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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 AI Labs
Expires: December 25, 2015                                    U. Herberg
                                                                H. Rogge
                                                           June 23, 2015


  Rules For Designing Protocols Using the RFC5444 Generalized Packet/
                             Message Format
                   draft-ietf-manet-rfc5444-usage-00

Abstract

   This document updates the generalized MANET packet/message format,
   specified in RFC5444, by providing prescriptive guidelines for how
   protocols can use that packet/message format.  In particular, these
   mandatory guidelines prohibit a number of uses of RFC5444 that have
   been suggested in various proposals, and which would have lead to
   interoperability problems, to impediment of protocol extension
   development, and to inability to use generic RFC5444 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 December 25, 2015.

Copyright Notice

   Copyright (c) 2015 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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 . . . . . . . . . . . .  8
     4.4.  Addresses Require Attributes . . . . . . . . . . . . . . .  9
     4.5.  Information Representation . . . . . . . . . . . . . . . . 10
     4.6.  Message Integrity  . . . . . . . . . . . . . . . . . . . . 11
   5.  Structure  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Message Efficiency . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Addressesblock compression . . . . . . . . . . . . . . . . 13
     6.2.  TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.3.  TLV Values . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.4.  Automation . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17















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1.  Introduction

   [RFC5444] specifies a generalized packet/message format, designed for
   use by MANET routing protocols.  [RFC5498] mandates the use of this
   format by protocols operating over the manet IP protocol and port
   numbers whose allocation it requested.

   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 by the MANET
   working group as a common building block for use by both proactive
   and reactive MANET routing protocols.

1.1.  History and Purpose

   Since the publication of [RFC5444] in 2009, several RFCs have been
   published, including [RFC5497], [RFC6130], [RFC6621], [RFC7181],
   [RFC7182], [RFC7183], and [RFC7188], which 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 (i) enable the use of an efficient and
   generic parser for all protocols using [RFC5444], (ii) ensure
   "forward compatibility" of a protocol with future extensions, and
   (iii) enable the creation of efficient messages.

   During the same time period, other suggestions have been made to use
   [RFC5444] in a manner that would lead to incompatibilities with
   generic RFC 5444 parsers, would inhibit the development of
   interoperable protocol extensions, or would potentially lead to
   inefficiencies.  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.

   o  It defines a packet as a packet header with a set of packet TLVs,
      followed by a set of messages.  Each message has a well-defined
      structure consisting of a message header (designed for making



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      processing and forwarding decisions) followed by set of message
      TLVs (Type-Length-Value structures), 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 packets, 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 packet over UDP or directly over IP,
      the responsibility of this multiplexing process.

   o  A packet is 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 type without that
      information rendering the message un-parseable 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.

   o  It uses address aggregation into compact address blocks by
      exploiting commonalities between addresses.  In many deployments,



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      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 a common interface Identifiers, also,
      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 type 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 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, make desirable forms of generic parsers impossible, or make
   forms of extensions of those protocols impossible, or impedes on the
   ability to generate efficient messages.


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],
   specifically 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, make desirable
   forms of generic parsers impossible, or make forms of extensions of
   those protocols impossible, or impedes on the ability to generate
   efficient (small) messages.  The use of this format is mandated by
   [RFC5498] for all protocols running over the MANET protocol and port



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   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]. #### SEE COMMENTS IN SVN COMMIT MESSAGE AND ON LIST ####
   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
   considered.

   TLV space, although greater than message space, SHOULD also be used



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   efficiently.  The full type of 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 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.
   [RFC7181] contains a general purpose process for doing that, 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 the
   following section.  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., HW addresses when
   the address block is recording IP addresses) MAY be carried as TLV
   values.  Note that a message contains a Message Address Length (MAL)
   field that can be used to allow carrying alternative message sizes,
   but only one length of addresses in all address blocks can be used in
   a single message.

4.2.  Packets and Messages

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

   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.





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   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 MAY be created by the 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 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 MAY combine messages from multiple
      protocols in a packet.

   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.




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   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 message (Message TLV) or any value of any
   address (Address 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
   described in [RFC7188].  This specification extends their
   applicability to other uses of [RFC5444].

   The following points indicate the reasons for these rules, based on
   considerations of extensibility and efficiency.

   A protocol MUST NOT assign any meaning to the presence, or absence,
   of an address, as this would prevent the addition of addresses with
   other meanings.  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
   below).  An unmodified extension to NHDP would ignore such addresses,
   as required, as it does not support that specialized purpose.  If



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   NHDP had been designed so that just the presence of an address
   indicated a neighbor, that extension would not have been possible.

