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Packet Voice
Backbone Network
Design
Matt Kolon
February 23rd, 2004
APRICOT 2004 - Kuala Lumpur
Copyright © 2003 Juniper Networks, Inc.
Proprietary and Confidential
www.juniper.net
1
Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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Proprietary and Confidential
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2
Business: IP Voice Trunking

Service Provided: Point-to-Point “IP trunk” with low-latency QoS
and guaranteed bandwidth. Usually to replace a pure FR service.

SP implements it with circuit-oriented access network(s) and a
Traffic Engineered MPLS tunnel in the IP/MPLS backbone

All VoIP “application” intelligence resides in enterprise private
devices (e.g. IAD/Media Gateway, IP PBX, SIP phones, etc)
IP trunk
Enterprise HQ
Enterprise
Remote Site
MPLS LSP
FR/TDM
DSLAM
IP PBX
SIP
IAD
ATM
IP/MPLS
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POTS
ETH/VLAN
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3
Business: [Vo]IP transport VPNs

Service Provided: Multipoint IP VPN with low-latency QoS and guaranteed
bandwidth, suitable for voice traffic. Often part of a multi traffic class IP
VPN offering (VoIP being only one traffic class).

SP implements it with circuit-oriented access network(s) and a mesh of
Traffic Engineered MPLS tunnels in the backbone. Or pure Diffserv
approach with traffic trend monitoring. Or Layer 2 VPLS. Or IPSec…

All VoIP “application” intelligence resides in enterprise private devices
Enterprise
Remote Sites
(Vo)IP VPN
Enterprise HQ
DSLAM
FR/TDM
IAD
POTS
IAD
POTS
IP PBX
SIP
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DSLAM
IP VPNs
ETH/VLAN
Proprietary and Confidential
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4
Business: IP VPNs + Managed VoIP

Service Provided: Multipoint IP VPN with low-latency QoS and guaranteed
bandwidth; Managed VoIP equipment in customer premises.

SP implements it with circuit-oriented access network(s) and a mesh of Traffic
Engineered MPLS tunnels in the backbone. Or pure Diffserv approach with traffic
trend monitoring. Or Layer 2 VPLS. Or IPSec. Or private line (e.g. FR) links. Etc.

All VoIP “application” intelligence resides in managed devices (e.g. IAD/Media
Gateway, IP PBX, etc) located in customer premises.
IP Telephony
Enterprise
Remote Sites
(Vo)IP VPN
Enterprise HQ
DSLAM
FR/TDM
IAD
POTS
IAD
POTS
IP PBX
SIP
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DSLAM
IP VPNs
ETH/VLAN
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Business: TDM/telephony, VoIP core

Service Provided: regular TDM Telephony (transport and application)

SP implements it with a TDM access network, Media Gateways, an IP Core, a
PSTN core, and PSTN mediation mechanisms. This is a Class 4/5
replacement application, not directly visible to the end users.

VoIP “application” intelligence (servers and gateways) hosted by the SP,
overlaid on IP backbone, coupled with PSTN “intelligence”.
Enterprise Site 1
TDM / Telephony
TDM / Telephony
Enterprise Site 2
IP/MPLS
POTS
GE
CSU/DSU
MPLS LSP
TDM
GE
TDM
TDM PBX
TDM
POTS
TDM
Softswitch
SIP
Softphone
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PSTN/SS7
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Carrier: signaling transport

Service Provided: IP VPN to convey IP-based signaling & control
messages (SS7-over-IP, SIP, H.323, MGCP/Megaco, TCAP/IN, etc) with
proper CoS and insulation.

SP implements it with an IP/MPLS Core. Could be operated by the voice
carrier, or outsourced to an IP provider.

VoIP “application” intelligence
(servers and gateways) hosted
by the SP, overlaid on IP
backbone, coupled with PSTN
intelligence.
Media
Gateway
Softswitch
Class 4/5
Signaling
IP/MPLS
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Carrier: inter-domain VoIP peering

Service Provided (to end users): public telephony (VoIP or POTS)

Main goal is to create a VoIP peering point between carriers

SP implements it with “virtual” IP-to-IP gateways, plus inter-domain
signaling (e.g. SIP or SS7). May require true media/codec transcoding, or
“simple” IP forwarding.

