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Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc6706 (if approved) May 27, 2014
Intended status: Standards Track
Expires: November 28, 2014
Transmission of IPv6 Packets over AERO Links
draft-templin-aerolink-21.txt
Abstract
This document specifies the operation of IPv6 over tunnel virtual
Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
Route Optimization (AERO). Nodes attached to AERO links can exchange
packets via trusted intermediate routers on the link that provide
forwarding services to reach off-link destinations and/or redirection
services to inform the node of an on-link neighbor that is closer to
the final destination. Operation of the IPv6 Neighbor Discovery (ND)
protocol over AERO links is based on an IPv6 link local address
format known as the AERO address.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 28, 2014.
Copyright Notice
Copyright (c) 2014 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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5
3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 5
3.2. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 6
3.3. AERO Interface Characteristics . . . . . . . . . . . . . 6
3.3.1. Coordination of Multiple Underlying Interfaces . . . 9
3.4. AERO Interface Neighbor Cache Maintenace . . . . . . . . 9
3.5. AERO Interface Data Origin Authentication . . . . . . . . 11
3.6. AERO Interface MTU Considerations . . . . . . . . . . . . 11
3.7. AERO Interface Encapsulation, Re-encapsulation and
Decapsulation . . . . . . . . . . . . . . . . . . . . . . 13
3.8. AERO Router Discovery, Prefix Delegation and Address
Configuration . . . . . . . . . . . . . . . . . . . . . . 14
3.8.1. AERO Client Behavior . . . . . . . . . . . . . . . . 14
3.8.2. AERO Server Behavior . . . . . . . . . . . . . . . . 15
3.9. AERO Redirection . . . . . . . . . . . . . . . . . . . . 16
3.9.1. Reference Operational Scenario . . . . . . . . . . . 16
3.9.2. Classical Redirection Approaches . . . . . . . . . . 18
3.9.3. Concept of Operations . . . . . . . . . . . . . . . . 19
3.9.4. Message Format . . . . . . . . . . . . . . . . . . . 19
3.9.5. Sending Predirects . . . . . . . . . . . . . . . . . 20
3.9.6. Processing Predirects and Sending Redirects . . . . . 21
3.9.7. Re-encapsulating and Relaying Redirects . . . . . . . 22
3.9.8. Processing Redirects . . . . . . . . . . . . . . . . 23
3.10. Neighbor Reachability Maintenance . . . . . . . . . . . . 24
3.11. Mobility and Link-Layer Address Change Considerations . . 25
3.12. Underlying Protocol Version Considerations . . . . . . . 26
3.13. Multicast Considerations . . . . . . . . . . . . . . . . 26
3.14. Operation on AERO Links Without DHCPv6 Services . . . . . 26
3.15. Operation on Server-less AERO Links . . . . . . . . . . . 26
3.16. Other Considerations . . . . . . . . . . . . . . . . . . 27
4. Implementation Status . . . . . . . . . . . . . . . . . . . . 27
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
6. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. Normative References . . . . . . . . . . . . . . . . . . 28
8.2. Informative References . . . . . . . . . . . . . . . . . 29
Appendix A. AERO Server and Relay Interworking . . . . . . . . . 31
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
This document specifies the operation of IPv6 over tunnel virtual
Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
Route Optimization (AERO). Nodes attached to AERO links can exchange
packets via trusted intermediate routers on the link that provide
forwarding services to reach off-link destinations and/or redirection
services to inform the node of an on-link neighbor that is closer to
the final destination. This redirection provides a route
optimization capability that addresses the requirements outlined in
[RFC5522].
Nodes on AERO links use an IPv6 link-local address format known as
the AERO Address. This address type has properties that avoid
duplication and statelessly link IPv6 Neighbor Discovery (ND) to IPv6
routing. The AERO link can be used for tunneling to neighboring
nodes on either IPv6 or IPv4 networks, i.e., AERO views the IPv6 and
IPv4 networks as equivalent links for tunneling. The remainder of
this document presents the AERO specification.
2. Terminology
The terminology in the normative references applies; the following
terms are defined within the scope of this document:
AERO link
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 IPv6.
AERO interface
a node's attachment to an AERO link.
AERO address
an IPv6 link-local address assigned to an AERO interface and
constructed as specified in Section 3.6.
AERO node
a node that is connected to an AERO link and that participates in
IPv6 Neighbor Discovery over the link.
AERO Client ("client")
a node that configures either a host interface or a router
interface on an AERO link.
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AERO Server ("server")
a node that configures a router interface on an AERO link over
which it can provide default forwarding and redirection services
for other AERO nodes.
AERO Relay ("relay")
a node that relays IPv6 packets between Servers on the same AERO
link, and/or that forwards IPv6 packets between the AERO link and
the IPv6 Internet. An AERO Relay may or may not also be
configured as an AERO Server.
ingress tunnel endpoint (ITE)
an AERO interface endpoint that injects tunneled packets into an
AERO link.
egress tunnel endpoint (ETE)
an AERO interface endpoint that receives tunneled packets from an
AERO link.
underlying network
a connected IPv6 or IPv4 network routing region over which AERO
nodes tunnel IPv6 packets.
underlying interface
an AERO node's interface point of attachment to an underlying
network.
underlying address
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 underlying address. Underlying
addresses are used as the source and destination addresses of the
AERO encapsulation header.
link-layer address
the same as defined for "underlying address" above, and formed
from the concatenation of the UDP port number and underlying
address as specified in Section 3.3.
network layer address
an IPv6 address used as the source or destination address of the
inner IPv6 packet header.
end user network (EUN)
an IPv6 network attached to a downstream interface of an AERO
Client (where the AERO interface is seen as the upstream
interface).
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Throughout the document, the simple terms "Client", "Server" and
"Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
respectively. Capitalization is used to distinguish these terms from
DHCPv6 client/server/relay. This is an important distinction, since
an AERO Server may be a DHCPv6 relay, and an AERO Relay may be a
DHCPv6 server.
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. Asymmetric Extended Route Optimization (AERO)
The following sections specify the operation of IPv6 over Asymmetric
Extended Route Optimization (AERO) links:
3.1. AERO Node Types
AERO Relays relay packets between nodes connected to the same AERO
link and also forward packets between the AERO link and the native
IPv6 network. The relaying process entails re-encapsulation of IPv6
packets that were received from a first AERO node and are to be
forwarded without modification to a second AERO node.
AERO Servers configure their AERO interfaces as router interfaces,
and provide default routing services to AERO Clients. AERO Servers
configure a DHCPv6 relay or server function and facilitate DHCPv6
Prefix Delegation (PD) exchanges. An AERO Server may also act as an
AERO Relay.
