Internet DRAFT - draft-kaliraj-idr-multinexthop-attribute
draft-kaliraj-idr-multinexthop-attribute
Network Working Group K. Vairavakkalai, Ed.
Internet-Draft M. Jeyananth
Intended status: Standards Track Juniper Networks, Inc.
Expires: 21 May 2024 M. Nanduri
Microsoft
Lingala
AT&T
18 November 2023
BGP MultiNexthop Attribute
draft-kaliraj-idr-multinexthop-attribute-11
Abstract
Today, a BGP speaker can advertise one nexthop for a set of NLRIs in
an Update. This nexthop can be encoded in either the top-level BGP-
Nexthop attribute (code 3), or inside the MP_REACH_NLRI attribute
(code 14).
This document defines a new optional non-transitive BGP attribute
called "MultiNexthop (MNH)" with IANA BGP attribute type code TBD,
that can be used to carry an ordered set of one or more Nexthops in
the same route, with forwaring information scoped on a per nexthop
basis.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 21 May 2024.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Operations . . . . . . . . . . . . . . . . . . . . . 6
4.1. Scope of Use, and Propagation . . . . . . . . . . . . . . 6
4.2. Interaction of MNH with Nexthop (in attr codes 3, 14) . . 7
4.3. Interaction with Addpath . . . . . . . . . . . . . . . . 7
4.4. Path Selection Considerations . . . . . . . . . . . . . . 8
4.4.1. Determining IGP Cost . . . . . . . . . . . . . . . . 8
4.5. Denoting Upstream or Downstream Semantics . . . . . . . . 8
5. Encoding of BGP MultiNexthop (MNH) Attribute . . . . . . . . 9
5.1. MNH TLV . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. Primary Forwarding Path . . . . . . . . . . . . . . . 13
5.1.2. Backup Forwarding Path . . . . . . . . . . . . . . . 13
5.1.3. Label Descriptor . . . . . . . . . . . . . . . . . . 14
5.2. Nexthop Forwarding Information TLV . . . . . . . . . . . 15
5.3. Forwarding Instruction TLV . . . . . . . . . . . . . . . 16
5.4. Forwarding Argument TLV . . . . . . . . . . . . . . . . . 18
5.4.1. Endpoint Identifier . . . . . . . . . . . . . . . . . 19
5.4.2. Path Constraints . . . . . . . . . . . . . . . . . . 20
5.4.3. Payload Encapsulation Info . . . . . . . . . . . . . 26
5.4.4. Endpoint Attributes . . . . . . . . . . . . . . . . . 31
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 32
7. Scaling Considerations . . . . . . . . . . . . . . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
8.1. BGP Path Attributes . . . . . . . . . . . . . . . . . . . 34
8.2. Capability Codes . . . . . . . . . . . . . . . . . . . . 34
8.3. BGP MultiNextHop Attribute . . . . . . . . . . . . . . . 34
8.3.1. MultiNextHop (MNH) TLV Types . . . . . . . . . . . . 34
8.3.2. Forwarding Action Types . . . . . . . . . . . . . . . 35
8.3.3. Forwarding Argument Types . . . . . . . . . . . . . . 35
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8.3.4. Endpoint Types . . . . . . . . . . . . . . . . . . . 36
8.3.5. Path Constrain Types . . . . . . . . . . . . . . . . 37
8.3.6. Encapsulation Types . . . . . . . . . . . . . . . . . 38
8.3.7. Endpoint Attribute Types . . . . . . . . . . . . . . 39
9. Security Considerations . . . . . . . . . . . . . . . . . . . 40
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 41
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Normative References . . . . . . . . . . . . . . . . . . . . . 41
Informative References . . . . . . . . . . . . . . . . . . . . 42
Appendix A. Example of Usecases . . . . . . . . . . . . . . . . 43
A.1. Signaling WECMP to Ingress Node . . . . . . . . . . . . . 43
A.2. Signaling Optimal Forwarding Exitpoints to Ingress
Node . . . . . . . . . . . . . . . . . . . . . . . . . . 44
A.3. Load balancing to multiple CEs in a VRF . . . . . . . . . 44
A.4. Choosing a Received Label Based on it's Forwarding Semantic
at Advertising Node . . . . . . . . . . . . . . . . . . 45
A.5. Signaling Desired Forwarding Behavior for MPLS Upstream
labels at Receiving Node . . . . . . . . . . . . . . . . 45
A.6. Load Balancing over EBGP Parallel Links . . . . . . . . . 45
A.7. Flowspec Routes with Multiple "Redirect IP" next hops . . 46
A.8. Color-Only Resolution next hop . . . . . . . . . . . . . 46
A.9. Avoid Label Advertisement Oscillation Between Multihomed
PEs. . . . . . . . . . . . . . . . . . . . . . . . . . . 47
A.10. Signaling Intent over PE-CE Attachment Circuit . . . . . 47
A.10.1. Using DSCP in MultiNexthop Attribute . . . . . . . . 48
A.10.2. MPLS-enabled CE . . . . . . . . . . . . . . . . . . 48
A.11. 4PE - Signal MPLS Label for IPv4 Unicast routes . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51
1. Introduction
Today, a BGP speaker can advertise one nexthop for a set of NLRIs in
an Update. This nexthop can be encoded in either the top-level BGP-
Nexthop attribute (code 3), or inside the MP_REACH_NLRI attribute
(code 14).
This document defines a new optional non-transitive BGP attribute
called "MultiNexthop (MNH)" with IANA BGP attribute type code TBD,
that can be used to carry an ordered set of one or more Nexthops in
the same route, with forwaring information scoped on a per nexthop
basis.
2. Terminology
SN: Service Node
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iSN: Ingress Service Node
eSN: Egress Service Node
NLRI: Network Layer Reachability Information
AFI: Address Family Identifier
SAFI: Subsequent Address Family Identifier
PE : Provider Edge
RT : Route-Target extended community
RD : Route-Distinguisher
MPLS: Multi Protocol Label Switching
ECMP: Equal Cost Multi Path
WECMP: Weighted Equal Cost Multi Path
FRR: Fast Re Route
PNH : Protocol Next hop address carried in a BGP Update message
MNH: BGP MultiNextHop attribute
NFI: Nexthop Forwarding Information
FI: Forwarding Instruction
FA: Forwarding Argument
2.1. Definitions
MULTI_NEXT_HOP (aka MNH): BGP MultiNexthop attribute. The new
attribute defined by this document.
MNH TLV: Top level TLV contained in a MULTI_NEXT_HOP.
NFI TLV: Nexthop Forwarding Information TLV, contained in a MNH TLV.
FI TLV: Forwarding Instruction TLV, contained in a NFI TLV.
FA TLV: Forwarding Argument TLV, contained as an argument to a FI in
the FI TLV.
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Service Family : BGP address family used for advertising routes for
"data traffic" as opposed to tunnels (e.g. AFI/SAFIs 1/1 or 1/128).
Transport Family : BGP address family used for advertising tunnels,
which are in turn used by service routes for resolution (e.g. AFI/
SAFIs 1/4 or 1/76).
3. Motivation
For cases where multiple nexthops need to be advertised, BGP Addpath
[RFC7911] is used with some address families. On some other address
families like Flowspec, nexthop addresses are carried in one or more
extended communities of specific type.
Though Addpath allows basic ability to advertise multiple-nexthops,
it does not allow the sender to express the desired relationship
between the multiple nexthops being advertised e.g., relative
ordering, type of load balancing, fast reroute. These are local
decisions at the receiving node based on local configuration and path
selection between the various additional paths, which may tie-break
on some arbitrary step like Router-Id or BGP nexthop address. Some
scenarios with a BGP free core may benefit from having a mechanism,
where egress node can signal multiple nexthops along with their
relationship to ingress nodes.
