Author: F. Templin, Ed.
Boeing Research & Technology
August 2012
Asymmetric Extended Route Optimization (AERO)
Abstract
Nodes attached to common multi-access link types (e.g., multicast-
capable, shared media, non-broadcast multiple access (NBMA), etc.)
can exchange packets as neighbors on the link, but they may not
always be provisioned with sufficient routing information for optimal
neighbor selection. Such nodes should therefore be able to discover
a trusted intermediate router on the link that provides both
forwarding services to reach off-link destinations and redirection
services to inform the node of an on-link neighbor that is closer to
the final destination. This redirection can provide a useful route
optimization, since the triangular path from the ingress link
neighbor, to the intermediate router, and finally to the egress link
neighbor may be considerably longer than the direct path from ingress
to egress. However, ordinary redirection may lead to operational
issues on certain link types and/or in certain deployment scenarios.
This document therefore introduces an Asymmetric Extended Route
Optimization (AERO) capability that addresses the issues.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6706.
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................4
2. Terminology .....................................................6
3. Motivation ......................................................7
4. Example Use Cases ...............................................8
5. Requirements ....................................................9
6. Asymmetric Extended Route Optimization (AERO) ..................10
6.1. AERO Link Dynamic Routing .................................10
6.2. AERO Node Behavior ........................................11
6.2.1. AERO Node Types ....................................11
6.2.2. AERO Host Behavior .................................11
6.2.3. Edge AERO Router Behavior ..........................11
6.2.4. Intermediate AERO Router Behavior ..................12
6.3. AERO Reference Operational Scenario .......................12
6.4. AERO Specification ........................................14
6.4.1. Traditional Redirection Approaches .................14
6.4.2. AERO Concept of Operations .........................15
6.4.3. Conceptual Data Structures and Protocol Constants ..16
6.4.4. Data Origin Authentication .........................17
6.4.5. AERO Redirection Message Format ....................18
6.4.6. Sending Predirects .................................20
6.4.7. Processing Predirects and Sending Redirects ........21
6.4.8. Forwarding Redirects ...............................22
6.4.9. Processing Redirects ...............................23
6.4.10. Sending Periodic Predirect Keepalives .............24
6.4.11. Neighbor Reachability Considerations ..............26
6.4.12. Mobility Considerations ...........................26
6.4.13. Link-Layer Address Change Considerations ..........27
6.4.14. Prefix Re-provisioning Considerations .............28
6.4.15. Backward Compatibility ............................29
7. IANA Considerations ............................................29
8. Security Considerations ........................................29
9. Acknowledgements ...............................................29
10. References ....................................................30
10.1. Normative References .....................................30
10.2. Informative References ...................................30
Appendix A. Intermediate Router Interworking ......................32
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1. Introduction
Nodes attached to common multi-access link types (e.g., multicast-
capable, shared media, non-broadcast multiple access (NBMA), etc.)
can exchange packets as neighbors on the link, but they may not
always be provisioned with sufficient routing information for optimal
neighbor selection. Such nodes should therefore be able to discover
a trusted intermediate router on the link that provides both default
forwarding services to reach off-link destinations and redirection
services to inform the node of an on-link neighbor that is closer to
the final destination.
+--------------+
| Router A |
| (D->C) |
+--------------+
|
X--------+--------+--------+------X
| |
+----------+---+ +---+----------+
| Node B | | Router C |
| (default->A) | +-------+------+
+--------------+ .-.
,-( _)-.
.-(_ IPv6 )-.
(__ EUN )
`-(______)-'
+-------+------+
| Node D |
+--------------+
Figure 1: Traditional Multi-Access Link Redirection
Figure 1 shows a traditional multi-access link redirection scenario.
In this figure, node ('B') is provisioned with only a default route
with router ('A') as the next hop. Router ('A'), in turn, has a more
specific route that lists router ('C') as the next-hop neighbor on
the link for the End User Network (EUN) attached to node ('D').
If node ('B') has a packet to send to node ('D'), node ('B') is
obliged to send its initial packets via router ('A'). Router ('A')
then forwards the packet to router ('C') and also returns a
redirection control message to inform ('B') that ('C') is, in fact,
an on-link neighbor that is closer to the final destination ('D').
After receiving the redirection control message, node ('B') can place
a more specific route in its forwarding table so that future packets
destined to node ('D') can be sent directly via router ('C'), as
shown in Figure 2.
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+--------------+
| Router A |
| (D->C) |
+--------------+
|
X--------+--------+--------+------X
| |
+----------+---+ +---+----------+
| Node B | | Router C |
| (default->A) | +-------+------+
| (D->C) | .-.
+--------------+ ,-( _)-.
.-(_ IPv6 )-.
