Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720, September 18, 2014
rfc6179, rfc6706 (if
approved)
Intended status: Standards Track
Expires: March 22, 2015

Transmission of IP Packets over AERO Links
draft-templin-aerolink-37.txt

Abstract

This document specifies the operation of IP over tunnel virtual links using Asymmetric Extended Route Optimization (AERO). Nodes attached to AERO links can exchange packets via trusted intermediate routers that provide forwarding services to reach off-link destinations and redirection services for route optimization. AERO provides an IPv6 link-local address format known as the AERO address that supports operation of the IPv6 Neighbor Discovery (ND) protocol and links IPv6 ND to IP forwarding. Admission control and provisioning are supported by the Dynamic Host Configuration Protocol for IPv6 (DHCPv6), and node mobility is naturally supported through dynamic neighbor cache updates. Although DHCPv6 and IPv6 ND messaging is used in the control plane, both IPv4 and IPv6 are supported in the data plane.

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 March 22, 2015.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

This document specifies the operation of IP over tunnel virtual links using Asymmetric Extended Route Optimization (AERO). The AERO link can be used for tunneling to neighboring nodes over either IPv6 or IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as equivalent links for tunneling. Nodes attached to AERO links can exchange packets via trusted intermediate routers that provide forwarding services to reach off-link destinations and redirection services for route optimization that addresses the requirements outlined in [RFC5522].

AERO provides an IPv6 link-local address format known as the AERO address that supports operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and links IPv6 ND to IP forwarding. Admission control and provisioning are supported by the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315], and node mobility is naturally supported through dynamic neighbor cache updates. Although DHCPv6 and IPv6 ND message signalling is used in the control plane, both IPv4 and IPv6 can be used in the data plane. 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 the virtual overlay.
AERO interface

a node's attachment to an AERO link.
AERO address

an IPv6 link-local address constructed as specified in Section 3.2 and applied to a Client's AERO interface.
AERO node

a node that is connected to an AERO link and that participates in IPv6 ND and DHCPv6 messaging over the link.
AERO Client ("Client")

a node that applies an AERO address to an AERO interface and receives an IP prefix via a DHCPv6 Prefix Delegation (PD) exchange with one or more AERO Servers.
AERO Server ("Server")

a node that configures an AERO interface to provide default forwarding and DHCPv6 services for AERO Clients. The Server applies the IPv6 link-local subnet router anycast address (fe80::) to the AERO interface and also applies an administratively assigned IPv6 link-local unicast address used for operation of DHCPv6 and the IPv6 ND protocol.
AERO Relay ("Relay")

a node that configures an AERO interface to relay IP packets between nodes on the same AERO link and/or forward IP packets between the AERO link and the native Internetwork. The Relay applies an administratively assigned IPv6 link-local unicast address to the AERO interface the same as for a 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 the tunnel virtual overlay is configured. A typical example is an enterprise network.
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 IP packet.
end user network (EUN)

an internal virtual or external edge IP network that an AERO Client connects to the rest of the network via the AERO interface.
AERO Service Prefix (ASP)

an IP prefix associated with the AERO link and from which AERO Client Prefixes (ACPs) are derived (for example, the IPv6 ACP 2001:db8:1:2::/64 is derived from the IPv6 ASP 2001:db8::/32).
AERO Client Prefix (ACP)

a more-specific IP prefix taken from an ASP and delegated to a Client.

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.

Throughout the document, it is said that an address is "applied" to an AERO interface since the address is not "assigned" to the interface from the perspective of the IP layer. However, the address must at least be bound to the interface in some fashion to support the operation of DHCPv6 and the IPv6 ND protocol.

The terminology of [RFC4861] (including the names of node variables and protocol constants) applies to this document. Also throughout the document, the term "IP" is used to generically refer to either Internet Protocol version (i.e., IPv4 or IPv6).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Asymmetric Extended Route Optimization (AERO)

The following sections specify the operation of IP over Asymmetric Extended Route Optimization (AERO) links:

3.1. AERO Link Reference Model

                           .-(::::::::)
                        .-(:::: IP ::::)-.
                       (:: Internetwork ::)
                        `-(::::::::::::)-'
                           `-(::::::)-' 
                                |
    +--------------+   +--------+-------+   +--------------+
    |AERO Server S1|   | AERO Relay R1  |   |AERO Server S2|
    |  Nbr: C1; R1 |   |   Nbr: S1; S2  |   |  Nbr: C2; R1 |
    |  default->R1 |   |(H1->S1; H2->S2)|   |  default->R1 |
    |    H1->C1    |   +--------+-------+   |    H2->C2    |
    +-------+------+            |           +------+-------+
            |                   |                  |
    X---+---+-------------------+------------------+---+---X
        |                  AERO Link                   |
  +-----+--------+                            +--------+-----+
  |AERO Client C1|                            |AERO Client C2|
  |    Nbr: S1   |                            |   Nbr: S2    |
  | default->S1  |                            | default->S2  |
  +--------------+                            +--------------+
        .-.                                         .-.
     ,-(  _)-.                                   ,-(  _)-.
  .-(_   IP  )-.                              .-(_   IP  )-.
 (__    EUN      )                           (__    EUN      )
    `-(______)-'                                `-(______)-'
         |                                           |
     +--------+                                  +--------+
     | Host H1|                                  | Host H2|
     +--------+                                  +--------+

Figure 1: AERO Link Reference Model

Figure 1 above presents the AERO link reference model. In this model:

In common operational practice, there may be many additional Relays, Servers and Clients.

3.2. AERO Node Types

AERO Relays provide default forwarding services to AERO Servers. Relays forward packets between Servers connected to the same AERO link and also forward packets between the AERO link and the native Internetwork. Relays present the AERO link to the native Internetwork as a set of one or more AERO Service Prefixes (ASPs). Each Relay advertises the ASPs for the AERO link into the native IP Internetwork and serves as a gateway between the AERO link and the Internetwork. AERO Relays maintain an AERO interface neighbor cache entry for each AERO Server, and maintain an IP forwarding table entry for each AERO Client Prefix (ACP).

