In early Internet routing, what was the main purpose of the Gateway to Gateway Protocol (GGP) used between core routers?

Introduction / Context:
The Gateway to Gateway Protocol, or GGP, is a historical Internet protocol that appears in exam questions about the evolution of routing on the Internet. Before modern routing protocols like BGP were widely deployed, GGP was used among core gateways to exchange routing information. This question asks you to recall the primary role of GGP in that early architecture.

Given Data / Assumptions:

- We are dealing with core Internet routers, historically called gateways.- The goal of any routing protocol is to help routers build and maintain routing tables.- GGP is described as gateway to gateway, suggesting router to router communication.

Concept / Approach:
GGP was one of the first routing protocols used in the Internet to allow core gateways to share information about reachable networks. It enabled gateways to advertise which networks they could reach and to receive similar information from other gateways. Using this information, each gateway could build a routing table that allowed it to forward IP packets toward their destinations. Over time, more advanced protocols like EGP and BGP replaced GGP, but its main function remained the same: distributing routing information among routers in the core of the network.

Step-by-Step Solution:
Step 1: Recognise that the name Gateway to Gateway Protocol clearly suggests communication between gateways, meaning routers.Step 2: Consider what gateways need to exchange: they need reachability information so that they can populate and update routing tables.Step 3: Option A states that GGP was used to exchange routing information between core routers so that gateways could build and update routing tables, which matches this understanding.Step 4: Option B talks about secure end to end encryption for application data, which is the domain of protocols such as TLS or IPsec, not GGP.Step 5: Option C describes dynamic IP address assignment, which is handled by DHCP, not by routing protocols.Step 6: Option D describes converting between Ethernet and Wi Fi frame formats, which is handled by access points at the link layer, not by Internet routing protocols.Step 7: Option E refers to compression for email, which is an application level function unrelated to gateway routing.

Verification / Alternative check:
Historical Internet engineering documents describe GGP as the protocol that carried routing information among core gateways. It was based on a distance vector approach and operated at the network layer. Later, when the Internet grew, GGP was replaced by the Exterior Gateway Protocol and then by the Border Gateway Protocol. In all these cases, the essential function remained routing information exchange, not encryption, address assignment, or frame translation. This confirms that option A correctly captures the role of GGP.

Why Other Options Are Wrong:
Encryption, dynamic address assignment, frame conversion, and file compression are functions handled by very different protocols that operate at different layers. GGP does not provide security services, does not hand out IP addresses, and does not manipulate link layer frames. It exists specifically so that gateways can coordinate and agree on paths to various networks.

Common Pitfalls:
Because GGP is an older protocol, many students have never configured or seen it and may mistakenly associate any unfamiliar protocol with security or address management. Another pitfall is to confuse similarly named protocols like EGP and BGP. To avoid this, remember that any protocol with gateway to gateway in its name is very likely a routing protocol focused on sharing reachability information between routers.

Final Answer:
The Gateway to Gateway Protocol was used to exchange routing information between Internet core routers so that gateways could build and update their routing tables, as described in option A.