RFC 8306
Extensions to the Path Computation Element Communication Protocol (PCEP) for Point-to-Multipoint Traffic Engineering Label Switched Paths
3.6. P2MP Objective Functions and Metric Types
3.6.1. Objective Functions
Six objective functions have been defined in [RFC5541] for P2P path computation. This document defines two additional objective functions -- namely, SPT (Shortest-Path Tree) and MCT (Minimum-Cost Tree) -- that apply to P2MP path computation. Hence, two objective function codes are defined as follows: Objective Function Code: 7 Name: Shortest-Path Tree (SPT) Description: Minimize the maximum source-to-leaf cost with respect to a specific metric or to the TE metric used as the default metric when the metric is not specified (e.g., TE or IGP metric). Objective Function Code: 8 Name: Minimum-Cost Tree (MCT) Description: Minimize the total cost of the tree (i.e., the sum of the costs of tree links) with respect to a specific metric or to the TE metric used as the default metric when the metric is not specified. Processing these two objective functions is subject to the rules defined in [RFC5541].
3.6.2. METRIC Object-Type Values
There are three types defined for the METRIC object in [RFC5440] -- namely, the IGP metric, the TE metric, and Hop Counts. This document defines three additional types for the METRIC object: the P2MP IGP metric, the P2MP TE metric, and the P2MP hop count metric. They encode the sum of the metrics of all links of the tree. The following values for these metric types have been assigned; see Section 6.4. o P2MP IGP metric: T=8 o P2MP TE metric: T=9 o P2MP hop count metric: T=103.7. Non-Support of P2MP Path Computation
o If a PCE receives a P2MP path computation request and it understands the P2MP flag in the RP object, but the PCE is not capable of P2MP computation, the PCE MUST send a PCErr message with a PCEP-ERROR object and corresponding Error-value. The request MUST then be cancelled at the PCC. The Error-Types and Error-values have been assigned; see Section 6 ("IANA Considerations") of this document. o If the PCE does not understand the P2MP flag in the RP object, then the PCE would send a PCErr message with Error-Type=2 (Capability not supported) as per [RFC5440].3.8. Non-Support by Back-Level PCE Implementations
If a PCE receives a P2MP request and the PCE does not understand the P2MP flag in the RP object, and therefore the PCEP P2MP extensions, then the PCE SHOULD reject the request.3.9. P2MP TE Path Reoptimization Request
A reoptimization request for a P2MP TE path is specified by the use of the R-bit within the RP object as defined in [RFC5440] and is similar to the reoptimization request for a P2P TE path. The only difference is that the PCC MUST insert the list of Record Route Objects (RROs) and SRROs after each instance of the END-POINTS object in the PCReq message, as described in Section 3.4 ("Request Message Format") of this document.
An example of a reoptimization request and subsequent PCReq message is described below: Common Header RP with P2MP flag/R-bit set END-POINTS for leaf type 3 RRO list OF (optional) Figure 5: PCReq Message Example 1 for Optimization In this example, we request reoptimization of the path to all leaves without adding or pruning leaves. The reoptimization request would use an END-POINTS object with leaf type 3. The RRO list would represent the P2MP LSP before the optimization, and the modifiable path leaves would be indicated in the END-POINTS object. It is also possible to specify distinct leaves whose path cannot be modified. An example of the PCReq message in this scenario would be: Common Header RP with P2MP flag/R-bit set END-POINTS for leaf type 3 RRO list END-POINTS for leaf type 4 RRO list OF (optional) Figure 6: PCReq Message Example 2 for Optimization3.10. Adding and Pruning Leaves to/from the P2MP Tree
When adding new leaves to or removing old leaves from the existing P2MP tree, by supplying a list of existing leaves, it is possible to optimize the existing P2MP tree. This section explains the methods for adding new leaves to or removing old leaves from the existing P2MP tree. To add new leaves, the PCC MUST build a P2MP request using END-POINTS with leaf type 1. To remove old leaves, the PCC MUST build a P2MP request using END-POINTS with leaf type 2. If no type-2 END-POINTS exist, then the PCE MUST send Error-Type 17, Error-value 1: the PCE cannot satisfy the request due to no END-POINTS with leaf type 2.
