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, address/prefix 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 especially
well suited to mobile Virtual Private Networks (VPNs) and other
applications 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 .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, address/prefix 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 applicable to a wide variety of use cases. For example, it
can be used to coordinate the Virtual Private Network (VPN) links of
mobile devices (e.g., cellphones, tablets, laptop computers, etc.) that
connect into a home enterprise network via public access networks. AERO
can also be applied to aviation applications for both manned and
unmanned aircraft where the aircraft is treated as a mobile host or
router that can connect an Internet of Things (IoT). Numerous other use
cases are also in scope. 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
even though they may be separated by many underlying network hops.
AERO can also operate over native multiple access link types (e.g.,
Ethernet, WiFi etc.) when a tunnel virtual overlay is not
needed.a node's attachment to an AERO
link. Since the addresses assigned to an AERO interface are obtained
from the unique prefix delegations it receives, AERO interfaces do
not require Duplicate Address Detection (DAD) and therefore set the
administrative variable DupAddrDetectTransmits to zero .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 to receive IP Prefix Delegations (PDs) from
one or more AERO Servers. Following PD, the Client assigns an AERO
address to the AERO interface for use in DHCPv6 and IPv6 ND
exchanges with other AERO nodes.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.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; otherwise, UDP port number is set to the constant value
'0'. 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 more-specific AERO
Client Prefixes (ACPs) are derived.an IP prefix derived
from an ASP and delegated to a Client, where the ACP prefix length
must be no shorter than the ASP prefix length and must be no longer
than 64 for IPv6 or 32 for IPv4.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 DHCPv6 and IPv6 ND (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:AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as
a default router for its associated Servers S1 and S2, and
connects the AERO link to the rest of the IP Internetwork.AERO Servers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive AERO Client Prefix (ACP) delegations P1
and P2, and also act as default routers for their associated
physical or internal virtual EUNs. (Alternatively, clients can act
as multi-addressed hosts without serving any EUNs).Simple hosts H1 and H2 attach to the EUNs served by Clients C1
and C2, respectively.Each AERO node maintains an AERO interface neighbor cache and
an IP forwarding table. For example, AERO Relay R1 in the diagram has
neighbor cache entries for Servers S1 and S2 as well as IP forwarding
table entries for the ACPs delegated to Clients C1 and C2. In common
operational practice, there may be many additional Relays, Servers and
Clients. (Although not shown in the figure, AERO Forwarding Agents may
also be provided for data plane forwarding offload services.)AERO Relays provide default forwarding services to AERO Servers.
Relays forward packets between neighbors 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, and maintain
an AERO interface neighbor cache entry for each AERO Relay. They also
maintain both neighbor cache entries and IP forwarding table entries
for each of their associated Clients. AERO Servers can also be
configured to act as AERO Relays.AERO Clients act as requesting routers to receive ACPs through
DHCPv6 PD exchanges with AERO Servers over the AERO link. Each Client
MAY associate with a single Server or with multiple Servers, e.g., for
fault tolerance, load balancing, etc. Each IPv6 Client receives at
least a /64 IPv6 ACP, and may receive even shorter prefixes.
Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
singleton IPv4 address), and may receive even shorter prefixes. AERO
Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.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 remains
stable as the Client moves between topological locations, i.e., even
if its link-layer addresses change.For IPv6, the AERO address begins with the prefix fe80::/64 and
includes in its interface identifier (i.e., the lower 64 bits) 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 based on an IPv4-mapped IPv6
address formed from the ACP and with a Prefix
Length of 96 plus the ACP prefix length. For example, for the IPv4 ACP
192.0.2.32/28 the IPv4-mapped IPv6 address is:0:0:0:0:0:FFFF:192.0.2.32/124The Client then constructs its AERO address with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as:fe80::FFFF:192.0.2.32NOTE: In some cases, prospective neighbors may not have
advanced knowledge of the Client's ACP length and may therefore send
initial IPv6 ND messages with an AERO destination address that matches
the ACP but does not correspond to the base prefix. For example, if
the Client receives the IPv6 ACP 2001:db8:1000:2000::/56 then
subsequently receives an IPv6 ND message with destination address
fe80::2001:db8:1000:2001, it accepts the message as though it were
addressed to fe80::2001:db8:1000:2000.AERO interfaces use encapsulation (see: ) to exchange packets with neighbors attached to
the AERO link.AERO interfaces maintain a neighbor cache, and AERO nodes use both
DHCPv6 and IPv6 ND control messaging to manage the creation,
modification and deletion of neighbor cache entries.AERO Clients send DHCPv6 Solicit, Rebind, Renew and Release
messages to AERO Servers, which respond with DHCPv6 Reply messages.
