Protocols are a set of rules and standards stablished by an external regulating body, whereas SOPs, are the methods used to achieve or comply with those protocols. Protocols do not necessary have an SOP and also, you can develop a SOP regardless of whether there is a protocol that needs to be complied. In addition, protocols are goal-oriented or problem-oriented, since they describe what has to be achieved while SOPs are the practical instructions that an individual needs to follow to achieve that goal. For instance, a protocol may indicate the accuracy that a process requires, meanwhile the SOP is the procedure a lab uses to conduct the process.
For more information about protocols and SOPs see the following materials: