# Gunter Van de Velde, RTG AD, comments for
draft-ietf-bess-evpn-mh-split-horizon-08
Hi Authors,
Many thanks to Himanshu Shah for the RTG-DIR review and Jouni Korhonen for
GENART review.
Please find some review notes below. Before processing the draft further with
the IESG and requesting IETF Last-Call, please have a look into these
observations. I believe the document is well written. Mostly the observations
are efforts to further enhance grammar/readablity and try to clarify some
formal procedures.
There is some need to expand with more content on the flags field for which the
IANA section requests a new 8 bit flags registery. I also wonder if the 'RFC
Required' is strict enough to contol the 8 bits. I am wondering if we should
not enforce IETF consensus/review to allocate any of them? When the registry
policy is set to "IETF Review" we make sure that there is an IETF track
document. Individual submissions are not valid for requesting a code point with
"IETF Review" policy. Also, would authors investigate if the not-yet-allocated
flag bits are unused or reserved, and what the implied actions are for
senders/recipients of the bits within the flags field are. And finally, where
in the existing IANA registry will the new flags field registry be placed? It
is not mentioned in the draft.
Thanks for all the hard work on this document. Before progressing i will be
waiting for your feedback and your revised drafts.
Please find below my observations when going through
draft-ietf-bess-evpn-mh-split-horizon-08
#Review Observations
#===================
16 Ethernet Virtual Private Network (EVPN) is commonly used along with
17 Network Virtualization Overlay (NVO) tunnels, as well as MPLS and
18 Segment Routing tunnels. The EVPN multi-homing procedures may be
19 different depending on the tunnel type used in the EVPN Broadcast
20 Domain. In particular, there are two multi-homing Split Horizon
21 procedures to avoid looped frames on the multi-homed CE: ESI Label
22 based and Local Bias. ESI Label based Split Horizon is used for
23 MPLSoX tunnels, E.g., MPLSoUDP, whereas Local Bias is used for other
24 tunnels, E.g., VXLAN tunnels. The existing specifications do not
25 allow the operator to decide which Split Horizon procedure to use for
26 tunnel encapsulations that could support both. Examples of tunnels
27 that may support both procedures are MPLSoGRE, MPLSoUDP, GENEVE or
28 SRv6. This document updates the EVPN Multihoming procedures in
29 RFC8365 and RFC7432 so that an operator can decide the Split Horizon
30 procedure for a given tunnel depending on their own requirements.
Maybe a proposed readability re-edit as follows helps reders of the abstract to
understand the document intent better:
"
Ethernet Virtual Private Network (EVPN) is commonly used with Network
Virtualization Overlay (NVO) tunnels, as well as MPLS and Segment Routing
tunnels. The multi-homing procedures in EVPN may vary based on the type of
tunnel used within the EVPN Broadcast Domain. Specifically, there are two
multi-homing Split Horizon procedures designed to prevent looped frames on
multi-homed Customer Edge (CE) devices: the ESI Label-based procedure and the
Local Bias procedure. The ESI Label-based Split Horizon is applied to
MPLS-based tunnels, such as MPLSoUDP, while the Local Bias procedure is used
for other tunnels, such as VXLAN.
Current specifications do not allow operators to choose which Split Horizon
procedure to use for tunnel encapsulations that support both methods. Examples
of tunnels that may support both procedures include MPLSoGRE, MPLSoUDP, GENEVE,
and SRv6. This document updates the EVPN multi-homing procedures described in
RFC 8365 and RFC 7432, enabling operators to select the appropriate Split
Horizon procedure for a given tunnel based on their specific requirements.
"
85 Ethernet Virtual Private Network (EVPN) is commonly used with the
86 following tunnel encapsulations:
88 * Network Virtualization Overlay (NVO) tunnels as specified in
89 [RFC8365]
91 * MPLS and Segment Routing with MPLS data plane (SR-MPLS), as
92 specified in [RFC7432]
94 * Segment Routing with IPv6 data plane (SRv6), as specified in
95 [RFC9252].
The abstract mentions: MPLSoGRE, MPLSoUDP, GENEVE or SRv6
The list here does not explicit calls these out. Can relevant references be
added?
Also is an MPLS lsp considered as a tunnel in the context of this document? if
yes, it maybe should be added next to referencing SRv6?
97 The EVPN multihoming procedures may be different depending on the
98 tunnel type used in the EVPN Broadcast Domain. In particular, there
99 are two multihoming Split Horizon procedures to avoid looped frames
100 on the multihomed CE: ESI Label based and Local Bias. ESI Label
101 based Split Horizon is used for MPLS or MPLSoX tunnels, E.g.,
102 MPLSoUDP [RFC7510], and its procedures described in [RFC7432]. Local
103 Bias is used by IP tunnels, E.g., VXLAN tunnels, and it is described
104 in [RFC8365].
>From readability perspective, the following can be used to improve the text:
"
The EVPN multihoming procedures may vary depending on the type of tunnel
utilized within the EVPN Broadcast Domain. Specifically, there are two
multihoming Split Horizon procedures employed to prevent looped frames on
multihomed Customer Edge (CE) devices: the ESI Label-based procedure and the
Local Bias procedure.
