Internet DRAFT - draft-ietf-pim-ecmp
draft-ietf-pim-ecmp
Network Working Group Yiqun Cai
Internet-Draft Microsoft
Intended status: Standards Track Liming Wei
Expires: February 1, 2013 Heidi Ou
Cisco Systems, Inc.
Vishal Arya
Sunil Jethwani
DIRECTV Inc.
July 31, 2012
Protocol Independent Multicast ECMP Redirect
draft-ietf-pim-ecmp-05.txt
Abstract
A Protocol Independent Multicast (PIM) router uses the Reverse Path
Forwarding (RPF) procedure to select an upstream interface and router
to build forwarding state. When there are equal cost multiple paths
(ECMP), existing implementations often use hash algorithms to select
a path. Such algorithms do not allow the spread of traffic among the
ECMPs according to administrative metrics. This usually leads to
inefficient or ineffective use of network resources. This document
introduces the ECMP Redirect, a mechanism to improve the RPF
procedure over ECMPs. It allows ECMP path selection to be based on
administratively selected metrics, such as data transmission delays,
path preferences and routing metrics.
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
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This Internet-Draft will expire on February 1, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Protocol Specification . . . . . . . . . . . . . . . . . . . . 6
5.1. Sending ECMP Redirect . . . . . . . . . . . . . . . . . . 6
5.2. Receiving ECMP Redirect . . . . . . . . . . . . . . . . . 7
5.3. Transient State . . . . . . . . . . . . . . . . . . . . . 7
5.4. Interoperability . . . . . . . . . . . . . . . . . . . . . 8
5.5. Packet Format . . . . . . . . . . . . . . . . . . . . . . 8
5.5.1. PIM ECMP Redirect Hello Option . . . . . . . . . . . . 8
5.5.2. PIM ECMP Redirect Format . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative Reference . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
A PIM router uses the RPF procedure to select an upstream interface
and a PIM neighbor on that interface to build forwarding state. When
there are equal cost multiple paths (ECMP) upstream, existing
implementations often use hash algorithms to select a path. Such
algorithms do not allow the spread of traffic among the ECMP
according to administrative metrics. This usually leads to
inefficient or ineffective use of network resources. This document
introduces the ECMP Redirect, a mechanism to improve the RPF
procedure over ECMP. It allows ECMP path selection to be based on
administratively selected metrics, such as data transmission delays,
path preferences and routing metrics, or a combination of metrics.
ECMPs are frequently used in networks to provide redundancy and to
increase available bandwidth. A PIM router selects a path in the
ECMP based on its own implementation specific choice. The selection
is a local decision. One way is to choose the PIM neighbor with the
highest IP address, another is to pick the PIM neighbor with the best
hash value over the destination and source addresses.
While implementations supporting ECMP have been deployed widely, the
existing RPF selection methods have weaknesses. The lack of
administratively effective ways to allocate traffic over alternative
paths is a major issue. For example, there is no straightforward way
to tell two downstream routers to select either the same or different
RPF neighbor routers for the same traffic flows.
With the ECMP Redirect mechanism introduced here, the upstream
routers use a PIM ECMP Redirect message to instruct the downstream
routers on how to tie-break among the upstream neighbors. The PIM
ECMP Redirect message conveys the tie-break information based on
metrics selected administratively.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses terms defined in [RFC4601] to describe actions
taken by PIM routers.
The following terms have special significance for ECMP Redirect:
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o Equal Cost Multiple Path (ECMP). In this document, the term ECMP
refers to parallel, single-hop, equal cost links between adjacent
nodes.
o ECMP Bundle. An ECMP bundle is a set of PIM enabled interfaces on
a router, where all interfaces belonging to the same bundle share
the same routing metric. The next hops for the ECMP are all 1 hop
away.