AERO Servers provide default forwarding services to AERO Clients. Each Server also peers with each Relay in a dynamic routing protocol instance to advertise its list of associated ACPs. Servers configure a DHCPv6 server function to facilitate Prefix Delegation (PD) exchanges with Clients. Each delegated prefix becomes an ACP taken from an ASP. Servers forward packets between Clients and Relays, as well as between Clients and other Clients associated with the same Server. AERO Servers maintain an AERO interface neighbor cache entry for each AERO Relay. They also maintain both a neighbor cache entry and an IP forwarding table entry for each of their associated Clients.

AERO Clients act as requesting routers to receive ACPs through DHCPv6 PD exchanges with AERO Servers over the AERO link and sub-delegate portions of their ACPs to EUN interfaces. (Each Client MAY associate with a single Server or with multiple Servers, e.g., for fault tolerance and/or load balancing.) Each IPv6 Client receives at least a /64 IPv6 ACP, and may receive even shorter prefixes. Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a singleton IPv4 address), and may receive even shorter prefixes. AERO Clients maintain an AERO interface neighbor cache entry for each of their associated Servers as well as for each of their correspondent Clients.

AERO Clients that act as hosts typically configure a TUN/TAP interface as a point-to-point linkage between the IP layer and the AERO interface. The IP layer therefore sees only the TUN/TAP interface, while the AERO interface provides an intermediate conduit between the TUN/TAP interface and the underlying interfaces. AERO Clients that act as hosts assign one or more IP addresses from their ACPs to the TUN/TAP interface.

3.3. AERO Addresses

An AERO address is an IPv6 link-local address with an embedded ACP and applied to a Client's AERO interface. The AERO address is formed as follows:

For IPv6, the AERO address begins with the prefix fe80::/64 and includes in its interface identifier the base prefix taken from the Client's IPv6 ACP. The base prefix is determined by masking the ACP with the prefix length. For example, if the AERO Client receives the IPv6 ACP:

it constructs its AERO address as:

[RFC4291] that includes the base prefix taken from the Client's IPv4 ACP. For example, if the AERO Client receives the IPv4 ACP:

For IPv4, the AERO address is formed from the lower 64 bits of an IPv4-mapped IPv6 address

it constructs its AERO address as:

The AERO address remains stable as the Client moves between topological locations, i.e., even if its link-layer addresses change.

NOTE: In some cases, prospective neighbors may not have advanced knowledge of the Client's ACP length and may therefore send initial IPv6 ND messages with an AERO destination address that matches the ACP but does not correspond to the base prefix. In that case, the Client MUST accept the address as equivalent to the base address, but then use the base address as the source address of any IPv6 ND message replies. For example, if the Client receives the IPv6 ACP 2001:db8:1000:2000::/56 then subsequently receives an IPv6 ND message with destination address fe80::2001:db8:1000:2001, it accepts the message but uses fe80::2001:db8:1000:2000 as the source address of any IPv6 ND replies.

3.4. AERO Interface Characteristics

AERO interfaces use IP-in-IPv6 encapsulation [RFC2473] to exchange tunneled packets with AERO neighbors attached to an underlying IPv6 network, and use IP-in-IPv4 encapsulation [RFC2003][RFC4213] to exchange tunneled packets with AERO neighbors attached to an underlying IPv4 network. AERO interfaces can also coordinate secured tunnel types such as IPsec [RFC4301] or TLS [RFC5246]. 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.

AERO interfaces maintain a neighbor cache, and AERO Clients and Servers use an adaptation of standard unicast IPv6 ND messaging. AERO interfaces use unicast 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 known as a Predirect message and the second being the standard Redirect message (see Section 3.9). AERO links further use link-local-only addressing; hence, AERO nodes ignore any Prefix Information Options (PIOs) they may receive in RA messages over an AERO interface.

AERO interface ND messages include one or more Target Link-Layer Address Options (TLLAOs) formatted as shown in Figure 2:

      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 2: AERO Target Link-Layer Address Option (TLLAO) Format

In this format, Link ID is an integer value between 0 and 255 corresponding to an underlying interface of the target node, and Preference is an integer value between 0 and 255 indicating the node's preference for this underlying interface (with 255 being the highest preference, 1 being the lowest, and 0 meaning "link disabled"). UDP Port Number and IP Address are set to the addresses used by the target node when it sends encapsulated packets over the underlying interface. 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-mapped IPv6 address [RFC4291].

When a Relay enables an AERO interface, it applies an administratively assigned link-local address fe80::ID to the interface. Each fe80::ID address MUST be unique among all Relays and Servers on the link, and MUST NOT collide with any potential AERO addresses. The addresses are typically taken from the range fe80::/96, e.g., as fe80::1, fe80::2, fe80::3, etc. The Relay also maintains an IP forwarding table entry for each Client-Server association and maintains a neighbor cache entry for each Server on the link. Relays do not require the use of IPv6 ND messaging for reachability determination since Relays and Servers engage in a dynamic routing protocol over the AERO interface. At a minimum, however, Relays respond to NS messages by returning an NA.

When a Server enables an AERO interface, it applies the address fe80:: to the interface as a link-local Subnet Router Anycast address, and also applies an administratively assigned link-local address fe80::ID the same as for Relays. (The Server then accepts DHCPv6 and IPv6 ND solicitation messages destined to either the fe80:: or fe80::ID addresses, but always uses fe80::ID as the source address in the replies it generates.) The Server further configures a DHCPv6 server function to facilitate DHCPv6 PD exchanges with AERO Clients. The Server maintains a neighbor cache entry for each Relay on the link, and manages per-Client neighbor cache entries and IP forwarding table entries based on DHCPv6 exchanges. When the Server receives an NS/RS message on the AERO interface it returns an NA/RA message but does not update the neighbor cache. Each Server also engages in a dynamic routing protocol with all Relays on the link. Finally, the Server provides a simple conduit between Clients and Relays, or between Clients and other Clients. Therefore, packets enter the Server's AERO interface from the link layer and are forwarded back out the link layer without ever leaving the AERO interface and therefore without ever disturbing the network layer.