The ESI Label-based Split Horizon procedure is used for MPLS or MPLS-over-X
(MPLSoX) tunnels, such as MPLS-over-UDP as specified in [RFC 7510], and its
procedures are detailed in [RFC 7432]. Conversely, the Local Bias procedure is
used for IP-based tunnels, such as VXLAN tunnels, and it is described in [RFC
8365].
"
112 When EVPN is used for MPLS transport tunnels, an MPLS label
113 enables the Split Horizon filtering capability to support All-
114 Active multihoming. The ingress Network Virtualization Edge (NVE)
115 device adds a label corresponding to the source ES (an ESI label)
116 when encapsulating the packet. The egress NVE checks the ESI
117 label when attempting to forward a multi-destination frame out of
118 a local ES interface, and if the label corresponds to the same
119 site identifier (ESI) associated with that ES interface, the
120 packet is not forwarded. This prevents the occurrence of
121 forwarding loops for BUM traffic.
123 The ESI Label Split Horizon filtering SHOULD also be used with
124 Single-Active multihoming to avoid transient loops for in-flight
125 packets when the egress NVE takes over as Designated Forwarder for
126 an ES.
I am wondering if the SHOULD in the above paragraph is something this document
is adding to its formal procedures? if not, then i wonder why RFC2119 language
is used. In the contex above it seems to be more a suggestion then a formal
procedure description. In the below roposed rewrite, i used lowercase should.
>From readability perspective, the following can be used to improve the text:
"
When EVPN is employed for MPLS transport tunnels, an MPLS label facilitates
Split Horizon filtering to support All-Active multihoming. The ingress Network
Virtualization Edge (NVE) device appends a label corresponding to the source
Ethernet Segment Identifier (ESI label) during packet encapsulation. The egress
NVE verifies the ESI label when attempting to forward a multi-destination frame
through a local Ethernet Segment (ES) interface. If the ESI label matches the
site identifier (ESI) associated with that ES interface, the packet is not
forwarded. This mechanism effectively prevents forwarding loops for Broadcast,
Unknown Unicast, and Multicast (BUM) traffic.
The ESI Label Split Horizon filtering should also be utilized with
Single-Active multihoming to prevent transient loops for in-flight packets when
the egress NVE assumes the role of Designated Forwarder for an ES.
"
130 Since IP tunnels (such as VXLAN or NVGRE) do not support the ESI
131 label (or any MPLS label at all), a different Split Horizon
132 filtering procedure must be used for All-Active multihoming. This
133 mechanism is called Local Bias and relies on the tunnel source IP
134 address to decide whether to forward BUM traffic to a local ES
135 interface at the egress NVE.
136
137 In a nutshell, every NVE tracks the IP address(es) associated with
138 the other NVE(s) with which it has shared multihomed ESs. When
139 the egress NVE receives a BUM frame encapsulated in a IP tunnel,
140 it examines the source IP address in the tunnel header (which
141 identifies the ingress NVE) and filters out the frame on all local
142 interfaces connected to ESes that are shared with the ingress NVE.
143
144 Due to this behavior at the egress NVE, the ingress NVE's behavior
145 is also changed to perform replication locally to all directly
146 attached ESes (regardless of the Designated Forwarder election
147 state) for all BUM ingress from the access ACs. Because of this
148 "local" replication at the ingress NVE, this approach is referred
149 to as Local Bias.
150
151 Local Bias cannot be used for Single-Active multihoming, since the
152 ingress NVE brings operationally down the Attachment Circuits
153 (ACs) for which it is non-Designated Forwarder (hence local
154 replication to non-Designated Forwarder ACs cannot be done). This
155 means transient in-flight BUM packets may be looped back to the
156 originating site by new elected Designated Forwarder egress NVEs.
>From readability perspective, the following can be used to improve the text:
"
Since IP tunnels, such as VXLAN or NVGRE, do not support the ESI label or any
MPLS label, an alternative Split Horizon filtering procedure must be
implemented for All-Active multihoming. This mechanism, known as Local Bias,
relies on the source IP address of the tunnel to determine whether to forward
Broadcast, Unknown Unicast, and Multicast (BUM) traffic to a local Ethernet
Segment (ES) interface at the egress Network Virtualization Edge (NVE).
In summary, each NVE monitors the IP address(es) of other NVEs with which it
shares multihomed ESs. Upon receiving a BUM frame encapsulated in an IP tunnel,
the egress NVE inspects the source IP address in the tunnel header, which
identifies the ingress NVE. The egress NVE then filters out the frame on all
local interfaces connected to ESs that are shared with the ingress NVE.
Due to this behavior at the egress NVE, the ingress NVE is required to perform
local replication to all directly attached ESs, regardless of the Designated
Forwarder election state, for all BUM traffic ingressing from the access
Attachment Circuits (ACs). This local replication at the ingress NVE is the
basis for the term Local Bias.
Local Bias is not suitable for Single-Active multihoming, as the ingress NVE
deactivates the ACs for which it is not the Designated Forwarder. Consequently,
local replication to non-Designated Forwarder ACs cannot occur, leading to the
potential for transient in-flight BUM packets to be looped back to the
originating site by newly elected Designated Forwarder egress NVEs.
