Network Working Group Dave Thaler Internet-Draft Mohit Talwar October 2004 Amit Aggarwal Expires: April 22, 2005 Microsoft Lorenzo Vicisano Cisco Dirk Ooms OneSparrow Automatic IP Multicast Without Explicit Tunnels (AMT) draft-ietf-mboned-auto-multicast-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract Automatic Multicast Tunneling (AMT) allows multicast communication amongst isolated multicast-enabled sites or hosts, attached to a network which has no native multicast support. It also enables them to exchange multicast traffic with the native multicast Thaler, et al. [Page 1] Internet-Draft Automatic Multicast October 2004 infrastructure (MBone) and does not require any manual configuration. AMT uses an encapsulation interface so that no changes to a host stack or applications are required, all protocols (not just UDP) are handled, and there is no additional overhead in core routers. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 1. Introduction The primary goal of this document is to foster the deployment of native IP multicast by enabling a potentially large number of nodes to connect to the already present multicast infrastructure. Therefore, the techniques discussed here should be viewed as an interim solution to help in the various stages of the transition to a native multicast network. To allow fast deployment, the solution presented here only requires small and concentrated changes to the network infrastructure, and no changes at all to user applications or to the socket API of end- nodes' operating systems. The protocol introduced in this specification is deployed in a few strategically-placed network nodes and in user-installable software modules (pseudo device drivers and/or user-mode daemons) that reside underneath the socket API of end-nodes' operating systems. This mechanism is very similar to that used by "6to4" [6TO4, ANYCAST] to get automatic IPv6 connectivity. Effectively, AMT treats the unicast-only internetwork as a large non- broadcast multi-access (NBMA) link layer, over which we require the ability to multicast. To do this, multicast packets being sent to or from a site must be encapsulated in unicast packets. If the group has members in multiple sites, AMT encapsulation of the same multicast packet will take place multiple times by necessity. The following problems are addressed: 1. Allowing isolated sites/hosts to receive the SSM flavor of multicast ([SSM]). 2. Allowing isolated sites/hosts to transmit the SSM flavor of multicast. 3. Allowing isolated sites/hosts to receive general multicast (ISM [RFC1112]). This document does not address allowing isolated sites/hosts to Thaler, et al. [Page 2] Internet-Draft Automatic Multicast October 2004 transmit general multicast. We expect that other solutions (e.g., Tunnel Brokers, a la [BROKER]) will be used for sites that desire this capability. 2. Requirements Terminology The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC-2119]. 3. Definitions +---------------+ Internet +---------------+ | AMT Site | | MBone | | | AMT | | | +------+----+ Relay +----+----+ AMT | | |AMT Gateway| Anycast |AMT Relay| Subnet | | | +-----+-+ Prefix +-+-----+ | Prefix | | | |AMT IF | <--------|AMT IF | |--------> | | | +-----+-+ +-+-----+ | | | +------+----+ +----+----+ | | | | | +---------------+ +---------------+ Figure 1: Automatic Multicast Definitions. AMT Pseudo-Interface AMT encapsulation of multicast packets inside unicast packets occurs at a point that is logically equivalent to an interface, with the link layer being the unicast-only network. This point is referred to as a pseudo-interface. Some implementations may treat it exactly like any other interface and others may treat it like a tunnel end-point. AMT Gateway A host, or a site gateway router, supporting an AMT Pseudo- Interface. It does not have native multicast connectivity to the native multicast backbone infrastructure. It is simply referred to in this document as a "gateway". AMT Site A multicast-enabled network not connected to the multicast backbone served by an AMT Gateway. It could also be a stand- alone AMT Gateway. Thaler, et al. [Page 3] Internet-Draft Automatic Multicast October 2004 AMT Relay Router A multicast router configured to support transit routing between AMT Sites and the native multicast backbone infrastructure. The relay router has one or more interfaces connected to the native multicast infrastructure, zero or more interfaces connected to the non-multicast capable internetwork, and an AMT pseudo-interface. It is simply referred to in this document as a "relay". As with [6TO4], we assume that normal multicast routers do not want to be tunnel endpoints (especially if this results in high fanout), and similarly that service providers do not want encapsulation to arbitrary routers. Instead, we assume that special-purpose routers will be deployed that are suitable for serving as relays. AMT Relay Anycast Prefix A well-known address prefix used to advertise (into the unicast routing infrastructure) a route to an available AMT Relay Router. This could also be private (i.e., not well-known) for a private relay. Prefixes for both IPv4 and IPv6 will be assigned in a future version of this draft. AMT Relay