SPRING S. Hegde Internet-Draft C. Bowers Intended status: Informational Juniper Networks Inc. Expires: August 26, 2021 X. Xu Alibaba Inc. A. Gulko Refinitiv A. Bogdanov Google Inc. J. Uttaro ATT L. Jalil Verizon M. Khaddam Cox communications A. Alston Liquid Telecom February 22, 2021 Seamless Segment Routing Architecture draft-hegde-spring-seamless-sr-architecture-00 Abstract In order to operate networks with large numbers of devices, network operators organize networks into multiple smaller network domains. Each network domain typically runs an IGP which has complete visibility within its own domain, but limited visibility outside of its domain. Seamless Segment Routing (Seamless SR) provides flexible, scalable and reliable end-to-end connectivity for services across independent network domains. Seamless SR accommodates domains using SR, LDP, and RSVP for MPLS label distribution as well as domains running IP without MPLS (IP-Fabric).It also provides seamless connectivity across domains having different IPv6 technologies such as SRv6 and SRm6. 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. Hegde, et al. Expires August 26, 2021 [Page 1] Internet-Draft Seamless Segment Routing February 2021 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 August 26, 2021. Copyright Notice Copyright (c) 2021 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/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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Seamless Segment Routing architecture . . . . . . . . . . . . 4 2.1. Solution Concepts . . . . . . . . . . . . . . . . . . . . 4 2.2. BGP Classful Transport . . . . . . . . . . . . . . . . . 5 2.3. Solution to usecases . . . . . . . . . . . . . . . . . . 10 2.3.1. Data sovereignty . . . . . . . . . . . . . . . . . . 10 2.3.2. Interconnecting IP Fabric Data Centers . . . . . . . 12 2.4. Inter-domain SLA Requirements . . . . . . . . . . . . . . 13 2.4.1. Inter-domain flex-algo with BGP-CT . . . . . . . . . 13 2.4.2. Applicability to color-only policies . . . . . . . . 13 2.4.3. SLA Guarantee . . . . . . . . . . . . . . . . . . . . 14 2.5. Operation and automation . . . . . . . . . . . . . . . . 15 2.5.1. Automatically Creating Transport Classes . . . . . . 15 2.5.2. Operations . . . . . . . . . . . . . . . . . . . . . 16 2.6. Scalability . . . . . . . . . . . . . . . . . . . . . . . 16 2.6.1. Access node scalability . . . . . . . . . . . . . . . 16 2.6.2. Label stack depth . . . . . . . . . . . . . . . . . . 17 2.6.3. Label Resources . . . . . . . . . . . . . . . . . . . 18 2.7. Availability . . . . . . . . . . . . . . . . . . . . . . 21 Hegde, et al. Expires August 26, 2021 [Page 2] Internet-Draft Seamless Segment Routing February 2021 2.7.1. Intra domain link and node protection . . . . . . . . 21 2.7.2. Egress link and node protection . . . . . . . . . . . 21 2.7.3. Border Node protection . . . . . . . . . . . . . . . 21 2.8. Service Mapping . . . . . . . . . . . . . . . . . . . . . 22 2.9. Mergers and Migrations . . . . . . . . . . . . . . . . . 22 2.9.1. SRv6 interworking with MPLS domains . . . . . . . . . 23 2.9.2. Translating Transport Classes across Domains . . . . 26 2.10. Service Function Chaining . . . . . . . . . . . . . . . . 27 2.11. BGP based Multicast . . . . . . . . . . . . . . . . . . . 29 3. Backward Compatibility . . . . . . . . . . . . . . . . . . . 29 4. Security Considerations . . . . . . . . . . . . . . . . . . . 29 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 8.1. Normative References . . . . . . . . . . . . . . . . . . 30 8.2. Informative References . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 1. Introduction Evolving wireless access technology and cloud applications are expected to place new requirements on the packet transport networks. These services are contributing to significantly higher bandwidth throughput which in turn leads to a growing number of transport network devices. As an example, 5G networks are expected to require up to 250Gbps in the fronthaul and up to 400Gbps in the backhaul. There is a desire to allow many network functions to be virtualized and cloud native. In order to support latency-sensitive cloud-native network functions, packet transport networks should be capable of providing low-latency paths end-to-end. Some services will require low-latency paths while others may require different QoS properties. The network should be able to differentiate between the services and provide corresponding SLA transport paths. In addition, as these applications become more sensitive and less loss tolerant, more and more emphasis is placed on overall service availability and reliability. The Seamless SR architecture builds upon the Seamless MPLS architecture and caters to new requirements imposed by the 5G transport networks and the cloud applications. [I-D.ietf-mpls-seamless-mpls], contains a good description of the Seamless MPLS architecture. Although [I-D.ietf-mpls-seamless-mpls] has not been published as an RFC, it serves as a useful description of the Seamless MPLS architecture. [I-D.ietf-mpls-seamless-mpls] describes the Seamless MPLS architecture, which uses LDP and/or RSVP for intra-domain label distribution, and BGP-LU [RFC3107] for end-to- end label distribution. Seamless SR focuses on using segment routing Hegde, et al. Expires August 26, 2021 [Page 3] Internet-Draft Seamless Segment Routing February 2021 for intra-domain label distribution. The mechansims described in this document are equally applicable to intra-domain tunneling mechanisms deployed using RSVP and/or LDP. By using segment routing for intra-domain label distribution, Seamless SR is able to easily support both SR-MPLS on IPv4 and IPv6 networks. This overcomes a limitation of the classic Seamless MPLS architecture, which was limited to run MPLS on IPv4 networks in practice. Seamless SR (like Seamless MPLS) can use BGP-LU (RFC 3107) to stitch different domains. However, Seamless SR can also take advantage of BGP Prefix-SID [RFC8669] to provide predictable and deterministic labels for the inter-domain connectivity. The basic functionality of the Seamless SR architecture does not require any enhancements to existing protocols. However, in order to support end-to-end service requirements across multiple domains, protocol extensions may be needed. This draft discusses use cases, requirements, and potential protocol enhancements. This document provides solutions to the problem statement and requirements described in draft [I-D.hegde-spring-mpls-seamless-sr]. There may be alternative solutions available to solve the same usecases. This document does not exclude other possible solutions. [I-D.hegde-spring-mpls-seamless-sr] section 4.9 refers to possible alternative solutions and describes how the different archictures can co-exist in the same network or be deployed independently. 2. Seamless Segment Routing architecture 2.1. Solution Concepts Hegde, et al. Expires August 26, 2021 [Page 4] Internet-Draft Seamless Segment Routing February 2021 The solution described below makes use of the following concepts. The definitions from the draft-kaliraj-idr-bgp-classful-transport-planes have been reproduced here for readability. In case of any conflicts, text from draft-kaliraj-idr-bgp-classful-transport-planes should be used. o Transport Class (TC): A Transport Class is defined as a collection of end-to-end MPLS paths that satisfy a set of constraints or Service Level Agreements. o BGP-Classful Transport (BGP-CT): A new BGP family used to establish Transport Class paths across different domains. o Route Distinguisher (RD): The Route Distinguisher is defined in RFC4364. In BGP-CT, the RD is used in BGP advertisements to differentiate multiple paths to the same loopback address. It may be useful to automatically generate RDs in order to simplify configuration. o Route Target (RT): The Route Target extended community is carried in BGP-CT advertisements. The RT represents the Transport Class of an advertised path. Note that the RT is only carried in the BGP-CT advertisements. No BGP-VPN related configuration or VPN family advertisements are needed when BGP-CT transport paths are used to carry non-VPN traffic. o Mapping Community (MC): The Mapping Community is the BGP extended community as defined in RFC4360. In the Seamless SR architecture, an MC is carried by a BGP-CT route and/or a service route. The MC is used to identify the specific local policy used to map traffic for a service route to different Transport Class paths. When a mapping community is advertised in a BGP-CT route it identifies the specific local policy used to map the BGP-CT route to the intra-domain tunnels.The local policy can include additional traffic steering properties for placing traffic on different Transport Class paths. The values of the MCs and the corresponding local policies for service mapping are defined by the network operator. Figure 1: Solution Concepts 2.2. BGP Classful Transport Hegde, et al. Expires August 26, 2021 [Page 5] Internet-Draft Seamless Segment Routing February 2021 ----IBGP------EBGP----IBGP------EBGP-----IBGP--- | | | | | | +-----------+ +-----------+ +-----------+ | | | | | | | ASBR1+--+ASBR2 ASBR3+--+ASBR4 | PE1+ D1 | X | D2 | X | D3 +PE2 | ASBR5+--+ASBR6 ASBR7+--+ASBR8 | | | | | | | +-----+-----+ +-----------+ +-----------+ PE3 |---ISIS1---| |---ISIS2---| |---ISIS3---| Figure 2: WAN Network The above diagram shows a WAN network divided into 3 different domains. Within each domain, BGP sessions are established between the PE nodes and the border nodes as well as between border nodes. BGP sessions are also established between border nodes across domains. The goal is for PE1 to have MPLS connectivity to PE2, satisfying specific characteristics. Multiple MPLS paths from PE1 to PE2 are required in order to satisfy different SLAs. [I-D.kaliraj-idr-bgp-classful-transport-planes] defines a new BGP family called BGP-Classful Transport. The NLRI for this new family consists of a prefix and a Route Distinguisher. The prefix corresponds to the loopback of the destination PE, and RD is used to distinguish different paths to the same PE loopback. The BGP-CT advertisement also carries a Route Target. The RT specifies the Transport Class to which the BGP-CT advertisement belongs. BGP-CT mechanisms are applicable to single ownership networks that are organized into multiple domains. It is also applicable to multiple ASes with different ownership but closely co-operating administration. BGP-CT mechansims are not expected to be applied on the internet peering or between domains that have completely independent administrations. Hegde, et al. Expires August 26, 2021 [Page 6] Internet-Draft Seamless Segment Routing February 2021 BGP-CT advertisements for red Transport Class Prefix:PE2 Prefix:PE2 Prefix:PE2 Prefix:PE2 Prefix:PE2 RD:RD1 RD:RD1 RD:RD1 RD:RD1 RD:RD1 RT:Red RT:Red RT:Red RT:Red RT:Red(100) nh:ASBR1 nh:ASBR2 nh:ASBR3 nh:ASBR4 nh:PE2 Label:L1 Label:L2 Label:L3 Label:L4 Label:L5 PE1-------ASBR1------ASBR2---------ASBR3-------ASBR4--------PE2 VPNa Prefix: 10.1.1.1/32 RD: RD50 RT: RT-VPNa ext-community: Red(100) nh: PE2 Label: S1 +------+ +------+ +------+ | IL71 | | IL72 | | IL73 | +------+ +------+ +------+ +------+ +------+ | L1 | | L2 | | L3 | | L4 | | L5 | +------+ +------+ +------+ +------+ +------+ | S1 | | S1 | | S1 | | S1 | | S1 | +------+ +------+ +------+ +------+ +------+ Label stacks along end-to-end path S1 is the end-to-end service label. IL71, IL72, and IL73 are intra-domain labels corresponding to red intra-domain paths. Figure 3: BGP-CT Advertisements and Label Stacks Hegde, et al. Expires August 26, 2021 [Page 7] Internet-Draft Seamless Segment Routing February 2021 BGP-CT advertisements for blue Transport Class Prefix:PE2 Prefix:PE2 Prefix:PE2 Prefix:PE2 Prefix:PE2 RD:RD2 RD:RD2 RD:RD2 RD:RD2 RD:RD2 RT:Blue RT:Blue RT:Blue RT:Blue RT:Blue(200) nh:ASBR1 nh:ASBR2 nh:ASBR3 nh:ASBR4 nh:PE2 Label:L11 Label:L12 Label:L13 Label:L14 Label:L15 PE1-------ASBR1----ASBR2----------ASBR3-------ASBR4--------PE2 VPNb Prefix: 10.1.1.1/32 RD: RD51 RT: RT-VPNb ext-community: Blue(200) nh: PE2 Label: S2 +------+ +------+ +------+ | IL81 | | IL82 | | IL83 | +------+ +------+ +------+ +------+ +------+ | L11 | | L12 | | L13 | | L14 | | L15 | +------+ +------+ +------+ +------+ +------+ | S2 | | S2 | | S2 | | S2 | | S2 | +------+ +------+ +------+ +------+ +------+ Label stacks along end-to-end path S2 is the end-to-end service label. IL81, IL82, and IL83 are intra-domain labels corresponding to blue intra-domain paths. Figure 4: BGP-CT Advertisements and Label Stacks For example, consider the diagram in Figure 3 and Figure 4 . The diagram shows the BGP-CT advertisements corresponding to two different end-to-end paths between PE1 and PE2. The two different paths belong to two different Transport Classes, red and blue. The inter-domain paths created by BGP-CT Transport Classes can be used by any traffic that can be steered using BGP next-hop resolution, including vanilla IPv4 and IPv6, L2VPN, L3VPN, and eVPN. In the example above, we show how traffic from two different L3VPNs (VPNa and VPNb) is mapped onto two different BGP-CT Transport Classes (Red and Blue). The L3VPN advertisements for VPNa and VPNb are originated by PE2 as usual. PE1 receives these L3VPN advertisements Hegde, et al. Expires August 26, 2021 [Page 8] Internet-Draft Seamless Segment Routing February 2021 and uses the next-hop in the L3VPN advertisements to determine the path to use. In the absence of any BGP-CT Transport Classes in the network, PE1 would likely resolve the L3VPN next-hop over BGP-LU routes corresponding to the BGP best path. However, when BGP-CT Transport Classes are used, PE1 will resolve the L3VPN next-hop over a BGP-CT route. In the example above, PE2 originates BGP-CT advertisements for the Red and Blue Transport Classes. These BGP-CT advertisements propagate across the multiple domains, causing forwarding state for the two Transport Classes to be installed at ABRs along the way. In order to create unique NLRIs for the two advertisements, PE2 uses two different RDs. In the example above, the red BGP-CT advertisement has an RD of RD1 and the blue BGP-CT advertisement has an RD of RD2. Note that the RD values used in the BGP-CT advertisement are completely independent of the RD values used in the L3VPN advertisements. In both cases, the RD values are simply a mechanism to guarantee uniqueness of a prefix/RD pair. The RT values used in the BGP-CT advertisements are unrelated to the RT values used on the L3VPN advertisements. The L3VPN RT values identify VPN membership, as usual. The BGP-CT RT values identify Transport Class membership. In order to be able to easily map VPN traffic into BGP-CT Transport classes, it can be useful however to make an association between BGP-CT RT values and color extended community values in the L3VPN advertisements. In the example above,the RT value carried in the BGP-CT advertisement originated from PE2 for the red Transport Class is configured to correspond to the color extended community advertised in the VPN advertisement for VPNa. Similarly, the RT value for the blue Transport Class corresponds to the color extended community for VPNb. In this way, traffic on PE1 for each VPN can be mapped to a tranport class path by associating the value of the color extended community carried in the VPN advertisement with an RT value carried in a BGP-CT advertisement. The example above also shows the label stacks at different points along the end-to-end paths for the forwarding entries which are established by the two advertisements. Labels L1-L4 are red BGP-CT labels advertised by border nodes ASBR1,2,3,and 4, while label L5 is advertised by PE2 for the red Transport Class. Labels L11-L14 are blue BGP-CT labels advertised by border nodes ASBR1,2,3,and 4, while label L15 is advertised by PE2 for the blue Transport Class. IL71, IL72, and IL73 represent tunnels internal to the domains 1, 2, and 3 which correspond to the red Transport Class. IL81, IL82, and IL83 represent tunnels internal to the domains 1, 2, and 3 which correspond to the blue Transport Class. In this example, we assume that the intra-domain tunnels correspond to SRTE policies having red Hegde, et al. Expires August 26, 2021 [Page 9] Internet-Draft Seamless Segment Routing February 2021 SRTE-policy-color and blue SRTE-policy-color. Service labels are represented by S1 and S2. Note that this example focuses on how signalling originated by PE2 results in forwarding state used by PE1 to reach PE2 on a specific Transport Class path. The solution supports the establishment of forwarding state for an arbitrary number of PEs to reach PE2. For example, PE3 in Figure 3 can reach PE2 on a red Transport Class path established using the same BGP-CT signalling. The signalling and forwarding state from ASBR1 all the way to PE2 is common to the paths used by both PE1 and PE3. This merging of signalling and forwarding state is essentially to the good scaling properties of the Seamless SR architecture. Millions of end-to-end Transport Class paths can be established in a scalable manner. 