### Instruction: What has the IETF ALTO working group accomplished?
### Input: Starting around 2005, the widespread use of peer-to-peer applications such as BitTorrent was a serious concern to many network operators, as the massive amounts of network traffic caused by these applications had a significant impact on traffic engineering and revenues. Some network operators tried to throttle this traffic.

In May 2008, in an IETF Workshop on Peer-to-Peer Infrastructure, several areas of work were identified:

1. A standardized interface for the exchange of information between the underlying IP network and an overlay network, such as a peer-to-peer network. The basic idea is, that if the overlay network was aware of the topology and the cost for sending traffic through the underlying IP network, it could optimize decisions with respect to the overlay network's topology (e.g., peer selection) and routing of traffic through the overlay network. The result would be better performance or Quality of Experience in the application while reducing the utilization of the underlying network infrastructure. This work item led to the establishment of the IETF ALTO working group.
2. Content caches in the network. This has been studied in the IETF DECADE working group. However, no new protocol has been developed and standardized.
3. A new congestion control mechanism in the transport layer for background traffic, which "yields" to standard TCP. This was worked on in the IETF LEDBAT working group and has been standardized in RFC 6817.
4. A new DiffServ code point to mark IP packets to have a lower priority than the default "best effort" category has been standardized in RFC 8622.

The IETF ALTO working group was established in November 2008. The first deliverables were the problem statement, the requirements document, the specification of the core ALTO protocol and an ALTO server discovery mechanism. Since then, various extensions have been specified (see below) or are still work in progress (see IETF ALTO Datatracker).

Originally designed to support peer-to-peer file sharing, the concept is broadly applicable to many network problems. However, as of 2021 it has not achieved widespread deployment in the internet. Nevertheless, there have been experiments in Internet service provider (ISP) networks and a deployment to support large data transfers for the Large Hadron Collider at CERN.
### Response: The IETF ALTO working group was formed to solve the problems caused by the widespread use of peer-to-peer applications such as BitTorrent. The working group has provided a problem statement, requirements document, and the specifications of the core ALTO protocol and ALTO server discovery mechanism. There is still no widespread adoption of the ALTO protocol as of 2021 although some Internet service provider (ISP) networks have experimented with it, and CERN has deployed it to support large data transfers for the Large Hadron Collider.