Pisces: a programmable, protocol independent software switch (summary)

PISCES: A Programmable, Protocol-Independent Software Switch

Importance

The virtual machine monitor uses a software switch to guide packets in and out of the virtual machine . These switches often need to be upgraded and customized , To support new protocol headers or encapsulation for tunneling and coverage , To improve the measurement and commissioning functions , Even add functions similar to intermediate devices . Software switches are usually based on a lot of code , Including kernel code , Changing switches is a daunting task , Need to be proficient in the design and development of network protocols 、 test , And maintaining a large and complex code base . Changing the way software switches forward packets does not require an in-depth understanding of the current situation . contrary , It should be possible to specify how to use high-level domain specific languages (DSL)( Such as P4) Process and forward packets , And compile it to run on the software switch . The assessment shows that ,PISCES Performance and OVS Quite a ,PISCES The length of the program is about OVS Source code 1/40.

Challenge

How to realize and optimize

programme

The forwarding behavior of software switch is specified in domain specific language . This example is based on Open vSwitch (OVS) Software switches , Use P4 Domain specific language . Implementation of protocol independent software switches .
The benefits of the scheme are ：

• Add a new standard or private protocol header ： Data center owners can compile and deploy new protocols faster
• Delete the standard protocol header ： Deleting unused protocols is as simple as deleting unused parts of the protocol specification and recompiling the switch source code
• Better visualization ： Can quickly modify forwarding and monitoring logic , It can reduce the time of diagnosing and repairing network faults .
• New functions can be added ： Users and network owners can modify forwarding behavior , Will improve their network faster .
  This example is right OVS Improved
• Arbitrary encapsulation and unpacking ：OVS I won't support it P4 Any encapsulation and de encapsulation that the program may need . So two new primitives are added , Encapsulation and de encapsulation .
• Conditions based on header field comparison ：OpenFlow Only the bitwise equality test of header field is directly supported . stay OVS Added direct support for unequal relationships , In the form of conditional operation .
• General checksum verification / to update ：OVS Only incremental checksum is supported , But in order to support the use of checksums in the way programmers want , The incremental verification is optimized .
  And this example optimizes the back-end of the compiler ：
• Inline editor / Pipeline editing : The optimizer analyzes the intermediate representation , To determine how many times a package may need to be modified in the pipeline . If the value is below a certain threshold , Then the optimizer performs inline editing ; otherwise , It will perform post pipeline editing .
• Motion optimization ： Use optimizations such as dead code elimination to identify and eliminate redundancy in setup operations
• Incremental checksum ： By using high-level programs to describe ( Such as P4) To represent the checksum operation , Programmers can provide the compiler with the necessary context information , So as to realize checksum more effectively
• Parser optimization ： Specifying forwarding behavior in a high-level language can provide the compiler with information that can be used to optimize the parser .
• Action merge ： Through analysis P4 Control flow and matching operation processing in the program , The compiler can find out which fields have actually been modified , And can generate an effective 、 A single operation to update these fields directly .
• Cache field modification ： Through automatic generation operation P4 Specific arithmetic operations required by the program , Avoid matching and setting when possible
• Assign in stages ： Enhanced P4 Language , Enables the user to annotate each header file with a phase number

opinion

Designed and implemented the first one to allow in advanced DSL A software switch that customizes the protocol specification without directly modifying the switch source code . The average ratio is equivalent to OVS The source code is about 40 times , And forwarding performance ( Throughput ) The cost is only 2% about . Prove that in the virtual machine monitor switch , The cost of programmability is negligible , The research results can inspire the design of new protocol independent software switches .