2.3. Solution to usecases 2.3.1. Data sovereignty +-----------+ +-----------+ +-----------+ | | | +-+ AS2 | | | | A1+--+A2 | | A3+--+A4 | PE1+ AS1 | | |Z| | | AS3 +PE3 | A5+--+A6 | | A7+--+A8 | | | | +-+ | | | +--A13--A15-+ +-A17--A19--+ +-----------+ | | | | | | | | | | | | +--A14--A16-+ +-A18--A20--+ | | | | | A9+--+A10 | PE4+ AS4 | | AS5 | | A11+-+A12 | | | | | +-----------+ +-----------+ Figure 5: Multi domain Network Consider a WAN network with multiple ASes as shown in the diagram Figure 5. The ASes roughly correspond to the geographical location of the nodes. In this example, we assume that each AS corresponds to a continent. The data sovereignty requirement in this example is that certain traffic from PE1(in AS1) to PE3(in AS3) must not cross through country Z in AS2. As indicate by the location of country Z in the diagram, all paths that go directly from AS1 to AS3 through Hegde, et al. Expires August 26, 2021 [Page 10] Internet-Draft Seamless Segment Routing February 2021 AS2 necessarily passes through country Z. Using BGP-LU to provide connectivity from PE1 to PE3 would generally result in a path that goes from AS1 to AS2 to AS3, which does not satisfy the data sovereignty requirement in this example. Instead, the solution using BGP-CT will go from AS1 to AS4 to AS5 to AS2 to AS3. BGP-CT will ensure that when the traffic passes through AS2, only intra-domain paths satisfying the data sovereignty requirement will be used. Within AS2, there are several different intra-domain TE mechanisms that can be used to exclude links that pass through country Z. For example, RSVP-TE or flex-algo can be used to create intra-domain paths that satisfy the data sovereignty requirement. BGP-CT allows the constrained intra-domain paths to satisfy requirements for end- to-end inter-domain paths. LSPs created by RSVP-TE or Flex-algo that satisfy the "exclude country Z" constraint are associated with a color Green. A Green Transport Class is defined on border nodes in all ASes. This Green Transport Class is associated with a mapping community called Not-Z. In AS2, the ASBRs are configured such that the presence of the mapping community Not-Z in BGP-CT routes results in a strict route resolution mechanism for those routes. A BGP-CT route carrying the color extended community Not-Z will only resolve on the Green Tranport Class. So it will only use Green intra-domain tunnels. In AS1, AS3, AS4, and AS5, no links pass through country Z, so all intra-domain paths automatically satisfy the data sovereignty requirement. So there is no need for the creation of Green intra- domain tunnels. In these ASes, the presence of the mapping community Not-Z in BGP-CT routes results in resolution on best-effort paths. Even though the ASBRs in these ASes do not need to create Green intra-domain tunnels, they still need to allocate labels to identify traffic using the Green Transport Class. These labels will be used by the ASBRs in AS2 to put traffic on the Green intra-domain tunnels in AS2. The requirement is that only a subset of traffic honor the data sovereignty requirement. The service prefixes from PE1 to PE2 that need to honor the data sovereignty requirement will be associated with Green extended color community in the service advertisements. This will result in PE1 using the BGP-CT labels corresponding to {PE2, Green} to forward the traffic. BGP-CT labels corresponding to {PE2, Green} will exist at every ASBR along the path. The traffic originating on PE1, will be associated with Green color community. The bottom-most label in the packet consists of a VPN label.Above the VPN label, BGP-CT label is imposed. Above BGP-CT label, the intra- domain transport label is imposed. Let us assume the traffic from Hegde, et al. Expires August 26, 2021 [Page 11] Internet-Draft Seamless Segment Routing February 2021 PE1 needs to go to PE2 through AS1, AS4, AS5, AS2, and AS3.The BGP-CT label for {PE2, Green} will be swapped at the border nodes. Note that end-to-end inter-domain data sovereignty can in principle be accomplished using BGP-LU with multiple loopbacks and associating those loopbacks to appropriate transport tunnels at every border node in every domain. This is very configuration intensive and require multiple loopbacks. BGP-CT builds on the basic mechanisms of BGP-LU while greatly simplifying such use cases. 2.3.2. Interconnecting IP Fabric Data Centers Prefix:TOR2 Prefix:TOR2 Prefix:TOR2 Prefix:TOR2 Prefix:TOR2 RD:RD2 RD:RD2 RD:RD2 RD:RD2 RD:RD2 RT:Blue RT:Blue RT:Blue RT:Blue RT:Blue nh:ASBR1 nh:ASBR2 nh:ASBR3 nh:ASBR4 nh:TOR2 Label:L11 Label:L12 Label:L13 Label:L14 Label:L15 +-----------+ +-----------+ +-----------+ | ASBR1 ASBR2 ASBR3 ASBR4 | | | | | | | TOR1+ DC1 +-------+ CORE +--------+ DC2 +TOR2 | ASBR11 ASBR22 ASBR33 ASBR44 | | | | | | | +-----------+ +-----------+ +-----------+ +------+ +------+ +------+ | UDP | | IL82 | | UDP | +------+ +------+ +------+ +------+ +------+ | L11 | | L12 | | L13 | | L14 | | L15 | +------+ +------+ +------+ +------+ +------+ | S2 | | S2 | | S2 | | S2 | | S2 | +------+ +------+ +------+ +------+ +------+ Label stacks along end-to-end path S2 is the end-to-end service label. IL82, is intra-domain labels corresponding to blue intra-domain paths. Figure 6: Operation in IP fabric Many data center networks consist of IP fabrics which do not have MPLS packet processing capability. A common requirement is that Hegde, et al. Expires August 26, 2021 [Page 12] Internet-Draft Seamless Segment Routing February 2021 traffic originated from an IP Fabric data center needs to satisfy certain constraints in the MPLS-enable core, for example, only using a subset of links (blue links). It is useful for the traffic originating in an IP Fabric DC to carry information that allows the MPLS-enable core to treat it accordingly. MPLSoUDP, as defined in [RFC7510], is a mechanism where a UDP header is imposed on an MPLS packets on the border nodes. In Figure 6 above, the traffic needs to take blue paths in the core. The Blue Transport Class is defined on the ASBRs. In the core, Blue intra-domain tunnels are created. The BGP-CT advertisements for the Blue Transport Class are as shown in the diagram. The BGP-CT advertisements originate at TOR2 and propagate through all the ASBRs, until finally reaching TOR1. Within DC1, traffic is encapsulated with a UDP header. Traffic with the UDP header gets decapsulated at ASBR1. The traffic follows Blue paths in the core. At ASBR4, the MPLS packet gets encapsulated with a UDP header. The UDP header is removed at TOR2, and the lookup will be done for the service label. 