AERO Clients act as requesting routers to receive IPv6 prefixes
through a DHCPv6 PD exchange via an AERO Server over the AERO link.
(Clients typically associate with a single Server at a time; Clients
MAY associate with multiple Servers, but associating with many
Servers may result in excessive control message overhead.) Each AERO
Client receives at least a /64 prefix delegation, and may receive
even shorter prefixes.
AERO Clients that act as routers configure their AERO interfaces as
router interfaces and sub-delegate portions of their received prefix
delegations to links on EUNs. End system applications on AERO
Clients that act as routers bind to EUN interfaces (i.e., and not the
AERO interface).
AERO Clients that act as ordinary hosts configure their AERO
interfaces as host interfaces and assign one or more IPv6 addresses
taken from their received prefix delegations to the AERO interface
but DO NOT assign the delegated prefix itself to the AERO interface.
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Instead, the host assigns the delegated prefix to a "black hole"
route so that unused portions of the prefix are nullified. End
system applications on AERO Clients that act as hosts bind directly
to the AERO interface.
3.2. AERO Addresses
An AERO address is an IPv6 link-local address assigned to an AERO
interface and with an IPv6 prefix embedded within the interface
identifier. The AERO address is formatted as:
fe80::[IPv6 prefix]
Each AERO Client configures an AERO address based on the prefix it
has received from the AERO link prefix delegation authority (e.g.,
the DHCPv6 server). The address begins with the prefix fe80::/64 and
includes in its interface identifier the base /64 prefix taken from
the Client's delegated IPv6 prefix. The base prefix is determined by
masking the delegated prefix with the prefix length. For example, if
an AERO Client has received the prefix delegation:
2001:db8:1000:2000::/56
it would construct its AERO address as:
fe80::2001:db8:1000:2000
The AERO address remains stable as the Client moves between
topological locations, i.e., even if its underlying address changes.
Each AERO Server configures the AERO address 'fe80::0'; this
corresponds to the IPv6 "default" prefix (i.e., ::/0) and provides a
handle for Clients to insert into a neighbor cache entry.
3.3. AERO Interface Characteristics
AERO interfaces use IPv6-in-IPv6 encapsulation [RFC2473] to exchange
tunneled packets with AERO neighbors attached to an underlying IPv6
network, and use IPv6-in-IPv4 encapsulation [RFC4213] to exchange
tunneled packets with AERO neighbors attached to an underlying IPv4
network. AERO interfaces can also operate over secured tunnel types
such as IPsec [RFC4301] or TLS [RFC5246] in environments where strong
authentication and confidentiality are required. When NAT traversal
and/or filtering middlebox traversal may be necessary, a UDP header
is further inserted immediately above the outer IP encapsulation
header.
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Servers assign the link-local address fe80::0 to their AERO
interfaces. Servers and Relays also use (non-AERO) administratively-
assigned link-local addresses to support the operation of the inter-
Server/Relay routing system (see: [IRON]).
Clients initially use a temporary IPv6 link-local address in the
DHCPv6 PD exchanges used to receive an IPv6 prefix and derive an AERO
address. If the Client is pre-provisioned with an IPv6 prefix
associated with the AERO service, it SHOULD use the AERO address
derived from the prefix as the temporary address. Otherwise, the
Client SHOULD use "fe80::1" as the temporary address since this
address will not conflict with any valid AERO addresses and will thus
not be used in any AERO neighbor discovery messaging. After the
Client receives a prefix delegation, it assigns the corresponding
AERO address to the AERO interface. DHCPv6 is therefore used to
bootstrap the assignment of unique link-local addresses on the AERO
link.
AERO interfaces maintain a neighbor cache and use an augmentation of
standard unicast IPv6 ND messaging. AERO interfaces use Redirect,
Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router
Solicitation (RS) and Router Advertisement (RA) messages the same as
for any IPv6 link. AERO links further use link-local-only
addressing; hence, Clients ignore any Prefix Information Options
(PIOs) they may receive in RA messages.
AERO Redirect messages include TLLAOs the same as for any IPv6 link.
The TLLAO includes the link-layer address for the target node, which
is formed from the concatenation of a 1-octet Link ID value followed
by a 1-octet Preference value followed by the 2-octet UDP port number
used by the target when it sends UDP-encapsulated packets over the
AERO interface (or 0 when the target does not use UDP encapsulation)
followed by a 16-octet IP address. The TLLAO format is shown in
Figure 1:
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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 = 2 | Length = 3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID | Preference | UDP Port Number (or 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-- --+
| |
+-- IP Address --+
| |
+-- --+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: AERO TLLAO Format for IPv6
In this format, Link ID is an integer value between 0 and 255
corresponding to the underlying interface by the target node, and
Preference is an integer value between 0 and 255 indicating the
target node's preference for this underlying interface (with 0 being
highest preference and 255 being lowest). UDP Port Number and IP
Address are the addresses that appear in the outer encapsulating
headers of packets sent over the target node's underlying interface.
(Note that when the encapsulation IP address family is IPv4 the IP
address is formed as an IPv4-compatible IPv6 address (see:
[RFC4291]). Note also that multiple TLLAO options may appear in an
ND message, e.g., if the target node wishes to publish multiple
underlying interfaces.)
AERO NS/NA/RS/RA messages include Source/Target Link Layer Address
Options (S/TLLAOs) formatted as shown in Figure 1 with Link ID and
Preference fields set to values corresponding to the underlying
interface that will convey (encapsulated) messages but with UDP Port
Number and IP Address values set to 0 (since the source may have no
way of knowing whether there is a NAT on the path and hence may be
unable to convey the correct values). Instead, AERO nodes ignore the
address information in these S/TLLAOs and determine the link-layer
addresses of neighbors by examining the link-layer source address of
any NS/NA/RS/RA messages they receive. Therefore, Redirect messages
alone convey non-zero address information in TLLAOs.
Finally, AERO interface NS/NA/RS/RA messages only update existing
neighbor cache entires and do not create new neighbor cache entries,
whereas Redirect messages can both update and create neighbor cache
entries.
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3.3.1. Coordination of Multiple Underlying Interfaces
AERO interfaces may be configured over multiple underlying
interfaces. From the perspective of IPv6 Neighbor Discovery, the
AERO interface therefore appears as a single logical interface with
multiple link-layer addresses the same as described for "Inbound Load
Balancing" in Section 3 of [RFC4861]. The load balancing paradigm
applies to AERO Servers that are connected to stable backhaul
networks, but may not necessarily be appropriate for AERO Clients
that connect to multiple diverse media types.
