computer networks graduate level n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Computer Networks (Graduate level) PowerPoint Presentation
Download Presentation
Computer Networks (Graduate level)

Loading in 2 Seconds...

play fullscreen
1 / 85

Computer Networks (Graduate level) - PowerPoint PPT Presentation


  • 146 Views
  • Uploaded on

Computer Networks (Graduate level). Lecture 7: Inter-domain Routing. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Inter-Domain Routing. Border Gateway Protocol (BGP) Assigned reading [LAB00] Delayed Internet Routing Convergence Sources

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Computer Networks (Graduate level)' - creda


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
computer networks graduate level

Computer Networks(Graduate level)

Lecture 7: Inter-domain Routing

University of Tehran

Dept. of EE and Computer Engineering

By:

Dr. Nasser Yazdani

Computer Network

inter domain routing
Inter-Domain Routing
  • Border Gateway Protocol (BGP)
  • Assigned reading
    • [LAB00] Delayed Internet Routing Convergence
  • Sources
    • RFC1771: main BGP RFC
    • RFC1772-3-4: application, experiences, and analysis of BGP
    • RFC1965: AS confederations for BGP
    • Christian Huitema: “Routing in the Internet”, chapters 8 and 9.
    • John Stewart III: “BGP4 - Inter-domain routing in the Internet”

Computer Network

outline
Outline
  • External BGP (E-BGP)
  • Internal BGP (I-BGP)
  • Multi-Homing
  • Stability Issues

Computer Network

internet s area hierarchy
Internet’s Area Hierarchy
  • What is an Autonomous System (AS)?
    • A set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to route packets within the AS and using an exterior gateway protocol (EGP) to route packets to other AS’s
    • Sometimes AS’s use multiple IGPs and metrics, but appear as single AS’s to other AS’s
  • Each AS assigned unique ID
  • AS’s peer at network exchange routing information.

Computer Network

example
Example

1

2

IGP

2.1

2.2

IGP

EGP

1.1

2.2.1

1.2

EGP

EGP

EGP

3

4.2

4.1

IGP

EGP

4

IGP

5

3.2

3.1

IGP

5.2

5.1

Computer Network

history
History
  • Mid-80s: EGP
    • Reachability protocol (no shortest path)
    • Did not accommodate cycles (tree topology)
    • Evolved when all networks connected to NSF backbone
  • Result: BGP introduced as routing protocol
    • Latest version = BGP 4
    • BGP-4 supports CIDR
    • Primary objective: connectivity not performance

Computer Network

choices
Choices
  • 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

Computer Network

solution distance vector with path
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 is 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

Computer Network

interconnecting bgp peers
Interconnecting BGP Peers
  • BGP uses TCP to connect peers
  • Advantages:
    • Simplifies BGP
    • No need for periodic refresh - routes are valid until withdrawn, or the connection is lost
    • Incremental updates
  • Disadvantages
    • Congestion control on a routing protocol?
    • Poor interaction during high load

Computer Network

hop by hop model
Hop-by-hop Model
  • BGP advertises to neighbors only those routes that it uses
    • Consistent with the hop-by-hop Internet paradigm
    • e.g., AS1 cannot tell AS2 to route to other AS’s in a manner different than what AS2 has chosen (need source routing for that)

Computer Network

as categories
AS Categories
  • Stub: an AS that has only a single connection to one other AS - carries only local traffic.
  • Multi-homed: an AS that has connections to more than one AS, but does not carry transit traffic
  • Transit: an AS that has connections to more than one AS, and carries both transit and local traffic (under certain policy restrictions)

Computer Network

as categories1
AS Categories

AS1

AS3

AS1

AS2

AS1

AS3

AS2

Transit

Stub

AS2

Multi-homed

Computer Network

policy with bgp
Policy with BGP
  • BGP provides capability for enforcing various policies
  • Policies are not part of BGP: they are provided to BGP as configuration information
  • BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s

Computer Network

examples of bgp policies
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
  • An AS can favor or disfavor certain AS’s for traffic transit from itself

