RFC 5440
Path Computation Element (PCE) Communication Protocol (PCEP)
5. Transport Protocol
PCEP operates over TCP using a registered TCP port (4189). This allows the requirements of reliable messaging and flow control to be met without further protocol work. All PCEP messages MUST be sent using the registered TCP port for the source and destination TCP port.6. PCEP Messages
A PCEP message consists of a common header followed by a variable- length body made of a set of objects that can either be mandatory or optional. In the context of this document, an object is said to be mandatory in a PCEP message when the object MUST be included for the message to be considered valid. A PCEP message with a missing mandatory object MUST trigger an Error message (see Section 7.15). Conversely, if an object is optional, the object may or may not be present. A flag referred to as the P flag is defined in the common header of each PCEP object (see Section 7.2). When this flag is set in an object in a PCReq, the PCE MUST take the information carried in the object into account during the path computation. For example, the METRIC object defined in Section 7.8 allows a PCC to specify a bounded acceptable path cost. The METRIC object is optional, but a PCC may set a flag to ensure that the constraint is taken into account. In this case, if the constraint cannot be taken into account by the PCE, the PCE MUST trigger an Error message. For each PCEP message type, rules are defined that specify the set of objects that the message can carry. We use the Backus-Naur Form (BNF) (see [RBNF]) to specify such rules. Square brackets refer to optional sub-sequences. An implementation MUST form the PCEP messages using the object ordering specified in this document.
6.1. Common Header
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ver | Flags | Message-Type | Message-Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: PCEP Message Common Header Ver (Version - 3 bits): PCEP version number. Current version is version 1. Flags (5 bits): No flags are currently defined. Unassigned bits are considered as reserved. They MUST be set to zero on transmission and MUST be ignored on receipt. Message-Type (8 bits): The following message types are currently defined: Value Meaning 1 Open 2 Keepalive 3 Path Computation Request 4 Path Computation Reply 5 Notification 6 Error 7 Close Message-Length (16 bits): total length of the PCEP message including the common header, expressed in bytes.6.2. Open Message
The Open message is a PCEP message sent by a PCC to a PCE and by a PCE to a PCC in order to establish a PCEP session. The Message-Type field of the PCEP common header for the Open message is set to 1. Once the TCP connection has been successfully established, the first message sent by the PCC to the PCE or by the PCE to the PCC MUST be an Open message as specified in Appendix A. Any message received prior to an Open message MUST trigger a protocol error condition causing a PCErr message to be sent with Error-Type "PCEP session establishment failure" and Error-value "reception of an invalid Open message or a non Open message" and the PCEP session establishment attempt MUST be terminated by closing the TCP connection.
The Open message is used to establish a PCEP session between the PCEP
peers. During the establishment phase, the PCEP peers exchange
several session characteristics. If both parties agree on such
characteristics, the PCEP session is successfully established.
The format of an Open message is as follows:
<Open Message>::= <Common Header>
<OPEN>
The Open message MUST contain exactly one OPEN object (see
Section 7.3).
Various session characteristics are specified within the OPEN object.
Once the TCP connection has been successfully established, the sender
MUST start an initialization timer called OpenWait after the
expiration of which, if no Open message has been received, it sends a
PCErr message and releases the TCP connection (see Appendix A for
details).
Once an Open message has been sent to a PCEP peer, the sender MUST
start an initialization timer called KeepWait after the expiration of
which, if neither a Keepalive message has been received nor a PCErr
message in case of disagreement of the session characteristics, a
PCErr message MUST be sent and the TCP connection MUST be released
(see Appendix A for details).
The OpenWait and KeepWait timers have a fixed value of 1 minute.
Upon the receipt of an Open message, the receiving PCEP peer MUST
determine whether the suggested PCEP session characteristics are
acceptable. If at least one of the characteristics is not acceptable
to the receiving peer, it MUST send an Error message. The Error
message SHOULD also contain the related OPEN object and, for each
unacceptable session parameter, an acceptable parameter value SHOULD
be proposed in the appropriate field of the OPEN object in place of
the originally proposed value. The PCEP peer MAY decide to resend an
Open message with different session characteristics. If a second
Open message is received with the same set of parameters or with
parameters that are still unacceptable, the receiving peer MUST send
an Error message and it MUST immediately close the TCP connection.
Details about error messages can be found in Section 7.15.
Successive retries are permitted, but an implementation SHOULD make
use of an exponential back-off session establishment retry procedure.
If the PCEP session characteristics are acceptable, the receiving
PCEP peer MUST send a Keepalive message (defined in Section 6.3) that
serves as an acknowledgment.
