1. CSE 461: Interdomain Routing

2. TCP Review: Thought Questions
1. Why do wireless networks often do link-layer re-tx?
Hint: is loss always correlated with load?
Write a server that accepts new TCP connections but never reads data. Then write a client that opens a TCP connection to that server, and writes forever. Why does write() finally stop working? Where is data stored at that point?

3. Routing: Scalability Concerns
Routing burden grows with size of an inter-network
Size of routing tables
Volume of routing messages
CPU time required for routing computation
To scale to the size of the Internet, apply:
Hierarchical addressing
Route aggregation

4. Advantages: We can now aggregate routing information.
1/nth as many networks as hosts  fewer updates, smaller tables.
Local changes don’t cause global updates.
Networks 2, 3

5. 2 2.2 2.3 2.4 2.1 Network 1 Network 3 Network 5 Network 4
The original Internet had exactly 1 level of hierarchy:
Network Address and Host Address (Class A, B, C…)
From the mid-90’s: CIDR allows arbitrary sub-networking.
Further improves route aggregation in the Internet core.
Network 4
Network 3
Network 5
Network 1 2
2.2
2.1.1
2.3
2.4
2.1
2.1.2
2.1.3

6. CIDR Example 128.220.192.0/20 128.220.192.0/19 Decimal Binary
X and Y routes can be aggregated because they form a bigger contiguous range.
Can only aggregate powers of 2 
Corporation X
( > )
/20
Border gateway
Regional network
(advertises path to
> )
Corporation Y
/19
( > )
Decimal Binary
/20

7. IP Forwarding Revisited
Routing table now contains routes to “prefixes”
IP address and length indicating what bits are fixed
Now need to “search” routing table for longest matching prefix, only at routers
Search routing table for the prefix that the destination belongs to, and use that to forward as before
There can be multiple matches; take the longest prefix
This is the IP forwarding routine used at routers.

8. Structure of the Internet
Many (~30k) Autonomous Systems (ASes) or domains
To scale, use hierarchy: separate inter-domain and intra-domain routing
IGP (Interior gateway, within an AS) = RIP, OSPF
EGP (Exterior gateway, between Ass) = BGP, EGP (obsolete)
Large corporation “
Consumer ”
ISP
Peering
point
Backbone service provider
Peering
point “
Consumer ”
ISP “
Consumer ”
ISP
Large corporation
Small
corporation

9. Internet Routing Architecture
Divided into Autonomous Systems
Distinct regions of administrative control
Routers/links managed by a single “institution”
Service provider, company, university, …
Hierarchy of Autonomous Systems
Large, tier-1 provider with a nationwide backbone
Medium-sized regional provider with smaller backbone
Small network run by a single company or university
Interaction between Autonomous Systems
Internal topology is not shared between ASes
… but, neighboring ASes interact to coordinate routing

10. Inter-Domain Routing AS1 AS2 Border router Border router
Border routers summarize and advertise internal routes to external neighbors and vice-versa
Border routers apply policy
Internal routers can use notion of default routes
Core is “default-free”; routers must have a route to all networks in the world
AS1
Border router
Border router
AS2

11. AS Topology Node: Autonomous System
Edge: Two ASes that connect to each other 1 2 3 4 5 6 7

12. Interdomain Paths Path: 6, 5, 4, 3, 2, 1 Web server Client 4 3 5 2 7 6

13. Hierarchical Routing May Pay a Price for Path Quality
AS path may be longer than shortest AS path
Router path may be longer than shortest path
2 AS hops,
8 router hops d s
3 AS hops, 7 router hops

14. Border Gateway Protocol (BGP-4)
Features:
Path vector routing
Application of policy
Operates over reliable transport (TCP)
Uses route aggregation (CIDR)

15. Internet Interdomain Routing: BGP
BGP (Border Gateway Protocol): the de facto standard
Path Vector protocol:
similar to Distance Vector protocol
a border gateway sends to a neighbor entire path (i.e., a sequence of ASes) to a destination, e.g.,
gateway X sends to neighbor N its path to dest. Z:
path (X,Z) = X,Y1,Y2,Y3,…,Z
if N selects path(X, Z) advertised by X, then: path (N,Z) = N, path (X,Z)
Question: what are the implications of path vector? Z N X

16. Convergence Prefix P In AS X 1 2 X View from here 4 3
Recently, it was realized that BGP convergence can undergo a process analogous to count-to-infinity!
AS 4 uses path 4 1 X. A link fails and 1 withdraws 4 1 X.
So 4 uses X, which is soon withdrawn, then X, …
Result is many invalid paths can be explored before convergence
Prefix P
In AS X 1 2 X
View from
here 4 3