Anycast Address An anycast address which is used to reach the nearest AMT Relay Router. This address corresponds to the lowest address in the AMT Relay Anycast Prefix. AMT Unicast Autonomous System ID A 16-bit Autonomous System ID, for use in BGP in accordance to this memo. AS 10888 might be usable for this, but for now we'll assume it's different, to avoid confusion. This number represents a "pseudo-AS" common to all AMT relays using the well known AMT Relay Anycast Prefix (private relays use their own ID). To protect themselves from erroneous advertisements, managers of border routers often use databases to check the relation between the advertised network and the last hop in the AS path. Associating a specific AS number with the AMT Relay Anycast Address allows us to enter this relationship in the databases used to check inter-domain routing [ANYCAST]. AMT Subnet Prefix A well-known address prefix used to advertise (into the M-RIB of the native multicast-enabled infrastructure) a route to AMT Sites. Thaler, et al. [Page 4] Internet-Draft Automatic Multicast October 2004 This prefix will be used to enable sourcing SSM traffic from an AMT Gateway. AMT Gateway Anycast Address An anycast address in the AMT Subnet Prefix range, which is used by an AMT Gateway to enable sourcing SSM traffic from local applications. AMT Multicast Autonomous System ID A 16-bit Autonomous system ID, for use in MBGP in accordance to this memo. This number represents a "pseudo-AS" common to all AMT relays using the well known AMT Subnet Prefix (private relays use their own ID). We assume that the existing AS 10888 is suitable for this purpose. (Note: if this is a problem, a different one would be fine.) 4. Overview 4.1. Receiving Multicast in an AMT Site Internet +---------------+ +---------------+ | AMT Site | 2. 3-way Membership | MBone | | | Handshake | | | 1. Join +---+---+ =================> +---+---+ | | +---->|Gateway| | Relay | | | | +---+---+ <================= +---+---+ | | R-+ | 3. Receive Data | | +---------------+ +---------------+ Figure 2: Receiving Multicast in an AMT Site. AMT relays and gateways cooperate to transmit multicast traffic sourced within the native multicast infrastructure to AMT sites: relays receive the traffic natively and unicast-encapsulate it to gateways; gateways decapsulate the traffic and possibly forward it into the AMT site. Each gateway has an AMT pseudo-interface that serves as a default multicast route. Requests to join a multicast session are sent to this interface and encapsulated to a particular relay reachable across the unicast-only infrastructure. Each relay has an AMT pseudo-interface too. Multicast traffic sent on this interface is encapsulated to zero or more gateways that have joined to the relay. The AMT recipient-list is determined for each Thaler, et al. [Page 5] Internet-Draft Automatic Multicast October 2004 multicast session. This requires the relay to keep state for each gateway which has joined a particular group or (source, group) pair). Multicast packets from the native infrastructure behind the relay will be sent to each gateway which has requested them. All multicast packets (data and control) are encapsulated in unicast packets. To work across NAT's, the encapsulation is done over UDP using a reserved port number [TBD IANA]. Each relay, plus the set of all gateways using the relay, together are thought of as being on a separate logical NBMA link. This implies that the AMT recipient- list is a list of "link layer" addresses which are (IP address, UDP port) pairs. Since the number of gateways using a relay can be quite large, and we expect that most sites will not want to receive most groups, an explicit-joining protocol is required for gateways to communicate group membership information to a relay. The two most likely candidates are the IGMP/MLD [IGMPv3/MLDv2] protocol, and the PIM- Sparse Mode [PIMSM] protocol. Since an AMT gateway may be a host, and hosts typically do not implement routing protocols, gateways will use IGMP/MLD as described in Section 5 below. This allows a host kernel (or a pseudo device driver) to easily implement AMT gateway behavior, and obviates the relay from the need to know whether a given gateway is a host or a router. From the relay's perspective, all gateways are indistinguishable from hosts on an NBMA leaf network. 4.1.1. Scalability Considerations The requirement that a relay keep group state per gateway that has joined the group introduces potential scalability concerns. However, scalability of AMT can be achieved by adding more relays, and using an appropriate relay discovery mechanism for gateways to discover relays. The solution we adopt is to assign an anycast address to relays. However, simply sending periodic membership reports to the anycast address can cause duplicates. Specifically, if routing changes such that a different relay receives a periodic membership report, both the new and old relays will encapsulate data to the AMT site until the old relay's state times out. This is obviously undesirable. Instead, we use the anycast address merely to find the unicast address of a relay to which membership reports are sent. Since adding another relay has the result of adding another independent NBMA link, this allows the gateways to be spread out Thaler, et al. [Page 6] Internet-Draft Automatic Multicast October 2004 among more relays so as to keep the number of gateways per relay at a reasonable level. 