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 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

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.
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).

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. 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:

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:

it would construct its AERO address as:

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.

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:

      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.

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 .

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.

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 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.

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 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.

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 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'):