Network Working Group J. Macker, editor Internet-Draft NRL Expires: December 3, 2005 SMF Design Team IETF MANET WG June 2005 Simplified Multicast Forwarding for MANET draft-ietf-manet-smf-01 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 BCP 79. 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. This Internet-Draft will expire on December 3, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document describes the Simplified Multicast Forwarding (SMF) protocol that provides a basic IP multicast forwarding capability within mobile ad-hoc networks (MANET). SMF is designed to have limited applicability as a forwarding mechanism for multicast packets within MANET routing areas. In addition, it provides mechanisms to support interoperability with a connected wired infrastructure. SMF uses a simplified forwarding mechanism that delivers multicast Macker, editor & SMF Design Team Expires December 3, 2005 [Page 1] Internet-Draft SMF for MANET June 2005 packets to all MANET multicast receivers within a MANET routing area. The core design does not use group specific information in favor of reduced complexity and state maintenance within the mobile topology. The design accounts for the unique nature and behavior of MANET interfaces and takes advantage of efficient relay set algorithms previously designed and applied in the MANET routing control plane. This document described the SMF forwarding process in detail and describes several efficient relay set algorithms that have been implemented in conjunction with SMF. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 2] Internet-Draft SMF for MANET June 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 6 2. SMF Design Considerations . . . . . . . . . . . . . . . . . . 7 2.1. Previous Related Work . . . . . . . . . . . . . . . . . . 8 3. SMF Packet Processing and Forwarding . . . . . . . . . . . . . 9 4. SMF Duplicate Packet Detection . . . . . . . . . . . . . . . . 12 4.1. SMF IPv4 Duplicate Packet Detection . . . . . . . . . . . 13 4.2. SMF IPv6 Duplicate Packet Detection . . . . . . . . . . . 14 4.2.1. IPv6 SMF-DPD Header Option Format . . . . . . . . . . 15 5. Relay Set Selection . . . . . . . . . . . . . . . . . . . . . 16 6. SMF Multicast Border Gateway Considerations . . . . . . . . . 17 6.1. Forwarded Multicast Groups . . . . . . . . . . . . . . . . 17 6.2. Multicast Group Scoping . . . . . . . . . . . . . . . . . 17 6.3. Duplicate Packet Detection Marking . . . . . . . . . . . . 17 6.4. Interface with Exterior Multicast Routing Protocols . . . 18 6.5. Multiple Gateways . . . . . . . . . . . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . . 22 Appendix A. Source-based Multipoint Relay (S-MPR) . . . . . . . . 23 A.1. S-MPR Relay Set Selection . . . . . . . . . . . . . . . . 23 A.2. S-MPR Forwarding Rules . . . . . . . . . . . . . . . . . . 24 A.3. Neighborhood Discovery Requirements . . . . . . . . . . . 24 Appendix B. Essential Connecting Dominating Set (E-CDS) Algorithm . . . . . . . . . . . . . . . . . . . . . . 25 B.1. E-CDS Relay Set Selection . . . . . . . . . . . . . . . . 25 B.2. E-CDS Forwarding Rules . . . . . . . . . . . . . . . . . . 25 B.3. Neighborhood Discovery Requirements . . . . . . . . . . . 25 Appendix C. Multipoint Relay Connected Dominating Set (MPR-CDS) Algorithm . . . . . . . . . . . . . . . . . 26 C.1. MPR-CDS Relay Set Selection . . . . . . . . . . . . . . . 26 C.2. MPR-CDS Forwarding Rules . . . . . . . . . . . . . . . . . 26 C.3. Neighborhood Discovery Requirements . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 Intellectual Property and Copyright Statements . . . . . . . . . . 28 Macker, editor & SMF Design Team Expires December 3, 2005 [Page 3] Internet-Draft SMF for MANET June 2005 1. Introduction Design and implementation progress has been made in developing and implementing more efficient ways to flood control packets within MANET wireless routing protocol designs. For example, algorithms within MANET RFC 3626 [RFC3626]and RFC 3684 [RFC3684] specify distributed methods of dynamically electing reduced relay sets for the purposes of optimizing the flooding routing control packets to all MANET nodes. In this document, we specify the Simplified Multicast Forwarding (SMF) framework. The main purpose of SMF is to adapt efficient flooding designs in MANET environments and apply these mechanisms to IP multicast packet forwarding. When efficient flooding is a sufficient technique, SMF can make multicast forwarding available to data flows within a MANET routing area. The SMF baseline design limits the scope to basic, best effort multicast forwarding and its applicability is intended to be constrained within a MANET routing area. Figure 1 provides an overview of the logical SMF node architecture, consisting of "Neighborhood Discovery", "Relay Set Selection" and "Forwarding Process" components. Typically, relay set selection (or even self-election) will occur based on input from a neighborhood discovery process, and the forwarding process will be controlled by status based upon relay set selection. In some cases, the forwarding decision for a packet may also depend on previous hop or incoming interface information. The asterisks (*) in Figure 1 mark the primitives and relationships needed by relay set algorithms requiring previous-hop packet forwarding knowledge. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 4] Internet-Draft SMF for MANET June 2005 ______________ _____________ | | | | | Neighborhood | | Relay Set | | Discovery |------------->| Selection | | Protocol | neighbor | Algorithm | |______________| info |_____________| \ / \ / neighbor\ /fowarding info* \ ____________ / status \ | | / `-->| Forwarding |<--' | Process | ~~~~~~~~~~~~~~~~~>|____________|~~~~~~~~~~~~~~~~~> incoming packet, forwarded packets interface id, and previous hop* Fig. 1 - SMF Node Architecture This document describes a network layer multicast forwarding process compatible with different neighborhood discovery protocols and relay set selection algorithms. Different discovery mechanisms or relay set algorithms may be applicable for different MANET routing protocols and deployments. In the simplest case, Classical Flooding (CF) can be supported, eliminating the need for any relay set algorithm or neighborhood topology information. However, it is expected that more efficient flooding techniques will be preferred due to expected gains in network efficiency and reductions in wireless congestion and contention. Efficient flooding is realized by selecting a _subset_ of all possible nodes in a MANET area as the forwarding relay set. Algorithms exist that can make use of local network neighborhood topology information to determine an appropriate relay set in a distributed, dynamic fashion. These relay set selection algorithms can be used to provide a distribution tree for user multicast data[MDC04]. A few such relay set selection algorithms are described in Appendices of this document, but it is possible that additional relay set algorithms or extensions may be specified in the future for use with SMF. As noted, neighborhood topology information is usually needed for a relay set selection algorithm to determine an appropriate set of forwarding nodes. In many cases, it is expected that neighborhood topology discovery functions may be provided by a MANET unicast routing protocol or neighbor discovery implementation running in concurrence with SMF. Or, in other cases, it is possible that some wireless link layer or multiple access protocols may provide the necessary neighborhood information through an enhanced interface. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 5] Internet-Draft SMF for MANET June 2005 Different relay set algorithms and associated forwarding decision logic may have differing neighborhood discovery demands than other relay set algorithms. While this document states specific requirements for neighborhood discovery with respect to the forwarding process and relay set selection algorithms described herein, it is left to other documents or protocol specifications to define a specific neighborhood discovery protocol [TBD - REF "MANET Neighborhood Discovery Protocol" document]. Note that Figure 1 provides a notional architecture for _typical_ MANET SMF-capable nodes. However, a goal is that simple end-system (non-forwarding) wireless nodes may also participate in multicast traffic transmission and reception with standard network layer semantics. Also, a multicast border gateway mechanism MUST be used when interoperating with other IP multicast routing in the wired world (e.g., PIM). In present experiments, MLD proxying methods have been used to enable some gateway functionality at MANET border gateways operating with wired multicast routing protocol interfaces. Although SMF may be extended or combined with other protocols to provide increased reliability and group specific forwarding state, the details of those methods will be discussed in other documents. 1.1. Terminology MANET : Mobile Ad hoc Networks SMF : Simplified Multicast Forwarding CF : Classical Flooding CDS : Connected Dominating Set MPR : Multi-point Relay DPD: Duplicate Packet Detection 1.2. Applicability A basic packet forwarding service that reaches all destinations participating within a MANET environment can be a useful generic group communication mechanism for an application layer. While the design requirements for this are similar to those often needed by the control plane of many MANET unicast routing protocol layers, it is desirable to provide a more generic forwarding function for use by other applications. There are a number of application areas that could take advantage of a simple ALL_NODES delivery service within a MANET routing region (e.g., multimedia streaming, peer-to-peer middleware multicasting, auto-configuration, and discovery services). Macker, editor & SMF Design Team Expires December 3, 2005 [Page 6] Internet-Draft SMF for MANET June 2005 2. SMF Design Considerations The simplest design often conceived and adapted for MANET packet flooding is something we designate as the "classical flooding" (CF) algorithm. In CF, each participating forwarder node is required to rebroadcast packets intended for dissemination once and only once. This approach is extremely simple and only requires only some means of duplicate packet detection (DPD) and a basic forwarding mechanism. However, it is well known that using CF, especially within dense networks, results in a significant number of redundant transmissions often referred to as the broadcast storm problem [NTSC99]. Within wireless multi-hop network direct contention and interference is often experienced beyond the single hop interface and reducing unnecessary channel contention within a MANET can also significantly improve network performance. Therefore, minimizing the number of required relay nodes is a heightened design goal in this environment. Unfortunately, reducing the number of relay nodes in a MANET environment may also decrease the robustness of packet delivery in