AERO nodes use unicast IPv6 ND Neighbor Solicitation (NS), Neighbor
Advertisement (NA), Router Solicitation (RS) and Router Advertisement
(RA) messages the same as for any IPv6 link.AERO interfaces use two IPv6 ND redirection message types -- the
first known as a Predirect message and the second being the standard
Redirect message (see ).AERO interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in :In this format:Type is set to '1' for SLLAO or '2' for TLLAO the same as for
IPv6 ND.Length is set to the constant value '5' (i.e., 5 units of 8
octets).Both Reserved fields are set to the value '0' on transmission
and ignored on receipt.Interface ID is set to an integer value between 0 and 255
corresponding to an underlying interface of the AERO node.UDP Port Number and IP Address are set to the addresses used by
the AERO node when it sends encapsulated packets over the
underlying interface. When UDP is not used as part of the
encapsulation, UDP Port Number is set to the value '0'. When the
encapsulation IP address family is IPv4, IP Address is formed as
an IPv4-mapped IPv6 address as specified in .P[i] is a set of 64 Preference values that correspond to the 64
Differentiated Service Code Point (DSCP) values . Each P(i) is set to the value '0'
("disabled"), '1' ("low"), '2' ("medium") or '3' ("high") to
indicate a preference level for packet forwarding purposes.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, cellular, 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 IPv6 ND messages include only a single
S/TLLAO with Interface 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 multiple link-layer
addresses. In that case, IPv6 ND messages MAY include multiple
S/TLLAOs -- each with an Interface ID that corresponds to a specific
underlying interface of the AERO node.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 ASPs into the native IP Internetwork.Each Relay subsequently maintains an IP forwarding table entry
for each ACP covered by its ASP(s), and maintains a neighbor cache
entry for each Server on the link. Relays exchange NS/NA messages
with AERO link neighbors the same as for any AERO node, however they
typically do not perform explicit Neighbor Unreachability Detection
(NUD) (see: ) 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-ACP neighbor cache entries and IP forwarding table
entries based on control message exchanges. Each Server also engages
in a dynamic routing protocol with each Relay on the link (see:
).When the Server receives an NS/RS message from a Client on the
AERO interface it returns an NA/RA message. The Server further
provides a simple link-layer conduit between AERO interface
neighbors. In particular, when a packet sent by a source Client
arrives on the Server's AERO interface and is destined to another of
the Server's Clients, the Server forwards the packet at the link
layer without ever disturbing the network layer and without ever
leaving the AERO interface.When a Client enables an AERO interface, it uses the special
address fe80::ffff:ffff:ffff:ffff to obtain one or more ACPs from an
AERO Server via DHCPv6 PD. Next, it assigns the corresponding AERO
address(es) to the AERO interface and creates a neighbor cache entry
for the Server, i.e., the DHCPv6 PD exchange bootstraps
autoconfiguration of unique link-local address(es). The Client
maintains a neighbor cache entry for each of its Servers and each of
its active correspondent Clients. When the Client receives
Redirect/Predirect messages on the AERO interface it updates or
creates neighbor cache entries, including link-layer address
information.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.The AERO routing system is based on a private instance of the
Border Gateway Protocol (BGP) that is
coordinated between Relays and Servers and does not interact with
either the public Internet BGP routing system or the native IP
Internetwork interior routing system. Relays advertise only a small
and unchanging set of ASPs to the native routing system instead of the
full dynamically changing set of ACPs.In a reference deployment, each AERO Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP instance,
and each Server further peers with each Relay but does not peer with
other Servers. Similarly, Relays do not peer with each other, since
they will reliably receive all updates from all Servers and will
therefore have a consistent view of the AERO link ACP delegations.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 of its ASPs. By
black-holing the ASPs, the Relay will maintain forwarding table
entries only for the ACPs that are currently active, and all other
ACPs will correctly result in destination unreachable failures due to
the black hole route. Relays do not send BGP updates for ACPs to
Servers, but instead originate a default route. In this way, Servers
have only partial topology knowledge (i.e., they know only about the
ACPs of their directly associated Cliens) and they forward all other
packets to Relays which have full topology knowledge.Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. Assuming O(10^6)
as a reasonable maximum number of BGP routes, this means that O(10^6)
Clients can be serviced by a single set of Relays. A means of
increasing scaling would be to assign a different set of Relays for
each set of ASPs. In that case, each Server still peers with each
Relay, but the Server institutes route filters so that each set of
Relays only receives BGP updates for the ASPs they aggregate. For
example, if the ASP for the AERO link is 2001:db8::/32, a first set of
Relays could service the ASP segment 2001:db8::/40, a second set of
Relays could service 2001:db8:0100::/40, a third set could service
2001:db8:0200::/40, etc.Assuming up to O(10^3) sets of Relays, the AERO routing system can
then accommodate O(10^9) ACPs with no additional overhead for Servers
and Relays (for example, it should be possible to service 4 billion
/64 ACPs taken from a /32 ASP and even more for shorter ASPs). In this
way, each set of Relays services a specific set of ASPs that they
advertise to the native routing system, and each Server configures
ASP-specific routes that list the correct set of Relays as next hops.
This arrangement also allows for natural incremental deployment, and
can support small scale initial deployments followed by dynamic
deployment of additional Clients, Servers and Relays without
disturbing the already-deployed base.Note that in an alternate routing arrangement each set of Relays
could advertise the aggregated ASP for the link into the native
routing system even though each Relay services only a segment of the
ASP. In that case, a Relay upon receiving a packet with a destination
address covered by the ASP segment of another Relay can simply tunnel
the packet to the correct Relay. The tradeoff then is the penalty for
Relay-to-Relay tunneling compared with reduced routing information in
the native routing system.Finally, Realys can express preferences for ACPs learned from
multiple Servers by assigning a BGP weight to each Server's peering
configuration. In this way Relays can choose the Serevr with the
highest weight as the preferred path, and then fail over to a Server
with lower weight in case of ACP withdrawl or Server failure.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 through DHCPv6 PD
exchanges as specified in and remain in place
for durations bounded by prefix lifetimes. AERO Servers maintain
static neighbor cache entries for the ACPs of each of their associated
Clients, and AERO Clients maintain a static neighbor cache entry for
each of their associated Servers. When an AERO Server sends a Reply
message response to a Client's Solicit, Rebind or Renew message, it
creates or updates a static neighbor cache entry based on the Client's
DHCP Unique Identifier (DUID) as the Client identifier, the AERO
address(es) corresponding to the Client's ACP(s) as the network-layer
address(es), the prefix lifetime as the neighbor cache entry lifetime,
the Client's encapsulation IP address and UDP port number as the
link-layer address and the prefix length(s) as the length to apply to
the AERO address(es). When an AERO Client receives a Reply message
from a Server, it creates or updates a static neighbor cache entry
based on the Reply message link-local source address as the
network-layer address, the prefix lifetime as the neighbor cache entry
lifetime, and the encapsulation IP source address and UDP source port
number as the link-layer address.Dynamic neighbor cache entries are created or updated based on
receipt of a Predirect/Redirect message as specified in , and are garbage-collected when keepalive timers
expire. 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.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.When there may be a Network Address Translator (NAT) between the
Client and the Server, or if the path from the Client to the Server
should be tested for reachability, the Client can send periodic RS
messages to the Server to receive RA replies. The RS/RA messaging will
keep NAT state alive and test Server reachability without disturbing
the DHCPv6 server.IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system), or
from the link layer (i.e., from the AERO tunnel virtual link). Packets
that enter the AERO interface from the network layer are encapsulated
and admitted into the AERO link, i.e., they are tunnelled to an AERO
interface neighbor. Packets that enter the AERO interface from the
link layer are either re-admitted into the AERO link or delivered to
the network layer where they are subject to either local delivery or
IP forwarding. Since each AERO node may have only partial information
about neighbors on the link, AERO interfaces may forward packets with
link-local destination addresses at a layer below the network layer.
