Just as a long outstanding note.
I made an extension to EnsLib.FTP.OutboundAdapter that adds handling for Use Session Resumption as a parameter, but it appears as of the 2024 version of IRIS this has been added to the core IRIS code.

We've only recently been able to use it as we had an old OpenSSL version that did not support the flags required, so this has been left and not looked at in a few years.
For reference the extension I wrote is listed below:

Include EnsembleClass Custom.Operations.FTP.OutboundAdapter Extends EnsLib.FTP.OutboundAdapter [ ClassType = "", Inheritance = right, ProcedureBlock, System = 4 ]{Property SSLUseSessionResumption As %Boolean [ InitialExpression = 0 ];/// These properties can be configured or set by the associated Business OperationParameter SETTINGS = "SSLUseSessionResumption:Connection";/// Connect to the FTP server and log in, setting the directory and transfer modeMethod Connect(pTimeout As %Numeric = 30, pInbound As %Boolean = 0) As %Status{Set $ZT="Trap", tSC=$$$OK, tFTPPort=..FTPPortDo {If ..Connected { Do ..TestConnection(pInbound) Quit:..Connected }#; Connect to the FTP serverIf '$IsObject(..%CredentialsObj) Do ..CredentialsSet(..Credentials) If '$IsObject(..%CredentialsObj) { Set tSC=$$$ERROR($$$EnsErrNoCredentials,..Credentials) Quit }Set ..%LastSetFilePath=""#; find FTP type; get a configuration-settings instance, use it to open an FTP Session instanceIf ..%isSFTP {#; Connect using an SFTP objectSet:""=tFTPPort tFTPPort=22Set tIOAddr=..FTPServer_":"_tFTPPort_"/"_..Credentials_"/SSL='"_..SSLConfig_"'/PubKey='"_..SFTPPublicKeyFile_"'/PrivKey='"_..SFTPPrivateKeyFile_"'"$$$catTRACE("connwait","Connecting to "_tIOAddr_"/"_..Credentials_"'/PubKey='"_..SFTPPublicKeyFile_"'/PrivKey='"_..SFTPPrivateKeyFile_"' with timeout="_pTimeout)Set:'$IsObject(..FTP)||'..FTP.%Extends("EnsLib.FTP.CommonSSH") ..FTP=$thisSet ..FTP.SSLUseSessionResumption=..SSLUseSessionResumptionSet t0=$zhSet tSC=..FTP.ConnectSSH(pTimeout, pInbound, tFTPPort)} Else {#; Connect using standard FTP, or FTPS with SSLConfigSet:""=tFTPPort tFTPPort=21Set tIOAddr=..FTPServer_":"_tFTPPort_"/"_..Credentials_"/SSL='"_..SSLConfig$$$catTRACE("connwait","Connecting to "_tIOAddr_"/"_..Credentials_" with timeout="_pTimeout)Set:'$IsObject(..FTP)||'..FTP.%Extends("%Net.FtpSession") ..FTP=##class(%Net.FtpSession).%New()Set ..FTP.SSLUseSessionResumption=..SSLUseSessionResumptionSet t0=$zh, ..FTP.Timeout=pTimeout, ..FTP.UsePASV=..UsePASV, ..FTP.LegacySSL=("*"=$E(..SSLConfig,*)), ..FTP.SSLConfiguration=$S("*"=$E(..SSLConfig,*):$E(..SSLConfig,1,*-1),1:..SSLConfig)If (..FTP.SSLConfiguration'="") {Set ..FTP.SSLCheckServerIdentity = ..SSLCheckServerIdentitySet ..FTP.SSLUseSessionResumption = ..SSLUseSessionResumption}If '..FTP.Connect(..FTPServer,..%CredentialsObj.Username,..