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 that supports operation of the IPv6 Neighbor
Discovery (ND) protocol and links IPv6 ND to IP forwarding. Admission
control and address/prefix provisioning are supported by the Dynamic
Host Configuration Protocol for IPv6 (DHCPv6), while mobility management
and route optimization are 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 that supports
operation of the IPv6 Neighbor Discovery (ND)
protocol and links IPv6 ND to IP forwarding. Admission control and
address/prefix provisioning are supported by the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) , while
mobility management and route optimization are 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.A node's AERO interface can be configured over multiple underlying
interfaces. From the standpoint of IPv6 ND, AERO interface neighbors
therefore may appear to have multiple link-layer addresses. Each
link-layer address is subject to change due to mobility, and link-layer
address changes are signaled by IPv6 ND messaging the same as for any
IPv6 link.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 nodes (e.g., cellphones, tablets, laptop computers, etc.) that
connect into a home enterprise network via public access networks using
services such as OpenVPN . AERO is also applicable
to aviation applications for both manned and unmanned aircraft where the
aircraft is treated as a mobile node that can connect an Internet of
Things (IoT). Numerous other use cases are also in scope.The AERO mobile VPN capability and Border Gateway Protocol
(BGP)-based core routing system can further be employed either in
conjunction or separately according to the specific use case (see ). This allows for correct fitting of the (modular)
AERO components to match the specific application. 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.
The AERO mechanisms can also operate over native 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 managed
for uniqueness, 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 .a node that is connected to an AERO
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 IPv6 ND exchanges with
other AERO nodes. A node that acts as an AERO Client on one AERO
interface can also act as an AERO Server on a different AERO
interface.a node that
configures an AERO interface to provide default forwarding services
for AERO Clients. The Server assigns an administratively provisioned
IPv6 link-local unicast address to the AERO interface 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.an AERO
interface endpoint that injects encapsulated packets into an AERO
link.an AERO
interface endpoint that receives encapsulated 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. 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. The Client sees each EUN as a
"downstream" network and sees the AERO interface as its point of
attachment to the "upstream" network.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.the lowest-numbered AERO
address from the first ACP delegated to the Client (see ).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 node on the AERO link maintains an AERO interface
neighbor cache and an IP forwarding table the same as for any link. In
common operational practice, there may be many additional Relays,
Servers and Clients.AERO Relays provide default forwarding services to AERO Servers.
Each Relay also peers with each Server in a dynamic routing protocol
instance to discover the Server's list of associated ACPs (see ). 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. 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 (see ). Servers configure a DHCPv6 server function and
act as delegating routers 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 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
can 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.
Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.The AERO routing system comprises 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 uses eBGP to peer with one or more Relays but
does not peer with other Servers. All Relays are members of the same
hub AS using a common ASN, and use iBGP to maintain a consistent view
of all active ACPs currently in service.Each Server maintains a working set of associated ACPs, and
dynamically announces new ACPs and withdraws departed ACPs in its eBGP
updates to Relays. Clients are expected to remain associated with
their current Servers for extended timeframes, however Servers SHOULD
selectively suppress 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 packets
destined to all other ACPs will correctly incur Destination
Unreachable messages due to the black hole route. Relays do not send
eBGP 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
Clients) 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. At the time of
this writing, the global public Internet BGP routing system manages
more than 500K routes with linear growth and no signs of router
resource exhaustion . Network emulation studies
have also shown that a single Relay can accommodate at least 1M
dynamically changing BGP routes even on a lightweight virtual machine,
i.e., and without requiring high-end dedicated router hardware.Therefore, assuming each Relay can carry 1M or more routes, this
means that at least 1M 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 one or more Relays, but the Server institutes route filters
so that it only sends BGP updates to the specific set of Relays that
aggregate the ASP. 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 1K sets of Relays, the AERO routing system can then
accommodate 1B or more ACPs with no additional overhead for Servers
and Relays (for example, it should be possible to service 1B /64 ACPs
taken from a /34 ASP and evne more for shorter prefixes). 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 an aggregated ASP for the link into the native routing
system even though each Relay services only smaller segments 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, Relays may have multiple Routing Information Base (RIB)
entries for a single ACP advertised by multiple Servers, but will
place only one entry in the Forwarding Information Base (FIB). Servers
can assign a weight to their eBGP peering configurations so that
Relays can determine preferences for ACPs learned from multiple
Servers. In this way, Relays can choose the Server with the highest
weight and insert the corresponding RIB route into the FIB. The Relay
can then fail over to a Server with lower weight in case of ACP
withdrawal or Server failure.AERO interface link-local address types include
administratively-provisioned addresses and AERO addresses.Administratively-provisioned addresses are allocated from the range
fe80::/96 and assigned to a Server or Relay's AERO interface.
Administratively-provisioned addresses MUST be managed for uniqueness
by the administrative authority for the AERO link. (Note that fe80::
is the IPv6 link-local subnet router anycast address, and
fe80::ffff:ffff is the address used by Clients to bootstrap AERO
address autoconfiguration. These special addresses are therefore not
available for administrative provisioning.)An AERO address is an IPv6 link-local address with an embedded
prefix based on an ACP and associated with a Client's AERO interface.
