Presentation by Cengiz Alaettinoglu at APRICOT 2017 on Monday, 1 March 2017.

1. Segment Routing
A Control Plane Simplification
Cengiz Alaettinoglu

2. Motivation
Forwarding traffic over non-shortest paths is necessary for:
• Traffic engineering
• Protection against failures (link/node/srlg)
• BGP exit selection
• SLA conforming service paths (e.g. low delay paths)
• …
Network operators have two choices:
• RSVP-TE
• Segment routing
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3. Segment Routing Provides Uncompromised
Functionality With Simplicity
Segment routing simplifies the IP/MPLS control plane
• No need to run LDP or RSVP-TE
• No signaling overhead
• Small overhead in IGP and BGP in comparison to RSVP-TE overhead in IGP
Functionality is not compromised
• Traffic engineering
• Shortest and non-shortest paths
• ECMP-aware TE
• Protection against link/node/SRLG failures
• Primary/secondary paths
• SLA conforming service paths (e.g. L2/L3 VPNs)
• Egress BGP exit selection (mainly for content providers/data centers)
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4. Why Simplify?
It is a challenge to operate LDP and RSVP-TE at large scale
• N^2 RSVP-TE tunnels
• N^2 LDP sessions over these tunnels
• RSVP-TE signaling overhead on the routers and race conditions
• Excessive IGP churn due to available link bandwidth changes
• IGP/LDP synchronization issues
As a result, many SPs are reluctant to run RSVP-TE
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5. State of RSVP-TE in the Network
More than 6% of the
tunnels are down due
to race conditions
Traffic is forced to IGP
paths or stuck on FRR
paths
• Creates congestion
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6. Segment Routing (by SPRING WG)
Each packet contains its path as a list of path segments
• Similar to “source” routing
• Any router along the path can insert segments
• Typically ingress router or protecting router
6
A B
Z
DC
V W YX
3 Segments
• Segment to C
• Segment to X
• Segment to Z
All link costs are 1
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7. IGP Distributes SIDs and SRGBs
Z announces SID 9 to reach Z
C announces SID 3 to reach C
X announces SID 7 to reach X
Each router announces its Segment Routing Global Block (SRGB)
• MPLS label = SID + offset
• Let’s assume that offset is 0
• MPLS label = SID
All routers set up MPLS forwarding state for these labels
• ECMP-aware shortest paths to each Z, C and X
7
A B
Z
DC
V W YX
3 Segments
• Go to C on shortest path
• Go to X on shortest path
• Go to Z on shortest path
All link costs are 1
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8. A Wants Path A B C X Y Z
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A B
Z
DC
V W YX
3 Segments: 3 -> 7-> 9
• Go to C on shortest path (SID 3)
• Go to X on shortest path (SID 7)
• Go to Z on shortest path (SID 9)
OSPF says SIDs for
• C is 3
• X is 7
• Z is 9
7
9
Payload
9
Payload
9
Payload
Payload
3
7
9
Payload
MPLS Label Stack

9. Any Path Can Be Encoded
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A B
Z
DC
V W YX
1 Segment (shortest IGP path)
• Go to Z on shortest path (node segment)
A B
Z
DC
V W YX
5 Segments
• Go to B on shortest path
• Go to W on shortest path
• Go to Y on shortest path
• Go to D on shortest path
• Go to Z on shortest path
A B
Z
DC
V W YX
3 Segments
• Go to C on shortest path
• Go to X on link 3 (adjacency segment)
• Go to Z on shortest path

10. How Does SPRING Eliminate LDP?
LDP distributes MPLS labels for prefixes
• Usually these are BGP next hops
IGP distributes prefix-SID for BGP next hops
(I am conveniently ignoring PWE signaling)
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11. SDN TE Use Case
Using on-device RSVP-TE for optimization is problematic
• Not network-wide optimization
• Race conditions lead to failed tunnels or stuck FRRs
• See my Apricot talk yesterday
SDN application centralizes bandwidth allocation
• No need to signal intermediate routers these bandwidth reservations
• No need to carry available bandwidth in IGP and interfere with IGP convergence
• No need for a reservation protocol
• No need for refreshes
Still need to encode shortest and non-shortest paths
• Segment routing can do this without any of this overhead
• Segment routing IGP overhead is small and not changing with bw reservations; hence small
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12. TI-LFA
Topology-independent loop-free alternative
Protects against link/node/SRLG failures
Protects any IP/MPLS traffic
• Segment routed or not
Same protection path as post-convergence path
100% coverage
For most networks, very few segments necessary
• Usually just one more
• Up to 3 in some networks
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13. Operational Challenges
Each router needs to have a large enough SRGB for the network
• Network is growing
Max SID depth (very large label stack) is a problem for old routers
• For most networks, TE paths can be encoded by less than 5 labels
• TI-LFA needs even less labels than this
• Unfortunately, some old hardware can handle only 4 or 5 labels
• They can swap but can not push these many labels
• Recent hardware can handle more than 15 labels
• Location of entropy label is also problematic with deep stack
• Binding SID can remedy these problems
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