2.4. Inter-domain SLA Requirements 2.4.1. Inter-domain flex-algo with BGP-CT Flex-algo (defined in [I-D.ietf-lsr-flex-algo]) provides a mechanism to separate routing planes. Multiple algorithms are defined and prefix-SIDs are advertised for each algorithm. BGP-CT can be used to advertise these flex-algo SIDs in BGP-CT. BGP Prefix-SID (RFC 8669) is an attribute and can be carried in the BGP-CT NLRI. Multiple transport classes that correspond to each of the flex-algo in IGP domain are defined. These Transport Classes advertise the IGP flex- algo SIDs in the prefix-SIDs attribute in the BGP-CT NLRI. 2.4.2. Applicability to color-only policies Color-only policies consist of (nullEndpont, color) as specified in [I-D.ietf-spring-segment-routing-policy]. Special steering mechanisms are defined with "CO" flags defined in the color extended community [I-D.ietf-idr-segment-routing-te-policy]. Color-only policies can be advertised in BGP-CT with the prefix being NULL (0.0.0.0/32 or 0::0/128). Seperate RD will be advertised for each NULL advertisement with different color. The Route target carries the Policy Color contained in SR Policy NLRI. The steering mechanisms defined in [I-D.ietf-spring-segment-routing-policy] MUST be honoured while resolving services prefixes on the BGP-CT advertisements. Hegde, et al. Expires August 26, 2021 [Page 13] Internet-Draft Seamless Segment Routing February 2021 2.4.3. SLA Guarantee 2.4.3.1. Low latency Many network functions are virtualized and distributed. Certain functions are time and latency sensitive. In inter-domain networks, End-to-End latency measurement is required. Inside a domain, latency measurement mechanisms such as TWAMP [RFC5357] are used and link latency is advertised in IGP using extensions described in [RFC8570]and [RFC7471] . [I-D.ietf-idr-performance-routing] extends the BGP AIGP attribute [RFC7311] by adding a sub TLV to carry an accumulated latency metric. The BGP best path selection algorithm used for a Transport Class requiring low latency will consider the accumulated latency metric to choose the lowest latency path. 2.4.3.2. Traffic Engineering (TE) constraints TE constraints generally include the ability to send traffic via certain nodes or links or avoid using certain nodes or links. In the Seamless SR architecture, the intra-domain transport technology is responsible for ensuring the TE constraints inside the domain, BGP-CT ensures that the end-to-end path is constructed from intra-domain paths and inter-AS links that individually satisfy the TE constraints. For example, in order to construct a pair of diverse paths, we can define a red and a blue Transport Class. Within each domain, the red and blue Transport Class path are realized using intra-domain path diversity mechanisms. For example, in a domain using flex-algo, red and blue Transport Classes are realized using red and blue flex-algo definitions (FAD) which don't share any links. To maintain path diversity on inter-AS links, BGP policies are used to associate two inter-AS peers with the red Transport Class and another two inter-AS peers with the blue Transport Class. 2.4.3.3. Bandwidth constraints The Seamless SR architecture does not natively support end-to-end bandwidth reservations. In this architecture, the bandwidth utilization characteristics of each domain are managed independently. The intra-domain bandwidth management can make use of a variety of tools. Link bandwidth extended community as defined in [I-D.ietf-idr-link-bandwidth] allows for efficient weighted load- balancing of traffic on multiple BGP-CT paths that belong to the same Hegde, et al. Expires August 26, 2021 [Page 14] Internet-Draft Seamless Segment Routing February 2021 Transport Class. For optimized path placement, a centralized TE system may be deployed with BGP policies/communities used for path placement. 2.5. Operation and automation 2.5.1. Automatically Creating Transport Classes In order to simplify the creation of inter-domain paths, it may be desirable to automatically advertise a BGP-CT Transport Class based on the existence of an intra-domain tunnel. The RT value used on the BGP-CT advertisement is automatically derived from a property of the intra-domain tunnel that triggered its creation. How the Transpor Class RT value is derived for different types of intra-domain tunnels is discussed below. 2.5.1.1. Automatically Creating Transport Classes for BGP-SR-TE Intra- domain Tunnels When the intra-domain tunnel is a BGP-SR-TE policy [I-D.ietf-idr-segment-routing-te-policy], the value of the Transport Class RT in the corresponding BGP-CT advertisement is derived from the Policy Color contained in SR Policy NLRI. The 32-bit Policy Color is directly converted to a 32-bit Transport Class RT. 2.5.1.2. Automatically Creating Transport Classes for Flex-Algo Tunnels When the intra-domain tunnel is created using Flex-Algo [I-D.ietf-lsr-flex-algo], the value of the Transport Class RT in the corresponding BGP-CT advertisement is derived from the 8-bit Algorithm value carried in SR-Algorithm sub-TLV (RFC8667). The conversion from 8-bit Algorithm value to 32-bit Transport Class RT is done by treating both as unsigned integers. Note that this definition allows for intra-domain tunnels created via standardized algorithm (0-127) as well as flex-algo (128-255). 2.5.1.3. Auto-deriving Transport Classes for PCEP When the intra-domain tunnel is created using PCEP, the value of the Transport Class RT in the corresponding BGP-CT advertisement is derived from the Color of the SR Policy Identifiers TLV defined in [I-D.ietf-pce-segment-routing-policy-cp]. The 32-bit Color is directly converted to a 32-bit Transport Class RT. Hegde, et al. Expires August 26, 2021 [Page 15] Internet-Draft Seamless Segment Routing February 2021 2.5.2. Operations 2.5.2.1. MPLS ping and Traceroute The Seamless SR Architecture consists of 3 layers: the service layer, intra-domain transport, and BGP-CT transport. Within each layer, connectivity can be verified independently. Within the BGP-CT transport layer, end-to-end connectivity can be verified using a new OAM FEC for BGP-CT defined in draft [I-D.kaliraj-idr-bgp-classful-transport-planes]. The draft describes end-to-end connectivity verification as well as fault isolation. BGP-CT verification happens only on the BGP nodes. The intra-domain connectivity verification and fault isolation will be based on the technology deployed in that domain as defined in [RFC8029] and [RFC8287]. 2.5.2.2. Counters and Statistics Traffic accounting and the ability to build demand matrix for PE to PE traffic is very important. With BGP-CT, per-label transit counters should be supported on every transit router. Per-label transit counters provide details of total traffic towards a remote PE measured at every BGP transit router. Per-label egress counters should be supported on ingress PE router. Per-label egress counters provide total traffic from ingress PE to the specific remote PE. 2.6. Scalability 2.6.1. Access node scalability The Seamless SR architecture needs to be able to accommodate very large numbers of access devices. These access devices are expected to be low-end devices with limited FIB capacity. The Seamless MPLS architecture, as described in [I-D.ietf-mpls-seamless-mpls], recommends the use of LDP DOD mode to limit the size of both the RIB and the FIB needed on the access devices. In the Seamless SR architecture, networks use IGP-based label distribution and do not have this selective label request mechanism. However, RIB scalability of access nodes has not been a problem for real seamless MPLS deployments. In cases where access devices are low on CPU and memory and unable to support large a RIB, BGP filtering policies can be applied at the ABR/ASBR routers to restrict the number of BGP-CT advertisements towards the access devices. The access devices will receive only the PE loopbacks that it needs to connect to. Hegde, et al. Expires August 26, 2021 [Page 16] Internet-Draft Seamless Segment Routing February 2021 2.6.1.1. Automating Filtering of BGP-CT Advertisements using Route Target Constraints When access devices have CPU and memory constraints, it is useful to be able to filter BGP-CT advertisements using policies on border nodes so that only a subset of BGP-CT advertisements are sent to a given access device. While this filtering of