Internet DRAFT - draft-ietf-bier-mld
draft-ietf-bier-mld
Network Working Group P. Pfister
Internet-Draft IJ. Wijnands
Intended status: Standards Track S. Venaas
Expires: 3 January 2024 Cisco Systems
C. Wang
Z. Zhang
ZTE Corporation
M. Stenberg
2 July 2023
BIER Ingress Multicast Flow Overlay using Multicast Listener Discovery
Protocols
draft-ietf-bier-mld-08
Abstract
This document specifies the ingress part of a multicast flow overlay
for BIER networks. Using existing multicast listener discovery
protocols, it enables multicast membership information sharing from
egress routers, acting as listeners, toward ingress routers, acting
as queriers. Ingress routers keep per-egress-router state, used to
construct the BIER bit mask associated with IP multicast packets
entering the BIER domain.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 January 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 4
5. Querier and Listener Specifications . . . . . . . . . . . . . 5
5.1. Configuration Parameters . . . . . . . . . . . . . . . . 5
5.2. MLDv2 instances. . . . . . . . . . . . . . . . . . . . . 6
5.2.1. Sending Queries . . . . . . . . . . . . . . . . . . . 6
5.2.2. Sending Reports . . . . . . . . . . . . . . . . . . . 7
5.2.3. Receiving Queries . . . . . . . . . . . . . . . . . . 8
5.2.4. Receiving Reports . . . . . . . . . . . . . . . . . . 8
5.3. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 8
6. BIER MLD/IGMP Extension Type . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. BIER Use Case in Data Centers . . . . . . . . . . . 13
A.1. Convention and Terminology . . . . . . . . . . . . . . . 14
A.2. BIER in data centers . . . . . . . . . . . . . . . . . . 15
A.3. A BIER MLD solution for Virtual Network information . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
The Bit Index Explicit Replication (BIER - [RFC8279]) forwarding
technique enables IP multicast transport across a BIER domain. When
receiving or originating a packet, ingress routers have to construct
a bit mask indicating which BIER egress routers located within the
same BIER domain will receive the packet. A stateless approach would
consist of forwarding all incoming packets toward all egress routers,
which would in turn make a forwarding decision based on local
information. But any more efficient approach would require ingress
routers to keep some state about egress routers multicast membership
information, hence requiring state sharing from egress routers toward
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ingress routers.
This document specifies how to use the Multicast Listener Discovery
protocol version 2 [RFC3810] (resp. the Internet Group Management
protocol version 3 [RFC3376]) as the ingress part of a BIER multicast
flow overlay (BIER layering is described in [RFC8279]) for IPv6
(resp. IPv4). It enables multicast membership information sharing
from egress routers, acting as listeners, toward ingress routers,
acting as queriers. Ingress routers keep per-egress-router state,
used to construct the BIER bit mask associated with IP multicast
packets entering the BIER domain.
This document defines an MLDv2 and IGMPv3 extension type, using the
extension scheme defined in [I-D.ietf-pim-igmp-mld-extension], that
is used to provide BIER specific information about the message
originator.
This specification is applicable to both IP version 4 and version 6.
It therefore specifies two separate mechanisms operating
independently. For the sake of simplicity, the rest of this document
uses IPv6 terminology. It can be applied to IPv4 by replacing
'MLDv2' with 'IGMPv3', and following specific requirements when
explicitly stated.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The terms "Bit-Forwarding Router" (BFR), "Bit-Forwarding Egress
Router" (BFER), "Bit-Forwarding Ingress Router" (BFIR), "BFR-id" and
"BFR-Prefix" are to be interpreted as described in [RFC8279].
Additionally, the following definitions are used:
BIER Multicast Listener Discovery (BMLD): The modified version of
MLD specified in this document.
BMLD Querier: A BFR implementing the Querier part of this
specification. A BMLD Node MAY be both a Querier and a Listener.
BMLD Listener: A BFR implementing the Listener part of this
specification. A BMLD Node MAY be both a Querier and a Listener.
