1) The document discusses using segment routing for traffic engineering. It provides an overview of segment routing technology, use cases, control and data plane operations, and how segment routing can be used for traffic engineering. 2) Key aspects covered include how segment routing works by encoding a path as an ordered list of segments, different types of segments (IGP prefixes, adjacencies, BGP), and how this allows for application-engineered end-to-end paths. 3) Traffic engineering with segment routing provides explicit routing, supports constraint-based routing without needing RSVP-TE, and uses existing IGP extensions to advertise link attributes.

1. Traffic Engineering using Segment Routing
Santiago Álvarez

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3. § Technology Overview
§ Use Cases
§ A Closer Look to Control and Data Plane
§ Traffic Engineering
§ Conclusions
Agenda

4. Technology Overview

5. § Source Routing
§ the source chooses a path and encodes it in the packet header as an ordered
list of segments
§ the rest of the network executes the encoded instructions without any further
per-flow state
§ Segment: an identifier for any type of instruction
§ forwarding or service
Segment Routing

6. § Shortest-path to the IGP prefix
§ Global
§ 16000 + Index
§ Signaled by ISIS/OSPF
IGP Prefix Segment
DC (BGP-SR)
10
11
12
13
14
2 4
6 5
7
WAN (IGP-SR)
3
1
PEER
16005

7. § Forward on the IGP adjacency
§ Local
§ 1XY
§ X is the “from”
§ Y is the “to”
§ Signaled by ISIS/OSPF
IGP Adjacency Segment
DC (BGP-SR)
10
11
12
13
14
2 4
6 5
7
WAN (IGP-SR)
3
1
PEER
124

8. § Shortest-path to the BGP prefix
§ Global
§ 16000 + Index
§ Signaled by BGP
BGP Prefix Segment
DC (BGP-SR)
10
11
12
13
14
2 4
6 5
7
WAN (IGP-SR)
3
1
PEER
16001

9. § Forward to the BGP peer
§ Local
§ 1XY
§ X is the “from”
§ Y is the “to”
§ Signaled by BGP-LS
(topology information) to
the controller
BGP Peering Segment
DC (BGP-SR)
10
11
12
13
14
2
6
7
WAN (IGP-SR)
3
1
PEER
Low Lat, Low BW
4
5
High Lat, High BW
147

10. § WAE collects via BGP-LS
§ IGP segments
§ BGP segments
§ Topology
WAN Controller
DC (BGP-SR)
10
11
12
13
14
2 4
6 5
7
WAN (IGP-SR)
3
1
PEER
Low Lat, Low BW
BGP-LS
BGP-LS
BGP-LS

11. § WAE computes that
the green path can be
encoded as
§ 16001
§ 16002
§ 124
§ 147
§ WAE programs a
single per-flow state to
create an application-
engineered end-to-end
policy
An end-to-end path as a list of segments
DC (BGP-SR)
10
11
12
13
14
2 4
6 5
7
WAN (IGP-SR)
3
1
PEER
Low Lat, Low BW
50
Default ISIS cost metric: 10
{16001,
16002,
124,
147}
PCEP, Netconf,
BGP

12. § IETF standardization in SPRING
working group
§ Protocol extensions progressing in
multiple groups
§ IS-IS
§ OSPF
§ PCE
§ IDR
§ 6MAN
§ Broad vendor and customer
support
Segment Routing Standardization
Sample IETF Documents
Segment Routing Architecture
(draft-ietf-spring-segment-routing)
Problem Statement and Requirements
(draft-ietf-spring-problem-statement)
IPv6 SPRING Use Cases
(draft-ietf-spring-ipv6-use-cases)
Segment Routing Use Cases
(draft-filsfils-spring-segment-routing-use-cases)
Topology Independent Fast Reroute using Segment Routing
(draft-francois-spring-segment-routing-ti-lfa)
IS-IS Extensions for Segment Routing
(draft-ietf-isis-segment-routing-extensions)
OSPF Extensions for Segment Routing
(draft-ietf-ospf-segment-routing-extensions)
PCEP Extensions for Segment Routing
(draft-ietf-pce-segment-routing)
Close to 30 IETF drafts in progress

