Network Working Group T. Clausen Internet-Draft Ecole Polytechnique Updates: 5444 (if approved) C. Dearlove Intended status: Standards Track BAE Systems Expires: November 5, 2016 U. Herberg H. Rogge Fraunhofer FKIE May 4, 2016 Rules For Designing Protocols Using the RFC 5444 Generalized Packet/ Message Format draft-ietf-manet-rfc5444-usage-04 Abstract RFC 5444 specifies a generalized MANET packet/message format and describes an intended use to multiplex MANET routing protocol messages that is mandated for use by RFC 5498. This document updates RFC 5444 by providing rules and recommendations for how the multiplexer operates and how protocols can use the packet/message format. In particular, the mandatory rules prohibit a number of uses of RFC 5444 that have been suggested in various proposals, and which would have led to interoperability problems, to the impediment of protocol extension development, and to an inability to use optional generic RFC 5444 parsers. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 5, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the Clausen, et al. Expires November 5, 2016 [Page 1] Internet-Draft RFC 5444 Usage May 2016 document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. History and Purpose . . . . . . . . . . . . . . . . . . . 3 1.2. RFC 5444 Features . . . . . . . . . . . . . . . . . . . . 3 1.2.1. Packet/Message Format . . . . . . . . . . . . . . . . 4 1.2.2. Multiplexing and Demultiplexing . . . . . . . . . . . 6 1.3. Status of This Document . . . . . . . . . . . . . . . . . 6 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Applicability Statement . . . . . . . . . . . . . . . . . . . 7 4. Information Transmission . . . . . . . . . . . . . . . . . . . 7 4.1. Where to Record Information . . . . . . . . . . . . . . . 7 4.2. Message Multiplexing and Packets . . . . . . . . . . . . . 9 4.3. Messages, Addresses and Attributes . . . . . . . . . . . . 11 4.4. Addresses Require Attributes . . . . . . . . . . . . . . . 12 4.5. Information Representation . . . . . . . . . . . . . . . . 14 4.6. TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.7. Message Integrity . . . . . . . . . . . . . . . . . . . . 15 5. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6. Message Efficiency . . . . . . . . . . . . . . . . . . . . . . 17 6.1. Address Block Compression . . . . . . . . . . . . . . . . 17 6.2. TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.3. TLV Values . . . . . . . . . . . . . . . . . . . . . . . . 19 6.4. Automation . . . . . . . . . . . . . . . . . . . . . . . . 20 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Clausen, et al. Expires November 5, 2016 [Page 2] Internet-Draft RFC 5444 Usage May 2016 1. Introduction [RFC5444] specifies a generalized packet/message format, designed for use by MANET routing protocols. [RFC5444] was designed following experiences with [RFC3626], which attempted, but did not quite succeed in, providing a packet/message format accommodating for diverse protocol extensions. [RFC5444] was designed as a common building block for use by both proactive and reactive MANET routing protocols. [RFC5498] mandates the use of this packet/message format, and of the packet multiplexing process described in an Appendix to [RFC5444], by protocols operating over the manet IP protocol and port numbers that were allocated following [RFC5498]. 1.1. History and Purpose Since the publication of [RFC5444] in 2009, several RFCs have been published, including [RFC5497], [RFC6130], [RFC6621], [RFC7181], [RFC7182], [RFC7183], [RFC7188], [RFC7631], and [RFC7722], that use the format of [RFC5444]. The ITU-T recommendation [G9903] also uses the format of [RFC5444] for encoding some of its control signals. In developing these specifications, experience with the use of [RFC5444] has been acquired, specifically with respect to how to write specifications using [RFC5444] so as to ensure "forward compatibility" of a protocol with future extensions, to enable the creation of efficient messages, and to enable the use of an efficient and generic parser for all protocols using [RFC5444]. During the same time period, other suggestions have been made to use [RFC5444] in a manner that would inhibit the development of interoperable protocol extensions, that would potentially lead to inefficiencies, or that would lead to incompatibilities with generic parsers for [RFC5444]. While these uses were not all explicitly prohibited by [RFC5444], they should be strongly discouraged. This document is intended to prohibit such uses, to present experiences from designing protocols using [RFC5444], and to provide these as guidelines (with their rationale) for future protocol designs using [RFC5444]. 1.2. RFC 5444 Features [RFC5444] performs two main functions: o It defines a packet/message format for use by MANET routing protocols. As far as [RFC5444] is concerned, it is up to each protocol that uses it to implement the required message parsing Clausen, et al. Expires November 5, 2016 [Page 3] Internet-Draft RFC 5444 Usage May 2016 and formation. It is natural, especially when implementing more than one such protocol, to implement these processes using protocol-independent packet/message creation and parsing procedures, however this is not required by [RFC5444]. Some comments in this document may be particularly applicable to such a case, but all that is required is that the messages passed to and from protocols are correctly formatted, and that packets containing those messages are correctly formatted as described in the following point. o It specifies, in its Appendix A combined with the intended usage in its Appendix B, a multiplexing and demultiplexing process whereby an entity which may be referred to as the "RFC 5444 multiplexer" (in this document simply as the multiplexer, or the demultiplexer when performing that function) manages packets that travel a single (logical) hop, and which contain messages that are owned by individual protocols. A packet may contain messages from more than one protocol. This process, and its usage, is mandated for use on the manet UDP port and IP protocol (alternative means for the transport of packets) by [RFC5498]. The multiplexer is responsible for creating packets and for parsing packet headers, extracting messages, and passing them to the appropriate protocol according to their type (the first octet in the message). 