Advertising IGP Active Measurement Groups in Router Capabilities

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Table of Contents

3. Active Measurement Groups

Each AMG supported by an IGP router is identified by a Group ID. Each AMG has an associated IP endpoint, AMP, and IP address family. IGP routers will discover measurement partners in common AMGs identified by Group ID and attempt to establish AMP sessions in the AMG. Additionally, unique non-default parameters may be associated with an AMP. For example, the UDP ports supported by STAMP may be associated with an AMG.¶

AMGs and their Group IDs are configured within a single administrative domain. This is normally a single IGP domain unless inter-domain discovery is required (refer to Section 6). Each AMG will have an associated AMP and optionally non-default parameters associated with the AMG. The AMP and non-default parameters are not advertised in the IGPs as the protocol extensions specified herein are solely to discover the IP endpoints participating in the AMG. Assuring consistent configuration of the AMP and associated non-default AMP parameters is beyond the scope of this specification. Distinct AMGs are required for distinct AMPs and for distinct IP address families. It is understood that unique AMGs will also be required for unique permutations of asymmetric non-default parameters (e.g., different UDP ports for STAMP endpoints). However, this is not seen as the predominant use case.¶

4. AMP Measurement Group Sub-TLV

Since loopback support is required, and there is no adjacency created over loopback interfaces to carry Application Specific Link Attribute (ASLA) information, the solution defines an IS-IS capability sub-TLV similar to seamless BFD discriminators [RFC7883]. This approach allows the advertisement of measurement group membership information in the Router CAPABILITY TLV, which is flooded domain-wide.¶

This document defines a new IS-IS capability sub-TLV for advertising AMP measurement group membership. The sub-TLV is carried in the IS-IS Router CAPABILITY TLV (TLV 242) as defined in [RFC7981].¶

The AMP Measurement Group sub-TLV 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     |         Group ID              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|          IPv4/IPv6 Endpoint Address (4 or 16 octets)          |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:¶

Type:

TBD1 (to be assigned by IANA, see Section 7)¶

Length:

1 octet. The length of the value field in octets. The address family will be inferred from the length of the sub-TLV. For IPv4 addresses, the length MUST be 6. For IPv6 addresses, the length MUST be 18. A sub-TLV with a length other than 6 or 18 MUST be considered malformed. Consistent with the handling of unsupported sub-TLVs in Section 4 of [RFC7981], a receiving router MUST silently ignore such a sub-TLV and MUST continue processing the other sub-TLVs carried in the Router CAPABILITY TLV.¶

Group ID:

2 octets. Group ID used to associate the endpoint's membership with other routers in the AMG. An IS-IS router can advertise multiple AMP Measurement Group sub-TLVs with different Group IDs. Each AMG is associated with a single AMP and a single IP address family, so a distinct Group ID is required for each AMG within the administrative domain. The Group ID values 0 and 65535 are reserved and MUST NOT be used to identify an AMG; the range of Group IDs available to identify an AMG is 1 through 65534. A router MUST ignore an AMP Measurement Group sub-TLV advertising a reserved Group ID. These values are reserved for possible future definition of well-known Group IDs; no such Group IDs are defined by this document.¶

IPv4/IPv6 Endpoint Address:

4 octets for IPv4 or 16 octets for IPv6. The interface address (IPv4 or IPv6) that identifies this endpoint's usage in the measurement group indicated by the Group ID. This address MAY be associated with a physical interface or a loopback interface.¶

Multiple instances of this sub-TLV MAY be included in the Router CAPABILITY TLV, each advertising the membership of one IP endpoint in one AMG. The AMP associated with an AMG is not carried in the sub-TLV; it is determined from the local configuration of the AMG identified by the Group ID, as described in Section 3. Since an AMG is associated with a single IP address family, all AMP Measurement Group sub-TLVs advertising a given Group ID MUST carry an endpoint address of that AMG's address family; a sub-TLV whose endpoint address family does not match that of the AMG identified by the Group ID MUST be ignored. Within a single Router CAPABILITY TLV, if multiple sub-TLVs have the same Group ID and IP endpoint, only the first is used. Since two such sub-TLVs convey identical membership information, this has no effect on the resulting measurement group membership. Section 3 of [RFC7981] leaves the choice undefined when a receiving system holds two copies of a Router CAPABILITY TLV from the same system with conflicting information for a given sub-TLV; no additional procedure is defined here.¶

The AMP Measurement Group sub-TLV MUST be advertised in an IS-IS Router CAPABILITY TLV with the S flag set, so that the TLV is flooded across the entire routing domain as specified in Section 2 of [RFC7981]. As specified in Section 3 of [RFC7981], a router advertising capabilities with different flooding scopes originates a separate Router CAPABILITY TLV for each scope; consequently, the AMP Measurement Group sub-TLV MUST NOT be advertised in a Router CAPABILITY TLV with the S flag clear. A receiving router processes the sub-TLV as described in Section 5, subject to the elements of procedure in Section 3 of [RFC7981].¶

Leaking of the Router CAPABILITY TLV between IS-IS levels, including the setting of the D bit when the TLV is leaked from Level 2 to Level 1 and the prohibition on leaking a TLV with the D bit set from Level 1 to Level 2, is performed as specified in Sections 2 and 3 of [RFC7981]. This document defines no additional leaking procedures. As specified in Section 4 of [RFC7981], a router performing the leaking leaks the entire Router CAPABILITY TLV without change even if it does not support the AMP Measurement Group sub-TLV; a leaking router therefore MUST NOT remove AMP Measurement Group sub-TLVs from a TLV it leaks.¶