When a Client enables an AERO interface, it invokes DHCPv6 PD to receive an ACP from an AERO Server. Next, it applies the corresponding AERO address to the AERO interface and creates a neighbor cache entry for the Server, i.e., the PD exchange bootstraps the provisioning of a unique link-local address. The Client maintains a neighbor cache entry for each of its Servers and each of its active correspondent Clients. When the Client receives Redirect/Predirect messages on the AERO interface it updates or creates neighbor cache entries, including link-layer address information. Unsolicited NA messages update the cached link-layer addresses for correspondent Clients (e.g., following a link-layer address change due to node mobility) but do not create new neighbor cache entries. NS/NA messages used for Neighbor Unreachability Detection (NUD) update timers in existing neighbor cache entires but do not update link-layer addresses nor create new neighbor cache entries. Finally, the Client need not maintain any IP forwarding table entries for its Servers or correspondent Clients. Instead, it can set a single "route-to-interface" default route in the IP forwarding table pointing to the AERO interface, and all forwarding decisions can be made within the AERO interface based on neighbor cache entries. (On systems in which adding a default route would violate security policy, the default route could instead be installed via a "synthesized RA", e.g., as discussed in Section 3.11.2.)

3.4.1. Coordination of Multiple Underlying Interfaces

AERO interfaces may be configured over multiple underlying interfaces. For example, common mobile handheld devices have both wireless local area network ("WLAN") and cellular wireless links. These links are typically used "one at a time" with low-cost WLAN preferred and highly-available cellular wireless as a standby. In a more complex example, aircraft frequently have many wireless data link types (e.g. satellite-based, terrestrial, air-to-air directional, etc.) with diverse performance and cost properties.

If a Client's multiple underlying interfaces are used "one at a time" (i.e., all other interfaces are in standby mode while one interface is active), then Redirect, Predirect and unsolicited NA messages include only a single TLLAO with Link ID set to a constant value.

If the Client has multiple active underlying interfaces, then from the perspective of IPv6 ND it would appear to have a single link-local address with multiple link-layer addresses. In that case, Redirect, Predirect and unsolicited NA messages MAY include multiple TLLAOs -- each with a different Link ID that corresponds to a specific underlying interface of the Client.

3.5. 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 [RFC4861]. AERO interface neighbor cache entires are said to be one of "permanent", "static" or "dynamic".

Permanent neighbor cache entries are created through explicit administrative action; they have no timeout values and remain in place until explicitly deleted. AERO Relays maintain a permanent neighbor cache entry for each Server on the link, and AERO Servers maintain a permanent neighbor cache entry for each Relay on the link.

Static neighbor cache entries are created though DHCPv6 PD exchanges and remain in place for durations bounded by prefix lifetimes. AERO Servers maintain a static neighbor cache entry for each of their associated Clients, and AERO Clients maintain a static neighbor cache for each of their associated Servers. When an AERO Server sends a DHCPv6 Reply message response to a Client's DHCPv6 Solicit/Request or Renew message, it creates or updates a static neighbor cache entry based on the Client's AERO address as the network-layer address, the prefix lifetime as the neighbor cache entry lifetime, the Client's encapsulation IP address and UDP port number as the link-layer address and the prefix length as the length to apply to the AERO address. When an AERO Client receives a DHCPv6 Reply message from a Server, it creates or updates a static neighbor cache entry based on the Reply message link-local source address as the network-layer address, the prefix lifetime as the neighbor cache entry lifetime, and the encapsulation IP source address and UDP source port number as the link-layer address.

Dynamic neighbor cache entries are created based on receipt of an IPv6 ND message, and are garbage-collected if not used within a short timescale. AERO Clients maintain dynamic neighbor cache entries for each of their active correspondent Clients with lifetimes based on IPv6 ND messaging constants. When an AERO Client receives a valid Predirect message it creates or updates a dynamic neighbor cache entry for the Predirect target network-layer and link-layer addresses plus prefix length. The node then sets an "AcceptTime" variable in the neighbor cache entry and uses this value to determine whether packets received from the correspondent can be accepted. When an AERO Client receives a valid Redirect message it creates or updates a dynamic neighbor cache entry for the Redirect target network-layer and link-layer addresses plus prefix length. The Client then sets a "ForwardTime" variable in the neighbor cache entry and uses this value to determine whether packets can be sent directly to the correspondent. The Client also maintains a "MaxRetry" variable to limit the number of keepalives sent when a correspondent may have gone unreachable.

For dynamic neighbor cache entries, when an AERO Client receives a valid NS message it (re)sets AcceptTime for the neighbor to ACCEPT_TIME. When an AERO Client receives a valid solicited NA message, it (re)sets ForwardTime for the neighbor to FORWARD_TIME and sets MaxRetry to MAX_RETRY. When an AERO Client receives a valid unsolicited NA message, it updates the correspondent's link-layer addresses but DOES NOT reset AcceptTime, ForwardTime or MaxRetry.

It is RECOMMENDED that FORWARD_TIME be set to the default constant value 30 seconds to match the default REACHABLE_TIME value specified for IPv6 ND [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 AcceptTime decrements below FORWARD_TIME.

It is RECOMMENDED that MAX_RETRY be set to 3 the same as described for IPv6 ND address resolution in Section 7.3.3 of [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. Most importantly, ACCEPT_TIME SHOULD be set to a value that is sufficiently longer than FORWARD_TIME to allow the AERO redirection procedure to converge.

3.6. AERO Interface Sending Algorithm

IP packets enter a node's AERO interface either from the network layer (i.e., from a local application or the IP forwarding system), or from the link layer (i.e., from the AERO tunnel virtual link). Packets that enter the AERO interface from the network layer are encapsulated and admitted into the AERO link (i.e., they are tunnelled to an AERO interface neighbor). Packets that enter the AERO interface from the link layer are either re-admitted into the AERO link or delivered to the network layer where they are subject to either local delivery or IP forwarding. Since each AERO node has only partial information about neighbors on the link, AERO interfaces may forward packets with link-local destination addresses at a layer below the network layer. This means that AERO nodes act as both IP routers and sub-IP layer forwarding agents. AERO interface sending considerations for Clients, Servers and Relays are given below.

