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© 1999, Cisco Systems, Inc.
IXC Architecture Session 1503
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Agenda • What Architecture? • Engineering and Design • Exchange Points • ISP Backbones • Services • Summary 1503 0925_04F9_c1
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The Free On-line Dictionary of Computing Architecture: Design; the way components fit together; it may also be used for any complex system, e.g. “software architecture”, “network architecture”
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Network Design and Architecture… • … can be critical • … can contribute to the success of the network • … can contribute to the failure of the network
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© 1999, Cisco Systems, Inc.
Ferguson’s Law of Engineering
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No amount of magic knobs will save a sloppily designed network Paul Ferguson—Consulting Engineer, Cisco Systems
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What Is a Well-Designed Network? • One that takes into consideration some main factors Topological/protocol hierarchy Redundancy Addressing aggregation Scaling Policy implementation (core/edge) Management/maintenance/operations Cost 1503 0925_04F9_c1
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One Must Acknowledge that… • Two different worlds exist One world revolves around private organizational networks and another concerns the global Internet
• Growth in the Internet is faster than any other technology introduced to the public-at-large 1503 0925_04F9_c1
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Technology Adoption Radio TV VCR Car Microwave PC
Telephone Electricity
Internet Airplane Cell Phone Source: Forbes Magazine July 7th— 1997 1503 0925_04F9_c1
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Thus—It Makes Sense that…
• Scaling is a major concern for ISP’s/IXC’s • Network stability is a critical component of scaling a network
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Scaling and Stability
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Focus • Network hierarchy • Scalability • Addressing and summarization • Stability • Convergence • Policy 1503 0925_04F9_c1
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“
In the Internet— scaling is the only problem Mike O’Dell—Chief Scientist, UUnet
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Network Hierarchy
• Full meshes in very large networks do not scale very well • Combine meshing and hierarchy • Link speeds should reflect topological significance • Avoid vast over-subscription 1503 0925_04F9_c1
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Full Meshing
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• N(n-1) peers • Limited scalability • No hierarchy 15
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Structured Hierarchy (Partial Mesh)
• Fewer peers • Virtually unlimited scalability • Hierarchy 1503 0925_04F9_c1
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Hierarchy Provides for Scaling Core OC3/155Mbs
Distribution T3/45Mbs
Access T1/1.5Mbs
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Layer 3 Scaling Considerations
• Convergence time • Routing instability • iBGP scaling
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Reducing IGP Convergence Time
• Segment large networks into interior routing sub-domains (e.g. OSPF areas) • Decrease peer count • Route summarization • Increase CPU and/or bandwidth 1503 0925_04F9_c1
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Routing Sub-Domains (Areas)
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Route Summarization • IP addressing is crucial and should be done in contiguous blocks which can be advertised (aggregated) as a single prefix • Based on the concepts of classless routing—where summarization can occur on any bit boundary • Greatly reduces the amount of route table bloat
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© 1999, Cisco Systems, Inc.
Classful Addressing
xxxx.
xxxx.
xxxx.
32 Bit Address Space
xxxx
24 Bit Boundary “Class C”
16 Bit Boundary “Class B”
8 Bit Boundary “Class A” 1503 0925_04F9_c1
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Classless Addressing
• Distinction of address “class” is defunct • Subnet mask can be applied on any bit boundary • Contiguous blocks of addresses can be expressed by a single prefix 1503 0925_04F9_c1
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Classless Addressing
• /32 = 255.255.255.255 = host route • /24 = 255.255.255.0 = 1 “class C” • /23 = 255.255.254.0 = 2 “class C’s” • See also—RFC1878
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Classless Routing Protocols • Supports Variable Length Subnet Masks (VLSM’s) and noncontiguous subnetting • OSPF, integrated IS-IS, EIGRP, RIPv2 and BGP • Classful routing protocols are effectively deprecated 1503 0925_04F9_c1
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© 1999, Cisco Systems, Inc.
Intra-Domain Route Summarization 199.1.0/19 Injected into Area 0
Area 0 199.2.0.0/24
Area 1
199.1.0.0/24 through 199.1.3.1.0/24
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Inter-Domain Route Summarization 199.1.0.0/16
199.1.0.0/16 199.1.0.0/24
199.1.1.0/24
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199.1.2.0/24 199.1.3.0/24 199.1.4.0/24 199.1.5.0/24
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Effects of CIDR in the Internet
Source: Telstra Internet (Australia)
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BGP4 General Operation • Learns multiple paths via internal (iBGP) and external (eBGP) speakers/peers • Picks the best path for installation into the IP routing table • Policies applied by influencing the best path selection 1503 0925_04F9_c1
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BGP Path Selection Algorithm (Summarized) • Best-match search—if equal prefer lowest protocol “distance” 1. As_sequence or as_set contain local ASN -> ignore/discard 2. Next_hop inaccessible -> ignore prefix/path 3. Synch enabled—path internal—route not in IGP -> ignore 4. Largest weight 5. Largest local_pref 6. Locally originated 7. Shortest as_path 8. Lowest origin type 9. Lowest med 10. eBGP derived path preferred over iBGP path 11. Select route with lowest metric to next_hop 12. Existing best route and this route = and maxpaths > 1 ->add route If maxpaths = 1 (default) -> prefer lowest router ID 1503 0925_04F9_c1
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BGP and IGP • BGP carries route information external to AS • BGP forwarding-based on external or gateway router next-hop; de-coupled from IGP forwarding • IGP carries next-hops and internal routes • Allows internal routing to be based on IGP decisions 1503 0925_04F9_c1
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When Should I Use BGP?
