Hierarchical Routing EECS 228 Abhay Parekh parekh@eecs.berkeley.edu
Hierarchical Routing � Is a natural way for routing 5 7 to scale 4 4 8 RIP � Size 6 6 � Network Administration � Governance 11 2 2 10 � Exploits address Inter Domain OSPF aggregation and allocation Routing 3 13 3 1 13 12 � Allows multiple metrics at IGRP different levels of the hierarchy October 16, 2002 Abhay K. Parekh: Topics in Routing 2
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 October 16, 2002 Abhay K. Parekh: Topics in Routing 3
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) October 16, 2002 Abhay K. Parekh: Topics in Routing 4
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 October 16, 2002 Abhay K. Parekh: Topics in Routing 5
Interconnections occur at many levels B 5 7 4 4 RIP 8 6 6 11 2 2 10 Inter Domain OSPF Routing 3 13 3 1 13 12 IGRP C A October 16, 2002 Abhay K. Parekh: Topics in Routing 6
Interconnections occur at many levels B A.1 D.2 5 7 4 4 RIP 8 6 6 11 2 2 10 OSPF D.1 D.3 3 13 3 1 13 12 IGRP C A October 16, 2002 Abhay K. Parekh: Topics in Routing 7
Interconnections occur at many levels B A.1 D.2 E.1 5 7 4 4 RIP 8 6 6 E.2 11 2 2 10 OSPF D.1 D.3 3 13 3 1 13 E.3 12 IGRP C A October 16, 2002 Abhay K. Parekh: Topics in Routing 8
Interconnections occur at many levels B A.1 D.2 E.1 5 7 4 4 RIP 8 6 6 E.2 11 2 2 10 OSPF D.1 D.3 3 13 3 1 13 E.3 12 IGRP C A October 16, 2002 Abhay K. Parekh: Topics in Routing 9
Interconnections occur at many levels E A.1 D.2 E.1 5 7 E.2 D 11 D.1 D.3 13 1 13 E.3 12 October 16, 2002 Abhay K. Parekh: Topics in Routing 10
Examples of overlaid interconnecting networks � IP over ATM � Multicast over IP � DSL over POTS � IP over CATV � Etc., � Each involves routing October 16, 2002 Abhay K. Parekh: Topics in Routing 11
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” October 16, 2002 Abhay K. Parekh: Topics in Routing 12
Peering and Transit Figures from William B. Norton, “Internet Service Providers and Peering” October 16, 2002 Abhay K. Parekh: Topics in Routing 13
Benefits of Transit v/s Peering William B. Norton, “Internet Service Providers and Peering” October 16, 2002 Abhay K. Parekh: Topics in Routing 14
Moving from Transit to Peering William B. Norton, “Internet Service Providers and Peering” October 16, 2002 Abhay K. Parekh: Topics in Routing 15
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 October 16, 2002 Abhay K. Parekh: Topics in Routing 16
Undifferentiated Network here to stay? Undifferentiated Network here to stay? Internet Infrastructure provides undifferentiated service No Business Model Cop-out No way to charge, peer or deliver high speed/ quality sensitive applications 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!! � October 16, 2002 Abhay K. Parekh: Topics in Routing 17
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… October 16, 2002 Abhay K. Parekh: Topics in Routing 18
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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 October 16, 2002 Abhay K. Parekh: Topics in Routing 20
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 October 16, 2002 Abhay K. Parekh: Topics in Routing 21
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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 October 16, 2002 Abhay K. Parekh: Topics in Routing 23
Four message types � Open: Session establishment id exchange � Notification: exception driven information � Keep Alive: soft state � Update: path vector information October 16, 2002 Abhay K. Parekh: Topics in Routing 24
Update Message � Withdrawn Routes: No Infeasible Route Length longer valid Withdrawn Routes � Attributes: Path Vector, (variable) weights and other Total Path Attribute Len information about each of Path Attributes the destinations (variable) � <length,prefix>: CIDR Length|Prefix notation for the destination <length,prefix> . . October 16, 2002 Abhay K. Parekh: Topics in Routing 25
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: 2 32-Y addresses in the range � Example 128.5.10/23 � Common prefix is 23 bits: 01000000 00000101 0000101 � Number of addresses: 2 9 = 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 � October 16, 2002 Abhay K. Parekh: Topics in Routing 26
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 October 16, 2002 Abhay K. Parekh: Topics in Routing 27
Example B b 5 7 4 4 RIP 8 IBGP 6 6 11 2 2 10 a IBGP OSPF BGP Session 3 13 3 c 1 13 12 b via A IGRP C A Announce a October 16, 2002 Abhay K. Parekh: Topics in Routing 28
Multihoming � Two or more B interdomain 5 7 connections between 4 4 RIP 8 the same AS’s IBGP 6 6 � Two or more interdomain 2 2 connections between a a IBGP customer and ISPs 3 3 1 IGRP A October 16, 2002 Abhay K. Parekh: Topics in Routing 29
Multiexit Discriminators (MEDs) Way for one AS to influence B � routing decisions of another 5 7 AS 4 4 AS_A wants to tell AS_B that RIP 8 � network a is closer to router 2 IBGP than to router 3 6 6 Router 2 advertises a smaller � MED value for a than Router 3 AS_B prefers the path to a that 2 2 � does not go through 6 and 3 a IBGP AS_B does not propagate � 3 3 MEDS from AS_A any further 1 IGRP A October 16, 2002 Abhay K. Parekh: Topics in Routing 30
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 October 16, 2002 Abhay K. Parekh: Topics in Routing 31
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