BIER WG Z. Zhang
Internet-Draft C. Wang
Intended status: Standards Track ZTE Corporation
Expires: June 23, 2016 December 21, 2015
Automatic Assignment of BIER BFR-ids in OSPF
draft-zhang-bier-bfrid-assignment-00
Abstract
[I-D.ietf-bier-architecture] has introduced a new method to forward
multicast flow, without explicit multicast states storing in every
node along the multicast paths. This document introduces a method to
allocate BFR-id automatically through OSPF extension.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 23, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. IANA considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. D-BFR Election Algorithm . . . . . . . . . . . . . . . . . 6
4.2. D-BFR Procedures . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Assignment of BMPs to BFRs . . . . . . . . . . . . . . 8
4.3. BD-BFR Procedures . . . . . . . . . . . . . . . . . . . . 8
4.4. BFER Procedures . . . . . . . . . . . . . . . . . . . . . 8
5. Special Considerations . . . . . . . . . . . . . . . . . . . . 9
5.1. BD-BFR to D-BFR Transition . . . . . . . . . . . . . . . . 9
5.2. Election FSM for BFR . . . . . . . . . . . . . . . . . . . 9
5.2.1. States . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.2. Events . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.3. Rules . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2.4. Warning and Logging . . . . . . . . . . . . . . . . . 12
6. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. BIER-PE BIER Protocol Election Sub-sub-TLV . . . . . . . . 13
6.2. Reuse of the Reserved Bits in BIER Info sub-TLV . . . . . 13
6.3. BIER-PE-BMP: BIER PE BMP Assignments TLV . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Normative References . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
[I-D.ietf-bier-architecture] defines a new efficient forwarding way
for multicast flow. The crucial object in this method is BFR-id for
every BFERs. All nodes in the BIER domain learn the BFR-ids of
BFERs, and forward the packet according to the BIER header that are
composed by the BFR-ids.
Although the BFR-id can be acquired by central controllers or
statically, it will be more convenient if there is a way to allocate
the BFR-id automatically.
This document introduces a new method to allocate BFR-id for BFERs in
BIER domain. And this document also defines the format of OSPF
extension for BFR-id auto assignment.
This document gets benefit from the DR election algorithm that is
defined in RFC2328. And the main idea of this document is the same
as that is defined in "draft-prz-bier-bfrid-assignment". The only
difference between the two documents is the protocol format.
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2. Terminology
Some of the terminology specified in [I-D.ietf-bier-architecture] is
replicated here and extended by necessary definitions:
BIER: Bit Index Explicit Replication (The overall architecture of
forwarding multicast using a Bit Position).
BIER-OL: BIER Overlay Signaling. (The method for the BFIR to learn
about BFER's).
BFR: Bit Forwarding Router (A router that participates in Bit Index
Multipoint Forwarding). A BFR is identified by a unique BFR-prefix
in a BIER domain.
BFIR: Bit Forwarding Ingress Router (The ingress border router that
inserts the BM into the packet).
BFER: Bit Forwarding Egress Router. A router that participates in
Bit Index Forwarding as leaf. Each BFER must be a BFR. Each BFER
must have a valid BFR-id assigned.
BFT: Bit Forwarding Tree used to reach all BFERs in a domain.
BIFT: Bit Index Forwarding Table.
BMS: Bit Mask Set. Set containing bit positions of all BFER
participating in a set.
BMP: Bit Mask Position, a given bit in a BMS.
Invalid BMP: Unassigned Bit Mask Position, consisting of all 0s.
IGP signaled BIER domain: A BIER underlay where the BIER
synchronization information is carried in IGP. Observe that a multi-
topology is NOT a separate BIER domain in IGP.
BIER sub-domain: A further distinction within a BIER domain
identified by its unique sub-domain identifier. A BIER sub-domain
can support multiple BitString Lengths.
BFR-id: An optional, unique identifier for a BFR within a BIER sub-
domain.
Invalid BFR-id: Unassigned BFR-id, consisting of all 0s.
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3. IANA considerations
This document adds the following new sub-sub-TLVs to the registry of
sub-TLVs for BIER Info sub-TLV.
BIER Protocol Election sub-sub-TLV Value: TBD (suggested - to be
assigned by IANA)
This document adds the following new TLV to the registery of OSPF
TLVs.