• Dual- or multi-homed • Partial or full exterior routing is required downstream • Anytime the AS path information is required to be propagated
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Multi-Homed AS
AS 100
AS 300 A
D B
C AS 200
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BGP Is Not Needed…
• …if singularly homed • …if not providing downstream routing • Use a default route
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BGP Is Not Needed… Upstream Provider AS100 Static Route(s)
Default Route
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Interior BGP (iBGP) • Same routing protocol as BGP, different application • iBGP should be used when AS_PATH information must remain intact between multiple eBGP peers (AS border routers) • All iBGP peers must be fully meshed, logically; an iBGP peer will not advertise a route learned by one iBGP peer to another iBGP peer (readvertisement restriction to prevent looping) 1503 0925_04F9_c1
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Interior BGP (iBGP)
iBGP
iBGP iBGP
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Scaling the iBGP Mesh
• Two methods Confederations Route reflectors
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BGP Confederations
• Method to subdivide a single AS into multiple, internal sub-AS’s, yet still advertise a single AS to external peers • Reduces iBGP mesh • Experience has shown confederations to be somewhat less than optimal 1503 0925_04F9_c1
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© 1999, Cisco Systems, Inc.
BGP Confederations
AS300
AS10
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AS30
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BGP Route Reflectors (RR’s) • Another method to reduce iBGP mesh • iBGP re-advertisement restrictions are relaxed • Single iBGP peer advertises (reflects) routes to subordinate iBGP peers • Clients must not peer with RR’s outside of cluster 1503 0925_04F9_c1
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© 1999, Cisco Systems, Inc.
BGP Route Reflectors
AS300 (RR)
(RR)
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(RR)
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Stability
• Using loop-backs • BGP/IGP interaction • Peer groups • Route-flap dampening • BGP soft reconfiguration 1503 0925_04F9_c1
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Stable iBGP Peering
• Peer with loop-back addresses • iBGP session is not dependent on a single interface • Loop-back interface does not go down
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Peering with Loop-Back Interfaces iBGP AS 100 D A B
iBGP iBGP E
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Rules of Thumb— BGP/IGP Interaction • (i)BGP carries exterior routing information within the AS • IGP is used to carry next hop and interior network information • Routes should never be redistributed from BGP into IGP (and vice versa) • Recursive route lookup 1503 0925_04F9_c1
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Peer Groups
• iBGP neighbors receive same update • Group peers with same outbound policy • Updates are generated once per group • Makes configuration easier • Members could have different inbound policy 1503 0925_04F9_c1
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© 1999, Cisco Systems, Inc.
BGP Communities • Method to group destinations into communities and apply routing decisions (accept, prefer, redistribute, etc.) using route-maps • Each destination could be member of multiple communities • Community attribute carried across AS’s • Useful in applying policies (e.g. influencing ingress traffic flow) 1503 0925_04F9_c1
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BGP Communities
Community 10:200 Community 10:300
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Community 10:200
Community 10:300
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© 1999, Cisco Systems, Inc.