Complex business peering issues are addressed by the signaling layer.
IP-to-IP
“Virtual”
Gateways
IP/MPLS
IP/MPLS
MPLS LSPs
SIP/H.323
Gatekeeper
MPLS LSPs
Softswitch
Softswitch
SIP/H.323
Gatekeeper
Peering Signaling
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Business: IP Centrex, Softswitch

Service Provided: IP Centrex (a.k.a. Hosted IP Telephony) to Small &
Medium VoIP-enabled Businesses.

SP implements it with a broadband access network(s), a VPN enabled
IP/MPLS backbone, softswitches with Centrex intelligence, and PSTN
gateways (transport & signaling).

All VoIP “application” intelligence is hosted by the SP, as well as PSTN
gateway mechanisms.
“Virtual PBX”
IP Centrex
Enterprise Site
IP VPN
SIP
Sig. Gateway
MG
Modem
DSLAM
Softswitch
FR/TDM
IP/MPLS with VPNs
SS7
PSTN
Media Gateway
POTS
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9
Residential: VoIP / telephony




Service Provided: regular telephony services (transport and application),
via VoIP, in addition to regular Broadband Internet.
SP implements it with a broadband access network, an IP/MPLS Core, a
PSTN core, and PSTN mediation mechanisms.
VoIP “application” intelligence hosted by the SP, overlaid on IP backbone,
and coupled with PSTN “intelligence”
CPE could be a mere bridge, or an IP router, or a full-blown media
gateway (POTS phones). Home network could be ETH, WLAN, etc.
Household/SOHO
IP / Telephony
INTERNET
IP / Telephony
Household/SMB
IP/MPLS
CPE
CPE
MPLS LSP (hierarchical)
SIP or
H.323
DSLAM
POTS
DSLAM
CMTS
CMTS
SIP or
H.323
POTS
Softswitch
PSTN/SS7
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Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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11
Goals for Packet Voice Networks
 Quality
• Deliver a grade of voice service equivalent to
that provided by the current Public Switched
Telephone Network (PSTN).
 Multiservice
• Voice service must live on a common IP
backbone with a set of other services.
 Flexibility
• Must be capable of supporting future
applications that may not yet be fully defined.
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Quality: MOS Model
 Voice quality in the PSTN network has historically
been measured using ‘mean opinion score’ (MOS).
 The mean opinion score measures the subjective
quality of a voice call.
 Historically the telephony providers invited people
and used various call types (with delay, echo etc.)
and recorded the results.
 MOS scores for “acceptable” voice have been
dropping, but quality is still important.
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Quality: Voice-worthy IP Backbones
 Sufficient bandwidth for voice + other services
 Delay: Less than 40msec
 Jitter: Less than 20msec
 Loss: Less than 2%
 Availability: Better than 4 9s, less than 1% blocking
 Security: No unauthorized intrusion or DoS effects
 Predictability: None of this changes in unforseeable
ways
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Engineering VoIP Experience Levels
Over-Provisioned
Network
Best Effort
Over-Subscribed
Network
Enhanced Delivery
Carrier-Grade
Multi-Service
Network
Assured Experience
Experience Levels
None (State-less)
Planning/Reporting (Historical)
Reactive (Real-time)
Service Level State
Best Effort
Diff-Serv
MPLS (Core) / Static (Access)
MPLS (Core) / Dynamic (Access)
QoS
Flat
Access / Core
Integrated End-to-End
Network Resources
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Core Domain VoIP Solutions
Over-Provisioned
Network
Core
Access
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Best Effort
Access
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Core Domain VoIP Architecture
Best Effort
 A Best Effort Experience is achieved by transporting voice over IP networks
without special treatment
• All packets delivered according to equal prioritization router queuing
throughout network
 Best effort engineered networks require over-provisioning to account for
peak traffic bursts associated with data applications and busy voice hours
 Studies and experience both show that today’s well engineered overprovisioned networks based on current routing technologies can support
most voice services
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Core Domain VoIP Architecture
Best Effort
Failure Detection ~ 300 ms – 1+ sec (without optimizations)
Route Convergence ~ 10+ sec (area size dependant)
Causes temporary service interruption, degradation of capacity
Router
Failure
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Core Domain VoIP Architecture
Best Effort
Routing protocols unable to detect route around congestion
Causes temporary service interruption, degradation of capacity
O
Link
Congestion
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Core Domain VoIP Architecture
Best Effort – Pros & Cons
Pros
 Inexpensive
 Simple
 Studies show that overprovisioning provides satisfactory
delay and jitter performance
 Sufficient strategy for voice-only
and over-provisioned networks
Cons
 Performance levels not
maintainable across failures
