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8/13/2019 Inter-Domain Routing Lecture
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15744 - Fall 2004 Lecture 3 1
Inter-Domain Routing:
BGP, Overlay Routing,
Multihoming
Vyas Sekar
Based on slides from:Srini Seshan, Tim Griffin
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Readings
• Assigned – [Mit notes] – Overview of BGP/ Interdomain routing
– [Gao01] – Inferrring AS relationships
– [Lab00] – BGP Convergence
– [S+99] -- Suboptimality in Internet routing• Optional
– [Griffin01] – BGP Tutorial
– [SARK01] – Characterizing the Internet Hierarchy from multiplevantage points
– [APMS+04] – A Comparison of Overlay routing and multihoming – [AWBKM] – Resilient Overlay Networks
– [GW02] – iBGP configuration
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Outline
• Need for hierarchical routing
• BGP – ASes, Policies
– BGP Attributes
– BGP Path Selection – iBGP
– Inferring AS relationships
• Problems with BGP – Convergence
– Sub optimal routing
• Overlay Routing and Multihoming
• Summary
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Routing Hierarchies
• Flat routing doesn’t scale – Each node cannot be expected to have routes
to every destination (or destination network)
• Key observation
– Need less information with increasing distanceto destination
• Two radically different approaches for
routing – The area hierarchy?
– The landmark hierarchy (discuss in routingalternatives)?
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Areas
• Divide network into areas – Areas can have nested
sub-areas
– Constraint: no pathbetween two sub-areas of
an area can exit that area• Hierarchically address
nodes in a network – Sequentially number top-
level areas
– Sub-areas of area arelabeled relative to that area
– Nodes are numberedrelative to the smallestcontaining area
1 2
3
1.1
1.2
2.1 2.2
3.1 3.2
2.2.1
2.2.2
1.2.1
1.2.2
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Routing
• Within area
– Each node has routes to every other node
• Outside area – Each node has routes for other top-level
areas only
– Inter-area packets are routed to nearest
appropriate border router
• Can result in sub-optimal paths
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Path Sub-optimality
1 2
3
1.1
1.2
2.1 2.2
3.1 3.2
2.2.1
3 hop red path
vs.
2 hop green path
start
end
3.2.1
1.2.1
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A Logical View of the Internet
Tier 1 Tier 1
Tier 2Tier 2
Tier 2
Tier 3
• National (Tier 1 ISP) – “Default-free” with
global reachability info
Eg: AT & T, UUNET,
Sprint• Regional (Tier 2 ISP)
– Regional or country-
wide
Eg: Pacific Bell• Local (Tier 3 ISP)
Eg: Telerama DSL
Customer
Provider
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Landmark Routing: Basic Idea
Source wants to reach LM0[a], whose address is c.b.a: •Source can see LM2[c], so sends packet towards c
•Entering LM1[b] area, first router diverts packet to b
•Entering LM0[a] area, packet delivered to a
- Not shortest path
- Packet does not necessarily
follow specified landmarks
- No policy routing
- Not source routing
-Why
-Small routing tables-Dynamic, self
configuring algorithms
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Outline
• Need for hierarchical routing
• BGP – ASes, Policies
– BGP Attributes
– BGP Path Selection – iBGP
– Inferring AS relationships
• Problems with BGP – Convergence
– Sub optimal routing
• Overlay Routing and Multihoming
• Summary
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Autonomous Systems (ASes)
• Autonomous Routing Domain – Glued together by a common administration, policies etc
• Autonomous system – is a specific case of an ARD – ARD is a concept vs AS is an actual entity that participates in
routing
– Has an unique 16 bit ASN assigned to it and typicallyparticipates in inter-domain routing
• Examples: – MIT: 3, CMU: 9
– AT&T: 7018, 6341, 5074, …
– UUNET: 701, 702, 284, 12199, … – Sprint: 1239, 1240, 6211, 6242, …
• How do ASes interconnect to provide global connectivity
• How does routing information get exchanged
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Nontransit vs. Transit ASes
ISP 1
ISP 2
Nontransit AS
might be a corporateor campus network.Could be a “content
provider”
NET ATraffic NEVERflows from ISP 1through NET A to ISP 2(At least not intentionally!)