BIER PE BMP Assignments TLV Value: TBD (suggested - to be assigned by
IANA)
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4. Procedures
At first, all the BIER nodes collect the information of D-BFR
candidates and BD-BFR candidates through the advertisements of BIER
protocol election sub-sub-TLVs. All the BFRs flood the sub-sub-TLVs
per sub-domain or per BMS to all other nodes.
The D-BFR election algorithm is most like the DR elect function in
OSPF protocol. And the FSM is also like the function in OSPF
protocol. The algorithm described below is most from RFC2328.
OSPF floods the DR/BDR information through OSPF hello packets. BIER
nodes flood the BIER protocol election sub-sub-TLVs along with BIER
information sub-TLV.
ALL the BIER nodes elect the D-BFR and BD-BFR through the Designated
Router BFR function. And the D-BFR assigns BFR-ids according to the
received BIER information sub-TLV which request to allocate a BFR-id.
After D-BFR assigns all the BFR-ids and floods the assignment to all
the BIER nodes, BD-BFR mirrors the assignment as its base assignment.
If there are some collisions existing, the BFRs that are not
allocated BFR-id negotiate the BFR-id assignment procedure with D-BFR
again.
4.1. D-BFR Election Algorithm
The Designated Router BFR election algorithm proceeds as follows:
o Call the router doing the calculation Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router X is examined.
o The state of BFRs that may be D-BFR or BD-BFR should be examined
by SPF computation.
o Router X itself is also considered to be on the list.
o Discard all routers from the list that are ineligible to become
DR-BDR. (Routers having Router Priority of 0 are ineligible to
become D-BFR.) The following steps are then executed, considering
only those routers that remain on the list:
(1) Note the current values for the network's D-BFR and BD-BFR.
This is used later for comparison purposes.
(2) Calculate the new BD-BFR for the network as follows. Only
those routers on the list that have not declared themselves to be
D-BFR are eligible to become BD-BFR. If one or more of these routers
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have declared themselves BD-BFR (i.e., they are currently listing
themselves as BD-BFR, but not as D-BFR, in their sub-sub-TLVs) the
one having highest Router Priority is declared to be BD-BFR. In case
of a tie, the one having the highest Router ID is chosen. If no
routers have declared themselves BD-BFR, choose the router having
highest Router Priority, (again excluding those routers who have
declared themselves D-BFR), and again use the Router ID to break
ties.
(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared themselves
D-BFR (i.e., they are currently listing themselves as D-BFR in their
BIER PE sub-sub-TLV) the one having highest Router Priority is
declared to be D-BFR. In case of a tie, the one having the highest
Router ID is chosen. If no routers have declared themselves D-BFR,
assign the D-BFR to be the same as the newly elected BD-BFR.
(4) If Router X is now newly the D-BFR or newly the BD-BFR, or
is now no longer the D-BFR or no longer the BD-BFR, repeat steps 2
and 3, and then proceed to step 5. For example, if Router X is now
the DR-BDR, when step 2 is repeated X will no longer be eligible for
BD-BFR election. Among other things, this will ensure that no router
will declare itself both BD-BFR and D-BFR.
(5) As a result of these calculations, the router itself may now
be D-BFR or BD-BFR. See Section4.2 and Section4.3 for the additional
duties this would entail.
(6) If the above calculations have caused the identity of either
the D-BFR or BD-BFR to change, all the routers must re-evaluate
whether they have been selected D-BFR or BD-BFR and initiate
according procedures. In case the new D-BFR is not advertising
according bitmask assignment and they are needed, they initiate
according procedures in Section4.2.
The reason behind the election algorithm's complexity is the desire
for an orderly transition from BD-BFR to D-BFR, when the current
D-BFR fails. This orderly transition is ensured through the
introduction of hysteresis: no new BD-BFR can be chosen until the old
Backup accepts its new D-BFR responsibilities.
The above procedure may elect the same router to be both D-BFR and
BD-BFR, although that router will never be the calculating router
(Router X) itself. The elected D-BFR may not be the router having
the highest Router Priority, nor will the BD-BFR necessarily have the
second highest Router Priority. If Router X is not itself eligible
to become D-BFR, it is possible that neither a BD-BFR nor a D-BFR
will be selected in the above procedure. Note also that if Router X
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is the only attached router that is eligible to become D-BFR, it will
select itself as D-BFR and there will be no BD-BFR for the network.
