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Versions: (draft-clausen-manet-rfc5444-usage) 00 01 02 03 04 05 06 07 RFC 8245

Network Working Group                                         T. Clausen
Internet-Draft                                  LIX, Ecole Polytechnique
Updates: 5444 (if approved)                                  C. Dearlove
Intended status: Standards Track                             BAE Systems
Expires: September 1, 2016                                    U. Herberg

                                                                H. Rogge
                                                         Fraunhofer FKIE
                                                       February 29, 2016


  Rules For Designing Protocols Using the RFC 5444 Generalized Packet/
                             Message Format
                   draft-ietf-manet-rfc5444-usage-02

Abstract

   This document updates the generalized MANET packet/message format,
   specified in RFC 5444, by providing rules and recommendations for how
   protocols can use that 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 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 September 1, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   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
     1.3.  Status of This Document  . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  5
   4.  Information Transmission . . . . . . . . . . . . . . . . . . .  6
     4.1.  Where to Record Information  . . . . . . . . . . . . . . .  6
     4.2.  Packets and Messages . . . . . . . . . . . . . . . . . . .  7
     4.3.  Messages, Addresses and Attributes . . . . . . . . . . . .  9
     4.4.  Addresses Require Attributes . . . . . . . . . . . . . . .  9
     4.5.  Information Representation . . . . . . . . . . . . . . . . 11
     4.6.  TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.7.  Message Integrity  . . . . . . . . . . . . . . . . . . . . 13
   5.  Structure  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Message Efficiency . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Address Block Compression  . . . . . . . . . . . . . . . . 14
     6.2.  TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.3.  TLV Values . . . . . . . . . . . . . . . . . . . . . . . . 16
     6.4.  Automation . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19












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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], and [RFC7631], 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, would potentially lead to
   inefficiencies, or 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

   Among the characteristics, and design criteria, of the packet/message
   format of [RFC5444] are:

   o  It is designed for carrying MANET routing protocol control
      signals.



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   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.  [RFC5444]
      defines a multiplexing process to achieve this that is mandated by
      [RFC5498] for use on the manet IP port and UDP port.  This makes
      the contents of the Packet Header, which may also contain Packet
      TLVs, and the transmission of packets over UDP or directly over
      IP, the responsibility of this multiplexing process.

   o  Its packets are designed to travel between two neighboring
      interfaces, which will result in a single decrement/increment of
      the IPv4 TTL or IPv6 hop limit.  The Packet Header and any Packet
      TLVs should convey information relevant to that link (for example,
      the Packet Sequence Number can be used to count transmission
      successes across that link).  Packets are not retransmitted, a
      packet transmission following a successful packet reception may
      include all, some, or none of the received messages, plus possibly
      additional messages received in separate packets or generated 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] multiplexing process).

      Information 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 may also add addresses to a message.



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   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
      processing the body of the message.

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
   "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.


3.  Applicability Statement

   This document does not specify a protocol, but documents constraints
   on how to design protocols which 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



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   generate efficient (small) messages, or makes desirable forms of
   generic parsers impossible.  The use of this format is mandated by
   [RFC5498] for all protocols running over the manet protocol and port
   number, defined therein.  Thus, the constraints in this document
   apply to all protocols running over the manet protocol and port
   number.


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.

   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 the 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 most 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



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   done.

   TLV space, although greater than message space, SHOULD also be used
   efficiently.  The extended type of a TLV occupies two octets, thus
   there are many more available TLVs.  However, in some cases
   (currently LINK_METRIC from [RFC7181] and ICV and TIMESTAMP from
   [RFC7182] in the global TLV space) a full set of 256 TLVs is defined
   (but not necessarily allocated).  Each message also has a block of
   message specific TLV Types (128 to 233, each with 256 type
   extensions), these SHOULD be used in preference to the common TLV
   Types (0 to 127, each with 256 type extensions) when a TLV is
   message-specific.

