Internet DRAFT - draft-ietf-bess-evpn-fast-df-recovery
draft-ietf-bess-evpn-fast-df-recovery
BESS Working Group P. Brissette, Ed.
Internet-Draft A. Sajassi
Updates: 8584 (if approved) LA. Burdet
Intended status: Standards Track Cisco
Expires: 27 September 2023 J. Drake
Juniper
J. Rabadan
Nokia
26 March 2023
Fast Recovery for EVPN Designated Forwarder Election
draft-ietf-bess-evpn-fast-df-recovery-07
Abstract
The Ethernet Virtual Private Network (EVPN) solution provides
Designated Forwarder election procedures for multihomed Ethernet
Segments. These procedures have been enhanced further by applying
Highest Random Weight (HRW) algorithm for Designated Forwarder (DF)
election in order to avoid unnecessary DF status changes upon a
failure. This document improves these procedures by providing a fast
Designated Forwarder election upon recovery of the failed link or
node associated with the multihomed Ethernet Segment. The solution
is independent of the number of EVIs associated with that Ethernet
Segment and it is performed via a simple signaling between the
recovered PE and each of the other PEs in the multihoming 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/.
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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 27 September 2023.
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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Challenges with Existing Solution . . . . . . . . . . . . 3
1.3. Advantages to Proposed Solution . . . . . . . . . . . . . 4
2. DF Election Synchronization Solution . . . . . . . . . . . . 5
2.1. BGP Encoding . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Updates to RFC8584 . . . . . . . . . . . . . . . . . . . 7
3. Synchronization Scenarios . . . . . . . . . . . . . . . . . . 7
3.1. Concurrent Recoveries . . . . . . . . . . . . . . . . . . 9
4. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The 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.
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[RFC7432] describes DF election procedures for multihomed Ethernet
Segments. These procedures are enhanced further in [RFC8584] by
applying Highest Random Weight Algorithm for DF election in order to
avoid unnecessary DF status changes upon a link or node failure
associated with the multihomed Ethernet Segment. This document makes
further improvements to the 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 multihomed 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 of the other PEs in the multihomed
group. The solution is based on simple one-way signaling mechanism.
1.1. Terminology
Designated Forwarder (DF): A PE that is currently forwarding
(encapsulating/decapsulating) traffic for a given VLAN in and out
of a site.
1.2. 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 split-horizon filtering (Section 8.3 of [RFC7432]) 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 updated DF procedures in [RFC8584] use the well known Highest
Random Weight (HRW) algorithm to avoid reshuffling of VLANs among PE
devices in the redundancy group upon failure/recovery. This reduces
the impact to VLANs not assigned to the failed/recovered ports and
eliminates loops or duplicates at failure/recovery events.
However, upon PE insertion or a port being newly added to a
multihomed Ethernet Segment, HRW also cannot help as a transfer of DF
role to the new port must occur while the old DF is still active.
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+---------+
+-------------+ | |
| | | |
/ | PE1 |----| | +-------------+
/ | | | MPLS/ | | |---CE3
/ +-------------+ | VxLAN/ | | PE3 |
CE1 - | Cloud | | |
\ +-------------+ | |---| |
\ | | | | +-------------+
\ | PE2 |----| |
| | | |
+-------------+ | |
+---------+
Figure 1: CE1 multihomed to PE1 and PE2.
In Figure 1, when PE2 is inserted or booted up, PE1 will transfer the
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 given VLAN is possible. Duplication of
DF roles may eventually lead to duplication of traffic as well as L2
loops.
Current EVPN specifications [RFC7432] and [RFC8584] rely on a timer-
based approach 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
1.3. Advantages to Proposed Solution
There are multiples advantages of using the proposed clock-
synchronization approach, namely:
* A simple uni-directional signaling is all that is needed, no
complicated handshake or state machine.
* Solution is backwards-compatible: PEs supporting only older
[RFC7432] shall simply discard the unrecognized new "Service
Carving Timestamp" BGP Extended Community
* Many of the existing DF Election algorithms can be supported:
- [RFC7432] default ordered list ordinal algorithm (Modulo),
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- [RFC8584] highest-random weight, etc.
* Solution is independent of any BGP propagation delay of Ethernet
Segment route (Route Type 4)
* Solution is agnostic of the actual time synchronization mechanism
used (e.g. NTP, PTP, etc.), while normalizing the exchange format
in an NTP-based encoding.
2. DF Election Synchronization Solution
The solution relies on the concept of common clock alignment between
partner PEs participating to a common Ethernet Segment i.e. PE1 and
PE2 in Figure 1. The main idea is to have all peering PEs of that
Ethernet Segment perform DF election, and apply their resulting
carving state, at a same pre-announced 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.). When a
new PE is inserted or an existing PE device, that PE communicates the
current time to peering partners plus the remaining peering timer
time left. This constitutes an "end time" or "absolute time" as seen
from local PE. That absolute time is called "Service Carving Time"
(SCT).
A new BGP Extended Community, the Service Carving Timestamp 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 can
determine exactly the anticipated carving time. The notion of skew
is introduced to eliminate any potential duplicate traffic or loops.
The receiving partner PEs 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.
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2.1. 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 0x0F is advertised along with Ethernet Segment route.
The expected Service Carving Time 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(0x0F)| Timestamp Seconds ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Timestamp Seconds | Timestamp Fractional Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The timestamp exchanged uses the NTP epoch of January 1, 1900
[RFC5905]. The 64-bit timestamp of the NTP protocol consists of a
32-bit part for seconds and a 32-bit part for fractional second:
* Timestamp Seconds: 32-bit NTP seconds are encoded in this field.
