SPRING S. Agrawal, Ed.
Internet-Draft Z. Ali
Intended status: Standards Track C. Filsfils
Expires: 12 March 2023 Cisco Systems
D. Voyer
Bell Canada
G. Dawra
LinkedIn
Z. Li
Huawei Technologies
8 September 2022
SRv6 and MPLS interworking
draft-agrawal-spring-srv6-mpls-interworking-09
Abstract
This document describes SRv6 and MPLS/SR-MPLS interworking and co-
existence procedures.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 March 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Interworking(IW) scenarios . . . . . . . . . . . . . . . . . 3
2.1. IW scenarios . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Transport IW . . . . . . . . . . . . . . . . . . . . 4
2.1.2. Service IW . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. SRv6 SID behavior . . . . . . . . . . . . . . . . . . . . . . 6
4.1. End.DTM . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. End.DPM . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. SRv6 Policy Headend Behaviors . . . . . . . . . . . . . . . . 8
5.1. H.Encaps.M: H.Encaps applied to MPLS label stack . . . . 8
5.2. H.Encaps.M.Red: H.Encaps.Red applied to MPLS label
stack . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Interconnecting Binding SIDs . . . . . . . . . . . . . . . . 8
7. Interworking Procedures . . . . . . . . . . . . . . . . . . . 8
7.1. Transport IW . . . . . . . . . . . . . . . . . . . . . . 9
7.1.1. SR-PCE multi-domain On Demand Nexthop . . . . . . . . 9
7.1.2. BGP inter domain routing procedures . . . . . . . . . 12
7.2. Service IW . . . . . . . . . . . . . . . . . . . . . . . 17
7.2.1. Gateway Interworking . . . . . . . . . . . . . . . . 18
7.2.2. Translation between Service labels and SRv6 service
SID . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Migration and co-existence . . . . . . . . . . . . . . . . . 19
9. Availability . . . . . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10.1. BGP Prefix-SID TLV Types registry . . . . . . . . . . . 19
10.2. SRv6 Endpoint Behaviors . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The incremental deployment of SRv6 into existing networks require
SRv6 to interwork and co-exist with SR-MPLS/MPLS. This document
introduces interworking scenarios and building blocks for solutions
to inter connect them.
Document assumes SR-MPLS-IPv4 for MPLS domains but equally work for
SR-MPLS-IPv6, LDP-IPv4/IPv6 and RSVP-TE-MPLS label binding protocols.
1.1. 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.
2. Interworking(IW) scenarios
A multi-domain network (Figure 1) can be generalized as a central
domain C with many leaf domains around it. Specifically, document
look at a service flow from an ingress PE in an ingress leaf domain
(LI), through the C domain and up to an egress PE of the egress leaf
domain (LE). Each domain runs its own IGP instance. A domain has a
single data plane type applicable both for its overlay and its
underlay.
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+-----+ +-----+ RD:V/v via 10 +-----+
.......|S-RR1|<...............|S-RR2|<.................|S-RR3| <..
: +-----+ +-----+ +-----+ :
: :
: :
+--:-------------------+----------------------+---------------------:-+
| : | 2 | | | 5 | | | 8 | : |
| : +---+ | +---+ | +---+ : |
| : | | : |
| : | | : |
| : | | : |
|----+ IGP1 +---+ IGP2 +---+ IGP3 +----|
| 1 | | 4 | | 7 | | 10 |
|----+ +---+ +---+ +----|
| | | |
| | | |
| | | |
| +---+ | +---+ | +---+ |
| | 3 | | | 6 | | | 9 | |
+----------------------+----------------------+-----------------------+
iPE iBR eBR ePE
<----------LI---------><----------C----------><-----------LE---------->
Figure 1: Reference multi-domain network topology
There are various SRv6 and SR-MPLS-IPv4 interworking scenarios
possible.
Below scenarios cover various cascading of SRv6/MPLS network, e.g.,
SR-MPLS-IPv4 <-> SRv6 <-> SR-MPLS-IPv4 <-> SRv6 <-> SR-MPLS-IPv4,
etc.
2.1. IW scenarios
2.1.1. Transport IW
Provider edge run MPLS based [RFC4364] or SRv6 Service SID based
[I-D.ietf-bess-srv6-services] BGP L3(e.g.VPN) or L2(e.g.EVPN)
services through service Route Reflectors. Service endpoint
signaling and forwarding state provide interworking over intermediate
transport.