IP traffic
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Customers and Providers
Customer pays provider for access to the Internet
provider
customer
IP trafficprovider customer
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The Peering Relationship
peer peer
customerprovider
Peers provide transit between
their respective customers
Peers do not provide transitbetween peers
Peers (often) do not exchange $$$trafficallowed
traffic NOTallowed
A
B
C
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Peering Wars
• Reduces upstream transitcosts
• Can increase end-to-endperformance
• May be the only way toconnect your customersto some part of theInternet (“Tier 1”)
• You would rather have
customers
• Peers are usually your
competition
• Peering relationships
may require periodic
renegotiation
Peering struggles are by far the mostcontentious issues in the ISP world!
Peering agreements are often confidential.
Peer Don’t Peer
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Routing in the Internet
• Link state or distance vector?
– No universal metric – policy decisions
• Problems with distance-vector:
– Bellman-Ford algorithm may not converge
• Problems with link state:
– Metric used by routers not the same – loops
– LS database too large – entire Internet
– May expose policies to other AS’s
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Solution: Distance Vector with
Path• Each routing update carries the entire path
• Loops are detected as follows:
– When AS gets route check if AS already in
path
• If yes, reject route
• If no, add self and (possibly) advertise route further
• Advantage: – Metrics are local - AS chooses path, protocol
ensures no loops
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BGP-4• BGP = Border Gateway Protocol• Is a Policy-Based routing protocol
• Is the EGP of today’s global Internet
• Relatively simple protocol, but configuration is complex and the
entire world can see, and be impacted by, your mistakes.
• 1989 : BGP-1 [RFC 1105]
– Replacement for EGP (1984, RFC 904)
• 1990 : BGP-2 [RFC 1163]
• 1991 : BGP-3 [RFC 1267]
• 1995 : BGP-4 [RFC 1771]
– Support for Classless Interdomain Routing (CIDR)
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BGP Operations (Simplified)
Establish session onTCP port 179
Exchange all
active routes
Exchange incremental
updates
AS1
AS2
While connection
is ALIVE exchange
route UPDATE messages
BGP session
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Interconnecting BGP Peers
• BGP uses TCP to connect peers
• Advantages: – Simplifies BGP
– No need for periodic refresh - routes are valid untilwithdrawn, or the connection is lost
– Incremental updates
• Disadvantages
– Congestion control on a routing protocol? – Inherits TCP vulnerabilities!
– Poor interaction during high load
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Four Types of BGP Messages
• Open : Establish a peering session.• Keep Alive : Handshake at regular intervals.
• Notification : Shuts down a peering session.
• Update : Announcing new routes or withdrawing
previously announced routes.
announcement=
prefix + attributes values
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Policy with BGP
• BGP provides capability for enforcing various policies
• Policies are not part of BGP: they are provided to BGPas configuration information
• BGP enforces policies by choosing paths from multiple
alternatives and controlling advertisement to other AS’s • Import policy
– What to do with routes learned from neighbors?
– Selecting best path
• Export policy – What routes to announce to neighbors?