   This example can be taken further.  NHDP must also not reject a HELLO
   message because it contains an unrecognized TLV.  This also applies
   to unrecognized TLV values, where a TLV supports only a limited set
   of values.  For example, the blacklisting described in the previous
   paragraph 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.

   Information may also be added to addresses recognized by the base
   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 SMF [RFC6621]) must still process the HELLO
   message, ignoring the LINK_METRIC TLVs.

   This does 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 which is not permitted by [RFC7181].  A receiving
   OLSRv2-aware implementation of NHDP should 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.

   The restrictions on the use of address ordering and an address
   presence or absence in given address blocks for carrying information
   are for two reasons.  First use of those prevents the approach to
   information representation described in Section 4.5.  Second, it
   reduces the options available for message optimization described in
   Section 6.

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



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   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
   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, the likely
      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.  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 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



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      of immutable (apart from hop count and/or 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 (4 bits, 2 used) that manages the contents of the
      packet header.

   o  Message flags (4 bits, 4 used) that manages the contents of the
      message header.

   o  Address Block flags (8 bits, 4 used) that manages the contents of
      an Address Block.

   o  TLV flags (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
   fields "are RESERVED and SHOULD each be cleared ('0') on transmission
   and SHOULD be ignored on reception.".

   If a specification 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.

   During the development of [RFC5444], and since publication hereof,
   some proposals have been made to use these RESERVED flags to specify
   behavior rather than structure, in particular message forwarding.
   These were, after due consideration, not accepted, for a number of
   reasons.  These 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



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   field later left no spare flags bits at the message level for such
   use.


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, enables avoiding unnecessary repetition of information
   - and, consequently, generation of smaller messages.

6.1.  Addressesblock compression

   Addresses in an address block can be compressed, and SHOULD be.
   While no algorithm for compression is given in [RFC5444], an
   efficient compression algorithm given a set of addresses, has to obey
   certain contraints.

   The protocol using RFC5444 sets the constraints by defining the list
   of addresses and a list of addressblock TLV types and values for each
   of the addresses.  A compression strategy has to decide two
   additional things which will have a major influence on the
   compression efficiency.

   o  the split of the addresses into address blocks

   o  the order of the addresses within the address blocks.

   The order of addresses can be as simple as sorting the addresses, but
   if a lot of addresses have the same TLV types attached, it might be
   more useful to group the messages by sections with same or similar
   TLV types (e.g.  RFC6130 HELLO messages with local interface
   addresses first and neighbor addresses later).

   Compression of address blocks is obtained by considering 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
   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 a
   possible common Head.  Note that when, for example, IPv4 addresses
   have a common Head, their Tail will be empty.  For example 192.0.2.1
   and 192.0.2.2 would have a 3 octet Head, a 1 octet Mid, and a 0 octet
   Tail.

   Address blocks with few similar addresses will save more bytes by



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   using longer Head and Tails in the address block header.  Address
   blocks with a lot of addresses will reduce the overhead created by
   the address block header and TLV headers for multivalue TLVs.  The
   compression strategy will have to select the tradeof between these
   two optimizations that will lead to a minimal number of bytes.

6.2.  TLVs

   The main opportunities for efficient messages when considering TLVs
   are 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-
   valued (associating the same attribute with each address) or multi-
   valued (associating a separate attribute with each address).

   The simplest to implement approach is to use multi-valued 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-valued.  In a typical case where a
      LINK_STATUS TLV uses only the values HEARD and SYMMETRIC, with
      enough addresses, sorted appropriately, two single-valued TLVs can
      be more efficient than one multi-valued TLV.  (When only one value
      is involved, such as NHDP in a steady state with LINK_STATUS equal
      to SYMMETRIC in all cases, a single single-valued 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



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   TLVs.  It is however more efficient to do this with a single
   multivalue LINK_STATUS TLV, assigning the third address the value
   UNSPECIFIED.  This approach was specified in [RFC7188], and required
   for protocols that extend [RFC6130] and [RFC7181].  It is here
   RECOMMNDED 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) has 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 has two octet values
   whose first four bits are flags indicating whether the metric value
   applies in four cases; if these are all zero then the metric value
   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 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 while protocol-independent, this cannot be entirely TLV-
   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 guidelines 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.






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8.  IANA Considerations

   This document has no actions for IANA.


9.  Acknowledgments

   TBD


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.




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   [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
              Protocol version 2 (OLSRv2) and MANET Neighborhood
              Discovery Protocol (NHDP) Extension TLVs", RFC 7188,
              April 2014.


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

   Phone: +44 1245 242194
   Email: chris.dearlove@baesystems.com
   URI:   http://www.baesystems.com/


   Ulrich Herberg

   Email: ulrich@herberg.name
   URI:   http://www.herberg.name


   Henning Rogge

   Email: henning.rogge@fkie.fraunhofer.de




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