PIM WG Zheng. Zhang
Internet-Draft ZTE Corporation
Intended status: Standards Track Fangwei. Hu
Expires: February 16, 2020 Individual
Benchong. Xu
ZTE Corporation
Mankamana. Mishra
Cisco Systems
August 15, 2019
PIM DR Improvement
draft-ietf-pim-dr-improvement-08.txt
Abstract
Protocol Independent Multicast - Sparse Mode (PIM-SM) is widely
deployed multicast protocol. As deployment for PIM protocol is
growing day by day, user expects lower traffic loss and faster
convergence in case of any network failure. This document provides
an extension to the existing protocol which would improve the
stability of the PIM protocol with respect to traffic loss and
convergence time when the PIM DR role changes.
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-
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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 February 16, 2020.
Copyright Notice
Copyright (c) 2019 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
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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PIM hello message format . . . . . . . . . . . . . . . . . . 4
3.1. DR Address Option format . . . . . . . . . . . . . . . . 4
3.2. BDR Address Option format . . . . . . . . . . . . . . . . 4
4. Protocol Specification . . . . . . . . . . . . . . . . . . . 5
4.1. Deployment Choice . . . . . . . . . . . . . . . . . . . . 6
4.2. Election Algorithm . . . . . . . . . . . . . . . . . . . 6
4.3. Sending Hello Messages . . . . . . . . . . . . . . . . . 7
4.4. Receiving Hello Messages . . . . . . . . . . . . . . . . 8
4.5. The treatment . . . . . . . . . . . . . . . . . . . . . . 9
4.6. Sender side . . . . . . . . . . . . . . . . . . . . . . . 10
5. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Multicast technology is used widely. Many modern technologies, such
as IPTV, Net-Meeting, use PIM-SM to facilitate multicast service.
There are many events that will influence the quality of multicast
services. Like the change of unicast routes, the change of the PIM-
SM DR may cause the loss of multicast packets too.
After a DR on a shared-media LAN goes down, other routers will elect
a new DR after the expiration of Hello-Holdtime. The default value
of Hello-Holdtime is 105 seconds. Although the minimum Hello
interval can be adjusted to 1 second, the Hello-Holdtime is 3.5 times
Hello interval. Thus, the detection of DR Down event cannot be
guaranteed in less than 3.5 seconds. And it is too long for modern
multicast services. Still, many multicast packets will be lost. The
quality of IPTV and Net-Meeting will be influenced.
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\ /
\ /
------- -------
| A | | B |
------- -------
| DR |
| |
------- -------
| SW |-----------------------------| SW |
------- -------
| |
Figure 1: An example of multicast network
For example, there are two routers on one LAN. Two SWs (Layer 2
switches) provide shared-media LAN connection. RouterA is elected as
DR. When RouterA goes down, the multicast packets are discarded
until the RouterB is elected to DR and RouterB imports the multicast
flows successfully.
We suppose that there is only a RouterA in the LAN at first in
Figure 1. RouterA is the DR which is responsible for forwarding
multicast flows. When RouterB connects to the LAN, RouterB will be
elected as DR because of its higher priority. RouterA will stop
forwarding multicast packets. The multicast flows will not recover
until RouterB pulls the multicast flows after it is elected to DR.
So if we want to increase the stability of DR, carrying DR/ BDR role
information in PIM hello packet is a feasible way to show the DR/ BDR
roles explicitly. It avoids the confusion caused by newcomers which
have a higher priority.
1.1. Keywords
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.
2. Terminology
Backup Designated Router (BDR): Like DR (Designated Router), a BDR
which acts on behalf of directly connected hosts in a shared-media
LAN. But BDR must not forward the flows when DR works normally.
When DR goes down, the BDR will forward multicast flows immediately.
A single BDR MUST be elected per interface like the DR.
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Designated Router Other (DROther): A router which is neither DR nor
BDR.