The PCEP session is considered as established once both PCEP peers have received a Keepalive message from their peer.6.3. Keepalive Message
A Keepalive message is a PCEP message sent by a PCC or a PCE in order to keep the session in active state. The Keepalive message is also used in response to an Open message to acknowledge that an Open message has been received and that the PCEP session characteristics are acceptable. The Message-Type field of the PCEP common header for the Keepalive message is set to 2. The Keepalive message does not contain any object. PCEP has its own keepalive mechanism used to ensure the liveness of the PCEP session. This requires the determination of the frequency at which each PCEP peer sends Keepalive messages. Asymmetric values may be chosen; thus, there is no constraint mandating the use of identical keepalive frequencies by both PCEP peers. The DeadTimer is defined as the period of time after the expiration of which a PCEP peer declares the session down if no PCEP message has been received (Keepalive or any other PCEP message); thus, any PCEP message acts as a Keepalive message. Similarly, there are no constraints mandating the use of identical DeadTimers by both PCEP peers. The minimum Keepalive timer value is 1 second. Deployments SHOULD consider carefully the impact of using low values for the Keepalive timer as these might not give rise to the expected results in periods of temporary network instability. Keepalive messages are sent at the frequency specified in the OPEN object carried within an Open message according to the rules specified in Section 7.3. Because any PCEP message may serve as Keepalive, an implementation may either decide to send Keepalive messages at fixed intervals regardless of whether other PCEP messages might have been sent since the last sent Keepalive message, or may decide to differ the sending of the next Keepalive message based on the time at which the last PCEP message (other than Keepalive) was sent. Note that sending Keepalive messages to keep the session alive is optional, and PCEP peers may decide not to send Keepalive messages once the PCEP session is established; in which case, the peer that does not receive Keepalive messages does not expect to receive them and MUST NOT declare the session as inactive. The format of a Keepalive message is as follows: <Keepalive Message>::= <Common Header>
6.4. Path Computation Request (PCReq) Message
A Path Computation Request message (also referred to as a PCReq message) is a PCEP message sent by a PCC to a PCE to request a path computation. A PCReq message may carry more than one path computation request. The Message-Type field of the PCEP common header for the PCReq message is set to 3. There are two mandatory objects that MUST be included within a PCReq message: the RP and the END-POINTS objects (see Section 7). If one or both of these objects is missing, the receiving PCE MUST send an error message to the requesting PCC. Other objects are optional. The format of a PCReq message is as follows: <PCReq Message>::= <Common Header> [<svec-list>] <request-list> where: <svec-list>::=<SVEC>[<svec-list>] <request-list>::=<request>[<request-list>] <request>::= <RP> <END-POINTS> [<LSPA>] [<BANDWIDTH>] [<metric-list>] [<RRO>[<BANDWIDTH>]] [<IRO>] [<LOAD-BALANCING>] where: <metric-list>::=<METRIC>[<metric-list>] The SVEC, RP, END-POINTS, LSPA, BANDWIDTH, METRIC, RRO, IRO, and LOAD-BALANCING objects are defined in Section 7. The special case of two BANDWIDTH objects is discussed in detail in Section 7.7. A PCEP implementation is free to process received requests in any order. For example, the requests may be processed in the order they are received, reordered and assigned priority according to local policy, reordered according to the priority encoded in the RP object (Section 7.4.1), or processed in parallel.
6.5. Path Computation Reply (PCRep) Message
The PCEP Path Computation Reply message (also referred to as a PCRep message) is a PCEP message sent by a PCE to a requesting PCC in response to a previously received PCReq message. The Message-Type field of the PCEP common header for the PCRep message is set to 4. The bundling of multiple replies to a set of path computation requests within a single PCRep message is supported by PCEP. If a PCE receives non-synchronized path computation requests by means of one or more PCReq messages from a requesting PCC, it MAY decide to bundle the computed paths within a single PCRep message so as to reduce the control plane load. Note that the counter side of such an approach is the introduction of additional delays for some path computation requests of the set. Conversely, a PCE that receives multiple requests within the same PCReq message MAY decide to provide each computed path in separate PCRep messages or within the same PCRep message. A PCRep message may contain positive and negative replies. A PCRep message may contain a set of computed paths corresponding to either a single path computation request with load-balancing (see Section 7.16) or multiple path computation requests originated by a requesting PCC. The PCRep message may also contain multiple acceptable paths corresponding to the same request. The PCRep message MUST contain at least one RP object. For each reply that is bundled into a single PCReq message, an RP object MUST be included that contains a Request-ID-number identical to the one specified in the RP object carried in the corresponding PCReq message (see Section 7.4 for the definition of the RP object). If the path computation request can be satisfied (i.e., the PCE finds a set of paths that satisfy the set of constraints), the set of computed paths specified by means of Explicit Route Objects (EROs) is inserted in the PCRep message. The ERO is defined in Section 7.9. The situation where multiple computed paths are provided in a PCRep message is discussed in detail in Section 7.13. Furthermore, when a PCC requests the computation of a set of paths for a total amount of bandwidth by means of a LOAD-BALANCING object carried within a PCReq message, the ERO of each computed path may be followed by a BANDWIDTH object as discussed in section Section 7.16. If the path computation request cannot be satisfied, the PCRep message MUST include a NO-PATH object. The NO-PATH object (described in Section 7.5) may also contain other information (e.g, reasons for the path computation failure).