"
158 [RFC8365] states that Local Bias is used only for IP tunnels, and ESI
159 Label based Split Horizon for IP-based MPLS tunnels. However, IP-
160 based MPLS tunnels, such as MPLSoGRE or MPLSoUDP, are also IP tunnels
161 and can potentially support both procedures, since they can carry ESI
162 Labels and they also use a tunnel IP header where the source IP
163 address identifies the ingress NVE.
164
165 Similarly, some IP tunnels that carry an identifier of the source ES
166 in the tunnel header, may potentially follow either procedure too.
167 Some examples are GENEVE or SRv6:
>From readability perspective, the following can be used to improve the text:
"
[RFC 8365] specifies that Local Bias is exclusively utilized for IP tunnels,
while ESI Label-based Split Horizon is employed for IP-based MPLS tunnels.
However, IP-based MPLS tunnels, such as MPLS over GRE (MPLSoGRE) or MPLS over
UDP (MPLSoUDP), are also categorized as IP tunnels and have the potential to
support both procedures. These tunnels are capable of carrying ESI labels and
also utilize a tunnel IP header in which the source IP address identifies the
ingress Network Virtualization Edge (NVE).
Similarly, certain IP tunnels that include an identifier for the source
Ethernet Segment (ES) in the tunnel header may also potentially support either
procedure. Examples of such tunnels include GENEVE and SRv6.
"
174 * In an SRv6 tunnel, the source IP address also identifies the
175 ingress NVE, however, by default, and as described in [RFC9252]
176 the ingress PE will add information in the SRv6 packet so that the
177 egress PE can identify the source ES of the BUM packet. That
178 information is the ESI filtering argument (Arg.FE2) of the service
179 Segment Identifier (SID) received on an A-D per ES route from the
180 egress PE.
It would be good to mention that Arg.FE2 is described in RFC 9252/8986.
Reference added to the below revised textblob edit:
>From readability perspective, the following can be used to improve the text:
"
In an SRv6 tunnel, the source IP address identifies the ingress NVE. By
default, and as outlined in [RFC 9252], the ingress PE adds specific
information to the SRv6 packet to enable the egress PE to identify the source
ES of the BUM packet. This information is the ESI filtering argument (Arg.FE2)
[RFC9252][RFC8986] of the service Segment Identifier (SID) received on an A-D
per ES route from the egress PE.
"
182 Table 1 shows different tunnel encapsulations and their supported and
183 default Split Horizon method. In the case of GENEVE, the default
184 Split Horizon Type (SHT) depends on whether the Ethernet Option with
185 Source ID TLV is negotiated. In the case of SRv6, the default SHT is
186 listed as ESI label filtering in the Table, since the behavior is
187 equivalent to that of ESI Label filtering. In this document, ESI
188 Label filtering refers to the Split Horizon filtering based on the
189 existence of a source ES identifier in the tunnel header.
>From readability perspective, the following can be used to improve the text:
"
Table 1 presents various tunnel encapsulations along with their supported and
default Split Horizon methods. For GENEVE, the default Split Horizon Type (SHT)
is contingent upon the negotiation of the Ethernet Option with the Source ID
TLV. In the case of SRv6, the default SHT is specified as ESI Label filtering
in the table, as its behavior is analogous to that of ESI Label filtering. In
this document, ESI Label filtering refers to the Split Horizon filtering based
on the presence of a source Ethernet Segment (ES) identifier in the tunnel
header.
"
201 Any other tunnel encapsulation (different from the encapsulations in
202 Table 1) that can be classified into any of the four encapsulation
203 groups above, supports Split Horizon based on the following rules:
204
205 * IP-based MPLS tunnels and SRv6 tunnels can support both Split
206 Horizon filtering methods
207
208 * (SR-)MPLS tunnels only support ESI Label based Split Horizon
209 filtering
210
211 * IP tunnels support Local Bias Split Horizon and may support ESI
212 Label based Split Horizon, if they include a method to identify
213 the source ESI in the header.
>From readability perspective, the following can be used to improve the text:
"
Any tunnel encapsulation not listed in Table 1 that can be categorized into one
of the four encapsulation groups mentioned above will support Split Horizon
filtering based on the following rules:
* IP-based MPLS tunnels and SRv6 tunnels are capable of supporting both Split
Horizon filtering methods.
* (SR-)MPLS tunnels only support ESI Label-based Split Horizon filtering.
* IP tunnels support Local Bias Split Horizon filtering and may also support
ESI Label-based Split Horizon filtering, provided they incorporate a mechanism
to identify the source ESI in the header.
"
244 The ESI Label method works for All-Active and Single-Active, while
245 Local Bias only works for All-Active. In addition, the ESI Label
246 method works across different network domains, whereas Local Bias is
247 limited to networks with no next hop change between the NVEs attached
248 to the same ES. However, some operators prefer the Local Bias
249 method, since it simplifies the encapsulation, consumes less
250 resources on the NVEs and the ingress NVE always forwards locally to
251 other interfaces, reducing the delay to reach multihomed hosts.