1.2.1. Packet/Message Format Among the characteristics and design objectives of the packet/message format of [RFC5444] are: o It is designed for carrying MANET routing protocol control signals. o It defines a packet as a Packet Header with a set of Packet TLVs (Type-Length-Value structures), followed by a set of messages. Each message has a well-defined structure consisting of a Message Header (designed for making processing and forwarding decisions) followed by a set of Message TLVs, and a set of (address, type, value) associations using Address Blocks and their Address Block TLVs. The [RFC5444] packet/message format then enables the use of simple and generic parsing logic for Packet Headers, Message Headers, and message content. A packet may include messages from different protocols, such as [RFC6130] and [RFC7181], in a single transmission. This was observed in [RFC3626] to be beneficial, especially in wireless networks where media contention may be significant. Clausen, et al. Expires November 5, 2016 [Page 4] Internet-Draft RFC 5444 Usage May 2016 o Its packets are designed to travel between two neighboring interfaces, which will result in a single decrement of the IPv4 TTL or IPv6 hop limit. The Packet Header and any Packet TLVs may thus convey information relevant to that link (for example, the Packet Sequence Number can be used to count transmission successes across that link). Packets are designed to be constructed for a single hop transmission; a packet transmission following a successful packet reception is by design of a new packet that may include all, some, or none of the received messages, plus possibly additional messages either received in separate packets, or generated locally at that router. Messages may thus travel more than one hop, and are designed to carry end-to-end protocol signals. o It supports "internal extensibility" using TLVs; an extension can add information to an existing message without that information rendering the message unparseable or unusable by a router that does not support the extension. An extension is typically of the protocol that created the message to be extended, for example [RFC7181] adds information to the HELLO messages created by [RFC6130]. However an extension may also be independent of the protocol, for example [RFC7182] can add ICV (Integrity Check Value) and timestamp information to any message (or to a packet, thus extending the [RFC5444] multiplexer). Information, in the form of TLVs, can be added to the message as a whole, such as the [RFC7182] integrity information, or may be associated with specific addresses in the message, such as the MPR selection and link metric information added to HELLO messages by [RFC7181]. An extension can also add addresses to a message. o It uses address aggregation into compact Address Blocks by exploiting commonalities between addresses. In many deployments, addresses (IPv4 and IPv6) used on interfaces share a common prefix that need not be repeated. Using IPv6, several addresses (of the same interface) may have common interface identifiers that need not be repeated. o It sets up common namespaces, formats, and data structures for use by different protocols, where common parsing logic can be used. For example, [RFC5497] defines a generic TLV format for representing time information (such as interval time or validity time). o It contains a minimal Message Header (a maximum of five elements: type, originator, sequence number, hop count and hop limit) that permit decisions whether to locally process a message, or forward a message (thus enabling MANET-wide flooding of a message) without Clausen, et al. Expires November 5, 2016 [Page 5] Internet-Draft RFC 5444 Usage May 2016 processing the body of the message. 1.2.2. Multiplexing and Demultiplexing The primary purposes of the multiplexer are to: o Accept messages from MANET protocols, which also indicate over which interface(s) the messages are to be sent, and to which destination address. The latter may be a unicast address or the "LL-MANET-Routers" link local multicast address defined in [RFC5498]. o Collect messages, possibly from multiple protocols, for the same interface and destination, into packets to be sent one logical hop, and to send packets using the manet UDP port or IP protocol defined in [RFC5498]. o Extract messages from received packets, and pass them to their owning protocols. The multiplexer's relationship is with the protocols that own the corresponding Message Types. Where those protocols have their own relationships, for example as extensions, this is the responsibility of the protocols. For example OLSRv2 [RFC7181] extends the HELLO messages created by NHDP [RFC6130]. However the multiplexer will deliver HELLO messages to NHDP and will expect to receive HELLO messages from NHDP, the relationship between NHDP and OLSRv2 is between those two protocols. The multiplexer is also responsible for the Packet Header, including any Packet Sequence Number and Packet TLVs. It may accept some additional instructions from protocols, pass additional information to protocols, and must follow some additional rules, see Section 4.2. 1.3. Status of This Document This document updates [RFC5444], and is intended for publication as a Proposed Standard (rather than as Informational) because it specifies and mandates constraints on the use of [RFC5444] which, if not followed, makes forms of extensions of those protocols impossible, impedes the ability to generate efficient messages, or makes desirable forms of generic parsers impossible. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and Clausen, et al. Expires November 5, 2016 [Page 6] Internet-Draft RFC 5444 Usage May 2016 "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. This document uses the terminology and notation defined in [RFC5444], in particular the terms "packet", "Packet Header", "message", "Message Header", "address", "Address Block", "TLV" and "TLV Block" are to be interpreted as described therein. Additionally, this document uses the following terminology: Full Type (of TLV) - As per [RFC5444], the 16-bit combination of the TLV Type and Type Extension is given the symbolic name , but is not assigned the term "Full Type", which is however assigned by this document as standard terminology. Owning Protocol - As per [RFC5444], for each Message Type, a protocol -- unless specified otherwise, the one making the IANA reservation for that Message Type -- is designated as the "owning protocol" of that Message Type. The (de)multiplexer inspects the Message Type of each received message, and delivers each message to its corresponding "owning protocol". 3. Applicability Statement This document does not specify a protocol, but documents constraints on how to design protocols that are using the generic packet/message format defined in [RFC5444] which, if not followed, makes forms of extensions of those protocols impossible, impedes the ability to generate efficient (small) messages, or makes desirable forms of generic parsers impossible. The use of the [RFC5444] format is mandated by [RFC5498] for all protocols running over the manet protocol and port, defined therein. Thus, the constraints in this document apply to all protocols running over the manet protocol and port. 4. Information Transmission Protocols need to transmit information from one instance implementing the protocol to another. 4.1. Where to Record Information A protocol has the following choices as to where to put information for transmission: Clausen, et al. Expires November 5, 2016 [Page 7] Internet-Draft RFC 5444 Usage May 2016 o In a TLV to be added to the Packet Header. o In a message of a type owned by another protocol. o In a message of a type owned by the protocol. The first case (a Packet TLV) can only be used when the information is to be carried one hop. It SHOULD only be used either where the information relates to the packet as a whole (for example packet integrity check values and timestamps, as specified in [RFC7182]) or if the information is of expected wider application than a single protocol. A protocol can also request that the Packet Header include Packet Sequence Numbers, but does not control those numbers. The second case (in a message of a type owned by another protocol) is only possible if the adding protocol is an extension to the owning protocol; for example OLSRv2 [RFC7181] is an extension of NHDP [RFC6130]. While this is not the most common case, protocols SHOULD be designed to enable this to be possible, and some of the rules in this document are to help facilitate that. An extension to [RFC5444], such as [RFC7182], is considered to be an extension to all protocols in this regard. The third case is the normal case for a new protocol. Protocols MUST be conservative in the number of new Message Types that they require, as the total available number of allocatable Message Types is only 224. Protocol design SHOULD consider whether different functions can be implemented by differences in TLVs carried in the same Message Type, rather than using multiple Message Types. If a protocol's needs can be covered by use of the second case, then this SHOULD be done. The TLV type space, although greater than the Message Type space, SHOULD also be used efficiently. The Full Type of a TLV occupies two octets, thus there are many more available TLV Full Types than there are Message Types. However, in some cases (currently LINK_METRIC from [RFC7181] and ICV and TIMESTAMP from [RFC7182], all in the global TLV type space) a TLV Type with a complete set of 256 TLV Full Types is defined (but not necessarily allocated). Each Message Type has an associated block of Message-Type-specific TLV Types (128 to 233, each of with 256 type extensions), both for Address Block TLV Types and Message TLV Types. TLV Types from within these blocks SHOULD be used in preference to the Message-Type- independent Message TLV Types (0 to 127, each with 256 type extensions) when a TLV is specific to a message. A message contains a Message Header and a Message Body; note that the Clausen, et al. Expires November 5, 2016 [Page 8] Internet-Draft RFC 5444 Usage May 2016 Message TLV Block is considered as part of the latter. The Message Header contains information whose primary purpose is to decide whether to process the message, and whether to forward the message. A message MUST be recognized by the combination of its Message Type, Originator Address and Message Sequence Number. This allows each protocol to manage its own Message Sequence Numbers, and also allows for the possibility that different Message Types may have greatly differing transmission rates. [RFC7181] contains a general purpose process for managing processing and forwarding decisions, albeit one presented as for use with MPR flooding. (Blind flooding can be handled similarly by assuming that all other routers are MPR selectors; it is not necessary in this case to differentiate between interfaces on which a message is received.) Most protocol information is thus contained in the Message Body. A model of how such information may be viewed is described in Section 4.3 and Section 4.4. To use that model, addresses (for example of neighboring or otherwise known routers) SHOULD be recorded in Address Blocks, not as data in TLVs. Recording addresses in TLV Value fields both breaks the model of addresses as identities and associated information (attributes) and also inhibits address compression. However in some cases alternative addresses (e.g., hardware addresses when the Address Block is recording IP addresses) MAY be carried as TLV Values. Note that a message contains a Message Address Length field that can be used to allow carrying alternative message sizes, but only one length of addresses can be used in a single message, in all Address Blocks and the Originator Address, and is established by the router and protocol generating the message. 