IS-IS Extensions to Support Segment Routing over IPv6 Dataplane

With Segment Routing (SR) , a node steers a packet through an ordered list of instructions, called segments. Segments are identified through Segment Identifiers (SIDs). Segment Routing can be directly instantiated on the IPv6 data plane through the use of the Segment Routing Header defined in . SRv6 refers to this SR instantiation on the IPv6 dataplane. The network programming paradigm is central to SRv6. It describes how any behavior can be bound to a SID and how any network program can be expressed as a combination of SIDs. This document specifies IS-IS extensions that allow the IS-IS protocol to encode some of these SIDs and their behaviors. Familiarity with the network programming paradigm is necessary to understand the extensions specified in this document. The new SRv6 Locator top level TLV announces SRv6 locators - a form of summary address for the set of topology/algorithm-specific SIDs instantiated at the node. The SRv6 Capabilities sub-TLV announces the ability to support SRv6. Several new sub-TLVs are defined to advertise various SRv6 Maximum SID Depths. The SRv6 End SID sub-TLV, the SRv6 End.X SID sub-TLV, and the SRv6 LAN End.X SID sub-TLV are used to advertise which SIDs are instantiated at a node and what Endpoint behavior is bound to each instantiated SID. This document updates by modifying an existing registry ().

A node indicates that it supports the SR Segment Endpoint Node functionality as specified in by advertising a new SRv6 Capabilities sub-TLV of the router capabilities TLV [RFC7981]. The SRv6 Capabilities sub-TLV may contain optional sub-sub-TLVs. No sub-sub-TLVs are currently defined. The SRv6 Capabilities sub-TLV has the following format:

An SRv6 capable router indicates supported algorithm(s) by advertising the Segment Routing Algorithm sub-TLV as defined in .

defines the means to advertise node/link specific values for Maximum SID Depths (MSD) of various types. Node MSDs are advertised in a sub-TLV of the Router Capabilities TLV . Link MSDs are advertised in a sub-TLV of TLVs 22, 23, 25, 141, 222, and 223. This document defines the relevant SRv6 MSDs and requests MSD type assignments in the MSD Types registry created by .

The Maximum Segments Left MSD Type signals the maximum value of the "Segments Left" field in the SRH of a received packet before applying the Endpoint behavior associated with a SID.

The Maximum End Pop MSD Type signals the maximum number of SIDs in the SRH to which the router can apply "Penultimate Segment Pop of the SRH" or "Ultimate Segment Pop of the SRH" behavior, as defined in flavors.

The Maximum H.Encaps MSD Type signals the maximum number of SIDs that can be added to the Segment List of an SRH as part of the "H.Encaps" behavior as defined in .

The Maximum End D MSD Type specifies the maximum number of SIDs present in an SRH when performing decapsulation. As specified in the permitted SID types include, but are not limited to End.DX6, End.DT4, End.DT46, End with USD, End.X with USD.