253 This document extends the EVPN multihoming procedures so that an
254 operator can decide the Split Horizon procedure for a given NVO
255 tunnel depending on their own specific requirements. The choice of
256 Local Bias or ESI Label Split Horizon is now allowed for tunnel
257 encapsulations that support both methods, and it is advertised along
258 with the EVPN A-D per ES route. IP tunnels that do not support both
259 methods, E.g., VXLAN or NVGRE, will keep following [RFC8365]
260 procedures
>From readability perspective, the following can be used to improve the text:
"
The ESI Label method is applicable for both All-Active and Single-Active
configurations, whereas the Local Bias method is suitable only for All-Active
configurations. Moreover, the ESI Label method is effective across different
network domains, while Local Bias is constrained to networks where there is no
change in the next hop between the NVEs attached to the same ES. Nonetheless,
some operators favor the Local Bias method due to its simplification of the
encapsulation process, reduced resource consumption on NVEs, and the fact that
the ingress NVE always forwards traffic locally to other interfaces, thereby
decreasing the delay in reaching multihomed hosts.
This document extends the EVPN multihoming procedures to allow operators to
select the preferred Split Horizon method for a given NVO tunnel according to
their specific requirements. The choice between Local Bias and ESI Label Split
Horizon is now permissible for tunnel encapsulations that support both methods,
and this selection is advertised along with the EVPN A-D per ES route. IP
tunnels that do not support both methods, such as VXLAN or NVGRE, will continue
to adhere to the procedures specified in [RFC 8365].
"
295 * EVI and EVI-RT: EVPN Instance and EVI Route Target. A group of
296 NVEs attached to the same EVI will share the same EVI-RT.
Why not split out in two entries instead of one entry in the terminology list?
307 * MPLSoUDP: Multi-Protocol Label Switching over User Datagram
308 Protocol, [RFC7510]
Should the terminology not be extended to MPLS-over-UDP in the description?
Same is valid for the next few entries in the list.
343 EVPN extensions are needed so that NVEs can advertise their
344 preference for the Split Horizon method to be used in the ES.
345 Figure 1 shows the ESI Label extended community that is always
346 advertised along with the EVPN A-D per ES route. All the NVEs
347 attached to an ES advertise an A-D per ES route for the ES, including
348 this extended community that conveys the information for the
349 multihoming mode (All-active or Single-Active), as well as the ESI
350 Label to be used (if needed).
It would be helpful to specify where the ESI Label extended community is
defined:
section "7.5. ESI Label Extended Community" of RFC7432.
>From readability perspective, the following can be used to improve the text:
"Extensions to EVPN are required to enable NVEs to advertise their preferred
Split Horizon method for a given ES. Figure 1 illustrates the ESI Label
extended community (RFC7432 Section7.5), which is consistently advertised
alongside the EVPN A-D per ES route. All NVEs connected to an ES advertise an
A-D per ES route for that ES, including the extended community, which
communicates information regarding the multihoming mode (either All-Active or
Single-Active) and, if necessary, specifies the ESI Label to be utilized."
363 * A value of 0 means that the multihomed ES is operating in All-
364 Active mode.
365
366 * A value of 1 means that the multihomed ES is operating in Single-
367 Active mode.
This is not entirely correct. section7.5 defines indeed the "Single-Active"
bit, but
is is All-Active "redundancy" mode and the Single-Active "redundancy" mode. I
suggest to be formally correct and add the word "redundancy".
369 Section 5 creates a registry for the Flags octet, where the "Single-
370 Active" bit is the low-order bit of the RED (multihoming redundancy
371 mode) field
It is not entirely clear that the RED field is new to a reader. We should
consider
making this more explicit. Not sure if the remainder of this document provides
extra context on the motivation why a new field is defined?. What about the
following:
"
Section 5 establishes a registry for the Flags octet, designating the
"Single-Active" bit as the low-order bit of the newly defined Redundancy (RED)
field for multihoming modes.
"
375 [RFC8365] does not add any explicit indication about the Split
376 Horizon method in the A-D per ES route. In this document, the
377 [RFC8365] Split Horizon procedure is the default behavior and assumes
378 that Local Bias is used only for IP tunnels, and ESI Label based
379 Split Horizon for IP-based MPLS tunnels. This document defines the
380 two high-order bits in the Flags octet (bits 6 and 7) as the "Split
381 Horizon Type" (SHT) field, where:
>From readability perspective, the following can be used to improve the text:
"
[RFC 8365] does not include any explicit indication regarding the Split Horizon
method in the A-D per Ethernet Segment (ES) route. In this document, the Split
Horizon procedure defined in [RFC 8365] is considered the default behavior,
presuming that Local Bias is employed exclusively for IP tunnels, while ESI
Label-based Split Horizon is used for IP-based MPLS tunnels. This document
specifies that the two high-order bits in the Flags octet (bits 6 and 7)
constitute the "Split Horizon Type" (SHT) field, where:
"
391 0 0 --> Default SHT. Backwards compatible with [RFC8365]
Is it not required to mention that there is backwards compatibility with
RFC7432 also?
Theflags field is defined in sectio 7.5 of RFC7432?
421 * An SHT value different than 00 expresses the intent to use a
422 specific Split Horizon method, but does not reflect the actual
423 operational SHT used by the advertising NVE, unless all the NVEs
424 attached to the ES advertise the same SHT.
I believe that formal RFC2119 language to explain this is desired.
439 As an example, egress NVEs that support IP-based MPLS tunnels, E.g.,
440 MPLSoGRE or MPLSoUDP, will advertise A-D per ES route(s) for the ES
441 along with the BGP Encapsulation extended community [RFC9012]
442 indicating the encapsulation (MPLSoGRE or MPLSoUDP) and MAY use the
443 SHT = 01 or 10 to indicate the intent to use Local Bias or ESI Label,
444 respectively.