4.2. D-BFR Procedures
Similar to the D-BFR and BD-BFR election procedure, the assignment of
D-BFR is also base on a sub-domian or a BMS.
4.2.1. Assignment of BMPs to BFRs
The procedure is initiated by a BFER announcing R bit in BIER Info
sub-TLV. The D-BFR assigns BMPs to such BFER or announces
collisions.
Observe that BFERs can request (or announce) the R bits even before a
D-BFR has been chosen so the election and assignment are largely
orthogonal sets of procedures.
The BFR-ids in one sub-domain or a BMS should be assigned one by one
as far as possible.
4.3. BD-BFR Procedures
BD-BFR mirrors the BIER PE BMP Assignments TLV from the advertisement
of D-BFR. And BD-BFR uses the existing assignment as the initial
input of probably allocation.
4.4. BFER Procedures
BFER sends the BIER protocol Election sub-sub-TLV at first. If the
BFER wants itself to be a D-BFR or BD-BFR, it should adjust the D-BFR
priority in advance. After BFER receives the BIER protocol Election
sub-sub-TLVs from other BIER nodes, it elects the D-BFR and BD-BFR
according to the function defined in Section4.1.
If the BFER finds that itself is the D-BFR, it should do the
assignment of D-BFR. If the BFER finds that itself is the BD-BFR, it
mirrors the assignment advertisement of D-BFR. If the BFER is
neither D-BFR nor BD-BFR, it should only care the interaction between
itself and D-BFR.
BFER which need be allocated BFR-id sends the request in BIER info
sub-TLV. If one certain BFR-id is pre-configured, BFER sends this
BFR-id to D-BFR along with BIER info sub-TLV. And D-BFR takes the
certain BFR-id into account preferential. If BFER can't receive the
satisfied result from the PE BMP assignments TLV, it should log the
error and negotiate with D-BFR again.
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5. Special Considerations
5.1. BD-BFR to D-BFR Transition
BD-BFR stores the assignments of D-BFR advertisement. And BD-BFR
treats this existing allocation as initial state. When BD-BFR should
take charge of D-BFR and continue allocating BFR-ids, it MUST NOT
change existing allocation, in other words, BD-BFR should allocate
new BFR-ids to the new nodes of the network.
5.2. Election FSM for BFR
This section describes the finite state machine that runs on every
BFR.
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+------+
| ==== | E1 = PE Expired OR
| Init | PI Expired New Admin
| ==== | Pref
+-+----+ +--+
| | |
| Joined SD Lost DR/ +--------++ |
| Rcvd First PE for SD New Admin Pref | ======= <-+
| +-------------------+ Passive |
+-v----+ | | ======= |
| ==== | | +^--------+
| Wait |Timer +--------v-+ Lost |
| ==== +------------> ======== +-----------------+
+------+ | Election |
+---------+ ======== +--------+
| Won BDR +^-------^-+ Won DR |
| | | |
| | |New DR |
+----v+ | |Seen +v---+
| === +---------+ +---------+ == |
| BDR | New BDR | DR |
+--> === | Lost DR +---+ == |
| ++----+ | +^---+
| | E1 | |
+---+ Diff R Flag | |
New DR PE Diff A Flag | |
New Admin Pref +-------v+ |
| BMP +---+
| Assign |
+--------+
Figure 1: D-BFR/BD-BFR election FSM
5.2.1. States
Init: Initial State of the Machine
Wait: State waiting for routers to update their PEs for the sub-
domian on startup
Election: State that runs the election procedures and generates
according events that progress it into another state immediately
Passive: State entered when lost both DR and BDR in election.
Elected D-BFR
Elected BD-BFR
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BMP Assign: State in which the assignment of bits happens upon
requests from BFERs.
5.2.2. Events
Timer: Initial timer waiting for s of other routers before election
is triggered.
Signalling/Rcvd First PE: First PE for has been received or signaling
enabled for the set S on BFR.
Lost DR: Current D-BFR cannot be reached anymore via SPF computation
in standard topology.
Lost: Lost election for D-BFR and BD-BFR.
Won BDR: Won election for BD-BFR.
Won DR: Won election for D-BFR.
