RFC 6621
Simplified Multicast Forwarding
Appendix A. Essential Connecting Dominating Set (E-CDS) Algorithm
The "Essential Connected Dominating Set" (E-CDS) algorithm [RFC5614] forms a single CDS mesh for the SMF operating region. It allows routers to use 2-hop neighborhood topology information to dynamically perform relay self-election to form a CDS. Its packet-forwarding rules are not dependent upon previous hop knowledge. Additionally, E-CDS SMF forwarders can be easily mixed without problems with CF SMF forwarders, even those not participating in NHDP. Another benefit is that packets opportunistically received from non-symmetric neighbors may be forwarded without compromising flooding efficiency or correctness. Furthermore, multicast sources not participating in NHDP may freely inject their traffic, and any neighboring E-CDS relays will properly forward the traffic. The E-CDS-based relay set selection algorithm is based upon [RFC5614]. E-CDS was originally discussed in the context of forming partial adjacencies and efficient flooding for MANET OSPF extensions work, and the core algorithm is applied here for SMF. It is RECOMMENDED that the SMF_TYPE:E-CDS Message TLV be included in NHDP_HELLO messages that are generated by routers conducting E-CDS SMF operation. It is also RECOMMENDED that the SMF_NBR_TYPE:E-CDS Address Block TLV be used to advertise neighbor routers that are also conducting E-CDS SMF operation.A.1. E-CDS Relay Set Selection Overview
The E-CDS relay set selection requires 2-hop neighborhood information collected through NHDP or another process. Relay routers, in E-CDS SMF selection, are "self-elected" using a Router Identifier (Router ID) and an optional nodal metric, referred to here as Router Priority for all 1-hop and 2-hop neighbors. To ensure proper relay set self- election, the Router ID and Router Priority MUST be consistent among participating routers. It is RECOMMENDED that NHDP be used to share Router ID and Router Priority through the use of SMF_TYPE:E-CDS TLVs as described in this appendix. The Router ID is a logical identification that MUST be consistent across interoperating SMF neighborhoods, and it is RECOMMENDED to be chosen as the numerically largest address contained in a router's "Neighbor Address List" as defined in NHDP. The E-CDS self-election process can be summarized as follows: 1. If an SMF router has a higher ordinal (Router Priority, Router ID) than all of its symmetric neighbors, it elects itself to act as a forwarder for all received multicast packets.
2. Else, if there does not exist a path from the neighbor with
largest (Router Priority, Router ID) to any 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 provide for redundant relay set selection
(e.g., bi-connected), but such capability is supported by the basic
E-CDS design.
A.2. E-CDS Forwarding Rules
With E-CDS, any SMF router that has selected itself as a relay
performs DPD and forwards all non-duplicative multicast traffic
allowed by the present forwarding policy. Packet previous-hop
knowledge is not needed for forwarding decisions when using E-CDS.
1. Upon packet reception, DPD is performed. Note E-CDS requires a
single duplicate table for the set of interfaces associated with
the relay set selection.
2. If the packet is a duplicate, no further action is taken.
3. If the packet is non-duplicative:
A. A DPD entry is made for the packet identifier.
B. The packet is forwarded out to all interfaces associated with
the relay set selection.
As previously mentioned, even packets sourced (or relayed) by routers
not participating in NHDP and/or the E-CDS relay set selection may be
forwarded by E-CDS forwarders without problem. A particular
deployment MAY choose to not forward packets from previous hop
routers that have been not explicitly identified via NHDP or other
means as operating as part of a different relay set algorithm (e.g.,
S-MPR) to allow coexistent deployments to operate correctly. Also,
E-CDS relay set selection may be configured to be influenced by
statically configured CF relays that are identified via NHDP or other
means.