4.2. Message Multiplexing and Packets The multiplexer has to handle message multiplexing into packets and their transmission, and packet reception and demultiplexing into messages. The multiplexer and the protocols that use it are subject to the following rules. Packets are formed for transmission by: o Outgoing messages are created by their owning protocol, and MAY be modified by any extending protocols if the owning protocol permits this. Messages MAY also be forwarded by their owning protocol. It is strongly RECOMMENDED that messages are not modified in the latter case, other than to their hop count and hop limit fields. This is because it enables authentication using [RFC7182], which ignores (zeros) those two fields (only) for its end to end Message TLV ICV (Integrity Check Value) calculations. Clausen, et al. Expires November 5, 2016 [Page 9] Internet-Draft RFC 5444 Usage May 2016 o Outgoing messages are then sent to the multiplexer. The owning protocol MUST indicate which interface(s) the messages are to be sent on and their destination address, and MAY request that messages are kept together in a packet; the multiplexer SHOULD respect this request if at all possible. o The multiplexer SHOULD combine messages from multiple protocols that are sent on the same interface in a packet, provided that in so doing the multiplexer does not cause an IP packet to exceed the current MTU (Maximum Transmission Unit). Note that the multiplexer cannot fragment messages; creating suitable sized messages that will not cause the MTU to be exceeded if sent in a single message packet is the responsibility of the protocol generating the message. If a larger message is created then only IP fragmentation is available to allow the packet to be sent, and this is generally considered undesirable, especially when transmission may be unreliable. o The multiplexer MAY delay messages briefly in order to assemble more efficient packets. It SHOULD respect any constraints on such delays requested by the protocol. o If requested by a protocol, the multiplexer SHOULD, and otherwise MAY, include a Packet Sequence Number in the packet. Note that, as per the errata to [RFC5444], this Packet Sequence Number MUST be specific to the interface on which the packet is sent. Separate sequence numbers MUST be maintained for each destination to which packets are sent with included Packet Sequence Numbers. (Note that packets travel one hop; the destination is therefore either a link local multicast address, if the packet is being multicast, or the address of the neighbor interface to which the packet is sent.) Addition of Packet Sequence Numbers MUST be consistent, i.e., for each interface and destination the Packet Sequence Number MUST be added to all packets or to none. o An extension to the multiplexer MAY add TLVs to the packet and/or the messages. For example [RFC7182] MAY be used by the multiplexer to add Packet TLVs or Message TLVs, or by the protocol to add Message TLVs. (Whether [RFC7182] Message TLVs are added and verified by the multiplexer or by the protocol is an implementation detail.) When a packet is received, the following steps are required to be performed by the demultiplexer: o The packet and/or the messages it contains MAY be verified by an extension to the demultiplexer, such as [RFC7182]. Clausen, et al. Expires November 5, 2016 [Page 10] Internet-Draft RFC 5444 Usage May 2016 o Each message MUST be sent to its owning protocol. The demultiplexer MUST also make the Packet Header, and the source and destination addresses in the IP datagram that included the packet, available to the protocol. o The demultiplexer MUST remove any TLVs added to the message by the multiplexer. The message MUST be passed on to the protocol exactly as received from (another instance of) the protocol. o The owning protocol SHOULD verify each message for correctness, it SHOULD allow any extending protocol(s) to also contribute to this verification. o The owning protocol MUST process each message, or make an informed decision not to do so. In the former case an owning protocol that permits this MUST allow any extending protocols to process or ignore the message. o The owning protocol is responsible for managing the hop count and/or hop limit in the message. It is RECOMMENDED that these are handled as described in Appendix B of [RFC5444]; they MUST be so handled if using hop count dependent TLVs such as those defined in [RFC5497]. 4.3. Messages, Addresses and Attributes The information in a Message Body, including Message TLVs and Address Block TLVs, can be considered to consist of: o Attributes of the message, each attribute consisting of a Full Type, a length, and a Value (of that length). o A set of addresses, carried in one or more Address Blocks. o Attributes of each address, each attribute consisting of an Full Type, a length, and a Value (of that length). Attributes are carried in TLVs. For Message TLVs the mapping from TLV to attribute is one to one. For Address Block TLVs the mapping from TLV to attribute is one to many: one TLV can carry attributes for multiple addresses, but only one attribute per address. Attributes for different addresses may be the same or different. A TLV Full Type MAY be (and this is RECOMMENDED whenever possible) defined so that there MUST only be one TLV of that Full Type associated with the packet (Packet TLV), message (Message TLV), or any value of any address (Address Block TLV). Note that an address may appear more than once in a message, but the restriction on Clausen, et al. Expires November 5, 2016 [Page 11] Internet-Draft RFC 5444 Usage May 2016 associating TLVs with addresses covers all copies of that address. It is RECOMMENDED that addresses are not repeated in a message. 4.4. Addresses Require Attributes It is not mandatory in [RFC5444] to associate an address with attributes using Address Block TLVs. Information about an address could thus, in principle, be carried using: o The simple presence of an address. o The ordering of addresses in an Address Block. o The use of different meanings for different Address Blocks. This specification, however, requires that those methods of carrying information MUST NOT be used for any protocol using [RFC5444]. Information about the meaning of an address MUST only be carried using Address Block TLVs. In addition, rules for the extensibility of OLSRv2 and NHDP are described in [RFC7188]. This specification extends their applicability to other uses of [RFC5444]. These rules are: o A protocol MUST NOT assign any meaning to the presence or absence of an address (either in a Message, or in a given Address Block in a Message), to the ordering of addresses in an Address Block, or to the division of addresses among Address Blocks. o A protocol MUST NOT reject a message based on the inclusion of a TLV of an unrecognized type. The protocol MUST ignore any such TLVs when processing the message. The protocol MUST NOT remove or change any such TLVs if the message is to be forwarded unchanged. o A protocol MUST NOT reject a message based on the inclusion of an unrecognized Value in a TLV of a recognized type. The protocol MUST ignore any such Values when processing the message, but MUST NOT ignore recognized Values in such a TLV. The protocol MUST NOT remove or change any such TLVs if the message is to be forwarded unchanged. o Similar restrictions to the two preceding points apply to the demultiplexer, which also MUST NOT reject a packet based on an unrecognized message; although it will reject any such messages, it MUST deliver any other messages in the packet to their owning protocols. Clausen, et al. Expires November 5, 2016 [Page 12] Internet-Draft RFC 5444 Usage May 2016 The following points indicate the reasons for these rules, based on considerations of extensibility and efficiency. Assigning a meaning to the presence, absence or location, of an address would reduce the extensibility of the protocol, prevent the approach to information representation described in Section 4.5, and reduce the options available for message optimization described in Section 6. To consider how the simple presence of an address conveying information would have restricted the development of an extension, two examples, one actual (included in the base specification, but could have been added later) and one hypothetical, are considered. The basic function of NHDP's HELLO messages [RFC6130] is to indicate that addresses are of neighbors, using the LINK_STATUS and OTHER_NEIGHB TLVs. (The message may also indicate the routers own addresses, which could also serve as a further example.) An extension to NHDP might decide to use the HELLO message to report that an address is one that could be used for a specialized purpose rather than for normal NHDP-based purposes. Such an example already exists in the use of LOST Values in the LINK_STATUS and OTHER_NEIGHB TLVs to report that an address is of a router known not to be a neighbor. A future example could be to indicate that an address is to be added to a "blacklist" of addresses not to be used. This would use a new TLV (or a new Value of an existing TLV, see below). Assuming that no other TLVs are attached to such blacklisted addresses, then an unmodified extension to NHDP would ignore those addresses, as required. (If however, for example, a LINK_STATUS or OTHER_NEIGHB TLV with Value LOST were also attached to that address, then the receiving router would process that address for that TLV.) If NHDP had been designed so that just the presence of an address indicated a neighbor, this blacklist extension would not be possible. Rejecting a message because it contains an unrecognized TLV Type, or an unrecognized TLV Value, reduces the extensibility of the protocol. For example, OLSRv2 [RFC7181] is, among other things, an extension to NHDP. It adds information to addresses in an NHDP HELLO message using a LINK_METRIC TLV. A non-OLSRv2 implementation of NHDP, for example to support Simplified Multicast Flooding (SMF) [RFC6621], must still process the HELLO message, ignoring the LINK_METRIC TLVs. Also, the blacklisting described in the example above could be signaled not with a new TLV, but with a new Value of a LINK_STATUS or Clausen, et al. Expires November 5, 2016 [Page 13] Internet-Draft RFC 5444 Usage May 2016 OTHER_NEIGHB TLV (requiring an IANA allocation as described in [RFC7188]), as is already done in the LOST case. The creation of Multi-Topology OLSRv2 (MT-OLSRv2) [RFC7722], as an extension to OLSRv2 that can interoperate with unextended instances of OLSRv2, would not have been possible without these restrictions, which were applied to NHDP and OLSRv2 by [RFC7181]. These restrictions do not, however, mean that added information is completely ignored for purposes of the base protocol. Suppose that a faulty implementation of OLSRv2 (including NHDP) creates a HELLO message that assigns two different values of the same link metric to an address, something that is not permitted by [RFC7181]. A receiving OLSRv2-aware implementation of NHDP MUST reject such a message, even though a receiving OLSRv2-unaware implementation of NHDP will process it. This is because the OLSRv2-aware implementation has access to additional information, that the HELLO message is definitely invalid, and the message is best ignored, as it is unknown what other errors it may contain. 