(__ EUN )
`-(______)-'
+-------+------+
| Node D |
+--------------+
Figure 2: More Specific Route Following Redirection
This traditional redirection can provide a useful route optimization,
since the triangular path from the ingress link neighbor, to the
intermediate router, and finally to the egress link neighbor may be
considerably longer than the direct path from ingress to egress.
However, ordinary redirection may lead to operational issues on
certain link types and/or in certain deployment scenarios.
For example, when an ingress link neighbor accepts an ordinary
redirection control message, it has no way of knowing whether the
egress link neighbor is ready and willing to accept packets directly
without forwarding through an intermediate router. Likewise, the
egress has no way of knowing that the ingress is authorized to
forward packets from the claimed network-layer source address. (This
is especially important for very large links, since any node on the
link can spoof the network-layer source address with low probability
of detection even if the link-layer source address cannot be
spoofed.) Additionally, the ingress would have no way of knowing
whether the direct path to the egress has failed, nor whether the
final destination has moved away from the egress to some other
network attachment point.
Therefore, a new approach is required that can enable redirection
signaling from the egress to the ingress link node under the
mediation of a trusted intermediate router. The mechanism is
asymmetric (since only the forward direction from the ingress to the
egress is optimized) and extended (since the redirection extends
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forward to the egress before reaching back to the ingress). This
document therefore introduces an Asymmetric Extended Route
Optimization (AERO) capability that addresses the issues.
While the AERO mechanisms were initially designed for the specific
purpose of NBMA tunnel virtual interfaces (e.g., see [RFC2529],
[RFC5214], [RFC5569], and [VET]), they can also be applied to any
multiple access link types that support redirection. The AERO
techniques are discussed herein with reference to IPv6
[RFC2460][RFC4861][RFC4862][RFC3315]; however, they can also be
applied to any other network-layer protocol (e.g., IPv4
[RFC0791][RFC0792][RFC2131], etc.) that provides a redirection
service (details of operation for other network-layer protocols are
out of scope).
This document is an Experimental RFC; therefore, it does not seek to
define a new standard for the Internet. Experimental status instead
of Standards Track has been used since the document proposes a new
and different dynamic routing mechanism. Experimentation will focus
on candidate multi-access link types that can connect large numbers
of neighboring nodes where the use of existing dynamic routing
protocols may be impractical. Examples include NBMA tunnel virtual
links, large bridged campus LANs, etc.
2. Terminology
The terminology in the normative references applies; the following
terms are defined within the scope of this document:
AERO link
any link (either physical or virtual) over which the AERO
mechanisms can be applied. (For example, a virtual overlay of
tunnels can serve as an AERO link.)
AERO interface
a node's attachment to an AERO link.
AERO node
a router or host that is connected to an AERO link and that
participates in the AERO protocol on that link.
intermediate AERO router ("intermediate router")
a router that configures an advertising router interface on an
AERO link over which it can provide default forwarding and
redirection services for other AERO nodes.
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edge AERO router ("edge router")
a router that configures a non-advertising router interface on an
AERO link over which it can connect End User Networks (EUNs) to
the AERO link.
AERO host
a simple host on an AERO link.
ingress AERO node ("ingress node")
a node that injects packets into an AERO link.
egress AERO node ("egress node")
a node that receives packets from an AERO link.
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 [RFC2119].
3. Motivation
AERO was designed to operate as an on-demand route optimization
function for nodes attached to a single multi-access link, i.e.,
similar to the standard IPv6 redirection mechanism based on ICMPv6
messaging [RFC4443][RFC4861]. However, AERO differs in that the
target of the redirection first receives a pre-authorization
notification, after which it returns route optimization information
to the source of the original packet. This scenario calls into
question whether a standard dynamic routing protocol could be used
instead of AERO, but a number of considerations indicate that
standard routing protocols may be poorly suited for the use cases
AERO was designed to address.
First, AERO is designed to work on very large multiple access links
that may connect a mix of many thousands of routers and hosts.
Traditional proactive dynamic routing protocols such as OSPF, IS-IS,
RIP, OLSR (Optimized Link State Routing), and TBRPF (Topology
Dissemination Based on Reverse-Path Forwarding) may be inefficient in
such environments due to the control message overhead scaling when
large numbers of routers are present and/or when link capacity is
low.
Second, AERO is designed to work on-demand of data packet arrival,
but it only seeks to discover neighbors on the same link and not
distant nodes that may be located many link hops away. Reactive
dynamic routing protocols such as Ad hoc On-Demand Distance Vector
(AODV) and Dynamic Source Routing (DSR) also operate on-demand;
however, they flood specialized route discovery messages that reach
all nodes on the link and may further traverse multiple link hops
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before a route reply is received. This requires a multicast-capable
network and does not ensure delivery of the original data packet,
which may be dropped or delayed during route discovery.
