4. Network Architecture Evolution
IP Forwarding
MPLS
??
Strict SLA
(Packet,
jitter, delay)
High Performance
Converged
N/w
Application
centric
Network
simplification
Programm
able
network
• IP header lookup
• Shortest path forwarding
• Lacks path control
• Lacks Flexibility
• Lacks Performance
•Label lookup
•Traffic Engineering
•Flexible to accommodate services
•Control Plane complexity
•Scalability
•Distributed Path computing
• Flexible
• Scalable
• Simple to Operate
5. Segment Routing
• Source Routing
Source chooses a path and encodes it in the packet header as an ordered list
of segments
Rest of the network executes the encoded instructions without any further per-
flow state
• Segment
Identifier for any type of instruction
Forwarding or service
• Control Plane Paradigm
Distributed intelligence is used to build these segments
Centralized intelligence maps application to path for resource optimization
6. Segment Routing
• Very briefly, the objective is to support end-to-end per-flow
policies in terms of
Connectivity
Resiliency
Path preference (traffic engineering)
Service selection
8. Segment Routing Components
• Segment ID
– Identifier for any type of instruction
– Node wide or Domain wide Uniqueness
• Data Plane
– Encode the sequence of segment to be applied to packet
– Associated forward semantic for the segment
– MPLS and IPv6
• Control Plane
– Distribution of segment among the devices in domain
– Flow to Segment sequence mapping instruction
9. IGP Prefix Segment
• Aka Node Segment ID
• Shortest-path to the
IGP prefix
• Global
• Signaled by ISIS/OSPF
• Manually assigned or
using centralized
controller.
11
1 2 3
15
654
Illustration:
Prefix-SID NodeX = 16000+X
16015
16015
payload
16015
payload
10. 11
1 2 3
15
654
24025
payload
IGP Adjacency Segment
• Forward on the IGP
adjacency
• Locally Assigned
• Local significance
• Signaled by ISIS/OSPF
• Programmed only in
originator’s forwarding
table
Illustration:
Adj-SID XY = 24nXY, n is index
24023
2412524025
24021
24125
payload
11. Service Segment
• Forward on the IGP
adjacency
• Locally Assigned
• Local significance
• Signaled by ISIS/OSPF
• Programmed only in
originator’s forwarding
table
Illustration:
Service-SID X = 30000+X
30020
Firewall
20
11
1 2 3
15
654
30020
payload
12. SR Data Plane
• Data Plane Forwarding actions:
– CONTINUE: Forwarding action based on active segment
– PUSH: Add segment to SR header
– NEXT: Mark the next segment as “Active”
Segment Routing MPLS Data Plane
SR Header Label Stack
Active Segment Topmost Label
PUSH Label Push
NEXT Label Pop
CONTINUE Label Swap
13. SR Control Plane
• Segment ID distribution
– OSPF, ISIS
• Flow to Segment sequence mapping
instruction
– Distributed CSPF
– SDN Controller based approach
– Operator defined
• Controller collects via BGP-LS
– IGP segments
– BGP segments
– Topology
Ctrl
BGP-LSBGP-LS
11
1 2 3
15
654
15. Traffic Engineering using SR Tunnels
11
1
14
2 3
15
654
1312
High BW
High delay
Low BW
Low delay
Ctrl
Application or Controller
triggered
Flow mapping
Instruction
• Voice Traffic with low latency
requirement uses {16002, 24125,
16013, 30021}
• File Transfer Traffic with High BW
requirement uses {16002, 24025,
16013, 30021}
DPI
21
Voice Traffic
File Transfer
Illustration:
Prefix-SID NodeX = 16000+X
Adj-SID XY = 24nXY, n is index
Service-SID X = 30000+X
Low BW
High delay
30021
Voice
16013
24125
16002
30021
File Data
16013
24125
16002
16. Service Function Chaining
11
1
14
2 3
15
654
1312
High BW
High delay
Low BW
Low delay
Ctrl
• Traffic to be applied with Service
Chain comprising FW and DPI
uses {16014, 30020, 16013,
30021, 16015}
21
Traffic
Illustration:
Prefix-SID NodeX = 16000+X
Adj-SID XY = 24nXY, n is index
Service-SID X = 30000+XFirewall
20
Flow mapping
Instruction
DPI
16015
payload
30021
16013
30020
16014
17. Segment Routing OAM
• OAM probe originates
and terminates at
Monitoring server.
• OAM probe to validate
path between 11 to 15
uses {16011, 16015,
16022}
• Path validation
performed with
no control plane
involvement
11
1
14
2 3
15
654
1312
Illustration:
Prefix-SID NodeX = 16000+X
Adj-SID XY = 24nXY, n is index
Service-SID X = 30000+X
Monitoring
server
2216022
payload
16015
16011
16022
payload
16015
16022
payload
18. More details are available in below links:
www.segment-routing.net
www.segment-routing.net/home/tutorial
www.linkedin.com/groups/Segment-Routing-8266623
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