a mobile topology. There exists an interdependent design tradeoff between relay efficiency and delivery robustness that is scenario and system dependent and should be considered carefully. If needed, additional reliability mechanisms MAY be considered for use with reduced relay sets (e.g., backup and redundant relay set) but the use of limited hop-by-hop retransmission schemes at the network layer are considered out of scope for this document. If needed by an application, the use of IETF reliable multicast transport layer protocols (e.g., RMT) should be transparently supported by SMF's best effort delivery mechanism. The basic design goal of SMF is support both a simple CF-based forwarding capability and to supplement this with reduced relay set algorithms for increased efficiency. At a theoretic level, work in the area of minimizing packet forwarders, or relay node sets, is often related to basic graph theory problems. In graph theory, a dominating set (DS) for a graph is a set of vertices that, along with their neighbors, constitute all the vertices in the graph. A connected DS (CDS) is a DS where the subgraph induced is connected. A minimum CDS (MCDS) is a set such that the number of vertices is the minimum required to form a CDS. Finding a small dominating set is one of the most fundamental problems of traditional graph theory and is often related to the problem of optimizing flooding algorithms in MANET routing protocols. Finding an MCDS in a given graph is known to be NP-hard [GJ79]. These basic static graph theoretic issues are important to apply in developing efficient relay sets, but in addition MANET relay set designs require distributed and dynamic operation. To better explain the design requirement, we formulated the following characteristics desired of an effective MANET flooding algorithm solution for use in SMF: Macker, editor & SMF Design Team Expires December 3, 2005 [Page 7] Internet-Draft SMF for MANET June 2005 1. A resultant cover set that is small compared to the total number of nodes as the network scales in size and density. 2. A robust approach somewhat resilient to network mobility and link dynamics. 3. A cover set election/maintenance mechanism that is lightweight, distributed, and adaptive in nature. 2.1. Previous Related Work Previous novel work on MANET flooding and reduced relay set mechanisms has been done and this document borrows from and builds off previous work accomplished. In [WC02], a taxonomy of flooding algorithms for use in MANET environments was presented and the work examined performance issues related to various approaches. Other important work has developed distributed mechanisms that select and maintain reduced relay node sets. As we have already mentioned, the design tradeoffs are further complicated by wireless contention, topological classes, and the robustness of packet delivery and set election under mobility scenarios. In addition, the actual protocol implementation for IP multicast forwarding based upon these flooding algorithms raises additional design tradeoffs and issues. This includes maintenance of protocol state, duplicate packet detection mechanisms, packet processing requirements, expected traffic patterns (e.g., one -to-many vs. many-to-many), requirements for robust performance, and associated protocol signaling required by any associated mechanism. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 8] Internet-Draft SMF for MANET June 2005 3. SMF Packet Processing and Forwarding The SMF Packet Processing and Forwarding actions are conducted with the following packet handling activities: 1. Interception of outbound, locally-generated multicast packets. 2. Reception of inbound packets on a specific network interface(s). In the case that sequence-based DPD as described later (see Section 4)is used, the purpose of intercepting outbound, locally- generated multicast packets is to apply resequencing of the IPv4 ID header field or add options headers as needed (e.g. IPv6). In the case that resequencing is deemed necessary, it is RECOMMENDED that sequence numbering be applied such that a different sequence number space per duple be used. For initial SMF purposes where no distinct routing path decisions for different IP Multicast address destinations occur, it might appear to be sufficient to use sequence number spaces aggregated across all IP Multicast destinations (or across all IP destinations for a source as is the default implementation of the IPv4 ID field in many operating systems). Future SMF extensions, beyond the present discussion, may contain dynamic forwarding state dependent on the multicast destination address. The future possibility that different destinations may be routed differently suggests "per source/ destination" numbering be used. It should be noted that the default global IPv4 ID sequence space may be sufficient for some SMF deployments and interception of outbound packets may not be required if end systems have numbered the IPv4 ID field in an acceptable manner. In other cases, such as when IPSec headers have been applied to packets, other sequence information (perhaps even "per flow") may be available for the SMF process to make use of in its duplicate table management. Inbound multicast packets will be received by the SMF implementation and processed for possible forwarding. Well-known multicast groups for flooding to all nodes of an ad hoc network are specified for use with the network-layer flooding provided by SMF. These multicast groups are specified to contain