%CredentialsObj.Password,tFTPPort) {Set tSC=$$$ERROR($$$EnsErrFTPConnectFailed,tIOAddr_"/"_..Credentials,..FTP.ReturnMessage,..FTP.ReturnCode)}#; Set after connect since FTP class will query server if empty stringSet ..FTP.CommandTranslateTable = ..CommandTranslateTable}If $$$ISERR(tSC) {Set tSC=$S((-1'=pTimeout)&&(t0+pTimeout<=$zh): $$$ADDSC($$$ERROR($$$EnsErrOutConnectExpired,pTimeout,$S(..%isSFTP:"SFTP",1:"FTP"),tIOAddr),tSC), 1: $$$ERROR($$$EnsErrOutConnectFailed,$$$StatusDisplayString(tSC),$S(..%isSFTP:"SFTP",1:"FTP"),tIOAddr))Set ..FTP=$$$NULLOREFQuit}#; Get the system declaration from the FTP serverSet ..%Syst=""  Set:..FTP.System(.tSystem) ..%Syst=tSystemIf ""'=..%Syst {Set ..%isVMS = ("VMS " = $E(..%Syst,1,$L("VMS ")))$$$catTRACE("connwait","Detected FTP server system type '"_..%Syst_"'")}#; Set the current directorySet ..%LastSetFilePath=..fixSvrPath(..FilePath,0)If ""=..%LastSetFilePath {$$$catTRACE("connwait","Not setting FTP working directory because FilePath is empty")} Else {If ..FTP.SetDirectory(..%LastSetFilePath) {$$$catTRACE("connwait","Set FTP working directory to "_..%LastSetFilePath)} Else {Set tSC=$$$ERROR($$$EnsErrFTPDirectoryChangeFailed,..%LastSetFilePath,..FTP.ReturnMessage,..FTP.ReturnCode) Set ..%LastSetFilePath=""Quit}}#; Set the transfer modeSet tTable = "RAW"Set csetlc=$ZCVT(..Charset,"L")Set tAscii=$Case($E(csetlc,1,5),"":1,"defau":1,"ascii":1,"latin":1,"iso-8":1,"utf-8":1,:0)If 'tAscii {If '..FTP.Binary() {Set tSC=$$$ERROR($$$EnsErrFTPModeChangeFailed,"Binary",..FTP.ReturnMessage,..FTP.ReturnCode)Set ..FTP.TranslateTable = ""Quit}If "binary"'=csetlc {Set tEnc=..Charset   Set:"*"=$E(tEnc) $E(tEnc)=""Set tTable = ##class(%IO.I.TranslationDevice).GetCharEncodingTable(tEnc)Set:tTable="" tTable="RAW"}} Else {If '..FTP.Ascii() {Set tSC=$$$ERROR($$$EnsErrFTPModeChangeFailed,"Ascii",..FTP.ReturnMessage,..FTP.ReturnCode)Quit}If "ascii"'=csetlc {If $Case(csetlc,"":0,"default":0,"native":0,:1) {Set tTable = ##class(%IO.I.TranslationDevice).GetCharEncodingTable(..Charset)} Else { Set tTable = "" }Set:tTable="" tTable=$$DefIO^%NLS(5)}}#; SuccessSet ..FTP.TranslateTable = tTableSet tTxt="Connected to FTP Server '"_tIOAddr_"' at path '"_..%LastSetFilePath_"' using Credentials '"_..Credentials_"'"If ..StayConnected<0 { $$$LOGINFO(tTxt) }Else {If pInbound&&'..StayConnected { $$$catTRACE("connwait",tTxt) }ElseIf ..%logTransfers { $$$LOGINFO(tTxt) }Else { $$$sysTRACE(tTxt) }}Set ..Connected=1$$$ASSERT(..FTP.Connected)If (..BusinessHost.%LastReportedError [ "ERROR <Ens>ErrOutConnect")||(..BusinessHost.%LastReportedError [ ..%LastNetErr) {Set ..BusinessHost.%LastReportedError=""$$$SetHostMonitor(..BusinessHost.%ConfigName,$$$eMonitorStatus,"OK")}Set ..%LastNetErr="%%%%%"Set i%%IOAddr=tIOAddr} While 0ExitIf $$$ISERR(tSC) {$$$ASSERT('..Connected)Do:..FTP.Connected ..FTP.Logout() ; force FTP class into sync in case it made a mistake}Quit tSCTrapSet $ZT="",tSC=$$$EnsSystemErrorSet tSC =$$$ERROR($$$EnsErrOutConnectException,$$$StatusText(tSC),$S(..%isSFTP:"SFTP",1:"FTP"),tIOAddr_"/"_..Credentials)Goto Exit}}