When an IP packet enters a Client's AERO interface from the network layer, if the destination is covered by an ASP the Client searches for a dynamic neighbor cache entry with a non-zero ForwardTime and an AERO address that matches the packet's destination address. (The destination address may be either an address covered by the neighbor's ACP or the (link-local) AERO address itself.) If there is a match, the Client uses a link-layer address in the entry as the link-layer address for encapsulation then admits the packet into the AERO link. If there is no match, the Client instead uses the link-layer address of a neighboring Server as the link-layer address for encapsulation.

When an IP packet enters a Server's AERO interface from the link layer, if the destination is covered by an ASP the Server searches for a static neighbor cache entry with an AERO address that matches the packet's destination address. (The destination address may be either an address covered by the neighbor's ACP or the AERO address itself.) If there is a match, the Server uses a link-layer address in the entry as the link-layer address for encapsulation and re-admits the packet into the AERO link. If there is no match, the Server instead uses the link-layer address in any permanent neighbor cache entry as the link-layer address for encapsulation.

When an IP packet enters a Relay's AERO interface from the network layer, the Relay searches its IP forwarding table for an entry that is covered by an ASP and also matches the destination. If there is a match, the Relay uses the link-layer address in the neighbor cache entry for the next-hop Server as the link-layer address for encapsulation and admits the packet into the AERO link. When an IP packet enters a Relay's AERO interface from the link-layer, if the destination is not a link-local address and is not covered by an ASP the Relay removes the packet from the AERO interface and uses IP forwarding to forward the packet to the Internetwork. If the destination address is covered by an ASP, and there is a more-specific IP forwarding table entry that matches the destination, the Relay uses the link-layer address in the neighbor cache entry for the next-hop Server as the link-layer address for encapsulation and re-admits the packet into the AERO link. When an IP packet enters a Relay's AERO interface from either the network layer or link-layer, and the packet's destination address matches an ASP but there is no more-specific ACP entry, the Relay drops the packet and returns an ICMP Destination Unreachable message (see: Section 3.10).

When an AERO Server receives a packet from a Relay via the AERO interface, the Server MUST NOT forward the packet back to the same or a different Relay.

When an AERO Relay receives a packet from a Server via the AERO interface, the Relay MUST NOT forward the packet back to the same Server.

When an AERO node re-admits a packet into the AERO link without involving the network layer, the node MUST NOT decrement the network layer TTL/Hop-count.

Note that in the above that the link-layer address for encapsulation may be determined through consulting either the neighbor cache or the IP forwarding table. IP forwarding is therefore linked to IPv6 ND via the AERO address.

3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation

AERO interfaces encapsulate IP packets according to whether they are entering the AERO interface from the network layer or if they are being re-admitted into the same AERO link they arrived on. This latter form of encapsulation is known as "re-encapsulation".

AERO interfaces encapsulate packets per the base tunneling specifications (e.g., [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246], etc.) except that the interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion Experienced" values in the packet's IP 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 base tunneling specification except that it inserts a UDP header between the encapsulation header and the packet's IP header. The AERO interface sets the UDP source port to a constant value that it will use in each successive packet it sends and sets the UDP length field to the length of the IP 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 correspondent Client, the AERO interface sets the UDP destination port to the port value stored in the neighbor cache entry for this correspondent.

The AERO interface also sets the UDP checksum field to zero (see: [RFC6935][RFC6936]) for packets that do not require assurance against reassembly errors. For packets that require reassembly checks (see Section 3.9), the AERO interface instead (re)calculates the UDP checksum and writes the resulting value in the UDP checksum field.

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., IPv4, 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 3.9.

AERO interfaces decapsulate packets destined either to the node itself or to a destination reached via an interface other than the AERO interface the packet was received on. When AERO UDP encapsulation is used (i.e., when a UDP header with destination port 8060 is present) the interface first verifies the UDP checksum if the UDP checksum was non-zero, then examines the first octet of the encapsulated packet. The packet is accepted if the most significant four bits of the first octet encode the value '0110' (i.e., the version number value for IPv6) or the value '0100' (i.e., the version number value for IPv4). Otherwise, the packet is accepted if the first octet encodes a valid IP protocol number per the IANA "protocol-numbers" registry that matches a supported encapsulation type. Otherwise, the packet is discarded.

Further decapsulation then proceeds according to the appropriate base tunneling specification.

3.8. AERO Interface Data Origin Authentication

AERO nodes employ simple data origin authentication procedures for encapsulated packets they receive from other nodes on the AERO link. In particular:

  • AERO Relays and Servers accept encapsulated packets with a link-layer source address that matches a permanent neighbor cache entry.
  • AERO Servers accept authentic encapsulated DHCPv6 messages, and create or update a static neighbor cache entry for the source based on the specific message type.
  • AERO Servers accept encapsulated packets if there is a static neighbor cache entry with an AERO address that matches the packet's network-layer source address and with a link-layer address that matches the packet's link-layer source address.
  • AERO Clients accept encapsulated packets if there is a static neighbor cache entry with a link-layer source address that matches the packet's link-layer source address.
  • AERO Clients and Servers accept encapsulated packets if there is a dynamic neighbor cache entry with an AERO address that matches the packet's network-layer source address, with a link-layer address that matches the packet's link-layer source address, and with a non-zero AcceptTime.

Note that this simple data origin authentication only applies to environments in which link-layer addresses cannot be spoofed. Additional security mitigations may be necessary in other environments.

3.9. AERO Interface MTU Considerations

The AERO interface is the node's point of attachment to the AERO link. AERO links over IP networks have a maximum link MTU of 64KB minus the encapsulation overhead (i.e., "64KB-ENCAPS"), since the maximum packet size in the base IP specifications is 64KB [RFC0791][RFC2460]. AERO links over IPv6 networks have a theoretical maximum link MTU of 4GB-ENCAPS [RFC2675], however IPv6 Jumbograms are considered optional for IPv6 nodes [RFC6434] and therefore out of scope for this document.

