Network Working Group J. Macker, editor
Internet-Draft NRL
Intended status: Experimental SMF Design Team
Expires: December 28, 2007 IETF MANET WG
June 26, 2007
Simplified Multicast Forwarding for MANET
draft-ietf-manet-smf-05
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Abstract
This document describes the Simplified Multicast Forwarding (SMF)
protocol. SMF provides a basic IP multicast forwarding capability
suitable for mobile ad-hoc networks (MANET). SMF applicability is
limited to providing multicast forwarding within MANET routing
regions. SMF specifies mechanisms to provide temporally unique
multicast packet identification for the purpose of enabling MANET-
specific duplicate packet detection (DPD). SMF also specifies the
operation of DPD maintenance and checking for both sequence-based and
hash-based methods. For IPv6, a hop-by-hop option header is
specified that assists in the overall DPD process. The optional
operation of intermediate devices, called taggers, is also specified
and relates to potential use with multiple border routers. SMF is
designed to take advantage of reduced relay sets for efficient MANET
multicast forwarding and the document describes use and interaction
with a number of approaches. Pseudocode and additional educed relay
set discussion is provided in the Appendices. Basic issues relating
to the operation of multicast MANET border routers are discussed but
ongoing work in this area remains and is beyond the scope of this
document.
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Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
2. Introduction and Scope . . . . . . . . . . . . . . . . . . . . 6
2.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 8
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
4. SMF Packet Processing and Forwarding . . . . . . . . . . . . . 10
5. SMF Duplicate Packet Detection . . . . . . . . . . . . . . . . 13
5.1. SMF IPv4 Packet Identification . . . . . . . . . . . . . . 14
5.2. SMF IPv6 Packet Identification . . . . . . . . . . . . . . 15
5.2.1. IPv6 SMF-DPD Header Option Format . . . . . . . . . . 16
5.2.2. IPv6 Sequence Based DPD (S-DPD) Header Mode . . . . . 17
5.2.3. IPv6 Hash Based DPD (H-DPD) Header Mode . . . . . . . 19
5.2.4. H-DPD Mode Operation . . . . . . . . . . . . . . . . . 20
6. Reduced Relay Set Forwarding and Relay Selection Capability . 22
7. SMF Neighborhood Discovery Requirements . . . . . . . . . . . 24
8. SMF Multicast Border Gateway Considerations . . . . . . . . . 26
8.1. Forwarded Multicast Groups . . . . . . . . . . . . . . . . 26
8.2. Multicast Group Scoping . . . . . . . . . . . . . . . . . 27
8.3. Duplicate Packet Detection Marking . . . . . . . . . . . . 28
8.4. Interface with Exterior Multicast Routing Protocols . . . 28
8.5. Multiple Border Routers . . . . . . . . . . . . . . . . . 29
8.6. Non-SMF MANET Nodes . . . . . . . . . . . . . . . . . . . 31
9. Security Considerations . . . . . . . . . . . . . . . . . . . 32
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
12.1. Normative References . . . . . . . . . . . . . . . . . . . 35
12.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Source-based Multipoint Relay (S-MPR) . . . . . . . . 37
A.1. S-MPR Relay Set Selection . . . . . . . . . . . . . . . . 38
A.2. Neighborhood Discovery Requirements . . . . . . . . . . . 38
Appendix B. Essential Connecting Dominating Set (E-CDS)
Algorithm . . . . . . . . . . . . . . . . . . . . . . 39
B.1. E-CDS Relay Set Selection . . . . . . . . . . . . . . . . 39
B.2. E-CDS Forwarding Rules . . . . . . . . . . . . . . . . . . 39
B.3. Neighborhood Discovery Requirements . . . . . . . . . . . 40
Appendix C. Multipoint Relay Connected Dominating Set
(MPR-CDS) Algorithm . . . . . . . . . . . . . . . . . 41
C.1. MPR-CDS Relay Set Selection . . . . . . . . . . . . . . . 41
C.2. MPR-CDS Forwarding Rules . . . . . . . . . . . . . . . . . 41
C.3. Neighborhood Discovery Requirements . . . . . . . . . . . 41
Appendix D. Pseudo Code for Relay Set Selection Algorithms . . . 42
D.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 42
D.2. S-MPR Selection Algorithm . . . . . . . . . . . . . . . . 42
D.2.1. Procedure to Select a Node as an MPR . . . . . . . . . 43
D.3. NS-MPR Selection Algorithm . . . . . . . . . . . . . . . . 43
D.4. MPR-CDS Selection Algorithm . . . . . . . . . . . . . . . 43
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D.5. 1-Hop E-CDS Selection Algorithm . . . . . . . . . . . . . 43
D.6. 2-Hop E-CDS Selection Algorithm . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . . . . 46
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1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Introduction and Scope
Past MANET unicast routing protocol designs have demonstrated
effective and efficient mechanisms to flood routing control packets
throughout a wireless routing region. For example, algorithms
specified within MANET RFC 3626 [RFC3626]and RFC 3684 [RFC3684]
provide distributed methods of dynamically electing reduced relay
sets that attempt to optimize control packet flooding of routing
control packets amongst MANET routing peers. The Simplified
Multicast Forwarding (SMF) extends this concept to the forwarding of
data plane IP multicast packets. The main goals of the SMF
specification are to define IPv4 and IPv6 duplicate multicast packet
detection (DPD) mechanisms and to adapt efficient reduced relay set
designs in MANET environments. The intent is to apply these
mechanisms to IP multicast packet forwarding within a MANET routing
region. SMF is intended for use when localized efficient flooding is
deemed an effective technique for multicast forwarding in dynamic
wireless networks. The SMF baseline design limits the scope to best
effort multicast forwarding and its applicability is also intended to
be constrained within a MANET routing region. Figure 1 provides an
overview of the logical SMF node architecture, consisting of optional
"Neighborhood Discovery", "Relay Set Selection" and "Forwarding
Process" components. Typically, relay set selection (or even self-
election) will occur based on input from a neighborhood discovery
process, and the forwarding process will be controlled by status
based upon relay set selection. This relay set information may be
obtained from a coexistent process (e.g., MANET unicast routing
protocol using relay sets). In some cases, the forwarding decision
for a packet may also depend on previous hop or incoming interface
information. The asterisks (*) in Figure 1 mark the primitives and
relationships needed by relay set algorithms requiring previous-hop
packet forwarding knowledge.
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______________ _____________
| | | |
| Neighborhood | | Relay Set |
| Discovery |------------->| Selection |
| Protocol | neighbor | Algorithm |
|______________| info |_____________|
\ /
\ /
neighbor\ /fowarding
info* \ ____________ / status
\ | | /
`-->| Forwarding |<--'
| Process |
~~~~~~~~~~~~~~~~~>|____________|~~~~~~~~~~~~~~~~~>
incoming packet, forwarded packets
interface id, and
previous hop*
Fig. 1 - SMF Node Architecture
SMF is a network layer multicast forwarding process compatible with
different neighborhood discovery protocols and relay set selection
algorithms. Different discovery mechanisms or relay set algorithms
may be applicable for different MANET routing protocols and
deployments and it is not the intent that SMF dictate a single
approach. However, interoperable SMF implementations must conform to
the specified DPD approach and the related header options. In the
simplest case of multicast forwarding, Classical Flooding (CF) with
DPD is supported. This mode eliminates the need for any relay set
algorithm or neighborhood topology information. However, a reduced
relay set mechanism will typically be preferred in a deployment. A
reduced relay set is realized by selecting a _subset_ of all possible
nodes in a MANET routing region as the forwarding relay set. Known
relay set selection algorithms can be used to provide and maintain a
dynamic distribution mesh for forwarding user multicast data[MDC04].