– Depends on relationship with neighbor
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Examples of BGP Policies
• A multi-homed AS refuses to act as transit
– Limit path advertisement
• A multi-homed AS can become transit for
some AS’s – Only advertise paths to some AS’s
– Eg: A Tier-2 provider multi-homed to Tier-1providers
• An AS can favor or disfavor certain AS’sfor traffic transit from itself
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Export Policy
• An AS exports only best paths to its neighbors – Guarantees that once the route is announced the AS
is willing to transit traffic on that route
• To Customers
– Announce all routes learned from peers, providersand customers, and self-origin routes
• To Providers – Announce routes learned from customers and self-
origin routes
• To Peers – Announce routes learned from customers and self-
origin routes
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Import Routes
From
peer
From
peer
From
provider
From
provider
From
customerFrom
customer
provider route customer routepeer route ISP route
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Export Routes
To
peer
To
peer
To
customer
To
customer
To
provider
From
provider
provider route customer routepeer route ISP route
filters
block
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BGP Route Processing
Best Route
Selection
Apply Import
Policies
Best Route
Table
Apply Export
Policies
Install forwarding
Entries for best
Routes.
Receive
BGP
Updates
Best
Routes
Transmit
BGP
Updates
Apply Policy =
filter routes &
tweak attributes
Based on
Attribute
Values
IP Forwarding Table
Apply Policy =
filter routes &
tweak attributes
Open ended programming.
Constrained only by vendor configuration language
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BGP UPDATE Message
• List of withdrawn routes
• Network layer reachability information
– List of reachable prefixes
• Path attributes
– Origin
– Path
– Metrics
• All prefixes advertised in message have
same path attributes
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Path Selection Criteria
• Information based on path attributes
• Attributes + external (policy) information
• Examples: – Hop count
– Policy considerations
• Preference for AS
• Presence or absence of certain AS
– Path origin
– Link dynamics
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Important BGP Attributes
• Local Preference
• AS-Path
• MED• Next hop
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LOCAL PREF
• Local (within an AS) mechanism to provide relative
priority among BGP routers
R1 R2
R3 R4I-BGP
AS 256
AS 300
Local Pref = 500 Local Pref =800
AS 100
R5
AS 200
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LOCAL PREF – Common Uses
• Handle routes advertised to multi-homed
transit customers
– Should use direct connection (multihoming
typically has a primary/backup arrangement)
• Peering vs. transit
– Prefer to use peering connection, why?
• In general, customer > peer > provider
– Use LOCAL PREF to ensure this
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AS_PATH
• List of traversed AS’s
• Useful for loop checking and for path-based route selection (length, regexp)
AS 500
AS 300
AS 200 AS 100
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
170.10.0.0/16 180.10.0.0/16
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Multi-Exit Discriminator (MED)
• Hint to external neighbors about the
preferred path into an AS
– Non-transitive attribute
– Different AS choose different scales
• Used when two AS’s connect to each
other in more than one place
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MED
• Typically used when two ASes peer at multiple locations
• Hint to R1 to use R3 over R4 link
• Cannot compare AS40’s values to AS30’s
R1 R2
R3 R4
AS 30
AS 40
180.10.0.0
MED = 120180.10.0.0
MED = 200
AS 10
180.10.0.0
MED = 50
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MED• MED is typically used in provider/subscriber scenarios
• It can lead to unfairness if used between ISP because it
may force one ISP to carry more traffic:
SF
NY
• ISP1 ignores MED from ISP2
• ISP2 obeys MED from ISP1• ISP2 ends up carrying traffic most of the way
ISP1
ISP2
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Other Attributes
• ORIGIN
– Source of route (IGP, EGP, other)
• NEXT_HOP
– Address of next hop router to use
• Check out http://www.cisco.com for full
explanation
• Question: Too many choices/ attributes
how to select routes !
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Route Selection Process
Highest Local Preference
Shortest ASPATHLowest MED
i BGP < e BGP
Lowest IGP cost
to BGP egress
Lowest router ID
traffic engineering
Enforce relationships
Throw up hands and
break ties
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Internal vs. External BGP
R3 R4
R1
R2
E-BGP
•BGP can be used by R3 and R4 to learn routes
•How do R1 and R2 learn routes?