A.3. E-CDS Neighborhood Discovery Requirements
It is possible to perform E-CDS relay set selection without
modification of NHDP, basing the self-election process exclusively on
the "Neighbor Address List" of participating SMF routers, for
example, by setting the Router Priority to a default value and
selecting the Router ID as the numerically largest address contained
in the "Neighbor Address List". However, steps MUST be taken to ensure that all NHDP-enabled routers not using SMF_TYPE:E-CDS full type Message TLVs are, in fact, running SMF E-CDS with the same methods for selecting Router Priority and Router ID; otherwise, incorrect forwarding may occur. Note that SMF routers with higher Router Priority values will be favored as relays over routers with lower Router Priority. Thus, preferred relays MAY be administratively configured to be selected when possible. Additionally, other metrics (e.g., nodal degree, energy capacity, etc.) may also be taken into account in constructing a Router Priority value. When using Router Priority with multiple interfaces, all interfaces on a router MUST use and advertise a common Router Priority value. A router's Router Priority value may be administratively or algorithmically selected. The method of selection does not need to be the same among different routers. E-CDS relay set selection may be configured to be influenced by statically configured CF relays that are identified via NHDP or other means. Nodes advertising CF through NHDP may be considered E-CDS SMF routers with maximal Router Priority. To share a router's Router Priority with its 1-hop neighbors, the SMF_TYPE:E-CDS Message TLV's <value> field is defined as shown in Table 14. +----------------+---------+-----------------+ | Length (bytes) | Value | Router Priority | +----------------+---------+-----------------+ | 0 | N/A | 64 | | 1 | <value> | 0-127 | +----------------+---------+-----------------+ Table 14: E-CDS Message TLV Values Where <value> is a one-octet-long bit field that is defined as: bit 0: the leftmost bit is reserved and SHOULD be set to 0. bits 1-7: contain the unsigned Router Priority value, 0-127, which is associated with the "Neighbor Address List". Combinations of value field lengths and values other than specified here are NOT permitted and SHOULD be ignored. Figure 6 shows an example SMF_TYPE:E-CDS Message TLV.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | SMF_TYPE |1|0|0|1|0|0| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | E-CDS |0|0|0|0|0|0|0|1|R| priority | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: E-CDS Message TLV Example To convey Router Priority values among 2-hop neighborhoods, the SMF_NBR_TYPE:E-CDS Address Block TLV's <value> field is used. Multi- index and multivalue TLV layouts as defined in [RFC5444] are supported. SMF_NBR_TYPE:E-CDS value fields are defined thus: +---------------+--------+----------+-------------------------------+ | Length(bytes) | # Addr | Value | Router Priority | +---------------+--------+----------+-------------------------------+ | 0 | Any | N/A | 64 | | 1 | Any | <value> | <value> is for all addresses | | N | N | <value>* | Each address gets its own | | | | | <value> | +---------------+--------+----------+-------------------------------+ Table 15: E-CDS Address Block TLV Values Where <value> is a one-byte bit field that is defined as: bit 0: the leftmost bit is reserved and SHOULD be set to 0. bits 1-7: contain the unsigned Router Priority value, 0-127, which is associated with the appropriate address(es). Combinations of value field lengths and # of addresses other than specified here are NOT permitted and SHOULD be ignored. A default technique of using nodal degree (i.e., count of 1-hop neighbors) is RECOMMENDED for the value field of these TLV types. Below are two example SMF_NBR_TYPE:E-CDS Address Block TLVs. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | SMF_NBR_TYPE |1|0|0|1|0|0| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | E-CDS |0|0|0|0|0|0|0|1|R| priority | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: E-CDS Address Block TLV Example 1
The single value example TLV, depicted in Figure 7, specifies that all address(es) contained in the address block are running SMF using the E-CDS algorithm and all address(es) share the value field and therefore the same Router Priority. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | SMF_NBR_TYPE |1|0|1|1|0|1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | E-CDS | index-start | index-end | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R| priority0 |R| priority1 | ... |R| priorityN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: E-CDS Address Block TLV Example 2 The example multivalued TLV, depicted in Figure 8, specifies that address(es) contained in the address block from index-start to index- end inclusive are running SMF using the E-CDS algorithm. Each address is associated with its own value byte and therefore its own Router Priority.A.4. E-CDS Selection Algorithm
This section describes an algorithm for E-CDS relay selection (self- election). The algorithm described uses 2-hop information. Note that it is possible to extend this algorithm to use k-hop information with added computational complexity and mechanisms for sharing k-hop topology information that are not described in this document or within the NHDP specification. It should also be noted that this algorithm does not impose the hop limit bound described in [RFC5614] when performing the path search that is used for relay selection. However, the algorithm below could be easily augmented to accommodate this additional criterion. It is not expected that the hop limit bound will provide significant benefit to the algorithm defined in this appendix. The tuple of Router Priority and Router ID is used in E-CDS relay set selection. Precedence is given to the Router Priority portion, and the Router ID value is used as a tiebreaker. The evaluation of this tuple is referred to as "RtrPri(n)" in the description below where "n" references a specific router. Note that it is possible that the Router Priority portion may be optional and the evaluation of "RtrPri()" be solely based upon the unique Router ID. Since there MUST NOT be any duplicate Router ID values among SMF routers, a comparison of "RtrPri(n)" between any two routers will always be an inequality. The use of nodal degree for calculating Router Priority is RECOMMENDED as default, and the largest IP address in the
"Neighbor Address List" as advertised by NHDP MUST be used as the
Router ID. NHDP provides all interface addresses throughout the
2-hop neighborhood through HELLO messages, so explicitly conveying a
Router ID is not necessary. The following steps describe a basic
algorithm for conducting E-CDS relay selection for a router "n0":
1. Initialize the set "N1" with tuples ("Router Priority", "Router
ID", "Neighbor Address List") for each 1-hop neighbor of "n0".
