LSR Working Group U. Chunduri Internet-Draft Intel Corporation Intended status: Standards Track R. Li Expires: May 16, 2022 Futurewei R. White Juniper Networks L. Contreras Telefonica J. Tantsura Microsoft Y. Qu Futurewei November 12, 2021 Preferred Path Routing (PPR) in IS-IS draft-chunduri-lsr-isis-preferred-path-routing-07 Abstract This document specifies a Preferred Path Routing (PPR), a routing protocol mechanism to simplify the path description using IS-IS protocol. PPR builds on existing encapsulation to add the path identity to the packet and supports further extensions along the preferred paths. PPR aims to provide path steering, services and support further extensions along the paths. Preferred path routing is achieved through the addition of path descriptions to the IS-IS advertised prefixes, and mapping those to a PPR data-plane identifier. 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 RFC2119 [RFC2119], RFC8174 [RFC8174] when, and only when they appear in all capitals, as shown here. 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/. Chunduri, et al. Expires May 16, 2022 [Page 1] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 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 May 16, 2022. 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 1.1. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3 2. PPR Details . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. PPR-ID and Data Plane Extensibility . . . . . . . . . . . 4 2.2. PPR Path Description . . . . . . . . . . . . . . . . . . 4 2.3. ECMP Considerations . . . . . . . . . . . . . . . . . . . 5 3. PPR Related TLVs . . . . . . . . . . . . . . . . . . . . . . 5 3.1. PPR-Prefix Sub-TLV . . . . . . . . . . . . . . . . . . . 7 3.2. PPR-ID Sub-TLV . . . . . . . . . . . . . . . . . . . . . 8 3.3. PPR-PDE Sub-TLV . . . . . . . . . . . . . . . . . . . . . 9 3.4. PPR-Attributes Sub-TLV . . . . . . . . . . . . . . . . . 12 4. PPR Processing Procedure Example . . . . . . . . . . . . . . 13 4.1. PPR TLV Processing . . . . . . . . . . . . . . . . . . . 14 4.2. Path Fragments . . . . . . . . . . . . . . . . . . . . . 15 5. PPR Data Plane aspects . . . . . . . . . . . . . . . . . . . 15 5.1. SR-MPLS with PPR . . . . . . . . . . . . . . . . . . . . 15 5.2. PPR Native IP Data Planes . . . . . . . . . . . . . . . . 16 5.3. SRv6 with PPR . . . . . . . . . . . . . . . . . . . . . . 16 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 7.1. PPR Sub-TLVs . . . . . . . . . . . . . . . . . . . . . . 17 7.2. IGP Parameters . . . . . . . . . . . . . . . . . . . . . 18 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 9. Contributing Authors . . . . . . . . . . . . . . . . . . . . 18 Chunduri, et al. Expires May 16, 2022 [Page 2] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . 19 10.2. Informative References . . . . . . . . . . . . . . . . . 19 Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 22 A.1. Challenges with Increased SID Depth . . . . . . . . . . . 22 A.2. Mitigation with MSD . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 1. Introduction PPR involves associating path descriptions to IS-IS advertised prefixes, mapping those to a data-plane identifier and specifying a mechanism to route packets with the abstracted identifier (PPR-ID), as opposed to individual segments on the packet. This is specified in detail in [I-D.chunduri-rtgwg-preferred-path-routing], along with key use cases and deployment scenarios. PPR allows the traffic along an engineered path through the network by replacing the label stack with a path identifier, PPR-ID, in the packet. The PPR-ID can either be a single label or a native destination address. To facilitate the use of a single label to describe an entire path, a new TLV is added to IS-IS, as described below in Section 3. A PPR could be an SR path, a traffic engineered path computed based on some constraints, an explicitly provisioned Fast Re-Route (FRR) path or a service chained path. A PPR can be signaled by any node, computed by a central controller, or manually configured by an operator. PPR extends the source routing and path steering capabilities to native IP (IPv4 and IPv6) data planes without hardware upgrades; see Section 5. 1.1. Acronyms EL - Entropy Label ELI - Entropy Label Indicator LSP - IS-IS Link State PDU MPLS - Multi Protocol Label Switching MSD - Maximum SID Depth MTU - Maximum Transferrable Unit NH - Next-Hop PPR - Preferred Path Routing/Route Chunduri, et al. Expires May 16, 2022 [Page 3] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 PPR-ID - Preferred Path Route Identifier, a data plane identifier SID - Segment Identifier SPF - Shortest Path First SR-MPLS - Segment Routing with MPLS data plane SRH - Segment Routing Header - IPv6 routing Extension header SRv6 - Segment Routing with IPv6 data plane with SRH TE - Traffic Engineering 2. PPR Details 2.1. PPR-ID and Data Plane Extensibility The PPR-ID describes a path through the network. A data plane type and corresponding data plane identifier as specified in Section 3.2 is mapped to PPR-ID to allow data plane extensibility. For SR-MPLS, PPR-ID is mapped to an MPLS Label/SID and for SRv6, this is mapped to an IPv6-SID. For native IP data planes, this is mapped to either IPv4 or IPv6 address/prefix. 