all nodes of a contiguous ad hoc network, so that packets transmitted to the multicast address associated with the group will be delivered to all nodes as desired. In other words, any node that is reachable, is automatically granted membership in these multicast groups. For IPv6, the multicast address is specified to be "site-local". The names of the multicast groups are given as "ALL_IPv4_MANET_NODES" (TBD) and "ALL_IPv6_MANET_NODES" (TBD). This document does not support transmissions to any directed broadcast address ranges. Minimally SMF SHALL forward unique (non-duplicate) packets received for these Macker, editor & SMF Design Team Expires December 3, 2005 [Page 9] Internet-Draft SMF for MANET June 2005 well-known group addresses when the TTL or hop count value in the IP header is greater than 1. Optionally, SMF deployments may choose to forward packets for additional "global scope" multicast groups to support application needs or to distribute multicast packets that have ingressed the MANET area via border gateways. Additional addresses will be specified by an a priori list or possible through a dynamic address management interface. In all cases, the following rules SHALL be observed for SMF multicast forwarding: 1. Multicast packets with TTL <= 1 MUST NOT be forwarded*. 2. IPv6 Link Local multicast packets MUST NOT be forwarded regardless of TTL. 3. Multicast packets with an IP source address matching one of those of the local host interface(s) MUST NOT be forwarded. 4. Received packet frames with the MAC source address matching the local host interface(s) MUST NOT be forwarded. _(* An exception may be needed for rule #1 if IGMP or MLD proxy functions dictate flooding of such messages)_ Note that rule #3 is important because in wireless networks, the local host may receive re-transmissions of its own packets when they are forwarded by neighboring nodes. This rule avoids unnecessary retransmission of locally-generated packets even when other forwarding decision rules would apply. Once these criteria have been met, the implementation should reference a forwarding decision algorithm, possibly in concert with duplicate packet detection, to determine the next step in packet processing. The forwarding decision may be implicit if the SMF implementation is configured to perform classic flooding (CF) of IP multicast packets. Otherwise, the forwarding decision may be controlled by another process. Neighborhood discovery protocols coupled with the source-based multi-point relay (S-MPR) or other CDS selection algorithms described below MAY be used to determine the local host's status with respect to forwarding. For example, some algorithms may control forwarding based on a previous hop indentifier (e.g. S-MPR forwarding), while others may designate the local host as a forwarder of all packets (or at least those generated by known neighbors) based on the neighborhood broadcast topology (e.g. Essential CDS (E-CDS)). DPD is the perhaps the most complex portion of the SMF forwarding process. In general, detection of received duplicate packets is necessary to avoid forwarding the same packet multiple times. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 10] Internet-Draft SMF for MANET June 2005 However, in some cases (e.g., S-MPR), duplicate detection of some non-forwarded packets is also needed to maintain efficient forwarding. Details on different duplicate packet detection and forwarding rules for the S-MPR, and E-CDS algorithms are given in Appendices of his document. The details for these classes of algorithms may also apply to other similar algorithms. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 11] Internet-Draft SMF for MANET June 2005 4. SMF Duplicate Packet Detection One important design difference between routing for MANET interfaces and many wired network interfaces is that forwarding out the same physical interface a packet arrived upon is a normal operation. This operation is often disallowed or discouraged in wired multicast routing designs to avoid possible looping. Similarly, any Reverse Path Forwarding (RPF) logic also used needs to be softened when operating over a single MANET interface. This MANET interface characteristic leads to DPD as a common requirement in MANET packet flooding. While this requires increased per packet processing, it is necessary in MANET-specific multicasting because packets must be potentially be forwarded out the same physical interface they may arrive on and nodes can even receive copies of previously-transmitted packets from other forwarding neighbors. This section describes a basic SMF DPD mechanism and some alternative operational options as considerations. SMF SHOULD implement explicit detection of duplicate multicast packets by a temporal packet identification scheme. This is typically implemented by keeping a history of previous received and forwarded packet identifiers for comparison against recently forwarded multicast packets. There are different possible approaches to packet identification that have been considered. Possibilities include unique markings within packet header fields, such as packet sequence numbering, or application of hash algorithms or similar techniques to compactly and uniquely describe the history of recently received packets. This document RECOMMENDS simple, sequence-based schemes that can be accomplished without additional (non-IP) encapsulation of packets and/or their content. Encapsulation approaches are considered out-of-scope so that non-forwarding edge nodes