Computer Network

routing information bases rib
Routing Information Bases (RIB)
  • Routes are stored in RIBs
  • Adj-RIBs-In: routing info that has been learned from other routers (unprocessed routing info)
  • Loc-RIB: local routing information selected from Adj-RIBs-In (routes selected locally)
  • Adj-RIBs-Out: info to be advertised to peers (routes to be advertised)

Computer Network

bgp common header
BGP Common Header

1

2

3

0

Marker (security and message delineation)

16 bytes

Length (2 bytes)

Type (1 byte)

Types: OPEN, UPDATE, NOTIFICATION, KEEPALIVE

Computer Network

bgp open message
BGP OPEN message

1

2

3

0

Marker (security and message delineation)

Length

Type: open

version

My autonomous system

Hold time

BGP identifier

Optional parameters <type, length, value>

Parameter length

My AS: id assigned to that AS

Hold timer: max interval between KEEPALIVE or UPDATE messages interval implies no keep_alive.

BGP ID: IP address of one interface (same for all messages)

Computer Network

bgp update message
BGP UPDATE message

1

2

3

0

Marker (security and message delineation)

Length

Type: update

Withdrawn..

..routes len

Withdrawn routes (variable)

...

Path attribute len

Path attributes (variable)

Network layer reachability information (NLRI) (variable)

  • Many prefixes may be included in UPDATE, but must
    • share same attributes.
  • UPDATE message may report multiple withdrawn routes.

Computer Network

bgp update message1
BGP UPDATE Message
  • List of withdrawn routes
  • Network layer reachability information
    • List of reachable prefixes
  • Path attributes
    • Origin
    • Path
    • Metrics
  • All prefixes advertised in a message have same path attributes

Computer Network

slide20
NLRI
  • Network Level Reachability Information
    • list of IP address prefixes encoded as follows:

Length (1 byte)

Prefix (variable)

Computer Network

path attributes
Path attributes

Type-Length-Value encoding

Attribute type (2 bytes)

Attribute length (1-2 bytes)

Attribute Value (variable length)

Attribute type field

Attribute flags (1 byte)

Attribute type code (1 byte)

Flags: optional, v.s. well-known

transitive, partial, extended length

Computer Network

bgp notification message
BGP NOTIFICATION message

1

2

3

0

Marker (security and message delineation)

Length

Type: NOTIFICATION

Error code

Data

Error sub-code

  • Used for error notification
    • TCP connection is closed immediately after notification

Computer Network

bgp keepalive message
BGP KEEPALIVE message

1

2

3

0

Marker (security and message delineation)

Length

Type: KEEPALIVE

Sent periodically to peers to ensure connectivity.

If hold_time is zero, messages are not sent..

Sent in place of an UPDATE message

Computer Network

path selection criteria
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

Computer Network

route selection summary
Route Selection Summary

Highest Local Preference

Enforce relationships

Shortest ASPATH

Lowest MED

traffic engineering

i-BGP < e-BGP

Lowest IGP cost

to BGP egress

Throw up hands and

break ties

Lowest router ID

Computer Network

back to frank

peer

peer

provider

customer

Back to Frank …

Local preference only used in iBGP

AS 4

local pref = 80

AS 3

local pref = 90

local pref = 100

AS 2

AS 1

Higher Local

preference values

are more preferred

13.13.0.0/16

implementing backup links with local preference outbound traffic
Implementing Backup Links with Local Preference (Outbound Traffic)

AS 1

primary link

backup link

Set Local Pref = 100

for all routes from AS 1

Set Local Pref = 50

for all routes from AS 1

AS 65000

Forces outbound traffic to take primary link, unless link is down.

We’ll talk about inbound traffic soon …

multihomed backups outbound traffic
Multihomed Backups (Outbound Traffic)

AS 1

AS 3

provider

provider

primary link

backup link

Set Local Pref = 100

for all routes from AS 1

Set Local Pref = 50

for all routes from AS 3

AS 2

Forces outbound traffic to take primary link, unless link is down.

aspath attribute

AS 1239

Sprint

ASPATH Attribute

AS 1129

135.207.0.0/16

AS Path = 1755 1239 7018 6341

Global Access

AS 1755

135.207.0.0/16

AS Path = 1239 7018 6341

135.207.0.0/16

AS Path = 1129 1755 1239 7018 6341

Ebone

AS 12654

RIPE NCC

RIS project

135.207.0.0/16

AS Path = 7018 6341

AS7018

135.207.0.0/16

AS Path = 3549 7018 6341

135.207.0.0/16

AS Path = 6341

AT&T

AS 3549

AS 6341

135.207.0.0/16

AS Path = 7018 6341

AT&T Research

Global Crossing

135.207.0.0/16

Prefix Originated

community attribute to the rescue
COMMUNITY Attribute to the Rescue!