For example, common handheld devices of the modern era have both
wireless local area network (aka "WiFi") and cellular wireless links.
These links are typically used "one at a time" with low-cost WiFi
preferred and highly-available cellular wireless as a cold standby.
In a more complex example, aircraft frequently have many wireless
data link types (e.g. satellite-based, terrestrial, directional
point-to-point, 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 quiescent when one interface is
active), then the S/TLLAOs in ND messages SHOULD use the same Link ID
and Preference values regardless of the underlying interface. If
multiple underlying interfaces may be used simultaneously, the Client
instead MAY use a different Link ID and Preference value for each
interface. In that case, the Client would need to send separate RS
messages to each of its Servers for each underlying interface it
wishes to activate so that the Server can convey correct addressing
information in the TLLAOs of Redirect messages.
3.4. AERO Interface Neighbor Cache Maintenace
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]).
Neighbor cache entries are created and maintained as follows:
When an AERO Server relays a DHCPv6 Reply message to an AERO Client,
it creates or updates a neighbor cache entry for the Client based on
the information in the IA_PD option as the Client's network layer
address and the Client's outer IP address and UDP port number as the
link-layer address.
When an AERO Client receives a DHCPv6 Reply message from an AERO
Server, it creates or updates a neighbor cache entry for the Server
based on fe80::0 as the network layer address and the Server's outer
IP address and UDP port number as the link-layer address .
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When an AERO Client receives a valid Predirect message (See
Section 3.10.5) it creates or updates a neighbor cache entry for the
Predirect target network-layer and link-layer addresses, and also
creates an IPv6 forwarding table entry for the Predirected (source)
prefix. The node then sets an "ACCEPT_TIME" timer in the neighbor
cache entry and uses this timer to validate any messages received
from the Predirected neighbor.
When an AERO Client receives a valid Redirect message (see
Section 3.10.7) it creates or updates a neighbor cache entry for the
Redirect target network-layer and link-layer addresses, and also
creates an IPv6 forwarding table entry for the redirected
(destination) prefix. The node then sets a "FORWARD_TIME" timer in
the neighbor cache entry and uses this timer to determine whether
packets can be sent directly to the redirected neighbor. The node
also maintains a constant value MAX_RETRY to limit the number of
keepalives sent when a neighbor has gone unreachable.
When an AERO Client receives a valid NS message it (re)sets the
ACCEPT_TIME timer for this neighbor.
When an AERO Client receives a valid NA message, it (re)sets the
FORWARD_TIME timer for this neighbor.
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 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, 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.
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3.5. AERO Interface Data Origin Authentication
AERO nodes use a simple data origin authentication for encapsulated
packets they receive from other nodes. In particular, AERO Clients
accept encapsulated packets with a link-layer source address
belonging to one of their current AERO Servers and all AERO nodes
accept encapsulated packets with a link-layer source address that is
correct for the network-layer source address.
The AERO node considers the link-layer source address correct for the
network-layer source address if there is an IPv6 forwarding table
entry that matches the network-layer source address as well as a
neighbor cache entry corresponding to the next hop that includes the
link-layer address and the ACCEPT_TIME timer is non-zero. An
exception is that neighbor discovery messages may include a different
link-layer address than the one currently in the neighbor cache, and
the new link-layer address updates the neighbor cache entry.
3.6. AERO Interface MTU Considerations
The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
encapsulation overhead for IPv4 [RFC0791] and 4GB minus the
encapsulation overhead for IPv6 [RFC2675]. This is the most that
IPv4 and IPv6 (respectively) can convey within the constraints of
protocol constants, but actual sizes available for tunneling will
frequently be much smaller.
The base tunneling specifications for IPv4 and IPv6 typically set a
static MTU on the tunnel interface to 1500 bytes minus the
encapsulation overhead or smaller still if the tunnel is likely to
incur additional encapsulations on the path. This can result in path
MTU related black holes when packets that are too large to be
accommodated over the AERO link are dropped, but the resulting ICMP
Packet Too Big (PTB) messages are lost on the return path. As a
result, AERO nodes use the following MTU mitigations to accommodate
larger packets.
AERO nodes set their AERO interface MTU to the larger of the
underlying interface MTU minus the encapsulation overhead, and 1500
bytes. (If there are multiple underlying interfaces, the node sets
the AERO interface MTU according to the largest underlying interface
MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU
cannot be determined.) AERO nodes optionally cache other per-
neighbor MTU values in the underlying IP path MTU discovery cache
initialized to the underlying interface MTU.
AERO nodes admit packets that are no larger than 1280 bytes minus the
encapsulation overhead (*) as well as packets that are larger than
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1500 bytes into the tunnel without fragmentation, i.e., as long as
they are no larger than the AERO interface MTU before encapsulation
and also no larger than the cached per-neighbor MTU following
encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit
to 0 for packets no larger than 1280 bytes minus the encapsulation
overhead (*) and sets the DF bit to 1 for packets larger than 1500
bytes. If a large packet is lost in the path, the node may
optionally cache the MTU reported in the resulting PTB message or may
ignore the message, e.g., if there is a possibility that the message
is spurious.
For packets destined to an AERO node that are larger than 1280 bytes
minus the encapsulation overhead (*) but no larger than 1500 bytes,
the node uses outer IP fragmentation to fragment the packet into two
pieces (where the first fragment contains 1024 bytes of the
fragmented inner packet) then admits the fragments into the tunnel.
If the outer protocol is IPv4, the node admits the packet into the
tunnel with DF set to 0 and subject to rate limiting to avoid
reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, the
node also sends a 1500 byte probe message (**) to the neighbor,
subject to rate limiting. To construct a probe, the node prepares an
ICMPv6 Neighbor Solicitation (NS) message with trailing padding
octets added to a length of 1500 bytes but does not include the
length of the padding in the IPv6 Payload Length field. The node
then encapsulates the NS in the outer encapsulation headers (while
including the length of the padding in the outer length fields), sets
DF to 1 (for IPv4) and sends the padded NS message to the neighbor.
If the neighbor returns an NA message, the node may then send whole
packets within this size range and (for IPv4) relax the rate limiting
requirement.
AERO nodes MUST be capable of reassembling packets up to 1500 bytes
plus the encapsulation overhead length. It is therefore RECOMMENDED
that AERO nodes be capable of reassembling at least 2KB.
(*) Note that if it is known without probing that the minimum Path
MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then
MINMTU can be used instead of 1280 in the fragmentation threshold
considerations listed above.
(**) It is RECOMMENDED that no probes smaller than 1500 bytes be used
for MTU probing purposes, since smaller probes may be fragmented if
there is a nested tunnel somewhere on the path to the neighbor.
