Internet DRAFT - draft-ietf-bess-bgp-vpls-control-flags
draft-ietf-bess-bgp-vpls-control-flags
BESS Working Group R. Singh
INTERNET-DRAFT K. Kompella
Intended Status: Proposed Standard Juniper Networks
Updates: 4761 (if approved) S. Palislamovic
Nokia
Expires: October 20, 2019 April 18, 2019
Updated processing of Control Flags for BGP VPLS
draft-ietf-bess-bgp-vpls-control-flags-08
Abstract
This document updates the meaning of the Control Flags field in the
Layer2 Info Extended Community used for BGP-VPLS NLRI as defined in
RFC4761. This document updates RFC4761.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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Copyright and License Notice
Copyright (c) 2018 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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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Updated meaning of Control Flags in the Layer2 Info Extended
Community . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Control word (C-bit) . . . . . . . . . . . . . . . . . . . . 4
3.2 Sequence flag (S-bit) . . . . . . . . . . . . . . . . . . . 4
4 Using Point-to-MultiPoint (P2MP) LSPs as transport for BGP
VPLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5 Illustrative diagram . . . . . . . . . . . . . . . . . . . . . 6
6 Treatment of C and S bits in multi-homing scenarios . . . . . . 7
6.1 Control word (C-bit) . . . . . . . . . . . . . . . . . . . . 7
6.2 Sequence flag (S-bit) . . . . . . . . . . . . . . . . . . . 7
7 Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1 Normative References . . . . . . . . . . . . . . . . . . . 8
9.2 Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1 Introduction
"Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling" ([RFC4761]) describes the concepts and signaling for using
Border Gateway Protocol (BGP) to setup a VPLS. It specifies the BGP
VPLS Network Layer Reachability Information (NLRI) by which a
provider-edge router (PE) may require other PEs in the same VPLS to
include (or not) the control-word and sequencing information in VPLS
frames sent to this PE.
The use of the Control Word (CW) helps prevent mis-ordering of IPv4
or IPv6 Pseudo-Wire (PW) traffic over Equal Cost Multi-Path (ECMP)
paths or Link Aggregation Group (LAG) bundles. [RFC4385] describes
the format for CW that may be used over Point-to-Point PWs and over a
VPLS. Along with [RFC3985], the document also describes sequence
number usage for VPLS frames.
However, [RFC4761] does not specify the behavior of PEs in a mixed
environment where some PEs support Control Word/sequencing and others
do not.
1.1 Terminology
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
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2 Problem
[RFC4761] specifies the VPLS BGP NLRI by which a given PE advertises
the behavior expected by the multiple PEs participating in the same
VPLS. The NLRI indicates the VPLS label that the various PE routers,
which are referred to in the NLRI, should use when forwarding VPLS
traffic to this PE. Additionally, by using the Control Flags this PE
specifies whether the other PEs (in the same VPLS) should use Control
Word or sequenced-delivery for frames forwarded to this PE. These are
respectively indicated by the C and the S bits in the Control Flags
as specified in section 3.2.4 in [RFC4761].
[RFC4761] requires that if the advertising PE sets the C and S bits,
the receiving PE MUST, respectively, insert control word (CW) and
include sequence numbers when forwarding VPLS traffic to the
advertising PE.
However, in a BGP VPLS deployment there would often be cases where a
PE receiving the VPLS BGP NLRI may not have the ability to insert a
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CW or include sequencing information inside PW frames. Thus, the
behavior of processing CW and sequencing needs to be further
specified.
This document updates the meaning of the Control Flags in layer2
extended community in the BGP VPLS NLRI. It also specifies the
forwarding behavior for a mixed-mode environment where not every PE
in a VPLS has the ability or the configuration to honor the control
flags received from the PE advertising the BGP NLRI.
3 Updated meaning of Control Flags in the Layer2 Info Extended
Community
The current specification does not allow for the CW setting to be
negotiated. In a typical implementation, if a PE sets the C-bit, it
expects to receive VPLS frames with a control word, and will send
frames the same way. If the PEs at the two ends of a PW do not agree
on the setting of the C-bit, the PW does not come up. The behavior
is similar for the S-bit.
This memo updates the meaning of the C-bit and the S-bit in the
control flags.
3.1 Control word (C-bit)
If a PE sets the C-bit in its NLRI, it means that the PE has ability
to send and receive frames with a control word. If the PEs at both
ends of a PW set the C-bit, control words MUST be used in both
directions of the PW. If both PEs send a C-bit of 0, Control Words
MUST NOT be used on the PW. These two cases behave as before.
