EECS 228 Abhay Parekh [email protected]/228a02... · October 16, 2002 Abhay K. Parekh: Topics...
Transcript of EECS 228 Abhay Parekh [email protected]/228a02... · October 16, 2002 Abhay K. Parekh: Topics...
Hierarchical Routing
EECS 228Abhay [email protected]
October 16, 2002 Abhay K. Parekh: Topics in Routing 2
Hierarchical Routing
� Is a natural way for routing to scale� Size� Network Administration� Governance
� Exploits address aggregation and allocation
� Allows multiple metrics at different levels of the hierarchy
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Inter DomainRouting OSPF
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Why is hierarchical routing important?
� The internet is an interconnection of unequal networks
� Interconnection arrangements drive � the competitive landscape� the robustness of the network� end-to-end performance
� Interconnection is central to all large networks� Voice� Data� Wireless� Cable
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Why are there so many players?
� www.thelist.com� How many ISP�s in the 415 area code?
� That start with A-C: about 200�� Just DSL that start with A-C: about 80
� In the telephone network� How many independent telephone companies in
1894-1902 in the US?� 3039 commercial companies, 979 co-operatives
� By controlling interconnection Bell got rid of them� Interconnection is now regulated (CLECs)
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What is an Interconnection
� Access to sites reachable via routing and transport facilities� But could also include:
� Wire� SLA + Lease
� Space� Size� Space
� Access to OSS� Dispute Resolution Process� Term
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Interconnections occur at many levels
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Inter DomainRouting OSPF
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Interconnections occur at many levels
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Interconnections occur at many levels
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Interconnections occur at many levels
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Interconnections occur at many levels
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Examples of overlaid interconnecting networks� IP over ATM� Multicast over IP� DSL over POTS� IP over CATV� Etc., � Each involves routing
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Two ways to interconnect IP Networks…
� Peering� The business relationship whereby ISPs
reciprocally provide to each other connectivity to each others� transit customers
� Transit� The business relationship whereby one ISP
provides (usually sells) access to all destinations in it�s routing table
William B. Norton, “Internet Service Providers and Peering”
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Peering and Transit Figures fromWilliam B. Norton, “Internet Service Providers and Peering”
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Benefits of Transit v/s Peering
William B. Norton, “Internet Service Providers and Peering”
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Moving from Transit to Peering
William B. Norton, “Internet Service Providers and Peering”
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Peering Attributes
� Bandwidth Pricing: Everything you can think of� Traffic may be asymmetric (web servers)� Clout may vary� Some existing and suggested methods
� Zero Charge (Bill and Keep)� Average Cost� Fully distributed cost pricing� Ramsey Pricing� Wholesale Pricing � Marginal Cost Pricing
� Bandwidth is undifferentiated (can�t peer for video quality bw)� Connection Method
� Direct Connection� Internet Exchange
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Internet Infrastructure providesundifferentiated service
� More capacity is thrown at the undifferentiated network, and emphasis continues on �speeding up the internet�, but this just speeds up existing applications
� No future for internet media or other bandwidth intensive applications� No future for significant high speed access penetration� These are huge lost opportunities!!
Undifferentiated Network here to stay?Undifferentiated Network here to stay?
No BusinessModel Cop-out No way to charge, peer or deliver
high speed/quality sensitive applications
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Name of the Game: Reachabilty
� BGP is the way by which ISPs co-operate on reachability
� Routing efficiency and performance is important, but not essential� E.g. Path Vector uses many messages
� Power of BGP is that it can express many different ISP routing policies without exposing internal network statistics such as load and topology
� Tremendous growth in the last 10 years�
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Skitter Legend
� Plot the AS based on polar co-ordinates (r,θ):� r = 1- log (As degree +1 / Max Degree+1)
� Higher the degree lower the radius� Θ = longitude of AS headquarters
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4/1-4/16 2002�1,224,733 IP addresses, �2,093,194 IP links, �932,000 destinations, �70% of globally routable network prefixes; �10,999 ASes (84% of ASes),�34,209 peering sessions
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BGP
� Runs over TCP port 179� One Border Routers can be involved in multiple
sessions� Border Routers
� from the same AS speak IBGP� from different AS�s speak EBGP
� EBGP and IBGP are essentially the same protocol� IBGP can only propagate routes it has learned directly from
its EBGP neighbors� All routers in the same AS form an IBGP mesh� Important to keep IBGP and EBGP in sync
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Four message types
� Open: Session establishment id exchange� Notification: exception driven information� Keep Alive: soft state � Update: path vector information
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Update Message
� Withdrawn Routes: No longer valid
� Attributes: Path Vector, weights and other information about each of the destinations
� <length,prefix>: CIDR notation for the destination
Infeasible Route Length
Withdrawn Routes (variable)
Total Path Attribute Len
Path Attributes (variable)
Length|Prefix
<length,prefix>..
