shown, a corresponding
example for IPv4 addressing can be easily constructed). The figure
shows an AERO Relay ('R1'), two AERO Servers ('S1', 'S2'), two AERO
Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1', 'H2'):In , Relay ('R1') assigns
the address fe80::1 to its AERO interface with link-layer address
L2(R1), Server ('S1') assigns the address fe80::2 with link-layer
address L2(S1),and Server ('S2') assigns the address fe80::3 with
link-layer address L2(S2). Servers ('S1') and ('S2') next arrange to
add their link-layer addresses to a published list of valid Servers
for the AERO link.AERO Client ('C1') receives the ACP 2001:db8:0::/48 in a DHCPv6
PD exchange via AERO Server ('S1') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(C1). Client ('C1') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::2 and
link-layer address L2(S1), then sub-delegates the ACP to its
attached EUNs. IPv6 host ('H1') connects to the EUN, and configures
the address 2001:db8:0::1.AERO Client ('C2') receives the ACP 2001:db8:1::/48 in a DHCPv6
PD exchange via AERO Server ('S2') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C2). Client ('C2') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::3 and
link-layer address L2(S2), then sub-delegates the ACP to its
attached EUNs. IPv6 host ('H1') connects to the EUN, and configures
the address 2001:db8:1::1.Again, with reference to ,
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').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').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').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').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.AERO Redirect/Predirect messages use the same format as for
ICMPv6 Redirect messages depicted in Section 4.5 of , 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 :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.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 ). Client ('C1') prepares the
Predirect message as follows:the link-layer source address is set to 'L2(C1)' (i.e., the
link-layer address of Client ('C1')).the link-layer destination address is set to 'L2(S1)' (i.e.,
the link-layer address of Server ('S1')).the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('C1')).the network-layer destination address is set to
fe80::2001:db8:1:0 (i.e., the AERO address of Client
('C2')).the Type is set to 137.the Code is set to 1 to indicate "Predirect".the Prefix Length is set to the length of the prefix to be
assigned to the Target Address.the Target Address is set to fe80::2001:db8:0:0 (i.e., the
AERO address of Client ('C1')).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).the message includes one or more TLLAOs with Link ID and
DSCPs set to appropriate values for Client ('C1')'s underlying
interfaces, and with UDP Port Number and IP Address set to
0'.the message SHOULD include a Timestamp option and a Nonce
option.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.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').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 , 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.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.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').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').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 , 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.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.After processing the message, Client ('C2') prepares a Redirect
message response as follows:the link-layer source address is set to 'L2(C2)' (i.e., the
link-layer address of Client ('C2')).the link-layer destination address is set to 'L2(S2)' (i.e.,
the link-layer address of Server ('S2')).the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C2')).the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the AERO address of Client
('C1')).the Type is set to 137.the Code is set to 0 to indicate "Redirect".the Prefix Length is set to the length of the prefix to be
applied to the Target Address.the Target Address is set to fe80::2001:db8:1:0 (i.e., the
AERO address of Client ('C2')).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).the message includes one or more TLLAOs with Link ID and
DSCPs set to appropriate values for Client ('C2')'s underlying
interfaces, and with UDP Port Number and IP Address set to
'0'.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).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.After Client ('C2') prepares the Redirect message, it sends the
message to Server ('S2').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 . 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.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.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').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').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 , 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.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.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.)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: ), except
that it writes its own link-layer address in the TLLAO option. The
Server must then maintain a dynamic neighbor cache entry for the
redirected source Client.AERO nodes perform Neighbor Unreachability Detection (NUD) by
sending unicast NS messages to elicit solicited NA messages from
neighbors the same as described in . NUD is
performed either reactively in response to persistent L2 errors (see
) or proactively to refresh existing neighbor
cache entries.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.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 ) periodically in order to keep dynamic neighbor
