]> Huawei Technologies

Divyashree Techno Park, Whitefield Bangalore, Karnataka 560066 India dhruv.ietf@gmail.com

Juniper Networks

adrian@olddog.co.uk

Huawei Technologies

Huawei Bld., No.156 Beiqing Rd. Beijing 100095 China lizhenbin@huawei.com

The Path Computation Element (PCE) is a functional component capable of selecting the paths through a traffic engineered network. These paths may be supplied in response to requests for computation, or may be unsolicited instructions issued by the PCE to network elements. Both approaches use the PCE Communication Protocol (PCEP) to convey the details of the computed path. Traffic flows may be categorized and described using "Flow Specifications". RFC 5575 defines the Flow Specification and describes how it may be distributed in BGP to allow specific traffic flows to be associated with routes. This document specifies a set of extensions to PCEP to support dissemination of Flow Specifications. This allows a PCE to indicate what traffic should be placed on each path that it is aware of.

defines the Path Computation Element (PCE), a functional component capable of computing paths for use in traffic engineering networks. PCE was originally conceived for use in Multiprotocol Label Switching (MPLS) for Traffic Engineering (TE) networks to derive the routes of Label Switched Paths (LSPs). However, the scope of PCE was quickly extended to make it applicable to Generalized MPLS (GMPLS) networks, and more recent work has brought other traffic engineering technologies and planning applications into scope (for example, Segment Routing (SR) ). describes the Path Computation Element Communication Protocol (PCEP). PCEP defines the communication between a Path Computation Client (PCC) and a PCE, or between PCE and PCE, enabling computation of path for MPLS-TE LSPs. Stateful PCE specifies a set of extensions to PCEP to enable control of TE-LSPs by a PCE that retains state about the the LSPs provisioned in the network (a stateful PCE). describes the setup, maintenance, and teardown of LSPs initiated by a stateful PCE without the need for local configuration on the PCC, thus allowing for a dynamic network that is centrally controlled. introduces the architecture for PCE as a central controller and describes how PCE can be viewed as a component that performs computation to place 'flows' within the network and decide how these flows are routed. Dissemination of traffic flow specifications (Flow Specifications) was introduced for BGP in . A Flow Specification is comprised of traffic filtering rules and actions. The routers that receive a Flow Specification can classify received packets according to the traffic filtering rules and can direct packets based on the actions. When a PCE is used to initiate tunnels (such as TE-LSPs or SR paths) using PCEP, it is important that the head end of the tunnels understands what traffic to place on each tunnel. The data flows intended for a tunnel can be described using Flow Specifications, and when PCEP is in use for tunnel initiation it makes sense for that same protocol to be used to distribute the Flow Specifications that describe what data is to flow on those tunnels. This document specifies a set of extensions to PCEP to support dissemination of Flow Specifications. The extensions include the creation, update, and withdrawal of Flow Specifications via PCEP, and can be applied to tunnels initiated by the PCE or to tunnels where control is delegated to the PCE by the PCC. Furthermore, a PCC requesting a new path can include Flow Specifications in the request to indicate the purpose of the tunnel allowing the PCE to factor this in during the path computation. Flow Specifications are carried in TLVs within a new Flow Spec Object defined in this document. The flow filtering rules indicated by the Flow Specifications are mainly defined by BGP Flow Specifications.

This document uses the following terms defined in : PCC, PCE, PCEP Peer. The following term from is used frequently throughout this document: Flow Specification (FlowSpec): A Flow Specification is an n-tuple consisting of several matching criteria that can be applied to IP traffic, including filters and actions. Each FlowSpec consists of a set of filters and a set of actions. This document uses the terms "stateful PCE" and "active PCE" as advocated in . 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.

There are three elements of procedure: A PCE and a PCC must be able to indicate whether or not they support the use of Flow Specifications. A PCE or PCC must be able to include Flow Specifications in PCEP messages with clear understanding of the applicability of those Flow Specifications in each case including whether the use of such information is mandatory, constrained, or optional, and how overlapping Flow Specifications will be resolved. Flow Specification information/state must be synchronized between PCEP peers so that, on recovery, the peers have the same understanding of which Flow Specifications apply. The following subsections describe these points.

As with most PCEP capability advertisements, the ability to support Flow Specifications can be indicated in the PCEP OPEN message or in IGP PCE capability advertisements.

