| PCE Working Group | C. Li |
| Internet-Draft | M. Chen |
| Intended status: Standards Track | Huawei Technologies |
| Expires: January 25, 2020 | W. Cheng |
| China Mobile | |
| J. Dong | |
| Z. Li | |
| Huawei Technologies | |
| R. Gandhi | |
| Cisco Systems, Inc. | |
| Q. Xiong | |
| ZTE Corporation | |
| July 24, 2019 |
Path Computation Element Communication Protocol (PCEP) Extension for Path Segment in Segment Routing (SR)
draft-li-pce-sr-path-segment-07
The Path Computation Element (PCE) provides path computation functions in support of traffic engineering in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.
The Source Packet Routing in Networking (SPRING) architecture describes how Segment Routing (SR) can be used to steer packets through an IPv6 or MPLS network using the source routing paradigm. A Segment Routed Path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), explicit configuration, or a Path Computation Element (PCE).
Path identification is needed for several use cases such as performance measurement in Segment Routing (SR) network. This document specifies extensions to the Path Computation Element Protocol (PCEP) to support requesting, replying, reporting and updating the Path Segment ID (Path SID) between PCEP speakers.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 25, 2020.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
[RFC5440] describes the Path Computation Element (PCE) Communication Protocol (PCEP). PCEP enables the communication between a Path Computation Client (PCC) and a PCE, or between PCE and PCE, for the purpose of computation of Multiprotocol Label Switching (MPLS) as well as Generalzied MPLS (GMPLS) Traffic Engineering Label Switched Path (TE LSP) characteristics.
[RFC8231] specifies a set of extensions to PCEP to enable stateful control of TE LSPs within and across PCEP sessions in compliance with [RFC4657]. It includes mechanisms to effect LSP State Synchronization between PCCs and PCEs, delegation of control over LSPs to PCEs, and PCE control of timing and sequence of path computations within and across PCEP sessions. The model of operation where LSPs are initiated from the PCE is described in [RFC8281].
[I-D.ietf-pce-pcep-extension-for-pce-controller] specify the procedures and PCEP protocol extensions for using the PCE as the central controller for static LSPs, where LSPs can be provisioned as explicit label instructions at each hop on the end-to-end path.
Segment routing (SR) [RFC8402] leverages the source routing and tunneling paradigms and supports steering packets into an explicit forwarding path at the ingress node.
An SR path needs to be identified in some use cases such as performance measurement. For identifying an SR path, [I-D.ietf-spring-mpls-path-segment] introduces a new segment that is referred to as Path Segment.
[I-D.ietf-pce-segment-routing] specifies extensions to the Path Computation Element Protocol (PCEP) [RFC5440] for SR networks, that allow a stateful PCE to compute and initiate SR-TE paths, as well as a PCC to request, report or delegate SR paths. [I-D.ietf-pce-segment-routing-ipv6] extend PCEP to support SR paths for IPv6 data plane.
[I-D.zhao-pce-pcep-extension-pce-controller-sr] specifies the procedures and PCEP protocol extensions when a PCE-based controller is also responsible for configuring the forwarding actions on the routers (SR SID distribution in this case), in addition to computing the paths for packet flows in a segment routing network and telling the edge routers what instructions to attach to packets as they enter the network.
This document specifies a mechanism to carry the SR path identification information in PCEP messages [RFC5440] [RFC8231] [RFC8281]. The SR path identifier can be a Path Segment in SR-MPLS [I-D.ietf-spring-mpls-path-segment], or a Path Segment in SRv6 [I-D.li-spring-srv6-path-segment] or other IDs that can identify an SR path. This document also extends the PCECC-SR mechanism to inform the Path Segment to the egress PCC.
This memo makes use of the terms defined in [RFC4655], [I-D.ietf-pce-segment-routing], and [RFC8402].
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This document specifies a mechanism of encoding (and allocating) Path Segment in PCEP extensions. For supporting Path Segment in PCEP, several TLVs and flags are defined. The formats of the objects and TLVs are described in Section 4. The procedures of Path Segment allocation are described in Section 5.