4.1.2. Spoofing Considerations An attacker could affect the group state in the relay or gateway by spoofing the source address in the join or leave reports. This can be used to launch reflection or denial of service attacks on the target. Such attacks can be mitigated by using a three way handshake between the gateway and the relay for each multicast membership report or leave. When a gateway or relay wants to send a membership report, it first sends an AMT Request with a request nonce in it. The receiving side (the respondent) can calculate a message authentication code (MAC) based on the source IP address of the Request, the request nonce, and a secret key known only to the respondent. An AMT Membership Query is sent back including the request nonce and the MAC to the originator of the Request. The originator then sends the IGMP/MLD Membership/Listener Report or Leave/Done along with the request nonce and the received MAC back to the respondent finalizing the 3-way handshake. Upon reception, the respondent can recalculate the MAC based on the source IP address, the request nonce, and the local secret. The IGMP/MLD message is only accepted if the received MAC matches the calculated MAC. The local secret never has to be shared with the other side. It is only used to verify return routability of the originator. 4.2. Sourcing Multicast from an AMT site Two cases are discussed below: multicast traffic sourced in an AMT site and received in the MBone, and multicast traffic sourced in an AMT site and received in another AMT site. In both cases only SSM sources are supported. Furthermore this specification only deals with the source residing directly in the gateway. To enable a generic node in an AMT site to source multicast, additional coordination between the gateway and the source-node is required. The general mechanism used to join towards AMT sources is based on the following: Thaler, et al. [Page 7] Internet-Draft Automatic Multicast October 2004 1. Applications residing in the gateway use addresses in the AMT Subnet Prefix to send multicast, as a result of sourcing traffic on the AMT pseudo-interface. 2. The AMT Subnet Prefix is advertised for RPF reachability in the M- RIB by relays and gateways. 3. Relays or gateways that receive a join for a source/group pair use information encoded in the address pair to rebuild the address of the gateway (source) to which to encapsulate the join (see Section 5 for more details). The membership reports use the same three way handshake as outlined in Section 4.1.2. 4.2.1. Supporting Site-MBone Multicast Internet +---------------+ +---------------+ | AMT Site | 2. 3-way Membership | MBone | | | Handshake | | | +---+---+ <================= +---+---+ 1. Join | | |Gateway| | Relay |<-----+ | | +---+---+ =================> +---+---+ | | | | 3. Receive Data | +-R | +---------------+ +---------------+ Figure 3: Site-MBone Multicast. If a relay receives an explicit join from the native infrastructure, for a given (source, group) pair where the source address belongs to the AMT Subnet Prefix, then the relay will periodically (using the rules specified in Section 4.1.2) encapsulate membership updates for the group to the gateway. The gateway must keep state per relay from which membership reports have been sent, and forward multicast traffic from the site to all relays from which membership reports have been received. The choice of whether this state and replication is done at the link-layer (i.e., by the tunnel interface) or at the network-layer is implementation dependent. If there are multiple relays present, this ensures that data from the AMT site is received via the closest relay to the receiver. This is necessary when the routers in the native multicast infrastructure employ Reverse-Path Forwarding (RPF) checks against the source address, such as occurs when [PIMSM] is used by the multicast infrastructure. The solution above will scale to an arbitrary number of relays, as Thaler, et al. [Page 8] Internet-Draft Automatic Multicast October 2004 long at the number of relays requiring multicast traffic from a given AMT site remains reasonable enough to not overly burden the site's gateway. 4.2.2. Supporting Site-Site Multicast Internet +---------------+ +---------------+ | AMT Site | 2. 