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As an application developer my entire understanding of networks was a high level 'acceptance' of http request/response pairs along with some DNS addressing.

Somewhere in the back of my mind I understood that HTTP was built on top of TCP and that TCP packets need to be routed across networks, and that the routing of these packets can vary. I have never, until now, considered the complexity that must be present in routing packets via 'hops' from origin/destination.

How much control do ISPs and other entities have over this process? i.e. my understanding is that an ISP can conceptually be seen as a destination within a multi-hop route - does an ISP have the ability/authority to decide on the next destination of a particular packet? If so, how is this controlled?

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TCP in turn runs on top of IP which is then routed from the source to the destination host.

The routing of each packet is a local decision which is entirely up to the forwarding router. This routing decision is controlled by the local routing table and possible policies imposed by its administrator (based on source IP, protocol, transport-layer ports, tags, bandwidth, load, QoS, ...).

So, an ISP has total control over what their routers do. The same is true for anyone else controlling routers along the path (carriers, IX operators, your local network admin).

Technically, it's totally possible that anyone controlling a router on your path decides to route and terminate packets "somewhere else". Therefore, it's essential that routers and other important infrastructure components (switches, DNS servers, ...) are kept very safe.

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  • does that mean that it's at least theoretically possible that my request reaches a router, and then instead of forwarding my request to that address - say chase.com - instead forwards my request to a server on their local network that is setup with a hostname of 'chase.com'? Commented Sep 5, 2018 at 8:52
  • Theoretically, yes. Routing is based on IP addresses though but an admin (or attacker) could route the destination IP to another server (directly, NATed, through tunneling) or manipulate DNS so you end up on a totally different IP address. Commented Sep 5, 2018 at 9:53
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ISPs connect between them directly or using IXPs. The routers use a protocol to exchange information about the networks they are connected to and some of their features as speed and latency.

The routers store that information as a routing table, so when an IP packet comes, it checks the routing table and decides through which interface it will forward the packet.

For example, a user connecting to YouTube from an Access ISP will be routed to its destination through Regional or Global ISPs because they have lower latency and higher speeds than any other Access ISP.

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Internet service provider (ISP) also considered as a network by itself . ISP has controll over the traffic to route towards destination traffic that has reaches to ISP layer 3 device from its previous hops .

ISP don't have any controll to route traffic of other networks.

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