3. PIM hello message format
The PIM hello message format is defined in [RFC7761]. In this
document, we define two new option values which are including Type,
Length, and Value.
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
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| Hello message format |
| |
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| OptionType + OptionLength |
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| OptionValue |
| |
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Figure 2: Hello message format
3.1. DR Address Option format
o OptionType : The value is TBD1.
o OptionLength: If the IP version of PIM message is IPv4, the
OptionLength is 4 octets. If the IP version of PIM message is
IPv6, the OptionLength is 16 octets.
o OptionValue: The OptionValue is IP address of DR. If the IP
version of PIM message is IPv4, the value is the IPv4 address of
DR. If the IP version of PIM message is IPv6, the value is the
IPv6 address of DR.
3.2. BDR Address Option format
o OptionType : The value is TBD2.
o OptionLength: If the IP version of PIM message is IPv4, the
OptionLength is 4 octets. If the IP version of PIM message is
IPv6, the OptionLength is 16 octets.
o OptionValue: The OptionValue is IP address of BDR. If the IP
version of PIM message is IPv4, the value is the IPv4 address of
BDR. If the IP version of PIM message is IPv6, the value is the
IPv6 address of BDR.
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4. Protocol Specification
Carrying DR/ BDR role information in PIM hello packet is a feasible
way to keep the stability of DR. It avoids the confusion caused by
newcomers which have a higher priority. So there are some changes in
PIM hello procedure and interface state machine.
A new router starts to send hello messages with the values of DR and
BDR are all set to 0 after its interface is enabled in PIM on a
shared-media LAN. When the router receives hello messages from other
routers on the same shared-media LAN, the router will check if the
value of DR is filled. If the value of DR is filled with IP address
of the router which is sending hello messages, the router will store
the IP address as the DR address of this interface.
Then the new router compares the priority and IP address itself to
the stored information of DR and BDR according to the algorithm of
[RFC7761]. If the new router notices that it is better to be DR than
the current DR or BDR, the new router will make itself the BDR, and
send new hello messages with its IP address as BDR and current DR.
If the router notices that the current DR has the highest priority in
the shared-media LAN, but the current BDR is set to 0.0.0.0 if IPv4
addresses are in use or 0:0:0:0:0:0:0:0/128 if IPv6 addresses are in
use in the received hello messages (To simplify, we use 0x0 in
abbreviation in following parts of the draft), or the current BDR is
not better than the new router, the new router will elect itself to
BDR. If the router notices that it is not better to be DR than
current DR and BDR, the router will follow the current DR and BDR.
When the new router becomes the new BDR, the router will join the
current multicast groups, and import multicast flows from upstream
routers. But the BDR must not forward the multicast flows to avoid
the duplicate multicast packets in the shared-media LAN. The new
router will monitor the DR. The method that BDR monitors the DR may
be Bidirectional Forwarding Detection (BFD) for Multi-point Networks
and Protocol Independent Multicast [I-D.ietf-pim-bfd-p2mp-use-case]
technology, BFD (Bidirectional Forwarding Detection) [RFC5880]
technology, or other ways that can be used to detect link/node
failure quickly. When the DR becomes unavailable because of the down
or other reasons, the BDR will forward multicast flows immediately.
BFD for PIM function defined in [I-D.ietf-pim-bfd-p2mp-use-case], or
asynchronous mode defined in BFD [RFC5880] are suggested to be used
for the DR failure detection. BDR monitors DR after the BFD session
between DR and BDR is established. For example, an aggressive BFD
session that achieves a detection time of 300 milliseconds, by using
a transmit interval of 100 milliseconds and a detect multiplier of 3.
So BDR can replace DR to forward flows when DR goes down within sub
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second. The other BFD modes can also be used to monitor the failure
of DR, the network administrator should choose the most suitable
function.
4.1. Deployment Choice
DR / BDR election SHOULD be handled in two ways. Selection of which
procedure to use would be totally dependent on deployment scenario.