   A message contains a Message Header and a Message Body; note that the
   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 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.

4.2.  Packets and Messages

   The [RFC5444] multiplexing process has to handle packet reception and
   message demultiplexing, and message transmission and packet
   multiplexing.



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   When a packet arrives, the following steps are required:

   o  The packet and/or the messages it contains MAY be verified by an
      extension to the demultiplexer, such as [RFC7182].

   o  Each message MUST be sent to its owning protocol, which MAY also
      view the Packet Header, and the source address in the IP datagram
      that included the packet.

   o  The owning protocol SHOULD verify each message, it SHOULD allow
      any extending protocol(s) to also contribute to this.

   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.

   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 RECOMMENDED that messages are not modified in the latter
      case.

   o  Outgoing messages are then sent to the [RFC5444] multiplexing
      process.  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
      multiplexing process SHOULD respect this request if possible.  A
      protocol MAY also request that a Packet Sequence Number and/or
      specified Packet TLVs are included, such requests SHOULD also be
      respected if possible.

   o  The multiplexing process SHOULD combine messages from multiple
      protocols that are sent on the same interface in a packet,
      provided that in so doing the multiplexing process does not cause
      an IP packet to exceed the current MTU (Maximum Transmission
      Unit).  (Note that the multiplexing process cannot fragment
      messages; creating suitable sized messages is the responsibility
      of 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.





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   o  An extension to the multiplexing process MAY add TLVs to the
      packet and/or the messages (for example as by [RFC7182]).

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 an
      extended 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
      extended 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 extended type MAY be (and this is RECOMMENDED whenever
   possible) defined so that there may only be one TLV of that extended
   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 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



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   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, 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 the 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
      packet multiplexing process, 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 Section 4.5, and
   reduce the options available for message optimization described in
   Section 6.

   For example, consider NHDP's HELLO messages [RFC6130].  The basic
   function of a HELLO message is to indicate that an address is of a
   neighbor, using the LINK_STATUS and OTHER_NEIGHB TLVs.  An extension
   to NHDP might decide to use the HELLO message to report that, for
   example, an address is one that could be used for a specialized
   purpose, but not for normal NHDP-based purposes.  Such an example
   already exists (but within the basic specification, rather than as an
   extension) 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 might be to list an address to be
   added to a "blacklist" of addresses not to be used.  This would be
   indicated by a new TLV (or a new Value of an existing TLV, see



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   below).  An unmodified extension to NHDP would ignore such addresses,
   as required, as it does not support that specialized purpose.  If
   NHDP had been designed so that just the presence of an address
   indicated a neighbor, that extension would not have been 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
   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

   A message (excluding the Message Header) can thus be represented by
   two, possibly multivalued, maps:

   o  Message: (extended type) -> (length, value)

   o  Address: (address, extended 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



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   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
   purposes only, for the address mapping:

   o  Input: address and extended type.  Output: list of (length, value)
      pairs.  Note that for most extended 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: extended 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).



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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:

   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 <version> field of the Packet Header, unused bits in these flags



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   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:

   o  The version number contained in the <version> 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.

6.1.  Address Block Compression

   Addresses in an Address Block can be compressed, and SHOULD be.

   Compression of addresses in an Address Block considers addresses to
   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.  An



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   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 be empty.  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 include:

   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 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).




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   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) 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 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 is followed when defining any Address Block TLV that
   may be used by a protocol using [RFC5444].

   It might be argued that this is not 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,



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   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 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.


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.


9.  Acknowledgments

   The authors thank Cedric Adjih (INRIA) and Justin Dean (NRL) for
   their contributions as authors of RFC 5444.










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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



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              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
   LIX, 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












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   Henning Rogge
   Fraunhofer FKIE
   Fraunhofer Strasse 20
   53343 Wachtberg
   Germany

   Email: henning.rogge@fkie.fraunhofer.de












































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