IDR WorkGroup D. Rao, Ed.
Internet-Draft S. Agrawal, Ed.
Intended status: Experimental Cisco Systems
Expires: 14 September 2023 Co-authors
section 11
13 March 2023
BGP Color-Aware Routing (CAR)
draft-ietf-idr-bgp-car-01
Abstract
This document describes a BGP based routing solution to establish
end-to-end intent-aware paths across a multi-domain service provider
transport network. This solution is called BGP Color-Aware Routing
(BGP CAR).
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Illustration . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 7
2. BGP CAR SAFI . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Data Model . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Extensible encoding . . . . . . . . . . . . . . . . . . . 8
2.3. BGP CAR Route Origination . . . . . . . . . . . . . . . . 8
2.4. BGP CAR Route Validation . . . . . . . . . . . . . . . . 8
2.5. BGP CAR Route Resolution . . . . . . . . . . . . . . . . 9
2.6. AIGP Metric Computation . . . . . . . . . . . . . . . . . 10
2.7. Path Availability . . . . . . . . . . . . . . . . . . . . 10
2.8. BGP CAR signaling through different color domains . . . . 11
2.9. Format and Encoding . . . . . . . . . . . . . . . . . . . 12
2.9.1. BGP CAR SAFI NLRI Format . . . . . . . . . . . . . . 12
2.9.2. Color-Aware Routes NLRI Type . . . . . . . . . . . . 13
2.9.3. Local-Color-Mapping (LCM) Extended Community . . . . 18
2.10. LCM and BGP Color Extended Community usage . . . . . . . 19
2.11. Error Handling . . . . . . . . . . . . . . . . . . . . . 20
3. Service route Automated Steering on Color-Aware path . . . . 22
4. Intents . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5. (E, C) Subscription and Filtering . . . . . . . . . . . . . . 23
5.1. Illustration . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Definition . . . . . . . . . . . . . . . . . . . . . . . 24
6. Scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1. Ultra-Scale Reference Topology . . . . . . . . . . . . . 24
6.2. Deployment model . . . . . . . . . . . . . . . . . . . . 26
6.2.1. Flat . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2.2. Hierarchical Design with next-hop-self at ingress
domain BR . . . . . . . . . . . . . . . . . . . . . . 27
6.2.3. Hierarchical Design with Next Hop Unchanged at ingress
domain BR . . . . . . . . . . . . . . . . . . . . . . 29
6.3. Scale Analysis . . . . . . . . . . . . . . . . . . . . . 30
6.4. Scaling Benefits of the (E, C) BGP Subscription and
Filtering . . . . . . . . . . . . . . . . . . . . . . . . 32
6.5. Anycast SID . . . . . . . . . . . . . . . . . . . . . . . 32
6.5.1. Anycast SID for transit inter-domain nodes . . . . . 32
6.5.2. Anycast SID for transport color endpoints (e.g.,
PEs) . . . . . . . . . . . . . . . . . . . . . . . . 33
7. Routing Convergence . . . . . . . . . . . . . . . . . . . . . 33
8. VPN CAR . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
9.1. BGP CAR NLRI Types Registry . . . . . . . . . . . . . . . 35
9.2. BGP CAR NLRI TLV Registry . . . . . . . . . . . . . . . . 35
9.3. Guidance for Designated Experts . . . . . . . . . . . . . 36
9.4. BGP Extended Community Registry . . . . . . . . . . . . . 36
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10. Manageability Considerations . . . . . . . . . . . . . . . . 36
11. Co-authors . . . . . . . . . . . . . . . . . . . . . . . . . 37
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 37
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
14.1. Normative References . . . . . . . . . . . . . . . . . . 38
14.2. Informative References . . . . . . . . . . . . . . . . . 40
Appendix A. Illustrations of Service Steering . . . . . . . . . 41
A.1. E2E BGP transport CAR intent realized using IGP
FlexAlgo . . . . . . . . . . . . . . . . . . . . . . . . 41
A.2. E2E BGP transport CAR intent realized using SR Policy . . 43
A.3. BGP transport CAR intent realized in a section of the
network . . . . . . . . . . . . . . . . . . . . . . . . . 45
A.3.1. Provide intent for service flows only in core domain
running ISIS FlexAlgo . . . . . . . . . . . . . . . . 45
A.3.2. Provide intent for service flows only in core domain
over TE tunnel mesh . . . . . . . . . . . . . . . . . 47
A.4. Transit network domains that do not support CAR . . . . . 49
A.5. Resource Avoidance using BGP CAR and IGP Flex-Algo . . . 50
A.6. Per-Flow Steering over CAR routes . . . . . . . . . . . . 52
A.7. Advertising BGP CAR routes for shared IP addresses . . . 53
Appendix B. Color Mapping Illustrations . . . . . . . . . . . . 55
B.1. Single color domain containing network domains with N:N
color distribution . . . . . . . . . . . . . . . . . . . 55
B.2. Single color domain containing network domains with N:M
color distribution . . . . . . . . . . . . . . . . . . . 55
B.3. Multiple color domains . . . . . . . . . . . . . . . . . 56
Appendix C. CAR SAFI NLRI update packing efficiency
calculation . . . . . . . . . . . . . . . . . . . . . . . 57
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57
1. Introduction
This document specifies a new BGP SAFI called BGP Color-Aware Routing
(BGP CAR). BGP CAR fulfills the transport and VPN problem statement
and requirements described in
[I-D.hr-spring-intentaware-routing-using-color].
1.1. Terminology
+=============+=============================================+
+=============+=============================================+
| Intent | Any combination of the following behaviors: |
| | a/ Topology path selection (e.g. minimize |
| | metric, avoid resource), b/ NFV service |
| | insertion (e.g. service chain steering), c/ |
| | per-hop behavior (e.g. 5G slice). |
+-------------+---------------------------------------------+
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+-------------+---------------------------------------------+
| Color | A 32-bit numerical value associated with an |
| | intent (e.g. low-cost , low-delay, avoid |
| | some resources etc.) as defined in |
| | [RFC9256] |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| Colored | An egress PE (e.g. E2) colors its BGP |
| Service | service (e.g. VPN) route V/v to indicate |
| Route | the intent that it requests for the traffic |
| | bound to V/v. The color is encoded as a |
| | BGP Color Extended community [RFC9012]. |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| Color-Aware | A routed path to E2 which satisfies the |
| Path to | intent associated with color C. Several |
| (E2, C) | technologies may provide a Color-Aware Path |
| | to (E2, C): SR Policy [RFC9256], IGP Flex- |
| | Algo [RFC9350], BGP CAR [specified in this |
| | document]. |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| Color-Aware | A distributed or signaled route that builds |
| Route (E2, | a color-aware path to E2 for color C. |
| C) | |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| Service | E1 automatically steers a C-colored service |
| Route | route V/v from E2 onto an (E2, C) path. If |
| Automated | several such paths exist, a preference |
| Steering on | scheme is used to select the best path (for |
| Color-aware | example, IGP Flex-Algo first then SR Policy |
| path | then BGP CAR. |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| Color | A set of nodes which share the same Color- |
| Domain | to-Intent mapping, typically under single |
| | administration. This set can be organized |
| | in one or several IGP instances or BGP |
| | domains. Color re-mapping may happen at |
| | color domain boundaries. |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| Resolution | An inter-domain BGP CAR route (E, C) from N |
| of a BGP | is resolved on an intra-domain color-aware |
| CAR route | path (N, C) where N is the next-hop of the |
| (E, C) | BGP CAR route. |
+-------------+---------------------------------------------+
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+-------------+---------------------------------------------+
| Resolution | In this document and consistently with the |
| vs Steering | terminology of the SR Policy document |
| | [RFC9256], steering is used to describe the |
| | mapping of a service route onto a BGP CAR |
| | path while the term resolution is preserved |
| | for the mapping of an inter-domain BGP CAR |
| | route on an intra-domain color-aware path. |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| | Service Steering: Service route maps to BGP |
| | CAR path (or other Color-Aware Routed |
| | Paths: e.g. SR Policy) |
+-------------+---------------------------------------------+
+-------------+---------------------------------------------+
| | Intra-Domain Resolution: BGP CAR route maps |
| | to intra-domain color aware path (e.g. SR |
| | Policy, IGP Flex-Algo, BGP CAR) or |
| | traditional routing/TE path (e.g. RSVP-TE, |
| | IGP/LDP, BGP-LU) |
+-------------+---------------------------------------------+
Table 1
1.2. Illustration
Here is a brief illustration of the salient properties of the BGP CAR
solution.
+-------------+ +-------------+ +-------------+
| | | | | | V/v with C1
|----+ |------| |------| +----|/
| E1 | | | | | | E2 |\
|----+ | | | | +----| W/w with C2
| |------| |------| |
| Domain 1 | | Domain 2 | | Domain 3 |
+-------------+ +-------------+ +-------------+
Figure 1
All the nodes are part of an interdomain network under a single
authority and with a consistent color-to-intent mapping:
* C1 is mapped to "low-delay"
- Flex-Algo FA1 is mapped to "low delay" and hence to C1
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* C2 is mapped to "low-delay and avoid resource R"
- Flex-Algo FA2 is mapped to "low delay and avoid resource R" and
hence C2
E1 receives two service routes from E2:
* V/v with BGP Color Extended-Community C1
* W/w with BGP Color Extended-Community C2
E1 has the following color-aware paths:
* (E2, C1) provided by BGP CAR with the following per-domain
support:
- Domain1: over IGP FA1
- Domain2: over SR Policy bound to color C1
- Domain3: over IGP FA1
* (E2, C2) provided by SR Policy
E1 automatically steers the received service routes as follows:
* V/v via (E2, C1) provided by BGP CAR
* W/w via (E2, C2) provided by SR Policy
Illustrated Properties:
* Leverage of the BGP Color Extended-Community
- The service routes are colored with widely-used BGP Color
Extended-Community
* (E, C) Automated Steering
- V/v and W/w are automatically steered on the appropriate color-
aware path
* Seamless co-existence of BGP CAR and SR Policy
- V/v is steered on BGP CAR color-aware path
- W/w is steered on SR Policy color-aware path
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* Seamless interworking of BGP CAR and SR Policy
- V/v is steered on a BGP CAR color-aware path that is itself
resolved within domain 2 onto an SR Policy bound to the color
of V/v
Other properties:
* MPLS dataplane: with 300k PE's and 5 colors, the BGP CAR solution
ensures that no single node needs to support a dataplane scaling
in the order of Remote PE * C. This would otherwise exceed the
MPLS dataplane.