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3. Overview
This document proposes to use the mechanisms described in MLDv2 in
order to enable multicast membership information sharing from BFERs
toward BFIRs within a given BIER domain. BMLD queries (resp.
reports) are sent over BIER toward all BMLD Nodes (resp. BMLD
Queriers) using modified MLDv2 messages which IP destination is set
to a configured 'all BMLD Nodes' (resp. 'all BMLD Queriers') IP
multicast address.
By running MLDv2 instances with per-listener explicit tracking, BMLD
Queriers are able to map BMLD Listeners with MLDv2 membership states.
This state is then used to construct the set of BFERs associated with
each incoming IP multicast data packet.
4. Applicability Statement
BMLD runs on top of a BIER Layer and provides the ingress part of a
BIER multicast flow overlay, i.e, it specifies how BFIRs construct
the set of BFERs for each ingress IP multicast data packet. The BFER
part of the Multicast Flow Overlay is out of scope of this document.
The BIER Layer MUST be able to transport BMLD messages toward all
BMLD Queriers and Listeners. Such packets are IP multicast packets
with a BFR-Prefix as source address, a multicast destination address,
and containing a MLDv2 message.
BMLD only requires state to be kept by Queriers, and is therefore
more scalable than PIMv2 [RFC7761] in terms of overall state, but is
also likely to be less scalable than PIMv2 in terms of the amount of
control traffic and the size of the state that is kept by individual
routers.
This specification is applicable to both IP version 4 and version 6.
It therefore specifies two separate mechanisms operating
independently. For the sake of simplicity, this document uses IPv6
terminology. It can be applied to IPv4 by replacing 'MLDv2' with
'IGMPv3', and following specific requirements when explicitly stated.
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If multiple BFIRs have connectivity to the same source, a mechanism
is needed to determine which BFIR should be the forwarder, that is
not specified in this document. As a special case, if BIER is used
end-to-end such that sources would be directly connected to the
BFIRs, then an election mechanism is needed if there are multiple
BFIRs on the same link as the source. One option is to utilize PIM
DR Election where the DR is the BIER forwarder, but other election
mechanisms could be used. In order to allow quick failover, the
BFIRs that are not forwarders should still track BFER interest so
that they have the correct state in case they become forwarders.
5. Querier and Listener Specifications
Routers desiring to receive IP multicast traffic (e.g., for their own
use, or for forwarding) MUST behave as BMLD Listeners. Routers
receiving IP multicast traffic from outside the BIER domain, or
originating multicast traffic, MUST behave as BMLD Queriers.
BMLD Queriers (resp. BMLD Listeners) MUST act as MLDv2 Queriers
(resp. MLDv2 Listeners) as specified in [RFC3810] unless stated
otherwise in this section.
5.1. Configuration Parameters
Both Queriers and Listeners MUST operate as BFIRs and BFERs within
the BIER domain in order to send and receive BMLD messages. They
MUST therefore be configured accordingly, as specified in [RFC8279].
All Listeners MUST be configured with an 'all BMLD Queriers'
multicast address and the BFR-ids of all the BMLD Queriers. This is
used by Listeners to send BMLD reports over BIER toward all Queriers.
All Queriers MUST be configured to accept BMLD reports sent to this
address.
All Queriers MUST be configured with an 'all BMLD Nodes' multicast
address and the BFR-ids of all the Queriers and Listeners. This
information is used by Queriers to send BMLD queries over BIER toward
all BMLD Nodes. All BMLD Nodes MUST be configured to accept BMLD
queries sent to this address.
It may be cumbersone to configure the exact set of BFR-ids for
Queriers and Listeners. One MAY configure the set of BFR-ids to
contain any potentially used BFR-id, perhaps having all bit positions
set. There is no harm in configuring unused BFR-ids. Configuring
the BFR-ids of additional routers would in most cases cause no harm,
as a router would drop the BMLD message unless it is configured as a
Querier or a Listener.