and congestion
 Not adequate for oversubscribed networks
 Challenges inherent with
building over-provisioned
networks
 Does not provide admission
control constructs
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Core Domain VoIP Solutions
Over-Subscribed
Network
Core
Access
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Enhanced Delivery
Differentiated Services
Access
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Core Domain VoIP Architecture
Enhanced Delivery
 Differentiated Services (Diff-Serv) facilitates the ability to
provision separate service classes such that they receive
particular treatment levels
 Packets marked accordingly before entering the network
 Participating routers process packets according to Diff-Serv
marking
 Router Diff-Serv processing variables
• Queuing (priority levels)
• Scheduling (strict, weighted, round-robin, etc)
• Congestion avoidance (RED, WRED)
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Core Domain VoIP Architecture
Enhanced Delivery
 DiffServ markings (DSCP) scale well
 DSCP’s can be AS-Dependant
• Router DSCP mediation requirement
 DSCP may be mapped to other QoS technologies across
network
• QoS migration
• Network segment QoS interworking
 DiffServ adds deterministic behavior to packet class
transport
• This benefit enhances transport behavior in secondary
path re-route optimizations
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Core Domain VoIP Architecture
Enhanced Delivery
•Cycle through output
queues emptying from
highest to lowest priority
•DiffServ markings map to
queue level
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High Priority Queue
Medium Priority Queue
Low Priority Queue
•Queuing schedulers
typically allow for variable
weighting/emptying
•Queue sizes typically
variable/provisionable
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Core Domain VoIP Architecture
Enhanced Delivery
Failure Detection ~ 300 ms – 1+ sec (without optimizations)
Route Convergence ~ 10+ sec (area size dependant)
Re-Route performance doesn’t benefit from DiffServ treatment
Causes temporary service interruption, degradation of capacity
Router
Failure
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Core Domain VoIP Architecture
Enhanced Delivery
Routing protocols unable to detect route around congestion
High-priority-marked VoIP flows will take longer to be affected by
congestion than lower priority flows
May cause temporary VoIP service interruption, degradation of
capacity, will affect other services
O
Link
Congestion
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Core Domain VoIP Architecture
Enhanced Delivery – Pros & Cons
Pros
Cons
 Adequate for oversubscribed networks
 Performance levels not
guaranteed across failures
and congestion
 Enhanced flow treatment
for VoIP across failure reroute paths
 Lowers per-router hop
latency
 Link bandwidth statistics not
maintained or usable
 Does not enable admission
control constructs
 Adds flow-based traffic
engineering model
 Scales easily
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Core Domain VoIP Solutions
Carrier-Grade
MultiService Network
Core
Access
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Assured Experience
MPLS-TE
Access
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Core Domain VoIP Architecture
Assured Experience
 Assured Experience networks are built upon an
intelligent network resource plane
 Allow the service provider to guarantee
deterministic performance to its customers under
all network conditions
• Even during network congestion and element
failures
 Facilitate multi-service network infrastructures
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Core Domain VoIP Architecture
Assured Experience
 The Intelligent Network Resource Plane…
• Maintains resource state, such as
• Link Bandwidth – up/down, total and current allocation
• Facilitates connection-oriented traffic engineering constructs, such
as…
• Constraint Based Routing Control
• Flow Classification and Forwarding
• Supports fault tolerance constructs, such as
• Fail-over Resources – routes, network elements
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Core Domain VoIP Architecture
Assured Experience – MPLS
 MPLS supports the requirements of Intelligent
Network Resource Plane
 MPLS was designed to ease the provisioning and
maintenance of efficient packet data networks
 IGP and BGP routing protocols building forwarding
tables based on shortest path only
 MPLS separates the route control and packet
forwarding such that policy-based paths may be
constructed
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Core Domain VoIP Architecture
Assured Experience – MPLS
 MPLS is based on…
• Label Switched Paths (LSP)
• Link Attribute Distribution (IGP/BGP protocol extensions)
• Traffic Engineering Databases (TED)
• Constrained-Shortest-Path-First Algorithm (CSPF)
• Label Distribution Protocols (LDP)
• Label Edge Routers (LER) and Label Switch Routers
(LSR)
 MPLS-TE facilitates constraint-based routing
