diff --git a/lib/log-courier/client.rb b/lib/log-courier/client.rb
index 0a443a0c..b77eb1a8 100644
--- a/lib/log-courier/client.rb
+++ b/lib/log-courier/client.rb
@@ -30,21 +30,60 @@ class ProtocolError < StandardError; end
 
   # Describes a pending payload
   class PendingPayload
-    attr_accessor :ack_events
-    attr_accessor :events
-    attr_accessor :nonce
-    attr_accessor :data
+    class << self
+      @json_adapter
+      def get_json_adapter
+        @json_adapter = MultiJson.adapter.instance if @json_adapter.nil?
+        return @json_adapter
+      end
+    end
 
-    attr_accessor :previous
     attr_accessor :next
+    attr_accessor :nonce
+    attr_accessor :events
+    attr_accessor :last_sequence
+    attr_accessor :sequence_len
+    attr_accessor :payload
 
-    def initialize(options = {})
-      @ack_events = 0
+    def initialize(events, nonce)
+      @events = events
+      @nonce = nonce
+
+      generate
+    end
+
+    def generate
+      fail ArgumentError, 'Corrupt payload' if @events.length == 0
+
+      buffer = Zlib::Deflate.new
 
-      options.each do |k, v|
-        fail ArgumentError unless self.respond_to?(k)
-        instance_variable_set "@#{k}", v
+      # Write each event in JSON format
+      events.each do |event|
+        json_data = self.class.get_json_adapter.dump(event)
+        # Add length and then the data
+        buffer << [json_data.length].pack('N') << json_data
       end
+
+      # Generate and store the payload
+      @payload = nonce + buffer.flush(Zlib::FINISH)
+      @last_sequence = 0
+      @sequence_len = @events.length
+    end
+
+    def ack(sequence)
+      if sequence <= @last_sequence
+        return 0, false
+      elsif sequence >= @sequence_len
+        lines = @sequence_len - @last_sequence
+        @last_sequence = sequence
+        @payload = nil
+        return lines, true
+      end
+
+      lines = sequence - @last_sequence
+      @last_sequence = sequence
+      @payload = nil
+      return lines, false
     end
   end
 
@@ -63,9 +102,6 @@ def initialize(options = {})
       require 'log-courier/client_tls'
       @client = ClientTls.new(@options)
 
-      # Load the json adapter
-      @json_adapter = MultiJson.adapter.instance
-
       @event_queue = EventQueue.new @options[:spool_size]
       @pending_payloads = {}
       @first_payload = nil
@@ -100,7 +136,7 @@ def shutdown
       @io_thread.raise ShutdownSignal
       @spooler_thread.join
       @io_thread.join
-      return
+      return @pending_payloads.length == 0
     end
 
     private
@@ -274,14 +310,9 @@ def send_ping
     def send_jdat(events)
       # Generate the JSON payload and compress it
       nonce = generate_nonce
-      data = buffer_jdat_data(events, nonce)
 
       # Save the pending payload
-      payload = PendingPayload.new(
-        :events => events,
-        :nonce  => nonce,
-        :data   => data
-      )
+      payload = PendingPayload.new(events, nonce)
 
       @pending_payloads[nonce] = payload
 
@@ -294,27 +325,7 @@ def send_jdat(events)
       end
 
       # Send it
-      @client.send 'JDAT', payload.data
-      return
-    end
-
-    def buffer_jdat_data(events, nonce)
-      buffer = Zlib::Deflate.new
-
-      # Write each event in JSON format
-      events.each do |event|
-        buffer_jdat_data_event(buffer, event)
-      end
-
-      # Generate and return the message
-      nonce + buffer.flush(Zlib::FINISH)
-    end
-
-    def buffer_jdat_data_event(buffer, event)
-      json_data = @json_adapter.dump(event)
-
-      # Add length and then the data
-      buffer << [json_data.length].pack('N') << json_data
+      @client.send 'JDAT', payload.payload
       return
     end
 
@@ -322,6 +333,8 @@ def process_pong(message)
       # Sanity
       fail ProtocolError, "Unexpected data attached to pong message (#{message.length})" if message.length != 0
 
+      @logger.debug 'PONG message received' unless @logger.nil? || !@logger.debug?
+
       # No longer pending a PONG
       @ping_pending = false
       return
@@ -332,29 +345,27 @@ def process_ackn(message)
       fail ProtocolError, "ACKN message size invalid (#{message.length})" if message.length != 20
 
       # Grab nonce
-      sequence, nonce = message[0...4].unpack('N').first, message[4..-1]
+      nonce, sequence = message.unpack('A16N')
+
+      if !@logger.nil? && @logger.debug?
+        nonce_str = nonce.each_byte.map do |b|
+          b.to_s(16).rjust(2, '0')
+        end
+
+        @logger.debug 'ACKN message received', :nonce => nonce_str.join, :sequence => sequence
+      end
 
       # Find the payload - skip if we couldn't as it will just a duplicated ACK
       return unless @pending_payloads.key?(nonce)
 
       payload = @pending_payloads[nonce]
+      lines, complete = payload.ack(sequence)
 
-      # Full ACK?
-      # TODO: protocol error if sequence too large?
-      if sequence >= payload.events.length
-        @client.resume_send if @client.send_paused?
-
+      if complete
         @pending_payloads.delete nonce
-        payload.previous.next = payload.next
-      else
-        # Partial ACK - only process if something was actually processed
-        if sequence > payload.ack_events
-          payload.ack_events = sequence
-          payload.events = payload.events[0...sequence]
-          payload.data = nil
-        end
+        @first_payload = payload.next
+        @client.resume_send if @client.send_paused?
       end
-      return
     end
   end
 end
diff --git a/lib/log-courier/client_tls.rb b/lib/log-courier/client_tls.rb
index c2122685..67439e7e 100644
--- a/lib/log-courier/client_tls.rb
+++ b/lib/log-courier/client_tls.rb
@@ -89,6 +89,25 @@ def send(signature, message)
       return
     end
 
+    def pause_send
+      return if @send_paused
+      @send_paused = true
+      @send_q << nil
+      return
+    end
+
+    def send_paused?
+      @send_paused
+    end
+
+    def resume_send
+      if @send_paused
+        @send_paused = false
+        @send_q << nil
+      end
+      return
+    end
+
     private
 
     def run_send(io_control)
@@ -171,25 +190,6 @@ def run_recv(io_control)
       return
     end
 
-    def pause_send
-      return if @send_paused
-      @send_paused = true
-      @send_q << nil
-      return
-    end
-
-    def send_paused
-      @send_paused
-    end
-
-    def resume_send
-      if @send_paused
-        @send_paused = false
-        @send_q << nil
-      end
-      return
-    end
-
     def tls_connect
       # TODO: Implement random selection - and don't use separate :port - remember to update post_connection_check too
       address = @options[:addresses][0]
diff --git a/spec/gem_spec.rb b/spec/gem_spec.rb
index a642749a..aeb3da2c 100644
--- a/spec/gem_spec.rb
+++ b/spec/gem_spec.rb
@@ -64,6 +64,6 @@ def shutdown
       i += 1
     end
 
-    shutdown
+    expect(shutdown).to eq true
   end
 end