AS 3: normal

customer local

pref is 100,

peer local pref is 90

AS 1

AS 3

provider

provider

192.0.2.0/24

ASPATH = 2

COMMUNITY = 3:70

192.0.2.0/24

ASPATH = 2

primary

backup

Customer import policy at AS 3:

If 3:90 in COMMUNITY then

set local preference to 90

If 3:80 in COMMUNITY then

set local preference to 80

If 3:70 in COMMUNITY then

set local preference to 70

customer

192.0.2.0/24

AS 2

hot potato routing go for the closest egress point
Hot Potato Routing: Go for the Closest Egress Point

192.44.78.0/24

egress 2

egress 1

IGP distances

56

15

This Router has two BGP routes to 192.44.78.0/24.

Hot potato: get traffic off of your network as

Soon as possible. Go for egress 1!

getting burned by the hot potato

Heavy

Content

Web Farm

Getting Burned by the Hot Potato

2865

High bandwidth

Provider backbone

17

SFF

NYC

Low bandwidth

customer backbone

56

15

San Diego

Many customers want

their provider to

carry the bits!

tiny http request

huge http reply

cold potato routing with meds multi exit discriminator attribute

Heavy

Content

Web Farm

Cold Potato Routing with MEDs(Multi-Exit Discriminator Attribute)

Prefer lower

MED values

2865

17

192.44.78.0/24

MED = 56

192.44.78.0/24

MED = 15

56

15

192.44.78.0/24

This means that MEDs must be considered BEFORE

IGP distance!

Note1 : some providers will not listen to MEDs

Note2 : MEDs need not be tied to IGP distance

route selection summary1
Route Selection Summary

Highest Local Preference

Enforce relationships

Shortest ASPATH

Lowest MED

traffic engineering

i-BGP < e-BGP

Lowest IGP cost

to BGP egress

Throw up hands and

break ties

Lowest router ID

This is somewhat simplified. Hey, what happened to ORIGIN??

Computer Network

policies can interact strangely route pinning example
Policies Can Interact Strangely(“Route Pinning” Example)

backup

customer

1

2

Install backup link using community

3

Disaster strikes primary link

and the backup takes over

4

Primary link is restored but some

traffic remains pinned to backup

Computer Network

path attributes1
Path Attributes
  • Categories (recall flags):
    • well-known mandatory (passed on)
    • well-known discretionary (passed on)
    • optional transitive (passed on)
    • optional non-transitive (if unrecognized, not passed on)
  • Optional attributes allow for BGP extensions

Computer Network

path attribute message format repeated
Path attribute message format (repeated)

Attribute flags

Attribute type code

O T P E

0

O: optional or well-known

T: transitive or local

P: partially evaluated

E: length in 1 or 2 bytes

Origin

AS_path

Next hop

etc.

Computer Network

origin path attribute
ORIGIN path attribute
  • Well-known, mandatory attribute.
  • Describes how a prefix was generated at the origin AS. Possible values:
    • IGP: prefix learned from IGP
    • EGP: prefix learned through EGP
    • INCOMPLETE: none of the above (often seen for static routes)

Computer Network

as path attribute
AS_PATH attribute
  • Well-known, mandatory attribute.
  • Important components:
    • list of traversed AS’s
  • If forwarding to internal peer:
    • do not modify AS_PATH attribute
  • If forwarding to external peer:
    • prepend self into the path

Computer Network

next hop path attribute
Next hop path attribute
  • Well-known, mandatory attribute
  • NEXT_HOP: IP address of border router to be used as next hop
  • Usually, next hop is the router sending the UPDATE message
  • Useful when some routers do not speak BGP

Computer Network

example of next hop
Example of NEXT_HOP

A

(BGP)

UPDATE MSG through BGP

B

(BGP)