17. BGP Routing Decision Process
route selection policy: rank paths
select best path
routing cache
export policy: which paths to export to which neighbors
export path to neighbors

18. Business Relationships
Neighboring ASes have business contracts
How much traffic to carry
Which destinations to reach
How much money to pay
Common business relationships
Provider
Customer
E.g., UW is a customer of NTT
E.g., MIT is a customer of Level 3
Peer
E.g., AT&T is a peer of Sprint

19. Customer-Provider Relationship
Customer needs to be reachable from everyone
Provider tells all neighbors how to reach the customer
Customer does not want to provide transit service
Customer does not let its providers route through it
Traffic to the customer
Traffic from the customer
advertisements d
provider
traffic
provider
customer d
customer

20. Multi-Homing: Two or More Providers
Motivations for multi-homing
Extra reliability, survive single ISP failure
Financial leverage through competition
Better performance by selecting better path
Gaming the 95th-percentile billing model
What implication does this have for routing table size?
Provider 1
Provider 2

21. Peer-Peer Relationship
Peers exchange traffic between customers
AS exports only customer routes to a peer
AS exports a peer’s routes only to its customers
Often the relationship is settlement-free (i.e., no $$$)
Traffic to/from the peer and its customers
advertisements
peer
peer
traffic d

22. Implication of Business Relationship on Policies
Route selection (ranking) policy:
the typical route selection policy is to prefer customers over peers/providers to reach a destination, i.e., Customer > Peer > Provider
Route export policy:
since the export of a path to a neighbor is an indication that the AS is willing to transport traffic for the neighbor, an AS may not export routes to all neighbors

23. Typical Export Policies
customer
provider
peer
case 1: routes learned from customer
routes learned from a customer are sent to all other neighbors
case 2: routes learned from provider
case 3: routes learned from peer
provider
customer
peer
peer
provider
customer
routes learned from a provider are sent only to customers
routes learned from a peer are sent only to customers

24. Example Export Policy: No-Valley Routing
IP traffic
A advertises path to C, but not P2
A advertises path to C, but not P1
A learns
paths to C, P1, P2 A
A advertises to C paths to P1 and P2 C
Suppose P1 and P2 are providers of A; A is a provider of C

25. AS Structure: Tier-1 Providers
Has no upstream provider of its own
Typically has a national or international backbone
UUNET, Sprint, AT&T, Level 3, …
Top of the Internet hierarchy of 9-15 ASes
Full peer-peer connections between tier-1 providers

26. AS Structure: Other ASes
Tier-2 and Tier-3 providers
Provide transit service to downstream customers
… but, need at least one provider of their own
Typically have national or regional scope
E.g., Minnesota Regional Network
Includes a few thousand of the Ases
Stub ASes
Do not provide transit service to others
Connect to one or more upstream providers
Includes vast majority (e.g., 85-90%) of the ASes

27. Characteristics of the AS Graph
AS graph structure
High variability in node degree (“power law”)
A few very highly-connected ASes
Many ASes have only a few connections 1
All ASes have 1 or more neighbors
0.1
CCDF
0.01
Very few have degree >= 100
0.001
AS degree 1 10
100
1000

28. www.cidr-report.org November 2008
Core BGP Table Growth
November 2008

29. % of Entries that advertise /24s
November 2008

30. Begging for Aggregation
--- 13Nov08 ---
ASnum  
NetsNow
  NetsAggr
  NetGain
  % Gain  
Description
Table  
287512
  176594
  110918
  38.6%  
All ASes
AS4538  
5057
  874
  4183
  82.7%  
ERX-CERNET-BKB China Education and Research Network Center
AS6389  
4372
  358
  4014
  91.8%  
BELLSOUTH-NET-BLK - BellSouth.net Inc.
AS209  
3038
  1350
  1688
  55.6%  
ASN-QWEST - Qwest Communications Corporation
AS6298  
2100
  729
  1371
  65.3%  
ASN-CXA-PH-6298-CBS - Cox Communications Inc.
AS17488  
1409
  290
  1119
  79.4%  
HATHWAY-NET-AP Hathway IP Over Cable Internet
AS4755  
1320
  239
  1081
  81.9%  
TATACOMM-AS TATA Communications formerly VSNL is Leading ISP
AS1785  
1695
  618
  1077
  63.5%  
AS-PAETEC-NET - PaeTec Communications, Inc.
November 2008

31. Key Concepts Internet is a collection of Autonomous Systems (ASes)
Policy dominates routing at the AS level
Structural hierarchy helps make routing scalable
BGP routes between autonomous systems (ASes)