Internet DRAFT - draft-clausen-manet-rfc5444-usage
draft-clausen-manet-rfc5444-usage
Network Working Group T. Clausen
Internet-Draft LIX, Ecole Polytechnique
Updates: RFC5444 (if approved) C. Dearlove
Intended status: Standards Track BAE Systems AI Labs
Expires: November 29, 2015 U. Herberg
H. Rogge
May 28, 2015
Rules For Designing Protocols Using the RFC5444 Generalized Packet/
Message Format
draft-clausen-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 November 29, 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. Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. TLV Values . . . . . . . . . . . . . . . . . . . . . . . . 14
6.4. Automation . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
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], [RFC6622],
[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 as 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 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. Addresses
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, is
straightforward to implement.
Compression of addresses 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 10.0.0.1
and 10.0.0.2 would have a 3 octet Head, a 1 octet Mid, and a 0 octet
Tail.
Separate address blocks will be compressed separately. One possible
use of multiple address blocks (as well as the ability to have more
than 255 addresses) is for organizing addresses that compress
separately, e.g., from the same interface (same tail) or from the
same prefix (common head), or both.
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).
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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
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
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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.
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,
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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.
[RFC6622] Herberg, U. and T. Clausen, "Integrity Check Value and
Timestamp TLV Definitions for Mobile Ad Hoc Networks
(MANETs)", RFC 6622, 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
Protocol version 2 (OLSRv2) and MANET Neighborhood
Discovery Protocol (NHDP) Extension TLVs", RFC 7188,
April 2014.
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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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