There can be one or more ECMP bundles on any router, while one
individual interface can only belong to a single bundle. ECMP
bundles are created on a router via configuration.
o RPF. RPF stands for Reverse Path Forwarding.
o Upstream. Towards the root of the multicast forwarding tree. An
upstream router refers to a router that is forwarding, or
potentially capable of forwarding data packets onto interfaces in
an ECMP bundle.
When there are multiple routers forwarding packets onto interfaces
in the ECMP bundle, all these routers are called upstream routers.
o Downstream. Away from the root of the multicast forwarding tree.
A downstream router is a router that uses an interface in the ECMP
bundle as an RPF interface for a multicast forwarding entry.
3. Overview
The existing PIM Assert mechanism allows the upstream router to
detect the existence of multiple forwarders for the same multicast
flow onto the same downstream interface. The upstream router sends a
PIM Assert message containing a routing metric for the downstream
routers to use for tie-breaking among the multiple upstream
forwarders on the same RPF interface.
With ECMP interfaces between the downstream and upstream routers, the
PIM ECMP Redirect mechanism works in a similar way, but extends the
ability to resolve the selection of forwarders among different
interfaces in the ECMP.
When a PIM router downstream of the ECMP interfaces creates a new
(*,G) or (S,G) entry, it will populate the RPF interface and RPF
neighbor information according to the rules specified by [RFC4601].
This router will send its initial PIM Joins to that RPF neighbor.
When the RPF neighbor router receives the Join message and finds that
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the receiving interface is one of the ECMP interfaces, it will check
if the same flow is already being forwarded out of another ECMP
interface. If so, this RPF neighbor router will send a PIM ECMP
Redirect message onto the interface the Join was received on. The
PIM ECMP Redirect message contains the address of the desired RPF
neighbor, an interface ID [RFC6395], along with other parameters used
as tie breakers. In essence, a PIM ECMP Redirect message is sent by
an upstream router to notify downstream routers to redirect PIM Joins
to the new RPF neighbor via a different interface. When the
downstream routers receive this message, they SHOULD trigger PIM
Joins toward the new RPF neighbor specified in the packet.
This PIM ECMP Redirect message has similar functions as the existing
PIM Assert message,
1. It is sent by an upstream router;
2. It is used to influence the RPF selection by downstream routers;
3. A tie breaker metric is used.
However, the existing Assert message is used to select an upstream
router within the same multi-access network (such as a LAN) while the
Redirect message is used to select both a network and an upstream
router.
One advantage of this design is that the control messages are only
sent when there is need to "re-balance" the traffic. This reduces
the amount of control traffic.
4. Applicability
The use of ECMP Redirect applies to shared trees or source trees
built with procedures described in [RFC4601]. The use of ECMP
Redirect in "Protocol Independent Multicast - Dense Mode" [RFC3973]
or in "Bidirectional Protocol Independent Multicast" [RFC5015] is not
considered in this document.
The enhancement described in this document can be applicable to a
number of scenarios. For example, it allows a network operator to
use ECMP paths and have the ability to perform load splitting based
on bandwidth. To do this, the downstream routers perform RPF
selection with bandwidth instead of IP addresses as a tie breaker.
The ECMP Redirect mechanism assures that all downstream routers
select the desired network link and upstream router whenever
possible. Another example is for a network operator to impose a
transmission delay limit on certain links. The ECMP Redirect
mechanism provides a means for an upstream router to instruct a
downstream router to choose a different RPF path.
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This specification does not dictate the scope of applications of this
mechanism.
5. Protocol Specification
5.1. Sending ECMP Redirect
ECMP Redirects are sent by an upstream router in a rate limited
fashion, under the following conditions,
o It detects a PIM Join on a non-desired outgoing interface; or
o It detects multicast traffic on a non-desired outgoing interface.
In both cases, an ECMP Redirect is sent to the non-desired interface.
An outgoing interface is considered "non-desired" when,
o The upstream router is already forwarding the same flow out of
another interface belonging to the same ECMP bundle;
o The upstream router is not forwarding the flow yet out any
interfaces of the ECMP bundle, but there is another interface with
more desired attributes.
