diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index a1ff334be..2c13221db 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -241,32 +241,32 @@ jobs:
           ACCEPTANCE_SET: lastseqnumprocessed
         run: make generate-ci && make build && make $FIX_TEST
 
-  # nextexpectedseqnum:
-  #   name: NextExpectedSeqNum Suite
-  #   runs-on: ubuntu-latest
-  #   strategy:
-  #     matrix:
-  #       go: [1.21]
-  #       fix-version:
-  #         - fix44
-  #         - fix50
-  #         - fix50sp1
-  #         - fix50sp2
-  #   steps:
-  #     - name: Setup
-  #       uses: actions/setup-go@v2
-  #       with:
-  #         go-version: ${{ matrix.go }}
-  #     - name: Check out source
-  #       uses: actions/checkout@v2
-  #     - name: Install ruby
-  #       uses: ruby/setup-ruby@v1
-  #       with:
-  #         ruby-version: '3.0'
-  #     - name: Acceptance test
-  #       env:
-  #         GO111MODULE: on
-  #         FIX_TEST: ${{ matrix.fix-version }}
-  #         STORE_TYPE: memory
-  #         ACCEPTANCE_SET: nextexpectedseqnum
-  #       run: make generate-ci && make build && make $FIX_TEST
\ No newline at end of file
+  nextexpectedseqnum:
+    name: NextExpectedSeqNum Suite
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go: [1.21]
+        fix-version:
+          - fix44
+          - fix50
+          - fix50sp1
+          - fix50sp2
+    steps:
+      - name: Setup
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{ matrix.go }}
+      - name: Check out source
+        uses: actions/checkout@v2
+      - name: Install ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.0'
+      - name: Acceptance test
+        env:
+          GO111MODULE: on
+          FIX_TEST: ${{ matrix.fix-version }}
+          STORE_TYPE: memory
+          ACCEPTANCE_SET: nextexpectedseqnum
+        run: make generate-ci && make build && make $FIX_TEST
\ No newline at end of file
diff --git a/_test/Reflector.rb b/_test/Reflector.rb
index fe9535f39..3e86af7a1 100644
--- a/_test/Reflector.rb
+++ b/_test/Reflector.rb
@@ -61,6 +61,8 @@ def processLine(lineNum, line, body, cid)
         connectAction(cid)
       elsif body == "DISCONNECT"
         disconnectAction(cid)
+      elsif body.index("SET_SESSION") == 0
+        setSeqnum(body)
       else
         raise "Syntax error: " + body
       end
diff --git a/_test/ReflectorClient.rb b/_test/ReflectorClient.rb
index acc8acc72..148867b40 100644
--- a/_test/ReflectorClient.rb
+++ b/_test/ReflectorClient.rb
@@ -19,6 +19,8 @@
 require 'Reflector'
 require 'FixParser'
 require "socket"
+require 'uri'
+require 'net/http'
 
 class ReflectorClient
 
@@ -62,6 +64,12 @@ def @reflector.disconnectAction(cid)
       @sockets.delete(cid)
       @parsers.delete(cid)
     end
+    
+    def @reflector.setSeqnum(body)
+      left_array = body.split(" ")
+      uri = URI('http://localhost:8095/seqnum?SESSION='+left_array[1]+'&'+left_array[2])
+      Net::HTTP.get_response(uri)
+    end
 