2. If "N1" has less than 2 tuples, then "n0" does not elect itself
as a relay, and no further steps are taken.
3. Initialize the set "N2" with tuples ("Router Priority", "Router
ID", "2-hop address") for each "2-hop address" of "n0", where
"2-hop address" is defined in NHDP.
4. If "RtrPri(n0)" is greater than that of all tuples in the union
of "N1" and "N2", then "n0" selects itself as a relay, and no
further steps are taken.
5. Initialize all tuples in the union of "N1" and "N2" as
"unvisited".
6. Find the tuple "n1_Max" that has the largest "RtrPri()" of all
tuples in "N1".
7. Initialize queue "Q" to contain "n1_Max", marking "n1_Max" as
"visited".
8. While router queue "Q" is not empty, remove router "x" from the
head of "Q", and for each 1-hop neighbor "n" of router "x"
(excluding "n0") that is not marked "visited".
A. Mark router "n" as "visited".
B. If "RtrPri(n)" is greater than "RtrPri(n0)", append "n" to
"Q".
9. If any tuple in "N1" remains "unvisited", then "n0" selects
itself as a relay. Otherwise, "n0" does not act as a relay.
Note these steps are re-evaluated upon neighborhood status changes.
Steps 5 through 8 of this procedure describe an approach to a path
search. The purpose of this path search is to determine if paths
exist from the 1-hop neighbor with maximum "RtrPri()" to all other
1-hop neighbors without traversing an intermediate router with a
"RtrPri()" value less than "RtrPri(n0)". These steps comprise a
breadth-first traversal that evaluates only paths that meet that
criteria. If all 1-hop neighbors of "n0" are "visited" during this traversal, then the path search has succeeded, and router "n0" does not need to provide relay. It can be assumed that other routers will provide relay operation to ensure SMF connectivity. It is possible to extend this algorithm to consider neighboring SMF routers that are known to be statically configured for CF (always relaying). The modification to the above algorithm is to process such routers as having a maximum possible Router Priority value. It is expected that routers configured for CF and participating in NHDP would indicate this with use of the SMF_TYPE:CF and SMF_NBR_TYPE:CF TLV types in their NHDP_HELLO message and address blocks, respectively.Appendix B. Source-Based Multipoint Relay (S-MPR) Algorithm
The source-based multipoint relay (S-MPR) set selection algorithm enables individual routers, using 2-hop topology information, to select relays from their set of neighboring routers. Relays are selected so that forwarding to the router's complete 2-hop neighbor set is covered. This distributed relay set selection technique has been shown to approximate a minimal connected dominating set (MCDS) in [JLMV02]. Individual routers must collect 2-hop neighborhood information from neighbors, determine an appropriate current relay set, and inform selected neighbors of their relay status. Note that since each router picks its neighboring relays independently, S-MPR forwarders depend upon previous hop information (e.g., source MAC address) to operate correctly. 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 it has been demonstrated operationally in dynamic network environments. It is RECOMMENDED that the SMF_TYPE:S-MPR Message TLV be included in NHDP_HELLO messages that are generated by routers conducting S-MPR SMF operation. It is also RECOMMENDED that the SMF_NBR_TYPE:S-MPR Address Block TLV be used to specify which neighbor routers are conducting S-MPR SMF operation.B.1. S-MPR Relay Set Selection Overview
The S-MPR algorithm uses bi-directional 1-hop and 2-hop neighborhood information collected via NHDP to select, from a router's 1-hop neighbors, a set of relays that will cover the router's entire 2-hop neighbor set upon forwarding. The algorithm described uses a "greedy" heuristic of first picking the 1-hop neighbor who will cover the most 2-hop neighbors. Then, excluding those 2-hop neighbors that have been covered, additional relays from its 1-hop neighbor set are