13. § Platforms: ASR9000, CRS-1/CRS-3 (shipping)
§ IS-IS IPv4 (shipping)
§ Node/Adjacency SID advertisement
§ LDP interworking (mapping server/client)
§ Traffic protection (topology independent LFA link protection)
§ OSPFv2 (shipping)
§ Node SID advertisement
§ Traffic protection (LFA)
§ Upcoming
§ IS-IS IPv6
§ IS-IS / OSPFv2 parity
§ SR Traffic Engineering (manual provisioning and PCEP)
§ OAM (Ping/Trace)
Segment Routing Product Support

14. Use Cases

15. § IGP only
§ No LDP, no RSVP-TE
§ ECMP
IPv4/6 VPN/Service transport
1
2 3
4
6 5
7
Site1 Site2
pkt
16007
vpn
pkt
16007
vpn
pkt
pkt
vpn
pkt

16. § Seamless deployment
Seamless interworking with LDP
1
2 3
4
6 5
7
Site1 Site2
pkt
pkt
vpn
pkt
pkt
16007
vpn
pkt
16007
vpn
pkt
vpn
LDP(7)

17. § 50msec FRR in any topology
§ IGP Automated
§ No LDP, no RSVP-TE
§ Optimum
§ Post-convergence path
§ No midpoint backup state
§ Detailed operator report
§ S. Litkowski, B. Decraene, Orange
§ Mate Design
§ How many backup segments
§ Capacity analysis
Topology-Independent LFA (TI-LFA FRR)
1
2 3
4
6 5
7
pkt
16007
16005
pkt
16007
pkt
16007

18. § Traffic Matrix is fundamental for
§ capacity planning
§ centralized traffic engineering
§ IP/Optical optimization
§ Most operators do not have an
accurate traffic matrix
§ With SR, the traffic matrix
collection is automated
Automated Traffic Matrix Collection
1 2 3 4
1
2
3
4
1
2
4
3

19. § On a per-content, per-user basis,
the content delivery application
can engineer
§ the path within the AS
§ the selected border router
§ the selected peer
§ Also applicable for engineering
egress traffic from DC to peer
§ BGP Prefix and Peering Segments
Optimized Content Delivery
1 2
6
4 3
AS1
5
7
AS6AS5
AS7
pkt
16003
16002
126

20. § Per-application
flow engineering
§ End-to-End
§ DC, WAN, AGG, PEER
§ Millions of flows
§ No signaling
§ No midpoint state
§ No reclassification at
boundaries
Application Engineered Routing
DC (or AGG)
10
11
12
13
14
Push
{16001,
200, 147}
Low-Latency to 7
for application A12
2 4
6 5
7
Default ISIS cost metric: 10
Default Latency metric: 10
ISIS: 35
WAN
3
1
BSID: 200
200: pop
and push
{16002,
16004}
PEER
Low Lat, Low BW
Low-Lat to 4
PeerSID: 147, Low Lat, Low BW
PeerSID: 147, High Lat, High BW

21. § Per-application
flow engineering
§ End-to-End
§ DC, WAN, AGG, PEER
§ Millions of flows
§ No signaling
§ No midpoint state
§ No reclassification at
boundaries
Application Engineered Routing
DC (or AGG)
10
11
12
13
14
Push
{16010,
16001,
200, 147}
Low-Latency to 7,
DC Plane 0 only,
for application A12
2 4
6 5
7
Default ISIS cost metric: 10
Default Latency metric: 10
ISIS: 35
WAN
3
1
BSID: 200
200: pop
and push
{16002,
16004}
PEER
Low Lat, Low BW
Low-Lat to 4
PeerSID: 147, Low Lat, Low BW
PeerSID: 147, High Lat, High BW

22. A Closer Look to Control and
Data Plane

23. MPLS Control and Forwarding Operation with Segment
Routing
PE1 PE2
IGPPE1 PE2
Services
IPv4 IPv6
IPv4
VPN
IPv6
VPN VPWS VPLS
Packet
Transport LDP
MPLS Forwarding
RSVP BGP Static IS-IS OSPF
No changes to
control or
forwarding plane
IGP label
distribution for IPv4
and IPv6, same
forwarding plane
BGP / LDP

24. § Prefix SID
§ SID encoded as an index
§ Index represents an offset from SRGB base
§ Index globally unique
§ SRGB may vary across LSRs
§ SRGB (base and range) advertised with router capabilities
§ Adjacency SID
§ SID encoded as absolute (i.e. not indexed) value
§ Locally significant
§ Automatically allocated for each adjacency
SID Encoding
SRGB = [ 16000 - 23999 ]. Advertised as base = 16,000, range = 7,999
Prefix SID = 16041. Advertised as Prefix SID Index = 41
Adjacency SID = 24000. Advertised as Adjacency SID = 24000
SR-enabled Node

