diff --git a/lib/net/protocol.rb b/lib/net/protocol.rb
index 822bc00..c676854 100644
--- a/lib/net/protocol.rb
+++ b/lib/net/protocol.rb
@@ -120,6 +120,7 @@ def initialize(io, read_timeout: 60, write_timeout: 60, continue_timeout: nil, d
       @continue_timeout = continue_timeout
       @debug_output = debug_output
       @rbuf = ''.b
+      @rbuf_offset = 0
     end
 
     attr_reader :io
@@ -154,14 +155,15 @@ def read(len, dest = ''.b, ignore_eof = false)
       LOG "reading #{len} bytes..."
       read_bytes = 0
       begin
-        while read_bytes + @rbuf.size < len
-          s = rbuf_consume(@rbuf.size)
-          read_bytes += s.size
-          dest << s
+        while read_bytes + rbuf_size < len
+          if s = rbuf_consume_all_shareable!
+            read_bytes += s.bytesize
+            dest << s
+          end
           rbuf_fill
         end
         s = rbuf_consume(len - read_bytes)
-        read_bytes += s.size
+        read_bytes += s.bytesize
         dest << s
       rescue EOFError
         raise unless ignore_eof
@@ -175,9 +177,10 @@ def read_all(dest = ''.b)
       read_bytes = 0
       begin
         while true
-          s = rbuf_consume(@rbuf.size)
-          read_bytes += s.size
-          dest << s
+          if s = rbuf_consume_all_shareable!
+            read_bytes += s.bytesize
+            dest << s
+          end
           rbuf_fill
         end
       rescue EOFError
@@ -188,14 +191,16 @@ def read_all(dest = ''.b)
     end
 
     def readuntil(terminator, ignore_eof = false)
+      offset = @rbuf_offset
       begin
-        until idx = @rbuf.index(terminator)
+        until idx = @rbuf.index(terminator, offset)
+          offset = @rbuf.bytesize
           rbuf_fill
         end
-        return rbuf_consume(idx + terminator.size)
+        return rbuf_consume(idx + terminator.bytesize - @rbuf_offset)
       rescue EOFError
         raise unless ignore_eof
-        return rbuf_consume(@rbuf.size)
+        return rbuf_consume
       end
     end
 
@@ -208,12 +213,16 @@ def readline
     BUFSIZE = 1024 * 16
 
     def rbuf_fill
-      tmp = @rbuf.empty? ? @rbuf : nil
+      tmp = @rbuf_empty ? @rbuf : nil
       case rv = @io.read_nonblock(BUFSIZE, tmp, exception: false)
       when String
-        return if rv.equal?(tmp)
-        @rbuf << rv
-        rv.clear
+        @rbuf_empty = false
+        if rv.equal?(tmp)
+          @rbuf_offset = 0
+        else
+          @rbuf << rv
+          rv.clear
+        end
         return
       when :wait_readable
         (io = @io.to_io).wait_readable(@read_timeout) or raise Net::ReadTimeout.new(io)
@@ -228,13 +237,50 @@ def rbuf_fill
       end while true
     end
 
-    def rbuf_consume(len)
-      if len == @rbuf.size
+    def rbuf_flush
+      if @rbuf_empty
+        @rbuf.clear
+        @rbuf_offset = 0
+      end
+      nil
+    end
+
+    def rbuf_size
+      @rbuf.bytesize - @rbuf_offset
+    end
+
+    # Warning: this method may share the buffer to avoid
+    # copying. The caller must no longer use the returned
+    # string once rbuf_fill has been called again
+    def rbuf_consume_all_shareable!
+      @rbuf_empty = true
+      buf = if @rbuf_offset == 0
+        @rbuf
+      else
+        @rbuf.byteslice(@rbuf_offset..-1)
+      end
+      @rbuf_offset = @rbuf.bytesize
+      buf
+    end
+
+    def rbuf_consume(len = nil)
+      if @rbuf_offset == 0 && (len.nil? || len == @rbuf.bytesize)
         s = @rbuf
         @rbuf = ''.b
+        @rbuf_offset = 0
+        @rbuf_empty = true
+      elsif len.nil?
+        s = @rbuf.byteslice(@rbuf_offset..-1)
+        @rbuf = ''.b
+        @rbuf_offset = 0
+        @rbuf_empty = true
       else
-        s = @rbuf.slice!(0, len)
+        s = @rbuf.byteslice(@rbuf_offset, len)
+        @rbuf_offset += len
+        @rbuf_empty = @rbuf_offset == @rbuf.bytesize
+        rbuf_flush
       end
+
       @debug_output << %Q[-> #{s.dump}\n] if @debug_output
       s
     end