FRA vs STIR Fitures

Hi! For FRA, I understand when calculating its payoff we need to take the time value over the notional borrowing period into consideration, i.e. by discounting the difference between rates at the end of the notional borrowing period to FRA’s maturity.

However, for SITR futures, for example Eurodollar, most of the time I saw in examples that the calculation is just to calculate the rate differences multiplied by 25 and by no. of contracts.

From the perspective of MTM, I understand that futures’ pnl comes from its price difference.

However, from no-arbitrage perspective, it seems that FRA and SITR futures are essentially the same, however the former considers the time value of notional borrowing period while the latter does not? Or, the price / reference rate of SITR futures has already considers this? If so, the forward rate in FRA should differ from the forward reference rate of an otherwise the same SITR futures?

Many thanks!

FRAs and STIRS are similar however the STIR being a future has a linear payoff whereas FRA exhibits some convexity. FRAs payoff occurs at expiry, which is why the amount is discounted whereas STIRs receive the payoff at the end of the underlying interest period. if you ignore the issue of convexity then the only difference reflects the timing of the payoff.