     def @reflector.waitConnectAction(cid)
     end
diff --git a/_test/cfg/nextexpectedseqnum/fix44.cfg b/_test/cfg/nextexpectedseqnum/fix44.cfg
new file mode 100644
index 000000000..b31219439
--- /dev/null
+++ b/_test/cfg/nextexpectedseqnum/fix44.cfg
@@ -0,0 +1,10 @@
+[DEFAULT]
+SocketAcceptPort=5005
+SenderCompID=ISLD
+TargetCompID=TW
+FileLogPath=tmp
+
+[SESSION]
+BeginString=FIX.4.4
+DataDictionary=../spec/FIX44.xml
+EnableNextExpectedMsgSeqNum=Y
diff --git a/_test/cfg/nextexpectedseqnum/fix50.cfg b/_test/cfg/nextexpectedseqnum/fix50.cfg
new file mode 100644
index 000000000..c60f608e2
--- /dev/null
+++ b/_test/cfg/nextexpectedseqnum/fix50.cfg
@@ -0,0 +1,12 @@
+[DEFAULT]
+SocketAcceptPort=5006
+SenderCompID=ISLD
+TargetCompID=TW
+FileLogPath=tmp
+
+[SESSION]
+BeginString=FIXT.1.1
+DefaultApplVerID=FIX.5.0
+TransportDataDictionary=../spec/FIXT11.xml
+AppDataDictionary=../spec/FIX50.xml
+EnableNextExpectedMsgSeqNum=Y
diff --git a/_test/cfg/nextexpectedseqnum/fix50sp1.cfg b/_test/cfg/nextexpectedseqnum/fix50sp1.cfg
new file mode 100644
index 000000000..f1ec2103b
--- /dev/null
+++ b/_test/cfg/nextexpectedseqnum/fix50sp1.cfg
@@ -0,0 +1,12 @@
+[DEFAULT]
+SocketAcceptPort=5007
+SenderCompID=ISLD
+TargetCompID=TW
+FileLogPath=tmp
+
+[SESSION]
+BeginString=FIXT.1.1
+DefaultApplVerID=FIX.5.0SP1
+TransportDataDictionary=../spec/FIXT11.xml
+AppDataDictionary=../spec/FIX50SP1.xml
+EnableNextExpectedMsgSeqNum=Y
diff --git a/_test/cfg/nextexpectedseqnum/fix50sp2.cfg b/_test/cfg/nextexpectedseqnum/fix50sp2.cfg
new file mode 100644
index 000000000..4a1db4eef
--- /dev/null
+++ b/_test/cfg/nextexpectedseqnum/fix50sp2.cfg
@@ -0,0 +1,12 @@
+[DEFAULT]
+SocketAcceptPort=5008
+SenderCompID=ISLD
+TargetCompID=TW
+FileLogPath=tmp
+
+[SESSION]
+BeginString=FIXT.1.1
+DefaultApplVerID=FIX.5.0SP2
+TransportDataDictionary=../spec/FIXT11.xml
+AppDataDictionary=../spec/FIX50SP2.xml
+EnableNextExpectedMsgSeqNum=Y
diff --git a/_test/definitions/nextexpectedseqnum/fix44/2a_NextExpectedMsgSeqNumCorrect.def b/_test/definitions/nextexpectedseqnum/fix44/2a_NextExpectedMsgSeqNumCorrect.def
new file mode 100644
index 000000000..3cadb8ca2
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix44/2a_NextExpectedMsgSeqNumCorrect.def
@@ -0,0 +1,16 @@
+iCONNECT
+
+#------------------------
+# logon message and response
+# send logon with sequence number equals to 1
+# expect for a logon response with NextExpectedMsgSeqNum field equals to 2 
+#------------------------
+
+I8=FIX.4.4
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
108=2

+E8=FIX.4.4
9=66
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=2
789=2
10=0

+
+# logout message and response
+I8=FIX.4.4
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIX.4.4
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix44/2b_NextExpectedMsgSeqNumTooHigh.def b/_test/definitions/nextexpectedseqnum/fix44/2b_NextExpectedMsgSeqNumTooHigh.def
new file mode 100644
index 000000000..dea8ea56b
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix44/2b_NextExpectedMsgSeqNumTooHigh.def
@@ -0,0 +1,10 @@
+# If message NextExpectedMsgSeqNum field is too high, we must disconnect
+
+iCONNECT
+iSET_SESSION FIX.4.4:ISLD->TW NEXTSENDERSEQNUM=1
+iSET_SESSION FIX.4.4:ISLD->TW NEXTTARGETSEQNUM=1
+
+I8=FIX.4.4
35=A
34=1000
49=TW
52=<TIME>
56=ISLD
98=0
789=1200
108=30

