How to parse the tcp data with fragments in lua

edit

I tried to write the Lua plugin, but it’s always Not right

local xnet_proto = Proto("SL", "SL Protolcol")

local XProto_wHeader = ProtoField.uint8("xnet.wHeader", "wHeader", base.HEX)
local XProto_xhToken = ProtoField.uint64("xnet.xhToken", "xhToken", base.DEC)
local XProto_Type = ProtoField.uint32("xnet.unOperatorType", "unOperatorType", base.DEC)
local XProto_Code = ProtoField.uint32("xnet.unOperatorCode", "unOperatorCode", base.HEX)
local XProto_Size = ProtoField.uint32("xnet.unPacketSize", "unPacketSize", base.DEC)
local XProto_wSerial = ProtoField.uint8("xnet.wPacketSerial", "wPacketSerial", base.DEC)
local XProto_wVersion = ProtoField.uint8("xnet.wVersion", "wVersion", base.DEC)
local XProto_wIsReply = ProtoField.uint8("xnet.wIsReply", "wIsReply", base.DEC)
local XProto_wTail = ProtoField.uint8("xnet.wTail", "wTail", base.HEX)
local XProto_Payload = ProtoField.bytes("xnet.Payload", "Payload", base.NONE)

xnet_proto.fields = 
{
    XProto_wHeader,
    XProto_xhToken,
    XProto_Type,
    XProto_Code,
    XProto_Size,
    XProto_wSerial,
    XProto_wVersion,
    XProto_wIsReply,
    XProto_wTail,
    XProto_Payload
}

function xnet_proto.dissector(tvb, pinfo, treeitem)

    pinfo.cols.protocol:set("SL")
    pinfo.cols.info:set("SL Protocol")

    local offset = pinfo.desegment_offset or 0
    local tvb_len = tvb:len()

    local xnet_tree = treeitem:add(xnet_proto, tvb:range(tvb_len))
    xnet_tree:add_le(XProto_wHeader, tvb(offset, 2))   
    offset = offset + 2
    xnet_tree:add_le(XProto_xhToken, tvb(offset, 8))
    offset = offset + 8
    xnet_tree:add_le(XProto_Type, tvb(offset, 4))
    offset = offset + 4
    xnet_tree:add_le(XProto_Code, tvb(offset, 4))
    offset = offset + 4
    xnet_tree:add_le(XProto_Size, tvb(offset, 4))
    offset = offset + 4
    xnet_tree:add_le(XProto_wSerial, tvb(offset, 2))
    offset = offset + 2
    xnet_tree:add_le(XProto_wVersion, tvb(offset, 2))
    offset = offset + 2
    xnet_tree:add_le(XProto_wIsReply, tvb(offset, 2))
    offset = offset + 2
    xnet_tree:add_le(XProto_wTail, tvb(offset, 2))
    offset = offset + 2

    offset = pinfo.desegment_offset or 0
    local frameLength = XProto_Size

    while true
    do
        local nextFrame = offset + frameLength;

        if  tvb:len() <  nextFrame then
            pinfo.desegment_len = nextFrame - tvb:len()
            return
        end
        if tvb:len() == nextFrame then 
            xnet_tree:add(XProto_Payload, XProto_Size)
            break
        end
    end
end

local tcp_port_table = DissectorTable.get("tcp.port")
--tcp_port_table:add(6000, xnet_proto)
--tcp_port_table:add(6001, xnet_proto)
tcp_port_table:add(6002, xnet_proto