Internet DRAFT - draft-ietf-pim-assert-packing
draft-ietf-pim-assert-packing
PIM Y. Liu, Ed.
Internet-Draft China Mobile
Intended status: Standards Track T. Eckert, Ed.
Expires: 21 October 2023 M. McBride
Futurewei
Z. Zhang
ZTE Corporation
19 April 2023
PIM Assert Message Packing
draft-ietf-pim-assert-packing-12
Abstract
In PIM-SM shared LAN networks, there is often more than one upstream
router. When PIM Sparse Mode (PIM-SM), including PIM Source
Specific-Specific Multicast (PIM-SSM), is used, this can lead to
duplicate IP multicast packets being forwarded by these PIM routers.
PIM Assert messages are used to elect a single forwarder for each IP
multicast traffic flow between these routers.
This document defines a mechanism to send and receive information for
multiple IP multicast flows in a single PackedAssert message. This
optimization reduces the total number of PIM packets on the LAN and
can therefore speed up the election of the single forwarder, reducing
the number of duplicate IP multicast packets incurred.
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 21 October 2023.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (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
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. PIM Assert Packing Hello Option . . . . . . . . . . . . . 5
3.2. Assert Packing Message Formats . . . . . . . . . . . . . 5
3.3. PackedAssert Mechanism . . . . . . . . . . . . . . . . . 6
3.3.1. Sending PackedAssert messages . . . . . . . . . . . . 7
3.3.1.1. Handling of reception-triggered assert
records. . . . . . . . . . . . . . . . . . . . . . 8
3.3.1.2. Handling of timer expiry-triggered assert
records. . . . . . . . . . . . . . . . . . . . . . 9
3.3.1.3. Beneficial delay in sending PackedAssert
messages . . . . . . . . . . . . . . . . . . . . . 9
3.3.1.4. Handling Assert/PackedAssert message loss . . . . 9
3.3.1.5. Optimal degree of assert record packing . . . . . 10
3.3.2. Receiving PackedAssert messages . . . . . . . . . . . 10
4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. PIM Assert Packing Hello Option . . . . . . . . . . . . . 10
4.2. Assert Message Format . . . . . . . . . . . . . . . . . . 11
4.3. Simple PackedAssert Message Format . . . . . . . . . . . 11
4.4. Aggregated PackedAssert Message Format . . . . . . . . . 13
4.4.1. Source Aggregated Assert Record . . . . . . . . . . . 15
4.4.2. RP Aggregated Assert Record . . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
8. Working Group considerations . . . . . . . . . . . . . . . . 19
8.1. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 19
8.2. Changelog . . . . . . . . . . . . . . . . . . . . . . . . 19
8.2.1. draft-ietf-pim-assert-packing-12 . . . . . . . . . . 19
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8.2.2. draft-ietf-pim-assert-packing-11 . . . . . . . . . . 19
8.2.3. draft-ietf-pim-assert-packing-10 . . . . . . . . . . 20
8.2.4. draft-ietf-pim-assert-packing-09 . . . . . . . . . . 20
8.2.5. draft-ietf-pim-assert-packing-08 . . . . . . . . . . 21
8.2.6. draft-ietf-pim-assert-packing-07 . . . . . . . . . . 21
8.2.7. draft-ietf-pim-assert-packing-06 . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Use case examples . . . . . . . . . . . . . . . . . 23
A.1. Enterprise network . . . . . . . . . . . . . . . . . . . 24
A.2. Video surveillance . . . . . . . . . . . . . . . . . . . 24
A.3. Financial Services . . . . . . . . . . . . . . . . . . . 24
A.4. IPTV broadcast Video . . . . . . . . . . . . . . . . . . 24
A.5. MVPN MDT . . . . . . . . . . . . . . . . . . . . . . . . 24
A.6. Special L2 services . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
In PIM-SM shared LAN networks, there is typically more than one
upstream router. When duplicate data packets appear on the LAN, from
different upstream routers, assert packets are sent from these
routers to elect a single forwarder according to [RFC7761]. The PIM
assert messages are sent periodically to keep the assert state. The
PIM assert message carries information about a single multicast
source and group, along with the corresponding metric-preference and
metric of the route towards the source or PIM Rendezvous Point (RP).