During PCEP session establishment, a PCC or PCE that supports the procedures described in this document announces this fact by including the "PCE FlowSpec Capability" TLV (described in ) in the OPEN Object carried in the PCEP Open message. The presence of the PCE FlowSpec Capability TLV in the OPEN Object in a PCE's OPEN message indicates that the PCE can distribute FlowSpecs to PCCs and can receive FlowSpecs in messages from the PCCs. The presence of the PCE FlowSpec Capability TLV in the OPEN Object in a PCC's OPEN message indicates that the PCC supports the FlowSpec functionality described in this document. If either one of a pair of PCEP peers does not indicate support of the functionality described in this document by not including the PCE FlowSpec Capability TLV in the OPEN Object in its OPEN message, then the other peer MUST NOT include a FlowSpec object in any PCEP message sent to the peer that does not support the procedures. If a FlowSpec object is received even though support has not been indicated, the receiver will respond with a PCErr message reporting the objects containing the FlowSpec as described in : that is, it will use 'Unknown Object' if it does not support this specification, and 'Not supported object' if it supports this specification but has not chosen to support FlowSpec objects on this PCEP session.

The ability to advertise support for PCEP and PCE features in IGP advertisements is provided for OSPF in and for IS-IS in . The mechanism uses the PCE Discovery TLV which has a PCE-CAP-FLAGS sub-TLV containing bit-flags each of which indicates support for a different feature. This document defines a new PCE-CAP-FLAGS sub-TLV bit, the FlowSpec Capable flag (bit number TBD1). Setting the bit indicates that an advertising PCE supports the procedures defined in this document. Note that while PCE FlowSpec Capability may be advertised during discovery, PCEP speakers that wish to use Flow Specification in PCEP MUST negotiate PCE FlowSpec Capability during PCEP session setup, as specified in . A PCC MAY initiate PCE FlowSpec Capability negotiation at PCEP session setup even if it did not receive any IGP PCE capability advertisement, and a PCEP peer that advertised support for FlowSpec in the IGP is not obliged to support these procedures on any given PCEP session.

This section describes the procedures to support Flow Specifications in PCEP messages. The primary purpose of distributing Flow Specification information is to allow a PCE to indicate to a PCC what traffic it should place on a path (such as an LSP or an SR path). This means that the Flow Specification may be included in: PCInitiate messages so that an active PCE can indicate the traffic to place on a path at the time that the PCE instantiates the path. PCUpd messages so that an active PCE can indicate or change the traffic to place on a path that has already been set up. PCRpt messages so that a PCC can report the traffic that the PCC plans to place on the path. PCReq messages so that a PCC can indicate what traffic it plans to place on a path at the time it requests the PCE to perform a computation in case that information aids the PCE in its work. PCRep messages so that a PCE that has been asked to compute a path can suggest which traffic could be placed on a path that a PCC may be about to set up. PCErr messages so that issues related to paths and the traffic they carry can be reported to the PCE by the PCC, and so that problems with other PCEP messages that carry Flow Specifications can be reported. To carry Flow Specifications in PCEP messages, this document defines a new PCEP object called the PCEP FLOWSPEC Object. The object is OPTIONAL in the messages described above and MAY appear more than once in each message. The PCEP FLOWSPEC Object carries zero or one Flow Filter TLV which describes a traffic flow. The inclusion of multiple PCEP FLOWSPEC Objects allows multiple traffic flows to be placed on a single path. Once a PCE and PCC have established that they can both support the use of Flow Specifications in PCEP messages, such information may be exchanged at any time for new or existing paths. The application and prioritization of Flow Specifications is described in .

The Flow Specifications are carried along with the LSP State information as per making the Flow Specifications part of the LSP database (LSP-DB). Thus, the synchronization of the Flow Specification information is done as part of LSP-DB synchronization. This may be achieved using normal state synchronization procedures as described in or enhanced state synchronization procedures as defined in . The approach selected will be implementation and deployment specific and will depend on issues such as how the databases are constructed and what level of synchronization support is needed.

The PCE-FLOWSPEC-CAPABILITY TLV is an optional TLV that can be carried in the OPEN Object to exchange PCE FlowSpec capabilities of PCEP speakers. The format of the PCE-FLOWSPEC-CAPABILITY TLV follows the format of all PCEP TLVs as defined in and is shown in .

The type of the PCE-FLOWSPEC-CAPABILITY TLV is TBD2 and it has a fixed length of 2 octets. The Value field is set to default value 0. The two bytes of padding MUST be set to zero and ignored on receipt. The inclusion of this TLV in an OPEN object indicates that the sender can perform FlowSpec handling as defined in this document.

The PCEP FLOWSPEC object defined in this document is compliant with the PCEP object format defined in . It is OPTIONAL in the PCReq, PCRep, PCErr, PCInitiate, PCRpt, and PCUpd messages and MAY be present zero, one, or more times. Each instance of the object specifies a traffic flow. The PCEP FLOWSPEC object carries a FlowSpec filter rule encoded in a TLV (as defined in . The FLOWSPEC Object-Class is TBD3 (to be assigned by IANA). The FLOWSPEC Object-Type is 1. The format of the body of the PCEP FLOWSPEC object is shown in

FS-ID (32-bits): A PCEP-specific identifier for the FlowSpec information. A PCE or PCC creates an FS-ID for each FlowSpec that it originates, and the value is unique within the scope of that PCE or PCC and is constant for the lifetime of a PCEP session. All subsequent PCEP messages can identify the FlowSpec using the FS-ID. The values 0 and 0xFFFFFFFF are reserved and MUST NOT be used. AFI (16-bits): Address Family Identifier as used in BGP (AFI=1 for IPv4 or VPNv4, AFI=2 for IPv6 and VPNv6 as per as per ). Reserved (8-bits): MUST be set to zero on transmission and ignored on receipt. Flags (8-bits): One flag is currently assigned - R bit: The Remove bit is set when a PCEP FLOWSPEC Object is included in a PCEP message to indicate removal of the Flow Specification from the associated tunnel. If the bit is clear, the Flow Specification is being added or modified. Unassigned bits MUST be set to zero on transmission and ignored on receipt. If the PCEP speaker receives a message with R bit set in FLOWSPEC object and the Flow Specification identified with a FS-ID does not exist, it MUST generate a PCErr with Error-type TBD8 (FlowSpec Error), error-value 4 (Unknown FlowSpec). If the PCEP speaker does not understand or support the AFI in the FLOWSPEC message, the PCEP peer MUST respond with a PCErr message with error-type TBD8 (FlowSpec Error), error-value 2 (Malformed FlowSpec). Flow Filter TLV (variable): One TLV MAY be included. The Flow Filter TLV is OPTIONAL when the R bit is set. The TLV MUST be present when the R bit is clear. If the TLV is missing when the R bit is clear, the PCEP peer MUST respond with a PCErr message with error-type TBD8 (FlowSpec Error), error-value 2 (Malformed FlowSpec).

A new PCEP TLV is defined to convey Flow Specification filtering rules that specify what traffic is carried on a path. The TLV follows the format of all PCEP TLVs as defined in . The Type field values come from the codepoint space for PCEP TLVs and has the value TBD4. The Value field contains one or more sub-TLVs (the Flow Specification TLVs) as defined in . Only one Flow Filter TLV can be present and represents the complete definition of a Flow Specification for traffic to be placed on the tunnel indicated by the PCEP message in which the PCEP Flow Spec Object is carried. The set of Flow Specification TLVs in a single instance of a Flow Filter TLV are combined to indicate the specific Flow Specification. Further Flow Specifications can be included in a PCEP message by including additional Flow Spec objects.

The Flow Filter TLV carries one or more Flow Specification TLV. The Flow Specification TLV follows the format of all PCEP TLVs as defined in , however, the Type values are selected from a separate IANA registry (see ) rather than from the common PCEP TLV registry. Type values are chosen so that there can be commonality with Flow Specifications defined for use with BGP . This is possible because the BGP Flow Spec encoding uses a single octet to encode the type where as PCEP uses two octets. Thus the space of values for the Type field is partitioned as shown in .

created the registry "Flow Spec Component Types" and made allocations to it. requested for another registry "Flow Spec IPv6 Component Types" and requested initial allocations in it. If the AFI (in the FLOWSPEC object) is set to IPv4, the range 1..255 is as per "Flow Spec Component Types" ; if the AFI is set to IPv6, the range 1..255 is as per "Flow Spec IPv6 Component Types" . When future BGP specifications (such as ) make further allocations to the aforementioned registries, they are also inherited to be used in PCEP. The content of the Value field in each TLV is specific to the type/AFI and describes the parameters of the Flow Specification. The definition of the format of many of these Value fields is inherited from BGP specifications as shown in . Specifically, the inheritance is from and , but may also be inherited from future BGP specifications. This is a non-exhaustive list for illustration purpose. When multiple Flow Specification TLVs are present in a single Flow Filter TLV they are combined to produce a more detailed description of a flow. For examples and rules about how this is achieved, see . An implementation that receives a PCEP message carrying a Flow Specification TLV with a type value that it does not recognize or does not support MUST respond with a PCErr message with error-type TBD8 (FlowSpec Error), error-value 1 (Unsupported FlowSpec) and MUST NOT install the Flow Specification. When used in other protocols (such as BGP) these Flow Specifications are also associated with actions to indicate how traffic matching the Flow Specification should be treated. In PCEP, however, the only action is to associate the traffic with a tunnel and to forward matching traffic on to that path, so no encoding of an action is needed. describes how overlapping Flow Specifications are prioritized and handled. All Flow Specification TLVs with Types in the range 1 to 255 have Values defined for use in BGP (for example, in , , and ) and are set using the BGP encoding, but without the type or length octets (the relevant information is in the Type and Length fields of the TLV). The Value field is padded with trailing zeros to achieve 4-byte alignment. This document defines following new types -

defines a way to convey identification of a VPN in PCEP via a Route Distinguisher (RD) encoded in ROUTE-DISTINGUISHER TLV. A Flow Specification TLV with Type TBD5 carries a Value field matching that present in the ROUTE-DISTINGUISHER TLV and is used to identify that other flow filter information (for example, an IPv4 destination prefix) is associated with a specific VPN identified by the RD. See for further discussion of VPN identification. Although it may be possible to describe a multicast Flow Specification from the combination of other Flow Specification TLVs with specific values, it is more convenient to use a dedicated Flow Specification TLV. Flow Specification TLVs with Type values TBD6 and TBD7 are used to identify a multicast flow for IPv4 and IPv6 respectively. The Value field is encoded as shown in .

The fields of the two Multicast Flow Specification TLVs are as described in Section 4.9.1 of noting that the two address fields are 32 bits for the IPv4 Multicast Flow and 128 bits for the IPv6 Multicast Flow. Reserved fields (RSVD) MUST be set to zero and ignored on receipt.

This section outlines some specific detailed procedures for using the protocol extensions defined in this document.

The default behavior is that no Flow Specification is applied to a tunnel. That is, the default is that the Flow Spec object is not used as is the case in all systems before the implementation of this specification. In this case it is a local matter (such as through configuration) how tunnel head ends are instructed what traffic to place on a tunnel. describes how receivers respond when they see unknown PCEP objects.

Flow Specifications may be represented by a single Flow Specification TLV or may require a more complex description using multiple Flow Specification TLVs. For example, a flow indicated by a source-destination pair of IPv6 addresses would be described by the combination of Destination IPv6 Prefix and Source IPv6 Prefix Flow Specification TLVs.

A PCE may want to modify a Flow Specification associated with a tunnel, or a PCC may want to report a change to the Flow Specification it is using with a tunnel. It is important that the specific Flow Specification is identified so that it is clear that this is a modification of an existing flow and not the addition of a new flow as described in . The FS-ID field of the PCEP Flow Spec Object is used to identify a specific Flow Specification. When modifying a Flow Specification, all Flow Specification TLVs for the intended specification of the flow MUST be included in the PCEP Flow Spec Object and the FS-ID MUST be retained from the previous description of the flow.

It is possible that multiple flows will be place on a single tunnel. In some cases it is possible to to define these within a single PCEP Flow Spec Object: for example, two Destination IPv4 Prefix TLVs could be included to indicate that packets matching either prefix are acceptable. PCEP would consider this as a single Flow Specification identified by a single FS-ID. In other scenarios the use of multiple Flow Specification TLVs would be confusing. For example, if flows from A to B and from C to D are to be included then using two Source IPv4 Prefix TLVs and two Destination IPv4 Prefix TLVs would be confusing (are flows from A to D included?). In these cases, each Flow Specification is carried in its own PCEP Flow Spec Object with multiple objects present on a single PCEP message. Use of separate objects also allows easier removal and modification of Flow Specifications.

The Remove bit in the the PCEP Flow Spec Object is left clear when a Flow Specification is being added or modified. To remove a Flow Specification, a PCEP Flow Spec Object is included with the FS-ID matching the one being removed, and the R bit set to indicate removal. In this case it is not necessary to include any Flow Specification TLVs. If the R bit is set and Flow Specification TLVs are present an implementation MAY ignore them. If the implementation checks the Flow Specification TLVs against those recorded for the FS-ID of the Flow Specification being removed and finds a mismatch, the Flow Specification MUST still be removed and the implementation SHOULD record a local exception or log.

VPN instances are identified in BGP using Route Distinguishers (RDs) . These values are not normally considered to have any meaning outside of the network, and they are not encoded in data packets belonging to the VPNs. However, RDs provide a useful way of identifying VPN instances and are often manually or automatically assigned to VPNs as they are provisioned. Thus the RD provides a useful way to indicate that traffic for a particular VPN should be placed on a given tunnel. The tunnel head end will need to interpret this Flow Specification not as a filter on the fields of data packets, but using the other mechanisms that it already uses to identify VPN traffic. This could be based on the incoming port (for port-based VPNs) or may leverage knowledge of the VRF that is in use for the traffic.

TBD An implementation that receives a PCEP message carrying a Flow Specification that it cannot resolve against other Flow Specifications already installed MUST respond with a PCErr message with error-type TBD8 (FlowSpec Error), error-value 3 (Unresolvable conflict) and MUST NOT install the Flow Specification.

The figures in this section use the notation defined in . The FLOWSPEC Object is OPTIONAL and MAY be carried in the PCEP messages. The PCInitiate message is defined in and updated as below:

::= Where: ::= [] ::= ( | ) ::= [] [] [] Where: ::= [] ]]>

The PCUpd message is defined in and updated as below:

::= Where: ::= [] ::= [] Where: ::= ::= [] ]]>

The PCRpt message is defined in and updated as below:

::= Where: ::= [] ::= [] [] Where: ::= [] ::= [] ]]>

The PCReq message is defined in and updated in , it is further updated below for flow specification:

::= [] Where: ::= [] ::= [] ::= [] [] [] [] [[]] [] [] [] Where: ::= [] ]]>

The PCRep message is defined in and updated in , it is further updated below for flow specification:

::= Where: ::=[] ::= [] [] [] [] [] Where: ::= [] ]]>

IANA maintains the "Path Computation Element Protocol (PCEP) Numbers" registry. This document requests IANA actions to allocate code points for the protocol elements defined in this document.

Each PCEP object has an Object-Class and an Object-Type. IANA maintains a subregistry called "PCEP Objects". IANA is requested to make an assignment from this subregistry as follows:

This document requests that a new sub-registry, named "FLOW SPEC Object Flag Field", is created within the "Path Computation Element Protocol (PCEP) Numbers" registry to manage the Flag field of the FLOWSPEC object. New values are to be assigned by Standards Action . Each bit should be tracked with the following qualities: Bit number (counting from bit 0 as the most significant bit) Capability description Defining RFC The following values are defined in this document:

IANA maintains a subregistry called "PCEP TLV Type Indicators". IANA is requested to make an assignment from this subregistry as follows:

IANA is requested to create a new subregistry call the "PCEP Flow Specification TLV Type Indicators" registry. Allocations from this registry are to be made according to the following assignment policies :

IANA is requested to pre-populate this registry with values defined in this document as follows, taking the new values from the range 256 to 64506:

IANA maintains a subregistry called "PCEP-ERROR Object Error Types and Values". Entries in this subregistry are described by Error-Type and Error-value. IANA is requested to make the following assignment from this subregistry:

IANA maintains a subregistry called "Open Shortest Path First v2 (OSPFv2) Parameters" with a sub-registry called "Path Computation Element (PCE) Capability Flags". IANA is requested to assign a new capability bit from this registry as follows:

We may assume that a system that utilizes a remote PCE is subject to a number of vulnerabilities that could allow spurious LSPs or SR paths to be established or that could result in existing paths being modified or torn down. Such systems, therefore, apply security considerations as described in , , and . The description of Flow Specifications associated with paths set up or controlled by a PCE add a further detail that could be attacked without tearing down LSPs or SR paths, but causing traffic to be misrouted within the network. Therefore, the use of the security mechanisms for PCEP referenced above is important. Visibility into the information carried in PCEP does not have direct privacy concerns for end-users' data, however, knowledge of how data is routed in a network may make that data more vulnerable. Of course, the ability to interfere with the way data is routed also makes the data more vulnerable. Furthermore, knowledge of the connected end-points (such as multicast receivers or VPN sites) is usually considered private customer information. Therefore, implementations or deployments concerned to protect privacy MUST apply the mechanisms described in the documents referenced above. Experience with Flow Specifications in BGP systems indicates that they can become complex and that the overlap of Flow Specifications installed in different orders can lead to unexpected results. Although this is not directly a security issue per se, the confusion and unexpected forwarding behavior may be engineered or exploited by an attacker. Therefore, implementers and operators SHOULD pay careful attention to the Manageability Considerations described in .

TBD

Thanks to Julian Lucek and Sudhir Cheruathur for useful discussions.

&RFC2119; &RFC4760; &RFC5440; &RFC5511; &RFC5575; &RFC8174; &RFC8253; &RFC4364; &RFC4655; &RFC5088; &RFC5089; &RFC6952; &RFC7399; &RFC7761; &RFC8126; &RFC8231; &RFC8232; &RFC8281; &RFC8283;

[Editor's Note: This section is added for illustration of various Types supported, some are inherited from BGP and others are defined in this document. This section might be removed at the time of final publication.]