SR-PCE

Before diving into how SR-PCE works, let's first understand it through real-life examples:

First Example:

• When we seek help, we often turn to the universe or God and ask for guidance ( PCRequest) . Whatever plan the universe or God has for us, it guides us accordingly (PCReply) and in turn we thanks to the Universe or God ( PCReport)
• Many times, your intuition (6th sense or subconscious mind) may tell you not to follow a different path or move to different place or ?not to go at that place ( PCUpdate) and later you might find out that a big tragedy occurred on that very path ( links up/down event , network change events , updated to SR-PC by BGP-Link State) and you say thank you to Universe or God (PCReport)

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Second Example:

• ?When you want to travel, you use a map to guide you. First, you select your destination and then ask for help in finding the best (shortest) path ( PCRequest).
• Based on your request, the map software figures out the best route for you, showing the shortest path (PCReply).
• Once your vehicle starts moving it starts updating the location ( PCUpdate and PC report)

• However, if you want to take certain routes, you can add stops between the source and destination, and the map will guide you via that explicit path.
• If there is any congestion or road closure, the map updates and diverts you to the best available path (PCUpdate) and you take the newly calculated path.

SR-PCE functions in the same way as described in the two real-life examples above. Let's dive into the details.

SR-PCE :-

Segment Routing Path Computation Element (SR-PCE) is a centralized controller that computes optimal SR-TE (Segment Routing Traffic Engineering) paths for network devices known as SR-PCCs (Path Computation Clients). The communication between SR-PCC and SR-PCE happens via PCEP (Path Computation Element Protocol) using specific message types.

Let's discuss the packet flow between the PCC and PCE devices.

1. PCRequest (Path Computation Request - PCReq)

• Sent by SR-PCC to SR-PCE when it needs an optimized SR path.
• It includes constraints such as: Bandwidth Delay Exclude/include links Segment list preferences (Prefix SIDs, Adjacency SIDs, Binding SIDs)

Example: PCC (Router) → PCE (Controller): "Compute an SR path from PE1 to PE2 with low latency and avoid congested links."

2. PCReply (Path Computation Reply - PCRep)

• Based on the latest database, the SR-PCE begins calculating the optimal path while considering the defined constraints.
• Sent by SR-PCE in response to a PCReq from the SR-PCC.
• Contains the computed path with a list of SIDs (Segment Identifiers) that should be used.
• Can include multiple path options, primary and backup.

Example: PCE (Controller) → PCC (Router): "Here is an SR path: PE1 → (Adj-SID 101) → P1 → (Prefix-SID 5002) → PE2."

3. PCReport (Path Report - PCReport)

• Sent by SR-PCC to SR-PCE to report active SR-TE paths.
• Used when the PCC has locally computed a path or when a delegated path is being monitored.
• Helps PCE maintain synchronization with network state.

Example: PCC (Router) → PCE (Controller): "I have installed SR policy 100 with path PE1 → P1 → PE2.by setting delegate flags towards PCE"

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Let's explore what happens when there are changes in the network, such as link up, link down, or metric modifications.

1 Link Down or Link Up Event

• A link failure (or recovery) occurs in the network.
• This event is detected by the adjacent routers, which then trigger the required mechanisms for fast reroute and path recalculation.

2? Traffic Switchover via TI-LFA Backup Path

• Upon detecting a link failure, the affected routers immediately switch traffic to a TI-LFA backup path, ensuring minimal disruption.
• TI-LFA precomputes these backup paths to enable sub-50ms convergence time.

3? BGP Link-State Update to SR-PCE

• The link-state information is updated in the BGP-LS (Link State) database, which feeds the SR-PCE with the latest topology changes.
• The SR-PCE then recalculates an optimal path based on updated constraints.

4? Path Computation Update (PCUpd)

• The SR-PCE computes the new best path considering the failure scenario.
• It sends a PCUpd message to the SR-PCC, providing a new segment list for forwarding.

Example: PCE (Controller) → PCC (Router) (PCUpdate Message): "A more optimal path is available. Update to PE1 → (Adj-SID 104) → P4 → (Prefix-SID 5004) → PE2."

5? Path Computation Report (PCReport)

• Once the new path is installed, the SR-PCC sends a PCReport message to the SR-PCE, confirming that the updated segment-routing policy is in effect.

Example: PCC (Router) → PCE (Controller) (PCReport Message): "New path installed successfully. Traffic now following PE1 → P3 → PE2."

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SR-PCE Workflow with PCEP Messages

1? PCC sends PCReq to request a path.

2? PCE responds with PCRep containing the computed path.

3? PCC installs the path and sends PCReport to confirm.

4? PCE monitors the network and may send a PCUpdate if a better path is found.

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Glossary:

• SR-PCC: Path Computation Client
• SR-PCE: Path Computation Element
• PCEP: Path Computation Element Protocol
• PCRequest: Path Computation Request
• PCReply: Path Computation Reply
• PCReport: Path Computation Report
• PCUpdate: Path Computation Update

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