LDP Extensions to Support Maximally Redundant Trees

This document describes the LDP signaling extension and associated behavior necessary to support the architecture that defines how IP/LDP Fast-Reroute can use MRTs . It is necessary to read the architecture in to understand how and why the LDP extensions for behavior are needed. At least one common standardized algorithm, such as the lowpoint algorithm explained and fully documented in , is required so that the routers supporting MRT computation consistently compute the same MRTs. LDP depends on the IGP to compute the MRTs and alternates. Extensions to OSPF are defined in . Extension to IS-IS are defined in MRT can also be used to protect multicast traffic via either global protection or local protection. An MRT path can be used to provide node-protection for mLDP traffic via the mechanisms described in ; an MRT path can also be use to provide link protection for mLDP traffic. For each destination, IP/LDP Fast-Reroute with MRT (MRT-FRR) creates two alternate destination-based trees separate from the primary next-hop forwarding used during stable operation. LDP uses the multi-topology extensions to signal FECs for these two new forwarding topologies, known as MRT-Blue and MRT-Red. In order to create MRT paths and support IP/LDP Fast-Reroute, a new capability extension is needed for LDP. An LDP implementation supporting MRT must also follow the described rules for originating and managing FECs related to MRT, as indicated by their multi-topology ID. Network reconvergence is described in and the worst-cast network convergence time can be flooded via the extension in Section 7 of . IP/LDP Fast-Reroute using MRTs can provide 100% coverage for link and node failures in an arbitrary network topology where the failure doesn't split the network. It can also be deployed incrementally; an MRT Island is formed of connected supporting routers and the MRTs are computed inside that island.

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

For ease of reading, some of the terminology defined in is repeated here. A pair of trees where the path from any node X to the root R along the first tree is node-disjoint with the path from the same node X to the root along the second tree. These can be computed in 2-connected graphs. A pair of trees where the path from any node X to the root R along the first tree and the path from the same node X to the root along the second tree share the minimum number of nodes and the minimum number of links. Each such shared node is a cut-vertex. Any shared links are cut-links. Any RT is an MRT but many MRTs are not RTs. The two MRTs are referred to as MRT-Blue and MRT-Red. From the computing router, the set of routers that support a particular MRT profile and are connected via MRT-eligible links. MRT-Red is used to describe one of the two MRTs; it is used to described the associated forwarding topology and MT-ID. Specifically, MRT-Red is the decreasing MRT where links in the GADAG are taken in the direction from a higher topologically ordered node to a lower one. MRT-Blue is used to describe one of the two MRTs; it is used to described the associated forwarding topology and MT-ID. Specifically, MRT-Blue is the increasing MRT where links in the GADAG are taken in the direction from a lower topologically ordered node to a higher one. It is useful to have an MT-ID that refers to the multiple MRT topologies and to the default topology. This is referred to as the Rainbow MRT MT-ID and is used by LDP to reduce signaling and permit the same label to always be advertised to all peers for the same (MT-ID, Prefix).

Routers need to know which of their neighbors support MRT. Supporting MRT indicates several different aspects of behavior, as listed below. Support for Multi-Topology (MT) - this MAY also be indicated via the Multi-Capability MT Capability . Understand the Rainbow MRT MT-ID and apply the associated labels to all relevant MT-IDs. Advertise the Rainbow MRT MT-ID to the appropriate neighbors for the associated prefix. If acting as LDP egress for a prefix in the default topology, also advertise and act as egress for the same prefix in MRT-Red and MRT-Blue. For a FEC learned from a neighbor that does not support MRT, originate FECS for MRT-Red and MRT-Blue with the same prefix. This MRT Island egress behavior is to support an MRT Island that does not include all routers in the area/level.

It is not possible to support MRT without supporting the LDP multi-topology extensions, but it is possible that the only use of the multi-topology extensions is for MRT. In that case, a router MAY not negotiate the multi-topology capability and only negotiate the MRT Capability with its LDP peer. Negotiation of the MT capability is not required with negotiation of the MRT capability. [EDITOR NOTE: How do we deal with different abilities for IPv4 and IPv6? The MT capability has the Wildcard FEC to indicate this. Do we just assume??] A new MRT Capability Parameter TLV is defined, which is defined in accordance with LDP Capability definition guidelines. The LDP MRT capability can be advertised during the LDP session initialization or after the LDP session is established. Advertisement of the MRT capability indicates support of the procedures for establishing the MRT-Blue and MRT-Red LSP paths detailed in this document. If the peer has not advertised the MRT capability, then it indicates that LSR does not support MRT procedures. If a router advertises the LDP MRT capability to its peer, but the peer has not advertised the MRT capability, then the router MUST NOT advertise MRT-related FEC-label bindings to that peer, until that peer starts to advertise the MRT capability. The following is the format of the MRT Capability Parameter.

Where: MUST be 1 and 0, respectively, as per Section 3. (Signaling Extensions) of LDP Capabilities . TBA-MRT-LDP-1 (To Be Allocated by IANA) MUST be 1 if used in LDP "Initialization" message. MAY be set to 0 or 1 in dynamic "Capability" message to advertise or withdraw the capability respectively. The length (in octets) of TLV. Its value is 1.

