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Interdomain Issues for IP networks. Henning Schulzrinne (with lots of borrowed slides...). Overview. Architecture review Interdomain routing Multicast VPNs Interdomain QoS signaling charging and settlements Interdomain application signaling Carrier selection and multihoming.

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interdomain issues for ip networks

Interdomain Issues for IP networks

Henning Schulzrinne

(with lots of borrowed slides...)

  • Architecture review
  • Interdomain routing
  • Multicast
  • VPNs
  • Interdomain QoS
    • signaling
    • charging and settlements
  • Interdomain application signaling
  • Carrier selection and multihoming
architecture review
Architecture review
  • Classical view of ISP food chain
  • Tier-1, tier-2
inter regional internet backbone

357 Mbit/s

USA &Canada

56’241 Mbit/s

19’716 Mbit/s



2’638 Mbit/s

468 Mbit/s

171 Mbit/s

LatinAmerica &


Arab States, Africa

127 Mbit/s

Inter-regional Internet backbone

Source: TeleGeography Inc., Global Backbone Database. Data valid for Sept. 2000.

examples of carriers
Examples of carriers
  • Tier 1: UUNet, Cable & Wireless (C&W), Sprint, Qwest, Genuity, AT&T
  • Tier 2: America Online, Broadwing, @home
  • Tier 3: RCN, Verizon, Log On America
  • Peering: exchange of data between ISPs on a sender-keeps-all basis
  • Access provider (IAP): provide dial-up and leased line access, buy Internet access from tier-1/2 providers
  • Transit: Using ISP A to reach customers of ISP B, C, ...
  • Hot potato routing: find earliest exit point to destination network  asymmetric routes
nap or ixps

Internet eXchange Points

  • "An Internet Exchange (IX)acts as a junction between multiple points of Internet presence. Here, peers are able to directly connect to each other to exchange local Internet traffic. Typically, the IX owns and operates the switching platforms used to interconnect the various users/subscribers."
  • Also known as Metropolitan Area Exchanges (MAEs)
  • see
  • governed by Multi-Lateral Peering Agreements (MLPA)
some european ixps
Austria - The Vienna Internet eXchange (VIX)

Belgium - Belnet (BNIX)

Cyprus - The Cyprus Internet eXchange (CyIX)

Denmark - Danish Internet eXchange (DIX) Lyngby

Finland - Finnish Commercial Internet eXchange (FCIX) Helsinki)

France - Paris Internet eXchange (PARIX)

France - French Global Internet eXchange (SFINX)

Germany - The Deutsche Central Internet eXchange (DE-CIX) Frankfurt

Greece - The Athens Internet eXchange (AIX)

Ireland - The Internet Neutral eXchange (INEX)

Italy - The Milan Internet eXchange (MIX)

Italy - NAP Nautilus (CASPUR)

Luxembourg - The Luxembourg Internet eXchange (LIX)

Netherlands - The Amsterdam Internat eXchange (AMS-IX)

Norway - Norwegian Internet eXchange (NIX)

Portugal - The Portuguese Internet eXchange (PIX)

Scotland - Scottish Internet Exchange (ScotIX)

Spain - El Punto Neutral Espanol (ESPANIX)

Sweden - The Netnod Internet eXchange (D-GIX)

Switzerland - The Swiss Internet eXchange (SIX)

Switzerland - Geneva Cern (CIXP)

Switzerland - Zürich Telehouse Internet Exchange (TIX)

United Kingdom - The London INternet eXchange (LINX)

United Kingdom - Manchester Network Access Point (MaNAP)

United Kingdom - London Network Access Point (LoNAP)

Bulgaria - The Sofia Internet eXchange (SIX - GoCIS)

Czech Rep. - Neutral Internet eXchange (NIX) Prague

Latvia - The Global Internet eXchange (GIX) LatNet

Romania - The Bucharest Internet eXchange (BUHIX)

Slovakia - The Slovak Internet eXchange (SIX)

Ukraine - The Central Ukrainian Internet eXchange

Some European IXPs
the view from elsewhere
The view from elsewhere
  • Looking glass sites show BGP routes:

$ whois -h AS14

aut-num: AS14

as-name: COLUMBIA

descr: Columbia University in the City of New York

Network Operations

Academic Information Systems

612 West 115th Street

New York, NY 10025

admin-c: CU-NOC

tech-c: CU239-ORG

import: from AS1785 action med=100; # ApTh commodity

accept ANY

import: from AS701 action med=200; # UUnet commodity

accept ANY

import: from AS14:AS-ISPPEERS action pref=10; # private ISP peers

accept <^PeerAS+$>

import: from AS14:AS-NNPEERS action pref=10; # private NN peers

accept <^PeerAS+$>

import: from AS145 action med=75; # vBNS I2

accept ANY AND NOT {}

import: from AS11537 action med=50; # Abilene I2

accept ANY AND NOT {}

import: from AS3754 action med=100; # NYSERNet I2

accept <^AS3754+ AS11537+> AND NOT {}

  • Interdomain SLAs are rare (or non-existent)
  • Large difference between inter- and intradomain performance?
interdomain qos issues
Interdomain QoS Issues
  • Request authentication
  • Uniform service levels – my "gold" is your "bronze"...
  • Payment
    • NJ Turnpike?
    • Gardenstate Parkway?
carrier selection
Carrier selection
  • Allow selection of carrier
  • Easy for multi-homed sites
  • but everything else requires loose source route – but what IP address?
  • will work in both directions
interdomain multicast
Interdomain multicast
  • Any-source multicast (ASM) has many operational problems:
    • PIM-SM/DM are only intradomain
    • PIM-SM complex
    • RP has scaling and reliability problems
    • interdomain never got off the ground
    • no deployed multicast address allocation mechanism
    • spam problem – anybody can send to group
interdomain multicast16
Interdomain multicast
  • Single-source multicast (SSM)
    • source-filtered  IGMPv3
    • {S,G} as group
    • avoid address allocation
    • match many applications:
      • Internet radio/TV
      • conferences with single active source
distributed denial of service ddos
Distributed Denial of Service (DDOS)
  • Need packet tracing (in progress)
  • Need push-back to filter DOS stream
    • at source
    • or close to source
  • Authentication of filter request to prevent malicious blackouts
  • = payments between providers
  • long history in telephone network
  • e.g., 4.6b US$ in 2000 net settlements
total accounting rate tar
Total Accounting Rate (TAR)
  • Traditional conceptual cost of connecting a call from country A to country B
  • Each end contribute the building cost of half circuit to the midpoint
  • Based on “cost” of early tiny capacity submarine cable
  • Settlement A to B at 1/2 TAR

S. Cheng/ITU

total accounting rate tar cont d
Total Accounting Rate (TAR) - cont’d
  • Same Rate for the opposite direction
  • Apply to all PSTN services
  • When the accounting rate change in one direction the other direction must follow

S. Cheng/ITU

termination rate
  • Usually based on cost of terminating call by destination carrier
  • Accounting rate may not be the same for the other direction
  • Accounting rate in each direction can change independent of each other
  • May deliver traffic at mid-point or FOB on either end of circuit.

S. Cheng/ITU

sender keeps all peering
Sender keeps all (Peering)
  • Sender keeps all revenue from calling party
  • No settlement between carriers
  • Applicable if average cost and traffic volume are virtually identical in each direction
  • Usually based on half circuit ownership

S. Cheng/ITU

us domestic telephony settlements
US domestic telephony settlements
  • Doesn't quite fit SKA
  • Long-distance company collects
  • Pays fixed charge/minute to originating and terminating local exchange carrier (LEC)

$19.95 per month subscription

$7.50-$10.50 Wholesale PoP Access

$2.00 - $3.00

Customer Care

$3.50-$7.50 margin per customer

$3.00 amortised customer marketing

Where does the money go? Typical US ISP cash-flow

Source: Adapted from Paul Stapleton, ISP$ Market Report, Boardwatch Magazine.


