IMPACT OF HOT-POTATO ROUTING CHANGES IN IP NETWORKS

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Oct 29, 2013 (3 years and 7 months ago)

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IMPACT OF
HOT
-
POTATO ROUTING
CHANGES IN
IP NETWORKS

Authors

Renata

Teixeira
,
Aman

Shaikh

and Jennifer Rexford(AT&T),


Tim Griffin(Intel)


http
://www
-
cse.ucsd.edu/~
teixeira


Presenter :
Farrukh

Shahzad

SIGMETRICS’04

Internet Routing
Architecture

2

UCSD

Sprint

AT&T

Verio

AOL

interdomain routing (BGP)

intradomain routing (OSPF,IS
-
IS)

Changes in one AS

may impact traffic

and routing in other ASes

User

Web

Server

End
-
to
-
end performance

depends on all ASes

along the path

SIGMETRICS’04

Distance
-
vector &Link state routing


Distance vector

routing



It

involves

two

factors
:

the

distance

or

metric,

of

a

destination,

and

the

vector
,

or

direction

to

take

to

get

there
.



Routing

information

is

only

exchanged

between

directly

connected

neighbors
.



A

router

knows

from

which

neighbor

a

route

was

learned,

but

it

does

not

know

where

that

neighbor

learned

the

route


A

router

can't

see

beyond

its

own

neighbors
.

This

aspect

of

distance

vector

routing

is

sometimes

referred

to

as

"routing

by

rumor
.
"



Measures

like

split

horizon

and

poison

reverse

are

employed

to

avoid

routing

loops
.


3

SIGMETRICS’04


Link
-
state

routing


In

contrast,

requires

that

all

routers

know

about

the

paths

reachable

by

all

other

routers

in

the

network
.



Link
-
state

information

is

flooded

throughout

the

link
-
state

domain

(an

area

in

OSPF

)

to

ensure

all

routers

possess

a

synchronized

copy

of

the

area's

link
-
state

database
.



From

this

common

database,

each

router

constructs

its

own

relative

shortest
-
path

tree,

with

itself

as

the

root,

for

all

known

routes
.



BGP

is

DV

routing

protocol


OSPF

is

LS

Routing

Protocol

4

Distance
-
vector &Link state routing

SIGMETRICS’04

5


A router combines the BGP and IGP information
to construct a forwarding table.


BGP exchanges route advertisements with neighboring
domains, and propagate
reachability

information within
AS.


IGP protocol, such as OSPF, computes shortest paths
based on configurable link weights.


The interaction between IGP and BGP


Hot potato routing.

Interaction between IGP and BGP

SIGMETRICS’04

Hot
-
Potato Routing

6

San Francisco

Dallas

New York

Hot
-
potato routing = route to closest egress point



when there is more than




one route to destination

ISP network

9

10

dst

multiple connections

to the same peer

SIGMETRICS’04

Hot
-
Potato Routing Change

7

San Francisco

Dallas

New York

ISP network

dst

9

10

-

failure

-

planned maintenance

-

traffic engineering

11

Routes to thousands

of destinations switch

exit point!!!

Consequences:


Transient forwarding instability


Traffic shift


Inter
-
domain routing changes

11

SIGMETRICS’04

Approach In Paper


Understanding impact in real networks


How often hot
-
potato changes happen?


How many destinations do they affect?


What are the convergence delays?


Main contributions


Methodology for measuring hot
-
potato
changes


Characterization on AT&T’s IP backbone


8

SIGMETRICS’04

Challenges for Identifying

Hot
-
Potato Changes


Cannot

collect
data from all routers


OSPF: flooding gives complete view of topology


BGP: multi
-
hop sessions to several vantage points


A single event may cause multiple messages


Group related routing messages in time


Router implementation affects message
timing(
PDelay
)


Real Time & Controlled
experiments of router in the lab


Many BGP updates caused by external events


Classify BGP routing changes by possible causes

9

SIGMETRICS’04

Measurement Methodology

10

Replay routing decisions from

vantage point

A and B to identify

hot
-
potato changes

AT&T

backbone

BGP monitor

BGP updates

OSPF Monitor

OSPF

messages

A

B

SIGMETRICS’04

Measurement Methodology

12

SIGMETRICS’04

Sections Details


Section III
-

A: presents the measurement infrastructure
used to collect BGP updates and OSPF LSAs.



Section III
-
B : describe how to compute the distance vector
from the OSPF LSAs in.



Section III
-
C : explains the classification of BGP routing
changes in terms of the possible causes. This sets the stage
for the discussion in next section.



Section III
-
D : How to associate BGP routing changes with
related distance changes that occur close in time


13

SIGMETRICS’04

Measurement Infrastructure



iBGP

session
allows the monitor to see changes in the “egress point” of BGP
routes.



The BGP monitor also dumps a snapshot of its routes four times a day to provide
an initial view of the best route for each prefix for each vantage point, For later
classify the type of BGP change as discussed in Section III
-
C


14

SIGMETRICS’04

Classifying BGP
Rt

Changes

15

SIGMETRICS’04


The large volume of BGP updates shows
exploration of multiple alternate routes when a
router switches from one best path to another .


IGP distance changes cause a router inside the
AS to switch from one stable route to another
with a different egress point.



16

SIGMETRICS’04

BGP Reaction Time to Distance Changes


17

(i)
Rerun
the IGP shortest
-
path
computation

(ii)
Apply the BGP
decision process to select the best
route

(iii)

Send
update messages to BGP neighbors for the routes
that
have
changed.

SIGMETRICS’04

Transfer

Delay for Multiple Prefixes


18

1
-
The
LSA is flooded throughout the network and each router

computes new
distances.
For example,
A and B compute new
distances of 21
and 11, respectively
.


2
-

After their scan timers elapse, and rerun the BGP decision

process. If runs first, selects the egress
point with
a distance of 20, since this is
smaller than 21.
Sometime afterwards
,
A selects
egress point
C.


3
-

B sends
the new route (with egress
point E
) to

A, A and selects
egress
point with a distance of 19.


SIGMETRICS’04

Temporal and Spatial Variability(distance
changes effect)



19

SIGMETRICS’04

Hot
-
Potato Variation Across
Prefixes


20

SIGMETRICS’04

IMPLICATIONS OF HOT POTATOES



Performance Degradation


Routing and Traffic Shifts


Slow Forwarding
-
Plane Convergence


Measurement Inaccuracies


Active Probes of the Forwarding Plane


External Analysis of BGP Updates





21

SIGMETRICS’04

Conclusion


Hot
-
potato routing plays an important role in


BGP routing changes, and that BGP updates can
lag 60 seconds (or more!) behind the related IGP
events


The frequency and impact of hot
-
potato routing
depends on the topology and configuration of
the network under study


routing is usually a major contributor to large
traffic variations. In particular, hot
-
potato
routing changes are responsible for the largest
shifts in the traffic matrix





22

SIGMETRICS’04

…conclusion


Setting IGP link weights without accounting for
possible changes in the egress points can lead to
routing configurations that cause unnecessary
congestion.


23

SIGMETRICS’04

Thanks

24