A few such relay set selection algorithms are described in Appendices
of this document. Additional relay set algorithms or extensions may
be specified in the future for use with SMF.
Dynamic neighborhood topology information is often needed to
determine and maintain an optimized set of forwarding nodes. It is
expected that neighborhood topology discovery functions will be
provided by a MANET unicast routing protocol or a MANET NeighborHood
Discovery Protocol (NHDP) implementation running in concurrence with
SMF. This specification does not preclude a lower link layer from
providing necessary neighborhood information through an enhanced
interface if available. An SMF implementation SHOULD provide the
ability for relay state to be dynamically managed per operating
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interface. Some of the relay state maintenance options and
interactions are outlined later in Section 6. This document states
specific requirements for neighborhood discovery with respect to the
forwarding process and relay set selection algorithms described
herein. SMF relies on the MANET NHDP specification to assist in
relay set maintenance in the absence of any MANET unicast protocol or
lower layer information interface.
2.1. Abbreviations
MANET : Mobile Ad hoc Network
SMF : Simplified Multicast Forwarding
CF : Classical Flooding
CDS : Connected Dominating Set
MCDS : Minimum Connected Domination Set
MPR : Multi-point Relay
S-MPR: Source-based MPR
CDS-MPR: CDS-based MPR
E-CDS: Essential Connected Dominating Set
DPD: Duplicate Packet Detection
NHDP: Neighborhood Discovery Protocol
S-DPD: Sequence-based Duplicate Packet Detection
H-DPD: Hash-based Duplicate Packet Detection
HAV: Hash Assist Value
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3. Applicability
In highly dynamic mobile topologies, a more traditional tree-based
multicast routing protocol may not always be sensible or needed. A
basic packet forwarding service that reaches all MANET SMF routers
participating within a localized MANET routing region can provide a
useful group communication mechanism for various classes of
applications. Applications that MAY take advantage of a simple
multicast forwarding service within a MANET routing region include
multimedia streaming, interactive group application, peer-to-peer
middleware multicasting, and multi-hop discovery services.
Note again that Figure 1 provides a notional architecture for
_typical_ MANET SMF-capable nodes. However, a goal is that simple
end-system (non-forwarding) wireless nodes may also participate in
multicast traffic transmission and reception with standard network
layer semantics. Also, a multicast border router or proxying
mechanism MUST be used when interoperating with other IP multicast
routing such as that for fixed-infrastructure networks (e.g.,
Protocol Independent Multicast (PIM)). In present experiments,
proxying methods have demonstrated gateway functionality at MANET
border routers operating with external IP multicast routing
protocols. Although SMF may be extended or combined with other
protocols to provide increased reliability and group specific
forwarding state, the details of such enhanced methods will be
discussed in future documents.
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4. SMF Packet Processing and Forwarding
The SMF Packet Processing and Forwarding actions are conducted with
the following packet handling activities:
1. Processing of outbound, locally-generated multicast packets.
2. Reception and processing of inbound packets on a specific network
interface(s).
In the case that sequence-based DPD as described in Section 5 is
used, the purpose of intercepting outbound, locally-generated
multicast packets is to apply resequencing of the IPv4 ID header
field or add options headers as needed (e.g. IPv6). In the case
that resequencing is deemed necessary, it is RECOMMENDED that
sequence numbering be applied such that a different sequence number
space per <sourceAddress::destinationAddress> tuple be used. For
initial SMF purposes where no distinct routing path decisions for
different IP Multicast address destinations occur, it might appear to
be sufficient to use sequence number spaces aggregated across all IP
Multicast destinations (or across all IP destinations for a source as
is the default implementation of the IPv4 ID field in many operating
systems). However, future SMF extensions, beyond the present
discussion, may contain dynamic forwarding state dependent on the
multicast destination address. The future possibility that different
multicast address destinations may be routed differently suggests
that "per source/destination" identification be used. The default
global IPv4 ID sequence space may be sufficient for some SMF
deployments and interception of outbound packets may not be required
if end systems have numbered the IPv4 ID field in an acceptable
manner. In other cases, such as when IPSec headers have been applied
to packets, other sequence information may be available for the SMF
process to make use of in its duplicate table management.
Inbound multicast packets will be received by the SMF implementation
and processed for possible forwarding. There will be some well-known
multicast groups for flooding to all routers of an ad hoc network
specified for use with the network-layer flooding provided by SMF.
These multicast groups are specified to contain all MANET routers of
a contiguous MANET routing region, so that packets transmitted to the
multicast address associated with the group will be delivered to all
nodes as desired. For IPv6, the multicast address is specified to be
"site-local". The names of the multicast groups are given as
"SL_MANET_ROUTERS". This document does not support transmissions to
any directed broadcast address ranges. Minimally SMF SHALL forward,
as instructed by the relay set selection algorithm, unique (non-
duplicate) packets received for these well-known group addresses when
the TTL or hop count value in the IP header is greater than 1.
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Optionally, SMF deployments SHOULD forward packets for additional
"global scope" multicast groups to support application needs or to
distribute multicast packets that ingress the MANET routing region
via border routers. These additional addresses will be specified by
an _a priori_ list or possibly through a implementation of a dynamic
address management interface that may interact with a yet to be
defined MANET dynamic group membership extension. In all cases, the
following rules SHALL be observed for SMF multicast forwarding:
1. Multicast packets with TTL <= 1 MUST NOT be forwarded*.
2. Link Local multicast packets MUST NOT be forwarded
3. Incoming multicast packets with an IP source address matching one
of those of the local host interface(s) MUST NOT be forwarded.
4. Received packet frames with the MAC source address matching the
local host interface(s) MUST NOT be forwarded.
5. Received packets for which SMF cannot ensure DPD uniqueness MUST
NOT be forwarded.
Note that rule #3 is important because in wireless networks, the
local host may receive re-transmissions of its own packets when they
are forwarded by neighboring nodes. This rule avoids unnecessary
retransmission of locally-generated packets even when other
forwarding decision rules would apply.
Once these criteria have been met, the implementation should
reference a forwarding decision algorithm, possibly in concert with
duplicate packet detection, to determine the next step in packet
processing. The forwarding decision may be implicit, dependent upon
DPD results, only if the SMF implementation is configured to perform
classical flooding (CF) of IP multicast packets. Otherwise, a
forwarding decision is controlled using additional information
including relay set state. Neighborhood discovery protocols coupled
with the Source-based Multi-Point Relay (S-MPR) or other CDS
selection algorithms described later MAY be used to determine the
local host's status with respect to forwarding. For example,
algorithms may control forwarding based on a relay set election and
previous hop identifier (e.g. S-MPR forwarding), while others may
designate the local host as a forwarder of all neighbor packets based
on the neighborhood broadcast topology (e.g. Essential CDS (E-CDS)).
DPD is a fundamental and critical portion of the SMF forwarding
process. In general, detection of received duplicate packets is
required to avoid forwarding the same packet multiple times.
However, in some cases (e.g., S-MPR), duplicate detection of some
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non-forwarded packets is also needed to maintain efficient
forwarding. Details on different duplicate packet detection and
forwarding rules for the S-MPR, and E-CDS algorithms are given in
Appendices of his document.
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5. SMF Duplicate Packet Detection
MANET topologies are often mesh-based and the maintenance of any tree
structure can be complex under mobile and dynamic conditions. These
MANET characteristics lead to DPD being a common requirement in MANET
packet flooding. While this requires increased per-packet
processing, it is often necessary in MANET-specific multicasting
because packets may be forwarded out the same physical interface upon
which they arrived and nodes can receive copies of previously-
transmitted packets from other forwarding neighbors. This section
describes a basic SMF DPD mechanism and some alternative operational
options as considerations.
