Internet Routing (COS Internet Routing (COS 598A)598A)

Today: BGP Routing Table SizeToday: BGP Routing Table Size

Jennifer RexfordJennifer Rexford

http://www.cs.princeton.edu/~jrex/teaching/http://www.cs.princeton.edu/~jrex/teaching/spring2005spring2005

Tuesdays/Thursdays 11:00am-12:20pmTuesdays/Thursdays 11:00am-12:20pm

Outline

• IP prefixes– Review of CIDR and hierarchical allocation– Resource constraints on IP routers– Impact of increasing number of prefixes

• Growth in BGP routing table size– Growth of global prefixes over time– Characterizing the causes of growth

• Limiting the number of prefixes– Techniques for limiting the size– Fundamental challenges of limiting size

04/21/23

Classless InterDomain Routing (CIDR)

IP Address : 12.4.0.0 IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000

11111111 11111110 00000000 00000000

Address

Mask

for hosts Network Prefix

Use two 32-bit numbers to represent a network. Network number = IP address + Mask

Usually written as 12.4.0.0/15

04/21/23

Hierarchy in Allocating Address Blocks

12.0.0.0/8

12.0.0.0/16

12.254.0.0/16

12.1.0.0/1612.2.0.0/1612.3.0.0/16

:::

12.253.0.0/16

12.3.0.0/2412.3.1.0/24

::

12.3.254.0/24

12.253.0.0/1912.253.32.0/1912.253.64.0/1912.253.96.0/1912.253.128.0/1912.253.160.0/1912.253.192.0/19

:::

• Prefixes are key to Internet scalability– Address allocation by ARIN/RIPE/APNIC and by ISPs– Routing protocols and packet forwarding based on

prefixes– Today, routing tables contain ~150,000-200,000

prefixes

Resource Constraints on a High-End Router

SwitchingFabric

Processor

Line card

Line card

Line card

Line card

Line card

Line card

Store routing table and process routing protocol messages

Store forwarding table and forward

data packets

Routing Information Base (RIB)

• Routing table for the routing protocol– E.g., BGP routes learned from each neighbor– Typically managed in software in router CPU

• Factors affecting RIB size– Number of destination prefixes– Number of BGP routes per prefix– Size of each route (e.g., BGP attributes)

• Impact of a large RIB– Higher delay to index or scan the table– Ungraceful reaction to table overflow

Ungraceful Overload Behavior in BGP

• BGP is an incremental protocol– Announcement when new route available– Withdrawal when route no longer available– No messages when nothing is changing

• Cannot discard or delete state– … because you won’t receive the message

again– When table is full, router must drop session(s)

• Router reaction in practice may be worse– E.g., drop all BGP sessions and reestablish– E.g., interface lock-up till router is rebooted– Reactions place heavy BGP load on neighbors

Forwarding Information Base

• Forwarding tables in IP routers– Maps each IP prefix to next-hop link(s)– Longest prefix match look-up for data packets– Hardware on line card in high-end routers

• Impact of a large FIB– Higher delay to construct/update the table– Higher delay for packet lookup– Incomplete table or router crash on overflow

4.0.0.0/84.83.128.0/1712.0.0.0/812.34.158.0/24126.255.103.0/24

12.34.158.5destination

forwarding table (FIB)

Serial0/0.1outgoing link

Impact of Table Size: Message Overhead

• More BGP update messages– More prefixes means more update

messages– … and more bandwidth and CPU

consumption– … and longer delays for bringing up a

session

• More BGP route flapping– More likely to have one or more flapping

prefixes– … which consumes even more resources– … and makes the routing system less stable

Growth in BGP Routing Table Size

http://www.cisco.com/en/US/about/ac123/ac147/ac174/ac176/about_cisco_ipj_archive_article09186a00800c83cc.html

http://www.cs.princeton.edu/~jrex/teaching/spring2005/reading/bu02.pdf

Pre-CIDR (1988-1994): Steep Growth Rate

Growth faster than improvements in equipment capability

CIDR Deployment (1994-1996): Much Flatter

Efforts to aggregate (even decreases after IETF meetings!)

CIDR Growth (1996-1998): Roughly Linear

Good use of aggregation, and peer pressure in CIDR report

Boom Period (1998-2001): Steep Growth

Internet boom and increased multi-homing

Long-Term View (1989-2005): Post-Boom

Cause of Growth #1: Multi-Homing

• Connecting to multiple providers– All providers must advertise the prefix– Hole-punching: subnet contained in a

supernet

• Detecting hole-punching– Stub AS connects to two or more ASes– Prefix is contained in one provider’s supernet

ISP #1 ISP #2

Stub

12.1.1.0/24

12.0.0.0/812.1.1.0/24

3.0.0.0/812.1.1.0/24

Cause of Growth #2: Failure to Aggregate

• Prefixes could be coalesced– Advertised exactly the same way– Adjacent prefixes or subnet/supernet relationship

• Detecting failure to aggregate– Prefixes with same attributes in set of BGP tables– Could be reduced to fewer prefixes by combining

ISP #1 ISP #2

Stub

12.1.1.0/24

12.0.0.0/812.1.1.0/24

12.1.2.0/2412.1.3.0/24

Stub

12.1.2.0/24

Stub

12.1.3.0/24

12.1.2.0/23

Cause of Growth #3: Load Balancing

• Larger block sub-divided for more control– Advertise multiple subnets of a larger prefix– Treat differently to influence incoming traffic