1. The algorithm defined in [RFC7761] should be used if it is ok to
adopt with new DR as and when they are available, and the loss caused
by DR changing is acceptable.
2. If the deployment requirement is to have minimum packets loss
when DR changing, the mechanism defined in this draft should be used.
That is, if the new router notices that it is better to be DR than
the current DR or BDR, the new router will make itself the BDR, and
send new hello message with its IP address as BDR and current DR.
According to section 4.9.2 defined in [RFC7761], the device receives
unknown options Hello packet will ignore it. So the new extension
defined in this draft will not influence the stability of neighbor.
But if the router which has the ability defined in this draft
receives non-DR/BDR capable Hello messages defined in [RFC7761], the
router MAY stop sending DR/BDR capable Hello messages in the LAN and
go back to use the advertisement and election algorithm defined in
[RFC7761].
4.2. Election Algorithm
The DR and BDR election is according to the rules defined below, the
algorithm is similar to the DR election defined in [RFC2328].
(1) Note the current values for the network's Designated Router and
Backup Designated Router. This is used later for comparison
purposes.
(2) Calculate the new Backup Designated Router for the network as
follows. The router that has not declared itself to be Designated
Router is eligible to become Backup Designated Router. The one which
has the highest priority will be chosen to be Backup Designated
Router. In case of a tie, the one having the highest primary address
is chosen.
(3) Calculate the new Designated Router for the network as follows.
If one or more of the routers have declared themselves Designated
Router (i.e., they are currently listing themselves as Designated
Router in their Hello Packets) the one having highest Router Priority
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is declared to be Designated Router. In case of a tie, the one
having the highest primary address is chosen. If no routers have
declared themselves Designated Router, assign the Designated Router
to be the same as the newly elected Backup Designated Router.
(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated Router
or no longer the Backup Designated Router, repeat steps 2 and 3, and
then proceed to step 5. For example, if Router X is now the
Designated Router, when step 2 is repeated X will no longer be
eligible for Backup Designated Router election. Among other things,
this will ensure that no router will declare itself both Backup
Designated Router and Designated Router.
(5) As a result of these calculations, the router itself may now be
Designated Router or Backup Designated Router.
The reason behind the election algorithm's complexity is the desire
for the DR stability.
The above procedure may elect the same router to be both Designated
Router and Backup Designated Router, although that router will never
be the calculating router (Router X) itself. The elected Designated
Router may not be the router having the highest Router Priority. If
Router X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a Designated
Router will be selected in the above procedure. Note also that if
Router X is the only attached router that is eligible to become
Designated Router, it will select itself as Designated Router and
there will be no Backup Designated Router for the network.
4.3. Sending Hello Messages
According to Section 4.3.1 in [RFC7761], when a new router's
interface is enabled in PIM protocol, the router sends Hello messages
with the values of DR and BDR are filled with 0x0. Then the
interface is in Waiting state and starts the hold-timer which is
equal to the Neighbor Liveness Timer. When the timer is expired, the
interface will elect the DR and BDR according to the DR election
rules.
When a new router sets itself BDR after receives hello messages from
other routers, the router sends hello messages with the value of DR
is set to the IP address of current DR and the value of BDR is set to
the IP address of the router itself.
A current BDR MUST set itself DROther after it receives Hello
messages from other router which is eligible to be BDR/DR, the router
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will send hello messages with the value of DR is set to current DR
and the value of BDR is set to the new BDR.
DR newcomer
\ /
----- ----- -----
| A | | B | | C |
----- ----- -----
| | |
| | |
------------------------------------------- LAN
Figure 3
For example, there is a stable LAN that includes RouterA and RouterB.
RouterA is the DR which has the highest priority. RouterC is a
newcomer. RouterC sends hello packet with the DR and BDR are all set
to zero.
If RouterC cannot send hello packet with the DR/BDR capability,
Router C MAY send the hello packet according to the rule defined in
[RFC7761].