AERO addresses remain stable as the Client moves between topological
locations, i.e., even if its link-layer addresses change.For IPv6, AERO addresses begin with the prefix fe80::/64 and
include in the interface identifier (i.e., the lower 64 bits) a 64-bit
prefix taken from one of the Client's IPv6 ACPs. For example, if the
AERO Client receives the IPv6 ACP:2001:db8:1000:2000::/56it constructs its corresponding AERO addresses as:fe80::2001:db8:1000:2000fe80::2001:db8:1000:2001fe80::2001:db8:1000:2002... etc. ...fe80::2001:db8:1000:20ffFor IPv4, AERO addresses are based on an IPv4-mapped IPv6
address formed from an IPv4 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 ACP is:0:0:0:0:0:FFFF:192.0.2.16/124The Client then constructs its AERO addresses with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as:fe80::FFFF:192.0.2.16fe80::FFFF:192.0.2.17fe80::FFFF:192.0.2.18... etc. ...fe80:FFFF:192.0.2.31When the Server delegates ACPs to the Client, both the Server
and Client use the lowest-numbered AERO address from the first ACP
delegation as the "base" AERO address. (For example, for the ACP
2001:db8:1000:2000::/56 the base address is 2001:db8:1000:2000.) The
Client then assigns the base AERO address to the AERO interface and
uses it for the purpose of maintaining the neighbor cache entry. If
the Client has multiple AERO addresses (i.e., when there are multiple
ACPs and/or ACPs with short prefix lengths), the Client originates
IPv6 ND messages using the base AERO address as the source address and
accepts and responds to IPv6 ND messages destined to any of its AERO
addresses as equivalent to the base AERO address. In this way, the
Client maintains a single neighbor cache entry that may include
multiple AERO addresses.AERO interfaces use encapsulation (see: ) to exchange packets with neighbors attached to
the AERO link.AERO interfaces maintain a neighbor cache, and use both DHCPv6 and
IPv6 ND control messaging to manage the creation, modification and
deletion of neighbor cache entries. AERO interfaces use standard
DHCPv6 messaging for prefix delegation, admission control and neighbor
cache entry management. AERO interfaces use unicast IPv6 ND Neighbor
Solicitation (NS), Neighbor Advertisement (NA), Router Solicitation
(RS) and Router Advertisement (RA) messages for neighbor cache
management 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).Reserved is set to the value '0' on transmission and ignored on
receipt.Interface ID is set to an integer value between 0 and 65535
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. In that case, the
Client would appear to have a single underlying interface but with a
dynamically changing link-layer address.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 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 is taken from the range fe80::/96 but excluding the
special addresses fe80:: and fe80::ffff:ffff. 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 active 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-Client neighbor cache entries and IP forwarding table
entries based on control message exchanges. Each Server also engages
in a dynamic routing protocol with each Relay on the link (see:
).When the Server receives an NS/RS message from a Client on the
AERO interface it returns an NA/RA message. 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
AERO node, 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
administratively-provisioned link-local address fe80::ffff:ffff as
the source network-layer address in DHCPv6 PD messages to obtain one
or more ACPs from an AERO Server. Next, the Client assigns the base
AERO address to the AERO interface and sends an RS to the Server to
receive an RA. In this way, the DHCPv6 PD exchange securely
bootstraps autoconfiguration of unique link-local address(es) while
the RS/RA exchange establishes link-layer addresses and
autoconfigures AERO link parameters. The Client maintains a neighbor
cache entry for each of its Servers and each of its active
correspondent Clients. When the Client receives IPv6 ND messages on
the AERO interface it updates or creates neighbor cache entries,
including link-layer address information.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 and maintained through
DHCPv6 PD and IPv6 ND exchanges as specified in , and remain in place for durations bounded by prefix
delegation 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 delegates prefixes via DHCPv6
PD, it creates a static neighbor cache entry for the Client using the
Client's base AERO address as the network-layer address and associates
all of the Client's other AERO addresses with the neighbor cache
entry. When the Client receives the prefix delegation, it creates a
static neighbor cache entry for the Server based on the DHCPv6 Reply
message link-local source address as the network-layer address and the
encapsulation IP source address and UDP source port number as the
link-layer address. The Client then sends an RS message to inform the
Server of its link-layer addresses and to solicit an RA. When the
Server returns an RA message, the Client uses the autoconfiguration
information in the RA message to configure AERO interface
parameters.Dynamic neighbor cache entries are created or updated based on
receipt of Predirect/Redirect messages as specified in , and are garbage-collected when keepalive timers
expire. AERO Clients maintain dynamic neighbor cache entries for each
of their active correspondent Clients with lifetimes based on IPv6 ND
messaging constants.When an AERO Client receives a valid Predirect message it creates
or updates a dynamic neighbor cache entry for the Predirect target
network-layer and link-layer addresses. 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. The node resets AcceptTime when it
receives a new Predirect, and otherwise decrements AcceptTime while no
Predirects have been received. 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 (see below).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. 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 node resets ForwardTime when it
receives a new Redirect, and otherwise decrements ForwardTime while no
Redirects have been received. 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 .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 MAX_RETRY be set to 3 the same as
described for IPv6 ND address resolution in Section 7.3.3 of .Different values for ACCEPT_TIME, FORWARD_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 forwarded 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 forwarded to
the network layer where they are subject to either local delivery or
IP forwarding. In all cases, the AERO interface itself MUST NOT
decrement the network layer TTL/Hop-count since its forwarding actions
occur below the network layer.AERO interfaces may have multiple underlying interfaces and/or
neighbor cache entries for neighbors with multiple Interface ID
registrations (see ). The AERO node uses
each packet's DSCP value to select an outgoing underlying interface