within a MANET area may easily receive flooded content without any additional software beyond that of a typical IP stack. Packet hashing approaches for DPD may be applicable in some cases, yet early examination of these approaches indicated the computation complexity may be prohibitive for per-packet processing on many candidate MANET platforms (e.g., PDAs). Additionally, the unavoidable "cache-miss" rates, while possibly low for some algorithms, result in the severe penalty of false DPD (and thus packet loss) rather than the more benign penalty of additional computation cycles as associated with most applications of hashing. Implementations will need to age and/or timeout duplicate packet state as new packets are received and forwarded. In the case that sequence-based packet identification is used, implementations SHOULD timeout stale histories for entries where new, non-duplicate packets have not been recently received. The proper duration of any timeout delay is a function of Macker, editor & SMF Design Team Expires December 3, 2005 [Page 12] Internet-Draft SMF for MANET June 2005 expected possible network traversal time, roughly NET_DIAMETER (in hops) times NODE_TRAVERSAL_TIME. NET_DIAMETER measures the maximum possible number of hops given any two nodes in the network. NODE_TRAVERSAL_TIME is a conservative estimate of the average one hop traversal time for packets and should include queueing delays, interrupt processing times, medium access delays, and propagation delays. If the timeout is reset only upon reception of non-duplicate packets, it also limits the time that packets might be incorrectly dropped if a source node is stopped and restarted in the case of sequence-based packet identification. The required size of the duplication detection cache is similarly governed and is also a function of the maximum expected packet rate. Of course, the duplicate packet detection mechanism SHOULD avoid keeping unnecessary state for packet flows such as those that are locally generated or link local destinations that would not be considered for forwarding. 4.1. SMF IPv4 Duplicate Packet Detection IPv4 multicast packets from a particular source are assumed to be marked with a temporally unique identification number in the ID field of the IPv4 packet header [RFC0791] that can serve as a "packetIdentifier" for SMF purposes. Unfortunately, in present operating system networking kernels, this IP header field value is not always generated or applied in a consistent manner with respect to SMF needs. In order to build a working implementation without encapsulating packets, an SMF implementation SHOULD provide a sequence generation and marking module that can maintain and set a monotonically increasing IP ID field for locally-generated multicast packets with independent sequence number spaces applied on a basis. When needed this process will also recalculate and replace a proper IP header checksum for the formulated header. For gateways injecting external IPv4 traffic into an SMF MANET area, the gateways SHOULD perform this same IP ID field re-sequencing. Note the presence of IPSec may prevent such resequencing, but does provide its own organic means for duplicate packet detection. The use of IPSec for candidate packet flows presents the opportunity to make use of the additional, perhaps more reliable, sequencing information of the IPSec header for unique packet identification. The IPSec header provides a packet identifier field that can be used on a "per-security assocation" basis. The IP addressing and IPSec Security Parameters Index (SPI) fields are used to identify security associations and, hence, packet flows. So, if the packet is IPSec encapsulated, SMF will check the where the or Macker, editor & SMF Design Team Expires December 3, 2005 [Page 13] Internet-Draft SMF for MANET June 2005 from the IPSec header serves as the "packetIdentifier" value. Although it would be possible to support IP layer fragmentation , SMF traffic sources or gateways SHOULD set the don't fragment bit for traffic intended to be carried by SMF. This is recommended to avoid the additional complexity and inefficiencies arising from supporting IP layer fragmentation. To perform duplicate detection, SMF will check the combination against a cache history of received packet identifiers. Some forwarding algorithms may require that unique packets are only noted when received from certain neighbor nodes. Multiple interface semantics may also add some additional considerations to the forwarding process depending upon the specific relay set selection forwarding rules. Although its use is demonstrated in working prototype code, the adoption of the IPv4 ID field for widespread packet duplication detection has some disadvantages that should be discussed. The main disadvantage is that, as mentioned, the use and interpretation of the field is known to be non-consistent across operating systems. The ID field is also limited and may provide less robust detection for high bandwidth applications since sequence wrap-around may occur relatively frequently if it is not possible to achieve "per source/ destination" sequencing. As an alternative, the use of a header option or encapsulation header in future implementations may provide more flexibility and consistency (see IPv6 DPD). Another advantage of using a header option (or other encapsulation, if determined absolutely necessary) is that it would be possible for MANET gateway nodes to assess whether packets ingressing a MANET area have already been properly sequenced to avoid unnecessary re-injection of packets. We leave these design alternatives to be further defined and discussed in future work or later revisions of this ID. A basic sequencing and marking design similar to the one we formulate here can be easily adapted to work with future approaches or can be bypassed when not needed. 