* SRv6 over MPLS (6oM)
- LI and LE domains are SRv6 data plane, C is MPLS data plane
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- L3/L2 BGP SRv6 services [I-D.ietf-bess-srv6-services] between
PEs. The ingress PE encapsulates the payload in an outer IPv6
header where the SRv6 Service SID the is last segment or
destination address(DA).
- Forward SRv6 encapsulated traffic destined to egress PE over
MPLS C domain.
* MPLS over SRv6 (Mo6)
- LI and LE domains are MPLS data plane, C is SRv6 data plane
- L3/L2 BGP MPLS services [RFC4364], [RFC7432]. The ingress PE
encapsulates the payload in an MPLS service label and sends it
MPLS LSP to egress PE.
- Forward encapsulated label stack to egress PE over SRv6 C
domain.
Note: Easiest and most probable deployment is ship in the night i.e.
supporting dual stack and IPv4 MPLS in each domain.
2.1.2. Service IW
L3/L2 service signaling discontinuity i.e. SRv6 service SID based PE
interworks with BGP MPLS based PE for service connectivity. L3/L2
service BGP signaling and forwarding state provide interworking over
intermediate domain.
* SRv6 to MPLS(6toM): The ingress PE encapsulates the payload in an
outer IPv6 header where the destination address is the SRv6
Service SID[I-D.ietf-bess-srv6-services]. Payload is delivered to
egress PE with MPLS service label[RFC4364] that it advertised with
service prefixes.
* MPLS to SRv6 (Mto6): The ingress PE encapsulates the payload in an
MPLS service label. Payload is delivered to egress PE with IPv6
header with destination address as SRv6 service SID that it
advertised with service prefixes.
3. Terminology
The following terms used within this document are defined in
[RFC8402]: Segment Routing, SR-MPLS, SRv6, SR Domain, Segment ID
(SID), SRv6 SID, Prefix-SID.
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Domain: Without loss of the generality, domain is assumed to be
instantiated by a single IGP instance or a network within IGP if
there is clear separation of data plane.
Node k has a classic IPv6 loopback address Ak::1/128.
A SID at node k with locator block B and function F is represented by
B:k:F::
A SID list is represented as <S1, S2, S3> where S1 is the first SID
to visit, S2 is the second SID to visit and S3 is the last SID to
visit along the SR path.
(SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:
IPv6 header with source address SA, destination addresses DA and SRH
as next-header
SRH with SID list <S1, S2, S3> with SegmentsLeft = SL
Note the difference between the <> and () symbols: <S1, S2, S3>
represents a SID list where S1 is the first SID and S3 is the last
SID to traverse. (S3, S2, S1; SL) represents the same SID list but
encoded in the SRH format where the rightmost SID in the SRH is the
first SID and the leftmost SID in the SRH is the last SID. When
referring to an SR policy in a high-level use-case, it is simpler to
use the <S1, S2, S3> notation. When referring to an illustration of
the detailed packet behavior, the (S3, S2, S1; SL) notation is more
convenient.
4. SRv6 SID behavior
This document introduces a new SRv6 SID behavior. This behavior is
executed on border routers between the SRv6 and MPLS domain.
4.1. End.DTM
The "Endpoint with decapsulation and MPLS table lookup" behavior.
The End.DTM SID MUST be the last segment in a SR Policy, and a SID
instance is associated with an MPLS table.
When N receives a packet destined to S and S is a local End.DTM SID,
N does:
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S01. When an SRH is processed {
S02. If (Segments Left != 0) {
S03. Send an ICMP Parameter Problem to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
interrupt packet processing and discard the packet.
S04. }
S05. Proceed to process the next header in the packet
S06. }
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an End.DTM SID, N does:
S01. If (Upper-Layer Header type == 137(MPLS) ) {
S02. Remove the outer IPv6 Header with all its extension headers
S03. Set the packet's associated FIB table to T
S04. Submit the packet to the MPLS FIB lookup for
transmission according to the lookup result.
S05. } Else {
S06. Process as per [ietf-spring-srv6-network-programming] section 4.1.1
S07. }
4.2. End.DPM
The "Endpoint with decapsulation and MPLS label push" behavior.
The End.DPM SID MUST be the last segment and a SID instance is
associated with label stack.