cache entries alive.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.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.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.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.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 address and with an ALREQ that includes the correct Link
ID and DSCP 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 prefix delegation
lifetime expires (e.g., if the Client has been out of touch with the
Server for a considerable amount of time), the Server's Reply will
report NoBinding and the Client must re-initiate the DHCPv6 PD
procedure.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 , 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.With reference to , when
Client ('C2') needs to change its link-layer address it sends
unicast unsolicited NA messages to Client ('C1') via Server ('S2')
as follows:the link-layer source address is set to 'L2(C2)' (i.e., the
link-layer address of Client ('C2')).the link-layer destination address is set to 'L2(S2)' (i.e.,
the link-layer address of Server ('S2')).the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C2')).the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the AERO address of Client
('C1')).the Type is set to 136.the Code is set to 0.the Solicited flag is set to 0.the Override flag is set to 1.the Target Address is set to fe80::2001:db8:1:0 (i.e., the
AERO address of Client ('C2')).the message includes one or more TLLAOs with Link ID and
DSCPs set to appropriate values for Client ('C2')'s underlying
interfaces, and with UDP Port Number and IP Address set to
'0'.the message SHOULD include a Timestamp option.When Server ('S1') receives the NA message, it relays the message
in the same way as described for relaying Redirect messages in . 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.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. Next, Client ('C1') SHOULD initiate
the NUD procedures specified in to provide
Client ('C2') with an indication that the link-layer source address
has been updated, and to refresh ('C2')'s AcceptTime and ('C1')'s
ForwardTime timers.If Client ('C2') receives an NS message from Client ('C1')
indicating that an unsolicited NA has updated its neighbor cache,
Client ('C2') need not send additional unsolicited NAs. If Client
('C2')'s unsolicited NA messages are somehow lost, however, Client
('C1') will soon learn of the mobility event via NUD.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 an ALREQ that
includes the new Link ID and DSCP 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 .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 an ALDEL that includes the deprecated
Link ID. 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
.When a Client associates with a new Server, it performs the
Client procedures specified in .When a Client disassociates with an existing Server, it sends a
DHCPv6 Release message via a new Server to the unicast link-local
network layer address of the old Server. The new Server then writes
its own link-layer address in the DHCPv6 Release message IP source
address and forwards the message to the old Server.When the old Server receives the DHCPv6 Release, it first
authenticates the message. The Server then resets the Client's
neighbor cache entry lifetime to 5 seconds, rewrites the link-layer
address in the neighbor cache entry to the address of the new
Server, then returns a DHCPv6 Reply message to the Client via the
old Server. When the lifetime expires, the old Server withdraws the
IP route from the AERO routing system and deletes the neighbor cache
entry for the Client. The Client can then use the Reply message to
verify that the termination signal has been processed, and can
delete both the default route and the neighbor cache entry for the
old Server. (Note that since Release/Reply messages may be lost in
the network the Client MUST retry until it gets Reply indicating
that the Release was successful.)Clients SHOULD NOT move rapidly between Servers in order to avoid
causing excessive oscillations in the AERO routing system. Such
oscillations could result in intermittent reachability for the
Client itself, while causing little harm to the network. Examples of
when a Client might wish to change to a different Server include a
Server that has gone unreachable, topological movements of
significant distance, etc.Proxy Mobile IPv6 (PMIPv6) presents a localized
mobility management scheme for use within an access network domain. It
is typically used in WiFi and cellular wireless access networks, and
allows Mobile Nodes (MNs) to receive and retain an IP address that
remains stable within the access network domain without needing to
implement any special mobility protocols. In the PMIPv6 architecture,
access network devices known as Mobility Access Gateways (MAGs)
provide MNs with an access link abstraction and receive prefixes for
the MNs from a Local Mobility Anchor (LMA).In a proxy AERO domain, a proxy AERO Client (acting as a MAG) can
similarly provide proxy services for MNs that do not participate in
AERO messaging. The proxy Client presents an access link abstraction
to MNs, and performs DHCPv6 PD exchanges over the AERO interface with
an AERO Server (acting as an LMA) to receive ACPs for address
provisioning of new MNs that come onto an access link. This scheme
assumes that proxy Clients act as fixed (non-mobile) infrastructure
elements under the same administrative trust basis as for Relays and
Servers.When an MN comes onto an access link within a proxy AERO domain for