* Control-Plane: a node should not install a (E, C) path if it does
not need it
* Incongruent Color-Intent mapping: the solution supports the
signaling of a BGP CAR route across different color domains
The keys to this simplicity are:
* the leverage of the BGP Color Extended-Community to color service
routes
* the definition of the automated steering: a C-colored service
route V/v from E2 is steered onto a color-aware path (E2, C)
* the definition of the data model of a BGP CAR path: (E, C)
- natural extension of BGP IP/LU data model (E)
- consistent with SR Policy data model
* the definition of the recursive resolution of a BGP CAR route: a
BGP CAR (E2, C) via N is resolved onto the color-aware path (N, C)
which may itself be provided by BGP CAR or via another color-aware
routing solution: SR Policy, IGP Flex-Algo.
* Native support for multiple transport encapsulations (e.g., MPLS,
SR, SRv6, IP)
1.3. Requirements Language
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. BGP CAR SAFI
2.1. Data Model
The BGP CAR data model is:
* NLRI Key: IP Prefix, Color
* NLRI non-key encapsulation data: MPLS label stack, Label index,
SRv6 SID list etc.
* BGP Next Hop
* AIGP Metric: accumulates color/intent specific metric across
domains
* Local-Color-Mapping Extended-Community (LCM-EC): Optional 32-bit
Color value used when a CAR route propagates between different
color domains
2.2. Extensible encoding
Extensible encoding is ensured by:
* NLRI Route-Type field: provides extensibility to add new NLRI
formats for new route-types
* Key length: field enables handling of unsupported route-types
opaquely, enabling transitivity via RRs
* TLV-based encoding of non-key part of NLRI: enables flexible
support for multiple encapsulations with efficient update packing
* AIGP Attribute provides extensibility via TLVs, enabling
definition of additional metric semantics for a color as needed
for an intent
2.3. BGP CAR Route Origination
A BGP CAR route may be originated locally (e.g., loopback) or through
redistribution of an (E, C) color-aware path provided by another
routing solution: SR Policy, IGP Flex-Algo, RSVP-TE or BGP-LU
[RFC8277].
2.4. BGP CAR Route Validation
A BGP CAR path (E, C) from N with encapsulation T is valid if color-
aware path (N, C) exists with encapsulation T available in dataplane.
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A local policy may customize the validation process:
* the color constraint in the first check may be relaxed: instead N
is reachable via alternate color(s) or in the default routing
table
* the dataplane availability constraint of T may be relaxed, to use
an alternate encapsulation
* a performance-measurement verification may be added to ensure that
the intent associated with C is met (e.g. delay < bound)
A path that is not valid MUST NOT be considered for path selection.
2.5. BGP CAR Route Resolution
A BGP color-aware route (E2, C1) from N is automatically resolved
over a color-aware route (N, C1) by default. The color-aware route
(N, C1) is provided by color aware mechanisms such as IGP Flex-Algo,
SR policy or recursively by BGP CAR. When multiple producers of
(N,C1) are available, the default preference is: IGP Flex-Algo, SR
Policy, BGP CAR.
Local policy can provide additional control and options:
* A BGP color-aware route (E2, C1) from N may be resolved over a
color-aware route (N, C2): i.e. the local policy maps the
resolution of C1 over C2.
- For example, in a domain where resource R is known to not be
present, the inter-domain intent C1="low delay and avoid R" may
be resolved over an intra-domain path of intent C2="low delay".
- Another example is, if no (N, C1) path is available, and the
user has allowed resolution to fallback via C2
* Resolution may be mapped to traditional mechanisms that are
unaware of color, such as RSVP-TE, IGP/LDP, BGP LU (e.g.,
Appendix A.3.2).
Route resolution via different color may be automated by attaching
BGP Color extended Community C2 to CAR route (E2, C1), leveraging
Automated steering as described section 8.4 of Segment Routing Policy
Architecture [RFC9256] for BGP CAR routes. This mechanism is
illustrated in section B.2.
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Local policy takes precedence over default color based automated
resolution. For a CAR route, Color-EC color takes precedence over
route NLRI color.
The color-aware route (N, C1) may have a different dataplane
encapsulation than the one of (E2, C1): e.g. a BGP CAR route (E2, C1)
with SR-MPLS encapsulation may be transported over an intermediate
SRv6 domain.
2.6. AIGP Metric Computation
The Accumulated IGP (AIGP) Attribute [RFC7311] is updated as the BGP
CAR route propagates across the network.
The value set (or appropriately incremented) in the AIGP TLV
corresponds to the metric associated with the underlying intent of
the color. For example, when the color is associated with a low-
latency path, the metric value is set based on the delay metric.
Information regarding the metric type used by the underlying intra-
domain mechanism can also be set.
If BGP CAR routes traverse across a discontinuity in the transport
path for a given intent, add a penalty in accumulated IGP metric.
AIGP metric computation is recursive.
To avoid continuous IGP metric changes causing end to end BGP CAR
churn, an implementation should provide thresholds to trigger AIGP
update.
Additional AIGP extensions may be defined to signal state for
specific use-cases: MSD along the BGP CAR advertisement, Minimum MTU
along the BGP CAR advertisement.
2.7. Path Availability
The (E, C) route inherently provides availability of redundant paths
at every hop, identical to BGP-LU or BGP IP. For instance, BGP CAR
routes originated by two or more egress ABRs in a domain are
advertised as multiple paths to ingress ABRs in the domain, where
they become equal-cost or primary-backup paths. A failure of an
egress ABR is detected and handled by ingress ABRs locally within the
domain for faster convergence, without any necessity to propagate the
event to upstream nodes for traffic restoration.
BGP ADD-PATH should be enabled for BGP CAR to signal multiple next
hops through a transport RR.
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2.8. BGP CAR signaling through different color domains
[Color Domain 1 A]-----[B Color Domain 2 E2]
[C1=low-delay ] [C2=low-delay ]
Let us assume a BGP CAR route (E2, C2) is signaled from B to A; two
border routers of respectively domain 2 and domain 1. Let us assume
that these two domains do not share the same color-to-intent mapping.
Low-delay in domain 2 is color C2 while C1 in domain 1 (C1 <> C2).
The BGP CAR solution seamlessly supports this (rare) scenario while
maintaining the separation and independence of the administrative
authority in different color domains.
The solution works as follows:
* Within domain 2, the BGP CAR route is (E2, C2) via E2
* B signals to A the BGP CAR route as (E2, C2) via B with Local-
Color-Mapping-Extended-Community (LCM-EC) of color C2
* A is aware (classic peering agreement) of the intent-to-color
mapping within domain 2 ("low-delay" in domain 2 is C2)
* A maps C2 in LCM-EC to C1 and signals within domain 1 the received
BGP CAR route as (E2, C2) via A with LCM-EC(C1)
* The nodes within the receiving domain 1 use the local color
encoded in the LCM-EC for next-hop resolution and service steering
Salient properties:
* The NLRI never changes
* E is globally unique, which makes E-C in that order unique
* In the vast majority of the cases, the color of the NLRI is used
for resolution and steering
* In the rare case of color incongruence, the local color encoded in
LCM-EC takes precedence
Further illustrations are provided in Appendix B.
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2.9. Format and Encoding
BGP CAR leverages the BGP multi-protocol extensions [RFC4760] and
uses the MP_REACH_NLRI and MP_UNREACH_NLRI attributes for route
updates by using the SAFI value TBD1 along with AFI 1 for IPv4
prefixes and AFI 2 for IPv6 prefixes.
BGP speakers MUST use BGP Capabilities Advertisement to ensure
support for processing of BGP CAR updates. This is done as specified
in [RFC4760], by using capability code 1 (multi-protocol BGP), with
AFI 1 and 2 (as required) and SAFI TBD1.
The sub-sections below specify the generic encoding of the BGP CAR
NLRI followed by the encoding for specific NLRI types introduced in
this document.
2.9.1. BGP CAR SAFI NLRI Format
The generic format for the BGP CAR SAFI NLRI is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLRI Length | Key Length | NLRI Type | //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ //
| Type-specific Key Fields //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-specific Non-Key Fields (if applicable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* NLRI Length: 1 octet field that indicates the length in octets of
the NLRI excluding the NLRI Length field itself.
* Key Length: 1 octet field that indicates the length in octets of
the NLRI type-specific key fields. Key length MUST be at least 2
less than the NLRI length.
* NLRI Type: 1 octet field that indicates the type of the BGP CAR
NLRI.
* Type-Specific Key Fields: Depend on the NLRI type and of length
indicated by the Key Length.
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* Type-Specific Non-Key Fields: optional and variable depending on
the NLRI type. The NLRI definition allows for encoding of
specific non-key information associated with the route (i.e. the
key) as part of the NLRI for efficient packing of BGP updates.
The indication of the key length enables BGP Speakers to determine
the key portion of the NLRI and use it along with the NLRI Type field
in an opaque manner for handling of unknown or unsupported NLRI
types. This can help deployed Route Reflectors (RR) to propagate
NLRI types introduced in the future in a transparent manner.
It also helps make error handling more resilient and minimally
disruptive as described in section Section 2.11.
A route (NLRI) can carry more than one non-key TLV (of different
types). This provides significant benefits such as signaling
multiple encapsulations simultaneously for the same route, each with
a different value (label/SID etc). This enables simpler, efficient
migrations with low overhead :
* avoids need for duplicate routes to signal different
encapsulations
* avoids need for separate control planes for distribution
* preserves update packing (e.g. Appendix C)
The non-key portion of the NLRI MUST be omitted while carrying it
within the MP_UNREACH_NLRI when withdrawing the route advertisement.
2.9.2. Color-Aware Routes NLRI Type
The Color-Aware Routes NLRI Type is used for advertisement of color-
aware routes and has the following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLRI Length | Key Length | NLRI Type |Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Prefix (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Followed by optional TLVs encoded as below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* NLRI Length: variable
* Key Length: variable. It indicates the total length comprised of
the Prefix Length field, IP Prefix field, and the Color field, as
described below. For IPv4 (AFI=1), the minimum length is 5 and
maximum length is 9. For IPv6 (AFI=2), the minimum length is 5
and maximum length is 21.
* NLRI Type: 1
* Type-Specific Key Fields: as below
- Prefix Length: 1 octet field that carries the length of prefix
in bits. Length MUST be less than or equal to 32 for IPv4
(AFI=1) and less than or equal to 128 for IPv6 (AFI=2).