3-way Membership | AMT Site | | | Handshake | | | +---+---+ <================= +---+---+ 1. Join | | |Gateway| |Gateway|<-----+ | | +---+---+ =================> +---+---+ | | | | 3. Receive Data | +-R | +---------------+ +---------------+ Figure 4: Site-Site Multicast. Since we require gateways to accept membership reports, as described above, it is also possible to support multicast among AMT sites, without requiring assistance from any relays. When a gateway wants to join a given (source, group) pair, where the source address belongs to the AMT Subnet Prefix, then the gateway will periodically unicast encapsulate an IGMPv3/MLDv2 [IGMPv3/MLDv2] Report directly to the site gateway for the source. We note that this can result in a significant amount of state at a site gateway sourcing multicast to a large number of other AMT sites. However, it is expected that this is not unreasonable for two reasons. First, the gateway does not have native multicast connectivity, and as a result is likely doing unicast replication at present. The amount of state is thus the same as what such a site already deals with. Secondly, any site expecting to source traffic to a large number of sites could get a point-to-point tunnel to the native multicast infrastructure, and use that instead of AMT. 5. Message Formats 5.1. AMT Relay Discovery The AMT Relay Discovery message is a UDP packet sent from the AMT gateway unicast address to the AMT relay anycast address to discover the unicast address of an AMT relay. Thaler, et al. [Page 9] Internet-Draft Automatic Multicast October 2004 The UDP source port is uniquely selected by the local host operating system. The UDP destination port is the IANA reserved AMT port number. The payload of the UDP packet contains the following fields. 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=0x1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Discovery Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields: Type The type of the message. Reserved A 24-bit reserved field. Sent as 0, ignored on receipt. Discovery Nonce A 32-bit random value generated by the gateway and replayed by the relay. 5.2. AMT Relay Advertisement The AMT Relay Advertisement message is a UDP packet sent from the AMT relay anycast address to the source of the discovery message. The UDP source port is the IANA reserved AMT port number and the UDP destination port is the source port received in the Discovery message. The payload of the UDP packet contains the following fields. Fields: Type The type of the message. Reserved A 24-bit reserved field. Sent as 0, ignored on receipt. Thaler, et al. [Page 10] Internet-Draft Automatic Multicast October 2004 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=0x2 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Discovery Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Relay Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Discovery Nonce A 32-bit random value replayed from the discovery message. Relay Address The unicast IPv4 or IPv6 address of the AMT relay. The family can be determined by the length of the Advertisement. 5.3. AMT Request A Request packet is sent to begin a 3-way handshake for sending an IGMP/MLD Membership/Listener Report or Leave/Done. It can be sent from a gateway to a relay, from a gateway to another gateway, or from a relay to a gateway. It is sent from the originator's unique unicast address to the respondents' unique unicast address. The UDP source port is uniquely selected by the local host operating system. It can be different for each Request and different from the source port used in Discovery messages but does not have to be. The UDP destination port is the IANA reserved AMT 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=0x3 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Request Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields: Type The type of the message. Thaler, et al. [Page 11] Internet-Draft Automatic Multicast October 2004 Reserved A 24-bit reserved field. Sent as 0, ignored on receipt. Request Nonce A 32-bit identifier used to distinguish this request. 5.4. AMT Membership Query An AMT Membership Query packet is sent from the relay back to the originator to solicit an AMT Membership Update while confirming the source of the original request. It contains a relay Message Authentication Code (MAC) that is a non-cryptographic hash of a private secret, the originators address, and the request nonce. It is sent from the destination address received in the Request to the source address received in the Request. The UDP source port is the IANA reserved AMT port number and the UDP destination port is the source port received in the Request message. 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=0x4 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Request Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Response MAC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields: Type The type of the message. Reserved A 24-bit reserved field. Sent as 0, ignored on receipt. Request Nonce A 32-bit identifier used to distinguish this request echoed back to the originator. Response MAC A 32-bit hash generated by the respondent and sent to the originator for inclusion in the AMT Membership Update. This MUST be calculated as HMAC-MD5-32 [HMAC]. Thaler, et al. [Page 12] Internet-Draft Automatic Multicast October 2004 5.5. AMT Membership Update An AMT Membership Update is sent from the originator to the respondent containing the original IGMP/MLD Membership/Listener Report or Leave/Done received over the AMT pseudo-interface. It echoes the Response MAC received in the AMT Membership Query so the respondent can verify return routability to the originator. It is sent from the destination address received in the Query to the source address received in the Query which should both be the same as the original Request. The UDP source and destination port numbers should be the same ones sent in the original Request. 