the first time, the proxy Client authenticates the MN and obtains a
unique identifier that it can use as a DHCPv6 DUID then issues a
DHCPv6 PD Solicit/Request to its Server. When the Server delegates an
ACP, the proxy Client creates an AERO address for the MN and assigns
the ACP to the MN's access link. The proxy Client then configures
itself as a default router for the MN and provides address
autoconfiguration services (e.g., SLAAC, DHCPv6, DHCPv4, etc.) for
provisioning MN addresses from the ACP over the access link. Since the
proxy Client may serve many such MNs simultaneously, it may receive
multiple ACP prefix delegations and configure multiple AERO addresses,
i.e., one for each MN.When two MNs are associated with the same proxy Client, the Client
can forward traffic between the MNs without involving a Server since
it configures the AERO addresses of both MNs and therefore also has
the necessary routing information. When two MNs are associated with
different proxy Clients, the source MN's Client can initiate standard
AERO route optimization to discover a direct path to the target MN's
Client through the exchange of Predirect/Redirect messages.When an MN in a proxy AERO domain leaves an access link provided by
an old proxy Client, the MN issues an access link-specific "leave"
message that informs the old Client of the link-layer address of a new
Client on the planned new access link. This is known as a "predictive
handover". When an MN comes onto an access link provided by a new
proxy Client, the MN issues an access link-specific "join" message
that informs the new Client of the link-layer address of the old
Client on the actual old access link. This is known as a "reactive
handover".Upon receiving a predictive handover indication, the old proxy
Client sends a DHCPv6 PD Solicit/Request message directly to the new
Client and queues any arriving data packets addressed to the departed
MN. The Solicit/Request message includes the MN's ID as the DUID, the
ACP in an IA_PD option, the old Client's address as the link-layer
source address and the new Client's address as the link-layer
destination address. When the new Client receives the Solicit/Request
message, it changes the link-layer source address to its own address,
changes the link-layer destination address to the address of its
Server, and forwards the message to the Server. At the same time, the
new Client creates access link state for the ACP in anticipation of
the MN's arrival (while queuing any data packets until the MN
arrives), creates a neighbor cache entry for the old Client with
AcceptTime set to ACCEPT_TIME, then sends a Redirect message back to
the old Client. When the old Client receives the Redirect message, it
creates a neighbor cache entry for the new Client with ForwardTime set
to FORWARD_TIME, then forwards any queued data packets to the new
Client. At the same time, the old Client sends a DHCPv6 PD Release
message to its Server. Finally, the old Client sends unsolicited NA
messages to any of the ACP's correspondents with a TLLAO containing
the link-layer address of the new Client. This follows the procedure
specified in , except that it is the old
Client and not the Server that supplies the link-layer address.Upon receiving a reactive handover indication, the new proxy Client
creates access link state for the MN's ACP, sends a DHCPv6 PD
Solicit/Request message to its Server, and sends a DHCPv6 PD Release
message directly to the old Client. The Release message includes the
MN's ID as the DUID, the ACP in an IA_PD option, the new Client's
address as the link-layer source address and the old Client's address
as the link-layer destination address. When the old Client receives
the Release message, it changes the link-layer source address to its
own address, changes the link-layer destination address to the address
of its Server, and forwards the message to the Server. At the same
time, the old Client sends a Predirect message back to the new Client
and queues any arriving data packets addressed to the departed MN.
When the new Client receives the Predirect, it creates a neighbor
cache entry for the old Client with AcceptTime set to ACCEPT_TIME,
then sends a Redirect message back to the old Client. When the old
Client receives the Redirect message, it creates a neighbor cache
entry for the new Client with ForwardTime set to FORWARD_TIME, then
forwards any queued data packets to the new Client. Finally, the old
Client sends unsolicited NA messages to correspondents the same as for
the predictive case.When a Server processes a DHCPv6 Solicit/Request message, it
creates a neighbor cache entry for this ACP if none currently exists.
If a neighbor cache entry already exists, however, the Server changes
the link-layer address to the address of the new proxy Client (this
satisfies the case of both the old Client and new Client using the
same Server).When a Server processes a DHCPv6 Release message, it resets the
neighbor cache entry lifetime for this ACP to 5 seconds if the cached
link-layer address matches the old proxy Client's address. Otherwise,
the Server ignores the Release message (this satisfies the case of
both the old Client and new Client using the same Server).When a correspondent Client receives an unsolicited NA message, it
changes the link-layer address for the ACP's neighbor cache entry to
the address of the new proxy Client. The correspondent Client then
issues a Predirect/Redirect exchange to establish a new neighbor cache
entry in the new Client.From an architectural perspective, in addition to the use of DHCPv6
PD and IPv6 ND signaling the AERO approach differs from PMIPv6 in its
use of the NBMA virtual link model instead of point-to-point tunnels.