The IP layer sees the AERO interface as an ordinary interface that configures an MTU that is no larger than the link MTU, i.e., the same as for any interface. Routers MAY set an AERO interface MTU up to the maximum link MTU. Hosts SHOULD set a more conservative MTU so that upper layer protocols will see an appropriate maximum packet size, for example when setting an initial TCP Maximum Segment Size (MSS). In all cases, routers and hosts MUST set an MTU of at least 1500 bytes.

IPv6 specifies a minimum link MTU of 1280 bytes [RFC2460]. This is the minimum packet size an AERO interface MUST be capable of forwarding without returning an ICMP Packet Too Big (PTB) message. Although IPv4 specifies a smaller minimum link MTU of 68 bytes [RFC0791], AERO interfaces also observe a 1280 byte minimum for IPv4. Additionally, the vast majority of links in the Internet configure an MTU of at least 1500 bytes. Hosts have therefore become conditioned to expect that IP packets up to 1500 bytes in length will either be delivered to the final destination or a suitable ICMP Packet Too Big (PTB) message returned, however such PTB messages are often lost [RFC2923]. Therefore, AERO interfaces MUST pass IP packets of at least 1500 bytes even if the encapsulated packet must be fragmented.

PTB messages may be generated by the IP layer of the AERO node if the packet is too large to enter the AERO interface, from within the AERO interface itself if the packet is larger than 1500 bytes and also larger than the MTU of the underlying interface to be used for tunneling minus ENCAPS, or from a router within the AERO link (i.e., the "tunnel") after the encapsulated packet has been admitted into the tunnel. The latter condition would result in a link-layer (L2) PTB message delivered to the AERO interface, while the former two conditions would result in a network-layer (L3) PTB message delivered to the original source.

For AERO links over IPv4, the IP ID field is only 16 bits in length, meaning that fragmentation at high data rates could result in dangerous reassembly misassociations [RFC6864][RFC4963]. For this reason, AERO interfaces that send fragmented IPv4-encapsulated packets MUST either institute rate limiting to ensure that the IP ID field will not wrap before all earlier fragments have been processed, or include an integrity check to detect reassembly errors.

The AERO interface therefore admits encapsulated packets into the tunnel (using fragmentation as necessary) as follows:

  • For IP packets that are no larger than (1280-ENCAPS) bytes, the AERO interface admits the packet into the tunnel without fragmentation. For IPv4 AERO links, the AERO interface sets the Don't Fragment (DF) bit to 0 so that these packets will be deterministically delivered even if there is a restricting link in the path. The AERO interface need not perform rate limiting or include integrity checks for these packets, since any IPv4 links in the path that configure an MTU smaller than 1280 bytes are very likely to be slow links [RFC3819].
  • For IP packets that are larger than (1280-ENCAPS) bytes but no larger than 1500 bytes, the AERO interface encapsulates the packet. (For IPv4 AERO links, the AERO interface then sets the DF bit to 0 and calculates the UDP checksum for the encapsulated packet as an integrity check to account for the potential for reassembly misassociations. If the encapsulation does not include a UDP header or other integrity check, the AERO interface instead MUST institute rate limiting.) Next, the AERO interface uses IP fragmentation to fragment the encapsulated packet into two fragments where the first fragment is no larger than 1024 bytes and the other fragment is no larger than the first fragment. The AERO interface then admits both fragments into the tunnel.
  • For IPv4 packets that are larger than 1500 bytes and with the DF bit set to 0, the AERO interface fragments the unencapsulated packet into a minimum number of fragments where the first fragment is no larger than 1024 bytes and all other fragments are no larger than the first fragment. The AERO interface then encapsulates each fragment (and for IPv4 sets the DF bit to 0) and admits the fragments into the tunnel. These encapsulated fragments will be deterministically delivered to the final destination. (The AERO interface need not perform rate limiting or include integrity checks for these packets since it is not the original source of the unencapsulated packet.)
  • For all other IP packets, if the packet is larger than the AERO interface MTU the AERO node drops the packet and returns an L3 PTB message with MTU set to the AERO interface MTU; otherwise, the node admits the packet into the AERO interface. Next, if the packet length is larger than the MTU of the underlying interface to be used for tunneling minus ENCAPS, the AERO interface drops the packet and returns an L3 PTB message with MTU set to the larger of 1500 or the underlying interface MTU minus ENCAPS. Otherwise, the AERO interface encapsulates the packet and admits it into the tunnel without fragmentation (and for IPv4 sets the DF bit to 1) and translates any L2 PTB messages it may receive from the network into corresponding L3 PTB messages to send to the original source as specified in Section 3.10. Since both L2 and L3 PTB messages may be either lost or contain insufficient information, however, it is RECOMMENDED that sources that send unfragmentable IP packets larger than 1500 bytes use Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821].

While sending packets according to the above specifications, the AERO interface (i.e., the tunnel ingress) MAY also send 1500 byte probe packets to the tunnel egress to determine whether the probes can traverse the tunnel without fragmentation. If the probes succeed, the tunnel ingress can begin sending packets that are larger than 1280-ENCAPS bytes but no larger than 1500 bytes without fragmentation (and for IPv4 with DF set to 1). Since the path MTU within the tunnel may fluctuate due to routing changes, the tunnel ingress SHOULD continually send additional probes subject to rate limiting in case L2 PTB messages are lost. If the path MTU within the tunnel later becomes insufficient, the tunnel ingress must resume fragmentation.

To construct a probe, the tunnel ingress prepares an NS message with 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 tunnel ingress then encapsulates the padded NS message in the encapsulation headers (and for IPv4 sets DF to 1) then sends the message to the tunnel egress. If the tunnel egress returns a solicited NA message with a matching Nonce option, the tunnel ingress deems the probe successful.. Note that the tunnel ingress SHOULD NOT include the trailing padding 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.