This means that AERO nodes act as both IP routers/hosts and sub-IP
layer forwarding nodes. AERO interface sending considerations for
Clients, Servers and Relays are given below.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
selected through longest-prefix-match 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 the corresponding
neighbor cache entry as the link-layer address for encapsulation and
admits the packet into the AERO link. When an IP packet enters a
Relay's AERO interface from the link-layer, if the destination is not
a link-local address and does not match an ASP the Relay removes the
packet from the AERO interface and uses IP forwarding to forward the
packet to the Internetwork. If the destination address is a link-local
address or a non-link-local address that matches an ASP, and there is
a more-specific ACP entry in the IP forwarding table, the Relay uses
the link-layer address in the corresponding neighbor cache entry as
the link-layer address for encapsulation and re-admits the packet into
the AERO link. When an IP packet enters a Relay's AERO interface from
either the network layer or link-layer, and the packet's destination
address matches an ASP but there is no more-specific ACP entry, the
Relay drops the packet and returns an ICMP Destination Unreachable
message (see: ).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 nodes may have multiple underlying interfaces and/or neighbor
cache entries for Clients with multiple Interface ID registrations
(see ). The AERO node uses the packet's DSCP
value to select the outgoing underlying interface based on its own
Interface ID preference values and to select the destination
link-layer address based on the neighbor's Interface ID with the
highest preference value. If multiple Interface IDs have a preference
of "high", the AERO node sends one copy of the packet to each of the
link-layer addresses (i.e., it replicates the packet); otherwise, the
node sends a single copy of the packet.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 , or through an alternate
encapsulation format (see: ). For packets
entering the AERO link from the IP layer, the AERO interface copies
the "TTL/Hop Limit", "Type of Service/Traffic Class" , "Flow Label".(for
IPv6) and "Congestion Experienced" values in
the packet's IP header into the corresponding fields in the
encapsulation IP header. For packets undergoing re-encapsulation
within the AERO link, the AERO interface instead copies the "TTL/Hop
Limit", "Type of Service/Traffic Class", "Flow Label" and "Congestion
Experienced" values in the original encapsulation IP header into the
corresponding fields in the new encapsulation IP header, i.e., the
values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header.When GUE encapsulation is used, the AERO interface next sets the
UDP source port to a constant value that it will use in each
successive packet it sends, and sets the UDP length field to the
length of the encapsulated packet plus 8 bytes for the UDP header
itself plus the length of the GUE header (or 0 if GUE direct IP
encapsulation is used). For packets sent to a Server, the AERO
interface sets the UDP destination port to 8060, i.e., the
IANA-registered port number for AERO. For packets sent to a
correspondent Client, the AERO interface sets the UDP destination port
to the port value stored in the neighbor cache entry for this
correspondent. The AERO interface then either includes or omits the
UDP checksum according to the GUE specification.For IPv4 encapsulation, the AERO interface sets the DF bit as
discussed in .AERO interfaces decapsulate packets destined either to the AERO
node itself or to a destination reached via an interface other than
the AERO interface the packet was received on. Decapsulation is per
the procedures specified for the appropriate encapsulation format.AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:AERO Servers and Relays 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 Client based on the specific DHCPv6 message type.AERO Clients and Servers accept encapsulated packets if there
is a static neighbor cache entry with a link-layer address that
matches the packet's link-layer source address.AERO Clients, Servers and Relays accept encapsulated packets if
there is a dynamic neighbor cache entry with an AERO address that
matches the packet's network-layer source address, with a
link-layer address that matches the packet's link-layer source
address, and with a non-zero AcceptTime.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 attachment to the AERO link. The
AERO interface acts as a tunnel ingress when it sends a packet to an
AERO link neighbor and as a tunnel egress when it receives a packet
from an AERO link neighbor. AERO interfaces observe the packet sizing
considerations for tunnels discussed in and as specified below.IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of
1280 bytes . Although IPv4 specifies a smaller
minimum link MTU of 68 bytes , AERO interfaces
also observe the IPv6 minimum for IPv4 even if the packet may incur
fragmentation in the network.IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500
bytes , while the minimum MRU for IPv4 is only
576 bytes (note that IPv6 over IPv4 tunnels
assume a larger MRU than the IPv4 minimum).Original sources expect that IP packets will either be delivered to
the final destination or a suitable Packet Too Big (PTB) message
returned. However, PTB messages may be crafted for malicious purposes
such as denial of service, or lost in the network resulting in failure of the IP Path MTU Discovery
(PMTUD) mechanisms .
For these reasons, AERO links employ operational procedures that avoid
all interactions with PMTUD.AERO Servers advertise an MTU that MUST be no smaller than 1280
bytes, MUST be no larger than the minimum MRU among all nodes on the
AERO link minus the encapsulation overhead ("ENCAPS"), and SHOULD be
no smaller than 1500 bytes. AERO Servers advertise a Maximum Fragment
Unit (MFU) as the maximum size for the fragments of an encapsulated
packet that require fragmentation. The MFU value MUST NOT be larger
than (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there
is operational assurance that a larger size can traverse the link
along all paths without fragmentation.AERO Clients set the AERO interface MTU/MFU based on the values
advertised by their Server, and configure an MRU large enough to
reassemble packets up to (MTU+ENCAPS) bytes.All AERO nodes on the link MUST configure the same MTU/MFU values
for reasons cited in
(in particular, multicast support requires a common MTU value among
all nodes on the link).All AERO nodes on the link MUST configure a minimum MRU of
(1500+ENCAPS) bytes, and SHOULD be capable of setting a larger MRU
accoding to the Server's advertised MTU.In accordnace with these requirements, the ingress accommodates
packets of various sizes as follows:First, for each original IPv4 packet that is larger than the
AERO interface MTU and with the DF bit set to 0, the ingress uses
IPv4 fragmentation to break the packet into a minimum number of
non-overlapping fragments where the first fragment is no larger
than (MFU-ENCAPS) bytes and the remaining fragments are no larger
than the first.Next, for each original IP packet or fragment that is no larger
than (MFU-ENCAPS) bytes, the ingress encapsulates the packet and
admits it into the tunnel. For IPv4 AERO links, the ingress sets
the Don't Fragment (DF) bit to 0 so that these packets will be
delivered to the egress even if some fragmentation occurs in the
network.For all other original IP packets or fragments, if the packet
is larger than the AERO interface MTU, the ingress drops the
packet and returns a PTB message to the original source.