iteratively selected until the entire 2-hop neighborhood is covered. Note that 1-hop neighbors also identified as 2-hop neighbors are considered as 1-hop neighbors only. NHDP HELLO messages supporting S-MPR forwarding operation SHOULD use the TLVs defined in Section 8.1 using the S-MPR extended type. The value field of an Address Block TLV that has a full type value of SMF_NBR_TYPE:S-MPR is defined in Table 17 such that signaling of MPR selections to 1-hop neighbors is possible. The value field of a message block TLV that has a full type value of SMF_TYPE:S-MPR is defined in Table 16 such that signaling of Router Priority (described as "WILLINGNESS" in [RFC3626]) to 1-hop neighbors is possible. It is important to note that S-MPR forwarding is dependent upon the previous hop of an incoming packet. An S-MPR router MUST forward packets only for neighbors that have explicitly selected it as a multipoint relay (i.e., its "selectors"). There are also some additional requirements for duplicate packet detection to support S-MPR SMF operation that are described below. For multiple interface operation, MPR selection SHOULD be conducted on a per-interface basis. However, it is possible to economize MPR selection among multiple interfaces by selecting common MPRs to the extent possible.B.2. S-MPR Forwarding Rules
An S-MPR SMF router MUST only forward packets for neighbors that have explicitly selected it as an MPR. The source-based forwarding technique also stipulates some additional duplicate packet detection operations. For multiple network interfaces, independent DPD state MUST be maintained for each separate interface. The following provides the procedure for S-MPR packet forwarding given the arrival of a packet on a given interface, denoted <srcIface>. There are three possible actions, depending upon the previous-hop transmitter: 1. If the previous-hop transmitter has selected the current router as an MPR, A. The packet identifier is checked against the DPD state for each possible outbound interface, including the <srcIface>. B. If the packet is not a duplicate for an outbound interface, the packet is forwarded on that interface and a DPD entry is made for the given packet identifier for the interface. C. If the packet is a duplicate, no action is taken for that interface.
2. Else, if the previous-hop transmitter is a 1-hop symmetric
neighbor, a DPD entry is added for that packet for the
<srcIface>, but the packet is not forwarded.
3. Otherwise, no action is taken.
Action number two in the list above is non-intuitive but important to
ensure correctness of S-MPR SMF operation. The selection of source-
based relays does not result in a common set among neighboring
routers, so relays MUST mark, in their DPD state, packets received
from non-selector, symmetric, 1-hop neighbors (for a given interface)
and not forward subsequent duplicates of that packet if received on
that interface. Deviation here can result in unnecessary, repeated
packet forwarding throughout the network or incomplete flooding.
Nodes not participating in neighborhood discovery and relay set
selection will not be able to source multicast packets into the area
and have SMF forward them, unlike E-CDS or MPR-CDS where forwarding
may occur dependent on topology. Correct S-MPR relay behavior will
occur with the introduction of repeaters (non-NHDP/SMF participants
that relay multicast packets using duplicate detection and CF), but
the repeaters will not efficiently contribute to S-MPR forwarding as
these routers will not be identified as neighbors (symmetric or
otherwise) in the S-MPR forwarding process. NHDP/SMF participants
MUST NOT forward packets that are not selected by the algorithm, as
this can disrupt network-wide S-MPR flooding, resulting in incomplete
or inefficient flooding. The result is that non-S-MPR SMF routers
will be unable to source multicast packets and have them forwarded by
other S-MPR SMF routers.