Settlements-based traffic

Settlements-based traffic

PTO = Public

PTOs A & Bsplit the cost ofthe int’l circuit



Delivers traffic



Pays settlement fees









User 1

User 2

User 3

User 1

User 2

User 3

For accounting rate traffic, a direct bilateral

relationship is established between the origin and

termination operators. Intermediate transit operators

are compensated from the accounting rate which is

usually split 50:50. PTO B retains net settlement.



Internet Peering traffic (Web)

ISP = Internet

PTO B pays the full cost ofthe int’l circuit



One-way (thick pipe)



Two-way (thin pipe)

Requestsand terminates






Web 1

Web 1

Web 1

User 1

User 2

User 3

For Internet Peering traffic, ISP B pays for

both halves of the International circuit(s) which are

used for peering with ISP A. ISP B also pays for traffic exchange.

ISP B may pay for the circuit directly, or in

conjunction with one or more PTOs.

settlements and peering what s the difference
Settlements and Peering: What’s the difference?
  • Settlement-payment traffic
    • Substantial revenue transfers, from core to periphery of network
    • Promotes “organic” network growth
    • So, Operators generating less traffic than they receive have an incentive to keep prices high
  • Peering traffic
    • Some revenue transfers, from periphery to core of network
    • Promotes “spontaneous” network growth
    • So, ISPs generating less traffic than they receive have an incentive to force prices down

Internet traffic flows are highly asymmetric

  • Public switched telephone service
  • Traffic flows are bilateral and broadly match value flow in that caller, who initiates the call, also pays for it
  • Call-back reverses the direction of the call, from a statistical viewpoint, but caller still pays & benefits
  • Traffic flows unbalanced between developed and developing countries
  • Public Internet service
  • Traffic flows are multi-lateral: A single session may poll many countries
  • Web-browsing is dominant form of traffic: traffic flow is dominantly towards user who initiates the call. Web traffic highly asymmetric
  • Newer forms of Internet traffic (telephony, push media, streaming video etc) reverses traffic flow to be from user which initiates the call
interdomain aaa
Interdomain AAA
  • Roaming user identified by NAI (RFC 2486), e.g.,
  • Generic problem different:
    • User Alice@A from ISP A visits ISP B
    • ISP B needs to determine whether Alice is a valid customer of A
    • Alice needs to authorize B to query A
    • Needs to get authorization for maximum € amount
    •  very similar to credit card authorization!
clearinghouse models
Clearinghouse models
  • e.g., iPass or GRIC for roaming dial-up and wireless users
    • member company charges subscribers
    • gets access to other dial-up ports via clearinghouse
    • gets reimbursed for "visitors"
  • GSM roaming is not a good model
    • no price transparency
    • inefficient routing
sip interdomain
SIP interdomain
  • Designed to find proxies by request URI
  • Authentication and anonymity are issues:
    • how can callee ascertain identity of random caller?
    • how can caller know that she's talking to the right person?
    • trust provider to remove privacy-compromising information
bgp problems
BGP problems
  • Trust
  • Need route filtering:

In April 1997, a small ISP in Florida made a mistake in configuring the router that joined its small network to Sprint. This ISP, known as AS number 7007, allowed all the routes it learned from Sprint using BGP to be exported back to Sprint as its own routes. This is easy to do, because BGP implementations can take routes from IGP and convert them into EGP routes. In this case, the IGP converted CIDR routes into classful routes. The Sprint BGP speaker wasn't filtering properly either and began sending out updates that added AS7007 as the correct route for a portion of every CIDR block (essentially, the first class C, 24-bit-long network prefix).

This misinformation first spread through Sprint's network, then to neighboring NSPs, including ANS, MCI, UUNet, and others. Many routers crashed because their routing tables suddenly doubled in size (an additional route was added for each CIDR block), and the routing instability spread throughout the Internet. Remember that, when a router crashes, it drops its BGP connection with its peer, which then sends out an update withdrawing all the routes announced previously by the crashed router. (Network Magazine, March 2002)

alternatives to improving routing
Alternatives to improving routing
  • "Resilient Overlay Networks" (Andersen/Balakrishnan/Kashoek/Morris2001)
    • application-layer routing with one hop
  • Multihoming:
    • treat networks like cheap PCs
    • 99.5% reliability² 99.9975% reliability
multihoming problems
Multihoming problems
  • Need either an ASN or two IP address ranges
  • Only for larger networks  don't allow advertisements for /24
  • Network impact: two /22 entries for each subnet
  • Alternative: NAT
    • doesn't help reachability of servers advertised in DNS
bgp issues