An upstream router MAY choose not to send ECMP Redirects if it
becomes aware that some of the downstream routers are unreachable via
some links in ECMP bundle.
An upstream router uses the "Neighbor Address" or the "Interface ID"
field in the ECMP Redirect message to indicate the interface it wants
traffic to be directed to. This Neighbor Address MUST be associated
with an interface in the same ECMP bundle as the ECMP Redirect
message's outgoing interface. If the Interface ID field is ignored,
this Neighbor Address field uniquely identifies a LAN and an upstream
router to which a downstream router SHOULD redirect its Join
messages, and an ECMP Redirect message MUST be discarded if the
Neighbor Address field in the message does not match the cached
neighbor address.
The Interface ID field is used in IPv4 when one or more RPF neighbors
in the ECMP bundle are unnumbered, or in IPv6 where link local
addresses are in use. For other IPv4 usage, this field is zero'ed
when sent, and ignored when received. If the "Router ID" part of the
Interface ID is zero, the field MUST be ignored. See [RFC6395] for
details of its assignment and usage in PIM Hellos. If the Interface
ID is not ignored, the receiving router of this message MUST use the
Interface ID, instead of Neighbor Address, to identify the new RPF
neighbor, and an ECMP Redirect message MUST be discarded if the
Interface ID field in the message does not match the cached interface
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ID.
5.2. Receiving ECMP Redirect
When a downstream router receives an ECMP Redirect, and detects that
the desired RPF path from its upstream router's point of view is
different from its current one, it should choose to join the newly
suggested path and prune from the current path. The exact order of
such actions is implementation specific.
If a downstream router receives multiple ECMP Redirects sent by
different upstream routers, it SHOULD use the Preference, Metric, or
other fields as specified below, as the tie breakers to choose the
most preferred RPF interface and neighbor. The tie break procedure
is the same as that used in PIM Assert processing described by
[RFC4601].
If an upstream router receives an ECMP Redirect, it SHOULD NOT change
its forwarding behavior even if the ECMP Redirect makes it a less
preferred RPF neighbor on the receiving interface.
5.3. Transient State
During a transient network outage with a single link cut in an ECMP
bundle, a downstream router may lose connection to its RPF neighbor
and the normal ECMP Redirect operation may be interrupted
temporarily. In such an event, the following actions are
RECOMMENDED.
The downstream router SHOULD select a new RPF neighbor. Among all
ECMP upstream routers, the one on the LAN the previous RPF neighbor
resided on is preferred.
If there is no upstream router reachable on the LAN the previous RPF
neighbor resided on, the downstream router will select a new RPF
neighbor on a different LAN. Among all ECMP upstream routers, the
one that served as RPF neighbor before the link failure is preferred.
Such a router can be identified by the Router ID, which is part of
the Interface ID in the PIM ECMP Redirect Hello option.
During normal ECMP Redirect operations, when PIM Joins for the same
(*,G) or (S,G) are received on a different LAN, an upstream router
will send ECMP Redirect to prune the non-preferred LAN. Such ECMP
Redirects during partial network outage can be suppressed if the
upstream router decides that the non-preferred PIM Join is from a
router that is not reachable via the preferred LAN. This check can
be performed by retrieving the downstream's Router ID, using the
source address in the PIM Join, and searching neighbors on the
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preferred LAN for one with the same router ID.
5.4. Interoperability
If a PIM router supports this specification, it MUST send the Hello
option ECMP-Redirect-Supported TLV in its PIM Hello messages.
A PIM router sends ECMP Redirects on an interface only when it
detects that all neighbors on that interface have sent this Hello
option. If a PIM router detects that any of its neighbors on an ECMP
bundle does not support this Hello option, it SHOULD NOT send ECMP
Redirects to any interface in that bundle, however, it SHOULD still
process any ECMP Redirects received from any interface in the same
bundle.
If a PIM router does not support this specification, it will ignore
the ECMP-Redirect-Supported TLV in Hello and ECMP Redirects in PIM
packets, which it receives.
