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Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc6706 (if approved) May 29, 2014
Intended status: Standards Track
Expires: November 30, 2014
Transmission of IPv6 Packets over AERO Links
draft-templin-aerolink-22.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 30, 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 . . . 8
3.4. AERO Interface Neighbor Cache Maintenace . . . . . . . . 9
3.5. AERO Interface Data Origin Authentication . . . . . . . . 10
3.6. AERO Interface MTU Considerations . . . . . . . . . . . . 11
3.7. AERO Interface Encapsulation, Re-encapsulation and
Decapsulation . . . . . . . . . . . . . . . . . . . . . . 12
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. Encapsulation 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 3.2.
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.
link-layer 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 link-layer address. Link-layer
addresses are used as the encapsulation header source and
destination addresses.
network layer address
the source or destination address of the encapsulated IPv6 packet.
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).
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.
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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 AERO Servers 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.
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.
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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 Server configures the AERO address 'fe80::'; this
corresponds to the IPv6 prefix '::/0' (i.e., "default") and provides
a handle for Clients to insert into a neighbor cache entry.
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.
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 Network
Address Translator (NAT) traversal and/or filtering middlebox
traversal may be necessary, a UDP header is further inserted
immediately above the IP encapsulation header.
Servers assign the AERO address fe80:: 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]).
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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 can use any randomly-selected link-local address as the
temporary address, as it is merely a placeholder in DHCPv6 messages
and not used to create neighbor cache entries. 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 adaptation of
standard unicast IPv6 ND messaging. AERO interfaces use Neighbor
Solicitation (NS), Neighbor Advertisement (NA), Router Solicitation
(RS) and Router Advertisement (RA) messages the same as for any IPv6
link. AERO interfaces use two redirection message types -- the first
being the standard Redirect message and the second known as a
Predirect message (see Section 3.9). AERO links further use link-
local-only addressing; hence, Clients ignore any Prefix Information
Options (PIOs) they may receive in RA messages.
AERO interfaces use Source/Target Link Layer Address Options (S/
TLLAOs) the same as for any IPv6 interface. The S/TLLAO option
format is shown in Figure 1:
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 = 1/3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID | Preference | UDP Port Number (or 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-- --+
| |
+-- IP Address (Redirect/Predirect messages only) --+
| |
+-- --+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: AERO S/TLLAO Format
In this format, Link ID is an integer value between 0 and 255
corresponding to an underlying interface of the source/target node,
and Preference is an integer value between 0 and 255 indicating the
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node's preference for this underlying interface (with 0 being highest
preference and 255 being lowest). For Redirect/Predirect messages,
Length is set to 3 and UDP Port Number/IP Address are set to the
addresses used by the target node when it sends encapsulated packets
over the underlying interface. For other ND messages, Length is set
to 1, UDP Port Number is set to 0 and IP Address is omitted.
Each AERO Redirect/Predirect message includes one or more TLLAOs,
i.e., one for each underlying interface the target node wishes to
publish. When no UDP encapsulation is used, UDP Port Number is set
to 0. When the encapsulation IP address family is IPv4, IP Address
is formed as an IPv4-compatible IPv6 address [RFC4291].
AERO interface Redirect/Predirect messages can both update and create
neighbor cache entries. Redirect/Predirect messages SHOULD include a
Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes
can use to verify the message time of origin.
Each AERO NS/NA/RS/RA message includes a single S/TLLAO with UDP Port
Number set to 0 and IP Address omitted (since the node has no way of
knowing whether it is behind a NAT and hence may be unable to convey
the correct values). Instead, AERO nodes determine the link-layer
addresses of neighbors by examining the encapsulation header IP
address and UDP port number of any NS/NA/RS/RA messages they receive.
AERO interface NS/NA/RS/RA messages only update existing neighbor
cache entires and do not create new neighbor cache entries. NS/RS
messages SHOULD include a Nonce option (see Section 5.3 of
[RFC3971]). If an NS/RS message contains a Nonce option, the
recipient MUST echo the option back in the corresponding NA/RA
response. Unsolicited NA/RA messages are not used on AERO
interfaces, and SHOULD be ignored on receipt.
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.
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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 disabled 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 and
Predirect/Redirect messages SHOULD include only a single TLLAO. If
the Client enables multiple underlying interfaces, it instead SHOULD
use a different Link ID and Preference value for the S/TLLAO
corresponding to each interface and Predirect/Redirect messages MAY
include multiple TLLAOs. In that case, the Client would need to send
separate NS/RS messages to each of its neighbors for each active
underlying interface.
3.4. AERO Interface Neighbor Cache Maintenace
Each AERO interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the AERO
link, the same as for any IPv6 interface (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 AERO address corresponding to the prefix in the IA_PD option as
the Client's network layer address and with the Client's
encapsulation 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:: as the network layer address and the Server's
encapsulation IP address and UDP port number as the link-layer
address.
When an AERO Client receives a valid Predirect message 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" timer for the neighbor and uses this timer to determine
whether messages received from the predirected neighbor can be
accepted.
