Implementing flow control without using unique symbols or designated packets, comprising: sending, from a first device to a second device, high throughput packet communication. Temporarily storing the high throughput packet communication in a buffer of the second device. Calculating, by the second device, a basic idle code word sequence known to the first device. Producing an idle sequence by replacing certain M code words of the basic idle sequence with M bitwise complement code words. Transmitting the idle sequence, wherein the M bitwise complement code words are indicative of the fullness of the buffer. Receiving the idle sequence by the first device, and determining, based on a difference between the idle sequence and the basic idle sequence, that the buffer is full or expected to get full, and thus the first device stop sending packets to the second device.