FAF (Forward Attachment Function)

Forward Attachment Function (FAF) is a concept in computer networking that refers to the process of routing packets through a network. Specifically, FAF is used in Content-Centric Networking (CCN), a proposed architecture for the future Internet that is based on the idea of retrieving content based on its name or identifier, rather than its location.

In this architecture, content is given a unique identifier, called a Content Name, and is distributed across the network in the form of named Data packets. When a user requests content, they send a request packet containing the Content Name. The request packet is then forwarded through the network until it reaches the content's source, where the content is retrieved and sent back to the user in the form of a response packet.

FAF plays a crucial role in this process, as it determines how packets are routed through the network. Specifically, FAF is responsible for deciding which interface a packet should be forwarded out of at each hop in the network. This decision is made based on the packet's Content Name, as well as other factors such as network topology, available bandwidth, and congestion.

To understand FAF in more detail, it is useful to first consider how packets are forwarded in traditional IP-based networks. In these networks, packets are routed based on their destination IP address. When a packet arrives at a router, the router looks up the destination IP address in a routing table to determine which interface to forward the packet out of. The routing table is typically populated using a protocol such as OSPF or BGP, which exchange routing information between routers to build a view of the network topology.

In contrast, FAF-based routing is based on the Content Name, rather than the destination IP address. When a request packet arrives at a router in a CCN network, the router looks up the Content Name in a Content Store (CS), which is a cache of recently accessed content. If the content is in the CS, the router forwards the response packet directly from the CS to the requester, without needing to traverse the network further.

If the content is not in the CS, the router must forward the request packet to other routers in the network until it reaches the content's source. At each hop, the router uses FAF to determine which interface to forward the packet out of. FAF takes into account the Content Name of the packet, as well as other factors such as the network topology, available bandwidth, and congestion.

One of the key benefits of FAF-based routing is that it enables efficient and scalable content distribution. In traditional IP-based networks, the routing table can become very large and complex as the network grows, making it difficult to manage and prone to errors. In contrast, FAF-based routing is more lightweight and scalable, as it only needs to consider the Content Name and a few other factors when making routing decisions.

Another benefit of FAF-based routing is that it can improve network performance and reliability. By distributing content based on its name rather than its location, FAF can take advantage of caching and replication to reduce network traffic and improve response times. In addition, FAF can adapt to changes in the network topology and traffic patterns, allowing it to avoid congestion and route around failures.

There are several different approaches to implementing FAF in a CCN network. One approach is to use a distributed algorithm, such as the Gradient-based Routing Algorithm (GRA), to make routing decisions. GRA is based on the idea of using gradients to estimate the distance and direction to the content's source, and using this information to make forwarding decisions.

Another approach is to use a centralized controller, such as the Named Data Networking Forwarding Daemon (NFD), to manage the routing decisions. NFD uses a table-based approach, where each router maintains a table of routing entries that map Content Names to interfaces and next hops. When a request packet arrives at a router, the router looks up the Content Name in its table and forwards the packet to the next hop specified in the table entry.

In addition to these approaches, there are also hybrid approaches that combine both distributed and centralized elements. For example, the Content Centric Internet Routing Protocol (CCIRP) uses a hybrid approach, where each router maintains a local routing table based on distributed information, but can also query a centralized directory service for additional information when needed.

Overall, FAF is a key component of Content-Centric Networking, and plays a crucial role in enabling efficient and scalable content distribution. By routing packets based on Content Name, rather than location, FAF-based routing can improve network performance and reliability, and adapt to changes in the network topology and traffic patterns. As CCN continues to evolve and mature, FAF is likely to remain a central component of this architecture.