2.2. PPR Path Description The path identified by the PPR-ID is described as a set of Path Description Elements (PDEs), each of which represents a segment of the path. Each node determines its location in the path as described, and forwards to the next segment/hop or label of the path description (see the Forwarding Procedure Example later in this document). These PPR-PDEs as defined in Section 3.3, like SR SIDs, can represent topological elements like links/nodes, backup nodes, as well as non- topological elements such as a service, function, or context on a particular node. A PPR path can be described as a Strict-PPR or a Loose-PPR. In a Strict-PPR all nodes/links on the path are described with SR SIDs for SR data planes or IPv4/IPv6 addresses for native IP data planes. In a Loose-PPR only some of the nodes/links from source to destination are described. More specifics and restrictions around Strict/Loose PPRs are described in respective data planes in Section 5. Each PDE is described as either an MPLS label towards the Next-Hop (NH) in MPLS enabled networks, or as an IP NH, in the case of either Chunduri, et al. Expires May 16, 2022 [Page 4] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 "plain"/"native" IP or SRv6 enabled networks. A PPR path is related to a set of PDEs using the TLVs as specified in Section 3. 2.3. ECMP Considerations PPR inherently supports Equal Cost Multi Path (ECMP) for both strict and loose paths. If a path is described using nodes, it would have ECMP NHs established for PPR-ID along the path. However, one can avoid ECMP on any segment of the path by pinning the path using a link identifier to the next segment. 3. PPR Related TLVs This section describes the encoding of PPR TLV. This TLV can be seen as having 4 logical sections viz, encoding of the PPR-Prefix (IS-IS Prefix), encoding of PPR-ID, encoding of path description with an ordered PDE Sub-TLVs and a set of optional PPR attribute Sub-TLVs, which can be used to describe one or more parameters of the path. Multiple instances of this TLV MAY be advertised in IS-IS LSPs with different PPR-ID Type (data plane) and with corresponding PDE Sub- TLVS. The PPR TLV has Type TBD (suggested value xxx), and has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | PPR-Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Fragment-ID | MT-ID | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPR-Prefix Sub-TLV (variable size) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPR-ID Sub-TLV (variable size) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPR-PDE Sub-TLVs (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPR-Attribute Sub-TLVs (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: PPR TLV Format o Type: 155 (Suggested Value, TBD IANA) from IS-IS top level TLV registry. o Length: Total length of the value field in bytes. o PPR-Flags: 2 Octet bit-field of flags for this TLV; described below. Chunduri, et al. Expires May 16, 2022 [Page 5] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 o Fragment-ID: This is an 8-bit Identifier value (0-255) of the TLV fragment. If fragments are not needed to represent the complete path, 'U' bit MUST be set and this value MUST be set to 0. o MT-ID: The multi-topology identifier defined in [RFC5120]; the 4 most significant bits MUST be set to 0 on transmit and ignored on receive. The remaining 12-bit field contains the MT-ID. o Algorithm: 1 octet value represents the route computation algorithm. Algorithm registry is as defined in [RFC8667]. Computation towards PPR-ID (Section 3.2) happens per MT-ID/ Algorithm pair. o PPR-Prefix: A variable size Sub-TLV representing the destination of the path being described. This is defined in Section 3.1. o PPR-ID: A variable size Sub-TLV representing the data plane or forwarding identifier of the PPR. Defined in Section 3.2. o PPR-PDEs: Variable number of ordered PDE Sub-TLVs which represents the path. This is defined in Section 3.3. o PPR-Attributes: Variable number of PPR-Attribute Sub-TLVs which represent the path attributes. These are defined in Section 3.4. The Flags field has the following flag bits defined: PPR TLV Flags Format 0 1 2 3 4 5 6 7 15 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F|D|A|U|Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1. F: Flood bit. If set, the PPR TLV MUST be flooded across the entire routing domain. If the F bit is not set, the PPR TLV MUST NOT be leaked between IS-IS levels. This bit MUST NOT be altered during the TLV leaking 2. D: Down Bit. When the PPR TLV is leaked from IS-IS level-2 to level-1, the D bit MUST be set. Otherwise, this bit MUST be clear. PPR TLVs with the D bit set MUST NOT be leaked from level-1 to level-2. This is to prevent TLV looping across levels. 3. A: Attach bit. The originator of the PPR TLV MUST set the A bit in order to signal that the prefix and PPR-ID advertised in the Chunduri, et al. Expires May 16, 2022 [Page 6] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 PPR TLV are directly connected to the originators. If this bit is not set, this allows any other node in the network to advertise this TLV on behalf of the originating node of the PPR- Prefix. If PPR TLV is leaked to other areas/levels the A-flag MUST be cleared. In case if the originating node of the prefix must be disambiguated for any reason including, if it is a Multi Homed Prefix (MHP) or leaked to a different IS-IS level or because [RFC7794] X-Flag is set, then PPR-Attribute Sub-TLV Source Router ID SHOULD be included. 