•Option 1: Inject routes in IGP
•Only works for small routing tables
•Option 2: Use I-BGP
AS1 AS2
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Internal BGP (I-BGP)
• Same messages as E-BGP
• Different rules about re-advertising
prefixes:
– Prefix learned from E-BGP can be advertised
to I-BGP neighbor and vice-versa, but
– Prefix learned from one I-BGP neighbor
cannot be advertised to another I-BGPneighbor
– Reason: no AS PATH within the same AS and
thus danger of looping.
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Internal BGP (I-BGP)
R3 R4
R1
R2
E-BGP
I-BGP
• R3 can tell R1 and R2 prefixes from R4
• R3 can tell R4 prefixes from R1 and R2• R3 cannot tell R2 prefixes from R1
R2 can only find these prefixes through a direct connection to R1
Result: I-BGP routers must be fully connected (via TCP)!
• contrast with E-BGP sessions that map to physical links
AS1 AS2
R t R fl t
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Route Reflector
eBGP update
iBGP updates
Mesh does not scale
RR RR
RR
Each RR passes only best routes, no longer
N^2 scaling problem
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Policy Impact
• Different relationships – Transit, Peering
• Export policies selective export
• “Valley-free” routing – Number links as (+1, 0, -1) for customer-to-
provider, peer and provider-to-customer
– In any path should only see sequence of +1,
followed by at most one 0, followed bysequence of -1
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How to infer AS relationships?
• Can we infer relationship from the AS graph – From routing information
– From size of ASes /AS topology graph
– From multiple views and route announcements
• [Gao01] – Three-pass heuristic
– Data from University of Oregon RouteViews
• [SARK01]
– Data from multiple vantage points – Formulate TOR problem
– Heuristic for solving the relationship assignment
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[Gao00] Basic Algorithm
• Phase 1: Identify the degrees of the ASes fromthe tables
• Phase 2: Annotate edges with “transit” relation – AS u transits traffic for AS v if it provides its
provider/peer routes to v.
• Phase 3: Identify P2C, C2P, Sibling edges – P2C -> If and only if u transits for v, and v does not,
Sibling otherwise
– Peering relationship ?
• Refined Algorithm : Another parameter L
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How does Phase 2 work?
• Notion of Valley free routing
– Each AS path can be
• Uphill
• Downhill
• Uphill – Downhill
• Uphill – P2P
• P2P -- Downhill
• Uphill – P2P – Downhill
• How to identify Uphill/Downhill
– Heuristic: Identify the highest degree AS to be the
end of the uphill path (path starts from source)
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Observations from [Gao00]
• Heuristic to identify top provider does not work
• Algorithm inferences verified from sources within AT & T.
• Majority are P2C, few peering, few sibling – Peering is few because of the dataset used?
– Sibling relationships are becoming more common• Mergers, takeovers, backup relationships
• AS relationships are often complex – inferredrelationships are “dominant commercialrelationships”
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Questions..
• Is inter-AS relationship – Prefix based
– Customer based
– Independent?
• Is the degree based heuristic valid?
• Are peering relationships underestimated?
• How useful is inferring the relationships
– Policy violations – Anomaly detection?
– Is this information revealed against the providers will?
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BGP Inefficiencies,Overlays and
Multihoming
Aditya Akella
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BGP Complexity
• BGP is a very complicated protocol – Too many knobs
– Need to accommodate (sub-optimal)ISP policies
– Requires complex, humanconfiguration
• For all its complexity, BGP offersno guarantees – Performance??
– Reliability?? – Correctness??
– Reachability??
• All of BGPs complexity begets…
Headache!
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BGP Pitfalls and Problems
• Pitfalls and problems
– Misconfiguration
– Convergence
– Performance
– Reliability
– Stability
– Security
– And the list goes on…
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Favorite Scapegoat!