It would be desirable to have a common way to carry one or more
nexthops on a BGP route of any family.
This document defines a new optional non-transitive BGP attribute
"MultiNexthop (MNH)" that can be used for this purpose.
The MNH attribute can be used in any BGP family that wants to carry
one or more nexthops, with forwaring information scoped on a per
nexthop basis. E.g. The MNH can be used to advertise MPLS label
along with nexthop for labeled and unlabeled families (e.g. Inet
Unicast, Inet6 Unicast, Flowspec) alike. Such that, mechanisms at
the transport layer can work uniformly on labeled and unlabled BGP
families to realize various usecases.
The MNH plays different role in "downstream allocation" scenario than
"upstream allocation" scenario. E.g. for [RFC8277] families that
advertise downstream allocated labels, the MNH can play the "Label
Descriptor" role, describing the forwarding semantics of the label
being advertised. This can be useful in network visualization and
controller based traffic engineering (e.g. EPE).
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4. Protocol Operations
4.1. Scope of Use, and Propagation
The MNH attribute is intended to be used in a BGP free core, between
egress and ingress BGP speakers that understand this attribute.
These BGP speakers may have an intra-AS or inter-AS BGP session
between them.
To avoid un-intentionally leaking the MNH to another AS, via a BGP
speaker that does not understand MNH attribute, it is defined as
"optional non-transitive". But this also means that a RR needs to be
upgraded to support this attribute before any PEs in the network can
make use of it.
Use of MNH on a BGP session is disabled by default. An
implementation MUST provide configuration control on a per BGP
neighbor address family basis, to enable MNH support. A sending BGP
speaker MUST NOT advertise MNH on a BGP address family route update
if MNH support is not enabled for the address family on the BGP
session.
If the MNH attribute is received on a BGP session where MNH support
is not enabled, the attribute MUST be treated as Unrecognized non-
transitive. This rule provides additional protection against
unintended propagation of this attribute, when both BGP speakers
understand MNH but receiver has not enabled the support. A RFC3392
Capability is not used for this purpose, because it would cause
session reset whenever 'MNH support' config is changed.
When a BGP speaker receives the MNH attribute on a BGP route in an
address family that is MNH enabled, it propagates the attribute
unchanged when readvertising the route with nexthop unchanged on a
BGP session address family that is also MNH enabled. The BGP speaker
excludes the MNH attribute when readvertising the route with nexthop
unchanged on a BGP session that is not MNH enabled.
When a BGP speaker receives the MNH attribute on a BGP route in an
address family that is MNH enabled, it processes the attribute but
does not propagate it further when readvertising the route with
nexthop altered. The BGP speaker MAY attach a new instance of MNH
attribute when readvertising the route (with nexthop unchanged or
altered) on a BGP session that is MNH enabled.
A BGP speaker re-advertising a BGP route with nexthop unchanged MAY
add the MNH attribute on the reflected BGP route, on behalf of the
originating BGP speaker. The "Advertising PNH field" in the MNH is
set to the Nexthop field in BGP route being re-advertised.
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4.2. Interaction of MNH with Nexthop (in attr codes 3, 14)
When adding a MultiNexthop attribute to an advertised BGP route, the
speaker MUST put the same next-hop address in the Advertising PNH
field as it put in the Nexthop field inside MP_REACH_NLRI attribute
if one exists, or the NEXT_HOP attribute.
A speaker that recognizes the MNH attribute and does not change the
PNH while readvertising the route, e.g. a Route Reflector, MUST
propagate unchanged the MultiNexthop attribute in the
readvertisement, satisfying the propagation scope constraints
described in previous section.
A speaker that recognizes MNH attribute and changes the PNH while
readvertising the route MUST remove the MNH attribute in the
readvertisement. The speaker MAY however add a new MNH attribute to
the re-dvertisement. While doing so the speaker MUST record in the
"Advertising PNH" field the same next-hop address as used in
MP_REACH_NLRI attribute if one exists, or the NEXT_HOP attribute.
A speaker receiving a MNH attribute SHOULD ignore it if the next-hop
address contained in 'Advertising PNH' field is not the same as the
nexthop address contained in MP_REACH_NLRI attribute if one exists,
or the NEXT_HOP attribute.
In case of [RFC2545], the global (non link-local) IPv6 address should
be used for this purpose.
As specified in [RFC7606] BGP update message can contain no more than
one instance of MP_REACH attribute or NEXT_HOP attribute. Similarly,
a BGP update MUST contain only one instance of MNH attribute. If the
MNH attribute (whether recognized or unrecognized) appears more than
once in an UPDATE message, then all the occurrences of the attribute
other than the first one SHALL be discarded and the UPDATE message
will continue to be processed.
4.3. Interaction with Addpath
[ADDPATH-GUIDELINES] suggests the following:
"Diverse path: A BGP path associated with a different BGP next-hop
and BGP router than some other set of paths. The BGP router
associated with a path is inferred from the ORIGINATOR_ID attribute
or, if there is none, the BGP Identifier of the peer that advertised
the path."
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When selecting "diverse paths" for ADD_PATH as specified above, the
MNH attribute should also be compared if it exists, to determine if
two routes have "different BGP next-hop".
4.4. Path Selection Considerations
4.4.1. Determining IGP Cost
While tie breaking in the path-selection as described in [RFC4271],
9.1.2.2. step (e) viz. the "IGP cost to nexthop", consider the
highest cost among the nexthop-legs present in this attribute.
The IGP cost thus calculated is also used when constructing AIGP TLV
([RFC7311])
4.5. Denoting Upstream or Downstream Semantics
MultiNexthop attribute may describe to a receiving speaker what the
forwarding semantics of an Upstream-allocated label should be. This
can be used with either labeled or unlabled BGP families.
A MultiNexthop attribute may also play "Downstream signaled Label
Descriptor" role. A BGP speaker advertising a route carrying
downstream allocated MPLS label MAY add this attribute to the BGP
route, to "describe" to the receiving speaker what the label's
forwarding semantics is at the Egress node.
Today semantics of a downstream-allocated label is known only to the
egress node advertising the label. The speaker receiving the label-
binding doesn't know what the label's forwarding semantic at the
advertiser is. In some environments, it may be useful to convey this
information to the receiving speaker. This may help in better
debugging and manageability, or enable the receiving speaker, which
could also be some centralized controller, make better decisions
about which label to use, based on the label's forwarding-semantic.
While doing upstream-label allocation, this attribute can be used to
convey the forwarding-semantics at the receiving node should be.
Details of the BGP protocol extensions required for signaling
upstream-label allocation are out of scope of this document, and are
described in [MPLS-NAMESPACES].
In rest of this document, the use of term "Label" will mean
downstream allocated label, unless specified otherwise as upstream-
allocated label.
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When using the MultiNexthop attribute for IP-routes, the Upstream
role is used. Since IP prefixes are by nature upstream allocated,
global scope.
5. Encoding of BGP MultiNexthop (MNH) Attribute
"MultiNexthop (MNH)" is a new BGP optional non-transitive attribute
(code TBD), that can be used to carry an ordered set of one or more
Nexthops in the same route, with forwaring information scoped on a
per nexthop basis. This attribute describes forwarding instructions
using TLVs described in this document.
This section describes the organization and encoding of the MNH
attribute.
MNH Attribute: {
PrimaryPath {
[Forwarding Instruction 1],
..