Otherwise, the ingress encapsulates the packet and fragments the
encapsulated packet into a minimum number of non-overlapping
fragments where the first fragment is no larger than MFU bytes and
the remaining fragments are no larger than the first. The ingress
then admits the fragments into the tunnel, and for IPv4 sets the
DF bit to 0 in the IP encapsulation header. These fragmented
encapsulated packets will be delivered to the egress, which
reassembles them into a whole packet.Several factors must be considered when fragmentation of the
encapsulated packet is needed. For AERO links over IPv4, the IP ID
field is only 16 bits in length, meaning that fragmentation at high
data rates could result in data corruption due to reassembly
misassociations . For
AERO links over both IPv4 and IPv6, studies have also shown that IP
fragments are dropped unconditionally over some network paths
[I-D.taylor-v6ops-fragdrop]. In environments where IP fragmentation
issues could result in operational problems, the ingress SHOULD employ
intermediate-layer fragmentation (see: and
) before appending the
outer encapsulation headers to each fragment.Since the encapsulation fragment header reduces the room available
for packet data, but the original source has no way to control its
insertion, the ingress MUST include the fragment header length in the
ENCAPS length even for packets in which the header is absent.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. Valid type values include "Destination
Unreachable", "Time Exceeded" and "Parameter Problem" . (AERO interfaces ignore
all L2 IPv4 "Fragmentation Needed" and IPv6 "Packet Too Big" messages
since they only emit packets that are guaranteed to be no larger than
the IP minimum link MTU.)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
integrity checks and/or institute rate limits for subseqent
packets.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 AERO routing system is likely in a temporary transitional
state that will soon re-converge. In case of a prolonged outage,
however, the AERO routing system will compensate for Relays or
Servers that have fallen silent.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 an encapsulated packet for which the
reassembly buffer it too small, it drops the packet and returns an L3
Packet To Big (PTB) message. The node first 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 and forwards the message to the next hop within the
AERO interface. Otherwise, the node uses one of its non link-local
addresses as the source address of the L3 PTB 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.AERO Router Discovery, Prefix Delegation and Address Configuration
are coordinated by the DHCPv6 control messaging protocol as discussed
in the following Sections.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 state synchronization is
necessary, and Clients can optionally hold separate PDs for the same
ACPs from multiple Servers. In this way, Clients can associate with
multiple Servers, and can receive new PDs from new Servers before
deprecating PDs received from existing Servers. This provides the
Client with a natural fault-tolerance and/or load balancing
profile.AERO Clients and Servers exchange configuration information using
an AERO Vendor-Specific Information Option (AVSIO) formatted as
follows:In this format, "enterprise-number" is set to 45282
(i.e., the IANA-reserved enterprise number for AERO) and
"option-length" is set to the total length of the option. A single
AVSIO may include one or more AERO-specific (sub)options as defined
in the following sections.AERO Clients MUST include an AVSIO in DHCPv6 Solicit and Rebind
messages to manage the Server's cached link-layer addresses and
preferences. AERO Servers MUST include an AVSIO in DHCPv6 Reply
messages that correspond to a Client's DHCPv6 message that also
included an AVSIO option.The following sections specify the Client and Server behavior in
more detail.AERO Clients discover the link-layer addresses of AERO Servers
via static configuration (e.g., from a flat-file map of Server
addresses and locations), or through an automated means such as 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 a
DNS suffix for the Client's underlying network (e.g.,
"example.com"). After discovering the link-layer addresses, the
Client associates with one or more of the corresponding Servers.To associate with a Server, the Client acts as a requesting
router to request ACPs through a two-message (i.e., Solicit/Reply)
DHCPv6 PD exchange .
The Client's includes fe80::ffff:ffff:ffff:ffff as the IPv6 source
address of the Solicit message, 'All_DHCP_Relay_Agents_and_Servers'
as the IPv6 destination address, an underlying interface address of
the Client (i.e., the link-layer address) as the link-layer source
address and the link-layer address of the Server as the link-layer
destination address. The Client also includes a Rapid Commit option,
a Client Identifier option with the Client's DUID, and an Identity
Association for Prefix Delegation (IA_PD) option. If the Client is
pre-provisioned with ACPs associated with the AERO service, it MAY
also include the ACPs in the IA_PD to indicate its preferences to
the DHCPv6 server.The Client also includes an AVSIO option with one or more AERO
Client Link-Layer Address Options (ACLLAOs) to register its
link-layer address(es) with the Server. The first ACLLAO MUST be
specific to the underlying interface over which the Client will send
the Solicit. The Client MAY include additonal ACLLAOs specific to
other underlying interfaces, but if so it MUST have assurance that
there will be no NATs on the paths to the Server via those
interfaces. (Otherwise, the Client MAY issue subsequent Rebind
messages after the initial Solicit/Reply exchange to register
additional link-layer addresses). The Server will echo the ACLLAOs
in the corresponding Reply message as specified in .The format for the ACLLAO is shown in :In the above format, the Client sets 'opt-code' to 0
("OPTION_ACLLAO") and sets 'option-len' to 36 (i.e., the length of
the option following this field). The Client then includes an "AERO
Client Link-Layer Address" in the same format as for S/TLLAOs in
beginning with the 'Reserved2' field and
extending to the end of the S/TLLAO. The Client then sets
'Reserved2', 'Interface ID', 'UDP Port Number', 'IP address' and
'P(i)' values for the specific underlying interface the same as for
S/TLLAO options (see ). The Client finally
includes any additional DHCPv6 options (including any necessary
authentication options to identify itself to the DHCPv6 server), and
sends the encapsulated Solicit message via the underlying interface
corresponding to the Interface ID of the first ACLLAO.When the Client attempts to perform a DHCPv6 PD exchange with a
Server that is too busy to service the request, the Client may
receive an error status code such as "NoPrefixAvail" in the Server's
Reply or no Reply at all. In that case, the
Client SHOULD discontinue DHCPv6 PD attempts through this Server and
try another Server.When the Client receives a Reply from the AERO Server with an
AVSIO option and no error status codes, it can compare the UDP Port
Number and IP Address values in the first ACLLAO with the values the
Client provided in its request. If the values are different, the
Client can infer that there is a NAT on the path to the Server via
that underlying interface. If the AVSIO option also includes an
ALINFO sub-option, the Client also assigns the MTU/MFU values in the
ALINFO option to its AERO interface, then caches any ASPs included
in the ALINFO option as ASPs to associate with the AERO link (see
). This configuration information
applies to the AERO link as a whole, and all Clients will receive