Additionally, AERO is designed to override an existing route to a
destination if the existing route directs traffic along a sub-optimal
path via an extraneous router on the shared link. AERO nodes send
data packets over a preexisting working route, and they may
subsequently receive notification of a better route based on route
optimization feedback from a trusted on-link neighbor. This stands
in contrast to on-demand routing protocols that were designed to
operate when no preexisting working routes are present and that
multicast explicit route request messages to receive a route reply
rather than simply unicast forwarding the data packet via a
preexisting route.
Finally, AERO requires less control message and/or processing
overhead than standard dynamic routing protocols on links for which
the number of routes that must be maintained by each router is far
smaller than the total number of routers on the link, and the routes
maintained by each router may be changing over time. For example, on
a link that connects N nodes, it will often be the case that each
node will only communicate with a small number of link neighbors, and
the set of neighbors may change dynamically over time. Therefore,
the number of active neighbor pairs on the link is V*N (where V is a
small variable number) instead of N**2. This is especially important
on very large links, e.g., for values of N such as 1,000 or more.
4. Example Use Cases
AERO was designed to satisfy numerous operational use cases. As a
first example, a hypothetical major airline has deployed an overlay
network on top of the global Internet to track the aircraft in its
fleet. The global Internet therefore acts as the "link" over which
the overlay network is configured. Each aircraft acts as a mobile
router that fronts for an internal network that includes various
devices controlled and monitored by the airline. However, it would
be impractical for each aircraft to track the changing locations of
all other aircraft in the fleet due to control message overhead on
limited capacity communication links.
In this example, an aircraft ('A') en route to its destination needs
to report its ETA and communicate passenger itineraries to other en
route aircraft that will be servicing passenger connections. ('A')
knows the overlay network addresses of the other aircraft, but does
not know the current underlay address mappings. ('A') sends its
initial messages targeted to the other aircraft via an airline
central dispatch router ('D'), which may be located in a far away
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location. ('D') forwards the messages, but also initiates the AERO
redirection procedure to step out of the triangular path and allow
direct aircraft-to-aircraft communications.
In a second example, Mobile Ad hoc Networks (MANETs) are often
deployed in environments with a high degree of mobility, attrition,
and very limited wireless communications link bandwidth. Such
environments typically also require the use of network-layer security
mechanisms that view the MANET as a "link" over which encrypted
messages are forwarded in an overlay network. In such environments,
a dynamic routing protocol running in the overlay network may serve
to add unacceptable additional congestion to the already overtaxed
wireless links. In that case, the AERO route optimization mechanism
can eliminate costly extraneous routing hops without imparting
additional control message overhead.
In a further example, a large campus LAN that is joined by Layer 2
(L2) bridges may connect many thousands of routers and hosts that
appear to share a single common multi-access link. In that case, the
AERO mechanisms can be applied to satisfy the necessary intra-link
route optimization functions without employing an adjunct dynamic
routing protocol that may be inefficient for reasons mentioned above.
5. Requirements
The route optimization mechanism must satisfy the following
requirements:
Req 1: Off-load traffic from performance-critical gateways.
The mechanism must offload sustained transit though an
intermediate AERO router that would otherwise become a
traffic concentrator.
Req 2: Support route optimization.
The ingress AERO node should be able to send packets directly
to the egress node without forwarding through an intermediate
router for route optimization purposes.
Req 3: Support scaling.
For scaling purposes, support interworking and control
message forwarding between multiple intermediate routers (see
Appendix A).
Req 4: Do not circumvent ingress filtering.
The mechanism must not open an attack vector where network-
layer source address spoofing is enabled even when link-layer
source address spoofing is disabled.
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Req 5: Do not expose packets to loss due to filtering.
The ingress AERO node must have a way of knowing that the
egress AERO node will accept its forwarded packets.
Req 6: Do not expose packets to loss due to path failure.
The ingress AERO node must have a way of discovering whether
the AERO egress node has gone unreachable on the route
optimized path.
Req 7: Do not introduce routing loops.
Intermediate routers must not invoke a route optimization
that would cause a routing loop to form.
Req 8: Support mobility.
The mechanism must continue to work even if the final
destination node/network moves from a first egress node and
re-associates with a second egress node.
Req 9: Support link layer address changes.
The mechanism must continue to work even if the Layer 2
addresses of ingress and/or egress AERO nodes change.
Req 10: Support network renumbering.
The mechanism must provide graceful transition when an AERO
node's attached EUN is renumbered.
6. Asymmetric Extended Route Optimization (AERO)
The following sections specify an Asymmetric Extended Route
Optimization (AERO) capability that fulfills the requirements
specified in Section 5.
6.1. AERO Link Dynamic Routing
In many AERO link use case scenarios (e.g., small enterprise
networks, small and stable MANETs, etc.), routers can engage in a
traditional dynamic routing protocol so that routing/forwarding
tables can be populated and standard forwarding between routers can
be used. In other scenarios (e.g., large enterprise/ISP networks,
cellular service provider networks, dynamic MANETs, etc.), this might
be impractical due to routing protocol control message scaling
issues.
