| Network Working Group | F. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Obsoletes: rfc5320, rfc5558, rfc5720, | February 13, 2019 |
| rfc6179, rfc6706 (if | |
| approved) | |
| Intended status: Standards Track | |
| Expires: August 17, 2019 |
Asymmetric Extended Route Optimization (AERO)
draft-templin-intarea-6706bis-05.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. 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). 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 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 may be many additional Relays, Servers, Clients and Proxies.
AERO Relays are standard IP routers that provide default forwarding services for AERO Servers. Each Relay also peers with Servers and other Relays in a dynamic routing protocol instance to discover 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 tunnels with neighboring Servers, and maintain an IP forwarding table entry for each AERO Client Prefix (ACP).
AERO Servers provide default forwarding services 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 AERO interface neighbor cache entries for Relays. They also maintain both neighbor cache entries and IP forwarding table entries for each of their associated Clients.
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 transparent conduit for AERO Clients connected to secured enclaves to associate with AERO link Servers. The Client sends all of its control plane messages to the Server's link-layer address and the Proxy 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 more direct route toward a target destination via AERO route optimization, in which case future outbound data packets would be forwarded via the more direct route. The Proxy function is specified in Section 4.
AERO Relays, Servers and Proxies are critical infrastructure elements in fixed (i.e., non-mobile) deployments. AERO Relays and Servers must use public 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 AERO 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. All Relays 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.
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 and Server 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:
[RFC4291], and the address fe80::ffff:ffff is reserved as the prefix solicitation address; hence, these values are not available for administrative assignment.)
Relay and Server AERO addresses are allocated from the range fe80::/96, and MUST be managed for uniqueness by the administrative authority for the link. For interfaces that assign static IPv4 addresses, the lower 32 bits of the AERO address includes the IPv4 address, e.g., for the IPv4 address 192.0.2.1 the corresponding AERO address is fe80::192.0.2.1. For other interfaces, the lower 32 bits of the AERO address includes a unique integer value, e.g., fe80::1, fe80::2, fe80::3, etc. (Note that the address fe80:: is reserved as the IPv6 link-local Subnet Router Anycast address
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.
AERO links additionally reserve an IPv6 prefix to support encapsulated forwarding of IPv6 ND messages between Servers on the link. Although any non-link-local IPv6 prefix could be reserved for this purpose, a Unique Local Address (ULA) prefix [RFC4389] would be a good candidate since it is not routable outside of the AERO link. For example, if the reserved (ULA) prefix is fd00:db8::/64 the AERO Server Subnet Router Anycast Address is fd00:db8::.
A full discussion of the AERO addressing service is found in [I-D.templin-6man-aeroaddr].
AERO interfaces use encapsulation (see: Section 3.9) 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), Neighbor Advertisement (NA) and Redirect 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 2:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID | UDP Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IP 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 2: 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 NAT or Proxy 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 sets both UDP Port Number and IP 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 fe80::ID to the interface. Each fe80::ID address MUST be unique among all AERO nodes on the link. The Relay then engages in a dynamic routing protocol session with one or more Servers and all 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).
When a Server enables an AERO interface, it assigns an administratively-provisioned AERO address fe80::ID 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 dynamic routing protocol 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 an 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 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. 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 correspondent 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 a Proxy enables an AERO interface, it maintains per-Client proxy neighbor cache entries based on control message exchanges. Proxies forward packets between their associated Clients and each Client's associated Servers.
When the Proxy receives an RS message from a Client in the secured enclave, it creates an incomplete proxy neighbor cache entry and sends a proxyed RS message to a Server selected by the Client 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.
Each AERO interface maintains a conceptual neighbor cache that includes an entry for each neighbor it communicates with on the AERO link, the same as for any IPv6 interface [RFC4861]. AERO interface neighbor cache entries are said to be one of "permanent", "static", "proxy" or "dynamic".
Permanent neighbor cache entries are created through explicit administrative action; they have no timeout values and remain in place until explicitly deleted. AERO Relays maintain permanent neighbor cache entries for their associated Relays and Servers on the link, and AERO Servers maintain permanent neighbor cache entries for their associated Relays. Each entry maintains the mapping between the neighbor's fe80::ID network-layer address and corresponding link-layer address.
Static neighbor cache entries are created and maintained through ND/PD exchanges as specified in Section 3.14, and remain in place for durations bounded by ND/PD lifetimes. AERO Servers maintain static neighbor cache entries for each of their associated Clients, and AERO Clients maintain static neighbor cache entries for each of their associated Servers.
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.
Dynamic neighbor cache entries are created or updated based on receipt of route optimization messages as specified in Section 3.15, and are garbage-collected when keepalive timers expire. AERO nodes maintain dynamic neighbor cache entries for each of their active correspondents with lifetimes based on ND messaging constants.
When a target AERO node receives a valid NS message used for route optimization, it returns an NA message and also creates or updates a dynamic neighbor cache entry for the source network-layer and link-layer addresses. The node then sets a "ReportTime" variable in the neighbor cache entry to REPORT_TIME seconds. The node resets ReportTime when it receives a new NS message, and otherwise decrements ReportTime while no NS messages have been received. It is RECOMMENDED that REPORT_TIME be set to the default constant value 40 seconds to allow a 10 second window so that the AERO route optimization procedure can converge before ReportTime decrements below FORWARD_TIME (see below).
When a source AERO node receives a valid NA message response to its NS message, it creates or updates a dynamic neighbor cache entry for the target network-layer and link-layer addresses. The node then sets a "ForwardTime" variable in the neighbor cache entry to FORWARD_TIME seconds and uses this value to determine whether packets can be forwarded directly to the correspondent, i.e., instead of via a default route. The node resets ForwardTime when it receives a new NA, and otherwise decrements ForwardTime while no further NA messages arrive. It is RECOMMENDED that FORWARD_TIME be set to the default constant value 30 seconds to match the default REACHABLE_TIME value specified in [RFC4861].
