An Autonomous System (AS) refers to a collection of IP networks and routers under the control of one entity that presents a common. It is clearly defined routing policy to the internet. These systems typically use an interior gateway protocol (IGP) for internal routing. An exterior gateway protocol (EGP) to exchange routing information with other autonomous systems.
Key characteristics of an Autonomous System include:
Single Administration:
An AS is managed and controlled by a single administrative entity such as an internet service provider (ISP) or a large organization.
Routers and Networks:
An AS consists of routers and networks that are interconnected and operated by the same entity. These networks could be connected via various means like cables, fiber optics, or wireless connections.
Routing Policies:
Each AS maintains its own routing policies which dictate how data packets should be forwarded within the system and exchanged with other autonomous systems. These policies help in determining the best path for data to traverse from one network to another.
Autonomous System Number (ASN):
Every AS is assigned a unique Autonomous System Number by regional internet registries (RIRs) such as ARIN, RIPE NCC, or APNIC. The ASN is used to identify and differentiate between different ASes when routing information is exchanged.
Interior Gateway Protocols (IGP):
Within an AS, routers communicate using Interior Gateway Protocols (IGP) such as OSPF (Open Shortest Path First) or IS-IS (Intermediate System to Intermediate System). These protocols handle the routing of traffic within the boundaries of the AS.
Exterior Gateway Protocol (EGP):
For communication between different ASes, the Border Gateway Protocol (BGP) is used. BGP exchanges routing information between ASes and helps in determining the best path for traffic to reach its destination. BGP allows ASes to make decisions based on various attributes such as path length, policies, and network reachability.
Autonomous System Number (ASN):
An ASN is a unique identifier allocated to each AS by regional internet registries (RIRs) like ARIN, RIPE NCC, or APNIC. There are two types of ASNs:
2-byte ASNs: Ranging from 1 to 65,535.
4-byte ASNs: Ranging from 65,536 to 4,294,967,295, introduced to accommodate the increasing demand for AS numbers due to the growth of the internet.
Peering and Transit Relationships:
ASes establish different relationships to exchange traffic:
Peering: When two ASes interconnect to exchange traffic between their networks without charging each other. This is common among large ISPs.
Transit: A transit relationship is established when one AS pays another AS to carry its traffic and provide access to the rest of the internet.
Internet Exchange Points (IXPs):
These physical locations facilitate the interconnection between multiple ASes. They allow networks to exchange traffic directly rather than through a third-party provider, improving performance and reducing latency.
Routing Policies:
ASes define routing policies to determine how traffic is routed within their network and how they interact with other ASes. Policies may involve preferences for certain routes, traffic filtering, or prioritization of certain types of traffic.
ASes play a crucial role in the operation of the Border Gateway Protocol (BGP), which is the primary protocol used to exchange routing information between different autonomous systems on the internet. BGP enables ASes to communicate and decide the most efficient paths for data to traverse across the interconnected networks, thereby facilitating global internet connectivity.