[Forwarding Instruction n]
}
BackupPath {
[Forwarding Instruction 1],
..
[Forwarding Instruction n]
}
LabelDescriptor {
[Forwarding Instruction 1],
..
[Forwarding Instruction n]
}
}
Forwarding Instruction: {
{FwdAction, Forwarding Arguments}
}
Figure 1: Overview of MNH Attribute Layout - Eye candy summary
A MNH attribute consists of one of more "MNH TLVs". A MNH TLV
contains a Type and one unit of Nexthop Forwarding Information (NFI
TLV).
A NFI TLV contains one or more Forwarding Instructions (FI TLV).
A Forwarding Instruction TLV contains a "Forwarding Action" and one
more "Forwarding Arguments" (FA TLVs). The Forwarding Arguments
describe the parameters required to complete a Forwarding Action.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNH-Flags | Advt-PNH-Len | Advertising PNH .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNH TLV ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ MNH TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: MultiNexthop - BGP Attribute
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- Attr. Flags (1 octet)
BGP Path-attribute flags. indicating an Optional Non-Transitive
attribute. i.e. Optional bit set, Transitive bit reset.
- Attr. Type Code (1 octet)
Type code allotted by IANA. TBD.
- Length (1 or 2 octets)
One or Two bytes field stating length of attribute value in bytes.
- MNH-Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
All bits are reserved.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Advt-PNH-Len (1 octet)
Length in octets (4 for IPv4, 16 for IPv6, 12 for VPN-IPv4,
24 for VPN-IPv6) of Advertising PNH Address.
- Advertising PNH Address (Advt-PNH-Len octets)
BGP Protocol Nexthop address advertised in NEXT_HOP or MP_REACH_NLRI attr.
Used to sanity-check the MNH attribute. In case of RFC-2545, this will be
the global (non link-local) IPv6 address.
- MNH TLVs: One or more MNH TLVs are carried in a MNH attr.
MNH TLV is described in subsequent sections.
5.1. MNH TLV
The type of MNH TLV describes how the forwarding information carried
in the MNH TLV is used.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNH-TLV Flags| MNH. Type Code| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 3: MNH TLV
- MNH-TLV Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
All bits are reserved.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
MNH Type Code Meaning
-------------- -------------
0 Reserved
1 Primary forwarding path
2 Backup forwarding path
3 Label Descriptor
- Length
Length of Value portion in octects.
- Value
Value portion contains the NFI TLV.
Type codes 1 and 2 are applicable for upstream allocated prefixes,
example IP, MPLS, Flowspec routes.
Type code 4 describes the forwarding behavior given to downstream
allocated MPLS label, adveritsed in BGP route.
Usage of Type code 1 in a BGP route containing IP prefix gives
similar result as advertising the route with nexthop contained in BGP
path-attributes: Nexthop (code 3) or MP_REACH_NLRI (code 14).
Upstream allocation for MPLS routes is achieved by using mechanisms
explained in [MPLS-NAMESPACES].
If an invalid Type Code (like 0) is received, the TLV is ignored
gracefully handing the error.
If an unknown Type Code is received, it SHOULD be ignored but
propagated further when the MNH attribute is propagated, because
nexthop is not changed.
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If the received Type Code is incompatible for the prefix in BGP NLRI,
the TLV should be ignored.
5.1.1. Primary Forwarding Path
Type Code = 1 means the TLV describes forwarding state to be
programmed at receiving speaker as primary path nexthop leg. This
TLV is used with Upstream allocated or global scope prefixes carried
in BGP NLRI. Value part of this TLV contains Nexthop Forwarding
Information TLV.
A BGP speaker uses the nexthop forwarding information received in
this TLV as a primary path nexthop leg when programming the route for
the NLRI prefix in its Forwarding table.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNH-TLV Flags| MNH Type = 1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nexthop Forwarding Information TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Primary forwarding path TLV
5.1.2. Backup Forwarding Path
Type Code = 2 means the TLV describes forwarding state to be
programmed at receiving speaker as backup-path nexthop leg. This TLV
is used with Upstream allocated prefixes or global scoped prefixes.
Value part contains Nexthop Forwarding Information TLV.
Signaling a different nexthop for use as backup path is desired in
some labeled forwarding scenarios, where two multihomed edge devices
use each other as backup path to protect traffic when primary path
fails.
This is required to avoid label advertisement oscillation between the
multihomed PEs when they implement per-nexthop label allocation mode.
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The label advertised by a PE1 for primary path advertisement is
allocated/forwarded using external paths as primary leg and backup-
path label from other multihomed PE2 as backup-path label. Such that
primary-path label allocation at PE1 is not a function of the
primary-path label advertised by PE2. Thus the primary path label
remains stable at a PE and does not change when a new primary path
label is received from the other multihomed PE. This prevents the
label oscillation problem.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNH-TLV Flags| MNH Type = 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nexthop Forwarding Information TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Backup forwarding path TLV
The backup path label allocated and advertised by a PE is a function
of only the primary path. E.g. path to the CE device. So this label
value does not change when a new label is received from the other
multihomed PE
5.1.3. Label Descriptor
Type Code = 3 means the TLV describes forwarding state associated
with downstream allocated MPLS label at the egress node identified in
Endpoint FA TLV. Value part of this TLV contains Endpoint FA-TLV,
Payload Info FA-TLV to identify the label being described, along with
Nexthop Forwarding Information TLV that describes the forwarding
state.
Signaling what a label advertised in BGP route signifies is helpful
for debugging. The information provided by label descriptor can
enable new usecases like network visualization and off box EPE
decisions.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MNH-TLV Flags| MNH Type = 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Fwd Argument TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encap Info. Fwd Argument TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nexthop Forwarding Information TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Endpoint Fwd Argument TLV:
Specifies the IP endpoint. Section 5.4.1.
Encap Info. Fwd Argument TLV:
Specifies the Label value being described. Section 5.4.3.
Nexthop Forwarding Information TLV:
Indicates the forwarding state. Described in Section 5.2.
Figure 6: Label Descriptor TLV
5.2. Nexthop Forwarding Information TLV
A Nexthop Forwarding Information TLV describes a MNH TLV. It
contains one or more Forwarding Instruction TLVs. These Forwarding
Instructions are the Forwarding Legs of the MNH.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NFI Flags | Num-Nexthops |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding Instruction TLV (F.I. TLV) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Forwarding Instruction TLV (F.I. YLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Nexthop Forwarding Information TLV
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- NFI Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
All bits are reserved.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Num-Nexthops
Number of F.I. TLVs.
- Forwarding Instruction TLV
Each F.I. TLV describes a Nexthop Leg.
Layout of Forwarding Instruction TLV is described in next section.
5.3. Forwarding Instruction TLV
Each Forwarding Instruction TLV describes a Nexthop Leg. It expresses
a "Forwarding Action" (FwdAction) along with arguments required to
complete the action. The type of actions defined by this TLV are
given below. The arguments are denoted by "Forwarding Argument
TLVs". The Forwarding Argument TLVs takes appropriate values based
on the FwdAction.
Each FwdAction should note the Arguments needed to complete the
action. Any extranous arguments should be ignored. If the minimum
set of arguments required to complete an action is not received, the
Forwarding Instruction TLV should be ignored. Appropriate logging
and diagnostic info MAY be provided by an implementation to help
troubleshoot such scenarios.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.I. Flags | Relative Pref | FwdAction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fwd Argument TLV ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Fwd Argument TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 8: Forwarding Instruction TLV
- F.I. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
All bits are reserved.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Relative Pref (2 octets)
Unsigned 2 octet integer specifying relative order or preference, among
the many forwarding instructions, to use in FIB. All usable nexthop legs
with lowest relative-pref are installed in FIB as primary-path. Thus if
multiple legs exist with that lowest relative-pref, ECMP is formed.