When a traditional dynamic routing protocol cannot be used, the
mechanisms specified in this section can provide a useful on-demand
route discovery capability. When both traditional dynamic routing
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protocols and the AERO mechanism are active on the same link, routes
discovered by the dynamic routing protocol should take precedence
over those discovered by AERO.
6.2. AERO Node Behavior
The following sections discuss characteristics of nodes attached to
links over which AERO can be used.
6.2.1. AERO Node Types
Intermediate AERO routers configure their AERO link interfaces as
advertising router interfaces (see [RFC4861], Section 6.2.2);
therefore, they may send Router Advertisement (RA) messages that
include non-zero Router Lifetimes.
Edge AERO routers configure their AERO link interfaces as non-
advertising router interfaces.
AERO hosts configure their AERO link interfaces as simple host
interfaces.
6.2.2. AERO Host Behavior
AERO hosts observe the IPv6 host requirements defined in [RFC6434],
except that AERO hosts also engage in the AERO route optimization
procedure as specified in Section 6.4.
6.2.3. Edge AERO Router Behavior
Edge AERO routers observe the IPv6 router requirements defined in
[RFC6434] except that they act as "hosts" on their non-advertising
AERO link router interfaces in the same fashion as for IPv6 Customer
Premises Equipment (CPE) routers [RFC6204]. Edge routers can then
acquire managed prefix delegations aggregated by an intermediate
router through the use of, e.g., DHCPv6 Prefix Delegation [RFC3633],
administrative configuration, etc.
After the edge router acquires prefixes, it can sub-delegate them to
nodes and links within its attached EUNs, then it can forward any
outbound packets coming from its EUNs via the intermediate router.
The edge router also engages in the AERO route optimization procedure
as specified in Section 6.4.
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6.2.4. Intermediate AERO Router Behavior
Intermediate AERO routers observe the IPv6 router requirements
defined in [RFC6434] and respond to Router Solicitation (RS) messages
from AERO hosts and edge routers on their advertising AERO link
router interfaces by returning an RA message. Intermediate routers
further configure a DHCP relay/server function on their AERO links
and/or provide an administrative interface for delegation of network-
layer addresses and prefixes.
When the intermediate router completes a stateful network-layer
address or prefix delegation transaction (e.g., as a DHCPv6 relay/
server, etc.), it establishes forwarding table entries that list the
link-layer address of the client AERO node as the link-layer address
of the next hop toward the delegated network-layer addresses/
prefixes.
When the intermediate router forwards a packet out the same AERO
interface on which it arrived, it initiates an AERO route
optimization procedure as specified in Section 6.4.
6.3. AERO Reference Operational Scenario
Figure 3 depicts the AERO reference operational scenario. The figure
shows an intermediate AERO router ('A'), two edge AERO routers ('B',
'D'), an AERO host ('F'), and three ordinary IPv6 hosts ('C', 'E',
'G'):
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.-(::::::::)
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::)--| Host G |
`-(::::::::::::)-' +-------------+
`-(::::::)-' 2001:db8:3::1
|
+--------------+ +--------------+
| Intermediate | | AERO Host F |
| AERO Router A| | (default->A) |
| (C->B; E->D) | +--------------+
+--------------+ 2001:db8:2:1
L3(A) L3(F)
L3(A) L2(F)
| |
X-----+-----------+-----------+-----------+---X
| AERO Link |
L2(B) L2(D)
L3(B) L3(D)
+--------------+ +--------------+ .-.
| AERO Edge | | AERO Edge | ,-( _)-.
| Router B | | Router D | .-(_ IPv6 )-.
| (default->A) | | (default->A) |--(__ EUN )
+--------------+ +--------------+ `-(______)-'
2001:db8:0::/48 2001:db8:1::/48 |
| 2001:db8:1::1
.-. +-------------+
,-( _)-. 2001:db8:0::1 | Host E |
.-(_ IPv6 )-. +-------------+ +-------------+
(__ EUN )--| Host C |
`-(______)-' +-------------+
Figure 3: AERO Reference Operational Scenario
In Figure 3, the intermediate AERO router ('A') connects to the AERO
link and connects to the IPv6 Internet, either directly or via other
IPv6 routers (not shown). Intermediate router ('A') configures an
AERO link interface with a link-local network-layer address L3(A) and
with link-layer address L2(A). The intermediate router ('A') next
arranges to add L2(A) to a published list of valid intermediate
routers for the link.
AERO node ('B') is an AERO edge router that connects to the AERO link
via an interface with link-local network-layer address L3(B) and with
link-layer address L2(B). Node ('B') configures a default route with
next-hop network-layer address L3(A) via the AERO interface, and it
assigns the network-layer prefix 2001:db8:0::/48 to its attached EUN
link. IPv6 host ('C') attaches to the EUN, and it configures the
network-layer address 2001:db8:0::1.