However, if the PEs at both ends of the PW do not agree on the
setting of the C-bit, control words MUST NOT be used in either
direction on that PW but the PW MUST NOT be prevented from coming up
due to this mismatch. So, the PW will still come up but not use
control word in either direction. This behavior is changed from the
behavior described in [RFC4761] where the PW does not come up.
3.2 Sequence flag (S-bit)
If a PE sets the S-bit in its NLRI, it means that the PE has ability
to set sequence numbers as listed in section 4.1 in [RFC4385] and
process sequence numbers as listed in section 4.2 in [RFC4385]. If
the PEs at both ends of a PW set the S-bit, non-zero sequence numbers
MUST be used in both directions of the PW. If both PEs send a S-bit
of 0, sequence numbers MUST NOT be used on the PW. These two cases
behave as before.
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Current BGP VPLS specification does not allow for S-bit setting to be
negotiated either. In a typical implementation, if the PE sets the
S-bit in the advertised NLRI, it expects to receive VPLS frames with
non-zero sequence numbers, and will send outgoing frames over the PW
with non-zero sequence numbers.
This memo further specifies the expected behavior when the PEs at the
ends of the PW advertise differing S-bit values. If the PEs at both
ends of the PW do not agree on the setting of the S-bit, then the PW
SHOULD NOT come up. This is to avoid running into out-of-sequence
ordering scenarios when the multiple PEs that are enabling multi-
homing for a site have differing S-bit advertisements as listed in
section 4.2 in [RFC4385]. However, if a deployment is known to not
utilize multi-homing, a user-configurable way to override this
recommendation MAY BE provided by an implementation whereby the PW is
allowed to come up. In that case the PE advertising S-bit as 0 should
set sequence numbers in the frames as zero and the PW receiving the
frames should not have an expectation to receive non-zero sequence
numbers.
4 Using Point-to-MultiPoint (P2MP) LSPs as transport for BGP VPLS
BGP VPLS can be used over point-2-point LSPs acting as transport
between the VPLS PEs. Alternately, BGP VPLS may also be used over
P2MP Label Switched Path (LSPs) with the source of the P2MP LSP
rooted at the PE advertising the VPLS BGP NLRI.
In a network that uses P2MP LSPs as transport for a VPLS, there may
be some PEs that support CW while others may not. Similarly, for the
sequencing of VPLS frames.
In such a setup, a source PE that supports CW should setup two
different P2MP LSPs such that:
- One P2MP LSP will transport CW-marked frames to those PEs
that advertised the C-bit as 1.
- The other P2MP LSP will transport frames without CW to those
PEs that advertised C-bit as 0.
Using two different P2MP LSPs to deliver frames with and without
the CW to different PEs ensures that a P2MP root PE honors the C-
bit advertised by the other P2MP PEs.
However, the set of leaves on the two P2MP LSPs (rooted at the
given PE) MUST NOT contain any PEs that advertised a value for the
S-bit different from what the root PE itself is advertising. PEs
that advertised their S-bit value differently (from what the P2MP
root PE advertised) will not be on either of the P2MP LSPs. This
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ensures that the P2MP root PE is sending VPLS frames only to those
PEs that agree on the setting of S-bit.
The ingress router for the P2MP LSP should send separate NLRIs for
the cases of using control-word and for not using control-word.
5 Illustrative diagram
-----
/ A1 \
---- ____CE1 |
/ \ -------- -------- / | |
| A2 CE2- / \ / PE1 \ /
\ / \ / \___/ | \ -----
---- ---PE2 | \
| | \ -----
| Service Provider Network | \ / \
| | CE5 A5
| ___ | / \ /
\ / \ PE4_/ -----
PE3 / \ /
|------/ \------- -------
---- / | ----
/ \/ \ / \ CE = Customer Edge Device
| A3 CE3 --CE4 A4 | PE = Provider Edge Router
\ / \ /
---- ---- A<n> = Customer site n
Figure 1: Example of a VPLS
In the above topology, let there be a VPLS configured with the PEs as
displayed. Let PE1 be the PE under consideration that is CW enabled
and sequencing enabled. Let PE2 and PE3 also be CW enabled and
sequencing enabled. Let PE4 not be CW enabled or have the ability to
include sequence numbers. PE1 will advertise a VPLS BGP NLRI,
containing the C/S bits marked as 1. PE2 and PE3 on learning of NLRI
from PE1, will include the CW and non-zero sequence numbers in the
VPLS frames being forwarded to PE1 as listed in section 4 in
[RFC4385]. However, PE4 which does not have the ability to include CW
or include non-zero sequence numbers, will not.
