Internet Engineering Task Force (IETF) T. Clausen Request for Comments: 8245 Ecole Polytechnique Updates: 5444 C. Dearlove Category: Standards Track BAE Systems ISSN: 2070-1721 U. Herberg H. Rogge Fraunhofer FKIE October 2017 Rules for Designing Protocols Using the Generalized Packet/Message Format from RFC 5444 Abstract RFC 5444 specifies a generalized Mobile Ad Hoc Network (MANET) packet/message format and describes an intended use for multiplexed MANET routing protocol messages; this use is mandated by RFC 5498 when using the MANET port or protocol number that it specifies. 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 that have been suggested in various proposals and that would have led to interoperability problems, to the impediment of protocol extension development, and/or to an inability to use optional generic parsers. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8245.
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Table of Contents 1. Introduction ....................................................4 1.1. History and Purpose ........................................4 1.2. Features of RFC 5444 .......................................4 1.2.1. Packet/Message Format ...............................5 1.2.2. Multiplexing and Demultiplexing .....................7 1.3. Status of This Document ....................................8 2. Terminology .....................................................8 3. Applicability Statement .........................................9 4. Information Transmission ........................................9 4.1. Where to Record Information ................................9 4.2. Message and TLV Type Allocation ...........................10 4.3. Message Recognition .......................................11 4.4. Message Multiplexing and Packets ..........................11 4.4.1. Packet Transmission ................................12 4.4.2. Packet Reception ...................................13 4.5. Messages, Addresses, and Attributes .......................15 4.6. Addresses Require Attributes ..............................16 4.7. TLVs ......................................................18 4.8. Message Integrity .........................................19 5. Structure ......................................................19 6. Message Efficiency .............................................20 6.1. Address Block Compression .................................21 6.2. TLVs ......................................................22 6.3. TLV Values ................................................23 7. Security Considerations ........................................24 8. IANA Considerations ............................................24 9. References .....................................................25 9.1. Normative References ......................................25 9.2. Informative References ....................................25 Appendix A. Information Representation ............................27 Appendix B. Automation ............................................28 Acknowledgments ...................................................28 Authors' Addresses ................................................29
1. Introduction [RFC5444] specifies a generalized packet/message format that is designed for use by MANET routing protocols. [RFC5444] was designed following experiences with [RFC3626], which attempted to provide a packet/message format accommodating diverse protocol extensions but did not fully succeed. [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 UDP port numbers that were allocated by [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 are 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. Features of RFC 5444 [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
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 might 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 Appendix A of [RFC5444], combined with the intended usage described in Appendix B of [RFC5444], specifies a multiplexing and demultiplexing process whereby an entity that can be referred to as the "RFC 5444 multiplexer" manages packets that travel a single (logical) hop and contain messages that are owned by individual protocols. Note that in this document, the "RFC 5444 multiplexer" is referred to as the "multiplexer", or as the "demultiplexer" when performing that function. A packet can contain messages from more than one protocol. This process 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 the following: 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 packet/message format from [RFC5444] then enables the use of simple and generic parsing logic for Packet Headers, Message Headers, and message content. A packet can include messages from different protocols, such as the Neighborhood Discovery Protocol (NHDP) [RFC6130] and the Optimized Link State Routing Protocol version 2 (OLSRv2)
[RFC7181], in a single transmission. This was observed in [RFC3626] to be beneficial, especially in wireless networks where media contention can be significant. 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 can 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) a new packet that can 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 can 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 can also be independent of the protocol; for example, [RFC7182] can add Integrity Check Value (ICV) and timestamp information to any message (or to a packet, thus extending the multiplexer). Information, in the form of TLVs, can be added to the message as a whole (such as the integrity information specified in [RFC7182]) or can be associated with specific addresses in the message (such as the Multipoint Relay (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) might 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 regarding whether to locally process a message or forward a message (thus enabling MANET-wide flooding of a message) without processing the body of the message. 1.2.2. Multiplexing and Demultiplexing The multiplexer (and demultiplexer) is defined in Appendix A of [RFC5444]. Its purpose is to allow multiple protocols to share the same IP protocol or UDP port. That sharing was made necessary by the separation of [RFC6130] from [RFC7181] as separate protocols and by the allocation of a single IP protocol and UDP port to all MANET protocols, including those protocols following [RFC5498], which states: All interoperable protocols running on these well-known IANA allocations MUST conform to [RFC5444]. [RFC5444] provides a common format that enables one or more protocols to share the IANA allocations defined in this document unambiguously. The multiplexer is the mechanism in [RFC5444] that enables that sharing. 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 can 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 local 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 can accept some additional instructions from protocols, can pass additional information to protocols, and will follow some additional rules; see Section 4.4. 1.3. Status of This Document This document updates [RFC5444] and is published on the Standards Track (rather than as Informational) because it specifies and mandates constraints on the use of [RFC5444] that, if not followed, make forms of extensions of those protocols impossible, impede the ability to generate efficient messages, or make desirable forms of generic parsers impossible. Each use of key words from [RFC2119] (see Section 2) can be considered an update to [RFC5444]. In most cases, these codify obvious best practice or constrain the use of [RFC5444] in the circumstances where this specification is applicable (see Section 3). In a few circumstances, operation of [RFC5444] is modified. These are all circumstances that do not occur in its main and current uses, specifically by [RFC6130] and [RFC7181] (that might already include the requirement, particularly through [RFC7188]). That such modifying cases are an update to [RFC5444] is explicitly indicated in this specification. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Use of those key words applies directly to existing and future implementations of [RFC5444]. It also applies to existing and future protocols that use or update that RFC. This document uses the terminology and notation defined in [RFC5444]; the terms "packet", "Packet Header", "message", "Message Header", "address", "Address Block", "TLV", "TLV Block", and other related terms 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 <tlv-fulltype>. This document uses the term "Full Type", which is not used in [RFC5444], but is 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 demultiplexer 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 use the generic packet/message format defined in [RFC5444] that, 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 IP protocol or UDP port. The constraints are strongly recommended for other uses of [RFC5444]. 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: o in a TLV to be added to the Packet Header; o in a message of a type owned by another protocol; or 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 expected to have a 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]. The third case is the normal case for a new protocol. A protocol extension can either be simply an update of the protocol (the third case) or be a new protocol that also updates another protocol (the second case). An example of the latter is that OLSRv2 [RFC7181] is a protocol that also extends the HELLO message owned by NHDP [RFC6130]; it is thus an example of both the second and third cases (the latter using the OLSRv2 owned Topology Control (TC) message). An extension to [RFC5444], such as [RFC7182], is considered to be an extension to all protocols. Protocols SHOULD be designed to enable extension by any of these means to be possible, and some of the rules in this document (in Sections 4.6 and 4.8, specifically) are to help facilitate that. 4.2. Message and TLV Type Allocation Protocols SHOULD 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. 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 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. The Expert Review guidelines in [RFC5444] are updated accordingly, as described in Section 8.
4.3. Message Recognition A message contains a Message Header and a Message Body; note that the Message TLV Block is considered 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 protocol might need to recognize whether a message, especially a flooded message, is one that it has previously received (for example, to determine whether to process and/or forward it, or to discard it). A message can be recognized as one that has been previously seen if it contains sufficient information in its Message Header. A message MUST be so recognized by the combination of its Message Type, Originator Address, and Message Sequence Number. The inclusion of Message Type allows each protocol to manage its own Message Sequence Numbers and also allows for the possibility that different Message Types can have greatly differing transmission rates. As an example of such use, [RFC7181] contains a general purpose process for managing processing and forwarding decisions, although specifically 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 can be viewed is described in Sections 4.5 and 4.6. 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) can 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. 4.4. Message Multiplexing and Packets The multiplexer has to handle message multiplexing into packets and the transmission of said packets, as well as packet reception and demultiplexing into messages. The multiplexer and the protocols that use it are subject to the following rules.
4.4.1. Packet Transmission Packets are formed for transmission through the following steps: 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 updates to their hop count and hop limit fields, as described in Section 7.1.1 of [RFC5444]. Note that this includes having an identical octet representation, including not allowing a different TLV representation of the same information. This is because it enables end-to-end authentication that ignores (zeros) those two fields (only), as is done in the Message TLV ICV (Integrity Check Value) calculations in [RFC7182]. Protocols MUST document their behavior with regard to modifiability of messages. 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. 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. o The owning protocol MAY request that messages are kept together in a packet; the multiplexer SHOULD respect this request if at all possible. The multiplexer SHOULD combine messages that are sent on the same interface in a packet, whether from the same or different protocols, provided that in so doing the multiplexer does not cause an IP packet to exceed the current Maximum Transmission Unit (MTU). Note that the multiplexer cannot fragment messages; creating suitably 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; this is generally considered undesirable, especially when transmission can be unreliable. o The multiplexer MAY delay messages in order to assemble more efficient packets. It MUST respect any constraints on such delays requested by the protocol if it is practical to do so. o If requested by a protocol, the multiplexer MUST (and otherwise MAY) include a Packet Sequence Number in the packet. Such a request MUST be respected as long as the protocol is active. Note that the errata to [RFC5444] indicates that the Packet Sequence Number SHOULD be specific to the interface on which the packet is
sent. This specification updates [RFC5444] by requiring that this sequence number MUST be specific to that interface and also that separate sequence numbers MUST be maintained for each destination to which packets are sent with included Packet Sequence Numbers. 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. It MAY also add TLVs to the messages, in which case it is considered as also extending the corresponding protocols. For example, [RFC7182] can be used by the multiplexer to add Packet TLVs or Message TLVs, or it can be used by the protocol to add Message TLVs. 4.4.2. Packet Reception When a packet is received, the following steps are performed by the demultiplexer and by protocols: o The Packet Header and the organization into the messages that it contains MUST be verified by the demultiplexer. o The packet and/or the messages it contains MAY also be verified by an extension to the demultiplexer, such as [RFC7182]. o Each message MUST be sent to its owning protocol or discarded if the Message Type is not recognized. The demultiplexer MUST also make available to the protocol the Packet Header and the source and destination addresses in the IP datagram that included the packet. o The demultiplexer MUST remove any Message TLVs that were added by an extension to the multiplexer. The message MUST be passed on to the protocol exactly as received from (another instance of) the protocol. This is, in part, an implementation detail. For example, an implementation of the multiplexer and of [RFC7182] could add a Message TLV either in the multiplexer or in the protocol and remove it in the same place on reception. An implementation MUST ensure that the message passed to a protocol is as it would be passed from that protocol by the same implementation, i.e., that the combined implementation on a router is self-consistent, and that messages included in packets by the multiplexer are independent of this implementation detail. o The owning protocol MUST verify each message for correctness; it MUST allow any extending protocol(s) to also contribute to this verification.