+
+E8=FIX.4.4
9=135
35=5
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
58=Tag 789 (NextExpectedMsgSeqNum) is higher than expected. Expected 1, Received 1200
10=0

+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix44/2c_NextExpectedMsgSeqNumTooLow.def b/_test/definitions/nextexpectedseqnum/fix44/2c_NextExpectedMsgSeqNumTooLow.def
new file mode 100644
index 000000000..85b7a5e19
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix44/2c_NextExpectedMsgSeqNumTooLow.def
@@ -0,0 +1,14 @@
+iCONNECT
+iSET_SESSION FIX.4.4:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIX.4.4:ISLD->TW NEXTTARGETSEQNUM=1
+
+I8=FIX.4.4
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
108=30

+
+E8=FIX.4.4
9=70
35=A
34=2000
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
789=2
10=0

+E8=FIX.4.4
9=94
35=4
34=1
43=Y
49=ISLD
52=<TIME>
56=TW
122=<TIME>
36=2001
123=Y

+
+# logout message and response
+I8=FIX.4.4
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIX.4.4
9=52
35=5
34=2001
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix44/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def b/_test/definitions/nextexpectedseqnum/fix44/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
new file mode 100644
index 000000000..f8a24d30f
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix44/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
@@ -0,0 +1,14 @@
+# FIX Logon using tag 141/ResetSeqNumFlag and tag 789/NextExpectedSeqMsgNum
+
+iCONNECT
+iSET_SESSION FIX.4.4:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIX.4.4:ISLD->TW NEXTTARGETSEQNUM=2000
+
+I8=FIX.4.4
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
141=Y
108=30

+E8=FIX.4.4
9=73
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
141=Y
789=2
10=0

+
+# logout message and response
+I8=FIX.4.4
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIX.4.4
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50/2a_NextExpectedMsgSeqNumCorrect.def b/_test/definitions/nextexpectedseqnum/fix50/2a_NextExpectedMsgSeqNumCorrect.def
new file mode 100644
index 000000000..72ee93f93
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50/2a_NextExpectedMsgSeqNumCorrect.def
@@ -0,0 +1,16 @@
+iCONNECT
+
+#------------------------
+# logon message and response
+# send logon with sequence number equals to 1
+# expect for a logon response with NextExpectedMsgSeqNum field equals to 2 
+#------------------------
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
1137=7
108=2

+E8=FIXT.1.1
9=73
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=2
789=2
1137=7
10=0

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50/2b_NextExpectedMsgSeqNumTooHigh.def b/_test/definitions/nextexpectedseqnum/fix50/2b_NextExpectedMsgSeqNumTooHigh.def
new file mode 100644
index 000000000..776b243a2
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50/2b_NextExpectedMsgSeqNumTooHigh.def
@@ -0,0 +1,9 @@
+# If message NextExpectedMsgSeqNum field is too high, we must disconnect
+
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=1
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=1
+I8=FIXT.1.1
35=A
34=1000
49=TW
52=<TIME>
56=ISLD
98=0
789=1200
1137=7
108=30

+
+E8=FIXT.1.1
9=135
35=5
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
58=Tag 789 (NextExpectedMsgSeqNum) is higher than expected. Expected 1, Received 1200
10=0

+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50/2c_NextExpectedMsgSeqNumTooLow.def b/_test/definitions/nextexpectedseqnum/fix50/2c_NextExpectedMsgSeqNumTooLow.def
new file mode 100644
index 000000000..e3d1e3bb1
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50/2c_NextExpectedMsgSeqNumTooLow.def
@@ -0,0 +1,14 @@
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=1
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
1137=7
108=30

+
+E8=FIXT.1.1
9=77
35=A
34=2000
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
789=2
1137=7
10=0