This document defines a mechanism to encode the information of
multiple PIM Assert messages into a single PackedAssert message.
This allows to send and receive information for multiple IP multicast
flows in a single PackedAssert message without changing the PIM
Assert state machinery. It reduces the total number of PIM packets
on the LAN and can therefore speed up the election of the single
forwarder, reducing the number of duplicate IP multicast packets.
This can particularly be helpful when there is traffic for a large
number of multicast groups or SSM channels and PIM packet processing
performance of the routers is slow.
1.1. Requirements Language
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.
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1.2. Terminology
The reader is expected to be familiar with the terminology of
[RFC7761]. The following lists the abbreviations repeated in this
document.
AT: Assert Timer
RP: Rendezvous Point
RPF: Reverse Path Forwarding
SPT: Shortest Path Tree
RPT: RP Tree
DR: Designated Router
2. Problem Statement
PIM Asserts occur in many deployments. See Appendix A for explicit
examples and explanations of why it is often not possible to avoid.
PIM assert state depends mainly on the network topology. As long as
there is a layer 2 network with more than 2 PIM routers, there may be
multiple upstream routers, which can cause duplicate multicast
traffic to be forwarded and assert process to occur.
As the multicast services become widely deployed, the number of
multicast entries increases, and a large number of assert messages
may be sent in a very short period when multicast data packets
trigger PIM assert processing in the shared LAN networks. The PIM
routers need to process a large number of PIM assert small packets in
a very short time. As a result, the device load is very large. The
assert packet may not be processed in time or even discarded, thus
extending the time of traffic duplication in the network.
The PIM Assert mechanism can only be avoided by designing the network
to be without transit subnets with multiple upstream routers. For
example, an L2 ring between routers can sometimes be reconfigured to
be a ring of point-to-point subnets connected by the routers. These
L2/L3 topology changes are undesirable though, when they are only
done to enable IP multicast with PIM because they increase the cost
of introducing IP multicast with PIM.
These designs are also not feasible when specific L2 technologies are
needed. For example various L2 technologies for rings provide sub 50
msec failover mechanisms, something not possible equally with an L3
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subnet based ring. Likewise, IEEE Time Sensitive Networking
mechanisms would require an L2 topology that can not simply be
replaced by an L3 topology. L2 sub-topologies can also significantly
reduce the cost of deployment.
3. Specification
This document defines three elements in support of PIM assert
packing:
1. The PIM Assert Packing Hello Option.
2. The encoding of PackedAssert messages.
3. How to send and receive PackedAssert messages.
3.1. PIM Assert Packing Hello Option
The PIM Assert Packing Hello Option (Section 4.1) is used to announce
support for the assert packing mechanisms specified in this document.
PackedAssert messages (Section 3.2) MUST NOT be used unless all PIM
routers in the same subnet announce this option.
3.2. Assert Packing Message Formats
The PIM Assert message, as defined in Section 4.9.6 of [RFC7761],
describes the parameters of a (*,G) or (S,G) assert through the
following information elements: Rendezvous Point Tree flag (R),
Source Address, Group Address, Metric and Metric Preference. This
document calls this information an assert record.
Assert packing introduces two new PIM Assert message encodings
through the allocation and use of two flags in the PIM Assert message
header [I-D.ietf-pim-rfc8736bis], the Packed (P) and the Aggregated
(A) flags.
If the (P)acked flag is 0, the message is a (non-packed) PIM Assert
message as specified in [RFC7761]. See Section 4.2. In this case,
the (A) flag MUST be set to 0, and MUST be ignored on receipt.
If the (P) flag is 1, then the message is called a PackedAssert
message and the type and hence encoding format of the payload is
determined by the (A) flag.
If A=0, then the message body is a sequence of assert records. This
is called a "Simple PackedAssert" message. See Section 4.3.
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If A=1, then the message body is a sequence of aggregated assert
records. This is called an "Aggregated PackedAssert". See
Section 4.4.
Two aggregated assert record types are specified.
The "Source Aggregated Assert Record", see Section 4.4.1, encodes one
(common) Source Address, Metric and Metric Preference as well as a
list of one or more Group Addresses. Source Aggregated Assert
Records provide a more compact encoding than the Simple PackedAssert
message format when multiple (S,G) flows share the same source S. A
single Source Aggregated Assert Record with n Group Addresses
represents the information of assert records for (S,G1)...(S,Gn).