BGP
Networkingcommunity
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Misconfiguration[Mahajan02Sigcomm]
• Origin misconfiguration: accidentally inject
routes for prefixes into global BGP tables
Old Route New Route
Self deaggregation(failure to
aggregate)
a.b.0.0/16 X Y Z a.b.c.0/24 X Y Z
Related origin
(likely connected to
the network –
human error)
a.b.0.0/16 X Y Z a.b.0.0/16 X Y
a.b.0.0/16 X Y Z O
a.b.c.0/24 X Y
a.b.c.0/24 X Y Z O
Foreign origin
(address space
hijack!)
a.b.0.0/16 X Y Z a.b.0.0/16 X Y O
a.b.c.0/24 X Y O
e.f.g.h/i X Y O
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Misconfiguration
• Export misconfiguration: export route to a peerin violation of policy
Export Policy Violation
Provider AS Provider Route exported to provider wasimported from a provider
Provider AS Peer Route exported to peer was
imported from a provider
Peer
AS
Provider Route exported to provider wasimported from a peer
Peer AS Peer Route exported to peer was
imported from a peer
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Interesting Observations
• Origin misconfig – 72% of new routes may be misconfig
– 11-13% of misconfig incidents affect connectivity• Pings and e-mail checks
– Self de-aggregation is the main cause
• Export misconfig – Upto 500 misconfiguration incidents per day
– All forms are prevalent, although provider-AS-provideris more likely
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Effects and Causes
• Effects – Routing load
– Connectivity disruption
– Extra traffic
– Policy violation
• Causes (Origin misconfig) – Router vendor software bugs:
announce and withdraw routeson reboot
– Reliance on upstream filtering
– New configuration not saved tostable storage (separatecommand and no autosave!)
– Hijacks of address spaces
– Forgotten to install filter
– Human operators and poorinterface
P1 P2
A
C
• Intended policy: Provide transit to Cthrough link A-C
• Configured policy: Export all routes
originated by C to P1 and P2
• Correct policy: export only when AS
path is “C”
Export Misconfig
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BGP Convergence[Labovitz00Sigcomm]
• Conventional beliefs – Path vector converges faster than traditional DV(eliminates the count to infinity problem)
– Internet path restoration takes order of 10s of seconds
• Convergence
– Recovery after a fault may take as much as ten minutes – Single routing fault could result in multiple
announcements and withdrawals
– Loss and RTT around times of faults are much worse
• Upon route withdrawal, explore paths of increasing
length – In the worst case, could explore n! paths
– Depends which messages are processed and when
• Limit between update message could reducemessages
– Forces all outstanding messages to be processed
IBGP P bl
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IBGP Problems[Griffin02Sigcomm]
• Route reflectors could impose signaling
and forwarding anamolies instability!
Ri is a reflector for Ci (updates sent between Ri and Ci, i=1, 2)
Ri is a BGP
router
announcingPi into the
network
Ci will only
know about Pi
and it as bestpath
But Ci---Pi shortest path is Ci Ci+1Ri and this causes a forwarding loop!
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End-to-End Routing Behavior[Paxson96Sigcomm]
• Large scale routing behavior as seen by end-hosts, based on analysis of traceroutes
• Pathologies: persistent routing loops, routingfailures and long connectivity outages
• Stability: 9% or routes changed every 10s of
minutes, 30% about ~6hrs and 68% took a fewdays
• Symmetry: more than half of paths probed were
asymmetric at router level
Inefficiencies in BGP &
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Inefficiencies in BGP &
Internet Routing
• Route convergence and oscillations
• Poor reliability
– No way to exploit redundancy in Internet
paths• Inefficiency: sub-optimal RTTs and
throughputs
– What are some of the causes?• Policies in routing: Inter-domain and Intra-domain
• Lack of direct routes, “sparseness” of the Internetgraph
I ffi i f R t
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Inefficiency of Routes[Spring03Sigcomm]
• Three classes of reasons for poor performance(“inflation”) – Intra-domain topology and policy
• Topology: no direct link between all cities
• Routing policy: “shortest paths” may be avoided due toengineering
– ISP Peering• Peeering topology: limited peering between ISPs
• Peering policy: hot-potato routing or early-exit routing
– Inter-domain• Topology: AS graph is sparse
• Inter-domain policies: policies are policies
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Path Inflation Summary
Performance: End to End
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Performance: End-to-End
Perspective
• From an end-to-end view…
– Is there a way of extracting better
performance?
