The Internet is a vast and complicated network connecting billions of devices worldwide, which operates efficiently through a well-structured hierarchy of smaller networks known as Autonomous Systems (AS). Guidelines for the creation, selection, and registration of an Autonomous System (AS) are presented in RFC 1930.
An AS is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. Each AS is assigned a unique Autonomous System Number (ASN) by the Internet Assigned Numbers Authority (IANA) or a Regional Internet Registry (RIR). ASNs enable different networks to identify each other and route information efficiently. A deeper explanation of ASNs is presented later in this article.
The routing of data packets within and between ASes is governed by routing protocols, which can be categorized into Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). IGPs are used for traffic routing within an AS. Such protocols include EIGRP OSPF and IS-IS. These protocols optimize data routing within a single AS, ensuring efficient internal network traffic management.
EGPs are used for traffic routing between ASes. BGP is currently the only used EGP in any production network today and is the routing protocol used on the Internet. This protocol is essential for the Internet’s scalability, allowing thousands of ASes to communicate and exchange routes effectively.
Autonomous Systems can be broadly classified into several categories based on their size, function, and connectivity:
1. Tier 1 Networks — the largest ASes, forming the backbone of the Internet. They have extensive networks and peer with each other without paying for the exchange of traffic. Examples include AT&T, Deutsche Telecom, CenturyLink, and NTT Communications.
2. Tier 2 Networks — ASes often purchase transit from Tier 1 networks and peer with other Tier 2 networks to reduce costs. They typically serve regional ISPs and large enterprises.
3. Tier 3 Networks — the smallest ASes, primarily purchasing transit from Tier 2 networks. They include local ISPs and smaller organizations.
4. Enterprise Networks — many large organizations operate their internal routing policies and control network traffic flow.
Autonomous Systems play a crucial role in the structure and functionality of the Internet for several reasons:
1. Scalability — the hierarchical structure of ASes allows the Internet to scale efficiently, managing the growing number of connected devices and networks.
2. Resilience — by distributing routing responsibilities across multiple ASes, the Internet can withstand outages and maintain connectivity even if parts of the network fail.
3. Performance — ASes optimize routing paths to ensure data packets travel the most efficient routes, enhancing overall Internet performance.
4. Security —ASes can implement security policies and measures to protect against various cyber threats, including Distributed Denial of Service (DDoS) attacks and routing hijacks (for example using BGPSec and RPKI).
Autonomous System Numbers (ASNs)
AS Number (ASN) is a unique identifier assigned to each AS on the Internet. ASNs enable these systems to identify themselves and interact with each other, facilitating efficient data routing.
ASNs serve several critical functions in the operation of the Internet:
1. Routing Identification — ASNs uniquely identify each AS, allowing them to exchange routing information with other ASes. This identification is crucial for maintaining the structure and order of the Internet.
2. Routing Policy Management — organizations can define and manage their routing policies by assigning ASNs, helping optimize the flow of data within their networks and to and from other ASes.
3. Path Selection — ASNs play a key role in path selection, enabling routers to determine the most efficient routes for data packets. This selection is based on the policies and preferences defined by each AS.
The allocation of ASNs is managed by the Internet Assigned Numbers Authority (IANA) in coordination with five Regional Internet Registries (RIRs). These RIRs are responsible for distributing ASNs within their respective regions:
1. ARIN (American Registry for Internet Numbers) — North America.
2. RIPE NCC (Réseaux IP Européens Network Coordination Centre) — Europe, the Middle East, and parts of Central Asia.
3. APNIC (Asia-Pacific Network Information Centre) — Asia and the Pacific region.
4. LACNIC (Latin American and Caribbean Internet Addresses Registry) — Latin America and the Caribbean.
5. AFRINIC (African Network Information Centre) — Africa.
Organizations needing an ASN must apply to their regional RIR, justifying the request. The RIR evaluates the application based on criteria such as the need for unique routing policies and the use of multiple connections to other ASes.
Conclusion
Autonomous Systems are the building blocks of the Internet, enabling the seamless and efficient routing of data across the globe. They provide scalability, resilience, performance, and security, ensuring the Internet remains robust and reliable. As the Internet continues to grow and evolve, so too must Autonomous Systems, addressing challenges and embracing new technologies to maintain the seamless connectivity we rely on every day.
Bibliography:
[1] https://www.thousandeyes.com/learning/techtorials/isp-tiers
[2] https://www.cloudflare.com/learning/network-layer/what-is-an-autonomous-system/