RJ-45 with magnetics transformer (Ethernet port) connections
Without R17 to R20 there will be no line terminations - note the values - 51 ohms on each - these are 100 ohm (102 ohm) line terminators for the data - they ensure there are few reflections when the signal is sent down the cable and reach your RJ45.

In a little bit more depth; imagine you have some pretty normal 50 ohm coax and you put a 1 V pulse from a signal generator at one end - the cable has a characteristic impedance of 50 ohms and it elicits 20 mA from the signal generator when the 1V pulse occurs. Ohms law 1 V = 0.02 A * 50 ohms etc..

This pulse of voltage and current (a power of 20 mW) travel down the cable and reaches the far end where it is expected to find a terminating 50 ohm resistor but finds an open circuit. The power has to go somewhere so it all reflects back up the cable to the signal generator. To do this the voltage at the receiving end doubles in order to force the power back to the source. When you look at it on an o-scope it looks a bit of a mess and mucks-up the data pretty badly.

This is why you use terminators on cables.