New BDR: A new BD-BFR has been elected by the D-BFR.
New DR PE: New BIER-PE Instance from D-BFR.
New Admin Pref: Changed Administrative preference. And it triggers
the election of BD-BFR.
Diff R Flag: R flag has been announced by a BFR which was not present
before. In case of a new R flag, an assignment should be attempted.
In case of R flag being deleted
if the A flag is set, the validity of the copied BFR-id with the
assignment is checked
if the A flag is clear, the value is assumed non-negotiable and
re-assignments may be necessary
Diff A Flag: A flag has been withdrawn or announced.
If A flag was present before and
R flag is clear, the value is assumed non-negotiable and re-
assignments may be necessary.
R flag is set, a new assignment is requested.
If A flag was not present before and
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R flag is clear, the validity of the copied BFR-id with the
assignment is checked
R flag is set, the client MUST be declared faulty and
disregarded.
5.2.3. Rules
When a new BFR originates its BIER protocol election advertisement,
and it is one candidate to be D-BFR or BD-BFR, it should announce
itself to be BD-BFR instead of D-BFR. If the administrative priority
is set to 0, it MUST NOT announce itself to be D-BFR or BD-BFR.
5.2.4. Warning and Logging
Election failure If there is no candidate for D-BFR and BD-BFR after
election timer expired, D-BFR and BD-BFR can't be elected, it should
trigger a warning or an error log.
D-BFR reachability After a D-BFR is elected, if the D-BFR is
unreachable, it should trigger a warning or an error log.
Flag error If the C bit, R bit and A bit are used incorrect, it
should trigger a warning or an error log.
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6. Packet Formats
The BIER information is advertised in a sub-TLV, and this information
is associated with the BFR-prefix, this defination is described in
[I-D.ietf-bier-ospf-bier-extensions] .
A new sub-sub-TLV that is defined for BIER DR election algorithm is
included in the BIER Info sub-TLV of the according sub-domain as
specified by [I-D.ietf-bier-ospf-bier-extensions]. It MUST be
included in the BIER Info sub-TLV only once, otherwise the first
instance is used. As the [I-D.ietf-bier-architecture] said, the
middle nodes that will not be BFER do not need the BFR-id. But in
some situations, one of the middle node will be used to be treated as
D-BFR to allocate BFR-ids, the middle node should also send the sub-
sub-TLV with the BIER info sub-TLV to indicate that it should be
treated as one of the candidate of D-BFR.
6.1. BIER-PE BIER Protocol Election Sub-sub-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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | D-BFR Priority| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| D-BFR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BD-BFR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 BIER Protocol Election sub-sub-TLV
o Type: TBD.
o Length: 1 octet.
o Priority: Priority at which this router is set to become D-BFR for
the sub-domain.
o D-BFR ID: ID of the router chosen as D-BFR. If the router elected
itself as D-BFR it MUST set it to its own ID.
o BD-BFR ID: ID of the router chosen as BD-BFR. If the router
elected itself as BD-BFR it MUST set it to its own ID.
6.2. Reuse of the Reserved Bits in BIER Info sub-TLV
The format listed here may seem more like the format that is defined
in [I-D.ietf-bier-isis-extensions]than that is defined in
[I-D.ietf-bier-ospf-bier-extensions], because the
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[I-D.ietf-bier-isis-extensions] has been discusssed more sufficient,
and the format of BIER info sub-TLV will be uniform later between
ISIS and OSPF.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|C|0 0 0 A R| subdomain-id | BFR-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 Reuse of the Reserved Bits in BIER Info sub-TLV
Version: Version of the protocol. It remains at 0.
C: The compatiblity bit. It is set according to following rules:
If the R bit is set, C is set to 0, i.e. the TLV is not
compatible with version 0 of the BIER information. This will prevent
routers not implementing this specification from looking at this
advertisement.
If the R bit is clear, C is set to 1. In case the BFR-id has
been obtained without an error by requesting it from a D-BFR, the
value is copied into BFR-id of this sub-TLV, otherwise it is set to
invalid BFR-id.
R: Request Bit. When set, this bit advertises that the BFER is
willing to accept another BMP than the one administratively desired
from D-BFR. The value of BMP is then determined by the according
element in BIER-PE-BMP of the D-BFR.