SMF MUST implement detection of duplicate multicast packets by a
temporal packet identification scheme. It is RECOMMENDED this be
implemented by keeping a history of previous received and forwarded
packet identifiers for comparison against recently forwarded
multicast packets. In the IPv6 case, SMF specifies two approaches to
multicast duplicate packet identification: a sequence numbering
mechanism and a hash-based ID mechanism. In the IPv4 case, SMF
specifies a sequence-based approach but does not necessarily preclude
hashing. To enforce proper avoidance of duplicate forwarding, SMF
implementations MUST manage DPD packet state for received and
forwarded packets. In the case that sequence-based packet
identification is used, implementations SHOULD timeout stale
histories for <sourceAddress::destinationAddress> entries where new,
_non-duplicate_ packets have not been recently received. The proper
minimum duration of any timeout delay SHOULD cover the expected
maximum network traversal time, MAX_PACKET_LIFETIME. We define
MAX_PACKET_LIFETIME as a system dependent estimate of the maximum
lifetime of a multicast packet being forwarded between any source and
destination nodes in the SMF network region. If the timeout is reset
only upon reception of non-duplicate packets, it also limits the time
that packets might be incorrectly dropped if a source node is stopped
and restarted in the case of sequence-based packet identification.
The required size of the DPD cache is similarly governed and is also
a function of the maximum expected packet rate. It should be noted
that less stateful bitmask approaches to marking packet status can be
used if there is a contiguous space of tuple-based sequence numbers
rather than explicit lists of arbitrary packet identifiers.
The DPD mechanism SHOULD avoid keeping unnecessary state for packet
flows such as those that are locally generated or link local
destinations that would not be considered for forwarding.
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5.1. SMF IPv4 Packet Identification
For SMF purposes, IPv4 multicast packets from a particular source are
assumed to be marked with a temporally unique identification number
in the ID field of the IPv4 packet header that can serve as a
"packetIdentifier" for SMF purposes. Unfortunately, in present
operating system networking kernels, the IP ID header field value is
not always generated or applied in a consistent manner with respect
to SMF needs. In order to build a working implementation without
encapsulating packets, an SMF implementation SHOULD provide a
sequence generation and marking module that can maintain and set a
monotonically increasing IP ID field for locally-generated multicast
packets with independent sequence number spaces applied on a
<sourceAddress::destinationAddress> tuple basis. This process will
also need to recalculate and replace a proper IP header checksum for
the modified header. For border routers ingressing external IPv4
traffic into an SMF MANET routing region, the border routers SHOULD
perform this same IP ID field re-sequencing. Note the presence of
IPSec may prevent such resequencing, but fortunately, IPSec does
provide its own organic means for duplicate packet detection that is
defined for use by SMF.
The use of IPSec for candidate packet flows presents the opportunity
to make use of the additional, perhaps more reliable, sequencing
information of the IPSec header for unique packet identification.
The IPSec header provides a packet identifier field that can be used
on a "per-security association" basis. The IP addressing and IPSec
Security Parameters Index (SPI) fields are used to identify security
associations and, hence, packet flows. So, if the packet is IPSec
encapsulated, SMF will check the <sourceAddress::destinationAddress::
SPI::packetIdentifier> where the <ESP_seq_number> or <AH_seq_number>
from the IPSec header serves as the "packetIdentifier" value.
Although it would be possible to support IPv4 network layer multicast
packet fragmentation, we presently do not specify the details of
managing such a DPD approach and we recommend having SMF intended
sources set the don't fragment bit and have detected IPv4 fragments
dropped SMF. This recommendation avoids the complexity and
inefficiencies arising from an implementation supporting IP layer
fragmentation of multicast packets and is often recommended best
practice in general for multicast.
To perform IPv4 duplicate detection for multicast packets, SMF will
check the <sourceAddress::destinationAddress::
[SPI::]packetIdentifier> combination against a history of received
packet identifiers. SMF use of the IPv4 ID field has been
demonstrated in running prototype code. The adoption of the IPv4 ID
field for widespread packet duplication detection has some
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disadvantages that need discussion. A main disadvantage is the use
and interpretation of the field is known to be inconsistent across
operating systems. The IPv4 ID field is also limited and may provide
less robust detection for high bandwidth applications since sequence
wrap-around may occur relatively frequently if it is not possible to
achieve "per source/destination" sequencing. As an alternative, the
use of a header option or encapsulation header in future
implementations may provide more flexibility and consistency (see
IPv6 DPD). Another advantage of using a header option (or other
encapsulation, if determined absolutely necessary) is that it would
be possible for MANET SMF border router to assess whether packets
ingressing a MANET routing region have already been properly
sequenced to avoid unnecessary re-injection of packets. We leave
these design alternatives to be further defined and discussed in
future work. A basic sequencing and marking design similar to the
one we formulate here can be easily adapted to work with future
approaches or can be bypassed when not needed.
5.2. SMF IPv6 Packet Identification
The following section describes the mechanism and options for SMF
IPv6 DPD. The core IPv6 header does not provide an explicit
identification header field that can be exploited for DPD. SMF
defines the following two areas to aid in DPD identification:
1. a hop-by-hop DPD options header (supporting sequencing and hash
assistance), and
2. the use of IPSec sequencing (in sequencing DPD mode) when an
IPSec header is detected.
SMF MUST provide a DPD marking module that can insert the hop-by-hop
IPv6 header option defined for locally generated multicast packets.
If the packet is _not_ IPSec encapsulated, SMF may use the IPv6
packet header and IPv6 DPD option to form the <sourceAddress::
destinationAddress::packetIdentifier> tuple that is checked against a
cache history of received IPv6 packet identifiers. Alternatively,
SMF may perform general DPD functionality by generating packet
hashing identifiers at the initial ingress and forwarders and
comparing these against a hash history cache. In this case, the IPv6
DPD header option can add a short hash assist value (HAV) when the
source detects a duplicate hash value has been generated. In this
case, implementations SHOULD use a common hash identifier calculation
to ensure the false positive detection method is robust at all nodes.
A header option is defined in Section 5.2.3 to help with the case of
false positives and to ensure a more robust approach in the case of a
weaker hashing algorithm or in the case of temporal packet content
similarities (e.g., multimedia streams, keep alives). It is also
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RECOMMENDED that any cache history be managed on a tuple
<sourceAddress::hash value> basis.
In sequence-based DPD deployments, it MAY be necessary for an MANET
multicast border router to apply the DPD marking on ingressing
packets. In this document, such an intermediate system is referred
to as a "tagger" (i.e., "tagging" the packet with DPD information)
and a "Tagger ID" field is provided in the IPv6 DPD marking mechanism
described below. In the case that a packet is tagged, SMF SHOULD
perform DPD processing on a <taggerID:destinationAddress> or
<taggerID::sourceAddress::destinationAddress::packetIdentifier>
depending upon the policy handling the case of multiple border
routers ingressing and tagging multiple copies of the same packet
flow. The usage of the "Tagger ID" is described in further detail in
Section 8. Similarly to the case for IPv4, the presence of IPSec may
prevent the intermediate addition of a hop-by-hop options header.
Again, the IPSec header provides a packet identifier field that can
be used on a "per-security association" basis. The IP addressing
fields and IPSec Security Parameters Index (SPI) fields are used to
identify security associations and, hence, packet flows. So, if the
packet is IPSec encapsulated, SMF will check the <sourceAddress::
destinationAddress::SPI::packetIdentifier> where the <ESP_seq_number>
or <AH_seq_number> from the IPv6 IPSec header serves as a
"packetIdentifier" value.
