Hierarchical Routing EECS 228 Abhay Parekh - - PowerPoint PPT Presentation
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
October 16, 2002 Abhay K. Parekh: Topics in Routing 2
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
6 4 3 2 13
2 4 3 6 13
7 8 5 1 12 10 11
Inter Domain Routing
OSPF RIP IGRP
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The internet is an interconnection of unequal
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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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
3039 commercial companies, 979 co-operatives
By controlling interconnection Bell got rid of them Interconnection is now regulated (CLECs)
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Access to sites reachable via routing and transport facilities But could also include:
Wire
Space
Access to OSS Dispute Resolution Process Term
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6 4 3 2 13
A B C 2 4 3 6 13
7 8 5 1 12 10 11
Inter Domain Routing
OSPF RIP IGRP
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6 4 3 2 13
A B C 2 4 3 6 13
7 8 5 1 12 10 11
OSPF RIP IGRP
A.1 D.1 D.2 D.3
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6 4 3 2 13
A B C 2 4 3 6 13
7 8 5 1 12 10 11
OSPF RIP IGRP
A.1 D.1 D.2 D.3 E.1 E.2 E.3
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6 4 3 2 13
A B C 2 4 3 6 13
7 8 5 1 12 10 11
OSPF RIP IGRP
A.1 D.1 D.2 D.3 E.1 E.2 E.3
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13
13
7 5 1 12 11
A.1 D.1 E.2 D.2 D.3 E.1 E.3
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IP over ATM Multicast over IP DSL over POTS IP over CATV Etc., Each involves routing
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Peering
The business relationship whereby ISPs
Transit
The business relationship whereby one ISP
William B. Norton, “Internet Service Providers and Peering”
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Figures from William B. Norton, “Internet Service Providers and Peering”
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William B. Norton, “Internet Service Providers and Peering”
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William B. Norton, “Internet Service Providers and Peering”
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Bandwidth Pricing: Everything you can think of
Traffic may be asymmetric (web servers) Clout may vary Some existing and suggested methods
Bandwidth is undifferentiated (can’t peer for video quality bw) Connection Method
Direct Connection Internet Exchange
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Internet Infrastructure provides undifferentiated service
continues on “speeding up the internet”, but this just speeds up existing applications
No Business Model Cop-out No way to charge, peer or deliver high speed/ quality sensitive applications
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BGP is the way by which ISPs co-operate on
Routing efficiency and performance is important, but
E.g. Path Vector uses many messages
Power of BGP is that it can express many different
Tremendous growth in the last 10 years…
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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
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Runs over TCP port 179 One Border Routers can be involved in multiple
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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Open: Session establishment id exchange Notification: exception driven information Keep Alive: soft state Update: path vector information
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Withdrawn Routes: No
Attributes: Path Vector,
<length,prefix>: CIDR
Infeasible Route Length Withdrawn Routes (variable) Total Path Attribute Len Path Attributes (variable) Length|Prefix <length,prefix> . .
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One router telling another one
The prefix IP address of the next hop Path list of AS’s that the announcement has
Since announcement propagates from destination, this
yields the path
No refresh messages required The announcing router will follow the path
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6 4 3 2 13
A B C 2 4 3 6 13
7 8 5 1 12 10 11
OSPF RIP IGRP b via A
b
IBGP IBGP
BGP Session
a c
Announce a
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Two or more
Two or more
6 4 3 2
A B 2 4 3 6
7 8 5 1
RIP IGRP
IBGP IBGP
a
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routing decisions of another AS
network a is closer to router 2 than to router 3
MED value for a than Router 3
does not go through 6 and 3
MEDS from AS_A any further
6 4 3 2
A B 2 4 3 6
7 8 5 1
RIP IGRP
IBGP IBGP
a
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Similar to MEDs but rather than being part of
Example
Choose the slower path when the prefix is to a
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Advantages
Redundancy Load Sharing Lowest Cost Routing Lowest Latency Routing
Disadvantages
Aggregation (if one customer has two ISPs)
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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
Many others…
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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
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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Most updates were bogus withdrawals This was to a large extent due to bad
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AADup is the advertising of a route that was
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Initially direct route
R 1 2
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Initially direct route
R has a fault so
R 1 2
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Initially direct route
R has a fault so
R 1 2
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Initially direct route
R has a fault so
R 1 2
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Initially direct route
R has a fault so
Nodes choose
R 1 2
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Initially direct route
R has a fault so sends
Nodes choose clockwise
Detecting loops they
R 1 2
October 16, 2002 Abhay K. Parekh: Topics in Routing 43
Initially direct route
R has a fault so sends
Nodes choose clockwise
Detecting loops they
R 1 2
October 16, 2002 Abhay K. Parekh: Topics in Routing 44
Initially direct route
R has a fault so sends
Nodes choose clockwise
Detecting loops they
And so on… N! steps, and even more
R 1 2
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Most router messages bogus Routing table oscillations worse under path
Timer interactions with router vendor
Network Administrator configuration errors
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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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Hierarchical Routing is a core component of most
Internet interconnection is complex and multi-
The ability for the network to expand its range of
Internet routing is becoming less hierarchical and
BGP stability is important and a work in progress
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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)