When adding new leaves to or removing old leaves from the existing P2MP tree, the PCC MUST also provide the list of old leaves, if any, including END-POINTS with leaf type 3, leaf type 4, or both. Specific PCEP-ERROR objects and types are used when certain conditions are not satisfied (i.e., when there are no END-POINTS with leaf type 3 or 4, or in the presence of END-POINTS with leaf type 1 or 2). A generic "Inconsistent END-POINTS" error will be used if a PCC receives a request that has an inconsistent END-POINTS setting (i.e., if a leaf specified as type 1 already exists). These IANA assignments are documented in Section 6.6 ("PCEP-ERROR Objects and Types") of this document. For old leaves, the PCC MUST provide the old path as a list of RROs that immediately follows each END-POINTS object. This document specifies Error-values when specific conditions are not satisfied. The following examples demonstrate full and partial reoptimization of existing P2MP LSPs: Case 1: Adding leaves with full reoptimization of existing paths Common Header RP with P2MP flag/R-bit set END-POINTS for leaf type 1 RRO list END-POINTS for leaf type 3 RRO list OF (optional) Case 2: Adding leaves with partial reoptimization of existing paths Common Header RP with P2MP flag/R-bit set END-POINTS for leaf type 1 END-POINTS for leaf type 3 RRO list END-POINTS for leaf type 4 RRO list OF (optional)
Case 3: Adding leaves without reoptimization of existing paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 1
RRO list
END-POINTS for leaf type 4
RRO list
OF (optional)
Case 4: Pruning leaves with full reoptimization of existing paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 2
RRO list
END-POINTS for leaf type 3
RRO list
OF (optional)
Case 5: Pruning leaves with partial reoptimization of existing paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 2
RRO list
END-POINTS for leaf type 3
RRO list
END-POINTS for leaf type 4
RRO list
OF (optional)
Case 6: Pruning leaves without reoptimization of existing paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 2
RRO list
END-POINTS for leaf type 4
RRO list
OF (optional)
Case 7: Adding and pruning leaves with full reoptimization of
existing paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 1
END-POINTS for leaf type 2
RRO list
END-POINTS for leaf type 3
RRO list
OF (optional)
Case 8: Adding and pruning leaves with partial reoptimization of
existing paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 1
END-POINTS for leaf type 2
RRO list
END-POINTS for leaf type 3
RRO list
END-POINTS for leaf type 4
RRO list
OF (optional)
Case 9: Adding and pruning leaves without reoptimization of existing
paths
Common Header
RP with P2MP flag/R-bit set
END-POINTS for leaf type 1
END-POINTS for leaf type 2
RRO list
END-POINTS for leaf type 4
RRO list
OF (optional)
3.11. Discovering Branch Nodes
Before computing the P2MP path, a PCE may need to be provided means to know which nodes in the network are capable of acting as branch LSRs. A PCE can discover such capabilities by using the mechanisms defined in [RFC5073].3.11.1. Branch Node Object
The PCC can specify a list of nodes that can be used as branch nodes or a list of nodes that cannot be used as branch nodes by using the Branch Node Capability (BNC) object. The BNC object has the same format as the Include Route Object (IRO) as defined in [RFC5440], except that it only supports IPv4 and IPv6 prefix sub-objects. Two Object-Type parameters are also defined: o Branch node list: List of nodes that can be used as branch nodes. o Non-branch node list: List of nodes that cannot be used as branch nodes. The object can only be carried in a PCReq message. A path computation request may carry at most one Branch Node object. The Object-Class and Object-Type values have been allocated by IANA. The IANA assignments are documented in Section 6.5 ("PCEP Objects").3.12. Synchronization of P2MP TE Path Computation Requests
There are cases when multiple P2MP LSPs' computations need to be synchronized. For example, one P2MP LSP is the designated backup of another P2MP LSP. In this case, path diversity for these dependent LSPs may need to be considered during the path computation. The synchronization can be done by using the existing SVEC functionality as defined in [RFC5440].
An example of synchronizing two P2MP LSPs, each having two leaves for Path Computation Request messages, is illustrated below: Common Header SVEC for sync of LSP1 and LSP2 OF (optional) RP for LSP1 END-POINTS1 for LSP1 RRO1 list RP for LSP2 END-POINTS2 for LSP2 RRO2 list Figure 7: PCReq Message Example for Synchronization This specification also defines two flags for the SVEC Object Flag Field for P2MP path-dependent computation requests. The first flag allows the PCC to request that the PCE should compute a secondary P2MP path tree with partial path diversity for specific leaves or a specific S2L sub-path to the primary P2MP path tree. The second flag allows the PCC to request that partial paths should be link direction diverse. The following flags are added to the SVEC object body in this document: o P (Partial Path Diverse bit - 1 bit): When set, this would indicate a request for path diversity for a specific leaf, a set of leaves, or all leaves. o D (Link Direction Diverse bit - 1 bit): When set, this would indicate a request that a partial path or paths should be link direction diverse. The IANA assignments are referenced in Section 6.8 of this document.3.13. Request and Response Fragmentation
The total PCEP message length, including the common header, is 16 bytes. In certain scenarios, the P2MP computation request may not fit into a single request or response message. For example, if a tree has many hundreds or thousands of leaves, then the request or response may need to be fragmented into multiple messages.