based on the node's own preference values, and also to select a
destination link-layer address based on the neighbor's underlying
interface with the highest preference value. If multiple outgoing
interfaces and/or neighbor interfaces have a preference of "high", the
AERO node sends one copy of the packet via each of the (outgoing /
neighbor) interface pairs; otherwise, the node sends a single copy of
the packet.The following sections discuss the AERO interface forwarding
algorithms for Clients, Servers and Relays. In the following
discussion, a packet's destination address is said to "match" if it is
a non-link-local address with a prefix covered by an ASP/ACP, or if it
is an AERO address that embeds an ACP, or if it is the same as an
administratively-provisioned link-local address.When an IP packet enters a Client's AERO interface from the
network layer the Client searches for a neighbor cache entry that
matches the destination. If there is a match, the Client uses one or
more link-layer addresses in the entry as the link-layer addresses
for encapsulation and admits the packet into the AERO link.
Otherwise, the Client uses the link-layer address in a static
neighbor cache entry for a Server as the encapsulation address.When an IP packet enters a Client's AERO interface from the
link-layer, if the destination matches one of the Client's ACPs or
link-local addresses the Client decapsulates the packet and delivers
it to the network layer. Otherwise, the Client drops the packet
silently.When an IP packet enters a Server's AERO interface from the
network layer, the Server searches for a static or dynamic neighbor
cache entry that matches the destination. If there is a match, the
Server uses one or more link-layer addresses in the entry as the
link-layer addresses for encapsulation and admits the packet into
the AERO link. Otherwise, the Server 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 Server's AERO interface from the link
layer, the Server processes the packet as follows:if the destination matches one of the Server's own addresses
the Server decapsulates the packet and forwards it to the
network layer for local delivery.else, if the destination matches a static or dynamic neighbor
cache entry the Server first determines whether the neighbor is
the same as the one it received the packet from. If so, the
Server MUST drop the packet silently to avoid looping;
otherwise, the Server uses the neighbor's link-layer address(es)
as the destination for encapsulation and re-admits the packet
into the AERO link.else, the Server 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 ACP
entry that matches the destination and otherwise searches for a
neighbor cache entry that 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
forwards the packet into the AERO link. Otherwise, the Relay drops
the packet and (for non-link-local addresses) returns an ICMP
Destination Unreachable message subject to rate limiting (see: ).When an IP packet enters a Relay's AERO interface from the
link-layer, the Relay processes the packet as follows:if the destination does not match an ASP, or if the
destination matches one of the Relay's own addresses, the Relay
decapsulates the packet and forwards it to the network layer
where it will be subject to either local delivery or IP
forwarding.else, if the destination matches an ACP entry in the IP
forwarding table, or if the destination matches the link-local
address in a permanent neighbor cache entry, the Relay first
determines whether the neighbor is the same as the one it
received the packet from. If so the Relay MUST drop the packet
silently to avoid looping; otherwise, the Relay uses the
neighbor's link-layer address as the destination for
encapsulation and re-admits the packet into the AERO link.else, the Relay drops the packet and (for non-link-local
addresses) returns an ICMP Destination Unreachable message
subject to rate limiting (see: ).AERO interfaces encapsulate IP packets according to whether they
are entering the AERO interface from the network layer or if they are
being re-admitted into the same AERO link they arrived on. This latter
form of encapsulation is known as "re-encapsulation".The AERO interface encapsulates packets per the Generic UDP
Encapsulation (GUE) procedures in , or through an alternate
encapsulation format (see: ). For packets
entering the AERO interface from the network 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, the
AERO interface instead copies these values from the original
encapsulation IP header into the new encapsulation header, i.e., the
values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header. (Note especially
that by copying the TTL/Hop Limit between encapsulation headers the
value will eventually decrement to 0 if there is a (temporary) routing
loop.) For IPv4 encapsulation/re-encapsulation, the AERO interface
sets the DF bit as discussed in .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 or Relay, the
AERO interface sets the UDP destination port to 8060, i.e., the
IANA-registered port number for AERO. For packets sent to a Client,
the AERO interface sets the UDP destination port to the port value
stored in the neighbor cache entry for this Client. The AERO interface
then either includes or omits the UDP checksum according to the GUE
specification.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 and IPv6 ND
messages from Clients, and create or update a static neighbor
cache entry for the Client based on the specific 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 and Servers accept encapsulated packets if there
is a dynamic neighbor cache entry with an AERO address that
matches the packet's network-layer source address, with a
link-layer address that matches the packet's link-layer source
address, and with a non-zero AcceptTime.Note that this simple data origin authentication is effective
in environments in which link-layer addresses cannot be spoofed. In
other environments, each AERO message must include a signature that
the recipient can use to authenticate the message origin, e.g., as for
common VPN systems such as OpenVPN . In
environments where end systems use end-to-end security, however, it
may be sufficient to require signatures only for AERO DHCPv6, IPv6 ND
and ICMP control plane messages and omit signatures for data plane
messages.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.The Internet Protocol expects that IP packets will either be
delivered to the destination or a suitable Packet Too Big (PTB)
message returned to support the process known as IP Path MTU Discovery
(PMTUD) . However, PTB
messages may be crafted for malicious purposes such as denial of
service, or lost in the network . This can be
especially problematic for tunnels, where a condition known as a PMTUD
"black hole" can result. For these reasons, AERO interfaces employ
operational procedures that avoid interactions with PMTUD, including
the use of fragmentation when necessary.AERO interfaces observe two different types of fragmentation.