5.5. Packet Format
5.5.1. PIM ECMP Redirect Hello Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 32 | Length = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ECMP Redirect Hello Option
Type: 32, temporarily assigned by IANA
Length: 0
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5.5.2. PIM ECMP Redirect 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 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+- ............ Interface ID ........... -+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-- ... Metric ... -+-+-+-+-+-+-+-+-+
| |
+- .. Metric .. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 2: ECMP Redirect Message Format
PIM Ver: See section 4.9 in [RFC4601].
Type: To be assigned by IANA.
Reserved: See section 4.9 in [RFC4601].
Checksum: See section 4.9 in [RFC4601].
Group Address (64/160 bits): Encoded-Group address as specified in
section 4.9.1 of [RFC4601].
Source Address (48/144 bits): Encoded-Unicast address as specified
in section 4.9.1 of [RFC4601].
Neighbor Address (32/128 bits): Address of desired upstream
neighbor where the downstream receiver redirects PIM Joins.
Interface ID (64 bits): See [RFC6395] for details.
Preference (8 bits): The first tie breaker when ECMP Redirects from
multiple upstream routers are compared against each other.
Numerically smaller value is preferred. A reserved value (15) is
used to indicate the metric value following the "Preference" field
is a Network Time Protocol (NTP) timestamp, encoded in the format
specified in [RFC5905], taken at the moment the sending router
started to forward out of this interface.
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Metric (64 bits): The second tie breaker if the "Preference" values
are the same. Numerically smaller metric is preferred. This
"Metric" can contain path parameters defined by users. When both
"Preference" and "Metric" values are the same, "Neighbor Address"
or "Interface ID" field is used as the third tie-breaker,
depending on which field is used to identify the RPF neighbor, and
the bigger value wins.
6. IANA Considerations
A PIM Hello Option Type is requested to be assigned to the PIM ECMP
Redirect Hello Option. According to [HELLO-OPT], 32 (0x20) has been
temporarily assigned by IANA to the "PIM ECMP Redirect Hello Option
Type".
A PIM Message Type (TBD) is requested to be assigned to the ECMP
Redirect message. According to [RFC6166], the next available Type
value is 11 (0xB).
7. Security Considerations
Security of the ECMP Redirect is only guaranteed by the security of
the PIM packet, the security considerations for PIM Assert packets as
described in [RFC4601] apply here. Spoofed ECMP Redirect packets may
cause the downstream routers to send PIM Joins to an undesired
upstream router, and trigger more ECMP Redirect messages. Security
considerations for PIM packets described in [RFC4601] also apply to
the new hello option defined here.
8. Acknowledgement
The authors would like to thank Apoorva Karan for helping with the
original idea, Eric Rosen, Isidor Kouvelas, Toerless Eckert, Stig
Venaas, Jeffrey Zhang, Bill Atwood and Adrian Farrel for their review
comments.
9. References
9.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
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"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
9.2. Informative References
[RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, January 2005.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, October 2007.
[RFC6166] Venaas, S., "A Registry for PIM Message Types", RFC 6166,
April 2011.
[RFC6395] Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
Hello Option for PIM", RFC 6395, October 2011.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2012.
[HELLO-OPT]
IANA, "PIM Hello Options", PIM-HELLO-OPTIONS per [RFC4601]
http://www.iana.org/assignments/pim-hello-options,
October 2011.
Authors' Addresses
Yiqun Cai
Microsoft
1065 La Avenida
Mountain View, CA 94043
USA
Email: yiqunc@microsoft.com
Liming Wei
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
USA
Email: lwei@cisco.com
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Heidi Ou
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
USA
Email: hou@cisco.com
Vishal Arya
DIRECTV Inc.
2230 E Imperial Hwy
El Segundo, CA 90245
USA
Email: varya@directv.com
Sunil Jethwani
DIRECTV Inc.
2230 E Imperial Hwy
El Segundo, CA 90245
USA
Email: sjethwani@directv.com
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