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=0x5 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Request Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Response MAC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IGMP/MLD Message | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields: Type The type of the message. Reserved A 24-bit reserved field. Sent as 0, ignored on receipt. Request Nonce A 32-bit identifier used to distinguish this request. Response MAC The MAC received from the relay and echoed back in the AMT Membership Update. Thaler, et al. [Page 13] Internet-Draft Automatic Multicast October 2004 5.6. AMT UDP Data The AMT UDP Data message is a UDP packet encapsulating the data requested by the originator based on a previous AMT Membership Update message. Currently, it is only defined for original UDP multicast data packets. This prevents the tunnel from being used as an arbitrary tunnel mechanism and greatly reduces the possibility of exploitation. It is sent from the unicast destination address of the Membership update to the source address of the Membership Update. The UDP source and destination port numbers should be the same ones sent in the original Query. The payload of the UDP packet contains the following fields. 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=0x6 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP Multicast Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields: Type The type of the message. Reserved A 24-bit reserved field. Sent as 0, ignored on receipt. UDP Multicast Data The original Multicast UDP data packet that is being replicated by the relay to the gateways. 6. AMT Gateway Details This section details the behavior of an AMT Gateway, which may be a router serving an AMT site, or the site may consist of a single host, serving as its own gateway. Thaler, et al. [Page 14] Internet-Draft Automatic Multicast October 2004 6.1. At Startup Time At startup time, the AMT gateway will bring up an AMT pseudo- interface, to be used for encapsulation. The gateway will then send a AMT Relay Discovery message to the AMT Relay Anycast Address, and note the unicast address (which is treated as a link-layer address to the encapsulation interface) from the AMT Relay Advertisement message. This discovery should be done periodically (e.g., once a day) to re-resolve the unicast address of a close relay. The gateway also initializes a timer used to send periodic membership reports to a random value from the interval [0, [Query Interval]] before sending the first periodic report, in order to prevent startup synchronization (e.g., after a power outage). If the gateway is serving as a local router, it SHOULD also function as an IGMP/MLD Proxy, as described in [PROXY], with its IGMP/MLD host-mode interface being the AMT pseudo-interface. This enables it to translate group memberships on its downstream interfaces into IGMP/MLD Reports. The gateway MUST also advertise itself as the default route for multicast in the M-RIB (or it must be the default unicast router if unicast and multicast topologies are congruent). Also, if a shared tree routing protocol is used inside the AMT site, each tree-root must be a gateway, e.g., in PIM-SM each RP must be a gateway. Finally, to support sourcing traffic to SSM groups by a gateway with a global unicast address, the AMT Subnet Prefix is treated as the subnet prefix of the AMT pseudo-interface, and an anycast address is added on the interface. This anycast address is formed by concatenating the AMT Subnet Prefix followed by the high bits of the gateway's global unicast address. For example, if IANA assigns the IPv4 prefix x.y/16 as the AMT Subnet Prefix, and the gateway has global unicast address a.b.c.d, then the AMT Gateway's Anycast Address will be x.y.a.b. Note that multiple gateways might end up with the same anycast address assigned to their pseudo-interfaces. 6.2. Joining Groups with MBone Sources The IGMP/MLD protocol usually operates by having the Querier multicast an IGMP/MLD Query message on the link. This behavior does not work on NBMA links which do not support multicast. Since the set of gateways is typically unknown to the relay (and potentially quite large), unicasting the queries is also impractical. The following behavior is used instead. Applications residing in a gateway should join groups on the AMT pseudo-interface, causing IGMP/MLD Membership/Listener Reports to be Thaler, et al. [Page 15] Internet-Draft Automatic Multicast October 2004 sent over that interface. When UDP encapsulating the membership reports (and in fact any other messages, unless specified otherwise in this document), the destination address in the outer IP header is the relay's unicast address. Robustness is provided by the underlying IGMP/MLD protocol messages sent on the AMT pseudo- interface. In other words, the gateway does not need to retransmit IGMP/MLD Membership/Listener Reports and Leave/Done messages received on the pseudo-interface since IGMP/MLD will already do this. The gateway simply needs to encapsulate each IGMP/MLD Membership/Listener Report and Leave/Done message it receives. However, since periodic IGMP/MLD Membership/Listener Reports are sent in response to IGMP/MLD Queries, some mechanism to trigger periodic Membership/Listener Reports and Leave/Done messages are necessary. This can be achieved in any implementation-specific manner. Some possibilities include: 1. The AMT pseudo-interface might periodically manufacture IGMPv3/MLDv2 Queries as if they had been received from an IGMP/MLD Querier, and deliver them to the IP layer, after which normal IGMP/MLD behavior will cause the appropriate reports to be sent. 2. The IGMP/MLD module itself might provide an option to operate in periodic mode on specific interfaces. 