The format of a PCRep message is as follows:
<PCRep Message> ::= <Common Header>
<response-list>
where:
<response-list>::=<response>[<response-list>]
<response>::=<RP>
[<NO-PATH>]
[<attribute-list>]
[<path-list>]
<path-list>::=<path>[<path-list>]
<path>::= <ERO><attribute-list>
where:
<attribute-list>::=[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<IRO>]
<metric-list>::=<METRIC>[<metric-list>]
6.6. Notification (PCNtf) Message
The PCEP Notification message (also referred to as the PCNtf message)
can be sent either by a PCE to a PCC, or by a PCC to a PCE, to notify
of a specific event. The Message-Type field of the PCEP common
header for the PCNtf message is set to 5.
The PCNtf message MUST carry at least one NOTIFICATION object and MAY
contain several NOTIFICATION objects should the PCE or the PCC intend
to notify of multiple events. The NOTIFICATION object is defined in
Section 7.14. The PCNtf message MAY also contain RP objects (see
Section 7.4) when the notification refers to particular path
computation requests.
The PCNtf message may be sent by a PCC or a PCE in response to a
request or in an unsolicited manner.
The format of a PCNtf message is as follows:
<PCNtf Message>::=<Common Header>
<notify-list>
<notify-list>::=<notify> [<notify-list>]
<notify>::= [<request-id-list>]
<notification-list>
<request-id-list>::=<RP>[<request-id-list>]
<notification-list>::=<NOTIFICATION>[<notification-list>]
6.7. Error (PCErr) Message
The PCEP Error message (also referred to as a PCErr message) is sent
in several situations: when a protocol error condition is met or when
the request is not compliant with the PCEP specification (e.g.,
reception of a malformed message, reception of a message with a
mandatory missing object, policy violation, unexpected message,
unknown request reference). The Message-Type field of the PCEP
common header for the PCErr message is set to 6.
The PCErr message is sent by a PCC or a PCE in response to a request
or in an unsolicited manner. If the PCErr message is sent in
response to a request, the PCErr message MUST include the set of RP
objects related to the pending path computation requests that
triggered the error condition. In the latter case (unsolicited), no
RP object is inserted in the PCErr message. For example, no RP
object is inserted in a PCErr when the error condition occurred
during the initialization phase. A PCErr message MUST contain a
PCEP-ERROR object specifying the PCEP error condition. The PCEP-
ERROR object is defined in Section 7.15.
The format of a PCErr message is as follows:
<PCErr Message> ::= <Common Header>
( <error-obj-list> [<Open>] ) | <error>
[<error-list>]
<error-obj-list>::=<PCEP-ERROR>[<error-obj-list>]
<error>::=[<request-id-list>]
<error-obj-list>
<request-id-list>::=<RP>[<request-id-list>]
<error-list>::=<error>[<error-list>] The procedure upon the receipt of a PCErr message is defined in Section 7.15.6.8. Close Message
The Close message is a PCEP message that is either sent by a PCC to a PCE or by a PCE to a PCC in order to close an established PCEP session. The Message-Type field of the PCEP common header for the Close message is set to 7. The format of a Close message is as follows: <Close Message>::= <Common Header> <CLOSE> The Close message MUST contain exactly one CLOSE object (see Section 6.8). If more than one CLOSE object is present, the first MUST be processed and subsequent objects ignored. Upon the receipt of a valid Close message, the receiving PCEP peer MUST cancel all pending requests, it MUST close the TCP connection and MUST NOT send any further PCEP messages on the PCEP session.6.9. Reception of Unknown Messages
A PCEP implementation that receives an unrecognized PCEP message MUST send a PCErr message with Error-value=2 (capability not supported). If a PCC/PCE receives unrecognized messages at a rate equal or greater than MAX-UNKNOWN-MESSAGES unknown message requests per minute, the PCC/PCE MUST send a PCEP CLOSE message with close value="Reception of an unacceptable number of unknown PCEP message". A RECOMMENDED value for MAX-UNKNOWN-MESSAGES is 5. The PCC/PCE MUST close the TCP session and MUST NOT send any further PCEP messages on the PCEP session.7. Object Formats
PCEP objects have a common format. They begin with a common object header (see Section 7.2). This is followed by object-specific fields defined for each different object. The object may also include one or more type-length-value (TLV) encoded data sets. Each TLV has the same structure as described in Section 7.1.