When N receives a packet destined to S and S is a local End.DPM SID,
N does:
S01. When an SRH is processed {
S02. If (Segments Left != 0) {
S03. Send an ICMP Parameter Problem to the Source Address,
Code 0 (Erroneous header field encountered),
Pointer set to the Segments Left field,
interrupt packet processing and discard the packet.
S04. }
S05. Proceed to process the next header in the packet
S06. }
When processing the Upper-layer header of a packet matching a FIB
entry locally instantiated as an End.DPM SID, N does:
S01. Remove the outer IPv6 Header with all its extension headers
S02. Push the MPLS label stack associated with S
S03. Submit the packet to the MPLS engine for transmission
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5. SRv6 Policy Headend Behaviors
5.1. H.Encaps.M: H.Encaps applied to MPLS label stack
The H.Encaps.M behavior encapsulates a received MPLS Label stack
[RFC3032] packet in an IPv6 header with an SRH. Together MPLS label
stack and its payload becomes the payload of the new IPv6 packet.
The Next Header field of the SRH MUST be set to 137 [RFC4023].
5.2. H.Encaps.M.Red: H.Encaps.Red applied to MPLS label stack
The H.Encaps.M.Red behavior is an optimization of the H.Encaps.M
behavior. H.Encaps.M.Red reduces the length of the SRH by excluding
the first SID in the SRH of the pushed IPv6 header. The first SID is
only placed in the Destination Address field of the pushed IPv6
header. The push of the SRH MAY be omitted when the SRv6 Policy only
contains one segment and there is no need to use any flag, tag or
TLV. In such case, the Next Header field of the IPv6 header MUST be
set to 137 [RFC4023].
6. Interconnecting Binding SIDs
Binding Segment (BSID) is bound to SR policy [RFC8402]. Further an
SR-MPLS label can be bound to an SRv6 Policy and an SRv6 SID can be
bound to an SR-MPLS Policy. The IW SR-PCE solution Section 7.1.1
leverage these BSIDs as segments of SR policy on headend domain to
represent intermediate domain of different dataplane type. In
summary, an intermediate domain of different data plane type is
represented by BSID of ingress domain data plane type in SID list.
7. Interworking Procedures
Figure 1 shows reference multi-domain network topology and Section 2
its description. The procedure in this section are illustrated using
the topology.
Following is assumed for data plane support of various nodes:
* Nodes 2,3,5,6,8,9 are provider(P) routers which need to support
single data plane type.
* 1 and 10 are PEs. They support single data plane type in overlay
and underlay.
* Border routers 4 and 7 need to support both the SRv6 and SR-MPLS-
IPv4 data plane.
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A VPN route is advertised via service RRs (S-RR) between an egress
PE(node 10) and an ingress PE (node 1).
For illustrations, the SRGB range starts from 16000 and prefix SID of
a node is 16000 plus node number
7.1. Transport IW
As described in Section 2.1.1, transport IW requires:
* For 6oM, tunnel traffic destined to SRv6 Service SID of egress PE
over MPLS C domain.
* For Mo6, Tunnel MPLS label stack bound to IPv4 loopback address of
egress PE over SRv6 C domain.
This draft enhances two well-known solutions to achieve above:
* An SR-PCE [RFC8664] multi-domain On Demand Next-hop (ODN) SR
policy [I-D.ietf-spring-segment-routing-policy] stitching end to
end across different data plane domains using interconnecting
binding SIDs. These procedures can be used when overlay prefixes
are signaled with a color extended community
[I-D.ietf-idr-tunnel-encaps].
* BGP Inter-Domain routing procedures advertising PE locator or IPv4
Loopback address for best effort end to end connectivity.
7.1.1. SR-PCE multi-domain On Demand Nexthop
This procedure provides a best-effort as well as a path that
satisfies the intent (e.g. low latency), across multiple domains.
Service routes (VPN/EVPN) are received on ingress PE with color
extended community from egress PE. A Color is a 32-bit numerical
value that associates an SR Policy with an intent
[I-D.ietf-spring-segment-routing-policy]. Ingress PE does not know
how to compute the traffic engineered path through the multi-domain
network to egress PE and request SR-PCE for it. The SR-PCE is aware
of interworking requirement at border nodes as its fed with BGP-LS
topological information from each domain. It programs intermediate
domain data plane specific policy on border nodes for the given
intent and represents it in end to end path SID list on ingress PE
leveraging Section 6.