6.3. Responding to Relay Changes When a gateway determines that its current relay is unreachable (e.g., upon receipt of a ICMP Unreachable message [ICMP] for the relay's unicast address), it may need to repeat relay address discovery. However, care should be taken not to abandon the current relay too quickly due to transient network conditions. Some implementations may find it difficult to send a discovery packet to the anycast relay address while the gateway has an address configured on the AMT pseudo-interface on the same anycast prefix. Therefore, it may be necessary to tear down the AMT pseudo-interface to rediscover a new relay. 6.4. Creating SSM groups When a gateway wants to create an SSM group (i.e., in 232/8) for which it can source traffic, the remaining 24 bits MUST be generated as described below. ([SSM] states that "the policy for allocating these bits is strictly locally determined at the sender's host.") When the gateway determined its AMT Gateway Anycast Address as Thaler, et al. [Page 16] Internet-Draft Automatic Multicast October 2004 described above, it used the high bits of its global unicast address. The remaining bits of its global unicast address are appended to the 232/8 prefix, and any spare bits may be allocated using any policy (again, strictly locally determined at the sender's host). For example, if the IPv4 AMT Subnet Prefix is x.y/16, and the device has global unicast address a.b.c.d, then it MUST allocate IPv4 SSM groups in the range 232.c.d/24. 6.5. Joining SSM Groups with AMT Sources An IGMPv3/MLDv2 Report for a given (source, group) pair MAY be encapsulated directly to the source, when the source address belongs to the AMT Subnet Prefix. The "link-layer" address to use as the destination address in the outer IP header is obtained as follows. The source address in the inclusion list of the IGMPv3/MLDv2 report will be an AMT Gateway Anycast Address with the high bits of the address, and the remaining bits will be in the middle of the group address. For example, if the IPv4 AMT Subnet Prefix is x.y/16, and the IGMPv3 Report is for (x.y.a.b, 232.c.d.e), then the "link layer" IPv4 destination address used for encapsulation is a.b.c.d. 6.6. Receiving IGMPv3/MLDv2 Reports at the Gateway When an AMT Request is received by the gateway, it follows the same 3-way handshake procedure a relay would follow if it received the AMT Request. It generates a MAC and responds with an AMT Membership Query. When the AMT Membership Update is received, it verifies the MAC and then processes the IGMP/MLD Membership/Listener Report or Leave/Done. At the gateway, the IGMP/MLD packet should be an IGMPv3/MLDv2 source specific (S,G) join or leave. If S is not the AMT Gateway Anycast Address, the packet is dropped. If G does not contain the low bits of the global unicast address (as described above), the packet is also dropped. The gateway adds the source address (from the outer IP header) and UDP port of the report to a membership list for G. Maintaining this membership list may be done in any implementation-dependent manner. For example, it might be maintained by the "link-layer" inside the AMT pseudo-interface, making it invisible to the normal IGMP/MLD Thaler, et al. [Page 17] Internet-Draft Automatic Multicast October 2004 module. 6.7. Sending data to SSM groups When multicast packets are sent on the AMT pseudo-interface, they are encapsulated as follows. If the group address is not an SSM group, then the packet is dropped (this memo does not currently provide a way to send to non-SSM groups). If the group address is an SSM group, then the packet is unicast encapsulated to each remote node from which the gateway has received an IGMPv3/MLDv2 report for the packet's (source, group) pair. 7. Relay Router Details 7.1. At Startup time At startup time, the relay router will bring up an NBMA-style AMT pseudo-interface. It shall also add the AMT Relay Anycast Address on some interface. The relay router shall then advertise the AMT Relay Anycast Prefix into the unicast-only Internet, as if it were a connection to an external network. When the advertisement is done using BGP, the AS path leading to the AMT Relay Anycast Prefix shall include the identifier of the local AS and the AMT Unicast Autonomous System ID. The relay router shall also enable IGMPv3/MLDv2 on the AMT pseudo- interface, except that it shall not multicast Queries (this might be done, for example, by having the AMT pseudo-device drop them, or by having the IGMP/MLD module not send them in the first place). Finally, to support sourcing SSM traffic from AMT sites, the AMT Subnet Prefix is assigned to the AMT pseudo-interface, and the AMT Subnet Prefix is injected into the M-RIB of MBGP. 7.2. Receiving Relay Discovery messages sent to the Anycast Address When a relay receives a AMT Relay Discovery message directed to the AMT Relay Anycast Address, it should respond with a AMT Relay Advertisement containing its unicast address. The source and destination addresses of the advertisement should be the same as the destination and source addresses of the discovery message respectively. Further, the nonce in the discovery message MUST be Thaler, et al. [Page 18] Internet-Draft Automatic Multicast October 2004 copied into the advertisement message. 