The "RP Aggregated Assert Record", see Section 4.4.2, encodes one
common Metric and Metric Preference as well as a list of "Group
Records", each of which encodes a Group Address and a list of zero or
more Source Addresses with a count. This is called an "RP Aggregated
Assert Record", because with standard RPF according to ([RFC7761]),
all the Group Addresses that use the same RP will have the same
Metric and Metric Preference.
RP Aggregation Records provide a more compact encoding than the
Simple PackedAssert message format for (*,G) flows. The Source
Address is optionally used by [RFC7761] assert procedures to indicate
the source(s) that triggered the assert, otherwise the Source Address
is set to 0 in the assert record.
Both Source Aggregated Assert Records and RP Aggregated Assert
Records also include the R flag which maintains its semantic from
[RFC7761] but also distinguishes the encodings. Source Aggregated
Assert Records have R=0, as (S,G) assert records do in [RFC7761]. RP
Aggregated Assert Records have R=1, as (*,G) assert records do in
[RFC7761].
3.3. PackedAssert Mechanism
PackedAsserts do not change the [RFC7761] PIM assert state machine
specification. Instead, sending and receiving of PackedAssert
messages as specified in the following subsections are logically new
packetization options for assert records in addition to the (not
packed) [RFC7761] Assert Message. There is no change to the assert
record information elements transmitted or their semantic. They are
just transmitted in fewer but larger packets and fewer total number
of bytes used to encode the information elements. In result, PIM
routers should be able to send/receive assert records faster and/or
with less processing overhead.
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3.3.1. Sending PackedAssert messages
When using assert packing, the regular [RFC7761] Assert message
encoding with A=0 and P=0 is still allowed to be sent. Routers are
free to choose which PackedAssert message format they send - simple
(Section 4.3) and/or aggregated (Section 4.4).
* When any PIM routers on the LAN have not signaled support for
Assert Packing, implementations MUST send only Asserts and MUST
NOT send PackedAsserts under any condition.
* Implementations SHOULD support sending of PackedAssert messages.
It is out of scope of this specification for which conditions,
such as data-triggered asserts or Assert Timer (AT) expiry-
triggered asserts, or under which conditions (such as high load)
an implementation will send PackedAsserts instead of Asserts.
* Implementations are expected to specify in documentation and/or
management interfaces (such as a YANG model), which PackedAssert
message formats they can send and under which conditions they will
send them.
* Implementations SHOULD be able to indicate to the operator (such
as through a YANG model) how many Assert and PackedAssert messages
were sent/received and how many assert records were sent/received.
* A configuration option SHOULD be available to disable PackedAssert
operations. Implementations that introduce support for assert
packing from day one of their [RFC7761] implementation MAY omit
this configuration option.
When a PIM router has an assert record ready to send according to
[RFC7761], it calls one of the following functions:
* send Assert(S,G) / send Assert(*,G) ([RFC7761], Section 4.2),
* Send Assert(S,G) / SendAssertCancel(S,G) ([RFC7761],
Section 4.6.1),
* Send Assert(*,G) / Send AssertCancel(*,G) ([RFC7761],
Section 4.6.2)
* send Assert(S,G) ([RFC7761], Section 4.8.2).
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If sending of PackedAsserts is possible on the network, instead of
sending an Assert message with an assert record, any of these calls
MAY instead result in the PIM implementation remembering the assert
record, and continuing with further processing for other flows which
may result in additional assert records.
PIM MUST then create PackedAssert messages from the remembered assert
records and schedule them for sending according to the considerations
of the following subsections.
3.3.1.1. Handling of reception-triggered assert records.
Avoiding additional delay because of assert packing compared to
immediate scheduling of Assert messages is most critical for assert
records that are triggered by reception of data or reception of
asserts against which the router is in the "I am Assert Winner"
state. In these cases the router SHOULD send out an Assert or
PackedAssert message containing this assert record as soon as
possible to minimize the time in which duplicate IP multicast packets
can occur.
To avoid additional delay in this case, the router should employ
appropriate assert packing and scheduling mechanisms, as explained
here.
Asserts/PackedAsserts created from reception-triggered assert records
should be scheduled for serialization with a higher priority than
those created from other reasons. They should also bypass other PIM
messages that can create significant bursts, such as PIM join/prune
messages.