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Note that BMLD (unlike MLDv2) makes use of per-instance configured
multicast group addresses rather than well-known addresses so that
multiple instances of BMLD (using different group addresses) can be
run simultaneously within the same BIER domain. Configured group
addresses MAY be obtained from allocated IP prefixes using [RFC3306].
One MAY choose to use the well-known MLDv2 addresses in one instance,
but different instances MUST use different addresses.
IP packets coming from outside of the BIER domain and having a
destination address set to the configured 'all BMLD Queriers' or the
'all BMLD Nodes' group address MUST be dropped. It is RECOMMENDED
that these configured addresses have a limited scope, enforcing this
behavior by scope-based filtering on BIER domain's egress interfaces.
5.2. MLDv2 instances.
BMLD Queriers MUST run a MLDv2 Querier instance with per-host
tracking, which means they keep track of the MLDv2 state associated
with each BMLD Listener. For that purpose, Listeners are identified
by their respective BFR-Prefix, used as IP source address in all BMLD
reports.
BMLD Listeners MUST run a MLDv2 Listener instance expressing their
interest in the multicast traffic they are supposed to receive for
local use or forwarding.
BMLD Listeners and Queriers MUST NOT run the MLDv1 (IGMPv2 and IGMPv1
for IPv4) backward compatibility procedures.
5.2.1. Sending Queries
BMLD Queries are IP packets sent over BIER by BMLD Queriers:
* Toward all BMLD Nodes (i.e., providing to the BIER Layer the BFR-
ids of all BMLD Nodes).
* Without the IPv6 router alert option [RFC2711] in the hop-by-hop
extension header [RFC8200] (or the IPv4 router alert option
[RFC2113] for IPv4).
* With the IP destination address set to the 'all BMLD Nodes' group
address.
* With a deterministic IP source address. It is RECOMMENDED that
the address is a BFR-Prefix of the sender, but it MAY be another
value. This address is only used for querier election.
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* With a TTL value large enough such that the packet can be received
by all BMLD Nodes, depending on the underlying BIER layer (whether
it decrements the IP TTL or not) and the size of the network. The
default value is 64.
* The extension type defined in Section 6 MUST be included once,
specifying the Sub-domain-id, BFR-id and BFR-Prefix of the sender.
This information may be useful for logging and debugging.
5.2.2. Sending Reports
BMLD Reports are IP packets sent over BIER by BMLD Listeners:
* Toward all BMLD Queriers (i.e., providing to the BIER layer the
BFR-ids of all BMLD Queriers).
* Without the IPv6 router alert option [RFC2711] in the hop-by-hop
extension header [RFC8200] (or the IPv4 router alert option
[RFC2113] for IPv4).
* With the IP destination address set to the 'all BMLD Queriers'
group address.
* With a deterministic IP source address. It is RECOMMENDED that
the address is a BFR-Prefix of the sender.
* With a TTL value large enough such that the packet can be received
by all BMLD Queriers, depending on the underlying BIER layer
(whether it decrements the IP TTL or not) and the size of the
network. The default value is 64.
* The extension type defined in Section 6 MUST be included once,
specifying the Sub-domain-id, BFR-id and BFR-Prefix of the sender.
This information is used to create the necessary forwarding state
for requested flows, and may be useful for logging and debugging.
Since the reports may contain a large number of records, they may
become larger than the maximum BIER payload that can be delivered to
all the BMLD Queriers. Hence an implementation will need to either
use a small default maximum size, allow configuration of a maximum
size, or rely on MTU discovery. MTU discovery may be done for a sub-
domain using BIER MTU Discovery [I-D.ietf-bier-mtud] or for the set
of BMLD Queriers using Path MTU Discovery
[I-D.ietf-bier-path-mtu-discovery].
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5.2.3. Receiving Queries
BMLD Queriers and Listeners MUST check the destination address of all
the IP packets that are received or forwarded over BIER whenever
their own BIER bit is set in the packet. If the destination address
is equal to the 'all BMLD Nodes' group address the packet is
processed as specified in this section.