Route Flap Dampening • Route flap Going up and down of path Change in attribute
• Ripples through the entire Internet • Consumes CPU • Reduce scope of route flap propagation 1503 0925_04F9_c1
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Route Flap Dampening 4
Suppress-Limit 3
Penalty
Reuse-Limit
2
1
0 0 1 2 3 4 5 6 7 8 9 10 1112 13 14 15 161718 19 20 21222324 25
Time 1503 0925_04F9_c1
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Route Flap Dampening—Operation • Add penalty for each flap • Exponentially decay penalty • Penalty above suppress-limit— do not advertise up route • Penalty decayed below reuse-limit— advertise route • Maintains a history per path 1503 0925_04F9_c1
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Route Flap Dampening—Operation
• Done only for external path • Alternate paths still usable • Suppress-limit, reuse-limit and half-life time give control • Less overhead 1503 0925_04F9_c1
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BGP Soft Reconfiguration • Soft reconfig allows BGP policies to be (re)configured and activated without clearing the BGP session • Does not invalidate forwarding cache—hence no short-term interruptions • Outbound preferable over inbound reconfig (CPU intensiveness) 1503 0925_04F9_c1
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Know Your Traffic Patterns…
• If you do not know what traffic looks like in your network—you can’t Respond effectively to growing congestion Do capacity planning Effectively offer a qualitative service
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The Problem of Congestion • Uncontrolled—congestion will seriously degrade system performance System buffers fill up Packets are dropped—resulting in retransmissions Results in additional packet loss and increased latency Problem builds on itself until congestion collapse 1503 0925_04F9_c1
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Backbone Utilization
Under Utilized
Over Subscribed
Time of Day Source: MAE East—WorldCom MAE Services
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Effects of Controlled/ Uncontrolled Congestion
Controlled Congestion
Throughput
Uncontrolled Congestion
Congestion 1503 0925_04F9_c1
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Random Early Detection (RED) • Based on the concept that TCP is “graceful” in the face of loss— response to packet loss is necessary for successful congestion prevention using RED • TCP is a sliding window protocol that uses a self-clocking mechanism to adjust its use of the network to match available bandwidth 1503 0925_04F9_c1
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Random Early Detection (RED)
• RED reduces overall network packet loss—maximizing throughput and minimizing latency • RED accomplishes this by fine-tuning the TCP slow start congestion window mechanism to avoid globally synchronized oscillation and minimize retransmission 1503 0925_04F9_c1
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Random Early Detection
Ethernet
Individual Flows RED Signals Traffic Methodically to Back off By Dropping Drop Random—Individual Traffic Flows Drop 1503 0925_04F9_c1
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Random Early Detection
Probability of Packet Discard
Slope Is Adjustable
Queue Length 1503 0925_04F9_c1
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Queue Max 62
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Selective Packet Discard (SPD) • Assigns higher priority to control traffic (layer 3 routing updates) to ensure routing system integrity in times of congestion • Based on the assumption that it is better to maintain stability than to black-hole packets in times of severe congestion 1503 0925_04F9_c1
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Multi-Vendor Interoperability • Theory: A lot of work has gone into designing protocol standards for multivendor interoperability • Practice: Sometimes multi-vendor interoperability can cause more problems than anticipated • Observation: ISP’s generally deploy single vendor solutions for a particular platform (e.g. routers—switches) 1503 0925_04F9_c1
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Policies and Network Services
• Policies are created by specifying how network layer (layer 3) services are utilized to control the network’s resources • External policy implementation should be pushed to edges of network, as much as possible 1503 0925_04F9_c1
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Policy Architecture— Edge Functions • Customer access aggregation • Traffic filtering • Route filtering
• Admission control • Bandwidth allocation • Shaping
• QoS/CoS marking
• Traffic classification and prioritization
• Route filtering
• Access control
• Accounting 1503 0925_04F9_c1
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Policy Architecture— Core Functions
• High speed transport • Congestion management • QoS/CoS administration (queuing/dropping) • Traffic management 1503 0925_04F9_c1
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Policy Architecture Implementation
POP POP
Edge
Backbone
POP
POP
Policy Implementations Require Cooperative Edge and Backbone Functions 1503 0925_04F9_c1
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An ISP—The Big Picture Content Services
Domestic Peering
Security Services Directory Services
Global Internet
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Non-Direct Individual Dial-Up Network
Satellite and Wireless 69
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Internet POP Topology Adjacent POP
Adjacent POP DS-3
…
DS-3
…
Service Node Routers
• Multiple levels of redundancy Access Interconnect Backbone 1503 0925_04F9_c1
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Routed POP Architecture Internet
Core POP Core Routers Gateway/Border Router
Core Interconnect Distribution Routers
Remote POP
Access Routers Private Customer Network
ISP Customer
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Switched POP Architecture Internet
Core POP Core Routers Gateway/Border Router
Core Interconnect Distribution Routers
Remote POP
Access Routers ISP Customer
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Private Customer Network 72
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Early Internet POP Architecture • Backbone trunks at 45 Mb/s • Shared media interconnect within POP FDDI—Ethernet— switched Ethernet
• Conventional T3 backbone Internet router 1503 0925_04F9_c1
Internet Backbone
45 Mb/s HSSI
FDDI
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Internet POP Architecture—’97/’98 • Backbone trunks at 622 Mb/s Packet over SONET OC12
Internet Backbone
ATM OC12
• Switched interconnect within POP
622 Mb/s POS/ATM
ATM at OC3 and OC12 Ethernet channel