Source fragmentation occurs when the AERO interface (acting as a
tunnel ingress) fragments the encapsulated packet into multiple
fragments before admitting each fragment into the tunnel. Network
fragmentation occurs when an encapsulated packet admitted into the
tunnel by the ingress is fragmented by an IPv4 router on the path to
the egress. Note that a packet that incurs source fragmentation may
also incur network fragmentation.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 encapsulated packets
may incur network fragmentation.IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500
bytes , while the minimum MRU for IPv4 is only
576 bytes (note that common IPv6 over IPv4
tunnels already assume a larger MRU than the IPv4 minimum).AERO interfaces therefore configure an MTU that MUST NOT be smaller
than 1280 bytes, MUST NOT be larger than the minimum MRU among all
nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also
configure a Maximum Segment Unit (MSU) as the maximum-sized
encapsulated packet that the ingress can inject into the tunnel
without source fragmentation. The MSU 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.All AERO nodes MUST configure the same MTU/MSU values for reasons
cited in ; in
particular, multicast support requires a common MTU value among all
nodes on the link. All AERO nodes MUST configure an MRU large enough
to reassemble packets up to (MTU+ENCAPS) bytes in length; nodes that
cannot configure a large-enough MRU MUST NOT enable an AERO
interface.The network layer proceeds as follow when it presents an IP packet
to the AERO interface. For each IPv4 packet that is larger than the
AERO interface MTU and with the DF bit set to 0, the network layer
uses IPv4 fragmentation to break the packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MTU and the remaining fragments are no larger than the first. For
all other IP packets, if the packet is larger than the AERO interface
MTU, the network layer drops the packet and returns a PTB message to
the original source. Otherwise, the network layer admits each IP
packet or fragment into the AERO interface.For each IP packet admitted into the AERO interface, the interface
(acting as a tunnel ingress) encapsulates the packet. If the
encapsulated packet is larger than the AERO interface MSU the ingress
source-fragments the encapsulated packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MSU and the remaining fragments are no larger than the first. The
ingress then admits each encapsulated packet or fragment into the
tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
header in case any network fragmentation is necessary. The
encapsulated packets will be delivered to the egress, which
reassembles them into a whole packet if necessary.Several factors must be considered when fragmentation is needed.
For AERO links over IPv4, the IP ID field is only 16 bits in length,
meaning that fragmentation at high data rates could result in data
corruption due to reassembly misassociations . 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
source 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 or network-layer error
indications.A link-layer 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 link-layer 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 as discussed in .)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 link-layer error message format is shown in (where, "L2" and "L3" refer to link-layer and
network-layer, respectively):The AERO node rules for processing these link-layer error
messages are as follows:When an AERO node receives a link-layer 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 link-layer Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
awaiting reassembly have been processed. In that case, the node
SHOULD begin including integrity checks and/or institute rate
limits for subsequent packets.When an AERO node receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its dynamic neighbor correspondents, the node
SHOULD test the path to the correspondent using Neighbor
Unreachability Detection (NUD) (see ). If NUD
fails, the node SHOULD set ForwardTime for the corresponding
dynamic neighbor cache entry to 0 and allow future packets
destined to the correspondent to flow through a default route.When an AERO Client receives persistent link-layer Destination
Unreachable messages in response to encapsulated 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 associate with a new Server and send a DHCPv6 Release
message to the old Server as specified in .When an AERO Server receives persistent link-layer Destination
Unreachable messages in response to encapsulated 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 link-layer Destination
Unreachable messages in response to an encapsulated packet that it
sends to one of its permanent neighbors, it treats the messages as
an indication that the path to the neighbor may be failing.