There are various modes of operations, such as -
The path information to the ingress PCC and PCE is exchanged via an extension to [I-D.ietf-pce-segment-routing] and [I-D.ietf-pce-segment-routing-ipv6]. The Path Segment information to the egress PCC can be informed via an extension to the PCECC-SR procedures [I-D.zhao-pce-pcep-extension-pce-controller-sr].
For the PCE to allocate a Path Segment, the PCE SHOULD be aware of the MPLS label space from the PCCs. This is done via mechanism as described in [I-D.li-pce-controlled-id-space]. Otherwise, the PCE should request the egress PCC for Path Segment allocation.
[I-D.ietf-pce-segment-routing] defined a new Path Setup Type (PST) and SR-PCE-CAPABILITY sub-TLV for SR. PCEP speakers use this sub-TLV to exchange information about their SR capability. The TLV defines a Flags field that includes one bit (L-flag) to indicate Local Significance [I-D.ietf-pce-segment-routing].
This document adds an additional flag for Path Segment allocation, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD11 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |P|N|X| MSD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: P-flag in SR-PCE-CAPABILITY TLV
The figure is included for the ease of the reader and can be removed at the time of publication.
[I-D.ietf-pce-segment-routing-ipv6] defined a new Path Setup Type (PST) and SRv6-PCE-CAPABILITY sub-TLV for SRv6. PCEP speakers use this sub-TLV to exchange information about their SRv6 capability. The TLV includes a Flags field and one bit (L-flag) was allocated in [I-D.ietf-pce-segment-routing-ipv6].
This document adds an additional flag for Path Segment allocation, 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Flags |P|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | MSD-Type | MSD-Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: P-flag in SRv6-PCE-CAPABILITY TLV
The figure is included for the ease of the reader and can be removed at the time of publication.
Along with the SR sub-TLVs, the PCECC Capability as per [I-D.zhao-pce-pcep-extension-pce-controller-sr] should be advertised if the PCE allocates the Path Segment and acts as a Central Controller that manages the Label space.
The PCECC Capability should also be advertised on the egress PCEP session, along with the SR sub-TLVs. This is needed to ensure that the PCE can use the PCECC objects/mechanism to request/inform the egress PCC of the Path Segment as described in this document.
The LSP Object is defined in Section 7.3 of [RFC8231]. This document adds the following flags to the LSP Object:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PLSP-ID | Flag|P|C| O |A|R|S|D| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: P-flag in LSP Object
The figure is included for the ease of the reader and can be removed at the time of publication.
The PATH-SEGMENT TLV is an optional TLV for use in the LSP Object for Path Segment allocation. The type of this TLV is to be allocated by IANA (TBA4). The format is shown below.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ST | Flag |L| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ (Variable length) Path Segment ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: The PATH-SEGMENT TLV Format
The type (16-bit) of the TLV is TBA4 (to be allocated by IANA). The length (16-bit) has a fixed value of 8 octets. The value contains the following fields:
In general, only one instance of PATH-SEGMENT TLV will be included in LSP object. If more than one PATH-SEGMENT TLV is included, the first one is processed and others MUST be ignored. Multiple Path Segment allocation for use cases like alternate-making will be considered in future version of this draft.
When the Path Segment allocation is enable, a PATH-SEGMENT TLV MUST be included in the LSP object.
If the label space is maintained by PCC itself, and the Path Segment is allocated by Egress PCC, then the PCE should request the Path Segment from Egress PCC as described in Section 5.1.1. In this case, the PCE should send a PCUpdate or PCInitiate message to the egress PCC to request the Path Segment. The P-flag in LSP should be unset in this case.
If a PCEP node does not recognize the PATH-SEGMENT TLV, it would behave in accordance with [RFC5440] and ignore the TLV. If a PCEP node recognizes the TLV but does not support the TLV, it MUST send PCErr with Error-Type = 2 (Capability not supported).
The FEC Object [I-D.zhao-pce-pcep-extension-pce-controller-sr] is used to specify the FEC information and MAY be carried within PCInitiate or PCRpt message for the PCECC-SR operations. The PCE MUST inform the Path Identification information to the Egress PCC. To do this, this document extends the procedures of [I-D.zhao-pce-pcep-extension-pce-controller-sr] by defining a new FEC object type for Path.