BGP-CT advertisements could be done via explicit configuration, it is desirable to have an automated filtering mechanism. When a service prefix advertisement is received on an access device, the protocol nexthop of the service prefix indicates the remote loopback address from which the service prefix is originated. An access device only needs to receive the subset of BGP-CT advertisements corresponding to the originators of the service prefixes recieved by that access device. When an access node receives a service prefix with a particular remote loopback address as the protocol nexthop, it can selectively request the BGP-CT advertisement for this particular loopback address from the Route Reflector. This mechanism is similar to how Route Target Constraints are used to selectively filter VPN advertisements. [RFC4684]. The Route Target Constraint defined in [RFC4684] currently allows for filtering based on Route Target information. Applying a similar mechanism to the filtering of BGP-CT advertisements based on individual loopback addresses requires an extension. The minor protocol enhancements required to achieve this are described in section 11 of [I-D.kaliraj-idr-bgp-classful-transport-planes] 2.6.2. Label stack depth The ability for a device to push multiple MPLS labels on a packet depends on hardware capabilities. Access devices are expected to have limited label stack push capabilities. Assuming shortest path SR-MPLS in the access domain, the access domain transport will use a single label. Lightweight traffic-engineering and slicing could also be achieved with a single label as described in [I-D.ietf-lsr-flex-algo]. The Seamless SR architecture can provide cross-domain MPLS connectivity with a single label. Assuming the use of a service label, end-to-end connectivity is provided by pushing one service label, one BGP-CT label, and one intra-domain transport label (which could also be a Binding-SID). Therefore, access nodes will only need to be able to push 3 labels for most applications. Hegde, et al. Expires August 26, 2021 [Page 17] Internet-Draft Seamless Segment Routing February 2021 2.6.3. Label Resources Hegde, et al. Expires August 26, 2021 [Page 18] Internet-Draft Seamless Segment Routing February 2021 -----IBGP----- -----IBGP----- -----IBGP------ | | | | BGP-CT Advt: Prefix: 2.2.2.2 (PE2 loopback) RD:20000 RT: 128 Label:100 Label:100 Label:101 Next hop:ABR3 Next hop:ABR3 Next hop: PE2 ---------------------------------------------------------------- BGP-CT Advt: Prefix: 30.30.30.30 (ABR3 loopback) RD:30000 RT:128 Label:2000 Label:2001 Nexthop:ABR1 Nexthop:ABR3 +-----------+ +------------+ +-----------+ / \ / \/ \ | ABR1 ABR3 | | | | | PE1+ Metro1 + Core + Metro2 +PE2 | | | | | ABR2 ABR4 | \ /\ /\ / +------------+ +-----------+ +------------+ |-ISIS1-| |-ISIS2-| |-ISIS3-| +------+ +------+ +------+ | 11111| | 22222| | 33333| IGP-labels: +------+ +------+ +------+ 11111,22222,33333 | 2000 | | 2001 | | 101 | BGP-CT label: +------+ +------+ + -----+ For ABR3: | 100 | | 100 | | VPN | 2000,2001 +------+ +------+ +------+ For PE2: | VPN | | VPN | 100, 101 +------+ +------+ Figure 7: Recursive Route Resolution Hegde, et al. Expires August 26, 2021 [Page 19] Internet-Draft Seamless Segment Routing February 2021 The label resources are an important consideration in MPLS networks. On access devices, labels are consumed by services as well as for transport loopbacks inside IGP domain where the access device resides. For example, in the above diagram PE1 would have to allocate label resources equal to the number of customers connecting (i.e. the number of L2/L3 VPNs). Based on the size of the IGP domain that PE1 resides in, it will also have to allocate labels for IGP loopbacks. This number is at most a few thousands. So overall a typical access device should have adequate label resources in Seamless SR architecture. The P routers need to allocate labels for IGP loopbacks. This number again is small. At most it will be a few thousand based on number of nodes in the largest IGP domains. The metro networks connect to the core network through ABRs. It is possible that a given ABR may end up having to maintain forwarding entries for a large subset of the transport loopback routes. There may be a large number of metro networks connecting to a given ABR, and in this case, the ABR will need forwarding entries for every access node in the directly connected metros. So, this ABR may have to maintain on the order of 100k routes. With BGP-CT each Transport Class will have to be separately allocated a label. So, in the above example, the ABR1 would have to use 300k labels if there were 3 Transport Classes. This large number of label forwarding entries could be problematic. In highly scaled scenarios, it is therefore desirable to reduce the forwarding state on the ABRs. This reduction can be achieved with label stacking as a result of recursive route resolution. Figure 7 illustrates how the forwarding state on ABRs can be greatly reduced by removing forward state for PEs in remote domains from the ABRs. In this example, we assume that we are setting up end-to-end paths for a single Transport Class, for example red. PE2 advertises a BGP- CT prefix of 2.2.2.2 with nexthop of 2.2.2.2 and label 101. 2.2.2.2 is PE2's loopback. ABR3 advertises label 100 for BGP-CT prefix 2.2.2.2 and changes the nexthop to self. When ABR1 receives the BGP- CT advertisement for 2.2.2.2, it does not change the nexthop and advertises same label advertised by ABR3. When PE1 receives the BGP- CT advertisement for 2.2.2.2 with a nexthop of ABR3, it resolves the route using reachability to ABR3. The reachability of ABR3 has been learned by PE1 as the result of a BGP-CT advertisement originated by ABR3. As shown in Figure 7, ABR3 advertises BGP-CT prefix 30.30.30.30 with label 2001. ABR1 advertises label 2000 for BGP-CT prefix 30.30.30.30 and sets nexthop to self. PE1 constructs the service data packet with a VPN label at the bottom followed by 2 BGP-CT labels 100 and 2000. The top most label 2000 is the transport label for the metro1 domain. Removing the forwarding state for PEs in remote domains on the ABRs comes at the expense of one additional BGP-CT label on the data packet. Hegde, et al. Expires August 26, 2021 [Page 20] Internet-Draft Seamless Segment Routing February 2021 Recursive route resolution provides significant forwarding state reduction on the ABRs. ABRs have to allocate label resources only for the PEs in their local domain. The number of PEs in the same domain as a given ABR is much lower than the total number of PEs in the network. The examples in this draft generally show VPN routes resolving on BGP-CT prefixes. However, the mechanisms are equally applicable to non-VPN routes. 2.7. Availability Transport layer availability is very important in latency and loss sensitive networks. Any link or node failure must be repaired with 50ms convergence time. 50 ms convergence time can be achieved with Fast ReRoute (FRR) mechanisms. The seamless SR architecture provides protection against intra-domain link and node failures, Protection against border node failures and the egress link and node failures are also provided. Details of the FRR techniques are described in the sections below. 2.7.1. Intra domain link and node protection In the seamless SR architecture, protection against node and link failure is achieved with the relevant FRR techniques for the corresponding transport mechanism used inside the domain. In the case of an IP fabric, ECMP FRR or LFA can be used. In SR networks, TI-LFA [I-D.ietf-rtgwg-segment-routing-ti-lfa] provides link and node protection. For SR-TE transport ([I-D.ietf-spring-segment-routing-policy]), link and node protection can be achieved using TI-LFA, combined with mechanisms described in [I-D.hegde-spring-node-protection-for-sr-te-paths]. 2.7.2. Egress link and node protection [RFC8679] describes the mechanisms for providing protection for border nodes and PE devices where services are hosted. The mechanism can be further simplified operationally with anycast SIDs and anycast service labels, as described in [I-D.hegde-rtgwg-egress-protection-sr-networks]. 