Probe sizes larger than 1500 bytes MAY be used, but may be
unnecessary since original sources are expected to implement
[RFC4821] when sending large packets.
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3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation
AERO interfaces encapsulate IPv6 packets according to whether they
are entering the AERO interface for the first time or if they are
being forwarded out the same AERO interface that they arrived on.
This latter form of encapsulation is known as "re-encapsulation".
AERO interfaces encapsulate packets per the specifications in
[RFC2473][RFC4213][RFC4301][RFC5246] except that the interface copies
the "Hop Limit", "Traffic Class" and "Congestion Experienced" values
in the inner IPv6 header into the corresponding fields in the outer
IP header. For packets undergoing re-encapsulation, the AERO
interface instead copies the "TTL/Hop Limit", "Type of Service/
Traffic Class" and "Congestion Experienced" values in the original
outer IP header into the corresponding fields in the new outer IP
header (i.e., the values are transferred between outer headers and
*not* copied from the inner network-layer header).
When AERO UDP encapsulation is used, the AERO interface encapsulates
the packet per the specifications in [RFC2473][RFC4213] except that
it inserts a UDP header between the outer IP header and inner IPv6
header. The AERO interface sets the UDP source port to a constant
value that it will use in each successive packet it sends, sets the
UDP checksum field to zero (see: [RFC6935][RFC6936]) and sets the UDP
length field to the length of the inner packet plus 8 bytes for the
UDP header itself. For packets sent via a Server, the AERO interface
sets the UDP destination port to 8060 (i.e., the IANA-registered port
number for AERO) when AERO-only encapsulation is used. For packets
sent to a neighboring Client, the AERO interface sets the UDP
destination port to the port value stored in the neighbor cache entry
for this neighbor.
The AERO interface next sets the outer IP protocol number to the
appropriate value for the first protocol layer within the
encapsulation (e.g., IPv6, UDP, IPsec, etc.). When IPv6 is used as
the outer IP protocol, the interface then sets the flow label value
in the outer IPv6 header the same as described in [RFC6438]. When
IPv4 is used as the outer IP protocol, the AERO interface sets the DF
bit as discussed in Section 3.6.
AERO interfaces decapsulate packets destined either to the node
itself or to a destination reached via an interface other than the
receiving AERO interface. When AERO UDP encapsulation is used (i.e.,
when a UDP header with destination port 8060 is present) the
interface examines the first octet of the encapsulated packet. If
the most significant four bits of the first octet encode the value
'0110' (i.e., the version number value for IPv6), the packet is
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accepted and the encapsulating UDP header is discarded; otherwise,
the packet is discarded.
Further decapsulation then proceeds according to the appropriate
tunnel type [RFC2473][RFC4213][RFC4301][RFC5246].
3.8. AERO Router Discovery, Prefix Delegation and Address Configuration
3.8.1. AERO Client Behavior
AERO Clients observe the IPv6 node requirements defined in [RFC6434].
AERO Clients first discover the link-layer addresses of AERO Servers
via static configuration, or through an automated means such as DNS
name resolution. In the absence of other information, the Client
resolves the Fully-Qualified Domain Name (FQDN)
"linkupnetworks.domainname", where "domainname" is the DNS domain
appropriate for the Client's attached underlying network. The Client
then creates a neighbor cache entry with fe80::0 as the link-local
address and the discovered address of one or more Servers as the
link-layer addresses. The neighbor cache entry is created with both
ACCEPT_TIME and FORWARD_TIME set to infinity, since the Client will
remain with its current Server(s) unless it explicitly switches to
different Server(s).
Next, the Client acts as a requesting router to request an IPv6
prefix through DHCPv6 PD [RFC3633] via each AERO Server it wishes to
associate with using a temporary link-local address (see:
Section 3.3) as the IPv6 source address and fe80::0 as the IPv6
destination address. The Client includes a DHCPv6 Unique Identifier
(DUID) in the Client Identifier option of its DHCPv6 messages
[RFC3315][RFC6355] and includes any additional authenticating
information necessary to authenticate itself to the DHCPv6 server.
If the Client is pre-provisioned with an IPv6 prefix associated with
the AERO service, it MAY also include the prefix in an IA_PD option
in its DHCPv6 Request command to indicate its preferred prefix to the
DHCPv6 server.
After the Client receives its prefix delegation, it assigns the link-
local AERO address taken from the prefix to the AERO interface (see:
Section 3.3) and sub-delegates the prefix to nodes and links within
its attached EUNs (the AERO link-local address thereafter remains
stable as the Client moves). The Client further renews its prefix
delegation via standard DHCPv6 procedures by sending DHCPv6 Renew
messages with its AERO address as the IPv6 source address, fe80::0 as
the IPv6 destination address and the same DUID value in the Client
Identifier option.
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The Client then sends an RS message to each of its associated Servers
to receive an RA message with a default router lifetime and any other
link-specific parameters. When the Client receives an RA message, it
configures a default route according to the default router lifetime
but ignores any Prefix Information Options (PIOs) included in the RA
message since the AERO link is link-local-only. The Client further
ignores any RS messages it might receive, since only Servers may
process RS messages.
The Client then sends periodic RS messages to each Server (subject to
rate limiting) to obtain new RA messages for Neighbor Unreachability
Detection (NUD), to refresh any network state, and to update the
default router lifetime and any other link-specific parameters. The
Client can also forward IPv6 packets destined to networks beyond its
local EUNs via a Server as an IPv6 default router. The Server may in
turn return a redirection message informing the Client of a neighbor
on the AERO link that is topologically closer to the final
destination as specified in Section 3.9.
Note that, since the Client's AERO address is configured from the
unique DHCPv6 prefix delegation it receives, there is no need for
Duplicate Address Detection (DAD) on AERO links. Other nodes
maliciously attempting to hijack an authorized Client's AERO address
will be denied due to an unacceptable link-layer address and/or
security parameters (see: Security Considerations).
3.8.2. AERO Server Behavior
AERO Servers observe the IPv6 router requirements defined in
[RFC6434] and further configure a DHCPv6 relay function on their AERO
links. When the AERO Server relays a Client's DHCPv6 PD messages to
the DHCPv6 server, it wraps each message in a "Relay-forward" message
per [RFC3315] and includes a DHCPv6 Interface Identifier option that
encodes a value that identifies the AERO link to the DHCPv6 server.