Below sections describe 6oM and Mo6 IW with SR-PCE
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7.1.1.1. 6oM
Service prefix (e.g. VPN or EVPN) is received on head-end (node 1)
with color extended community (C1) from egress PE (node 10) with SRv6
service SID. The PCE computes (C1,10) path via node 2, 5 and 8. It
programs an SR policy at border node 4 with segment list node 5 and 7
bounded to an End.BM BSID [RFC8986]. SR-PCE responds back to node 1
with SRv6 segments along required SLA including End.BM at node 4 to
traverse SR-MPLS-IPv4 C domain.
For example, SR-PCE create SR-MPLS policy (C1,7) at node 4 with
segments <16005,16007>. It is bound to End.BM behavior with SRv6
BSID as B:4:BM-C1-7::
The data plane operations for the above-mentioned interworking
example are described in the following:
* Node 1 performs SRv6 function H.Encaps.Red with VPN service SID
and SRv6 Policy (C1,10):
Packet leaving node 1 IPv6 ((A:1::, B:2:E::) (B:10::DT4, B:8:E::,
B:4:BM-C1-7:: ; SL=3))
* Node 2 performs End function
Packet leaving node 2 IPv6 ((A:1::, B:4:BM-C1-7::) (B:10::DT4,
B:8:E::, B:4:BM-C1-7:: ; SL=2))
* Node 4(border rout4er) performs End.BM function
Packet leaving node 4 MPLS (16005,16007,2)((A:1::, B:8:E::)
(B:10::DT4, B:8:E::, B:4:BM-C1-7-:: ; SL=1)).
* Node 7 performs a native IPv6 lookup on due PHP behavior for 16007
Packet leaving node 7 IPv6 ((A:1::, B:8:E::) (B:10::DT4, B:8:E::,
B:4:BM-C1-7:: ; SL=1))
* Node 8 performs End(PSP) function
Packet leaving node 8 IPv6 ((A:1::, B:10::DT4))
* Node 10 performs End.DT function and lookups IP in VRF and send
traffic to CE.
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7.1.1.2. Mo6
Refer Section 2.1.1 for Mo6 scenario. MPLS Service prefix (e.g. VPN
or EVPN) is received on head-end(node 1) with color extended
community(C1) from egress PE(node 10). The PCE computes color-aware
C1 path via node 2, 5 and 8. It programs a SRv6 policy bound to MPLS
BSID at border node 4 with SRv6 segment list along required color-
aware path with last segment of behavior End.DTM Section 4.1. SR-PCE
responds back to node 1 with MPLS segment list including MPLS BSID of
SRv6 policy at node 4 to traverse SRv6 core domain.
For example, SR-PCE create SRv6 policy (C1,7) at node 4 with segments
<B:5:E::,B:7:DTM::>. It is bound to MPLS BSID 24407.
The data plan operations for the above-mentioned interworking example
are described in the following:
1. Node 1 performs MPLS label stack encapsulation with VPN label and
SR-MPLS Policy (C1,10):
Packet leaving node 1 towards 2 (Note: PHP of node 2 prefix SID):
MPLS packet (16004,24407,16008,16010,vpn_label)
2. Node 2 forwards traffic towards 4 (PHP of 16004)
Packet leaving node 2 MPLS packet (24407,16008,16010,vpn_label)
3. Node 4 steers MPLS traffic into SRv6 policy bound to 24407
Packet leaving node 4 IPv6(A:4::, B:5:E::) (B:7:DTM:: ;
SL=1)NH=137) MPLS((16008,16010,vpn_label)
4. Node 7 receive IPv6 packet with DA=B:7:DTM::. It performs DTM
behavior to remove IPv6 header and perform 16008 lookup in MPLS
table.
Packet leaves node 7 towards node 8(PHP of 16008) MPLS packet
(16010,vpn_label)
5. Node 8 forwards traffic towards 10 (PHP of 16010)
Packet leaving node 8 MPLS packet (vpn_label)
6. Node 10 performs vpn_label lookup and send traffic to CE.
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7.1.2. BGP inter domain routing procedures
Procedures similar to BGP 3107 [I-D.ietf-mpls-seamless-mpls] to
advertise PE locators or IPv4 loopbacks transport reachability in
multi-domain network. Also Next hop self on border routers which
provide independence of intra domain tunnel technology.