4.2. SMF IPv6 Duplicate Packet Detection The following section describes the mechanism and options for SMF IPv6 DPD. The core IPv6 header does not provide an explicit identification header field that we can exploit for DPD. SMF defines two methods for IPv6 use: a hop-by-hop DPD options header and the use of IPSec sequencing when an IPSec header is detected. SMF MUST provide a DPD marking module that can insert the hop-by-hop IPv6 header option defined for locally generated MANET multicast. If the packet is _not_ IPSec encapsulated, SMF will use the IPv6 packet header and IPv6 DPD option to form that is checked against a cache history of received IPv6 packet identifiers. Similarly to the case for IPv4, the presence of IPSec may prevent the intermediate addition of a hop-by-hop options header. Fortunately, the IPSec header provides a packet identifier field that can be used on a "per- security assocation" basis. The IP addressing fields and IPSec Security Parameters Index (SPI) fields are used to identify security associations and, hence, packet flows. So, if the packet is IPSec encapsulated, SMF will check the where the or from the IPv6 IPSec header serves as a "packetIdentifier" value. 4.2.1. IPv6 SMF-DPD Header Option Format Figure 2 illustrates the format of the IPv6 SMF Duplication Packet Detection (SMF-DPD) hop-by-hop header option. (Discussion: Is there any value in specifying this as a "destination" option instead?). If this is the only hop-by-hop option present, this will result in the addition of 8 bytes to the IPv6 packet header including the "Next Header", "Header Extension Length", SMF-DPD option fields, and padding. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Option Type | Opt. Data Len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DPD packet identifier | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fig. 2 - IPv6 SMF-DPD Hop-by-hop Header Option Option Type = (TBD) Opt. Data Len = 2 DPD packet identifier = monotonically increasing 16-bit sequence number assigned on a per basis. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 15] Internet-Draft SMF for MANET June 2005 5. Relay Set Selection As mentioned SMF MUST support CF-based forwarding as a basic forwarding mechanism when optimized relay set information is not available or not selected. In CF mode, each node transmits a locally generated or newly received packet exactly once. The DPD technique mentioned in the previous section is critical to proper operation and avoids any duplicate packet retransmissions. In the requirements sections, it was stated that SMF MUST support the ability to modify forwarding rules based upon relay set information that is received dynamically during operation. In this way, SMF can operate more efficiently within the MANET multicast area. In the section we define the interface and processing semantics to allow SMF to support a variety of different relay set algorithms and approaches. Some recommended criteria for relay set selection algorithm candidates: 1. Robust with respect to mobility or other network dynamics 2. Require minimal signaling for neighbor discovery and/or control 3. Allow nodes to express preference for/against selection as relay Some relay set algorithms that meet this criteria are described in the Appendices of this document. Different algorithms may be more suitable for different MANET routing types or deployments. Additional relay set selection algorithms may be specified in separate documents in the future. The Appendices in this document can serve as a template for the content of such potential future specifications. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 16] Internet-Draft SMF for MANET June 2005 6. SMF Multicast Border Gateway Considerations Typically, it is expected that SMF will be used to distribute multicast traffic within a MANET area. However, it is also envisioned to allow interconnection of SMF operation with networks using other multicast routing protocols if appropriate conditions are met. Some primary considerations for further defintion (TBD) include: 1. Determining which multicast groups should transit the gateway whether entering or exiting the attached MANET area(s). 2. TTL threshold or other scoping policies. 3. Any marking or labeling to enable DPD on ingressing packets. 4. Interface with exterior multicast routing protocols. 5. Possible operation with multiple gateways. The behavior of gateway nodes is the same as that of non-gateway nodes when forwarding packets to interfaces within the MANET area. Packets that are passed to interfaces operating typical multicast routing protocols SHOULD be evaluated for duplicate packet status since standard multicast forwarding for wired interfaces do not usually perform this function. 6.1. Forwarded Multicast Groups Determining which groups should be forwarded to or from a MANET SMF area presents challenges. Ideally, only groups for which there is active group membership should be injected into the SMF domain. This might be accomplished by providing an IPv4 Internet Group Membership Protocol (IGMP) or IPV6 Multicast Listener Discovery (MLD) proxy function so that MANET SMF nodes can inform attached gateways (and hence multicast networks) of their current group membership status. 