5.2.1. IPv6 SMF-DPD Header Option Format
Figure 2 illustrates the format of the IPv6 SMF Duplication Packet
Detection (SMF-DPD) hop-by-hop header option. If this is the only
hop-by-hop option present, the optional "Tagger ID" field is not
included, and the size of the DPD packet identifier (sequence number)
or hash token is 24 bits or less, this will result in the addition of
8 bytes to the IPv6 packet header including the "Next Header",
"Header Extension Length", SMF-DPD option fields, and padding.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Option Type | Opt. Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|H| Sequence-based DPD Option Fields or Hash Assist Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig. 2 - IPv6 SMF-DPD Hop-by-hop Header Option
"Option Type" = (TBD pending IANA assignment)
"Opt. Data Len" = Length of option content (I.e., 1 + (<IDType> ?
(<IDLen> + 1): 0) + Length(DPD ID)).
"H-bit" = a hash indicator bit value identifying DPD marking type. 0
== sequence-based approach w/ optional taggerID and a tuple-based
sequence number. 1 == indicates a hash assist value (HAV) field
follows to aid in avoiding hash-based DPD collisions.
The following sections will provide specification for both sequence-
based DPD (S-DPD) and hash-based DPD (H-DPD) header option
implementations.
5.2.2. IPv6 Sequence Based DPD (S-DPD) Header Mode
Figure 3 illustrates the format of the IPv6 SMF-DPD hop-by-hop header
option for supporting S-DPD. For S-DPD mode header options the H-bit
must be set to zero as shown in Figure 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Option Type | Opt. Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|IDTyp| IDLen | Tagger ID (optional |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | DPD Sequence Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig. 3 - IPv6 SMF-DPD Header Option in S-DPD mode
"IDType" = 3-bit type indicating either optional "Tagger ID" field or
basic sequencing. An enumeration of initial type values is given
below.
"ID Len" = 4-bit length of optional "Tagger ID" field in bytes minus
one iff non-zero IdType (I.e., if IdType is non-zero and IdLen==0,
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then the length of the "Tagger ID" field is one byte). The "IDLen"
field MUST be set to ZERO if "IDType" is ZERO.
Tagger ID = identifies an intermediate node that has applied the SMF-
DPD option to the packet (instead of the source).
DPD packet identifier = monotonically increasing n-bit sequence
number assigned on a tuple basis as per <sourceAddress::
destinationAddress> or <taggerID::sourceAddress::destinationAddress>
basis. If "IdType" is non-zero, the length of this field is (<Opt.
Data Len> - <IdLen> - 2). Otherwise, the length of this field is
(<Opt. Data Len> - 1).
The following list of Tagger ID type values are defined below.
Additional Tagger "IDType" values may be assigned in subsequent
specifications. Tagger IDTypes only valid when H bit is set to 0.
+---------+-------+-------------------------------------------------+
| Name | Value | Purpose |
+---------+-------+-------------------------------------------------+
| NULL | 0 | Indicates no "Tagger ID" field is present. |
| | | IdLen MUST be also set to ZERO.` |
| | | |
| DEFAULT | 1 | The "Tagger ID" field of unknown context is |
| | | present. "ID Len + 1" defines field length in |
| | | bytes. |
| | | |
| IPv4 | 2 | The "Tagger ID" represents an IPv4 address. |
| | | The "ID Len" MUST be set to 3. |
| | | |
| IPv6 | 3 | The "Tagger ID" represents an IPv4 address. |
| | | The "ID Len" MUST be set to 15 |
+---------+-------+-------------------------------------------------+
This specified format allows a quick check of the "IdType" field to
detect whether a "Tagger ID" is present. If "IdType" is NULL, then
the length of the "DPD packet identifier" (sequence number)
corresponds to (<Opt. Data Len> - 1). If the "IdType" is not NULL,
then the length of the "Tagger ID" field is equal to (<IdLen> + 1)
and the remainder of the option content comprises the "DPD packet
identifier" field. When the "Tagger ID" field is present, duplicate
packet detection SHALL be conducted using the <taggerID::
sourceAddress::destinationAddress> tuple from the packet to identify
the applicable sequence space. When the "Tagger ID" field is not
present, then it is assumed that the source host applied the DPD
option and the packet's <sourceAddress::destinationAddress> SHALL be
used to identify the sequence space for duplicate packet detection.
Thus "Tagger ID" fields sized up to 16 bytes may be applied and the
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size of the DPD packet identifier is configurable to meet the needs
of different network environments. In fact, if an 8-bit "Tagger ID"
and a 16-bit "DPD packet identifier" are used, the size of the SMF-
DPD hop-by-hop header extension would still be the minimum possible
IPv6 size if no other options are present.
The rationale for providing "Tagger ID" context (i.e., the "IdType"
field) is that the tagger identifier may correspond to some commonly-
available identifier such as an IP address so that management of a
specific identifier space for border routers possibly applying the
SMF-DPD may not be necessary. While the context of the "Tagger ID"
(what the "Tagger ID" actually represents) is not necessary for SMF
DPD processing, it is possible that this context may be useful as
part of additional network management or policies that might be
operate in concert with SMF.
Figure 4 illustrates a specific example of the SMF-DPD option format
when the "Tagger ID" is _not_ applied and a 16-bit "DPD packet
identifier" is used. It is RECOMMENDED that a 16-bit "DPD packet
identifier" be used for most purposes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Option Type |OptDataLen = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IdType&IdLen=0 | DPD packet identifier | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig. 4 - Example SMF-DPD Option w/ no "Tagger ID" and 16-bit DPD ID
5.2.3. IPv6 Hash Based DPD (H-DPD) Header Mode
Figure 5 illustrates the format of the IPv6 SMF-DPD hop-by-hop header
option for supporting hash-based DPD (H-DPD) approaches. Within a
H-DPD mode header, the H-bit must be set to 1 as shown in Figure 5.
The length of the H-bit + Hash Assist Value (HAV) is equal to
(<Opt.Data Len> -1) bytes.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Option Type | Opt. Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Hash Assist Value (HAV) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig. 5 - IPv6 SMF-DPD Header Option in H-DPD mode
The H-DPD mode header option SHOULD be applied when SMF is operating
in a hash-based DPD mode and the source calculates a duplicate hash
value within the tuple-based <sourceAddress:: hash value> cache
history. The hash marking default approach is RECOMMENDED to be a
non-zero monotonically increasing value of 31 bits maintained within
the tuple history. This should provide enough variability to resolve
envisioned ambiguities given a reasonable strong hash index approach.
5.2.4. H-DPD Mode Operation
To ensure the robustness of the H-DPD method and the consistent use
of the H-DPD option header we specify a default hashing approach for
use by H-DPD SMF nodes.
The default mode of SMF H-DPD is as follows. SMF SHOULD perform an
MD5 [RFC1321]hash of the immutable fields and header options of the
IPv6 multicast header and data contents resulting in a 128 bit digest
result. The parsing exception is that SMF should include any
detected H-DPD HAV option header in its hash calculation. Other
mutable header options should be skipped. SMF H-DPD mode SHOULD
maintain a cache history of lower 64 bits of the digest (MD5_64)
based upon the tuple <sourceAddress::MD5_64). This approach is
likely to have a reasonably low probability of digest collision when
packet headers and content are varying. In the SMF case, MD5 is only
be applied to provide a reasonable low probability of collision and
is not being used for cryptographic or authentication purposes.
If a collision is detected, an H-DPD option header is generated and
retested for collision. If rehashing with the generated options
header provides a non-collision event after testing the cache
history, then the multicast packet is forwarded with the addition of
an the IPv6 H-DPD header option. If a collision is detected with the
option header in place, the header option should generate a new HAV
and retest for collision until a non-colliding digest entry results.
