| Network Working Group | F. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Obsoletes: rfc5320, rfc5558, rfc5720, | April 4, 2019 |
| rfc6179, rfc6706 (if | |
| approved) | |
| Intended status: Standards Track | |
| Expires: October 6, 2019 |
Asymmetric Extended Route Optimization (AERO)
draft-templin-intarea-6706bis-11.txt
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 route optimization services for improved performance. AERO provides an IPv6 link-local address format that supports operation of the IPv6 Neighbor Discovery (ND) protocol and links ND to IP forwarding. Prefix delegation services are employed to manage the routing system. Dynamic link selection, mobility management, quality of service (QoS) signaling and route optimization are naturally supported through dynamic neighbor cache updates. AERO is a widely-applicable tunneling solution especially well-suited to aviation services, mobile Virtual Private Networks (VPNs) and other applications as described in this document.
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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 between 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 route optimization services for improved performance [RFC5522].
AERO provides an IPv6 link-local address format that supports operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and links ND to IP forwarding. Dynamic link selection, mobility management, quality of service (QoS) signaling and route optimization are naturally supported through dynamic neighbor cache updates, while IPv6 Prefix Delegation (PD) is supported by network services such as the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC8415].
A node's AERO interface can be configured over multiple underlying interfaces. From the standpoint of ND, AERO interface neighbors therefore may appear to have multiple link-layer addresses (i.e., the IP addresses assigned to underlying interfaces). Each link-layer address is subject to change due to mobility and/or QoS fluctuations, and link-layer address changes are signaled by ND messaging the same as for any IPv6 link.
AERO is applicable to a wide variety of use cases. For example, it can be used to coordinate the Virtual Private Network (VPN) links of mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that connect into a home enterprise network via public access networks using services such as OpenVPN [OVPN]. AERO is also applicable to aviation services for both manned and unmanned aircraft where the aircraft is treated as a mobile node that can connect an Internet of Things (IoT). Other applicable use cases are also in scope.
The following numbered sections present the AERO specification. The appendices at the end of the document are non-normative.
The terminology in the normative references applies; the following terms are defined within the scope of this document:
Throughout the document, the simple terms "Client", "Server", "Relay" and "Proxy" refer to "AERO Client", "AERO Server", "AERO Relay" and "AERO Proxy", respectively. Capitalization is used to distinguish these terms from DHCPv6 client/server/relay
The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including the names of node variables, messages and protocol constants) is used throughout this document. Also, the term "IP" is used to generically refer to either Internet Protocol version, i.e., IPv4 [RFC0791] or IPv6 [RFC8200].
The terms Mobility Anchor Point (MAP) and Distributed Mobility Management (DMM) are used in the same sense as standard Internetworking terminology.
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]. Lower case uses of these words are not to be interpreted as carrying RFC2119 significance.
The following sections specify the operation of IP over Asymmetric Extended Route Optimization (AERO) links:
.-(::::::::)
.-(::::::::::::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +--------+-------+ +--------------+
|AERO Server S1| | AERO Relay R1 | |AERO Server S2|
| Nbr: C1, R1 | | Nbr: S1, S2 | | Nbr: C2, R1 |
| default->R1 | |(X1->S1; X2->S2)| | default->R1 |
| X1->C1 | | ASP A1 | | X2->C2 |
+-------+------+ +--------+-------+ +------+-------+
| AERO Link | |
X---+---+-------------------+-+----------------+---+---X
| | |
+-----+--------+ +----------+------+ +--------+-----+
|AERO Client C1| | AERO Proxy P1 | |AERO Client C2|
| Nbr: S1 | |(Proxy Nbr Cache)| | Nbr: S2 |
| default->S1 | +--------+--------+ | default->S2 |
| ACP X1 | | | ACP X2 |
+------+-------+ .--------+------. +-----+--------+
| (- Proxyed Clients -) |
.-. `---------------' .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. +-------+ +-------+ .-(_ IP )-.
(__ EUN )--|Host H1| |Host H2|--(__ EUN )
`-(______)-' +-------+ +-------+ `-(______)-'
Figure 1: AERO Link Reference Model
Figure 1 presents the AERO link reference model. In this model:
Each node on the AERO link maintains an AERO interface neighbor cache and an IP forwarding table the same as for any link. Although the figure shows a limited deployment, in common operational practice there will normally be many additional Relays, Servers, Clients and Proxies.
AERO Relays provide both layer-3 routing and layer-2 bridging services (as well as a security trust anchor) for nodes on an AERO link. As a router, the Relay forwards data packets using standard IP forwarding. As a bridge, the Relay securely forwards control messages between Proxies and Servers both within the same partition and between disjoint partitions. Each Relay also peers with Servers and other Relays in a dynamic routing protocol instance to provide a Distributed Mobility Management (DMM) service for the list of active ACPs (see Section 3.3). Relays forward packets between neighbors 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) and serve as a gateway between the AERO link and the Internetwork. Relays maintain neighbor cache entries for Servers and Proxies, and maintain an IP forwarding table entry for each AERO Client Prefix (ACP).
AERO Servers provide default forwarding services and a Mobility Anchor Point (MAP) for AERO Clients. Each Server also peers with Relays in a dynamic routing protocol instance to advertise its list of associated ACPs (see Section 3.3). Servers facilitate PD exchanges with Clients, where each delegated prefix becomes an ACP taken from an ASP. Servers forward packets between AERO interface neighbors, and maintain neighbor cache entries for Relays. They also maintain both neighbor cache entries and IP forwarding table entries for each of their associated Clients, and track each Client's mobility profiles.
AERO Clients act as requesting routers to receive ACPs through PD exchanges with AERO Servers over the AERO link. Each Client can associate with a single Server or with multiple Servers, e.g., for fault tolerance, load balancing, etc. 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. Clients maintain an AERO interface neighbor cache entry for each of their associated Servers as well as for each of their correspondent Clients.
AERO Proxies provide a conduit for AERO Clients in secured enclaves to associate with AERO Servers. The Client sends all of its control plane messages to the Server via the Proxy, which intercepts them before they leave the secured enclave. The Proxy forwards the Client's control and data plane messages to and from the Client's current Server(s). The Proxy may also discover a better route toward a target destination via AERO route optimization, in which case future outbound data packets would be forwarded via the more direct route. Proxies maintain AERO interface neighbor cache entries for Relays, i.e., the same as Servers. The Proxy function is specified in Section 3.16.
AERO Relays, Servers and Proxies are critical infrastructure elements in fixed (i.e., non-mobile) deployments. Relays, Servers and Proxies must use link-layer addresses that do not change and can be reached from any correspondent in the underlying Internetwork (i.e., in the same fashion as for popular Internet services). AERO Clients may be mobile, and may not have any public link-layer addresses, e.g., if they are located behind NATs or Proxies.
The AERO routing system comprises a private instance of the Border Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays and Servers and does not interact with either the public Internet BGP routing system or the native Internetwork routing system. Relays advertise only a small and unchanging set of ASPs to the native Internetwork routing system instead of the full dynamically changing set of ACPs.
In a reference deployment, each Server is configured as an Autonomous System Border Router (ASBR) for a stub Autonomous System (AS) using an AS Number (ASN) that is unique within the BGP instance, and each Server further uses eBGP to peer with one or more Relays but does not peer with other Servers. Each segment of a multi-segment AERO link must include one or more Relays, which peer with the Servers and Proxies within that segment. All Relays within the same segment are members of the same hub AS using a common ASN, and use iBGP to maintain a consistent view of all active ACPs currently in service. The Relays of different segments peer with one another using eBGP.
Each Server maintains a working set of associated ACPs, and dynamically announces new ACPs and withdraws departed ACPs in its eBGP updates to Relays. Clients are expected to remain associated with their current Servers for extended timeframes, however Servers SHOULD selectively suppress updates for impatient Clients that repeatedly associate and disassociate with them in order to dampen routing churn.
Each Relay configures a black-hole route for each of its ASPs. By black-holing the ASPs, the Relay will maintain forwarding table entries only for the ACPs that are currently active, and packets destined to all other ACPs will correctly incur Destination Unreachable messages due to the black hole route. Relays do not send eBGP updates for ACPs to Servers, but instead only originate a default route. In this way, Servers have only partial topology knowledge (i.e., they know only about the ACPs of their directly associated Clients) and they forward all other packets to Relays which have full topology knowledge.
Scaling properties of the AERO routing system are limited by the number of BGP routes that can be carried by Relays. As of 2015, the global public Internet BGP routing system manages more than 500K routes with linear growth and no signs of router resource exhaustion [BGP]. More recent network emulation studies have also shown that a single Relay can accommodate at least 1M dynamically changing BGP routes even on a lightweight virtual machine, i.e., and without requiring high-end dedicated router hardware.
Therefore, assuming each Relay can carry 1M or more routes, this means that at least 1M Clients can be serviced by a single set of Relays. A means of increasing scaling would be to assign a different set of Relays for each set of ASPs. In that case, each Server still peers with one or more Relays, but the Server institutes route filters so that it only sends BGP updates to the specific set of Relays that aggregate the ASP. For example, if the ASP for the AERO link is 2001:db8::/32, a first set of Relays could service the ASP segment 2001:db8::/40, a second set of Relays could service 2001:db8:0100::/40, a third set could service 2001:db8:0200::/40, etc.
Assuming up to 1K sets of Relays, the AERO routing system can then accommodate 1B or more ACPs with no additional overhead for Servers and Relays (for example, it should be possible to service 1B /64 ACPs taken from a /34 ASP and even more for shorter prefixes). In this way, each set of Relays services a specific set of ASPs that they advertise to the native Internetwork routing system, and each Server configures ASP-specific routes that list the correct set of Relays as next hops. This arrangement also allows for natural incremental deployment, and can support small scale initial deployments followed by dynamic deployment of additional Clients, Servers and Relays without disturbing the already-deployed base.
In an alternate routing arrangement, each set of Relays could advertise an aggregated ASP for the link into the native Internetwork routing system even though each Relay services only smaller segments of the ASP. In that case, a Relay upon receiving a packet with a destination address covered by the ASP segment of another Relay can simply tunnel the packet to the other Relay. The tradeoff then is the penalty for Relay-to-Relay tunneling compared with reduced routing information in the native routing system.
A full discussion of the BGP-based routing system used by AERO is found in [I-D.ietf-rtgwg-atn-bgp]. The system provides for Distributed Mobility Management (DMM) per the distributed mobility anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring].
A Client's AERO address is an IPv6 link-local address with an interface identifier based on the Client's delegated ACP. Relay, Server and Proxy AERO addresses are assigned from the range fe80::/96 and include an administratively-provisioned value in the lower 32 bits.
For IPv6, Client AERO addresses begin with the prefix fe80::/64 and include in the interface identifier (i.e., the lower 64 bits) a 64-bit prefix taken from one of the Client's IPv6 ACPs. For example, if the AERO Client receives the IPv6 ACP:
it constructs its corresponding AERO addresses as:
For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6 address formed from an IPv4 ACP and with a Prefix Length of 96 plus the ACP prefix length. For example, for the IPv4 ACP 192.0.2.32/28 the IPv4-mapped IPv6 ACP is:
The Client then constructs its AERO addresses with the prefix fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address in the interface identifier as:
Section 3.5). The address fe80:: is reserved as the IPv6 link-local Subnet Router Anycast address [RFC4291], and the address fe80::ffff:ffff is reserved as the unspecified AERO address; hence, these values are not available general assignment.
