PIM DR Improvement
draft-ietf-pim-dr-improvement-02
The information below is for an old version of the document.
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|---|---|---|---|
| Authors | Zheng Zhang , fangwei hu , BenChong Xu | ||
| Last updated | 2016-12-05 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
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draft-ietf-pim-dr-improvement-02
PIM WG Zheng. Zhang
Internet-Draft Fangwei. Hu
Intended status: Standards Track BenChong. Xu
Expires: June 8, 2017 ZTE Corporation
December 5, 2016
PIM DR Improvement
draft-ietf-pim-dr-improvement-02.txt
Abstract
PIM is worldly deployed multicast protocol. This document will
improve the stability of PIM protocol, decrease the lost of multicast
packets when the PIM DR (Designed Router) is down.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 8, 2017.
Copyright Notice
Copyright (c) 2016 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PIM hello message format . . . . . . . . . . . . . . . . . . 3
3.1. DR Address Option format . . . . . . . . . . . . . . . . 3
3.2. BDR Address Option format . . . . . . . . . . . . . . . . 4
4. The Protocol Treatment . . . . . . . . . . . . . . . . . . . 4
4.1. Election Algorithm . . . . . . . . . . . . . . . . . . . 5
4.2. Sending Hello Messages . . . . . . . . . . . . . . . . . 6
4.3. Receiving Hello Messages . . . . . . . . . . . . . . . . 6
4.4. The treatment . . . . . . . . . . . . . . . . . . . . . . 7
5. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Deployment suggestion . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Multicast technology is used widely. Many modern technology use PIM
technology, such as IPTV, Net-Meeting, and so on. There are many
events that will influence the quality of multicast services. The
change of unicast routes will cause the lost of multicast packets.
The change of DR cause the lost of multicast packets too.
When a DR on a share-media LAN is down, other routers will elect a
new DR until the expiration of Hello-Holdtime. The default value of
Hello-Holdtime is 105 seconds. Although the value of Hello-Holdtime
can be changed by manual, when the DR is down, there are still many
multicast packets will be lost. The quality of IPTV and Net-Meeting
will be influenced.
\ /
\ /
------- -------
| A | | B |
------- -------
| DR |
| |
------- -------
| SW |-----------------------------| SW |
------- -------
| |
Figure 1: An example of multicast network
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For example, there were two routers on one Ethernet. RouterA was
elected to DR. When RouterA is 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 Ethernet at first in
Figure 1. RouterA is the DR which is responsible for forwarding
multicast flows. When RouterB connects the Ethernet, RouterB will be
elected to DR because a higher priority. So RouterA will stop
forwarding multicast packets. The multicast flows will not recover
until RouterB joins the multicast group after it is elected to DR.
2. Terminology
Backup Designated Router (BDR): A shared-media LAN like Ethernet may
have multiple PIM-SM routers connected to it. Except for DR, a
router which will act on behalf of directly connected hosts with
respect to the PIM-SM protocol. But BDR will not forward the flows.
When DR is down, the BDR will forward multicast flows immediately. A
single BDR is elected per interface like the DR.
3. PIM hello message format
In [RFC4601] and [RFC7761], the PIM hello message format was defined.
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 TBD.
o OptionLength: If the network is support IPv4, the OptionLength is
4 octets. If the network is support IPv6, the OptionLength is 16
octets.
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o OptionValue: The OptionValue is IP address of DR. If the network
is support IPv4, the value is IPv4 address of DR. If the network
is support IPv6, the value is IPv6 address of DR.
3.2. BDR Address Option format
o OptionType : The value is TBD.
o OptionLength: If the network is support IPv4, the OptionLength is
4 octets. If the network is support IPv6, the OptionLength is 16
octets.
o OptionValue: The OptionValue is IP address of BDR. If the network
is support IPv4, the value is IPv4 address of BDR. If the network
is support IPv6, the value is IPv6 address of BDR.
4. The Protocol Treatment
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
share-media LAN. When the router receives hello messages from other
routers on the same share-media LAN, the router will check if the
value of DR is filled. If the value of DR is filled with IP address
of router which is sending hello messages, the router will store the
IP address as the DR address of this interface.
Then the new router compare the priority and IP address itself to the
stored IP address of DR and BDR according to the algorithm of
[RFC4601] and [RFC7761]. If the new router notices that it is better
to be DR than the existed DR or BDR. The router will make itself the
BDR, and send new hello messages with its IP address as BDR and
existed DR. If the router notices that the existed DR has the
highest priority in the share-media LAN, but the existed BDR is set
to 0x0 in the received hello messages, or the existed 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 existed DR
and BDR, the router will respect the existed DR and BDR.
When the new router becomes the new BDR, the router will join the
existed multicast groups, import multicast flows from upstream
routers. But the BDR MUST not forward the multicast flows to avoid
the duplicate multicast packets in the share-media LAN. The new
router will monitor the DR. The method that BDR monitors the DR may
be BFD 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.
