P4tutorial实战

Tutorial样例实战


  • GitHub仓库地址
  • 参考博客

实验一:SIGCOMM_2015/Sourse_Routing

实验环境:

  • OS:Ubuntu16.04
  • bmv2:behavioral-model
  • p4c-bm:p4c-bm

实验准备:

  • 认真阅读readme文件,按照提示做下去!
  • 修改环境变量脚本env.sh,该脚本记录了bmv2和p4c-bm的位置。
  • 我的脚本信息如下:
THIS_DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )

# ---------------- EDIT THIS ------------------
BMV2_PATH=/home/liuhy/behavioral-model
# e.g. BMV2_PATH=$THIS_DIR/../bmv2
P4C_BM_PATH=/home/liuhy/p4c-bm
# e.g P4C_BM_PATH=$THIS_DIR/../p4c-bm
# ---------------- END ------------------
  • 其实就是注明bmv2的路径和p4c-bm的路径,为了保险起见我选择了绝对路径

实验描述

  • 本次实验是为了实现一个名为Sourse_Routing的协议,其协议格式如下:
preamble (8 bytes) | num_valid (4 bytes) | port_1 (1 byte) | port_2 (1 byte) | ... | port_n (1 byte) | payload
  • 其中各字段含义为:
    • preamble:8bytes,恒为0,用于区分Sourse_Routing协议的数据包,为parser的编码提供了思路。
    • num_valid:4bytes,可用端口数,这各字段的值代表头部中端口字段的数目,假定一个Sourse_Routing数据包要通过三个switch,把它的num_valid字段设置为3,意味着端口列表长度为3bytes。每经过一个switch,该字段值减1并移除第一个端口字段,第一个端口字段表明该数据包的出端口。为action提供了编码思路。
    • payload:该字段代表我们要发送的信息,目前不用管。
  • 举个例子,如果想利用Sourse_Routing协议从h1主机发送hello p4到主机h3,根据本次实验拓扑,可以设置数据包格式为:
00000000 00000000 | 00000002 | 03 | 01 | Hello
  • 经过sw1转发后:
00000000 00000000 | 00000001 | 01 | Hello
  • 经过sw3转发后:
00000000 00000000 | 00000000 | Hello
  • 注意:最后一个交换机不能移除Easyroute头部,因为这会导致主机无法解析数据包。而其他交换机可以,因为它会把数据包给交换机而不是主机,交换机会自动丢弃该数据包。

实验要求

  • 非Easyroute数据包直接丢弃,无Easyroute头部或者其他协议数据包。
  • num_vaild字段值为0的数据包直接丢弃。

Tips

  1. in the start parse state, you can use current() to check if the packet is
    an EasyRoute packet. A call to current(0, 64) will examine the first 64 bits
    of the packet, without shifting the packet pointer.
  2. do not forget that a table can match on the validity of a header. Furthermore
    if a header is not valid, our software switch will set all its fields to 0.
  3. a table can "match" on an empty key, which means the default action will
    always be executed - if configured correctly by the runtime. Just omit the
    "reads" attribute to achieve this.
  4. you can remove a header with a call to remove_header()
  5. when parsing the EasyRoute header, you do not have to parse the whole port
    list. Actually P4 is currently missing language constructs needed to parse a
    general Type-Length-Value style header1, and hence
    you’ll need to simply extract the first port of the list and ignore the rest
    (including the payload). Also preamble, num_valid and the port number don't have
    to all be placed in the same header type.
  6. finally, we advise you to put all your logic in the ingress control flow and
    leave the egress empty. You will not need more than 1 or 2 tables to implement
    EasyRoute.

填充流表

  • readme中给出了必要的两个bmv2命令:
    • table_set_default [action_data]: this is used to
      set the default action of a given table
    • table_add => [action_data]: this
      is used to add an entry to a table

Run!

P4tutorial实战_第1张图片

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