- IP Prefix: IPv4 or IPv6 prefix (based on the AFI). A variable
size field that contains the most significant octets of the
prefix, i.e., 0 octet for prefix length 0, 1 octet for prefix
length 1 to 8, 2 octets for prefix length 9 to 16, 3 octets for
prefix length 17 up to 24, 4 octets for prefix length 25 up to
32, and so on. Last octet has enough trailing bits to make the
end of the field fall on an octet boundary. Note that the
value of the trailing bits is irrelevant. The size of the
field MUST be less than or equal to 4 for IPv4 (AFI=1) and less
than or equal to 16 for IPv6 (AFI=2).
- Color: 4 octets that contains color value associated with the
prefix.
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* Type-Specific Non-Key Fields: specified in the form of optional
TLVs as below:
- Type: 1 octet that contains the type code and flags. It is
encoded as shown below:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R|T| Type code |
+-+-+-+-+-+-+-+-+
where:
o R: Bit is reserved and MUST be set to 0 and ignored on
receive.
o T: Transitive bit, applicable to speakers that change the
BGP CAR next hop
+ T bit set to indicate TLV is transitive. An unrecognized
transitive TLV MUST be propagated by a speaker that
changes the next hop
+ T bit unset to indicate TLV is non-transitive. An
unrecognized non-transitive TLV MUST not be propagated by
a speaker that changes next hop
A speaker that does not change next hop SHOULD propagate all
received TLVs.
o Type code: Remaining 6 bits contain the type of the TLV.
- Length: 1 octet field that contains the length of the value
portion of the non-key TLV in terms of octets
- Value: variable length field as indicated by the length field
and to be interpreted as per the type field.
The prefix is routable across the administrative domain where BGP
transport CAR is deployed. It is possible that the same prefix is
originated by multiple BGP CAR speakers in the case of anycast
addressing or multi-homing.
The Color is introduced to enable multiple route advertisements for
the same prefix. The color is associated with an intent (e.g. low-
latency) in originator color-domain.
The following sub-sections specify the non-key TLVs associated with
the Color-Aware Routes NLRI type.
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2.9.2.1. Label TLV
The Label TLV is used for advertisement of color-aware routes along
with their MPLS labels and has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Followed by one (or more) Labels encoded as below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label |Rsrv |S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* Type : Type code is 1. T bit MUST be unset
* Length: variable, MUST be a multiple of 3
* Label Information: multiples of 3 octet fields to convey the MPLS
label(s) associated with the advertised color-aware route. It is
used for encoding a single label or a stack of labels as per
procedures specified in [RFC8277].
When a BGP transport CAR speaker is propagating the route further
after setting itself as the nexthop, it allocates a local label for
the specific prefix and color combination which it updates in this
TLV. It also MUST program a label cross-connect that would result in
the label swap operation for the incoming label that it advertises
with the label received from its best-path router(s).
2.9.2.2. Label Index TLV
The Label Index TLV is used for advertisement of Segment Routing MPLS
(SR-MPLS) Segment Identifier (SID) [RFC8402] information associated
with the labeled color-aware routes and has the following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved | Flags ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ | Label Index ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+
where:
* Type : Type code is 2. T bit MUST be set
* Length: 7
* Reserved: 1 octet field that MUST be set to 0 and ignored on
receipt.
* Flags: 2 octet field that maps to the Flags field of the Label-
Index TLV of the BGP Prefix SID Attribute [RFC8669].
* Label Index: 4 octet field that maps to the Label Index field of
the Label-Index TLV of the BGP Prefix SID Attribute [RFC8669].
This TLV provides the equivalent functionality as Label-Index TLV of
[RFC8669] for Transport CAR route in SR-MPLS deployments. It
provides much better packing efficiency by carrying label Index in
NLRI instead of the BGP Prefix SID attribute. The BGP Prefix SID
Attribute SHOULD be omitted from the labeled color-aware routes when
the attribute is being used to only convey the Label Index TLV.
When a BGP Transport CAR speaker is propagating the route further
after setting itself as the nexthop, it allocates a local label for
the specific prefix and color combination. When the received update
has the Label Index TLV, it SHOULD use that hint to allocate the
local label from the SR Global Block (SRGB) using procedures as
specified in [RFC8669].
2.9.2.3. SRv6 SID TLV
BGP Transport CAR can be also used to setup end-to-end color-aware
connectivity using Segment Routing over IPv6 (SRv6) [RFC8402].
[RFC8986] specifies the SRv6 Endpoint behaviors (e.g. End PSP) which
MAY be leveraged for BGP CAR with SRv6.The SRv6 SID TLV is used for
advertisement of color-aware routes along with their SRv6 SIDs and
has the following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | SRv6 SID Info (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* Type : Type code is 3. T bit MUST be unset
* Length: variable, MUST be either less than or equal to 16, or be a
multiple of 16
* SRv6 SID Information: field of size as indicated by the length
that either carries the SRv6 SID(s) for the advertised color-aware
route as one of the following:
- A single 128-bit SRv6 SID or a stack of 128-bit SRv6 SIDs
- A transposed portion (refer [RFC9252]) of the SRv6 SID that
MUST be of size in multiples of one octet and less than 16.
BGP CAR SRv6 SID TLV definitions provide the following benefits:
* Native encoding of SIDs avoids robustness issue caused by
overloading of MPLS label fields.
* Simple encoding to signal Unique SIDs (non-transposition),
maintaining BGP update prefix packing
* Highly efficient transposition scheme (12-14 bytes saved per
NLRI), also maintaining BGP update prefix packing
The BGP color-aware route update for SRv6 MUST include the BGP
Prefix-SID attribute along with the TLV carrying the SRv6 SID
information as specified in [RFC9252] when using the transposition
scheme of encoding for packing efficiency of BGP updates.
2.9.3. Local-Color-Mapping (LCM) Extended Community
This document defines a new BGP Extended Community called "LCM". The
LCM is a Transitive Opaque Extended Community with the following
encoding:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x3 | Sub-Type=TBD2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
* Type: 0x3
* Sub-Type: TBD2.
* Reserved: 2 octet of reserved field that MUST be set to zero on
transmission and ignored on reception.
* Color: 4-octet field that carries the 32-bit color value.
When a CAR route crosses the originator's color domain boundary, LCM-
EC is added. LCM-EC conveys the local color mapping for the intent
(e.g. low latency) in other (transit or destination) color domains.
An implementation SHOULD NOT send more than one instance of the LCM-
EC. However, if more than one instance is received, an
implementation MUST disregard all instances other than the one with
the numerically highest value.
If present, LCM-EC is the effective intent of a BGP CAR route.
LCM-EC Color is used instead of the Color in CAR route NLRI for
procedures described in earlier sections such as route validation,
resolution, AIGP calculation and steering.
The LCM-EC MAY be used for filtering of BGP CAR routes and/or for
applying routing policies for the intent, when present.
2.10. LCM and BGP Color Extended Community usage
There are 2 distinct requirements to be supported as stated in
[I-D.hr-spring-intentaware-routing-using-color]":
1. Domains with different intent granularity (section 6.3.1.9)
2. Network domains under different administration, i.e. color
domains (section 4.1)
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Requirement 1 is the case where within the same administrative or
color domain, BGP CAR routes for N end-to-end intents may need to
traverse across an intermediate domain where only M intents are
available, N >= M. Example: a multi-domain network is designed as
Access-Core-Access. The core may have the most granular N intents,
whereas the access only has fewer M intents. So, the BGP nexthop
resolution for a CAR route in the access domain must be via a color-
aware path for one of these M intents. As described in Section 2.5
and Appendix B.2, BGP Color Extended Community is used to automate
the CAR route resolution.
For requirement 2, where CAR routes traverse across different color
domains, LCM-EC is used to carry the local color mapping for the NLRI
color in other color domains as already described in Section 2.8 and
Appendix.B.3
Both LCM-EC and BGP Color Extended Community may be present with a BGP
CAR route. Example: BGP CAR route (E, C1) from color domain D1, with
LCM-EC C2 in color domain D2, may also carry Color-EC C3 and next-hop
N in a transit network domain within D2 that has intent C3 available
and resolves C2 via C3.
Default order of processing for resolution in presence of LCM-EC is
local policy, then BGP Color-EC color, and finally LCM-EC color.
2.11. Error Handling
The error handling actions as described in [RFC7606] are applicable
for handling of BGP update messages for BGP-CAR. In general, as
indicated in [RFC7606], the goal is to minimize the disruption of a
session reset or 'AFI/SAFI disable' to the extent possible.
When the error determined allows for the router to skip the malformed
NLRI(s) and continue processing of the rest of the update message,
then it MUST handle such malformed NLRIs as 'Treat-as-withdraw'. In
other cases, where the error in the NLRI encoding results in the
inability to process the BGP update message, then the router SHOULD
handle such malformed NLRIs as 'AFI/SAFI disable' when other AFI/SAFI
besides BGP-CAR are being advertised over the same session.
Alternately, the router MUST perform 'session reset' when the session
is only being used for BGP-CAR.
The CAR NLRI definition encodes NLRI length and key length
explicitly. The NLRI length MUST be relied upon to enable the
beginning of the next NLRI field to be located. Key length MUST be
relied upon to extract the key and perform 'treat-as-withdraw' for
malformed information.
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A sender MUST ensure that the NLRI and key lengths are number of
actual bytes encoded in NLRI and key fields respectively, regardless
of content being encoded.
Given NLRI length and Key length MUST be valid, failures in following
checks result in 'AFI/SAFI disable' or 'session reset':
* Minimum NLRI length (must be atleast 2, as key length and NLRI
type are required fields)
* Key Length MUST be at least two less than NLRI Length
NLRI Type specific error handling:
* By default, a speaker SHOULD discard unrecognized or unsupported
NLRI type and move to next NLRI.
* Key length and key errors of known NLRI type SHOULD result in
discard of NLRI similar to unrecognized NLRI type.(This MUST be
logged for trouble shooting).
Transparent propagation of unrecognized NLRI type:
* Key length allows unrecognized route types to transit through RR
transparently without a software upgrade. Such RR does not need
to interpret key portion of NLRI and works on opaque key of given
length. An implementation SHOULD provide a knob that controls the
RR unrecognized route type propagation behavior and possibly at
granularity of route type values allowed. This gives ability to
operator to allow specific route type transparent reflection based
on client speaker support.
* In such a case RR may reflect NLRIs with NLRI type specific key
length and field errors. Clients of such RR that consume the
route for installation will perform the key error handling of
known NLRI type or discard unrecognized type. This prevents
propagation of routes with NLRI errors any further in network.