When there is no reception-triggered Assert/PackedAssert messages
currently being serialized on the interface or scheduled to be sent,
the router should immediately generate and schedule an Assert or
PackedAssert message without further assert packing.
If there are one or more reception-triggered Assert/PackedAssert
messages already serializing and/or scheduled to be serialized on the
outgoing interface, then the router can use the time until the last
of those messages will have finished serializing for PIM processing
of further conditions that may result in additional reception-
triggered assert records as well as packing of these assert records
without introducing additional delay.
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3.3.1.2. Handling of timer expiry-triggered assert records.
Asserts triggered by expiry of the AT on an assert winner are not
time-critical because they can be scheduled in advance and because
the Assert_Override_Interval parameter of [RFC7761] already creates a
3 second window in which such assert records can be sent, received,
and processed before an assert loser's state would expire and
duplicate IP multicast packets could occur.
An example mechanism to allow packing of AT expiry-triggered assert
records on assert winners is to round the AT to an appropriate
granularity such as 100 msec. This will cause AT for multiple (S,G)
and/or (*,G) states to expire at the same time, thus allowing them to
be easily packed without changes to the assert state machinery.
AssertCancel messages have assert records with an infinite metric and
can use assert packing as any other Assert. They are sent on
Override Timer (OT) expiry and can be packed for example with the
same considerations as AT expiry-triggered assert records.
3.3.1.3. Beneficial delay in sending PackedAssert messages
Delay in sending PackedAsserts beyond what was discussed in prior
subsections can still be beneficial when it causes the overall amount
of (possible) duplicate IP multicast packets to decrease in a
condition with large number of (S,G) and/or (*,G), compared to the
situation in which an implementation only sends Assert messages.
This delay can simply be used in implementations because it can not
support the (more advanced) mechanisms described above, and this
longer delay can be achieved by some simpler mechanism (such as only
periodic generation of PackedAsserts) and still achieves an overall
reduction in duplicate IP multicast packets compared to sending only
Asserts.
3.3.1.4. Handling Assert/PackedAssert message loss
When Asserts are sent, a single packet loss will result only in
continued or new duplicates from a single IP multicast flow. Loss of
(non AssertCancel) PackedAssert impacts duplicates for all flows
packed into the PackedAssert and may result in the need for re-
sending more than one Assert/PackedAssert, because of the possible
inability to pack the assert records in this condition. Therefore,
routers SHOULD support mechanisms allowing for PackedAsserts and
Asserts to be sent with an appropriate Differentiated Services Code
Point (DSCP, [RFC2475]), such as Expedited Forwarding (EF), to
minimize their loss, especially when duplicate IP multicast packets
could cause congestion and loss.
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Routers MAY support a configurable option for sending PackedAssert
messages twice in short order (such as 50 msec apart), to overcome
possible loss, but only when the following two conditions are met.
1. The total size of the two PackedAsserts is less than the total
size of equivalent Assert messages,
2. The condition of the assert records flows in the PackedAssert is
such that the router can expect that their reception by PIM
routers will not trigger Assert/PackedAsserts replies. This
condition is true for example when sending an assert record while
becoming or being Assert Winner (Action A1/A3 in [RFC7761]).
3.3.1.5. Optimal degree of assert record packing
The optimal target packing size will vary depending on factors
including implementation characteristics and the required operating
scale. At some point, as the target packing size is varied from the
size of a single non-packed Assert, to the MTU size, a size can be
expected to be found where the router can achieve the required
operating scale of (S,G) and (*,G) flows with minimum duplicates.
Beyond this size, a further increase in the target packing size would
not produce further benefits, but might introduce possible negative
effects such as the incurrence of more duplicates on loss.
For example, in some router implementations, the total number of
packets that a control plane function such as PIM can send/receive
per unit of time is a more limiting factor than the total amount of
data across these packets. As soon as the packet size is large
enough for the maximum possible payload throughput, increasing the
packet size any further may still reduce the processing overhead of
the router, but may increase latency incurred in creating the packet
in a way that may increase duplicates compared to smaller packets.