445
446 An egress NVE MUST NOT use an SHT value different from 00 when
447 advertising an A-D per ES route with encapsulation VXLAN, NVGRE, MPLS
448 or no BGP tunnel encapsulation extended community [RFC9012]. We
449 assume that, in all these cases, there is no Split Horizon method
450 choice, and therefore the SHT value MUST be 00. A received route
451 with one of the above encapsulation options and SHT value different
452 from 00 SHOULD be treat-as-withdraw.
453
454 An egress NVE advertising A-D per ES route(s) for an ES with
455 encapsulation GENEVE MAY use an SHT value of 01 or 10. A value of 01
456 indicates the intent to use Local Bias, irrespective of the presence
457 of an Ethernet option TLV with a non-zero Source-ID
458 [I-D.ietf-bess-evpn-geneve]. A value of 10 indicates the intent to
459 use ESI Label based Split Horizon. A value of 00 indicates the
460 default behavior in Table 1, that is, use Local Bias if no ESI-Label
461 exists in the Ethernet option TLV or no Ethernet option TLV
462 whatsoever. Otherwise the ESI Label Split Horizon method is used.
463
464 The above procedures assume a single encapsulation supported in the
465 egress NVE. Section 3 describes additional procedures for NVEs
466 supporting multiple encapsulations.
How do some of the rules apply to routers not aware or not supporting SHT?
A brief rewording for readability and removing the usage of the word 'we':
"As an example, egress NVEs that support IP-based MPLS tunnels, such as
MPLSoGRE or MPLSoUDP, will advertise A-D per ES routes for the ES along with
the BGP Encapsulation extended community, as defined in [RFC 9012]. This
extended community indicates the encapsulation type (MPLSoGRE or MPLSoUDP) and
may use the SHT value of 01 or 10 to signify the intent to use Local Bias or
ESI Label, respectively.
An egress NVE MUST NOT use an SHT value other than 00 when advertising an A-D
per ES route with encapsulation types such as VXLAN, NVGRE, MPLS, or no BGP
tunnel encapsulation extended community, as specified in [RFC 9012]. In all
these cases, it is presumed that there is no choice for the Split Horizon
method; therefore, the SHT value MUST be set to 00. If a route with any of the
mentioned encapsulation options is received and has an SHT value different from
00, it SHOULD be treated as a withdrawal.
An egress NVE advertising A-D per ES route(s) for an ES with GENEVE
encapsulation MAY use an SHT value of 01 or 10. A value of 01 indicates the
intent to use Local Bias, regardless of the presence of an Ethernet option TLV
with a non-zero Source-ID, as described in [I-D.ietf-bess-evpn-geneve]. A value
of 10 indicates the intent to use ESI Label-based Split Horizon. A value of 00
indicates the default behavior outlined in Table 1, which is to use Local Bias
if no ESI-Label is present in the Ethernet option TLV, or if there is no
Ethernet option TLV. Otherwise, the ESI Label Split Horizon method is applied.
These procedures assume a single encapsulation supported in the egress NVE.
Section 3 describes additional procedures for NVEs supporting multiple
encapsulations."
470 This document also updates [RFC8365] in the value that is advertised
471 in the ESI Label field of the ESI Label extended community, as
472 follows:
474 * The A-D per ES route(s) for an ES MAY have an ESI Label value of
475 zero if the SHT value is 01. Section 2.2 specifies the cases
476 where the SHT can be 01. An ESI Label value of zero avoids the
477 allocation of Labels in the cases where they are not used (Local
478 Bias).
480 * The A-D per ES route(s) for an ES MAY have an ESI Label value of
481 zero for VXLAN or NVGRE encapsulations.
A rewording from readability perspective
"
This document also updates [RFC 8365] regarding the value that is advertised in
the ESI Label field of the ESI Label extended community, as follows:
* The A-D per ES route(s) for an ES MAY have an ESI Label value of zero if the
SHT value is 01. Section 2.2 specifies the scenarios where the SHT can be 01.
An ESI Label value of zero eliminates the need to allocate labels in cases
where they are not utilized, such as in the Local Bias method.
* The A-D per ES route(s) for an ES MAY have an ESI Label value of zero for
VXLAN or NVGRE encapsulations.
"
490 An NVE has an administrative SHT value for an ES (the one that is
491 advertised along with the A-D per ES route) and an operational SHT
492 value (the one that is actually used irrespective of what the NVE
493 advertised). The administrative SHT matches the operational SHT if
494 all the NVEs attached to the ES have the same administrative SHT.
495
496 This document assumes that an [RFC7432] or [RFC8365] implementation
497 that does not support this document, ignores the value of all the
498 Flags in the ESI Label extended community except for the Single-
499 Active bit. Based on this assumption, a non-upgraded NVE will ignore
500 an SHT different from 00. As soon as an upgraded NVE receives at
501 least one A-D per ES route for the ES with SHT value of 00, it MUST
502 revert its operational SHT to the default Split Horizon method, as in
503 Table 1, and irrespective of its administrative SHT.
504
505 As an example, consider an NVE attached to ES N that receives two A-D
506 per ES routes for N from different NVEs, NVE1 and NVE2. If the route
507 from NVE1 has SHT = 00 and the one from NVE2 an SHT = 01, the NVE
508 MUST use the default Split Horizon method in Table 1 as operational
509 SHT, irrespective of its administrative SHT.
510
511 All the NVEs attached to an ES with operational SHT value of 10 MUST
512 advertise a valid non-zero ESI Label. If the operational SHT value
513 is 01, the ESI Label MAY be zero. If the operational SHT value is
514 00, the ESI Label MAY be zero only if the default encapsulation
515 supports Local Bias only and the NVEs do not check the presence of a
516 valid non-zero ESI Label.
