Tutorial: Segment Routing

https://www.youtube.com/watch?v=BEo5MdB3o3Y

Take-away points:

• SR中最重要的是TE
• Segment (1-hop or n-hop) vs SID
• SR是hybrid TE approaches: path info in packet and network
• CSPF - attribute(b/w, color), requirement and shortest path
• CP = controller

What's SR?

• A tunneling tech - nothing fancy
• A TE! - steer packet instead of routing path. ~= RSVP TE

Terminology:

• Domain
• SR path
• Segment = SID, can be multiple hops

Traditional TE Approaches

• path info in packet
• like IPv4 strict routing option
• path info in the network
• RSVP-signaled MPLS

SR is hybrid TE approach

• Segment types:
• 1 router hop
• multiple router hops
• Types:
• Adjacency (IGP adjacency, 1-hop)
• Prefix = IGP least cost path to a prefix

SR Encapsulation

• MPLS
• Label = segment
• IPv6:
• Segment Routing Extension Header (SRH)
• A list of ipv6 addr
• each ipv6 addr = a segment

Local Labels

• Some SIDs have node-local significance
• Adjancency
• Why important
• Stack can be too big, ASIC cannot handle
• MTU

Global Labels

• Some have domain-wide significance
• Each node reserves a block of labels. 
• SRGB base

IPv6 forwarding

• SRH, segment routing hdr
• A list of ipv6 address as SID

Binding SIDs

• Pointing to another SR paths or tunnels
• Reasons:
• Label stack
• MTU size too big

Control Plane

• Path computation: on SR ingress or central controller. 

CSPF, 这个讲的清楚！

• Attributes to segment: color, b/w, SRLG
• Req to each path
• Shortest path meeting req.
• LSDB, TED = extension to carry info. 
• Alternative path to protect
• All info in LSDB, no need for RSVP or LDP

SR convergence after failures

• Fast recovery by IGP reconvergence. 
• TI-FLA, speed up convergence if not fast enuf
• Use anycast SID (ecmp)

SR benefits:

• If using SR to reserve b/w, MUST go for controller
• Central controller has global view 

Controller Protocol options:

• pull LSDB:
• controller: a passive mbr in IGP
• BGP-LS
• push segment list to ingress SR
• PCEP
• BGP
• push policy
• binding what traffic to which path
• PCEP or BGP

Conclusion:

• SR moves state from network to packet - simplified
• Some open issues: OAM, Fast Reroute
• Need  experience