2.7.3. Border Node protection Border node protection is very important in a network consisting of multiple domains. Seamless SR architecture can achieve 50ms FRR protection in the event of node failure using anycast addresses for the ABR/ASBRs. The requires that a set of ABRs advertise the same Hegde, et al. Expires August 26, 2021 [Page 21] Internet-Draft Seamless Segment Routing February 2021 label for a given BGP-CT Prefix. The detailed mechanism is described in [I-D.hegde-rtgwg-egress-protection-sr-networks]. 2.8. Service Mapping Service mapping is an important aspect of any architecture. It provides means to translate end users SLA requirements into operator's network configurations. Seamless SR architecture supports automatic steering with extended color community. The Transport Class and the route target carried by the BGP-CT advertisement directly map to the extended color community. Services that require specific SLA carry the extended color community which maps to the Transport Class to which the BGP-CT advertisement belongs. Other types of traffic steering such as DSCP based forwarding is expressed with mapping-community. Mapping community is a standard BGP community and is completely generic and user defined. The mapping community will have a specific service mapping feature associated with it along with required fallback behaviour when the primary transport goes down. The below list provides a general guideline into the different service mapping features and fallback options an implementation should provide. DSCP based mapping with each DSCP mapping to a Transport Class. DSCP based mapping with default mapping to a best-effort transport DSCP based mapping with fallback to best-effort when primary transport tunnel goes down. Extended color community based mapping with fallback to best effort Fallback options with specific protocol during migrations Fallback options to a different Transport Class. No Fallback permitted. 2.9. Mergers and Migrations Networks that migrate from Seamless MPLS architecture to Seamless SR architecture, require that all the border nodes and PE devices be upgraded and enabled with new family on the BGP session. In cases where legacy nodes that cannot be upgraded, exporting from BGP-LU into BGP-CT and vice versa SHOULD be supported. Once the entire network is migrated to support BGP-CT, there is no need to run BGP-LU Hegde, et al. Expires August 26, 2021 [Page 22] Internet-Draft Seamless Segment Routing February 2021 family on the BGP sessions. BGP-CT itself can advertise a best effort Transport Class and BGP-LU family can be removed. 2.9.1. SRv6 interworking with MPLS domains SRv6 defines the Segment Routing architecture for IPv6 data plane with a new extension header as described in [RFC8402]. As described in sec 2.3 of [I-D.hegde-spring-mpls-seamless-sr] , data center and access/aggregation networks may deploy SRv6 and connect to the WAN networks. Since current WAN networks predominantly use MPLS, it is important to provide solutions that interconnect SRv6 and MPLS domains. The seamless SR architecture supports interconnecting domains that deploy SRv6 and MPLS. The SRv6 Network Programming draft [I-D.ietf-spring-srv6-network-programming] defines an SRv6 SID as consisting of locator, function, and argument bits. The locator part of the SRv6 SID is routable, and the route leads to the node that instantiates the SID. The seamless SR architecture builds on this concept to enable interworking between SRv6 and other domains. In the Seamless SR architecture, different domains are loosely coupled, and prefixes are not leaked from the IGP in one domain into the IGP of another domain. BGP is used to stitch the different domains together and build an end-to-end path. In SRv6, a seperate locator is allocated for each color. The service SIDs that need to use the particular colored path will be derived based on corresponding locator. Locators are IPv6 prefixes of length less than 128 bits. These locators are advertised in BGP in AFI 2/ SAFI 1 family (IPv6 unicast). BGP will install these locator routes on each border node, so each border node will have reachability for the SRv6 SIDs. In order to transparently traverse an MPLS domain, the SRv6 traffic is encapsulated with MPLS headers at the ingress MPLS border node and decapsulated at the egress MPLS border node. The association of the SRv6 locator with a particular color is also carried in the IPv6 unicast advertisement so that specific transport class paths can be used when desired. This is illustrated in the following example. Hegde, et al. Expires August 26, 2021 [Page 23] Internet-Draft Seamless Segment Routing February 2021 Locator for Red Transport Class : 5:6::/96 Locator for Blue Transport Class: 5:7::/96 BGP AFI2/SAFI 1 advertisements for Red transport class Pfx:5:6::/96 Pfx:5:6::/96 Pfx:5:6::/96 Pfx:5:6::/96 Pfx:5:6::/96 Ext-Com: Red Ext-Com:Red Ext-Com:Red Ext-Com:Red Ext-Com:Red nh:ASBR1 nh:ASBR2 nh:ASBR3 nh:ASBR4 nh:PE2 PE1------------ASBR1-----------ASBR2---------ASBR3-------ASBR4--------PE2 | | | | | | ------SRv6------ -----MPLS----- ----SRv6----- VPNa Prefix: 10.1.1.0/24 RD: RD50 RT: RT-VPNa ext-community: Red(100) nh: PE2 END.DT4 SID: 5:6::16/128 +-----------+ | IL1 | +-----------+ | IL2 | +---------+ +------------+ +-----------+ +-----------+ |src:PE1 | | src:PE1 | |src:PE1 | |src:PE1 | |dst:ASBR1| | dst:5:6::16| |dst:5:6::16| |dst:5:6::16| |SRH: SL=1| |SRH: SL = 0 | |SRH: SL=0 | |SRH: SL=0 | |5:6::16 | |5:6::16 | |5:6::16 | |5:6::16 | +---------+ +------------+ +-----------+ +-----------+ +----------+ | orig | | orig | | Orig | | Orig | | Orig | +---------+ +------------+ +-----------+ +-----------+ +----------+ Packet format along end-to-end path Orig is the original packet destined to 10.1.1.1 IL1, IL2, intra-domain labels corresponding to red intra-domain paths in MPLS domain. Figure 8: SRv6 and MPLS interworking In the diagram above Figure 8 describes an example where the core is MPLS domain and the datacenters deploy SRv6. In the example above, an end-to-end path is built for the Red transport class. The SRv6 domains in this example use best effort paths. On PE2, locator Hegde, et al. Expires August 26, 2021 [Page 24] Internet-Draft Seamless Segment Routing February 2021 5:6::/96 represents the Red transport class. PE2 would like for traffic for service prefix 10.1.1.0/24 to use a Red tranport class path. To accomplish this PE2 creates two BGP advertisements, a VPN advertisement and an IPv6 unicast advertisement. PE2 creates a VPN advertisement using an END.DT4 SID derived from its Red locator 5:6::/96.(END.DT4 SID = 5:6::16/128 in this example.) The VPN advertisement also associates the Red extended color community with the service prefix 10.1.1.0/24. PE2 also creates a IPv6 unicast BGP advertisement that associates the IPv6 prefix of the Red locator (5:6::/96) with the Red extended community. This advertisement allows PE1 as well as the ASBRs to have routes for 5:6::/96, and to associate those routes with the Red transport class where appropriate. The routes that make up the end-to-end path from PE1 to PE2 are described below. On PE1, the VPN prefix 10.1.1.0/24. will resolve on the locator prefix 5:6::/96. The prefix 5:6::/96 will then resolve on an SRv6/IPv6 tunnel to ASBR1. ASBR1 will have a normal IPv6 route for 5:6::/96 installed by BGP to reach ASBR2. On ASBR2, the prefix 5:6::/96 will resolve on an MPLS tunnel belonging to Red transport class terminating on ASBR3. The route for 5:6::/96 from ASBR3 to ASBR4 is again a simple IPv6 route installed by BGP.On ASBR4, both BGP and the IGP will provide a route for 5:6::/96. In general, the active route will be derived from the IGP which will normally be preferred. In cases where a traffic engineered path is needed in the last SRv6 domain, the preference needs to be set appropriately by the administrator. Below