Below sections describe 6oM and Mo6 IW with BGP procedures
7.1.2.1. 6oM
Refer Section 2.1.1 for 6oM scenario. SRv6 based L3/L2 BGP services
are signaled with SRv6 Service SID between PEs through Service RRs
with no color extended community. Ingress PEs need reachability to
remote locator to send traffic to SRv6 service SID.
* Egress border router learns local PE locators through IGP. These
should be redistributed in BGP like any IPv6 global prefixes.
Alternatively, locator is advertised by PE in the BGP iPv6 unicast
address family (AFI=2,SAFI=1) to border nodes.
* Egress border router advertise LE domain PE locators in BGP IPv6
LU[AFI=2/SAFI=4] with local label (explicit NULL) to ingress
border router with IPv4 next hops. These next hops have SR-MPLS-
IPv4 LSP paths built in C domain. It may advertise summary prefix
covering all locators in LE domain.
* If ingress border router advertise remote locators in LI domain to
ingress PE in BGP address family (AFI=2,SAFI=1), it attaches local
End behavior as SRv6 SID in Prefix-SID attribute TLV type 5
[I-D.ietf-bess-srv6-services]. Alternatively, it may leak remote
locators in LI IGP domain such that P routers also have
reachability
* Ingress PE learn remote locator over BGP iPv6 address family
AFI=2, SAFI=1 or through LI IGP. When learnt through BGP, SRv6
SID carried in Prefix-SID attribute TLV 5 tunnels traffic to
ingress border node in LI domain as P routers (node 2 and 3) will
not be aware of remote locator.
Control plane example:
1. Routing Protocol(RP) @10:
* In ISIS advertise locator B:10::/48
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* BGP AFI=1,SAFI=128 originates a VPN route RD:V/v via B:10::1
and Prefix-SID attribute B:10:DT4::. This route is advertised
to service RR.
2. RP @ 7:
* ISIS redistribute B:10::/48 into BGP
* BGP Originates B:10::/48 in AFI=2/SAFI=4 with next hop node 7
and label explicit null among border routers.
3. RP @ 4:
* BGP learns B:10::/48 with next hop node 7 and outgoing label.
* BGP advertise B:10::/48 in AFI=2/SAFI=1 with next hop B:4::1
and Prefix-SID attribute tlv type 5 carrying local End
behavior function B:4:END:: to node 1
* Alternatively, BGP redistributes remote locator or summary
route in LI domain IGP.
4. RP @ 1:
* BGP learns B:10::/48 via B:4::1 and Prefix-SID attribute TLV
type 5 with SRv6 SID B:4:END::
* Alternatively, B:10::/48 or summary route reachability is
learned through ISIS
* BGP AFI=1, SAFI=128 learn service prefix RD:V/v, next hop
B:10::1 and PrefixSID attribute TLV type 5 with SRv6 SID
B:10:DT4
FIB state
@1: IPv4 VRF V/v => H.Encaps.red <B:4:END::, B:10:DT4::> with SRH, SRH.NH=IPv4
@4: IPv6 Table: B:4:END:: => Update DA with B:10:DT4::,set IPv6.NH=IPv4, pop the SRH
@4: IPv6 Table: B:10::/48 => push MPLS label 2 (Explicit NULL), push MPLS Label 16007
@7: MPLS label 2 => pop and lookup next IPv6 DA
@7: IPv6 Table B:10::/48 => forward via ISIS path to 10
@10: IPv6 Table B:10:DT4:: => pop the outer header and lookup the inner IPv4 DA in the VRF
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7.1.2.2. Mo6
Refer Section 2.1.1 for Mo6 scenario. MPLS based L3/L2 BGP services
are signaled with IPv4 next-hop of PE through Service RRs with no
color extended community. Ingress PE need labelled reachability to
remote PE IPv4 loopback address advertised as next hop with service
routes.
BGP LU [RFC8277] advertise IPv4 PE loopbacks. Next hop self-
performed on border routers.
Following are options and protocol extensions to tunnel IPv4 PE
loopback LSP through SRv6 C domain
7.1.2.2.1. Tunnel BGP LU LSP across SRv6 C domain
Intuitive solution for an MPLS-minded operator
* Existing BGP-LU label cross-connect on border routers for each PE
IPv4 loopback address.