As specified in Section 2 of [RFC7981], a single IS-IS Router CAPABILITY TLV carries at most 250 octets of sub-TLVs, and more than one Router CAPABILITY TLV from the same source may be present. A router with more measurement group memberships than will fit in one TLV MUST advertise additional instances of the Router CAPABILITY TLV, each with the S flag set. A receiving router MUST process the AMP Measurement Group sub-TLVs carried in all such instances as a single set of advertisements. See [RFC9885] for the multi-part TLV semantics corresponding to the MP value registered in Section 7.¶

5. Operations

A router that participates in one or more AMGs MUST advertise the AMP Measurement Group sub-TLV in its Router CAPABILITY TLV. For each IP endpoint that participates in an AMG, the router MUST include an instance of the sub-TLV with the Group ID corresponding to that AMG.¶

If an endpoint participates in AMGs associated with different AMPs (e.g., one AMG for TWAMP and another for STAMP), the router advertises one sub-TLV instance per AMG, each with the corresponding Group ID. The protocols themselves are not advertised.¶

An AMP session is identified by the AMG and the pair of IP endpoints between which it is established. A router MUST NOT establish more than one AMP session for a given AMG between the same pair of IP endpoints. Where two routers have IP endpoints in more than one common AMG, a separate AMP session is established for each such AMG, since distinct AMGs may be associated with distinct AMPs or with distinct non-default AMP parameters.¶

A router MUST NOT use an AMP Measurement Group sub-TLV carried in a Router CAPABILITY TLV that, per Section 3 of [RFC7981], is not to be used: that is, one present in an LSP of a system that is not currently reachable via Level x paths, where "x" is the level in which the sending system advertised the TLV, or one whose Router ID is 0.0.0.0 with no IPv6 TE Router ID sub-TLV present. Where an AMP session has already been established as a result of such an advertisement, the receiving router SHOULD tear the session down.¶

When a router receives a usable AMP Measurement Group sub-TLV, it MUST determine whether it has an IP endpoint configured as a member of the AMG identified by the Group ID, and whether the advertised endpoint address is of that AMG's address family. If either is not the case, the sub-TLV MUST be ignored. Otherwise, if no AMP session for that AMG has already been established between the two IP endpoints, the receiving router attempts to establish one. When both IP endpoints attempt to establish a session for the same AMG, the session initiated by the endpoint with the greater IP address takes precedence, where the two addresses are compared as unsigned octet strings. Since both endpoints belong to the same AMG, they are of the same address family and the comparison is therefore always between addresses of equal length.¶

Because the AMP Measurement Group sub-TLV is advertised in a Router CAPABILITY TLV with the S flag set, it is flooded across the entire routing domain and is leaked between levels by L1/L2 routers as specified in [RFC7981]. This satisfies the requirement for discovery of measurement group members that are not in the same IS-IS level as the advertising router. As noted in Section 4 of [RFC7981], at least one L1/L2 router in every area of the domain must support the Router CAPABILITY TLV for domain-wide flooding of TLVs originated by L1 routers to work.¶

If a router's measurement group membership changes, it MUST update its Router CAPABILITY TLV advertisement accordingly. Routers receiving updated information MUST process the changes and update their record of measurement group membership.¶

6. Inter-IGP Domain Discovery of Active Measurement Groups

Inter-IGP domain discovery of AMGs is expected to use BGP-LS [RFC9552], with IGP capability TLVs defined for BGP-LS similar to what has been done for Segment Routing [RFC9085]. Specification of those extensions is out of scope for this document.¶

7. IANA Considerations

8. Security Considerations

This document defines a mechanism for advertising measurement group membership in IS-IS. The security considerations for IS-IS as specified in [ISO10589] and [RFC1195] apply.¶

An attacker that can inject false AMP Measurement Group sub-TLVs could cause routers to attempt to establish measurement sessions with incorrect endpoints, potentially leading to:¶

• Failed measurement sessions¶
• Misconfiguration of measurement infrastructure¶
• Resource exhaustion if many false endpoints are advertised¶
• Measurement traffic directed at a third party, if an attacker advertises an address belonging to a victim as an endpoint of a widely deployed AMG¶

To mitigate these risks, and as recommended in Section 5 of [RFC7981] for information carried in the Router CAPABILITY TLV, an integrity mechanism such as those specified in [RFC5304] or [RFC5310] SHOULD be applied to IS-IS protocol exchanges. Additionally, operators SHOULD configure appropriate access controls and monitoring to detect and prevent unauthorized advertisements.¶

The information advertised in the AMP Measurement Group sub-TLV reveals which routers are participating in measurement groups and which interface addresses are used for measurement purposes. This information may be considered sensitive in some deployments. Since the sub-TLV is advertised with the S flag set, this information is flooded throughout the IS-IS routing domain rather than being confined to the level in which it originates, which widens the set of routers to which it is disclosed. Operators should consider the implications of this information disclosure when deploying this mechanism.¶

Acknowledgements

Thanks to Rakesh Gandhi for discussion of requirements.¶

Authors' Addresses