However, neighbor reachability will be determined by the dynamic
routing protocol.When an AERO Relay receives a packet for which the
network-layer 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 a network-layer Destination Unreachable message
subject to rate limiting. The Relay first writes the network-layer
source address of the original packet as the destination address of
the 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 source address of
the message, then encapsulates the message and forwards it 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 message and
forwards it 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
network-layer Packet Too Big (PTB) message. The node first writes the
MRU value into the PTB message MTU field, writes the network-layer
source address of the original packet as the destination address of
the 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 source address of
the message, then encapsulates the message and forwards it 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 message and forwards
it via a link outside the AERO interface.When an AERO node receives any network-layer error message via the
AERO interface, it examines the network-layer destination address. If
the next hop toward the destination is via the AERO interface, the
node re-encapsulates and forwards the message to the next hop within
the AERO interface. Otherwise, if the network-layer source address 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,
recalculates the IP and/or ICMP checksums then forwards the message
via a link outside the AERO interface.AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated by the DHCPv6 and IPv6 ND control messaging protocols 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) , via static
configuration, etc.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 use unicast IPv6 ND messages to maintain
neighbor cache entries the same as for any link. AERO Servers act as
default routers for AERO Clients, and therefore send unicast RA
messages with configuration information in response to a Client's RS
message.The following sections specify the Client and Server
behavior.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 DHCPv6 PD exchange . The Client's DHCPv6
Solicit message includes fe80::ffff:ffff as the IPv6 source address,
'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address,
the address of the Client's underlying interface 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 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 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 any available underlying interface.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 it creates a static neighbor cache entry with the Server's
link-local address as the network-layer address and the Server's
encapsulation address as the link-layer address. Next, the Client
autoconfigures AERO addresses for each of the delegated ACPs and
assigns the base AERO address to the AERO interface.The Client then prepares a unicast RS message to send to the
Server in order to obtain a solicited RA. The Client includes its
base AERO address as the network-layer source address, the Server's
link-local address as the network-layer destination address, the
Client's link-layer address as the link-layer source address, and
Server's link-layer address as the link-layer destination address.
The Client also includes one or more SLLAOs formatted as described
in to register its link-layer address(es)
with the Server.The first SLLAO MUST correspond to the underlying interface over
which the Client will send the RS. The Client MAY include additional
SLLAOs 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 can register additional
link-layer addresses with the Server by sending subsequent
unsolicited NA messages after the initial RS/RA exchange). The
Server will use the S/TLLAOs to populate its link-layer address
information for the Client.When the Client receives an RA from the AERO Server (see ), it configures a default route with the
Server as the next hop via the AERO interface. The Client next
examines the Code value in the RA message; if Code was 1 the Client
can assume there was a NAT on the path to the Server. The Client
also caches any ASPs included in Prefix Information Options (PIOs)
as ASPs to associate with the AERO link, and assigns the MTU/MSU
values in the MTU options to its AERO interface while configuring an
appropriate MRU. This configuration information applies to the AERO
link as a whole, and all AERO nodes will use the same values.Following autoconfiguration, the Client sub-delegates the ACPs to
its attached EUNs and/or the Client's own internal virtual
interfaces. In the former case, the Client acts as a router for
nodes on its attached EUNs. In the latter case, the Client acts as a
host and can configure as many addresses as it wants from /64
prefixes taken from the ACPs and assign them to either an internal
virtual interface ("weak end-system") or to the AERO interface
itself ("strong end-system") while
black-holing the remaining portions of the /64s. The Client
subsequently renews its ACP delegations through each of its Servers
by sending DHCPv6 Renew messages.After the Client registers its Interface IDs and their associated
'P(i)' values, it 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 an
unsolicited NA message to send over any available underlying
interface. The NA MUST include a S/TLLAO specific to the selected
available underlying interface as the first S/TLLAO and MAY include
any additional S/TLLAOs specific to other underlying interfaces. The
Client includes fresh 'P(i)' values in each S/TLLAO 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 S/TLLAO
with 'P(i)' values set to 0 ("disabled").If the Client wishes to discontinue use of a Server it issues a
DHCPv6 Release message to both delete the Server's neighbor cache
entry and release the DHCPv6 PD.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.Next, the Server prepares a Reply message to send to the Client
while using fe80::ID as the network-layer source address, the
link-local address taken from the Client's Solicit as the
network-layer destination address, the Server's link-layer address
as the source link-layer address, and the Client's link-layer
address as the destination link-layer address. The Server also
includes an IA_PD option with the delegated 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 .When the Server sends the Reply message, it creates a static
neighbor cache entry for the Client using the base AERO address as
the network-layer address and with lifetime set to no more than the
smallest PD lifetime. The Client will subsequently issue an RS
message with one or more SLLAO options and with the Client's base
AERO address as the source address.When the Server receives the RS message, it first verifies that a
neighbor cache entry for the Client exists (otherwise, it discards
the RS). The Server then updates the neighbor cache entry link-layer
address(es) by recording the information in each SLLAO option
indexed by the Interface ID and including the UDP port number, IP