If the IPv6 (resp. IPv4) packet contains an ICMPv6 (resp. IGMP)
message of type 'Multicast Listener Query' (resp. of type 'Membership
Query'), and include the extension defined in Section 6), it is
processed by the MLDv2 (resp. IGMPv3) instance run by the BMLD
Querier. It MUST be dropped otherwise.
During the MLDv2 processing, the packet MUST NOT be checked against
the MLDv2 consistency conditions (i.e., the presence of the router
alert option, the TTL equaling 1 and, for IPv6 only, the source
address being link-local).
5.2.4. Receiving Reports
BMLD Queriers MUST check the destination address of all the IP
packets that are received or forwarded over BIER whenever their own
BIER bit is set. If the destination address is equal to the 'all
BMLD Queriers' the packet is processed as specified in this section.
If the IPv6 (resp. IPv4) packet contains an ICMPv6 (resp. IGMP)
message of type 'Multicast Listener Report Message v2' (resp.
'Version 3 Membership Report'), and include the extension defined in
Section 6), it is processed by the MLDv2 (resp. IGMPv3) instance run
by the BMLD Querier. It MUST be dropped otherwise.
During the MLDv2 processing, the packet MUST NOT be checked against
the MLDv2 consistency conditions (i.e., the presence of the router
alert option, the TTL equaling 1 and, for IPv6 only, the source
address being link-local).
5.3. Packet Forwarding
BMLD Queriers configure the BIER Layer using the information obtained
using BMLD, and the extension Section 6), to track membership state,
including the Sub-domain-id, BFR-id and BFR-Prefix of the members.
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More specifically, the membership state associated with each BMLD
Listener is provided to the BIER layer such that whenever a multicast
packet enters the BIER domain, if that packet matches the membership
information from a BMLD Listener, its Sub-domain-id and BFR-id is
added to the set of Sub-domains and BFR-ids the packet should be
forwarded to by the BIER-Layer.
6. BIER MLD/IGMP Extension Type
A new MLD/IGMP extension type adds BIER specific information to IGMP/
MLD messages, using the extension scheme defined in
[I-D.ietf-pim-igmp-mld-extension]). The BIER specific information is
the same as the PTA tunnel identifier in [RFC8556] and is shown in
Figure 1. Note that, as defined in the MLD (resp. IGMP), existing
implementations are supposed to ignore this additional data.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ext Type TBD | Extension Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-domain ID | Reserved | BFR-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BFR-Prefix 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BFR-Prefix n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: MLD/IGMP Extension Type for BIER
* Ext Type: Assigned by IANA, identifying this BIER extension.
* Extension Length: The length in octets of the data after this
field. If there are n IPv4 prefixes, the length would be 4 + 4 *
n, if there are n IPv6 prefixes, the length would be 4 + 16 * n.
* Sub-domain-id: A single octet containing a BIER sub-domain-id (see
[[RFC8279]]). This indicates the BIER sub-domain of the router
originating the message.
* Reserved: A single octect, MUST be set to 0 when sending and
ignored when receiving.
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* BFR-id: A two-octet field containing the BFR-id, in the specified
sub-domain, of the router originating the message.
* BFR-prefix: The BFR-prefix (see [[RFC8279]]) of the router that is
originating the message. The BFR-prefix will either be a /32 IPv4
address or a /128 IPv6 address.
This extension type MUST be present once in all IGMP and MLD messages
when originated with a BIER header to identify the BIER originator.
It is expected that any BIER router originating IGMP/MLD messages in
BIER supports this specification. Any IGMP/MLD messages that do not
contain the extension Section 6) MUST be dropped by the decapsulating
router with no processing other than potentially logging or
debugging. It is expected that any BIER router processing IGMP/MLD
messages with BIER encapsulation supports this specification. If
they do not, they will likely ignore the report since they cannot
identify the BIER receiver, but they may be able to derive some of
the receiver information from the BIER header.