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AERO node ('D') is an AERO edge router that connects to the AERO link
via an interface with link-local network-layer address L3(D) and with
link-layer address L2(D). Node ('D') configures a default route with
next-hop network-layer address L3(A) via the AERO interface, and it
assigns the network-layer prefix 2001:db8:1::/48 to its attached EUN
link. IPv6 host ('E') attaches to the EUN, and it configures the
network-layer address 2001:db8:1::1.
AERO host ('F') connects to the AERO link via an interface with link-
local network-layer address L3(F) and with link-layer address L2(F).
Host ('F') configures a default route with next-hop network-layer
address L3(A) via the AERO interface, and it assigns the network-
layer address 2001:db8:2::1 to the AERO interface.
Finally, IPv6 host ('G') connects to an IPv6 network outside of the
AERO link domain. Host ('G') configures its IPv6 interface in a
manner specific to its attached IPv6 link, and it assigns the
network-layer address 2001:db8:3::1 to its IPv6 link interface.
In these arrangements, intermediate router ('A') must maintain state
that associates the delegated network-layer addresses/prefixes with
the link-local network-layer addresses of the correct edge routers
and/or hosts on the AERO link. The nodes must, in turn, maintain at
least a default route that points to intermediate router ('A'), and
they can discover more-specific routes either via a proactive dynamic
routing protocol or via the AERO mechanisms specified in Section 6.4.
6.4. AERO Specification
Section 6.3 describes the AERO reference operational scenario. We
now discuss the operation and protocol details of AERO with respect
to this reference scenario.
6.4.1. Traditional Redirection Approaches
With reference to Figure 3, when the IPv6 source host ('C') sends a
packet to an IPv6 destination host ('E'), the packet is first
forwarded via the EUN to ingress AERO node ('B'). The ingress node
('B') then forwards the packet over its AERO interface to
intermediate router ('A'), which then forwards the packet to egress
AERO node ('D'), where the packet is finally forwarded to the IPv6
destination host ('E'). When intermediate router ('A') forwards the
packet back out on its advertising AERO interface, it must arrange to
redirect ingress node ('B') toward egress node ('D') as a better
next-hop node on the AERO link that is closer to the final
destination. However, this redirection process should only occur if
there is assurance that both the ingress and egress nodes are willing
participants.
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Consider a first alternative in which intermediate router ('A')
informs ingress node ('B') only and does not inform egress node ('D')
(i.e., "traditional redirection"). In that case, the egress node has
no way of knowing that the ingress is authorized to forward packets
from their claimed source network-layer addresses, and it may simply
elect to drop the packets. Also, the ingress node has no way of
knowing whether the egress is performing some form of source address
filtering that would reject packets arriving from a node other than a
trusted default router, nor whether the egress is even reachable via
a direct path that does not involve the intermediate router.
Finally, the ingress node has no way of knowing whether the final
destination has moved away from the egress node.
Consider a second alternative in which intermediate router ('A')
informs both ingress node ('B') and egress node ('D') separately, via
independent redirection control messages (i.e., "augmented
redirection"). In that case, several conditions can occur that could
result in communication failures. First, if the ingress receives the
redirection control message but the egress does not, subsequent
packets sent by the ingress could be dropped due to filtering since
the egress would not have neighbor state to verify their source
network-layer addresses. Second, if the egress receives the
redirection control message but the ingress does not, subsequent
packets sent in the reverse direction by the egress would be lost.
Finally, timing issues surrounding the establishment and garbage
collection of neighbor state at the ingress and egress nodes could
yield unpredictable behavior. For example, unless the timing were
carefully coordinated through some form of synchronization loop,
there would invariably be instances in which one node has the correct
neighbor state and the other node does not resulting in non-
deterministic packet loss.
Since neither of these alternatives can satisfy the requirements
listed in Section 5, a new redirection technique (i.e., "AERO
redirection") is needed.
6.4.2. AERO Concept of Operations
AERO redirection is used on links for which the traditional
redirection approaches described in Section 6.4.1 are insufficient to
satisfy all requirements. We now discuss the concept of operations
for this new approach.
Again, with reference to Figure 3, when source host ('C') sends a
packet to destination host ('E'), the packet is first forwarded over
the source host's attached EUN to ingress node ('B'), which then
forwards the packet via its AERO interface to intermediate router
('A').