7.1. PCEP TLV Format
A PCEP object may include a set of one or more optional TLVs. All PCEP TLVs have the following format: Type: 2 bytes Length: 2 bytes Value: variable A PCEP object TLV is comprised of 2 bytes for the type, 2 bytes specifying the TLV length, and a value field. The Length field defines the length of the value portion in bytes. The TLV is padded to 4-bytes alignment; padding is not included in the Length field (so a 3-byte value would have a length of 3, but the total size of the TLV would be 8 bytes). Unrecognized TLVs MUST be ignored. IANA management of the PCEP Object TLV type identifier codespace is described in Section 9.7.2. Common Object Header
A PCEP object carried within a PCEP message consists of one or more 32-bit words with a common header that 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Object-Class | OT |Res|P|I| Object Length (bytes) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // (Object body) // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: PCEP Common Object Header Object-Class (8 bits): identifies the PCEP object class. OT (Object-Type - 4 bits): identifies the PCEP object type. The Object-Class and Object-Type fields are managed by IANA. The Object-Class and Object-Type fields uniquely identify each PCEP object.
Res flags (2 bits): Reserved field. This field MUST be set to zero
on transmission and MUST be ignored on receipt.
P flag (Processing-Rule - 1-bit): the P flag allows a PCC to specify
in a PCReq message sent to a PCE whether the object must be taken
into account by the PCE during path computation or is just
optional. When the P flag is set, the object MUST be taken into
account by the PCE. Conversely, when the P flag is cleared, the
object is optional and the PCE is free to ignore it.
I flag (Ignore - 1 bit): the I flag is used by a PCE in a PCRep
message to indicate to a PCC whether or not an optional object was
processed. The PCE MAY include the ignored optional object in its
reply and set the I flag to indicate that the optional object was
ignored during path computation. When the I flag is cleared, the
PCE indicates that the optional object was processed during the
path computation. The setting of the I flag for optional objects
is purely indicative and optional. The I flag has no meaning in a
PCRep message when the P flag has been set in the corresponding
PCReq message.
If the PCE does not understand an object with the P flag set or
understands the object but decides to ignore the object, the entire
PCEP message MUST be rejected and the PCE MUST send a PCErr message
with Error-Type="Unknown Object" or "Not supported Object" along with
the corresponding RP object. Note that if a PCReq includes multiple
requests, only requests for which an object with the P flag set is
unknown/unrecognized MUST be rejected.
Object Length (16 bits): Specifies the total object length including
the header, in bytes. The Object Length field MUST always be a
multiple of 4, and at least 4. The maximum object content length
is 65528 bytes.
7.3. OPEN Object
The OPEN object MUST be present in each Open message and MAY be
present in a PCErr message. There MUST be only one OPEN object per
Open or PCErr message.
The OPEN object contains a set of fields used to specify the PCEP
version, Keepalive frequency, DeadTimer, and PCEP session ID, along
with various flags. The OPEN object may also contain a set of TLVs
used to convey various session characteristics such as the detailed
PCE capabilities, policy rules, and so on. No TLVs are currently
defined.
OPEN Object-Class is 1. OPEN Object-Type is 1. The format of the OPEN object body 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ver | Flags | Keepalive | DeadTimer | SID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Optional TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: OPEN Object Format Ver (3 bits): PCEP version. Current version is 1. Flags (5 bits): No flags are currently defined. Unassigned bits are considered as reserved. They MUST be set to zero on transmission and MUST be ignored on receipt. Keepalive (8 bits): maximum period of time (in seconds) between two consecutive PCEP messages sent by the sender of this message. The minimum value for the Keepalive is 1 second. When set to 0, once the session is established, no further Keepalive messages are sent to the remote peer. A RECOMMENDED value for the keepalive frequency is 30 seconds. DeadTimer (8 bits): specifies the amount of time after the expiration of which the PCEP peer can declare the session with the sender of the Open message to be down if no PCEP message has been received. The DeadTimer SHOULD be set to 0 and MUST be ignored if the Keepalive is set to 0. A RECOMMENDED value for the DeadTimer is 4 times the value of the Keepalive. Example: A sends an Open message to B with Keepalive=10 seconds and DeadTimer=40 seconds. This means that A sends Keepalive messages (or any other PCEP message) to B every 10 seconds and B can declare the PCEP session with A down if no PCEP message has been received from A within any 40-second period.
SID (PCEP session ID - 8 bits): unsigned PCEP session number that
identifies the current session. The SID MUST be incremented each
time a new PCEP session is established. It is used for logging
and troubleshooting purposes. Each increment SHOULD have a value
of 1 and may cause a wrap back to zero.
The SID is used to disambiguate instances of sessions to the same
peer. A PCEP implementation could use a single source of SIDs
across all peers, or one source for each peer. The former might
constrain the implementation to only 256 concurrent sessions. The
latter potentially requires more states. There is one SID number
in each direction.
Optional TLVs may be included within the OPEN object body to specify
PCC or PCE characteristics. The specification of such TLVs is
outside the scope of this document.
When present in an Open message, the OPEN object specifies the
proposed PCEP session characteristics. Upon receiving unacceptable
PCEP session characteristics during the PCEP session initialization
phase, the receiving PCEP peer (PCE) MAY include an OPEN object
within the PCErr message so as to propose alternative acceptable
session characteristic values.
7.4. RP Object
The RP (Request Parameters) object MUST be carried within each PCReq
and PCRep messages and MAY be carried within PCNtf and PCErr
messages. The RP object is used to specify various characteristics
of the path computation request.