As discussed in , an SRv6 Segment Identifier (SID) is 128 bits and consists of Locator, Function and Argument parts. A node is provisioned with topology/algorithm specific locators for each of the topology/algorithm pairs supported by that node. Each locator is a covering prefix for all SIDs provisioned on that node which have the matching topology/algorithm. Locators MUST be advertised in the SRv6 Locator TLV (see ). Forwarding entries for the locators advertised in the SRv6 Locator TLV MUST be installed in the forwarding plane of receiving SRv6 capable routers when the associated topology/algorithm is supported by the receiving node. The processing of the prefix advertised in the SRv6 Locator TLV, the calculation of its reachability and the installation in the forwarding plane follows the process defined for the Prefix Reachability TLV 236 , or TLV 237 . Locators associated with algorithm 0 and 1 (for all supported topologies) SHOULD be advertised in a Prefix Reachability TLV (236 or 237) so that legacy routers (i.e., routers which do not support SRv6) will install a forwarding entry for algorithm 0 and 1 SRv6 traffic. In cases where the same prefix, with the same prefix-length, Multi Topology ID (MT ID), and algorithm is received in both a Prefix Reachability TLV and an SRv6 Locator TLV, the Prefix Reachability advertisement MUST be preferred when installing entries in the forwarding plane. This is to prevent inconsistent forwarding entries between SRv6 capable and SRv6 incapable routers. Such preference of Prefix Reachability advertisement does not have any impact on the rest of the data advertised in the SRv6 Locator TLV. Locators associated with Flexible Algorithms (see Section 4 of ) SHOULD NOT be advertised in Prefix Reachability TLVs (236 or 237). Advertising the Flexible Algorithm locator in regular Prefix Reachability TLV (236 or 237) would make the forwarding for it to follow algo 0 path. SRv6 SIDs are advertised as sub-TLVs in the SRv6 Locator TLV except for SRv6 SIDs which are associated with a specific Neighbor/Link and are therefore advertised as sub-TLVs in TLVs 22, 23, 25, 141, 222, and 223. SRv6 SIDs received from other nodes are not directly routable and MUST NOT be installed in the forwarding plane. Reachability to SRv6 SIDs depends upon the existence of a covering locator. Adherence to the rules defined in this section will assure that SRv6 SIDs associated with a supported topology/algorithm pair will be forwarded correctly, while SRv6 SIDs associated with an unsupported topology/algorithm pair will be dropped. NOTE: The drop behavior depends on the absence of a default/summary route covering a given locator. In order for forwarding to work correctly, the locator associated with SRv6 SID advertisements must be the longest match prefix installed in the forwarding plane for those SIDs. In order to ensure correct forwarding, network operators should take steps to make sure that this requirement is not compromised. For example, the following situations should be avoided: Another locator associated with a different topology/algorithm is the longest match Another prefix advertisement (i.e., from TLV 236 or 237) is the longest match

Both prefixes and SRv6 Locators may be configured as anycast and as such the same value can be advertised by multiple routers. It is useful for other routers to know that the advertisement is for an anycast identifier. A new flag in Prefix Attribute Flags Sub-TLV is defined to advertise the anycast property:

Bit #: 4 Name: Anycast Flag (A-flag) When the prefix/SRv6 locator is configured as anycast, the A-flag SHOULD be set. Otherwise, this flag MUST be clear. The A-flag MUST be preserved when the advertisement is leaked between levels. The A-flag and the N-flag MUST NOT both be set. If both N-flag and A-flag are set in the prefix/SRv6 Locator advertisement, the receiving routers MUST ignore the N-flag. The same prefix/SRv6 Locator can be advertised by multiple routers. If at least one of them sets the A-Flag in its advertisement, the prefix/SRv6 Locator SHOULD be considered as anycast. A prefix/SRv6 Locator that is advertised by a single node and without an A-Flag is considered node specific. All the nodes advertising the same anycast locator MUST instantiate the exact same set of SIDs under that anycast locator. Failure to do so may result in traffic being black-holed or mis-routed. The Prefix Attribute Flags Sub-TLV can be carried in the SRv6 Locator TLV as well as the Prefix Reachability TLVs. When a router originates both the Prefix Reachability TLV and the SRv6 Locator TLV for a given prefix, and the router is originating the Prefix Attribute Flags Sub-TLV in one of the TLVs, the router SHOULD advertise the same flags in the Prefix Attribute Flags Sub-TLV in both TLVs. However, unlike TLVs 236 and 237 the X-flag in the Prefix Attributes Flags sub-TLV is valid when sent in the SRv6 Locator TLV. The state of the X-flag in the Prefix Attributes Flags sub-TLV when included in the Locator TLV MUST match the setting of the embedded "X-bit" in any advertisement for the same prefix in TLVs 236 and 237 . In case of any inconsistency between the Prefix Attribute Flags advertised in the Locator TLV and in the Prefix Reachability TLV, the ones advertised in Prefix Reachability TLV MUST be preferred.

The SRv6 Locator TLV is introduced to advertise SRv6 Locators and End SIDs associated with each locator. This new TLV shares the sub-TLV space defined for TLVs 135, 235, 236 and 237.

