This code system http://hl7.org/fhir/message-transport defines the following codes:

CodeDisplayDefinition
http HTTPThe application sends or receives messages using HTTP POST (may be over http: or https:).
ftp FTPThe application sends or receives messages using File Transfer Protocol.
mllp MLLPThe application sends or receives messages using HL7's Minimal Lower Level Protocol.
<status value="active"/><experimental value="false"/><date value="2020-12-28T16:55:11+11:00"/><publisher value="HL7 (FHIR Project)"/><contact><telecom><system value="url"/><value value="http://hl7.org/fhir"/></telecom><telecom><system value="email"/><value value="fhir@lists.hl7.org"/></telecom></contact><description value="The protocol used for message transport."/><jurisdiction><coding><system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/><code value="001"/><display value="World"/></coding></jurisdiction><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/message-transport"/><content value="complete"/><concept><code value="http"/><display value="HTTP"/><definition value="The application sends or receives messages using HTTP POST (may be over http: or https:)."/></concept><concept><code value="ftp"/><display value="FTP"/><definition value="The application sends or receives messages using File Transfer Protocol."/></concept><concept><code value="mllp"/><display value="MLLP"/><definition value="The application sends or receives messages using HL7's Minimal Lower Level Protocol."/></concept></CodeSystem>