FEC Object-Type is TBA6 'Path'.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // TLV(s) // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The path FEC object Format
One or more following TLV(s) are allowed in the 'path' FEC object -
Either SYMBOLIC-PATH-NAME TLV or LSP-IDENTIFIERS TLV MUST be included. SPEAKER-ENTITY-ID TLV is optional. Only one instance of each TLV is processed, if more than one TLV of each type is included, the first one is processed and others MUST be ignored.
The Central Control Instructions (CCI) Object is used by the PCE to specify the forwarding instructions is defined in [I-D.ietf-pce-pcep-extension-for-pce-controller]. Further [I-D.zhao-pce-pcep-extension-pce-controller-sr] defined a CCI object type for SR.
The Path Segment information is encoded directly in the CCI SR object. The Path Segment TLV as described in the Section 4.2.1, MUST also be included in the CCI SR object as the TLV (as it includes additional information regarding the Path Segment identifier).
This document adds the following flags to the CCI Object:
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CC-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MT-ID | Algorithm | Flags |C|N|E|V|L|O| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // SID/Label/Index (variable) // +---------------------------------------------------------------+ | | // Optional TLV // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: The CCI object for SR
(Editor's Note - An update is planned for [I-D.zhao-pce-pcep-extension-pce-controller-sr] in the next revision detailing this procedure, and the above text might move there.)
The Path Segment allocation and encoding is as per the stateful PCE operations for segment routing. The procedures are as per the corresponding extensions defined in [I-D.ietf-pce-segment-routing] and [I-D.ietf-pce-segment-routing-ipv6] (which are further based on [RFC8231] and [RFC8281]). The additional operations for Path Segment are defined in this section.
To notify (or request) the Path Segment to the Egress PCC, the procedures are as per the PCECC-SR [I-D.zhao-pce-pcep-extension-pce-controller-sr] (which is based on [I-D.ietf-pce-pcep-extension-for-pce-controller]). The additional operations are defined in this section.
As defined in [I-D.ietf-spring-mpls-path-segment], a Path Segment can be allocated by the egress PCC. In this case, the label space is maintained on the PCC itself.
This section describes the mechanism of Path Segment allocation by using PCInitiate and PCUpd message in Stateful PCE model.
The ingress PCC could request the Path Segment to be allocated by the PCE via PCRpt message. The delegate flag (D-flag) MUST also be set for this LSP. Also, the P-flag in the LSP object MUST be set.
On receiving a stateful path computation request with Path Segment allocation request from an ingress PCC, a stateful PCE requests the egress PCC to allocate a Path Segment.
The PATH-SEGMENT TLV MUST be included in an LSP object in the PCInitiate message sent from the PCE to the egress to request Path Segment allocation by the egress PCC. The P flag in LSP object MUST be set to 0. This PCInitiate message to egress PCC would be the similar to the one sent to ingress PCC as per [I-D.ietf-pce-segment-routing], but the egress PCC would only allocate the Path Segment and would not trigger the initiation/update operation.
If the value of Path Segment is 0x0 it indicates that the PCE is requesting a Path Segment for this LSP. If the Path Segment is set to a value 'n' and the P flag is unset in the LSP object, it indicates that the PCE requests a specific value 'n' of Path Segment. If the Path Segment is allocated successfully, the egress PCC reports the Path Segment via PCRpt message with PATH-SEGMENT TLV in LSP object. Else, it MUST send a PCErr message with Error-Type = TBA7 ("Path SID failure") and Error Value = 1 ("Invalid SID"). If the value of Path Segment is valid, but the PCC is unable to allocate the Path Segment, it MUST send a PCErr message with Error-Type = TBA7 ("Path label/SID failure") and Error Value = 2 ("Unable to allocate the specified label/SID").
Once the PCE receives the PCRpt message, it can obtain the Path Segment information from the egress PCC and then update the path with Path Segment by sending PCUpd message.
If the Path Segment is updated successfully, the ingress PCC will acknowledge with a PCRpt message to the PCE. In case of error, as described in [I-D.ietf-pce-segment-routing], an PCErr message with Error-Type = TBA7 ("Path SID failure") and Error Value = 1 ("Invalid SID") will be sent back to the PCE. The PCE MUST roll back the Path Segment vlaue to the previous value by sending a PCUpd message to synchronize with the egress PCC.
Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
1) LSP State | ---- PCRpt ----> | |
Delegate | Delegate=1 | |
| P=1 |2) PCE update |
| | the LSP-DB and |
| | request Path SID |
| | |
| | --- PCInitiate ---> | Egress
| | PATH-SEGMENT | allocates
| | TLV in LSP | a Path-SID
| | | from its
| | <----- PCRpt ------ | space
| | Path SID |
| | |
|<---- PCUpd ---- |3)Paths update with |
| PATH-SEGMENT TLV | Path SID |
| | |
4) LSP State | ----- PCRpt ---> | |
Report | | |
| | |
Figure 7: Ingress PCC-Initiated Path Segment Allocation
If the ingress PCC wishes to withdraw or modify a previously reported Path Segment value, it MUST send a PCRpt message without any PATH-SEGMENT TLV or with the PATH-SEGMENT TLV containing the new Path Segment respectively. In this case, the PCE should synchronize with egresss PCC via PCUpd message.
The Path Segment MUST be withdrawn when the corresponding LSP is removed. When the LSP is deleted, the PCE MUST request the egress PCC to withdraw the LSP and associated Path Segment via PCInitiate message with the R flag is set in the SRP object.
If an egress PCC receives a valid Path Segment value from a PCE which is different than the current Path Segment, it MUST try to allocate the new value. If the new Path Segment is successfully allocated, the egress PCC MUST report the new value to the PCE. Otherwise, it MUST send a PCErr message with Error-Type = TBA7 ("Path label/SID failure") and Error Value = 2 ("Unable to allocate the specified label/SID").
A stateful PCE also can initiate or update an LSP with Path Segment actively via requesting the egress PCC to allocate a Path Segment.
If a PCE wishes to modify a previously requested Path Segment value or allocate a Path Segment for an PCE-Initiated LSP, it MUST request the egress PCC to allocate a new value by sending a PCUpd message to the egress PCC with PATH-SEGMENT TLV containing the new Path Segment value. Also, the P flag in LSP object is unset. Absence of the PATH-SEGMENT TLV in PCUpd message means that the PCE wishes to withdraw the Path Segment.
The mechanism of requesting Path Segment is as per Section 5.1.1.
Once the PCE receives the PCRpt message, it can obtain the Path Segment information from the egress PCC and then update or initiate an LSP with Path Segment.
If the SR-Path is setup, the ingress PCC will acknowledge with a PCRpt message to the PCE. In case of error, as described in [I-D.ietf-pce-segment-routing], an PCErr message will be sent back to the PCE. The PCE MUST request the egress PCC to withdraw the LSP and associated Path Segment via PCInitiate message with the R flag is set in the SRP object.
If the Path Segment is updated successfully, the ingress PCC will acknowledge with a PCRpt message to the PCE. In case of error, an PCErr message with Error-Type = TBA7 ("Path SID failure") and Error Value = 1 ("Invalid SID") will be sent back to the PCE. The PCE MUST roll back the Path Segment vlaue to the previous value by sending a PCUpd message to synchronize with the egress PCC.
Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
1) LSP State | ---- PCRpt ----> | |
Delegate if| Delegate=1 | |
the LSP exists| |2)PCE actively update|
| | the LSP-DB and |
| | request Path SID |
| | |
| | --- PCInitiate ---> | Egress
| | PATH-SEGMENT | allocates
| | TLV in LSP | a Path-SID
| | | from its
| | <----- PCRpt ------ | space
| | Path SID |
| | |
|<-- PCUpd/PCInit -- |3)Paths update with |
| PATH-SEGMENT TLV | Path SID |
| | |
4) LSP State | ----- PCRpt ---> | |
Report | | |
| | |
Figure 8: Stateful PCE-Initiated Path Segment Allocation
For allocating the Path Segments to SR paths by the PCEs, the PCE controlled label space MUST be known at PCEs via configurations or any other mechanisms. The PCE controlled label spaces MAY be advertised as described in [I-D.li-pce-controlled-id-space].
The PCE could allocate the Path Segment on its own for a PCE-Initiated (or delegated LSP). The allocated Path Segment needs to be informed to the Ingress and Egress PCC. The PCE would use the PCInitiate message [RFC8281] or PCUpd message [RFC8231] towards the Ingress PCC and MUST include the PATH-SEGMENT TLV in the LSP object. The PCE would further inform the egress PCC about the Path Segment allocated by the PCE using the PCInitiate message as described in [I-D.zhao-pce-pcep-extension-pce-controller-sr].
Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
| | |
| <--PCInitiate--- |1)Initiate LSP with |
| PATH-SEGMENT TLV | Path SID |
| | |
2)LSP delegation |---PCRpt, D=1---> | (Confirm) |
| | |
|3) PCE informs the | --- PCInitiate ---> |
| Path SID to Egress| FEC=Path |
| | |
| | <-------- PCRpt --- |
| | |
Figure 9: PCE allocated Path Segment on its own
The ingress PCC could request the Path Segment to be allocated by the PCE via PCRpt message as per [RFC8231]. The delegate flag (D-flag) MUST also be set for this LSP. Also, the P-flag in the LSP object MUST be set.
A PATH-SEGMENT TLV MUST be included in the LSP object. If the value of Path Segment is 0x0, it indicates that the Ingress PCC is requesting a Path Segment for this LSP. If the Path Segment is set to a value 'n', it indicates that the ingress PCC requests a specific value 'n' of Path Segment.
If the Path Segment is allocated successfully, the PCE would further respond to Ingress PCC with PCUpd message as per [RFC8231] and MUST include the PATH-SEGMENT TLV in a LSP object. Else, it MUST send a PCErr message with Error-Type = TBA7 ("Path SID failure") and Error Value = 1 ("Invalid SID"). If the value of Path Segment is valid, but the PCC is unable to allocate the Path Segment, it MUST send a PCErr message with Error-Type = TBA7 ("Path label/SID failure") and Error Value = 2 ("Unable to allocate the specified label/SID").
The active PCE would allocate the Path Segment as per the PATH-SEGMENT flags and in case PATH-SEGMENT is not included, the PCE MUST act based on the local policy.
The PCE would further inform the egress PCC about the Path Segment allocated by the PCE using the PCInitiate message as described in [I-D.zhao-pce-pcep-extension-pce-controller-sr].
Ingress Egress
+-+-+ +-+-+ +-+-+
|PCC| |PCE| |PCC|
+-+-+ +-+-+ +-+-+
1) LSP State | ---- PCRpt ----> | |
Delegate | Delegate=1 | |
| P=1 |2) PCE update |
| | the LSP-DB and |
| | allocate Path SID |
|<---- PCUpd ---- |3)Paths update with |
| PATH-SEGMENT TLV | Path SID |
| | |
4) LSP State Report | ----- PCRpt ---> | |
| | |
|5) PCE informs the | --- PCInitiate ---> |
| Path SID to Egress| FEC=Path |
| | |
| | <-------- PCRpt --- |
| | |
Figure 10: Ingress PCC request Path Segment to PCE
As described in [I-D.ietf-spring-mpls-path-segment], in an SR-MPLS network, when a packet is transmitted along an SR path, the labels in the MPLS label stack will be swapped or popped. So that no label or only the last label may be left in the MPLS label stack when the packet reaches the egress node. Thus, the egress node cannot determine from which SR path the packet comes. For this reason, it introduces the Path Segment.
Apart from allocation and encoding of the Path Segment (described in this document) for the LSP, it would also be included in the SID/Label stack of the LSP (usually for processing by the egress). To support this, the Path Segment MAY also be a part of SR-ERO as prepared by the PCE as per [I-D.ietf-pce-segment-routing]. The PCC MAY also include the Path Segment while preparing the label stack based on the local policy and use-case.
It is important that the PCE learns the Maximum SID Depth (MSD) that can be imposed at each node/link of a given SR path to ensure that the SID stack depth does not exceed the number of SIDs the node is capable of imposing. As a new type of segment, Path Segment will be inserted in the SID list just like other SIDs. Thus, the PCE needs to consider the affect of Path Segment when computing a LSP with Path Segment allocation.
SR PCE Capability TLV is defined in [I-D.ietf-pce-segment-routing], and the registry to manage the Flag field of the SR PCE Capability TLV is requested in [I-D.ietf-pce-segment-routing]. IANA is requested to make the following allocation in the aforementioned registry.
Bit Description Reference TBA1 Path Segment Allocation is supported(P) This document
SRv6 PCE Capability TLV is defined in defined in [I-D.ietf-pce-segment-routing-ipv6], and the registry to manage the Flag field of the SRv6 PCE Capability Flags is requested in [I-D.ietf-pce-segment-routing-ipv6]. IANA is requested to make the following allocation in the aforementioned registry.
Bit Description Reference TBA2 Path Segment Allocation is supported(P) This document
[RFC8231] defines the LSP object; per that RFC, IANA created a registry to manage the value of the LSP object's Flag field. IANA has allocated a new bit in the "LSP Object Flag Field" subregistry, as follows:
Bit Description Reference TBA3 Request for Path Segment Allocation(P) This document
IANA is requested to add the assignment of a new allocation in the existing "PCEP TLV Type Indicators" subregistry as follows:
Value Description Reference TBA4 PATH-SEGMENT TLV This document
This document requests that a new subregistry named "PATH-SEGMENT TLV Segment Type (ST) Field" to be created to manage the value of the ST field in the PATH-SEGMENT TLV.
Value Description Reference 0 MPLS Path Segment(MPLS label) This document 1 SRv6 Path Segment(IPv6 address) This document
Further, this document also requests that a new subregistry named "PATH-SEGMENT TLV Flag Field" to be created to manage the Flag field in the PATH-SEGMENT TLV. New values are assigned by Standards Action [RFC8126]. Each bit should be tracked with the following qualities:
Bit Description Reference 7 Local Signification(L) This document
CCI object is defined in defined in [I-D.ietf-pce-pcep-extension-for-pce-controller], further [I-D.zhao-pce-pcep-extension-pce-controller-sr] defined a CCI object type for SR. and the subregistry to manage the Flag field of the CCI object for SR is requested in [I-D.zhao-pce-pcep-extension-pce-controller-sr]. IANA is requested to make the following allocation in the aforementioned subregistry.
Bit Description Reference
TBA5 PCC is requested to This document
allocate resource(C)
A new PCEP object called FEC is defined in [I-D.zhao-pce-pcep-extension-pce-controller-sr]. IANA is requested to allocate a new Object-Type for FEC object in the "PCEP Objects" subregistry.
Value Description Reference TBA6 SR path This document
IANA is requested to allocate code-points in the "PCEP-ERROR Object Error Types and Values" subregistry for the following new error-types and error-values:
Error-Type Meaning Reference
TBA7 Path SID failure: This document
Error-value = 1
Invalid SID
Error-value = 2
Unable to allocate
Path SID
TBA
The following people have substantially contributed to this document:
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
Email: dhruv.ietf@gmail.com
Zafar Ali
Cisco Systems, Inc.
Email: zali@cisco.com
| [RFC4655] | Farrel, A., Vasseur, J. and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006. |
| [RFC4657] | Ash, J. and J. Le Roux, "Path Computation Element (PCE) Communication Protocol Generic Requirements", RFC 4657, DOI 10.17487/RFC4657, September 2006. |
| [RFC8402] | Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., Litkowski, S. and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018. |
| [I-D.li-pce-controlled-id-space] | Li, C., Chen, M., Dong, J., Li, Z., Wang, A., Cheng, W. and C. Zhou, "PCE Controlled ID Space", Internet-Draft draft-li-pce-controlled-id-space-03, June 2019. |
| [I-D.ietf-spring-mpls-path-segment] | Cheng, W., Li, H., Chen, M., Gandhi, R. and R. Zigler, "Path Segment in MPLS Based Segment Routing Network", Internet-Draft draft-ietf-spring-mpls-path-segment-00, March 2019. |