Traffic to 138.39.0.0/16

C

(no BGP)

138.39.0.0/16

Computer Network

local pref
LOCAL PREF
  • Local (within an AS) mechanism to provide relative priority among BGP routers

R5

R1

AS 200

R2

AS 100

AS 300

R3

Local Pref = 500

Local Pref =800

R4

I-BGP

AS 256

Computer Network

as path
AS_PATH
  • List of traversed AS’s

AS 200

AS 100

170.10.0.0/16

180.10.0.0/16

AS 300

180.10.0.0/16 300 200 100

170.10.0.0/16 300 200

AS 500

Computer Network

cidr and bgp
CIDR and BGP

AS X

197.8.2.0/24

AS T (provider)

197.8.0.0/23

AS Z

AS Y

197.8.3.0/24

What should T announce to Z?

Computer Network

options
Options
  • Advertise all paths:
    • Path 1: through T can reach 197.8.0.0/23
    • Path 2: through T can reach 197.8.2.0/24
    • Path 3: through T can reach 197.8.3.0/24
  • But this does not reduce routing tables! We would like to advertise:
    • Path 1: through T can reach 197.8.0.0/22

Computer Network

sets and sequences
Sets and Sequences
  • Problem: what do we list in the route?
      • List T: omitting information not acceptable, may lead to loops
      • List T, X, Y: misleading, appears as 3-hop path
  • Solution: restructure AS Path attribute as:
      • Path: (Sequence (T), Set (X, Y))
      • If Z wants to advertise path:
        • Path: (Sequence (Z, T), Set (X, Y))
      • In practice used only if paths in set have same attributes

Computer Network

multi exit discriminator med
Multi-Exit Discriminator (MED)
  • Hint to external neighbors about the preferred path into an AS
    • Non-transitive attribute (we will see later why)
    • Different AS choose different scales
  • Used when two AS’s connect to each other in more than one place

Computer Network

slide48
MED
  • Hint to R1 to use R3 over R4 link
  • Cannot compare AS40’s values to AS30’s

180.10.0.0

MED = 50

R1

R2

AS 10

AS 40

180.10.0.0

MED = 120

180.10.0.0

MED = 200

R3

R4

AS 30

Computer Network

slide49
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:

ISP1

SF

ISP2

NY

  • ISP1 ignores MED from ISP2
  • ISP2 obeys MED from ISP1
  • ISP2 ends up carrying traffic most of the way

Computer Network

other attributes
Other Attributes
  • ORIGIN
    • Source of route (IGP, EGP, other)
  • NEXT_HOP
    • Address of next hop router to use
    • Used to direct traffic to non-BGP router
  • Check out http://www.cisco.com for full explanation

Computer Network

decision process
Decision Process
  • Processing order of attributes:
    • Select route with highest LOCAL-PREF
    • Select route with shortest AS-PATH
    • Apply MED (if routes learned from same neighbor)

Computer Network

outline1
Outline
  • External BGP (E-BGP)
  • Internal BGP (I-BGP)
  • Multi-Homing
  • Stability Issues

Computer Network

internal vs external bgp
Internal vs. External 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

R1

E-BGP

AS1

R3

R4

AS2

R2

Computer Network

i bgp
I-BGP

Computer Network

internal bgp i bgp
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-BGP neighbor
    • Reason: no AS PATH within the same AS and thus danger of looping.

Computer Network

internal bgp i bgp1
Internal 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

R1

E-BGP

AS1

R3

R4

AS2

R2

I-BGP

Computer Network

link failures
Link Failures
  • Two types of link failures:
    • Failure on an E-BGP link
    • Failure on an I-BGP Link
  • These failures are treated completely different in BGP
  • Why?

