The document provides a comprehensive tutorial on segment routing (SR), detailing its relationship with multi-protocol label switching (MPLS) and software-defined networking (SDN). It discusses the limitations of traditional MPLS, highlights the benefits of SR, and outlines implementations in various network environments, including ONOS and trellis use cases. Additionally, it covers the architecture and functionality of SR, showcasing its traffic engineering capabilities without requiring mid-point state.

1. Segment Routing Tutorial
Steven Chiu
Winlab, NCTU CS

2. Agenda
• Background
• Multi-Protocol Label Switching (MPLS)
• Software-Defined Network (SDN)
• Segment Routing (SR)
• SR in ONOS
• Trellis Usecase
• 2x2 Leaf Spine Fabric
2

3. Background
Multi-Protocol Label Switching (MPLS)
Software-Defined Network (SDN)
3

4. Background
• In legacy network, MPLS with 1RSVP-TE is used to provide
• Traffic engineering (TE)
• Fast rerouting (FRR)
• Software-defined network (SDN) was proposed for centralized network
intelligence
Segment routing was introduced based on the two network technologies
41 Resource Reservation Protocol - Traffic Engineering

5. Multi-Protocol Label Switching (MPLS)
• Source Routing
• Label Switching
• Between layer 2 and layer 3
Layer 2 Header MPLS Header Layer 3 Header Layer 3 Payload
Label Exp S TTL
0 19 22 23 31
Label: Label Value
Exp: Experimental Use for QoS
S: Bottom of Stack
TTL: Time to Live
5

6. Terminology
• Label Edge Router (LER)
• Label Switching Router (LSR)
• Label Switching Path (LSP)
• Label Distribution Protocol (LDP)
6

7. How MPLS works?
ℎ1
ℎ2
7
Add labels1
MPLS lookup2 MPLS lookup3
MPLS & IP lookup4

8. Penultimate Hop Popping
• Egress LSR double lookup problem
• MPLS table lookup
• IP table lookup
Pop labels at penultimate nodes
• Only IP table Lookup at egress nodes
8

9. Software-Defined Network (SDN)
9
Underlying infrastructure is abstracted
for applications and network services
* Cited from https://www.opennetworking.org/sdn-definition/
Network control become directly
programmable
Network intelligence is (logically)
centralized with a global view of network

10. Segment Routing (SR)
10

11. What problems are we trying to solve?
• Two main control protocols for MPLS
• LDP for scaling and simplicity
• RSVP-TE for traffic engineering (TE) and fast rerouting (FRR)
• RSVP-TE is hard to deploy, manage and scale
11

12. Segment Routing (SR)
• A source routing mechanism support traffic engineering
• Without requiring mid-point state
• Addressed by IETF SPRING (Source Packet Routing In NetworkinG)
• 1RFC 8402
121 Ref. [1]

13. More Precise…
A source-routing architecture that seeks the right balance between
distributed intelligence and centralized optimization.
The application steers its packets through an ordered list of instructions and
realizes end-to-end policy without creating any per-flow state in the network.
13* Cited from http://www.segment-routing.net/

14. Implementation
• Two implementations on data plane
• MPLS (SR-MPLS)
• IPv6 (SRv6)
• Two types of 1IGP segment
• Prefix/Node
• Adjacency
141 Interior Gateway Protocol

15. IGP Prefix/Node Segment
• Global unique within a SR domain
• Typically multi-hop
• ECMP-aware shortest-path first (SPF) route to designated node
15

16. IGP Adjacency Segment
• Locally unique within a SR router
• Typically single-hop
16

17. Combining IGP Segments
17
A
F
C
B
E
D
24134 14006
14003
ℎ1
ℎ2
Packet
14006
24134
14003
Packet
14006
Packet
14006
24134
14003
Packet
14006
Packet
POP [14003, 24134]
POP [14006]
Packet
Prefix/Node Segment
Adjacency Segment

18. Comparison – MPLS
18
• Each node on the new path should update MPLS table
• Per-flow state

19. • Only the edge switch has to be updated
• Update added labels
Comparison – SR
19

20. SR in ONOS
20

21. 1Network Programming API in ONOS
211 Ref. [4]
Topology Independent
Abstraction
Pipeline Independent
Abstraction
Control Protocol Independent
Abstraction

22. 1SR in ONOS Architecture
22
OpenFlow NetconfP4Runtime ...more
OVS Arista Barefoot Cavium Mellanox Ciena Cisco Corsa
Fujitsu HP Huawei Juniper Lumentum Microsemi Polatis ...
FlowRule APITopology API FlowObjective API Intent API Packet API ...
Distributed core
State management, notifications, high-availability & scale-out
Northbound API
Device/protocol-agnostic
Java, REST, CLI, gRPC
Device driver
Allow device-specific variants of
standard protocols
Shared protocol libraries
AppsAppsApps
Control and configure the network
using a global topology view
and independently of the device-specific details
Segment
Routing
Take OpenFlow and
OvS for example
1 Ref. [4]

23. Trellis Usecase
2x2 Leaf Spine Fabric
23

24. Trellis Usecase (I)
• 2x2 leaf spine fabric
24
Spine Switch
Leaf Switch
1 Node segment ID (SID)
1

25. Trellis Usecase (II)
25
Dst B

26. 1OF-DPA Pipeline (Simplified View)
1 Ref. [5] 26

27. L3 Unicast at Source Leaf
27

28. OF-DPA Pipeline – Source Leaf (I)
28
MPLS
Interface
Group

29. Ingress Port Table
Table ID Selector Treatment
0 (No traffic selector criteria for this flow) Transition: TABLE: 10 (VLAN Table)
29

30. OF-DPA Pipeline – Source Leaf (II)
30
MPLS
Interface
Group

31. VLAN Table
Table ID Selector Treatment
10
IN_PORT: 3 (Edge port)
VLAN_VID: None
VLAN_PUSH: vlan
VLAN_ID: 20 (vlan-untagged
configuration from users)
Transition: TABLE: 20 (Termination
MAC Table)
31

32. OF-DPA Pipeline – Source Leaf (III)
32
MPLS
Interface
Group

33. Termination MAC Table
Table ID Selector Treatment
20
IN_PORT: 3
ETH_DST: 00:00:00:00:01:80 (Current node
MAC)
ETH_TYPE: ipv4
VLAN_VID: 20
Transition: TABLE: 30 (Unicast
Routing Table)
33

34. OF-DPA Pipeline – Source Leaf (IV)
34
MPLS
Interface
Group

35. Unicast Routing Table
Table ID Selector Treatment
30
ETH_TYPE: ipv4
IPV4_DST: 10.6.2.0/24 (Destination IP subnet)
Def[GROUP:0x70000021] (Deferred)
Transition: TABLE: 60 (ACL Policy
Table)
35

36. OF-DPA Pipeline – Source Leaf (V)
36
MPLS
Interface
Group

37. ACL Policy Table
Table ID Selector Treatment
60 (No traffic selector criteria for this flow) NOACTION
37

38. OF-DPA Pipeline – Source Leaf (VI)
38
MPLS
Interface
Group

39. L3 ECMP Group
Group ID Type Bucket Action
0x70000021 Select
GROUP: 0x92000020 (MPLS Label Group)
GROUP: 0x9200002d (MPLS Label Group)
39

40. OF-DPA Pipeline – Source Leaf (VII)
40
MPLS
Interface
Group

41. MPLS Label Group
Group ID Type Bucket Action
0x92000020 Indirect
VLAN_POP
MPLS_PUSH: mpls_unicast
MPLS_LABEL: 202 (Destination node SID configuration from user)
GROUP: 0x9000001f (MPLS Interface Group)
VLAN_PUSH: vlan
VLAN_ID: 4095
41

42. OF-DPA Pipeline – Source Leaf (VIII)
42
MPLS
Interface
Group

43. MPLS Interface Group
Group ID Type Bucket Action
0x9000001f Indirect
ETH_DST: 00:00:00:00:03:80 (Next hop spine node MAC)
ETH_SRC: 00:00:00:00:01:80 (Current leaf node MAC)
VLAN_ID: 4094
GROUP: 0xffe0001 (L2 Interface Group)
VLAN_ID 4094 is served as internal default VLAN tag 43

44. OF-DPA Pipeline – Source Leaf (IX)
44
MPLS
Interface
Group

45. L2 Interface Group
Group ID Type Bucket Action
0xffe0001 Indirect
VLAN_POP
OUTPUT: 1 (Next hop output port)
45

46. L3 Unicast at Spine
46

47. OF-DPA Pipeline – Spine (I)
47
MPLS
Interface
Group

48. Ingress Port Table
Table ID Selector Treatment
0 (No traffic selector criteria for this flow) Transition: TABLE: 10 (VLAN Table)
48

49. OF-DPA Pipeline – Spine (II)
49
MPLS
Interface
Group

50. VLAN Table
Table ID Selector Treatment
10
IN_PORT: 1 (Leaf node connected port)
VLAN_VID: None
VLAN_PUSH: vlan
VLAN_ID: 4094
Transition: TABLE: 20 (Termination
MAC Table)
50

51. OF-DPA Pipeline – Spine (III)
51
MPLS
Interface
Group

52. Termination MAC Table
Table ID Selector Treatment
20
IN_PORT: 1
ETH_DST: 00:00:00:00:03:80 (Current node
MAC)
ETH_TYPE: mpls_unicast
VLAN_VID: 4094
Transition: TABLE: 23 (MPLS Table)
52

53. OF-DPA Pipeline – Spine (IV)
53
MPLS
Interface
Group

54. MPLS 0 Flow Table
Table ID Selector Treatment
23 (No traffic selector criteria for this flow)
Transition: TABLE: 24 (MPLS 1 Flow
Table)
54

