BESS Working Group P. Brissette, Ed.
Internet-Draft A. Sajassi
Intended status: Standards Track LA. Burdet
Expires: January 7, 2022 Cisco
J. Drake
Juniper
J. Rabadan
Nokia
July 6, 2021
Fast Recovery for EVPN DF Election
draft-ietf-bess-evpn-fast-df-recovery-02
Abstract
Ethernet Virtual Private Network (EVPN) solution provides Designated
Forwarder election procedures for multi-homing Ethernet Segments.
These procedures have been enhanced further by applying Highest
Random Weight (HRW) Algorithm for Designated Forwarded election in
order to avoid unnecessary DF status changes upon a failure. This
draft improves these procedures by providing a fast Designated
Forwarder (DF) election upon recovery of the failed link or node
associated with the multi-homing Ethernet Segment. The solution is
independent of number of EVIs associated with that Ethernet Segment
and it is performed via a simple signaling between the recovered PE
and each PEs in the multi-homing group.
Requirements Language
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 RFC 2119 [RFC2119] and
RFC 8174 [RFC8174].
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
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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 January 7, 2022.
Copyright Notice
Copyright (c) 2021 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Challenges with Existing Solution . . . . . . . . . . . . . . 3
3. DF Election Synchronization Solution . . . . . . . . . . . . 4
3.1. Advantages . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. BGP Encoding . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Note on NTP-based synchronization . . . . . . . . . . . . 6
3.4. Synchronization Scenarios . . . . . . . . . . . . . . . . 7
3.5. Backwards Compatibility . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Normative References . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Ethernet Virtual Private Network (EVPN) solution [RFC7432] is
becoming pervasive in data center (DC) applications for Network
Virtualization Overlay (NVO) and DC interconnect (DCI) services, and
in service provider (SP) applications for next generation virtual
private LAN services.
EVPN solution [RFC7432] describes DF election procedures for multi-
homing Ethernet Segments. These procedures are enhanced further in
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[RFC8584] by applying Highest Random Weight Algorithm for DF election
in order to avoid DF status change unnecessarily upon a link or node
failure associated with the multi-homing Ethernet Segment. This
draft makes further improvement to DF election procedures in
[RFC8584] by providing an option for a fast DF election upon recovery
of the failed link or node associated with the multi-homing Ethernet
Segment. This DF election is achieved independent of number of EVIs
associated with that Ethernet Segment and it is performed via a
simple signaling between the recovered PE and each PE in the multi-
homing group. The solution is based on simple one-way signaling
mechanism.
1.1. Terminology
Provider Edge (PE): A device that sits in the boundary of Provider
and Customer networks and performs encap/decap of data from L2 to
L3 and vice-versa.
Designated Forwarder (DF): A PE that is currently forwarding
(encapsulating/decapsulating) traffic for a given VLAN in and out
of a site.
2. Challenges with Existing Solution
In EVPN technology, multiple PE devices have the ability to encap and
decap data belonging to the same VLAN. In certain situations, this
may cause L2 duplicates and even loops if there is a momentary
overlap of forwarding roles between two or more PE devices, leading
to broadcast storms.
EVPN [RFC7432] currently uses timer based synchronization among PE
devices in redundancy group that can result in duplications (and even
loops) because of multiple DFs if the timer is too short or
blackholing if the timer is too long.
Using ESI label Split Horizon filtering can prevent loops (but not
duplicates), however if there are overlapping DFs in two different
sites at the same time for the same VLAN, the site identifier will be
different upon re-entry of the packet and hence the split horizon
check will fail, leading to L2 loops.
The current state of art [RFC8584] uses the well known HRW
(Highest Random Weight) algorithm to avoid reshuffling of VLANs among
PE devices in the redundancy group upon failure/recovery and thus
reducing the impact of failure/recovery to VLANs not on the
failed/recovered ports. This eliminates loops/duplicates in failure
scenarios.
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However, upon PE insertion or port bring-up, HRW cannot help as a
transfer of DF role need to happen to the newly inserted device/port
while the old DF is still active.
+---------+
+-------------+ | |
| | | |
/ | PE1 |----| | +-------------+
/ | | | MPLS/ | | |---H3
/ +-------------+ | VxLAN/ | | PE10 |
CE1 - | Cloud | | |
\ +-------------+ | |---| |
\ | | | | +-------------+
\ | PE2 |----| |
| | | |
+-------------+ | |
+---------+
Figure 1: CE1 multi-homed to PE1 and PE2.