B.3. S-MPR Neighborhood Discovery Requirements
Nodes may optionally signal a Router Priority value to their 1-hop
neighbors by using the SMF_TYPE:S-MPR message block TLV value field.
If the value field is omitted, a default Router Priority value of 64
is to be assumed. This is summarized here:
+---------------+---------+-----------------+
| Length(bytes) | Value | Router Priority |
+---------------+---------+-----------------+
| 0 | N/A | 64 |
| 1 | <value> | 0-127 |
+---------------+---------+-----------------+
Table 16: S-MPR Message TLV Values
Where <value> is a one-octet-long bit field defined as: bit 0: the leftmost bit is reserved and SHOULD be set to 0. bits 1-7: contain the Router Priority value, 0-127, which is associated with the "Neighbor Address List". Router Priority values for S-MPR are interpreted in the same fashion as "WILLINGNESS" ([RFC3626]), with the value 0 indicating a router will NEVER forward and value 127 indicating a router will ALWAYS forward. Values 1-126 indicate how likely a S-MPR SMF router will be selected as an MPR by a neighboring SMF router, with higher values increasing the likelihood. Combinations of value field lengths and values other than those specified here are NOT permitted and SHOULD be ignored. Below is an example SMF_TYPE:S-MPR Message TLV. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | SMF_TYPE |1|0|0|1|0|0| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S-MPR |0|0|0|0|0|0|0|1|R| priority | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: S-MPR Message TLV Example S-MPR election operation requires 2-hop neighbor knowledge as provided by NHDP [RFC6130] or from external sources. MPRs are dynamically selected by each router, and selections MUST be advertised and dynamically updated within NHDP or an equivalent protocol or mechanism. For NHDP use, the SMF_NBR_TYPE:S-MPR Address Block TLV value field is defined as such: +---------------+--------+----------+-------------------------------+ | Length(bytes) | # Addr | Value | Meaning | +---------------+--------+----------+-------------------------------+ | 0 | Any | N/A | NOT MPRs | | 1 | Any | <value> | <value> is for all addresses | | N | N | <value>* | Each address gets its own | | | | | <value> | +---------------+--------+----------+-------------------------------+ Table 17: S-MPR Address Block TLV Values
Where <value>, if present, is a one-octet bit field defined as: bit 0: The leftmost bit is the M bit that, when set, indicates MPR selection of the relevant interface, represented by the associated address(es), by the originator router of the NHDP HELLO message. When unset, it indicates the originator router of the NHDP HELLO message has not selected the relevant interfaces, represented by the associated address(es), as its MPR. bits 1-7: These bits are reserved and SHOULD be set to 0. Combinations of value field lengths and number of addresses other than specified here are NOT permitted and SHOULD be ignored. All bits, excepting the leftmost bit, are RESERVED and SHOULD be set to 0. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | SMF_NBR_TYPE |1|1|0|1|0|0| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S-MPR | start-index |0|0|0|0|0|0|0|1|M| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: S-MPR Address Block TLV Example The single index TLV example, depicted in Figure 10, indicates that the address specified by the <start-index> field is running SMF using S-MPR and has been selected by the originator of the NHDP HELLO message as an MPR forwarder if the M bit is set. Multivalued TLVs may also be used to specify MPR selection status of multiple addresses using only one TLV. See Figure 8 for a similar example on how this may be done.B.4. S-MPR Selection Algorithm
This section describes a basic algorithm for the S-MPR selection process. Note that the selection is with respect to a specific interface of the router performing selection, and other router interfaces referenced are reachable from this reference router interface. This is consistent with the S-MPR forwarding rules described above. When multiple interfaces per router are used, it is possible to enhance the overall selection process across multiple interfaces such that common routers are selected as MPRs for each interface to avoid unnecessary inefficiencies in flooding. The following steps describe a basic algorithm for conducting S-MPR selection for a router interface "n0":
1. Initialize the set "MPR" to empty.
2. Initialize the set "N1" to include all 1-hop neighbors of "n0".
3. Initialize the set "N2" to include all 2-hop neighbors, excluding
"n0" and any routers in "N1". Nodes that are only reachable via
"N1" routers with router priority values of NEVER are also
excluded.
4. For each interface "y" in "N1", initialize a set "N2(y)" to
include any interfaces in "N2" that are 1-hop neighbors of "y".