The node also sets a "MaxRetry" variable to MAX_RETRY to limit the number of keepalives sent when a correspondent may have gone unreachable. It is RECOMMENDED that MAX_RETRY be set to 3 the same as described for address resolution in Section 7.3.3 of [RFC4861].
Different values for REPORT_TIME, FORWARD_TIME and MAX_RETRY MAY be administratively set; however, if different values are chosen, all nodes on the link MUST consistently configure the same values. Most importantly, REPORT_TIME SHOULD be set to a value that is sufficiently longer than FORWARD_TIME to allow the AERO route optimization procedure to converge.
When there may be a NAT or Proxy between the Client and the Server, or if the path from the Client to the Server should be tested for reachability, the Client can send periodic RS messages to the Server without PD parameters to receive RA replies. The RS/RA messaging will keep NAT/Proxy state alive and test Server reachability without disturbing the PD service.
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 3.5). The AERO node uses each packet's DSCP value 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 a dynamic 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. Otherwise, the Client uses the link-layer address in a static neighbor cache entry for a Server as the encapsulation address (noting that there may be a Proxy on the path to the real Server).
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 3.13).
When the Proxy receives a packet from a Client within the secured enclave, the Proxy searches for a dynamic neighbor cache entry that matches the destination. If there is a match, the Proxy 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. Otherwise, the Proxy uses the link-layer address for one of the Client's Servers as the encapsulation address.
When the Proxy receives a packet from an AERO interface neighbor, it searches for a proxy neighbor cache entry for a Client within the secured enclave that matches the destination. If there is a match, the Proxy forwards the packet to the Client. Otherwise, the Proxy returns the packet to the neighbor, i.e., by reversing the source and destination link-layer addresses and re-admitting the packet into the AERO link.
When an IP packet enters a Server's AERO interface from the network layer, the Server searches for a static neighbor cache entry for a Client that matches the destination. If there is a match, the Server uses one or more link-layer addresses in the entry as the link-layer addresses for encapsulation and admits the packet into the AERO link. Otherwise, the Server uses the link-layer address in a permanent neighbor cache entry for a Relay (selected through longest-prefix match) as the link-layer address for encapsulation.
When an IP packet enters a Server's AERO interface from the link layer, the Server processes the packet according to the network-layer destination address as follows:
When an IP packet enters a Relay's AERO interface from the network layer, the Relay searches its IP forwarding table for an ACP entry that matches the destination the same as for any IP router. If there is a match, the Relay uses the link-layer address in the corresponding neighbor cache entry as the link-layer address for encapsulation and forwards the packet to the AERO neighbor. Otherwise, the Relay drops the packet and returns a network-layer ICMP Destination Unreachable message subject to rate limiting (see: Section 3.13).
When an IP packet enters a Relay's AERO interface from the link-layer, (i.e., when it receives an encapsulated packet from a Server) the Relay processes the packet as follows:
When an AERO Server receives a return packet from an AERO Proxy (see Section 3.8.2), it proceeds according to the AERO link trust basis. Namely, the return packets have the same trust profile as for link-layer Destination Unreachable messages. If the Server has sufficient trust basis to accept link-layer Destination Unreachable messages, it can then process the return packet by searching for a dynamic neighbor cache entry that matches the destination. If there is a match, the Server marks the corresponding link-layer address as "unreachable", selects the next-highest priority "reachable" link-layer address in the entry as the link-layer address for encapsulation then (re)admits the packet into the AERO link. If there are no "reachable" link-layer addresses, the Server instead sets ForwardTime in the dynamic neighbor cache entry to 0 (noting that ReportTime may still be non-zero). Otherwise, the Server SHOULD drop the packet and treat it as an indication that a path may be failing, and MAY use Neighbor Unreachability Detection (NUD) (see: Section 3.13) to test the path for reachability.
When an AERO Relay receives a return packet from an AERO Server, it searches its routing table for an entry that matches the inner destination address. If there is a routing table entry that lists a different Server as the next hop, the Relay forwards the packet to the different Server; otherwise, the Relay drops 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 3.12.
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.
Clients normally use the IP address of the underlying interface as the encapsulation source address. If the underlying interface does not have an IP address, however, the Client uses an IP address taken from an ACP as the encapsulation source address (assuming the node has some way of injecting the ACP into the underlying network routing system). For IPv6 addresses, the Client normally uses the ACP Subnet Router Anycast address [RFC4291].
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][RFC1981]. 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 a 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] (note that common IPv6 over IPv4 tunnels 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/MSU values 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]. For AERO links over both IPv4 and IPv6, studies have also shown that IP fragments are dropped unconditionally over some network paths [I-D.taylor-v6ops-fragdrop]. 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 3.12.)
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 3 (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 3: 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 the RS/RA messages they exchange (see: [I-D.templin-6man-dhcpv6-ndopt]). The unified ND/PD messages are exchanged between Client and Server according to the prefix management schedule required by the PD service.
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 explicit PD options. Instead, the RS/RA messages 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 common to all ACPs for the link).
The following sections specify the Client and Server behavior.
AERO Clients discover the link-layer addresses of AERO Servers in the Potential Router List (PRL) 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 through an ND/PD message exchange. The Client sends an RS message with PD parameters and with all-routers multicast as the IPv6 destination address, 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 its own AERO address, it uses the AERO address as the IPv6 source address; otherwise, it uses the prefix-solicitation address as the source address. If the Client's underlying interface connects to a subnetwork that supports ACP injection, the Client can use the ACP's Subnet Router Anycast address as the link-layer source address.