517
518 If an NVE changes its operational SHT value from 01 (Local Bias) to
519 00 (Default SHT) as a result of a new non-upgraded NVE present in the
520 ES, and it previously advertised a zero ESI Label, it MUST send an
521 update with a non-zero valid ESI Label, unless all the non-upgraded
522 NVEs in the ES support Local Bias only. As an example, suppose NVE1
523 and NVE2 use MPLSoUDP as encapsulation, they are attached to the same
524 Ethernet Segment ES1 and advertise an SHT value of 01 (Local Bias)
525 and a zero ESI label value. Suppose NVE3 does not support this
526 specification and joins ES1, therefore advertises an SHT of 00
527 (default). Upon receiving NVE3's A-D per ES route, NVE1 and NVE2
528 MUST send an update of their A-D per ES route for ES1 with a non-zero
529 valid ESI label value. The assumption is that NVE3 supports only the
530 default ESI label based Split Horizon filtering.
A rewording from readability perspective
"
A NVE maintains an administrative SHT value for an Ethernet Segment (ES), which
is advertised alongside the A-D per ES route, and an operational SHT value,
which is the one actually used regardless of what the NVE has advertised. The
administrative SHT matches the operational SHT if all the NVEs attached to the
ES have the same administrative SHT.
This document assumes that an implementation of [RFC 7432] or [RFC 8365] that
does not support the specifications in this document will ignore the values of
all the Flags in the ESI Label extended community, except for the Single-Active
bit. Based on this assumption, a non-upgraded NVE will disregard any SHT value
other than 00. If an upgraded NVE receives at least one A-D per ES route for
the ES with an SHT value of 00, it MUST revert its operational SHT to the
default Split Horizon method, as described in Table 1, irrespective of its
administrative SHT.
For instance, consider an NVE attached to ES N that receives two A-D per ES
routes for N from different NVEs, NVE1 and NVE2. If the route from NVE1 has an
SHT value of 00 and the one from NVE2 has an SHT value of 01, the NVE MUST use
the default Split Horizon method specified in Table 1 as its operational SHT,
regardless of its administrative SHT.
All NVEs attached to an ES with an operational SHT value of 10 MUST advertise a
valid, non-zero ESI Label. If the operational SHT value is 01, the ESI Label
MAY be zero. If the operational SHT value is 00, the ESI Label may be zero only
if the default encapsulation supports Local Bias exclusively, and the NVEs do
not require the presence of a valid, non-zero ESI Label.
If an NVE changes its operational SHT value from 01 (Local Bias) to 00 (Default
SHT) due to the presence of a new non-upgraded NVE in the ES, and it previously
advertised a zero ESI Label, it MUST send an update with a valid, non-zero ESI
Label, unless all the non-upgraded NVEs in the ES support only Local Bias. For
example, consider NVE1 and NVE2 using MPLSoUDP as encapsulation, attached to
the same Ethernet Segment ES1, and advertising an SHT value of 01 (Local Bias)
with a zero ESI Label value. Suppose NVE3, which does not support this
specification, joins ES1 and advertises an SHT value of 00 (default). Upon
receiving NVE3's A-D per ES route, NVE1 and NVE2 MUST update their A-D per ES
routes for ES1 to include a valid, non-zero ESI Label value. The assumption
here is that NVE3 only supports the default ESI Label-based Split Horizon
filtering.
"
534 As specified by [RFC8365], an egress NVE that supports multiple data
535 plane encapsulations (I.e., VXLAN, NVGRE, MPLS, MPLSoUDP, GENEVE)
536 needs to indicate all the supported encapsulations using BGP
537 Encapsulation extended communities defined in [RFC9012] with all EVPN
538 routes. This section clarifies the multihoming Split Horizon
539 behavior for NVEs advertising and receiving multiple BGP
540 Encapsulation extended communities along with the A-D per ES routes.
541 This section uses a notation of {x,y} to indicate the encapsulations
542 advertised in BGP Encapsulation extended communities [RFC9012], with
543 x and y being different encapsulation values.
544
545 It is important to remember that an NVE MAY advertise multiple A-D
546 per ES routes for the same ES (and not only one), each route
547 conveying a number of Route Targets (RT). We refer to the total
548 number of Route Targets in a given ES as RT-set for that ES. Any of
549 the EVIs represented in the RT-set will have its RT included in one
550 (and only one) A-D per ES route for the ES. When multiple A-D per ES
551 routes are advertised for the same ES, each route MUST have a
552 different Route Distinguisher.