The SRv6 Locator TLV has the following format:

Type: 27. Single octet as defined in section 9 of [ISO10589]. Length: Single octet as defined in section 9 of [ISO10589]. The length value is variable. R bits: reserved for future use. They MUST be set to zero on transmission and MUST be ignored on receipt. MT ID: Multitopology Identifier as defined in [RFC5120]. Note that the value 0 is legal. Followed by one or more locator entries of the form:

Metric: 4 octets. As described in Section 4 of [RFC5305]. Flags: 1 octet. The following flags are defined:

D-flag: Same as described in section 4.1. of [RFC5305]. The remaining bits are reserved for future use. They MUST be set to zero on transmission and MUST be ignored on receipt. Algorithm: 1 octet. As defined in IGP Algorithm Types registry . Loc-Size: 1 octet. Number of bits in the SRv6 Locator field. MUST be from the range (1 - 128). The TLV MUST be ignored if the Loc-Size is outside this range. Locator: 1-16 octets. This field encodes the advertised SRv6 Locator. The Locator is encoded in the minimal number of octets for the given number of bits. Trailing bits MUST be set to zero and ignored when received. Sub-TLV-length: 1 octet. Number of octets used by sub-TLVs. Optional sub-TLVs: Supported sub-TLVs are specified in . Any Sub-TLV that is not allowed in the SRv6 Locator TLV MUST be ignored. Prefix Attribute Flags Sub-TLV SHOULD be included in the Locator TLV. Prefix Attribute Flags Sub-TLV MUST be included in the the Locator TLV when it is leaked upwards in the hierarchy or originated as a result of the redistribution from another protocol or another ISIS instance. If the Prefix Attribute Flags Sub-TLV is not included in these cases, receivers will be unable to determine the correct source of the advertisement. The receivers will be unable to detect the violation.

The SRv6 End SID sub-TLV is introduced to advertise SRv6 Segment Identifiers (SID) with Endpoint behaviors which do not require a particular neighbor in order to be correctly applied. SRv6 SIDs associated with a neighbor are advertised using the sub-TLVs defined in . Supported behavior values, together with parent TLVs in which they are advertised, are specified in of this document. Please note that not all behaviors defined in are defined in this document, e.g. END.T is not. This new sub-TLV is advertised in the SRv6 Locator TLV defined in the previous section. SRv6 End SIDs inherit the topology/algorithm from the parent locator. The SRv6 End SID sub-TLV has the following format:

Type: 5. Single octet as defined in section 9 of [ISO10589]. Length: Single octet as defined in section 9 of [ISO10589]. The length value is variable. Flags: 1 octet. No flags are currently defined. All bits are reserved for future use. They MUST be set to zero on transmission and MUST be ignored on receipt. Endpoint Behavior: 2 octets, as defined in . Supported behavior values for this sub-TLV are defined in of this document. Unsupported or unrecognized behavior values are ignored by the receiver. SID: 16 octets. This field encodes the advertised SRv6 SID. Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub-TLVs. Optional Sub-sub-TLVs: Supported Sub-sub-TLVs are specified in . Any Sub-sub-TLV that is not allowed in SRv6 End SID sub-TLV MUST be ignored. The SRv6 End SID MUST be allocated from its associated locator. SRv6 End SIDs that are not allocated from the associated locator MUST be ignored. Multiple SRv6 End SIDs MAY be associated with the same locator. In cases where the number of SRv6 End SID sub-TLVs exceeds the capacity of a single TLV, multiple Locator TLVs for the same locator MAY be advertised. For a given MTID/Locator the algorithm MUST be the same in all TLVs. If this restriction is not met all TLVs for that MTID/Locator MUST be ignored.

Certain SRv6 Endpoint behaviors are associated with a particular adjacency. This document defines two new sub-TLVs of TLV 22, 23, 25, 141, 222, and 223 - namely "SRv6 End.X SID sub-TLVs" and "SRv6 LAN End.X SID sub-TLVs". IS-IS Neighbor advertisements are topology specific - but not algorithm specific. SIDs advertised in SRv6 End.X SID and SRv6 LAN End.X SID sub-TLVs therefore inherit the topology from the associated neighbor advertisement, but the algorithm is specified in the individual SID. All SIDs advertised in SRv6 End.X SID and SRv6 LAN End.X SID sub-TLVs MUST be a subnet of a Locator with matching topology and algorithm which is advertised by the same node in an SRv6 Locator TLV. SIDs that do not meet this requirement MUST be ignored. This ensures that the node advertising these SIDs is also advertising its corresponding Locator with the algorithm that will be used for computing paths destined to the SID.