 We’ll talk more about MPLS items later…
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Core Domain VoIP Architecture
Assured Experience – MPLS Route Protection
 Primary LSP / Secondary LSP Configuration
• Allows for backup physical path TE
 Fast Rerouting
• Facilitates dynamic routing around link / node failures
 Fate Sharing
• Limit backup LSP crossing of the same physical
elements as primary LSP
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Core Domain VoIP Architecture
Assured Experience – MPLS
•Traffic Engineering creates LSP’s
•Labels are distributed to construct LSP’s
LER
•Packets are classified / Labels added
•L2/L3 policy application
•Upstream flows policed, downstream
flows shaped
LSR
LER
•LSR’s only inspect label
•Label is removed from packets
•Label and interface table lookup
•Packets are routed to
destination
•Output label and interface
•Queue and drop accordingly
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Core Domain VoIP Architecture
Assured Experience – MPLS
Failure Detection ~ 20 – 30 ms
Fast Reroute < 50 ms
Small amount of packet loss during failover
Service interruption not noticeable, minimal capacity degradation
Router
Failure
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Core Domain VoIP Architecture
Assured Experience – Pros & Cons
Pros
Cons
 State-full, intelligent network
resource plane
 Fully meshed topologies
suffer from n2 scaling issues
 Designed to ease TE design,
maintenance and management
 Facilitates class-based
forwarding for multi-service
networks
 Interworks with disparate QoS
mechanisms and transport
technologies
 Supports hierarchical forwarding
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Multiservice: Service Classes
 From this…
Control
To this….
Data
Control
Internet
Voice
VPN
 Easy to think of as “CoS”, but actually involves
much more than traditional router CoS or QoS
mechanisms.
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Multiservice: Bundled Service Offerings
 Plain VoIP service model is proven to be non-sustainable
• First generation of pure VoIP carriers are gone
• Price of 1 min of voice has fallen through the floor
 VoIP with other services is the way to go
• Value-add: Unified messaging, voice accessible content,
video telephony
• Additional non-voice: Broadcast video, surveillance, etc.
VPNs and other business services
• Generate more revenue, key differentiator from
competitors
• Can be offered at minimum additional cost
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Final Thought on Goals:
Who Really Knows?
 Future service revenue
• By definition unknowable, will always surprise us…
• Immense possibility in diverse areas such as mobile,
micropayment, handheld videoconferencing…
• Infrastructure must have:
• Unrestricted future service rollout
– Vendors must design flexible hardware and software platforms
• Upgradeable without forklift
• Capability to support many services at one time, without the
services affecting each other
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Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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QoS: Bandwidth
 VoIP traffic is constant bit stream, bandwidth required
varies with which codec used, # of voice sample per
packet, and transport media used.
 Even G.711 packets are only ~80 bytes, each call only
~112 kbps.
 VoIP packet is very small for compressed codecs
• G.729 with two 10ms samples/frame yields 24Kbps
without layer2 headers
 Line rate processing of VoIP packets is crucial!
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QoS: Delay
 ITU G.114: <150ms for one-way, e2e
 Delay Budget:
• T f Packet formation delay, O(10ms)
• Tsf Packet switching delay, O(10us) per Hop
Si
•
Serialization delay, (#bits/link rate*#Hop)
Pi
• Q Propagation delay, (1ms/100mile)
•
Queuing delay, (variable)
 typical backbone delay requirement: <30ms
max
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QoS: Jitter
 Definition: Variations in packet arrival time
 Causes:
• Queuing variation under changing network load condition
• Load sharing over changing paths
 De-jitter (“playout”) buffer in gateways
• Static or dynamic
• Adds to the overall delay
 Best to avoid causes of Jitter rather than trying to buffer it away.
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QoS: Packet Loss
 UDP as transport
• no flow control
• doesn’t tolerate packet loss very well
 <1% to avoid quality degradation
 <5% if VoIP gateway provides concealment
mechanism
 Higher compression rates demand lower loss
budgets
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Network Availability & Recovery
 Availability
• Common SLA for VoIP network: 99.995% or 26 min/yr
• Availability needs continue to increase
 Recovery
• O(sub-second) to avoid session timeout and new call
setup
• VoIP gateway to gateway recovery usually spans over
several segments
• Layer 3 based network recovery is generally
unacceptable
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Network Security