Relay, Server and Proxy AERO addresses are allocated from the range fe80::/96, and MUST be managed for uniqueness. The lower 32 bits of the AERO address includes a unique integer value (e.g., fe80::1, fe80::2, fe80::3, etc.) as assigned by the administrative authority for the link. If the link comprises multiple segments, the AERO addresses are assigned to each segment in correspondence with the SPAN addresses assigned to the segment (see:
When the Server delegates ACPs to the Client, the lowest-numbered AERO address from the first ACP delegation serves as the "base" AERO address (for example, for the ACP 2001:db8:1000:2000::/56 the base AERO address is fe80::2001:db8:1000:2000). The Client then assigns the base AERO address to the AERO interface and uses it for the purpose of maintaining the neighbor cache entry. The Server likewise uses the AERO address as its index into the neighbor cache for this Client.
If the Client has multiple AERO addresses (i.e., when there are multiple ACPs and/or ACPs with prefix lengths shorter than /64), the Client originates ND messages using the base AERO address as the source address and accepts and responds to ND messages destined to any of its AERO addresses as equivalent to the base AERO address. In this way, the Client maintains a single neighbor cache entry that may be indexed by multiple AERO addresses.
AERO addresses that embed an IPv6 prefix can be statelessly transformed into an IPv6 Subnet Router Anycast address and vice-versa. For example, for the AERO address fe80::2001:db8:2000:3000 the corresponding Subnet Router Anycast address is 2001:db8:2000:3000::. In the same way, for the IPv6 Subnet Router Anycast address 2001:db8:1:2:: the corresponding AERO address is fe80::2001:db8:1:2. In other words, the low-order 64 bits of an AERO address can be used as the high-order 64 bits of a Subnet Router Anycast address, and vice-versa.
In the simplest case, an AERO link configured over a single administrative domain (e.g., an enterprise network) appears as a single unified link with a consistent underlying network addressing plan. In that case, all nodes on the link can exchange packets directly since the underlying network is connected.
In a more complex case, an AERO link may be partitioned into multiple "segments", where each segment is configured over a different administrative domain (e.g., as for regional aviation networks). In that case, the underlying network addressing plan of each segment is consistent internally but will often bear no relation to the addressing plans of other segments. Each segment is also likely to be separated from other segments by network security devices (e.g., firewalls, proxies, packet filtering gateways, etc.), and in many cases disjoint segments may not even have any common physical link connections at all. Therefore, the nodes within each segment can only be assured of exchanging packets directly with nodes in the same segment, and not with nodes in other segments. The only means for joining the segments therefore is through inter-domain peerings between segment border routers.
The same as for traditional campus LANs, multiple AERO link segments can be joined into a single unified link via bridging in an underlay network termed "The SPAN". The SPAN performs link-layer packet forwarding between segments so that nodes on segment A can exchange packets with nodes on segment B via bridging without decrementing the network-layer TTL/Hop Limit. To support the SPAN, AERO links require a reserved /96 IPv6 "SPAN Service Prefix (SSP)". Although any routable IPv6 prefix can be reserved, use of a Unique Local Address (ULA) prefix (e.g., fd00::/96) [RFC4389] is RECOMMENDED since packets with ULAs cannot be injected into the AERO link by an external IPv6 node and cannot leak out of the AERO link to the outside world.
Each partition in the SPAN assigns a unique sub-prefix of the SSP, i.e., a "SPAN Partition Prefix (SPP)". For example, a first partition could assign fd00::/116, a second partition could assign fd00::1000/116, a third could assign fd00::2000/116, etc. The administrative authorities for each partition must therefore coordinate to assure mutually-exclusive SPP assignments, but internal provisioning of the SPP is a local consideration for each administrative authority.
A "SPAN address" is an address taken from a SPP and assigned to a Relay, Server or Proxy AERO interface. SPAN addresses are formed by simply replacing the upper portion of an administratively-assigned AERO address with the SPP. For example, if the SPP is fd00::/116, the SPAN address formed from the AERO address fe80::1 is simply fd00::1. (As with AERO addresses, the values ::0 and ::ffff:ffff are reserved and not available for general assignment.)
AERO Relays serve as bridges to join multiple segments into a unified AERO link over multiple diverse administrative domains. They support the bridging function by first exchanging their SPPs via standard BGP routing. For example, if three Relays (Relays 'A', 'B' and 'C') from different administrative domains advertised the SPPs fd00::1000/116, fd00::2000/116 and fd00::3000/116 respectively, then the forwarding tables in each Relay are as follows:
These forwarding table entries remain in place indefinitely and never change, since they correspond to fixed infrastructure elements in their respective partitions. This point is of critical importance, since it provides the basis for a link-layer forwarding service that cannot be disrupted by routing updates due to node mobility.
With the SPPs in place in each Relay's forwarding table, control and data packets sent between AERO nodes in different partitions can therefore be carried over the SPAN via encapsulation. For example, when a node in partition A forwards a packet with IPv6 address 2001:db8:1:2::1 to a node in partition C with IPv6 address 2001:db8:1000:2000::1, it first encapsulates the packet in a SPAN header with source address from fd00::1000/116 (e.g., fd00::1001) and destination address from fd00::3000/116 (e.g., fd00::3001).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Header(s) |
| src = L2(X) |
| dst = L2(Y) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPAN Header (RFC2473) |
| src = fd00::1001 |
| dst = fd00::3001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header |
| src = 2001:db8:1:2::1 |
| dst = 2001:db8:1000:2000::1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Inner Packet Body ~
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SPAN Encapsulation
SPAN encapsulation is based on Generic Packet Tunneling in IPv6 [RFC2473]; the encapsulation format in this example is shown inFigure 2:
In this format, the inner IP header and packet body are the original IP packet, the SPAN header is an IPv6 header prepared according to [RFC2473], and the outer header is added by the same node ('X') that added the SPAN header according to Section 3.10. The source and destination addresses of the outer header are the link-layer addresses of nodes 'X' and 'Y' (where 'Y' is a Relay connected to the SPAN).
An (inner) IP packet is said to be "sent into the SPAN" or "sent via the SPAN" when it is encapsulated as described above then forwarded to a neighboring Relay. This terminology appears throughout the remaining sections of the document.
This gives rise to a routing system that contains both ACP routes that may change dynamically due to regional node mobility and SPAN routes that never change. The Relays can therefore provide link-layer bridging by sending packets via the SPAN instead of network-layer routing according to ACP routes. As a result, opportunities for packet loss due to node mobility are mitigated.
AERO interfaces use encapsulation (see: Section 3.10) to exchange packets with neighbors attached to the AERO link.
AERO interfaces maintain a neighbor cache for tracking per-neighbor state the same as for any interface. AERO interfaces use ND messages including Router Solicitation (RS), Router Advertisement (RA), Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for neighbor cache management.
AERO interface ND messages include one or more Source/Target Link-Layer Address Options (S/TLLAOs) formatted as shown in Figure 3:
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 | Length = 5 | Prefix Length |R|D|X|T| Resvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID | Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Link Layer Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: AERO Source/Target Link-Layer Address Option (S/TLLAO) Format
In this format:
AERO interfaces may be configured over multiple underlying interface connections to underlying links. 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, cellular, terrestrial, air-to-air directional, etc.) with diverse performance and cost properties.
A Client's underlying interfaces are classified as follows:
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 ND messages include only a single S/TLLAO with Interface ID set to a constant value. In that case, the Client would appear to have a single underlying interface but with a dynamically changing link-layer address.
If the Client has multiple active underlying interfaces, then from the perspective of ND it would appear to have multiple link-layer addresses. In that case, ND messages MAY include multiple S/TLLAOs -- each with an Interface ID that corresponds to a specific underlying interface of the AERO node.
When the Client includes an S/TLLAO for an underlying interface for which it is aware that there is a Translator on the path to the Server, or when a node includes an S/TLLAO solely for the purpose of announcing new QoS preferences, the node MAY set both Port Number and Link-Layer Address to 0 to indicate that the addresses are unspecified at the network layer and must instead be derived from the link-layer encapsulation headers.
When an ND message includes multiple S/TLLAOs, the first S/TLLAO MUST correspond to the AERO node's underlying interface used to transmit the message.
When a Relay enables an AERO interface, it first assigns an administratively-provisioned AERO address (e.g., fe80::1) and its companion SPAN address (e.g., fd00::1) to the interface, where each address MUST be unique among all AERO nodes on the link. The Relay also configures a neighbor cache entry for Servers and Proxys on the local segment. The Relay then engages in a BGP routing protocol session with Servers on the local segment and other Relays on the link (see: Section 3.3), and advertises its assigned ASPs into the native Internetwork. Each Relay subsequently maintains an IP forwarding table entry for each active ACP covered by its ASP(s) as well as for each SPAN prefix.
When a Server enables an AERO interface, it assigns AERO and SPAN addresses the same as for Relays. The Server further configures a service to facilitate ND/PD exchanges with AERO Clients. The Server maintains neighbor cache entries for one or more Relays on the link, and manages per-Client neighbor cache entries and IP forwarding table entries based on control message exchanges. The Server also engages in a BGP routing protocol session with its neighboring Relays (see: Section 3.3).
When the Server receives an NS/RS message on the AERO interface it authenticates the message and returns a solicited NA/RA message. (When the Server receives an unsolicited NA message, it likewise authenticates the message and processes it locally.) The Server further provides a simple link-layer conduit between AERO interface neighbors. In particular, when a packet sent by a source Client arrives on the Server's AERO interface and is destined to another AERO node, the Server forwards the packet from within the AERO interface at the link layer without ever disturbing the network layer.
When a Proxy enables an AERO interface, it assigns AERO and SPAN addresses the same as for Relays and Servers, and maintains neighbor cache entires for one or more Relays. The Proxy further maintains per-Client neighbor cache entries based on control message exchanges. Proxies forward packets between each Client and their associated Servers and neighbors.
When the Proxy receives an RS message from a Client in the secured enclave, it creates an incomplete neighbor cache entry and sends a proxyed RS message to a Server via the SPAN while using its own link-layer address as the source address. When the Server returns an RA message, the Proxy completes the proxy neighbor cache entry based on autoconfiguration information in the RA and sends a proxyed RA to the Client while using its own link-layer address as the source address. The Client, Server and Proxy will then have the necessary state for managing the proxy neighbor association.
When a Client enables an AERO interface, it sends RS messages with ND/PD parameters over an underlying interface to one or more AERO Servers, which return RA messages with corresponding PD parameters. (The RS/RA messages may pass through a Proxy on the path in the case of a Client's Proxyed interface.) See [I-D.templin-6man-dhcpv6-ndopt] for the types of ND/PD parameters that can be included in the RS/RA message exchanges.
After the initial ND/PD message exchange, the Client assigns AERO addresses to the AERO interface based on the delegated prefix(es). The Client can then register additional underlying interfaces with the Server by sending a simple RS message (i.e., one with no PD parameters) over each underlying interface using its base AERO address as the source network layer address. The Server will update its neighbor cache entry for the Client and return a simple RA message.