The MD5, indexing, and HAV approach are specified at present for
consistency and robustness. Future approaches and experimentation
may discover designs tradeoffs in hash robustness and efficiency
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worth considering for future versions of SMF. This MAY include
reducing the maximum payload length that is processed, determining
shorter indexes, or applying more efficient hashing algorithms.
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6. Reduced Relay Set Forwarding and Relay Selection Capability
SMF MUST support CF-based forwarding as a basic forwarding mechanism
when reduced relay set information is not available or not selected
for operation. In CF mode, each node transmits a locally generated
or newly received packet exactly once. The DPD techniques mentioned
in the previous section are critical to proper operation and prevent
duplicate packet retransmissions by the same forwarding node.
In addition to DPD specification, an additional goal of SMF is to
support reduced relay sets as a forwarding mechanisms within dynamic
topologies. To accomplish this SMF MUST support the ability to
modify local multicast packet forwarding rules based upon relay set
state received dynamically during operation. In this way, SMF
forwarding can continue to operate effectively as adjacencies within
the topology mesh change.
In early SMF implementations, the relay set information has been
derived from a coexistent unicast MANET protocol (e.g., OLSR and
MANET-OSPFv3) calculation of a reduced relay set for control plane
traffic. From current experience, extra pruning considerations are
sometimes required when utilizing a relay set from a independent
routing protocol process, since a relay set formed for the unicast
control plane flooding MAY include a more redundant set of nodes than
desired for multicast forwarding use.
Here is a recommended criteria list for relay set selection algorithm
candidates:
1. Robustness to topological dynamics and mobility
2. Localized election or coordination of any relay sets
3. Heuristic support for preference or election metrics (Better
enables management of relay set)
Some relay set algorithms meeting these criteria are described in the
Appendices of this document. Different algorithms may be more
suitable for different MANET routing types or deployments.
Additional relay set selection algorithms may be specified in
separate documents in the future. The Appendices in this document
can serve as a template for the content of such potential future
specifications.
Figure 6 depicts a state information diagram of possible relay set
control options. There are basically three style of SMF operation
with reduced relay sets as follows.
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1. Unicast dependent operation with a coexisting MANET unicast
routing protocol (e.g., MANET-OSPF, OLSR) in which the relay set
state is derived from the unicast CDS information.
2. Unicast independent operation in which SMF performs its own Relay
Set Selection and derives dynamic neighborhood information from a
MANET NHDP process. Additional TLV definitions for related CDS
collection may be required as specified in the Appendices.z
3. Possible crosslayer implementation that uses L2 neighborhood
information and possible triggers to assist the dynamic relay set
selection and maintenance process.
Possible L2 Trigger/Information
|
|
______________ _______v_____ __________________
| MANET | | | | |
| Neighborhood | | Relay Set | | Other Heuristics |
| Discovery |------------->| Selection |<------| (Preference,etc) |
| Protocol | neighbor | Algorithm | | |
|______________| info |_____________| |__________________|
\ /
\ /
neighbor\ / Dynamic Relay
info* \ ____________ / Set Status
\ | SMF | / (State, {neighbor info})
`-->| Relay Set |<--'
| State |
-->|____________|
/
/
______________
| |
| Coexistent |
| MANET |
| Unicast |
| Process |
|______________|
Fig. 6 - SMF Relay Set Control Options
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7. SMF Neighborhood Discovery Requirements
In absence of a compatible, coexisting unicast routing protocol or
lower layer protocol providing neighborhood topology information
sufficient for relay set selection, this section defines the issues
and additional requirements for a MANET Neighborhood Discovery
Protocol (NHDP) that MAY be operational between SMF nodes.
With respect to neighborhood topology knowledge and/or discovery,
there are three basic modes of SMF operation:
1. Classical Flooding (CF) mode: with no requirements for discovery
or knowledge of neighborhood topology,
2. External CDS control mode: an external process dynamically
determines the local SMF relay status (e.g., SMF prototypes have
leveraged neighborhood topology information collected by MANET
unicast routing protocols such as OLSRv2 or Manet-OSPF ), and
3. Independent CDS control mode: SMF uses the MANET Neighborhood
Discovery Protocol (NHDP) [NHDP] to collect localized link
information required for the various CDS algorithm modes
discussed in the Appendices.
We have previously discussed modes 1 and 2. This section will
describe mode 3, using NHDP to support CDS relay set capability
independent of any MANET unicast routing protocol process. This
design uses and is consistent with the Generalized MANET Packet/
Message Format [PacketBB] and NHDP protocol work in progress within
the MANET WG.
Core NHDP messages and the neighborhood information base are
described separately within the NHDP specification (IETF work in
progress). In this mode, SMF uses and relies upon an implementation
of NHDP. The NHDP protocol provides the following basic functions:
1. 1-hop neighbor link sensing: maintaining neighbor lists and
performing a basic bidirectionality check of neighbor links
2. 2-hop Neighborhood Discovery: collecting 2-hop bidirectional
neighborhood information and any information relevant to relay
set election
3. The collection and maintenance of the above information across
multiple interfaces.
4. Relay Set Signaling: signal relay set selection to neighbor nodes
if the relay set algorithm requires such information
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The Appendices discuss a set of implemented SMF CDS approaches and
the related TLV requirements that may be needed by an NHDP
implementation to support each approach.
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8. SMF Multicast Border Gateway Considerations
Typically, it is expected that SMF will be used to provide a more
simplified forwarding of multicast traffic within a MANET mesh
routing topology. However, a border router method should be used to
allow interconnection of SMF operation with networks using other
multicast routing protocols (e.g., PIM). It is important to note
that there are many scenario specific issues that should be addressed
when discussing border routers. At the present time, working
deployments of SMF and PIM border router approaches are being
experimented with.
In general, some of the functionality border routers may need to
address include the following.
1. Determining which multicast groups should transit the border
router whether entering or exiting the attached MANET routing
region(s).
2. Enforcement of TTL threshold or other scoping policies.
3. Any marking or labeling to enable DPD on ingressing packets.
4. Interface with exterior multicast routing protocols.
5. Possible operation with multiple border routers (presently beyond
scope of this document).
6. Provisions for participating non-SMF nodes.
Note the behavior of border router nodes is the same as that of non-
border routers when forwarding packets on interfaces within the MANET
routing region. And packets that are passed outbound to interfaces
operating more fixed Internet multicast routing protocols SHOULD be
evaluated for duplicate packet status since present standard
multicast forwarding mechanisms do not usually perform this function.
8.1. Forwarded Multicast Groups
Determining which groups should be forwarded into a MANET SMF routing
region is an evolving technology area. Ideally, only groups for
which there is active group membership should be injected into the
SMF domain. This might be accomplished by providing an IPv4 Internet
Group Membership Protocol (IGMP) or IPV6 Multicast Listener Discovery
(MLD) proxy protocol so that MANET SMF nodes can inform attached
border routers (and hence multicast networks) of their current group
membership status. For specific systems and services it may be
possible to statically configure group membership joins in border
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border routers, but it is RECOMMENDED that some form of IGMP/MLD
proxy or other explicit, dynamic control of membership be provided.
Specification of such an IGMP/MLD proxy protocol is beyond the scope
of this document.
Outbound traffic is less problematic. SMF border routers can perform
duplicate packet detection and forward non-duplicate traffic that
meets TTL/hop limit and scoping criteria to other interfaces.
Appropriate IP multicast routing (PIM, etc) on those interfaces can
then make further forwarding decisions with respect to the given
traffic and its MANET source address. Note that the presence of
multiple border routers associated with a MANET routing region may
create some additional issues. This is further discussed in
Section 8.5.