Type-Specific Non-Key TLV handling:
* Either the length of a TLV would cause the NLRI length to be
exceeded when parsing the TLV, or fewer than 2 bytes remain when
beginning to parse the TLV. In either of these cases, an error
condition exists and the 'treat-as-withdraw' approach MUST be used
* Type specific length constraints should be verified. The TLV MUST
be discarded if there is an error.
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* If multiple instances of same type are encountered, all but the
first instance MUST be ignored.
* If multiple instances of same type are encountered, all but the
first instance MUST be ignored.
* A TLV is not considered malformed because of failing any semantic
validation of its Value field.
* Speaker modifying the BGP next-hop MUST recognize at least one of
the forwarding information TLVs (such as label and SRv6 SID). If
it is not able to, such NLRI is considered invalid and not
eligible for best path selection.
3. Service route Automated Steering on Color-Aware path
E1 automatically steers a C-colored service route V/v from E2 onto an
(E2, C) color-aware path. If several such paths exist, a preference
scheme is used to select the best path: E.g. IGP Flex-Algo first
then SR Policy then BGP CAR.
This is consistent with the automated service route steering on SR
Policy (a routing solution providing color-aware path) defined in
[RFC9256]. All the steering variations defined in [RFC9256] are
applicable to BGP CAR color-aware path: on-demand steering, per-
destination, per-flow, CO-only. For brevity, in this revision, we
refer the reader to the [RFC9256] text.
Salient property: Seamless integration of BGP CAR and SR Policy.
Service steering via BGP CAR is applicable to any BGP SAFI, including
SAFIs for IPv4/IPv6, L3VPN, PW, EVPN, FlowSpec, and BGP-LU.
Appendix A provides illustrations of service route automated
steering.
4. Intents
The widely deployed color-aware path SR Policy solution demonstrates
that the following intents can easily be associated with a color:
1. Minimization of a cost metric vs a latency metric
* Minimization of different metric types, static and dynamic
2. Exclusion/Inclusion of SRLG and/or Link Affinity and/or minimum
MTU/number of hops
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3. Bandwidth management
4. In the inter-domain context, exclusion/inclusion of entire
domains, and border routers
5. Inclusion of one or several virtual network function chains
* Located in a regional domain and/or core domain, in a DC
6. Localization of the virtual network function chains
* Some functions may be desired in the regional DC or vice versa
7. Per-Destination and Per-Flow steering
It is straightforward to note that the BGP CAR color-aware
alternative supports intents 1, 2, 4 and 7.
Future revisions of this document will analyze the BGP CAR supports
for 3, 5 and 6.
5. (E, C) Subscription and Filtering
This section defines an (E, C) BGP subscription model that allows to
filter the (E, C) routes learned by a BGP CAR node.
5.1. Illustration
E1-----------------A-------------------B-------------------E2
<--- (E2, C1) ----
-- F (E2, C1) --> --- F (E2, C1) -->
| |
<-- (E2, C1) ---- <--- (E2, C1) ----
* BGP CAR route (E2, C1) advertised by E2 is not unconditionally
distributed beyond a certain point (e.g., B)
* E1 subscribes to (E2, C1) by advertising a filter route F (E2, C1)
to its upstream peer A
* If A has (E2, C1) in its BGP RIB, it will advertise (E2, C1) to E1
* If A does not have (E2, C1), it will advertise F (E2, C1) to its
peer B
* B will advertise (E2, C1) to A, which will distribute it to E1
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E1 may trigger a subscription for BGP CAR route (E2, C1) as a result
of receiving a C1-colored service route V/v from E2, for on-demand
steering via (E2, C1).
5.2. Definition
future version of this document
6. Scaling
This section analyses the key scale requirement of [ref:dskc-bess-
bgp-car-problem-statement], specifically:
* No intermediate node dataplane should need to scale to (Colors *
PEs)
* No node should learn and install a BGP CAR route to (E,C) if it
does not install a Colored service route to E
Two key principles used to address the scaling requirements are a
hierarchical network and routing design, and on-demand route
subscription and filtering.
Figure 2 provides an ultra-scale reference topology. Section 6.2
presents three design models to deploy BGP CAR in the reference
topology, including hierarchical options. Section 6.3 analyses the
scaling properties of each model. Section 6.4 illustrates the
scaling benefits of the (E, C) BGP subscription and filtering.
6.1. Ultra-Scale Reference Topology
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RD:V/v via E2
+-----+ +-----+ vpn label:30030 +-----+
....... |S-RR1| <........... |S-RR2| <...............|S-RR3| <......
: +-----+ +-----+ Color C1 +-----+ :
: :
: :
: :
+:------------+--------------+--------------+--------------+--------:-+
|: | | | | : |
|: | | | | : |
|: +---+ +---+ +---+ +---+ : |
|: |121| |231| |341| |451| : |
|: +---+ +---+ +---+ +---+ : |
|---+ | | | | +---|
| E1| | | | | | E2|
|---+ | | | | +---|
| +---+ +---+ +---+ +---+ |
| |122| |232| |342| |452| |
| +---+ +---+ +---+ +---+ |
| Access | Metro | Core | Metro | Access |
| domain 1 | domain 2 | domain 3 | domain 4 | domain 5 |
+-------------+--------------+--------------+--------------+----------+
iPE iBRM iBRC eBRC eBRM ePE
Figure 2: Ultra-Scale Reference Topology
The following applies to the reference topology above:
* Independent ISIS/OSPF SR instance in each domain.
* Each domain has Flex Algo 128. Prefix SID for a node is SRGB
168000 plus node number.
* A BGP CAR route (E2, C1) is advertised by egress BRM node 451.The
route is sourced locally from redistribution from IGP-FA 128.
* Not shown for simplicity, node 452 will also advertise (E2, C1).
* When a transport RR is used within the domain or across domains,
ADD-PATH is enabled to advertise paths from both egress BRs to
it's clients.
* Egress PE E2 advertises a VPN route RD:V/v with BGP Color extended
community C1 that propagates via service RRs to ingress PE E1.
* E1 steers V/v prefix via color-aware path (E2,C1) and VPN label
30030
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6.2. Deployment model
6.2.1. Flat
RD:V/v via E2
+-----+ +-----+ vpn label:30030 +-----+
....... |S-RR1| <........... |S-RR2| <...............|S-RR3| <......
: +-----+ +-----+ Color C1 +-----+ :
: :
: :
: :
+:------------+--------------+--------------+--------------+--------:-+
|: | | | | : |
|: | (E2,C1) | (E2,C1) | (E2,C1) | : |
|: +---+ via 231 +---+ via 341 +---+ via 451 +---+ : |
|:(E2,C1) |121|<---------|231|<---------|341|<---------|451| : |
|: via 121 /+---+ L=168002 +---+ L=168002 +---+ L=168002 +---+ : |
|---+ / | | | | +---|
| E1| <--/ | | | | | E2|
|---+ L=168002| | | | +---|
| +---+ +---+ +---+ +---+ |
| |122| |232| |342| |452| |
| +---+ +---+ +---+ +---+ |
| Access | Metro | Core | Metro | Access |
| domain 1 | domain 2 | domain 3 | domain 4 | domain 5 |
+-------------+--------------+--------------+--------------+----------+
iPE iBRM iBRC eBRC eBRM ePE
168121 168231 168341 168451
168002 168002 168002 168002 168002
30030 30030 30030 30030 30030 30030
Figure 3
1. Node 451 advertises BGP CAR route (E2, C1) to 341, from which it
goes to 231 then to 121 and finally to E1
2. Each BGP hop allocates local label and programs swap entry in
forwarding for (E2, C1)
3. E1 receives BGP CAR route (E2, C1) via 121 with label 168002
1. Let's assume E1 selects that path
4. E1 resolves BGP CAR route (E2, C1) via 121 on color-aware path
(121, C1)
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1. Color-aware path (121, C1) is FA128 path to 121 (label
168121)
5. E1's imposition color-aware label-stack for V/v is thus
1. 30030 <=> V/v
2. 168002 <=> (E2, C1)
3. 168121 <=> (121, C1)
6. Each BGP hop performs swap operation on 168002 bound to color-
aware path (E2,C1)
6.2.2. Hierarchical Design with next-hop-self at ingress domain BR
(E2,C1)
+-----+ via 451 +-----+
|T-RR1| <-------------- |T-RR2|
/ +-----+ L=168002 +-----+\
/ \
+-------------+---/----------+--------------+-----------\--+----------+
| | / | | \ | |
| (E2,C1) | / (451,C1) | (451,C1) | \| |
| via 121 +---+ via 231 +---+ via 341 +---+ +---+ |
| L=168002 |121| <======= |231| <========|341| <======= |451| |
| / +---+ L=168451 +---+ L=168451 +---+ +---+ |
|---+ / | | | | +---|
| E1|<--/ | | | | | E2|
|---+ | | | | +---|
| +---+ +---+ +---+ +---+ |
| |122| |232| |342| |452| |
| +---+ +---+ +---+ +---+ |
| Access | Metro | Core | Metro | Access |
| domain 1 | domain 2 | domain 3 | domain 4 | domain 5 |
+-------------+--------------+--------------+--------------+----------+
iPE iBRM iBRC eBRC eBRM ePE
168231 168341
168121 168451 168451 168451
168002 168002 168002 168002 168002
30030 30030 30030 30030 30030 30030
Figure 4: Heirarchical BGP transport CAR, NHS at iBR
1. Node 451 advertises BGP CAR route (451, C1) to 341, from which
it goes to 231 and finally to 121
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2. Each BGP hop allocates local label and programs swap entry in
forwarding for (451, C1)
3. 121 resolves received BGP CAR route (451, C1) via 231 (label
168451) on color-aware path (231, C1)
1. Color-aware path (231, C1) is FA128 path to 231 (label
168231)
4. 451 advertises BGP CAR route (E2, C1) via 451 to Transport RR
T-RR2, which reflects it to T-RR1, which reflects it to 121
5. 121 receives BGP CAR route (E2, C1) via 451 with label 168002
1. Let's assume 121 selects that path
6. 121 resolves BGP CAR route (E2, C1) via 451 on color-aware path
(451, C1)
1. Color-aware path (451, C1) is BGP CAR path to 451 (label
168451)
7. 121 imposition of color-aware label stack for (E2, C1) is thus
1. 168002 <=> (E2, C1)
2. 168451 <=> (451, C1)
3. 168231 <=> (231, C1)
8. 121 advertises (E2, C1) to E1 with next hop self (121) and label
168002
9. E1 constructs same imposition color-aware label-stack for V/v
via (E2, C1) as in the flat model:
1. 30030 <=> V/v
2. 168002 <=> (E2, C1)
3. 168121 <=> (121, C1)
10. 121 performs swap operation on 168002 with hierarchical color-
aware label stack for (E2, C1) via 451 from step 7
11. Nodes 231 and 341 perform swap operation on 168451 bound to
color-aware path (451, C1)
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12. 451 performs swap operation on 168002 bound to color-aware path
(E2, C1)
Note: E1 does not need the BGP CAR (451, C1) route
6.2.3. Hierarchical Design with Next Hop Unchanged at ingress domain BR
(E2,C1)
+-----+ via 451 +-----+
|T-RR1| <-------------- |T-RR2|
/ +-----+ L=168002 +-----+\
/ \
+-------------+---/----------+--------------+-----------\--+----------+
| | / | | \ | |
| (E2,C1) | / (451,C1) | (451,C1) | \| |
| via 451 +---+ via 231 +---+ via 341 +---+ +---+ |
| L=168002/|121| <======= |231| <========|341| <======= |451| |
| / +---+ L=168451 +---+ L=168451 +---+ +---+ |
|---+ <--/ //| | | | +---|
| E1| // | | | | | E2|
|---+ <===// | | | | +---|
| (451,C1) +---+ +---+ +---+ +---+ |
| via 121 |122| |232| |342| |452| |
| L=168451 +---+ +---+ +---+ +---+ |
| | | | | |
| Access | Metro | Core | Metro | Access |
| domain 1 | domain 2 | domain 3 | domain 4 | domain 5 |
+-------------+--------------+--------------+--------------+----------+
iPE iBRM iBRC eBRC eBRM ePE
168121 168231 168341
168451 168451 168451 168451
168002 168002 168002 168002 168002
30030 30030 30030 30030 30030 30030
Figure 5: Heirarchical BGP transport CAR, NHU at iBR
1. Nodes 341, 231 and 121 receive and resolve BGP CAR route (451,
C1) the same as in the previous model
2. Node 121 allocates local label and programs swap entry in
forwarding for (451, C1)