4. U: Ultimate fragment bit. bit MUST be set if a path has only one fragment or if it is the last Fragment of the path. PPR-ID value for all fragments of the same path MUST be the same. 5. Reserved: For future use; MUST be set to 0 on transmit and ignored on receive. PPR path description for each IS-IS level is computed and given to one of the nodes for L1 and L2 respectively. Similarly path information when crossing the level boundaries MUST be relevant to the destination level. If there is no path information available for the destination level PPR TLV MUST NOT be leaked regardless of F and D bits as defined above. The following Sub-TLVs draw from a new registry for Sub-TLV numbers as specified in Section 7.1 and Section 7.2. 3.1. PPR-Prefix Sub-TLV The structure of PPR-Prefix is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Prefix Length | Mask Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // IS-IS Prefix (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: PPR-Prefix Sub-TLV Format o Type: 1 (IANA to assign from Sub-TLV registry described above). o Length: Total length of the value field in bytes. o Prefix Length: The length of the IS-IS Prefix being encoded in bytes. For IPv4 it MUST be 4 and IPv6 it MUST be 16 bytes. Chunduri, et al. Expires May 16, 2022 [Page 7] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 o Mask Length: The length of the prefix in bits. Only the most significant octets of the Prefix are encoded. o IS-IS Prefix: The IS-IS prefix at the tail-end of the advertised PPR. This corresponds to a routable prefix of the originating node and it MAY have one of the [RFC7794] flags set (X-Flag/R- Flag/N-Flag) in the IS-IS reachability TLVs. Length of this field MUST be as per "Prefix Length". Encoding is same as TLV 135 [RFC5305] and TLV 236 [RFC5308] or MT-Capable [RFC5120] IPv4 (TLV 235) and IPv6 Prefixes (TLV 237) respectively. 3.2. PPR-ID Sub-TLV This is the actual data plane identifier in the packet header and could be of any data plane as defined in the PPR-ID Type field. Both PPR-Prefix and PPR-ID belongs to a same node in the network. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length |PPR-ID Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPR-ID Type | PPR-ID Length |PPR-ID Mask Len| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // PPR-ID (variable size) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: PPR-ID Sub-TLV Format o Type: 2 (IANA to assign from Sub-TLV registry described above). o Length: Total length of the value field in bytes. o PPR-ID Flags: 2 Octet field for PPR-ID flags: PPR-ID Flags Format 0 1 2 3 4 5 6 7.. 15 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Reserved: For future use; MUST be set to 0 on transmit and ignored on receive. Chunduri, et al. Expires May 16, 2022 [Page 8] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 o PPR-ID Type: Data plane type of PPR-ID. This is a new registry (TBD IANA - Suggested values as below) for this Sub-TLV and the defined types are as follows: Type: 1 SR-MPLS SID/Label Type: 2 Native IPv4 Address/Prefix Type: 3 Native IPv6 Address/Prefix Type: 4 IPv6 SID in SRv6 with SRH o PPR-ID Length: Length of the PPR-ID field in octets and this depends on the PPR-ID type. o PPR-ID Mask Len: It is applicable only for PPR-ID Type 2, 3 and 4. For Type 1 this value MUST be set to zero. It contains the length of the PPR-ID Prefix in bits. Only the most significant octets of the Prefix are encoded. This is needed, if PPR-ID followed by an IPv4/IPv6 Prefix instead of 4/16 octet Address respectively. o PPR-ID: This is the Preferred Path forwarding identifier that would be on the data packet. The value of this field is variable and it depends on the PPR-ID Type - for Type 1, this is encoded as SR-MPLS SID/Label. For Type 2 this is a 4 byte IPv4 address. For Type 3 this is a 16 byte IPv6 address. For Type 2 and Type 3 encoding is similar to "IS-IS Prefix" as specified in Section 3.1. For Type 4, this is encoded as 16 byte SRv6 SID. For PPR-ID Type 2, 3 or 4, PPR-ID MUST NOT be advertised as a routable prefix in TLV 135, TLV 235, TLV 236 and TLV 237. PPR-ID MUST belong to the node, from where the PPR-Prefix (Section 3.1) is advertised. 3.3. PPR-PDE Sub-TLV This Sub-TLV represents the PPR Path Description Element (PDE). PPR- PDEs are used to describe the path in the form of a set of contiguous and ordered Sub-TLVs, where first Sub-TLV represents (the top of the stack in MPLS data plane or) first node/segment of the path. These sets of ordered Sub-TLVs can have both topological elements and non- topological elements (e.g., service segments). Chunduri, et al. Expires May 16, 2022 [Page 9] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | PPR-PDE Type | PDE-ID Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PDE-ID Length | PPR-PDE Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // PDE-ID Value (variable size) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Sub-TLV Length | PPR-PDE Sub-TLVs (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: PPR-PDE Sub-TLV Format o Type: 3 (See IANA for suggested value) from IS-IS PPR TLV Section 3 Sub-TLV registry. o Length: Total length of the value field in bytes. o PPR-PDE Type: A new registry (TBD IANA) for this Sub-TLV and the defined types are as follows: Type: 1 Topological Type: 2 Non-Topological o PDE-ID Type: 1 Octet