This provides a more agile interface for Client/Server and
Client/Client coordinations, and also facilitates simple route
optimization. The AERO routing system is also arranged in such a
fashion that Clients get the same service from any Server they happen
to associate with. This provides a natural fault tolerance and load
balancing capability such as desired for distributed mobility
management.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.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.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.In order to support this virtual link grafting, the security
gateway (acting as an AERO Server) must keep static neighbor cache
entries for all of its associated Clients located on the public
Internet. The neighbor cache entry is keyed by the AERO Client's AERO
address the same as if the Client were located within the enterprise
internetwork. The neighbor cache is then managed in all ways as though
the Client were an ordinary AERO Client. This includes the AERO IPv6
ND messaging signaling for Route Optimization and Neighbor
Unreachability Detection.Note that the main difference between a security gateway acting as
an AERO Server and an enterprise-internal AERO Server is that the
security gateway has at least one enterprise-internal physical
interface and at least one public Internet physical interface.
Conversely, the enterprise-internal AERO Server has only
enterprise-internal physical interfaces. For this reason security
gateway proxying is needed to ensure that the public Internet
link-layer addressing space is kept separate from the
enterprise-internal link-layer addressing space. This is afforded
through a natural extension of the security association caching
already performed for each VPN client by the security gateway.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.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.Consider the case when host ('H1') has the IPv6 address
2001:db8:1::1, and Client ('C1') has the ACP 2001:db8:1::/64 with
underlying IPv6 Internet address of 2001:db8:1000::1. Also, host
('H2') has the IPv6 address 2001:db8:2::1, and Client ('C2') has the
ACP 2001:db8:2::/64 with underlying IPv6 address of 2001:db8:2000::1.
Client ('C1') can determine whether 'C2' is indeed also an AERO Client
willing to serve as a route optimization correspondent by resolving
the AAAA records for the DNS FQDN that matches ('H2')s prefix,
i.e.:'0.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net'If ('C2') is indeed a candidate correspondent, the FQDN lookup will
return a PTR resource record that contains the domain name for the
AERO link that manages ('C2')s ASP. Client ('C1') can then attempt
route optimization using an approach similar to the Return Routability
procedure specified for Mobile IPv6 (MIPv6) .
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.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:The encapsulating IPv6 header source address is set to
2001:db8:1:: (i.e., the IPv6 subnet router anycast address for
('C1')s ACP)The encapsulating IPv6 header destination address is set to
2001:db8:2:: (i.e., the IPv6 subnet router anycast address for
('C2')s ACP)The encapsulating IPv6 header is followed by a UDP header with
source and destination port set to 8060The encapsulated IPv6 header source address is set to
fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))The encapsulated IPv6 header destination address is set to
fe80::2001:db8:2:0 (i.e., the AERO address for ('C2'))The encapsulated AERO Predirect message includes all of the
securing information that would occur in a MIPv6 "Home Test Init"
message (format TBD)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').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:The encapsulating IPv6 header source address is set to
2001:db8:1000:1 (i.e., the Internet IPv6 address of ('C1'))The encapsulating IPv6 header destination address is set to
2001:db8:2:: (i.e., the IPv6 subnet router anycast address for
('C2')s ACP)The encapsulating IPv6 header is followed by a UDP header with
source and destination port set to 8060The encapsulated IPv6 header source address is set to
fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))The encapsulated IPv6 header destination address is set to
fe80::2001:db8:2:0 (i.e., the AERO address for ('C2'))The encapsulated AERO Predirect message includes all of the
securing information that would occur in a MIPv6 "Care-of Test
Init" message (format TBD)('C2') will receive both Predirect messages through its home
network then return a corresponding Redirect for each of the Predirect
messages with the source and destination addresses in the inner and
outer headers reversed. The first message includes all of the securing
information that would occur in a MIPv6 "Home Test" message, while the
second message includes all of the securing information that would