If deployment requirement is to adopt with a new DR when it is
available, a new router with the highest priority or the highest IP
address sends hello packet with DR and BDR are all set to zero at
first. It sends hello packet with itself set to DR after it finish
join all the existing multicast groups. Then current DR compares
with the new router, the new router will be the final DR.
4.4. Receiving Hello Messages
When the values of DR and BDR which are carried by hello messages
received are all set to 0x0, the router MUST elect the DR using
procedure defined in DR election algorithm after the hold-timer
expires. And elect a new BDR which is the best choice except DR.
The election cases can be executed as follows:
In case the value of DR which is carried by received hello messages
is not 0x0, and the value of BDR is set to 0x0, when the hold-timer
expires there is no hello packet from other router is received, the
router will elect itself to BDR.
In case either of the values of DR and BDR that are carried by
received hello messages is greater than 0x0. The router will mark
the current DR, and compare itself with the BDR in the message. When
the router notices that it is better to be DR than the current BDR.
The router will elect itself to the BDR.
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When a router receives a new hello message with the values of DR and
BDR are set to 0x0. The router will compare the new router with
current information. If the router noticed that the new router is
better to be DR than itself, or the new router is better to be BDR
than the current BDR, the router will set the BDR to the new router.
When current DR receives hello packet with the value of DR is set
larger than zero, the algorithm defined in section 4.2 can be used to
select the final DR.
As illustrated in Figure 3, after RouterC sends hello packet, RouterC
will not elect the DR until hold-timer expired. During the period,
RouterC should receive the hello packets from RouterA and RouterB.
RouterC accepts the result that RouterA is the DR. In case RouterC
has the lowest priority than RouterA and RouterB, RouterC will also
accept that Router B is the BDR. In case RouterC has the
intermediate priority among the three routers, RouterC will treat
itself as new BDR after the hold-timer expired. In case RouterC has
the highest priority among the three routers, RouterC will treat
RouterA which is the current DR as DR, and RouterC will treat itself
as the new BDR. If the network administrator thinks that RouterC
should be the new DR, the DR changing should be triggered manually.
That is RouterC will be elected as DR after it sends hello message
with DR is set to RouterC itself.
Exception: In case RouterC receives only the hello packet from
RouterA during the hold-timer period, when the hold-timer expired,
RouterC treats RouterA as DR, and RouterC treats itself as BDR. In
case RouterC only receives the hello packet from RouterB during the
hold-timer period, RouterC will compare the priority between RouterB
and itself to elect the new DR. In these situations, some interfaces
or links go wrong in the LAN.
4.5. The treatment
If all the routers on a shared-media LAN have started working at the
same time, then the election result of DR is same as the definition
in [RFC7761]. And all the routers will elect a BDR which is next
best to DR. The routers in the network MUST store the DR and BDR.
The hello messages sent by all the routers are the same with the
value of DR and BDR are all set. When a new router is activated on
the shared-media LAN and receives hello messages from other routers
with the value of DR is already set. The new router will not change
the current DR even if it is superior to the current DR. If the new
router is superior to current BDR, the new router will replace the
current BDR.
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When the routers receive a hello message from a new router, the
routers compare the new router and all the other routers on the LAN.
If the new router is superior to the current BDR, the new router will
be the new BDR. Then the "old" BDR will send the Prune message to
upstream routers.
As a result, the BDR is the one which has the highest priority except
for DR. Once the DR is elected, the DR will not change until it
fails or be manually adjusted. Once the DR and BDR are elected, the
routers in the network MUST store the address of DR and BDR.
4.6. Sender side
DR/BDR function is also used in source side that multiple routers and
source is in a same shared-media LAN. The algorithm is the same as
the receiver side. Only the BDR need not build multicast tree from a
downstream router.