+E8=FIXT.1.1
9=94
35=4
34=1
43=Y
49=ISLD
52=<TIME>
56=TW
122=<TIME>
36=2001
123=Y

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=52
35=5
34=2001
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def b/_test/definitions/nextexpectedseqnum/fix50/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
new file mode 100644
index 000000000..3a738b90d
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
@@ -0,0 +1,14 @@
+# FIX Logon using tag 141/ResetSeqNumFlag and tag 789/NextExpectedSeqMsgNum
+
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=2000
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
141=Y
1137=7
108=30

+E8=FIXT.1.1
9=80
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
141=Y
789=2
1137=7
10=0

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp1/2a_NextExpectedMsgSeqNumCorrect.def b/_test/definitions/nextexpectedseqnum/fix50sp1/2a_NextExpectedMsgSeqNumCorrect.def
new file mode 100644
index 000000000..0441f335c
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp1/2a_NextExpectedMsgSeqNumCorrect.def
@@ -0,0 +1,16 @@
+iCONNECT
+
+#------------------------
+# logon message and response
+# send logon with sequence number equals to 1
+# expect for a logon response with NextExpectedMsgSeqNum field equals to 2 
+#------------------------
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
1137=8
108=2

+E8=FIXT.1.1
9=73
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=2
789=2
1137=8
10=0

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp1/2b_NextExpectedMsgSeqNumTooHigh.def b/_test/definitions/nextexpectedseqnum/fix50sp1/2b_NextExpectedMsgSeqNumTooHigh.def
new file mode 100644
index 000000000..a29496672
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp1/2b_NextExpectedMsgSeqNumTooHigh.def
@@ -0,0 +1,9 @@
+# If message NextExpectedMsgSeqNum field is too high, we must disconnect
+
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=1
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=1
+I8=FIXT.1.1
35=A
34=1000
49=TW
52=<TIME>
56=ISLD
98=0
789=1200
1137=8
108=30

+
+E8=FIXT.1.1
9=135
35=5
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
58=Tag 789 (NextExpectedMsgSeqNum) is higher than expected. Expected 1, Received 1200
10=0

+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp1/2c_NextExpectedMsgSeqNumTooLow.def b/_test/definitions/nextexpectedseqnum/fix50sp1/2c_NextExpectedMsgSeqNumTooLow.def
new file mode 100644
index 000000000..4de8b8094
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp1/2c_NextExpectedMsgSeqNumTooLow.def
@@ -0,0 +1,14 @@
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=1
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
1137=10
108=30

+
+E8=FIXT.1.1
9=77
35=A
34=2000
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
789=2
1137=8
10=0

+E8=FIXT.1.1
9=94
35=4
34=1
43=Y
49=ISLD
52=<TIME>
56=TW
122=<TIME>
36=2001
123=Y

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=52
35=5
34=2001
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp1/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def b/_test/definitions/nextexpectedseqnum/fix50sp1/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
new file mode 100644
index 000000000..2fc6d76af
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp1/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
@@ -0,0 +1,14 @@
+# FIX Logon using tag 141/ResetSeqNumFlag and tag 789/NextExpectedSeqMsgNum
+
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=2000
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
141=Y
1137=8
108=30

+E8=FIXT.1.1
9=80
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
141=Y
789=2
1137=8
10=0

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp2/2a_NextExpectedMsgSeqNumCorrect.def b/_test/definitions/nextexpectedseqnum/fix50sp2/2a_NextExpectedMsgSeqNumCorrect.def
new file mode 100644
index 000000000..d106d7108
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp2/2a_NextExpectedMsgSeqNumCorrect.def
@@ -0,0 +1,16 @@
+iCONNECT
+
+#------------------------
+# logon message and response
+# send logon with sequence number equals to 1
+# expect for a logon response with NextExpectedMsgSeqNum field equals to 2 
+#------------------------
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
1137=9
108=2

+E8=FIXT.1.1
9=73
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=2
789=2
1137=9
10=0