8.2. Multicast Group Scoping
Multicast scoping is used by network administrators to control the
network routing regions which are reached by multicast packets. This
is usually done by configuring external interfaces of border routers
in the border of an routing region to not forward multicast packets
which must be kept within the routing region. This is commonly done
based on TTL of messages or the basis of group addresses. These
schemes are known respectively as:
1. TTL scoping.
2. Administrative scoping.
For IPv4, network administrators can configure border routers with
the appropriate TTL thresholds or administratively scoped multicast
groups in the router's interfaces as with any traditional multicast
router. However, for the case of TTL scoping it must be taken into
account that the packet could traverse multiple hops within the MANET
SMF routing region before reaching the border router. Thus, TTL
thresholds must be selected carefully.
For IPv6, multicast addresses themselves include information about
the scope of the group. Thus, border routers of an SMF routing
region know if they must forward a packet based on the IPv6 multicast
group address. For the case of IPv6, we recommend a MANET SMF
routing region be designated a site. Thus, all multicast packets in
the range FF05::/16 will be kept within the MANET SMF routing region
by border routers. Packets in any other wider range (i.e.
FF08::/16, FF0B::/16 and FF0E::16) MAY traverse border routers unless
other restrictions different from the scope applies.
Given that scoping of multicast packets is performed at the border
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routers, and given that existing scoping mechanisms are not designed
to work with mobile routers, we assume that non-border SMF routers,
will not stop forwarding multicast data packets because of their
scope. That is, we assume that the whole MANET SMF routing region is
a non-divisible scoping area except in the case of link-local
addresses that are not forwarded by SMF.
8.3. Duplicate Packet Detection Marking
Packets sourced external to an SMF routing region may not have
duplicate packet sequencing properly applied, or hash ID collision
may not have been previously checked, and the border router may need
to provide that sequencing information or hash collision detection
upon entry into the MANET routing region In the case of IPv6, the
border router can apply the SMF DPD Hop-by-Hop options header to
packets forwarded into the MANET routing region for those packets
that do not already have the option applied. If this option has been
applied, this indicates the packet has already been marked for
potential handling by SMF relays. Similarly, IP packets that have
been encapsulated with IPSec may also be treated as appropriately
marked for DPD and may be forwarded without modification. Both of
these indicators (the IPv6 SMF DPD option and IPSec encapsulation)
provide the side benefit for the border router to explicitly
determine if the packet has already been marked. In this case, the
border router can use the packet identification field to ensure it is
not re-injecting a duplicate packet into the MANET routing region.
For IPv4 packets that are not IPSec encapsulated, it is RECOMMENDED
that border routers re-sequence the ID field of packets injected into
the routing region. However, the IPv4 ID field does not provide the
border router with explicit information on whether the field has been
previously set for SMF purposes. Thus, the potential exists that
duplicate IPv4 packets may be re-injected by a border router into an
SMF routing region if a multicast routing loop has occurred. If
multiple multicast border routers are envisioned, additional future
considerations must be taken into account and solutions are
considered out of scope for this document. See Section 8.5 for more
discussion of related issues.
8.4. Interface with Exterior Multicast Routing Protocols
The traditional operation of multicast routing protocols is tightly
integrated with the group membership function. Leaf routers are
configured to periodically gather group membership information, while
intermediate routers conspire to create multicast trees connecting
routers with directly-connected multicast sources and routers with
active multicast receivers. In the concrete case of SMF, we can
consider border routers as leaf routers. Mechanisms for multicast
sources and receivers to interoperate with border routers over the
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multihop MANET SMF routing region as if they were directly connected
to the router need to be defined. The following issues need to be
addressed:
1. Mechanism by which border routers gather membership information.
2. Mechanism by which multicast sources are known by the border
router.
3. Exchange of exterior routing protocol messages across the MANET
routing region if the MANET routing region is to provide transit
connectivity for multicast traffic.
It is beyond the scope of this document to address implementation
solutions to these issues. As described in Section 8.1, IGMP/MLD
proxy mechanisms can be deployed to address some of these issues.
Similarly, exterior routing protocol messages could be tunneled or
conveyed across the MANET routing region. But, because MANET routing
regions are multi-hop and potentially unreliable, as opposed to the
single-hop LAN interconnection that neighboring IP Multicast routers
might typically enjoy, additional provisions may be required to
achieve successful operation.
The need for the border router to receive traffic from recognized
multicast sources within the MANET SMF routing region is important to
achieve a smooth interworking with existing routing protocols. For
instance, PIM-S requires routers with locally attached multicast
sources to register them to the Rendezvous Point (RP) so that other
people can join the multicast tree. In addition, if those sources
are not advertised to other autonomous systems (AS) using MSDP,
receivers in those external networks are not able to join the
multicast tree for that source.
8.5. Multiple Border Routers
A MANET might be deployed with multiple participating nodes having
connectivity to external (to the MANET), fixed-infrastructure
networks. Allowing multiple nodes to forward multicast traffic to/
from the MANET routing region can be beneficial since it can increase
reliability, and provide better service. For example, if the MANET
routing region were to fragment with different MANET nodes
maintaining connectivity to different border routers, multicast
service could still continue successfully. But, the case of multiple
border routers connecting a MANET routing region to external networks
presents several challenges for SMF:
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1. Detection/hash collision/sequencing of duplicate unmarked IPv4 or
IPv6 (without IPSec encapsulation or DPD option) packets possibly
injected by multiple border routers.
2. Source-based relay algorithms handling of duplicate traffic
injected by multiple border routers.
3. Determination of which border router(s) will forward outbound
multicast traffic.
4. Additional challenges with interfaces to exterior multicast
routing protocols.
One of the most obvious issues is when multiple borde routers are
present and may be alternatively (due to route changes) or
simultaneously injecting common traffic into the MANET routing region
that has not been previously marked for SMF DPD. Different border
routers would not be able to implicitly synchronize sequencing of
injected traffic since they may not receive exactly the same messages
due to packet losses. For IPv6 operation, the "Tagger ID" optional
field described for the SMF-DPD header option can be used to mitigate
this issue. When multiple border routers are injecting a flow into a
MANET routing region, there are two forwarding policies that SMF
DPD-S nodes may implement:
1. Redundantly forward the multicast flows (identified by
<sourceAddress:destinationAddress>) from each border router,
performing DPD processing on a <taggerID:destinationAddress> or
<taggerID:sourceAddress:destinationAddress> basis, or
2. Use some basis to select the flow of one tagger (border router)
over the others and forward packets for applicable flows
(identified by <sourceAddress:destinationAddress>) only for that
"Tagger ID" until timeout or some other criteria to favor another
tagger occurs.
It is RECOMMENDED that the first approach be used in the case of
DPD-S unless the SMF system is specifically designed to implement the
second option. Additional specification may be required to describe
an interoperable forwarding policy based on this second option. Note
that the implementation of the second option requires that per-flow
(i.e., <sourceAddress::destinationAddress>) state be maintained for
the selected "Tagger ID".
The use of DPD-H may actually improve the case of duplicate packet
detection when ingressing traffic comes from multiple border routers.
Non-colliding hash indexes (those not requiring the DPD-H options
header should be resolved effectively.
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8.6. Non-SMF MANET Nodes
There may be scenarios in which some peer wireless MANET nodes may
not wish to run the SMF protocol and/or conduct forwarding, but they
are interested in receiving multicast data. For example, a MANET
service might be deployed that is accessible to wireless edge devices
that do not participate in MANET routing and/or SMF forwarding
operation. These devices include:
1. Devices that opportunistically receive multicast traffic due to
proximity with SMF relays (possibly with asymmetric IP
connectivity e.g., sensor network device).
2. Devices that participate in NHDP (directly or via routing
protocol signaling) but do not forward traffic.