* Timestamp Fractional Seconds: 16 bits of the NTP fractional
seconds are encoded in this field. The use of a 16-bit fractional
seconds yields adequate precision of 15 microseconds (2^-16 s).
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* Bit 3: Time Synchronization (corresponds to Bit 27 of the DF
Election Extended Community). When set to 1, it indicates the
desire to use Time Synchronization capability with the rest of the
PEs in the Ethernet Segment.
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This capability is used in conjunction with the agreed upon DF Type
(DF Election Type). For example if all the PEs in the Ethernet
Segment indicate having Time Synchronization capability and are
requesting the DF type to be HRW, then the HRW algorithm is used in
conjunction with this capability.
2.2. Updates to RFC8584
This document introduces an additional delay to the events and
transitions defined for the default DF election algorithm FSM in
Section 2.1 of [RFC8584] without changing the FSM states or events
itself.
The peering PE's FSM in DF_DONE which receives a RECV_ES transitions
to DF_CALC. Because of the SCT carried in the Ethernet-Segment
update, the output of the DF_CALC and transition back into DF_DONE
are delayed, as are accompanying forwarding updates to DF/NDF state.
The corresponding actions when transitions are performed or states
are entered/exited is modified as follows:
9. DF_CALC on CALCULATED: Mark the election result for the VLAN or
Bundle.
9.1 Where SCT timestamp is present on the RECV_ES event of
Action 11, wait the remaining time before continuing to 9.2.
9.2 Assume a DF/NDF for the local PE for the VLAN or VLAN
Bundle, and transition to DF_DONE.
10. DF_DONE on exiting the state: If a new DF election is triggered
and the current DF is lost, then assume an NDF for the local PE
for the VLAN or VLAN Bundle.
11. DF_DONE on VLAN_CHANGE, RCVD_ES, or LOST_ES: Transition to
DF_CALC.
3. 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 the
DF-Election mechanism in [RFC7432].
Based on Section 8.5 of [RFC7432], using the default 3 second peering
timer:
1. Initial state: PE1 is in steady-state, PE2 is recovering
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2. PE2 recovers at (absolute) time t=99
3. PE2 advertises RT-4 (sent at t=100) to partner PE1
4. PE2 starts a 3 second peering timer
5. PE1 carves immediately on RT-4 reception, i.e. t=100 + minimal
BGP propagation delay
6. PE2 carves at time t=103
[RFC7432] aims of favouring traffic black hole over duplicate
traffic. With above procedure, traffic black holing will occur as
part of each PE recovery sequence since PE1 has transitioned some
VLANs to Non-Designated-Forwarder (NDF) immediately upon reception.
The peering timer value (default = 3 seconds) has a direct effect on
the duration of the blackholing. A shorter (esp. zero) peering timer
may, however, result in duplicate traffic or traffic loops.
Based on the Service Carving Time (SCT) approach:
1. Initial state: PE1 is in steady-state, PE2 is recovering
2. PE2 recovers at (absolute) time t=99
3. PE2 advertises RT-4 (sent at t=100) with target SCT value t=103
to partner PE1
4. PE2 starts 3 second peering timer
5. PE1 starts service carving timer, with remaining time until t=103
6. Both PE1 and PE2 carve at (absolute) time t=103
In fact, PE1 should carve slightly before PE2 (skew) to maintain the
preference of minimal loss over duplicate traffic. 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:
* 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 a good transition of DF role
with contained amount of loss.
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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 non-issue. The use of
SCT approach remedies the problem associated with the peering timer:
the 3 second timer window is shortened to the order of milliseconds.
3.1. Concurrent Recoveries
In the eventuality 2 or more PEs in a peering Ethernet Segment group
are recovering concurrently or roughly the same time, each will
advertise a Service Carving Timestamp. This SCT value would
correspond to what each recovering PE considers the "end time" for DF
Election. A similar situation arises in staggered recovering PEs,
when a second PE recovers at rougly a first PE's advertised SCT
expiry, and with its own new SCT-2 outside of the initial SCT window.
In the case of multiple outstanding DF elections, one requested by
each of the recovering PEs, the SCTs must simply be time-ordered and
all PEs execute only a single DF Election at the service carving time
corresponding to the largest received timestamp value. The DF
Election will involve all the active PEs in a single DF Election
update.
Example:
1. Initial state: PE1 is in steady-state, all services elected at
PE1.
2. PE2 recovers at time t=100, advertises RT-4 with target SCT value
t=103 to partners (PE1)
3. PE2 starts 3 second peering timer
4. PE1 starts service carving timer, with remaining time until t=103
5. PE3 recovers at time t=102, advertises RT-4 with target SCT value
t=105 to partners (PE1, PE2)
6. PE3 starts 3 second peering timer
7. PE2 cancels peering timer, starts service carving timer with
remaining time until t=105
8. PE1 updates service carving timer, with remaining time until
t=105
9. PE1, PE2 and PE3 carve at (absolute) time t=105
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4. 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 modulo-based DF
election and do not rely on the new SCT BGP extended community. PEs
running a baseline DF election mechanism will simply discard the new
SCT BGP extended community as unrecognized.
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 SHALL revert back to the [RFC7432] timer approach.
This is especially important in the context of the VLAN shuffling
with more than 2 PEs.
5. 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.
6. IANA Considerations
This document solicits the allocation of the following sub-type in
the "EVPN Extended Community Sub-Types" registry setup by [RFC7153]:
0x0F Service Carving Timestamp This document
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
7. 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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[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>.
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
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Appendix B. Acknowledgements
Authors would like to acknowledge helpful comments and contributions
of Satya Mohanty and Bharath Vasudevan. Also thank you to Anoop
Ghanwani for his thorough review with valuable comments and
corrections.
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
Jorge Rabadan
Nokia
Email: jorge.rabadan@nokia.com
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