7.3. Receiving Membership Updates from AMT Gateways The relay operates passively, sending no Queries but simply tracking membership information according to Reports and Leave messages, as a router normally would. In addition, the relay must also do explicit membership tracking, as to which gateways on the AMT pseudo- interface have joined which groups. Once an AMT Membership Update has been successfully received, it updates the forwarding state for the appropriate group and source (if provided). When data arrives for that group, the traffic must be encapsulated to each gateway which has joined that group. The explicit membership tracking and unicast replication may be done in any implementation-specific manner. Some examples are: 1. The AMT pseudo-device driver might track the group information and perform the replication at the "link-layer", with no changes to a pre-existing IGMP/MLD module. 2. The IGMP/MLD module might have native support for explicit membership tracking, especially if it supports other NBMA-style interfaces. 7.4. Receiving (S,G) Joins from the Native Side, for AMT Sources The relay encapsulates an IGMPv3/MLDv2 report to the AMT source as described above in Section 4.1.2. 8. IANA Considerations The IANA should allocate a prefix dedicated to the public AMT Relays to the native multicast backbone. The prefix length should be determined by the IANA; the prefix should be large enough to guarantee advertisement in the default- free BGP networks; a length of 16 will meet this requirement. This is a one time effort; there is no need for any recurring assignment after this stage. The IANA should also allocate an Autonomous System ID which can be used as a pseudo-AS when advertising routes to the above prefix. Furthermore, to support sourcing SSM traffic from AMT gateways, the IANA should allocate a subnet prefix dedicated to the AMT link. The prefix length should be determined by the IANA; the prefix should be large enough to guarantee advertisement in the default- free BGP Thaler, et al. [Page 19] Internet-Draft Automatic Multicast October 2004 networks; a length of 16 will meet this requirement. This is a one time effort; there is no need for any recurring assignment after this stage. It should also be noted that this prefix length directly affects the number of groups available to be created by the AMT gateway: a length of 16 gives 256 groups, and a length of 8 gives 65536 groups. For diagnostic purposes, it is helpful to have a prefix length which is a multiple of 8, although this is not required. An autonomous system number dedicated to a pseudo-AS for multicast is already in use today (AS 10888), and so it is expected that no additional AS number is required for this prefix. Finally, IANA should allocate a reserved UDP port number for AMT encapsulation. 9. Security Considerations The anycast technique introduces a risk that a rogue router or a rogue AS could introduce a bogus route to the AMT Relay Anycast Prefix, and thus divert the traffic. Network managers have to guarantee the integrity of their routing to the AMT Relay anycast prefix in much the same way that they guarantee the integrity of all other routes. Within the native MBGP infrastructure, there is a risk that a rogue router or a rogue AS could introduce a bogus route to the AMT Subnet Prefix, and thus divert joins and cause RPF failures of multicast traffic. Again, network managers have to guarantee the integrity of the MBGP routing to the AMT subnet prefix in much the same way that they guarantee the integrity of all other routes in the M-RIB. Gateways and relays will accept and decapsulate multicast traffic from any source from which regular unicast traffic is accepted. If this is for any reason felt to be a security risk, then additional source address based packet filtering MUST be applied: 1. To prevent a rogue sender (that can't do traditional spoofing because of e.g. access lists deployed by its ISP) from making use of AMT to send packets to an SSM tree, a relay that receives an encapsulated multicast packet MUST discard the multicast packet if the IPv4 source address in the outer header is not composed of the last 2 bytes of the source address and the 2 middle bytes of the destination address of the inner header (i.e., a.b.c.d must be composed of the a.b of x.y.a.b and the c.d of 232.c.d.e). Thaler, et al. [Page 20] Internet-Draft Automatic Multicast October 2004 2. A gateway MUST discard encapsulated multicast packets if the source address in the outer header is not the address to which the encapsulated join message was sent. An AMT Gateway that receives an encapsulated IGMPv3/MLDv2 (S,G)-Join MUST discard the message if the IPv4 destination address in the outer header is not composed of the last 2 bytes of S and the 2 middle bytes of G (i.e. the destination address a.b.c.d must be composed of the a.b of the multicast source x.y.a.b and the c.d of the multicast group 232.c.d.e). 