6.2. Multicast Group Scoping (TBD) 6.3. Duplicate Packet Detection Marking Packets sourced external to an SMF area may not have duplicate packet sequencing properly applied and the gateway may need to provide that sequencing information upon entry to the network. In the case of IPv6, the gateway can apply the SMF DPD Hop-by-Hop options header to packets forwarded into the MANET area for those packets that do not Macker, editor & SMF Design Team Expires December 3, 2005 [Page 17] Internet-Draft SMF for MANET June 2005 already have the option applied. This header option provides the side benefit that the gateway can explicitly determine if the packet has already been marked and can use the packet identification field to determine that it is not re-injecting a duplicate packet. For IPv4, it is RECOMMENDED that gateway nodes re-sequence the ID field of packets injected into the area. However, the IPv4 ID field does not provide the gateway with explicit information on whether the field has been previously set for SMF purposes. Thus, the potential exists that duplicate IPv4 packets may be re-injected by a gateway into an SMF area. Further discussion and experimentation is recommended in this area. _(If multiple gateways are injecting packets, how do we make sure that dups are sequenced the same way? )_ 6.4. Interface with Exterior Multicast Routing Protocols (TBD) 6.5. Multiple Gateways (TBD) Macker, editor & SMF Design Team Expires December 3, 2005 [Page 18] Internet-Draft SMF for MANET June 2005 7. Security Considerations Gratuitous use of option headers can cause problems in routers. Routers outside of MANET routing areas should ignore SMF header options if encountered. Authentication mechanisms to identify the source of an option header should be considered to reduce vulnerability to a variety of attacks. Additional security consideration TBD. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 19] Internet-Draft SMF for MANET June 2005 8. IANA Considerations There are number of discussions within this SMF specificat that will be subject to IANA registration. The IP Header Extensions being defined within this document MUST have an IANA registry established for them upon publication of the first RFC. Additionally, the well- known multicast addresses intended for default use by the SMF forwarding process should be registered and defined by the first RFC published. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 20] Internet-Draft SMF for MANET June 2005 9. Acknowledgments Many of the concepts and mechanisms used and adopted by SMF resulted from many years of discussion of related work within the MANET WG. There are obviously many people that have contributed to past discussions and related draft documents within the WG that have influenced the development of SMF concepts that deserve acknowledgment. In particular, the document is largely a direct product of the SMF design team within the IETF MANET WG and borrows text and implementation ideas from these individuals. SMF Design Team Contributors Thomas Clausen Charles Perkins Brian Adamson Justin Dean Brian Haberman Ian Chakeres Maoyu Wang Et al The RFC text was produced using Marshall Rose's xml2rfc tool. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 21] Internet-Draft SMF for MANET June 2005 10. References 10.1. Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. 10.2. Informative References [E-CDS] Ogier, R., "MANET Extension of OSPF Using CDS Flooding", Proceedings of the 62nd IETF , March 2005. [GJ79] Garey, M. and D. Johnson, "Computers and Intractability: A Guide to the Theory of NP-Completeness.", Freeman and Company , 1979. [JLMV02] Jacquet, P., Laouiti, V., Minet, P., and L. Viennot, "Performance of multipoint relaying in ad hoc mobile routing protocols", Networking , 2002. [MDC04] Macker, J., Dean, J., and W. Chao, "Simplified Multicast Forwarding in Mobile Ad hoc Networks", IEEE MILCOM 2004 Proceedings , 2004. [NTSC99] Ni, S., Tseng, Y., Chen, Y., and J. Sheu, "The Broadcast Storm Problem in Mobile Ad hoc Networks", Proceedings Of ACM Mobicom 99 , 1999. [RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing Protocol", 2003. [RFC3684] Ogier, R., Templin, F., and M. Lewis, "Topology Dissemination Based on Reverse-Path Forwarding", 2003. [WC02] Williams, B. and T. Camp, "Comparison of Broadcasting Techniques for Mobile Ad hoc Networks", Proceedings of ACM Mobihoc 2002 , 2002. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 22] Internet-Draft SMF for MANET June 2005 Appendix A. Source-based Multipoint Relay (S-MPR) The source-based multipoint relay (S-MPR) set selection algorithm enables individual nodes, using two-hop topology information to select a minimum set of neighboring nodes that can provide relay to all nodes within a two-hop radius. This distributed technique has been shown to approximate selection of a MCDS in [JLMV02]. Individual nodes must collect two-hop neighborhood information from neighbors, determine an appropriate current relay set, and then inform the resultant selected neighbors of their relay status. The Optimized Link State Routing (OLSR) protocol has used this algorithm and protocol for relay of link state updates and other control information[RFC3626] and has been shown to operate well even in dynamic network environments. Because a node's status as a relay is with respect to neighboring nodes who have selected it (i.e., its "selectors"), the relaying node must