7. Security Considerations
BMLD makes use of IGMPv3/MLDv2 messages transported over BIER in
order to configure the BIER Layer of BFIRs. BMLD messages MUST be
secured, either by relying on physical or link-layer security, by
securing the IP packets (e.g., using IPSec [RFC4301]), or by relying
on security features provided by the BIER Layer.
By spoofing the IP source address, an attacker could become the IGMP/
MLD querier. Once one becomes the querier, several attack vectors
are possible. This is similar to regular IGMP/MLD without BIER
encapsulation.
An attacker could send reports with the BIER IGMP/MLD extension
Section 6) specifying a BFR-ID and BIER prefix identifying another
router. This would allow the attacker to:
* Redirect undesired traffic toward the spoofed router by
subscribing to undesired multicast traffic.
* Prevent desired multicast traffic from reaching the spoofed router
by unsubscribing to some desired multicast traffic.
8. IANA Considerations
This document requests that IANA assigns a new type called BIER
information in the registry defined in
[I-D.ietf-pim-igmp-mld-extension].
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9. Acknowledgements
Comments concerning this document are very welcome.
10. References
10.1. Normative References
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
DOI 10.17487/RFC2113, February 1997,
<https://www.rfc-editor.org/info/rfc2113>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[I-D.ietf-pim-igmp-mld-extension]
Sivakumar, M., Venaas, S., Zhang, Z., and H. Asaeda,
"Internet Group Management Protocol Version 3 (IGMPv3) and
Multicast Listener Discovery Version 2 (MLDv2) Message
Extension", Work in Progress, Internet-Draft, draft-ietf-
pim-igmp-mld-extension-08, 3 June 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-pim-
igmp-mld-extension-08>.
10.2. Informative References
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[RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, DOI 10.17487/RFC2711, October 1999,
<https://www.rfc-editor.org/info/rfc2711>.
[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306,
August 2002, <https://www.rfc-editor.org/info/rfc3306>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
<https://www.rfc-editor.org/info/rfc5015>.
[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
<https://www.rfc-editor.org/info/rfc7348>.
[RFC7365] Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
Rekhter, "Framework for Data Center (DC) Network
Virtualization", RFC 7365, DOI 10.17487/RFC7365, October
2014, <https://www.rfc-editor.org/info/rfc7365>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8556] Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S.,
and A. Dolganow, "Multicast VPN Using Bit Index Explicit
Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April
2019, <https://www.rfc-editor.org/info/rfc8556>.
[I-D.ietf-bier-mtud]
Venaas, S., Wijnands, I., Ginsberg, L., and M. Sivakumar,
"BIER MTU Discovery", Work in Progress, Internet-Draft,
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draft-ietf-bier-mtud-00, 27 February 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
mtud-00>.
[I-D.ietf-bier-path-mtu-discovery]
Mirsky, G., Przygienda, T., and A. Dolganow, "Path Maximum
Transmission Unit Discovery (PMTUD) for Bit Index Explicit
Replication (BIER) Layer", Work in Progress, Internet-
Draft, draft-ietf-bier-path-mtu-discovery-14, 26 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
bier-path-mtu-discovery-14>.
Appendix A. BIER Use Case in Data Centers
In current data center virtualization, virtual eXtensible Local Area
Network (VXLAN) [RFC7348] is a kind of network virtualization overlay
technology which is overlaid between NVEs and is intended for multi-
tenancy data center networks, whose reference architecture is
illustrated as per Figure 2.
+--------+ +--------+
| Tenant +--+ +----| Tenant |
| System | | (') | System |
+--------+ | ................ ( ) +--------+
| +-+--+ . . +--+-+ (_)
| | NVE|--. .--| NVE| |
+--| | . . | |---+
+-+--+ . . +--+-+
/ . .
/ . L3 Overlay . +--+-++--------+
+--------+ / . Network . | NVE|| Tenant |
| Tenant +--+ . .--| || System |
| System | . . +--+-++--------+
+--------+ ................