This sub-TLV is used to advertise an SRv6 SID associated with a point to point adjacency. Multiple SRv6 End.X SID sub-TLVs MAY be associated with the same adjacency. The SRv6 End.X SID sub-TLV has the following format:

Type: 43. Single octet as defined in section 9 of [ISO10589]. Length: Single octet as defined in section 9 of [ISO10589]. The length value is variable. Flags: 1 octet.

0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B|S|P|Reserved | +-+-+-+-+-+-+-+-+ where: B-Flag: Backup flag. If set, the SID is eligible for protection, e.g., using IP Fast Re-route (IPFRR) , as described in . S-Flag. Set flag. When set, the S-Flag indicates that the SID refers to a set of adjacencies (and therefore MAY be assigned to other adjacencies as well). P-Flag. Persistent flag. When set, the P-Flag indicates that the SID is persistently allocated, i.e., the SID value remains consistent across router restart and/or interface flap. Reserved bits: MUST be zero when originated and MUST be ignored when received. Algorithm: 1 octet. As defined in IGP Algorithm Types registry . Weight: 1 octet. The value represents the weight of the SID for the purpose of load balancing. The use of the weight is defined in [RFC8402]. Endpoint Behavior: 2 octets. As defined in . Supported behavior values for this sub-TLV are defined in of this document. Unsupported or unrecognized behavior values are ignored by the receiver. SID: 16 octets. This field encodes the advertised SRv6 SID. Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub- TLVs. Optional Sub-sub-TLVs: Supported Sub-sub-TLVs are specified in . Any Sub-sub-TLV that is not allowed in SRv6 End.X SID sub-TLV MUST be ignored. Note that multiple TLVs for the same neighbor may be required in order to advertise all the SRv6 SIDs associated with that neighbor.

This sub-TLV is used to advertise an SRv6 SID associated with a LAN adjacency. Since the parent TLV is advertising an adjacency to the Designated Intermediate System (DIS) for the LAN, it is necessary to include the System ID of the physical neighbor on the LAN with which the SRv6 SID is associated. Given that many neighbors may exist on a given LAN, multiple SRv6 LAN END.X SID sub-TLVs may be associated with the same LAN. Note that multiple TLVs for the same DIS neighbor may be required in order to advertise all the SRv6 SIDs associated with that neighbor. The SRv6 LAN End.X SID sub-TLV has the following format:

Type: 44. Single octet as defined in section 9 of [ISO10589]. Length: Single octet as defined in section 9 of [ISO10589]. The length value is variable. Neighbor System-ID: IS-IS System-ID of length "ID Length" as defined in [ISO10589]. Flags: 1 octet.

0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |B|S|P|Reserved | +-+-+-+-+-+-+-+-+ where B,S, and P flags are as described in . Reserved bits MUST be zero when originated and MUST be ignored when received. Algorithm: 1 octet. As defined in IGP Algorithm Types registry . Weight: 1 octet. The value represents the weight of the SID for the purpose of load balancing. The use of the weight is defined in [RFC8402]. Endpoint Behavior: 2 octets. As defined in . Supported behavior values for this sub-TLV are defined in of this document. Unsupported or unrecognized behavior values are ignored by the receiver. SID: 16 octets. This field encodes the advertised SRv6 SID. Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub- TLVs. Optional Sub-sub-TLVs: Supported Sub-sub-TLVs are specified in . Any Sub-sub-TLV that is not allowed in SRv6 LAN End.X SID sub-TLV MUST be ignored. Note that multiple TLVs for the same neighbor, on the same LAN, may be required in order to advertise all the SRv6 SIDs associated with that neighbor.