address and P(i) values. For the first SLLAO in the list, however,
the Server records the actual encapsulation source UDP and IP
addresses instead of those that appear in the SLLAO in case there
was a NAT in the path.The Server then prepares a unicast RA message to send back to the
Client using fe80::ID as the network-layer source address, the
Client's base AERO address as the network-layer destination address,
the Server's link-layer address as the source link-layer address,
and the source link-layer address of the RS message as the
destination link-layer address. In the RA message, if the actual
encapsulation addresses in the RS message were the same as those
that appeared in the first SLLAO (see above), the Server sets the
Code field to 0; otherwise it sets Code to 1. The Server then
includes one or more PIOs that encode the ASPs for the AERO link,
and with flags A=0; L=1. The Server also includes two MTU options -
the first MTU option includes the MTU for the link and the second
MTU option includes the MSU for the link (see ).When the Server delegates the ACPs, it also creates an IP
forwarding table entry for each ACP so that the AERO BGP-based
routing system will propagate the ACPs to all Relays that aggregate
the corresponding ASP (see: ).After the initial DHCPv6 PD Solicit/Reply and IPv6 ND RS/RA
exchanges, the AERO Server maintains the neighbor cache entry for
the Client until the PD lifetimes expire. 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 addressed
to either 'All_DHCP_Relay_Agents_and_Servers' or the Server's
fe80::ID unicast address, it wraps the message in a Relay-Forward
message header and includes an Interface-ID option that includes
enough information to allow the LDRA to forward the resulting
Reply message back to the Client (this information may include the
Client's UDP and IP addresses, a security association identifier,
etc). The LDRA then forwards the message to the DHCPv6 server.When the DHCPv6 server prepares a Reply message, it wraps the
message in a Relay-Reply message and echoes the Interface-ID
option. 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 based on the information in the
Interface ID option.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.
The procedure is based on an exchange of IPv6 ND messages using a
chain of AERO Servers and Relays as a trust basis.Although the Client is responsible for initiating route
optimization, 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 simply discard any route optimization messages instead
of forwarding them.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 administratively-provisioned link-local 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 . AERO Redirect/Predirect messages formats are
identical except that Redirect messages use Code=0, while Predirect
messages use Code=1. The Redirect/Predirect message format is 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 base AERO address of Client ('C1')).the network-layer destination address is set to the AERO
address corresponding to the destination address of Client
('C2').the Type is set to 137.the Code is set to 1 to indicate "Predirect".the Target Address is set to fe80::2001:db8:0:0 (i.e., the
base 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.the message includes one or more Route Information Options
(RIOs) that include Client ('C1')'s
ACPs.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 ACPs encoded in the RIOs of the Predirect. If validation fails,
Server ('S1') discards the Predirect; otherwise, it copies the
correct UDP Port number and IP Address for Client ('C1')'s
underlying link into the first TLLAO in case the addresses have been
subject to NAT.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 ACPs encoded in the RIOs of the
Predirect as the ACPs for Client ('C1'). 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 base AERO address of Client ('C2')).the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the base AERO address of Client
('C1')).the Type is set to 137.the Code is set to 0 to indicate "Redirect".the Target Address is set to fe80::2001:db8:1:0 (i.e., the
base 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.the message includes one or more Route Information Options
(RIOs) that include Client ('C2')'s ACPs.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 ACPs encoded in the RIOs of the
Redirect message. If validation fails, Server ('S2') discards the
Redirect; otherwise, it copies the correct UDP Port number and IP
Address for Client ('C2')'s underlying link into the first TLLAO in
case the addresses have been subject to NAT.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 ACPs encoded in the RIOs of the
Redirect as the ACPs for Client ('C2').. 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 a proxy redirection target, e.g., if direct
Client-to-Client communications are not possible. In that case, when
the source Client sends a Predirect message the target Server
prepares a corresponding Redirect the same as if it were the target
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.Similarly, when the source Client must send all packets via its
own Server and cannot act on a route optimization request, the
source Server can send a Predirect message toward the target Client.
The target Client then prepares a corresponding Redirect message the
same as for Client-to-Client route optimization and sends the
Redirect message back to the source Server.Thereafter, if a Client moves to a new Server, the old Server
sends ICMP "Destination Unreachable" messages subject to rate
limiting in response to data packets received from a correspondent
node to report that the route optimization ForwardTime should be set
to 0. The correspondent Client (or Server) then allows future
packets destined to the departed Client to again flow through its
own Server (or Relay). Note however that the old Server retains the
neighbor cache entry and does not set AcceptTime to 0 since there
may be many packets in flight. When the old Server receives these
packets, it forwards them to a Relay which will forward them to the
departed Client's new Server. AcceptTime will then eventually
decrement to 0 once the correspondent node processes and acts on the
Destination Unreachables.In any case, a Server MUST NOT send a BGP update to its Relays
for Clients discovered through dynamic route optimization
redirection. BGP updates are only to be sent for the Server's
working set of statically-associated Clients.If neither the source nor target Clients are capable of sending
packets other than via their own Servers, a Server-to-Server route
optimization can still be employed. In that case, the source
Client's Server can send a Predirect message via a Relay to the AERO
address of the target Client, and the Relay will forward the message
to the target Client's Server. The target Server prepares the
Redirect message the same as if it were the target Client, except
that it writes its own link-layer address in the TLLAO option then
sends a Redirect message back to the source Server. (The target
Server can send the Redirect message back to the source Server
either directly or via the Relay according to the security model.)