When an AERO Client receives a valid Redirect message 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
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"FORWARD" timer for the neighbor 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 may have gone unreachable.
When an AERO Client receives a valid NS message it (re)sets the
ACCEPT timer for the neighbor to ACCEPT_TIME.
When an AERO Client receives a valid NA message, it (re)sets the
FORWARD timer for the neighbor to FORWARD_TIME.
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 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.
In particular, ACCEPT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO redirection
procedure to converge.
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 nodes
accept encapsulated packets with a link-layer source address
belonging to one of their current AERO Servers and 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 timer is non-zero. An exception is
that neighbor discovery messages may include a different link-layer
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address than the one currently in the neighbor cache for this Link
ID, 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
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 IP fragmentation to fragment the encapsulated packet
into two pieces (where the first fragment contains 1024 bytes of the
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original IPv6 packet) then admits the fragments into the tunnel. If
the encapsulation protocol is IPv4, the node admits each fragment
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 NS message with a SLLAO, a
Nonce option, plus trailing padding octets added to a length of 1500
bytes without including the length of the padding in the IPv6 Payload
Length field. The node then encapsulates the NS in the encapsulation
headers (while including the length of the padding in the
encapsulation header length fields), sets DF to 1 (for IPv4) and
sends the padded NS message to the neighbor. If the neighbor returns
an NA message with a correct Nonce value, the node may then send
whole packets within this size range and (for IPv4) relax the rate
limiting requirement. (Note that the trailing padding SHOULD NOT be
included within the Nonce option itself but rather as padding beyond
the last option in the NS message; otherwise, the (large) Nonce
option would be echoed back in the solicited NA message and may be
lost at a link with a small MTU along the reverse path.)
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. Also, if the neighbor can be
reached by multiple underlying interfaces the paths via each
interface must be probed independently.
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".
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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 packet's IPv6 header into the corresponding fields in the
encapsulation 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
encapsulation header into the corresponding fields in the new
encapsulation header (i.e., the values are transferred between
encapsulation headers and *not* copied from the encapsulated packet's
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 encapsulation header and IPv6
packet 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 IPv6 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 IP protocol number in the
encapsulation header to the appropriate value for the first protocol
layer within the encapsulation (e.g., IPv6, UDP, IPsec, etc.). When
IPv6 is used as the encapsulation protocol, the interface then sets
the flow label value in the encapsulation header the same as
described in [RFC6438]. When IPv4 is used as the encapsulation
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
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].
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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:: as the link-local
address and the discovered addresses of one or more Servers as the
link-layer addresses.
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:: 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 to indicate its preferred prefix to the DHCPv6
server. The Client then sends the encapsulated DHCPv6 request via
one of its active underlying interfaces (i.e., the "primary"
underlying interface for DHCPv6 transactions).
After the Client receives its prefix delegation, it assigns the link-
local AERO address taken from the prefix to the AERO interface 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 also sets both the ACCEPT and FORWARD timers for
each Server to infinity, since the Client will remain with this
Server unless it explicitly terminates the association. 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:: as the IPv6 destination address and the same
DUID value in the Client Identifier option.
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
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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. (If
the Client has multiple active underlying interfaces, it sends
periodic RS messages over each underlying interface.) 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
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 with the Client's link-layer
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address as the link-layer address for the neighbor cache entry. The
neighbor cache entry is created with both ACCEPT and FORWARD timers
set to infinity, since the Client will remain with this Server unless
it explicitly terminates the association.
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:: 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
the 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 received 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::
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:: 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 and neighbor cache
entry via the AERO interface with next-hop address fe80:: 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 and neighbor cache
entry via the AERO interface with next-hop address fe80:: 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') 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') 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 the 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:: (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 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 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 ACCEPT
timer 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::, 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 FORWARD timer 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
verify that the packets came from an acceptable source. (In order
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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
AERO nodes send unicast NS messages to elicit NA messages from
neighbors the same as described for Neighbor Unreachability Detection
(NUD) in [RFC4861]. 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 timer to ACCEPT_TIME 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 timer to FORWARD_TIME if a neighbor
cache entry exists; otherwise, it discards the NA message.
When the FORWARD 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 timer
on a neighbor cache entry expires, the target Client discards any
subsequent packets received directly from the source Client. When
both the FORWARD and ACCEPT timers on a neighbor cache entry expire,
the Client deletes both the neighbor cache entry and the
corresponding IPv6 forwarding table entry.
If the source Client is unable to elicit an NA response from the
target Client after MAX_RETRY attempts, it SHOULD consider the direct
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path unusable for forwarding purposes. Otherwise, the source Client
considers the path usable and SHOULD thereafter process any link-
layer errors as a hint that the direct path to the target Client has
either failed or has become intermittent.
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 encapsulation 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 and is ready to accept encapsulated packets with the new link-
layer address.
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. 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::.
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 MAY 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. Encapsulation 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
encapsulated 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, additional 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.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[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.
[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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