The F-bit is outlined in Section 3.3.1 ("The Extension of the RP Object") of this document. The F-bit is used in the RP object to signal that the initial request or response was too large to fit into a single message and will be fragmented into multiple messages. In order to identify the single request or response, each message will use the same request ID.3.13.1. Request Fragmentation Procedure
If the initial request is too large to fit into a single request message, the PCC will split the request over multiple messages. Each message sent to the PCE, except the last one, will have the F-bit set in the RP object to signify that the request has been fragmented into multiple messages. In order to identify that a series of request messages represents a single request, each message will use the same request ID. The assumption is that request messages are reliably delivered and in sequence, since PCEP relies on TCP.3.13.2. Response Fragmentation Procedure
Once the PCE computes a path based on the initial request, a response is sent back to the PCC. If the response is too large to fit into a single response message, the PCE will split the response over multiple messages. Each message sent by the PCE, except the last one, will have the F-bit set in the RP object to signify that the response has been fragmented into multiple messages. In order to identify that a series of response messages represents a single response, each message will use the same response ID. Again, the assumption is that response messages are reliably delivered and in sequence, since PCEP relies on TCP.3.13.3. Fragmentation Example
The following example illustrates the PCC sending a request message with Req-ID1 to the PCE, in order to add one leaf to an existing tree with 1200 leaves. The assumption used for this example is that one request message can hold up to 800 leaves. In this scenario, the original single message needs to be fragmented and sent using two smaller messages, which have Req-ID1 specified in the RP object, and with the F-bit set on the first message and the F-bit cleared on the second message.
Common Header RP1 with Req-ID1 and P2MP=1 and F-bit=1 OF (optional) END-POINTS1 for P2MP RRO1 list Common Header RP2 with Req-ID1 and P2MP=1 and F-bit=0 OF (optional) END-POINTS1 for P2MP RRO1 list Figure 8: PCReq Message Fragmentation Example To handle a scenario where the last fragmented message piece is lost, the receiver side of the fragmented message may start a timer once it receives the first piece of the fragmented message. If the timer expires and it still has not received the last piece of the fragmented message, it should send an error message to the sender to signal that it has received an incomplete message. The relevant error message is documented in Section 3.15 ("P2MP PCEP-ERROR Objects and Types").3.14. UNREACH-DESTINATION Object
The PCE path computation request may fail because all or a subset of the destinations are unreachable. In such a case, the UNREACH-DESTINATION object allows the PCE to optionally specify the list of unreachable destinations. This object can be present in PCRep messages. There can be up to one such object per RP.
The following UNREACH-DESTINATION objects (for IPv4 and IPv6) are
defined:
UNREACH-DESTINATION Object-Class is 28.
UNREACH-DESTINATION Object-Type for IPv4 is 1.
UNREACH-DESTINATION Object-Type for IPv6 is 2.
The format of the UNREACH-DESTINATION object body for IPv4
(Object-Type=1) is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IPv4 address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: UNREACH-DESTINATION Object Body for IPv4
The format of the UNREACH-DESTINATION object body for IPv6
(Object-Type=2) is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Destination IPv6 address (16 bytes) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Destination IPv6 address (16 bytes) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: UNREACH-DESTINATION Object Body for IPv6
3.15. P2MP PCEP-ERROR Objects and Types
To indicate an error associated with a policy violation, the Error-value "P2MP Path computation is not allowed" has been added to the existing error code for Error-Type 5 ("Policy violation") as defined in [RFC5440] (see also Section 6.6 of this document): Error-Type=5; Error-value=7: if a PCE receives a P2MP path computation request that is not compliant with administrative privileges (i.e., "The PCE policy does not support P2MP path computation"), the PCE MUST send a PCErr message with a PCEP-ERROR object (Error-Type=5) and an Error-value of 7. The corresponding P2MP path computation request MUST also be cancelled. To indicate capability errors associated with the P2MP path computation request, Error-Type (16) and subsequent Error-values are defined as follows for inclusion in the PCEP-ERROR object: Error-Type=16; Error-value=1: if a PCE receives a P2MP path computation request and the PCE is not capable of satisfying the request due to insufficient memory, the PCE MUST send a PCErr message with a PCEP-ERROR object (Error-Type=16) and an Error-value of 1. The corresponding P2MP path computation request MUST also be cancelled. Error-Type=16; Error-value=2: if a PCE receives a P2MP path computation request and the PCE is not capable of P2MP computation, the PCE MUST send a PCErr message with a PCEP-ERROR object (Error-Type=16) and an Error-value of 2. The corresponding P2MP path computation request MUST also be cancelled. To indicate P2MP message fragmentation errors associated with a P2MP path computation request, Error-Type (18) and subsequent Error-values are defined as follows for inclusion in the PCEP-ERROR object: Error-Type=18; Error-value=1: if a PCE has not received the last piece of the fragmented message, it should send an error message to the sender to signal that it has received an incomplete message (i.e., "Fragmented request failure"). The PCE MUST send a PCErr message with a PCEP-ERROR object (Error-Type=18) and an Error-value of 1.
3.16. PCEP NO-PATH Indicator
To communicate the reasons for not being able to find a P2MP path computation, the NO-PATH object can be used in the PCRep message. One bit is defined in the NO-PATH-VECTOR TLV carried in the NO-PATH object: bit 24: when set, the PCE indicates that there is a reachability problem with all or a subset of the P2MP destinations. Optionally, the PCE can specify the destination or list of destinations that are not reachable using the UNREACH-DESTINATION object defined in Section 3.14.4. Manageability Considerations
[RFC5862] describes various manageability requirements in support of P2MP path computation when applying PCEP. This section describes how manageability requirements mentioned in [RFC5862] are supported in the context of PCEP extensions specified in this document. Note that [RFC5440] describes various manageability considerations for PCEP, and most of the manageability requirements mentioned in [RFC5862] are already covered there.4.1. Control of Function and Policy
In addition to PCE configuration parameters listed in [RFC5440], the following additional parameters might be required: o The PCE may be configured to enable or disable P2MP path computations. o The PCE may be configured to enable or disable the advertisement of its P2MP path computation capability. A PCE can advertise its P2MP capability via the IGP discovery mechanism discussed in Section 3.1.1 ("IGP Extensions for P2MP Capability Advertisement") or during the Open Message Exchange discussed in Section 3.1.2 ("Open Message Extension").4.2. Information and Data Models
A number of MIB objects have been defined in [RFC7420] for general PCEP control and monitoring of P2P computations. [RFC5862] specifies that MIB objects will be required to support the control and monitoring of the protocol extensions defined in this document. A new document will be required to define MIB objects for PCEP control and monitoring of P2MP computations.
The "ietf-pcep" PCEP YANG module is specified in [PCEP-YANG]. The P2MP capability of a PCEP entity or a configured peer can be set using this YANG module. Also, support for P2MP path computation can be learned using this module. The statistics are maintained in the "ietf-pcep-stats" YANG module as specified in [PCEP-YANG]. This YANG module will be required to be augmented to also include the P2MP-related statistics.4.3. Liveness Detection and Monitoring
There are no additional considerations beyond those expressed in [RFC5440], since [RFC5862] does not address any additional requirements.4.4. Verifying Correct Operation
There are no additional requirements beyond those expressed in [RFC4657] for verifying the correct operation of the PCEP sessions. It is expected that future MIB objects will facilitate verification of correct operation and reporting of P2MP PCEP requests, responses, and errors.4.5. Requirements for Other Protocols and Functional Components
The method for the PCE to obtain information about a PCE capable of P2MP path computations via OSPF and IS-IS is discussed in Section 3.1.1 ("IGP Extensions for P2MP Capability Advertisement") of this document. The relevant IANA assignment is documented in Section 6.9 ("OSPF PCE Capability Flag") of this document.4.6. Impact on Network Operation
It is expected that the use of PCEP extensions specified in this document will not significantly increase the level of operational traffic. However, computing a P2MP tree may require more PCE state compared to a P2P computation. In the event of a major network failure and multiple recovery P2MP tree computation requests being sent to the PCE, the load on the PCE may also be significantly increased.
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