The Server then includes the Client's link-layer address in a DHCPv6
Client Link Layer Address Option (CLLAO) [RFC6939] with the link-
layer address format shown in Figure 1. The Server sets the CLLAO
'option-length' field to 22 (2 plus the length of the link-layer
address) and sets the 'link-layer type' field to TBD (see: IANA
Considerations). The Server finally includes a DHCPv6 Echo Request
Option (ERO) [RFC4994] that encodes the option code for the CLLAO in
a 'requested-option-code-n' field. The CLLAO information will
therefore subsequently be echoed back in the DHCPv6 Server's "Relay-
reply" message.
When the DHCPv6 server issues the IPv6 prefix delegation in a "Relay-
reply" message via the AERO Server (acting as a DHCPv6 relay), the
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AERO Server obtains the Client's link-layer address from the echoed
CLLAO option and obtains the Client's delegated prefix from the
included IA_PD option. The Server then creates a neighbor cache
entry for the Client's AERO address (see: Section 3.3) with the
Client's link-layer address as the link-layer address for the
neighbor cache entry. The neighbor cache entry is created with both
ACCEPT_TIME and FORWARD_TIME set to infinity, since the Client will
remain with this Server unless it explicitly disassociates with the
Server.
The Server also configures an IPv6 forwarding table entry that lists
the Client's AERO address as the next hop toward the delegated IPv6
prefix with a lifetime derived from the DHCPv6 lease lifetime. The
Server finally injects the Client's prefix as an IPv6 route into the
inter-Server/Relay routing system (see: [IRON]) then relays the
DHCPv6 message to the Client while using fe80::0 as the IPv6 source
address, the link-local address found in the "peer address" field of
the Relay-reply message as the IPv6 destination address, and the
Client's link-layer address as the destination link-layer address.
Servers respond to RS/NS messages from Clients on their AERO
interfaces by returning an RA/NA message. When the Server receives
an RS/NS message, it updates the neighbor cache entry using the
network-layer source address as the neighbor's network-layer address
and using the link-layer source address of the RS/NS message as the
neighbor's link-layer address. The Server SHOULD NOT include PIOs in
any RA messages it sends to Clients, since the Client will ignore any
such options.
Servers ignore any RA messages they may receive from a Client.
Servers MAY examine RA messages they may receive from other Servers
for consistency verification purposes.
When the Server forwards a packet via the same AERO interface on
which it arrived, it initiates an AERO route optimization procedure
as specified in Section 3.9.
3.9. AERO Redirection
3.9.1. Reference Operational Scenario
Figure 2 depicts the AERO redirection reference operational scenario.
The figure shows an AERO Server('A'), two AERO Clients ('B', 'D') and
three ordinary IPv6 hosts ('C', 'E', 'F'):
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.-(::::::::)
.-(::: IPv6 :::)-. +-------------+
(:::: Internet ::::)--| Host F |
`-(::::::::::::)-' +-------------+
`-(::::::)-' 2001:db8:2::1
|
+--------------+
| AERO Server A|
| (C->B; E->D) |
+--------------+
fe80::0
L2(A)
|
X-----+-----------+-----------+--------X
| AERO Link |
L2(B) L2(D)
fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-.
+--------------+ +--------------+ ,-( _)-.
| AERO Client B| | AERO Client 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 2: AERO Reference Operational Scenario
In Figure 2, AERO Server ('A') connects to the AERO link and connects
to the IPv6 Internet, either directly or via an AERO Relay (not
shown). Server ('A') assigns the address fe80::0 to its AERO
interface with link-layer address L2(A). Server ('A') next arranges
to add L2(A) to a published list of valid Servers for the AERO link.
AERO Client ('B') receives the IPv6 prefix 2001:db8:0::/48 in a
DHCPv6 PD exchange via AERO Server ('A') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(B). Client ('B') configures a default route via the AERO
interface with next-hop address fe80::0 and link-layer address L2(A),
then sub-delegates the prefix 2001:db8:0::/48 to its attached EUNs.
IPv6 host ('C') connects to the EUN, and configures the address
2001:db8:0::1.
AERO Client ('D') receives the IPv6 prefix 2001:db8:1::/48 in a
DHCPv6 PD exchange via AERO Server ('A') then assigns the address
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fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(D). Client ('D') configures a default route via the AERO
interface with next-hop address fe80::0 and link-layer address L2(A),
then sub-delegates the prefix 2001:db8:1::/48 to its attached EUNs.
IPv6 host ('E') connects to the EUN, and configures the address
2001:db8:1::1.
Finally, IPv6 host ('F') connects to an IPv6 network outside of the
AERO link domain. Host ('F') configures its IPv6 interface in a
manner specific to its attached IPv6 link, and assigns the address
2001:db8:2::1 to its IPv6 link interface.
3.9.2. Classical Redirection Approaches
With reference to Figure 2, when the IPv6 source host ('C') sends a
packet to an IPv6 destination host ('E'), the packet is first
forwarded via the EUN to AERO Client ('B'). Client ('B') then
forwards the packet over its AERO interface to AERO Server ('A'),
which then re-encapsulates and forwards the packet to AERO Client
('D'), where the packet is finally forwarded to the IPv6 destination
host ('E'). When Server ('A') re-encapsulates and forwards the
packet back out on its advertising AERO interface, it must arrange to
redirect Client ('B') toward Client ('D') as a better next-hop node
on the AERO link that is closer to the final destination. However,
this redirection process applied to AERO interfaces must be more
carefully orchestrated than on ordinary links since the parties may
be separated by potentially many underlying network routing hops.
Consider a first alternative in which Server ('A') informs Client
('B') only and does not inform Client ('D') (i.e., "classical
redirection"). In that case, Client ('D') has no way of knowing that
Client ('B') is authorized to forward packets from the claimed source
address, and it may simply elect to drop the packets. Also, Client
('B') has no way of knowing whether Client ('D') is performing some
form of source address filtering that would reject packets arriving
from a node other than a trusted default router, nor whether Client
('D') is even reachable via a direct path that does not involve
Server ('A').
Consider a second alternative in which Server ('A') informs both
Client ('B') and Client ('D') separately, via independent redirection
control messages (i.e., "augmented redirection"). In that case, if
Client ('B') receives the redirection control message but Client
('D') does not, subsequent packets sent by Client ('B') could be
dropped due to filtering since Client ('D') would not have a route to
verify the claimed source address. Also, if Client ('D') receives
the redirection control message but Client ('B') does not, subsequent
packets sent in the reverse direction by Client ('D') would be lost.
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Since both of these alternatives have shortcomings, a new redirection
technique (i.e., "AERO redirection") is needed.
3.9.3. Concept of Operations
Again, with reference to Figure 2, when source host ('C') sends a
packet to destination host ('E'), the packet is first forwarded over
the source host's attached EUN to Client ('B'), which then forwards
the packet via its AERO interface to Server ('A').