The P flag of the RP object MUST be set in PCReq and PCRep messages
and MUST be cleared in PCNtf and PCErr messages. If the RP object is
received with the P flag set incorrectly according to the rules
stated above, the receiving peer MUST send a PCErr message with
Error-Type=10 and Error-value=1. The corresponding path computation
request MUST be cancelled by the PCE without further notification.
7.4.1. Object Definition
RP Object-Class is 2.
RP Object-Type is 1.
The format of the RP object body 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags |O|B|R| Pri | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Request-ID-number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Optional TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: RP Object Body Format The RP object body has a variable length and may contain additional TLVs. No TLVs are currently defined. Flags (32 bits) The following flags are currently defined: o Pri (Priority - 3 bits): the Priority field may be used by the requesting PCC to specify to the PCE the request's priority from 1 to 7. The decision of which priority should be used for a specific request is a local matter; it MUST be set to 0 when unused. Furthermore, the use of the path computation request priority by the PCE's scheduler is implementation specific and out of the scope of this document. Note that it is not required for a PCE to support the priority field: in this case, it is RECOMMENDED that the PCC set the priority field to 0 in the RP object. If the PCE does not take into account the request priority, it is RECOMMENDED to set the priority field to 0 in the RP object carried within the corresponding PCRep message, regardless of the priority value contained in the RP object carried within the corresponding PCReq message. A higher numerical value of the priority field reflects a higher priority. Note that it is the responsibility of the network administrator to make use of the priority values in a consistent manner across the various PCCs. The ability of a PCE to support request prioritization MAY be dynamically discovered by the PCCs by means of PCE capability discovery. If not advertised by the PCE, a PCC may decide to set the request priority and will learn the ability of the PCE to support request prioritization by observing the Priority field of the RP object received in the PCRep message. If the value of the Pri field is set to 0, this means that the PCE does not support
the handling of request priorities: in other words, the path
computation request has been honored but without taking the
request priority into account.
o R (Reoptimization - 1 bit): when set, the requesting PCC specifies
that the PCReq message relates to the reoptimization of an
existing TE LSP. For all TE LSPs except zero-bandwidth LSPs, when
the R bit is set, an RRO (see Section 7.10) MUST be included in
the PCReq message to show the path of the existing TE LSP. Also,
for all TE LSPs except zero-bandwidth LSPs, when the R bit is set,
the existing bandwidth of the TE LSP to be reoptimized MUST be
supplied in a BANDWIDTH object (see Section 7.7). This BANDWIDTH
object is in addition to the instance of that object used to
describe the desired bandwidth of the reoptimized LSP. For zero-
bandwidth LSPs, the RRO and BANDWIDTH objects that report the
characteristics of the existing TE LSP are optional.
o B (Bi-directional - 1 bit): when set, the PCC specifies that the
path computation request relates to a bi-directional TE LSP that
has the same traffic engineering requirements including fate
sharing, protection and restoration, LSRs, TE links, and resource
requirements (e.g., latency and jitter) in each direction. When
cleared, the TE LSP is unidirectional.
o O (strict/loose - 1 bit): when set, in a PCReq message, this
indicates that a loose path is acceptable. Otherwise, when
cleared, this indicates to the PCE that a path exclusively made of
strict hops is required. In a PCRep message, when the O bit is
set this indicates that the returned path is a loose path;
otherwise (when the O bit is cleared), the returned path is made
of strict hops.
Unassigned bits are considered reserved. They MUST be set to zero on
transmission and MUST be ignored on receipt.
Request-ID-number (32 bits): The Request-ID-number value combined
with the source IP address of the PCC and the PCE address uniquely
identify the path computation request context. The Request-ID-
number is used for disambiguation between pending requests, and
thus it MUST be changed (such as by incrementing it) each time a
new request is sent to the PCE, and may wrap.
The value 0x00000000 is considered invalid.
If no path computation reply is received from the PCE (e.g., the
request is dropped by the PCE because of memory overflow), and the
PCC wishes to resend its request, the same Request-ID-number MUST
be used. Upon receiving a path computation request from a PCC
with the same Request-ID-number, the PCE SHOULD treat the request
as a new request. An implementation MAY choose to cache path
computation replies in order to quickly handle retransmission
without having to process a path computation request twice (in the
case that the first request was dropped or lost). Upon receiving
a path computation reply from a PCE with the same Request-ID-
number, the PCC SHOULD silently discard the path computation
reply.
Conversely, different Request-ID-numbers MUST be used for
different requests sent to a PCE.
The same Request-ID-number MAY be used for path computation
requests sent to different PCEs. The path computation reply is
unambiguously identified by the IP source address of the replying
PCE.
7.4.2. Handling of the RP Object
If a PCReq message is received that does not contain an RP object,
the PCE MUST send a PCErr message to the requesting PCC with Error-
Type="Required Object missing" and Error-value="RP Object missing".
If the O bit of the RP message carried within a PCReq message is
cleared and local policy has been configured on the PCE to not
provide explicit paths (for instance, for confidentiality reasons), a
PCErr message MUST be sent by the PCE to the requesting PCC and the
pending path computation request MUST be discarded. The Error-Type
is "Policy Violation" and Error-value is "O bit cleared".
When the R bit of the RP object is set in a PCReq message, this
indicates that the path computation request relates to the
reoptimization of an existing TE LSP. In this case, the PCC MUST
also provide the strict/loose path by including an RRO object in the
PCReq message so as to avoid/limit double-bandwidth counting if and
only if the TE LSP is a non-zero-bandwidth TE LSP. If the PCC has
not requested a strict path (O bit set), a reoptimization can still
be requested by the PCC, but this requires that the PCE either be
stateful (keep track of the previously computed path with the
associated list of strict hops), or have the ability to retrieve the
complete required path segment. Alternatively, the PCC MUST inform
the PCE about the working path and the associated list of strict hops
in PCReq. The absence of an RRO in the PCReq message for a non-zero-
bandwidth TE LSP (when the R bit of the RP object is set) MUST
trigger the sending of a PCErr message with Error-Type="Required
Object Missing" and Error-value="RRO Object missing for
reoptimization".
If a PCC/PCE receives a PCRep/PCReq message that contains an RP object referring to an unknown Request-ID-number, the PCC/PCE MUST send a PCErr message with Error-Type="Unknown request reference". This is used for debugging purposes. If a PCC/PCE receives PCRep/ PCReq messages with unknown requests at a rate equal or greater than MAX-UNKNOWN-REQUESTS unknown requests per minute, the PCC/PCE MUST send a PCEP CLOSE message with close value="Reception of an unacceptable number of unknown requests/replies". A RECOMMENDED value for MAX-UNKNOWN-REQUESTS is 5. The PCC/PCE MUST close the TCP session and MUST NOT send any further PCEP messages on the PCEP session. The reception of a PCEP message that contains an RP object referring to a Request-ID-number=0x00000000 MUST be treated in similar manner as an unknown request.7.5. NO-PATH Object
The NO-PATH object is used in PCRep messages in response to an unsuccessful path computation request (the PCE could not find a path satisfying the set of constraints). When a PCE cannot find a path satisfying a set of constraints, it MUST include a NO-PATH object in the PCRep message. There are several categories of issue that can lead to a negative reply. For example, the PCE chain might be broken (should there be more than one PCE involved in the path computation) or no path obeying the set constraints could be found. The "NI (Nature of Issue)" field in the NO-PATH object is used to report the error category. Optionally, if the PCE supports such capability, the NO-PATH object MAY contain an optional NO-PATH-VECTOR TLV defined below and used to provide more information on the reasons that led to a negative reply. The PCRep message MAY also contain a list of objects that specify the set of constraints that could not be satisfied. The PCE MAY just replicate the set of objects that was received that was the cause of the unsuccessful computation or MAY optionally report a suggested value for which a path could have been found (in which case, the value differs from the value in the original request). NO-PATH Object-Class is 3. NO-PATH Object-Type is 1.
The format of the NO-PATH object body 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Nature of Issue|C| Flags | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Optional TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: NO-PATH Object Format NI - Nature of Issue (8 bits): The NI field is used to report the nature of the issue that led to a negative reply. Two values are currently defined: 0: No path satisfying the set of constraints could be found 1: PCE chain broken The Nature of Issue field value can be used by the PCC for various purposes: * Constraint adjustment before reissuing a new path computation request, * Explicit selection of a new PCE chain, * Logging of the error type for further action by the network administrator. IANA management of the NI field codespace is described in Section 9. Flags (16 bits). The following flag is currently defined: o C flag (1 bit): when set, the PCE indicates the set of unsatisfied constraints (reasons why a path could not be found) in the PCRep message by including the relevant PCEP objects. When cleared, no failing constraints are specified. The C flag has no meaning and is ignored unless the NI field is set to 0x00. Unassigned bits are considered as reserved. They MUST be set to zero on transmission and MUST be ignored on receipt.
Reserved (8 bits): This field MUST be set to zero on transmission
and MUST be ignored on receipt.
The NO-PATH object body has a variable length and may contain
additional TLVs. The only TLV currently defined is the NO-PATH-
VECTOR TLV defined below.
Example: consider the case of a PCC that sends a path computation
request to a PCE for a TE LSP of X Mbit/s. Suppose that PCE cannot
find a path for X Mbit/s. In this case, the PCE must include in the
PCRep message a NO-PATH object. Optionally, the PCE may also include
the original BANDWIDTH object so as to indicate that the reason for
the unsuccessful computation is the bandwidth constraint (in this
case, the NI field value is 0x00 and C flag is set). If the PCE
supports such capability, it may alternatively include the BANDWIDTH
object and report a value of Y in the bandwidth field of the
BANDWIDTH object (in this case, the C flag is set) where Y refers to
the bandwidth for which a TE LSP with the same other characteristics
(such as Setup/Holding priorities, TE LSP attribute, local
protection, etc.) could have been computed.
When the NO-PATH object is absent from a PCRep message, the path
computation request has been fully satisfied and the corresponding
paths are provided in the PCRep message.
An optional TLV named NO-PATH-VECTOR MAY be included in the NO-PATH
object in order to provide more information on the reasons that led
to a negative reply.
The NO-PATH-VECTOR TLV is compliant with the PCEP TLV format defined
in Section 7.1 and is comprised of 2 bytes for the type, 2 bytes
specifying the TLV length (length of the value portion in bytes)
followed by a fixed-length 32-bit flags field.
Type: 1
Length: 4 bytes
Value: 32-bit flags field
IANA manages the space of flags carried in the NO-PATH-VECTOR TLV
(see Section 9).
The following flags are currently defined:
o Bit number: 31 - PCE currently unavailable
o Bit number: 30 - Unknown destination
o Bit number: 29 - Unknown source
7.6. END-POINTS Object
The END-POINTS object is used in a PCReq message to specify the source IP address and the destination IP address of the path for which a path computation is requested. The P flag of the END-POINTS object MUST be set. If the END-POINTS object is received with the P flag cleared, the receiving peer MUST send a PCErr message with Error-Type=10 and Error-value=1. The corresponding path computation request MUST be cancelled by the PCE without further notification. Note that the source and destination addresses specified in the END- POINTS object may correspond to the source and destination IP address of the TE LSP or to those of a path segment. Two END-POINTS objects (for IPv4 and IPv6) are defined. END-POINTS Object-Class is 4. END-POINTS Object-Type is 1 for IPv4 and 2 for IPv6. The format of the END-POINTS 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: END-POINTS Object Body Format for IPv4
The format of the END-POINTS object 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Source IPv6 address (16 bytes) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Destination IPv6 address (16 bytes) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13: END-POINTS Object Body Format for IPv6 The END-POINTS object body has a fixed length of 8 bytes for IPv4 and 32 bytes for IPv6. If more than one END-POINTS object is present, the first MUST be processed and subsequent objects ignored.7.7. BANDWIDTH Object
The BANDWIDTH object is used to specify the requested bandwidth for a TE LSP. The notion of bandwidth is similar to the one used for RSVP signaling in [RFC2205], [RFC3209], and [RFC3473]. If the requested bandwidth is equal to 0, the BANDWIDTH object is optional. Conversely, if the requested bandwidth is not equal to 0, the PCReq message MUST contain a BANDWIDTH object. In the case of the reoptimization of a TE LSP, the bandwidth of the existing TE LSP MUST also be included in addition to the requested bandwidth if and only if the two values differ. Consequently, two Object-Type values are defined that refer to the requested bandwidth and the bandwidth of the TE LSP for which a reoptimization is being performed. The BANDWIDTH object may be carried within PCReq and PCRep messages. BANDWIDTH Object-Class is 5.
Two Object-Type values are defined for the BANDWIDTH object:
o Requested bandwidth: BANDWIDTH Object-Type is 1.
o Bandwidth of an existing TE LSP for which a reoptimization is
requested. BANDWIDTH Object-Type is 2.
The format of the BANDWIDTH object body 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: BANDWIDTH Object Body Format
Bandwidth (32 bits): The requested bandwidth is encoded in 32 bits
in IEEE floating point format (see [IEEE.754.1985]), expressed in
bytes per second. Refer to Section 3.1.2 of [RFC3471] for a table
of commonly used values.
The BANDWIDTH object body has a fixed length of 4 bytes.
7.8. METRIC Object
The METRIC object is optional and can be used for several purposes.
In a PCReq message, a PCC MAY insert one or more METRIC objects:
o To indicate the metric that MUST be optimized by the path
computation algorithm (IGP metric, TE metric, hop counts).
Currently, three metrics are defined: the IGP cost, the TE metric
(see [RFC3785]), and the number of hops traversed by a TE LSP.
o To indicate a bound on the path cost that MUST NOT be exceeded for
the path to be considered as acceptable by the PCC.
In a PCRep message, the METRIC object MAY be inserted so as to
provide the cost for the computed path. It MAY also be inserted
within a PCRep with the NO-PATH object to indicate that the metric
constraint could not be satisfied.
The path computation algorithmic aspects used by the PCE to optimize
a path with respect to a specific metric are outside the scope of
this document.
It must be understood that such path metrics are only meaningful if used consistently: for instance, if the delay of a computed path segment is exchanged between two PCEs residing in different domains, consistent ways of defining the delay must be used. The absence of the METRIC object MUST be interpreted by the PCE as a path computation request for which no constraints need be applied to any of the metrics. METRIC Object-Class is 6. METRIC Object-Type is 1. The format of the METRIC object body 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |C|B| T | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | metric-value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 15: METRIC Object Body Format The METRIC object body has a fixed length of 8 bytes. Reserved (16 bits): This field MUST be set to zero on transmission and MUST be ignored on receipt. T (Type - 8 bits): Specifies the metric type. Three values are currently defined: * T=1: IGP metric * T=2: TE metric * T=3: Hop Counts Flags (8 bits): Two flags are currently defined: * B (Bound - 1 bit): When set in a PCReq message, the metric- value indicates a bound (a maximum) for the path metric that must not be exceeded for the PCC to consider the computed path as acceptable. The path metric must be less than or equal to the value specified in the metric-value field. When the B flag is cleared, the metric-value field is not used to reflect a bound constraint.
* C (Computed Metric - 1 bit): When set in a PCReq message, this
indicates that the PCE MUST provide the computed path metric
value (should a path satisfying the constraints be found) in
the PCRep message for the corresponding metric.
Unassigned flags MUST be set to zero on transmission and MUST be
ignored on receipt.
Metric-value (32 bits): metric value encoded in 32 bits in IEEE
floating point format (see [IEEE.754.1985]).
Multiple METRIC objects MAY be inserted in a PCRep or a PCReq message
for a given request (i.e., for a given RP). For a given request,
there MUST be at most one instance of the METRIC object for each
metric type with the same B flag value. If, for a given request, two
or more instances of a METRIC object with the same B flag value are
present for a metric type, only the first instance MUST be considered
and other instances MUST be ignored.
For a given request, the presence of two METRIC objects of the same
type with a different value of the B flag is allowed. Furthermore,
it is also allowed to insert, for a given request, two METRIC objects
with different types that have both their B flag cleared: in this
case, an objective function must be used by the PCE to solve a multi-
parameter optimization problem.
A METRIC object used to indicate the metric to optimize during the
path computation MUST have the B flag cleared and the C flag set to
the appropriate value. When the path computation relates to the
reoptimization of an exiting TE LSP (in which case, the R flag of the
RP object is set), an implementation MAY decide to set the metric-
value field to the computed value of the metric of the TE LSP to be
reoptimized with regards to a specific metric type.
A METRIC object used to reflect a bound MUST have the B flag set, and
the C flag and metric-value field set to the appropriate values.
In a PCRep message, unless not allowed by PCE policy, at least one
METRIC object MUST be present that reports the computed path metric
if the C flag of the METRIC object was set in the corresponding path
computation request (the B flag MUST be cleared). The C flag has no
meaning in a PCRep message. Optionally, the PCRep message MAY
contain additional METRIC objects that correspond to bound
constraints; in which case, the metric-value MUST be equal to the
corresponding computed path metric (the B flag MUST be set). If no
path satisfying the constraints could be found by the PCE, the METRIC
objects MAY also be present in the PCRep message with the NO-PATH
object to indicate the constraint metric that could be satisfied.
Example: if a PCC sends a path computation request to a PCE where the metric to optimize is the IGP metric and the TE metric must not exceed the value of M, two METRIC objects are inserted in the PCReq message: o First METRIC object with B=0, T=1, C=1, metric-value=0x0000 o Second METRIC object with B=1, T=2, metric-value=M If a path satisfying the set of constraints can be found by the PCE and there is no policy that prevents the return of the computed metric, the PCE inserts one METRIC object with B=0, T=1, metric- value= computed IGP path cost. Additionally, the PCE may insert a second METRIC object with B=1, T=2, metric-value= computed TE path cost.7.9. Explicit Route Object
The ERO is used to encode the path of a TE LSP through the network. The ERO is carried within a PCRep message to provide the computed TE LSP if the path computation was successful. The contents of this object are identical in encoding to the contents of the Resource Reservation Protocol Traffic Engineering Extensions (RSVP-TE) Explicit Route Object (ERO) defined in [RFC3209], [RFC3473], and [RFC3477]. That is, the object is constructed from a series of sub-objects. Any RSVP-TE ERO sub-object already defined or that could be defined in the future for use in the RSVP-TE ERO is acceptable in this object. PCEP ERO sub-object types correspond to RSVP-TE ERO sub-object types. Since the explicit path is available for immediate signaling by the MPLS or GMPLS control plane, the meanings of all of the sub-objects and fields in this object are identical to those defined for the ERO. ERO Object-Class is 7. ERO Object-Type is 1.7.10. Reported Route Object
The RRO is exclusively carried within a PCReq message so as to report the route followed by a TE LSP for which a reoptimization is desired. The contents of this object are identical in encoding to the contents of the Route Record Object defined in [RFC3209], [RFC3473], and [RFC3477]. That is, the object is constructed from a series of sub-
objects. Any RSVP-TE RRO sub-object already defined or that could be defined in the future for use in the RSVP-TE RRO is acceptable in this object. The meanings of all of the sub-objects and fields in this object are identical to those defined for the RSVP-TE RRO. PCEP RRO sub-object types correspond to RSVP-TE RRO sub-object types. RRO Object-Class is 8. RRO Object-Type is 1.
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