PCEP in SR Traffic Engineering

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PCEP (Path Computation Element Protocol) | In this article , we will discuss PCEP and its role in Segment Routing.

PCE (Path computation Element) is network entity responsible for computation and maintaining paths (traffic engineered paths) in those cases when headend node cannot compute by itself such as inter-domain or dis-joint paths. In other words, it extends SR-TE capability of headend node in segment routing domain. Path computation client (PCC) is the entity which uses PCE for the SR-TE path computation , that is, headend node. SR-PCE communicates with its clients (headend node PCC) using PCEP (path computation element protocol). Here we will focus on the role of PCEP in SRTE as we already discussed SR-PCE in detail in my earlier article and below is the link for the same,

SR-PCE

PCE and PCC works in server-client model, wherein PCC request PCE to compute a path, PCE computes and return the result. PCEP mechanism to communicate with PCE in this case is stateful, means , PCE not only computes a path but also learn SLA constraints using SR policies . PCE also updates SR policies and paths when there is any change or during any path or node failure.There is IETF draft which extends PCEP to support Segment Routing traffic engineering policy (earlier PCEP was supporting RSVP based traffic engineering). Here is the link to IETF draft ,

https://tools.ietf.org/html/draft-ietf-pce-segment-routing-16

PCEP mechanism in stateful PCE consists of “report-update-report” messages exchange between PCE and PCC. PCC initiated SR policy CLI configuration or NETCONF through orchestration or using ODN template. Here is the sequence of message exchange happens between PCE and PCC using PCEP , refer below sample topology for the explanation,

PCEP in segment routing traffic engineering
PCEP Session between S1 and SR-PCE
  1. PCE builds its SRTE database using IGP and BGP-LS (link , node, prefix, SR policy information)
  2. SR Policy is configured on PCE with SLA constraint parameters such a bandwidth or delay etc along with endpoint information.
  3. PCC sends “Report message” to PCE along with SR policy information and delegates control of the path to PCE .
  4. PCE accepts delegation, computes new path as per SR Policy parameters .
  5. PCE returns the path to PCC using “Update message”.
  6. Headend or PCC installs the path and sends “Report message” to PCE regarding the state of the new path.
  7. PCE controls the path updates during any change or failure scenarios.

This is how PCEP with SR extensions helps achieving dynamic SRTE . I hope this article is helpful. Please write comment if you have any clarification or query.

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