In the Figure 1, when PE2 is inserted or booted up, PE1 will transfer
DF role of some VLANs to PE2 to achieve load balancing. However,
because there is no handshake mechanism between PE1 and PE2,
duplication of DF roles for a give VLAN is possible. Duplication of
DF roles may eventually lead to L2 loops as well as duplication of
traffic.
Current state of EVPN art relies on a blackholing timer for
transferring the DF role to the newly inserted device. This can
cause the following issues:
* Loops/Duplicates if the timer value is too short
* Prolonged Traffic Blackholing if the timer value is too long
3. DF Election Synchronization Solution
The solution relies on the concept of common clock alignment between
partner PEs participating to a common Ethernet-Segment. The main
idea is to have them all to perform/apply their carving state,
resulting from DF election, at the well-known time.
The DF Election procedure, as described in [RFC7432] and as
optionally signalled in [RFC8584], is applied. All PEs attached to a
given Ethernet-Segment are clock-synchronized; using a networking
protocol for clock synchronization (e.g. NTP, PTP, etc.). Newly
inserted device PE or during failure recovery of a PE, that PE
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communicates the current time to peering partners plus the remaining
peering timer time left. This constitute an "end" or "absolute" time
as seen from local PE. That absolute time is called "Service Carving
Time" (SCT).
A new BGP Extended Community is advertised along with Ethernet-
Segment route (RT-4) to communicate to other partners the Service
Carving Time.
Upon reception of that new BGP Extended Community, partner PEs know
exactly its carving time. The notion of skew is introduced to
eliminate any potential duplicate traffic or loops. They add a skew
(default = -10ms) to the Service Carving Time to enforce this. The
previously inserted PE(s) must carve first, followed shortly(skew) by
the newly insterted PE.
To summarize, all peering PEs carve almost simultaneously at the time
announced by newly added/recovered PE. The newly inserted PE
initiates the SCT, and carves immediately on peering timer expiry.
The previously inserted PE(s) receiving Ethernet-Segment route (RT-4)
with a SCT BGP extended community, carve shortly before Service
Carving Time.
3.1. Advantages
There are multiples advantages of using the approach. Here is a non-
exhaustive list:
- A simple uni-directional signaling is all needed
- Backwards-compatible: PEs supporting only older [RFC7432] shall
simply discard unrecognized new "Service Carving Timestamp" BGP
Extended Community
- Multiple DF Election algorithms can be supported:
* [RFC7432] default ordered list ordinal algorithm (Modulo),
* [RFC8584] highest-random weight, etc.
- Independent of BGP transmission delay regarding Ethernet-Segment
route (RT-4)
- Agnostic of the time synchronization mechanism used (e.g .NTP, PTP,
etc.)
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3.2. BGP Encoding
A new BGP extended community needs to be defined to communicate the
Service Carving Timestamp for each Ethernet Segment.
A new transitive extended community where the Type field is 0x06, and
the Sub-Type is [TBD3] is advertised along with Ethernet Segment
route. Timestamp for expected Service carving is encoded as a
8-octet value as follows:
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=0x06 | Sub-Type(TBD3)| Timestamp Seconds ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Timestamp Seconds | Timestamp Fractional Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This document introduces a new flag called "T" (for Time
Synchronization) to the bitmap field of the DF Election Extended
Community defined in [RFC8584].
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 = 0x06 | Sub-Type(0x06)| RSV | DF Alg | |A| |T| ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Bitmap | Reserved = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
T: This flag is located in bit position 27 as shown above. When set
to 1, it indicates the desire to use Time Synchronization capability
with the rest of the PEs in the ES. This capability is used in
conjunction with the agreed upon DF Type (DF Election Type). For
example if all the PEs in the ES indicated that they have Time
Synchronization capability and they want the DF type be of HRW, then
HRW algorithm is used in conjunction with this capability.
3.3. Note on NTP-based synchronization
The 64-bit timestamp used by NTP protocol consists of a 32-bit part
for seconds and a 32-bit part for fractional second. The timestamp
exchanged uses the NTP epoch of January 1, 1900 [RFC5905]. The use
of a 32-bit seconds and 16-bit fractional seconds yields adequate
precision of 15 microseconds (2^-16 s).
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3.4. Synchronization Scenarios
Let's take Figure 1 as an example where initially PE2 had failed and
PE1 had taken over. This example shows the problem with known
mechanism.