* The lookups at the ingress border router are based on BGP3107
label as usual
* Just the SR-MPLS IGP label to next hop is replaced by an IPv6
tunnel with DA = SRv6 SID associated with DTM behavior in C
domain.
* Ingress border router forwarding perform 3107 label swap and
H.Encaps.M with DA = SRv6 SID associated with DTM behavior
* Similar to MPLS-over-IP
Following section describes how existing BGP LU updates between
border routers may carry SRv6 SID associated with DTM behavior to
tunnel LSP across SRv6 C domain
7.1.2.2.1.1. SRv6 label route tunnel TLV
This document introduces a new TLV called "SRv6 label route tunnel"
TLV of the BGP Prefix-SID Attribute to achieve signaling of SRv6 SIDs
to tunnel MPLS packet with label in NLRI at the top of its label
stack through SRv6/IPv6 domain. Behavior which may be encoded but
not limited to is End.DTM. SRv6 label route tunnel TLV signals "AND"
semantics i.e. push label signaled in NLRI and perform H.Encaps.M
with DA as SRv6 SID signaled in TLV.
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* Reminder: RFC 8669 introduced Prefix-SID attribute with TLV type 1
for label index and TLV type 3 for Originator SRGB for AFI=1/2 and
SAFI 4 (BGP LU)
* This document extends the BGP Prefix-SID attribute [RFC8669] to
carry new "SRv6 label route tunnel" TLV. This document limits the
usage of this new TLV to AFI=1/2 SAFI 4. The usage of this TLV
for other AFI/SAFI is out of scope of this document.
* "SRv6 label route tunnel" TLV is encoded exactly like SRv6 Service
TLVs in Prefix-SID Attribute [I-D.ietf-bess-srv6-services] with
following modification:
1. TLV Type (1 octet): This field is assigned values from the
IANA registry "BGP Prefix-SID TLV Types". It is set to 7 for
"SRv6 label route tunnel" TLV.
2. No transposition scheme is allowed i.e. transposition length
MUST be 0 in SRv6 SID Structure Sub-Sub-TLV
* Possibility of label encapsulation when dataplane has LSP to next
hop irrespective of SRv6 SID signaled in "SRv6 label route tunnel"
of Prefix-SID attribute. This allows existing MPLS data plane to
keep operating.
Control plane example
1. Routing Protocol(RP) @10:
* ISIS originates its IPv4 PE loopback with Node SID 16010
* BGP AFI=1,SAFI=4 originate IPv4 loopback address with next hop
node 10 and optionally label index=10 in Label-Index TLV of
Prefix-SID attribute.
* BGP AFI=1, SAFI=128 originates a VPN route RD:V/v next hop
node 10. This route is advertised to service RR.
2. RP @ 7:
* ISIS v6, advertise locator B:7::/48 in C domain
* BGP learns node 10 IPv4 loopback address with outgoing label.
It allocates local label (based on label index if present) and
programs label swap to outgoing label and MPLS LSP to next
hop.
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* BGP AFI=1, SAFI=4 advertise IPv4 loopback address of node 10
to node 4. NLRI label is set to local label and SRv6 SID
B:7:DTM:: carried in SRv6 SID Information Sub-TLV of "SRv6
label route tunnel" TLV in Prefix-Sid attribute. If received,
label index=10 in Label-Index TLV of Prefix-SID attribute is
also signaled.
3. RP @ 4:
* ISIS v4 originates its IPv4 loopback with prefix SID 16004 in
LI domain.
* BGP learns node10 IPv4 loopback address from node 7 with
outgoing label. It allocate local label (based on label index
if present) and programs label swap and H.Encaps.M.red with
IPv6 header destination address as SRv6 SID received in "SRv6
label route tunnel" TLV of Prefix-Sid attribute i.e.
B:7:DTM::.
* BGP AFI=1, SAFI=4 advertise IPv4 Loopback address of node 10
to node 1. NLRI label is set to local label and do not signal
"SRv6 label route tunnel" TLV in Prefix-SID attribute.
4. RP @ 1:
* BGP learns IPv4 loopback address of node 10 from node 4 with
outgoing label. It programs route to push outgoing label and
MPLS LSP to next hop i.e. node 4
* BGP AFI=1, SAFI=128 learn service prefix RD:V/v, next hop IPv4
loopback address of node 10 and service label.