The Client maintains a neighbor cache entry for each of its Servers and each of its active target Clients. When the Client receives ND messages on the AERO interface it updates or creates neighbor cache entries, including link-layer address and QoS preferences.
When there is a NAT on the path, the Client must send periodic messages to keep NAT state alive. If no other periodic messaging service is available, the Client can send RS messages to receive RA replies from its Server(s).
Each AERO interface maintains a conceptual neighbor cache that includes an entry for each neighbor it communicates with on the AERO link per [RFC4861]. AERO interface neighbor cache entries are said to be one of "permanent", "symmetric", "asymmetric" or "proxy".
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 permanent neighbor cache entries for their associated Relays, Servers and Proxys, and AERO Servers and Proxys maintain permanent neighbor cache entries for their associated Relays. Each entry maintains the mapping between the neighbor's network-layer AERO address and corresponding link-layer address.
Symmetric neighbor cache entries are created and maintained through ND/PD exchanges as specified in Section 3.15, and remain in place for durations bounded by ND/PD lifetimes. AERO Servers maintain symmetric neighbor cache entries for each of their associated Clients, and AERO Clients maintain symmetric neighbor cache entries for each of their associated Servers.
Asymmetric neighbor cache entries are created or updated based on route optimization messaging as specified in Section 3.17, and are garbage-collected when keepalive timers expire. AERO route optimization sources (ROSs) maintain asymmetric neighbor cache entries for each of their active target Clients with lifetimes based on ND messaging constants. Asymmetric neighbor cache entries are unidirectional since only the ROS (i.e., and not the route optimization responder (ROR)) creates an entry.
Proxy neighbor cache entries are created and maintained by AERO Proxies when they process Client/Server ND/PD exchanges, and remain in place for durations bounded by ND/PD lifetimes. AERO Proxies maintain proxy neighbor cache entries for each of their associated Clients. Proxy neighbor cache entries track the Client state and the state of each of the Client's associated Servers.
To the list of neighbor cache entry states in Section 7.3.2 of [RFC4861], AERO interfaces add an additional state DEPARTED that applies to symmetric and proxy neighbor cache entries for Clients that have recently departed. The interface sets a "DepartTime" variable for the neighbor cache entry to "DEPARTTIME" seconds. DepartTime is decremented unless a new ND message causes the state to return to REACHABLE. While a neighbor cache entry is in the DEPARTED state, packets destined to the target Client are forwarded to the Client's new location instead of being dropped. When DepartTime decrements to 0, the neighbor cache entry is deleted. It is RECOMMENDED that DEPARTTIME be set to the default constant value 40 seconds to allow for packets in flight to be delivered while stale route optimization state may be present.
When a target AERO Server (acting as a Mobility Anchor Point (MAP)) receives a valid NS message used for route optimization, it searches for a symmetric neighbor cache entry for the target Client. The Server then acts as an ROR and returns a solicited NA message without creating a neighbor cache entry for the ROS, but maintains a "Report List" for the Client's symmetric neighbor cache entry. When the ROR receives an authentic NS message it adds a Report list entry for the ROS and sets a "ReportTime" variable for the entry to REPORTTIME seconds. The ROR resets ReportTime when it receives a new authentic NS message, and otherwise decrements ReportTime while no NS messages have been received. It is RECOMMENDED that REPORTTIME be set to the default constant value 40 seconds to allow a 10 second window so that route optimization can converege before ReportTime decrements below REACHABLETIME.
When the ROS receives a solicited NA message response to its NS message, it creates or updates an asymmetric neighbor cache entry for the target network-layer and link-layer addresses. The ROS then (re)sets ReachableTime for the neighbor cache entry to REACHABLETIME seconds and uses this value to determine whether packets can be forwarded directly to the target, i.e., instead of via a default route. The ROS otherwise decrements ReachableTime while no further solicited NA messages arrive. It is RECOMMENDED that REACHABLETIME be set to the default constant value 30 seconds as specified in [RFC4861].
The ROS also uses the value MAX_UNICAST_SOLICIT to limit the number of NS keepalives sent when a correspondent may have gone unreachable, the value MAX_RTR_SOLICITATIONS to limit the number of RS messages sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT to limit the number of unsolicited NAs that can be sent based on a single event. It is RECOMMENDED that MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in [RFC4861].
Different values for DEPARTTIME, REPORTTIME, REACHABLETIME, MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if different values are chosen, all nodes on the link MUST consistently configure the same values. Most importantly, DEPARTTIME and REPORTTIME SHOULD be set to a value that is sufficiently longer than REACHABLETIME to avoid packet loss due to stale route optimization state.
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 forwarded into the AERO link, i.e., they are tunneled to an AERO interface neighbor. Packets that enter the AERO interface from the link layer are either re-admitted into the AERO link or forwarded to the network layer where they are subject to either local delivery or IP forwarding. In all cases, the AERO interface itself MUST NOT decrement the network layer TTL/Hop-count since its forwarding actions occur below the network layer.
AERO interfaces may have multiple underlying interfaces and/or neighbor cache entries for neighbors with multiple Interface ID registrations (see Section 3.6). The AERO node uses each packet's DSCP value (and/or port number) to select an outgoing underlying interface based on the node's own QoS preferences, and also to select a destination link-layer address based on the neighbor's underlying interface with the highest preference. AERO implementations SHOULD allow for QoS preference values to be modified at runtime through network management.
If multiple outgoing interfaces and/or neighbor interfaces have a preference of "high", the AERO node sends one copy of the packet via each of the (outgoing / neighbor) interface pairs; otherwise, the node sends a single copy of the packet via the interface with the highest preference. AERO nodes keep track of which underlying interfaces are currently "reachable" or "unreachable", and only use "reachable" interfaces for forwarding purposes.
The following sections discuss the AERO interface forwarding algorithms for Clients, Proxies, Servers and Relays. In the following discussion, a packet's destination address is said to "match" if it is a non-link-local address with a prefix covered by an ASP/ACP, or if it is an AERO address that embeds an ACP, or if it is the same as an administratively-provisioned AERO address.
When an IP packet enters a Client's AERO interface from the network layer the Client searches for an asymmetric neighbor cache entry that matches the destination. If there is a match, the Client uses one or more "reachable" link-layer addresses in the entry as the link-layer addresses for encapsulation and admits the packet into the AERO link (if the link-layer address is a SPAN address, the Client instead forwards the packet into the SPAN). If there is no asymmetric neighbor cache entry, the Client instead uses the link-layer address in a symmetric neighbor cache entry as the encapsulation address for interfaces other than Proxyed interfaces. For Proxyed interfaces, the Client simply forwards the unencapsulated packet to the first-hop access router.
When an IP packet enters a Client's AERO interface from the link-layer, if the destination matches one of the Client's ACPs or link-local addresses the Client decapsulates the packet and delivers it to the network layer. Otherwise, the Client drops the packet and MAY return a network-layer ICMP Destination Unreachable message subject to rate limiting (see: Section 3.14).
When the Proxy receives a packet from a Client within the secured enclave, the Proxy searches for an asymmetric neighbor cache entry that matches the network-layer destination. If there is a match, the Proxy uses one or more "reachable" link-layer addresses in the entry as the destination link-layer addresses for encapsulation and admits the packet into the AERO link (if the link-layer address is a SPAN address, the Proxy instead forwards the packet into the SPAN). Otherwise, the Proxy uses the link-layer address for one of the Client's Servers as the encapsulation address.
When the Proxy receives an encapsulated data packet from outside of the secured enclave, it searches for a proxy neighbor cache entry that matches the destination. If there is a proxy neighbor cache entry for the target Client, the Proxy forwards the packet according to the cached link-layer address. If the proxy neighbor cache entry is in the DEPARTED state, the Proxy instead forwards the packet to the Client's Server and may return an unsolicited NA message as discussed in Section 3.19. If there is no neighbor cache entry, the Proxy discards the packet.
When an IP packet enters a Server's AERO interface from the link-layer, it decapsulates the packet and processes it the same as if it entered from ethe network layer. The Server then processes the packet according to the network-layer destination address as follows:
Relays forward packets the same as any IP router. When the Relay receives an encapsulated packet from a Server via the AERO link, it removes the encapsulation header and searches for a forwarding table entry that matches the network layer destination address. When the Relay receives an unencapsulated packet from a node outside the AERO link, it performs the same forwarding table lookup. The Relay then processes the packet as follows:
As for any IP router, the Relay decrements the TTL/Hop Count when it forwards the packet.
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".
The AERO interface encapsulates packets per the Generic UDP Encapsulation (GUE) procedures in [I-D.ietf-intarea-gue][I-D.ietf-intarea-gue-extensions], or through an alternate encapsulation format (e.g., see: Appendix A, [RFC2784], [RFC8086], [RFC4301], etc.). For packets entering the AERO interface from the network layer, the AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class" [RFC2983], "Flow Label"[RFC6438] (for IPv6) and "Congestion Experienced" [RFC3168] values in the packet's IP header into the corresponding fields in the encapsulation IP header. For packets undergoing re-encapsulation, the AERO interface instead copies these values from the original encapsulation IP header into the new encapsulation header, i.e., the values are transferred between encapsulation headers and *not* copied from the encapsulated packet's network-layer header. (Note especially that by copying the TTL/Hop Limit between encapsulation headers the value will eventually decrement to 0 if there is a (temporary) routing loop.) For IPv4 encapsulation/re-encapsulation, the AERO interface sets the DF bit as discussed in Section 3.13.
When GUE encapsulation is used, the AERO interface next 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 encapsulated packet plus 8 bytes for the UDP header itself plus the length of the GUE header (or 0 if GUE direct IP encapsulation is used). For packets sent to a Server or Relay, the AERO interface sets the UDP destination port to 8060, i.e., the IANA-registered port number for AERO. For packets sent to a Client, the AERO interface sets the UDP destination port to the port value stored in the neighbor cache entry for this Client. The AERO interface then either includes or omits the UDP checksum according to the GUE specification.
When GUE encapsulation is not available, encapsulation between Servers and Relays can use standard mechanisms such as Generic Routing Encapsulation (GRE) [RFC2784], GRE-in-UDP [RFC8086] and IPSec [RFC4301] so that Relays can be standard IP routers with no AERO-specific mechanisms.
AERO interfaces decapsulate packets destined either to the AERO node itself or to a destination reached via an interface other than the AERO interface the packet was received on. Decapsulation is per the procedures specified for the appropriate encapsulation format.
AERO nodes employ simple data origin authentication procedures for encapsulated packets they receive from other nodes on the AERO link. In particular:
[OVPN]. In some environments, however, it may be sufficient to require signatures only for ND control plane messages (see: Section 10) and omit signatures for data plane messages.
Each packet should include a signature that the recipient can use to authenticate the message origin, e.g., as for common VPN systems such as OpenVPN
The AERO interface is the node's attachment to the AERO link. The AERO interface acts as a tunnel ingress when it sends a packet to an AERO link neighbor and as a tunnel egress when it receives a packet from an AERO link neighbor. AERO interfaces observe the packet sizing considerations for tunnels discussed in [I-D.ietf-intarea-tunnels] and as specified below.