The Client next includes one or more SLLAOs in the RS message formatted as described in Section 3.5 to register its link-layer address(es) 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 UDP Port Number and IP Address fields to 0. The Client can instead register 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 then sends the RS message to the AERO Server and waits for an RA message reply (see Section 3.14.3) while retrying MAX_RETRY times until an RA is received. If the Client receives no RAs, or if it receives an RA with Router Lifetime set to 0 and/or with no ACP PD parameters, the Client SHOULD discontinue autoconfiguration attempts through this Server and try another Server. Otherwise, the Client processes the ACPs found in the RA message.
Next, the Client creates a static neighbor cache entry with the Server's link-local address as the network-layer address and the Server's encapsulation source address as the link-layer address. The Client then autoconfigures AERO addresses for each of the delegated ACPs and assigns them to the AERO interface.
The Client next examines the P bit in the RA message flags field [RFC5175]. If the P bit value was 1, the Client infers that there is a NAT or Proxy on the path to the Server via the interface over which it sent the RS message. In that case, the Client sets UDP Port Number and IP Address to 0 in the S/TLLAOs of any subsequent ND messages it sends to the Server over that link.
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/MSU values in the MTU options 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.
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 RS messages with PD parameters to receive corresponding RA messages.
After the Client registers its Interface IDs and their associated UDP/IP 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 unsolicited NA message to send over any available underlying interface. The target address of the NA message is set to the Client's link-local address, and the destination address is set to all-nodes multicast. The NA MUST include a TLLAO specific to the selected available underlying interface as the first TLLAO and MAY include any additional TLLAOs specific to other underlying interfaces. The Client includes fresh 'P(i)' values in each TLLAO 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 UDP Port Number and IP Address fields to 0. If the Client wishes to disable the interface, it sets all 'P(i)' values to '0' ("disabled").
If the Client wishes to discontinue use of a Server it issues an RS message with PD parameters that will cause the Server to release the Client. When the Server processes the message, it releases the ACP, deletes its neighbor cache entry for the Client, withdraws the IP route from the routing system and returns an RA reply containing any necessary PD parameters.
AERO Servers act as IPv6 routers and support a PD service for Clients. AERO Servers arrange to add their encapsulation layer IP addresses (i.e., their link-layer addresses) 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 Potential Router List (PRL) for the AERO link.
When an AERO Server receives a prospective Client's RS message with PD parameters on its AERO interface, and the Server is too busy, it SHOULD return an immediate RA reply with no ACPs and with Router Lifetime set to 0. 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).
Next, the Server prepares an RA message that includes the delegated ACPs and any other PD parameters. The Server then returns the RA message using its link-local 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 P flag in the RA message flags field [RFC5175] 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 or Proxy on the path; otherwise it sets P to 0. The Server then includes one or more RIOs that encode the ASPs for the AERO link. The Server also includes two MTU options - the first MTU option includes the MTU for the link and the second MTU option includes the MSU for the link (see Section 3.12). The Server finally sends the RA message to the Client.
The Server next creates a static 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 option indexed by the Interface ID and including the UDP port number, IP address and P(i) values. For the first SLLAO in the list, however, the Server records the actual encapsulation source UDP and IP addresses instead of those that appear in the SLLAO in case there was a NAT or Proxy in the path.
After the initial RS/RA exchange, the AERO Server maintains the neighbor cache entry for the Client until the PD lifetimes expire. If the Client issues additional RS messages with PD renewal parameters, the Server extends the PD lifetimes. If the Client issues an RS with PD release parameters, or if the Client does not issue a renewal before the lifetime expires, the Server deletes the neighbor cache entry for the Client and withdraws the IP routes from the AERO routing system. The Server processes these and any other Client PD messages, and returns an RA reply. The Server may also issue an unsolicited RA message with PD reconfigure parameters to inform the Client that it needs to renegotiate its PDs.
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 fabricate 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 SLLAO options 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 fabricate 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.
When a source Client forwards packets to a prospective correspondent Client within the same AERO link domain (i.e., one for which the packet's destination address is covered by an ASP), the source Client MAY initiate an AERO link route optimization procedure. The procedure is based on an exchange of IPv6 ND messages using a chain of AERO Servers and Relays as a trust basis.
Although the Client is responsible for initiating route optimization, the Server is the policy enforcement point that determines whether route optimization is permitted. For example, on some AERO links route optimization would allow traffic to circumvent critical network-based traffic inspection points. In those cases, the Server can simply discard any route optimization messages instead of forwarding them.
The following sections specify the AERO route optimization procedure.
Figure 4 depicts the AERO route optimization reference operational scenario, using IPv6 addressing as the example (while not shown, a corresponding example for IPv4 addressing can be easily constructed). The figure shows an AERO Relay ('R1'), two AERO Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1', 'H2'):
+--------------+ +--------------+ +--------------+
| Server S1 | | Relay R1 | | Server S2 |
+--------------+ +--------------+ +--------------+
fe80::2 fe80::1 fe80::3
L2(S1) L2(R1) L2(S2)
| | |
X-----+-----+------------------+-----------------+----+----X
| AERO Link |
L2(C1) L2(C2)
fe80::2001:db8:0:0 fe80::2001:db8:1:0
+--------------+ +--------------+
|AERO Client C1| |AERO Client C2|
+--------------+ +--------------+
2001:DB8:0::/48 2001:DB8:1::/48
| |
.-. .-.
,-( _)-. 2001:db8:0::1 2001:db8:1::1 ,-( _)-.