In light of the above fragmentation and reassembly recommendations, the tunnel egress MUST be capable of reassembling encapsulated packets up to 1500+ENCAPS bytes in length. It is therefore RECOMMENDED that the tunnel egress be capable of reassembling at least 2KB. Also, in some environments there may be operational assurance that all links within the routing region spanned by the tunnel configure sufficiently large MTUs so that fragmentation and reassembly can be avoided. In those cases, specific tunnel specifications must explain the circumstances under which the above fragmentation and reassembly recommendations need not be applied.

Of possible concern is that some network middleboxes hold the fragments of a fragmented UDP packet until all fragments have arrived before forwarding the fragments to the final destination. This means that the network middlebox must also be able to accommodate fragmented UDP packets up to 1500+ENCAPS bytes in length, i.e., and not just the IP protocol minimum reassembly size. However, network middleboxes are already capable of passing fragmented UDP datagrams up to the maximum fragmented IP packet size as evidenced through actual operational experience (see the thread "PMTUD issue discussion" in the IETF v6ops archive dated September 10, 2014). Hence, there is no need for AERO to stipulate a minimum reassembly size for such devices.

3.10. AERO Interface Error Handling

When an AERO node admits encapsulated packets into the AERO interface, it may receive link-layer (L2) or network-layer (L3) error indications.

An L2 error indication is an ICMP error message generated by a router on the path to the neighbor or by the neighbor itself. The message includes an IP header with the address of the node that generated the error as the source address and with the link-layer address of the AERO node as the destination address.

The IP header is followed by an ICMP header that includes an error Type, Code and Checksum. For ICMPv6 [RFC4443], the error Types include "Destination Unreachable", "Packet Too Big (PTB)", "Time Exceeded" and "Parameter Problem". For ICMPv4 [RFC0792], the error Types include "Destination Unreachable", "Fragmentation Needed" (a Destination Unreachable Code that is analogous to the ICMPv6 PTB), "Time Exceeded" and "Parameter Problem".

The ICMP header is followed by the leading portion of the packet that generated the error, also known as the "packet-in-error". For ICMPv6, [RFC4443] specifies that the packet-in-error includes: "As much of invoking packet as possible without the ICMPv6 packet exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes). For ICMPv4, [RFC0792] specifies that the packet-in-error includes: "Internet Header + 64 bits of Original Data Datagram", however [RFC1812] Section 4.3.2.3 updates this specification by stating: "the ICMP datagram SHOULD contain as much of the original datagram as possible without the length of the ICMP datagram exceeding 576 bytes".

The L2 error message format is shown in Figure 3:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                               ~
     |        L2 IP Header of        |
     |         error message         |
     ~                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         L2 ICMP Header        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
     ~                               ~   P
     |   IP and other encapsulation  |   a
     | headers of original L3 packet |   c
     ~                               ~   k
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
     ~                               ~   t
     |        IP header of           |   
     |      original L3 packet       |   i
     ~                               ~   n
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   
     ~                               ~   e
     |    Upper layer headers and    |   r
     |    leading portion of body    |   r
     |   of the original L3 packet   |   o
     ~                               ~   r
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

Figure 3: AERO Interface L2 Error Message Format

  • When an AERO node receives an L2 "Parameter Problem", it processes the message the same as described as for ordinary ICMP errors in the normative references [RFC0792][RFC4443].
  • When an AERO node receives persistent L2 Time Exceeded messages, it SHOULD reduce its current rate of admitting fragmented encapsulated packets into the tunnel to ensure that the IP ID field will not wrap before all earlier fragments have been processed. If the AERO node includes an integrity check vector, however, it MAY ignore the messages and continue sending fragmented encapsulated packets without rate limiting.
  • When an AERO Client receives persistent L2 Destination Unreachable messages in response to tunneled packets that it sends to one of its dynamic neighbor correspondents, the Client SHOULD test the path to the correspondent using Neighbor Unreachability Detection (NUD) (see Section 3.14). If NUD fails, the Client SHOULD set ForwardTime for the corresponding dynamic neighbor cache entry to 0 and allow future packets destined to the correspondent to flow through a Server.
  • When an AERO Client receives persistent L2 Destination Unreachable messages in response to tunneled packets that it sends to one of its static neighbor Servers, the Client SHOULD test the path to the Server using NUD. If NUD fails, the Client SHOULD delete the neighbor cache entry and attempt to associate with a new Server.
  • When an AERO Server receives persistent L2 Destination Unreachable messages in response to tunneled packets that it sends to one of its static neighbor Clients, the Server SHOULD test the path to the Client using NUD. If NUD fails, the Server SHOULD cancel the DHCPv6 PD lease for the Client's ACP, withdraw its route for the ACP from the AERO routing system and delete the neighbor cache entry (see Sections 3.11 and 3.12).
  • When an AERO Relay or Server receives an L2 Destination Unreachable message in response to a tunneled packet that it sends to one of its permanent neighbors, it discards the message since the routing system is likely in a temporary transitional state that will soon re-converge.
  • When an AERO node receives an L2 PTB message, it translates the message into an L3 PTB message if possible (*) and forwards the message toward the original source as described below.

To translate an L2 PTB message to an L3 PTB message, the AERO node first caches the MTU field value of the L2 ICMP header. The node next discards the L2 IP and ICMP headers, and also discards the encapsulation headers of the original L3 packet. Next the node encapsulates the included segment of the original L3 packet in an L3 IP and ICMP header, and sets the ICMP header Type and Code values to appropriate values for the L3 IP protocol. In the process, the node writes the maximum of 1500 bytes and (L2 MTU - ENCAPS) into the MTU field of the L3 ICMP header.