3. 451 advertises BGP CAR route (E2, C1) to Transport RR T-RR2,
which reflects it to T-RR1, which reflects it to 121
4. Node 121 advertises (E2, C1) to E1 with next hop as 451 i.e.
next-hop unchanged
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5. 121 also advertises (451, C1) to E1 with next hop self (121) and
label 168451
6. E1 resolves BGP CAR route (451, C1) via 121 on color-aware path
(121, C1)
1. Color-aware path (121, C1) is FA128 path to 121 (label
168121)
7. E1 receives BGP CAR route (E2, C1) via 451 with label 168002
1. Let's assume E1 selects that path
8. E1 resolves BGP CAR route (E2, C1) via 451 on color-aware path
(451, C1)
1. Color-aware path (451, C1) is BGP CAR path to 451 (label
168451)
9. E1's imposition color-aware label-stack for V/v is thus
1. 30030 <=> V/v
2. 168002 <=> (E2, C1)
3. 168451 <=> (451, C1)
4. 168121 <=> (121, C1)
10. Nodes 121, 231 and 341 perform swap operation on 168451 bound to
(451, C1)
11. 451 performs swap operation on 168002 bound to color-aware path
(E2, C1)
6.3. Scale Analysis
The following two tables summarize the control-plane and dataplane
scale of these three models:
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| E1 | 121 | 231
-----+---------------------+---------------------+--------------------
FLAT | (E2,C) via (121,C) | (E2,C) via (231,C) | (E2,C) via (341,C)
-----+---------------------+---------------------+--------------------
H.NHS| (E2,C) via (121,C) | (E2,C) via (451,C) |
| | (451,C) via (231,C) | (451,C) via (341,C)
-----+---------------------+---------------------+--------------------
H.NHU| (E2,C) via (451,C) | |
| (451,C) via (121,C) | (451,C) via (231,C) | (451,C) via (341,C)
-----+---------------------+---------------------+--------------------
| E1 | 121 | 231
-----+---------------------+---------------------+--------------------
FLAT | V -> 30030 | 168002 -> 168002 | 168002 -> 168002
| 168002 | 168231 | 168341
| 168121 | |
-----+---------------------+---------------------+--------------------
H.NHS| V -> 30030 | 168002 -> 168002 | 168451 -> 168451
| 168002 | 168451 | 168341
| 168121 | 168231 |
-----+---------------------+---------------------+--------------------
H.NHU| V -> 30030 | 168451 -> 168451 | 168451 -> 168451
| 168002 | 168231 | 168341
| 168451 | |
| 168121 | |
-----+---------------------+---------------------+--------------------
* The flat model is the simplest design, with a single BGP transport
level. It results in the minimum label/SID stack at each BGP hop.
However, it significantly increases the scale impact on the core
BRs (e.g. 341), whose FIB capacity and even MPLS label space may
be exceeded.
- 341's dataplane scales with (E2,C) where there may be 300k E's
and 5 C's hence 1.5M entries > 1M MPLS dataplane
* The hierarchical models avoid the need for core BRs to learn
routes and install label forwarding entries for (E, C) routes.
- Whether NH self or unchanged at 121, 341's dataplane scales
with (451,C) where there may be thousands of 451's and 5 C's
hence well under the 1M MPLS dataplane
- They also aid faster convergence by allowing the PE routes to
be distributed via out-of-band RRs that can be scaled
independent of the transport BRs.
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* The next-hop-self option at ingress BRM (e.g. 121) hides the
hierarchical design from the ingress PE, keeping its outgoing
label programming as simple as the flat model. However, the
ingress BRM requires an additional BGP transport level recursion,
which coupled with load-balancing adds dataplane complexity. It
needs to support a swap and push operation. It also needs to
install label forwarding entries for the egress PEs that are of
interest to its local ingress PEs.
* With the next-hop-unchanged option at ingress BRM (e.g. 121), only
an ingress PE needs to learn and install output label entries for
egress (E, C) routes. The ingress BRM only installs label
forwarding entries for the egress ABR (e.g. 451). However, the
ingress PE needs an additional BGP transport level recursion and
pushes a BGP VPN label and two BGP transport labels. It may also
need to handle load-balancing for the egress ABRs. This is the
most complex dataplane option for the ingress PE.
6.4. Scaling Benefits of the (E, C) BGP Subscription and Filtering
The (E, C) subscription scheme from Section 5 provides the following
scaling benefits for the models in Section 6.2
* An ingress PE (E1) only learns (E, C) routes that it needs to
install into data plane for service route automated steering
* An ingress BRM (121) only learns (E, C) routes that it needs to
install into data plane (for Next-Hop-Self), or that it needs to
distribute towards it's ingress PEs (inline RR with Next-Hop-
Unchanged)
* An ingress BRM or a transport RR only needs to distribute the
necessary subset of (E, C) routes to each client (subscriber);
this minimizes their processing load for generating updates
* As a result, withdrawal of (E, C) routes when a remote node fails
(E2), may also be faster, aiding better convergence
6.5. Anycast SID
This section describes how Anycast SID complements and improves the
scaling designs above.
6.5.1. Anycast SID for transit inter-domain nodes
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* Redundant BRs (e.g. two egress BRMs, 451 and 452) advertise BGP
CAR routes for a local PE (e.g., E2) with the same SID (based on
label-index). Such egress BRMs may be assigned a common Anycast
SID, so that the BGP next-hops for these routes will also resolve
via a color-aware path to the Anycast SID.
* The use of Anycast SID naturally provides fast local convergence
upon failure of an egress BRM node. In addition, it decreases the
recursive resolution and load-balancing complexity at an ingress
BRM or PE in the hierarchical designs above.
6.5.2. Anycast SID for transport color endpoints (e.g., PEs)
The common Anycast SID technique may also be used for a redundant
pair of PEs that share an identical set of service (VPN) attachments.
* For example, assume a node E2' paired with E2 above. Both PEs
should be configured with the same static label/SID for the
services (e.g., per-VRF VPN label/SID), and will advertise
associated service routes with the Anycast IP as BGP next-hop.
* This design provides a convergence and recursive resolution
benefit on an ingress PE or ABR similar to the egress ABR case in
the previous section. But its applicability is limited to cases
where the constraints above can be met.
7. Routing Convergence
This section will analyze routing convergence.
8. VPN CAR
This section illustrates the extension of BGP CAR to address the VPN
CAR requirement stated in Section 3.2 of [dskc-bess-bgp-car-problem-
statement].
CE1 -------------- PE1 -------------------- PE2 -------------- CE2 - V
* BGP CAR is enabled between CE1-PE1 and PE2-CE2
* BGP VPN CAR is enabled between PE1 and PE2
* Provider publishes intent 'low-delay' is mapped to color CP on its
inbound peering links
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* Within its infrastructure, Provider maps intent 'low-delay' to
color CPT
* On CE1 and CE2, intent 'low-delay' is mapped to CC
(V, CC) is a Color-Aware route originated by CE2
1. CE2 sends to PE2 : [(V, CC), Label L1] via CE2 with LCM (CP)
2. PE2 installs in VRF A: [(V, CC), L1] via CE2 which resolves on (CE2, CP)
/ connected OIF
2.a. PE2 allocates VPN Label L2 and programs swap entry for (V, CC)
3. PE2 sends to PE1 : [(RD, V, CC), L2] via PE2 with regular Color Extended
Community (CPT)
4. PE1 installs in VRF A: [(V, CC), L2] via (PE2, CPT) steered on (PE2, CPT)
4.a. PE1 allocates Label L3 and programs swap entry for (V, CC)
5. PE1 sends to CE1 : [(V, CC), L3] via PE1 without any LCM
6. CE1 installs : [(V, CC), L3] via PE1 which resolves on (PE1, CC)
/ connected OIF
6.a. Label L3 is installed as the imposition label for (V, CC)
VPN CAR distribution for (RD, V, CC) requires a new SAFI that follows
same VPN semantics as defined in [RFC4364], the difference being that
the advertised routes carry CAR NLRI defined in Section 2.9.2 of this
document.
VPN CAR NLRI with RD has the format shown below
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLRI Length | Key Length | NLRI Type |Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Prefix (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Color (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Followed by optional TLVs encoded as below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
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Route Distinguisher: 8 octet field encoded according to [RFC4364]
9. IANA Considerations
IANA has assigned SAFI value 83 (BGP CAR) and SAFI value 84 (BGP VPN
CAR) from the "SAFI Values" sub-registry under the "Subsequent
Address Family Identifiers (SAFI) Parameters" registry with this
document as a reference.