.-(_ IP )-. +-------+ +-------+ .-(_ IP )-.
(__ EUN )--|Host H1| |Host H2|--(__ EUN )
`-(______)-' +-------+ +-------+ `-(______)-'
Figure 4: AERO Reference Operational Scenario
In Figure 4, Relay ('R1') assigns the administratively-provisioned AERO address fe80::1 to its AERO interface with link-layer address L2(R1), Server ('S1') assigns the address fe80::2 with link-layer address L2(S1), and Server ('S2') assigns the address fe80::3 with link-layer address L2(S2). Servers ('S1') and ('S2') next arrange to add their link-layer addresses to a published list of valid Servers for the AERO link.
AERO Client ('C1') receives the ACP 2001:db8:0::/48 in an ND/PD exchange via AERO Server ('S1') then assigns the address fe80::2001:db8:0:0 to its AERO interface with link-layer address L2(C1). Client ('C1') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80::2 and link-layer address L2(S1), then sub-delegates the ACP to its attached EUNs. IPv6 host ('H1') connects to the EUN, and configures the address 2001:db8:0::1.
AERO Client ('C2') receives the ACP 2001:db8:1::/48 in an ND/PD exchange via AERO Server ('S2') then assigns the address fe80::2001:db8:1:0 to its AERO interface with link-layer address L2(C2). Client ('C2') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80::3 and link-layer address L2(S2), then sub-delegates the ACP to its attached EUNs. IPv6 host ('H2') connects to the EUN, and configures the address 2001:db8:1::1.
Again, with reference to Figure 4, when source host ('H1') sends a packet to destination host ('H2'), the packet is first forwarded over the source host's attached EUN to Client ('C1'). Client ('C1') then forwards the packet via its AERO interface to Server ('S1') and also sends an NS message toward Client ('C2') via Server ('S1').
Server ('S1') then forwards both the packet and the NS message out the same AERO interface toward Client ('C2') via Relay ('R1'). When Relay ('R1') receives the packet and NS message, it consults its forwarding table to discover Server ('S2') as the next hop toward Client ('C2'). Relay ('R1') then forwards both the packet and the NS message to Server ('S2'), which then forwards them to Client ('C2').
After Client ('C2') receives the NS message, it process the message and creates or updates a dynamic neighbor cache entry for Client ('C1'), then sends the NA response to the link-layer address of Client ('C1').
After Client ('C1') receives the NA message, it processes the message and creates or updates a dynamic neighbor cache entry for Client ('C2'). Thereafter, forwarding of packets from Client ('C1') to Client ('C2') without involving any intermediate nodes is enabled. The mechanisms that support this exchange are specified in the following sections.
When a Client forwards a packet with a source address from one of its ACPs toward a destination address covered by an ASP (i.e., toward another AERO Client connected to the same AERO link), the source Client MAY send an NS message forward toward the destination Client via the Server.
In the reference operational scenario, when Client ('C1') forwards a packet toward Client ('C2'), it MAY also send an NS message forward toward Client ('C2'), subject to rate limiting (see Section 8.2 of [RFC4861]). Client ('C1') prepares the NS message as follows:
Note that the act of sending NS messages is cited as "MAY", since Client ('C1') may have advanced knowledge that the direct path to Client ('C2') would be unusable or otherwise undesirable. If the direct path later becomes unusable after the initial route optimization, Client ('C1') simply allows packets to again flow through Server ('S1').
When Server ('S1') receives an NS message from Client ('C1'), it first verifies that the SLLAOs in the NS are a proper subset of the link-layer addresses in Client ('C1')'s neighbor cache entry. If the Client's SLLAOs are not acceptable, Server ('S1') discards the message.
Server ('S1') then examines the network-layer destination address of the NS to determine the next hop toward Client ('C2') by searching for the AERO address in the neighbor cache. Since Client ('C2') is not one of its neighbors, Server ('S1') then inserts an additional layer of encapsulation between the outer IP header and the NS message. This mid-layer IP header uses the AERO Server Subnet Router Anycast address as the source address and the Subnet Router Anycast address corresponding to Client ("C2")'s AERO address as the destination address (in this case, C2's Subnet Router Anycast address is 2001:db8:1:0::). The Server then forwards this double-encapsulated NS message to Relay ('R1') by changing the link-layer source address of the message to 'L2(S1)' and changing the link-layer destination address to 'L2(R1)'. Server ('S1') finally forwards the message to Relay ('R1') without decrementing the network-layer TTL/Hop Limit field.
When Relay ('R1') receives the double-encapsulated NS message from Server ('S1') it discards the outer IP header and determines that Server ('S2') is the next hop toward Client ('C2') by consulting its standard IP forwarding table for the Client Subnet Router Anycast destination address. Relay ('R1') then encapsulates and forwards the message to Server ('S2') the same as for any IP router.
When Server ('S2') receives the double-encapsulated NS message from Relay ('R1') it removes the mid-layer IP header and determines that Client ('C2') is a neighbor on a native underlying interface by consulting its neighbor cache for Client ('C2')'s AERO address. Server ('S2') then re-encapsulates the NS while changing the link-layer source address to 'L2(S2)' and changing the link-layer destination address to 'L2(C2)'. Server ('S2') then forwards the message to Client ('C2').
When Client ('C2') receives the NS message, it accepts the NS only if the message has a link-layer source address of one of its Servers (e.g., L2(S2)). Client ('C2') further accepts the message only if it is willing to serve as a route optimization target.
In the reference operational scenario, when Client ('C2') receives a valid NS message, it either creates or updates a dynamic neighbor cache entry that stores the source address of the message as the network-layer address of Client ('C1') and stores the link-layer addresses found in the SLLAOs as the link-layer addresses of Client ('C1'). Client ('C2') then sets ReportTime for the neighbor cache entry to REPORT_TIME.