The node next writes the IP source address of the original L3 packet as the destination address of the L3 PTB message and determines the next hop to the destination. If the next hop is reached via the AERO interface, the node uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the IP source address of the L3 PTB message. Otherwise, the node uses one of its non link-local addresses as the source address of the L3 PTB message. The node finally calculates the ICMP checksum over the L3 PTB message and writes the Checksum in the corresponding field of the L3 ICMP header. The L3 PTB message therefore is formatted as follows:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                               ~
     |        L3 IP Header of        |
     |         error message         |
     ~                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         L3 ICMP Header        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ---
     ~                               ~   p
     |        IP header of           |   k
     |      original L3 packet       |   t
     ~                               ~ 
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   i  
     ~                               ~   n
     |    Upper layer headers and    |
     |    leading portion of body    |   e
     |   of the original L3 packet   |   r
     ~                               ~   r
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

Figure 4: AERO Interface L3 Error Message Format

When an AERO Relay receives an L3 packet for which the destination address is covered by an ASP, if there is no more-specific routing information for the destination the Relay drops the packet and returns an L3 Destination Unreachable message. The Relay first writes the IP source address of the original L3 packet as the destination address of the L3 Destination Unreachable message and determines the next hop to the destination. If the next hop is reached via the AERO interface, the Relay uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the IP source address of the L3 Destination Unreachable message and forwards the message to the next hop within the AERO interface. Otherwise, the Relay uses one of its non link-local addresses as the source address of the L3 Destination Unreachable message and forwards the message via a link outside the AERO interface.

When an AERO node receives any L3 error message via the AERO interface, it examines the destination address in the L3 IP header of the message. If the next hop toward the destination address of the error message is via the AERO interface, the node re-encapsulates and forwards the message to the next hop within the AERO interface. Otherwise, if the source address in the L3 IP header of the message is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node writes one of its non link-local addresses as the source address of the L3 message and recalculates the IP and/or ICMP checksums. The node finally forwards the message via a link outside of the AERO interface.

(*) Note that in some instances the packet-in-error field of an L2 PTB message may not include enough information for translation to an L3 PTB message. In that case, the AERO interface simply discards the L2 PTB message. It can therefore be said that translation of L2 PTB messages to L3 PTB messages can provide a useful optimization when possible, but is not critical for sources that correctly use PLPMTUD.

3.11. AERO Router Discovery, Prefix Delegation and Address Configuration

3.11.1. AERO DHCPv6 Service Model

Each AERO Server configures a DHCPv6 server function to facilitate PD requests from Clients. Each Server is pre-configured with an identical list of ACP-to-Client ID mappings for all Clients enrolled in the AERO system, as well as any information necessary to authenticate Clients. The configuration information is maintained by a central administrative authority for the AERO link and securely propagated to all Servers whenever a new Client is enrolled or an existing Client is withdrawn.

With these identical configurations, each Server can function independently of all other Servers, including the maintenance of active leases. Therefore, no Server-to-Server DHCPv6 state synchronization is necessary, and Clients can optionally hold separate leases for the same ACP from multiple Servers.

In this way, Clients can easily associate with multiple Servers, and can receive new leases from new Servers before deprecating leases held through old Servers. This enables a graceful "make-before-break" capability.

3.11.2. AERO Client Behavior

AERO Clients 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 "linkupnetworks" is a constant text string and "[domainname]" is the connection-specific DNS suffix for the Client's underlying network connection (e.g., "example.com"). After discovering the link-layer addresses, the Client associates with one or more of the corresponding Servers.

To associate with a Server, the Client acts as a requesting router to request an ACP through a DHCPv6 PD exchange[RFC3315][RFC3633] in which the Client's Solicit/Request messages use the IPv6 "unspecified" address (i.e., "::") as the IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address and the link-layer address of the Server as the link-layer destination address. The Client also includes a Client Identifier option with a DHCP Unique Identifier (DUID) plus any necessary authentication options to identify itself to the DHCPv6 server, and includes a Client Link Layer Address Option (CLLAO) [RFC6939] with the format shown in Figure 5:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | OPTION_CLIENT_LINKLAYER_ADDR  |           option-length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   link-layer type (16 bits)   |    Link ID    |   Preference  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: AERO Client Link-Layer Address Option (CLLAO) Format

Figure 2). If the Client is pre-provisioned with an ACP associated with the AERO service, it MAY also include the ACP in the Solicit/Request message Identity Association (IA) option to indicate its preferred ACP to the DHCPv6 server. The Client then sends the encapsulated DHCPv6 request via the underlying interface.

When the Client receives its ACP and the set of ASPs via a DHCPv6 Reply from the AERO Server, it creates a static neighbor cache entry with the Server's link-local address as the network-layer address and the Server's encapsulation address as the link-layer address. The Client then records the lifetime for the ACP in the neighbor cache entry and marks the neighbor cache entry as "default", i.e., the Client considers the Server as a default router. If the Reply message contains a Vendor-Specific Information Option (see: Section 3.10.3) the Client also caches each ASP in the option.

The Client then applies the AERO address to the AERO interface and sub-delegates the ACP to nodes and links within its attached EUNs (the AERO address thereafter remains stable as the Client moves). The Client also assigns a default IP route to the AERO interface as a route-to-interface, i.e., with no explicit next-hop. The next hop will then be determined after a packet has been submitted to the AERO interface by inspecting the neighbor cache (see above).

On some platforms (e.g., popular cell phone operating systems), the act of assigning a default IPv6 route to the AERO interface may not be permitted from a user application due to security policy. Typically, those platforms include a TUN/TAP interface that acts as a point-to-point conduit between user applications and the AERO interface. In that case, the Client can instead generate a "synthesized RA" message. The message conforms to [RFC4861] and is prepared as follows:

[RFC2131]. Note that in this method, the Client appears as a mobility proxy for applications that bind to the (point-to-point) TUN/TAP interface. The arrangement can be likened to a Proxy AERO scenario in which the mobile node and Client are located within the same physical platform (see Section 3.20 for further details on Proxy AERO).