3.3.2. Receiving PackedAssert messages
Upon reception of a PackedAssert message, the PIM router logically
converts its payload into a sequence of assert records that are then
processed as if an equivalent sequence of Assert messages were
received according to [RFC7761].
4. Packet Formats
This section describes the format of new PIM extensions introduced by
this document.
4.1. PIM Assert Packing Hello Option
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionType = TBD | OptionLength = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: PIM Assert Packing Hello Option
The PIM Assert Packing Hello Option is a new option for PIM Hello
Messages according to Section 4.9.2 of [RFC7761].
* OptionType TBD: PIM Packed Assert Capability Hello Option
Including the PIM OptionType TBD indicates support for the ability to
receive and process all the PackedAssert encodings defined in this
document.
4.2. Assert Message Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |7 6 5 4 3 2|A|P| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Assert Message Format
Figure 2 shows a PIM Assert message as specified in Section 4.9.6 of
[RFC7761]. The Encoded-Group and Encoded-Unicast address formats are
specified in Section 4.9.1 of [RFC7761] for IP and IPv6.
This common header is showing the "7 6 5 4 3 2" Flag Bits as defined
in Section 4 of [I-D.ietf-pim-rfc8736bis] and the location of the P
and A flags, as described in Section 5. As specified in
Section 3.2, both flags in a (non-packed) PIM Assert message are
required to be set to 0.
4.3. Simple PackedAssert Message Format
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |7 6 5 4 3 2|A|P| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Zero | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Assert Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Assert Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Assert Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Simple PackedAssert Message Format
* PIM Version, Type, Checksum:
As specified in Section 4.9.6 of [RFC7761].
* "7 6 5 4 3 2": IANA registry handled bits according to Section 4
of [I-D.ietf-pim-rfc8736bis].
* Zero: Set to zero on transmission. Serves to make non assert
packing capable PIM routers fail in parsing the message instead of
possible mis-parsing if this field was used.
* Reserved: Set to zero on transmission. Ignored upon receipt.
* P: packed flag. MUST be 1.
* A: aggregated flag. MUST be 0.
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* M: The number of Assert Records in the message. Derived from the
length of the packet carrying the message.
* Assert Record: formatted according to {FIG-MESSAGE-SIMPLE}}, which
is the same as the PIM assert message body as specified in
Section 4.9.6 of [RFC7761]. The number M of Assert Records is
determined from the packet size.
The format of each Assert Record is the same as the PIM assert
message body as specified in Section 4.9.6 of [RFC7761].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Assert Record
4.4. Aggregated PackedAssert Message Format
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |7 6 5 4 3 2|A|P| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Zero | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Aggregated Assert Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Aggregated Assert Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Aggregated Assert Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Aggregated PackedAssert Message Format
* PIM Version, Type, Reserved, Checksum:
As specified in Section 4.9.6 of [RFC7761].
* "7 6 5 4 3 2": IANA registry handled bits according to Section 4
of [I-D.ietf-pim-rfc8736bis].
* P: packed flag. MUST be 1.
* A: aggregated flag. MUST be 1.
* Zero: Set to zero on transmission. Serves to make non assert
packing capable PIM routers fail in parsing the message instead of
possible mis-parsing if this field was used.
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* Aggregated Assert Record: formatted according to Figure 5. The
number M of Aggregated Assert Records is determined from the
packet size.
4.4.1. Source Aggregated Assert Record
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Groups (N) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address 1 (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address 2 (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address N (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Source Aggregated Assert Record
* Reserved: Set to zero on transmission. Ignored upon receipt.
* R: MUST be 0.
R indicates both that the encoding format of the record is that of
a Source Aggregated Assert Record, but also that all assert
records represented by the Source Aggregated Assert Record have
R=0 and are therefore (S,G) assert records according to the
definition of R in [RFC7761], Section 4.9.6.
* Source Address, Metric Preference, Metric:
As specified in Section 4.9.6 of [RFC7761]. Source Address MUST
NOT be zero.
* Number of Groups:
The number of Group Address fields.
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* Group Address:
As specified in Section 4.9.6 of [RFC7761].
4.4.2. RP Aggregated Assert Record
An RP Aggregation Assert record aggregates (*,G) assert records with
the same Metric Preference and Metric. Typically this is the case
for all (*,G) using the same RP, but the encoding is not limited to
only (*,G) using the same RP because the RP address is not encoded as
it is also not present in [RFC7761] assert records.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Group Records (K) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [K] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: RP Aggregated Assert Record
* R: MUST be 1.