Figure 2: NVO3 Architecture
And there are two kinds of most common methods about how to forward
BUM packets in this virtualization overlay network. One is using PIM
as underlay multicast routing protocol to build explicit multicast
distribution tree, such as PIM-SM [RFC7761] or PIM-BIDIR [RFC5015]
multicast routing protocol. Then, when BUM packets arrive at NVE, it
requires NVE to have a mapping between the VXLAN Network Identifier
and the IP multicast group. According to the mapping, NVE can
encapsulate BUM packets in a multicast packet which group address is
the mapping IP multicast group address and steer them through
explicit multicast distribution tree to the destination NVEs. This
method has two serious drawbacks. It need the underlay network
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supports complicated multicast routing protocol and maintains
multicast related per-flow state in every transit nodes. What is
more, how to configure the ratio of the mapping between VNI and IP
multicast group is also an issue. If the ratio is 1:1, there should
be 16M multicast groups in the underlay network at maximum to map to
the 16M VNIs, which is really a significant challenge for the data
center devices. If the ratio is n:1, it would result in inefficiency
bandwidth utilization which is not optimal in data center networks.
The other method is using ingress replication to require each NVE to
create a mapping between the VXLAN Network Identifier and the remote
addresses of NVEs which belong to the same virtual network. When NVE
receives BUM traffic from the attached tenant, NVE can encapsulate
these BUM packets in unicast packets and replicate them and tunnel
them to different remote NVEs respectively. Although this method can
eliminate the burden of running multicast protocol in the underlay
network, it has a significant disadvantage: large waste of bandwidth,
especially in big-sized data center where there are many receivers.
BIER [RFC8279] is an architecture that provides optimal multicast
forwarding through a "BIER domain" without requiring intermediate
routers to maintain any multicast related per-flow state. BIER also
does not require any explicit tree-building protocol for its
operation. A multicast data packet enters a BIER domain at a "Bit-
Forwarding Ingress Router" (BFIR), and leaves the BIER domain at one
or more "Bit-Forwarding Egress Routers" (BFERs). The BFIR router
adds a BIER header to the packet. The BIER header contains a bit-
string in which each bit represents exactly one BFER to forward the
packet to. The set of BFERs to which the multicast packet needs to
be forwarded is expressed by setting the bits that correspond to
those routers in the BIER header. Specifically, for BIER-TE, the
BIER header may also contain a bit-string in which each bit indicates
the link the flow passes through.
The following sub-sections try to propose how to take full advantage
of overlay multicast protocol to carry virtual network information,
and create a mapping between the virtual network information and the
bit-string to implement BUM services in data centers.
A.1. Convention and Terminology
The terms about NVO3 are defined in [RFC7365]. The most common
terminology used in this appendix is listed below.
NVE: Network Virtualization Edge, which is the entity that
implements the overlay functionality. An NVE resides at the
boundary between a Tenant System and the overlay network.
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VXLAN: Virtual eXtensible Local Area Network
VNI: VXLAN Network Identifier
Virtal Network Context Identifier: Field in an overlay encapsulation
header that identifies the specific VN the packet belongs to.
A.2. BIER in data centers
This section tries to describe how to use BIER as an optimal scheme
to forward the broadcast, unknown and multicast (BUM) packets when
they arrive at the ingress NVE in data centers.
The principle of using BIER to forward BUM traffic is that: firstly,
it requires each ingress NVE to have a mapping between the Virtual
Network Context Identifier and the bit-string in which each bit
represents exactly one egress NVE to forward the packet to. And
then, when receiving the BUM traffic, the BFIR/Ingree NVE maps the
receiving BUM traffic to the mapping bit-string, encapsulates the
BIER header, and forwards the encapsulated BUM traffic into the BIER
domain to the other BFERs/Egress NVEs indicated by the bit-string.