• Detecting load balancing– Prefixes originated by the same AS– Could be collapsed (e.g., contiguous or

contained)– … but, have different attributes, such as AS path

ISP #1 ISP #2

Stub12.1.2.0/2312.1.2.0/24

12.1.2.0/2312.1.3.0/24

Cause of Growth #4: Address Fragmentation

• Different parts of the address space– Distinct address blocks allocated to same AS– Must be advertised separately in BGP

• Detecting address fragmentation– Prefixes announced the same way by same

AS– Cannot be collapsed into fewer prefixes

ISP #1

Stub

18.8.0.0/1612.1.1.0/24

Significance of the Four Causes

• Overall contribution– Address fragmentation is the most significant– The other three causes are all important as

well

• Growth over time– Increasing multi-homing– Increasing load balancing

• Architectural implications– Exploit commonality across non-contiguous

address blocks?– Multi-homing without hole-punching?– Load balancing without de-aggregating?

Transient Growth in Table Size: Routing Leaks

Transient spike due to neighbor’s BGP mistake

Techniques for Limiting Table Size

Hierarchical Address Allocation

• Regional Internet Registries– Allocate large address blocks to ISPs– Publish guidelines for minimum block sizes

• ARIN: in 63.0.0.0/8, no mask lengths more than /19• APNIC: in 211.0.0.0/8, no mask lengths more

than /23

• Internet Service Providers– Allocate smaller blocks to customers

• Reclaim address blocks when customers leave

– Hierarchical address allocation inside the ISP• Advertise subnets only when necessary• Customer-owned addresses and multi-homing

Hierarchical Allocation: Only One Router Knows

Stub

12.0.0.0/8

Stub

12.1.0.0/16

12.1.2.0/24 12.1.5.0/24

• Three-level hierarchy– ISP as a whole: 12.0.0.0/8– Edge router in ISP: 12.1.0.0/16– Customer at edge router: 12.1.2.0/24,

12.1.5.0/24Only this router

needs to know the small /24 blocks

Hierarchical Allocation: Only the ISP Knows

Stub

12.0.0.0/8

12.1.5.0/24

• Customer connecting in multiple places– All routers in the ISP need to know the subnet– Otherwise they can’t reach all egress points

– But the rest of the Internet doesn’t need to know

12.1.0.0/16

Hierarchical Allocation: Must Advertise

Stub

12.0.0.0/8

Stub

12.1.0.0/16

78.34.0.0/16 12.1.5.0/24

Another ISP

• Sometimes have to advertise the subnet– Customer doesn’t fall in ISP’s address block– Customer connects to multiple providers

Filtering Small Subnets on BGP Sessions

• Small address blocks– Larger mask than RIR guidelines

• E.g., filter /20 and longer in 63.0.0.0/8

– Or, all prefixes with mask longer than /24

• Trade-off on aggressive filtering– Don’t filter aggressively

• Risk of exceeding memory limits on the router

– Filter aggressively• Risk of disconnecting some parts of the Internet• Risk of thwarting stub ASes trying to load-balance

• Who should pay to store the small subnets???

Prefix Limits to Protect Against Route Leaks

• Vulnerability to other ASes– Sending many small subnets– Exporting address space they shouldn’t

• Filtering policies may not be enough– E.g., all /24s is still 224 prefixes is still a lot

• Max-prefix limit on BGP session– Per-session configurable limit on # of

prefixes– Tear down the session if number exceeded– Not great, but better than exceeding the

memory

Fundamental Problems: Not Easily Automated

• Dependence on “side information”– Customer prefix falls in provider’s address

space?– Customer connects to ISP in multiple places?– Customer connects to multiple providers?

• Auto-combining is hard in distributed system– Safe to combine 12.1.2.0/24 and 12.1.3.0/24???– Depends on whether other ASes need the details

12.1.2.0/24

12.1.3.0/24

seems safenot safe

Optimization: Reducing Forwarding Table Size

• Local FIB minimization– Router locally minimizes size of forwarding

table– E.g., purple router has FIB entry for 12.1.2.0/23– … while still keeping both subnets in BGP table

– But, the size of the RIB may still be an issue

12.1.2.0/24

12.1.3.0/24

Architectural Idea: Reducing BGP Table Size

• Separating BGP propagation from the routers– Exchange BGP updates via separate servers– Servers tell routers only the BGP routes they

need– … yet still propagate full details to neighbors

– We’ll return to this idea in the coming weeks

12.1.2.0/24

12.1.3.0/24

12.1.2.0/2312.1.2.0/2412.1.3.0/24

BGP BGP

Conclusions

• Scalability limitations– Resource constraints on routers– … impose limits on number of prefixes

• Growth in the number of prefixes– Historical trends toward increasing table size– Multi-homing, failure to aggregate, load

balancing, and address fragmentation

• Approaches to limiting growth– Hierarchical address allocation– Careful scoping of BGP route advertisements– Explicit minimization of FIB and RIB sizes

Next Time: Large Topologies

• Two papers– “Hierarchical routing for large networks:

Performance evaluation and optimization”– “BGP route reflection: An alternative to full

mesh IBGP”• Review only of first paper

– Summary– Why accept– Why reject– Avenues for future work

• Optional reading– Fun 1928 article “On Being the Right Size”