BGP Issues

Geoff Huston

why measure bgp
Why measure BGP?
  • BGP describes the structure of the Internet, and an analysis of the BGP routing table can provide information to help answer the following questions:
    • What is changing in the deployment environment?
    • Are these changes sustainable?
    • How do address allocation policies, BGP and the Internet inter-relate?
    • Are current address allocation policies still relevant?
    • What are sensible objectives for address allocation policies?
  • Passive Measurement
    • Takes measurements from a default-free router at the edge of the local network
    • Easily configured
    • Single (Filtered) view of the larger Internet
      • What you see is a collection of best paths from your immediate neighbours


Local AS

Measurement Point

  • Multiple Passive measurement points
    • Measure a number of locations simultaneously
    • Can be used to infer policy




Measurement Points

  • Single passive measurement point with multiple route feeds
    • Best example:
      • Operating since 1995
      • 42 peers
      • Uses eBGP multihop to pull in route views
  • Active Measurement Tests
    • Convergence
      • Announcement and withdrawal

Reporting Points

Monitoring Unit


Route Injection Point



  • BGP is not a link state protocol
  • There is no synchronized overview of the entire connectivity and policy state
  • Every BGP viewing point contains a filtered view of the network
    • Just because you can’t see it does not mean that it does not exist
  • BGP metrics are sample metrics
bgp table growth
BGP Table Growth

BGP Table Growth – 12 year history

prefixes by as
Prefixes by AS
  • Distribution of originating address sizes per AS
  • Address advertisements are getting smaller



Number of AS’s

Prefix Length

multi homing on the rise
Multi-homing on the rise?
  • Track rate of CIDR “holes” – currently 41% of all route advertisements are routing ‘holes”

This graph tracks the number of address prefix advertisements which are part of an advertised larger address prefix


Proportion of BGP advertisements which are

more specific advertisements of existing aggregates

  • Program bug! The number is larger than that.
  • More specific advertisement of existing aggregates account for 54% of the BGP selected route table from the perspective of AS1221
    • 56,799 entries from a total of 103,561
  • Older (mid Jan) data from AS286 has the number at 53,644 from a total of 95,036 (56%)
routed address space
Routed Address Space

Large fluctuation is due to announcement / withdrawals of /8 prefixes

12 months of data does not provide clear longer growth characteristic

routed address space 8 corrected
Routed Address Space (/8 Corrected)

Annual compound growth rate is 7% p.a.

Most address consumption today appears to be

ocurring behind NATs

/8 Corrected Data

as number use extrapolation
AS Number Use - Extrapolation

Continued exponential growth implies AS number exhaustion in 2005

average size of a routing table entry
Average size of a routing table entry



The BGP routing tale is growing at a faster rate than the rate of growth of announced address space

denser internet structure
Denser Internet Structure





AS Hops

denser internet structure59
Denser Internet Structure

90% point

Address Span



AS Hops

internet shape
Internet ‘Shape’
  • The network is becoming less ‘stringy’ and more densely interconnected
    • i.e. Transit depth is getting smaller





aggregation and specifics
Aggregation and Specifics
  • Is the prevalence of fine-grained advertisements the result of deliberate configuration or inadvertent leakage of advertisements?
publicity helps
Publicity helps ?
  • Efforts to illustrate the common problem of unconstrained table growth appear to have had an impact on growth of the table, as seen on the edge of AS1221 since Dec 2000
but the view from kpnqwest
But - the view from KPNQwest