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp2/2b_NextExpectedMsgSeqNumTooHigh.def b/_test/definitions/nextexpectedseqnum/fix50sp2/2b_NextExpectedMsgSeqNumTooHigh.def
new file mode 100644
index 000000000..f702d38ee
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp2/2b_NextExpectedMsgSeqNumTooHigh.def
@@ -0,0 +1,9 @@
+# If message NextExpectedMsgSeqNum field is too high, we must disconnect
+
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=1
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=1
+I8=FIXT.1.1
35=A
34=1000
49=TW
52=<TIME>
56=ISLD
98=0
789=1200
1137=9
108=30

+
+E8=FIXT.1.1
9=135
35=5
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
58=Tag 789 (NextExpectedMsgSeqNum) is higher than expected. Expected 1, Received 1200
10=0

+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp2/2c_NextExpectedMsgSeqNumTooLow.def b/_test/definitions/nextexpectedseqnum/fix50sp2/2c_NextExpectedMsgSeqNumTooLow.def
new file mode 100644
index 000000000..4a941eb53
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp2/2c_NextExpectedMsgSeqNumTooLow.def
@@ -0,0 +1,14 @@
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=1
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
1137=9
108=30

+
+E8=FIXT.1.1
9=77
35=A
34=2000
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
789=2
1137=9
10=0

+E8=FIXT.1.1
9=94
35=4
34=1
43=Y
49=ISLD
52=<TIME>
56=TW
122=<TIME>
36=2001
123=Y

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=52
35=5
34=2001
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/definitions/nextexpectedseqnum/fix50sp2/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def b/_test/definitions/nextexpectedseqnum/fix50sp2/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
new file mode 100644
index 000000000..c9c03b272
--- /dev/null
+++ b/_test/definitions/nextexpectedseqnum/fix50sp2/ResetSeqNumFlagAndNextExpectedMsgSeqNum.def
@@ -0,0 +1,14 @@
+# FIX Logon using tag 141/ResetSeqNumFlag and tag 789/NextExpectedSeqMsgNum
+
+iCONNECT
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTSENDERSEQNUM=2000
+iSET_SESSION FIXT.1.1:ISLD->TW NEXTTARGETSEQNUM=2000
+
+I8=FIXT.1.1
35=A
34=1
49=TW
52=<TIME>
56=ISLD
98=0
789=1
141=Y
1137=9
108=30

+E8=FIXT.1.1
9=80
35=A
34=1
49=ISLD
52=00000000-00:00:00.000
56=TW
98=0
108=30
141=Y
789=2
1137=9
10=0

+
+# logout message and response
+I8=FIXT.1.1
35=5
34=2
49=TW
52=<TIME>
56=ISLD

+E8=FIXT.1.1
9=49
35=5
34=2
49=ISLD
52=00000000-00:00:00.000
56=TW
10=0

+
+eDISCONNECT
\ No newline at end of file
diff --git a/_test/test-server/main.go b/_test/test-server/main.go
index b80bd4d6c..2679e889d 100644
--- a/_test/test-server/main.go
+++ b/_test/test-server/main.go
@@ -5,9 +5,11 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"net/http"
 	"os"
 	"os/signal"
 	"path"
+	"strconv"
 	"strings"
 	"time"
 
@@ -131,11 +133,15 @@ func main() {
 	}
 
 	fileLogFactory, err := quickfix.NewFileLogFactory(appSettings)
-
 	if err != nil {
 		fmt.Println("Error creating file log factory:", err)
 		return
 	}
+	//fileLogFactory := quickfix.NewScreenLogFactory()
+	// if err != nil {
+	// 	fmt.Println("Error creating file log factory:", err)
+	// 	return
+	// }
 
 	storeType := os.Args[2]
 