This section can use a textual readability and grammar update. THe proposed
rewrite removes the usage of the word 'we' as it is unclear in formla
procedures who exactly is 'we'? What about the following:
"
As specified in [RFC 8365], an NVE that supports multiple data plane
encapsulations (e.g., VXLAN, NVGRE, MPLS, MPLSoUDP, GENEVE) must indicate all
supported encapsulations using BGP Encapsulation extended communities as
defined in [RFC 9012] for all EVPN routes. This section provides clarification
on the multihoming Split Horizon behavior for NVEs that advertise and receive
multiple BGP Encapsulation extended communities along with the A-D per ES
routes. This section uses the notation {x, y} to denote the encapsulations
advertised in BGP Encapsulation extended communities, where x and y represent
different encapsulation values.
It is important to note that an NVE MAY advertise multiple A-D per ES routes
for the same ES, rather than a single route, with each route conveying a set of
Route Targets (RT). The total set of Route Targets associated with a given ES
is referred to as the RT-set for that ES. Each of the EVIs represented in the
RT-set will have its RT included in one, and only one, A-D per ES route for the
ES. When multiple A-D per ES routes are advertised for the same ES, each route
must have a distinct Route Distinguisher.
"
568 This document extends this behavior as follows:
570 a. An A-D per ES route for ES-x may be advertised with multiple
571 encapsulations where some support a single Split Horizon method.
572 In this case, the SHT value MUST be 00. As an example, {VXLAN,
573 NVGRE}, {VXLAN, GENEVE} or {MPLS, MPLSoGRE, MPLSoUDP} can be
574 advertised in an A-D per ES route. In all those cases SHT MUST
575 be 00.
576
577 b. An A-D per ES route for ES-y may be advertised with multiple
578 encapsulations where all of them support both Split Horizon
579 methods. In this case the SHT value MAY be 01 if the desired
580 method is Local Bias, or 10 if ESI Label based. For example,
581 {MPLSoGRE, MPLSoUDP, GENEVE} (or a subset) may be advertised in
582 an A-D per ES route with SHT value of 01. The ESI Label value in
583 this case MAY be zero.
584
585 c. If ES-z with RT-set composed of (RT1, RT2, RT3.. RTn) supports
586 multiple encapsulations that require a different Split Horizon
587 method, a different A-D per ES route (or group of routes) per
588 Split Horizon method MUST be advertised. For example, consider n
589 RTs in ES-z and:
590
591 * the EVIs corresponding to (RT1..RTi) support VXLAN,
592 * the ones for (RTi+1..RTm) (with i<m) support MPLSoUDP with
593 Local Bias,
594
595 * and the ones for (RTm+1..RTn) (with m<n) support GENEVE with
596 ESI Label based Split Horizon.
597
598 In this case, three groups of A-D per ES routes MUST be
599 advertised for ES-z:
600
601 * A-D per ES route group 1, including (RT1..RTi), with
602 encapsulation {VXLAN}, SHT = 00. The ESI Label MAY be zero.
603
604 * A-D per ES route group 2, including (RTi+1..RTm), with
605 encapsulation {MPLSoUDP}, SHT = 01. The ESI Label MAY be
606 zero.
607
608 * A-D per ES route group 3, including (RTm+1..RTn), with
609 encapsulation {GENEVE}, SHT = 10. The ESI Label MUST have a
610 valid value, different from zero, and the Ethernet option
611 [RFC8926] MUST be advertised.
612
613 As per [RFC8365], it is the responsibility of the operator of a given
614 EVI to ensure that all of the NVEs in that EVI support a common
615 encapsulation. If this condition is violated, it could result in
616 service disruption or failure.
I did an effort to improve readability of these formal rules and hope i kept
the original intent unchanged (i changed some words into uppercase RFC2119
language.. please confirm it was correctly processed):
"
This document extends the described behavior as follows:
a. An A-D per ES route for ES-x may be advertised with multiple encapsulations,
some of which support a single Split Horizon method. In this case, the Split
Horizon Type (SHT) value MUST be 00. For instance, encapsulations such as
{VXLAN, NVGRE}, {VXLAN, GENEVE}, or {MPLS, MPLSoGRE, MPLSoUDP} can be
advertised in an A-D per ES route. In all these cases, the SHT value MUST be 00.
b. An A-D per ES route for ES-y may be advertised with multiple encapsulations
that all support both Split Horizon methods. In this case, the SHT value MAY be
01 if the preferred method is Local Bias, or 10 if the ESI Label-based method
is desired. For example, encapsulations such as {MPLSoGRE, MPLSoUDP, GENEVE}
(or a subset) MAY be advertised in an A-D per ES route with an SHT value of 01.
The ESI Label value in this case MAY be zero.
c. If ES-z with an RT-set composed of (RT1, RT2, RT3... RTn) supports multiple
encapsulations requiring different Split Horizon methods, a distinct A-D per ES
route (or group of routes) for each Split Horizon method MUST be advertised.
For example, consider an ES-z with n Route Targets (RTs) where:
* The EVIs corresponding to (RT1...RTi) support VXLAN.
* The EVIs for (RTi+1...RTm) (with i < m) support MPLSoUDP with Local Bias.
* The EVIs for (RTm+1...RTn) (with m < n) support GENEVE with ESI Label-based
Split Horizon.
In this scenario, three groups of A-D per ES routes MUST be advertised for ES-z:
* A-D per ES route group 1, including (RT1...RTi), with encapsulation {VXLAN},
and an SHT value of 00. The ESI Label MAY be zero.