SRv6 SID Structure Sub-Sub-TLV is an optional Sub-Sub-TLV of: SRv6 End SID Sub-TLV () SRv6 End.X SID Sub-TLV () SRv6 LAN End.X SID Sub-TLV () SRv6 SID Structure Sub-Sub-TLV is used to advertise the structure of the SRv6 SID as defined in . It 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LB Length | LN Length | Fun. Length | Arg. Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where: Type: 1. Single octet as defined in section 9 of [ISO10589]. Length: Single octet as defined in section 9 of [ISO10589]. The length value is 4 octets. LB Length: 1 octet. SRv6 SID Locator Block length in bits. LN Length: 1 octet. SRv6 SID Locator Node length in bits. Fun. Length: 1 octet. SRv6 SID Function length in bits. Arg. Length: 1 octet. SRv6 SID Arguments length in bits. ISIS SRv6 SID Structure Sub-Sub-TLV MUST NOT appear more than once in its parent Sub-TLV. If it appears more than once in its parent Sub-TLV, the parent Sub-TLV MUST be ignored by the receiver. The sum of all four sizes advertised in ISIS SRv6 SID Structure Sub-Sub-TLV MUST be less than or equal to 128 bits. If the sum of all four sizes advertised in the ISIS SRv6 SID Structure Sub-Sub-TLV is larger than 128 bits, the parent Sub-TLV MUST be ignored by the receiver. The SRv6 SID Structure Sub-Sub-TLV is intended for informational use by the control and management planes. It MUST NOT be used at a transit node (as defined in ) for forwarding packets. As an example, this information could be used for: validation of SRv6 SIDs being instantiated in the network and advertised via ISIS. These can be learnt by controllers via BGP-LS and then be monitored for conformance to the SRv6 SID allocation scheme chosen by the operator as described in Section 3.2 of . verification and the automation for securing the SRv6 domain by provisioning filtering rules at SR domain boundaries as described in Section 5 of . The details of these potential applications are outside the scope of this document.

Endpoint behaviors are defined in . The codepoints for the Endpoint behaviors are defined in the "SRv6 Endpoint Behaviors" registry defined in . If a behavior is advertised it MUST only be advertised in the TLV[s] marked with "Y" in the table below, and MUST NOT be advertised in the TLV[s] marked with "N" in the table below.

This document requests allocation for the following TLVs, sub-TLVs, and sub-sub-TLVs by updating the existing registries and defining new registries under the IS-IS TLV Codepoints registry.

This document makes the following registrations in the "IS-IS Top-Level TLV Codepoints" registry:

The SRv6 Locator TLV shares sub-TLV space with TLVs advertising prefix reachability. This document updates the "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry initially defined in . IANA is asked to add this document as a reference to "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry and update the description of that registry to include the SRv6 Locator TLV (27). This document makes the following registrations in the "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry: Type: 5 Description: SRv6 End SID. Reference: This document ().

The revised table of sub-TLVs for the "IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability" registry is shown below:

This document makes the following registrations in the "IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV": Type: 25 Description: SRv6 Capabilities. Reference: This document ().

This document requests a new IANA registry be created under the IS-IS TLV Codepoints registry to control the assignment of sub-TLV types for the SRv6 Capability sub-TLV specified in this document - : Name: IS-IS Sub-Sub-TLVs for SRv6 Capability Sub-TLV Description: This registry defines sub-sub-TLVs for the SRv6 Capability Sub-TLV (25) advertised in the IS-IS Router Capabilities TLV (242). The registration procedure is "Expert Review" as defined in . Guidance for the Designated Experts is provided in the . No sub-sub-TLVs are defined by this document except for the reserved type 0.

This document makes the following registrations in the "IS-IS Sub-TLVs for TLVs Advertising Neighbor Information" registry: Type: 43 Description: SRv6 End.X SID. Reference: This document (). Type: 44 Description: SRv6 LAN End.X SID. Reference: This document ().

This document makes the following registrations in the IGP MSD-Types registry:

This document requests a new IANA registry be created under the IS-IS TLV Codepoints registry to control the assignment of sub-TLV types for the SID Sub-TLVs specified in this document - , , : Name: IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs Description: This registry defines Sub-Sub-TLVs for SRv6 SID Sub-TLVs. This includes the following sub-TLVs: SRv6 End SID (5) (Advertised in SRv6 Locator TLV (27) SRv6 End.X SID (43) (Advertised in TLVs advertising neighbor information) SRv6 LAN End.X SID (44) (Advertised in TLVs advertising neighbor information) The registration procedure is "Expert Review" as defined in . Guidance for the Designated Experts is provided in . The following assignments are made by this document:

This document adds a new bit in the "IS-IS Bit Values for Prefix Attribute Flags Sub-TLV" registry: Bit #: 4 Description: Anycast Flag (A-flag) Reference: This document ().