FwdAction Meaning
--------- -------------
0 Reserved
1 Forward
2 Pop-And-Forward
3 Swap
4 Push
5 Pop-And-Lookup
6 Replicate
Forwarding Instruction TLV with unknown FwdAction should be ignored, skipped
and rest of the attribute processed; gracefully handling the error. The event
may be appropriately logged for diagnosis.
- Length (2 octets)
Length in octets, of all Forwarding Argument TLVs.
Meaning of most of the above FwdAction semantics is well understood.
FwdAction 1 is applicable for both IP and MPLS routes. FwdActions
2-5 are applicable for encapsulated payloads (like MPLS) only.
FwdActions 1, 6 are applicable for Flowspec routes for Redirect and
Mirror actions. FwdAction 6 can also be used to indicate multicast
replication like functionality.
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The "Forward" action means forward the IP/MPLS packet with the
destination prefix (IP-dest-addr/MPLS-label) value unchanged. For IP
routes, this is the forwarding-action given for next-hop addresses
contained in BGP path-attributes: Nexthop (code 3) or MP_REACH_NLRI
(code 14). For MPLS routes, usage of this action is equivalent to
SWAP with same label-value; one such usage is explained in
[MPLS-NAMESPACES] when Upstream-label-allocation is in use.
The "Pop-And-Forward" action means Pop the payload header (e.g.
MPLS-label) and forward the payload towards the Nexthop IP-address
specified in the Endpoint Id TLV, using appropriate encapsulation to
reach the Nexthop.
When applied to MPLS packet, the "Pop-And-Lookup" action may result
in a MPLS-lookup or an upper-layer header (like IPv4, IPv6) lookup,
depending on whether the label that was popped was the bottom of
stack label.
If an incompatible FwdAction is received for a prefix-type, or an
unsupported FwdAction is received, it is considered a semantic-error
and MUST be dealt with as explained in "Error handling procedures"
section.
5.4. Forwarding Argument TLV
The Forwarding Argument TLV describes various parameters required to
execute a FwdAction.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Forwarding Argument TLV
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- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
All bits are reserved.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
F.A. Type Code Meaning
------------- ---------
0 Reserved
1 Endpoint Identifier
2 Path Constraints
3 Payload encapsulation info signaling
4 Endpoint attributes advertisement
- Length (2 octets)
Length in bytes of Value field.
5.4.1. Endpoint Identifier
F.A. Type Code = 1. This Forwarding Argument TLV identifies an
Endpoint of different types.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code =1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Endpoint Type | Endpoint Len | Endpoint Value|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Endpoint Identifier
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- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
Endpoint Type Value Len (octets)
------------- --------- ---------------------
0 Reserved
1 IPv4 Address 4
2 IPv6 Address 16
3 MPLS Label (Upstream 4
allocated or
Global scope)
4 Fwd Context RD 8
5 Fwd Context RT 8
- Endpoint Len (1 octet)
Length in bytes of Endpoint Value field.
5.4.2. Path Constraints
F.A. Type Code = 2. This Forwarding Argument TLV defines
constraints for path to the Endpoint.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | ConstrainType | Constrain Len | ConstrainValue|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ConstrainValue |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Path Constraints
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- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
ConstrainType Value Len (octets)
------------- ------------------------- ---------------------
0 Reserved
1 Proximity check 2
2 Transport Class ID (Color) 4
3 Load balance factor 2
- Constrain Len (1 octet)
Length in bytes of Constrain Value field.
- Proximity check Flags (2 octets)
Flags describing whether the nexthop endpoint is expected to be single hop
away, or multihop away. Format of flags is described in next section.
- Transport Class ID (Color):
This is a 32 bit identifier, associated with the Nexthop address.
The Nexthop IP-address specified in "Endpoint Identifier" TLVs
are resolved over tunnels of this color.
Defined in [BGP-CT] [draft-kaliraj-idr-bgp-classful-transport-planes]
- Load balance factor (2 octets)
Balance Percentage
5.4.2.1. Proximity Check
Usually EBGP singlehop received routes are expected to be one hop
away, directly connected. And IBGP received routes are expected to
be multihop away. Implementations today provide configuring
exceptions to this rule.
The 'expected proximity' of the Nexthop can be signaled to the
receiver using the Proximity check flags. Such that irrespective of
whether the route is received from IBGP/EBGP peer, it can be treated
as a single-hop away or multihop away nexthop.
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The format of the Proximity check Sub-TLV is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |ConstrainType=1| Len = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Proximity Check Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Proximity check Flags (2 octets)
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S M R R R R R R R R R R R R R R|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
S: Restrict to Singlehop path.
M: Expect Multihop path.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
Figure 12: Proximity check constrain
This TLV would be valid with Forwarding Instructions TLV with
FwdAction of Forward, Pop-And-Forward, Swap or Push.
When S bit is set, receiver considers the nexthop valid only if it is
directly connected to the receiver.
When M bit is set, receiver assumes that the nexthop can be multiple
hops away, and resolves the path to the nexthop via another route.
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When both S and M bits are set, M bit behavior takes precedence.
When both S and M bits are Clear, the current behavior of deriving
proximity from peer type (EBGP is singlehop, IBGP is multihop) is
followed.
5.4.2.2. Transport Class ID (Color)
The Nexthop can be associated with a Transport Class, so as to
resolve a path that satisfies required Transport tunnel
characteristics. Transport Class is defined in [BGP-CT]
Transport Class is a per-nexthop scoped attribute. Without MNH, the
Transport class is applied to the nexthop IP-address encoded in the
BGP-Nexthop attribute (code 3), or inside the MP_REACH_NLRI attribute
(code 14). With MNH, the Transport Class can be specified per
Nexthop-Leg (Forwarding Instruction TLV). It is applied to the IP-
address encoded in the Endpoint Identifier TLV of type "IPv4
Address", "IPv6 Address" , "MPLS Label (Upstream allocated or Global
scope)".
The format of the Transport Class ID Sub-TLV is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |ConstrainType=2| Len = 4 | Transport.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. Class ID (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Transport Class ID (Color):
This is a 32 bit identifier, associated with the Nexthop address.
The Nexthop specified in Endpoint Identifier TLVs
are resolved over tunnels of this color.
Defined in [BGP-CT] [draft-kaliraj-idr-bgp-classful-transport-planes]
Figure 13: Transport Class ID (Color)
This TLV would be valid with Forwarding Instructions TLV with
FwdAction of Forward, Swap or Push.
5.4.2.3. Load Balance Factor
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |ConstrainType=3| Len = 2 | Balance.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|.. Percentage |
+-+-+-+-+-+-+-+-+
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Len (1 octet)
Length of the Constrain Value field.
- Balance Percentage:
This is the explicit "balance percentage" requested by the sender,
for unequal load-balancing over these Nexthop-Descriptor-TLV legs.
This balance percentage would override the implicit
balance-percentage calculated using "Bandwidth" attribute
sub-TLV.
Figure 14: Load Balance Factor
This sub-TLV would be valid with Forwarding Instructions TLV with
FwdAction of Forward, Swap or Push.
This is the explicit "balance percentage" requested by the sender,
for unequal load-balancing over these Nexthop-Descriptor-TLV legs.