* A-D per ES route group 2, including (RTi+1...RTm), with encapsulation
{MPLSoUDP}, and an SHT value of 01. The ESI Label MAY be zero.
* A-D per ES route group 3, including (RTm+1...RTn), with encapsulation
{GENEVE}, and an SHT value of 10. The ESI Label MUST have a valid, non-zero
value, and the Ethernet option as defined in [RFC 8926] MUST be advertised.
As per [RFC 8365], it is the responsibility of the operator of a given EVI to
ensure that all NVEs within that EVI support a common encapsulation. Failure to
meet this condition may result in service disruption or failure.
"
618 4. Security Considerations
617
620 The same security considerations described in [RFC7432] relevant to
621 multihoming apply to this document.
622
623 In addition, this document modifies the [RFC8365] procedures for
624 Split Horizon filtering, providing the operator with a choice between
625 Local Bias and ESI Label based filtering for the tunnels that support
626 both methods. A misconfiguration of the desired SHT to be used may
627 result in a forwarding behavior that is different from the intended
628 one. Other than that, this document describes procedures so that all
629 the PEs or NVEs attached to the same ES agree on a common SHT method,
630 therefore an attacker changing the configuration of the SHT should
631 not cause traffic disruption, only a change in the forwarding
632 behavior.
The text here reads slightly odd. WHat about following readability rewrite:
"
The security considerations relevant to multihoming described in [RFC 7432] are
applicable to this document.
Additionally, this document modifies the procedures for Split Horizon filtering
as outlined in [RFC 8365], offering operators a choice between Local Bias and
ESI Label-based filtering for tunnels that support both methods.
Misconfiguration of the desired Split Horizon Type (SHT) could lead to
forwarding behaviors that differ from the intended configuration. Apart from
this risk, this document describes procedures to ensure that all Provider Edge
(PE) devices or Network Virtualization Edges (NVEs) connected to the same
Ethernet Segment (ES) agree on a common SHT method. Consequently, unauthorized
changes to the SHT configuration by an attacker should not cause traffic
disruption but may result in alterations to forwarding behavior.
"
634 5. IANA Considerations
In which parent IANA codepoint registration does the new flags registery sit?
What is with bits 2, 3, 4, 5? reserved or unused? Please document somewhere in
the document and prescribe what must happen by a sender and received of these
fields.
While i could not find a specific formal IETF normative description of unused
or reserved bits the difference between "unused bits" and "reserved bits" is
often as follows:
* Reserved Bits: These are explicitly set aside for future use. They must be
set to a specific value, usually zero, and ignored by receivers. Reserved bits
ensure compatibility and extensibility for future updates or enhancements to
the protocol.
* Unused Bits: These bits currently have no defined function or purpose. They
are not used in the protocol's operation and are typically ignored by both
senders and receivers. Unused bits provide flexibility for potential future
uses without specific handling requirements in the current specification.
* Summary
Reserved Bits: Explicitly set aside for future use with specific handling
instructions.
Unused Bits: Currently unassigned and ignored, providing flexibility for future
definition.
636 This document creates a registry called "EVPN ESI Label Extended
637 Community Flags" for the 1-octet Flags field in the ESI Label
638 Extended Community. New registrations will be made through the "RFC
639 Required" procedure defined in [RFC8126]. Initial registrations are
640 made for the "Multihoming redundancy mode" field in bits 0 and 1, as
641 follows:
643 +=====+=============================+
644 | RED | Multihoming redundancy mode |
645 +=====+=============================+
646 | 00 | All-Active mode |
647 +-----+-----------------------------+
648 | 01 | Single-Active mode |
649 +-----+-----------------------------+
651 Table 2
Is there a reason the registration procedure is "RFC Required". There are only
few (8) bits available and maybe there should be IETF consensus before
allocating any of them. RFC Required "The RFC need not be in the IETF stream,
but may be in any RFC stream (currently an RFC may be in the IETF, IRTF, IAB,
or Independent Submission streams [RFC5742])." If the setting is slightly more
strict "IETF Review" then the following applies:
" With the IETF Review policy, new values are assigned only
through RFCs in the IETF Stream -- those that have been shepherded
through the IESG as AD-Sponsored or IETF working group documents
[RFC2026] [RFC5378], have gone through IETF Last Call, and have been
approved by the IESG as having IETF consensus."
The motivation for 2 bits where only 1 bit is used is unusual. Is the
high-order bit of RED for future use or is is reserved (because some
implementation is squatting on it?) or are they unused? There should be
guidelines about what value the unuased bit is set and what recipients
should/must set the value.
653 In addition, this document requests the registration of the "Split
654 Horizon Type" field in bits 6 and 7 of the Flags Octet of the EVPN
655 ESI Label extended community. This field is called "Split Horizon
656 Type" bits and it is defined as follows:
658 +=====+===========================+
659 | SHT | Split Horizon Type value |
660 +=====+===========================+
661 | 00 | Default SHT |
662 +-----+---------------------------+
663 | 01 | Local Bias |
664 +-----+---------------------------+
665 | 10 | ESI Label based filtering |
666 +-----+---------------------------+
667 | 11 | Reserved |
668 +-----+---------------------------+
670 Table 3
Is the setting SHT setting 11 "Reserved" or "Unused"?
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