Based on [RFC7432]:
- Initial state: PE1 is in steady-state, PE2 is recovering
- PE2 recovers at (absolute) time t=99
- PE2 advertises RT-4 (sent at t=100) to partner PE1
- PE2, it starts its 3sec peering timer as per RFC7432
- PE1 carves immediately on RT-4 reception, i.e. t=100 + minimal BGP
propagation delay
- PE2 carves at time t=103
[RFC7432] aims of favouring traffic black hole over duplicate traffic
With above procedure, traffic black hole will occur as part of each
PE recovery sequence. The peering timer value (default = 3 seconds)
has a direct effect on the duration of the prolonged blackholing. A
short (esp. zero) peering timer may, however, result in duplicate
traffic or traffic loops.
Based on the Service Carving Time (SCT) approach:
- Initial state: PE1 is in steady-state, PE2 is recovering
- PE2 recovers at (absolute) time t=99
- PE2 advertises RT-4 (sent at t=100) with target SCT value t=103 to
partner PE1
- PE2 starts its 3 second peering timer as per [RFC7432]
- Both PE1 and PE2 carves at (absolute) time t=103
In fact, PE1 should carve slightly before PE2 (skew). The previously
inserted PE2 that is recovering performs both transitions DF to NDF
and NDF to DF per VLANs at the peering timer expiry. Since the goal
is to prevent duplicates, the original PE1, which received the SCT
will apply:
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- DF to NDF transition at t=SCT minus skew where both PEs are NDF for
'skew' amount of time
- NDF to DF transition at t=SCT
It is this split-behaviour which ensures good transition of DF role
with contained amount of loss.
Using SCT approach, the negative effect of the peering timer is
mitigated. Furthermore, the BGP Ethernet-Segment route (RT-4)
transmission delay (from PE2 to PE1) becomes a no-op. The usage of
SCT approach remedies to the exposed problem with the usage of
peering timer. The 3 seconds timer window is shorthen to few
milliseconds.
3.5. Backwards Compatibility
Per redundancy group, for the DF election procedures to be globally
convergent and unanimous, it is necessary that all the participating
PEs agree on the DF Election algorithm to be used. It is, however,
possible that some PEs continue to use the existing modulus based DF
election and do not rely on the new SCT BGP extended community. PEs
running an baseline DF election mechanism shall simply discard
unrecognized new SCT BGP extended community.
A PE can indicate its willingness to support clock-synched carving by
signaling the new 'T' DF Election Capability as well as including the
new Service Carving Time BGP extended community along with the
Ethernet-Segment Route (Type-4). In the case where one or more PEs
attached to the Ethernet-Segment do not signal T=1, all PEs in the
Ethernet-Segment may revert back to the RFC7432 timer approach.
4. Security Considerations
The mechanisms in this document use EVPN control plane as defined in
[RFC7432]. Security considerations described in [RFC7432] are
equally applicable. This document uses MPLS and IP-based tunnel
technologies to support data plane transport. Security
considerations described in [RFC7432] and in [RFC8365] are equally
applicable.
5. IANA Considerations
This document solicits the allocation of the following sub-type in
the "EVPN Extended Community Sub-Types" registry setup by [RFC7153]:
TBD3 Service Carving Timestamp This document
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This document solicits the allocation of the following values in the
"DF Election Capabilities" registry setup by [RFC8584]:
Bit Name Reference
---- ---------------- -------------
3 Time Synchronization This document
6. 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>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC7153] Rosen, E. and Y. Rekhter, "IANA Registries for BGP
Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
March 2014, <https://www.rfc-editor.org/info/rfc7153>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[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>.
[RFC8365] Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
Uttaro, J., and W. Henderickx, "A Network Virtualization
Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
DOI 10.17487/RFC8365, March 2018,
<https://www.rfc-editor.org/info/rfc8365>.
[RFC8584] Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake,
J., Nagaraj, K., and S. Sathappan, "Framework for Ethernet
VPN Designated Forwarder Election Extensibility",
RFC 8584, DOI 10.17487/RFC8584, April 2019,
<https://www.rfc-editor.org/info/rfc8584>.
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Appendix A. Contributors
In addition to the authors listed on the front page, the following
co-authors have also contributed substantially to this document:
Gaurav Badoni
Cisco
Email: gbadoni@cisco.com
Dhananjaya Rao
Cisco
Email: dhrao@cisco.com
Appendix B. Acknowledgements
Authors would like to acknowledge helpful comments and contributions
of Satya Mohanty and Bharath Vasudevan.
Authors' Addresses
Patrice Brissette (editor)
Cisco
Email: pbrisset@cisco.com
Ali Sajassi
Cisco
Email: sajassi@cisco.com
Luc Andre Burdet
Cisco
Email: lburdet@cisco.com
John Drake
Juniper
Email: jdrake@juniper.net
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Jorge Rabadan
Nokia
Email: jorge.rabadan@nokia.com
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