Update to Automatic Bandwidth Adjustment procedure of Stateful PCE

Introduction describes the Path Computation Element Communication Protocol (PCEP). PCEP defines the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between PCEs, enabling computation of Multiprotocol Label Switching (MPLS) for Traffic Engineering Label Switched Path (TE LSP) characteristics. specifies extensions to PCEP to enable stateful control of MPLS TE LSPs. It describes two modes of operations - Passive stateful PCE and Active stateful PCE. Further, describes the setup, maintenance and teardown of PCE-Initiated LSPs for the stateful PCE model. describes the auto-bandwidth feature that allows automatic and dynamic adjustment of the TE LSP bandwidth reservation based on the volume of traffic flowing through the LSP. It describes PCEP extensions for auto-bandwidth adjustment when employing an active stateful PCE for both PCE-initiated and PCC-initiated LSPs. It defines the AUTO-BANDWIDTH-ATTRIBUTES TLV that provides the 'configurable knobs' of the feature, and it can be included as an optional TLV in the LSPA object. The TLV is encoded in all PCEP messages for the LSP while the auto-bandwidth adjustment feature is enabled. The absence of the TLV indicates the PCEP speaker wishes to disable the feature. The TLV includes multiple AUTO-BANDWIDTH-ATTRIBUTES sub-TLVs defined in . The AUTO-BANDWIDTH-ATTRIBUTES sub-TLVs are included if there is a change since the last information sent in the PCEP message. It also states that in the case of a missing sub-TLV, as per the local policy, either the default value or some other operator-configured value is used. Since the missing sub-TLV in a subsequent PCEP message is considered as no change, there is no mechanism to remove a particular attribute encoded in the sub-TLV. This document updates to define such a procedure. Note that for the attributes that have an associated default value, they could simply encode the default value in the sub-TLV but this cannot be used for the attributes that do not have a default value. This document proposes to use a special value of all zeros to indicate "restore to default", which could mean going back to the default values or removal of the attribute itself. The following table includes the sub-TLVs and the default values as per .

Type	Len	Name	Default
1	4	Sample-Interval	300 seconds
2	4	Adjustment-Interval	86400 seconds
3	4	Down-Adjustment-Interval	Adjustment-Interval
4	4	Adjustment-Threshold	none
5	8	Adjustment-Threshold-Percentage	5%, 0
6	4	Down-Adjustment-Threshold	Adjustment-Threshold
7	8	Down-Adjustment-Threshold-Percentage	Adjustment-Threshold-Percentage
8	4	Minimum-Bandwidth	0
9	4	Maximum-Bandwidth	none
10	8	Overflow-Threshold	none
11	8	Overflow-Threshold-Percentage	none
12	8	Underflow-Threshold	none
13	8	Underflow-Threshold-Percentage	none

Thus, the use of the special value of all zeros in the value portion of the sub-TLV can be used to indicate "restore to default", which could mean :

if the default value is set for the sub-TLV:
- Restore to the default values
if there is no default value for the sub-TLV:
- Remove the associated attribute

Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Updated Procedures Section 5.2 of defines the AUTO-BANDWIDTH-ATTRIBUTES TLV and its associated sub-TLVs. This document updates by adding this text at the end of paragraph 3 in section 5.2:

Extensions The section 5.1.1 of defines the AUTO-BANDWIDTH-CAPABILITY TLV as an optional TLV for use in the OPEN Object for auto-bandwidth adjustment. This document adds a new flag - Z (TBD): The flag indicates that a PCEP speaker supports the use of the special value of all zeros in the value field as specified in this document. The presence of the Z flag can give a clear indication to the PCEP peer if they can use the updated procedures defined in this document.

Example Consider an LSP with the following information in the AUTO-BANDWIDTH-ATTRIBUTES TLV in the PCInitiate message:

Sample-Interval: 600 (in sec)
Adjustment-Interval: 172800 (2 days in sec)
Adjustment-Threshold: 0x49989680 (10 mbps in bps)

Now if the PCE would like to not use the Adjustment-Thresholds feature for the LSP and set the Adjustment-Interval to 1 day, it could send the AUTO-BANDWIDTH-ATTRIBUTES TLV in the PCUpd message with the following sub-TLVs:

Adjustment-Interval: 86400 (1 day in sec, the default value)
Adjustment-Threshold: 0x0

On receiving the special value of all zeros in the value portion of the Adjustment-Threshold sub-TLV, the PCEP speaker would consider that as removal of the Adjustment-Threshold feature. Note that, the PCE could also set the Adjustment-Interval: 0x0 instead of the default value to trigger the restore to default. The Sample-Interval remains unchanged.

Backward Compatibility Note that to achieve the same objective, an compliant implementation could send a PCEP message without AUTO-BANDWIDTH-ATTRIBUTES TLV first and then include the AUTO-BANDWIDTH-ATTRIBUTES TLV with the updated sub-TLV. An existing implementation of that does not support this update (Z flag is not set) could consider a special value of all zeros in the sub-TLV to be a malformed sub-TLV thus ignoring it and keeping the previous value. A PCEP speaker SHOULD use other techniques in this case.

Security Considerations This document does not add any substantial new security concerns beyond those already discussed in .

IANA Considerations

AUTO-BANDWIDTH-CAPABILITY TLV Flag Field defines the AUTO-BANDWIDTH-CAPABILITY TLV. IANA created a subregistry to manage the Flag field of the AUTO-BANDWIDTH-CAPABILITY TLV within the "Path Computation Element Protocol (PCEP) Numbers" registry. This document requests IANA to allocate a new bit in the AUTO-BANDWIDTH-CAPABILITY TLV Flag Field registry, as follows. IANA is requested to make allocations starting from the least significant bit (31).

Bit	Description	Reference
TBD	Z flag (special value of all zeros)	[This.I-D]