This document requests a new IANA registry be created under the IS-IS TLV Codepoints registry to control the assignment of bits 0 to 15 in the Flags field of the ISIS SRv6 Capabilities sub-TLV specified in this document (): Name: IS-IS SRv6 Capabilities Sub-TLV Flags Description: This registry defines bit values advertised in the flags field of the SRv6 Capabilities sub-TLV (25). This sub-TLV is advertised in the IS-IS Router CAPABILITY TLV (242). The registration procedure is "Expert Review" as defined in . Guidance for the Designated Experts is provided in . The following assignments are made by this document: Bit #: 1 Description: O-flag Reference: This document (). Bit #: 0, 2-15 Description: Unassigned

This document requests a new IANA registry be created under the IS-IS TLV Codepoints registry to control the assignment of bits 0 to 7 in the Flags field of the SRv6 Locator TLV specified in this document (): Name: IS-IS SRv6 Locator TLV Flags Description: This registry defines bit values advertised in the flags field of the SRv6 Locator TLV (27). The registration procedure is "Expert Review" as defined in . Guidance for the Designated Experts is provided in . The following assignments are made by this document: Bit #: 0 Description: D-flag Reference: This document (). Bit #: 1-7 Description: Unassigned

This document requests a new IANA registry be created under the IS-IS TLV Codepoints registry to control the assignment of bits 0 to 7 in the Flags field of the ISIS SRv6 End SID sub-TLV specified in this document (): Name: IS-IS SRv6 End SID Sub-TLV Flags Description: This registry defines bit values advertised in the flags field of the SRv6 End SID sub-TLV, which is advertised in the SRv6 Locator TLV (27). The registration procedure is "Expert Review" as defined in . Guidance for the Designated Experts is provided in . No assignments are made by this document. Bit #: 0-7 Description: Unassigned

This document requests a new IANA registry be created under the IS-IS TLV Codepoints registry to control the assignment of bits 0 to 7 in the Flags field of the ISIS SRv6 End.X SID and LAN End.X SID sub-TLVs ( and ): Name: IS-IS SRv6 Adjacency SID Sub-TLVs Flags. Description: This registry defines bit values advertised in the flags field of SRv6 SID sub-TLVs associated with adjacencies. These sub-TLVs are advertised in TLVs advertising neighbor information. The list of sub-TLVs includes: SRv6 End.X SID (43) SRv6 LAN End.X SID (44) The registration procedure is "Expert Review" as defined in . Guidance for the Designated Experts is provided in . The following assignments are made by this document: Bit #: 0 Description: B-flag Reference: This document (). Bit #: 1 Description: S-flag Reference: This document (). Bit #: 2 Description: P-flag Reference: This document (). Bit #: 3-7 Description: Unassigned

Security concerns for IS-IS are addressed in , , and . While IS-IS is deployed under a single administrative domain, there can be deployments where potential attackers have access to one or more networks in the IS-IS routing domain. In these deployments, the stronger authentication mechanisms defined in the aforementioned documents SHOULD be used. This document describes the IS-IS extensions required to support Segment Routing over an IPv6 data plane. The security considerations for Segment Routing are discussed in . defines the SRv6 Network Programming concept and specifies the main Segment Routing behaviors to enable the creation of interoperable overlays; the security considerations from that document apply too. The advertisement for an incorrect MSD value may have negative consequences, see for additional considerations. Security concerns associated with the setting of the O-flag are described in . Security concerns associated with the usage of Flex-Algorithms are described in ).

The following people gave a substantial contribution to the content of this document and should be considered as co-authors:

Robert Hanzl Cisco Systems Millenium Plaza Building, V Celnici 10, Prague 1, Prague, Czech Republic Email rhanzl@cisco.com Ketan Talaulikar Cisco Systems, Inc. Email: ketant@cisco.com ]]>

Thanks to Christian Hopps for his review comments and shepherd work. Thanks to Alvaro Retana and John Scudder for AD review and comments.