As per [RFC4761], PE1 would have an expectation that all other PEs
forward CW-containing frames which have non-zero sequence numbers.
That expectation cannot be met by PE4 in this example. Thus, as per
[RFC4761], the PW between PE1 and PE4 does not come up.
However, this document addresses how to support the mixed-CW and
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mixed sequencing-ability of PEs described above. PE1 will not bring
up the PW with PE4 due to the S-bit mismatch, unless overridden by
local configuration on PE1 and PE4 as specified in section 3.2. If
PE4 instead was to advertise a C-bit of 0 and an S-bit of 1, then
despite the CW mismatch the PW between PE1 and PE4 would come up.
Additionally PE1 would setup its data-plane such that it will strip
the CW only for those VPLS frames that are received from PEs that
have indicated their desire to receive CW marked frames. So, PE1 will
setup its data plane to strip the CW only for VPLs frames received
from PE2 and PE3 and it will expect to process PW frames containing
non-zero sequence numbers as listed in section 4.2 in [RFC4385]. PE1
will setup its data-plane to not strip the CW from frames received
from PE4 and it it would expect PE4 to send frames with non-zero
sequence numbers. All frames sent by PE4 to PE1 over the PW would
have a non-zero sequence number.
6 Treatment of C and S bits in multi-homing scenarios
6.1 Control word (C-bit)
In multi-homed environment, different PEs may effectively represent
the same service destination end-point. It could be assumed that the
end-to-end PW establishment process should follow the same rules when
it comes to control word requirement, meaning setting the C-bit would
be enforced equally toward both primary and backup designated
forwarders.
However, in the multi-homing case each PW SHOULD be evaluated
independently. Assuming the network topology specified in section 5,
there could be the case where PW between PE2 and PE1 could have CW
signaled via extended community and would be used in the VPLS frame,
while PE2 to PE4 PW would not insert the CW in the VPLS frame due to
C-bit mismatch. The rest of PEs multi-homing behavior should simply
follow the rules specified in [VPLS-MULTIHOMING].
6.2 Sequence flag (S-bit)
In a multi-homed environment, different PEs may effectively represent
the same service destination end-point. In this case, the rules for
end-to-end PW establishment SHOULD follow the same behavior as listed
in section 3.2 when it comes to sequence bit requirements. Consider
the case described in section 5 with CE5 being multi-homed to PE4 and
PE1. The PW behavior is similar to the CW scenario so that the
insertion of S-bit evaluation SHOULD be independent per PW. However,
because S-bit mismatch between two end-point PEs results in no PW
establishment, in the case where PE4 doesn't support S-bit. So, only
one PW would be established, between PE1 and PE2. Thus, even though
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CE5 is physically multi-homed, due to PE4's lack of support for
sending frames with non-zero sequence numbers there would be no PW
between PE2 and PE4. CE5 would effectively not be multi-homed.
7 Security Considerations
This document updates the behavior specified in [RFC4761]. The
security considerations listed in [RFC4761] apply. This document
essentially addresses BGP-VPLS behavior for PEs when the C-bit and/or
S-bit value advertised by a given PE are different from what another
PE in the VPLS is advertising. Any bit-flipping media errors leading
to causing this mismatch of C/S bits between PEs do not adversely
affect the availability of the PWs. Rather they cause control-words
to not be used or cause the NRLI-advertising PE to not expect non-
zero sequenced frames, for the C-bit and the S-bit respectively being
mismatched across PEs. This is no worse than the previous behavior
where any bit-flipping media errors leading to mismatch of C/S bit
between PEs would cause the PW to not come up.
8 IANA Considerations
This document does not make any requests from IANA.
9 References
9.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4761] Kompella, K., Y. Rekhter, Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling,
RFC 4761, January 2007.
[RFC4385] Bryant, S., Swallow G., Martini L., D. McPherson,
Pseudowire Emulation Edge-to-Edge (PWE3) Control Word,
RFC 4385, February 2006.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017.
9.2 Informative References
[RFC3985] Bryant, S., P. Pate, Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture, RFC3985, March 2005.
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[VPLS-MULTIHOMING] Kothari, B., et al, BGP based Multi-homing in
Virtual Private LAN Service,
draft-ietf-bess-vpls-multihoming-02, September 2018.
Authors' Addresses
Ravi Singh
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
US
EMail: ravis@juniper.net
Kireeti Kompella
Juniper Networks
1133 Innovation Way
Sunnyvale, CA 94089
US
EMail: kireeti@juniper.net
Senad Palislamovic
Nokia
600 Mountain Avenue
Murray Hill, NJ 07974-0636
US
EMail: Senad.palislamovic@nokia.com
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