Server ('A') then re-encapsulates and forwards the packet out the
same AERO interface toward Client ('D') and also sends an AERO
"Predirect" message forward to Client ('D') as specified in
Section 3.9.5. The Predirect message includes Client ('B')'s
network- and link-layer addresses as well as information that Client
('D') can use to determine the IPv6 prefix used by Client ('B') .
After Client ('D') receives the Predirect message, it process the
message and returns an AERO Redirect message destined for Client
('B') via Server ('A') as specified in Section 3.9.6. During the
process, Client ('D') also creates or updates a neighbor cache entry
for Client ('B') in the "ACCEPT" state, and creates an IPv6
forwarding table entry for Client ('B')'s IPv6 prefix.
When Server ('A') receives the Redirect message, it re-encapsulates
the message and forwards it on to Client ('B') as specified in
Section 3.9.7. The message includes Client ('D')'s network- and
link-layer addresses as well as information that Client ('B') can use
to determine the IPv6 prefix used by Client ('D'). After Client
('B') receives the Redirect message, it processes the message as
specified in Section 3.9.8. During the process, Client ('B') also
creates or updates a neighbor cache entry for Client ('D') in the
"FORWARD" state, and creates an IPv6 forwarding table entry for
Client ('D')'s IPv6 prefix.
Following the above Predirect/Redirect message exchange, forwarding
of packets from Client ('B') to Client ('D') without involving Server
('A) as an intermediary is enabled. The mechanisms that support this
exchange are specified in the following sections.
3.9.4. Message Format
AERO Redirect/Predirect messages use the same format as for ICMPv6
Redirect messages depicted in Section 4.5 of [RFC4861], but also
include a new "Prefix Length" field taken from the low-order 8 bits
of the Redirect message Reserved field (valid values for the Prefix
Length field are 0 through 64). The Redirect/Predirect messages are
formatted as shown in Figure 3:
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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 (=137) | Code (=0/1) | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 3: AERO Redirect/Predirect Message Format
3.9.5. Sending Predirects
When a Server forwards a packet out the same AERO interface that it
arrived on, the Server sends a Predirect message forward toward the
AERO Client nearest the destination instead of sending a Redirect
message back to Client nearest the source.
In the reference operational scenario, when Server ('A') forwards a
packet sent by Client ('B') toward Client ('D'), it also sends a
Predirect message forward toward Client ('D'), subject to rate
limiting (see Section 8.2 of [RFC4861]). Server ('A') prepares the
Predirect message as follows:
o the link-layer source address is set to 'L2(A)' (i.e., the
underlying address of Server ('A')).
o the link-layer destination address is set to 'L2(D)' (i.e., the
underlying address of Client ('D')).
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o the network-layer source address is set to fe80::0 (i.e., the
link-local address of Server ('A')).
o the network-layer destination address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('D')).
o the Type is set to 137.
o the Code is set to 1 to indicate "Predirect".
o the Prefix Length is set to the length of the prefix to be applied
to Target address.
o the Target Address is set to fe80::2001:db8:0::0 (i.e., the AERO
address of Client ('B')).
o the Destination Address is set to the IPv6 source address of the
packet that triggered the Predirection event.
o the message includes one or more TLLAOs set to 'L2(B)' and any
other underlying address(es) of Client ('B').
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.
Server ('A') then sends the message forward to Client ('D').
3.9.6. Processing Predirects and Sending Redirects
When Client ('D') receives a Predirect message, it accepts the
message only if it the message has a link-layer source address of the
Server, i.e. 'L2(A)'. Client ('D') further accepts the message only
if it is willing to serve as a redirection target. Next, Client
('D') validates the message according to the ICMPv6 Redirect message
validation rules in Section 8.1 of [RFC4861].
In the reference operational scenario, when the Client ('D') receives
a valid Predirect message, it either creates or updates a neighbor
cache entry that stores the Target Address of the message as the
network-layer address of Client ('B') and stores the link-layer
address(es) found in the TLLAO(s) as the link-layer address(es) of
Client ('B'). Client ('D') then sets the neighbor cache entry in the
ACCEPT state with timeout value ACCEPT-TIME. Next, Client ('D')
applies the Prefix Length to the Interface Identifier portion of the
Target Address and records the resulting IPv6 prefix in its IPv6
forwarding table.
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After processing the message, Client ('D') prepares a Redirect
message response as follows:
o the link-layer source address is set to 'L2(D)' (i.e., the link-
layer address of Client ('D')).
o the link-layer destination address is set to 'L2(A)' (i.e., the
link-layer address of Server ('A')).
o the network-layer source address is set to 'L3(D)' (i.e., the AERO
address of Client ('D')).
o the network-layer destination address is set to 'L3(B)' (i.e., the
AERO address of Client ('B')).
o the Type is set to 137.
o the Code is set to 0 to indicate "Redirect".
o the Prefix Length is set to the length of the prefix to be applied
to the Target and Destination address.
o the Target Address is set to fe80::2001:db8:1::1 (i.e., the AERO
address of Client ('D')).
o the Destination Address is set to the IPv6 destination address of
the packet that triggered the Redirection event.
o the message includes one or more TLLAOs with UDP port number and
IP address set to '0' and with Link ID and Preference values set
to the appropriate values for the underlying interfaces Client
('D') wishes to enable for accepting encapsulated packets from
Client ('B').
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.
After Client ('D') prepares the Redirect message, it sends the
message to Server ('A').
3.9.7. Re-encapsulating and Relaying Redirects
When Server ('A') receives a Redirect message from Client ('D'), it
accepts the message only if it has a neighbor cache entry that
associates the message's link-layer source address with the network-
layer source address. Next, Server ('A') validates the message
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according to the ICMPv6 Redirect message validation rules in
Section 8.1 of [RFC4861] and also verifies that Client ('D') is
authorized to use the Prefix Length in the Redirect message when
applied to the AERO address in the network-layer source of the
Redirect message. If validation fails, Server ('A') discards the
message; otherwise, it copies the correct UDP port numbers and IP
addresses into the TLLAOs supplied by Client ('D') according to the
Link ID in each TLLAO.
Server ('A') then re-encapsulates the Redirect and relays it on to
Client ('B') by changing the link-layer source address of the message
to 'L2(A)', changing the network-layer source address of the message
to fe80::0, and changing the link-layer destination address to
'L2(B)' . Server ('A') finally forwards the re-encapsulated message
to the ingress node ('B') without decrementing the network-layer IPv6
header Hop Limit field.