This balance percentage would override the implicit balance-
percentage calculated using "Bandwidth" attribute sub-TLV
When the sum of "balance percentage" on the nexthop legs does not
equal 100, it is scaled up or down to match 100. The individual
balance percentages in each nexthop leg are also scaled up or down
proportionally to determine the effective balance percentage per
nexthop leg.
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5.4.3. Payload Encapsulation Info
F.A. Type Code = 3. This Forwarding Argument TLV defines payload
encapsulation information.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code =3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Encap Type | Encap Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encap Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Payload Encapsulation Info
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
Endcap Type Value
------------- --------------
0 Reserved
1 MPLS Label Info
2 SR MPLS label Index Info
3 SRv6 SID info
4 DSCP code point
- Encap Len (2 octets)
Length in octets of Encap Value field.
5.4.3.1. MPLS Label Info
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code =3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Encap Type=1 | Encap Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags (2 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPLS Label (20 bits) |Rsrv |S~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ MPLS Label (20 bits) |Rsrv |S|
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: MPLS Label Info
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- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Encap Type
= 1, to signify MPLS Label Info.
- Encap Len (2 octets)
Length in bytes of following Encap Value field.
- Flags (2 octets):
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E R R R R R R R R R R R R R R R|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
E: ELC bit. Indicates if this egress NH is Entropy Label Capable.
1 means the Entropy Label capable.
0 means not capable to handle Entropy Label.
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- MPLS Label, Rsrv, S bit.
20 bit MPLS Label stack encoded as in RFC 8277.
S bit set on last label in label stack.
5.4.3.2. SR MPLS Label Index Info
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code =3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Encap Type=2 | Encap Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESERVED | LI Flags | Label .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: SR MPLS Label Index Info
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Encap Type
= 2, to signify SR MPLS SID Info.
- Encap Len (2 octets)
Length in bytes of following Encap Value field.
Rest of the value portion is encoded as specified in RFC-8669 sec 3.1.
- RESERVED: 8-bit field. MUST be set to zero, SHOULD be ignored by receiver.
- LI Flags: 16 bits of flags. None defined. MUST be set to zero, SHOULD be ignored by receiver.
- Label Index:
32-bit value representing the index value in the SRGB space.
5.4.3.3. SRv6 SID Info
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code =3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Encap Type=3 | Encap Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. SRv6 SID Info (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: SRv6 SID Info
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Encap Type
= 3, to signify SR MPLS SID Info.
- Encap Len (2 octets)
Length in bytes of following Encap Value field.
- SRv6 SID Info:
SRv6 SID Information, as specified in RFC-9252 sec 3.1.
5.4.3.4. DSCP
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Encap Type=4 | Encap Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|DSCP code point|
+-+-+-+-+-+-+-+-+
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Figure 19: DSCP
- F.A. Flags (1 octet)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R R R R R R R R|
+-+-+-+-+-+-+-+-+
R: Reserved. MUST be set to zero, SHOULD be ignored by receiver.
- Length (2 octets)
Length in bytes of Value field.
- Encap Type
= 4, to signify DSCP code point.
- Encap Len (2 octets)
= 1, Length in bytes of following Encap Value field.
- DSCP code point:
DS Field, as specified in RFC-2474 sec 3.
5.4.4. Endpoint Attributes
F.A. Type Code = 4. This Forwarding Argument TLV defines attributes
of an endpoint.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Attrib Type | Attr Len | Attr Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 20: Endpoint attributes
EP Attrib Type Attrib Value Attrib Len (octets)
---------------- ------------------ ---------------------
0 None
1 Endpoint Bandwidth 8
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5.4.4.1. Endpoint Bandwidth
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F.A. Flags | F.A. Type Code = 4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Attrib Type 1| Attr Len=8 | Attr Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth (contd.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- Len (2 octets)
Length in bytes of remaining portion of SubTLV.
- Bandwidth
The bandwidth to the endpoint expressed as 8 octets,
units being bits per second.
Figure 21: Endpoint Bandwidth
This sub-TLV would be valid with Forwarding Instruction TLV with
FwdAction of Forward, Swap or Push.
6. Error Handling
With MNH TLV Type = 4 (Label Descriptor), this attribute is used to
describe the label advertised by the BGP-peer. If the value in the
attribute is syntactically parse-able, but not semantically valid,
the receiving speaker should deal with the error gracefully and MUST
NOT tear down the BGP session. In such cases the rest of the BGP-
update can be consumed if possibe.
With other MNH TLV Types, this attribute is used to specify the
forwarding action at the receiving BGP-peer. If the value in the
attribute is syntactically parse-able, but not semantically valid,
the receiving speaker SHOULD deal with the error gracefully by
ignoring the MNH attribute, and continue processing the route. It
MUST NOT tear down the BGP session.
If a MNH TLV Type = 4 is received for an IP-route (SAFI Unicast), the
MNH attribute SHOULD be ignored. Because IP route prefixes are
upstream allocated by nature.
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If a MNH TLV Type = 4 is received for an [MPLS-NAMESPACES] route, the
MNH attribute SHOULD be ignored. Because the label prefix in MPLS-
NAMESPACE family routes is upstream allocated.
The receiving BGP speaker MAY consider the "Num-Nexthops" value in a
Nexthop Forwarding Information TLV not acceptable, based on it's
forwarding capabilities. In such cases, the MNH attribute SHOULD be
considered Unusable, and not be used, ignored on receipt. The
condition SHOULD be dealt gracefully and MUST NOT tear down the BGP
session.
A TLV or sub-TLV of a certain Type in a MNH attribute can occur only
once, unless specified otherwise by that type value. If multiple
instances of such TLV or sub-TLV is received, the instances other
than the first occurance are ignored.
If a TLV or sub-TLV of an unknown Type value is received, it is
ignored and skipped. Remaining part of the MNH attribute if
parseable is used
In case of length errors inside a TLV, such that the MNH attribute
cannot be used, but the length value in MNH attribute itself is
proper, the MNH attribute should be considered invalid and not used.
But rest of the route update if parseable should be used. This
follows the 'Attribute discard' approach described in [RFC7606]
Section 2.
7. Scaling Considerations
The MNH attribute allows receiving multiple nexthops on the same BGP
session. This flexibility also opens up the possibility that a peer
can send large number of multipath (ECMP/UCMP/FRR) nexthops that may
overwhelm the local system's forwarding plane. Prefix-limit based
checks will not avoid this situation.
To keep the scaling limits under check, a BGP speaker MAY keep
account of number of unique multipath nexthops that are received from
a BGP peer, and impose a configurable max-limit on that. This is
especially useful for EBGP peers.
A good scaling property of conveying multipath nexthops using the MNH
attribute with N nexthop legs on one BGP session, as against BGP
routes on N BGP sessions is that, it limits the amount of
transitionary multipath combinatorial state in the latter model.
Because the final multipath state is conveyed by one route update in
deterministic manner, there is no transitionary multipath
combinatorial explosion created during establishment of N sessions.
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8. IANA Considerations
This document makes request to IANA to allocate the following codes
in BGP attributes registry.
8.1. BGP Path Attributes
A new BGP attribute code TBD for "BGP MultiNexthop Attribute
(MULTI_NEXT_HOP)", in "BGP Path Attributes" registry.
8.2. Capability Codes
This document makes request to IANA to allocate a BGP capability code
TBD for "BGP MultiNexthop Attribute (MULTI_NEXT_HOP), in "Capability
Codes" registry.
8.3. BGP MultiNextHop Attribute
This document requests IANA to create a new registry group for
MultiNextHop attribute, and the following registries in it.