Guard against un-trusted network elements and networklevel attacks

Stateful and stateless firewall capabilities may be
necessary

Authentication to Prevent Fraud
• RADIUS most common deployment

Confidentiality is emerging as another basic security
requirement for VoIP
• Carry VoIP traffic within VPN, such as IPsec tunnel or
MPLS VPN
• Increased security vs. encryption overhead for VoIP
packet
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Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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Topological Assumptions
 Routers deployed in pairs at each site
• Primarily for fault tolerance
• Also useful for load sharing
 Intra-site connections required in all topologies
• Must be at least same capacity as inter-site
trunk links
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Core Topologies
 Star-connected Core
• “Outer core”
connected
to two
“super-routers”
• Simple routing and
forwarding
• Probably least
expensive
• Concerns about redundancy
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Core Topologies
 Fully-connected Mesh
• Each router connected
to every other site
• Also simple routing and
forwarding
• Perhaps most
expensive
• Mesh can always
be reduced!
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Core Topologies
 Half-mesh router groups
• Each router connected
to ~half of other sites
• More complex routing
and forwarding
• Many full-mesh benefits
without the expense
• Success depends on
engineering to
particular needs
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Edge-core Topologies 1
 Single uplink per router edge site
• Two connections
to two routers in one
core site
• Availability largely
dependent on physical
layout
Core
Router
Site 1
Ed g e
Ro u ter
S it e D
Co re
Ro u ter
• Usually lowest cost
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Edge-core Topologies 2
 Single uplink per router edge site
• Two site connections
to two separate
routers
• Availability depends on
physical media
• Somewhat low cost
Core
Router
Site 1
Ed g e
Ro u ter
S it e C
Co re
Ro u ter
S it e 2
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Edge-core Topologies 3
 Partially duplicated edge router uplinks
• Three connections
to three separate
routers
• One dual-homed,
Edge
one not
Router
Site B
• Particularly useful in
MPLS topologies
• High availability
• Somewhat high cost
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Ro u ter
S it e 1
Co re
Ro u ter
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Edge-core Topologies 4
 Fully duplicated edge router uplinks
• Four connections
to four separate
routers
• Both edge routers
dual-homed
• Highest availability
Ed g e
Core
Router
Site 1
Ro u ter
S it e A
Co re
Ro u ter
S it e 2
• Highest cost
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Site Connection to Edge Routers
 Many variants on dual-homed
designs possible
 Essential idea is suitable
for gateway or
softswitch sites
Media
 Best-effort traffic
may enter
through separate
aggregation
points
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IGP Selection

Two options:
• ISIS
• OSPF

Very close race!