While not shown in Figure 2, AERO Relays relay Redirect and Predirect
messages in exactly this same fashion described above. See Figure 4
in Appendix A for an extension of the reference operational scenario
that includes Relays.
3.9.8. Processing Redirects
When Client ('B') receives the Redirect message, it accepts the
message only if it has a link-layer source address of the Server,
i.e. 'L2(A)'. Next, Client ('B') validates the message according to
the ICMPv6 Redirect message validation rules in Section 8.1 of
[RFC4861]. Following validation, Client ('B') then processes the
message as follows.
In the reference operational scenario, when Client ('B') receives the
Redirect message, it either creates or updates a neighbor cache entry
that stores the Target Address of the message as the network-layer
address of Client ('D') and stores the link-layer address(es) found
in the TLLAO(s) as the link-layer address(es) of Client ('D').
Client ('D') then sets the neighbor cache entry in the FORWARD state
with timeout value FORWARD_TIME. Next, Client ('B') applies the
Prefix Length to the Interface Identifier portion of the Target
Address and records the resulting IPv6 prefix in its IPv6 forwarding
table.
Now, Client ('B') has an IPv6 forwarding table entry for
Client('D')'s prefix and a neighbor cache entry with a valid
FORWARD_TIME, while Client ('D') has an IPv6 forwarding table entry
for Client ('B')'s prefix with a valid ACCEPT_TIME. Thereafter,
Client ('B') may forward ordinary network-layer data packets directly
to Client ("D") without involving Server ('A') and Client ('D') can
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verify that the packets came from an acceptable source. (In order
for Client ('D') to forward packets to Client ('B') a corresponding
Predirect/Redirect message exchange is required in the reverse
direction.)
3.10. Neighbor Reachability Maintenance
Each AERO node uses its delegated prefix to create an AERO address
(see: Section 3.3). It can then send unicast NS messages to elicit
NA messages from neighbors the same as described for Neighbor
Unreachability Detection (NUD) in [RFC4861] except that the UDP port
and IP address fields of any included S/TLLAOs encode the value 0.
When an AERO node sends an NS/NA message, it MUST use its AERO
address as the IPv6 source address and the AERO address of the
neighbor as the IPv6 destination address. When an AERO node receives
an NS/NA message, it accepts the message if it has a neighbor cache
entry for the neighbor; otherwise, it ignores the message.
When a source Client is redirected to a target Client it SHOULD test
the direct path to the target by sending an initial NS message to
elicit a solicited NA response. While testing the path, the source
Client SHOULD continue sending packets via the Server until target
Client reachability has been confirmed. The source Client SHOULD
thereafter continue to test the direct path to the target Client (see
Section 7.3 of [RFC4861]) in order to keep neighbor cache entries
alive. In particular, the source Client sends NS messages to the
target Client subject to rate limiting in order to receive solicited
NA messages. If at any time the direct path over all underlying
interfaces appears to be failing, the source Client can resume
sending packets via the Server which may or may not result in a new
redirection event.
When a target Client receives an NS message from a source Client, it
resets the ACCEPT_TIME timer if a neighbor cache entry exists;
otherwise, it discards the NS message.
When a source Client receives a solicited NA message from a target
Client, it resets the FORWARD_TIME timer if a neighbor cache entry
exists; otherwise, it discards the NA message.
When the FORWARD_TIME timer on a neighbor cache entry expires, the
source Client resumes sending any subsequent packets via the Server
and may (eventually) receive a new Redirect message. When the
ACCEPT_TIME timer on a neighbor cache entry expires, the target
Client discards any subsequent packets received directly from the
source Client. When both the FORWARD_TIME and ACCEPT_TIME timers on
a neighbor cache entry expire, the Client deletes both the neighbor
cache entry and the corresponding IPv6 forwarding table entry.
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If the source Client is unable to elicit an NA response from the
target Client after MAX_RETRY attempts, it SHOULD consider the direct
path unusable for forwarding purpose.
After reachability of the target Client has been verified, the source
Client SHOULD thereafter process any link-layer errors as a hint that
the direct path to the target Client has either failed or has become
intermittent.
3.11. Mobility and Link-Layer Address Change Considerations
When a Client needs to change one of its link-layer addresses (e.g.,
due to a mobility event), it sends an immediate NS message to each of
its active neighbors (including the Server) using the new link-layer
address as the outer source address and with the correct Link ID and
Preference values in the SLLAO. The Client processes any NA messages
returned as an indication that the neighbor has received the update.
When a Client needs to associate with a new Server, it issues a new
DHCPv6 Request message via the new Server as the DHCPv6 relay. The
new Server then relays the message to the DHCPv6 server and processes
the resulting exchange the same as described in Section 3.9.2. After
the Client receives the resulting DHCPv6 Reply message, it sends an
RS message to the new Server to receive a new RA message and update
its neighbor cache entry for fe80::0.
When a Client disassociates with an existing Server, it sends a
"terminating RS" message to the old Server. The terminating RS
message is prepared exactly the same as for an ordinary RS message,
except that the Code field contains the value '1'. When the old
Server receives the terminating RS message, it withdraws the IPv6
route from the routing system and deletes the neighbor cache entry
and IPv6 forwarding table entry for the Client. The old Server then
returns an RA message with default router lifetime set to 0 which the
Client can use to verify that the termination signal has been
processed. The client then deletes both the default route and the
neighbor cache entry for the old Server. (Note that the Client and
the old Server can impose a small delay before deleting the neighbor
cache and IPv6 forwarding table entries so that any packets already
in the system can still be delivered to the Client.)
An alternative to sending a "terminating RS" message would be for the
Client to somehow perform a DHCPv6 exchange with the DHCPv6 relay
function on the old AERO Server but without involving the DHCPv6
server. This would be insecure because the Client only has a DHCPv6
security association with the DHCPv6 server and not the DHCPv6 relay.
Conversely, the Client and Server already require an authentic
exchange of IPv6 Neighbor Discovery messages.
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3.12. Underlying Protocol Version Considerations
A source Client may connect only to an IPvX underlying network, while
the target Client connects only to an IPvY underlying network. In
that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any redirection
messages and continue to send packets via the Server.
3.13. Multicast Considerations
When the underlying network does not support multicast, AERO nodes
map IPv6 link-scoped multicast addresses (including
"All_DHCP_Relay_Agents_and_Servers") to the underlying IP address of
a Server.
When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in [RFC2529] for IPv4
underlying networks and use a direct multicast mapping for IPv6
underlying networks. (In the latter case, "direct multicast mapping"
means that if the IPv6 multicast destination address of the inner
packet is "M", then the IPv6 multicast destination address of the
encapsulating header is also "M".)