8.3.1. MultiNextHop (MNH) TLV Types
This is a Registry for Type codes in Section 5.1 "MULTI_NEXT_HOP TLV"
Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: MultiNexthop (MNH) TLV Types
MNH Type Code Meaning
-------------- -------------
0 Reserved
1 Primary forwarding path
2 Backup forwarding path
3 Label Descriptor
4-254 Unassigned
255 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
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8.3.2. Forwarding Action Types
This is a Registry for Type codes in Section 5.3 "Forwarding
Instruction TLV"
Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: Forwarding Action Types
FwdAction Meaning
--------- -------------
0 Reserved
1 Forward
2 Pop-And-Forward
3 Swap
4 Push
5 Pop-And-Lookup
6 Replicate
7-254 Unassigned
255 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
8.3.3. Forwarding Argument Types
This is a Registry for Type codes in Section 5.4 "Forwarding
Arguments TLV"
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Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: Forwarding Argument Types
F.A. Type Code Meaning
--------------- ------------------
0 Reserved
1 Endpoint Identifier
2 Path Constraints
3 Payload encapsulation info signaling
4 Endpoint attributes advertisement
5-65534 Unassigned
65535 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
8.3.4. Endpoint Types
This is a Registry for Type codes in Section 5.4.1 "Endpoint
Identifier" Forwarding Argument.
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Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: Endpoint Types
Endpoint Type Value
------------- ---------
0 Reserved
1 IPv4 Address
2 IPv6 Address
3 MPLS Label
4 Fwd Context RD
5 Fwd Context RT
6-254 Unassigned
255 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
8.3.5. Path Constrain Types
This is a Registry for Type codes in Section 5.4.2 "Path Constrain"
Forwarding Argument.
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Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: Path Constrain Types
ConstrainType Value
------------- -------------------------
0 Reserved
1 Proximity check
2 Transport Class ID (Color)
3 Load balance factor
4-254 Unassigned
255 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
8.3.6. Encapsulation Types
This is a Registry for Type codes in Section 5.4.3 "Payload
Encapsulation Info" Forwarding Argument.
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Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: Encapsulation Types
Encap Type Value
------------- --------------
0 Reserved
1 MPLS Label Info
2 SR MPLS label Index Info
3 SRv6 SID info
4 DSCP code point
5-254 Unassigned
255 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
8.3.7. Endpoint Attribute Types
This is a Registry for Type codes in Section 5.4.4 "Endpoint
attributes" Forwarding Argument.
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Under "Border Gateway Protocol (BGP) Parameters",
Registry Group: BGP MultiNextHop Attribute
Registry Name: Endpoint Attribute Types
EP Attrib Type Attrib Value
---------------- ------------------
0 Reserved
1 Bandwidth
2-254 Unassigned
255 Reserved
Reference: This document.
Registration Procedure(s)
Future assignments are to be made using either the Standards Action
process defined in [RFC2434], or the Early IANA Allocation process
defined in [RFC4020].
Note to RFC Editor: this section may be removed on publication as an
RFC.
9. Security Considerations
The MNH attribute is defined as optional non-transitive BGP
attribute, such that it does not accidentally get propagated or
leaked via BGP speakers that dont support this feature, especially
does not unintentionally leak across EBGP boundaries.
MNH may be used to advertise nexthop with MPLS label in various BGP
families. In scenarios where MPLS is enabled on link to a device in
an untrusted domain, e.g. a PE-CE link or ASBR-ASBR inter-AS link,
security can be provided against MPLS label spoofing by using MPLS
context tables as described in MPLS enabled CE (Appendix A.10.2).
Such that only MPLS traffic with labels advertised to the BGP speaker
are allowed to forward. However, the PE may not be able to perform
any checks based on inner payload in the MPLS packet since it
performs label swap forwarding. Such 'inner payload' based checks
may be offloaded to a downstream node that forwards and processes
inner payload, e.g., an IP router having full FIB. These security
aspects should be considered when using MPLS enabled CE devices.
Contributors
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Reshma Das
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: dreshma@juniper.net
Natrajan Venkataraman
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: natv@juniper.net
Acknowledgements
Thanks to Jeff Haas, Robert Raszuk, Ron Bonica for the review,
discussions and input to the draft.
Thanks to Blaine Williams and Satya Mohanty for the discussions on
some usecases.
References
Normative References
[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
DOI 10.17487/RFC2545, March 1999,
<https://www.rfc-editor.org/info/rfc2545>.
[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>.
[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>.
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[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>.
[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>.
Informative References
[ADDPATH-GUIDELINES]
Uttaro, Ed., "BGP Flow-Spec Redirect to IP Action", 25
April 2016, <https://datatracker.ietf.org/doc/html/draft-
ietf-idr-add-paths-guidelines-08#section-2>.
[BGP-CT] Vairavakkalai, Ed. and Venkataraman, Ed., "BGP Classful
Transport Planes", 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ct-12>.
[FLWSPC-REDIR-IP]
Simpson, Ed., "BGP Flow-Spec Redirect to IP Action", 2
February 2015, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-flowspec-redirect-ip#section-3>.
[MPLS-NAMESPACES]
Vairavakkalai, Ed., "BGP Signaled MPLS Namespaces", 10
July 2023, <https://datatracker.ietf.org/doc/html/draft-
kaliraj-bess-bgp-sig-private-mpls-labels-06>.
[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>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[SRTE-COLOR-ONLY]
Filsfils, Ed., "BGP Flow-Spec Redirect to IP Action", 21
February 2018, <https://tools.ietf.org/html/draft-
filsfils-spring-segment-routing-policy-06#section-8.8.1>.
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Appendix A. Example of Usecases
This section describes various example usecases of the MNH attribute.
A.1. Signaling WECMP to Ingress Node
This section describes how MNH can be used to provide weighted equal
cost multipath in a network fabric, while not increasing RIB scale.
[RR1]
.
. +-[P21] |
. +-[P22] __
. +-[P23] _.( )..
[R1]. +-[P24] .. (_ _) .. [R2]
. +-[P25] (._..)
.
. +-[P2n]
<---- Traffic Direction ----
Figure 22: WECMP without increasing RIB scale
Figure 22 shows a network with BGP speaker R1 connected to a number
of routers P21 .. P2n in its region. R1 is eSN and R2 is iSN for the
IP traffic in consideration. BGP service families IPv4 Unicast (AFI/
SAFI: 1/1) and IPv6 Unicast (AFI/SAFI: 2/1) are negotiated on the BGP
sessions between RR1 - R1 and RR1 - R2. RR1 reflects the BGP routes
between R1 and R2 with next hop unchanged.
When MNH is not in use, R1 advertises "n" BGP Addpath routes for a
service prefix Pfx1, each having a distinct next hop, P21 .. P2n, and
desired Link Bandwidth Extended Community. These Addpath routes will
be received by R2, which can do WECMP based on the Link Bandwidth
Extended Communities attached on the routes. This model increases
RIB scale by "n" times, so that WECMP can be achieved.
When MNH is used in this network, R1 advertises a single BGP route
for prefix Pfx1, which contains a MNH attribute with "n" next hops,
each carrying the desired link bandwidth using Section 5.4.2.3 or
Section 5.4.4.1
This allows achieving WECMP in the network without increasing RIB
scale.
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A.2. Signaling Optimal Forwarding Exitpoints to Ingress Node
In a BGP free core, one can dynamically signal to the ingress-node,
how traffic should be load-balanced towards a set of exit nodes, in
one BGP-route containing this attribute.
Example, for prefix1, perform equal load balancing towards exit nodes
A, B; where as for prefix2, perform weighted load balancing (40%,
30%, 30%) towards exit nodes A, B, C.