Note there is no guarantee of traffic delivery with category 1 above,
but the election heuristics shown in Figure 2 may be adjusted via
management to better support such devices. It is RECOMMENDED that
nodes participate in NHDP when possible. Such devices may also
transmit multicast traffic, but it is important to note that SMF
routing regions using source-specific relay set algorithms such as
(S-MPR) may not forward such traffic. These devices SHOULD also
listen for any IGMP/MLD Queries that are provided and transmit IGMP/
MLD Reports for groups they have joined per usual IP Multicast
operation. While it is not in the scope of this document, IGMP/MLD
proxy mechanisms may be in place to convey group membership
information to any border routers or intermediate systems providing
IP Multicast routing functions.
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9. Security Considerations
Gratuitous use of option headers can cause problems in routers.
Routers outside of MANET routing regions should ignore SMF specific
header options if encountered.
Authentication mechanisms to identify the source of an option header
should be considered to reduce vulnerability to a variety of attacks.
A source may produce large quantities of multicast packets with the
same header and content value to force the collision detection for
every packet when operating in H-DPD. Well known packet types may be
spoofed intentionally apriori to corrupt temporal cache histories and
force collisions for key networking packets and in this case the
authentication of packet sources should be strongly considered.
Additional security considerations TBD.
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10. IANA Considerations
There are number of discussions within this SMF specification that
will be subject to IANA registration. The IP Header Extensions being
defined within this document MUST have an IANA registry established
for them upon publication of the first RFC. Additionally, the well-
known multicast addresses intended for default use by the SMF
forwarding process should be registered and defined by the first RFC
published.
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11. Acknowledgments
Many of the concepts and mechanisms used and adopted by SMF resulted
from many years of discussion and related work within the MANET WG
since the late 1990s. There are obviously many contributors to past
discussions and related draft documents within the WG that have
influenced the development of SMF concepts that deserve
acknowledgment. In particular, the document is largely a direct
product of the SMF design team within the IETF MANET WG and borrows
text and implementation ideas from the related individuals. Some of
the contributors who have been involved in key document content
editing, prototype implementation, and core discussions are listed
below. We appreciate input from others we may have missed in this
list as well.
SMF Core Design Team Contributors:
Brian Adamson
Ian Chakeres
Thomas Clausen
Justin Dean
Brian Haberman
Charles Perkins
Pedro Ruiz
Maoyu Wang
The RFC text was produced using Marshall Rose's xml2rfc tool.
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12. References
12.1. Normative References
[E-CDS] Ogier, R., "MANET Extension of OSPF Using CDS Flooding",
Proceedings of the 62nd IETF , March 2005.
[MPR-CDS] Adjih, C., Jacquet, P., and L. Viennot, "Computing
Connected Dominating Sets with Multipoint Relays", Ad Hoc
and Sensor Wireless Networks , January 2005.
[NHDP] Clausen, T. and et al, "Neighborhood Discovery Protocol",
draft-ietf-manet-nhdp-03, Work in progress , May 2007.
[OLSRv2] Clausen, T. and et al, "Optimized Link State Routing
Protocol version 2", draft-ietf-manet-olsrv2-03, Work in
progress , February 2007.
[PacketBB]
Clausen, T. and et al, "Generalized MANET Packet/Message
Format", draft-ietf-manet-packetbb-06, Work in progress ,
June 2007.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol", 2003.
12.2. Informative References
[GJ79] Garey, M. and D. Johnson, "Computers and Intractability: A
Guide to the Theory of NP-Completeness.", Freeman and
Company , 1979.
[JLMV02] Jacquet, P., Laouiti, V., Minet, P., and L. Viennot,
"Performance of multipoint relaying in ad hoc mobile
routing protocols", Networking , 2002.
[MDC04] Macker, J., Dean, J., and W. Chao, "Simplified Multicast
Forwarding in Mobile Ad hoc Networks", IEEE MILCOM 2004
Proceedings , 2004.
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[NTSC99] Ni, S., Tseng, Y., Chen, Y., and J. Sheu, "The Broadcast
Storm Problem in Mobile Ad hoc Networks", Proceedings Of
ACM Mobicom 99 , 1999.
[RFC2901] Macker, JP. and MS. Corson, "Mobile Ad hoc Networking
(MANET): Routing Protocol Performance Issues and
Evaluation Considerations", 1999.
[RFC3684] Ogier, R., Templin, F., and M. Lewis, "Topology
Dissemination Based on Reverse-Path Forwarding", 2003.
[WC02] Williams, B. and T. Camp, "Comparison of Broadcasting
Techniques for Mobile Ad hoc Networks", Proceedings of ACM
Mobihoc 2002 , 2002.
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Appendix A. Source-based Multipoint Relay (S-MPR)
The source-based multipoint relay (S-MPR) set selection algorithm
enables individual nodes, using two-hop topology information to
select a minimum set of neighboring nodes that can provide relay to
all nodes within a two-hop radius. This distributed technique has
been shown to approximate selection of a MCDS in [JLMV02].
Individual nodes must collect two-hop neighborhood information from
neighbors, determine an appropriate current relay set, and then
inform the resultant selected neighbors of their relay status. The
Optimized Link State Routing (OLSR) protocol has used this algorithm
and protocol for relay of link state updates and other control
information[RFC3626] and has been shown to operate well even in
dynamic network environments.
Because a node's status as a relay is with respect to neighboring
nodes who have selected it (i.e., its "selectors"), the relaying node
must know the previous-hop transmitter of packets it receives in
order to make an appropriate forwarding decision. Additionally, it
is important that relay nodes forward packets only for those nodes
currently identified as symmetric, one-hop neighbors to maintain
correctness. Also, because the selection of relays does not result
in a common set among neighboring nodes, relays MUST mark in their
duplicate table any transmissions from non-selector, symmetric, one-
hop neighbors (for a given interface) and not forward subsequent
received copies of that packet even if received from a selector
neighbor. Deviation here may result in unnecessary, even excessive,
repeat transmission of packets throughout the network. Or incorrect
duplicate table recording of packets received from non-symmetric
neighbors may result in incomplete flooding. In these respects,
flooding based on the S-MPR algorithm is more complex than that based
upon some other relay set selection algorithms.
When multiple interfaces are present, the S-MPR SMF forwarded must
keep some independent state for each interface with regards to
duplicate packets. For example, when a packet is received from a
non-selector, one-hop symmetric neighbor, an SMF forwarder using the
S-MPR algorithm must update its duplicate packet state with respect
to the interface on which the packet was received. If the SMF
forwarder receives that same packet from a selector neighbor on a
different interface, it MUST still forward that packet on all
interfaces it has not received that packet from a one-hop symmetric
neighbor. Once a packet has been forwarded in this fashion,
subsequent duplicates received on any interface are ignored.
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A.1. S-MPR Relay Set Selection
If SMF is operating S-MPR relay set election independent of
coexistent OLSR operation, based upon NHDP mechanisms, the election
algorithm defined within RFC3626 [RFC3626] should be used.
A.2. Neighborhood Discovery Requirements
S-MPR election operation requires 2-hop neighbor knowledge as
provided by the NHDP protocol[NHDP] or as available from external
sources. MPRs are dynamically selected by each node and selections
MUST be advertised and dynamically updated within the SMF NDP or
equivalent protocol. In this mode, the MPR specific TLVs defined in
OLSRv2 [OLSRv2]are also required to be implemented by NHDP.
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Appendix B. Essential Connecting Dominating Set (E-CDS) Algorithm
The "Essential Connected Dominating Set" (E-CDS) algorithm [E-CDS]
allows nodes to use two-hop topology information to appropriately
elect _themselves_ as relay nodes to form an efficient (for flooding)
CDS. While this algorithm does not tend to produce as small a set of
relay nodes (per forwarded packet) as the previously-described S-MPR
algorithm, it is not dependent upon previous-hop information to make
a forwarding decision; it simply forwards any received non-duplicate
packets. This property also allows relay nodes using the E-CDS
algorithm to be intermixed with nodes performing only classical
flooding. Additionally, the semantics for multiple interface support
are simplified as compared to S-MPR and even packets that are
received from non-symmetric neighbors may be forwarded without
compromising flooding efficiency or correctness.