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Using AERO redirection, intermediate router ('A') then forwards the
packet out the same AERO interface toward egress node ('D') and also
sends an AERO "Predirect" message forward to the egress node as
specified in Section 6.4.6. The AERO Predirect message includes the
identity of ingress node ('B') as well as information that egress
node ('D') can use to determine the longest-match prefixes that cover
the source and destination network-layer addresses of the packet that
triggered the predirection event. After egress node ('D') receives
the AERO Predirect message, it process the message and returns an
AERO Redirect message to the intermediate router ('A') as specified
in Section 6.4.7. (During the process, it also creates or updates
neighbor state for ingress node ('B'), and retains this (src, dst)
"prefix pair" as ingress filtering information to accept future
packets using addresses matched by the prefixes from ingress node
('B').)
When the intermediate router ('A') receives the AERO Redirect
message, it processes the message and forwards it on to ingress node
('B') as specified in Section 6.4.8. The message includes the
identity of egress node ('D') as well as information that ingress
node ('B') can use to determine the longest-match prefixes that cover
the source and destination network-layer addresses of the packet that
triggered the redirection event. After ingress node ('B') receives
the AERO Redirect message, it processes the message as specified in
Section 6.4.9. (During the process, it also creates or updates
neighbor state for egress node ('D'), and retains this prefix pair as
forwarding information to forward future packets using addresses
matched by the prefixes to the egress node ('D').)
Following the above AERO Predirect/Redirect message exchange,
forwarding of packets with source and destination network-layer
addresses covered by the longest-match prefix pair is enabled in the
forward direction from ingress node ('B') to egress node ('D'). The
mechanisms that enable this exchange are specified in the following
sections.
6.4.3. Conceptual Data Structures and Protocol Constants
Each AERO node maintains a per-AERO interface conceptual neighbor
cache that includes an entry for each neighbor it communicates with
on the AERO link, the same as for any IPv6 interface (see [RFC4861]).
Each AERO interface neighbor cache entry further maintains two lists
of (src, dst) prefix pairs. The AERO node adds a prefix pair to the
ACCEPT list if it has been informed by a trusted intermediate router
that it is safe to accept packets from the neighbor using network-
layer source and destination addresses covered by the prefix pair.
The AERO node adds a prefix pair to the FORWARD list if it has been
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informed by a trusted intermediate router that it is permitted to
forward packets to the neighbor using network-layer addresses covered
by the prefix pair.
When the node adds a prefix pair to a neighbor cache entry ACCEPT
list, it also sets an expiration timer for the prefix pair to
ACCEPT_TIME seconds. When the node adds a prefix pair to a neighbor
cache entry FORWARD list, it also sets an expiration timer for the
prefix pair to FORWARD_TIME seconds. The node further maintains a
keepalive interval KEEPALIVE_TIME used to limit the number of
keepalive control messages. Finally, the node maintains a constant
value MAX_RETRY to limit the number of keepalives sent when a
neighbor has gone unreachable.
It is RECOMMENDED that FORWARD_TIME be set to the default constant
value 30 seconds to match the default REACHABLE_TIME value specified
for IPv6 neighbor discovery [RFC4861].
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 the ACCEPT_TIME timer
decrements below FORWARD_TIME.
It is RECOMMENDED that KEEPALIVE_TIME be set to the default constant
value 5 seconds to providing timely reachability verification without
causing excessive control message overhead.
It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
for IPv6 neighbor discovery address resolution in Section 7.3.3 of
[RFC4861].
Different values for FORWARD_TIME, ACCEPT_TIME, KEEPALIVE_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. ACCEPT_TIME SHOULD further be set to a value that
is sufficiently longer than FORWARD time to allow the AERO
redirection procedure to converge.
6.4.4. Data Origin Authentication
AERO nodes MUST employ a data origin authentication check for the
packets they receive on an AERO interface. In particular, the node
considers the network-layer source address correct for the link-layer
source address if at least one of the following is true:
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o the network-layer source address is an on-link address that embeds
the link-layer source address, or
o the network-layer source address is explicitly linked to the link-
layer source address through per-neighbor state, or
o the link-layer source address is the address of a trusted
intermediate AERO router.
When the AERO node receives a packet on an AERO interface, it
processes the packet further if it satisfies one of these data origin
authentication conditions; otherwise, it drops the packet.
Note that on links in which link-layer address spoofing is possible,
AERO nodes may require additional securing mechanisms. To address
this, future work will define a strong data origin authentication
scheme such as the use of digital signatures.
6.4.5. AERO Redirection Message Format
AERO Redirect/Predirect messages use the same format as for ICMPv6
Redirect messages depicted in Section 4.5 of [RFC4861]; however, the
messages are encapsulated in a UDP header [RFC0768] to distinguish
them from ordinary ICMPv6 Redirect messages. AERO Redirect messages
therefore require a new UDP service port number 'AERO_PORT'.