the same information.The Client next 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. Next, the
Client autoconfigures an AERO address for each of the delegated
ACPs, assigns the address(es) to the AERO interface and
sub-delegates the ACPs to its attached EUNs and/or the Client's own
internal virtual interfaces. The Client can then configure as many
addresses as it wants from /64 prefixes taken from the ACPs and
assign them to either an internal virtual interface ("weak
end-system") or to the AERO interface itself ("strong end-system")
while black-holing the remaining portions
of the /64s. Finally, the Client assigns a default IP route to the
AERO interface with the link-local address of the Server as the next
hop and with the PD lifetime as the default router lifetime.After the initial Solicit/Reply exchange, the Client SHOULD begin
using the AERO address as the source address for further DHCPv6
messaging. The Client subsequently renews its ACP delegations
through each of its Servers by sending Renew messages with the
link-layer address of a Server as the link-layer destination
address. The Client MAY subsequently issue Rebind messages with
additional ACLLAOs if it wishes to register additional Interface IDs
and/or update the link-layer address information for existing
Interface IDs. In that case, the Rebind message MUST be sent over
the underlying interface corresponding to the first ACLLAO in the
message, i.e., the same as for Solicits.After an AERO Client registers its Interface IDs and their
associated 'P(i)' values with the AERO Server, the Client may wish
to change one or more Interface ID registrations, e.g., if an
underlying interface becomes unavailable, if cost profiles change,
etc. To do so, the Client prepares a Rebind message to send over any
available underlying interface. The Rebind MUST include the ACLLAO
specific to the selected avaialble underlying interface as the first
ACLLAO and MAY include any additional ACLLAOs specific to other
underlying interfaces. The Client includes fresh 'P(i)' values in
each ACLLAO to update the Server's neighbor cache entry. If the
Client wishes to disable some or all DSCPs for an underlying
interface, it includes an ACLLAO with 'P(i)' values set to 0
("disabled").If the Client wishes to discontinue use of a Server it issues a
Release to delete the Server's neighbor cache entry.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 a static map of
Server addresses for the link and/or the DNS resource records for
the FQDN "linkupnetworks.[domainname]" before entering service.When an AERO Server receives a prospective Client's Solicit on
its AERO interface, and the Server is too busy to service the
message, it SHOULD return a Reply with status code "NoPrefixAvail"
per . Otherwise, the Server authenticates
the message. If authentication succeeds, the Server determines the
correct ACPs to delegate to the Client by searching the Client
database.When the Server delegates the ACPs, it also creates IP forwarding
table entries so that the AERO BGP-based routing system will
propagate the ACPs to all Relays that aggregate the corresponding
ASP (see: ). Next, the Server prepares a
Reply message to send to the Client while using fe80::ID as the IPv6
source address, the link-local address taken from the Client's
Solicit as the IPv6 destination address, the Server's link-layer
address as the source link-layer address, and the Client's
link-layer address as the destination link-layer address. The Server
also includes IA_PD options with the delegated ACPs. For IPv4 ACPs,
the prefix included in the IA_PD option is in IPv4-mapped IPv6
address format and with prefix length set as specified in . For AERO links where a Client may
experience a fault that prevents it from issuing a Release before
departing from the network, Servers should set a short prefix
lifetime (e.g., 40 seconds) so that stale PD state can be flushed
out of the network.For Replies to Client DHCPv6 messages that include an AVSIO, the
Server prepares a new AVSIO to include in the Reply. The Server
first copies the ACLLAOs in the body of the Client's AVSIO into the
AVSIO that the Server will supply in the Reply message. For the
initial ACLLAO, the Server sets 'UDP Port Number' and 'IP address'
to the values observed in the outer encapsulating headers of the
Client's DHCPv6 message, i.e., even if these values are different
than the ones included by the Client.The Server next copies an ALINFO option into the body of the
AVSIO (i.e., following the ACLLAO options) formatted as shown in
:In the ALINFO option, the Server sets sets 'opt-code' to
1 ("OPTION_ALINFO") and sets 'option-len' to the length of the
remainder of the option. The Server next sets MTU and MFU according
to the considerations specified in . The
Server finally includes one or more ASPs with 'Prefix Len' set to
the ASP prefix length (between 0 and 64), and 'AERO Service Prefix'
set to the ASP (between 1 and 8 bytes).When the Server sends the Reply message, it creates or updates a
static neighbor cache entry for the Client based on the DUID and
AERO addresses with lifetime set to no more than the PD lifetimes
and updates the Client's link-layer addresses according to the
ACLLAOs. The Server then uses the Client link-layer addresses as the
link-layer addresses for encapsulation and uses the 'P(i)' values
included in ACLLAOs as preference levels for each DSCP value.After the initial DHCPv6 PD Solicit/Reply exchange, the AERO
Server maintains the neighbor cache entry for the Client until the
PD lifetimes expire. If the Client issues a Rebind, the Server uses
any included ACLLAOs to update the link-layer information in the
Client's neighbor cache entry. If the Client issues a Renew, the
Server extends the PD lifetimes. If the Client issues a Release, or
if the Client does not issue a Renew before the lifetime expires,
the Server deletes the neighbor cache entry for the Client and
withdraws the IP routes from the AERO routing system.AERO Clients and Servers are always on the same link (i.e., the
AERO link) from the perspective of DHCPv6. However, in some
implementations the DHCPv6 server and AERO interface driver may be
located in separate modules. In that case, the Server's AERO
interface driver module can act as a Lightweight DHCPv6 Relay
Agent (LDRA) to relay DHCPv6
messages to and from the DHCPv6 server module.When the LDRA receives a DHCPv6 message from a client, it
prepares an AVSIO (including any ACLLAO and ALINFO options as
described above) and copies the option into a DHCPv6
Relay-Supplied Option Option (RSOO) . The
LDRA then incorporates the RSOO 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 AVSIO included in the RSOO and discards the AVSIO
included within the Client's message itself. Next, the server
authenticates the Client's message and prepares a Reply message if
authentication succeeds.When the DHCPv6 server prepares a Reply message, it then
includes the relay-supplied AVSIO 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.AERO Servers MAY associate with one or more companion AERO
Forwarding Agents as platforms for offloading high-speed data plane
traffic. When an AERO Server receives a Client's
Solicit/Renew/Rebind/Release message, it services the message then
forwards the corresponding Reply message to the Forwarding Agent. When
the Forwarding Agent receives the Reply message, it creates, updates
or deletes a neighbor cache entry with the Client's AERO address and
link-layer information included in the Reply message. The Forwarding
Agent then forwards the Reply message back to the AERO Server, which
forwards the message to the Client. In this way, Forwarding Agent
state is managed in conjunction with Server state, with the Client