Forwarding state at different nodes:
@1: IPv4 VRF: V/v => out label=vpn_label, next hop=IPv4 address of node 10
@1: IPv4 table: IPv4 address of node 10 => out label=16010, next hop=node4
@1: IPv4 table: IPv4 address of node 4 => out label=16004, next hop=interface to reach 2
@4: MPLS Table: 16010 => out label=16010, H.Encaps.M.red with DA=B:7:DTM::
@4: IPv6 table: B:7::/48 => next hop=interface to reach 5
@7: SRv6 My SID table: B:7:DTM:: => decaps IPv6 header and lookup top label.
@7: MPLS table: 16010 => out label=16010, next hop=interface to reach 8
@10: MPLS table: vpn label => pop label and lookup the inner IPv4 DA in the VRF
7.1.2.2.2. Label and SRv6 SID translation per BGP LU route
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* For each PE IPv4 loopback address, existing BGP-LU label cross-
connect on area border routers is replaced by label to SRv6 SID
cross-connect or vice versa on border routers.
* Advertise SRv6 SID associated with End.DPM behavior for each BGP
LU route (IPv4 loopback address of PE) received from LE domain on
egress border router
* Lookup of SRv6 SID result in decaps of IPv6 header and push of BGP
LU outgoing label and MPLS LSP to next hop.
* Advertise BGP LU route with SRv6 SID to ingress border router.
* Ingress border router allocate local label and advertise to LI
domain.
* The lookups at the ingress border router are based on BGP3107
label as usual. Lookup results SRv6 SID of DPM behavior signaled
by egress border node. Decap BGP3107 label and perform H.Encaps.M
with DA = SRv6 SID.
Following section describes how existing BGP LU updates between
border routers may carry SRv6 SID associated with DPM behavior
* Reminder: RFC 8669 introduced Prefix-SID attribute with TLV type 1
for label index and TLV type 3 for Originator SRGB for AFI=1/2 and
SAFI 4 (BGP LU)
* This document extends the BGP Prefix-SID attribute [RFC8669] to
carry "SRv6 L3 Service TLV" defined in
[I-D.ietf-bess-srv6-services] with AFI=1/2 SAFI 4 route.
* TLV is encoded exactly like SRv6 Service TLVs in Prefix-SID
Attribute [I-D.ietf-bess-srv6-services]
7.2. Service IW
As described in Section 2.1.2 Service IW need BGP SRv6 based L2/L3 PE
interworking with BGP MPLS based L2/L3 PE.
There are a number of different ways of handling this scenario as
detailed below.
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7.2.1. Gateway Interworking
Gateway is router which supports both BGP SRv6 based L2/L3 services
and BGP MPLS based L2/L3 services for a service instance (e.g. L3
VRF, EVPN EVI). It terminates service encapsulation and perform L2/
L3 destination lookup in service instance.
* A border router between SRv6 domain and SR-MPLS-IPv4 domain is
suitable for Gateway role.
* Transport reachability to SRv6 PE and gateway locators in SRv6
domain or MPLS LSP to PE/gateway IPv4 Loopbacks can be exchanged
in IGP or through mechanism detailed in Section 2.1.1.
* Gateway exchange BGP L2/L3 service prefix with SRv6 based Service
PEs via set of service RRs. This session will learn/advertise L3/
L2 service prefixes with SRv6 service SID in prefix SID attribute
[I-D.ietf-bess-srv6-services].
* Gateway exchange BGP L2/L3 service prefix with MPLS based Service
PEs via set of distinct service RRs. This session will learn/
advertise L3/L2 service prefixes with service labels [RFC4364]
[RFC7432].
* L2/L3 prefix received from a domain is locally installed in
service instance and re advertised to other domain with modified
service encapsulation information.
* Prefix learned with SRv6 service SID from SRv6 PE is installed in
service instance with instruction to perform H.Encaps. It is
advertised to MPLS service PE with service label. When gateway
receives traffic with service label from MPLS service PE, it
perform destination lookup in service instance. Lookup result in
instruction to perform H.Encaps with DA being SRv6 Service SID
learnt with prefix from SRv6 PE.
* Prefix learned with MPLS service label from MPLS service PE is
installed in service instance with instruction to perform service
label encapsulation and send to MPLS LSP to nexthop. It is
advertised to SRv6 service PE with SRv6 service SID of behavior
(e.g. DT4/DT6/DT2U) [RFC8986]. When gateway receives traffic
with SRv6 Service SID as DA of IPv6 header from SRv6 service PE,
it perform destination lookup in service instance after decaps of
IPv6 header. Lookup result in instruction to push service label
and send it to nexthop.