occur in a MIPv6 "Care-of Test" message (formats TBD).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').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').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.('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.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.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.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 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.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.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.When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in 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".)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.In some AERO link scenarios, there may be no Servers on the link
and/or no need for Clients to use a Server as an intermediary trust
anchor. In that case, each Client acts as a Server unto itself to
establish neighbor cache entries by performing direct Client-to-Client
IPv6 ND message exchanges, and some other form of trust basis must be
applied so that each Client can verify that the prospective neighbor
is authorized to use its claimed ACP.When there is no Server on the link, Clients must arrange to
receive ACPs and publish them via a secure alternate prefix delegation
authority through some means outside the scope of this document.In addition to the dynamic neighbor discovery procedures for AERO
link neighbors described above, AERO encapsulation can be applied to
manually-configured tunnels. In that case, the tunnel endpoints use an
administratively-assigned link-local address and exchange NS/NA
messages the same as for dynamically-established tunnels.After a tunnel neighbor relationship has been established,
neighbors can use a traditional dynamic routing protocol over the
tunnel to exchange routing information without having to inject the
routes into the AERO routing system.User-level and kernel-level AERO implementations have been developed
and are undergoing internal testing within Boeing.A new Generic UDP Encapsulation (GUE) format has been specified in
. The GUE encapsulation format will
eventually supplant the native AERO UDP encapsulation format.Future versions of the spec will explore the subject of DSCP marking
in more detail.The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO . This document
obsoletes and claims the UDP port number "8060"
for all future use.No further IANA actions are required.AERO link security considerations are the same as for standard IPv6
Neighbor Discovery 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 ,
etc.) for Client authentication and network admission control. In
particular, Clients SHOULD include authenticating information on each
Solicit/Request/Rebind/Release message they send, but omit
authenticating information on Renew messages. Renew messages are exempt
due to the fact that the Renew must already be checked for having a
correct link-layer address and does not update any link-layer addresses.
Therefore, asking the Server to also authenticate the Renew message
would be unnecessary and could result in excessive processing
overhead.AERO Redirect, Predirect and unsolicited NA messages SHOULD include a
Timestamp option (see Section 5.3 of ) 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 ) that recipients echo back in
corresponding responses.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) , IPsec or TLS may be necessary.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.)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 to
establish a security association.An AERO Client's link-layer address could be rewritten by a
link-layer switching element on the path from the Client to the Server
and not detected by the DHCPv6 security mechanism. However, such a
condition would only be a matter of concern on unmanaged/unsecured links
where the link-layer switching elements themselves present a
man-in-the-middle attack threat. For this reason, IP security MUST be
used when AERO is employed over unmanaged/unsecured links.Discussions both on IETF lists and in private exchanges helped shape
some of the concepts in this work. Individuals who contributed insights
include Mikael Abrahamsson, Mark Andrews, Fred Baker, Stewart Bryant,
Brian Carpenter, Wojciech Dec, Ralph Droms, Adrian Farrel, Sri
Gundavelli, Brian Haberman, Joel Halpern, Tom Herbert, Sascha Hlusiak,
Lee Howard, Andre Kostur, Ted Lemon, Andy Malis, Satoru Matsushima,
Tomek Mrugalski, Alexandru Petrescu, Behcet Saikaya, Joe Touch, Bernie
Volz, Ryuji Wakikawa and Lloyd Wood. Members of the IESG also provided
valuable input during their review process that greatly improved the
document. Special thanks go to Stewart Bryant, Joel Halpern and Brian
Haberman for their shepherding guidance.This work has further been encouraged and supported by Boeing
colleagues including Dave Bernhardt, Cam Brodie, Balaguruna Chidambaram,
Bruce Cornish, Claudiu Danilov, Wen Fang, Anthony Gregory, Jeff Holland,
Ed King, Gen MacLean, Rob Muszkiewicz, Sean O'Sullivan, Kent Shuey,
Brian Skeen, Mike Slane, Brendan Williams, Julie Wulff, Yueli Yang, and
other members of the BR&T and BIT mobile networking teams.Earlier works on NBMA tunneling approaches are found in .Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:The Internet Routing Overlay Network (IRON) Virtual Enterprise Traversal (VET) The Subnetwork Encapsulation and Adaptation Layer (SEAL) AERO, First Edition Note that these works cite numerous earlier efforts that are
not also cited here due to space limitations. The authors of those
earlier works are acknowledged for their insights.http://en.wikipedia.org/wiki/TUN/TAP