B.1. E-CDS Relay Set Selection
This section provides a short description of the E-CDS based relay
set selection algorithm and is based upon Richard Ogier's original
summary within [E-CDS]. This was originally discussed in the context
of forming partial adjacencies and efficient flooding for MANET-OSPF
work but its core algorithm is applied here.
E-CDS requires two-hop neighbor information collected through the
SMF-NDP or other process. Each router has a Router Identifier (may
be represented by an interface address) and Router Priority value.
The Router Priority value may be dynamic and represent such metrics
as node degree. The fundamental election steps are as follows:
1. If an SMF node has a higher (Router Priority, Router ID) than all
of its symmetric neighbors, it elects itself to the relay set.
2. Else, if there does not exist a path from neighbor j with largest
(Router Priority, Router ID) to some other neighbor, via
neighbors with larger values of (Router Priority, Router ID),
then it elects itself to the relay set.
The basic form of E-CDS described and applied within this
specification does not at present define redundant relay set election
but such capability is supported by the E-CDS design.
B.2. E-CDS Forwarding Rules
E-CDS forwarding is quite simple and straightforward. As mentioned,
there is no need to check previous hop information during forwarding.
Upon electing itself as an E-CDS relay set forwarder, SMF nodes
perform DPD functions and forward all ranges of non-duplicative
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multicast traffic allowed by the present forwarding policy.
B.3. Neighborhood Discovery Requirements
To support functions required by the core E_CDS relay set algorithm
the following TLV is required to be transmitted by each node within a
NHDP HELLO message:
*Router Priority*: type=SMF_ROUTER_PRIORITY, length=1, value =
priority*
For E-CDS operation, some value of SMF_ROUTER_PRIORITY must be given
or assumed for each address in the <address-block> portion of the
SMF_HELLO message. If a SMF_HELLO message originator does not
provide a SMF_ROUTER_PRIORITY value for given address(es), a default
value SMF_RPRI_DEFAULT=(TBD) should be assumed. Local determination
of a node SMF_ROUTER_PRIORITY value can be done in multiple ways as
described in the [E-CDS]. An early implementation of SMF and E-CDS
has used node degree computed during neighbor discovery, yet it is
still unclear if this is the best method. Unlike the MPR method, the
E-CDS is a self-electing algorithm. SMF_ROUTER_PRIORITY needs to be
shared with all immediate neighbor nodes and 2-hop neighbor knowledge
is needed during the self election process. Further algorithm
examples and details are covered in the Appendices.
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Appendix C. Multipoint Relay Connected Dominating Set (MPR-CDS)
Algorithm
The MPR-CDS algorithm is an extension to the basic MPR election
algorithm and results in a shared relay set that forms a CDS. Its
forwarding rules within SMF are non-dependent upon previous hop
information similar to E-CDS.
C.1. MPR-CDS Relay Set Selection
An overview of the MPR-CDS selection algorithm is provided in
[MPR-CDS]. The basic requirements for election are similar to the
basic MPR algorithm with the addition that some node ordering
knowledge is required. This is similar to the E-CDS requirement and
can be based upon node IP address or some other unique router
identifier. The rules for election are as follows:
A node decides it is in the relay set if:
1. the node is smaller than all its neighbors (Rule 1)
2. or the node is an MPR of its smallest neighbor (Rule 2)
C.2. MPR-CDS Forwarding Rules
MPR-CDS forwarding are quite simple and straightforward. As with
E-CDS, there is no need to check previous hop information during
forwarding. Upon electing itself as a MPR-CDS relay set forwarder,
SMF nodes perform DPD functions and forward all ranges of multicast
traffic allowed.
C.3. Neighborhood Discovery Requirements
No additional discovery requirements are needed beyond the basic MPR-
related TLVs already discussed.
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Appendix D. Pseudo Code for Relay Set Selection Algorithms
D.1. Definitions
node : A MANET router which is implementing SMF Routing protocol.
n_0 : The node performing the SMF algorithm computation.
N_1 : A set of 1-hop neighbors of n_0. Initially set to all 1-hop
neighbors.
N_2 : A set of 2-hop neighbors reachable by n_0, excluding n_0 and
all nodes in N_1. Initially set to all 2-hop neighbors excluding
n_0 and all nodes in N_1.
N_2(y) : The subset of N_2 nodes which are 1-hop neighbors of node
y, where node y is in N_1.
N_1(z) : The subset of N_1 nodes which are 1-hop neighbors of node
z, where z is in N_2.
RtrPri : an expression of router priority. For example, use |N_1|
as n's router's priority then break ties with n_0's address.
rp_max_1 : The node with the largest RtrPri of N_1.
MPRs : The subset of N_1 which have been selected by n_0 to
forward packets from n_0.
MPR-Selectors : The subset of N_1 for whom n_0 has been selected
to forward packets.
D.2. S-MPR Selection Algorithm
1. Calculate N_1(z) for all nodes z in N_2.
2. Calculate N_2(y) for all nodes y in N_1.
3. For each z in N_2 where |N_1(z)| is equal to 1, select the node
in N_1(z) as an MPR by using Appendix D.2.1.
4. While N_2 is not empty select the node y, with the largest
|N_2(y)|, as MPR by using Appendix D.2.1.
5. Restore N_1 and N_2.
6. Node n_0 shares its MPRs with N_1.
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7. Each node in n_0's MPRs set add n_0 to their MPR-Selectors set.
8. Nodes forward all unique multicast packets which are first
received from a node in their MPR-Selectors set.
D.2.1. Procedure to Select a Node as an MPR
1. Add n to the MPRs set.
2. Remove node n from N_1.
3. For each y in N_2(n), remove y from N_2.
4. Calculate N_1(z) for all nodes z in N_2
5. Calculate N_2(y) for all nodes y in N_1.
D.3. NS-MPR Selection Algorithm
1. Perform steps 1-7 of Appendix D.2.
2. If |MPR-Selectors| > 0, then n_0 selects itself as a forwarder
for all nodes.
D.4. MPR-CDS Selection Algorithm
1. Perform steps 1-7 of Appendix D.2.
2. If n_0 RtrPri value is greater than rp_max_1's RtrPri value, and
|MPR-Selectors| > 0, then n_0 selects itself as a forwarder for
all nodes.
3. If rp_max_1 is in the MPR-Selectors set, then n_0 selects itself
as a forwarder for all nodes.
D.5. 1-Hop E-CDS Selection Algorithm
1. If n_0 has a larger value of RtrPri than pr_max_1, then n_0
selects itself as a forwarder for all nodes.
2. If there does not exist a path from pr_max_1 to every other node
in N_1 using only N_1 nodes that have RtrPri larger than n_0's,
then n_0 selects itself as a forwarder for all nodes.
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D.6. 2-Hop E-CDS Selection Algorithm
1. If n_0 has a larger value of RtrPri than all nodes in N_1 and
N_2, then n_0 selects itself as a forwarder for all nodes.
2. If there does not exist a path from r_max_1 to every other node
in N_1 and N_2 using only N_1 and N_2 nodes that have RtrPri
larger than n_0's, then n_0 selects itself as a forwarder for all
nodes.
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Authors' Addresses
Joseph Macker
NRL
Washington, DC 20375
USA
Email: macker@itd.nrl.navy.mil
SMF Design Team
IETF MANET WG
Email: manet@ietf.org
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