The Internet Protocol expects that IP packets will either be delivered to the destination or a suitable Packet Too Big (PTB) message returned to support the process known as IP Path MTU Discovery (PMTUD) [RFC1191][RFC8201]. However, PTB messages may be crafted for malicious purposes such as denial of service, or lost in the network [RFC2923]. This can be especially problematic for tunnels, where a condition known as a PMTUD "black hole" can result. For these reasons, AERO interfaces employ operational procedures that avoid interactions with PMTUD, including the use of fragmentation when necessary.
AERO interfaces observe two different types of fragmentation. Source fragmentation occurs when the AERO interface (acting as a tunnel ingress) fragments the encapsulated packet into multiple fragments before admitting each fragment into the tunnel. Network fragmentation occurs when an encapsulated packet admitted into the tunnel by the ingress is fragmented by an IPv4 router on the path to the egress. Note that an IPv4 packet that incurs source fragmentation may also incur network fragmentation.
IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280 bytes [RFC8200]. Although IPv4 specifies a smaller minimum link MTU of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum for IPv4 even if encapsulated packets may incur network fragmentation.
IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes [RFC8200], while the minimum MRU for IPv4 is only 576 bytes [RFC1122] (but, note that many standard IPv6 over IPv4 tunnel types already assume a larger MRU than the IPv4 minimum).
AERO interfaces therefore configure an MTU that MUST NOT be smaller than 1280 bytes, MUST NOT be larger than the minimum MRU among all nodes on the AERO link minus the encapsulation overhead ("ENCAPS"), and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also configure a Maximum Segment Unit (MSU) as the maximum-sized encapsulated packet that the ingress can inject into the tunnel without source fragmentation. The MSU value MUST NOT be larger than (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is operational assurance that a larger size can traverse the link along all paths.
All AERO nodes MUST configure the same MTU value for reasons cited in [RFC3819][RFC4861]; in particular, multicast support requires a common MTU value among all nodes on the link. All AERO nodes MUST configure an MRU large enough to reassemble packets up to (MTU+ENCAPS) bytes in length; nodes that cannot configure a large-enough MRU MUST NOT enable an AERO interface.
The network layer proceeds as follow when it presents an IP packet to the AERO interface. For each IPv4 packet that is larger than the AERO interface MTU and with the DF bit set to 0, the network layer uses IPv4 fragmentation to break the packet into a minimum number of non-overlapping fragments where the first fragment is no larger than the MTU and the remaining fragments are no larger than the first. For all other IP packets, if the packet is larger than the AERO interface MTU, the network layer drops the packet and returns a PTB message to the original source. Otherwise, the network layer admits each IP packet or fragment into the AERO interface.
For each IP packet admitted into the AERO interface, the interface (acting as a tunnel ingress) encapsulates the packet. If the encapsulated packet is larger than the AERO interface MSU the ingress source-fragments the encapsulated packet into a minimum number of non-overlapping fragments where the first fragment is no larger than the MSU and the remaining fragments are no larger than the first. The ingress then admits each encapsulated packet or fragment into the tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation header in case any network fragmentation is necessary. The encapsulated packets will be delivered to the egress, which reassembles them into a whole packet if necessary.
Several factors must be considered when fragmentation is needed. 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 data corruption due to reassembly misassociations [RFC6864][RFC4963]. In environments where IP fragmentation issues could result in operational problems, the ingress SHOULD employ intermediate-layer source fragmentation (see: [RFC2764] and [I-D.ietf-intarea-gue-extensions]) before appending the outer encapsulation headers to each fragment. Since the encapsulation fragment header reduces the room available for packet data, but the original source has no way to control its insertion, the ingress MUST include the fragment header length in the ENCAPS length even for packets in which the header is absent.
When an AERO node admits encapsulated packets into the AERO interface, it may receive link-layer or network-layer error indications.
A link-layer error indication is an ICMP error message generated by a router in the underlying network 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. Valid type values include "Destination Unreachable", "Time Exceeded" and "Parameter Problem" [RFC0792][RFC4443]. (AERO interfaces ignore all link-layer IPv4 "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they only emit packets that are guaranteed to be no larger than the IP minimum link MTU as discussed in Section 3.13.)
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 link-layer error message format is shown in Figure 4 (where, "L2" and "L3" refer to link-layer and network-layer, respectively):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| 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 4: AERO Interface Link-Layer Error Message Format
When an AERO Relay receives a packet for which the network-layer 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 a network-layer Destination Unreachable message subject to rate limiting. The Relay writes the network-layer source address of the original packet as the destination address and uses one of its non link-local addresses as the source address of the message.
When an AERO node receives an encapsulated packet for which the reassembly buffer it too small, it drops the packet and returns a network-layer Packet Too Big (PTB) message. The node first writes the MRU value into the PTB message MTU field, writes the network-layer source address of the original packet as the destination address and writes one of its non link-local addresses as the source address.
AERO Router Discovery, Prefix Delegation and Autoconfiguration are coordinated as discussed in the following Sections.
Each AERO Server configures a PD service to facilitate Client requests. Each Server is provisioned with a database of ACP-to-Client ID mappings for all Clients enrolled in the AERO system, as well as any information necessary to authenticate each Client. The Client database is maintained by a central administrative authority for the AERO link and securely distributed to all Servers, e.g., via the Lightweight Directory Access Protocol (LDAP) [RFC4511], via static configuration, etc. Therefore, no Server-to-Server PD state synchronization is necessary, and Clients can optionally hold separate PDs for the same ACPs from multiple Servers. In this way, Clients can associate with multiple Servers, and can receive new PDs from new Servers before releasing PDs received from existing Servers. This provides the Client with a natural fault-tolerance and/or load balancing profile.
AERO Clients and Servers use ND messages to maintain neighbor cache entries. AERO Servers configure their AERO interfaces as advertising interfaces, and therefore send unicast RA messages with configuration information in response to a Client's RS message. Thereafter, Clients send additional RS messages to the Server's unicast address to refresh prefix and/or router lifetimes.
AERO Clients and Servers include PD parameters in RS/RA messages to be used for Prefix Delegation (see [I-D.templin-6man-dhcpv6-ndopt] for ND/PD alternatives). The unified ND/PD messages are exchanged between Client and Server according to the prefix management schedule required by the PD service. If the Client knows its ACP in advance, it can include its AERO address as the source address of an RS message and with an SLLAO with a valid Prefix Length for the ACP. If the Server (and Proxy) accept the Client's ACP assertion, they inject the prefix into the routing system and establish the necessary neighbor cache state. If the Client does not know its ACP in advance, or if it wishes to engage in an explicit PD exchange, it can include ND/PD parameters for an ancillary service such as DHCPv6.
On Some AERO links, PD arrangements may be through some out-of-band service such as network management, static configuration, etc. In those cases, AERO nodes can use simple RS/RA message exchanges with no PD options. In other cases, the RS/RA messages can use AERO addresses as a means of representing the delegated prefixes, e.g., if a message includes a source address of "fe80::2001:db8:1:2" then the recipient can infer that the sender holds the prefix delegation "2001:db8:1:2::/N" (where 'N' is the Prefix Length included in the first SLLAO in the message).
The following sections specify the Client and Server behavior.
AERO Clients can discover the link-layer and AERO addresses of AERO Servers in the MAP list via static configuration (e.g., from a flat-file map of Server addresses and locations), or through an automated means such as Domain Name System (DNS) name resolution [RFC1035]. In the absence of other information, the Client resolves the DNS Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where "linkupnetworks" is a constant text string and "[domainname]" is a DNS suffix for the Client's underlying interface (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 ACPs. The Client prepares an RS message with PD parameters (e.g., with an SLLAO with non-zero Prefix Length), the address of the Client's underlying interface as the link-layer source address and the link-layer address of the Server as the link-layer destination address. If the Client already knows the Server's AERO address, it includes the AERO address as the network-layer destination address; otherwise, it includes all-routers multicast (ff02::2) as the network-layer destination address. If the Client already knows its own AERO address, it uses the AERO address as the network-layer source address; otherwise, it uses the unspecified AERO address (fe80::ffff:ffff) as the network-layer source address.
The Client next includes an SLLAO in the RS message formatted as described in Section 3.6 to register its link-layer address with the Server. The first SLLAO MUST correspond to the underlying interface over which the Client will send the RS message. The Client MAY include additional SLLAOs specific to other underlying interfaces, but if so it sets their Port Number and Link Layer Address fields to 0.
The Client then sends the RS message (either via a VPN for VPNed interfaces, via a Proxy for proxyed interfaces or via the SPAN for native interfaces) and waits for an RA message reply (see Section 3.15.3) while retrying up to MAX_RTR_SOLICITATIONS times until an RA is received. If the Client receives no RAs, or if it receives an RA with Router Lifetime set to 0, the Client SHOULD abandon this Server and try another Server. Otherwise, the Client processes the PD information found in the RA message.
Next, the Client creates a symmetric neighbor cache entry with the Server's AERO address as the network-layer address and the address in the first SLLAO as the link-layer address. The Client records the RA Router Lifetime field value in the cache entry as the time for which the Server has committed to maintaining the ACP in the routing system. The Client then autoconfigures AERO addresses for each of the delegated ACPs and assigns them to the AERO interface. The Client also caches any ASPs included in Route Information Options (RIOs) [RFC4191] as ASPs to associate with the AERO link, and assigns the MTU value in the MTU option to its AERO interface while configuring an appropriate MRU. This configuration information applies to the AERO link as a whole, and all AERO nodes will receive the same values.
The Client then registers additional link-layer addresses with the Server by sending additional RS messages including SLLAOs via other underlying interfaces after the initial RS/RA exchange. The Client sends the RS messages to the Server's AERO address (discovered in the initial RS/RA exchange), but omits PD parameters since the initial RS/RA exchange has already established PD state.
The Client examines the X and N bits in the first SLLAO of each RA message it receives. If the X bit value is '1' the Client infers that there is a Proxy on the path via the interface over which it sent the RS message, and if the N bit value is '1' the Client infers that there is a NAT on the path. If N is '1', the Client SHOULD set Port Number and Link-Layer Address to 0 in the first S/TLLAO of any subsequent ND messages it sends to the Server over that link.
Following autoconfiguration, the Client sub-delegates the ACPs to its attached EUNs and/or the Client's own internal virtual interfaces as described in [I-D.templin-v6ops-pdhost] to support the Client's downstream attached "Internet of Things (IoT)". The Client subsequently maintains its ACP delegations through each of its Servers by sending additional RS messages with PD parameters before Router Lifetime expires.
After the Client registers its Interface IDs and their associated port numbers, link-layer addresses and 'P(i)' values, it may wish to change one or more Interface ID registrations, e.g., if an underlying interface changes address or becomes unavailable, if QoS preferences change, etc. To do so, the Client prepares an RS message to send over any available underlying interface. The RS MUST include an SLLAO specific to the selected available underlying interface as the first SLLAO and MAY include any additional SLLAOs specific to other underlying interfaces. The Client includes fresh 'P(i)' values in each SLLAO to update the Server's neighbor cache entry. If the Client wishes to update 'P(i)' values without updating the link-layer address, it sets the Port Number and Link-Layer Address fields to 0. If the Client wishes to disable the underlying interface, it sets the 'D' bit to 1. When the Client receives the Server's RA response, it has assurance that the Server has been updated with the new information.