9.1. BGP CAR NLRI Types Registry
IANA is requested to create a "BGP CAR NLRI Types" sub-registry under
the "Border Gateway Protocol (BGP) Parameters" registry with this
document as a reference. The registry is for assignment of the one
octet sized code-points for BGP CAR NLRI types and populated with the
values shown below:
Type NLRI Type Reference
-----------------------------------------------------------------
0 Reserved (not to be used) [This document]
1 Color-Aware Routes NLRI [This document]
2-255 Unassigned
Allocations within the registry are to be made under the
"Specification Required" policy as specified in [RFC8126]).
9.2. BGP CAR NLRI TLV Registry
IANA is requested to create a "BGP CAR NLRI TLV Types" sub-registry
under the "Border Gateway Protocol (BGP) Parameters" registry with
this document as a reference. The registry is for assignment of the
one octet sized code-points for BGP-CAR NLRI non-key TLV types and
populated with the values shown below:
Type NLRI Type Reference
-----------------------------------------------------------------
0 Reserved (not to be used) [This document]
1 Label TLV [This document]
2 Label Index TLV [This document]
3 SRv6 SID TLV [This document]
4-255 Unassigned
Allocations within the registry are to be made under the
"Specification Required" policy as specified in [RFC8126]).
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9.3. Guidance for Designated Experts
In all cases of review by the Designated Expert (DE) described here,
the DE is expected to ascertain the existence of suitable
documentation (a specification) as described in [RFC8126]. The DE is
also expected to check the clarity of purpose and use of the
requested code points. Additionally, the DE must verify that any
request for one of these code points has been made available for
review and comment within the IETF: the DE will post the request to
the IDR Working Group mailing list (or a successor mailing list
designated by the IESG). If the request comes from within the IETF,
it should be documented in an Internet-Draft. Lastly, the DE must
ensure that any other request for a code point does not conflict with
work that is active or already published within the IETF.
9.4. BGP Extended Community Registry
IANA is requested to allocate the sub-type TBD2 for "Local Color
Mapping (LCM)" under the "BGP Transitive Opaque Extended Community"
registry under the "BGP Extended Community" parameter registry.
10. Manageability Considerations
Color assignments in a multi-domain network operating under a common
or cooperating administrative control (i.e., color domain) should be
managed similar to transport layer IP addresses, and ensure a unique
and non-conflicting color allocation across the different network
domains in that color domain.
When color-aware routes propagate across a color domain boundary,
there is typically no need for coordinating color assignments, since
the IP prefix is unique, and hence makes the color scope also unique
and non-conflicting. The color only needs to be re-mapped into a
local color assigned for the same intent (which is carried in the
LCM-EC).
However, if networks under different administrative control establish
a shared transport service between them, where the same transport IP
address is co-ordinated and shared across the two networks, then the
color assignments associated with that IP address should also be co-
ordinated to avoid any conflicts in either network.
It should be noted that the color assignments coordination are only
necessary for routes to the shared service IP. Colors used for
intra-domain or for inter-domain intents associated with the unique
IP addresses do not need any coordination.
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11. Co-authors
Clarence Filsfils
Cisco Systems
Belgium
Email: cfilsfil@cisco.com
Bruno Decraene
Orange
France
Email: bruno.decraene@orange.com
Luay Jalil
Verizon
USA
Email: luay.jalil@verizon.com
Yuanchao Su
Alibaba, Inc
Email: yitai.syc@alibaba-inc.com
Jim Uttaro
ATT
USA
Email: ju1738@att.com
Jim Guichard
Futurewei
USA
Email: james.n.guichard@futurewei.com
Ketan Talaulikar
Arrcus, Inc
India
Email: ketant.ietf@gmail.com
Keyur Patel
Arrcus, Inc
USA
Email: keyur@arrcus.com
Haibo Wang
Huawei Technologies
China
Email: rainsword.wang@huawei.com
12. Contributors
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Dirk Steinberg
Lapishills Consulting Limited
Germany
Email: dirk@lapishills.com
Israel Means
AT&T
USA
Email: im8327@att.com
Reza Rokui
Ciena
USA
Email: rrokui@ciena.com
13. Acknowledgements
The authors would like to acknowledge the review and inputs from many
people.TBD
14. References
14.1. Normative References
[I-D.hr-spring-intentaware-routing-using-color]
Hegde, S., Rao, D., Sangli, S. R., Agrawal, S., Filsfils,
C., Talaulikar, K., Patel, K., Uttaro, J., Decraene, B.,
Bogdanov, A., Jalil, L., Alston, A., Xu, X., Gulko, A.,
Khaddam, M., Contreras, L. M., Steinberg, D., Guichard,
J., Henderickx, W., and C. Bowers, "Problem statement for
Inter-domain Intent-aware Routing using Color", Work in
Progress, Internet-Draft, draft-hr-spring-intentaware-
routing-using-color-00, 15 July 2022,
<https://datatracker.ietf.org/doc/html/draft-hr-spring-
intentaware-routing-using-color-00>.
[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>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.
[RFC4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
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Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
November 2006, <https://www.rfc-editor.org/info/rfc4684>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5512] Mohapatra, P. and E. Rosen, "The BGP Encapsulation
Subsequent Address Family Identifier (SAFI) and the BGP
Tunnel Encapsulation Attribute", RFC 5512,
DOI 10.17487/RFC5512, April 2009,
<https://www.rfc-editor.org/info/rfc5512>.
[RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended Community
Attribute", RFC 5701, DOI 10.17487/RFC5701, November 2009,
<https://www.rfc-editor.org/info/rfc5701>.
[RFC7311] Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro,
"The Accumulated IGP Metric Attribute for BGP", RFC 7311,
DOI 10.17487/RFC7311, August 2014,
<https://www.rfc-editor.org/info/rfc7311>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
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[RFC8669] Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
A., and H. Gredler, "Segment Routing Prefix Segment
Identifier Extensions for BGP", RFC 8669,
DOI 10.17487/RFC8669, December 2019,
<https://www.rfc-editor.org/info/rfc8669>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
[RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
Based on Segment Routing over IPv6 (SRv6)", RFC 9252,
DOI 10.17487/RFC9252, July 2022,
<https://www.rfc-editor.org/info/rfc9252>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.
14.2. Informative References
[I-D.ietf-mpls-seamless-mpls]
Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
M., and D. Steinberg, "Seamless MPLS Architecture", Work
in Progress, Internet-Draft, draft-ietf-mpls-seamless-
mpls-07, 28 June 2014,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
seamless-mpls-07>.
[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
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[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <https://www.rfc-editor.org/info/rfc5462>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
Appendix A. Illustrations of Service Steering
The following sub-sections illustrate example scenarios of Colored
Service Route Steering over E2E BGP CAR resolving over different
intra-domain mechanisms
The examples use MPLS/SR for the transport data plane. Scenarios
specific to other encapsulations will be added in subsequent
versions.
A.1. E2E BGP transport CAR intent realized using IGP FlexAlgo
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RD:V/v via E2
+-----+ vpn label: 30030 +-----+
...... |S-RR1| <..................................|S-RR2| <.......
: +-----+ Color C1 +-----+ :
: :
: :
: :
+-:-----------------------+----------------------+------------------:--+
| : | | : |
| : | | : |
| : (E2,C1) via 121 | (E2,C1) via 231 | (E2,C1)via E2 : |
| : L=168002,AIGP=110 +---+ L=168002,AIGP=10 +---+ L=0x3,LI=8002 : |
| : |-------------------|121|<-----------------|231|<-------------| : |
| : V LI=8002 +---+ LI=8002 +---+ | : |
|----+ | | +-----|
| E1 | | | | E2 |
|----+(E2,C1) via 122 | (E2,C1) via 232 | (E2,C1)via E2+-----|
| ^ L=168002,AIGP=210 +---+ L=168002,AIGP=20 +---+ L=0x3 | |
| |---------------- |122|<-----------------|232|<-------------| |
| LI=8002 +---+ LI=8002 +---+ LI=8002 |
| | | |
| ISIS SR | ISIS SR | ISIS SR |
| FA 128 | FA 128 | FA 128 |
+-------------------------+----------------------+---------------------+
iPE iABR eABR ePE
+------+ +------+
|168121| |168231|
+------+ +------+
+------+ +------+ +------+
|168002| |168002| |168002|
+------+ +------+ +------+
+------+ +------+ +------+
|30030 | |30030 | |30030 |
+------+ +------+ +------+
Figure 6: BGP FA Aware transport CAR path
Use case: Provide end to end intent for service flows.
* With reference to the topology above:
- IGP FA 128 is running in each domain, and mapped to Color C1
- Egress PE E2 advertises a VPN route RD:V/v colored with (color
extended community) C1 to steer traffic to BGP transport CAR
(E2, C1). VPN route propagates via service RRs to ingress PE
E1.
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- BGP CAR route (E2, C1) with next-hop, label-index and label as
shown above are advertised through border routers in each
domain. When a RR is used in the domain, ADD-PATH is enabled
to advertise multiple available paths.
- On each BGP hop, (E2, C1) next-hop is resolved over IGP FA 128
of the domain. AIGP attribute influences BGP CAR route best
path decision as per [RFC7311]. BGP CAR label swap entry is
installed that goes over FA 128 LSP to next-hop providing
intent in each IGP domain. Update AIGP metric to reflect FA
128 metric to next-hop.
- Ingress PE E1 learns CAR route (E2, C1). It steers colored VPN
route RD:V/v into (E2, C1)
* Important:
- IGP FA 128 top label provides intent within each domain.
- BGP CAR label (e.g. 168002) carries end to end intent. Thus
stitches intent over intra domain FA 128.
A.2. E2E BGP transport CAR intent realized using SR Policy
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RD:1/8 via E2
+-----+ vpn label: 30030 +-----+
...... |S-RR1| <..................................|S-RR2| <......