Biggest issue is probably legacy deployment in current
network, and customer comfort.

ISIS has slight edge in terms of sub-second failure
detection

Main point is that a successful network can be built with
either.
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IGP Configuration
 Issues to consider
• Hierarchy (areas or levels)
• Hello Timers
•BFD changes things here!
• Authentication for security
• Addressing plan
•ISIS requires ISO NET lo0 addresses
• Metrics
• Load balancing
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Load-balancing Considerations

Two approaches to load balancing
• Per-destination
• Single path chosen from equal-cost next hops
• Simpler to predict
• Per-flow
• Flow distributed between equal-cost next hops
• Policy can restrict potential traffic path

Choice depends primarily on topology and other requirements

Most voice engineers more comfortable with per-destination
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Forwarding Protection Protocol Options
 Link Redundancy
• MLPPP – T1/E1 Link aggregation
• 802.3ad – Ethernet aggregation
• SONET/SDH aggregation
 SONET/SDH APS/MPS
 Virtual Router Redundancy Protocol (VRRP)
 Standard IGP protocols
• OSPF
• ISIS
 Bidirectional Forwarding Detection (BFD)
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Bidirectional Forwarding Detection
(BFD)
 IETF Draft co-authored by Juniper and Cisco
 Optimized timer-based link failure detection protocol
• Brings link failure detection in line with today’s highspeed transport technologies
 Reduces link failure recognition from seconds to 10’s of
milliseconds
• Provisionable for link/service requirements
 Operates at packet forwarding plane
• Independent from routing protocols and applications
 When run between edge router and media gateway,
provides network resource to VoIP service link
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MG to Router Connection with BFD
VoIP
Line Cards
MG
BFD-A1bu
BFD-B1bu
BFD-B2
BFD-A2
BFD-B2bu
BFD-A2bu

Line
Cards
BFD-B1
Line
Cards
BFD-A1
VoIP Line Card Failure
• Connectivity of A1 protected by B1 (vice-versa)
• Call preserved only under specific MG application control

Router PIC Failure
• Connectivity of A1 and B1 protected by A2 and B2 respectively (vice-versa)
• Call preserved with packet-loss period (dependant on detection and re-route times)

Router System Failure
• Connectivity of A and B protected by Abu and Bbu respectively (vice-versa)
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Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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IP CoS Functions
Per-flow Rate
Policing
Traffic
Classification
&
Marking
Priority
Queuing
Congestion
Avoidance
W
R
R
RED
• IP Flow
• IP Precedence bits, DSCP Byte
• MPLS CoS bits
100%
Stream
• Incoming Physical Interface
• Incoming Logical Interface
• Destination IP address
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100%
100%
PLP=1
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PLP=0
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Converged Network CoS Design

In a voice / best effort network, three classes (at least) of service are
necessary:
•
IP network control traffic
• Low bandwidth requirements, not sensitive to latency, jitter
• Must not be starved
•
Voice signaling and bearer traffic
• Highest latency and jitter requirements
•
Best effort data traffic
• Whatever capacity is left

More complex configurations may or may not be needed in other
network designs (e.g. with VPN service)

More classes = more complexity, no way around this.
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Converged Network CoS Design
 Queue 0 : IP Network Control traffic
• Allocated bandwidth : 5% (the default for
NC)
• Priority: High; this guarantees that NC
traffic will never be starved of bandwidth.
• No RED drop profile assigned, as NC traffic
should never be dropped.
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Converged Network CoS Design


Queue 3 : Voice Signaling and Bearer traffic
•
Initial requirement is 50% of total traffic.
•
Allocated bandwidth: 20%; although doesn’t really
matter as this queue gets strictly high priority.
•
Strictly High Priority: voice can take as much
bandwidth as it needs.
RED drop profile: drop nothing until queue is full, then
drop everything.
•
Dropping packets randomly is not very suitable on
voice traffic.
•
Forces head dropping (rather than tail dropping)
once queue is full.
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Converged Network CoS Design

