Bottleneck Link

networking

Definition

Bottleneck Link

Let $P=(e_1,\dots ,e_k)$ be a path of links from source $s$ to destination $t$ in a computer network, where each $e_i$ has capacity $R_i$ bits/s.

The bottleneck link of $P$ is the link $e^{\ast }$ with the smallest capacity:

$$e^{\ast }=\arg \underset{e_i\in P}{\min }{R}_i$$

Its capacity $R^{\ast }={\min }_i{R}_i$ is the bottleneck bandwidth of $P$. No flow along $P$ can exceed $R^{\ast }$, regardless of the capacities of other links on the path:

$$\forall \text{ flows }f\text{ on }P:\text{rate}(f)\le R^{\ast }$$

Role in End-to-End Throughput

The bottleneck link upper-bounds the throughput achievable on $P$. All other links may have spare capacity, but the bottleneck determines the ceiling.

Multiple flows

If $m$ flows share the bottleneck link, each flow $f_j$ receives at most $R^{\ast }/m$ under fair sharing, assuming no other bottleneck constrains any flow.