multilink

Multilink is a networking concept in which multiple physical communication links are combined to form a single logical data path. The goal is to increase available bandwidth, improve resilience, or both, by distributing traffic across several parallel connections.

In the context of PPP (Point-to-Point Protocol), Multilink PPP (MP) extends PPP to support aggregating several PPP links into one logical bundle. A multilink bundle contains multiple member links, each providing a physical path to the remote end. The multilink control protocol negotiates capabilities, manages the set of active links, and coordinates the aggregation. IP traffic is divided into fragments that are transmitted over the bundle and reassembled on the far side. Each fragment carries information used to reorder and reassemble the original datagram, and different implementations may use various methods to distribute fragments across member links.

Operation typically involves creating a virtual link entity that handles sequencing, reassembly, and load distribution across the member links. This enables bandwidth aggregation and can offer protection against the failure of individual links, since traffic can be shifted to remaining links without terminating the session. Implementations are found in some enterprise and service-provider routers and have historically been used in DSL and leased-line scenarios to maximize usable throughput.

Limitations include the need for both ends to support multilink technology, potential complexity in handling packet ordering and timing across heterogeneous paths, and the fact that not all higher-layer protocols or applications benefit equally from multilink. Modern alternatives for multipath connectivity include Multipath TCP and, at the link layer, standard Ethernet link aggregation (LACP) where appropriate.