55. OF-DPA Pipeline – Spine (V)
55
MPLS
Interface
Group

56. MPLS 1 Flow Table
Table ID Selector Treatment
24
ETH_TYPE: mpls_unicast
MPLS_LABEL: 202 (Destination node SID
configuration from user)
MPLS_BOS: true
Def[MPLS_POP:ipv4, DEC_TTL,
GROUP:0x7000001e] (Deferred)
Transition: TABLE: 60 (ACL Policy
Table)
56

57. OF-DPA Pipeline – Spine (VI)
57
MPLS
Interface
Group

58. ACL Policy Table
Table ID Selector Treatment
60 (No traffic selector criteria for this flow) NOACTION
58

59. OF-DPA Pipeline – Spine (VII)
59
MPLS
Interface
Group

60. L3 ECMP Group
Group ID Type Bucket Action
0x7000001e Select GROUP: 0x2000001c (MPLS Label Group)
60

61. OF-DPA Pipeline – Spine (VIII)
61
MPLS
Interface
Group

62. L3 Unicast Group
Group ID Type Bucket Action
0x2000001c Indirect
ETH_DST: 00:00:00:00:02:80 (Next hop leaf switch MAC)
ETH_SRC: 00:00:00:00:03:80 (Current spine switch MAC)
VLAN_ID: 4094
GROUP: 0xffe0002 (L2 Interface Group)
62

63. OF-DPA Pipeline – Spine (IX)
63
MPLS
Interface
Group

64. L2 Interface Group
Group ID Type Bucket Action
0xffe0002 Indirect
VLAN_POP
OUTPUT: 2 (Next hop output port)
64

65. L3 Unicast at Destination Leaf
65

66. OF-DPA Pipeline – Destination Leaf (I)
66
MPLS
Interface
Group

67. Ingress Port Table
Table ID Selector Treatment
0 (No traffic selector criteria for this flow) Transition: TABLE: 10 (VLAN Table)
67

68. OF-DPA Pipeline – Destination Leaf (II)
68
MPLS
Interface
Group

69. VLAN Table
Table ID Selector Treatment
10
IN_PORT: 1 (Spine node connected port)
VLAN_VID: None
VLAN_PUSH: vlan
VLAN_ID: 4094
Transition: TABLE: 20 (Termination
MAC Table)
69

70. OF-DPA Pipeline – Destination Leaf (III)
70
MPLS
Interface
Group

71. Termination MAC Table
Table ID Selector Treatment
20
IN_PORT: 1
ETH_DST: 00:00:00:00:02:80 (Current node
MAC)
ETH_TYPE: ipv4
VLAN_VID: 4094
Transition: TABLE: 30 (Unicast
Routing Table)
71

72. OF-DPA Pipeline – Destination Leaf (IV)
72
MPLS
Interface
Group

73. Unicast Routing Table
Table ID Selector Treatment
30
ETH_TYPE: ipv4
IPV4_DST: 10.6.2.1/32 (Destination IP)
Def[GROUP: 0x20000031] (Deferred)
Transition: TABLE: 60 (ACL Policy
Table)
73

74. OF-DPA Pipeline – Destination Leaf (V)
74
MPLS
Interface
Group

75. ACL Policy Table
Table ID Selector Treatment
60 (No traffic selector criteria for this flow) NOACTION
75

76. OF-DPA Pipeline – Destination Leaf (VI)
76
MPLS
Interface
Group

77. L3 Unicast Group
Group ID Type Bucket Action
0x20000031 Indirect
ETH_DST: 00:00:00:00:00:03 (Destination MAC)
ETH_SRC: 00:00:00:00:02:80 (Current leaf switch MAC)
VLAN_ID: 40
GROUP: 0x280003 (L2 Interface Group)
77

78. OF-DPA Pipeline – Destination Leaf (VII)
78
MPLS
Interface
Group

79. L2 Interface Group
Group ID Type Bucket Action
0x280003 Indirect
VLAN_POP
OUTPUT: 3 (Destination output port)
79

80. Thank You
80

81. Reference
[1] “Segment Routing Architecture.” IETF. [Online]. Available: rfc8402
[2] “Segment Routing: A Tutorial.” APNIC, 22 Feb. 2016. [Online]. Available:
https://www.slideshare.net/apnic/segment-routing-a-tutorial
[3] “Segment Routing with MPLS data plane.” IETF. [Online]. Available: draft-ietf-
spring-segment-routing-mpls-14
[4] “ONOS+P4 tutorial (master).” ONF. [Online]. Available: http://bit.ly/onos-p4-
tutorial-slides
[5] “Fabric Design Note.” OpenCORD Wiki. [Online]. Available:
https://wiki.opencord.org/display/CORD/Fabric+Design+Note
[6] “OpenFlow™ Data Plane Abstraction (OF-DPA): Abstract Switch Specification.”
Broadcom. [Online]. Available: https://docs.broadcom.com/docs/12378911
81