Who do I contact in case of missed connection?

Hi,

I am flying from SFO to IST next week.

SFO - BOS on United
BOS - ZRH on Swiss
ZRH - IST on Swiss

I bought the ticket on Travelocity.

If there were any delays/cancellation and I miss my connecting flights, who should I contact? United, Swiss, or Travelocity? If united cancels the flight, can they say they are responsible only for getting me to BOS? Or would Swiss refuse to put me on another flight because it was not their fault I missed the connection?

Looks like they are expecting a snow storm in the east coast early next week and I am little concerned.

Thanks.