Computer Networking Lecture Notes
2013 October 3 • Internetworking
Outline
Function
Hierarchy
Architecture
Piece parts.
Integration
Protocols
Border gateway protocols.
Where Are We?
How do we go from the left to the right?
The Internet, how does it work?
Internetworking
Internetworking
combines independent networks into a common network.
An
internetwork
or
internet
.
Note: an internet, the Internet.
Intenetworking Functions
An internet should provide
Uniform network services.
Homogeneous behavior on heterogeneous systems.
Transport-layer (at least) services.
There is no middle of an internetwork.
The independent networks maintain their separate identifies.
IP-Address Hierarchy
IP addresses provide a simple, two-level hierarchy.
Outside a network, only the network exists.
More Hierarchy
IP (v4) addresses don't scale well to 100,000s of networks.
Expensive to maintain and use.
Large scale needs intermediate structure between networks and an internet.
Extra-technical motivations also intrude.
Organizations and policies.
More Structure
Networks group themselves into
autonomous systems
(
AS
s,
rfc 1930
).
An AS is also a (relatively) small internet.
An AS is controlled by a single entity (a corporation, an agency, a government).
Each AS has a 32-bit
autonomous system number
(
ASN
)
assigned by the Internet Assigned Number Authority
(IANA).
Around 74,000 AS numbers
have been allocated.
AS Traffic
Local traffic
is traffic into or out of an autonomous system.
Transit traffic
is traffic through an autonomous system.
AS Types
A
stub AS
is singly-connected autonomous system.
A
multi-homed AS
is a multiply-connected AS that doesn't carry transit traffic.
A
transit AS
is a multi-homed AS that carries transit traffic.
AS Hierarchy
An internet of autonomous systems has (roughly) a three-level structure.
1. Backbone
2. Service providers.
3. Customers
Backbone and provider levels are sets of
peer AS
s (roughly).
Customers are individual ASs or smaller networks (roughly).
Recursion
An autonomous system may contain smaller versions of itself.
A non-AS group of networks may organize in an AS hierarchy.
Example: dear old MU.
Backbone: campus-wide ring.
Provider: each building wired into the ring.
Customers: each department wired into the building.
What About Scaling?
IP addresses aggregate hosts as a network address.
Autonomous systems aggregate networks as ASs.
But AS numbers are not addresses.
Each AS associates host addresses with ASs.
Unaccessed networks can be ignored.
Backbone ASs are still on the hook for everything.
Connecting
What is a transport-layer connection?
It is a path from one host through autonomous systems to the other host.
Finding that path is the
routing problem
.
Or just
routing
.
The path is the
route
.
AS Routing Policy
An AS has a set of rules governing traffic through the AS.
Multi-homed ASs have a no transit traffic rule.
Microsoft won't route through Apple's AS, and vice versa.
An AS's rule set is known as its
routing policy
.
AS policies can get complicated quickly.
And AS organizations tend to be competitive rather than cooperative.
AS Routing
Routing through autonomous systems in an internet should
Produce efficient and effective routes between hosts
Respect the constituent ASs routing policies.
Respecting policy takes precedence over effective routes.
AS Routing Protocols
Autonomous systems use several
routing protocols
to route through an internet.
The
border gateway protocol
(
BGP
,
rfc 4271
) is the main protocol used among ASs.
Border Routers
A
border router
is a host in an autonomous system through which traffic moves.
Sometimes known as an
edge router
or a
gateway
.
ASs are connected via border routers.
An AS has at least one border router.
BGP Routers
A
BGP router
is a host in an autonomous system that communicates using BGP.
Border routers are usually BGP routers, but don't have to be.
An AS has to have at least one BGP router.
BGP routers
collect and distribute routing information with connected ASs.
construct new routes based on collected routing information.
Internet Routes
A path through an internet is a sequence of autonomous systems ending at a network.
For example: AS
1
, AS
2
, …, AS
k
,
a
.
b
.
c
.
d
/
n
.
An AS's BGP routers maintain the path list.
Potentially one path for every network.
The BGP routers periodically
advertise
a subset of their path list to connected ASs.
The AS policy determines the list advertised.
Path Finding
A BGP router determines new paths from path information advertised by connected ASs.
Add paths to previously unknown networks.
Replace old paths with newer, better paths.
Drop old paths that now violate policy.
Internet path finding can be a difficult task.
Local policies constrain and complicate choice.
Connected ASs may advertise poor-quality information.
summary
An internet combines many networks into a single network.
This is a recursive definition.
A tree-shaped hierarchy scales up to global-sized internets.
For the most part.
The constituent networks still maintain their autonomy.
And that's what makes it interesting.
References
The Global Internet (Section 4.1) in
Computer Networking: A Systems Approach
by Larry Peterson and Bruce Davie, Morgan-Kaufman, 2011.
Internetworking (Section 5.5) in
Computer Networks
by Andrew Tanenbaum and David Wetherall, Prentice Hall, 2010.
This page last modified on 2011 October 9.