VPP API Language {#api_lang_doc}

The VPP binary API is a message passing API. The VPP API language is used to define a RPC interface between VPP and its control plane. The API messages supports shared memory transport and Unix domain sockets (SOCK_STREAM).

The wire format is essentially that of a network formatted (big-endian) packed C struct.

The VPP API compiler is located in src/tools/vppapigen and can currently compile to JSON or C (used by the VPP binary itself).

Language definition

Defining a messages

There are 3 types of message exchanges:

  • Request/Reply The client sends a request message and the server replies with a single reply message. The convention is that the reply message is named as method_name + _reply.

  • Dump/Detail The client sends a “bulk” request message to the server, and the server replies with a set of detail messages. These messages may be of different type. A dump/detail call must be enclosed in a control ping block (Otherwise the client will not know the end of the bulk transmission). The method name must end with method + “_dump”, the reply message should be named method + “_details”. The exception here is for the methods that return multiple message types (e.g. sw_interface_dump). The Dump/Detail methods are typically used for acquiring bulk information, like the complete FIB table.

  • Events The client can register for getting asynchronous notifications from the server. This is useful for getting interface state changes, and so on. The method name for requesting notifications is conventionally prefixed with “want_”. E.g. “want_interface_events”. Which notification types results from an event registration is defined in the service definition.

A message from a client must include the ‘client_index’, an opaque cookie identifying the sender, and a ‘context’ field to let the client match request with reply.

An example of a message definition. The client sends the show_version request, the server replies with the show_version_reply.

The client_index and context fields are required in all requests. The context is returned by the server and is used by the client to match up request and reply messages.

define show_version
  u32 client_index;
  u32 context;
define show_version_reply
  u32 context;
  i32 retval;
  string program [32];
  string version [32];
  string build_date [32];
  /* The final field can be a variable length argument */
  string build_directory [];

The flags are not used by the clients, but have special meaning for some of the tracing and debugging of the API. The autoreply flag is a shorthand for a reply message with just a retval field.

    define : DEFINE ID '{' block_statements_opt '}' ';'
    define : flist DEFINE ID '{' block_statements_opt '}' ';'
    flist : flag
          | flist flag
    flag : MANUAL_PRINT
         | MANUAL_ENDIAN
         | DONT_TRACE
         | AUTOREPLY

    block_statements_opt : block_statements
    block_statements : block_statement
                     | block_statements block_statement
    block_statement : declaration
                    | option
    declaration : type_specifier ID ';'
                | type_specifier ID '[' ID '=' assignee ']' ';'
    declaration : type_specifier ID '[' NUM ']' ';'
                | type_specifier ID '[' ID ']' ';'
    type_specifier : U8
                   | U16
                   | U32
                   | U64
                   | I8
                   | I16
                   | I32
                   | I64
                   | F64
                   | BOOL
                   | STRING
    type_specifier : ID


The option word is used to specify meta information. The only current use is to specify a semantic version of the .api file itself.


option version = "1.0.0";

    option : OPTION ID '=' assignee ';'
    assignee : NUM
             | TRUE
             | FALSE
             | STRING_LITERAL

Defining new types

New user defined types are defined just like messages. A typedef has two forms. It can either define an alias for a different type (or array).


typedef u8 ip4_address[4];
typedef u8 ip6_address[16];

Where the above defines two new types vl_api_ip4_address_t and vl_api_ip6_address_t. These are aliases for the underlying u8 array.

In the other form, it is used to specify an abstract data type.

enum address_family {
  ADDRESS_IP4 = 0,

union address_union {
  vl_api_ip4_address_t ip4;
  vl_api_ip6_address_t ip6;

typedef address {
  vl_api_address_family_t af;
  vl_api_address_union_t un;

Where the new type vl_api_address_t

    typedef : TYPEDEF ID '{' block_statements_opt '}' ';'
    typedef : TYPEDEF declaration

Importing Definitions

You can use definitions from other .api files by importing them. To import another .api’s definitions, you add an import statement to the top of your file:

import “vnet/ip/ip_types.api”;

By default you can only use definitions from directly imported .api files.

The API compiler searches for imported files in a set of directories specified on the API compiler command line using the –includedir flag.



The API language uses C style comments.

/* */


Enums are similar to enums in C.

Every enum definition must contain a constant that maps to zero as its first element. This is because:

There must be a zero value, so that we can use 0 as a numeric default value. The zero value needs to be the first element.

As in C, enums can be used as flags or just as numbers. The on-wire, and in memory representation size of an enum can be specified. Not all language bindings will support that. The default size is 4 (u32).


enum ip_neighbor_flags

Which generates the vl_api_ip_neighbor_flags_t in the C binding. In Python that is represented as an IntFlag object VppEnum.vl_api_ip_neighbor_flags_t.

    enum : ENUM ID '{' enum_statements '}' ';'
    enum : ENUM ID ':' enum_size '{' enum_statements '}' ';'
    enum_size : U8
              | U16
              | U32
    enum_statements : enum_statement
                    | enum_statements enum_statement
    enum_statement : ID '=' NUM ','
                   | ID ','


The service statement defines the relationship between messages. For request/response and dump/details messages it ties the request with the reply. For events, it specifies which events that can be received for a given want_* call.


service {
  rpc want_interface_events returns want_interface_events_reply
    events sw_interface_event;

Which states that the request want_interface_events returns a want_interface_events_reply and if enabled the client will receive sw_interface_event messages whenever interface states changes.

    service : SERVICE '{' service_statements '}' ';'
    service_statements : service_statement
                    | service_statements service_statement
    service_statement : RPC ID RETURNS NULL ';'
                         | RPC ID RETURNS ID ';'
                         | RPC ID RETURNS STREAM ID ';'
                         | RPC ID RETURNS ID EVENTS event_list ';'
    event_list : events
               | event_list events
    events : ID
           | ID ','


Scalar Value Types

.api type|size|C type|Python type ———|—-|——|———– i8 | 1|i8 |int u8 | 1|u8 |int i16 | 2|i16 |int u16 | 2|u16 |int i32 | 4|i32 |int u32 | 4|u32 |int i64 | 8|i64 |int u64 | 8|u64 |int f64 | 8|f64 |float bool | 1|bool |boolean string |variable|vl_api_string_t|str

User Defined Types


.api type|size|C type|Python type ———|—-|——|———– vl_api_address_t|20|vl_api_address_t|<class 'ipaddress.IPv4Address'> or <class 'ipaddress.IPv6Address'> vl_api_ip4_address_t|4|vl_api_ip4_address_t|<class 'ipaddress.IPv4Address'> vl_api_ip6_address_t|16|vl_api_ip6_address_t|<class 'ipaddress.IPv6Address'> vl_api_prefix_t|21|vl_api_prefix_t|<class 'ipaddress.IPv4Network'> or <class 'ipaddress.IPv6Network'> vl_api_ip4_prefix_t|5|vl_api_ip4_prefix_t|<class 'ipaddress.IPv4Network'> vl_api_ip6_prefix_t|17|vl_api_ip6_prefix_t|<class 'ipaddress.IPv6Network'> vl_api_ip4_address_with_prefix_t|5|vl_api_ip4_address_with_prefix_t|<class 'ipaddress.IPv4Interface'> vl_api_ip6_address_with_prefix_t|17|vl_api_ip6_address_with_prefix_t|<class 'ipaddress.IPv6Interface'>


.api type|size|C type|Python type ———|—-|——|———– vl_api_mac_address_t|6|vl_api_mac_address_t|class 'vpp_papi.MACAddress'>


.api type|size|C type|Python type ———|—-|——|———– vl_api_interface_index_t|4|vl_api_interface_index_t|int

New explicit types

String versus bytes

A byte string with a maximum length of 64:

u8 name[64];

Before the “string” type was added, text string were defined like this. The implications of that was the user would have to know if the field represented a \0 ended C-string or a fixed length byte string. The wire format of the ‘string’ type is a u32 length

An IPv4 or IPv6 address was previously defined like:

u8 is_ip6;
u8 address[16];

Which made it hard for language bindings to represent the address as anything but a byte string. The new explicit address types are shown above.

Language generators

The VPP API compiler currently has two output modules. One generating JSON and one generating C header files that are directly used by the VPP infrastructure and plugins.

The C/C++, Python, Go Lua, and Java language bindings are generated based on the JSON files.

Future considerations

  • [ ] Generate C/C++ (vapi) client code directly from vppapigen

  • [ ] Embed JSON definitions into the API server, so dynamic languages can download them directly without going via the filesystem and JSON files.