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R indicates both that the encoding format of the record is that of
an RP Aggregated Assert Record, and that all assert records
represented by the RP Aggregated Assert Record have R=1 and are
therefore (*,G) assert records according to the definition of R in
[RFC7761], Section 4.9.6.
* Metric Preference, Metric:
As specified in Section 4.9.6 of [RFC7761].
* Reserved: Set to zero on transmission. Ignored upon receipt.
* Number of Group Records (K):
The number of packed Group Records. A record consists of a Group
Address and a Source Address list with a number of sources.
The format of each Group Record is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Sources (P) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address 2 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address P (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Group Record
* Group Address and Reserved:
As specified in Section 4.9.6 of [RFC7761].
* Reserved: Set to zero on transmission. Ignored upon receipt.
* Number of Sources (P):
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The Number of Sources is corresponding to the number of Source
Address fields in the Group Record. If this number is 0, the
Group Record indicates one assert record with a Source Address of
0. If this number is not 0 and one of the (*,G) assert records to
be encoded should have the Source Address 0, then 0 needs to be
encoded as one of the Source Address fields.
* Source Address:
As specified in Section 4.9.6 of [RFC7761]. But there can be
multiple Source Address fields in the Group Record.
5. IANA Considerations
IANA has assigned the following code point value to the "PIM-Hello
Options" registry for the Packed Assert Capability.
+=======+========+=========================+================+
| Value | Length | Name | Reference |
+=======+========+=========================+================+
| 40 | 0 | Packed Assert Capability| [This Document]|
+=======+========+=========================+================+
Figure 9: IANA PIM-Hello Options
IANA has assigned the following two Flag Bits for PIM Assert messages
to the "PIM Message Types" registry.
+======+========+=================+====================+
| Type | Name | Flag Bits | Reference |
+======+========+=================+====================+
| 5 | Assert | 0: Packed | [This Document] |
| | | 1: Aggregated | [This Document] |
| | | 2-7: Unassigned | [RFC3973][RFC7761] |
+======+========+=================+====================+
Figure 10: IANA PIM Message Types
6. Security Considerations
The security considerations of [RFC7761] apply to the extensions
defined in this document.
This document packs multiple assert records in a single message. As
described in Section 6.1 of [RFC7761], a forged Assert message could
cause the legitimate designated forwarder to stop forwarding traffic
to the LAN. The effect may be amplified when using a PackedAssert
message.
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Like other optional extensions of [RFC7761] that are active only when
all routers indicate support for them, a single misconfigured or
malicious router emitting forged PIM Hello messages can inhibit
operations of this extension.
Authentication of PIM messages such as explained in [RFC7761],
Sections 6.2 and 6.3 can protect against the forged message attacks
attacks.
7. Acknowledgments
The authors would like to thank: Stig Venaas for the valuable
contributions of this document, Alvaro Retana for his thorough and
constructive RTG AD review, Ines Robles for her Gen-ART review, Tommy
Pauly for his transport area review, Robert Sparks for his SecDir
review, Shuping Peng for her RtgDir review, John Scudder for his RTG
AD review, Eric Vyncke for his INT AD review, Eric Kline for his INT
AD review, Paul Wouter for his SEC AD review, Zaheduzzaman Sarker for
his TSV AD review, Robert Wilton for his OPS AD review and Martin
Duke for his TSV AD review.
8. Working Group considerations
[RFC-Editor: please remove this section].
8.1. Open Issues
8.2. Changelog
This document is hosted starting with -06 on
https://github.com/toerless/pim-assert-packing.
8.2.1. draft-ietf-pim-assert-packing-12
Changed text of IANA considerations from request to assigned after
IANA has assigned the code points.
Fixed leftover nits from John Scudders review that where not done
right in -11.
8.2.2. draft-ietf-pim-assert-packing-11
Comprehensive 2 round AD review by John Scudder.
Functional enhancement: add requirement for existing implementation
to be able to disable assert packing so that any possible
compatibility issues introduced (which we think will not happen) can
be avoided when upgrading to a packedassert version of the software.
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Also to allow performance comparison. No making a requirement for
day 0 implementations because they may want to save the work of
having a non-packed-assert code path.