If the Client wishes to associate with multiple Servers, it repeats the same procedures above for each additional Server. If the Client wishes to discontinue use of a Server it issues an RS message over any underlying interface with the 'R' bit set to 1 in the first SLLAO. When the Server processes the message, it releases the ACP, sets the symmetric neighbor cache entry state for the Client to DEPARTED, withdraws the IP route from the routing system and returns an RA reply with Router Lifetime set to 0.
AERO Servers act as IPv6 routers and support a PD service for Clients. AERO Servers arrange to add their link-layer and AERO address to a static map of Server addresses for the link and/or the DNS resource records for the FQDN "linkupnetworks.[domainname]" before entering service. The list of Server addresses should be geographically and/or topologically referenced, and forms the MAP list for the AERO link.
When an AERO Server receives a prospective Client's RS message with PD parameters on its AERO interface, it SHOULD return an immediate RA reply with Router Lifetime set to 0 if it is currently too busy or otherwise unable to service the Client. Otherwise, the Server authenticates the RS message and processes the PD parameters. The Server first determines the correct ACPs to delegate to the Client by searching the Client database. When the Server delegates the ACPs, it also creates an IP forwarding table entry for each ACP so that the AERO BGP-based routing system will propagate the ACPs to the Relays that aggregate the corresponding ASP (see: Section 3.3).
The Server next creates a symmetric neighbor cache entry for the Client using the base AERO address as the network-layer address and with lifetime set to no more than the smallest PD lifetime. Next, the Server updates the neighbor cache entry link-layer address(es) by recording the information in each SLLAO in the RS indexed by the Interface ID and including the Port Number, Link Layer Address and P(i) values. For the first SLLAO in the list, however, the Server records the actual encapsulation source addresses instead of those that appear in the SLLAO in case there was a NAT in the path. The Server also records the value of the X bit to indicate whether there is a Proxy on the path.
Next, the Server prepares an RA message that includes the delegated ACPs, any other PD parameters and an SLLAO with the Server's link-layer address and with Interface ID set to 255. The Server uses its AERO address as the network-layer source address, the network-layer source address of the RS message as the network-layer destination address, the Server's link-layer address as the source link-layer address, and the source link-layer address of the RS message as the destination link-layer address. The Server next sets the N flag to 1 if the source link-layer address in the RS message was different than the address in the first SLLAO to indicate that there is a NAT on the path. The Server then includes one or more RIOs that encode the ASPs for the AERO link. The Server also includes an MTU option for the MTU for the link (see Section 3.13). The Server finally sends the RA message to the Client via the SPAN.
After the initial RS/RA exchange, the AERO Server maintains the symmetric neighbor cache entry for the Client. If the Client (or Proxy) issues additional NS/RS messages, the Server resets ReachableTime. If the Client (or Proxy) issues an RS with PD release parameters (e.g., by including an SLLAO with R set to 1), or if the Client becomes unreachable, the Server sets the Client's symmetric neighbor cache entry to the DEPARTED state and withdraws the IP routes from the AERO routing system.
The Server processes these and any other Client ND/PD messages, and returns an NA/RA reply. The Server may also issue an unsolicited RA message with PD reconfigure parameters to cause the Client to renegotiate its PDs, and may issue an unsolicited RA message with Router Lifetime set to 0 if it can no longer service this Client. Finally, If the symmetric neighbor cache entry is in the DEPARTED state, the Server deletes the entry after DepartTime expires.
When DHCPv6 is used as the ND/PD service back end, AERO Clients and Servers are always on the same link (i.e., the AERO link) from the perspective of DHCPv6. However, in some implementations the DHCPv6 server and ND function may be located in separate modules. In that case, the Server's AERO interface module can act as a Lightweight DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from the DHCPv6 server module.
When the LDRA receives an authentic RS message, it extracts the PD message parameters and uses them to construct an IPv6/UDP/DHCPv6 message. It sets the IPv6 source address to the source address of the RS message, sets the IPv6 destination address to 'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values that will be understood by the DHCPv6 server.
The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message header and includes an 'Interface-Id' option that includes enough information to allow the LDRA to forward the resulting Reply message back to the Client (e.g., the Client's link-layer addresses, a security association identifier, etc.). The LDRA also wraps the information in all of the SLLAOs from the RS message into the Interface-Id option, then forwards the message to the DHCPv6 server.
When the DHCPv6 server prepares a Reply message, it wraps the message in a 'Relay-Reply' message and echoes the Interface-Id option. The DHCPv6 server then delivers the Relay-Reply message to the LDRA, which discards the Relay-Reply wrapper and IPv6/UDP headers, then uses the DHCPv6 message to construct an RA response to the Client. The Server uses the information in the Interface-Id option to prepare the RA message and to cache the link-layer addresses taken from the SLLAOs echoed in the Interface-Id option.
In some environments, Clients may be located in secured enclaves that do not allow direct communications from the Client to a Server in the outside Internetwork. In that case, the secured enclave can employ an AERO Proxy.
The Proxy is located at the secured enclave perimeter and listens for encapsulated RS messages originating from or RA messages destined to Clients located within the enclave. The Proxy acts on these control messages as follows:
After the initial RS/RA exchange, the Proxy forwards any Client data packets for which there is no matching asymmetric neighbor cache entry to the "eldest" of the Client's Servers, i.e., the first among possibly multiple Servers selected by the Client. If the eldest Server becomes unreachable, the Proxy sends future data packets via the next-eldest Server, etc. Finally, the Proxy forwards any Client data destined to an asymmetric neighbor cache target directly to the target according to the link-layer information - the process of establishing asymmetric neighbor cache entries is specified in
While the Client is still active, the Proxy continues to send NS/RS messages to update each Server's symmetric neighbor cache entries on behalf of the Client and/or to convey QoS updates. If the Server ceases to send solicited NA/RA responses, the Proxy marks the Server as unreachable and sends an unsolicited RA to the Client with Router Lifetime set to zero so that the Client knows that this Server is no longer able to provide Service. If the Client becomes unreachable, the Proxy sets the neighbor cache entry state to DEPARTED and sends an RS message to each Server with an SLLAO with the 'D' bit set to 1 and with Interface ID set to the Client's interface ID so that the Server will de-register this Interface ID. Although the Proxy engages in these ND exchanges on behalf of the Client, the Client can also send ND messages on its own behalf, e.g., if it is in a better position than the Proxy to convey QoS changes, etc.
In some subnetworks that employ a Proxy, the Client's ACP can be injected into the underlying network routing system. In that case, the Client can send data messages without encapsulation so that the native underlying network routing system transports the unencapsulated packets to the Proxy. This can be very beneficial, e.g., if the Client connects to the network via low-end data links such as some aviation wireless links. In that case, however, the Client's control messages are still sent encapsulated so as to supply the Proxy with the address of the Server and to transport IPv6 ND messages without decrementing the hop-count. In summary, the interface becomes one where control messages are encapsulated while data messages are either unencapsulated or encapsulated according to the specific use case. This encapsulation avoidance represents a form of "header compression", meaning that the MTU should be sized based on the size of full encapsulated messages even if most messages are sent unencapsulated.
If the Client is aware that its data link interface connects to a secured enclave with an AERO-aware Access Router as the first-hop router, it can avoid encapsulation for its control messages as well as its data messages. When the Client comes onto the link, it can send an unencapsulated RS message with source address set to its AERO address and with destination address set to the AERO address of the Client's selected Server or to all-routers multicast.
The Client includes an SLLAO with Interface ID, Prefix Length and P(i) information but with Port Number and Link-Layer Address set to 0. The Client then sends the unencapsulated RS message, which will be intercepted by the on-link AERO-Aware Access Router. The Access Router then encapsulates the RS message in an outer header with its own address as the source address and the address of a Proxy as the destination address. The Access Router further remembers the address of the Proxy so that it can encapsulate future data packets from the Client via the same Proxy. If the Access Router needs to change to a new Proxy, it simply sends another RS message toward the Server via the new Proxy on behalf of the Client.
In this arrangement, the only control messages that would ever be sent by the Client are unencapsulated RS messages with its AERO address as the source address and the AERO address of the Server as the destination address. The Client will also receive unencapsulated RA messages from the Server via both the Proxy and Access Router.
In some cases, the Access Router and AERO Proxy may be one and the same node. In that case, the node would be located on the same physical link as the Client, but its messages exchanges with the Server would need to pass through a security gateway at the secured enclave ingress/egress. The method for deploying Access Routers and Proxys (i.e. as a single node or multiple nodes) is a subnetwork-local administrative consideration.
While data packets are flowing from a source Client to a target Client that are both holders of ACPs belonging to the same ASP, route optimization SHOULD be used to establish the best path(s). Route optimization is initiated by the first eligible Route Optimization Source (ROS) closest to the source Client as follows:
The route optimization procedure is conducted between the ROS and a Route Optimization Responder (ROR) in the same manner as for IPv6 ND Address Resolution, and using the same NS/NA messaging. The procedures are specified in the following sections.
While the data packets are flowing from the source Client toward a target Client, the ROS also sends an NS message to receive a solicited NA message from an ROR acting as a Mobility Anchor Point (MAP).
When the ROS sends an NS, it includes the AERO address of the ROS as the source address and the AERO address corresponding to the data packet's destination address as the destination address (for example, if the destination address is 2001:db8:1:2::1 then the target AERO address is fe80::2001:db8:1:2). The NS message includes no SLLAOs, but SHOULD include a Timestamp and Nonce option.
The ROS then sends the message into the SPAN (but with SPAN destination set to the inner packet destination) without decrementing the network-layer TTL/Hop Limit field.
When the Relay receives the (double-encapsulated) NS message from the ROS, it discards the outer IP header and determines that the ROR is the next hop by consulting its standard IP forwarding table for the SPAN header destination address. The Relay then forwards the SPAN message toward the ROR the same as for any IP router. The final-hop Relay in the SPAN will encapsulate the message in an outer IP header when it delivers the message to the ROR.
When the ROR receives the (double-encapsulated) NS message, it discards the outer IP and SPAN headers. The ROR next examines the AERO destination address to determine whether the target Client is one of its symmetric neighbors in the REACHABLE state. If so, the ROR adds the AERO source address to the target Client's symmetric neighbor cache entry Report list with time set to ReportTime.
Next, the ROR prepares a solicited NA message to send back to the ROS but does not create a neighbor cache entry. The ROR sets the NA source address to its own AERO address and sets the destination address to the AERO address of the ROS. The NA message includes the Nonce value received in the NS, the current Timestamp, and a first TLLAO with Interface ID set to 255, with all P(i) values set to "low" and with "Prefix Length" set to the prefix length of the target Client's ACP. If the ROR and ROS are on the same segment, the ROR sets the TLLAO Link Layer address to the ROR's own link-layer address; otherwise, set to the ROR's SPAN address.
If the ROS and ROR are on the same segment, the ROR next includes additional TLLAOs for all of the target Client's Interface IDs. For NATed, VPNed and Direct interfaces, the TLLAO addresses are the address of the ROR. For Proxyed interfaces, the TLLAO addresses are the addresses of the target Client's Proxies, and for native interfaces the TLLAO addresses are the addresses of the target Client.
The ROR then sends the message into the SPAN without decrementing the network-layer TTL/Hop Limit field.