Couple of border routers can act as gateway for redundancy. It can
scale horizontally by distributing service instance among them.
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7.2.2. Translation between Service labels and SRv6 service SID
This is similar to inter-as option B control plane procedures
described in [RFC4364].
This would be described in future version of draft.
8. Migration and co-existence
In addition, the draft also addresses migration and coexistence of
the SRv6 and SR-MPLS-IPv4. Co-existence means a network that
supports both SRv6 and MPLS in a given domain. This may be a
transient state when brownfield SR-MPLS-IPv4 network upgrades to SRv6
(migration) or permanent state when some devices are not capable of
SRv6 but supports native IPv6 and SR-MPLS-IPv4.
These procedures would be detailed in a future revision
9. Availability
* Failure within domain are taken care by existing FRR mechanisms
[I-D.ietf-rtgwg-segment-routing-ti-lfa].
* Procedures listed in [I-D.ietf-spring-segment-routing-policy]
provides protection in SR-PCE multi-domain On Demand Nexthop (ODN)
SR policy based approach.
* Convergence on failure of border routers can be achieved by well
known methods for BGP inter domain routing approach:
- BGP Add Path provide diverse path visibility
- BGP backup path pre-programming
- Sub-second convergence on border router failure notified by
local IGP.
10. IANA Considerations
10.1. BGP Prefix-SID TLV Types registry
This document introduce a new TLV Type of the BGP Prefix-SID
attribute. IANA is requested to assign Type value in the registry
"BGP Prefix-SID TLV Types" as follows
Value Type Reference
----------------------------------------------------------
TBD SRv6 label route tunnel TLV <this document>
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10.2. SRv6 Endpoint Behaviors
This document introduces a new SRv6 Endpoint behavior "End.DTM".
IANA is requested to assign identifier value in the "SRv6 Endpoint
Behaviors" sub-registry under "Segment Routing Parameters" registry.
+-------------+--------+-------------------------+------------------+
| Value | Hex | Endpoint behavior | Reference |
+-------------+--------+-------------------------+------------------+
| TBD | TBD | End.DTM | <this document> |
+-------------+--------+-------------------------+------------------+
11. Security Considerations
12. Acknowledgements
The authors would like to acknowledge Kamran Raza, Dhananjaya Rao,
Stephane Litkowski, Pablo Camarillo, Ketan Talaulikar
13. References
13.1. Normative References
[I-D.ietf-bess-srv6-services]
Dawra, G., Talaulikar, K., Raszuk, R., Decraene, B.,
Zhuang, S., and J. Rabadan, "SRv6 BGP based Overlay
Services", Work in Progress, Internet-Draft, draft-ietf-
bess-srv6-services-15, 22 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-bess-srv6-
services-15.txt>.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-22, 22 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-spring-
segment-routing-policy-22.txt>.
[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>.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
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[RFC4023] Worster, T., Rekhter, Y., and E. Rosen, Ed.,
"Encapsulating MPLS in IP or Generic Routing Encapsulation
(GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
<https://www.rfc-editor.org/info/rfc4023>.
[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>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
[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>.
13.2. Informative References
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[I-D.ietf-idr-tunnel-encaps]
Patel, K., Velde, G. V. D., Sangli, S. R., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", Work in
Progress, Internet-Draft, draft-ietf-idr-tunnel-encaps-22,
7 January 2021, <https://www.ietf.org/archive/id/draft-
ietf-idr-tunnel-encaps-22.txt>.
[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://www.ietf.org/archive/id/
draft-ietf-mpls-seamless-mpls-07.txt>.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
08, 21 January 2022, <https://www.ietf.org/archive/id/
draft-ietf-rtgwg-segment-routing-ti-lfa-08.txt>.
Authors' Addresses
Swadesh Agrawal (editor)
Cisco Systems
Email: swaagraw@cisco.com
Zafar ALI
Cisco Systems
Email: zali@cisco.com
Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
Gaurav dawra
LinkedIn
United States of America
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Email: gdawra.ietf@gmail.com
Zhenbin Li
Huawei Technologies
China
Email: lizhenbin@huawei.com
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