: +-----+ Color C1 +-----+ :
: :
: :
: :
+-:-----------------------+----------------------+------------------:-+
| : | | : |
| : | | : |
| : <-(E2,C1) via 121 | <-(E2,C1) via 231 | <-(E2,C1)via E2 : |
| : +---+ +---+ : |
| : ------------------>|121|----------------->|231|--------------| : |
| : | SR policy(C1,121) +---+ SR policy(C1,231)+---+ SR policy v : |
|----+ | | (C1,E2) +---|
| E1 | | | |E2 |
|----+ <-(E2,C1) via 122 | (E2,C1) via 232 | <-(E2,C1)via E2+---|
| | +---+ +---+ ^ |
| ------------------>|122|----------------->|232|---------------| |
| SR policy(C1,122) +---+ SR policy(C1,232)+---+ SR policy(C1,E2) |
| | | |
| | | |
| ISIS SR | ISIS SR | ISIS SR |
+-------------------------+----------------------+--------------------+
iPE iABR eABR ePE
+------+ +------+
| S1 | | S2 |
+------+ +------+
+------+ +------+ +------+
|160121| |160231| | S3 |
+------+ +------+ +------+
+------+ +------+ +------+
|168002| |168002| |168002|
+------+ +------+ +------+
+------+ +------+ +------+
|30030 | |30030 | |30030 |
+------+ +------+ +------+
Figure 7: BGP SR policy Aware transport CAR path
Use case: Provide end to end intent for service flows
* With reference to the topology above:
- SR Policy provide intra domain intent. Below are example SID
lists of SR policies in each domain corresponding to label
stack in Figure 7
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o SR policy (C1,121) segments <S1, 121>
o SR policy (C1,231) segments <S2, 231>
o SR policy (C1,E2) segments <S3, E2>
- Egress PE E2 advertises a VPN route RD:V/v colored with (color
extended community) C1 to steer traffic to BGP transport CAR
(E2, C1). VPN route propagates via service RRs to ingress PE
E1.
- BGP CAR route (E2, C1) with next-hop, label-index and label as
shown above are advertised through border routers in each
domain. When a RR is used in the domain, ADD-PATH is enabled
to advertise multiple available paths.
- On each BGP hop, CAR route (E2, C1) next-hop is resolved over
an SR policy(C1, next-hop). BGP CAR label swap entry is
installed that goes over SR policy segment list.
- Ingress PE E1 learns CAR route (E2, C1). It steers colored VPN
route RD:V/v into (E2, C1).
* Important:
- SR policy provides intent in each domain.
- BGP CAR label (e.g. 168002) carries end to end intent. Thus
stitches intent over intra domain SR policies.
A.3. BGP transport CAR intent realized in a section of the network
A.3.1. Provide intent for service flows only in core domain running
ISIS FlexAlgo
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RD:1/8 via E2
+-----+ vpn label: 30030 +-----+
...... |S-RR1| <..................................|S-RR2| <.......
: +-----+ Color C1 +-----+ :
: :
: :
: :
+-:-----------------------+----------------------+------------------:--+
| : | | : |
| : | | : |
| : (E2,C1) via 121 | (E2,C1) via 231 | (E2,C1) via E2 : |
| : L=168002,AIGP=1110+---+L=168002,AIGP=1010+---+ L=0x3 : |
| : |-------------------|121|<-----------------|231|<-------------| : |
| : V LI=8002 +---+ LI=8002 +---+ | : |
|----+ | | +-----|
| E1 | | | | E2 |
|----+(E2,C1) via 122 | (E2,C1) via 232 | (E2,C1) via E2+-----|
| ^ L=168002,AIGP=1210+---+L=168002,AIGP=1020+---+ L=0x3 | |
| |---------------- |122|<-----------------|232|<-------------| |
| LI=8002 +---+ LI=8002 +---+ |
| | | |
| ISIS SR | ISIS SR | ISIS SR |
| Algo 0 | FlexAlgo 128 | Algo 0 |
| Access | Core | Access
+-------------------------+----------------------+---------------------+
iPE iABR eABR ePE
+------+ +------+
|160121| |168231|
+------+ +------+
+------+ +------+ +------+
|168002| |168002| |160002|
+------+ +------+ +------+
+------+ +------+ +------+
|30030 | |30030 | |30030 |
+------+ +------+ +------+
Figure 8: BGP Hybrid FlexAlgo Aware transport CAR path
* With reference to the topology above:
- IGP FA 128 is only enabled in Core (e.g. WAN network), mapped
to C1. Access network domain only has base algo 0.
- Egress PE E2 advertises a VPN route RD:V/v colored with (color
extended community) C1 to steer traffic via BGP transport CAR
(E2, C1). VPN route propagates via service RRs to ingress PE
E1.
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- BGP CAR route (E2, C1) with next-hop, label-index and label as
shown above are advertised through border routers in each
domain. When a RR is used in the domain, ADD-PATH is enabled
to advertise multiple available paths.
- Local policy on 231 and 232 maps intent C1 to resolve CAR route
next-hop over IGP base algo 0 in right access domain. BGP CAR
label swap entry is installed that goes over algo 0 LSP to
next-hop. Update AIGP metric to reflect algo 0 metric to next-
hop with an additional penalty.
- On 121 and 122, CAR route (E2, C1) next-hop learnt from Core
domain is resolved over IGP FA 128. BGP CAR label swap entry
is installed that goes over FA 128 LSP to next-hop providing
intent in Core IGP domain.
- Ingress PE E1 learns CAR route (E2, C1). It maps intent C1 to
resolve CAR route next-hop over IGP base algo 0. It steers
colored VPN route RD:V/v via (E2, C1)
* Important:
- IGP FlexAlgo 128 top label provides intent in Core domain.
- BGP CAR label (e.g. 168002) carries intent from PEs which is
realized in core domain
A.3.2. Provide intent for service flows only in core domain over TE
tunnel mesh
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RD:1/8 via E2
+-----+ vpn label: 30030 +-----+
...... |S-RR1| <..................................|S-RR2| <.......
: +-----+ Color C1 +-----+ :
: :
: :
: :
+-:-----------------------+----------------------+------------------:--+
| : | | : |
| : | | : |
| : (E2,C1) via 121 | (E2,C1) via 231 | (E2,C1) via E2 : |
| : L=242003,AIGP=1110+---+L=242002,AIGP=1010+---+ L=0x3 : |
| : |-------------------|121|<-----------------|231|<-------------| : |
| : V +---+ TE tunnel(231) +---+ | : |
|----+ | | +-----|
| E1 | | | | E2 |
|----+(E2,C1) via 122 | (E2,C1) via 232 | (E2,C1) via E2+-----|
| ^ L=242004,AIGP=1210+---+L=242001,AIGP=1020+---+ L=0x3 | |
| |---------------- |122|<-----------------|232|<-------------| |
| +---+ TE tunnel(232) +---+ |
| | | |
| | | |
| ISIS/LDP | ISIS/RSVP-TE | ISIS/LDP |
| Access 0 | Core | Access 1 |
+-------------------------+----------------------+---------------------+
iPE iABR eABR ePE
+------+ +------+
|240121| |241231|
+------+ +------+
+------+ +------+ +------+
|242003| |242002| |240002|
+------+ +------+ +------+
+------+ +------+ +------+
|30030 | |30030 | |30030 |
+------+ +------+ +------+
Figure 9: BGP CAR over TE tunnel mesh in core network
* With reference to the topology above:
- RSVP-TE MPLS tunnel mesh is configured only in core (e.g. WAN
network). Access only has ISIS/LDP. (Figure does not show all
TE tunnels)
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- Egress PE E2 advertises a VPN route RD:V/v colored with (color
extended community) C1 to steer traffic via BGP transport CAR
(E2, C1). VPN route propagates via service RRs to ingress PE
E1.
- BGP CAR route (E2, C1) with next-hops and labels as shown above
is advertised through border routers in each domain. When a RR
is used in the domain, ADD-PATH is enabled to advertise
multiple available paths.
- Local policy on 231 and 232 maps intent C1 to resolve CAR route
next-hop over best effort LDP LSP in access domain 1. BGP CAR
label swap entry is installed that goes over LDP LSP to next-
hop. AIGP metric is updated to reflect best effort metric to
next- hop with an additional penalty.
- Local policy on 121 and 122 maps intent C1 to resolve CAR route
next-hop in Core domain over TE tunnels. BGP CAR label swap
entry is installed that goes over a TE tunnel to next-hop
providing intent in Core domain. AIGP metric is updated to
reflect TE tunnel metric.
- Ingress PE E1 learns CAR route (E2, C1). It maps intent C1 to
resolve CAR route next-hop over best effort LDP LSP in Access
domain 0. It steers colored VPN route RD:V/v via (E2, C1).
* Important:
- TE tunnel LSP provides intent in Core domain.
- Dynamic BGP CAR label carries intent from PEs which is realized
in core domain by resolution via TE tunnel.
A.4. Transit network domains that do not support CAR
* In a brownfield deployment, color-aware paths between two PEs may
need to go through a transit domain that does not support CAR.
Example include an MPLS LDP network with IGP best-effort; or a
BGP-LU based multi-domain network. MPLS LDP network with best
effort IGP can adopt above scheme. Below is the example for BGP
LU.
* Reference topology:
E1 --- BR1 --- BR2 ......... BR3 ---- BR4 --- E2
Ci <----LU----> Ci
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- Network between BR2 and BR3 comprises of multiple BGP-LU hops
(over IGP-LDP domains).
- E1, BR1, BR4 and E2 are enabled for BGP CAR, with Ci colors
- BR1 and BR2 are directly connected; BR3 and BR4 are directly
connected
* BR1 and BR4 form an over-the-top peering (via RRs as needed) to
exchange BGP CAR routes
* BR1 and BR4 also form direct BGP-LU sessions to BR2 and BR3
respectively, to establish labeled paths between each other
through the BGP-LU network
* BR1 recursively resolves the BGP CAR next-hop for CAR routes
learnt from BR4 via the BGP-LU path to BR4
* BR1 signals the transport discontinuity to E1 via the AIGP TLV, so
that E1 can prefer other paths if available
* BR4 does the same in the reverse direction
* Thus, the color-awareness of the routes and hence the paths in the
data plane are maintained between E1 and E2, even if the intent is
not available within the BGP-LU island
* A similar design can be used for going over network islands of
other types
A.5. Resource Avoidance using BGP CAR and IGP Flex-Algo
This example illustrates a case of resource avoidance within a domain
for a multi-domain color-aware path.
