add en/new_protocol.md

docs/cn/new_protocol.md

@@ -2,7 +2,7 @@
 
 brpc server在同端口支持所有的协议，大部分时候这对部署和运维更加方便。由于不同协议的格式大相径庭，严格地来说，同端口很难无二义地支持所有协议。出于解耦和可扩展性的考虑，也不太可能集中式地构建一个针对所有协议的分类器。我们的做法就是把协议归三类后逐个尝试：
 
-- 第一类协议：标记或特殊字符在最前面，比如[baidu_std](baidu_std.md)，hulu_pbrpc的前4个字符分别分别是PRPC和HULU，解析代码只需要检查前4个字节就可以知道协议是否匹配，最先尝试这类协议。这些协议在同一个连接上也可以共存。
+- 第一类协议：标记或特殊字符在最前面，比如[baidu_std](baidu_std.md)，hulu_pbrpc的前4个字符分别是PRPC和HULU，解析代码只需要检查前4个字节就可以知道协议是否匹配，最先尝试这类协议。这些协议在同一个连接上也可以共存。
 - 第二类协议：有较为复杂的语法，没有固定的协议标记或特殊字符，可能在解析一段输入后才能判断是否匹配，目前此类协议只有http。
 - 第三类协议：协议标记或特殊字符在中间，比如nshead的magic_num在第25-28字节。由于之前的字段均为二进制，难以判断正确性，在没有读取完28字节前，我们无法判定消息是不是nshead格式的，所以处理起来很麻烦，若其解析排在http之前，那么<=28字节的http消息便可能无法被解析，因为程序以为是“还未完整的nshead消息”。
 

docs/en/new_protocol.md

+# Multi-protocol support in server side
+
+brpc server supports all protocols in the same port, and it makes deployment and maintenance more convenient in most of the time. Since the format of different protocols is very different, it is hard to support all protocols in the same port unambiguously. In consider of decoupling and extensibility, it is also hard to build a multiplexer for all protocols. Thus our way is to classify all protocols into three categories and try one by one:
+
+- First-class protocol: Special characters are marked in front of the protocol data, for example, the data of protocol [baidu_std](baidu_std.md) and hulu_pbrpc begins with 'PRPC' and 'HULU' respectively. Parser just check first four characters to know whether the protocol is matched. This class of protocol is checked first and all these protocols can share one TCP connection.
+- Second-class protocol: Some complex protocols without special marked characters can only be detected after several input data are parsed. Currently only HTTP is classified into this category.
+- Third-class protocol: Special characters are in the middle of the protocol data, such as the magic number of nshead protocol is the 25th-28th characters. It is complex to handle this case because without reading first 28 bytes, we cannot determine whether the protocol is nshead. If it is tried before http, http messages less than 28 bytes may not be parsed, since the parser consider it as an uncomplete nshead message.
+
+Considering that there will be only one protocol in most connections, we record the result of last selection so that it will be tried first when further data comes. It reduces the overhead of matching protocols to nearly zero for long connections. Although the process of matching protocols will be run everytime for short connections, the bottleneck of short connections is not in here and this method is still fast enough. If there are lots of new protocols added into brpc in the future, we may consider some heuristic methods to match protocols.
+
+# Multi-protocol support in client side
+