@@ -175,6 +181,69 @@ func main() {
 		return
 	}
 
+	// http handler for seqnum resets for specific test defs. not applicable for most tests.
+	http.HandleFunc("/seqnum", func(w http.ResponseWriter, r *http.Request) {
+		defer w.WriteHeader(http.StatusOK)
+		queryParams := r.URL.Query()
+		sessionStr, ok :=  queryParams["SESSION"]
+		if !ok {
+			fmt.Println("cannot find session")
+			os.Exit(1)
+		}
+		splts := strings.Split(sessionStr[0], ":")
+		parties := strings.Split(splts[1], "->")
+		sessionID := quickfix.SessionID{
+			BeginString: splts[0],
+			SenderCompID: parties[0],
+			TargetCompID: parties[1],
+		}
+		num, ok :=  queryParams["NEXTTARGETSEQNUM"]
+		if ok {
+			seqnumInt, cErr := strconv.Atoi(num[0])
+			if cErr != nil {
+				fmt.Println("cannot find seqnum")
+				os.Exit(1)
+			}
+			setErr := quickfix.SetNextTargetMsgSeqNum(sessionID, seqnumInt)
+			if setErr != nil {
+				fmt.Println("err setting target seqnum: " + setErr.Error())
+				return
+			}
+			fmt.Println("set target seqnum to: " + num[0])
+			return
+		}
+
+		num, ok =  queryParams["NEXTSENDERSEQNUM"]
+		if ok {
+			seqnumInt, cErr := strconv.Atoi(num[0])
+			if cErr != nil {
+				fmt.Println("cannot find seqnum")
+				os.Exit(1)
+			}
+			setErr := quickfix.SetNextSenderMsgSeqNum(sessionID, seqnumInt)
+			if setErr != nil {
+				fmt.Println("err setting sender seqnum: " + setErr.Error())
+				return
+			}
+			fmt.Println("set sender seqnum to: "+num[0])
+			return
+		}
+		
+		fmt.Println("never found seqnum")
+		os.Exit(1)
+	})
+	websrvr := http.Server{Addr: ":8095"}
+	defer func() {
+		if err := websrvr.Close(); err != nil {
+			fmt.Println("unable to stop srver: ", err)
+		}
+	}()
+	go func() {
+		if err := websrvr.ListenAndServe(); err != http.ErrServerClosed {
+			fmt.Println("websrvr err: ", err)
+		}
+	}()
+
 	c := make(chan os.Signal, 1)
 	signal.Notify(c, os.Interrupt)
 	<-c
diff --git a/config/configuration.go b/config/configuration.go
index 754f587b1..d76a6dd0c 100644
--- a/config/configuration.go
+++ b/config/configuration.go
@@ -1019,4 +1019,17 @@ const (
 	//  - Y
 	//  - N
 	EnableLastMsgSeqNumProcessed string = "EnableLastMsgSeqNumProcessed"
+
+	// EnableNextExpectedMsgSeqNum tells the FIX engine to add tag NextExpectedMsgSeqNum (optional tag 789) on the 
+	// sent Logon message and use value of tag 789 on received Logon message to synchronize session. 
+	// This should not be enabled for FIX versions less than FIX.4.4.
+	//
+	// Required: No
+	//
+	// Default: N
+	//
+	// Valid Values:
+	//  - Y
+	//  - N
+	EnableNextExpectedMsgSeqNum  string = "EnableNextExpectedMsgSeqNum"
 )
diff --git a/internal/session_settings.go b/internal/session_settings.go
index e003f54e4..5cbdcfb06 100644
--- a/internal/session_settings.go
+++ b/internal/session_settings.go
@@ -14,6 +14,7 @@ type SessionSettings struct {
 	InitiateLogon                bool
 	ResendRequestChunkSize       int
 	EnableLastMsgSeqNumProcessed bool
+	EnableNextExpectedMsgSeqNum  bool
 	SkipCheckLatency             bool
 	MaxLatency                   time.Duration
 	DisableMessagePersist        bool
diff --git a/registry.go b/registry.go
index 236c795b6..145c710c3 100644
--- a/registry.go
+++ b/registry.go
@@ -93,6 +93,42 @@ func UnregisterSession(sessionID SessionID) error {
 	return errUnknownSession
 }
 