A: When this bit is set, the BFER advertises that the value indicated
in the BFR-id has been copied from the assignment provided by D-BFR.
If clear and BFR-id is set, the value is administratively assigned
and is non-negotiable.
BFR-id: If set and R bit is clear, it indicates the BFR-id the BFR is
occupying to the D-BFR. If the R bit is set, it indicates the
desired BFR-id to be assigned or no preference.
6.3. BIER-PE-BMP: BIER PE BMP Assignments TLV
This TLV is advertised only for a sub-domain or a BMS for which the
router has been elected to be D-BDR or BD-BDR. It can repeat
multiple times.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R R R R| BMS ID | subdomain-id |# of Assigments|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<---+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| AF |E|Stats| Assigned BFR-id | Prefix Length | # Bit
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Mask
| Address Prefix (variable) | Assgn
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
<---+
Figure 4 BIER PE BMP Assignments TLV
Type: TBD
BMS ID: BMS ID for which the assignments are provided
subdomain-id: subdomain-id for which the assignments are provided
AF: identifies address family of the prefix for which the assignment
is provided. It includes IPv4 and IPv6. Values TBD
Prefix Length: Prefix length of the prefix for which the assignment
is provided.
Prefix: The BFR-prefix of BIER nodes.
Assigned BFR-id: Bit Mask Position assigned by D-BFR, set to invalid
BMP on an error status. 2 octets.
E: Bit indicating assignment error, i.e. the BFER does NOT have a
valid assignment.
Status: Status of the assignment, 3 bits.
0 Assignment is OK and can be used (based on either
administratively requested BMP or chosen by D-BFR for the requesting
BFER). E-bit MUST be clear.
1 error: Unresolvable collision with other administratively set
values, Bit Mask Position cannot be used. E-bit MUST be set.
2 error: Out of Bit Mask Positions for the Topology and Set, Bit
Mask Position cannot be used. E-bit MUST be set.
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all other values reserved, MUST NOT be used.
The assignments SHOULD be sorted on BFER-ID. Assignments MUST NOT
repeat when the TLV is advertised multiple times and a router
discovering such condition MUST issue an adequate warning. When
multiple assignments for the same BFR are found, the first one in
first TLV MUST be used and all others disregarded.
The assignments MUST NOT repeat any BIER Info sub-TLVs that have the
R and A bit cleared, e.g. purely administrative assignments. A
router discovering such condition MUST issue an adequate warning and
disregard such assignments.
The assignments MUST repeat all assigned BIER Info sub-TLVs (that
have A bit set). When such assignment is not advertised anymore, the
according BFER MUST interpret that as loss as assignment, i.e. start
with R bit again or set the BFR-id to invalid BFR-id.
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7. Security Considerations
For general BIER Security Considerations.
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8. Normative References
[I-D.ietf-bier-architecture]
Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
S. Aldrin, "Multicast using Bit Index Explicit
Replication", draft-ietf-bier-architecture-02 (work in
progress), July 2015.
[I-D.ietf-bier-isis-extensions]
Ginsberg, L., Przygienda, T., Aldrin, S., and J. Zhang,
"BIER support via ISIS",
draft-ietf-bier-isis-extensions-01 (work in progress),
October 2015.
[I-D.ietf-bier-mpls-encapsulation]
Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J., and
S. Aldrin, "Encapsulation for Bit Index Explicit
Replication in MPLS Networks",
draft-ietf-bier-mpls-encapsulation-02 (work in progress),
August 2015.
[I-D.ietf-bier-ospf-bier-extensions]
Psenak, P., Kumar, N., Wijnands, I., Dolganow, A.,
Przygienda, T., Zhang, J., and S. Aldrin, "OSPF Extensions
For BIER", draft-ietf-bier-ospf-bier-extensions-01 (work
in progress), October 2015.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/
RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
Zhang & Wang Expires June 23, 2016 [Page 18]
Internet-Draft BFR-id Automatic Assignment in OSPF December 2015
Authors' Addresses
Zheng(Sandy) Zhang
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct
Nanjing,
China
Phone:
Email: zhang.zheng@zte.com.cn
Cui(Linda) Wang
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct
Nanjing,
China
Phone:
Email: wang.cui1@zte.com.cn
Zhang & Wang Expires June 23, 2016 [Page 19]