Queue 1 : Best effort
•
Allocated bandwidth: remaining 75%.
•
Not guaranteed
•
Priority: Low; this traffic is served only if there is no
voice traffic, and there is bandwidth available.
•
RED drop profile: medium. This can be fine tuned,
perhaps start to drop when queue is 70%, with a
probability of 30%, then drop 100% of the traffic
when queue fullness reaches 90%.
•
Medium RED drop profile will limit the TCP
congestion synchronization phenomena that would
occur otherwise.
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More Services Possible!
 Multiservice queuing
• Service VoIP Queue Aggressively to Avoid Filling the Queue
Best Effort Traffic
Queue 0 = 50%
WRR Service Rate = 15%
VPN Traffic
Queue 1 = 35%
WRR Service Rate = 15%
VoIP Traffic
Queue 2 = 10%
WRR Service Rate = 65%
Network Control
Traffic
Queue 3 = 5%
WRR Service Rate = 5%
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Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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Don’t Stop Sending the Voice!
 It doesn’t matter what happens otherwise…
• Customers only notice when the call is interrupted
 Many call this idea “Non-Stop Forwarding”
 Main Principles of NSF
• Data Plane should not be disrupted
• Control plane failures should not effect forwarding
• Failures happen but the infrastructure can recover gracefully
• Management/Routing sessions can be re-connected unnoticed
 Many Vendors Adopting this approach
• Not all, some favor fully redundant protocol state mirroring
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Graceful Restart - How ?
 Restarting node preserves the forwarding state
 Control plane failure known only to the Routing peers
 Routing peers preserve routing information of restarting node
 Restarting node (re)learns its routing information from its
routing peers
 No preservation of any of the protocol-related state across the
restart on restarting node
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Graceful Restart - How ?
Separate control
and data planes
P
P
PE 2
When router recovers,
neighbors sync up
without disturbing
forwarding.
PE 1
If router’s control
plane fails, data
plane can keep
forwarding packets
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PE 3
P
Neighbors hide
failure from all
others routers
in the network
P
Other routers
are never made
aware of failure
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Graceful Restart - How ? (cont.)
 Graceful restart mechanisms are protocol specific:
• BGP for Interdomain routing
• ISIS and OSPF for IGPs
• LDP and RSVP for LSP management
• BGP/MPLS specific to MPLS VPN management
• RIP – already built in, but a draft nonetheless
 All these are currently IETF drafts, but implemented by major vendors
 (this isn’t an unusual situation, many examples of this these days)
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Hitless RE Switchover
Routing Engines

Protects against Single Node Hardware Failure

Primary (REP) and Secondary (RES) utilize
keepalive process
Keep
Alive
• Automatic failover to RES
• Synchronized Configuration

REP and RES share:
• Forwarding info + PFE config

Packet Forwarding
Engines
REP failure does not reset PFE
• No forwarding interruption
• Only Management sessions lost
• Alarms, SNMP traps on failover
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Agenda
 Packet voice essentials
• Quick background: VoIP Applications
• Customer Goals for Voice
• VoIP Traffic Characteristics
 Packet voice backbone design
• Class of service
• High Availability
• MPLS
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IP-only Path Selection
 Largely dependent on routing protocols
 Adjustable only through metrics
• Changes tend to be global
• Difficult on per-application basis
• Extremely manual and labor-intensive in nature
• Requires offline path computation
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IP-only Network Reliability
Mechanisms
•Connection-oriented transport (TCP)
•Not used for realtime traffic like voice
•Dependence on underlying network infrastructure
•E.g. SONET/SDH APS, Ethernet VRRP, ATM
•Not IP-based, therefore not network-wide
•Routing protocol recovery
•Relatively slow convergence
•Potential system-wide effects
•BFD improves this, but not enough by itself
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Enter MPLS
•Low-overhead virtual circuits for IP!
•Gives many Voice-friendly attributes to IP
•DiffServ-compatible CoS
•Deterministic path selection
•Failure recovery via:
•Fast reroute
•Secondary LSPs
•Planning and determinism through circuit-like
traffic engineering
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MPLS-TE network optimization
•Traffic engineering
allows deterministic
paths for Voice and other
realtime data, similar to
circuit switched networks
•Constraint-based routing
can dynamically choose
paths best suited to
applications or types of
traffic
label-switched-path HK_to_Tokyo {
to Tokyo;
from Hong_Kong;
admin-group {exclude red}
cspf}
Seoul
Tokyo
Hong Kong
Taipei
Kuala Lumpur
Manilla
Singpore
Jakarta
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MPLS CoS Capabilities
•EXP field (and label) can be used for CoS
Label (20-bits)
L2 Header
MPLS Header
CoS S
TTL
IP Packet
32 bits
•DiffServ-compatible
•Consistent meanings can exist for MPLS EXP
(and label) and IP DiffServ per-hop behaviors
•Core (MPLS) and edge (IP/DiffServ) PHBs can
be related and consistent
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What is Diff-Serv TE ?

Diff-Serv: scheduling/queueing behaviour at each node depends on traffic type
(indicated by DSCP/EXP setting)

MPLS TE: use of constraints to control placement of LSPs. Typically, various
traffic classes share the same LSP. Bandwidth reservations do not take account
of the classes of traffic involved.

MPLS Diff-Serv TE:
• Traffic divided into up to eight Class-Types.
• CSPF and RSVP take the Class-Type into account when computing path of
LSP.
• Results in More granular bandwidth reservation.

On each link in network, can have separate bandwidth constraints for each type
of traffic
• E.g. limit the bandwidth taken by voice LSPs on a link to a maximum of
40%, data LSPs take the rest.
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CoS / QoS & Forwarding

Diff-Serv-aware MPLS Traffic Engineering

Guaranteed bandwidth for MPLS
• Combines MPLS Diffserv and Diffserv TE
• Provides strict point to point QoS guarantees
Aggregated State (DS)
Aggregate Admission Control (DS-TE)
Aggregate Constraint-based Routing (DS-TE)
No state
Aggregated state
Per-Flow state
MPLS Diff-Serv + MPLS
DS-TE
Best effort
Diff-Serv
MPLS
Guaranteed
Bandwidth
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& Int-Serv
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How DS-TE Operates

Extended IGP
Routing Table
Traffic Engineering
Database (TED)
Operations Performed by the
Ingress LSR
Constrained
Shortest Path First
User
Constraints
1) Store information from IGP flooding
2) Store traffic engineering information
Explicit Route
3) Examine user defined constraints
4) Calculate the physical path for the LSP
5) Represent path as an explicit route
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MPLS failure recovery
•Fast reroute allows rapid switching to alternate link
segments while longer-term repairs are made
•Secondary LSPs provide deterministic alternate
paths during link failure
•Possible in a consistent, network-wide manner
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MPLS Fast Reroute
Single user command
at head end to enable
Fast Reroute.
Detour
Detour
Primary
LSR1
Primary
LSR2
Detour
Primary
LSR3
Primary
LSR4
LSR5
• Fast reroute is signaled to each LSR in the path
• Each LSR computes and sets up a detour path
that avoids the next link and next LSR
• Each LSR along the path uses the same route
constraints used by head-end LSR
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MSeconds
MPLS Fast Reroute:Recovery Times
400
350
300
250
200
150
100
50
0
Max
Average
Min
5.0
5.1
5.2
5.3+
JUNOS version
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Summary
•VoIP deployments are going ahead
•Good for provider profits
•Good for customer services and needs
•The question is no longer “if”, but rather “how”
•Luckily:
•There are tools that make voice backbones
•Possible
•High-quality
•Profitable
•Diff-serv, NSF, and MPLS are up to the job
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Thank You