5. For each interface "x" in "N1" with a router priority value of
"ALWAYS" (or using the CF relay algorithm), select "x" as an MPR:
A. Add "x" to the set "MPR" and remove "x" from "N1".
B. For each interface "z" in "N2(x)", remove "z" from "N2".
C. For each interface "y" in "N1", remove any interfaces in
"N2(x)" from "N2(y)".
6. For each interface "z" in "N2", initialize the set "N1(z)" to
include any interfaces in "N1" that are 1-hop neighbors of "z".
7. For each interface "x" in "N2" where "N1(x)" has only one member,
select "x" as an MPR:
A. Add "x" to the set "MPR" and remove "x" from "N1".
B. For each interface "z" in "N2(x)", remove "z" from "N2" and
delete "N1(z)".
C. For each interface "y" in "N1", remove any interfaces in
"N2(x)" from "N2(y)".
8. While "N2" is not empty, select the interface "x" in "N1" with
the largest router priority that has the number of members in
"N_2(x)" as an MPR:
A. Add "x" to the set "MPR" and remove "x" from "N1".
B. For each interface "z" in "N2(x)", remove "z" from "N2".
C. For each interface "y" in "N1", remove any interfaces in
"N2(x)" from "N2(y)".
After the set of routers "MPR" is selected, router "n_0" must signal its selections to its neighbors. With NHDP, this is done by using the MPR Address Block TLV to mark selected neighbor addresses in NHDP_HELLO messages. Neighbors MUST record their MPR selection status and the previous hop address (e.g., link or MAC layer) of the selector. Note these steps are re-evaluated upon neighborhood status changes.Appendix C. Multipoint Relay Connected Dominating Set (MPR-CDS) Algorithm
The MPR-CDS algorithm is an extension to the basic S-MPR election algorithm that results in a shared (non-source-specific) SMF CDS. Thus, its forwarding rules are not dependent upon previous hop information, similar to E-CDS. An overview of the MPR-CDS selection algorithm is provided in [MPR-CDS]. It is RECOMMENDED that the SMF_TYPE Message TLV be included in NHDP_HELLO messages that are generated by routers conducting MPR-CDS SMF operation.C.1. MPR-CDS Relay Set Selection Overview
The MPR-CDS relay set selection process is based upon the MPR selection process of the S-MPR algorithm with the added refinement of a distributed technique for subsequently down-selecting to a common reduced, shared relay set. A router ordering (or "prioritization") metric is used as part of this down-selection process; like the E-CDS algorithm, this metric can be based upon router address(es) or some other unique router identifier (e.g., Router ID based on largest address contained within the "Neighbor Address List") as well as an additional Router Priority measure, if desired. The process for MPR- CDS relay selection is as follows: 1. First, MPR selection per the S-MPR algorithm is conducted, with selectors informing their MPRs (via NHDP) of their selection. 2. Then, the following rules are used on a distributed basis by selected routers to possibly deselect themselves and thus jointly establish a common set of shared SMF relays: A. If a selected router has a larger "RtrPri()" than all of its 1-hop symmetric neighbors, then it acts as a relay for all multicast traffic, regardless of the previous hop.
B. Else, if the 1-hop symmetric neighbor with the largest
"RtrPri()" value has selected the router, then it also acts
as a relay for all multicast traffic, regardless of the
previous hop.
C. Otherwise, it deselects itself as a relay and does not
forward any traffic unless changes occur that require re-
evaluation of the above steps.
This technique shares many of the desirable properties of the E-CDS
technique with regards to compatibility with multicast sources not
participating in NHDP and the opportunity for statically configured
CF routers to be present, regardless of their participation in NHDP.
C.2. MPR-CDS Forwarding Rules
The forwarding rules for MPR-CDS are similar to those for E-CDS. Any
SMF router that has selected itself as a relay performs DPD and
forwards all non-duplicative multicast traffic allowed by the present
forwarding policy. Packet previous hop knowledge is not needed for
forwarding decisions when using MPR-CDS.
1. Upon packet reception, DPD is performed. Note that MPR-CDS
requires one duplicate table for the set of interfaces associated
with the relay set selection.