is a description of packet forwarding operations along the end- to-end path. On PE1, the original packet destined to 10.1.1.1 will get encapsulated in IPv6 header with one segment END.DT4SID. The destination address is set to ASBR1. On ASBR1, segment left is decremented and the END.DT4 sid 5:6::16 is copied into destination address. On ASBR1, forwarding will be based on the locator route programmed by BGP. Between ASBR1 and ASBR2, it is normal ipv6 forwarding. On ASBR2, an MPLS header corresponding to Red transport Class is pushed on the packet. The MPLS header gets removed when packet reaches ASBR3 and normal ipv6 forwarding based on the locator route is performed. On ASBR4, since best effort path for locator 5:6::/96 is used which is created by IGP, normal IPv6 forwarding is used. The packet reaches PE2 with 5:6::16 as the destination which is present in MyLocalSID table. IPv6 header is decapsulated and lookup for 10.1.1.1 is performed in the VPN table. The example described above has complete domain seperation where SRv6 operations end on one border nodeand MPLS header operations are Hegde, et al. Expires August 26, 2021 [Page 25] Internet-Draft Seamless Segment Routing February 2021 performed on next border node. There may be cases where the a single border node needs to perform both SRv6 and MPLS operations. A goal for the Seamless SR architecture is to avoid service routes on border nodes and provide seamless end-to-end connectivity for the services. In order to satisfy this goal for the single border node use case, a new SID type is defined. The END.DTM SID decapsulates the IPv6 header and pushes an MPLS SID List. It is used to determine the MPLS labels for traffic flowing from a SRv6 domain to an MPLS domain. [draft-bonica-spring-srv6-end-dtm] provides details of this new SID and its operation in detail. 2.9.2. Translating Transport Classes across Domains Prefix:PE2 Prefix:PE2 Prefix:PE2 RD:RD2 RD:RD2 RD:RD2 RT:Red RT:Blue RT:Blue nh:ASBR1 nh:ASBR2 nh:PE2 Label:L11 Label:L12 Label:L13 +-----------+ +-----------+ | ASBR1 ASBR2 | | | | | PE1+ AS1 +----------------+ AS2 +PE2 | ASBR11 ASBR22 | | | | | +-----------+ +-----------+ +------+ +------+ | IL1 | | IL2 | +------+ +------+ +------+ +------+ | L11 | | L12 | | L13 | | L14 | +------+ +------+ +------+ +------+ | S2 | | S2 | | S2 | | S2 | +------+ +------+ +------+ +------+ Label stacks along end-to-end path S2 is the end-to-end service label. IL1 and IL2 are intra-domain labels corresponding to red intra-domain path in AS1 and Blue intra-domain path in AS2. Figure 9: Translating Transport Classes across Domains In certain scenarios, the TE intent represented by Transport Classes may differ from one domain to another. This could be the result of Hegde, et al. Expires August 26, 2021 [Page 26] Internet-Draft Seamless Segment Routing February 2021 two independent organizations merging into one. It could also occur when two ASes are under different administration, but use BGP-CT to provide an end-to-end service. In both scenarios, the same color may represent different intent in each domain. When the traffic needs to satisfy certain TE characteristic, the colors need to be mapped correctly at the border. In the example in Figure 9, there are two ASes. The low latency TE intent is represented with the Red Transport Class in AS1 and with the Blue Transport Class in AS2. PE2 advertises a BGP-CT prefix with RT of Blue. ASBR2 sets the nexthop to self and advertises a new label L12. On ASBR1, the Blue BGP-CT advertisement is imported into the Red Transport RIB and the advertisement from ASBR1 will carry a Red RT. This ensures that the BGP-CT prefix for PE2 resolves on a Red intra-domain path in AS1. The detailed protocol procedures for this usecase is described in section 10 of [I-D.kaliraj-idr-bgp-classful-transport-planes]. 2.10. Service Function Chaining Service Function Chaining involves steering traffic through an ordered set of service functions. Virtualized service functions may be deployed in a single Data Center location or across multiple Data Centers which are geographically separated. There are several different service function chaining solutions available. One set of solutions uses the source routing paradigm as described in [I-D.ietf-spring-sr-service-programming]. The source routing based solution may use SR-MPLS or SRv6 as described in above draft. Another set of solutions uses stitched tunnels to achieve the traffic steering through service functions. The tunneling technology can be MPLS tunneling or IP tunnelling. This set of solutions is described in [draft-hegde-spring-service-chaining-stitched-tunnel]. When a network deploys Seamless SR-based inter-domain solutions, it can deploy either of these solutions for service chaining. This section describes how service chaining is applied in a network that uses Seamless SR for inter-domain connectivity. For simplicity, the example below assumes service functions deployed in a single Data Center. The procedures are equally applicable when the service functions are spread across multiple geographically separated Data Centers. Hegde, et al. Expires August 26, 2021 [Page 27] Internet-Draft Seamless Segment Routing February 2021 ----------------------------- -------------------- | --- | | | | | S1| TOR1 | | | Z | --- SP1 DCGW1 | / | --- | | WAN PE2 | | S2| TOR2 | | | | --- SP2 DCGW2 | | --- | | | | | S3| TOR3 | | | | --- | | | |----------------------------- ------------------- BGP-CT BGP-CT |----------------|---------------------| Figure 10: SFC in a seamless SR based network Figure 10 shows a Data Center (DC) network connected to a WAN network. We assume the traffic is originating at S1 in the DC network and destined for Z in the WAN network. The traffic should go through service functions deployed on S2 and S3. The DCGW1 and DCGW2 are the border nodes between the DC domain and WAN domain. BGP-CT is deployed to provide seamless end-to-end connectivity. We also assume that DC network deploys a pure IP underlay, and that the WAN uses an MPLS underlay. BGP-CT is deployed on the Top-of-Rack switches (TORs), and BGP-CT sessions are running from the TORs to the DCGWs, and from the DCGWs to PE2. All the BGP-CT speakers will have an SLA- specific forwarding entries to reach PE2. When source routed SFC is used [I-D.ietf-spring-sr-service-programming], a packet originating at S1 will use an SR-MPLS or SRv6 SID-list to achieve service function chaining. In this example, the packet will have a SID-list corresponding to the service functions on S2 and S3. The SFC SID- list gets removed by the time the packets leaves S3. The packet arrives at TOR3 with its original IP header exposed. On TOR3 a lookup is done for destination Z. The packet follows SLA-specific BGP-CT paths in both the DC and the WAN. When the stitched tunnel mechanism is used for service chaining [draft-hegde-spring-service-chaining-stitched-tunnel], it is typical for an an overlay orchestrator to build the tunnels in the DC fabric for the S1->S2 and S2->S3. The overlay orchestrator also provisions the appropriate firewall filters to steer the traffic across these stitched tunnels. When the packet arrives at S3, all service functions have been applied and a lookup on the original IP header is done. In the case, the packet also follows SLA-specific BGP-CT paths in both the DC and the WAN. Hegde, et al. Expires August 26, 2021 [Page 28] Internet-Draft Seamless Segment Routing February 2021 2.11. BGP based Multicast BGP based multicast as described in draft [I-D.zzhang-bess-bgp-multicast] serves two main purposes. It can replace PIM/ mLDP inside a domain to natively do a BGP based multicast. It can also serve as an overlay stitching protocol to stitch multiple P2MP LSPs across the domain. This gives the ability to easily transition each domain independently from one technology to the other. BGP based multicast defines a new SAFI for carrying the MULTICAST TREE SAFI. Different route types are defined to support the various usecases. section 1.2.6 of [I-D.zzhang-bess-bgp-multicast] describes the use of new SAFI for stitching the multicast tunnels across different domains. 