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4.1. Election Algorithm
The DR and BDR election is according the rules defined below, the
algorithm is similar to the DR election definition 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. Those routers that have not declared themselves to be
Designated Router are eligible to become Backup Designated Router.
The one which have the highest priority will be chosen to be Backup
Designed Router. In case of a tie, the one having the highest Router
ID 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
is declared to be Designated Router. In case of a tie, the one
having the highest Router ID 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
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Designated Router, it will select itself as Designated Router and
there will be no Backup Designated Router for the network.
4.2. Sending Hello Messages
According to Section 4.3.1 in [RFC4601] and [RFC7761], when a new
router's interface is enabled in PIM protocol, the router send hello
messages with the values of DR and BDR are filled with 0x0. Then the
interface is in waiting state and start the hold-timer which is like
the neighbor hold-timer. When the hold-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 receive hello messages from
other routers, the router send hello messages with the value of DR is
set to the IP address of existed DR and the value of BDR is set to
the IP address of the router itself.
When a existed BDR sets itself non DR and non BDR after receive hello
messages from other routers, the router will send hello messages with
the value of DR is set to existed DR and the value of BDR is set to
new BDR.
DR newcomer
\ /
----- ----- -----
| A | | B | | C |
----- ----- -----
| | |
| | |
------------------------------------------- LAN
Figure 3
For example, there is a stable LAN that include RouterA and RouterB.
RouterA is the DR which has the best priority. RouterC is a
newcomer. RouterC sends hello packet with the DR and BDR is all set
to zero.
If RouterC cannot send hello packet with the DR/BDR capability,
Router C may send the hello packet according the rule defined
in[RFC4601] and [RFC7761].
4.3. Receiving Hello Messages
When the values of DR and BDR which are carried by hello messages are
received is all set to 0x0, the router MUST elect the DR due to the
algorithm of [RFC4601] and [RFC7761] after the hold-timer expires.
And elect a new BDR which are the best choice except DR.
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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 the values of DR and BDR that are carried by received hello
messages are all larger than 0x0. The router will mark the existed
DR, and compare itself and the BDR in message. When the router
notice that it is better to be DR than existed BDR. The router will
elect itself to the BDR.
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
existed 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 existed BDR, the router will set the BDR to the new router.
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 existed DR as DR, and RouterC will treat itself
as new BDR. If the network administrator thinks that RouterC should
be new DR, the DR changing should be triggered manually.
Exception: During the hold-timer period, RouterC receives only the
hello packet from RouterA. 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.
4.4. The treatment
When all the routers on a shared-media LAN are start to work on the
same time, the election result of DR is same as [RFC4601] and
[RFC7761]. And all the routers will elect a BDR which is suboptimum
to DR. The routers in the network will store the DR and BDR. The
hello messages sent by all the routers are same with the value of DR
and BDR are all set.
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When a new router start to work on a shared-media LAN and receive
hello messages from other routers that the value of DR is set. The
new router will not change the existed DR even if it is superior to
the existed DR. If the new router is superior to existed BDR, the
new router will replace the existed BDR.
When the routers receive hello message from a new router, the routers
will compare the new router and all the other routers on the LAN. If
the new router is superior to existed BDR, the new router will be new
BDR. Then the old BDR will send prune message to upstream routers.
As a result, the BDR is the one which has the highest priority except
DR. Once the DR is elected, the DR will not change until it fails or
manually adjustment. After the DR and BDR are elected, the routers
in the network will store the address of DR and BDR.
5. Compatibility
If the LAN is a hybrid network that there are some routers which have
DR/BDR capability and the other routers which have not DR/BDR
capability. In order to avoid duplicated multicast flows in the LAN,
all the routers should go backward to use the algorithm defined in
[RFC4601] and [RFC7761].
6. Deployment suggestion
If there are two and more routers on a share-media LAN, and the
multicast services is sensitive to the lost of multicast packets, the
function of DR and BDR defined in this document should be deployed.
7. Security Considerations
For general PIM Security Considerations.
8. IANA Considerations
IANA is requested to allocate OptionTypes in TLVs of hello message.
Include DR and BDR.
9. Normative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
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[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601,
DOI 10.17487/RFC4601, August 2006,
<http://www.rfc-editor.org/info/rfc4601>.
[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, <http://www.rfc-editor.org/info/rfc7761>.
Authors' Addresses
Zheng(Sandy) Zhang
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct
Nanjing
China
Email: zhang.zheng@zte.com.cn
Fangwei Hu
ZTE Corporation
No.889 Bibo Rd
Shanghai
China
Email: hu.fangwei@zte.com.cn
BenChong Xu
ZTE Corporation
No. 68 Zijinghua Road, Yuhuatai Distinct
Nanjing
China
Email: xu.benchong@zte.com.cn
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