Both Servers must then maintain a dynamic neighbor cache entry for
the redirected Clients.Thereafter, if a Client moves to a new Server, the old Server
sends ICMP "Destination Unreachable" messages subject to rate
limiting in response to data packets forwarded by the correspondent
Server to report that the route optimization ForwardTime should be
set to 0. The correspondent Server then allows future packets
destined to the departed Client to again flow through its own Relay.
Note however that the old Server retains the neighbor cache entry
and does not set AcceptTime to 0 since there may be many packets in
flight. When the old Server receives these packets, it forwards them
to a Relay which will forward them to the departed Client's new
Server. AcceptTime will then eventually decrement to 0 once the
correspondent node processes and acts on the Destination
Unreachables.In any case, a Server MUST NOT send a BGP update to its Relays
for Clients discovered through dynamic route optimization
redirection. BGP updates are only to be sent for the Server's
working set of statically-associated Clients..AERO nodes perform Neighbor Unreachability Detection (NUD) by
sending unicast NS messages with SLLAOs 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
update neighbor cache entry timers and/or link-layer address
information.When an AERO node sends an NS/NA message, it MUST use one of its
link-local addresses as the IPv6 source address and a 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 AERO node is redirected to a target AERO node 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 node can optionally continue sending packets via its Server (or
Relay), maintain a small queue of packets until target reachability is
confirmed, or (optimistically) allow packets to flow directly to the
target.While data packets are still flowing, the source node thereafter
periodically tests the direct path to the target node (see Section 7.3
of ) in order to keep dynamic neighbor cache
entries alive. When the target node receives a valid NS message, it
resets AcceptTime to ACCEPT_TIME and updates its cached link-layer
addresses (if necessary). When the source node receives a solicited NA
message, it resets ForwardTime to FORWARD_TIME and updates its cached
link-layer addresses (if necessary). If the source node is unable to
elicit a solicited NA response from the target node after MaxRetry
attempts, it SHOULD set ForwardTime to 0. Otherwise, the source node
considers the path usable and SHOULD thereafter process any link-layer
errors as a hint that the direct path to the target node has either
failed or has become intermittent.When ForwardTime for a dynamic neighbor cache entry expires, the
source node resumes sending any subsequent packets via a Server (or
Relay) and may (eventually) attempt to re-initiate the AERO
redirection process. When AcceptTime for a dynamic neighbor cache
entry expires, the target node discards any subsequent packets
received directly from the source node. When both ForwardTime and
AcceptTime for a dynamic neighbor cache entry expire, the node deletes
the neighbor cache entry.When a Client needs to change its link-layer addresses, e.g., due
to a mobility event, it sends unsolicited NAs to its neighbors using
the new link-layer address as the source address and with TLLAOs
that include the updated Client link-layer information.The Client MAY send up to MaxRetry unsolicited NA messages in
parallel with sending actual data packets in case one or more NAs
are lost. If all NAs are lost, the Client will eventually invoke NUD
by sending NS messages that include SLLAOs.When a Client needs to bring new underlying interfaces into
service (e.g., when it activates a new data link), it sends
unsolicited NAs to its neighbors using the new link-layer address as
the source address and with TLLAOs that include the new Client
link-layer information.When a Client needs to remove existing underlying interfaces from
service (e.g., when it de-activates an existing data link), it sends
unsolicited NAs to its neighbors with TLLAOs that include P(i)
values set to "disabled".If the Client needs to send the unsolicited NAs over a link other
than the one being removed from service, it MUST include a TLLAO for
the sending link as the first TLLAO and include the TLLAO for the
link being removed from service as an additional TLLAO.AERO interface neighbors MAY include a configuration knob that
allows them to perform implicit mobility management in which no IPv6
ND messaging is used. In that case, the Client only transmits
packets over a single interface at a time, and the neighbor 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 neighbor 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
DHCPv6 Release message via a new Server with its base AERO address
as the network-layer source address and the unicast link-local
address of the old Server as the network-layer destination address.