• Is there scope?• How do we realize this?
• Scope: Savage99, Akella03, Akella04• Reality: UW’s “Detour” system, MIT’s
RON, Akamai’s SureRoute, CMU’s Route
Control
Q tif i P f L
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Quantifying Performance Loss[Savage99Sigcomm]
• Measure round trip time (RTT) and loss
rate between pairs of hosts
• Alternate path characteristics
– 30-55% of hosts had lower latency
– 10% of alternate routes have 50% lower
latency
– 75-85% have lower loss rates
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Bandwidth Estimation
• RTT & loss for multi-hop path – RTT by addition
– Loss either worst or combine of hops – why?
• Large number of flows combination ofprobabilities
• Small number of flows worst hop
• Bandwidth calculation
– TCP bandwidth is based primarily on loss andRTT
• 70-80% paths have better bandwidth
• 10-20% of paths have 3x improvement
Possible Sources of Alternate
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Possible Sources of Alternate
Paths• A few really good or bad AS’s
– No, benefit of top ten hosts not great
• Better congestion or better propagation
delay?
– How to measure?
• Propagation = 5th percentile of delays
– Both contribute to improvement ofperformance
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Overlay Networks
• Basic idea:
– Treat multiple hops through IP network as one hop in overlay network
– Run routing protocol on overlay nodes
• Why?
– For performance – like the Savage 99 paper showed
– For efficiency – can make core routers very simple
• E.g. CSFQ,
• Also aid deployment. E.g. Active networks
– For functionality – can provide new features such as multicast, active
processing
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Future of Overlay
• Application specific overlays
– Why should overlay nodes only do routing?
• Caching
– Intercept requests and create responses
• Transcoding
– Changing content of packets to match
available bandwidth
• Peer-to-peer applications
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Overlay Challenges
• “Routers” no longer have complete knowledge about link they areresponsible for
• How do you build efficient overlay – Probably don’t want all N2 links – which links to create?
– Without direct knowledge of underlying topology how to know what’snearby and what is efficient?
• Do we need overlays for performance?
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Number of Route Choices
• Flexible control of end-
to-end pathmany route choices
Multiple candidatepaths
Single path
Multiple BGP
paths
• BGP: one path via each
ISPchoices linked to #ISPs
Few more routechoices…?
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Route Selection Mechanism
• BGP: simple, coarsemetrics such as least AShops, policy
Best performingpath
Least AS hopsPolicy compliant
Current best
performing
BGP path
• Overlays: complex,performance-orientedselection
Sophisticated selection among
multiple BGP routes
Smart
selection
“Multihoming route control”
O l R ti M ltih i R t C t l
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Overlay Routing vs Multihoming Route Control[Akella04Sigcomm]
>
>
~
~
1-multihoming
k-multihoming
k-multihoming
1-overlays
1-overlays
k-overlays
~
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Overlay Routing vs. Multihoming Route Control
Cost
Operational
issues
Route Control Overlay Routing
Sprint$$
Genuity$$
ATT
$$
Overlay provider$$
ATT
$$
Overlay node
forces inter-mediate ISPto providetransit
/18 netblock
Announce/20 sub-blocksto ISPs
If all multihomedends do this
Routing table expansion Bad interactions with policies
Connectivity fees Connectivity fees + overlay fee
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Summary
• Route control similar to overlay routing for most practical
purposes
• Overlays very useful for deploying functionality
– Multicast, VPNs, QoS, security
• But overlays may be overrated for end-to-end
performance and resilience
• Don’t abandon BGP – there’s still hope of extracting good
performance and availability