5. Compatibility
If the LAN is a hybrid network that there are some routers which
support DR/BDR capability and the other routers which do not support
DR/BDR capability. All the routers MUST go backward to use the
election algorithm defined in [RFC7761]. And the values of DR and
BDR carried in hello message MUST be set to zero. That is once a
router sends hello messages with no DR/BDR options, the DR election
MUST go backward to the definition in [RFC7761].
If the routers find that all the routers in the LAN support DR/BDR
capability by the hello messages with DR/BDR options set, they MUST
elect DR and BDR according the algorithm defined in this document.
And the routers MUST send hello messages with correct DR/BDR options
set.
In case there is only one router which does not support DR/BDR
capability in a shared-media LAN, the other routers in the LAN send
hello messages with the values of DR and BDR are set to zero, the
router which does not support DR/BDR capability ignores the options.
All the routers elect DR according to the algorithm defined in
[RFC7761]. When the router which does not support DR/BDR capability
goes away, the routers in the LAN MUST elect DR/BDR according to the
algorithm defined in this document, and send hello messages with
correct DR/BDR options set.
This draft allows DR election to be sticky by not unnecessarily
changing the DR when routers go down or come up. This is done by
introducing new PIM Hello options. Both this draft, and the draft
[I-D.mankamana-pim-bdr], introduce a backup DR. The latter draft
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does this without introducing new options, but does not consider the
sticky behavior.
6. Security Considerations
If an attacker which has the highest priority participates in the DR
election when a shared-media LAN starts to work, it will be elected
as DR, but it may not forward flows to receivers. And the attacker
remains DR position even if a legal router which has a higher
priority joins the LAN.
If an attacker is a newcomer which has a higher priority than the
existed BDR, it will be elected as the new BDR, but it may not
monitor DR, import multicast flows and forward flows to receiver when
DR is down.
In order to avoid these situations, source authentication should be
used to identify the validity of the DR/BDR candidates.
Authentication methods mentioned in section 6 RFC7761 can be used.
And the network administrator should consider the potential BFD
session attack if BFD is used between BDR and DR for DR failure
detection. The security function mentioned in section 9 RFC5880 can
be used.
7. IANA Considerations
IANA is requested to allocate two OptionTypes in TLVs of hello
message: DR Address Option and BDR Address Option. The strings TBD1
and TBD2 will be replaced by the assigned values.
8. Acknowledgements
The authors would like to thank Greg Mirsky, Jake Holland, Stig
Venaas for their valuable comments and suggestions.
9. References
9.1. Normative References
[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>.
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[RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., Jacobson, V., Liu, C., Sharma, P., and L.
Wei, "Protocol Independent Multicast-Sparse Mode (PIM-SM):
Protocol Specification", RFC 2362, DOI 10.17487/RFC2362,
June 1998, <https://www.rfc-editor.org/info/rfc2362>.
[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
[I-D.ietf-pim-bfd-p2mp-use-case]
Mirsky, G. and J. Xiaoli, "Bidirectional Forwarding
Detection (BFD) for Multi-point Networks and Protocol
Independent Multicast - Sparse Mode (PIM-SM) Use Case",
draft-ietf-pim-bfd-p2mp-use-case-02 (work in progress),
July 2019.
[I-D.mankamana-pim-bdr]
mishra, m., "PIM Backup Designated Router Procedure",
draft-mankamana-pim-bdr-02 (work in progress), April 2019.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
Authors' Addresses
Zheng(Sandy) Zhang
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct
Nanjing
China
Email: zhang.zheng@zte.com.cn
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Fangwei Hu
Individual
Shanghai
China
Email: hufwei@gmail.com
Benchong Xu
ZTE Corporation
No. 68 Zijinghua Road, Yuhuatai Distinct
Nanjing
China
Email: xu.benchong@zte.com.cn
Mankamana Mishra
Cisco Systems
821 Alder Drive,
MILPITAS, CALIFORNIA 95035
UNITED STATES
Email: mankamis@cisco.com
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