The new Server then encapsulates the Release message in a DHCPv6
Relay-Forward message header, writes the Client's source address in
the 'peer-address' field, and writes its own link-local address in
the IP source address (i.e., the new Server acts as a DHCPv6 relay
agent). The new Server then forwards the message to an Relay, which
forwards the message to the old Server based on the network-layer
destination address.When the old Server receives the Release, it first authenticates
the message then releases the DHCPv6 PDs and deletes the Client's
ACP routes. The old Server then deletes the Client's neighbor cache
entry so that any in-flight packets will be forwarded via a Relay to
the new Server, which will forward them to the Client. The old
Server finally returns a DHCPv6 Relay-Reply message via an Relay to
the new Server, which will decapsulate the DHCPv6 Reply message and
forward it to the Client.When the new Server forwards the Reply message, the Client 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 MaxRetry attempts, the old Server may have failed and
the Client should discontinue its Release attempts.)Note that this DHCPv6 relay-chaining approach is necessary to
avoid failures, e.g., due to temporary routing fluctuations. In
particular, the Client should always be able to forward messages via
its new Server but may not always be able to send messages directly
to an old Server. But, the new Server and Old Server should always
be able to exchange messages with one another.Finally, 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 small 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 delivered 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.In some environments, an AERO node may have no way of
authenticating any unsolicited NA messages it receives. In that
case, the target AERO node SHOULD ignore any unsolicited NA messages
it receives, and the source AERO node SHOULD inform the target of
its new link-layer addresses by sending a fresh Predirect message
via its Server (or Relay). The target AERO node can then accept the
Predirect message and update its link-layer addresses based on the
Predirect TLLAOs.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.AERO can be used in many different variations based on the specific
use case. The following sections discuss variations that adhere to the
AERO principles while allowing selective application of AERO
components.IPv6 AERO links typically have ASPs that cover many candidate ACPs
of length /64 or shorter. However, in some cases it may be desirable
to use AERO over links that have only a /64 ASP. This can be
accommodated by treating all Clients on the AERO link as simple hosts
that receive /128 prefix delegations.In that case, each Client configures an
administratively-provisioned link-local address instead of an AERO
address, i.e., the same as for Servers and Relays. The Client
discovers its link-local address by including an IA_NA option in its
DHCPv6 Solicit message to the Server. The Server responds by returning
the Client's administratively-provisioned link-local address in the
IA_NA option plus any IPv6 addresses for the Client in IA_PD options
with prefix length /128.For example, if the ASP for the host-only IPv6 AERO link is
2001:db8:1000:2000::/64, each Client will receive one or more /128
IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
2001:db8:1000:2000::2/128, etc. The Client then assigns the /128s to
the AERO interface as IPv6 addresses, and the Client's applications
treat the AERO interface as an ordinary host interface.In this arrangement, the Client conducts route optimization in the
same sense as discussed in , except that the
Predirect message network-layer source address is the Client's
administratively-assigned link-local address and the network-layer
destination address is the same as the destination address of the
packet that triggered the redirection. All other aspects of AERO
operation are the same as described in earlier sections.This has applicability for nodes that act as a Client on an
"upstream" AERO link, but also act as a Server on "downstream" AERO
links. More specifically, if the node acts as a Client to receive a
/64 prefix from the upstream AERO link it can then act as a Server to
provision /128s to Clients on downstream AERO links.Note that, due to the nature of the AERO address format, valid IPv6
ACP lengths are either /64 or shorter, or exactly /128 (i.e., prefix
lengths between /65 and /127 cannot be accommodated).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 subnetwork 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
subnetwork. The Server can then inject the ACP into the BGP routing
system the same as if an AERO Client/Server DHCPv6 PD exchange had
occurred. If the mobile node subsequently withdraws the ACP from the
access network routing system, the Server can then withdraw 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 subnetwork routing systems must be
capable of accommodating rapid ACP injections and withdrawals from
mobile nodes with the understanding that the information must be
propagated to all routers in the system. Operational experience has
shown that this kind of routing system "churn" can lead to overall
instability and routing system inconsistency.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-provisioned 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.Production user-level and kernel-level AERO implementations have been
developed and are undergoing internal testing within Boeing.An initial public release of the AERO proof-of-concept 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. In cases where spoofing cannot be mitigated
through other means, however, all AERO IPv6 ND messages should employ
Secure Neighbor Discovery (SeND) .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. Following authenticated DHCPv6 PD procedures,
AERO nodes MUST ensure that the source of data packets corresponds to
the node to which the prefixes were delegated.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 other nodes via some form of Internet connection sharing.)AERO Clients, Servers and Relays on the open Internet are susceptible
to the same attack profiles as for any Internet nodes. For this reason,
IP security SHOULD be used when AERO is employed over
unmanaged/unsecured links using securing mechanisms such as IPsec , IKE and/or TLS . In some environments, however, the use of end-to-end
security from Clients to correspondent nodes (i.e., other Clients and/or
Internet nodes) could obviate the need for IP security between AERO
Clients, Servers and Relays.AERO Servers and Relays present targets for traffic amplification 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 can be mitigated by
connecting Relays and Servers over dedicated links with no connections
to the Internet and/or when connections to the Internet are only
permitted through well-managed firewalls.Traffic amplification 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, Bob Braden,
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, Gene MacLean III, Rob
Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian Skeen, Mike Slane,
Carrie Spiker, 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.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.This work is aligned with the Boeing Research and Technology
(BR&T) autonomous systems networking program.http://en.wikipedia.org/wiki/TUN/TAPhttp://openvpn.netBGP in 2015, http://potaroo.netWhen 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.AERO is especially well suited for use with VPN system encapsulations
such as OpenVPN .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 .When an enterprise mobile node 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 node to the security
gateway. During this process, the mobile node supplies the security
gateway with its public Internet address as the link-layer address for
the VPN. The mobile node 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 node (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 nodes 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 nodes
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 node 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.Changes from -74 to -75:Bumped version number ahead of expiration deadline