# Extensions to the Path Computation Element Communication Protocol (PCEP) for Point-to-Multipoint Traffic Engineering Label Switched Paths

Pages: 43
Proposed Standard
Errata
Obsoletes:  6006
Part 2 of 3 – Pages 16 to 29

```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=10

3.7.  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:

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:

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 Optimization

3.10.  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

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

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

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

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

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

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

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

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

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:

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

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

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~                           ...                                 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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
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
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
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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