AERO Redirect/Predirect messages are formatted as shown in Figure 4:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (=0) | Code (=0) | Checksum (=0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 4: AERO Redirect/Predirect Message Format
The AERO Redirect/Predirect message sender sets the 'Type' field to 0
(since this is not an actual ICMPv6 message), and it also sets the
'Checksum' field to 0 (since the UDP checksum will provide protection
for the entire packet). The sender further sets the 'P' bit to 1 if
this is a 'Predirect' message and sets the 'P' bit to 0 if this is a
'Redirect' message (as described below).
The sender then encapsulates the AERO Redirect message in IP/UDP
headers as shown in Figure 5:
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+--------------------+
~ IP header ~
+--------------------+
~ UDP header ~
+--------------------+
| |
~ AERO Redirect ~
~ Message ~
| |
+--------------------+
Figure 5: AERO Message UDP Encapsulation Format
The AERO Redirect/Predirect message sender sets the UDP destination
port number to 'AERO_PORT' and sets the UDP source port number to a
(pseudo-)random value. The sender next sets the UDP length field to
the length of the UDP message, then calculates the checksum across
the message and writes the value into the UDP checksum field. Next,
the sender sets the IP TTL/Hop-limit field to a small integer value
chosen to provide a quick exit from any temporal routing loops. It
is RECOMMENDED that the sender set IP TTL/Hop-limit to the value 8
unless it has better knowledge of the AERO link characteristics.
6.4.6. Sending Predirects
When an intermediate AERO router forwards a packet out the same AERO
interface that it arrived on, the router sends an AERO Predirect
message forward toward the egress AERO node instead of sending an
ICMPv6 Redirect message back to the ingress AERO node.
In the reference operational scenario, when the intermediate router
('A') forwards a packet sent by the ingress node ('B') toward the
egress node ('D'), it also sends an AERO Predirect message forward
toward the egress, subject to rate limiting (see Section 8.2 of
[RFC4861]). The intermediate router ('A') prepares the AERO
Predirect message as follows:
o the link-layer source address is set to 'L2(A)' (i.e., the link-
layer address of the intermediate router).
o the link-layer destination address is set to 'L2(D)' (i.e., the
link-layer address of the egress node).
o the network-layer source address is set to 'L3(A)' (i.e., the
link-local network-layer address of the intermediate router).
o the network-layer destination address is set to 'L3(D)' (i.e., the
link-local network-layer address of the egress node).
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o the UDP destination port is set to 'AERO_PORT'.
o the Target and Destination Addresses are both set to 'L3(B)'
(i.e., the link-local network-layer address of the ingress node).
o on links that require stateful address mapping, the message
includes a Target Link Layer Address Option (TLLAO) set to 'L2(B)'
(i.e., the link-layer address of the ingress node).
o the message includes a Route Information Option (RIO) [RFC4191]
that encodes the ingress node's network-layer address/prefix
delegation that covers the network-layer source address of the
originating packet.
o the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated to ensure that at least
the network-layer header is included but the size of the message
does not exceed 1280 bytes.
o the 'P' bit is set to P=1.
The intermediate router ('A') then sends the message forward to the
egress node ('D').
6.4.7. Processing Predirects and Sending Redirects
When the egress node ('D') receives an AERO Predirect message, it
accepts the message only if it satisfies the data origin
authentication requirements specified in Section 6.4.4. The egress
further accepts the message only if it is willing to serve as a
redirection target.
Next, the egress node ('D') validates the message according to the
ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]
with the exception that the message includes a Type value of 0, a
Checksum value of 0 and a link-local address in the ICMP destination
field that differs from the destination address of the packet header
encapsulated in the RHO.
In the reference operational scenario, when the egress node ('D')
receives a valid AERO Predirect message, it either creates or updates
a neighbor cache entry that stores the Target address of the message
(i.e., the link-local network-layer address of the ingress node
('B')). The egress node ('D') then records the prefix found in the
RIO along with its own prefix that matches the network-layer
destination address in the packet header found in the RHO with the
neighbor cache entry as an acceptable (src, dst) prefix pair. The
egress node ('D') then adds the prefix pair to the neighbor cache
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entry ACCEPT list, and sets/resets an expiration timer for the prefix
pair to ACCEPT_TIME seconds. If the timer later expires, the egress
node ('D') deletes the prefix pair.
After processing the message, the egress node ('D') prepares an AERO
Redirect message response as follows:
o the link-layer source address is set to 'L2(D)' (i.e., the link-
layer address of the egress node).
o the link-layer destination address is set to 'L2(A)' (i.e., the
link-layer address of the intermediate router).
o the network-layer source address is set to 'L3(D)' (i.e., the
link-local network-layer address of the egress node).
o the network-layer destination address is set to 'L3(B)' (i.e., the
link-local network-layer address of the ingress node).
o the UDP destination port is set to 'AERO_PORT'.
o the Target and the Destination Addresses are both set to 'L3(D)'
(i.e., the link-local network-layer address of the egress node).
o on links that require stateful address mapping, the message
includes a Target Link Layer Address Option (TLLAO) set to
'L2(D)'.
o the message includes an RIO that encodes the egress node's
network-layer address/prefix delegation that covers the network-
layer destination address of the originating packet.
o the message includes as much of the RHO copied from the
corresponding AERO Predirect message as possible such that at
least the network-layer header is included but the size of the
message does not exceed 1280 bytes.
o the 'P' bit is set to P=0.