PDE-forwarding IDentifier Type. A new registry (Suggested Values as listed, IANA TBD) for this Sub-TLV and the defined types and corresponding PDE-ID Length, PDE-ID Value are as follows: Type 0: This value MUST be set only when PPR-PDE Type is Non- Topological. PDE-ID Length indicates the length of the PDE-ID Value field in bytes. For this type, PDE-ID value represents a service/function. This information is provisioned on the immediate topological PDE preceding this PDE based on the 'E' bit. Type 1: SID/Label type as defined in [RFC8667]. PDE-ID Length and PDE-ID Value fields are per Section 2.3 of the referenced document. Type 2: SR-MPLS Prefix SID. PDE-ID Length and PDE-ID Value are same as Type 1. Type 3: SR-MPLS Adjacency SID. PDE-ID Length and PDE-ID Value are same as Type 1. Chunduri, et al. Expires May 16, 2022 [Page 10] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 Type 4: IPv4 Node Loopback Address. PDE-ID Length 4 bytes and PDE-ID Value is full 4 bytes IPv4 address encoded as specified in "4-octet IPv4 address" of Sub-TLV 6/TLV 22 in [RFC5305]. Type 5: IPv4 Interface Address. PDE-ID Length is 4 bytes and PDE-ID Value is full 4 bytes IPv4 address encoded as specified in "4-octet IPv4 address" of Sub-TLV 6/TLV 22 in [RFC5305]. This PDE-ID in the path description represents the egress interface of the path segment and corresponding adjacency is set as nexthop for the PPR-ID. Type 6: IPv6 Node Loopback Address. PDE-ID Length and PDE-ID Value are encoded as specified in "Prefix Len" and "prefix" portion of TLV 236 in [RFC5308] respectively. Type 7: IPv6 Interface Address. PDE-ID Length is 16 bytes and PDE-ID Value is full 16 bytes IPv6 address encoded as specified in "Interface Address 1" portion of TLV 232 in [RFC5308]. This PDE-ID in the path description represents the egress interface of the path segment and corresponding adjacency is set as nexthop for the PPR-ID. Type 8: SRv6 Node SID as defined in [I-D.ietf-lsr-isis-srv6-extensions]. PDE-ID Length and PDE-ID Value are as defined in SRv6 SID from the referenced draft. Type 9: SRv6 Adjacency-SID. PDE-ID Length and PDE-ID Values are similar to SRv6 Node SID above. o PPR-PDE Flags: 2 Octet bit-field of flags; described below: PPR-PDE Flags Format 0 1 2 3 4 5 6 7 .. 15 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |L|N|E| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L: Loose Bit. Indicates the type of next "Topological PDE-ID" in the path description. This bit MUST be set for only Node/Prefix PDE type. If this flag is unset, the next Topological PDE is Strict Type. N: Node Bit. By default this bit MUST be unset. This bit MUST be set only for PPR-PDE Type is 1 i.e., Topological and this PDE represents the node, where PPR-Prefix (Section 3.1) belongs to Chunduri, et al. Expires May 16, 2022 [Page 11] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 (if there is no further PDE specific Sub-TLVs to override PPR- Prefix and PPR-ID values). E: Egress Bit. By default this bit MUST be unset. This bit MUST be set only for PPR-PDE Type is 2 i.e., Non-Topological and the service needs to be applied on the egress side of the topological PDE preceding this PDE. Reserved: Reserved bits for future use. Reserved bits MUST be reset on transmission and ignored on receive. o Sub-TLV Length: 1 byte length of all Sub-TLVs followed. It MUST be set to 0 if no further Sub-TLVs are present. o PPR-PDE Sub-TLVs: These have 1 octet type, 1 octet length and value field is defined per the type field. Types are as defined in PPR-TLV Sub-TLVs (Section 7), encoded further as sub-sub-TLVs of PPR-PDE and the length field represents the total length of the value field in bytes. IS-IS System-ID Sub-TLV: Type 4 (IANA TBD), Length Total length of value field in bytes, Value: IS-IS System-ID of length "ID Length" as defined in [ISO.10589.1992]. This Sub-TLV MUST NOT be present, if the PPR-PDE Type is not Topological. Though the type for this comes from the PPR Sub-TLV registry, here this is a sub-sub-TLV and is part of PPR-ID/PPR-PDE Sub-TLV. 3.4. PPR-Attributes Sub-TLV PPR-Attribute Sub-TLVs describe the attributes of the path. This document defines the following optional PPR-Attribute Sub-TLVs: o Type 5 (Suggested Value - IANA TBD): PPR-Prefix originating node's IPv4 Router ID Sub-TLV. Length and Value fields are as specified in [RFC7794]. o Type 6 (Suggested Value - IANA TBD): PPR-Prefix originating node's IPv6 Router ID Sub-TLV. Length and Value fields are as specified in [RFC7794]. o Type 7 (Suggested Value - IANA TBD): PPR-Metric Sub-TLV. Length 4 bytes, and Value is the metric of this path represented through the PPR-ID. Different nodes can advertise the same PPR-ID for the same Prefix with a different set of PPR-PDE Sub-TLVs and the receiving node MUST consider the lowest metric value. Chunduri, et al. Expires May 16, 2022 [Page 12] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 4. PPR Processing Procedure Example As specified in [I-D.chunduri-rtgwg-preferred-path-routing], a PPR can be a TE path, locally provisioned by the operator or by a controller. Consider the following IS-IS network to describe the operation of PPR TLV as defined in Section 3: 1 _______ / 1 \ +---R2-------R3---+ / \_______/ \ / 1 \ 1 / \ 1 / 1__R13__1 \ / / \ \ R1------R6 R7-----------R4 \ 2 \__R14__/ 2 /\ \ 2 2 / \ 3 \ / 3 \1 \ 4 / \ +----R8------R9-----R10------R12 \ 1 / 1 \ / 1 +----R11---+ Figure 5: IS-IS Network In the (Figure 5), consider node R1 as an ingress node, or a head-end node, and the node R4 may be an egress node or another head-end node. The numbers shown on links between nodes indicate the bi-directional IS-IS metric as provisioned. R1 may be configured to receive TE source routed path information from a central entity (PCE [RFC5440], Netconf [RFC6241] or a Controller) that comprises PPR information which relates to sources that are attached to R1. It is also possible to have a PPR provisioned locally by the operator for non-TE needs (e.g. FRR or for chaining certain services). The PPR TLV (as specified in Section 3) is encoded as an ordered list of PPR-PDEs from source to a destination node in the network and is represented with a PPR-ID (Section 3.2). The PPR TLV includes PPR- PDE Sub-TLVs Section 3.3, which represent both topological and non- topological elements and specifies the actual path towards a PPR- Prefix at R4. o The shortest path towards R4 from R1 are through the following sequence of nodes: R1-R2-R3-R4 based on the provisioned metrics. Chunduri, et al. Expires May 16, 2022 [Page 13] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 o The central entity can define a few PPRs from R1 to R4 that deviate from the shortest path based on other network characteristic requirements as requested by an application or service. For example, the network characteristics or performance requirements may include bandwidth, jitter, latency, throughput, error rate, etc. o A first PPR may be identified by PPR-ID = 1 (value) and may include the path of R1-R6-R7-R4 for a Prefix advertised by R4. This is an example for a Loose-PPR and 'L' bit MUST be set appropriately at Section 3.3. o A second PPR may be identified by PPR-ID = 2 (value) and may include the path of R1-R8-R9-R10-R4. This is an example for a Strict-PPR and 'L' bit MUST be unset appropriately at Section 3.3. Though this example shows PPR with all nodal SIDs, it is possible to have a PPR with combination of node and adjacency SIDs (local or global) or with PPR-PDE Type set to Non-Topological as defined in Section 3.3 elements along with these. 4.1. PPR TLV Processing The first topological sub-object or PDE (Section 3.3) relative to the beginning of PPR Path contains the information about the first node (e.g. in SR-MPLS it's the topmost label). The last topological sub- object or PDE contains information about the last node (e.g. in SR- MPLS it's the bottommost label). Each receiving node determines whether an advertised PPR includes information regarding the receiving node. Before processing any further, validation MUST be done to see if any PPR topological PDE is seen more than once (possible loop), if yes, this PPR TLV MUST be ignored. Processing of PPR TLVs may be done, during the end of the SPF computation (for MTID that is advertised in this TLV) and for each prefix described through PPR TLV. For example, node R9 receives the PPR information, and ignores the PPR-ID=1 (Section 4) because this PPR TLV does not include node R9 in the path description/ordered PPR-PDE list. However, node R9 may determine that the second PPR identified by PPR- ID = 2 does include the node R9 in its PDE list. Therefore, node R9 updates the local forwarding database to include an entry for the destination address that R4 indicates, so that when a data packet comprising a PPR-ID of 2 is received, forward the data packet to node R10 instead of R11. This is done, even though from R9 the shortest path to reach R4 via R11 (Cost 3: R9-R11-R12-R4) it chooses the NH to R10 to reach R4 as specified in the PPR path description. Same Chunduri, et al. Expires May 16, 2022 [Page 14] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 process happens to all nodes or all topological PDEs as described in the PPR TLV. In summary, the receiving node checks first, if this node is on the path by checking the node's topological elements (with PPR-PDE Type set to Topological) in the path list. If yes, it adds/adjusts the PPR-ID's shortest path NH towards the next topological PDE in the PPR's Path. 4.2. Path Fragments A complete PPR path may not fit into the maximum allowable size of the IS-IS TLV. To overcome this a 7 bit Fragment-ID field is defined in Section 3 . With this, a single PPR path is represented via one or more fragmented PPR path TLVs, with all having the same PPR-ID. Each fragment carries the PPR-ID as well as a numeric Fragment-ID from 0 to (N-1), when N fragments are needed to describe the PPR Graph (where N>1). In this case Fragment (N-1) MUST set the 'U' bit (PPR-Flags) to indicate it is the last fragment. If Fragment-ID is non-zero in the TLV, then it MUST not carry PPR-Prefix Sub-TLV. The optional PPR Attribute Sub-TLVs which describe the path overall MUST be included in the last fragment only (i.e., when the 'U' bit is set). 