Some rewrite to increase readibility, subdivided 3.3.1 into multiple
subsections to better structure it.
3.3.1 improved core requirements - added requirement for counters to
show assert/packedassert operations, documentation (e.g.: YANG) for
what/how it can send, config option to disable packedasserts.
Refined text: Bulletized cases of asserts in rfc7761,
Subdivided 3.3.1 into multiple subsections for readability improved
text based on review. Added reference for DSCP.
3.3.1.5 Added explicit example of improvement because of packet size/
throughput limits of router.
Added notion of attacks by wrong hellos to security section.
Eric Vyncke review:
Appendix A: Better elaboration of L2 ring vs L3 ring benefits. Nits.
Paul Wouter review:
Changed explanation of number "M" of records to be inline with
formatting of other data (sections 4.3 and 4.4).
Some nits.
8.2.3. draft-ietf-pim-assert-packing-10
Fixed up Reserved field of PackedAsserts to get back to 32 bit
alignment of the following fields (was down to 16 bits). Sorry, had
a misinterpretation reading rfc7761, though there ws something that
had only made it 16 bit aligned. Anyhow. Only this one change, 8 ->
24 bit for this field.
8.2.4. draft-ietf-pim-assert-packing-09
For details of review discussion/replies, see review reply emails in
(https://github.com/toerless/pim-assert-packing/tree/main/emails)j
review Alvaro Retana: Reintroduced ref to PIM-DM, fixed typos,
downgraded MAY->may for "sufficient".
IANA Expert Review / Stig Venaas:
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Removed Count field from message headers as it complicates parsing
and is unnecessary. Added "Zero" field to make packed asserts
received by a non-packed-assert-capable-router guaranteed to fail
("Reserved" address family type).
Changed from RFC8736 to RFC8736bis so that we can use the word
"Unassigned" in the IANA table.
Review Shuping Peng
Changed explanation of how assert packing works from "layer" to
"alternative to packetization via PIM Assert Message. Fixed various
typos, expanded term etc..
Review Robert Sparks:
Moved Intro explanations of how one could avoid asserts (but how its
problematic) to appendix. Applied textual nits found. Removed
quotes around term "sufficient" for easier readbility.
Review Tommy Paul:
Transport related concern explained in reply, but no additional
explanations in text because the question referred to basic PIM
behavior expected to be understood by readers: No discovery of loss/
trigger for retransmission, just restransmission of same message
element after discover of ongoing duplicates and/or expiry of timers.
8.2.5. draft-ietf-pim-assert-packing-08
Included changes from review of Alvaro Retana
(https://mailarchive.ietf.org/arch/msg/pim/
GsKq9bB2a6yDdM9DvAUGCWthdEI)
Please see the following emails discussing the changes:
https://raw.githubusercontent.com/toerless/pim-assert-packing/main/
emails/07-alvaro-review-reply.txt
8.2.6. draft-ietf-pim-assert-packing-07
Included changes from review of Alvaro Retana
(https://mailarchive.ietf.org/arch/msg/pim/
Cp4o5glUFge2b84X9CQMwCWZjAk/)
Please see the following emails discussing the changes:
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https://raw.githubusercontent.com/toerless/pim-assert-packing/main/
emails/05-alvaro-review-reply.txt
https://raw.githubusercontent.com/toerless/pim-assert-packing/main/
emails/07-pim-wg-announce.txt
8.2.7. draft-ietf-pim-assert-packing-06
This version was converted from txt format into markdown for better
editing later, but is otherwise text identical to -05. It was posted
to DataTracker to make diffs easier.
Functional changes:
9. References
9.1. Normative References
[I-D.ietf-pim-rfc8736bis]
Venaas, S. and A. Retana, "PIM Message Type Space
Extension and Reserved Bits", Work in Progress, Internet-
Draft, draft-ietf-pim-rfc8736bis-00, 2 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pim-
rfc8736bis-00>.
[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>.
[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>.
[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>.
9.2. Informative References
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
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[RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
January 2005, <https://www.rfc-editor.org/info/rfc3973>.
[RFC6037] Rosen, E., Ed., Cai, Y., Ed., and IJ. Wijnands, "Cisco
Systems' Solution for Multicast in BGP/MPLS IP VPNs",
RFC 6037, DOI 10.17487/RFC6037, October 2010,
<https://www.rfc-editor.org/info/rfc6037>.