Computer Network

failure on an e bgp link
Failure on an E-BGP Link
  • If the link R1-R2 goes down
    • The TCP connection breaks
    • BGP routes are removed
  • This is the desired behavior

AS1

AS2

E-BGP session

R1

R2

Physical link

138.39.1.1/30

138.39.1.2/30

Computer Network

failure on an i bgp link
Failure on an I-BGP Link
  • If link R1-R2 goes down, R1 and R2 should still be able to exchange traffic
  • The indirect path through R3 must be used
  • Thus, E-BGP and I-BGP must use different conventions with respect to TCP endpoints

R2

138.39.1.2/30

Physical link

138.39.1.1/30

R1

R3

I-BGP connection

Computer Network

outline2
Outline
  • External BGP (E-BGP)
  • Internal BGP (I-BGP)
  • Multi-Homing
  • Stability Issues

Computer Network

multi homing
Multi-homing
  • With multi-homing, a single network has more than one connection to the Internet.
  • Improves reliability and performance:
    • Can accommodate link failure
    • Bandwidth is sum of links to Internet
  • Challenges
    • Getting policy right (MED, etc..)
    • Addressing

Computer Network

multi homing to a single provider case 1

ISP

R1

Customer

R2

Multi-homing to a Single Provider Case 1
  • Easy solution:
    • Use IMUX or Multi-link PPP
  • Hard solution:
    • Use BGP
    • Makes assumptions about traffic (same amount of prefixes can be reached from both links)

Computer Network

multi homing to a single provider case 2
Multi-homing to a single provider: Case 2
  • If multiple prefixes, may use MED
    • good if traffic load from prefixes is equal
  • If single prefix, load may be unequal
    • break-down prefix and advertise different prefixes over different links

ISP

R1

Customer

R2

R3

138.39/16

204.70/16

Computer Network

multi homing to a single provider case 3
Multi-homing to a single provider: Case 3
  • For ISP-> customer traffic, same as before:
    • use MED
    • good if traffic load to prefixes is equal
  • For customer -> ISP traffic:
    • R3 alternates links
    • multiple default routes

ISP

R1

R2

Customer

R3

138.39/16

204.70/16

Computer Network

multi homing to a single provider case 4
Multi-homing to a single provider: Case 4
  • Most reliable approach
    • no equipment sharing
  • Customer -> ISP:
    • same as case 2
  • ISP -> customer:
    • same as case 3

ISP

R1

R2

Customer

R3

R4

138.39/16

204.70/16

Computer Network

multi homing to multiple providers
Multi-homing to Multiple Providers
  • Major issues:
    • Addressing
    • Aggregation
  • Customer address space:
    • Delegated by ISP1
    • Delegated by ISP2
    • Delegated by ISP1 and ISP2
    • Obtained independently
  • Advantage and disadvantage?

ISP3

ISP1

ISP2

Customer

Computer Network

address space from one isp
Address Space from one ISP
  • Customer uses address space from one, I.e ISP1
  • ISP1 advertises /16 aggregate
  • Customer advertises /24 route to ISP2
  • ISP2 relays route to ISP1 and ISP3
  • ISP2-3 use /24 route
  • ISP1 routes directly
  • Problems with traffic load?

ISP3

138.39/16

ISP1

ISP2

Customer

138.39.1/24

Computer Network

pitfalls
Pitfalls
  • ISP1 aggregates to a /19 at border router to reduce internal tables.
  • ISP1 still announces /16.
  • ISP1 hears /24 from ISP2.
  • ISP1 routes packets for customer to ISP2!
  • Workaround: ISP1 must inject /24 into I-BGP.

ISP3

138.39/16

ISP1

ISP2

138.39.0/19

Customer

138.39.1/24

Computer Network

address space from both isps
Address Space from Both ISPs
  • ISP1 and ISP2 continue to announce aggregates
  • Load sharing depends on traffic to two prefixes
  • Lack of reliability: if ISP1 link goes down, part of customer becomes inaccessible.
  • Customer may announce prefixes to both ISPs, but still problems with longest match as in case 1.

ISP3

ISP1

ISP2

204.70.1/24

Customer

138.39.1/24

Computer Network

address space obtained independently
Address Space Obtained Independently
  • Offers the most control, but at the cost of aggregation.
  • Still need to control paths
    • suppose ISP1 large, ISP2-3 small
    • customer advertises long path to ISP1, but local-pref attribute used to override
    • ISP3 learns shorter path from ISP2

ISP3

ISP1

ISP2

Customer

Computer Network

outline3
Outline
  • External BGP (e-BGP)
  • Internal BGP (i-BGP)
  • Multi-Homing
  • Stability Issues

Computer Network

signs of routing instability
Signs of Routing Instability
  • Record of BGP messages at major exchanges
  • Discovered orders of magnitude larger than expected updates
    • Bulk were duplicate withdrawals
      • Stateless implementation of BGP – did not keep track of information passed to peers
      • Impact of few implementations
    • Strong frequency (30/60 sec) components
      • Interaction with other local routing/links etc.