3. Backward Compatibility 4. Security Considerations TBD 5. IANA Considerations 6. Acknowledgements Many thanks to Kireeti Kompella, Ron Bonica, Krzysztof Szarcowitz, Srihari Sangli,Julian Lucek, Ram Santhanakrishnan for discussions and inputs. Thanks to Joel Halpern for review and comments. 7. Contributors 1.Kaliraj Vairavakkalai Juniper Networks kaliraj@juniper.net 2. Jeffrey Zhang Juniper Networks zzhang@juniper.net 8. References Hegde, et al. Expires August 26, 2021 [Page 29] Internet-Draft Seamless Segment Routing February 2021 8.1. Normative References [I-D.hegde-rtgwg-egress-protection-sr-networks] Hegde, S., Lin, W., and S. Peng, "Egress Protection for Segment Routing (SR) networks", draft-hegde-rtgwg-egress- protection-sr-networks-01 (work in progress), November 2020. [I-D.hegde-spring-mpls-seamless-sr] Hegde, S., Bowers, C., Xu, X., Gulko, A., Bogdanov, A., Uttaro, J., Jalil, L., Khaddam, M., and A. Alston, "Seamless Segment Routing", draft-hegde-spring-mpls- seamless-sr-04 (work in progress), January 2021. [I-D.ietf-idr-performance-routing] Xu, X., Hegde, S., Talaulikar, K., Boucadair, M., and C. Jacquenet, "Performance-based BGP Routing Mechanism", draft-ietf-idr-performance-routing-03 (work in progress), December 2020. [I-D.kaliraj-idr-bgp-classful-transport-planes] Vairavakkalai, K., Venkataraman, N., Rajagopalan, B., Mishra, G., Khaddam, M., and X. Xu, "BGP Classful Transport Planes", draft-kaliraj-idr-bgp-classful- transport-planes-06 (work in progress), January 2021. [I-D.zzhang-bess-bgp-multicast] Zhang, Z., Giuliano, L., Patel, K., Wijnands, I., mishra, m., and A. Gulko, "BGP Based Multicast", draft-zzhang- bess-bgp-multicast-03 (work in progress), October 2019. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001, . [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, . Hegde, et al. Expires August 26, 2021 [Page 30] Internet-Draft Seamless Segment Routing February 2021 8.2. Informative References [I-D.hegde-spring-node-protection-for-sr-te-paths] Hegde, S., Bowers, C., Litkowski, S., Xu, X., and F. Xu, "Node Protection for SR-TE Paths", draft-hegde-spring- node-protection-for-sr-te-paths-07 (work in progress), July 2020. [I-D.ietf-idr-link-bandwidth] Mohapatra, P. and R. Fernando, "BGP Link Bandwidth Extended Community", draft-ietf-idr-link-bandwidth-07 (work in progress), March 2018. [I-D.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., Rosen, E., Jain, D., and S. Lin, "Advertising Segment Routing Policies in BGP", draft-ietf-idr-segment-routing- te-policy-11 (work in progress), November 2020. [I-D.ietf-idr-tunnel-encaps] Patel, K., Velde, G., Sangli, S., and J. Scudder, "The BGP Tunnel Encapsulation Attribute", draft-ietf-idr-tunnel- encaps-21 (work in progress), January 2021. [I-D.ietf-lsr-flex-algo] Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex- algo-13 (work in progress), October 2020. [I-D.ietf-mpls-seamless-mpls] Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz, M., and D. Steinberg, "Seamless MPLS Architecture", draft- ietf-mpls-seamless-mpls-07 (work in progress), June 2014. [I-D.ietf-pce-segment-routing-policy-cp] Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H. Bidgoli, "PCEP extension to support Segment Routing Policy Candidate Paths", draft-ietf-pce-segment-routing-policy- cp-02 (work in progress), January 2021. [I-D.ietf-rtgwg-segment-routing-ti-lfa] Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B., and D. Voyer, "Topology Independent Fast Reroute using Segment Routing", draft-ietf-rtgwg-segment-routing-ti- lfa-05 (work in progress), November 2020. Hegde, et al. Expires August 26, 2021 [Page 31] Internet-Draft Seamless Segment Routing February 2021 [I-D.ietf-spring-segment-routing-policy] Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", draft- ietf-spring-segment-routing-policy-09 (work in progress), November 2020. [I-D.ietf-spring-sr-service-programming] Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca, d., Li, C., Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and S. Salsano, "Service Programming with Segment Routing", draft-ietf-spring-sr-service- programming-03 (work in progress), September 2020. [I-D.ietf-spring-srv6-network-programming] Filsfils, C., Camarillo, P., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "SRv6 Network Programming", draft-ietf-spring-srv6-network-programming-28 (work in progress), December 2020. [I-D.saad-sr-fa-link] Saad, T., Beeram, V., Barth, C., and S. Sivabalan, "Segment-Routing over Forwarding Adjacency Links", draft- saad-sr-fa-link-02 (work in progress), July 2020. [I-D.voyer-pim-sr-p2mp-policy] Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z. Zhang, "Segment Routing Point-to-Multipoint Policy", draft-voyer-pim-sr-p2mp-policy-02 (work in progress), July 2020. [RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996, . [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February 2006, . [RFC4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk, R., Patel, K., and J. Guichard, "Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684, November 2006, . Hegde, et al. Expires August 26, 2021 [Page 32] Internet-Draft Seamless Segment Routing February 2021 [RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J. Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)", RFC 5357, DOI 10.17487/RFC5357, October 2008, . [RFC6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B. Thomas, "Label Distribution Protocol Extensions for Point- to-Multipoint and Multipoint-to-Multipoint Label Switched Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011, . [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, . [RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S. Previdi, "OSPF Traffic Engineering (TE) Metric Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015, . [RFC7510] Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black, "Encapsulating MPLS in UDP", RFC 7510, DOI 10.17487/RFC7510, April 2015, . [RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/RFC7665, October 2015, . [RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., Aldrin, S., and M. Chen, "Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures", RFC 8029, DOI 10.17487/RFC8029, March 2017, . [RFC8287] Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya, N., Kini, S., and M. Chen, "Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR) IGP-Prefix and IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017, . [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, . Hegde, et al. Expires August 26, 2021 [Page 33] Internet-Draft Seamless Segment Routing February 2021 [RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March 2019, . [RFC8604] Filsfils, C., Ed., Previdi, S., Dawra, G., Ed., Henderickx, W., and D. Cooper, "Interconnecting Millions of Endpoints with Segment Routing", RFC 8604, DOI 10.17487/RFC8604, June 2019, . [RFC8679] Shen, Y., Jeganathan, M., Decraene, B., Gredler, H., Michel, C., and H. Chen, "MPLS Egress Protection Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019, . [TS.23.501-3GPP] 3rd Generation Partnership Project (3GPP), "System Architecture for 5G System; Stage 2, 3GPP TS 23.501 v16.4.0", March 2020. Authors' Addresses Shraddha Hegde Juniper Networks Inc. Exora Business Park Bangalore, KA 560103 India Email: shraddha@juniper.net Chris Bowers Juniper Networks Inc. Email: cbowers@juniper.net Xiaohu Xu Alibaba Inc. Beijing China Email: xiaohu.xxh@alibaba-inc.com Hegde, et al. Expires August 26, 2021 [Page 34] Internet-Draft Seamless Segment Routing February 2021 Arkadiy Gulko Refinitiv Email: arkadiy.gulko@refinitiv.com Alex Bogdanov Google Inc. Email: bogdanov@google.com James Uttaro ATT Email: ju1738@att.com Luay Jalil Verizon Email: luay.jalil@verizon.com Mazen Khaddam Cox communications Email: mazen.khaddam@cox.com Andrew Alston Liquid Telecom Email: andrew.alston@liquidtelecom.com Hegde, et al. Expires August 26, 2021 [Page 35]