When the Relay receives the (double-encapsulated) NA message from the ROR, it discards the outer IP header and determines that the ROS is the next hop by consulting its standard IP forwarding table for the SPAN header destination address. The Relay then forwards the SPAN message toward the ROS the same as for any IP router. The final-hop Relay in the SPAN will encapsulate the message in an outer IP header when it delivers the message to the ROS.
When the ROS receives the (double-encapsulated) solicited NA message, it discards the outer IP and SPAN headers. The ROS next verifies the Nonce and Timestamp values, then creates an asymmetric neighbor cache entry for the target Client and caches all information found in the solicited NA TLLAOs. The ROS finally sets the asymmetric neighbor cache entry lifetime to ReachableTime seconds.
Following route optimization, if the ROS and ROR are on the same SPAN segment the ROS forwards future data packets directly to the target Client using the cached link-layer information instead of through a dogleg route involving unnecessary Servers and/or Relays. Otherwise, the ROS forwards future data packets into the SPAN using the ROS's SPAN address as the source address and the ROR's SPAN address as the destination address. In both cases, the route optimization is shared by all sources that send packets to the target Client via the ROS, i.e., and not just the original source Client.
While new data packets destined to the target are flowing through the ROS, it sends additional NS messages to the ROR before ReachableTime expires to receive a fresh solicited NA message the same as described in the previous sections. The ROS then updates the asymmetric neighbor cache entry to refresh ReachableTime, while the ROR adds or updates the ROS address to the target Client's symmetric neighbor cache entry Report list and with time set to ReportTime. While no data packets are flowing, the ROS instead allows ReachableTime for the asymmetric neighbor cache entry to expire. When ReachableTime expires, the ROS deletes the asymmetric neighbor cache entry. Future data packets flowing through the ROS will again trigger a new route optimization exchange while initial data packets travel over a suboptimal route via Servers and/or Relays.
The ROS may also receive unsolicited NA messages from the ROR at any time. If there is an asymmetric neighbor cache entry for the target, the ROS updates the link-layer information but does not update ReachableTime since the receipt of an unsolicited NA does not confirm that the forward path is still working. If there is no asymmetric neighbor cache entry, the route optimization source simply discards the unsolicited NA. Cases in which unsolicited NA messages are generated are specified in Section 3.19.
In this arrangement, the ROS holds an asymmetric neighbor cache entry for the ROR, but the ROR does not hold an asymmetric neighbor cache entry for the ROS. The route optimization neighbor relationship is therefore asymmetric and unidirectional. If the target Client also has packets to send back to the source Client, then a separate route optimization procedure is required in the reverse direction. But, there is no requirement that the forward and reverse paths be symmetric.
AERO nodes perform Neighbor Unreachability Detection (NUD) the same as described in [RFC4861]. NUD is performed either reactively in response to persistent link-layer errors (see Section 3.14) or proactively to confirm bi-directional reachability. The NUD algorithm may further be seeded by neighbor discovery hints of forward progress, but care must be taken to avoid inferring reachability based on spoofed information.
When an AERO node sends an NS/NA message used for NUD, it uses one of its AERO addresses as the IPv6 source address and an AERO address of the neighbor as the IPv6 destination address, but does not include S/TLLAOs. When an ROR directs an ROS to one or more target addresses, the ROS SHOULD proactively test the direct path to each target address by sending an initial NS message to elicit a solicited NA response. While testing the path, the source node can optionally continue sending packets via its default router, maintain a small queue of packets until target reachability is confirmed, or (optimistically) allow packets to flow directly to the target.
Note that AERO nodes may have multiple underlying interface paths toward the target neighbor. In that case, NUD SHOULD be performed over each underlying interface individually and the node should only consider the neighbor unreachable if NUD fails over multiple underlying interface paths.
Underlying interface paths that pass NUD tests are marked as "reachable", while those that do not are marked as "unreachable". These markings inform the AERO interface forwarding algorithm specified in Section 3.9.
Proxies can perform NUD to verify Server reachability on behalf of their proxyed Clients so that the Clients need not engage in NUD messaging themselves.
AERO is an example of a Distributed Mobility Management (DMM) service. Each Server is responsible for only a subset of the Clients on the AERO link, as opposed to a Centralized Mobility Management (CMM) service where there is a single network mobility service for all Clients. Clients coordinate with their associated Servers via RS/RA exchanges to maintain the DMM profile, and the AERO routing system tracks all current Client/Server peering relationships.
Servers provide a Mobility Anchor Point (MAP) for their dependent Clients. Clients are responsible for maintaining neighbor relationships with their Servers through periodic RS/RA exchanges, which also serves to confirm neighbor reachability. When a Client's underlying interface address and/or QoS information changes, the Client is responsible for updating the Server with this new information. Note that for Proxyed interfaces, however, the Proxy can perform the RS/RA exchanges on the Client's behalf.
Mobility management considerations are specified in the following sections.
RORs (acting as MAPs) accommodate mobility and/or QoS change events by sending an unsolicited NA message to each ROS in the target Client's Report list. When an ROR sends an unsolicited NA message, it sets the IPv6 source address to the Client's AERO address and sets the IPv6 destination address to all-nodes multicast (ff02::1). The ROR also includes a first TLLAO for Interface ID 255 with Link Layer address set to the ROR link-layer address if the ROR and ROS are on the same segment; otherwise, set to the ROR SPAN address. If the ROS and ROR are on the same segment the ROR next includes additional TLLAOs for all of the target Client's Interface IDs. The ROR then finally sends the message into the SPAN.
As for the hot-swap of interface cards discussed in Section 7.2.6 of [RFC4861], the transmission and reception of unsolicited NA messages is unreliable but provides a useful optimization. In well-connected Internetworks with robust data links unsolicited NA messages will be delivered with high probability, but in any case the ROR can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to each ROS to increase the likelihood that at least one will be received.
When an ROS receives an unsolicited NA message, it ignores the message if there is no existing neighbor cache entry for the Client. Otherwise, it uses the included TLLAOs to update the address and QoS information in the neighbor cache entry, but does not reset ReachableTime since the receipt of an unsolicited NA message from the target Server does not provide confirmation that any forward paths to the target Client are working.
If unsolicited NA messages are lost, the ROS may be left with stale address and/or QoS information for the Client for up to ReachableTime seconds. During this time, the ROS can continue sending packets to the target Client according to its current neighbor cache information but may receive persistent unsolicited NA messages as discussed in Section 3.19.2.
When a Server receives packets with destination addresses that match a symmetric neighbor cache entry in the DEPARTED state, it forwards the packets according to the Client's cached link layer address information, noting that the information may be stale. If the encapsulation source is in the Report list (i.e., if it is an ROS), the Server also sends an unsolicited NA message via the SPAN (subject to rate limiting) with a TLLAO with Interface ID 255 and with D set to 1. The ROS will then realize that it needs to set its asymmetric neighbor cache entry state for the target to DEPARTED, and SHOULD re-initiate route optimization after a short delay.
When a Proxy receives packets with destination addresses that match a proxy neighbor cache entry in the DEPARTED state, it forwards the packets to one of the target Client's Servers. If the encapsulation source is neither one of the target Client's Servers nor one of its proxy neighbor Clients, the Proxy also returns an unsolicited NA message via the SPAN (subject to rate limiting) with a single TLLAO with the target Client's Interface ID and with D set to 1. The source will then realize that it needs to mark its neighbor cache entry Interface ID for the Proxy as "unreachable", and SHOULD re-initiate route optimization while continuing to forward packets according to the remaining neighbor cache entry state.
When a Server receives packets from a symmetric neighbor Client that are destined to the same Client, the Server marks the neighbor cache entry Interface ID for this path as "unreachable", and forwards the packets via a "reachable" Interface ID. If there are no "reachable" Interface IDs, the Server drops the packet.
When a Client receives packets with destination addresses that do not match one of its ACPs, it drops the packets silently.
When a Client needs to change its link-layer addresses and/or QoS preferences (e.g., due to a mobility event), either the Client or Proxy sends RS messages to its Servers via the SPAN using the new link-layer address as the source address and with SLLAOs that include the new Client Port Number, Link-Layer Address and P(i) values. If the RS messages are sent solely for the purpose of updating QoS preferences without updating the link-layer address, the Port Number and Link-Layer Address are set to 0. If the RS message is not sent for the purpose of asserting a PD, the Prefix Length is set to 0.
Up to MAX_RTR_SOLICITATION RS messages MAY be sent in parallel with sending actual data packets in case one or more RAs are lost. If all RAs are lost, the Client SHOULD re-associate with a new Server.
When a Client needs to bring new underlying interfaces into service (e.g., when it activates a new data link), it sends RS messages to its Servers using the new link-layer address as the source address and with SLLAOs that include the new Client link-layer information. If the RS message is not sent for the purpose of asserting a PD, the Prefix Length is set to 0.
When a Client needs to remove existing underlying interfaces from service (e.g., when it de-activates an existing data link), it sends RS messages to its Servers with SLLAOs with the D flag set to 1.
If the Client needs to send RS messages over an underlying interface other than the one being removed from service, it MUST include a current SLLAO for the sending interface as the first SLLAO and include SLLAOs for any underlying interfaces being removed from service as additional SLLAOs.
AERO interface neighbors MAY provide a configuration option that allows them to perform implicit mobility management in which no ND messaging is used. In that case, the Client only transmits packets over a single interface at a time, and the neighbor always observes packets arriving from the Client from the same link-layer source address.
If the Client's underlying interface address changes (either due to a readdressing of the original interface or switching to a new interface) the neighbor immediately updates the neighbor cache entry for the Client and begins accepting and sending packets to the Client's new link-layer address. This implicit mobility method applies to use cases such as cellphones with both WiFi and Cellular interfaces where only one of the interfaces is active at a given time, and the Client automatically switches over to the backup interface if the primary interface fails.
When a Client associates with a new Server, it performs the Client procedures specified in Section 3.15.2. The Client then sends an RS message with R set to 1 in the first SLLAO and with PD parameters over any working underlying interface to fully release itself from the old Server. The SLLAO also includes the link-layer address of the new Server if the new and old Servers are on the same segment; otherwise, it includes the SPAN address of the new Server. If the Client does not receive an RA reply after MAX_RTR_SOLICITATIONS attempts over multiple underlying interfaces, the old Server may have failed and the Client should discontinue its release attempts.
When the old Server processes the RS, it sends unsolicited NA messages with a single TLLAO with Interface ID set to 255 and with D set to 1 to all route optimization sources in the Client's Report list. The Server also changes the symmetric neighbor cache entry state to DEPARTED, sets the link-layer address of the Client to the address found in the RS SLLAO, and sets a timer to DepartTime seconds. The Server then returns an RA message to the Client with Router Lifetime set to 0. After DepartTime seconds expires, the Server deletes the symmetric neighbor cache entry.
When the Client receives the RA message with Router Lifetime set to 0, it still must inform each of its remaining Proxies that it has released the old Server from service. To do so, it sends an RS over each remaining proxyed underlying interface with destination set to the old Server's AERO address and with R set to 1 in the first SLLAO but with no PD parameters. The Proxy will mark this Server as DEAPARTED and return an immediate RA without first performing an RS/RA exchange with the old Server.