After processing the message, Client ('C2') prepares an NA message response as follows:
Client ('C2') then sends the NA message to Client ('C1').
When Client ('C1') receives the NA message, it first verifies that the NA matches the original NS message. Client ('C1') then processes the message as follows.
In the reference operational scenario, when Client ('C1') receives the NA message, it either creates or updates a dynamic neighbor cache entry that stores the source address of the message as the network-layer address of Client ('C2'), stores the link-layer addresses found in the TLLAOs as the link-layer addresses of Client ('C2') and stores the ACPs encoded in the RIOs of the NA as the ACPs for Client ('C2'). Client ('C1') then sets ForwardTime for the neighbor cache entry to FORWARD_TIME.
Now, Client ('C1') has a neighbor cache entry with a valid ForwardTime value, while Client ('C2') has a neighbor cache entry with a valid ReportTime value. Thereafter, Client ('C1') may forward ordinary network-layer data packets directly to Client ('C2') without involving any intermediate nodes, and Client ('C2') can dynamically report any changes in link-layer information by sending unsolicited NA messages. (In order for Client ('C2') to forward packets to Client ('C1'), a corresponding NS/NA message exchange is required in the reverse direction; hence, the mechanism is asymmetric.)
The source Client itself may initiate route optimization if it has only native interfaces. If the source Client has Direct, NATed, Proxyed or VPNed interfaces, route optimization must instead be initiated by the source Server. The source Server MUST include an SLLAO with a "Report-To" address in the route optimization NS messages it sends. The "Report-To" address must be one of the source Server's globally routable IP addresses.
In the same way, the target Client may serve as a route optimization target if it has only native interfaces. If some or all of the target Client's underlying interfaces are Direct, NATed, Proxyed or VPNed the target Server must instead serve as the route optimization target. In that case, when the source sends an NS message the target Server prepares an NA response the same as if it were the target Client (see: Section 3.15.5) and does not forward the NS.
When the target Server sends an NA response to a route optimization NS, it includes a Timestamp option, any necessary security options, and TLLAOs corresponding to the target Client's underlying interfaces. The target Server writes the link-layer address of the Client in TLLAOs corresponding to native underlying interfaces, writes the link-layer address of the Proxy in TLLAOs corresponding to Proxyed underlying interfaces and writes its own link-layer address in TLLAOs corresponding to other interfaces. The Interface ID and QoS Preference values in the TLLAOs are those supplied by the target Client during ND exchanges with the target Server. The target Server then establishes a dynamic neighbor cache entry for the source with ReportTime set to REPORT_TIME seconds and with a "Report-To" address set to the address of the source.
When the source Server receives the NA response, it creates or updates a dynamic neighbor cache entry for the target with ForwardTime set to FORWARD_TIME seconds and with the information provided in the TLLAOs as the link-layer addresses and preference values for the target. The source Server then translates the solicited NA message into an unsolicited NA message by changing the source address to its own link-local address, changing the destination address to all-nodes multicast, recalculating checksums and any security options, and including the Timestamp option as it appeared in the original solicited NA. The source Server then retains this message for subsequent on-demand transmission to any source neighbors that send packets to the target within the current ForwardTime window.
While ForwardTime is greater than 0, the source Server sends unsolicited NA messages (subject to rate limiting) in response to data packets from source Clients or Proxies that are destined to the target Client. The unsolicited NA messages update source Client and Proxy dynamic neighbor cache entries with ForwardTime set to FORWARD_TIME minus the difference between the current time and the NA Timestamp. Subsequent packets from the source destined to the target Client then travel via the route-optimized path instead of through the dogleg path through Servers and Relays.
Following route optimization, when the target Client (or Proxy) sends unsolicited NA messages to the target Server to update link-layer addresses and/or QoS preferences, the target Server translates the messages the same as described above and repeats them to any of its neighbors with non-zero ReportTime. The source Server in turn translates the messages and repeats them to any of their source Clients or Proxys to which they recently sent NAs.
If the target Client moves to a new Server, the old Server sends immediate unsolicited NA messages with no TLLAOs to any of its dynamic neighbors with non-zero ReportTime, and retains the dynamic neighbor cache entry until ReportTime expires. While ReportTime is non-zero, the old Server sends unsolicited NA messages with no TLLAOs (subject to rate limiting) back to the source in response to data packets received from a correspondent node while forwarding the packets themselves to a Relay. The Relay will then either forward the packets to the new Server if the target Client has moved, or drop the packets if the target Client is no longer in the network. When the source receives the unsolicited NAs with no TLLAOs, it allows future packets destined to the target Client to again flow through its own Server (or Relay).
AERO nodes perform Neighbor Unreachability Detection (NUD) by sending NS messages to elicit solicited NA messages from neighbors the same as described in [RFC4861]. NUD is performed either reactively in response to persistent link-layer errors (see Section 3.13) or proactively to update neighbor cache entry timers and/or link-layer address information. (NS messages may include SLLAOs and NA messages may include TLLAOs in order to update link-layer address information.)
When an AERO node sends an NS/NA message, it uses one of its link-local addresses as the IPv6 source address and a link-local address of the neighbor as the IPv6 destination address. When route optimization directs a source AERO node to a target AERO node, the source node SHOULD proactively test the direct path 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.