[RFC7431] Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
Decraene, "Multicast-Only Fast Reroute", RFC 7431,
DOI 10.17487/RFC7431, August 2015,
<https://www.rfc-editor.org/info/rfc7431>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<https://www.rfc-editor.org/info/rfc7490>.
Appendix A. Use case examples
The PIM Assert mechanism can only be avoided by designing the network
to be without transit subnets with multiple upstream routers. For
example, an L2 ring between routers can sometimes be reconfigured to
be a ring of point-to-point subnets connected by the routers. These
L2/L3 topology changes are undesirable though, when they are only
done to enable IP multicast with PIM because they increase the cost
of introducing IP multicast with PIM.
These L3 ring designs are specifically undesirable, when particular
L2 technologies are needed. For example various L2 technologies for
rings provide sub 50 msec failover mechanisms that will benefit IP
unicast and multicast alike without any added complexity to the IP
layer (forwarding or routing). If such L2 rings where to be replaced
by L3 rings just to avoid PIM asserts, then this would result in the
need for a complex choice of of a sub 50 msec IP unicast failover
solutions as well as a sub 50 msec IP multicast failover solution.
The mere fact that by operating at the IP layer, different solutions
for IP unicast and multicast are required makes them more difficult
to operate, they typically require more expensive hardware and
therefore most often, they are not even available on the target
equipment, such as [RFC7490] with IP repair tunnels for IP unicast or
[RFC7431] for IP multicast.
Likewise, IEEE Time Sensitive Networking mechanisms would require an
L2 topology that can not simply be replaced by an L3 topology. L2
sub-topologies can also significantly reduce the cost of deployment.
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The following subsections give examples of the type of network and
use-cases in which subnets with asserts have been observerd or are
expected to require scaling as provided by this specification.
A.1. Enterprise network
When an Enterprise network is connected through a layer-2 network,
the intra-enterprise runs layer-3 PIM multicast. The different sites
of the enterprise are equivalent to the PIM connection through the
shared LAN network. Depending upon the locations and amount of
groups there could be many asserts on the first-hop routers.
A.2. Video surveillance
Video surveillance deployments have migrated from analog based
systems to IP-based systems oftentimes using multicast. In the
shared LAN network deployments, when there are many cameras streaming
to many groups there may be issues with many asserts on first-hop
routers.
A.3. Financial Services
Financial services extensively rely on IP Multicast to deliver stock
market data and its derivatives, and current multicast solution PIM
is usually deployed. As the number of multicast flows grow, there
are many stock data with many groups may result in many PIM asserts
on a shared LAN network from publisher to the subscribers.
A.4. IPTV broadcast Video
PIM DR deployments are often used in host-side network for IPTV
broadcast video services. Host-side access network failure scenario
may be benefitted by assert packing when many groups are being used.
According to [RFC7761] the DR will be elected to forward multicast
traffic in the shared access network. When the DR recovers from a
failure, the original DR starts to send traffic, and the current DR
is still forwarding traffic. In the situation multicast traffic
duplication maybe happen in the shared access network and can trigger
the assert progress.
A.5. MVPN MDT
As described in [RFC6037], MDT (Multicast Distribution Tree) is used
as tunnels for MVPN. The configuration of multicast-enabled VRF (VPN
routing and forwarding) or interface that is in a VRF changing may
cause many assert packets to be sent in a same time.
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A.6. Special L2 services
Additionally, future backhaul, or fronthaul, networks may want to
connect L3 across an L2 underlay supporting Time Sensitive Networks
(TSN). The infrastructure may run DetNet over TSN. These transit L2
LANs would have multiple upstreams and downstreams. This document is
taking a proactive approach to prevention of possible future assert
issues in these types of environments.
Authors' Addresses
Yisong Liu (editor)
China Mobile
China
Email: liuyisong@chinamobile.com
Toerless Eckert (editor)
Futurewei
United States of America
Email: tte@cs.fau.de
Mike McBride
Futurewei
United States of America
Email: michael.mcbride@futurewei.com
Zheng(Sandy) Zhang
ZTE Corporation
China
Email: zhang.zheng@zte.com.cn
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