3.14. Operation on AERO Links Without DHCPv6 Services
When the AERO link does not provide DHCPv6 services, operation can
still be accommodated through administrative configuration of
prefixes on AERO Clients. In that case, administrative
configurations of IPv6 routes and AERO interface neighbor cache
entries on both the Server and Client are also necessary. However,
this may preclude 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.
3.15. Operation on Server-less AERO Links
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 can then act as its own Server to
establish neighbor cache entries and IPv6 forwarding table entries by
performing direct Client-to-Client Predirect/Redirect exchanges, and
some other form of trust basis must be applied so that each Client
can verify that the prospective neighbor is authorized to use its
claimed prefix.
When there is no Server on the link, Clients must arrange to receive
prefix delegations and publish the delegations via a secure alternate
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prefix delegation authority through some means outside the scope of
this document.
3.16. Other Considerations
IPv6 hosts serviced by an AERO Client can reach IPv4-only services
via a NAT64 gateway [RFC6146] within the IPv6 network.
AERO nodes can use the Default Address Selection Policy with DHCPv6
option [RFC7078] the same as on any IPv6 link.
All other (non-multicast) functions that operate over ordinary IPv6
links operate in the same fashion over AERO links.
4. Implementation Status
An application-layer implementation is in progress.
5. IANA Considerations
The IANA is instructed to assign a new 2-octet Hardware Type number
for AERO in the "arp-parameters" registry per Section 2 of [RFC5494].
The number is assigned from the 2-octet Unassigned range with
Hardware Type "AERO" and with this document as the reference.
6. Security Considerations
AERO link security considerations are the same as for standard IPv6
Neighbor Discovery [RFC4861] except that AERO improves on some
aspects. In particular, AERO is dependent on a trust basis between
Clients and Servers, where the Clients only engage in the AERO
mechanism when it is facilitated by a trust anchor.
AERO 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.,
WiFi networks) and links that provide physical security (e.g.,
enterprise network wired LANs) provide a first line of defense that
is often sufficient. In other instances, securing mechanisms such as
Secure Neighbor Discovery (SeND) [RFC3971], IPsec [RFC4301] or TLS
[RFC5246] may be necessary.
AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on EUNs to gain access to a protected network,
i.e., AERO Clients that act as IPv6 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
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"share" the address with unauthorized nodes via an IPv6/IPv6 NAT
function.)
On some AERO links, establishment and maintenance of a direct path
between neighbors requires secured coordination such as through the
Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a
security association.
7. Acknowledgements
Discussions both on IETF lists and in private exchanges helped shape
some of the concepts in this work. Individuals who contributed
insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant,
Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha
Hlusiak, Lee Howard, Joe Touch and Bernie Volz. Members of the IESG
also provided valuable input during their review process that greatly
improved the document. Special thanks go to Stewart Bryant, Joel
Halpern and Brian Haberman for their shepherding guidance.
This work has further been encouraged and supported by Boeing
colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie,
Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed
King, Gen MacLean, Kent Shuey, Mike Slane, Julie Wulff, Yueli Yang,
and other members of the BR&T and BIT mobile networking teams.
Earlier works on NBMA tunneling approaches are found in
[RFC2529][RFC5214][RFC5569].
8. References
8.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, December 2011.
8.2. Informative References
[IRON] Templin, F., "The Internet Routing Overlay Network
(IRON)", Work in Progress, June 2012.
[RFC0879] Postel, J., "TCP maximum segment size and related topics",
RFC 879, November 1983.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
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[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC4994] Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
"DHCPv6 Relay Agent Echo Request Option", RFC 4994,
September 2007.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines
for the Address Resolution Protocol (ARP)", RFC 5494,
April 2009.
[RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
Route Optimization Requirements for Operational Use in
Aeronautics and Space Exploration Mobile Networks", RFC
5522, October 2009.
[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, January 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O.
Troan, "Basic Requirements for IPv6 Customer Edge
Routers", RFC 6204, April 2011.
[RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based
DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August
2011.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, November 2011.
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[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, July 2012.
[RFC6706] Templin, F., "Asymmetric Extended Route Optimization
(AERO)", RFC 6706, August 2012.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, February 2013.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 2013.
[RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
Address Option in DHCPv6", RFC 6939, May 2013.
[RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation
with IPv6 Neighbor Discovery", RFC 6980, August 2013.
[RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
Address Selection Policy Using DHCPv6", RFC 7078, January
2014.
Appendix A. AERO Server and Relay Interworking
Figure 2 depicts a reference AERO operational scenario with a single
Server on the AERO link. In order to support scaling to larger
numbers of nodes, the AERO link can deploy multiple Servers and
Relays, e.g., as shown in Figure 4.
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.-(::::::::)
.-(::: IPv6 :::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +------+-------+ +--------------+
|AERO Server C | | AERO Relay D | |AERO Server E |
| (default->D) | | (A->C; G->E) | | (default->D) |
| (A->B) | +-------+------+ | (G->F) |
+-------+------+ | +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client B | |AERO Client F |
| (default->C) | | (default->E) |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IPv6 )-. .-(_ IPv6 )-.
(__ EUN ) (__ EUN )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host A | | Host G |
+--------+ +--------+
Figure 4: AERO Server/Relay Interworking
In this example, Client ('B') associates with Server ('C'), while
Client ('F') associates with Server ('E'). Furthermore, Servers
('C') and ('E') do not associate with each other directly, but rather
have an association with Relay ('D') (i.e., a router that has full
topology information concerning its associated Servers and their
Clients). Relay ('D') connects to the AERO link, and also connects
to the native IPv6 Internetwork.
When host ('A') sends a packet toward destination host ('G'), IPv6
forwarding directs the packet through the EUN to Client ('B'), which
forwards the packet to Server ('C') in absence of more-specific
forwarding information. Server ('C') forwards the packet, and it
also generates an AERO Predirect message that is then forwarded
through Relay ('D') to Server ('E'). When Server ('E') receives the
message, it forwards the message to Client ('F').
After processing the AERO Predirect message, Client ('F') sends an
AERO Redirect message to Server ('E'). Server ('E'), in turn,
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forwards the message through Relay ('D') to Server ('C'). When
Server ('C') receives the message, it forwards the message to Client
('B') informing it that host 'G's EUN can be reached via Client
('F'), thus completing the AERO redirection.
The network layer routing information shared between Servers and
Relays must be carefully coordinated in a manner outside the scope of
this document. In particular, Relays require full topology
information, while individual Servers only require partial topology
information (i.e., they only need to know the EUN prefixes associated
with their current set of Clients). See [IRON] for an architectural
discussion of routing coordination between Relays and Servers.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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