While data packets are still flowing, the source node thereafter periodically tests the direct path to the target node (see Section 7.3 of [RFC4861]) in order to keep dynamic neighbor cache entries alive. When the target node receives a valid NS message, it resets ReportTime to REPORT_TIME and updates its cached link-layer addresses (if necessary). When the source node receives a corresponding NA message, it resets ForwardTime to FORWARD_TIME and updates its cached link-layer addresses (if necessary). If the source node is unable to elicit an NA response from the target node after MaxRetry attempts, it SHOULD set ForwardTime to 0. Otherwise, the source node considers the path usable and SHOULD thereafter process any link-layer errors as an indication that the direct path to the target node may be failing.
When ForwardTime for a dynamic neighbor cache entry expires, the source node resumes sending any subsequent packets via a Server (or Relay) and may (eventually) attempt to re-initiate the AERO route optimization process. When ReportTime for a dynamic neighbor cache entry expires, the target node ceases to send dynamic mobility and QoS updates to the source node. When both ForwardTime and ReportTime for a dynamic neighbor cache entry expire, the node deletes the neighbor cache entry.
Note that an AERO node may have multiple underlying interface paths toward the target neighbor. In that case, the node SHOULD perform NUD over each underlying interface individually and only consider the neighbor unreachable if NUD fails over multiple underlying interface paths.
AERO is an example of a Distributed Mobility Management (DMM) service. Each AERO 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. AERO Clients coordinate with their associated AERO Servers via RS/RA exchanges to maintain the DMM profile, and the AERO routing system tracks the current AERO Client/Server peering relationships.
AERO interfaces accommodate mobility management by sending unsolicited NA messages the same as for announcing link-layer address changes for any interface that implements IPv6 ND [RFC4861]. (In environments where reliability is a concern, AERO interfaces can send immediate NS messages to receive solicited NA messages, i.e., they can skip the unreliable unsolicited NA messaging step and move directly to a reliable NS/NA exchange.)
When a node sends an unsolicited NA message, it sets the IPv6 source to its own link-local address, sets the IPv6 destination address to all-nodes multicast, sets the link-layer source address to its own address and sets the link-layer destination address to either a multicast address or the unicast link-layer address of a neighbor. In the latter case, if the unsolicited NA message must be received by multiple neighbors, the node sends multiple copies of the NA using a different unicast link-layer destination address for each neighbor. Mobility management considerations are specified in the following sections.
When a Server receives packets with destination addresses that do not match one of its static neighbor cache Clients, it forwards the packets to a Relay which delivers them to the target Client's current location. If the source is not one of its static neighbor Clients, the Server also returns an unsolicited NA message to the sender with no TLLAOs - the sender will then realize that it needs to delete its neighbor cache entry that associated the target with this Server.
When a Client needs to change its link-layer addresses, e.g., due to a mobility event, it sends unsolicited NAs to its neighbors using the new link-layer address as the source address and with TLLAOs that include the new Client UDP Port Number, IP Address and P(i) values. (For neighbors that are Servers, the Client can instead initiate an RS/RA exchange.) If the Client sends the NA solely for the purpose of updating QoS preferences without updating the link-layer address, the Client sets the UDP Port Number and IP Address to 0.
The Client MAY send up to MaxRetry unsolicited NA messages in parallel with sending actual data packets in case one or more NAs are lost. If all NAs are lost, the neighbor will eventually invoke NUD by sending NS messages that include SLLAOs.
When a Client needs to bring new underlying interfaces into service (e.g., when it activates a new data link), it sends unsolicited NAs to its neighbors using the new link-layer address as the source address and with TLLAOs that include the new Client link-layer information. (For neighbors that are Servers, the Client can instead initiate an RS/RA exchange.)
When a Client needs to remove existing underlying interfaces from service (e.g., when it de-activates an existing data link), it sends unsolicited NAs to its neighbors with TLLAOs with all P(i) values set to 0. (For neighbors that are Servers, the Client can instead initiate an RS/RA exchange.)
If the Client needs to send ND messages over an underlying interface other than the one being removed from service, it MUST include a current TLLAO for the sending interface as the first TLLAO and include TLLAOs for any underlying interface being removed from service as additional TLLAOs.
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 3.14.2. The Client then sends RS messages with PD release parameters to the old Server to release itself from that Server's domain. If the Client does not receive an RA reply after MaxRetry attempts, the old Server may have failed and the Client should discontinue its release attempts.
Clients SHOULD NOT move rapidly between Servers in order to avoid causing excessive oscillations in the AERO routing system. Such oscillations could result in intermittent reachability for the Client itself, while causing no harm to the network. 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, 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.
In some environments, AERO Clients may be located in secured subnetwork 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 AERO Proxy is located at the secured enclave perimeter and listens for encapsulated RS messages originating from or RA messages destined to AERO Clients located within the enclave. The Proxy acts on these control messages as follows:
After the initial RS/RA exchange, the Proxy forwards data packets between the Client and Server with the Server acting as the Client's default router. The Proxy can send ND messages to the Client's Server(s) to update Server neighbor cache entries on behalf of the Client. (For example, the Proxy can send unsolicited NA messages with a TLLAO with UDP Port Number and IP Address set to 0 and with valid P(i) values to update the Server(s) with the Client's new QoS preferences for the path that traverses the Proxy). The Proxy also forwards any control and data messages originating from the Client to the Client's primary Server.
At some time after data packets have been flowing from the Client to the Server, the Proxy may receive unsolicited NA messages from the Server with TLLAOs corresponding to a target Client. The Proxy establishes a dynamic neighbor cache entry for the target with ForwardTime set to FORWARD_TIME and allows future data packets destined to the target to flow directly according to the link-layer address information instead of through the Server. The Proxy may at some later point receive additional NA messages with TLLAOs, and if so resets ForwardTime and updates its cached link-layer address information. If the Proxy receives no further NA messages, or if it receives NA messages with no TLLAOs, it deletes the dynamic neighbor cache entry.