After the egress node ('D') prepares the AERO Redirect message, it
sends the message to the intermediate router ('A').
6.4.8. Forwarding Redirects
When the intermediate router ('A') receives an AERO Redirect message,
it accepts the message only if it satisfies the data origin
authentication requirements specified in Section 6.4.4. Next, the
intermediate router ('A') validates the message the same as described
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in Section 6.4.7. Following validation, the intermediate router
('A') processes the Redirect, and then forwards a corresponding
Redirect on to the ingress node ('B') as follows.
In the reference operational scenario, the intermediate router ('A')
receives the AERO Redirect message from the egress node ('D') and
prepares to forward a corresponding AERO Redirect message to the
ingress node ('B'). The intermediate router ('A') then verifies that
the RIO encodes a network-layer address/prefix that the egress node
('D') is authorized to use, and it discards the message if
verification fails. Otherwise, the intermediate router ('A') changes
the link-layer source address of the message to 'L2(A)', changes the
network-layer source address of the message to the link-local
network-layer address 'L3(A)', and changes the link-layer destination
address to 'L2(B)' . The intermediate router ('A') finally
decrements the IP TTL/Hop-limit and forwards the message to the
ingress node ('B').
6.4.9. Processing Redirects
When the ingress node ('B') receives an AERO Redirect message (i.e.,
one with P=0), it accepts the message only if it satisfies the data
origin authentication requirements specified in Section 6.4.4. Next,
the ingress node ('B') validates the message the same as described in
Section 6.4.6. Following validation, the ingress node ('B') then
processes the message as follows.
In the reference operational scenario, when the ingress node ('B')
receives the AERO Redirect message, it either creates or updates a
neighbor cache entry that stores the Target address of the message
(i.e., the link-local network-layer address of the egress node
'L3(D)'). The ingress node ('B') then records the (src, dst) prefix
pair associated with the triggering packet in the neighbor cache
entry FORWARD list, i.e., it records its prefix that matches the
redirected packet's network-layer source address and the prefix
listed in the RIO as the prefix pair. The ingress node ('B') then
sets/resets an expiration timer for the prefix pair to FORWARD_TIME
seconds. If the timer later expires, the ingress node ('B') deletes
the entry.
Now, the ingress node ('B') has a neighbor cache FORWARD list entry
for the prefix pair, and the egress node ('D') has a neighbor cache
ACCEPT list entry for the prefix pair. Therefore, the ingress node
('B') may forward ordinary network-layer data packets with network-
layer source and destination addresses that match the prefix pair
directly to the egress node ('D') without forwarding through the
intermediate router ('A'). Note that the ingress node must have a
way of informing the network layer of a route that associates the
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destination prefix with this neighbor cache entry. The manner of
establishing such a route (and deleting it when it is no longer
necessary) is left to the implementation.
To enable packet forwarding in the reverse direction, a separate AERO
redirection operation is required that is the mirror-image of the
forward operation described above but the link segments traversed in
the forward and reverse directions may be different, i.e., the
operations are asymmetric.
6.4.10. Sending Periodic Predirect Keepalives
In order to prevent prefix pairs from expiring while data packets are
actively flowing, the ingress node ('B') can send AERO Predirect
messages directly to the egress node ('D') as a "keepalive" to
solicit AERO Redirect messages. The node should send such keepalive
messages only when a data packet covered by the prefix pair has been
sent recently, and should wait for at least KEEPALIVE_TIME seconds
before sending each successive keepalive message in order to limit
control message overhead.
In the reference operational scenario, when the ingress node ('B')
needs to refresh the FORWARD timer for a specific prefix pair, it can
send an AERO Predirect message directly to the egress node ('D')
prepared as follows:
o the link-layer source address is set to 'L2(B)' (i.e., the link-
layer address of the ingress node).
o the link-layer destination address is set to 'L2(D)' (i.e., the
link-layer address of the egress node).
o the network-layer source address is set to 'L3(B)' (i.e., the
link-local network-layer address of the ingress node).
o the network-layer destination address is set to 'L3(D)' (i.e., the
link-local network-layer address of the egress node).
o the UDP destination port is set to 'AERO_PORT'.
o the Predirect Target and Destination Addresses are both set to
'L3(B)' (i.e., the link-local network-layer address of the ingress
node).
o the message includes an RHO that contains the originating packet
truncated to ensure that at least the network-layer header is
included but the size of the message does not exceed 1280 bytes.
Templin Experimen