25. § Level 1, level 2 and multi-level routing
§ Prefix Segment ID (Prefix-SID) for host prefixes on loopback interfaces
§ Adjacency SIDs for adjacencies
§ Prefix-to-SID mapping advertisements (mapping server)
§ MPLS penultimate hop popping (PHP) signaling
§ MPLS explicit-null label signaling
SR IS-IS Control Plane Overview

26. § Required
§ Wide metrics
§ SR enabled under unicast address family
§ Optional
§ Prefix-SID configured under loopback(s) AF IPv4
§ MPLS forwarding enabled automatically on all (non-passive) IS-IS
interfaces
§ Adjacency-SIDs are automatically allocated for each adjacency
IS-IS Configuration

27. § IPv4 Prefix Segment ID (Prefix-SID) for host prefixes on loopback
interfaces
§ MPLS penultimate hop popping (PHP) signaling
§ MPLS explicit-null label signaling
SR OSPF Control Plane Overview

28. § OSPFv2 control plane
§ Required
§ Enable segment-routing under instance or area(s)
§ Command has area scope, usual inheritance applies
§ Enable segment-routing forwarding under instance, area(s) or interface(s)
§ Command has interface scope, usual inheritance applies
§ Optional
§ Prefix-SID configured under loopback(s)
§ MPLS forwarding enabled on all OSPF interfaces with
segment-routing forwarding configured
OSPF Configuration

29. § Packet forwarded along IGP shortest path
§ Packet leverages ECMP load balancing
§ Swap operation performed on input label
§ Same top label if same/similar SRGB
§ PHP if signaled by egress LSR
MPLS Data Plane Operation
Payload
SRGB [16,000 – 23,999 ]
X
Payload
Swap
Y
Payload
SRGB [16,000 – 23,999 ]
Y
Payload
Pop
Y
Adjacency SID = X
X
Prefix SID Adjacency SID
§ Packet forwarded along IGP adjacency
§ Pop operation performed on input label
§ Top labels will likely differ
§ Penultimate hop always pops last adjacency SID

30. Payload
VPN Label
MPLS Data Plane Operation (Prefix SID)
SRGB [16,000 – 23,999 ] SRGB [16,000 – 23,999 ] SRGB [26,000 – 23,999 ] SRGB [16,000 – 23,999 ]
Loopback X.X.X.X
Prefix SID Index = 41
A B C D
Payload
16041
Payload
Push
Push
Swap Pop
Payload Payload
VPN Label
26041
VPN Label
Pop

31. Payload
VPN Label
MPLS Data Plane Operation (Adjacency SIDs)
MPLS Label Range
[ 24000– 265535 ]
MPLS Label Range
[ 24000– 265535 ]
MPLS Label Range
[ 24000– 265535 ]
MPLS Label Range
[ 24000– 265535 ]
Payload
24000
Payload
Push
Push
Push
Pop Pop
Payload Payload
VPN Label
24000
VPN Label
Pop
Adjacency SID = 24000Adjacency SID = 24000Adjacency SID = 24010
24000
A B C D

32. § LFIB populated by IGP (ISIS /
OSPF)
§ Forwarding table remains
constant (Nodes + Adjacencies)
regardless of number of paths
§ Other protocols (LDP, RSVP,
BGP) can still program LFIB
MPLS LFIB with Segment Routing
PE
PE
PE
PE
PE
PE
PE
PE
P
In
Label
Out
Label
Out
Interface
L1 L1 Intf1
L2 L2 Intf1
… … …
L8 L8 Intf4
L9 L9 Intf2
L10 Pop Intf2
… … …
Ln Pop Intf5
Network
Node
Segment Ids
Node
Adjacency
Segment Ids
Forwarding
table remains
constant

33. Traffic Engineering

34. § Provides explicit routing
§ Supports constraint-based routing
§ Supports centralized admission
control
§ No RSVP-TE to establish LSPs
§ Uses existing ISIS / OSPF
extensions to advertise link
attributes
§ Provides ECMP
Traffic Engineering with Segment Routing
TE LSP
Segment
Routing

35. § Link information Distribution
§ ISIS-TE
§ OSPF-TE
§ Path Calculation
§ Path Setup
§ Forwarding Traffic down path
§ Auto-route (announce / destinations)
§ Static route
§ PBR
§ PBTS / CBTS
§ Forwarding Adjacency
§ Pseudowire Tunnel select
How Traffic Engineering Works
IP/MPLS
Head end
Mid-point Tail end
TE LSP