+// SetNextTargetMsgSeqNum set the next expected target message sequence number for the session matching the session id.
+func SetNextTargetMsgSeqNum(sessionID SessionID, seqNum int) error {
+	session, ok := lookupSession(sessionID)
+	if !ok {
+		return errUnknownSession
+	}
+	return session.store.SetNextTargetMsgSeqNum(seqNum)
+}
+
+// SetNextSenderMsgSeqNum sets the next outgoing message sequence number for the session matching the session id.
+func SetNextSenderMsgSeqNum(sessionID SessionID, seqNum int) error {
+	session, ok := lookupSession(sessionID)
+	if !ok {
+		return errUnknownSession
+	}
+	return session.store.SetNextSenderMsgSeqNum(seqNum)
+}
+
+// GetExpectedSenderNum retrieves the expected sender sequence number for the session matching the session id.
+func GetExpectedSenderNum(sessionID SessionID) (int, error) {
+	session, ok := lookupSession(sessionID)
+	if !ok {
+		return 0, errUnknownSession
+	}
+	return session.store.NextSenderMsgSeqNum(), nil
+}
+
+// GetExpectedTargetNum retrieves the next target sequence number for the session matching the session id.
+func GetExpectedTargetNum(sessionID SessionID) (int, error) {
+	session, ok := lookupSession(sessionID)
+	if !ok {
+		return 0, errUnknownSession
+	}
+	return session.store.NextTargetMsgSeqNum(), nil
+}
+
 func registerSession(s *session) error {
 	sessionsLock.Lock()
 	defer sessionsLock.Unlock()
diff --git a/session.go b/session.go
index 29d0b89e9..8cc840e47 100644
--- a/session.go
+++ b/session.go
@@ -181,6 +181,27 @@ func (s *session) sendLogonInReplyTo(setResetSeqNum bool, inReplyTo *Message) er
 		logon.Body.SetField(tagDefaultApplVerID, FIXString(s.DefaultApplVerID))
 	}
 
+	// Evaluate tag 789.
+	if s.EnableNextExpectedMsgSeqNum {
+		if inReplyTo != nil {
+			targetWantsNextSeqNumToBe, getErr := inReplyTo.Body.GetInt(tagNextExpectedMsgSeqNum)
+			if getErr == nil {
+				actualNextNum := s.store.NextSenderMsgSeqNum()
+				// // Is the 789 we received too high ??
+				if targetWantsNextSeqNumToBe > actualNextNum {
+					// we can't resend what we never sent! something unrecoverable has happened.
+					return RejectLogon{fmt.Sprintf("Tag 789 (NextExpectedMsgSeqNum) is higher than expected. Expected %d, Received %d", actualNextNum, targetWantsNextSeqNumToBe)}
+				}
+				nextSeqNum := s.store.NextTargetMsgSeqNum()
+				logon.Body.SetField(tagNextExpectedMsgSeqNum, FIXInt(nextSeqNum+1))
+			}
+		} else {
+			// We are sending a logon.
+			nextseqnum := s.store.NextTargetMsgSeqNum()
+			logon.Body.SetField(tagNextExpectedMsgSeqNum, FIXInt(nextseqnum+1))
+		}
+	}
+
 	if err := s.dropAndSendInReplyTo(logon, inReplyTo); err != nil {
 		return err
 	}
@@ -188,6 +209,31 @@ func (s *session) sendLogonInReplyTo(setResetSeqNum bool, inReplyTo *Message) er
 	return nil
 }
 
