The Architecture of Global Connectivity

The Architecture of Global Connectivity

Interdomain routing operates across administrative, organizational, and geopolitical frontiers. To understand how routing breaks and how to secure it, we must first establish the physical, administrative, and economic foundations of global Internet connectivity.

IP Addressing and the Prefix Hierarchy

The Internet’s addressing model relies on hierarchical bit-level partitioning. An IP address identifies an interface on a host or network element, but routing at global scale operates entirely on prefixes—contiguous blocks of IP addresses represented in Classless Inter-Domain Routing (CIDR) notation.

In IPv4, an address comprises 32 bits (\(4\) bytes); in IPv6, 128 bits (\(16\) bytes). The prefix length indicates the size of the network mask:

\[\text{Total Addresses} = 2^{(32 - L)} \quad (\text{IPv4}) \qquad\text{or}\qquad 2^{(128 - L)} \quad (\text{IPv6})\]

Where \(L\) is the prefix length. For example: - A /24 in IPv4 encompasses \(2^{(32-24)} = 256\) individual host addresses. - A /48 in IPv6 provides \(2^{(128-48)} = 2^{80}\) individual host addresses, typically delegated to an enterprise or end-site.

The global routing system does not accept arbitrary prefix sizes into the Default-Free Zone (DFZ). By broad operator consensus: - In IPv4, the longest (most specific) prefix universally accepted across transit providers is /24. Any advertisement more specific than /24 (e.g., /25, /28) is dropped by upstream transit filters to protect router memory (TCAM). - In IPv6, the standard filter boundary is /48. Prefixes longer than /48 (such as /56 or /64) are filtered in the global DFZ.

Autonomous Systems and Administrative Boundaries

An Autonomous System (AS) is a collection of IP routing prefixes under the control of one or more network operators that presents a single, clearly defined routing policy to the Internet (RFC 1930, RFC 4271).

Every AS is identified by a globally unique 32-bit number known as an Autonomous System Number (ASN): - 16-bit ASNs (legacy): \(1\) to \(65,535\). (Private range: \(64,512\)–\(65,534\)). - 32-bit ASNs (RFC 4893 / RFC 6793): \(65,536\) to \(4,294,967,295\). (Private range: \(4,200,000,000\)–\(4,294,967,294\)).

An ASN serves as the identity token attached to BGP path announcements. Administrative boundaries are absolute: inside an AS, an Interior Gateway Protocol (IGP) like OSPF or IS-IS computes shortest paths based on link metrics and interface bandwidth. Across AS boundaries, policy rules supersede topology metrics completely.

The Transit-Peering Model and Its Economics

The global routing fabric is organized around two primary relationship types governed by commercial contracts:

                      +-------------------+
                      |   Tier-1 Transit  |
                      |   (e.g., AS174)   |
                      +---------+---------+
                                |  Transit ($$$)
                                v
+------------------+  Peering +-------------------+
| Large Peer AS    | <======> | Regional ISP AS   |
| (Settlement-Free)|          | (e.g., AS2856)    |
+------------------+          +---------+---------+
                                        |  Transit ($$$)
                                        v
                              +-------------------+
                              | Customer / Campus |
                              | (e.g., AS65000)   |
                              +-------------------+

1. Transit (Customer-Provider)

In a transit relationship, a customer pays an upstream provider for reachability to the entire Internet. The provider guarantees that packets addressed to any valid global IP destination will be forwarded through its network and uplinks. The provider advertises the customer’s prefixes to the entire world and delivers either a default route (0.0.0.0/0 or ::/0) or the full global routing table (over 950,000 IPv4 routes and 200,000 IPv6 routes) to the customer.

2. Peering (Settlement-Free or Paid)

In a peering relationship, two networks agree to exchange traffic between their respective customers directly, typically at an Internet Exchange Point (IXP) or private interconnect (PNI). Crucially: - Network \(A\) only advertises its own prefixes and its downstream customer prefixes to Network \(B\). - Network \(A\) does not advertise prefixes learned from its transit providers or other peers to Network \(B\). - Neither party pays the other; the exchange is mutually beneficial because it offloads expensive transit traffic.

Delegation: How Addresses and ASNs Become Someone’s Responsibility

IP addresses and ASNs are managed globally under the auspices of the Internet Assigned Numbers Authority (IANA), which delegates large allocation blocks to five Regional Internet Registries (RIRs):

RIR	Geographic Region
ARIN	North America and parts of the Caribbean
RIPE NCC	Europe, Middle East, and parts of Central Asia
APNIC	Asia-Pacific
LACNIC	Latin America and the Caribbean
AFRINIC	Africa

                     +-----------------------+
                     |         IANA          |
                     +-----------+-----------+
                                 |
        +------------+-----------+-----------+------------+
        |            |           |           |            |
        v            v           v           v            v
      AFRINIC      APNIC       ARIN       LACNIC      RIPE NCC
        |            |           |           |            |
        +------------+-----------+-----------+------------+
                                 |
                                 v
                     +-----------------------+
                     |  LIR / ISP / Cloud    |
                     +-----------+-----------+
                                 |
                                 v
                     +-----------------------+
                     |   Enterprise End-Site |
                     +-----------------------+

The RIRs allocate blocks to Local Internet Registries (LIRs), which include national telcos, cloud providers, and large enterprises. The LIRs in turn assign address space to end-users or deploy it across their own infrastructure.

This delegation chain represents the administrative authority over address space. Historically, this chain was documented only in text-based WHOIS databases without cryptographic integrity, leaving a massive gap between administrative ownership and operational routing assertions.

Scalability Through Hierarchical Aggregation

Without hierarchical aggregation, routers in the core of the Internet would need to hold routing table entries for every individual machine or small subnet on Earth, leading to immediate hardware collapse.

Hierarchical aggregation functions by announcing a covering supernet (e.g., 198.51.100.0/22) that represents multiple constituent subnets (198.51.100.0/24, 198.51.101.0/24, 198.51.102.0/24, and 198.51.103.0/24). Outside observers only need to route towards the aggregate /22.

However, hierarchical aggregation creates the very vulnerability that enables subprefix hijacking: because longest prefix match (LPM) is an immutable forwarding rule in IP routing, any announcement of a /24 will always override a covering /22, regardless of AS path length or administrative preference.