o The owning protocol MUST process each message. In some cases, which will be defined in the protocol specification, this processing will determine that the message will be ignored. Except in the latter case, the owning protocol MUST also allow any extending protocols to process the message. o The owning protocol MUST manage 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]. 22.214.171.124. Other Information In addition to the messages between the multiplexer and the protocols in each direction, the following additional information (summarized from other sections in this specification) can be exchanged. o The packet source and destination addresses MUST be sent from the demultiplexer to the protocol. o The Packet Header, including the Packet Sequence Number, MUST be sent from the (de)multiplexer to the protocol if present. (An implementation MAY choose to only do so or only report the Packet Sequence Number, on request.) o A protocol MAY require that all outgoing packets contain a Packet Sequence Number. o The interface over which a message is to be sent and its destination address MUST be sent from protocol to multiplexer. The destination address MAY be a multicast address, in particular, the LL-MANET-Routers link-local multicast address defined in [RFC5498]. o A request to keep messages together in one packet MAY be sent from protocol to multiplexer. o A requested maximum message delay MAY be sent from protocol to multiplexer. The protocol SHOULD also be aware of the MTU that will apply to its messages, if this is available.
4.5. Messages, Addresses, and Attributes The information in a Message Body, including Message TLVs and Address Block TLVs, consists of: o Attributes of the message, in which each attribute consists of a Full Type, a length, and a Value (of that length). o A set of addresses, which are carried in one or more Address Blocks. o Attributes of each address, in which each attribute consists of a 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 can be the same or different. [RFC5444] requires that when a TLV Full Type is defined, then it MUST also define how to handle the cases of multiple TLVs of the same type applying to the same information element - i.e., when more than one Packet TLV of the same TLV Full Type is included in the same Packet Header, when more than one Message TLV of the same TLV Full Type is included in the same Message TLV Block, or when more than one Address Block TLV of the same TLV Full Type applies to the same value of any address. It is RECOMMENDED that when defining a new TLV Full Type, a rule of the following form is adopted. o If used, there MUST be only 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 this applies to address values; an address can appear more than once in a message, but the restriction on associating TLVs with addresses covers all copies of that address. It is RECOMMENDED that addresses are not repeated in a message. A conceptual way to view this information is described in Appendix A.
4.6. 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. 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 Appendix A, 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 are considered: one actual (included in the base specification, but which could have been added later) and one hypothetical. 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 can also indicate the router's 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). If no other TLVs were attached to such a blacklisted address, then an unmodified implementation of NHDP would ignore that address, as required; if any other TLVs were attached to that address, then that implementation would process that address for those TLVs. Had NHDP been designed so that just the presence of an address indicated a neighbor, this blacklist extension would not be possible, as an unmodified implementation of NHDP would treat all blacklisted addresses as neighbors. 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]) will 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 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 [RFC7188]). 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 will 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 might contain. 4.7. TLVs Within a message, the attributes are represented by TLVs. Particularly for Address Block TLVs, different TLVs can 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], see Section 6.3. (This specification does not, however, relax the application of [RFC7188] where it is mandated.)
4.8. Message Integrity In addition to not rejecting a message due to unknown TLVs or TLV Values, 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. Such behavior may have the following consequences: 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_WILLING TLV to HELLO messages (created by NHDP [RFC6130], of which it is an extension) to recognize this case (and for other reasons). 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.