+func (session *session) generateSequenceReset(beginSeqNo int, endSeqNo int, inReplyTo Message) (err error) {
+	sequenceReset := NewMessage()
+	session.fillDefaultHeader(sequenceReset, &inReplyTo)
+
+	sequenceReset.Header.SetField(tagMsgType, FIXString("4"))
+	sequenceReset.Header.SetField(tagMsgSeqNum, FIXInt(beginSeqNo))
+	sequenceReset.Header.SetField(tagPossDupFlag, FIXBoolean(true))
+	sequenceReset.Body.SetField(tagNewSeqNo, FIXInt(endSeqNo))
+	sequenceReset.Body.SetField(tagGapFillFlag, FIXBoolean(true))
+
+	var origSendingTime FIXString
+	if err := sequenceReset.Header.GetField(tagSendingTime, &origSendingTime); err == nil {
+		sequenceReset.Header.SetField(tagOrigSendingTime, origSendingTime)
+	}
+
+	session.application.ToAdmin(sequenceReset, session.sessionID)
+
+	msgBytes := sequenceReset.build()
+
+	session.EnqueueBytesAndSend(msgBytes)
+	session.log.OnEventf("Sent SequenceReset TO: %v", endSeqNo)
+
+	return
+}
+
 func (s *session) buildLogout(reason string) *Message {
 	logout := NewMessage()
 	logout.Header.SetField(tagMsgType, FIXString("5"))
@@ -471,6 +517,8 @@ func (s *session) handleLogon(msg *Message) error {
 		}
 	}
 
+	nextSenderMsgNumAtLogonReceived := s.store.NextSenderMsgSeqNum()
+
 	if resetStore {
 		if err := s.store.Reset(); err != nil {
 			return err
@@ -499,6 +547,23 @@ func (s *session) handleLogon(msg *Message) error {
 	s.peerTimer.Reset(time.Duration(float64(1.2) * float64(s.HeartBtInt)))
 	s.application.OnLogon(s.sessionID)
 
+	// Evaluate tag 789 to see if we end up with an implied gapfill/resend.
+	if s.EnableNextExpectedMsgSeqNum && !msg.Body.Has(tagResetSeqNumFlag) {
+			targetWantsNextSeqNumToBe, getErr := msg.Body.GetInt(tagNextExpectedMsgSeqNum)
+			if getErr == nil {
+					if targetWantsNextSeqNumToBe != nextSenderMsgNumAtLogonReceived {
+						if !s.DisableMessagePersist {
+							seqResetErr := s.generateSequenceReset(targetWantsNextSeqNumToBe, nextSenderMsgNumAtLogonReceived+1, *msg)
+							if seqResetErr != nil {
+								return seqResetErr
+							}
+						} else {
+							return targetTooHigh{ReceivedTarget: targetWantsNextSeqNumToBe, ExpectedTarget: nextSenderMsgNumAtLogonReceived}
+						}
+					}
+			}
+	}
+
 	if err := s.checkTargetTooHigh(msg); err != nil {
 		return err
 	}
diff --git a/session_factory.go b/session_factory.go
index d875762de..effe6ed35 100644
--- a/session_factory.go
+++ b/session_factory.go
@@ -196,6 +196,12 @@ func (f sessionFactory) newSession(
 		}
 	}
 
+	if settings.HasSetting(config.EnableNextExpectedMsgSeqNum) {
+		if s.EnableNextExpectedMsgSeqNum, err = settings.BoolSetting(config.EnableNextExpectedMsgSeqNum); err != nil {
+			return
+		}
+	}
+
 	if settings.HasSetting(config.CheckLatency) {
 		var doCheckLatency bool
 		if doCheckLatency, err = settings.BoolSetting(config.CheckLatency); err != nil {
diff --git a/tag.go b/tag.go
index 01b28356f..909f8e5d0 100644
--- a/tag.go
+++ b/tag.go
@@ -44,6 +44,7 @@ const (
 	tagXMLData                Tag = 213
 	tagMessageEncoding        Tag = 347
 	tagLastMsgSeqNumProcessed Tag = 369
+	tagNextExpectedMsgSeqNum  Tag = 789
 	tagOnBehalfOfSendingTime  Tag = 370
 	tagApplVerID              Tag = 1128
 	tagCstmApplVerID          Tag = 1129