Furthermore, as for how each ingress NVE knows the other egress NVEs
that belong to the same virtual network and creates the mapping is
the main issue discussed below. Basically, BIER Multicast Listener
Discovery is an overlay solution to support ingress routers to keep
per-egress-router state to construct the BIER bit-string associated
with IP multicast packets entering the BIER domain. The following
section tries to extend BIER MLD to carry virtual network
information(such as Virtual Network Context identifier), and
advertise them between NVEs. When each NVE receive these
information, they create the mapping between the virtual network
information and the bit-string representing the other NVEs belonged
to the same virtual network.
A.3. A BIER MLD solution for Virtual Network information
The BIER MLD solution allows having multiple MLD instances by having
unique pairs of BMLD Nodes and BMLD Querier addresses for each
instance. Assume for now that we have a unique instance per VNI and
that all BMLD routers are using the same mapping between VNIs and
BMLD address pairs. Also for each VNI there is a multicast group
used for encapsulation of BUM traffic over BIER. This group may
potentially be shared by some or all of the VNIs.
Each NVE acquires the Virtual Network information, and advertises
this Virtual Network information to other NVEs through the MLD
messages. For a given VNI it sends BMLD reports to the BMLD nodes
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address used for that VNI, for the group used for delivering BUM
traffic for that VNI. This allows all NVE routers to know which
other NVE routers have interest in BUM traffic for a particular VNI.
If one attached virtual network is migrated, the NVE will withdraw
the Virtual Network information by sending an unsolicited BMLD
report. Note that NVEs also respond to periodic queries to BMLD
Nodes addresses corresponding to VNIs for which they have interest.
When ingress NVE receives the Virtual Network information
advertisement message, it builds a mapping between the receiving
Virtual Network Context Identifier in this message and the bit-string
in which each bit represents one egress NVE who sends the same
Virtual Network information. Subsequently, once this ingress NVE
receives some other MLD advertisements which include the same Virtual
Network information from some other NVEs , it updates the bit-string
in the mapping and adds the corresponding sending NVE to the updated
bit-string. Once the ingress NVE removes one virtual network, it
will delete the mapping corresponding to this virtual network as well
as send withdraw message to other NVEs.
After finishing the above interaction of MLD messages, each ingress
NVE knows where the other egress NVEs are in the same virtual
network. When receiving BUM traffic from the attached virtual
network, each ingress NVE knows exactly how to encapsulate this
traffic and where to forward them to.
This can be used in both IPv4 network and IPv6 network. In IPv4,
IGMP protocol does the similar extension for carrying Virtual Network
information TLV in Version 2 membership report message.
Note that it is possible to have multiple VNIs map to the same pair
of BMLD addresses. Provided VNIs that map to the same BMLD address
uses different multicast groups for encapsulation, this is not a
problem, because each instance is tracking interest for each
multicast group separately. If multiple VNIs map to the same pair
and the multicast group used is not unique, some NVEs may receive BUM
traffic for which they are not interested. An NVE would drop packets
for an unknown VNI, but it means wasting some bandwidth and
processing. This is similar to the non-BIER case where there is not
a unique multicast group for encapsulation. The improvement offered
by using BMLD is by using multiple instance, hence reducing the
problems caused by using the same transport group for multiple VNIs.
Authors' Addresses
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Pierre Pfister
Cisco Systems
Paris
France
Email: pierre.pfister@darou.fr
IJsbrand Wijnands
Cisco Systems
De Kleetlaan 6a
1831 Diegem
Belgium
Email: ice@cisco.com
Stig Venaas
Cisco Systems
Tasman Drive
San Jose, CA 95134
United States of America
Email: stig@cisco.com
Cui(Linda) Wang
Email: lindawangjoy@gmail.com
Zheng(Sandy) Zhang
ZTE Corporation
No.50 Software Avenue, Yuhuatai District
Nanjing
CA,
China
Email: zhang.zheng@zte.com.cn
Markus Stenberg
FI-00930 Helsinki
Finland
Email: markus.stenberg@iki.fi
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