5. PPR Data Plane aspects Data plane for PPR-ID is selected by the entity (e.g., a controller, locally provisioned by operator), which selects a particular PPR in the network. Section 3.2 defines various data plane identifier types and a corresponding data plane identifier is selected by the entity which selects the PPR. 5.1. SR-MPLS with PPR If PPR-ID Type is 1, then the PPR belongs to SR-MPLS data plane and the complete PPR stack is represented with a unique SR SID/Label and this gets programmed on the data plane of each node, with the appropriate NH computed as specified in Section 4. PPR-ID here is a label/index from the SRGB (like another node SID or global ADJ-SID). PPR path description here is a set of ordered SIDs represented with PPR-PDE (Section 3.2) Sub-TLVs. Non-Topological segments are also programmed in the forwarding to enable specific function/service, when the data packet hits with corresponding PPR-ID. Based on the 'L' flag in PPR-ID Flags (Section 3.2), for SR-MPLS data plane either 1 label or 2 labels need to be provisioned on individual nodes on the path description. For the example network in Section 4, for PPR-ID=1, which is a loose path, node R6 programs the bottom Chunduri, et al. Expires May 16, 2022 [Page 15] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 label as PPR-ID and the top label as the next topological PPR-PDE in the path, which is a node SID of R7. The NH computed at R6 would be the shortest path towards R7 i.e., the interface towards R13. If 'L' flag is unset, only PPR-ID is programmed on the data plane with NH set to the shortest path towards the next topological PPR-PDE. 5.2. PPR Native IP Data Planes If PPR-ID Type is 2 then source routing and packet steering can be done in IPv4 data plane (PPR-IPv4), along the path as described in PPR Path description. This is achieved by setting the destination IP address as PPR-ID, which is an IPv4 address in the data packet (tunneled/encapsulated). There is no data plane change or upgrade needed to support this. Similarly for PPR-ID Type is 3, then source routing and packet steering can be done in the IPv6 data plane (PPR-IPv6), along the path as described in PPR Path description. Whatever specified above for IPv4 applies here too, except that the destination IP address of the data packet is an IPv6 Address (PPR-ID). This doesn't require any IPv6 extension headers (EH), if there is no metadata/TLVs need to be carried in the data packet. Based on 'L' flag in PPR-ID Flags (Section 3.2), for PPR-ID Type 2 or 3 (Native IPv4 or IPv6 data planes respectively) the packet has to be encapsulated using the capabilities (either dynamically signaled through [I-D.ietf-isis-encapsulation-cap] or statically provisioned on the nodes) of the next loose PDE in the path description. For the example network in Section 4, for PPR-ID=1, which is a loose path, node R6 programs to encapsulate the data packet towards the next loose topological PPR-PDE in the path, which is R7. The NH computed at R6 would be the shortest path towards R7 i.e., the interface towards R13. If the 'L' flag is unset, only PPR-ID is programmed on the data plane with NH set to the shortest path towards the next topological PPR-PDE, with no further encapsulation of the data packet. 5.3. SRv6 with PPR If PPR-ID Type is 4, the PPR belongs to SRv6 with SRH data plane and the complete PPR stack is represented with IPv6 SIDs and this gets programmed on the data plane with the appropriate NH computed as specified in Section 4. PPR-ID here is a SRv6 SID. PPR path description here is a set of ordered SID TLVs similar to as specified in Section 5.1. One way PPR-ID would be used in this case is by setting it as the destination IPv6 address and SL field in SRH would Chunduri, et al. Expires May 16, 2022 [Page 16] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 be set to 0; however SRH [RFC8754] can contain any other TLVs and non-topological SIDs as needed. 6. Acknowledgements Thanks to Alex Clemm, Lin Han, Toerless Eckert, Asit Chakraborti, Stewart Bryant and Kiran Makhijani for initial discussions on this topic. Thanks to Kevin Smith and Stephen Johnson for various deployment scenarios applicability from ETSI WGs perspective. Authors also acknowledge Alexander Vainshtein for detailed discussions and few suggestions on this topic. Earlier versions of [RFC8667] have a mechanism to advertise EROs through Binding SID. 7. IANA Considerations This document requests the following new TLV in IANA IS-IS TLV code- point registry. TLV # Name ----- -------------- 155 PPR TLV (Suggested Value, IANA TBD) 7.1. PPR Sub-TLVs This document requests IANA to create a new Sub-TLV registry for PPR TLV Section 3 with the following initial entries (suggested values). Though these are defined as Sub-TLVs of PPR TLV, these can be part of another Sub-TLV as a nested sub-sub-TLV (e.g. IS-IS System-ID). Sub-TLV # Sub-TLV Name --------- --------------------------------------------------------- 1 PPR-Prefix (Section 3.1) 2 PPR-ID (Section 3.2) 3 PPR-PDE (Section 3.3) 4 IS-IS System-ID (Section 3.3) 5 PPR-Prefix Source IPv4 Router ID (Section 3.4) 6 PPR-Prefix Source IPv6 Router ID (Section 3.4) 7 PPR-Metric (Section 3.4) Chunduri, et al. Expires May 16, 2022 [Page 17] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 7.2. IGP Parameters This document requests additional IANA registries in an IANA managed registry "Interior Gateway Protocol (IGP) Parameters" for various PPR TLV parameters. The registration procedure is based on the "Expert Review" as defined in [RFC8126]. The suggested registry names are: o "PPR-Type" - Types are unsigned 8 bit numbers. Values are as defined in Section 3 of this document. o "PPR-Flags" - 1 Octet. Bits as described in Section 3 of this document. o "PPR-ID Type" - Types are unsigned 8 bit numbers. Values are as defined in Section 3.2 of this document. o "PPR-ID Flags" - 1 Octet. Bits as described in Section 3.2 of this document. o "PPR-PDE Type" - Types are unsigned 8 bit numbers. Values are as defined in Section 3.3 of this document. o "PPR-PDE Flags" - 1 Octet. Bits as described in Section 3.3 of this document. o "PDE-ID Type" - Types are unsigned 8 bit numbers. Values are as defined in Section 3.3 of this document. 