In Section 10.1 of , the need to advertise different MPLS labels to different neighbors for the same FEC is described. This can be shortly summarized as either advertising MRT MT-ID differentiated labels to a neighbor or just advertising the same MPLS label for the default topology, for MRT-Red and MRT-Blue. MRT-supporting neighbors in the same domain as the default SPT next-hop get the differentiated MPLS labels; all other neighbors do not. A second use for the Rainbow MRT MT-ID is for an egress LER to send the Rainbow MRT MT-ID with an IMPLICIT_NULL label to indicate penultimate-hop-popping for all three types of FECs (IP Prefix FEC, MRT-Blue MT-IP Prefix FEC, and MRT-Red MT-IP Prefix FEC). The use of the Rainbow-FEC by the ABR for non-best-area advertisements is RECOMMENDED. An ABR MAY advertise the label for the default topology in separate MRT-Blue and MRT-Red advertisements. An LSR advertising the MRT capability MUST recognize the Rainbow MRT MT-ID and associate the advertised label with the specific prefix with the MRT-Red and MRT-Blue MT-IDs associated with all MRT Profiles that advertise LDP as the forwarding mechanism. The value of the Rainbow MRT MT-ID (TBA-MRT-LDP-2) will be assigned by IANA from the LDP MT-ID space. Prototype experiments have used the value 3999.

To provide MRT support in LDP, the MT Prefix FEC is used. contains the IANA request for the MRT-Red and MRT-Blue MT-IDs associated with the Default MRT Profile. The MT Prefix FEC encoding is defined in and is used without alteration for signaling MRT-Blue, MRT-Red and Rainbow MRT FECs.

This sections describes how and when labels for MRT-Red and MRT-Blue FECs are advertised. The associated LSPs must be created before a failure occurs, in order to provide protection paths which are immediately usable by a PLR.

If the upstream session is negotiated with the MRT capability, the Egress LER advertises via a Rainbow MRT FEC an allocated MPLS label; this may be Explicit Null, Implicit Null, or another value. Based on the MRT algorithm , the IGP computes the MRT-Red and MRT-Blue disjoint paths at Ingress and Transit LSRs. Once the IGP computes the MRT-Red and MRT-Blue next-hops, LDP will advertise the Label Mapping for the MRT-Blue and MRT-Red FECs. If a label is received from a downstream LSR for an MRT-Red or MRT-Blue FEC where the downstream LSR is capable of MRT, the MRT-Red FEC or MRT-Blue FEC label is swapped according to the received downstream label. An LSR may also choose to use the MRT-Red or MRT-Blue path as an alternate for doing fast-reroute for the local traffic. When a downstream router is not capable of MRT, the LSR is an MRT Island Border Router (IBR) and SHOULD advertise Label Bindings for the MRT-Red FEC and MRT-Blue FEC as well as the associated normal topology. The normal topology's primary next-hops will be used to forward traffic received for the MRT-Red FEC or the MRT-Blue FEC where the FEC's destination is outside the MRT Island. This functionality is critical for partial deployment scenarios.

After the IGP computes the MRT-Red and MRT-Blue paths, the IGP MAY also decide to use either the MRT-Red or MRT-Blue path as a fast-reroute alternate for the particular FEC. If so, then when in Downstream On Demand Mode, the LSR sends a Label Request for either the MRT-Red or MRT-Blue FEC to the downstream LSR. The downstream LSR responds by either sending a Label Mapping if available or by sending a Label Request to its downstream LSR. Once a Label Mapping is received, the associated label may be used as a fast-reroute alternate to forward IP and LDP traffic. A Label Mapping may be available in the following circumstances: The LSR is acting as Egress A Label Mapping was already received from its downstream router A Label Mapping for the default topology FEC was received and the downstream router is not capable of MRT or is in a different MRT Island.

As discussed in , the Rainbow MRT FEC is defined to facilitate signaling the same label for multiple topologies. Section 9 of recommends that traffic leaving an OSPF area or IS-IS level SHOULD use the default topology's shortest-path-tree next-hops instead of remaining on the MRT-Red or MRT-Blue paths. If an LDP peer is in the same OSPF area or IS-IS level as the primary next-hop, then LDP SHOULD advertise different label values for a given set of MRT-Red FEC, MRT-Blue FEC, and FEC, unless Explicit-Null or Implicit-Null is appropriate. If an LDP peer is in a different OSPF area or IS-IS level from the primary next-hop, then LDP SHOULD either advertise the same label value for the given set of MRT-Red FEC, MRT-Blue FEC, and FEC or advertise a single label for the Rainbow MRT FEC, whose behavior is defined in .

This LDP extension is not believed to introduce new security concerns. It relies upon the security architecture already provided for LDP.

Please allocate a value for the new LDP Capability TLV from the LDP registry "TLV Type Name Space": MRT Capability TLV (TBA-MRT-LDP-1). Please allocate a value from the LDP Multi-Topology (MT) ID Name Space : Rainbow MRT MT-ID (TBA-MRT-LDP-2).

The authors would like to thank Ross Callon for his suggestions.