Data from James Aldridge, KPNQwest -

different views65
Different Views
  • Route views in prefix-length-filtered parts of the net do not show the same recent reduction in the size of the routing table.
  • It is likely that the reduction in routes seen by AS1221 appears to be in the prefix-length filtered ranges
    • Either more transit networks are prefix length filtering or origin AS’s are filtering at the edge, or both
  • The underlying growth trend in BGP table size remains strong
aggregation possibilities
Aggregation possibilities
  • What if all advertisements were maximally aggregated* ?
    • 27% reduction (103126 -> 74427) using AS Path aggregation
    • 33% reduction (103126 -> 68504) using AS Origin aggregation
    • This assumes that the specific advertisements are not matched by other specific advertisements which have been masked out closer to the origin AS – this is not a terribly good assumption, so these numbers are optimistic to some extent
the aggregation potential view from kpnqwest
The aggregation potential view from KPNQwest

Data from James Aldridge, KPNQwest -

AS Path

AS Origin

different views70
Different Views
  • Similar AS Origin, but different AS Path aggregation outcomes
  • Prevalence of the use of specifics for local inter-domain traffic engineering
  • A remote view of aggregation has two potential interpretations:
    • Propose aggregation to the origin AS
    • Propose a self-imposed proxy aggregation ruleset
  • Any aggregation reduces the information content in the routing table. Any such reduction implies a potential change in inter-domain traffic patterns.
  • Aggregation with preserved integrity of traffic flows is different from aggregation with potential changes in traffic flow patters
  • Origin AS aggregation is easier to perform at the origin, but harder to determine remotely IF traffic flows are to be preserved
  • Proxy Aggregation is only possible IF you know what your neighbors know

Yes this is a recursive statement

    • If an AS proxy aggregates will it learn new specifics in response?
bgp as a routing protocol
BGP as a Routing Protocol
  • How quickly can the routing system converge to a consistent state following dynamic change?
  • Is this time interval changing over time?
increased convergence time intervals for bgp
Increased convergence time intervals for BGP
  • Measured time to withdraw route:
    • Up to 2 minutes
  • Measured time to advertise new route:
    • Up to 30 minutes
withdraw convergence77
Withdraw Convergence
  • Probability distribution
  • Providers exhibit different, but related convergence behaviors
  • 80% of withdraws from all ISPs take more than a minute
  • For ISP4, 20% withdraws took more than three minutes to converge
failures fail overs and repairs79
Failures, Fail-overs and Repairs
  • Bad news does not travel fast…
  • Repairs (Tup) exhibit similar convergence properties as long-short ASPath fail-over
  • Failures (Tdown) and short-long fail-overs (e.g. primary to secondary path) also similar
    • Slower than Tup (e.g. a repair)
    • 60% take longer than two minutes
    • Fail-over times degrade the greater the degree of multi-homing!

BGP table size will continue to rise exponentially

  • Multi-homing at the edge of the Internet is on the increase
  • The interconnectivity mesh is getting denser
    • The number of AS paths is increasing faster than the number of AS’s
    • Average AS path length remains constant
  • AS number deployment growth will exhaust 64K AS number space in August 2005 if current growth trends continue
more conjecturing
More conjecturing….
  • Inter-AS Traffic Engineering is being undertaken through routing discrete prefixes along different paths -- globally (the routing mallet!)
    • AS Origin aggregation < AS Path aggregation
  • RIR allocation policy (/19, /20) is driving one area of per-prefix length growth in the aggregated prefix area of the table
  • BUT - NAT is a very common deployment tool
    • NAT, multihoming and Traffic Engineering is driving even larger growth in the /24 prefix area
and while we are having such a good time conjecturing
And while we are having such a good time conjecturing…
  • Over 12 months average prefix length in the table has shifted from /18.1 to /18.5
  • More noise (/25 and greater) in the table, but the absolute level of noise is low (so far)
  • Most routing table flux is in the /24 to /32 prefix space – as this space gets relatively larger so will total routing table flux levels
    • “Flux” here is used to describe the cumulative result of the withdrawals and announcements
    • This space appears to be susceptible to social pressure – at present
this is fun lets have even more conjectures
This is fun – lets have even more conjectures…
  • CIDR worked effectively for four years, but its effective leverage to support long term dampened route table growth and improved table stability has now finished
  • Provider-based service aggregation hierarchies as a model of Internet deployment structure is more theoretic than real these days

i.e. provider based route aggregation is leaking like a sieve!

