通信协议TLV的介绍及在python下的代码实现及仿真

　TLV协议是一种通讯协议，一般将数据封装成TLV的形式，即Tag，Length，Value。协议就是指通信双方对数据传输控制的一种规定，规定了数据格式，同步方式，传送速度，传送步骤的问题作出统一的规定。可以理解为两个节点之间为了协同工作，协商一定的规则和约定。例如我们会规定字节序，各个字段类型等。

TLV 是一种可变的格式，其中：

• T 可以理解为 Tag 或 Type ，用于标识标签或者编码格式信息；
• L 定义数值的长度；
• V 表示实际的数值。

T 和 L 的长度固定，一般是2或4个字节，V 的长度由 Length 指定。

图例帧格式如下所示：

由于用到这块，我就自己弄了一个python下的仿真代码，这里就给大家demo一下了：

　服务端:

import socket
import threading
import pickle

import time
from TLV import *
# 定义保存所有socket的列表
socket_list = []
# 创建socket对象
ss = socket.socket()
# 将socket绑定到本机IP和端口
ss.bind((‘localhost‘, 2333))
# 服务端开始监听来自客户端的连接
ss.listen()
tlv = TLV(t_ext=7, l_ext=7)

def server_target(s):
    try:
        # 采用循环不断地从socket中读取客户端发送过来的数据
        while True:
            line = input()
            if line is None or line == ‘exit‘:
                break

            time.sleep(2)
            tlv.add(8,line)
            data = pickle.dumps(tlv)
            s.send(data)
    except Exception:
        print(Exception.with_traceback())
while True:
    # 此行代码会阻塞，将一直等待别人的连接
    s, addr = ss.accept()
    #socket_list.append(s)
    # 每当客户端连接后启动一个线程为该客户端服务
    threading.Thread(target=server_target, args=(s, )).start()

   客户端：

import socket
import threading
import pickle
from TLV import *
# 创建socket对象
s = socket.socket()
# 连接远程主机
s.connect((‘localhost‘, 2333))

def read_from_server(s):
    try:
        data = pickle.loads(s.recv(2048))
        # test
        tlvp = TLVParser(data.buffer, t_ext=7, l_ext=7)
        for avp in tlvp.parse():
            print("%d(%d): %s" % (avp["type"], avp["length"], avp["value"]))
        # return s.recv(2048).decode(‘utf-8‘)
        return tlvp
    # 如果捕获到异常，则表明该socket对应的客户端已经关闭
    except:
        # 删除该socket
        socket_list.remove(s)   # ① 

def read_server(s):
    try:
        while True:
                contend = read_from_server(s)
                if contend is None:
                        break
    except:
        print(Exception.with_traceback())

# 客户端启动线程不断地读取来自服务器的数据
threading.Thread(target=read_server, args=(s, )).start()   # ①

　TLV的实现：

from scapy.all import *

class TLVError(Exception):
    pass


class TLV:
    def __init__(self, tl_in_l=False, t_ext=0, l_ext=0):
        self.buffer = ""
        self.tl_in_l = tl_in_l
        self.t_ext = t_ext
        self.l_ext = l_ext


    def _int(self, i, ext):
        maxi = 1<<8
        if ext > 0:
            maxi = (1 << ext)
        holdstr = ""
        holder = i 
        extend = 0 
        count = 1 
        while holder >= maxi:
            count += 1
            newnum = (holder & (maxi - 1)) 
            holdstr = chr(newnum | extend) + holdstr
            extend = maxi
            holder /= maxi

        holdstr = chr(holder | extend) + holdstr
        return holdstr

    def _t(self, t):
        if self.t_ext == 0 and t > 256:
            raise TLVError("type > 256 and no extension bit set")
        return self._int(t, self.t_ext)


    def _l(self, l):
        if self.l_ext == 0 and l > 256:
            raise TLVError("length > 256 and no extension bit set")
        return self._int(l, self.l_ext)


    def add(self, t, v, l=None):
        self.buffer += self._t(t)
        length = 0 if l is None else l

        if self.tl_in_l:
            length += t

        if l is None:
            length += len(v)

        self.buffer += self._l(length)
        self.buffer += v


    def __str__(self):
        return self.buffer

    def __repr__(self):
        return self.buffer


class TLVParser:
    def __init__(self, buffer, tl_in_l=False, t_ext=0, l_ext=0):
        self.buffer = buffer
        self.tl_in_l = tl_in_l
        self.t_ext = t_ext
        self.l_ext = l_ext
        self.offset = 0


    def _get_i(self, i_ext):
        try:
            byte = ord(self.buffer[self.offset])
        except IndexError:
            raise TLVError("Not enough data")
        ext = 1 << (i_ext if i_ext > 0 else 8)
        i = 0
        while byte & ext:
            i += (byte & (ext - 1))
            i <<= i_ext
            self.offset += 1
            try:
                byte = ord(self.buffer[self.offset])
            except IndexError:
                raise TLVError("Not enough data")
        i += byte
        self.offset += 1
        return i


    def _get_tlv(self):
        t = self._get_i(self.t_ext)
        l = self._get_i(self.l_ext)
        if self.offset + l > len(self.buffer):
            raise TLVError("Buffer not long enough to encompass TLV")
        v = self.buffer[self.offset:self.offset+l]
        self.offset += l
        return (t, l, v)


    def parse(self):
        while self.offset < len(self.buffer):
            t, l, v = self._get_tlv()
            yield {
                "type": t,
                "length": l,
                "value": v,
            }

# Test/example program for building TLVs and parsing the TLVs
if __name__ == "__main__":
    tlv = TLV(t_ext=7, l_ext=7)
    tlv.add(10, "Foobar")
    tlv.add(16, "Bladibla")
    # hexdump(tlv)
    tlvp = TLVParser(tlv.buffer, t_ext=7, l_ext=7)
    for avp in tlvp.parse():
        print ("%d(%d): %s" % (avp["type"], avp["length"], avp["value"]))

具体的代码运行结果就不贴了，对懂得python的同学来说，这个很简单的。