10. Acknowledgments Most of the mechanisms described in this document are based on similar work done by the NGTrans WG for obtaining automatic IPv6 connectivity without explicit tunnels ("6to4"). Tony Ballardie provided helpful discussion that inspired this document. Tom Pusateri helped flush out protocol details based on implementation experience and provided updates to this draft. 11. Normative References [HMAC] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [ICMP] Postel, J., "Internet Control Message Protocol", RFC 792, September 1981. [PROXY] Fenner, W., He, H., Haberman, B., Sandick, H., "IGMP/MLD- based Multicast Forwarding ("IGMP/MLD Proxying")", Work in progress, draft-ietf-magma-igmp-proxy-06.txt, April 2004. [IGMPv3] Cain, B., Deering, S., Fenner, B., Kouvelas, I., Thyagarajan A., "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [MLDv2] Vida, R., Costa, L., "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. [SSM] Holbrook, H., Cain, B., "Source-Specific Multicast for IP", Work in Progress, draft-ietf-ssm-arch-06.txt, September 2004. Thaler, et al. [Page 21] Internet-Draft Automatic Multicast October 2004 12. Informative References [Brad88] Braden, R., Borman, D., Partridge, C., "Computing the Internet Checksum", RFC 1071, September 1988. [6TO4] Carpenter, B., Moore, K., "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. [BROKER] Durand, A., Fasano, P., Guardini, I., Lento, D., "IPv6 Tunnel Broker", RFC 3053, January 2001. [ANYCAST] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers", RFC 3068, June 2001. [PIMSM] Estrin, D. Farinacci, D., Helmy, A., Thaler, D., Deering, S., Handley, M., Jacobson, V., Liu, C., Sharma, P., Wei, L., "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC 2362, June 1998. 13. Author's Address Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 Phone: +1 425 703 8835 EMail: dthaler@microsoft.com Mohit Talwar Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 Phone: +1 425 705 3131 EMail: mohitt@microsoft.com Amit Aggarwal Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 Phone: +1 425 706 0593 EMail: amitag@microsoft.com Thaler, et al. [Page 22] Internet-Draft Automatic Multicast October 2004 Lorenzo Vicisano Cisco Systems 170 West Tasman Dr. San Jose, CA 95134 Phone: +1 408 525 2530 EMail: lorenzo@cisco.com Dirk Ooms OneSparrow Belegstraat 13; 2018 Antwerp; Belgium EMail: dirk@onesparrow.com 14. Intellectual Property Rights Notice The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org." 15. Full Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Thaler, et al. [Page 23] Internet-Draft Automatic Multicast October 2004 16. Disclaimer This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Thaler, et al. [Page 24] Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Receiving Multicast in an AMT Site . . . . . . . . . . . . . 5 4.2. Sourcing Multicast from an AMT site . . . . . . . . . . . . 7 5. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. AMT Relay Discovery . . . . . . . . . . . . . . . . . . . . 9 5.2. AMT Relay Advertisement . . . . . . . . . . . . . . . . . . 10 5.3. AMT Request . . . . . . . . . . . . . . . . . . . . . . . . 11 5.4. AMT Membership Query . . . . . . . . . . . . . . . . . . . . 12 5.5. AMT Membership Update . . . . . . . . . . . . . . . . . . . 13 5.6. AMT UDP Data . . . . . . . . . . . . . . . . . . . . . . . . 14 6. AMT Gateway Details . . . . . . . . . . . . . . . . . . . . . 14 6.1. At Startup Time . . . . . . . . . . . . . . . . . . . . . . 15 6.2. Joining Groups with MBone Sources . . . . . . . . . . . . . 15 6.3. Responding to Relay Changes . . . . . . . . . . . . . . . . 16 6.4. Creating SSM groups . . . . . . . . . . . . . . . . . . . . 16 6.5. Joining SSM Groups with AMT Sources . . . . . . . . . . . . 17 6.6. Receiving IGMPv3/MLDv2 Reports at the Gateway . . . . . . . 17 6.7. Sending data to SSM groups . . . . . . . . . . . . . . . . . 18 7. Relay Router Details . . . . . . . . . . . . . . . . . . . . . 18 7.1. At Startup time . . . . . . . . . . . . . . . . . . . . . . 18 7.2. Receiving Relay Discovery messages sent to the Anycast Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.3. Receiving Membership Updates from AMT Gateways . . . . . . . 19 7.4. Receiving (S,G) Joins from the Native Side, for AMT Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 9. Security Considerations . . . . . . . . . . . . . . . . . . . 20 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 11. Normative References . . . . . . . . . . . . . . . . . . . . 21 12. Informative References . . . . . . . . . . . . . . . . . . . 22 13. Author's Address . . . . . . . . . . . . . . . . . . . . . . 22 14. Intellectual Property Rights Notice . . . . . . . . . . . . . 23 15. Full Copyright Statement . . . . . . . . . . . . . . . . . . 23 16. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 24 Thaler, et al. [Page i]