4.5. Information Representation This section describes a conceptual way to consider the information in a message. It may be used as the basis of an approach to parsing, or creating, a message to, or from, the information that it contains, or is to contain. However there is no requirement that a protocol does so. This approach may be used either to inform a protocol design, or by a protocol (or generic parser) implementer. A message (excluding the Message Header) can be represented by two, possibly multivalued, maps: o Message: (Full Type) -> (length, Value) o Address: (address, Full Type) -> (length, Value) These maps (plus a representation of the Message Header) can be the basis for a generic representation of information in a message. Such maps can be created by parsing the message, or can be constructed using the protocol rules for creating a message, and later converted into the octet form of the message specified in [RFC5444]. While of course any implementation of software that represents software in the above form can specify an application programming interface (API) for that software, such an interface is not proposed here. First, a full API would be programming language specific. Second, even within the above framework, there are alternative approaches to such an interface. For example, and for illustrative Clausen, et al. Expires November 5, 2016 [Page 14] Internet-Draft RFC 5444 Usage May 2016 purposes only, for the address mapping: o Input: address and Full Type. Output: list of (length, Value) pairs. Note that for most Full Types it will be known in advance that this list will have length zero or one. The list of addresses that can be used as inputs with non-empty output would need to be provided as a separate output. o Input: Full Type. Output: list of (address, length, Value) triples. As this list length may be significant, a possible output will be of one or two iterators that will allow iterating through that list. (One iterator that can detect the end of list, or a pair of iterators specifying a range.) Additional differences in the interface may relate to, for example, the ordering of output lists. 4.6. TLVs Within a message, the attributes are represented by TLVs. Particularly for Address Block TLVs, different TLVs may represent the same information. For example, using the LINK_STATUS TLV defined in [RFC6130], if some addresses have Value SYMMETRIC and some have Value HEARD, arranged in that order, then this information can be represented using two single value TLVs or one multivalue TLV. The latter can be used even if the addresses are not so ordered. A protocol MAY use any representation of information using TLVs that convey the required information. A protocol SHOULD use an efficient representation, but this is a quality of implementation issue. A protocol MUST recognize any permitted representation of the information; even if it chooses to (for example) only use multivalue TLVs, it MUST recognize single value TLVs (and vice versa). A protocol defining new TLVs MUST respect the naming and organizational rules in [RFC7631]. It SHOULD follow the guidance in [RFC7188], except where those requirements are ones that MUST be followed as required by this specification (or when extending [RFC6130] or [RFC7181], when these MUST also be followed). 4.7. Message Integrity In addition to not rejecting a message due to unknown TLVs or TLV Values, a protocol MUST NOT fail to forward a message (by whatever means of message forwarding are appropriate to that protocol) due to the presence of such TLVs or TLV Values, and MUST NOT remove such TLVs or TLV Values. Such behavior would have the consequences that: Clausen, et al. Expires November 5, 2016 [Page 15] Internet-Draft RFC 5444 Usage May 2016 o It might disrupt the operation of an extension of which it is unaware. Note that it is the responsibility of a protocol extension to handle interoperation with unextended instances of the protocol. For example OLSRv2 [RFC7181] adds an MPR_WILLNG TLV to HELLO messages (created by NHDP, [RFC6130], of which it is in part an extension) to recognize this case (and for other reasons). If an incompatible protocol extension were defined, it would be the responsibility of network management to ensure that incompatible routers were not both present in the MANET; this case is NOT RECOMMENDED. o It would prevent the operation of end to end message authentication using [RFC7182], or any similar mechanism. The use of immutable (apart from hop count and/or hop limit) messages by a protocol is strongly RECOMMENDED for that reason. 5. Structure The elements defined in [RFC5444] have structures that are managed by a number of flags fields: o Packet flags field (4 bits, 2 used) that manages the contents of the Packet Header. o Message flags field (4 bits, 4 used) that manages the contents of the Message Header. o Address Block flags field (8 bits, 4 used) that manages the contents of an Address Block. o TLV flags field (8 bits, 5 used) that manages the contents of a TLV. Note that all of these flags are structural, they specify which elements are present or absent, or field lengths, or whether a field has one or multiple values in it. In the current version of [RFC5444], indicated by version number 0 in the field of the Packet Header, unused bits in these flags fields "are RESERVED and SHOULD each be cleared ('0') on transmission and SHOULD be ignored on reception". If a specification updating [RFC5444] introduces new flags in one of the flags fields of a packet, message or Address Block, the following rules MUST be followed: Clausen, et al. Expires November 5, 2016 [Page 16] Internet-Draft RFC 5444 Usage May 2016 o The version number contained in the field of the Packet Header MUST NOT be 0. o The new flag(s) MUST indicate the structure of the corresponding packet, message, Address Block or TLV, and MUST NOT be used to indicate any other semantics, such as message forwarding behavior. An update that would be incompatible with the current specification of [RFC5444] SHOULD NOT be created unless there is a pressing reason for it that cannot be satisfied using the current specification (e.g., by use of a suitable Message TLV). During the development of [RFC5444], and since publication thereof, some proposals have been made to use these RESERVED flags to specify