Clients SHOULD NOT move rapidly between Servers in order to avoid causing excessive oscillations in the AERO routing system. Examples of when a Client might wish to change to a different Server include a Server that has gone unreachable, topological movements of significant distance, movement to a new geographic region, movement to a new segment, etc.
When the underlying network does not support multicast, AERO Clients map link-scoped multicast addresses to the link-layer address of a Server, which acts as a multicast forwarding agent. The AERO Client also serves as an IGMP/MLD Proxy for its EUNs and/or hosted applications per [RFC4605] while using the link-layer address of the Server as the link-layer address for all multicast packets.
When the underlying network supports multicast, AERO nodes use the multicast address mapping specification found in [RFC2529] for IPv4 underlying networks and use a TBD site-scoped multicast mapping for IPv6 underlying networks. In that case, border routers must ensure that the encapsulated site-scoped multicast packets do not leak outside of the site spanned by the AERO link.
When a Client's AERO interface is configured over a Direct underlying interface, the neighbor at the other end of the Direct link can receive packets without any encapsulation. In that case, the Client sends packets over the Direct link according to the QoS preferences associated with its underling interfaces. If the Direct underlying interface has the highest QoS preference, then the Client's IP packets are transmitted directly to the peer without going through an underlying network. If other underlying interfaces have higher QoS preferences, then the Client's IP packets are transmitted via a different underlying interface, which may result in the inclusion of Proxies, Servers and Relays in the communications path. Direct underlying interfaces must be tested periodically for reachability, e.g., via NUD.
IPv6 AERO links typically have ASPs that cover many candidate ACPs of length /64 or shorter. However, in some cases it may be desirable to use AERO over links that have only a /64 ASP. This can be accommodated by treating all Clients on the AERO link as simple hosts that receive /128 prefix delegations.
In that case, the Client sends an RS message to the Server the same as for ordinary AERO links. The Server responds with an RA message that includes one or more /128 prefixes (i.e., singleton addresses) that include the /64 ASP prefix along with an interface identifier portion to be assigned to the Client. The Client and Server then configure their AERO addresses based on the interface identifier portions of the /128s (i.e., the lower 64 bits) and not based on the /64 prefix (i.e., the upper 64 bits).
For example, if the ASP for the host-only IPv6 AERO link is 2001:db8:1000:2000::/64, each Client will receive one or more /128 IPv6 prefix delegations such as 2001:db8:1000:2000::1/128, 2001:db8:1000:2000::2/128, etc. When the Client receives the prefix delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to the AERO interface, and assigns the global IPv6 addresses (i.e., the /128s) to either the AERO interface or an internal virtual interface such as a loopback. In this arrangement, the Client conducts route optimization in the same sense as discussed in Section 3.17.
This specification has applicability for nodes that act as a Client on an "upstream" AERO link, but also act as a Server on "downstream" AERO links. More specifically, if the node acts as a Client to receive a /64 prefix from the upstream AERO link it can then act as a Server to provision /128s to Clients on downstream AERO links.
SEcure Neighbor Discovery (SEND) [RFC3971] and Cryptographically Generated Addresses (CGAs) [RFC3972] were designed to secure IPv6 ND messaging in environments where symmetric network and/or transport-layer security services are impractical (see: Section 10). AERO nodes that use SEND/CGA employ the following adaptations.
When a source AERO node prepares a SEND-protected ND message, it uses a link-local CGA as the IPv6 source address and writes the prefix embedded in its AERO address (i.e., instead of fe80::/64) in the CGA parameters Subnet Prefix field. When the neighbor receives the ND message, it first verifies the message checksum and SEND/CGA parameters while using the link-local prefix fe80::/64 (i.e., instead of the value in the Subnet Prefix field) to match against the IPv6 source address of the ND message.
The neighbor then derives the AERO address of the source by using the value in the Subnet Prefix field as the interface identifier of an AERO address. For example, if the Subnet Prefix field contains 2001:db8:1:2, the neighbor constructs the AERO address as fe80::2001:db8:1:2. The neighbor then caches the AERO address in the neighbor cache entry it creates for the source, and uses the AERO address as the IPv6 destination address of any ND message replies.
AERO Relays are low-end to midrange Commercial off-the Shelf (COTS) standard IP routers with no AERO code. Relays must be provisioned, supported and managed by the AERO Link Service Provider. Cost for purchasing, configuring and managing Relays is nominal even for very large AERO links.
AERO Servers can be standard dedicated server platforms, but most often will be deployed as virtual machines in the cloud. The only requirements for Servers are that they can run the AERO user-level code and have at least one network interface with a public IP address. As with Relays, Servers must be provisioned, supported and managed by the AERO Link Service Provider. Cost for purchasing, configuring and managing Servers is nominal especially for virtual Servers hosted in the cloud.
AERO Proxies are most often standard dedicated server platforms with one network interface connected to the secured enclave and a second interface connected to the public Internetwork. As with Servers, the only requirements are that they can run the AERO user-level code and have at least one interface with a public IP address. Proxies must be provisioned, supported and managed by the administrative authority for the secured enclave. Cost for purchasing, configuring and managing Proxies is nominal, and borne by the secured enclave administrative authority.
AERO Clients are most often mobile nodes, but fixed AERO Clients can also be used to attach large non-mobile networks to the AERO link. In that case, the AERO Client would be a fixed IPv6 router that would appear the same as for any Client, albeit with no mobility signaling requirements.
An AERO implementation based on OpenVPN (https://openvpn.net/) was announced on the v6ops mailing list on January 10, 2018. The latest version is available at: http://linkupnetworks.net/aero/AERO-OpenVPN-2.0.tgz.
An initial public release of the AERO proof-of-concept source code was announced on the intarea mailing list on August 21, 2015. The latest version is available at: http://linkupnetworks.net/aero/aero-4.0.0.tgz.
A survey of public domain and commercial SEND implementations is available at https://www.ietf.org/mail-archive/web/its/current/msg02758.html.
The IANA has assigned a 4-octet Private Enterprise Number "45282" for AERO in the "enterprise-numbers" registry.
The IANA has assigned the UDP port number "8060" for an earlier experimental version of AERO [RFC6706]. This document obsoletes [RFC6706] and claims the UDP port number "8060" for all future use.
No further IANA actions are required.
AERO link security considerations include considerations for both the data plane and the control plane.
Data plane security considerations are the same as for ordinary Internet communications. Application endpoints in AERO Clients and their EUNs SHOULD use application-layer security services such as TLS/SSL [RFC8446], DTLS [RFC6347] or SSH [RFC4251] to assure the same level of protection as for critical secured Internet services. AERO Clients that require host-based VPN services SHOULD use symmetric network and/or transport layer security services such as TLS/SSL, DTLS, IPsec [RFC4301], etc. AERO Proxies and Servers can also provide a network-based VPN service on behalf of the Client, e.g., if the Client is located within a secured enclave and cannot establish a VPN on its own behalf.
Control plane security considerations are the same as for standard IPv6 Neighbor Discovery [RFC4861], except that the MAP list also improves security by providing AERO Clients with an authentic list of trusted Servers. As fixed infrastructure elements, AERO Proxies and Servers SHOULD pre-configure security associations for one or more Relays on their SPAN segments (e.g., using pre-placed keys) and use symmetric network and/or transport layer security services such as IPsec, TLS/SSL or DTLS to secure ND messages. The AERO Relays of all SPAN segments in turn SHOULD pre-configure security associations for their neighboring AERO Relays. AERO Clients that connect to secured enclaves need not apply security to their ND messages, since the messages will be intercepted by an enclave perimeter Proxy. AERO Clients located outside of secured enclaves SHOULD use symmetric network and/or transport layer security to secure their ND exchanges with Servers, but when there are many prospective neighbors with dynamically changing connectivity an asymmetric security service such as SEND may be needed (see: Section 6).
AERO Servers and Relays present targets for traffic amplification Denial of Service (DoS) attacks. This concern is no different than for widely-deployed VPN security gateways in the Internet, where attackers could send spoofed packets to the gateways at high data rates. This can be mitigated by connecting Relays and Servers over dedicated links with no connections to the Internet and/or when connections to the Internet are only permitted through well-managed firewalls. Traffic amplification DoS attacks can also target an AERO Client's low data rate links. This is a concern not only for Clients located on the open Internet but also for Clients in secured enclaves. AERO Servers and Proxies can institute rate limits that protect Clients from receiving packet floods that could DoS low data rate links.
AERO Relays must implement ingress filtering to avoid a spoofing attack in which spurious SPAN messages are injected into an AERO link from an outside attacker. Also, since an AERO link spans one or Internetwork segments, restricting access to the link can be achieved by having Internetwork border routers implement ingress filtering to discard encapsulated packets injected into the link by an outside agent.
AERO Clients MUST ensure that their connectivity is not used by unauthorized nodes on their EUNs to gain access to a protected network, i.e., AERO Clients that act as routers MUST NOT provide routing services for unauthorized nodes. (This concern is no different than for ordinary hosts that receive an IP address delegation but then "share" the address with other nodes via some form of Internet connection sharing such as tethering.)
The MAP list MUST be well-managed and secured from unauthorized tampering, even though the list contains only public information. The MAP list can be conveyed to the Client, e.g., through secure upload of a static file, through DNS lookups, etc.
Although public domain and commercial SEND implementations exist, concerns regarding the strength of the cryptographic hash algorithm have been documented [RFC6273] [RFC4982].
Security considerations for accepting link-layer ICMP messages and reflected packets are discussed throughout the document.
Discussions in the IETF, aviation standards communities and private exchanges helped shape some of the concepts in this work. Individuals who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred Baker, Bob Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli, Brian Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak, Lee Howard, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt. Members of the IESG also provided valuable input during their review process that greatly improved the document. Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman for their shepherding guidance during the publication of the AERO first edition.
This work has further been encouraged and supported by Boeing colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam Brodie, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed King, Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg Saccone, Kent Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan Williams, Julie Wulff, Yueli Yang, Eric Yeh and other members of the BR&T and BIT mobile networking teams. Kyle Bae, Wayne Benson and Eric Yeh are especially acknowledged for implementing the AERO functions as extensions to the public domain OpenVPN distribution.
Earlier works on NBMA tunneling approaches are found in [RFC2529][RFC5214][RFC5569].
Many of the constructs presented in this second edition of AERO are based on the author's earlier works, including:
Note that these works cite numerous earlier efforts that are not also cited here due to space limitations. The authors of those earlier works are acknowledged for their insights.
This work is aligned with the NASA Safe Autonomous Systems Operation (SASO) program under NASA contract number NNA16BD84C.
This work is aligned with the FAA as per the SE2025 contract number DTFAWA-15-D-00030.
This work is aligned with the Boeing Information Technology (BIT) MobileNet program.