2. If the packet is a duplicate, no further action is taken.
3. If the packet is non-duplicative:
A. A DPD entry is added for the packet identifier
B. The packet is forwarded out to all interfaces associated with
the relay set selection.
As previously mentioned, even packets sourced (or relayed) by routers
not participating in NHDP and/or the MPR-CDS relay set selection may
be forwarded by MPR-CDS forwarders without problem. A particular
deployment MAY choose to not forward packets from sources or relays
that have been explicitly identified via NHDP or other means as
operating as part of a different relay set algorithm (e.g., S-MPR) to
allow coexistent deployments to operate correctly.
C.3. MPR-CDS Neighborhood Discovery Requirements
The neighborhood discovery requirements for MPR-CDS have commonality
with both the S-MPR and E-CDS algorithms. MPR-CDS selection
operation requires 2-hop neighbor knowledge as provided by NHDP
[RFC6130] or from external sources. Unlike S-MPR operation, there is no need for associating link-layer address information with 1-hop neighbors since MPR-CDS forwarding is independent of the previous hop similar to E-CDS forwarding. To advertise an optional Router Priority value or "WILLINGNESS", an originating router may use the Message TLV of type SMF_TYPE:MPR-CDS, which shares a common <value> format with both SMF_TYPE:E-CDS (Table 14) and SMF_TYPE:S-MPR (Table 16). MPR-CDS only requires 1-hop knowledge of Router Priority for correct operation. In the S-MPR phase of MPR-CDS selection, MPRs are dynamically determined by each router, and selections MUST be advertised and dynamically updated using NHDP or an equivalent protocol or mechanism. The <value> field of the SMF_NBR_TYPE:MPR-CDS type TLV shares a common format with SMF_NBR_TYPE:S-MPR (Table 17) to convey MPR selection.C.4. MPR-CDS Selection Algorithm
This section describes an algorithm for the MPR-CDS selection process. Note that the selection described is with respect to a specific interface of the router performing selection, and other router interfaces referenced are reachable from this reference router interface. An ordered tuple of Router Priority and Router ID is used in MPR-CDS relay set selection. The Router ID value should be set to the largest advertised address of a given router; this information is provided to one-hop neighbors via NHDP by default. Precedence is given to the Router Priority portion, and the Router ID value is used as a tiebreaker. The evaluation of this tuple is referred to as "RtrPri(n)" in the description below where "n" references a specific router. Note that it is possible that the Router Priority portion may be optional and the evaluation of "RtrPri()" be solely based upon the unique Router ID. Since there MUST NOT be any duplicate address values among SMF routers, a comparison of "RtrPri(n)" between any two routers will always be an inequality. The following steps, repeated upon any changes detected within the 1-hop and 2-hop neighborhood, describe a basic algorithm for conducting MPR-CDS selection for a router interface "n0": 1. Perform steps 1-8 of Appendix B.4 to select MPRs from the set of 1-hop neighbors of "n0" and notify/update neighbors of selections. 2. Upon being selected as an MPR (or any change in the set of routers selecting "n0" as an MPR):
A. If no neighbors have selected "n0" as an MPR, "n0" does not
act as a relay, and no further steps are taken until a change
in neighborhood topology or selection status occurs.
B. Determine the router "n1_max" that has the maximum "RtrPri()"
of all 1-hop neighbors.
C. If "RtrPri(n0)" is greater than "RtrPri(n1_max)", then "n0"
selects itself as a relay for all multicast packets.
D. Else, if "n1_max" has selected "n0" as an MPR, then "0"
selects itself as a relay for all multicast packets.
E. Otherwise, "n0" does not act as a relay.
It is possible to extend this algorithm to consider neighboring SMF
routers that are known to be statically configured for CF (always
relaying). The modification to the above algorithm is to process
such routers as having a maximum possible Router Priority value.
This is the same as the case for participating routers that have been
configured with a S-MPR "WILLINGNESS" value of "WILL_ALWAYS". It is
expected that routers configured for CF and participating in NHDP
would indicate their status with use of the SMF_TYPE TLV type in
their NHDP_HELLO message TLV block. It is important to note,
however, that CF routers will not select MPR routers and therefore
cannot guarantee connectedness.
Author's Address
Joseph Macker (editor)
NRL
Washington, DC 20375
USA
EMail: macker@itd.nrl.navy.mil