behavior rather than structure, in particular message forwarding. These proposals were, after due consideration, not accepted, for a number of reasons. These reasons include that message forwarding, in particular, is protocol-specific; for example [RFC7181] forwards messages using its MPR (Multi-Point Relay) mechanism, rather than a "blind" flooding mechanism. (The later addition of a 4 bit Message Address Length field later left no unused message flags bits, but other fields still have unused bits.) 6. Message Efficiency The ability to organize addresses into different, or the same, Address Blocks, as well as to change the order of addresses within an Address Block, and the flexibility of the TLV specification, enables avoiding unnecessary repetition of information, and consequently can generate smaller messages. No algorithms for address organization or compression or for TLV usage are given in [RFC5444], any algorithms that leave the information content unchanged MAY be used when generating a message. Note, however, that this does not apply when forwarding a message, a message that is (as strongly RECOMMENDED) forwarded unchanged MUST have an identical octet representation, other than that the owning protocol SHOULD increment and decrement, respectively, the hop count and hop limit, if present. 6.1. Address Block Compression Addresses in an Address Block can be compressed, and SHOULD be. Addresses in an Address Block consist of a Head, a Mid, and a Tail, where all addresses in an Address Block have the same Head and Tail, but different Mids. Each has a length that is greater than or equal to zero, the sum of the lengths being the address length. (The Mid length is deduced from this relationship.) Compression is possible Clausen, et al. Expires November 5, 2016 [Page 17] Internet-Draft RFC 5444 Usage May 2016 when the Head and/or the Tail have non-zero length. An additional compression is possible when the Tail consists of all zero-valued octets. Expected use cases are IPv4 and IPv6 addresses from within the same prefix and which therefore have a common Head, IPv4 subnets with a common zero-valued Tail, and IPv6 addresses with a common Tail representing an interface identifier, as well as having a possible common Head. Note that when, for example, IPv4 addresses have a common Head, their Tail will usually have length zero. For example 192.0.2.1 and 192.0.2.2 would, for greatest efficiency, have a 3 octet Head, a 1 octet Mid, and a 0 octet Tail. Putting addresses into a message efficiently also has to consider: o The split of the addresses into Address Blocks. o The order of the addresses within the Address Blocks. This split and/or ordering is for efficiency only, it does not provide any information. The split of the addresses affects both the address compression and the TLV efficiency (see Section 6.2), the order of the addresses within an Address Block affects only the TLV efficiency. However using more Address Blocks than is needed can increase the message size due to the overhead of each Address Block and the following TLV Block, and/or if additional TLVs are now required. The order of addresses can be as simple as sorting the addresses, but if many addresses have the same TLV Types attached, it might be more useful to put these addresses together, either within the same Address Block as other addresses, or in a separate Address Block. A separate Address Block might also improve address compression, for example if more than one address form is used (such as from independent subnets). An example of the possible use of address ordering is a HELLO message from [RFC6130] which MAY be generated with local interface addresses first and neighbor addresses later. These MAY be in separate Address Blocks. 6.2. TLVs The main opportunities for creating more efficient messages when considering TLVs are in Address Block TLVs, rather than Message TLVs. An Address Block TLV provides attributes for one address or a contiguous (as stored in the Address Block) set of addresses (with a special case for when this is all addresses in an Address Block). When associated with more than one address, a TLV may be single value (associating the same attribute with each address) or multivalue (associating a separate attribute with each address). Clausen, et al. Expires November 5, 2016 [Page 18] Internet-Draft RFC 5444 Usage May 2016 The simplest to implement approach is to use multivalue TLVs that cover all affected addresses. However unless care is taken to order addresses appropriately, these affected addresses may not all be contiguous. Approaches to this are to: o Reorder the addresses. It is, for example, possible (though not straightforward, and beyond the scope of this document to describe exactly how) to order all addresses in HELLO message as specified in [RFC6130] so that all TLVs used only cover contiguous addresses. This is even possible if the MPR TLV specified in OLSRv2 [RFC7181] is added; but it is not possible, in general, if the LINK_METRIC TLV specified in OLSRv2 [RFC7181] is also added. o Allow the TLV to span over addresses that do not need the corresponding attribute, using a Value that indicates no information, see Section 6.3. o Use more than one TLV. Note that this can be efficient when the TLVs thus become single value TLVs. In a typical case where a LINK_STATUS TLV uses only the Values HEARD and SYMMETRIC, with enough addresses, sorted appropriately, two single value TLVs can be more efficient than one multivalue TLV. (When only one Value is involved, such as NHDP in a steady state with LINK_STATUS equal to SYMMETRIC in all cases, one single value TLV SHOULD always be used.) 6.3. TLV Values If, for example, an Address Block contains five addresses, the first two and the last two requiring Values assigned using a LINK_STATUS TLV, but the third does not, then this can be indicated using two TLVs. It is however more efficient to do this with one multivalue LINK_STATUS TLV, assigning the third address the Value