  • the IPv6 source address is fe80::
  • the IPv6 destination address is all-nodes multicast
  • the Router Lifetime is set to a time that is no longer than the ACP DHCPv6 lifetime
  • the message does not include a Source Link Layer Address Option (SLLAO)
  • the message includes a Prefix Information Option (PIO) with a /64 prefix taken from the ACP as the prefix for autoconfiguration

The Client then sends the synthesized RA message via the TUN/TAP interface, where the operating system kernel will interpret it as though it were generated by an actual router. The operating system will then install a default route and use StateLess Address AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP interface. Methods for similarly installing an IPv4 default route and IPv4 address on the TUN/TAP interface are based on synthesized DHCPv4 messages

The Client subsequently renews its ACP delegation through each of its Servers by performing DHCPv6 Renew/Reply exchanges with its AERO address as the IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address, the link-layer address of a Server as the link-layer destination address and the same Client identifier, authentication options and CLLAO option as was used in the initial PD request. Note that if the Client does not issue a DHCPv6 Renew before the Server has terminated the lease (e.g., if the Client has been out of touch with the Server for a considerable amount of time), the Server's Reply will report NoBinding and the Client must re-initiate the DHCPv6 PD procedure. If the Client sends synthesized RA and/or DHCPv4 messages (see above), it also sends a new synthesized message when issuing a DHCPv6 Renew or when re-initiating the DHCPv6 PD procedure.

Since the Client's AERO address is configured from the unique ACP 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 access to the network by the DHCPv6 server due to an unacceptable link-layer address and/or security parameters (see: Security Considerations).

AERO Clients ignore the IP address and UDP port number in any S/TLLAO options in ND messages they receive directly from another AERO Client, but examine the Link ID and Preference values to match the message with the correct link-layer address information.

When a source Client forwards a packet to a prospective destination Client (i.e., one for which the packet's destination address is covered by an ASP), the source Client initiates an AERO route optimization procedure as specified in Section 3.13.

3.11.3. AERO Server Behavior

AERO Servers configure a DHCPv6 server function on their AERO links. AERO Servers arrange to add their encapsulation layer IP addresses (i.e., their link-layer addresses) to the DNS resource records for the FQDN "linkupnetworks.[domainname]" before entering service.

When an AERO Server receives a prospective Client's DHCPv6 PD Solicit/Request message, it first authenticates the message. If authentication succeeds, the Server determines the correct ACP to delegate to the Client by matching the Client's DUID within an online directory service (e.g., LDAP). The Server then delegates the ACP and creates a static neighbor cache entry for the Client's AERO address with lifetime set to no more than the lease lifetime and the Client's link-layer address as the link-layer address for the Link ID specified in the CLLAO option. The Server then creates an IP forwarding table entry so that the AERO routing system will propagate the ACP to all Relays (see: Section 3.12). Finally, the Server sends a DHCPv6 Reply message to the Client while using fe80::ID as the IPv6 source address, the Client's AERO address as the IPv6 destination address, and the Client's link-layer address as the destination link-layer address. The Server also includes a Server Unicast option with server-address set to fe80::ID so that all future Client/Server transactions will be link-local-only unicast over the AERO link.

When the Server sends the DHCPv6 Reply message, it also includes a DHCPv6 Vendor-Specific Information Option with 'enterprise-number' set to "TBD2" (see: IANA Considerations). The option is formatted as shown in[RFC3315] and with the AERO enterprise-specific format shown in Figure 6:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      OPTION_VENDOR_OPTS       |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   enterprise-number ("TBD2")                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Reserved                 | Prefix Length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                            ASP (1)                            +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Reserved                 | Prefix Length | 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                             ASP (2)                           +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Reserved                 | Prefix Length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                             ASP (3)                           +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                             (etc.)                            .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: AERO Vendor-Specific Information Option

Figure 6, the option includes one or more ASP. The ASP field contains the IP prefix as it would appear in the interface identifier portion of the corresponding AERO address (see: Section 3.3). For IPv6, valid values for the Prefix Length field are 0 through 64; for IPv4, valid values are 0 through 32.

After the initial DHCPv6 PD exchange, the AERO Server maintains the neighbor cache entry for the Client as long as the lease lifetime remains current. If the Client issues a Renew/Reply exchange, the Server extends the lifetime. If the Client issues a Release/Reply exchange, or if the Client does not issue a Renew/Reply within the lease lifetime, the Server deletes the neighbor cache entry for the Client and withdraws the IP route from the AERO routing system.

3.12. AERO Relay/Server Routing System

Relays require full topology information of all Client/Server associations, while individual Servers only require partial topology information, i.e., they only need to know the ACPs associated with their current set of associated Clients. This is accomplished through the use of an internal instance of the Border Gateway Protocol (BGP) [RFC4271] coordinated between Servers and Relays. This internal BGP instance does not interact with the public Internet BGP instance; therefore, the AERO link is presented to the IP Internetwork as a small set of ASPs as opposed to the full set of individual ACPs.

In a reference BGP arrangement, each AERO Server is configured as an Autonomous System Border Router (ASBR) for a stub Autonomous System (AS) (possibly using a private AS Number (ASN) [RFC1930]), and each Server further peers with each Relay but does not peer with other Servers. Similarly, Relays need not peer with each other, since they will receive all updates from all Servers and will therefore have a consistent view of the AERO link ACP delegations.

Each Server maintains a working set of associated Clients, and dynamically announces new ACPs and withdraws departed ACPs in its BGP updates to Relays (this is typically accomplished via a "redistribute static" routing directive). Relays do not send BGP updates to Servers, however, such that the BGP route reporting is unidirectional from the Servers to the Relays.

The Relays therefore discover the full topology of the AERO link in terms of the working set of ACPs associated with each Server, while the Servers only discover the ACPs of their associated Clients. Since Clients are expected to remain associated with their current set of Servers for extended timeframes, the amount of BGP control messaging between Servers and Relays should be minimal. However, BGP peers SHOULD dampen any route oscillations caused by impatient Clients that repeatedly associate and disassociate with Servers.

3.13. AERO Redirection

3.13.1. Reference Operational Scenario

Figure 7 depicts the AERO redirection reference operational scenario, using IPv6 addressing as the example (while not shown, a corresponding example for IPv4 addressing can be easily constructed). The figure shows an AERO Relay ('R1'), two AERO Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1', 'H2'):