This work is aligned with the Boeing autonomy program.
| [BGP] | Huston, G., "BGP in 2015, http://potaroo.net", January 2016. |
| [I-D.ietf-dmm-distributed-mobility-anchoring] | Chan, A., Wei, X., Lee, J., Jeon, S. and C. Bernardos, "Distributed Mobility Anchoring", Internet-Draft draft-ietf-dmm-distributed-mobility-anchoring-13, March 2019. |
| [I-D.ietf-intarea-gue] | Herbert, T., Yong, L. and O. Zia, "Generic UDP Encapsulation", Internet-Draft draft-ietf-intarea-gue-07, March 2019. |
| [I-D.ietf-intarea-gue-extensions] | Herbert, T., Yong, L. and F. Templin, "Extensions for Generic UDP Encapsulation", Internet-Draft draft-ietf-intarea-gue-extensions-06, March 2019. |
| [I-D.ietf-intarea-tunnels] | Touch, J. and M. Townsley, "IP Tunnels in the Internet Architecture", Internet-Draft draft-ietf-intarea-tunnels-09, July 2018. |
| [I-D.ietf-rtgwg-atn-bgp] | Templin, F., Saccone, G., Dawra, G., Lindem, A. and V. Moreno, "A Simple BGP-based Mobile Routing System for the Aeronautical Telecommunications Network", Internet-Draft draft-ietf-rtgwg-atn-bgp-01, January 2019. |
| [I-D.templin-6man-dhcpv6-ndopt] | Templin, F., "A Unified Stateful/Stateless Configuration Service for IPv6", Internet-Draft draft-templin-6man-dhcpv6-ndopt-07, December 2018. |
| [I-D.templin-intarea-grefrag] | Templin, F., "GRE Tunnel Level Fragmentation", Internet-Draft draft-templin-intarea-grefrag-04, July 2016. |
| [I-D.templin-intarea-seal] | Templin, F., "The Subnetwork Encapsulation and Adaptation Layer (SEAL)", Internet-Draft draft-templin-intarea-seal-68, January 2014. |
| [I-D.templin-intarea-vet] | Templin, F., "Virtual Enterprise Traversal (VET)", Internet-Draft draft-templin-intarea-vet-40, May 2013. |
| [I-D.templin-ironbis] | Templin, F., "The Interior Routing Overlay Network (IRON)", Internet-Draft draft-templin-ironbis-16, March 2014. |
| [I-D.templin-v6ops-pdhost] | Templin, F., "IPv6 Prefix Delegation and Multi-Addressing Models", Internet-Draft draft-templin-v6ops-pdhost-23, December 2018. |
| [OVPN] | OpenVPN, O., "http://openvpn.net", October 2016. |
| [RFC1035] | Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987. |
| [RFC1122] | Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989. |
| [RFC1191] | Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990. |
| [RFC1812] | Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, DOI 10.17487/RFC1812, June 1995. |
| [RFC2003] | Perkins, C., "IP Encapsulation within IP", RFC 2003, DOI 10.17487/RFC2003, October 1996. |
| [RFC2473] | Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, December 1998. |
| [RFC2529] | Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 Domains without Explicit Tunnels", RFC 2529, DOI 10.17487/RFC2529, March 1999. |
| [RFC2764] | Gleeson, B., Lin, A., Heinanen, J., Armitage, G. and A. Malis, "A Framework for IP Based Virtual Private Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000. |
| [RFC2784] | Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, DOI 10.17487/RFC2784, March 2000. |
| [RFC2890] | Dommety, G., "Key and Sequence Number Extensions to GRE", RFC 2890, DOI 10.17487/RFC2890, September 2000. |
| [RFC2923] | Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, DOI 10.17487/RFC2923, September 2000. |
| [RFC2983] | Black, D., "Differentiated Services and Tunnels", RFC 2983, DOI 10.17487/RFC2983, October 2000. |
| [RFC3168] | Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001. |
| [RFC3819] | Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for Internet Subnetwork Designers", BCP 89, RFC 3819, DOI 10.17487/RFC3819, July 2004. |
| [RFC4213] | Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, DOI 10.17487/RFC4213, October 2005. |
| [RFC4251] | Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006. |
| [RFC4271] | Rekhter, Y., Li, T. and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006. |
| [RFC4291] | Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006. |
| [RFC4301] | Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005. |
| [RFC4389] | Thaler, D., Talwar, M. and C. Patel, "Neighbor Discovery Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April 2006. |
| [RFC4443] | Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006. |
| [RFC4511] | Sermersheim, J., "Lightweight Directory Access Protocol (LDAP): The Protocol", RFC 4511, DOI 10.17487/RFC4511, June 2006. |
| [RFC4605] | Fenner, B., He, H., Haberman, B. and H. Sandick, "Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605, August 2006. |
| [RFC4963] | Heffner, J., Mathis, M. and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, DOI 10.17487/RFC4963, July 2007. |
| [RFC4982] | Bagnulo, M. and J. Arkko, "Support for Multiple Hash Algorithms in Cryptographically Generated Addresses (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007. |
| [RFC5214] | Templin, F., Gleeson, T. and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, DOI 10.17487/RFC5214, March 2008. |
| [RFC5320] | Templin, F., "The Subnetwork Encapsulation and Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320, February 2010. |
| [RFC5522] | Eddy, W., Ivancic, W. and T. Davis, "Network Mobility Route Optimization Requirements for Operational Use in Aeronautics and Space Exploration Mobile Networks", RFC 5522, DOI 10.17487/RFC5522, October 2009. |
| [RFC5558] | Templin, F., "Virtual Enterprise Traversal (VET)", RFC 5558, DOI 10.17487/RFC5558, February 2010. |
| [RFC5569] | Despres, R., "IPv6 Rapid Deployment on IPv4 Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569, January 2010. |
| [RFC6179] | Templin, F., "The Internet Routing Overlay Network (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011. |
| [RFC6221] | Miles, D., Ooghe, S., Dec, W., Krishnan, S. and A. Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, DOI 10.17487/RFC6221, May 2011. |
| [RFC6273] | Kukec, A., Krishnan, S. and S. Jiang, "The Secure Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273, DOI 10.17487/RFC6273, June 2011. |
| [RFC6347] | Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012. |
| [RFC6438] | Carpenter, B. and S. Amante, "Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011. |
| [RFC6706] | Templin, F., "Asymmetric Extended Route Optimization (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012. |
| [RFC6864] | Touch, J., "Updated Specification of the IPv4 ID Field", RFC 6864, DOI 10.17487/RFC6864, February 2013. |
| [RFC8086] | Yong, L., Crabbe, E., Xu, X. and T. Herbert, "GRE-in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086, March 2017. |
| [RFC8201] | McCann, J., Deering, S., Mogul, J. and R. Hinden, "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017. |
| [RFC8446] | Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018. |
When GUE encapsulation is not needed, AERO can use common encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic Routing Encapsulation (GRE) [RFC2784][RFC2890] and others. The encapsulation is therefore only differentiated from non-AERO tunnels through the application of AERO control messaging and not through, e.g., a well-known UDP port number.
As for GUE encapsulation, alternate AERO encapsulation formats may require encapsulation layer fragmentation. For simple IP-in-IP encapsulation, an IPv6 fragment header is inserted directly between the inner and outer IP headers when needed, i.e., even if the outer header is IPv4. The IPv6 Fragment Header is identified to the outer IP layer by its IP protocol number, and the Next Header field in the IPv6 Fragment Header identifies the inner IP header version. For GRE encapsulation, a GRE fragment header is inserted within the GRE header [I-D.templin-intarea-grefrag].
Figure 5 shows the AERO IP-in-IP encapsulation format before any fragmentation is applied:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IPv4 Header | | Outer IPv6 Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPv6 Frag Header (optional)| |IPv6 Frag Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header | | Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
~ ~ ~ ~
~ Inner Packet Body ~ ~ Inner Packet Body ~
~ ~ ~ ~
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Minimal Encapsulation in IPv4 Minimal Encapsulation in IPv6
Figure 5: Minimal Encapsulation Format using IP-in-IP
Figure 6 shows the AERO GRE encapsulation format before any fragmentation is applied:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Header |
| (with checksum, key, etc..) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GRE Fragment Header (optional)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner IP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Inner Packet Body ~
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Minimal Encapsulation Using GRE
Alternate encapsulation may be preferred in environments where GUE encapsulation would add unnecessary overhead. For example, certain low-bandwidth wireless data links may benefit from a reduced encapsulation overhead.
GUE encapsulation can traverse network paths that are inaccessible to non-UDP encapsulations, e.g., for crossing Network Address Translators (NATs). More and more, network middleboxes are also being configured to discard packets that include anything other than a well-known IP protocol such as UDP and TCP. It may therefore be necessary to determine the potential for middlebox filtering before enabling alternate encapsulation in a given environment.
In addition to IP-in-IP, GRE and GUE, AERO can also use security encapsulations such as IPsec, TLS/SSL, DTLS, etc. In that case, AERO control messaging and route determination occur before security encapsulation is applied for outgoing packets and after security decapsulation is applied for incoming packets.
AERO is especially well suited for use with VPN system encapsulations such as OpenVPN [OVPN].
The AERO S/TLLAO format specified in Section 3.6 includes a Length value of 5 (i.e., 5 units of 8 octets). However, special-purpose links may extend the basic format to include additional fields and a Length value larger than 5.
For example, adaptation of AERO to the Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS) includes link selection preferences based on transport port numbers in addition to the existing DSCP-based preferences. ATN/IPS nodes maintain a map of transport port numbers to 64 possible preference fields, e.g., TCP port 22 maps to preference field 8, TCP port 443 maps to preference field 20, UDP port 8060 maps to preference field 34, etc. The extended S/TLLAO format for ATN/IPS is shown in Figure 7, where the Length value is 7 and the 'Q(i)' fields provide link preferences for the corresponding transport port number.
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 | Length = 7 | Prefix Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID | Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Link-Layer Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q00|Q01|Q02|Q03|Q04|Q05|Q06|Q07|Q08|Q09|Q10|Q11|Q12|Q13|Q14|Q15|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q16|Q17|Q18|Q19|Q20|Q21|Q22|Q23|Q24|Q25|Q26|Q27|Q28|Q29|Q30|Q31|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q32|Q33|Q34|Q35|Q36|Q37|Q38|Q39|Q40|Q41|Q42|Q43|Q44|Q45|Q46|Q47|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q48|Q49|Q50|Q51|Q52|Q53|Q54|Q55|Q56|Q57|Q58|Q59|Q60|Q61|Q62|Q63|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: ATN/IPS Extended S/TLLAO Format
<< RFC Editor - remove prior to publication >>
Changes from draft-templin-intarea-6706bis-10 to draft-templin-intrea-6706bis-11:
Changes from draft-templin-intarea-6706bis-09 to draft-templin-intrea-6706bis-10:
Changes from draft-templin-intarea-6706bis-08 to draft-templin-intrea-6706bis-09:
Changes from draft-templin-intarea-6706bis-07 to draft-templin-intrea-6706bis-08:
Changes from draft-templin-intarea-6706bis-06 to draft-templin-intrea-6706bis-07:
Changes from draft-templin-intarea-6706bis-05 to draft-templin-intrea-6706bis-06:
Changes from draft-templin-intarea-6706bis-04 to draft-templin-intrea-6706bis-05:
Changes from draft-templin-intarea-6706bis-03 to draft-templin-intrea-6706bis-04:
Changes from draft-templin-intarea-6706bis-02 to draft-templin-intrea-6706bis-03:
Changes from draft-templin-intarea-6706bis-01 to draft-templin-intrea-6706bis-02:
Changes from draft-templin-intarea-6706bis-00 to draft-templin-intrea-6706bis-01:
Changes from draft-templin-aerolink-82 to draft-templin-intarea-6706bis-00: