Network Working Group F. Templin, Ed. Internet-Draft Boeing Research & Technology Obsoletes: rfc6706 (if approved) May 29, 2014 Intended status: Standards Track Expires: November 30, 2014 Transmission of IPv6 Packets over AERO Links draft-templin-aerolink-22.txt Abstract This document specifies the operation of IPv6 over tunnel virtual Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended Route Optimization (AERO). Nodes attached to AERO links can exchange packets via trusted intermediate routers on the link that provide forwarding services to reach off-link destinations and/or redirection services to inform the node of an on-link neighbor that is closer to the final destination. Operation of the IPv6 Neighbor Discovery (ND) protocol over AERO links is based on an IPv6 link local address format known as the AERO address. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 30, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Templin Expires November 30, 2014 [Page 1] Internet-Draft AERO May 2014 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5 3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 5 3.2. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 6 3.3. AERO Interface Characteristics . . . . . . . . . . . . . 6 3.3.1. Coordination of Multiple Underlying Interfaces . . . 8 3.4. AERO Interface Neighbor Cache Maintenace . . . . . . . . 9 3.5. AERO Interface Data Origin Authentication . . . . . . . . 10 3.6. AERO Interface MTU Considerations . . . . . . . . . . . . 11 3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation . . . . . . . . . . . . . . . . . . . . . . 12 3.8. AERO Router Discovery, Prefix Delegation and Address Configuration . . . . . . . . . . . . . . . . . . . . . . 14 3.8.1. AERO Client Behavior . . . . . . . . . . . . . . . . 14 3.8.2. AERO Server Behavior . . . . . . . . . . . . . . . . 15 3.9. AERO Redirection . . . . . . . . . . . . . . . . . . . . 16 3.9.1. Reference Operational Scenario . . . . . . . . . . . 16 3.9.2. Classical Redirection Approaches . . . . . . . . . . 18 3.9.3. Concept of Operations . . . . . . . . . . . . . . . . 19 3.9.4. Message Format . . . . . . . . . . . . . . . . . . . 19 3.9.5. Sending Predirects . . . . . . . . . . . . . . . . . 20 3.9.6. Processing Predirects and Sending Redirects . . . . . 21 3.9.7. Re-encapsulating and Relaying Redirects . . . . . . . 22 3.9.8. Processing Redirects . . . . . . . . . . . . . . . . 23 3.10. Neighbor Reachability Maintenance . . . . . . . . . . . . 24 3.11. Mobility and Link-Layer Address Change Considerations . . 25 3.12. Encapsulation Protocol Version Considerations . . . . . . 26 3.13. Multicast Considerations . . . . . . . . . . . . . . . . 26 3.14. Operation on AERO Links Without DHCPv6 Services . . . . . 26 3.15. Operation on Server-less AERO Links . . . . . . . . . . . 26 3.16. Other Considerations . . . . . . . . . . . . . . . . . . 27 4. Implementation Status . . . . . . . . . . . . . . . . . . . . 27 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 6. Security Considerations . . . . . . . . . . . . . . . . . . . 27 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 8.1. Normative References . . . . . . . . . . . . . . . . . . 28 8.2. Informative References . . . . . . . . . . . . . . . . . 29 Appendix A. AERO Server and Relay Interworking . . . . . . . . . 31 Templin Expires November 30, 2014 [Page 2] Internet-Draft AERO May 2014 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 33 1. Introduction This document specifies the operation of IPv6 over tunnel virtual Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended Route Optimization (AERO). Nodes attached to AERO links can exchange packets via trusted intermediate routers on the link that provide forwarding services to reach off-link destinations and/or redirection services to inform the node of an on-link neighbor that is closer to the final destination. This redirection provides a route optimization capability that addresses the requirements outlined in [RFC5522]. Nodes on AERO links use an IPv6 link-local address format known as the AERO Address. This address type has properties that avoid duplication and statelessly link IPv6 Neighbor Discovery (ND) to IPv6 routing. The AERO link can be used for tunneling to neighboring nodes on either IPv6 or IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as equivalent links for tunneling. The remainder of this document presents the AERO specification. 2. Terminology The terminology in the normative references applies; the following terms are defined within the scope of this document: AERO link a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay configured over a node's attached IPv6 and/or IPv4 networks. All nodes on the AERO link appear as single-hop neighbors from the perspective of IPv6. AERO interface a node's attachment to an AERO link. AERO address an IPv6 link-local address assigned to an AERO interface and constructed as specified in Section 3.2. AERO node a node that is connected to an AERO link and that participates in IPv6 Neighbor Discovery over the link. AERO Client ("client") a node that configures either a host interface or a router interface on an AERO link. Templin Expires November 30, 2014 [Page 3] Internet-Draft AERO May 2014 AERO Server ("server") a node that configures a router interface on an AERO link over which it can provide default forwarding and redirection services for other AERO nodes. AERO Relay ("relay") a node that relays IPv6 packets between Servers on the same AERO link, and/or that forwards IPv6 packets between the AERO link and the IPv6 Internet. An AERO Relay may or may not also be configured as an AERO Server. ingress tunnel endpoint (ITE) an AERO interface endpoint that injects tunneled packets into an AERO link. egress tunnel endpoint (ETE) an AERO interface endpoint that receives tunneled packets from an AERO link. underlying network a connected IPv6 or IPv4 network routing region over which AERO nodes tunnel IPv6 packets. underlying interface an AERO node's interface point of attachment to an underlying network. link-layer address an IP address assigned to an AERO node's underlying interface. When UDP encapsulation is used, the UDP port number is also considered as part of the link-layer address. Link-layer addresses are used as the encapsulation header source and destination addresses. network layer address the source or destination address of the encapsulated IPv6 packet. end user network (EUN) an IPv6 network attached to a downstream interface of an AERO Client (where the AERO interface is seen as the upstream interface). Throughout the document, the simple terms "Client", "Server" and "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay", respectively. Capitalization is used to distinguish these terms from DHCPv6 client/server/relay. This is an important distinction, since an AERO Server may be a DHCPv6 relay, and an AERO Relay may be a DHCPv6 server. Templin Expires November 30, 2014 [Page 4] Internet-Draft AERO May 2014 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Asymmetric Extended Route Optimization (AERO) The following sections specify the operation of IPv6 over Asymmetric Extended Route Optimization (AERO) links: 3.1. AERO Node Types AERO Relays relay packets between nodes connected to the same AERO link and also forward packets between the AERO link and the native IPv6 network. The relaying process entails re-encapsulation of IPv6 packets that were received from a first AERO node and are to be forwarded without modification to a second AERO node. AERO Servers configure their AERO interfaces as router interfaces, and provide default routing services to AERO Clients. AERO Servers configure a DHCPv6 relay or server function and facilitate DHCPv6 Prefix Delegation (PD) exchanges. An AERO Server may also act as an AERO Relay. AERO Clients act as requesting routers to receive IPv6 prefixes through a DHCPv6 PD exchange via AERO Servers over the AERO link. (Clients typically associate with a single Server at a time; Clients MAY associate with multiple Servers, but associating with many Servers may result in excessive control message overhead.) Each AERO Client receives at least a /64 prefix delegation, and may receive even shorter prefixes. AERO Clients that act as routers configure their AERO interfaces as router interfaces and sub-delegate portions of their received prefix delegations to links on EUNs. End system applications on AERO Clients that act as routers bind to EUN interfaces (i.e., and not the AERO interface). AERO Clients that act as ordinary hosts configure their AERO interfaces as host interfaces and assign one or more IPv6 addresses taken from their received prefix delegations to the AERO interface but DO NOT assign the delegated prefix itself to the AERO interface. Instead, the host assigns the delegated prefix to a "black hole" route so that unused portions of the prefix are nullified. End system applications on AERO Clients that act as hosts bind directly to the AERO interface. Templin Expires November 30, 2014 [Page 5] Internet-Draft AERO May 2014 3.2. AERO Addresses An AERO address is an IPv6 link-local address assigned to an AERO interface and with an IPv6 prefix embedded within the interface identifier. The AERO address is formatted as: fe80::[IPv6 prefix] Each AERO Server configures the AERO address 'fe80::'; this corresponds to the IPv6 prefix '::/0' (i.e., "default") and provides a handle for Clients to insert into a neighbor cache entry. Each AERO Client configures an AERO address based on the prefix it has received from the AERO link prefix delegation authority (e.g., the DHCPv6 server). The address begins with the prefix fe80::/64 and includes in its interface identifier the base /64 prefix taken from the Client's delegated IPv6 prefix. The base prefix is determined by masking the delegated prefix with the prefix length. For example, if an AERO Client has received the prefix delegation: 2001:db8:1000:2000::/56 it would construct its AERO address as: fe80::2001:db8:1000:2000 The AERO address remains stable as the Client moves between topological locations, i.e., even if its underlying address changes. 3.3. AERO Interface Characteristics AERO interfaces use IPv6-in-IPv6 encapsulation [RFC2473] to exchange tunneled packets with AERO neighbors attached to an underlying IPv6 network, and use IPv6-in-IPv4 encapsulation [RFC4213] to exchange tunneled packets with AERO neighbors attached to an underlying IPv4 network. AERO interfaces can also operate over secured tunnel types such as IPsec [RFC4301] or TLS [RFC5246] in environments where strong authentication and confidentiality are required. When Network Address Translator (NAT) traversal and/or filtering middlebox traversal may be necessary, a UDP header is further inserted immediately above the IP encapsulation header. Servers assign the AERO address fe80:: to their AERO interfaces. Servers and Relays also use (non-AERO) administratively-assigned link-local addresses to support the operation of the inter-Server/ Relay routing system (see: [IRON]). Templin Expires November 30, 2014 [Page 6] Internet-Draft AERO May 2014 Clients initially use a temporary IPv6 link-local address in the DHCPv6 PD exchanges used to receive an IPv6 prefix and derive an AERO address. If the Client is pre-provisioned with an IPv6 prefix associated with the AERO service, it SHOULD use the AERO address derived from the prefix as the temporary address. Otherwise, the Client can use any randomly-selected link-local address as the temporary address, as it is merely a placeholder in DHCPv6 messages and not used to create neighbor cache entries. After the Client receives a prefix delegation, it assigns the corresponding AERO address to the AERO interface. DHCPv6 is therefore used to bootstrap the assignment of unique link-local addresses on the AERO link. AERO interfaces maintain a neighbor cache and use an adaptation of standard unicast IPv6 ND messaging. AERO interfaces use Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router Solicitation (RS) and Router Advertisement (RA) messages the same as for any IPv6 link. AERO interfaces use two redirection message types -- the first being the standard Redirect message and the second known as a Predirect message (see Section 3.9). AERO links further use link- local-only addressing; hence, Clients ignore any Prefix Information Options (PIOs) they may receive in RA messages. AERO interfaces use Source/Target Link Layer Address Options (S/ TLLAOs) the same as for any IPv6 interface. The S/TLLAO option format is shown in Figure 1: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 2 | Length = 1/3 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | Preference | UDP Port Number (or 0) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-- --+ | | +-- IP Address (Redirect/Predirect messages only) --+ | | +-- --+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: AERO S/TLLAO Format In this format, Link ID is an integer value between 0 and 255 corresponding to an underlying interface of the source/target node, and Preference is an integer value between 0 and 255 indicating the Templin Expires November 30, 2014 [Page 7] Internet-Draft AERO May 2014 node's preference for this underlying interface (with 0 being highest preference and 255 being lowest). For Redirect/Predirect messages, Length is set to 3 and UDP Port Number/IP Address are set to the addresses used by the target node when it sends encapsulated packets over the underlying interface. For other ND messages, Length is set to 1, UDP Port Number is set to 0 and IP Address is omitted. Each AERO Redirect/Predirect message includes one or more TLLAOs, i.e., one for each underlying interface the target node wishes to publish. When no UDP encapsulation is used, UDP Port Number is set to 0. When the encapsulation IP address family is IPv4, IP Address is formed as an IPv4-compatible IPv6 address [RFC4291]. AERO interface Redirect/Predirect messages can both update and create neighbor cache entries. Redirect/Predirect messages SHOULD include a Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes can use to verify the message time of origin. Each AERO NS/NA/RS/RA message includes a single S/TLLAO with UDP Port Number set to 0 and IP Address omitted (since the node has no way of knowing whether it is behind a NAT and hence may be unable to convey the correct values). Instead, AERO nodes determine the link-layer addresses of neighbors by examining the encapsulation header IP address and UDP port number of any NS/NA/RS/RA messages they receive. AERO interface NS/NA/RS/RA messages only update existing neighbor cache entires and do not create new neighbor cache entries. NS/RS messages SHOULD include a Nonce option (see Section 5.3 of [RFC3971]). If an NS/RS message contains a Nonce option, the recipient MUST echo the option back in the corresponding NA/RA response. Unsolicited NA/RA messages are not used on AERO interfaces, and SHOULD be ignored on receipt. 3.3.1. Coordination of Multiple Underlying Interfaces AERO interfaces may be configured over multiple underlying interfaces. From the perspective of IPv6 Neighbor Discovery, the AERO interface therefore appears as a single logical interface with multiple link-layer addresses the same as described for "Inbound Load Balancing" in Section 3 of [RFC4861]. The load balancing paradigm applies to AERO Servers that are connected to stable backhaul networks, but may not necessarily be appropriate for AERO Clients that connect to multiple diverse media types. For example, common handheld devices of the modern era have both wireless local area network (aka "WiFi") and cellular wireless links. These links are typically used "one at a time" with low-cost WiFi preferred and highly-available cellular wireless as a cold standby. Templin Expires November 30, 2014 [Page 8] Internet-Draft AERO May 2014 In a more complex example, aircraft frequently have many wireless data link types (e.g. satellite-based, terrestrial, directional point-to-point, etc.) with diverse performance and cost properties. If a Client's multiple underlying interfaces are used "one at a time" (i.e., all other interfaces are disabled when one interface is active), then the S/TLLAOs in ND messages SHOULD use the same Link ID and Preference values regardless of the underlying interface and Predirect/Redirect messages SHOULD include only a single TLLAO. If the Client enables multiple underlying interfaces, it instead SHOULD use a different Link ID and Preference value for the S/TLLAO corresponding to each interface and Predirect/Redirect messages MAY include multiple TLLAOs. In that case, the Client would need to send separate NS/RS messages to each of its neighbors for each active underlying interface. 3.4. AERO Interface Neighbor Cache Maintenace Each AERO interface maintains a conceptual neighbor cache that includes an entry for each neighbor it communicates with on the AERO link, the same as for any IPv6 interface (see [RFC4861]). Neighbor cache entries are created and maintained as follows: When an AERO Server relays a DHCPv6 Reply message to an AERO Client, it creates or updates a neighbor cache entry for the Client based on the AERO address corresponding to the prefix in the IA_PD option as the Client's network layer address and with the Client's encapsulation IP address and UDP port number as the link-layer address. When an AERO Client receives a DHCPv6 Reply message from an AERO Server, it creates or updates a neighbor cache entry for the Server based on fe80:: as the network layer address and the Server's encapsulation IP address and UDP port number as the link-layer address. When an AERO Client receives a valid Predirect message it creates or updates a neighbor cache entry for the Predirect target network-layer and link-layer addresses, and also creates an IPv6 forwarding table entry for the Predirected (source) prefix. The node then sets an "ACCEPT" timer for the neighbor and uses this timer to determine whether messages received from the predirected neighbor can be accepted. When an AERO Client receives a valid Redirect message it creates or updates a neighbor cache entry for the Redirect target network-layer and link-layer addresses, and also creates an IPv6 forwarding table entry for the redirected (destination) prefix. The node then sets a Templin Expires November 30, 2014 [Page 9] Internet-Draft AERO May 2014 "FORWARD" timer for the neighbor and uses this timer to determine whether packets can be sent directly to the redirected neighbor. The node also maintains a constant value MAX_RETRY to limit the number of keepalives sent when a neighbor may have gone unreachable. When an AERO Client receives a valid NS message it (re)sets the ACCEPT timer for the neighbor to ACCEPT_TIME. When an AERO Client receives a valid NA message, it (re)sets the FORWARD timer for the neighbor to FORWARD_TIME. It is RECOMMENDED that FORWARD_TIME be set to the default constant value 30 seconds to match the default REACHABLE_TIME value specified for IPv6 neighbor discovery [RFC4861]. It is RECOMMENDED that ACCEPT_TIME be set to the default constant value 40 seconds to allow a 10 second window so that the AERO redirection procedure can converge before the ACCEPT timer decrements below FORWARD_TIME. It is RECOMMENDED that MAX_RETRY be set to 3 the same as described for IPv6 neighbor discovery address resolution in Section 7.3.3 of [RFC4861]. Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be administratively set, if necessary, to better match the AERO link's performance characteristics; however, if different values are chosen, all nodes on the link MUST consistently configure the same values. In particular, ACCEPT_TIME SHOULD be set to a value that is sufficiently longer than FORWARD_TIME to allow the AERO redirection procedure to converge. 3.5. AERO Interface Data Origin Authentication AERO nodes use a simple data origin authentication for encapsulated packets they receive from other nodes. In particular, AERO nodes accept encapsulated packets with a link-layer source address belonging to one of their current AERO Servers and accept encapsulated packets with a link-layer source address that is correct for the network-layer source address. The AERO node considers the link-layer source address correct for the network-layer source address if there is an IPv6 forwarding table entry that matches the network-layer source address as well as a neighbor cache entry corresponding to the next hop that includes the link-layer address and the ACCEPT timer is non-zero. An exception is that neighbor discovery messages may include a different link-layer Templin Expires November 30, 2014 [Page 10] Internet-Draft AERO May 2014 address than the one currently in the neighbor cache for this Link ID, and the new link-layer address updates the neighbor cache entry. 3.6. AERO Interface MTU Considerations The AERO link Maximum Transmission Unit (MTU) is 64KB minus the encapsulation overhead for IPv4 [RFC0791] and 4GB minus the encapsulation overhead for IPv6 [RFC2675]. This is the most that IPv4 and IPv6 (respectively) can convey within the constraints of protocol constants, but actual sizes available for tunneling will frequently be much smaller. The base tunneling specifications for IPv4 and IPv6 typically set a static MTU on the tunnel interface to 1500 bytes minus the encapsulation overhead or smaller still if the tunnel is likely to incur additional encapsulations on the path. This can result in path MTU related black holes when packets that are too large to be accommodated over the AERO link are dropped, but the resulting ICMP Packet Too Big (PTB) messages are lost on the return path. As a result, AERO nodes use the following MTU mitigations to accommodate larger packets. AERO nodes set their AERO interface MTU to the larger of the underlying interface MTU minus the encapsulation overhead, and 1500 bytes. (If there are multiple underlying interfaces, the node sets the AERO interface MTU according to the largest underlying interface MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU cannot be determined.) AERO nodes optionally cache other per- neighbor MTU values in the underlying IP path MTU discovery cache initialized to the underlying interface MTU. AERO nodes admit packets that are no larger than 1280 bytes minus the encapsulation overhead (*) as well as packets that are larger than 1500 bytes into the tunnel without fragmentation, i.e., as long as they are no larger than the AERO interface MTU before encapsulation and also no larger than the cached per-neighbor MTU following encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit to 0 for packets no larger than 1280 bytes minus the encapsulation overhead (*) and sets the DF bit to 1 for packets larger than 1500 bytes. If a large packet is lost in the path, the node may optionally cache the MTU reported in the resulting PTB message or may ignore the message, e.g., if there is a possibility that the message is spurious. For packets destined to an AERO node that are larger than 1280 bytes minus the encapsulation overhead (*) but no larger than 1500 bytes, the node uses IP fragmentation to fragment the encapsulated packet into two pieces (where the first fragment contains 1024 bytes of the Templin Expires November 30, 2014 [Page 11] Internet-Draft AERO May 2014 original IPv6 packet) then admits the fragments into the tunnel. If the encapsulation protocol is IPv4, the node admits each fragment into the tunnel with DF set to 0 and subject to rate limiting to avoid reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, the node also sends a 1500 byte probe message (**) to the neighbor, subject to rate limiting. To construct a probe, the node prepares an NS message with a SLLAO, a Nonce option, plus trailing padding octets added to a length of 1500 bytes without including the length of the padding in the IPv6 Payload Length field. The node then encapsulates the NS in the encapsulation headers (while including the length of the padding in the encapsulation header length fields), sets DF to 1 (for IPv4) and sends the padded NS message to the neighbor. If the neighbor returns an NA message with a correct Nonce value, the node may then send whole packets within this size range and (for IPv4) relax the rate limiting requirement. (Note that the trailing padding SHOULD NOT be included within the Nonce option itself but rather as padding beyond the last option in the NS message; otherwise, the (large) Nonce option would be echoed back in the solicited NA message and may be lost at a link with a small MTU along the reverse path.) AERO nodes MUST be capable of reassembling packets up to 1500 bytes plus the encapsulation overhead length. It is therefore RECOMMENDED that AERO nodes be capable of reassembling at least 2KB. (*) Note that if it is known without probing that the minimum Path MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then MINMTU can be used instead of 1280 in the fragmentation threshold considerations listed above. (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used for MTU probing purposes, since smaller probes may be fragmented if there is a nested tunnel somewhere on the path to the neighbor. Probe sizes larger than 1500 bytes MAY be used, but may be unnecessary since original sources are expected to implement [RFC4821] when sending large packets. Also, if the neighbor can be reached by multiple underlying interfaces the paths via each interface must be probed independently. 3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation AERO interfaces encapsulate IPv6 packets according to whether they are entering the AERO interface for the first time or if they are being forwarded out the same AERO interface that they arrived on. This latter form of encapsulation is known as "re-encapsulation". Templin Expires November 30, 2014 [Page 12] Internet-Draft AERO May 2014 AERO interfaces encapsulate packets per the specifications in [RFC2473][RFC4213][RFC4301][RFC5246] except that the interface copies the "Hop Limit", "Traffic Class" and "Congestion Experienced" values in the packet's IPv6 header into the corresponding fields in the encapsulation header. For packets undergoing re-encapsulation, the AERO interface instead copies the "TTL/Hop Limit", "Type of Service/ Traffic Class" and "Congestion Experienced" values in the original encapsulation header into the corresponding fields in the new encapsulation header (i.e., the values are transferred between encapsulation headers and *not* copied from the encapsulated packet's network-layer header). When AERO UDP encapsulation is used, the AERO interface encapsulates the packet per the specifications in [RFC2473][RFC4213] except that it inserts a UDP header between the encapsulation header and IPv6 packet header. The AERO interface sets the UDP source port to a constant value that it will use in each successive packet it sends, sets the UDP checksum field to zero (see: [RFC6935][RFC6936]) and sets the UDP length field to the length of the IPv6 packet plus 8 bytes for the UDP header itself. For packets sent via a Server, the AERO interface sets the UDP destination port to 8060 (i.e., the IANA- registered port number for AERO) when AERO-only encapsulation is used. For packets sent to a neighboring Client, the AERO interface sets the UDP destination port to the port value stored in the neighbor cache entry for this neighbor. The AERO interface next sets the IP protocol number in the encapsulation header to the appropriate value for the first protocol layer within the encapsulation (e.g., IPv6, UDP, IPsec, etc.). When IPv6 is used as the encapsulation protocol, the interface then sets the flow label value in the encapsulation header the same as described in [RFC6438]. When IPv4 is used as the encapsulation protocol, the AERO interface sets the DF bit as discussed in Section 3.6. AERO interfaces decapsulate packets destined either to the node itself or to a destination reached via an interface other than the receiving AERO interface. When AERO UDP encapsulation is used (i.e., when a UDP header with destination port 8060 is present) the interface examines the first octet of the encapsulated packet. If the most significant four bits of the first octet encode the value '0110' (i.e., the version number value for IPv6), the packet is accepted and the encapsulating UDP header is discarded; otherwise, the packet is discarded. Further decapsulation then proceeds according to the appropriate tunnel type [RFC2473][RFC4213][RFC4301][RFC5246]. Templin Expires November 30, 2014 [Page 13] Internet-Draft AERO May 2014 3.8. AERO Router Discovery, Prefix Delegation and Address Configuration 3.8.1. AERO Client Behavior AERO Clients observe the IPv6 node requirements defined in [RFC6434]. AERO Clients first discover the link-layer addresses of AERO Servers via static configuration, or through an automated means such as DNS name resolution. In the absence of other information, the Client resolves the Fully-Qualified Domain Name (FQDN) "linkupnetworks.domainname", where "domainname" is the DNS domain appropriate for the Client's attached underlying network. The Client then creates a neighbor cache entry with fe80:: as the link-local address and the discovered addresses of one or more Servers as the link-layer addresses. Next, the Client acts as a requesting router to request an IPv6 prefix through DHCPv6 PD [RFC3633] via each AERO Server it wishes to associate with using a temporary link-local address (see Section 3.3) as the IPv6 source address and fe80:: as the IPv6 destination address. The Client includes a DHCPv6 Unique Identifier (DUID) in the Client Identifier option of its DHCPv6 messages [RFC3315][RFC6355] and includes any additional authenticating information necessary to authenticate itself to the DHCPv6 server. If the Client is pre-provisioned with an IPv6 prefix associated with the AERO service, it MAY also include the prefix in an IA_PD option in its DHCPv6 Request to indicate its preferred prefix to the DHCPv6 server. The Client then sends the encapsulated DHCPv6 request via one of its active underlying interfaces (i.e., the "primary" underlying interface for DHCPv6 transactions). After the Client receives its prefix delegation, it assigns the link- local AERO address taken from the prefix to the AERO interface and sub-delegates the prefix to nodes and links within its attached EUNs (the AERO link-local address thereafter remains stable as the Client moves). The Client also sets both the ACCEPT and FORWARD timers for each Server to infinity, since the Client will remain with this Server unless it explicitly terminates the association. The Client further renews its prefix delegation via standard DHCPv6 procedures by sending DHCPv6 Renew messages with its AERO address as the IPv6 source address, fe80:: as the IPv6 destination address and the same DUID value in the Client Identifier option. The Client then sends an RS message to each of its associated Servers to receive an RA message with a default router lifetime and any other link-specific parameters. When the Client receives an RA message, it configures a default route according to the default router lifetime but ignores any Prefix Information Options (PIOs) included in the RA message since the AERO link is link-local-only. The Client further Templin Expires November 30, 2014 [Page 14] Internet-Draft AERO May 2014 ignores any RS messages it might receive, since only Servers may process RS messages. The Client then sends periodic RS messages to each Server (subject to rate limiting) to obtain new RA messages for Neighbor Unreachability Detection (NUD), to refresh any network state, and to update the default router lifetime and any other link-specific parameters. (If the Client has multiple active underlying interfaces, it sends periodic RS messages over each underlying interface.) The Client can also forward IPv6 packets destined to networks beyond its local EUNs via a Server as an IPv6 default router. The Server may in turn return a redirection message informing the Client of a neighbor on the AERO link that is topologically closer to the final destination as specified in Section 3.9. Note that, since the Client's AERO address is configured from the unique DHCPv6 prefix delegation it receives, there is no need for Duplicate Address Detection (DAD) on AERO links. Other nodes maliciously attempting to hijack an authorized Client's AERO address will be denied due to an unacceptable link-layer address and/or security parameters (see: Security Considerations). 3.8.2. AERO Server Behavior AERO Servers observe the IPv6 router requirements defined in [RFC6434] and further configure a DHCPv6 relay function on their AERO links. When the AERO Server relays a Client's DHCPv6 PD messages to the DHCPv6 server, it wraps each message in a "Relay-forward" message per [RFC3315] and includes a DHCPv6 Interface Identifier option that encodes a value that identifies the AERO link to the DHCPv6 server. The Server then includes the Client's link-layer address in a DHCPv6 Client Link Layer Address Option (CLLAO) [RFC6939] with the link- layer address format shown in Figure 1. The Server sets the CLLAO 'option-length' field to 22 (2 plus the length of the link-layer address) and sets the 'link-layer type' field to TBD (see: IANA Considerations). The Server finally includes a DHCPv6 Echo Request Option (ERO) [RFC4994] that encodes the option code for the CLLAO in a 'requested-option-code-n' field. The CLLAO information will therefore subsequently be echoed back in the DHCPv6 Server's "Relay- reply" message. When the DHCPv6 server issues the IPv6 prefix delegation in a "Relay- reply" message via the AERO Server (acting as a DHCPv6 relay), the AERO Server obtains the Client's link-layer address from the echoed CLLAO option and obtains the Client's delegated prefix from the included IA_PD option. The Server then creates a neighbor cache entry for the Client's AERO address with the Client's link-layer Templin Expires November 30, 2014 [Page 15] Internet-Draft AERO May 2014 address as the link-layer address for the neighbor cache entry. The neighbor cache entry is created with both ACCEPT and FORWARD timers set to infinity, since the Client will remain with this Server unless it explicitly terminates the association. The Server also configures an IPv6 forwarding table entry that lists the Client's AERO address as the next hop toward the delegated IPv6 prefix with a lifetime derived from the DHCPv6 lease lifetime. The Server finally injects the Client's prefix as an IPv6 route into the inter-Server/Relay routing system (see: [IRON]) then relays the DHCPv6 message to the Client while using fe80:: as the IPv6 source address, the link-local address found in the "peer address" field of the Relay-reply message as the IPv6 destination address, and the Client's link-layer address as the destination link-layer address. Servers respond to RS/NS messages from Clients on their AERO interfaces by returning an RA/NA message. When the Server receives an RS/NS message, it updates the neighbor cache entry using the network-layer source address as the neighbor's network-layer address and using the link-layer source address of the RS/NS message as the neighbor's link-layer address. The Server SHOULD NOT include PIOs in the RA messages it sends to Clients, since the Client will ignore any such options. Servers ignore any RA messages they may receive from a Client. Servers MAY examine RA messages received from other Servers for consistency verification purposes. When the Server forwards a packet via the same AERO interface on which it arrived, it initiates an AERO route optimization procedure as specified in Section 3.9. 3.9. AERO Redirection 3.9.1. Reference Operational Scenario Figure 2 depicts the AERO redirection reference operational scenario. The figure shows an AERO Server('A'), two AERO Clients ('B', 'D') and three ordinary IPv6 hosts ('C', 'E', 'F'): Templin Expires November 30, 2014 [Page 16] Internet-Draft AERO May 2014 .-(::::::::) .-(::: IPv6 :::)-. +-------------+ (:::: Internet ::::)--| Host F | `-(::::::::::::)-' +-------------+ `-(::::::)-' 2001:db8:2::1 | +--------------+ | AERO Server A| | (C->B; E->D) | +--------------+ fe80:: L2(A) | X-----+-----------+-----------+--------X | AERO Link | L2(B) L2(D) fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-. +--------------+ +--------------+ ,-( _)-. | AERO Client B| | AERO Client D| .-(_ IPv6 )-. | (default->A) | | (default->A) |--(__ EUN ) +--------------+ +--------------+ `-(______)-' 2001:DB8:0::/48 2001:DB8:1::/48 | | 2001:db8:1::1 .-. +-------------+ ,-( _)-. 2001:db8:0::1 | Host E | .-(_ IPv6 )-. +-------------+ +-------------+ (__ EUN )--| Host C | `-(______)-' +-------------+ Figure 2: AERO Reference Operational Scenario In Figure 2, AERO Server ('A') connects to the AERO link and connects to the IPv6 Internet, either directly or via an AERO Relay (not shown). Server ('A') assigns the address fe80:: to its AERO interface with link-layer address L2(A). Server ('A') next arranges to add L2(A) to a published list of valid Servers for the AERO link. AERO Client ('B') receives the IPv6 prefix 2001:db8:0::/48 in a DHCPv6 PD exchange via AERO Server ('A') then assigns the address fe80::2001:db8:0:0 to its AERO interface with link-layer address L2(B). Client ('B') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80:: and link- layer address L2(A), then sub-delegates the prefix 2001:db8:0::/48 to its attached EUNs. IPv6 host ('C') connects to the EUN, and configures the address 2001:db8:0::1. AERO Client ('D') receives the IPv6 prefix 2001:db8:1::/48 in a DHCPv6 PD exchange via AERO Server ('A') then assigns the address Templin Expires November 30, 2014 [Page 17] Internet-Draft AERO May 2014 fe80::2001:db8:1:0 to its AERO interface with link-layer address L2(D). Client ('D') configures a default route and neighbor cache entry via the AERO interface with next-hop address fe80:: and link- layer address L2(A), then sub-delegates the prefix 2001:db8:1::/48 to its attached EUNs. IPv6 host ('E') connects to the EUN, and configures the address 2001:db8:1::1. Finally, IPv6 host ('F') connects to an IPv6 network outside of the AERO link domain. Host ('F') configures its IPv6 interface in a manner specific to its attached IPv6 link, and assigns the address 2001:db8:2::1 to its IPv6 link interface. 3.9.2. Classical Redirection Approaches With reference to Figure 2, when the IPv6 source host ('C') sends a packet to an IPv6 destination host ('E'), the packet is first forwarded via the EUN to AERO Client ('B'). Client ('B') then forwards the packet over its AERO interface to AERO Server ('A'), which then re-encapsulates and forwards the packet to AERO Client ('D'), where the packet is finally forwarded to the IPv6 destination host ('E'). When Server ('A') re-encapsulates and forwards the packet back out on its advertising AERO interface, it must arrange to redirect Client ('B') toward Client ('D') as a better next-hop node on the AERO link that is closer to the final destination. However, this redirection process applied to AERO interfaces must be more carefully orchestrated than on ordinary links since the parties may be separated by potentially many underlying network routing hops. Consider a first alternative in which Server ('A') informs Client ('B') only and does not inform Client ('D') (i.e., "classical redirection"). In that case, Client ('D') has no way of knowing that Client ('B') is authorized to forward packets from the claimed source address, and it may simply elect to drop the packets. Also, Client ('B') has no way of knowing whether Client ('D') is performing some form of source address filtering that would reject packets arriving from a node other than a trusted default router, nor whether Client ('D') is even reachable via a direct path that does not involve Server ('A'). Consider a second alternative in which Server ('A') informs both Client ('B') and Client ('D') separately, via independent redirection control messages (i.e., "augmented redirection"). In that case, if Client ('B') receives the redirection control message but Client ('D') does not, subsequent packets sent by Client ('B') could be dropped due to filtering since Client ('D') would not have a route to verify the claimed source address. Also, if Client ('D') receives the redirection control message but Client ('B') does not, subsequent packets sent in the reverse direction by Client ('D') would be lost. Templin Expires November 30, 2014 [Page 18] Internet-Draft AERO May 2014 Since both of these alternatives have shortcomings, a new redirection technique (i.e., "AERO redirection") is needed. 3.9.3. Concept of Operations Again, with reference to Figure 2, when source host ('C') sends a packet to destination host ('E'), the packet is first forwarded over the source host's attached EUN to Client ('B'), which then forwards the packet via its AERO interface to Server ('A'). Server ('A') then re-encapsulates and forwards the packet out the same AERO interface toward Client ('D') and also sends an AERO "Predirect" message forward to Client ('D') as specified in Section 3.9.5. The Predirect message includes Client ('B')'s network- and link-layer addresses as well as information that Client ('D') can use to determine the IPv6 prefix used by Client ('B') . After Client ('D') receives the Predirect message, it process the message and returns an AERO Redirect message destined for Client ('B') via Server ('A') as specified in Section 3.9.6. During the process, Client ('D') also creates or updates a neighbor cache entry for Client ('B') and creates an IPv6 forwarding table entry for Client ('B')'s IPv6 prefix. When Server ('A') receives the Redirect message, it re-encapsulates the message and forwards it on to Client ('B') as specified in Section 3.9.7. The message includes Client ('D')'s network- and link-layer addresses as well as information that Client ('B') can use to determine the IPv6 prefix used by Client ('D'). After Client ('B') receives the Redirect message, it processes the message as specified in Section 3.9.8. During the process, Client ('B') also creates or updates a neighbor cache entry for Client ('D') and creates an IPv6 forwarding table entry for Client ('D')'s IPv6 prefix. Following the above Predirect/Redirect message exchange, forwarding of packets from Client ('B') to Client ('D') without involving Server ('A) as an intermediary is enabled. The mechanisms that support this exchange are specified in the following sections. 3.9.4. Message Format AERO Redirect/Predirect messages use the same format as for ICMPv6 Redirect messages depicted in Section 4.5 of [RFC4861], but also include a new "Prefix Length" field taken from the low-order 8 bits of the Redirect message Reserved field (valid values for the Prefix Length field are 0 through 64). The Redirect/Predirect messages are formatted as shown in Figure 3: Templin Expires November 30, 2014 [Page 19] Internet-Draft AERO May 2014 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type (=137) | Code (=0/1) | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Prefix Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Target Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Destination Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 3: AERO Redirect/Predirect Message Format 3.9.5. Sending Predirects When a Server forwards a packet out the same AERO interface that it arrived on, the Server sends a Predirect message forward toward the AERO Client nearest the destination instead of sending a Redirect message back to the Client nearest the source. In the reference operational scenario, when Server ('A') forwards a packet sent by Client ('B') toward Client ('D'), it also sends a Predirect message forward toward Client ('D'), subject to rate limiting (see Section 8.2 of [RFC4861]). Server ('A') prepares the Predirect message as follows: o the link-layer source address is set to 'L2(A)' (i.e., the underlying address of Server ('A')). o the link-layer destination address is set to 'L2(D)' (i.e., the underlying address of Client ('D')). Templin Expires November 30, 2014 [Page 20] Internet-Draft AERO May 2014 o the network-layer source address is set to fe80:: (i.e., the link- local address of Server ('A')). o the network-layer destination address is set to fe80::2001:db8:1:0 (i.e., the AERO address of Client ('D')). o the Type is set to 137. o the Code is set to 1 to indicate "Predirect". o the Prefix Length is set to the length of the prefix to be applied to Target address. o the Target Address is set to fe80::2001:db8:0::0 (i.e., the AERO address of Client ('B')). o the Destination Address is set to the IPv6 source address of the packet that triggered the Predirection event. o the message includes one or more TLLAOs set to 'L2(B)' and any other underlying address(es) of Client ('B'). o the message includes a Redirected Header Option (RHO) that contains the originating packet truncated to ensure that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. Server ('A') then sends the message forward to Client ('D'). 3.9.6. Processing Predirects and Sending Redirects When Client ('D') receives a Predirect message, it accepts the message only if the message has a link-layer source address of the Server, i.e. 'L2(A)'. Client ('D') further accepts the message only if it is willing to serve as a redirection target. Next, Client ('D') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]. In the reference operational scenario, when Client ('D') receives a valid Predirect message, it either creates or updates a neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('B') and stores the link-layer address(es) found in the TLLAO(s) as the link-layer address(es) of Client ('B'). Client ('D') then sets the neighbor cache entry ACCEPT timer with timeout value ACCEPT_TIME. Next, Client ('D') applies the Prefix Length to the Interface Identifier portion of the Target Address and records the resulting IPv6 prefix in its IPv6 forwarding table. Templin Expires November 30, 2014 [Page 21] Internet-Draft AERO May 2014 After processing the message, Client ('D') prepares a Redirect message response as follows: o the link-layer source address is set to 'L2(D)' (i.e., the link- layer address of Client ('D')). o the link-layer destination address is set to 'L2(A)' (i.e., the link-layer address of Server ('A')). o the network-layer source address is set to 'L3(D)' (i.e., the AERO address of Client ('D')). o the network-layer destination address is set to 'L3(B)' (i.e., the AERO address of Client ('B')). o the Type is set to 137. o the Code is set to 0 to indicate "Redirect". o the Prefix Length is set to the length of the prefix to be applied to the Target and Destination address. o the Target Address is set to fe80::2001:db8:1::1 (i.e., the AERO address of Client ('D')). o the Destination Address is set to the IPv6 destination address of the packet that triggered the Redirection event. o the message includes one or more TLLAOs with UDP port number and IP address set to '0' and with Link ID and Preference values set to the appropriate values for the underlying interfaces Client ('D') wishes to enable for accepting encapsulated packets from Client ('B'). o the message includes as much of the RHO copied from the corresponding AERO Predirect message as possible such that at least the network-layer header is included but the size of the message does not exceed 1280 bytes. After Client ('D') prepares the Redirect message, it sends the message to Server ('A'). 3.9.7. Re-encapsulating and Relaying Redirects When Server ('A') receives a Redirect message from Client ('D'), it accepts the message only if it has a neighbor cache entry that associates the message's link-layer source address with the network- layer source address. Next, Server ('A') validates the message Templin Expires November 30, 2014 [Page 22] Internet-Draft AERO May 2014 according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861] and also verifies that Client ('D') is authorized to use the Prefix Length in the Redirect message when applied to the AERO address in the network-layer source of the Redirect message. If validation fails, Server ('A') discards the message; otherwise, it copies the correct UDP port numbers and IP addresses into the TLLAOs supplied by Client ('D') according to the Link ID in each TLLAO. Server ('A') then re-encapsulates the Redirect and relays it on to Client ('B') by changing the link-layer source address of the message to 'L2(A)', changing the network-layer source address of the message to fe80::, and changing the link-layer destination address to 'L2(B)' . Server ('A') finally forwards the re-encapsulated message to the ingress node ('B') without decrementing the network-layer IPv6 header Hop Limit field. While not shown in Figure 2, AERO Relays relay Redirect and Predirect messages in exactly this same fashion described above. See Figure 4 in Appendix A for an extension of the reference operational scenario that includes Relays. 3.9.8. Processing Redirects When Client ('B') receives the Redirect message, it accepts the message only if it has a link-layer source address of the Server, i.e. 'L2(A)'. Next, Client ('B') validates the message according to the ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861]. Following validation, Client ('B') then processes the message as follows. In the reference operational scenario, when Client ('B') receives the Redirect message, it either creates or updates a neighbor cache entry that stores the Target Address of the message as the network-layer address of Client ('D') and stores the link-layer address(es) found in the TLLAO(s) as the link-layer address(es) of Client ('D'). Client ('D') then sets the neighbor cache entry FORWARD timer with timeout value FORWARD_TIME. Next, Client ('B') applies the Prefix Length to the Interface Identifier portion of the Target Address and records the resulting IPv6 prefix in its IPv6 forwarding table. Now, Client ('B') has an IPv6 forwarding table entry for Client('D')'s prefix and a neighbor cache entry with a valid FORWARD time, while Client ('D') has an IPv6 forwarding table entry for Client ('B')'s prefix with a valid ACCEPT time. Thereafter, Client ('B') may forward ordinary network-layer data packets directly to Client ("D") without involving Server ('A') and Client ('D') can verify that the packets came from an acceptable source. (In order Templin Expires November 30, 2014 [Page 23] Internet-Draft AERO May 2014 for Client ('D') to forward packets to Client ('B') a corresponding Predirect/Redirect message exchange is required in the reverse direction.) 3.10. Neighbor Reachability Maintenance AERO nodes send unicast NS messages to elicit NA messages from neighbors the same as described for Neighbor Unreachability Detection (NUD) in [RFC4861]. When an AERO node sends an NS/NA message, it MUST use its AERO address as the IPv6 source address and the AERO address of the neighbor as the IPv6 destination address. When an AERO node receives an NS/NA message, it accepts the message if it has a neighbor cache entry for the neighbor; otherwise, it ignores the message. When a source Client is redirected to a target Client it SHOULD test the direct path to the target by sending an initial NS message to elicit a solicited NA response. While testing the path, the source Client SHOULD continue sending packets via the Server until target Client reachability has been confirmed. The source Client SHOULD thereafter continue to test the direct path to the target Client (see Section 7.3 of [RFC4861]) in order to keep neighbor cache entries alive. In particular, the source Client sends NS messages to the target Client subject to rate limiting in order to receive solicited NA messages. If at any time the direct path over all underlying interfaces appears to be failing, the source Client can resume sending packets via the Server which may or may not result in a new redirection event. When a target Client receives an NS message from a source Client, it resets the ACCEPT timer to ACCEPT_TIME if a neighbor cache entry exists; otherwise, it discards the NS message. When a source Client receives a solicited NA message from a target Client, it resets the FORWARD timer to FORWARD_TIME if a neighbor cache entry exists; otherwise, it discards the NA message. When the FORWARD timer on a neighbor cache entry expires, the source Client resumes sending any subsequent packets via the Server and may (eventually) receive a new Redirect message. When the ACCEPT timer on a neighbor cache entry expires, the target Client discards any subsequent packets received directly from the source Client. When both the FORWARD and ACCEPT timers on a neighbor cache entry expire, the Client deletes both the neighbor cache entry and the corresponding IPv6 forwarding table entry. If the source Client is unable to elicit an NA response from the target Client after MAX_RETRY attempts, it SHOULD consider the direct Templin Expires November 30, 2014 [Page 24] Internet-Draft AERO May 2014 path unusable for forwarding purposes. Otherwise, the source Client considers the path usable and SHOULD thereafter process any link- layer errors as a hint that the direct path to the target Client has either failed or has become intermittent. 3.11. Mobility and Link-Layer Address Change Considerations When a Client needs to change one of its link-layer addresses (e.g., due to a mobility event), it sends an immediate NS message to each of its active neighbors (including the Server) using the new link-layer address as the encapsulation source address and with the correct Link ID and Preference values in the SLLAO. The Client processes any NA messages returned as an indication that the neighbor has received the update and is ready to accept encapsulated packets with the new link- layer address. When a Client needs to associate with a new Server, it issues a new DHCPv6 Request message via the new Server as the DHCPv6 relay. The new Server then relays the message to the DHCPv6 server and processes the resulting exchange. After the Client receives the resulting DHCPv6 Reply message, it sends an RS message to the new Server to receive a new RA message and update its neighbor cache entry for fe80::. When a Client disassociates with an existing Server, it sends a "terminating RS" message to the old Server. The terminating RS message is prepared exactly the same as for an ordinary RS message, except that the Code field contains the value '1'. When the old Server receives the terminating RS message, it withdraws the IPv6 route from the routing system and deletes the neighbor cache entry and IPv6 forwarding table entry for the Client. The old Server then returns an RA message with default router lifetime set to 0 which the Client can use to verify that the termination signal has been processed. The client then deletes both the default route and the neighbor cache entry for the old Server. (Note that the Client and the old Server MAY impose a small delay before deleting the neighbor cache and IPv6 forwarding table entries so that any packets already in the system can still be delivered to the Client.) An alternative to sending a "terminating RS" message would be for the Client to somehow perform a DHCPv6 exchange with the DHCPv6 relay function on the old AERO Server but without involving the DHCPv6 server. This would be insecure because the Client only has a DHCPv6 security association with the DHCPv6 server and not the DHCPv6 relay. Conversely, the Client and Server already require an authentic exchange of IPv6 Neighbor Discovery messages. Templin Expires November 30, 2014 [Page 25] Internet-Draft AERO May 2014 3.12. Encapsulation Protocol Version Considerations A source Client may connect only to an IPvX underlying network, while the target Client connects only to an IPvY underlying network. In that case, the target and source Clients have no means for reaching each other directly (since they connect to underlying networks of different IP protocol versions) and so must ignore any redirection messages and continue to send packets via the Server. 3.13. Multicast Considerations When the underlying network does not support multicast, AERO nodes map IPv6 link-scoped multicast addresses (including "All_DHCP_Relay_Agents_and_Servers") to the underlying IP address of a Server. When the underlying network supports multicast, AERO nodes use the multicast address mapping specification found in [RFC2529] for IPv4 underlying networks and use a direct multicast mapping for IPv6 underlying networks. (In the latter case, "direct multicast mapping" means that if the IPv6 multicast destination address of the encapsulated packet is "M", then the IPv6 multicast destination address of the encapsulating header is also "M".) 3.14. Operation on AERO Links Without DHCPv6 Services When the AERO link does not provide DHCPv6 services, operation can still be accommodated through administrative configuration of prefixes on AERO Clients. In that case, administrative configurations of IPv6 routes and AERO interface neighbor cache entries on both the Server and Client are also necessary. However, this may preclude the ability for Clients to dynamically change to new Servers, and can expose the AERO link to misconfigurations unless the administrative configurations are carefully coordinated. 3.15. Operation on Server-less AERO Links In some AERO link scenarios, there may be no Servers on the link and/ or no need for Clients to use a Server as an intermediary trust anchor. In that case, each Client can then act as its own Server to establish neighbor cache entries and IPv6 forwarding table entries by performing direct Client-to-Client Predirect/Redirect exchanges, and some other form of trust basis must be applied so that each Client can verify that the prospective neighbor is authorized to use its claimed prefix. When there is no Server on the link, Clients must arrange to receive prefix delegations and publish the delegations via a secure alternate Templin Expires November 30, 2014 [Page 26] Internet-Draft AERO May 2014 prefix delegation authority through some means outside the scope of this document. 3.16. Other Considerations IPv6 hosts serviced by an AERO Client can reach IPv4-only services via a NAT64 gateway [RFC6146] within the IPv6 network. AERO nodes can use the Default Address Selection Policy with DHCPv6 option [RFC7078] the same as on any IPv6 link. All other (non-multicast) functions that operate over ordinary IPv6 links operate in the same fashion over AERO links. 4. Implementation Status An application-layer implementation is in progress. 5. IANA Considerations The IANA is instructed to assign a new 2-octet Hardware Type number for AERO in the "arp-parameters" registry per Section 2 of [RFC5494]. The number is assigned from the 2-octet Unassigned range with Hardware Type "AERO" and with this document as the reference. 6. Security Considerations AERO link security considerations are the same as for standard IPv6 Neighbor Discovery [RFC4861] except that AERO improves on some aspects. In particular, AERO is dependent on a trust basis between Clients and Servers, where the Clients only engage in the AERO mechanism when it is facilitated by a trust anchor. AERO links must be protected against link-layer address spoofing attacks in which an attacker on the link pretends to be a trusted neighbor. Links that provide link-layer securing mechanisms (e.g., WiFi networks) and links that provide physical security (e.g., enterprise network wired LANs) provide a first line of defense that is often sufficient. In other instances, additional securing mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec [RFC4301] or TLS [RFC5246] may be necessary. AERO Clients MUST ensure that their connectivity is not used by unauthorized nodes on EUNs to gain access to a protected network, i.e., AERO Clients that act as IPv6 routers MUST NOT provide routing services for unauthorized nodes. (This concern is no different than for ordinary hosts that receive an IP address delegation but then Templin Expires November 30, 2014 [Page 27] Internet-Draft AERO May 2014 "share" the address with unauthorized nodes via an IPv6/IPv6 NAT function.) On some AERO links, establishment and maintenance of a direct path between neighbors requires secured coordination such as through the Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a security association. 7. Acknowledgements Discussions both on IETF lists and in private exchanges helped shape some of the concepts in this work. Individuals who contributed insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant, Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha Hlusiak, Lee Howard, Joe Touch and Bernie Volz. Members of the IESG also provided valuable input during their review process that greatly improved the document. Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman for their shepherding guidance. This work has further been encouraged and supported by Boeing colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie, Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed King, Gen MacLean, Kent Shuey, Mike Slane, Julie Wulff, Yueli Yang, and other members of the BR&T and BIT mobile networking teams. Earlier works on NBMA tunneling approaches are found in [RFC2529][RFC5214][RFC5569]. 8. References 8.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. Templin Expires November 30, 2014 [Page 28] Internet-Draft AERO May 2014 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6", RFC 3633, December 2003. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for IPv6 Hosts and Routers", RFC 4213, October 2005. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node Requirements", RFC 6434, December 2011. 8.2. Informative References [IRON] Templin, F., "The Internet Routing Overlay Network (IRON)", Work in Progress, June 2012. [RFC0879] Postel, J., "TCP maximum segment size and related topics", RFC 879, November 1983. [RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4 Domains without Explicit Tunnels", RFC 2529, March 1999. [RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", RFC 2675, August 1999. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. Templin Expires November 30, 2014 [Page 29] Internet-Draft AERO May 2014 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [RFC4994] Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski, "DHCPv6 Relay Agent Echo Request Option", RFC 4994, September 2007. [RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214, March 2008. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines for the Address Resolution Protocol (ARP)", RFC 5494, April 2009. [RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility Route Optimization Requirements for Operational Use in Aeronautics and Space Exploration Mobile Networks", RFC 5522, October 2009. [RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4 Infrastructures (6rd)", RFC 5569, January 2010. [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 5996, September 2010. [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, April 2011. [RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O. Troan, "Basic Requirements for IPv6 Customer Edge Routers", RFC 6204, April 2011. [RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August 2011. [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels", RFC 6438, November 2011. Templin Expires November 30, 2014 [Page 30] Internet-Draft AERO May 2014 [RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, July 2012. [RFC6706] Templin, F., "Asymmetric Extended Route Optimization (AERO)", RFC 6706, August 2012. [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field", RFC 6864, February 2013. [RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and UDP Checksums for Tunneled Packets", RFC 6935, April 2013. [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936, April 2013. [RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer Address Option in DHCPv6", RFC 6939, May 2013. [RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery", RFC 6980, August 2013. [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing Address Selection Policy Using DHCPv6", RFC 7078, January 2014. Appendix A. AERO Server and Relay Interworking Figure 2 depicts a reference AERO operational scenario with a single Server on the AERO link. In order to support scaling to larger numbers of nodes, the AERO link can deploy multiple Servers and Relays, e.g., as shown in Figure 4. Templin Expires November 30, 2014 [Page 31] Internet-Draft AERO May 2014 .-(::::::::) .-(::: IPv6 :::)-. (:: Internetwork ::) `-(::::::::::::)-' `-(::::::)-' | +--------------+ +------+-------+ +--------------+ |AERO Server C | | AERO Relay D | |AERO Server E | | (default->D) | | (A->C; G->E) | | (default->D) | | (A->B) | +-------+------+ | (G->F) | +-------+------+ | +------+-------+ | | | X---+---+-------------------+------------------+---+---X | AERO Link | +-----+--------+ +--------+-----+ |AERO Client B | |AERO Client F | | (default->C) | | (default->E) | +--------------+ +--------------+ .-. .-. ,-( _)-. ,-( _)-. .-(_ IPv6 )-. .-(_ IPv6 )-. (__ EUN ) (__ EUN ) `-(______)-' `-(______)-' | | +--------+ +--------+ | Host A | | Host G | +--------+ +--------+ Figure 4: AERO Server/Relay Interworking In this example, Client ('B') associates with Server ('C'), while Client ('F') associates with Server ('E'). Furthermore, Servers ('C') and ('E') do not associate with each other directly, but rather have an association with Relay ('D') (i.e., a router that has full topology information concerning its associated Servers and their Clients). Relay ('D') connects to the AERO link, and also connects to the native IPv6 Internetwork. When host ('A') sends a packet toward destination host ('G'), IPv6 forwarding directs the packet through the EUN to Client ('B'), which forwards the packet to Server ('C') in absence of more-specific forwarding information. Server ('C') forwards the packet, and it also generates an AERO Predirect message that is then forwarded through Relay ('D') to Server ('E'). When Server ('E') receives the message, it forwards the message to Client ('F'). After processing the AERO Predirect message, Client ('F') sends an AERO Redirect message to Server ('E'). Server ('E'), in turn, Templin Expires November 30, 2014 [Page 32] Internet-Draft AERO May 2014 forwards the message through Relay ('D') to Server ('C'). When Server ('C') receives the message, it forwards the message to Client ('B') informing it that host 'G's EUN can be reached via Client ('F'), thus completing the AERO redirection. The network layer routing information shared between Servers and Relays must be carefully coordinated in a manner outside the scope of this document. In particular, Relays require full topology information, while individual Servers only require partial topology information (i.e., they only need to know the EUN prefixes associated with their current set of Clients). See [IRON] for an architectural discussion of routing coordination between Relays and Servers. Author's Address Fred L. Templin (editor) Boeing Research & Technology P.O. Box 3707 Seattle, WA 98124 USA Email: fltemplin@acm.org Templin Expires November 30, 2014 [Page 33]