8. Security Considerations Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310]. Further security analysis for the IS-IS protocol is done in [RFC7645] with detailed analysis of various security threats and why [RFC5304] should not be used in the deployments. Advertisement of the additional information defined in this document introduces no new security concerns in IS-IS protocol. However, for extensions related ro SR-MPLS and SRH data planes, those particular data plane security considerations do apply here. 9. Contributing Authors The following people contributed substantially to the content of this document and should be considered co-authors. Chunduri, et al. Expires May 16, 2022 [Page 18] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 Yingzhen Qu Futurewei 2330 Central Expressway Santa Clara CA 95050 USA Email: yingzhen.qu@futurewei.com 10. References 10.1. Normative References [I-D.chunduri-rtgwg-preferred-path-routing] Bryant, S., Chunduri, U., and A. Clemm, "Preferred Path Routing Framework", draft-chunduri-rtgwg-preferred-path- routing-01 (work in progress), October 2021. [ISO.10589.1992] International Organization for Standardization, "Intermediate system to intermediate system intra-domain- routing routine information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473)", ISO Standard 10589, 1992. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 10.2. Informative References [I-D.bryant-rtgwg-plfa] Bryant, S., Chunduri, U., and T. Eckert, "Preferred Path Loop-Free Alternate (pLFA)", draft-bryant-rtgwg-plfa-02 (work in progress), June 2021. [I-D.chunduri-dmm-5g-mobility-with-ppr] Chunduri, U., Contreras, L. M., Bhaskaran, S., Tantsura, J., and P. Muley, "Transport aware 5G mobility with PPR", draft-chunduri-dmm-5g-mobility-with-ppr-00 (work in progress), November 2020. Chunduri, et al. Expires May 16, 2022 [Page 19] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 [I-D.ietf-dmm-tn-aware-mobility] Chunduri, U., Kaippallimalil, J., Bhaskaran, S., Tantsura, J., and P. Muley, "Mobility aware Transport Network Slicing for 5G", draft-ietf-dmm-tn-aware-mobility-02 (work in progress), October 2021. [I-D.ietf-isis-encapsulation-cap] Xu, X., Decraene, B., Raszuk, R., Chunduri, U., Contreras, L. M., and L. Jalil, "Advertising Tunnelling Capability in IS-IS", draft-ietf-isis-encapsulation-cap-01 (work in progress), April 2017. [I-D.ietf-isis-mpls-elc] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., and M. Bocci, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS", draft-ietf- isis-mpls-elc-13 (work in progress), May 2020. [I-D.ietf-lsr-isis-srv6-extensions] Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and Z. Hu, "IS-IS Extensions to Support Segment Routing over IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-18 (work in progress), October 2021. [I-D.ietf-mpls-sfc] Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based Forwarding Plane for Service Function Chaining", draft- ietf-mpls-sfc-07 (work in progress), March 2019. [I-D.ietf-teas-enhanced-vpn] Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A Framework for Enhanced Virtual Private Network (VPN+) Services", draft-ietf-teas-enhanced-vpn-09 (work in progress), October 2021. [I-D.xuclad-spring-sr-service-chaining] Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C., Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and S. Salsano, "Segment Routing for Service Chaining", draft- xuclad-spring-sr-service-chaining-01 (work in progress), March 2018. [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, DOI 10.17487/RFC5120, February 2008, . Chunduri, et al. Expires May 16, 2022 [Page 20] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2008, . [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, . [RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, DOI 10.17487/RFC5308, October 2008, . [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., and M. Fanto, "IS-IS Generic Cryptographic Authentication", RFC 5310, DOI 10.17487/RFC5310, February 2009, . [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, . [RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011, . [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and L. Yong, "The Use of Entropy Labels in MPLS Forwarding", RFC 6790, DOI 10.17487/RFC6790, November 2012, . [RFC7645] Chunduri, U., Tian, A., and W. Lu, "The Keying and Authentication for Routing Protocol (KARP) IS-IS Security Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015, . [RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794, March 2016, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . Chunduri, et al. Expires May 16, 2022 [Page 21] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 [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, . [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, DOI 10.17487/RFC8491, November 2018, . [RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., Bashandy, A., Gredler, H., and B. Decraene, "IS-IS Extensions for Segment Routing", RFC 8667, DOI 10.17487/RFC8667, December 