responsible for reliability.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 MAY initiate an AERO link 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 link route optimization procedure. depicts the AERO link route
optimization reference operational scenario, using IPv6 addressing
as the example (while not shown, a corresponding example for IPv4
addressing can be easily constructed). The figure shows an AERO
Relay ('R1'), two AERO 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 ('H2') 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 IPv6
ND 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-mapped IPv6 address format).the message includes one or more TLLAOs 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 Interface IDs in Client ('C1')'s neighbor cache
entry. If the Client's TLLAOs are not acceptable, Server ('S1')
discards the message. Otherwise, Server ('S1') validates the message
according to the Redirect message validation rules in Section 8.1 of
, 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 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-mapped IPv6 address format).the message includes one or more TLLAOs 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 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 Interface IDs in Client ('C2')'s neighbor cache
entry. If the Client's TLLAOs are not acceptable, Server ('S2')
discards the message. Otherwise, Server ('S2') validates the message
according to the 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 Redirect; 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 Redirect to determine the next hop toward Client ('C1') by
searching for the AERO address in the neighbor cache. Since Client
('C1') is not a neighbor, Server ('S2') re-encapsulates the Redirect
and relays it via Relay ('R1') by changing the link-layer source
address of the message to 'L2(S2)' and changing the link-layer
destination address to 'L2(R1)'. Server ('S2') finally forwards the
re-encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.When Relay ('R1') receives the Redirect message from Server
('S2') it determines that Server ('S1') is the next hop toward
Client ('C1') by consulting its forwarding table. Relay ('R1') then
re-encapsulates the Redirect while changing the link-layer source
address to 'L2(R1)' and changing the link-layer destination address
to 'L2(S1)'. Relay ('R1') then relays the Redirect via Server
('S1').When Server ('S1') receives the Redirect message from Relay
('R1') it determines that Client ('C1') is a neighbor by consulting
its neighbor cache. Server ('S1') then re-encapsulates the Redirect
while changing the link-layer source address to 'L2(S1)' and
changing the link-layer destination address to 'L2(C1)'. Server
('S1') then forwards the message to Client ('C1').When Client ('C1') receives the Redirect message, it accepts the
message only if it has a link-layer source address of one of its
Servers (e.g., ''L2(S1)'). Next, Client ('C1') validates the message
according to the Redirect message validation rules in Section 8.1 of
, 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.Although the Client is responsible for initiating route
optimization through the transmission of Predirect messages, the
Server is the policy enforcement point that determines whether route
optimization is permitted. For example, on some AERO links route
optimization would allow traffic to circumvent critical
network-based traffic interception points. In those cases, the
Server can deny route optimization requests by simply discarding any
Predirect messages instead of forwarding them.Clients that receive multiple non-contiguous ACP delegations must
perform route optimization for each of the individual ACPs based on
demand of traffic with source addresses taken from those prefixes.
For example, if Client C1 has already performed route optimization
for destination ACP X on behalf of its source ACP Y, it must also
perform route optimization for X on behalf of its source ACP Z. As a
result, source route optimization state cannot be shared between
non-contiguous ACPs and must be managed separately.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 test 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 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 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 issues an immediate Rebind to each of its
Servers using the new link-layer address as the source address and
with an ACLLAO that includes the updated client link-layer
information. If authentication succeeds, the Server then updates its
neighbor cache and sends a Reply. Note that if the Client does not
issue a Rebind before the PD 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 Predirect messages to each of its
correspondent Client neighbors using the same procedures as
specified in . The Client sends the
Predirect messages via a Server the same as if it was performing the
initial route optimization procedure with the correspondent. The
Predirect message will update the correspondent's link layer address
mapping for the Client.When a Client needs to bring a new underlying interface into
service (e.g., when it activates a new data link), it issues an
immediate Rebind to each of its Servers using the new link-layer
address as the source address and with an ACLLAO that includes the
new client link-layer information. If authentication succeeds, the
Server then updates its neighbor cache and sends a Reply. The Client
MAY then send Predirect messages to each of its correspondent
Clients to inform them of the new link-layer address as described in
.When a Client needs to remove an existing underlying interface
from service (e.g., when it de-activates an existing data link), it
issues an immediate Rebind to each of its Servers over any available
link with an ACLLAO that includes P(i) values set to "disabled". If
authentication succeeds, the Server then updates its neighbor cache
and sends a Reply. The Client SHOULD then send Predirect messages to
each of its correspondent Clients to inform them of the deprecated
link-layer address as described in .AERO Clients and Servers MAY include a configuration knob that
allows them to perform implicit mobility management in which no
DHCPv6 messaging is used. In that case, the Client only transmits
packets over a single interface at a time, and the Server always
observes packets arriving from the Client from the same link-layer
source address.If the Client's underlying interface address changes (either due
to a readdressing of the original interface or switching to a new
interface) the Server immediately updates the neighbor cache entry
for the Client and begins accepting and sending packets to the
Client's new link-layer address. This implicit mobility method
applies to use cases such as cellphones with both WiFi and Cellular
interfaces where only one of the interfaces is active at a given
time, and the Client automatically switches over to the backup
interface if the primary interface fails.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
Release message via a new Server to the unicast link-local network
layer address of the old Server. The new Server then writes its own
link-layer address in the Release message IP source address and
forwards the message to the old Server.When the old Server receives the Release, it first authenticates
the message. Next, it resets the Client's neighbor cache entry
lifetime to 3 seconds, rewrites the link-layer address in the
neighbor cache entry to the address of the new Server, then returns
a Reply message to the Client via the old Server. When the lifetime
expires, the old Server withdraws the IP route from the AERO routing