Example, for prefix1, use PE1 as primary-nexthop and use PE2 as a
backup-nexthop.
A.3. Load balancing to multiple CEs in a VRF
This section describes how MNH can be used to provide load balancing
and entropy in a provider network for traffic destined to multiple
CEs in a VRF, without increasing RIB scale.
[RR1]
.
[CE1]. |
. __
[CE2] . _.( )..
.[PE1] .. (_ _) .. [PE2]
[CE3] . (._..)
.
[CE4].
<---- Traffic Direction ----
Figure 23: Load balancing to multiple CEs in a VRF
Figure 23 shows a L3VPN network with multiple CE devices connected to
the same VRF at PE1. The VRF is configured with a RD: RD1, and uses
"per next hop" label allocation mode to advertise the CE routes to
L3VPN core. PE1 is eSN and PE2 is iSN for the IP traffic in
consideration. CE1..CE4 advertise route for same prefix Pfx1 in BGP
service families IPv4 Unicast (AFI/SAFI: 1/1) negotiated on the BGP
sessions between the CEs and PE1. BGP L3VPN address family (AFI/
SAFI: 1/128) is negotiated between PE1 - RR1, and RR1 - PE2. RR1
reflects the BGP routes between PE1 and PE2 with next hop unchanged.
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PE1 would typically advertise to RR1 only the best path for prefix
Pfx1 out of routes received from CE1..CE4. Using per CE RD or
Addpath for L3VPN family may allow PE1 to advertise all CE routes to
the RR, with an increase in RIB scale. This model increases RIB
scale by "n" times, where 'n' is the number of CEs.
When MNH is used in this network, PE1 advertises a single BGP L3VPN
route for prefix Pfx1, which contains a MNH attribute with "n" next
hops, each carrying the label pointing towards a particular CE, using
Section 5.4.3 along with the Section 5.4.1
This allows the network to direct traffic to a specific CE, and
better loadbalance traffic in the provider network, with entropy
provided by the per CE VPN labels, without increasing RIB scale.
A.4. Choosing a Received Label Based on it's Forwarding Semantic at
Advertising Node
In Downstream label allocation case, the MNH plays role of "Label
descriptor" and describes the forwarding treatment given to the label
at the advertising speaker. The receiving speaker can benefit from
this information as in the following examples:
- For a Prefix, a label with FRR enabled nexthop-set can be preferred
to another label with a nexthop-set that doesn't provide FRR.
- For a Prefix, a label pointing to 10g nexthop can be preferred to
another label pointing to a 1g nexthop
- Set of labels advertised can be aggregated, if they have same
forwarding semantics (e.g. VPN per-prefix-label case)
A.5. Signaling Desired Forwarding Behavior for MPLS Upstream labels at
Receiving Node
In Upstream label allocation case, the receiving speaker's
forwarding-state can be controlled by the advertising speaker, thus
enabling a standardized API to program desired MPLS forwarding-state
at the receiving node. This is described in the [MPLS-NAMESPACES]
A.6. Load Balancing over EBGP Parallel Links
Consider N parallel links between two EBGP speakers. There are
different models possible to do load balancing over these links:
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N single-hop EBGP sessions over the N links. Interface addresses
are used as next-hops. N copies of the RIB are exchanged to form
N-way ECMP paths. The routes advertised on the N sessions can be
attached with Link bandwidth comunity to perform weighted ECMP.
1 multi-hop EBGP session between loopback addresses, reachable via
static route over the N links. Loopback addresses are used as
next-hops. 1 copy of the RIB is exchanged with loopback address as
nexthop. And a static route can be configured to the loopback
address to perform desired N-way ECMP path. M loopbacks are
configured in this model, to achieve M different load balancing
schemes: ECMP, weighted ECMP, Fast-reroute enabled paths etc.
1 multi-hop EBGP session between loopback addresses, reachable via
static route over the N links. Interface addresses are used as
next-hops, without using additional loopbacks. 1 copy of the RIB
is exchanged with MNH attribute to form N-way ECMP paths, weighted
ECMP, Fast-reroute backup paths etc. BFD may be used to these
directly connected BGP nexthops to detect liveness.
A.7. Flowspec Routes with Multiple "Redirect IP" next hops
There are existing protocol machinery which can benefit from the
ability of MNH to clearly specify fallback behavior when multiple
nexthops are involved. One example is the scenario described in
[FLWSPC-REDIR-IP] where multiple Redirect-to-IP nexthop addresses
exist for a Flowspec prefix. In such a scenario, the receiving
speakers may redirect the traffic to different nexthops, based on
variables like IGP-cost. If instead, the MNH was used to specify the
redirect-to-IP nexthop, then the order of preference between the
different nexthops can be clearly specified using one flowspec route
carrying a MNH containing those different nexthop-addresses
specifying the desired preference-order. Such that, irrespective of
IGP-cost, the receiving speakers will redirect the flow towards the
same traffic collector device.
A.8. Color-Only Resolution next hop
Another existing protocol machinery that manufactures nexthop
addresses from overloaded extended color community is specified in
[SRTE-COLOR-ONLY]. In a way, the color field is overloaded to carry
one anycast BGP next-hop with pre-specified fallback options. This
approach gives us only two next-hops to play with. The 'BGP nexthop
address' and the 'Color-only nexthop'
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Instead, the MNH could be used to achieve the same result with more
flexibility. Multiple BGP nexthops can be carried, each resolving
over a desired Transport class (Color), and with customizable
fallback order. And the solution will work for non-SRTE networks as-
well.
A.9. Avoid Label Advertisement Oscillation Between Multihomed PEs.
In a MPLS network, a router may be multihomed to two PEs. The PEs
may re-advertise routes received from the router to the IBGP core
with self as nexthop and a "per nexthop" label. The PEs may also
protect failure of primary path to the router by using the IBGP path
via the other multihomed PE as a backup path.
In this scenario, label allocation oscillation may occur when one PE
advertises a new label to the other PE. Reception of a new label
results in change of nexthop, as the label is used as back nexthop
leg, and per-nexthop label allocation is in use. Thus a new label is
allocated and advertised. And when this new label is received by the
first PE, it allocates a new label in turn. This process repeats.
This oscillation can be stopped only if the primary path label
allocated by a PE does not depend on the primary path label
advertised by other PE. A PE needs to be able to advertise multiple
labels, one for use as primary path and another to be used as bacakup
path by the receiver.
MNH attribute allows to advertise a Backup forwarding path label
using Section 5.1.2 in addition to Primary forwarding path label
using Section 5.1.1
A.10. Signaling Intent over PE-CE Attachment Circuit
BGP CT specifies procedures for Intent Driven Service Mapping in a
service provider network, and defines 'Transport Class' construct to
represent an Intent.
It may be desirable to allow a CE device to indicate in the data
packet it sends what treatment it desires (the Intent) when the
packet is forwarded within the provider network.
This section describes the mechanisms that enable such signaling.
These procedures use existing AFIs 1 or 2, and service families (SAFI
1) on the PE-CE attachment circuit, with a new BGP attribute.
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---Gold----->
[CE1]-----[PE1]---[P]----[PE2]-----[CE2]
---Bronze--->
203.0.113.11 203.0.113.22
---- Traffic direction ---->
Figure 24: Example Topology with PE-CE Links
A.10.1. Using DSCP in MultiNexthop Attribute
Such an indication can be in form of DSCP code point ([RFC2474]) in
the IP header.
In RFC2474, a Forwarding Class Selector maps to a PHB (Per-hop
Behavior). The Transport Class construct is a PHB at transport
layer.