UNSPECIFIED. In general, use of UNSPECIFIED Values allows use of fewer TLVs and thus often an efficiency gain; however a long run of consecutive UNSPECIFIED Values (more than the overhead of a TLV) may make more TLVs more efficient. This approach was specified in [RFC7188], and REQUIRED for protocols that extend [RFC6130] and [RFC7181]. It is here RECOMMENDED that this approach (i.e., defining an UNSPECIFIED Value) is followed when defining any Address Block TLV with discrete Values that may be used by a protocol using [RFC5444], and that a modified approach is used where possible for other Address Block TLVs, as described below for the LINK_METRIC TLV defined in [RFC7181]. It might be argued that this (provision of an UNSPECIFIED Value to allow an Address Bloc TLV to cover unaffected addresses) is not Clausen, et al. Expires November 5, 2016 [Page 19] Internet-Draft RFC 5444 Usage May 2016 necessary in the example above, because the addresses can be reordered. However ordering addresses in such a way for all possible TLVs is not, in general, possible. As indicated, the LINK_STATUS TLV, and some other TLVs that take single octet Values (per address), have a Value UNSPECIFIED defined, as the Value 255, in [RFC7188]. A similar approach (and a similar Value) is RECOMMENDED in any similar cases. Some other TLVs may need a different approach. As noted in [RFC7188], but implicitly permissible before then, the LINK_METRIC TLV, defined in [RFC7181], has two octet Values whose first four bits are flags indicating whether the metric applies in four cases; if these are all zero then the metric does not apply in this case, which is thus the equivalent of an UNSPECIFIED Value. 6.4. Automation There is scope for creating a protocol-independent optimizer for [RFC5444] messages that performs appropriate address re-organization (ordering and Address Block separation) and TLV changes (of number, single- or multi- valuedness and use of UNSPECIFIED Values) to create more compact messages. The possible gain depends on the efficiency of the original message creation, and the specific details of the message. Note that this process cannot be TLV Type independent, for example a LINK_METRIC TLV has a more complicated Value structure than a LINK_STATUS TLV does if using UNSPECIFIED Values. Such a protocol-independent optimizer MAY be used by the router generating a message, but MUST NOT be used on a message that is forwarded unchanged by a router. 7. Security Considerations This document does not specify a protocol, but provides rules and recommendations for how to design protocols using [RFC5444]. This document does not introduce any new security considerations; protocols designed according to these rules and recommendations are subject to the security considerations detailed in [RFC5444]. In particular the applicability of the security framework for [RFC5444] specified in [RFC7182] is unchanged. 8. IANA Considerations This document has no actions for IANA. [This Section may be removed by the RFC Editor.] Clausen, et al. Expires November 5, 2016 [Page 20] Internet-Draft RFC 5444 Usage May 2016 9. Acknowledgments The authors thank Cedric Adjih (INRIA) and Justin Dean (NRL) for their contributions as authors of RFC 5444. Clausen, et al. Expires November 5, 2016 [Page 21] Internet-Draft RFC 5444 Usage May 2016 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [RFC5444] Clausen, T., Dearlove, C., Dean, J., and C. Adjih, "Generalized MANET Packet/Message Format", RFC 5444, February 2009. 10.2. Informative References [G9903] "ITU-T G.9903: Narrow-band orthogonal frequency division multiplexing power line communication transceivers for G3- PLC networks", May 2013. [RFC3626] Clausen, T. and P. Jacquet, "The Optimized Link State Routing Protocol", RFC 3626, October 2003. [RFC5497] Clausen, T. and C. Dearlove, "Representing Multi-Value Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497, March 2009. [RFC5498] Chakeres, I., "IANA Allocations for Mobile Ad Hoc Network (MANET) Protocols", RFC 5498, March 2009. [RFC6130] Clausen, T., Dean, J., and C. Dearlove, "Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)", RFC 6130, April 2011. [RFC6621] Macker, J., "Simplified Multicast Forwarding", RFC 6621, May 2012. [RFC7181] Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg, "The Optimized Link State Routing Protocol version 2", RFC 7181, April 2014. [RFC7182] Herberg, U., Clausen, T., and C. Dearlove, "Integrity Check Value and Timestamp TLV Definitions for Mobile Ad Hoc Networks (MANETs)", RFC 7182, April 2014. [RFC7183] Herberg, U., Dearlove, C., and T. Clausen, "Integrity Protection for the Neighborhood Discovery Protocol (NHDP) and Optimized Link State Routing Protocol Version 2 (OLSRv2)", RFC 7183, April 2014. [RFC7188] Dearlove, C. and T. Clausen, "Optimized Link State Routing Clausen, et al. Expires November 5, 2016 [Page 22] Internet-Draft RFC 5444 Usage May 2016 Protocol version 2 (OLSRv2) and MANET Neighborhood Discovery Protocol (NHDP) Extension TLVs", RFC 7183, April 2014. [RFC7631] Dearlove, C. and T. Clausen, "TLV Naming in the MANET Generalized Packet/Message Format", RFC 7631, January 2015. [RFC7722] Dearlove, C. and T. Clausen, "Multi-Topology Extension for the Optimized Link State Routing Protocol Version 2 (OLSRv2)", RFC 7722, December 2015. Authors' Addresses Thomas Clausen Ecole Polytechnique 91128 Palaiseau Cedex, France Phone: +33-6-6058-9349 Email: T.Clausen@computer.org URI: http://www.thomasclausen.org Christopher Dearlove BAE Systems Applied Intelligence Laboratories West Hanningfield Road Great Baddow, Chelmsford United Kingdom Email: chris.dearlove@baesystems.com URI: http://www.baesystems.com Ulrich Herberg Email: ulrich@herberg.name URI: http://www.herberg.name Clausen, et al. Expires November 5, 2016 [Page 23] Internet-Draft RFC 5444 Usage May 2016 Henning Rogge Fraunhofer FKIE Fraunhofer Strasse 20 53343 Wachtberg Germany Email: henning.rogge@fkie.fraunhofer.de Clausen, et al. Expires November 5, 2016 [Page 24]