Computer Network

route flap storm
Route Flap Storm
  • Overloaded routers fail to send Keep_Alive message and marked as down
  • I-BGP peers find alternate paths
  • Overloaded router re-establishes peering session
  • Must send large updates
  • Increased load causes more routers to fail!

Computer Network

route flap dampening
Route Flap Dampening
  • Routers now give higher priority to BGP/Keep_Alive to avoid problem
  • Associate a penalty with each route
    • Increase when route flaps
    • Exponentially decay penalty with time
  • When penalty reaches threshold, suppress route

Computer Network

bgp limitations oscillations
BGP Limitations: Oscillations

(*R,1R,2R)

AS 0

R

AS 1

AS 2

(0R,1R,*R)

(0R,*R,2R)

Computer Network

bgp limitations oscillations1
BGP Limitations: Oscillations

AS 0

(-,*1R,2R)

(*R,1R,2R)

W

R

W

W

AS 1

AS 2

(*0R,-,2R)

(0R,*R,2R)

(0R,1R,*R)

(*0R,1R,-)

Computer Network

bgp limitations oscillations2
BGP Limitations: Oscillations

AS 0

(-,*1R,2R)

(-,*1R,2R)

01R

01R

R

AS 1

AS 2

(-,-,*2R)

(*0R,-,2R)

(*0R,1R,-)

(01R,*1R,-)

Computer Network

bgp limitations oscillations3
BGP Limitations: Oscillations

AS 0

(-,-,*2R)

(-,*1R,2R)

10R

R

AS 1

AS 2

(-,-,*2R)

(-,-,*2R)

(01R,*1R,-)

(*01R,10R,-)

10R

Computer Network

bgp limitations oscillations4
BGP Limitations: Oscillations

AS 0

(-,-,*2R)

(-,-,-)

20R

R

AS 1

AS 2

(-,-,*20R)

(-,-,*2R)

(*01R,10R,-)

(*01R,10R,-)

20R

Computer Network

bgp limitations oscillations5
BGP Limitations: Oscillations

AS 0

(-,*12R,-)

(-,-,-)

12R

R

AS 1

AS 2

(*01R,10R,-)

12R

(-,-,*20R)

(-,-,*20R)

(*01R,-,-)

Computer Network

bgp limitations oscillations6
BGP Limitations: Oscillations

AS 0

(-,*12R,21R)

(-,*12R,-)

21R

R

AS 1

AS 2

(*01R,-,-)

21R

(-,-,*20R)

(*01R,-,-)

(-,-,-)

Computer Network

bgp oscillations
BGP Oscillations
  • Can possible explore every possible path through network  (n-1)! Combinations
  • Limit between update messages (MinRouteAdver) reduces exploration
    • Forces router to process all outstanding messages
  • Typical Internet failover times
    • New/shorter link  60 seconds
      • Results in simple replacement at nodes
    • Down link  180 seconds
      • Results in search of possible options
    • Longer link  120 seconds
      • Results in replacement or search based on length

Computer Network

problems
Problems
  • Routing table size
    • Need an entry for all paths to all networks
  • Required memory= O((N + M*A) * K)
    • N: number of networks
    • M: mean AS distance (in terms of hops)
    • A: number of AS’s
    • K: number of BGP peers

Computer Network

routing table size
Routing Table Size
  • Problem reduced with CIDR

Networks

Mean AS Distance

Number of AS’s

BGP Peers/Net

Memory

2,100

5

59

3

27,000

4,000

10

100

6

108,000

10,000

15

300

10

490,000

100,000

20

3,000

20

1,040,000

Computer Network

next lecture tcp
Next Lecture: TCP
  • Transport layer issues
  • Assigned reading
    • [S+99] The End-to-End Effects of Internet Path Selection
    • [Tsu88] The Landmark Hierarchy: A New Hierarchy for Routing in Very Large Networks

Computer Network