system and deletes the neighbor cache entry for the Client. The
Client can then use the Reply message to verify that the termination
signal has been processed, and can delete both the default route and
the neighbor cache entry for the old Server. (Note that since
Release/Reply messages may be lost in the network the Client SHOULD
retry until it gets a Reply indicating that the Release was
successful. If the Client does not receive a Reply after MAX_RETRY
attempts, the old Server may have failed and the Client should
discontinue its Release attempts.)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.AERO Clients and Servers should maintain a samll queue of packets
they have recently sent to an AERO neighbor, e.g., a 1 second
window. If the AERO neighbor moves, the AERO node MAY retransmit the
queued packets to ensure that they are delviered to the AERO
neighbor's new location.Note that this may have performance implications for asymmetric
paths. For example, if the AERO neighbor moves from a 50mbps link to
a 128kbps link, retransmitting a 1 second window could cause
significant congestion. However, any retransmission bursts will
subside after the 1 second window.Proxy Mobile IPv6 (PMIPv6) presents a network-based
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 sends a
Solicit message to its Server. When the Server delegates an ACP and
returns a Reply, the proxy Client creates an AERO address for the MN
and assigns the ACP to the MN's access link. The proxy Client then
configures itself as a default router for the MN and provides address
autoconfiguration services (e.g., SLAAC, DHCPv6, DHCPv4, etc.) for
provisioning MN addresses from the ACP over the access link. Since the
proxy Client may serve many such MNs simultaneously, it may receive
multiple ACP 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 link 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 Solicit message directly to the new Client and queues
any arriving data packets addressed to the departed MN. The Solicit
message includes the MN's ID as the DUID, the ACP in an IA_PD option,
the old Client's address as the link-layer source address and the new
Client's address as the link-layer destination address. When the new
Client receives the Solicit message, it changes the link-layer source
address to its own address, changes the link-layer destination address
to the address of its Server, and forwards the message to the Server.
At the same time, the new Client creates access link state for the ACP
in anticipation of the MN's arrival (while queuing any data packets
until the MN arrives), creates a neighbor cache entry for the old
Client with AcceptTime set to ACCEPT_TIME, then sends a Redirect
message back to the old Client. When the old Client receives the
Redirect message, it creates a neighbor cache entry for the new Client
with ForwardTime set to FORWARD_TIME, then forwards any queued data
packets to the new Client. At the same time, the old Client sends a
Release message to its Server. Finally, the old Client sends
unsolicited Redirect messages to any of the ACP's correspondents with
a TLLAO containing the link-layer address of the new Client.Upon receiving a reactive handover indication, the new proxy Client
creates access link state for the MN's ACP, sends a Solicit message to
its Server, and sends a Release message directly to the old Client.
The Release message includes the MN's ID as the DUID, the ACP in an
IA_PD option, the new Client's address as the link-layer source
address and the old Client's address as the link-layer destination
address. When the old Client receives the Release message, it changes
the link-layer source address to its own address, changes the
link-layer destination address to the address of its Server, and
forwards the message to the Server. At the same time, the old Client
sends a Predirect message back to the new Client and queues any
arriving data packets addressed to the departed MN. When the new
Client receives the Predirect, it creates a neighbor cache entry for
the old Client with AcceptTime set to ACCEPT_TIME, then sends a
Redirect message back to the old Client. When the old Client receives
the Redirect message, it creates a neighbor cache entry for the new
Client with ForwardTime set to FORWARD_TIME, then forwards any queued
data packets to the new Client. Finally, the old Client sends
unsolicited Redirect messages to correspondents the same as for the
predictive case.When a Server processes a Solicit message, it creates a neighbor
cache entry for this ACP if none currently exists. If a neighbor cache
entry already exists, however, the Server changes the link-layer
address to the address of the new proxy Client (this satisfies the
case of both the old Client and new Client using the same Server).When a Server processes a Release message, it resets the neighbor
cache entry lifetime for this ACP to 3 seconds if the cached
link-layer address matches the old proxy Client's address. Otherwise,
the Server ignores the Release message (this satisfies the case of
both the old Client and new Client using the same Server).When a correspondent Client receives an unsolicited Redirect
message, it changes the link-layer address for the ACP's neighbor
cache entry to the address of the new proxy Client.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 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 any additional
encapsulation headersThe 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 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
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 any additional
encapsulation headersThe 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 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, 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.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 PD authority
through some means outside the scope of this document.In some environments, the AERO service may be useful for mobile
nodes that do not implement the AERO Client function and do not
perform encapsulation. For example, if the mobile node has a way of
injecting its ACP into the access network routing system an AERO
Server connected to the same access network can accept the ACP prefix
injection as an indication that a new mobile node has come onto the
link. The Server can then inject the ACP into the BGP routing system
the same as if an AERO Client/Server DHCPv6 exchange had occurred. If
the mobile node subsequently withdraws the ACP from the access network
routing system, the Server can then withrdaw the ACP from the BGP
routing system.In this arrangement, AERO Servers and Relays are used in exactly
the same ways as for environments where DHCPv6 Client/Server exchanges
are supported. However, the access network routing systems must be
capable of accommodating rapid ACP injections and withrawls from
mobile nodes with the understanding that the information must be
propagated to all routers in the system. Operational expereince has
shown that this kind of routing system "churn" can lead to overall
instability and inconsistency in the routing system.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.In some environments, AERO Servers and Relays may be connected by
dedicated point-to-point links, e.g., high speed fiberoptic leased
lines. In that case, the Servers and Relays can participate in the
AERO link the same as specified above but can avoid encapsulation over
the dedicated links. In that case, however, the links would be
dedicated for AERO and could not be multiplexed for both AERO and
non-AERO communications.A source Client may connect only to an IPvX underlying network,
while the target Client connects only to an IPvY underlying network.