+-------------+ +-------------+
| | | | V/v with C1
|----+ |------| +----|/
| E1 | | | | E2 |\
|----+ | | +----| W/w with C2
| |------| IGP FA128 |
| IGP FA128 | | IGP FA129 |
| Domain 1 | | Domain 2 |
+-------------+ +-------------+
Figure 10: BGP CAR resolution over IGP FLex-Algo for resource
avoidance in a domain
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* C1 and C2 represent two unique intents in multi-domain network
- C1 is mapped to "minimize IGP metric"
- C2 is mapped to "minimize IGP metric and avoid resource R"
* Resource R represents link(s) or node(s) to be avoided
* Flex-Algo FA128 in Domain 2 is mapped to "minimize IGP metric" and
hence to C1
* Flex-Algo FA129 in Domain 2 is mapped to "minimize IGP metric and
avoid resource R" and hence to C2
* Flex-Algo FA128 in Domain 1 is mapped to "minimize IGP metric"
- There is no resource R to be avoided in Domain 1, hence both C1
and C2 are mapped to FA128
* E1 receives two service routes from E2:
- V/v with BGP Color Extended-Community C1
- W/w with BGP Color Extended-Community C2
* E1 has the following color-aware paths:
- (E2, C1) provided by BGP CAR with the following per-domain
resolution:
o Domain1: over IGP FA128
o Domain2: over IGP FA128
- (E2, C2) provided by BGP CAR with the following per-domain
resolution:
o Domain1: over IGP FA128
o Domain2: over IGP FA129, avoiding resource R
* E1 automatically steers the received service routes as follows:
- V/v via (E2, C1) provided by BGP CAR
- W/w via (E2, C2) provided by BGP CAR
Observations:
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* C1 and C2 are realized over a common intra-domain intent (FA128)
in one domain and distinct intents in another domain as required
* 32-bit Color space provides flexibility in defining a large number
of intents in a multi-domain network. They may be efficiently
realized by mapping to a smaller number of intra-domain intents in
different domains.
A.6. Per-Flow Steering over CAR routes
This section provides an example of ingress PE per-flow steering as
defined in section 8.6 of [RFC9256] onto BGP CAR routes.
With reference to the Figure 6
* Ingress PE E1 learns best effort BGP LU route E2
* Ingress PE E1 learns CAR route (E2, C1), C1 is mapped to "low
delay"
* Ingress PE E1 learns CAR route (E2, C2), C2 is mapped to "low
delay and avoid resource R"
* Ingress PE E1 is configured to instantiate an array of paths to E2
where the entry 0 is the BGP LU path to N, color C1 is the first
entry and color C2 is the second entry. The index into the array
is called a Forwarding Class (FC). The index can have values 0 to
7, especially when derived from the MPLS TC bits [RFC5462]
* E1 is configured to match flows in its ingress interfaces (upon
any field such as Ethernet destination/source/VLAN/TOS or IP
destination/source/DSCP or transport ports etc.) and color them
with an internal per-packet FC variable (0, 1 or 2 in this
example).
* This array is presented as composite candidate path of SR policy
(E2, C100) and acts as a container for grouping constituent paths
of different colors/best effort. This representation provide
automated steering for services colored with Color Extended
Community C100 via paths of different colors. Note that color
extended community C100 is used as indirection to the composite
policy configured on ingress PE.
* Egress PE E2 advertises a VPN route RD:V/v with Color Extended
community C100 to steer traffic via composite SR policy (E2, C100)
i.e. FC array of paths.
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E1 receives three packets K, K1, and K2 on its incoming interface.
These three packets matches on VPN route which recurses on E2. E1
colors these 3 packets respectively with forwarding-class 0, 1, and
2.
As a result
* E1 forwards K along the best effort path to E2 (i.e., for MPLS
data plane, it pushes the best effort label of E2).
* E1 forwards K1 along the (E2, C1) BGP CAR route
* E1 forwards K2 along the (E2, C2) BGP CAR route
A.7. Advertising BGP CAR routes for shared IP addresses
+-------------+ +--------------+
| | | +----|
| |------| | E2 |(IP1)
|----+ | | +----|
| E1 | | | Domain 2 |
|----+ | +--------------+
| | +--------------+
| | | +----|
| Domain 1 |------| | E3 |(IP1)
+-------------+ | +----|
| Domain 3 |
+--------------+
Figure 11: BGP CAR advertisements for shared IP addresses
This example describes a case where a route for the same transport IP
address is originated from multiple nodes in different network
domains.
One use of this scenario is an Anycast transport service, where
packet encapsulation (e.g., LSP) may terminate on any one among a set
of nodes. All the nodes are capable of forwarding the inner payload,
typically via an IP lookup in the global table for Internet routes.
A couple of variations of the use-case are described in the example
below.
One node is shown in each domain, but there will be multiple nodes in
practice for redundancy.
Example-1: Anycast with forwarding to nearest
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* Both E2 and E3 advertise Anycast (shared) IP (IP1, C1) with same
label L1
* An ingress PE E1 receives by default the best path(s) for (IP1,
C1) propagated through BGP hops across the network.
* The paths to (IP1, C1) from E2 and E3 may merge at a common node
along the path to E1, forming equal cost multipaths or active-
backup paths at that node
* Service route V/v is advertised from egress domains D1 and D2 with
color C1 and next-hop IP1.
* Traffic for V/v steered at E1 via (IP1, C1) is forwarded to either
E2 or E3 (or both) as determined by routing along the network
(nodes in the path).
Example-2: Anycast with egress domain visibility at ingress PE
* E2 advertises (IP1, C1) and E3 advertises (IP1, C2) CAR routes for
the Anycast IP IP1. C1 and C2 are colors assigned to distinguish
the egress domains originating the routes to IP1.
* An ingress PE E1 receives the best path(s) propagated through BGP
hops across the network for both (IP1, C1) and (IP1, C2).
* The CAR routes (IP1, C1) and (IP1, C2) do not get merged at any
intermediate node, providing E1 control over path selection and
load-balancing of traffic across these two routes. Each route may
itself provide multipathing or Anycast to a set of egress nodes.
* Service route V/v advertised from egress domain D1 and D2 with
colors C1 and C2 respectively, but with same next-hop IP1.
* E1 will resolve and steer V/v path from D1 via (IP1, C1) and path
from D2 via (IP2, C2). E1 will load-balance traffic to V/v across
the two paths as determined by a local load-balancing policy.
* Traffic for colored service routes steered at E1 is forwarded to
either E2 or E3 (or load-balanced across both) as determined by
E1.
In above example, D1 and D2 belonged to the same color or
administrative domain. If D1 and D2 belonged to different color
domains, the domains will coordinate the assignment of colors to be
used with shared IP IP1 such that they do not cause conflicts. For
instance, in Example-1 :
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* D1 and D2 may both use C1 when they originate CAR route for IP1.
- In this case, naturally neither D1 nor D2 will use C1 for some
other intent
* Alternatively, D1 may use C2 and D2 may use C3 for originating a
CAR route for IP1 for the same intent.
- In this case, D1 will not use C3 for originating CAR route for
IP1 for some other intent. Similarly, D2 will not use C2 for
originating CAR route for IP1 for some other intent.
Appendix B. Color Mapping Illustrations
There are a variety of deployment scenarios that arise w.r.t
different color mappings in an inter-domain environment. This
section attempts to enumerate them and provide clarity into the usage
of the color related protocol constructs.
B.1. Single color domain containing network domains with N:N color
distribution
* All network domains (ingress, egress and all transit domains) are
enabled for the same N colors.
- A color may of course be realized by different technologies in
different domains as described above.
* The N intents are both signaled end-to-end via BGP CAR routes; as
well as realized in the data plane.
* Appendix A.1 is an example of this case.
B.2. Single color domain containing network domains with N:M color
distribution
* Certain network domains may not be enabled for some of the colors
used for end-to-end intents, but may still be required to provide
transit for routes of those colors.
* When a (E, C1) route traverses a domain where color C1 is not
available, the operator may decide to use a different intent of
color C2 that is available in that domain to resolve the next-hop
and establish a path through the domain.
- The next-hop resolution may occur via paths of any intra-domain
protocol or even via paths provided by BGP CAR.
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- The next-hop resolution color C2 may be defined as a local
policy at ingress or transit nodes of the domain.
- It may also be automatically signaled from egress border nodes
by attaching a color extended community with value C2 to the
BGP CAR routes.
* Hence, routes of N end-to-end colors may be resolved over paths
from a smaller set of M colors in a transit domain, while
preserving the original color-awareness end-to-end.
* Any ingress PE that installs a service (VPN) route with a color
C1, must have C1 enabled locally to install IP routes to (E, C1)
and resolve the service route next-hop.
* A degenerate variation of this scenario is where a transit domain
does not support any color. Appendix A.3 describes an example of
this case.
B.3. Multiple color domains
When the routes are distributed between domains with different color-
to-intent mapping schemes, both N:N and N:M cases are possible,
although an N:M mapping is more likely to occur.
Reference topology:
D1 ----- D2 ----- D3
C1 C2 C3
* C1 in D1 maps to C2 in D2 and to C3 in D3
* BGP CAR is enabled in all three color domains
The reference topology above is used to elaborate on the design
described in Section 2.8
When the route originates in color domain D1 and gets advertised to a
different color domain D2, following procedures apply:
* The original intent in the BGP CAR route is preserved; i.e. route
is (E, C1)
* A BR of D1 attaches LCM-EC with value C1 when advertising to a BR
in D2
* A BR in D2 receiving (E, C1) maps C1 in received LCM-EC to local
color, say C2
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- A BR in D2 may receive (E, C1) from multiple D1 BRs which
provide equal cost or primary/backup paths
* Within D2, this LCM-EC value of C2 is used instead of the Color in
CAR route NLRI (E, C1). This applies to all procedures described
in the earlier section for a single color domain, such as next-hop
resolution and service steering.
* A colored service route V/v originated in color domain D1 with
next-hop E and color C1 will also have its color extended-
community value re-mapped to C2, typically at a service RR
* On an ingress PE in D2, V/v will resolve via C2
* When a BR in D2 advertises the route to a BR in D3, the same
process repeats.
Appendix C. CAR SAFI NLRI update packing efficiency calculation
CAR SAFI NLRI encoding is optimized for updating packing i.e. it
allows per prefix information (example label index, SRv6 SID) to be
carried in non key TLV part of NLRI. This allows multiple NLRIs to
be packed in single update message when other attributes are shared.
Example below shows gain convergence time and reduction in total BGP
data on the wire.
Consider 1.5 million routes and average 5 NLRIs sharing attributes:
Number of update messages:
1.5M/5 = 300K update messages instead of 1.5M without packing
Convergence time presuming 10k updates/second
300k/10k = 30 seconds instead of 2.5 minutes without packing
Reduction of BGP data:
Consider, each NLRI size 30 Bytes and 200 bytes of shared attributes
Update message size = (30 * 5 NLRIs) + 200 = 350 bytes;
Total BGP bytes with packing = 350 * 300k = 105MB
Total BGP bytes without packing (200 + 30) * 1.5M = 345MB
Authors' Addresses
Dhananjaya Rao (editor)
Cisco Systems
United States of America
Email: dhrao@cisco.com
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Swadesh Agrawal (editor)
Cisco Systems
United States of America
Email: swaagraw@cisco.com
Co-authors
section 11
Email: xyz@xyz.com
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