  • draft-iab-bgparch-00.txt
    • Exponential growth of BGP tables has resumed
    • AS number space exhaustion
    • Convergence issues
    • Traffic Engineering in a denser mesh
    • What are the inter-domain routing protocol evolutionary requirements?
objectives and requirements
Objectives and Requirements
  • Supporting a larger and denser interconnection topology
  • Scale by x100 over current levels in number of discrete policy entities
  • Fast Convergence
  • Security
  • Integration of Policy and Traffic Engineering as an overlay on basic connectivity
  • Control entropy / noise inputs
available options
Available Options
  • Social Pressure on aggregation
  • Economic Pressure on route advertisements
  • Tweak BGP4 behavior
  • Revise BGP4 community attributes
  • BGPng
  • New IDR protocol(s)
  • New IP routing architecture
social pressure
Social Pressure
  • Social pressure can reduce BGP noise
  • Social pressure cannot reduce pressures caused by
    • Denser interconnection meshing
    • Increased use of multi-homing
    • Traffic engineering of multiple connections
  • Limited utility and does not address longer term routing scaling
economic pressure on routing
Economic Pressure on Routing
  • Charge for route advertisements
    • Upstream charges a downstream per route advertisements
    • Peers charge each other
  • This topic is outside an agenda based on technology scope
  • Raises a whole set of thorny secondary issues:
    • Commercial
    • National Regulatory
    • International
  • Such measures would attempt to make multi-homing less attractive economically. It does not address why multi-homing is attractive from a perspective of enhanced service resilience.
tweaking bgp4
Tweaking BGP4
  • Potential tweak to BGP-4
    • Auto-Proxy-Aggregation
      • Automatically proxy aggregate bitwise aligned route advertisements
      • Cleans up noise – but reduces information
      • Cannot merge multi-homed environments unless the proxy aggregation process makes sweeping assumptions, or unless there is an overlay aggregation protocol to control proxy aggregation (this is then no longer a tweak)
extend bgp4 communities
Extend BGP4 Communities
  • We already need to extend community attributes to take on the 2 / 4 octet AS number transition.
  • Can we add further community attribute semantics to allow proxy aggregation and proxy sublimation under specified conditions?
  • Extend commonly defined transitive community attributes to allow further information to be attached to a routing advertisement
    • Limit of ‘locality’ of propagation
    • Aggregation conditions or constraints
  • If we could do this, will this be enough? Can this improve
    • Scaling properties
    • convergence properties
  • Preserve: AS concept, prefix + AS advertisements, distance vector operation, AS policy “opaqueness”
  • Alter: convergence algorithm (DUAL?), advertisement syntax (AS + prefix set + specifics + constraints), BGP processing algorithm
  • Issues:
    • Development time
    • Potential to reach closure on specification
    • Testing of critical properties
    • Deployment considerations
    • Transition mechanisms
  • A different IDR protocol?
    • Can we separate connectivity maintenance, application of policy constraints and sender- and/or receiver- managed traffic engineering?
      • SPF topology maintenance
      • Inter-Domain Policy Protocol to communicate policy preferences between policy “islands”
      • Multi-domain path maintenance to support traffic engineering requirements
    • Eliminate the need to advertise specifics to undertake traffic engineering
    • Multi-homing may still be an issue – is multi-homing a policy issue within an aggregate or a new distinct routing “entity”?
    • Can SPF scale? Will SPF routing hierarchies impose policy on the hierarchy elements?
new ip routing architecture
New IP Routing Architecture
  • Separate Identity, Location and Path at an architectural level?
  • Identity
    • How do you structure an entirely new unique identity label space? How do you construct the “identity lookup” mechanism?
  • Location
    • How can location be specified independent of network topology?
  • Path:
    • Is multi-homing an internal attribute within the network driven by inter-domain policies, or is multi-homing an end-host switching function
new ip routing architecture94
New IP Routing Architecture
  • Other approaches?
    • Realms and RSIP
    • Inter-Domain CRLDP approaches where policy is the constraint
slide credits
Slide credits
  • Geoff Huston
  • Tim Kelly, ITU