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Classless Inter-domain Routing Addresses
� 32 bits in the address divided into 4 8-bit parts, A.B.C.D � Each part takes value 0,1,2,�,255
� E.g. 128.23.9.0� Specify a range of addresses by a prefix: X/Y
� The prefix common to the entire range is the first Y bits of X.� X: The first address in the range has prefix X� Y: 232-Y addresses in the range
� Example 128.5.10/23� Common prefix is 23 bits: 01000000 00000101 0000101� Number of addresses: 29 = 512
� Prefix aggregation� 128.5.10/24 and 128.5.11/24 gives 128.5.10/23
� Addresses allocated by central authority: IANA � Routers match to longest prefix
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Advertising a prefix
� One router telling another one � The prefix� IP address of the next hop� Path list of AS�s that the announcement has
passed through� Since announcement propagates from destination, this
yields the path
� No refresh messages required� The announcing router will follow the path
itself
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Example
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Multihoming
� Two or more interdomainconnections between the same AS�s
� Two or more interdomainconnections between a customer and ISPs
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Multiexit Discriminators (MEDs)
� Way for one AS to influence routing decisions of another AS
� AS_A wants to tell AS_B that network a is closer to router 2 than to router 3
� Router 2 advertises a smaller MED value for a than Router 3
� AS_B prefers the path to a that does not go through 6 and 3
� AS_B does not propagate MEDS from AS_A any further
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Local Preference (for IBGP)
� Similar to MEDs but rather than being part of the EBGP announcement, is a way for IBGP within an AS to prefer one path over another for the same prefix
� Example� Choose the slower path when the prefix is to a
competitor�s network
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Multihoming
� Advantages� Redundancy� Load Sharing� Lowest Cost Routing� Lowest Latency Routing
� Disadvantages� Aggregation (if one customer has two ISPs)
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BGP Policies
� Multiple ways to implement a �policy�� Decide not propagate advertisements
� I�m not carrying your traffic� Use MEDs to improve performance� Decide not to consider MEDs but use shortest hop
� Hot potato routing� Prepend own AS# multiple times to fool BGP into
not thinking AS further away� Many others�
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BGP and Performance
� BGP isn�t designed for policy routing not performance� Hot Potato routing is most common but suboptimal� Performance isn�t the greatest
� 20% of internet paths inflated by at least 5 router hops� Very susceptible to router misconfiguration
� Blackholes: announce a route you cannot reach� October 1997 one router brought down the internet for 2 hours
� Flood update messages (don�t store routes, but keep asking your neighbors to clue you in). 3-5 million useless withdrawals!
� In principle, BGP could diverge � Various solutions proposed to limit the set of allowable policies� Focuses on avoiding �policy cycles�
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BGP Updates (Labovitz)
� Most updates were bogus withdrawals� This was to a large extent due to bad
implementations
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Non-Withdrawal Updates (Labovitz)
� AADup is the advertising of a route that was just withdrawn
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
� Nodes choose clockwise 2 hop paths
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
� Nodes choose clockwise 2 hop paths
� Detecting loops they choose anticlockwise 2 hop paths
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
� Nodes choose clockwise 2 hop paths
� Detecting loops they choose anticlockwise 2 hop paths
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BGP Bouncing Problem (Labovitz et al)
� Initially direct route preferred by each node
� R has a fault so sends withdrawal messages to each of its neighbors
� Nodes choose clockwise 2 hop paths
� Detecting loops they choose anticlockwise 2 hop paths
� And so on�� N! steps, and even more
messages
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Other problems
� Most router messages bogus� Routing table oscillations worse under path
vector when compared to distance vector� Timer interactions with router vendor
implementations create pathologies� Network Administrator configuration errors
can create catastrophic outages throughout the network
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Precautions
� Route Damping� Don�t believe updates unless they make sense� Improves oscillations but adds to convergence time
� More state� Keep history of received and sent messages� Improves situation but quite expensive
� Router Configuration Restrictions� Save Path Vector Protocols (Griffin et al)� Human error potential is still considerable
� Alternatives� Detour� Routing Feedback based approaches� Hard to change the installed base!
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Conclusions
� Hierarchical Routing is a core component of most large networks
� Internet interconnection is complex and multi-faceted
� The ability for the network to expand its range of services is tied to the modes of peering
� Internet routing is becoming less hierarchical and more driven by policy than performance
� BGP stability is important and a work in progress
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References
� Peering� Norton: �Internet Service Providers and Peering�� Eli Noam: �Interconnecting the Network of Networks�, MIT
Press, 2001.� BGP: Basics
� BGP Tutorial by Nina Taft, SprintLabs� Halabi, �Internet Routing Architectures�, Cisco Press 1997
� BGP: Instability� Labovitz et al (see readings)� Griffin et al (see readings)