know the previous-hop transmitter of packets it receives in order to make an appropriate forwarding decision. Additionally, it is important that relay nodes forward packets only for those nodes currently identified as symmetric, one-hop neighbors to maintain correctness. Also, because the selection of relays does not result in a common set among neighboring nodes, relays MUST mark in their duplicate table any transmissions from non-selector, symmetric, one- hop neighbors (for a given interface) and not forward subsequent received copies of that packet even if received from a selector neighbor. Deviation here may result in unecessary, even excessive, repeat transmission of packets throughout the network. Or incorrect duplicate table recording of packets received from non-symmetric neighbors may result in incomplete flooding. In these respects, flooding based on the S-MPR algorithm is more complex than that based upon some other relay set selection algorithms. When multiple interfaces are present, the S-MPR SMF forwarded must keep some independent state for each interface with regards to duplicate packets. For example, when a packet is received from a non-selector, one-hop symmetric neighbor, an SMF forwarder using the S-MPR algorithm must update its duplicate packet state with respect to the interface on which the packet was received. If the SMF forwarder receives that same packet from a selector neighbor on a different interface, it MUST still forward that packet on all interfaces it has not received that packet from a one-hop symmetric neighbor. Once a packet has been forwarded in this fashion, subsequent duplicates received on any interface are ignored. A.1. S-MPR Relay Set Selection (TBD - Succinct description of the relay set selection algorithm. In Macker, editor & SMF Design Team Expires December 3, 2005 [Page 23] Internet-Draft SMF for MANET June 2005 this case, nodes collect two-hop neighbor information, then selctexplicitly designate (via messaging) certain neighbors as relays.) A.2. S-MPR Forwarding Rules (TBD - Enumerate rules for S-MPR duplicate marking and forwarding) A.3. Neighborhood Discovery Requirements (TBD - describe any extensions (i.e., TLV fields) to the neighborhood discovery protocol to support the S-MPR algorithm. For example, S-MPR can possibly make use of relay "wilingness" factor. Also will need to specify TLV for messages sent (or broadcast) designating multipoint relay choices) Macker, editor & SMF Design Team Expires December 3, 2005 [Page 24] Internet-Draft SMF for MANET June 2005 Appendix B. Essential Connecting Dominating Set (E-CDS) Algorithm The "Essential connecting dominating set" (E-CDS) algorithm [E-CDS] allows nodes to use two-hop topology information to appropriately elect _themselves_ as relay nodes to form an efficient (for flooding) CDS. While this algorithm does not tend to produce as small a set of relay nodes (per forwarded packet) as the previously-described S-MPR algorithm, it is not dependent upon previous-hop information to make a forwarding decision; it simply forwards any received non-duplicate packets. This property also allows relay nodes using the E-CDS algorithm to be intermixed with nodes performing only classical flooding. Additionally, the semantics for multiple interface support are simplified as compared to S-MPR and even packets that are received from non-symmetric neighbors may be forwarded without compromising flooding efficiency or correctness. B.1. E-CDS Relay Set Selection (TBD - Succinct description of the relay set selection algorithm. In this case, nodes collect two-hop neighbor information and "self- elect" themselves as relays as needed) B.2. E-CDS Forwarding Rules (TBD - Enumerate rules for E-CDS duplicate marking and forwarding) B.3. Neighborhood Discovery Requirements (TBD - describe an extensions (i.e., TLV fields) to the neighborhood discovery protocol to support the E-CDS algorithm. For example, E-CDS can make use of node "degree" and/or possibly a "wilingness" or "reluctance" factor) Macker, editor & SMF Design Team Expires December 3, 2005 [Page 25] Internet-Draft SMF for MANET June 2005 Appendix C. Multipoint Relay Connected Dominating Set (MPR-CDS) Algorithm (TBD - Overview of MPR-CDS algorithm) C.1. MPR-CDS Relay Set Selection (TBD - Succinct description of the relay set selection algorithm. In this case, nodes collect two-hop neighbor information, MPR relay nodes are selected and designated, and finally, some selected MPRs "prune" themselves based on other information) C.2. MPR-CDS Forwarding Rules (TBD - Enumerate rules for MPR-CDS duplicate marking and forwarding) C.3. Neighborhood Discovery Requirements (TBD - describe an extensions (i.e., TLV fields) to the neighborhood discovery protocol to support the MPR-CDS algorithm. For example, MPR-CDS may use extensions similar to those of S-MPR and/or E-CDS to meet its needs.) Macker, editor & SMF Design Team Expires December 3, 2005 [Page 26] Internet-Draft SMF for MANET June 2005 Authors' Addresses Joseph Macker NRL Code 5522 Washington, DC 20375 USA Email: macker@itd.nrl.navy.mil SMF Design Team IETF MANET WG Email: manet@ietf.org Macker, editor & SMF Design Team Expires December 3, 2005 [Page 27] Internet-Draft SMF for MANET June 2005 Intellectual Property Statement 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. Disclaimer of Validity 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. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Macker, editor & SMF Design Team Expires December 3, 2005 [Page 28]