In that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any redirection
messages and continue to send packets via their Servers.When the underlying network does not support multicast, AERO
Clients map link-scoped multicast addresses to the link-layer address
of a Server, which acts as a multicast forwarding agent. The AERO
Client also serves as an IGMP/MLD Proxy for its EUNs and/or hosted
applications per while using the link-layer
address of the Server as the link-layer address for all multicast
packets.When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in for IPv4 underlying networks and use a TBD
site-scoped multicast mapping for IPv6 underlying networks. In that
case, border routers must ensure that the encapsulated site-scoped
multicast packets do not leak outside of the site spanned by the AERO
link.User-level and kernel-level AERO implementations have been developed
and are undergoing internal testing within Boeing.An initial public release of the AERO source code was announced on
the intarea mailing list on August 21, 2015, and a pointer to the code
is available in the list archives.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.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.
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, however AERO nodes
SHOULD also use DHCPv6 securing services (e.g., Secure DHCPv6 , etc.) for Client authentication and
network admission control.AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on their EUNs to gain access to a protected network,
i.e., AERO Clients that act as routers MUST NOT provide routing services
for unauthorized nodes. (This concern is no different than for ordinary
hosts that receive an IP address delegation but then "share" the address
with unauthorized nodes via some form of Internet connection
sharing.)AERO Clients, Servers and Relays on the open Internet are suceptible
to the same attack profiles as for any Internet nodes. For this reason,
IP security MUST be used when AERO is employed over unmanaged/unsecured
links using securing mechanisms such as IPsec ,
IKE and/or TLS .AERO Servers and Relays present targets for traffic amplification
Denial of Service (DoS) attacks. This concern is no different than for
widely-deployed VPN security gateways in the Internet, where attackers
could send spoofed packets to the gateways at high data rates. This
becomes less of a problem when Relays and Servers are connected by
dedicated links with no connections to the Internet and/or when
connections to the Internet asre only permitted through well-managed
firewalls.Traffic amplfication DoS attacks can also target an AERO Client's low
data rate links. This is a concern not only for Clients located on the
open Internet but also for Clients in protected enclaves. AERO Servers
can institute rate limits that protect Clients from receiving packet
floods that could DoS low data rate 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. Discussions on the v6ops list in the December 2015 through
January 2016 timeframe further helped clarify AERO multi-addressing
capabilities. Special thanks go to Stewart Bryant, Joel Halpern and
Brian Haberman for their shepherding guidance during the publication of
the AERO first edition.This work has further been encouraged and supported by Boeing
colleagues including M. Wayne Benson, Dave Bernhardt, Cam Brodie,
Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu Danilov, Wen
Fang, Anthony Gregory, Jeff Holland, Ed King, Gen MacLean, Rob
Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian Skeen, Mike Slane,
Brendan Williams, Julie Wulff, Yueli Yang, and other members of the
BR&T and BIT mobile networking teams. Wayne Benson is especially
acknowledged for his outstanding work in converting the AERO
proof-of-concept implementation into production-ready code.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/TAPWhen GUE encapsulation is not needed, AERO can use common
encapsulations such as IP-in-IP , Generic Routing
Encapsulation (GRE) and
others. The encapsulation is therefore only differentiated from non-AERO
tunnels through the application of AERO control messaging and not
through, e.g., a well-known UDP port number.As for GUE encapsulation, alternate AERO encapsulation formats may
require encapsulation layer fragmentation. For simple IP-in-IP
encapsulation, an IPv6 fragment header is inserted directly between the
inner and outer IP headers when needed, i.e., even if the outer header
is IPv4. The IPv6 Fragment Header is identified to the outer IP layer by
its IP protocol number, and the Next Header field in the IPv6 Fragment
Header identifies the inner IP header version. For GRE encapsulation, a
GRE fragment header is inserted within the GRE header . shows the AERO IP-in-IP encapsulation format
before any fragmentation is applied: shows the AERO GRE encapsulation format
before any fragmentation is applied:Alternate encapsulation may be preferred in environments where GUE
encapsulation would add unnecessary overhead. For example, certain
low-bandwidth wireless data links may benefit from a reduced
encapsulation overhead.GUE encapsulation can traverse network paths that are inaccessible to
non-UDP encapsulations, e.g., for crossing Network Address Translators
(NATs). More and more, network middleboxes are also being configured to
discard packets that include anything other than a well-known IP
protocol such as UDP and TCP. It may therefore be necessary to determine
the potential for middlebox filtering before enabling alternate
encapsulation in a given environment.In addition to IP-in-IP, GRE and GUE, AERO can also use security
encapsulations such as IPsec and SSL/TLS. In that case, AERO control
messaging and route determination occur before security encapsulation is
applied for outgoing packets and after security decapsulation is applied
for incoming packets.An encapsulation fragment header is inserted when the AERO tunnel
ingress needs to apply fragmentation to accommodate packets that must be
delivered without loss due to a size restriction. Fragmentation is
performed on the inner packet while encapsulating each inner packet
fragment in outer IP and encapsulation layer headers that differ only in
the fragment header fields.The fragment header can also be inserted in order to include a
coherent Identification value with each packet, e.g., to aid in
Duplicate Packet Detection (DPD). In this way, network nodes can cache
the Identification values of recently-seen packets and use the cached
values to determine whether a newly-arrived packet is in fact a
duplicate. The Identification value within each packet could further
provide a rough indicator of packet reordering, e.g., in cases when the
tunnel egress wishes to discard packets that are grossly out of
order.In some use cases, there may be operational assurance that no
fragmentation of any kind will be necessary, or that only occasional
large control messages will require fragmentation. In that case, the
encapsulation fragment header can be omitted and ordinary fragmentation
of the outer IP protocol version can be applied when necessary.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 .