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.
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 AERO Proxies, Servers and Relays in the communications path. Direct underlying interfaces must be tested periodically for reachability, e.g., via NUD, via periodic unsolicited NAs, etc.
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 3.15.
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.
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 [RFC5246], DTLS [RFC6347] and SSH [RFC4251] to assure the same level of protection as for critical secured Internet services such as online banking. AERO Clients that require VPN access to enterprise networks SHOULD use symmetric network and/or transport layer security services such as TLS/SSL, DTLS, IPsec [RFC4301], etc.
Control plane security considerations are the same as for standard IPv6 Neighbor Discovery [RFC4861], except that the PRL also provides AERO Clients with a list of trusted Servers. As fixed infrastructure elements, AERO Proxys and Servers SHOULD pre-configure security associations for one another (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. 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 7).
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 Proxys can institute rate limits that protect Clients from receiving packet floods that could DoS low data rate links.
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.)
Although public domain and commercial SEND implementations exist, concerns regarding the strength of the cryptographic hash algorithm have been documented [RFC6273] [RFC4982].
The PRL MUST be well-managed and secured from unauthorized tampering, even though the list includes only public information.
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.
| [RFC0768] | Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980. |
| [RFC0791] | Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981. |
| [RFC0792] | Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, DOI 10.17487/RFC0792, September 1981. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC2474] | Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998. |
| [RFC3971] | Arkko, J., Kempf, J., Zill, B. and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, DOI 10.17487/RFC3971, March 2005. |
| [RFC3972] | Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, DOI 10.17487/RFC3972, March 2005. |
| [RFC4191] | Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, November 2005. |
| [RFC4193] | Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005. |
| [RFC4861] | Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007. |
| [RFC4862] | Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007. |
| [RFC5175] | Haberman, B. and R. Hinden, "IPv6 Router Advertisement Flags Option", RFC 5175, DOI 10.17487/RFC5175, March 2008. |
| [RFC8200] | Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017. |
| [RFC8415] | Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T. and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018. |
| [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-12, January 2019. |
| [I-D.ietf-intarea-gue] | Herbert, T., Yong, L. and O. Zia, "Generic UDP Encapsulation", Internet-Draft draft-ietf-intarea-gue-06, August 2018. |
| [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-05, August 2018. |
| [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-aeroaddr] | Templin, F., "The AERO Address", Internet-Draft draft-templin-6man-aeroaddr-04, December 2018. |
| [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-6man-rio-redirect] | Templin, F. and j. woodyatt, "Route Information Options in IPv6 Neighbor Discovery", Internet-Draft draft-templin-6man-rio-redirect-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. |
| [RFC1981] | McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August 1996. |
| [RFC2003] | Perkins, C., "IP Encapsulation within IP", RFC 2003, DOI 10.17487/RFC2003, October 1996. |
| [RFC2131] | Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997. |
| [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. |
| [RFC5246] | Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 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. |
| [RFC5720] | Templin, F., "Routing and Addressing in Networks with Global Enterprise Recursion (RANGER)", RFC 5720, DOI 10.17487/RFC5720, February 2010. |
| [RFC5996] | Kaufman, C., Hoffman, P., Nir, Y. and P. Eronen, "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 5996, DOI 10.17487/RFC5996, September 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. |
| [RFC6422] | Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options", RFC 6422, DOI 10.17487/RFC6422, December 2011. |
| [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. |
| [TUNTAP] | Wikipedia, W., "http://en.wikipedia.org/wiki/TUN/TAP", October 2014. |
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 and SSL/TLS. 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].
An encapsulation fragment header is inserted when the AERO tunnel ingress needs to apply fragmentation to accommodate packets that must be delivered without loss due to a size restriction. Fragmentation is performed on the inner packet while encapsulating each inner packet fragment in outer IP and encapsulation layer headers that differ only in the fragment header fields.
The fragment header can also be inserted in order to include a coherent Identification value with each packet, e.g., to aid in Duplicate Packet Detection (DPD). In this way, network nodes can cache the Identification values of recently-seen packets and use the cached values to determine whether a newly-arrived packet is in fact a duplicate. The Identification value within each packet could further provide a rough indicator of packet reordering, e.g., in cases when the tunnel egress wishes to discard packets that are grossly out of order.
In some use cases, there may be operational assurance that no fragmentation of any kind will be necessary, or that only occasional large control messages will require fragmentation. In that case, the encapsulation fragment header can be omitted and ordinary fragmentation of the outer IP protocol version can be applied when necessary.
On some platforms (e.g., popular cell phone operating systems), the act of assigning a default IPv6 route and/or assigning an address to an interface may not be permitted from a user application due to security policy. Typically, those platforms include a TUN/TAP interface [TUNTAP] that acts as a point-to-point conduit between user applications and the AERO interface. In that case, the Client can instead generate a "synthesized RA" message. The message conforms to [RFC4861] and is prepared as follows:
The Client then sends the synthesized RA message via the TUN/TAP interface, where the operating system kernel will interpret it as though it were generated by an actual router. The operating system will then install a default route and use StateLess Address AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP interface. Methods for similarly installing an IPv4 default route and IPv4 address on the TUN/TAP interface are based on synthesized DHCPv4 messages
AERO can be used in many different variations based on the specific use case. The following sections discuss variations that adhere to the AERO principles while allowing selective application of AERO components.
When Servers on the AERO link do not provide DHCPv6 services, operation can still be accommodated through administrative configuration of ACPs on AERO Clients. In that case, administrative configurations of AERO interface neighbor cache entries on both the Server and Client are also necessary. However, this may interfere with the ability for Clients to dynamically change to new Servers, and can expose the AERO link to misconfigurations unless the administrative configurations are carefully coordinated.