36. § Addresses complex requirements for path computation in large, multi-domain and multi-layer networks
§ Path computation element (PCE)
§ Computes network paths based on network information (topology, paths, etc.)
§ Stores TE topology database (synchronized with network)
§ May reside on a network node or on out-of-network server
§ May initiate path creation
§ Stateful - stores path database included resources used (synchronized with network)
§ Stateless - no knowledge of previously established paths
§ Path computation client (PCC)
§ May send path computation requests to PCE
§ May send path state updates to PCE
§ PCC and PCE communicate via Path Computation Element Protocol (PCEP)
§ Cisco WAN orchestration provides network path instantiation driven by an out-of-network stateful PCE
PCE Architecture Introduction
H E L L O
my name is
PCE

37. § PCE maintains topology and path
database (established paths)
§ More optimal centralized path
computation
§ Enables centralized path initiation
and update control
§ Well suited for SDN deployments
Stateful PCE
PCEP
Stateful PCE
TED
LSP DB
PCC

38. § An external PCE requires some form of
topology acquisition
§ A PCE may learn topology using BGP-
LS, IGP, SNMP, etc.
§ BGP-LS characteristics
§ aggregates topology across one or more
domains
§ provides familiar operational model
§ New BGP-LS attribute TLVs for SR
§ IGP: links, nodes, prefixes
§ BGP: peer node, peer adjacency, peer set
Topology Acquisition
Domain 1 Domain 2
Domain 0
BGP-LS
TED
BGP-LS BGP-LS
RR
PCE

39. § PCC or PCE may initiate path
setup
§ PCC may delegate update control
to PCE
§ PCC may revoke delegation
§ PCE may return delegation
Active Stateful PCE
PCEP
Active Stateful PCE
TED
LSP DB
Stateful
PCC
PCE has update
control over
delegated paths

40. Active Stateful PCE
PCE-Initiated and PCC-Initiated LSPs
PCEP
PCC-Initiated (Active Stateful PCE)
TED
LSP DB
Stateful
PCC
PCEP
PCE-Initiated (Active Stateful PCE)
TED
LSP DB
Stateful
PCC
PCC initiates
LSP and
delegates
update
control
PCE initiates
LSP and
maintains
update control
§ PCE part of controller architecture
managing full path life cycle
§ Tighter integration with application
demands
§ PCC may initiate path setup based on
distributed network state
§ Can be used in conjunction with PCE-
initiated paths

41. § Segment routing enables source routing
based on segment ids distributed by IGP
§ PCE specifies path as list of segment ids
§ PCC forwards traffic by pushing segment
id list on packets
§ No path signaling required
§ Minimal forwarding state
§ Maximum network forwarding virtualization
§ The state is no longer in the network but in
the packet
§ Paths may be PCE- or PCC-initiated
PCE Extensions for Segment Routing (SR)
PCEP
Segment List:: 10,20,30,40
Stateful PCE
TED
LSP DB
Stateful
PCC
Node
SID
Adjacency
SID
Forwarding
table remains
constant
In Out Int
L1 L1 Intf1
… … …
L7 L7 Int3
L8 Pop Intf3
… … …
L9 Pop Intf5
Application
Path
Request

42. § Segment Routing capability (when opening PCEP
session)
§ Existing ERO object with new Segment Routing
Explicit Route Object (SR-ERO) sub-object
§ Sub-objects include a segment id (SID) and/or an
associated “Node or Adjacency Identifier”(NAI)
§ A NAI can specify a
§ IPv4 node
§ IPv6 node
§ IPv4 adjacency
§ IPv6 adjacency
§ Unnumbered adjacency with IPv4 node ids
§ Request parameters indicate path type (SR or RSVP)
PCEP Extensions for Segment Routing
SID / NAI
SID / NAI
SID / NAI
SR-ERO
subobject (1)
SR-ERO
subobject (2)
SR-ERO
subobject (n)
ERO Object

43. Conclusions

44. § Minimal midpoint forwarding state required
§ No extra protocol (LDP or RSVP-TE) required to signal path
§ Native ECMP
§ Few segments required (apply Cisco Mate Design on your data)
§ Distributed computation or centralized
§ Integration with IP/Optical optimization
Conclusions

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