Let PE1 be configured to map DSCP1 to Gold Transport class, and DSCP2
to Bronze Transport class. Based on the DSCP code point received on
the IP traffic from CE1, PE1 forwards the IP packet over a Gold or
Bronze tunnel. Thus, the forwarding is not based on just the
destination IP address, but also the DSCP code point. This is known
as Class Based Forwarding (CBF). Today CBF is configured at the PE1
device roles and CE1 doesn't receive any indication in BGP signaling
regarding what DSCP code points are being offered by the provider
network.
With a BGP MultiNexthop Attribute attached to a AFI/SAFI 1/1 service
route, it is possible to extend the PE-CE BGP signaling (if used) to
communicate such information to the CE1. In the preceding example,
the MNH contains two Next hop Legs, described by two Forwarding
Instruction TLVs. Each Next hop Leg contains PE1's peering self
address in Endpoint Identifier TLV ( Section 5.4.1), the color Gold
or Bronze encoded in the Transport class ID TLV (Section 5.4.2.2,
Figure 13), and associated DSCP code point indicating Gold or Bronze
transport class encoded in the Payload Encapsulation Info TLV
(Section 5.4.3.4, Section 5.4.3). This allows the CE to discover
what transport classes exist in the provider network, and which DSCP
codepoint to encode so that traffic is forwarded using the desired
transport class in the provided network.
A.10.2. MPLS-enabled CE
If the PE-CE link is MPLS enabled, a distinct MPLS label can also be
used to express Intent in data packets from CE. Enabling MPLS
forwarding on PE-CE links comes with some security implications.
This section gives details on these aspects.
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Consider the ingress PE1 receiving a VPN prefix RD:Pfx1 received with
VPN label VL1, next hop as PE2 and a mapping community containing TC1
as 'Transport class ID'. PE1 can allocate a MPLS Label PVL1 for the
tuple "VPN Label, PNH Address, Transport class ID" and advertise to
CE1.
Label PVL1 may identifies a service function at any node in the
network, e.g. a Firewall device or egress node PE2. And, for the
same service prefix, a distinct label may be advertised to different
CEs, such that incoming traffic from different CEs to the same
service prefix can be diverted to a distinct devices in the network
for further processing. This provides Ingress Peer Engineering
control to the network.
PE1 installs a MPLS FIB route for PVL1 with next hop as "Swap VL1,
Push TL1 towards PE2". TL1 is the BGP CT label received for the
tuple 'PE2, TC1'. In forwarding, when MPLS packet with label PVL1 is
received from CE1, PVL1 Swaps to label VL1 and pushes the BGP CT
label TL1. PE1 advertises the label "PVL1" in the MNH to CE1. PE1
forwards based on MPLS label without performing any IP lookup. This
allows for PE1 to be a low IP FIB device and still support CBF by
using MPLS Label inferred PHB. The number of MPLS Labels consumed at
PE1 for this approach will be proportional to the number of Service
functions and Intents that are exposed to CE1.
A BGP MultiNexthop Attribute is attached to a AFI/SAFI 1/1 service
route to convey the MPLS Label information to CE1. In the preceding
example, the MNH contains two Next hop Legs, described by two
Forwarding Instruction TLVs. Each Next hop Leg contains PE1's
peering self address in Endpoint Identifier TLV ( Section 5.4.1), the
color Gold or Bronze encoded in the Transport class ID TLV
(Figure 13), and associated MPLS Label "PVL1" or "PVL2" encoded in
the Payload Encapsulation Info TLV (Section 5.4.3.1, Section 5.4.3).
This allows the CE to discover what transport classes exist in the
provider network, and which MPLS Label to encode so that traffic is
forwarded using the desired transport class.
A.10.2.1. Secure MPLS Forwarding on Inter-AS Link
The MPLS enabled PE-CE attachment circuit is considered connecting to
an untrusted domain. Such interfaces can be secured against MPLS
label spoofing by a walled garden approach using "MPLS context
tables".
The PE1-CE1 interface can be confined to a specific MPLS context
table "A" corresponding to the BGP peer. Such that only the routes
for labels advertised to CE1 are installed in MPLS context table "A".
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This ensures that if CE1 sends MPLS packet with a label that was not
advertised to the CE1, the packet will be dropped.
Furthermore, the routes for labels PVL1, PVL2 installed in MPLS
context table "A" can match on 'Bottom of stack' bit being 'one',
ensuring a MPLS packet is accepted from CE1 only if it has no more
than one label in the label stack.
However, the PE itself may not be able to perform any checks based on
inner payload in the MPLS packet since it performs label swap
forwarding. Such inner payload based checks may be offloaded to a
downstream node that forwards and processes inner payload, e.g. a IP
FIB router. These security aspects should be considered when using
MPLS enabled CE devices.
A.11. 4PE - Signal MPLS Label for IPv4 Unicast routes
This section describes how MNH can be used to signal MPLS explicit
null label in AFI/SAFI: 1/1 routes in a pure IPv6 core environment,
to achieve 4PE.
[RR1]
.
|
__
_.( )..
[CE1] - [PE1]-[P1] .. (_ v6 _) .. [PE2] - [CE2]
(._..)
<---- Traffic Direction ----
P1: PHP node.
PE1: Egress PE.
Figure 25: 4PE Network with Pure IPv6 Core
Figure 25 shows a 4PE network with pure IPv6 core, PE1 is the egress
PE connected to penultimate hop node P1. PE1 to PE2 have some IPv6
core tunneling protocol like LDPv6. When PE1 has advertised Implicit
Null label in LDPv6, some implementations of P1 may not be able to
forward the inner IPv4 payload to PE1.
To solve this problem, PE1 needs to signal IPv4 Explicit NULL Label
(Special Label 0) to PE2. PE2 will push this IPv4 Explicit NULL
Label received in the MNH on the AFI/SAFI:1/1 route. Such that P1
does a MPLS Label swap operation and does not need to look into inner
payload.
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MNH can be used by PE1 on a AFI/SAFI: 1/1 route, to advertise the
IPv4 Explicit Null label for the IPv4 Unicast service route. MPLS
Label is encoded in the Payload Encapsulation Info TLV
(Section 5.4.3.1, Section 5.4.3).
This allows the network to provide clear separation of service and
transport routes, and not overloading AFI/SAFI: 1/4 to carry the IPv4
service routes. Not mixing service and transport routes improves
security and manageability aspects of the network.
An egress PE may not need to advertise IPv4 Explicit Null label for
the IPv4 service route, if it does UHP label in LDPv6. This model
using MNH provides a homogenous service layer (AFI/SAFI: 1/1) that
accomodates differences in requirement of different PE and P routers.
Only the PEs which are connected to P nodes that cannot handle the
PHP situation need to advertise Label using MNH. The service layer
is kept consistent in the network, and can seamlessly extend to
multiple domains without needing redistribution between AFI/SAFIs.
Not mixing service and transport routes improves security and
manageability aspects of the network.
Authors' Addresses
Kaliraj Vairavakkalai (editor)
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: kaliraj@juniper.net
Minto Jeyananth
Juniper Networks, Inc.
1133 Innovation Way,
Sunnyvale, CA 94089
United States of America
Email: minto@juniper.net
Mohan Nanduri
Microsoft
1 Microsoft Way,
Redmond, WA 98052
United States of America
Email: mohannanduri@microsoft.com
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Avinash Reddy
AT&T
3400 W Plano Pkwy,
Plano, TX 75075
United States of America
Email: ar977m@att.com
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