In some AERO link scenarios, there may be no Servers on the link and/or no need for Clients to use a Server as an intermediary trust anchor. In that case, each Client acts as a Server unto itself to establish neighbor cache entries by performing direct Client-to-Client IPv6 ND message exchanges, and some other form of trust basis must be applied so that each Client can verify that the prospective neighbor is authorized to use its claimed ACP.
When there is no Server on the link, Clients must arrange to receive ACPs and publish them via a secure alternate PD authority through some means outside the scope of this document.
In some environments, the AERO service may be useful for mobile nodes that do not implement the AERO Client function and do not perform encapsulation. For example, if the mobile node has a way of injecting its ACP into the access subnetwork routing system an AERO Server connected to the same access network can accept the ACP prefix injection as an indication that a new mobile node has come onto the subnetwork. The Server can then inject the ACP into the BGP routing system the same as if an AERO Client/Server PD exchange had occurred. If the mobile node subsequently withdraws the ACP from the access network routing system, the Server can then withdraw the ACP from the BGP routing system.
In this arrangement, AERO Servers and Relays are used in exactly the same ways as for environments where DHCPv6 Client/Server exchanges are supported. However, the access subnetwork routing systems must be capable of accommodating rapid ACP injections and withdrawals from mobile nodes with the understanding that the information must be propagated to all routers in the system. Operational experience has shown that this kind of routing system "churn" can lead to overall instability and routing system inconsistency.
In addition to the dynamic neighbor discovery procedures for AERO link neighbors described above, AERO encapsulation can be applied to manually-configured tunnels. In that case, the tunnel endpoints use an administratively-provisioned AERO address and exchange NS/NA messages the same as for dynamically-established tunnels.
In some environments, AERO Servers and Relays may be connected by dedicated point-to-point links, e.g., high speed fiberoptic leased lines. In that case, the Servers and Relays can participate in the AERO link the same as specified above but can avoid encapsulation over the dedicated links. In that case, however, the links would be dedicated for AERO and could not be multiplexed for both AERO and non-AERO communications.
A source Client may connect only to an IPvX underlying network, while the target Client connects only to an IPvY underlying network. In that case, the target and source Clients have no means for reaching each other directly (since they connect to underlying networks of different IP protocol versions) and so must ignore any route optimization messages and continue to send packets via their Servers.
When an enterprise mobile node moves from a campus LAN connection to a public Internet link, it must re-enter the enterprise via a security gateway that has both a physical interface connection to the Internet and a physical interface connection to the enterprise internetwork. This most often entails the establishment of a Virtual Private Network (VPN) link over the public Internet from the mobile node to the security gateway. During this process, the mobile node supplies the security gateway with its public Internet address as the link-layer address for the VPN. The mobile node then acts as an AERO Client to negotiate with the security gateway to obtain its ACP.
In order to satisfy this need, the security gateway also operates as an AERO Server with support for AERO Client proxying. In particular, when a mobile node (i.e., the Client) connects via the security gateway (i.e., the Server), the Server provides the Client with an ACP in a PD exchange the same as if it were attached to an enterprise campus access link. The Server then replaces the Client's link-layer source address with the Server's enterprise-facing link-layer address in all AERO messages the Client sends toward neighbors on the AERO link. The AERO messages are then delivered to other nodes on the AERO link as if they were originated by the security gateway instead of by the AERO Client. In the reverse direction, the AERO messages sourced by nodes within the enterprise network can be forwarded to the security gateway, which then replaces the link-layer destination address with the Client's link-layer address and replaces the link-layer source address with its own (Internet-facing) link-layer address.
After receiving the ACP, the Client can send IP packets that use an address taken from the ACP as the network layer source address, the Client's link-layer address as the link-layer source address, and the Server's Internet-facing link-layer address as the link-layer destination address. The Server will then rewrite the link-layer source address with the Server's own enterprise-facing link-layer address and rewrite the link-layer destination address with the target AERO node's link-layer address, and the packets will enter the enterprise network as though they were sourced from a node located within the enterprise. In the reverse direction, when a packet sourced by a node within the enterprise network uses a destination address from the Client's ACP, the packet will be delivered to the security gateway which then rewrites the link-layer destination address to the Client's link-layer address and rewrites the link-layer source address to the Server's Internet-facing link-layer address. The Server then delivers the packet across the VPN to the AERO Client. In this way, the AERO virtual link is essentially extended *through* the security gateway to the point at which the VPN link and AERO link are effectively grafted together by the link-layer address rewriting performed by the security gateway. All AERO messaging services (including route optimization and mobility signaling) are therefore extended to the Client.
In order to support this virtual link grafting, the security gateway (acting as an AERO Server) must keep static neighbor cache entries for all of its associated Clients located on the public Internet. The neighbor cache entry is keyed by the AERO Client's AERO address the same as if the Client were located within the enterprise internetwork. The neighbor cache is then managed in all ways as though the Client were an ordinary AERO Client. This includes the AERO IPv6 ND messaging signaling for Route Optimization and Neighbor Unreachability Detection.
Note that the main difference between a security gateway acting as an AERO Server and an enterprise-internal AERO Server is that the security gateway has at least one enterprise-internal physical interface and at least one public Internet physical interface. Conversely, the enterprise-internal AERO Server has only enterprise-internal physical interfaces. For this reason security gateway proxying is needed to ensure that the public Internet link-layer addressing space is kept separate from the enterprise-internal link-layer addressing space. This is afforded through a natural extension of the security association caching already performed for each VPN client by the security gateway.
The AERO S/TLLAO format specified in Section 3.5 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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID | UDP Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IP 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-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: