PCEP Extensions for Unifed Source Routing-based SFC

Service Function Chaining provides a flexible way to construct services. When applying a particular Service Function Chain (SFC) to the traffic classified by the Classifier, the traffic needs to be steered through an ordered set of Service Function Forwarders (SFF) and Service Functions (SF) in the network. This ordered set of SFFs and SFs in the network, referred to as a Service Function Path (SFP), is an instantiation of the SFC in the network. For example, as shown in Figure 1, an SFP corresponding to the SFC of {SF1, SF3} can be expressed as {SFF1, SF1, SFF2, SF3}.

|SFC +----+|----> | SFF1 |---->| +-----+ |----> ----+Classifier+------+ +-----+ SFF2 +-------- +----------+ +----------+ +--------------+ | | | +-------------------------------------------------+ Figure 1: PCE-based Service Function Chaining in MPLS-SR Network]]> MPLS-SPRING (a.k.a., MPLS Segment Routing) could be leveraged to realize a unified source routing mechanism across MPLS, IPv4 and IPv6 data planes by using a unified source routing instruction set while preserving backward compatibility with MPLS-SPRING as descried in . More specifically, the source routing instruction set information contained in a source routed packet could be uniformly encoded as an MPLS label stack no matter the underlay is IPv4, IPv6 or MPLS. The unified source routing mechanism in turn could be leveraged to realize a transport-independent service function chaining by encoding the service function path information or service function chain information as an MPLS label stack as described in . This document describes extensions to the Path Computation Element Protocol (PCEP) that allow a PCE to compute and instantiate service function paths in the MPLS source routing based service function chaining context. More specifically, the PCC provides an ordered list of SF IDs to the PCE and indicates to the PCE that what type SFs and paths are requested (e.g., an SFP, or a compact SFP, or an SR-specific SFP, or a compact SR-specific SFP) through the path computation request message, and then the PCE responds with a corresponding path through the path computation response message. The extensions specified in this document are applicable to both the stateless PCE model and the stateful PCE model .

This memo makes use of the terms defined in , and . In addition, this memo defines the following two additional terms: Compact SFP: An ordered list of SFFs. SR-specific SFP: An ordered list of node SIDs (representing SFFs) and Service Function SIDs. Compact SR-specific SFP: An ordered list of node SIDs (representing SFFs).

This document does not specify any changes to the PCReq message format. This document requires the PATH-SETUP-TYPE TLV to be carried in the RP Object in order for a PCC to request a particular type of path. Four new Path Setup Types need to be defined for MPLS source routing-based SFC (see Section 4.2). This document also requires the Include Route Object (IRO) to be carried in the PCReq message in order for a PCC to specify an SFC. A new IRO sub-object type needs to be defined for SF (see Section 4.3).

This document defines the format of the PCRep message carrying an SFP. The message is sent by a PCE to a PCC in response to a previously received PCReq message, where the PCC requested an SFP. The format of the SFC-specific PCRep message is defined as follows:

::=

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

The RP and NO-PATH Objects are defined in . The <SR-SFC- ERO> object contains an SFP and is defined in Section 4.4.

This document defines the format of the PCUpd message carrying an SFP update. The message is sent forwardly by a PCE to a PCC to update an previously computed SFP. The format of the PCUpd message is defined as follows:

::=

Where: ::=[] ::=

Where: ::=[] ]]>

PCPRpt message sent from a PCC to PCE as a respond to a PCUpd message or in an unsolicited manner (e.g., during state synchronization). The format of the PCUpd message is defined as follows:

::=

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

This document defines a new optional TLV for use in the OPEN Object.

The SR-SFC-PCE-CAPABILITY TLV is an optional TLV for use in the OPEN Object to negotiate SR-SFC capability on the PCEP session. The format of the SR-SFC-PCE-CAPABILITY TLV is shown in the following Figure 2:

The code point for the TLV type is to be defined by IANA. The TLV length is 4 octets. The 32-bit value is formatted as follows. The "Maximum SID Depth" (1 octet) field (MSD) specifies the maximum number of SIDs that a PCC is capable of imposing on a packet. The "Flags" (1 octet) and "Reserved" (2 octets) fields are currently unused, and MUST be set to zero and ignored on receipt.

The SR-SFC capability TLV is contained in the OPEN object. By including the TLV in the OPEN message to a PCE, a PCC indicates its support for SFPs. By including the TLV in the OPEN message to a PCC, a PCE indicates that it is capable of computing SFPs.

In order to setup an SFP, the RP or SRP object MUST carry a PATH- SETUP-TYPE TLV specified in . This document defines four new Path Setup Types (PST) for SR-SFC as follows: PST = 2: The path is an SFP. PST = 3: The path is a compact SFP. PST = 4: The path is an SR-specific SFP. PST = 5: The path is a compact SR-specific SFP.

The IRO (Include Route Object) MUST be carried within PCReq messages to indicate a particular SFC. Furthermore, the IRO MAY be carried in PCRep messages. When carried within a PCRep message with the NO-PATH object, the IRO indicates the set of service functions that cause the PCE to fail to find a path. This document defines a new sub-object type for the SR-SFC as follows:

Generally speaking, an SR-SFC-ERO object consists of one or more ERO subobjects described in the following sub-sections to represent a particular type of service function path. In the ERO subobject, each SID is associated with an identifier that represents either an SFF or an SF. This identifier is referred to as the 'Node or Service Identifier' (NSI). As described later, an NSI can be represented in various formats (e.g., IPv4 address, IPv6 address, SF identifier, etc). Specifically, in the SFP case, the NSI of every ERO subobject contained in the SR-SFC-ERO object represents an SFF or an SF while the SID of each ERO subobject is set to null. In the compact SFP case, the NSI of every ERO subobject contained in the SR-SFC-ERO object only represents an SFF meanwhile the SID of every ERO subobject is set to null. In the SR-specific SFP, the NSI of every ERO subobject contained in the SR-SFC-ERO object represents an SFF or an SF while the SID of every ERO subject MUST NOT be null. In the compact SR-specific SFP, the NSI of every ERO subobject contained in the SR-SFC-ERO object represents an SFF meanwhile the SID of every ERO subobject MUST NOT be null.

An SR-SFC-ERO subobject (as shown in Figure 3) consists of a 32-bit header followed by the SID and the NSI associated with the SID. The SID is a 32-bit or 128 bit number. The size of the NSI depends on its respective type, as described in the following sub-sections.

'L' Flag: indicates whether the subobject represents a loose-hop in the explicit route [RFC3209]. If this flag is unset, a PCC MUST not overwrite the SID value present in the SR-SFC-ERO subobject. Otherwise, a PCC MAY expand or replace one or more SID value(s) in the received SR-SFC-ERO based on its local policy. Type: is the type of the SR-SFC-ERO Subobject. This document defines the SR-SFC-ERO Subobject type. A new code point will be requested for the SR-SFC-ERO Subobject from IANA. Length: contains the total length of the subobject in octets, including the L, Type and Length fields. Length MUST be at least 4, and MUST be a multiple of 4. NSI Type (NSIT): indicates the type of NSI associated with the SID. The NSI-Type values are described later in this document. Flags: is used to carry any additional information pertaining to SID. Currently, the following flag bits are defined: M: When this bit is set, the SID value represents an MPLS label stack entry as specified in [RFC5462], where only the label value is specified by the PCE. Other fields (TC, S, and TTL) fields MUST be considered invalid, and PCC MUST set these fields according to its local policy and MPLS forwarding rules. C: When this bit as well as the M bit are set, then the SID value represents an MPLS label stack entry as specified in [RFC5462], where all the entry's fields (Label, TC, S, and TTL) are specified by the PCE. However, a PCC MAY choose to override TC, S, and TTL values according its local policy and MPLS forwarding rules. S: When this bit is set, the SID value in the subobject body is null. In this case, the PCC is responsible for choosing the SID value, e.g., by looking up its Traffic Engineering Database (TED) using node/service identifier in the subobject body. F: When this bit is set, the NSI value in the subobject body is null.// When will the NSI value is null? P: When this bit is set, the SID value represents an IPv6 address. SID: is the 4-octect or 16-octect Segment Identifier. NSI: contains the NSI associated with the SID. Depending on the value of NSIT, the NSI can have different format as described in the following sub-section.

This document defines the following NSIs: 'IPv4 Node ID': is specified as an IPv4 address. In this case, NSIT and Length are 1 and 12 respectively. 'IPv6 Node ID': is specified as an IPv6 address. In this case, NSIT and Length are 2 and 24 respectively. 'Service Function ID': is specified as an SF ID. In this case, NSIT and Length are TBD.

TBD

TBD.

IANA is requested to allocate an ERO subobject type (recommended value= 6) for the SR-SFC-ERO subobject.

TBD

This document defines the following new PCEP TLV:

This document defines the following four new setup types for the PATH-SETUP-TYPE TLV:

This document defines a new IRO sub-object type for SFC as follows:

This document does not introduce any new security considerations.