622 Mb/s ATM 1 Gb/s Ethernet
Gigabit Ethernet (early ’98) POSIP (late ’98)
• Gigabit OC12 backbone Internet router 1503 0925_04F9_c1
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155 or 622 Mb/s ATM 1 Gb/s Ethernet
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State of the Internet POP 5000
2 x OC48
4000
City-Pair Bandwidth Requirements
3000
Mbps
OC48 Tier 1 ISP
2000
2 x OC12 1000
OC12
Tier 2 ISP
500 0
Jan ’96 1503 0925_04F9_c1
Jan ’97
Jan ’98
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Example ISP/IXC Backbones
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WorldCom/UUnet Backbone
Source: WorldCom/UUnet
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Source: WorldCom/UUnet
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Source: WorldCom/UUnet
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Verio Backbone
Source: Verio
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Source: AT&T/TCG CERFnet
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Source: Intermedia/Digex
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Source: Bell Atlantic
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Internet Exchange Points
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ISP/IXC Interconnection Points
NAP
ISP Z
CIX
ISP D ISP C ISP B
MAE West
ISP A
NAP
MAE West
Source: WorldCom MAE Services
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Exchange Points Generally… • Do not include peering agreements with any of the ISP’s who are connected
Peering
• Do not include transit with any of the ISP’s who are connected
Transit
• Do not guarantee that there will be specific major ISP’s present • Do not provide IP addresses or AS (Autonomous System) numbers 1503 0925_04F9_c1
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Major ISPs
IP or AS Assignments
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Peering Arrangements
• It is usually a requirement for an incoming ISP to have peering or transit arrangements with the other ISP’s connected to the exchange point • The administrators of the exchange point generally cannot arrange peering 1503 0925_04F9_c1
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Exchange Points Generally… • Climate-controlled collocation space • Physical security for equipment • A place to interconnect with other IXC’s/ISP’s • Various connection options • Provide only “layer 2” connectivity— prefers to remain politically agnostic 1503 0925_04F9_c1
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Exchange Point Examples
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Pacific Bell NAP
Source: Pacific Bell
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Network Access Point ISPs
NSPs, vBNs
NAP ISPs
San Francisco Bay Area LATA Other LATAs LATAs,, States, or Countries
Inter-Exchange Carrier Point of Presence (POP)
Inter-Exchange Carrier Point of Presence (POP)
ISPs
ISPs
Source: Pacific Bell
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Ameritech (Chicago) NAP
ADDS Switch Site
ISP1 Backbone Router
ISP2
ATM
ATM Switch
ISP3 ATM
ATM
ATM
A D HSSI Router Backbone S U
ISP4
Router
Backbone Router
Backbone Router
Source: Ameritech
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Exchange Point Information Pointer
• http://www.isi.edu/div7/naps/
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Adding “Services” to the Architecture Cause and Effect
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Services? • Differentiated service • Voice over IP • Managed services • Dial—DSL—cable • Web hosting—caching
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Ask the Right Questions
• What am I differentiating? • Does the network have the right characteristics for VoIP? • Do I have the right technical expertise—staffing and knowledge base for managed services?
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What Will Service Offerings Impose On the Network?
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Who Knows? • What will be the impact on existing traffic loads/patterns? • Can the network deliver the performance that your customers/applications desire? delay? jitter (delay variation)? • Do not allow new services to oversubscribe the network capacity more than it may have already been over-subscribed prior to deployment of said services 1503 0925_04F9_c1
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Deployment of New Services • Is more of a business decision • The technical aspect is to ensure continued network performance— scalability and stability • Remember that services which might cross AS boundaries are statistically less probable of being successful 1503 0925_04F9_c1
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Deploying New Services • Don’t feed the hype fire • By all means—deploy new services as they are developed—but do so rationally and with consideration to the impact of existing connectivity • Don’t deploy new technologies and services just for the sake of it; have valid business and technical reasons 1503 0925_04F9_c1
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A Word on Real-Time Services
• Real-time services can be characterized by having sensitivities to network performance—namely, delay and jitter • Delay and jitter can be quite difficult to minimize and keep consistent 1503 0925_04F9_c1
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Why Is it so Important to Minimize Delay? Usability of Voice Circuit as a Function of End-to-End Delay • To hit the target
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Fax Relay— Fax Broadcast CB Radio I-Phone Today
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Summary • No amount of magic knobs will save a sloppily designed network • Propagation delay is bounded by the speed of light • Careful consideration is required in network design and architecture, both prior to deployment and on an on-going basis 1503 0925_04F9_c1
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Summary (2) • Scaling and stability is of utmost importance; you shouldn’t even begin to think of offering new services (beyond simple connectivity) unless these two requirements are satisfied • Building hierarchy into the network design allows virtually unlimited scaling 1503 0925_04F9_c1
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Summary (3)
• Design in the right amount of redundancy • Peering is more of a political and economic issue and less of a technical one • BGP is your friend 1503 0925_04F9_c1
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Thank You! Fin
[email protected] [email protected] 1503 0925_04F9_c1
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Please Complete Your Evaluation Form Session 1503
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