2019, . [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, . Appendix A. Appendix A.1. Challenges with Increased SID Depth SR label stacks carried in the packet header create challenges in the design and deployment of networks and networking equipment. Following examples illustrates the need for increased SID depth in various use cases: (a). Consider the following network where SR-MPLS data plane is in use and with same SRGB (5000-6000) on all nodes i.e., A1 to A11 and B1 to B7 for illustration: Chunduri, et al. Expires May 16, 2022 [Page 22] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 SID:10 SID:20 SID:30 SID:40 SID:50 SID:300(Ax) SID:60 SID:70 A1--------A2-------A3-------A4-------A5===============A6-- ----------A7 | \ / \5 5/ \ SID:310(Ay) \ / | 5 \ 10 10/ +-A10-+ \ \10 10/ | \ SID:80 / |SID:100 \ \ / A11 SID:111 \A8-----A9/ | \ 40 \ / | / SID:90 \ +-----+ +---+ \ / | 5 /10 \10 5 \ \ \ / | /SID:125(B2x) \ \ \ \/ B1-------B2==============B3----B4------B5-------=B6----------B7 SID:127(B2y) SID:110 SID:120 SID:130 SID:140 SID:150 SID:160 SID:170 === = Path with Parallel Adjacencies and ADJ-SIDs --- = Shortest Path Nodal SID Figure 6: SR-MPLS Network Global ADJ-SIDs are provisioned between A5-A6 and B2-B3 (with parallel adjecencies). All other SIDs shown are nodal SID indices. All metrics of the links are set to 1, unless marked otherwise. Shortest Path from A1 to A7: A2-A3-A4-A5-A6-A7 Path-x: From A1 to A7 - A2-A8-B2-B2x-A9-A10-Ax-A7; Pushed Label Stack @A1: 5020:5080:5120:5125:5090:5100:5300:5070 (where B2x is a local ADJ-SID and Ax is a global ADJ-SID). In this example, the traffic engineered path is represented with a combination of Adjacency and Node SIDs with a stack of 8 labels. However, this value can be larger, if the use of entropy label [RFC6790] is desired and based on the Readable Label Depth (Appendix A.2) capabilities of each node and additional labels required to insert ELI/EL at appropriate places. Though above network is shown with SR-MPLS data plane, if the network were to use SRv6 data plane, path size would be increased even more because of the size of the IPv6 SID (16 bytes) in SRH. (b). Apart from the TE case above, when deploying [I-D.ietf-mpls-sfc] or [I-D.xuclad-spring-sr-service-chaining], with the inclusion of services, or non-topological segments on the label stack, can also make the size of the stack much larger. Chunduri, et al. Expires May 16, 2022 [Page 23] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 Overall the additional path overhead in various SR deployments may cause the following issues: a. HW Capabilities: Not all nodes in the path can support the ability to push or read label stack (with additional non- topological and special labels) needed to satisfy user/operator requirements. Alternate paths, which meet these user/operator requirements may not be available. b. Line Rate: Potential performance issues in deployments, which use data plane with extension header as both size of the SIDs in the extension header and the fixed extension header size itself needs to be factored by the hardware. c. MTU: Larger SID stacks on the data packet can cause potential MTU/fragmentation issues (SRH). d. Header Tax: Some deployments, such as 5G, require minimal packet overhead in order to conserve network resources. Carrying 40 or 50 octets of data in a packet with hundreds of octet of header would be an unacceptable use of available bandwidth. With the solution proposed in this document, for Path-x in Figure 6 above, SID stack would be reduced from 8 SIDs to a single SID witout any additional overhead. A.2. Mitigation with MSD The number of SIDs in the stack a node can impose is referred as Maximum SID Depth (MSD) capability [RFC8491], which must be taken into consideration when computing a path to transport a data packet in a network implementing segment routing. [I-D.ietf-isis-mpls-elc] defines another MSD type, Readable Label Depth (RLD) that is used by a head-end to insert Entropy Label pair (ELI/EL) at appropriate depth, so it could be read by transit nodes. There are situations where the source routed path can be excessive as path represented by SR SIDs need to describe all the nodes and ELI/EL based on the readability of the nodes in that path. Registries setforth in [RFC8491] applicable for MPLS data plane and for IPv6 data plane with SRH. MSDs (and RLD type) capabilities advertisement help mitigate the problem for a central entity to create the right source routed path per application/operator requirements. However the availability of actual paths meeting these requirements are still limited by the underlying hardware and their MSD capabilities in the data path. Chunduri, et al. Expires May 16, 2022 [Page 24] Internet-Draft Preferred Path Routing (PPR) in IS-IS November 2021 Authors' Addresses Uma Chunduri Intel Corporation Email: umac.ietf@gmail.com Richard Li Futurewei 2330 Central Expressway Santa Clara, CA 95050 USA Email: richard.li@futurewei.com Russ White Juniper Networks Oak Island, NC 28465 USA Email: russ@riw.us Luis M. Contreras Telefonica Sur-3 building, 3rd floor Madrid 28050 Spain Email: luismiguel.contrerasmurillo@telefonica.com Jeff Tantsura Microsoft Email: jefftanti.ietf@gmail.com Yingzhen Qu Futurewei 2330 Central Expressway Santa Clara, CA 95050 USA Email: yingzhen.qu@futurewei.com Chunduri, et al. Expires May 16, 2022 [Page 25]