Python policy – GTPv2-C API

The system provides a Python object for input GTPv2-C packet: alc.gtp2c.

Shown in Table: GTPv2-C API , the alc.gtp2c packet has following attributes to represent the GTPv2-C header fields:

Table: GTPv2-C API
Class attributes	GTPv2-C header field	Access
alc.gtp2c.version	Integer version field in the header	Read
alc.gtp2c.pbit	boolean, true mean 1, false means 0; P bit in the header	Read
alc.gtp2c.tbit	boolean, T bit in the header	Read
alc.gtp2c.type	Integer message type field in the header	Read
alc.gtp2c.len	Integer message length field in the header	Read
alc.gtp2c.teid	Unsigned integer, or None if TEID does not exist	Read
alc.gtp2c.seq	Integer sequence number in the header	Read

The following is a list of alc.gtp2c functions:

alc.gtp2c.drop()

The system drops the resulting packet.
alc.gtp2c.getIEList()

The system returns a tuple of 2-element tuple (ie-type, ie-instance). Each 2-element tuple represent one IE instance in the packet. For example: a packet with the following IEs:
- type:2, instance: 1
- type:2, instance: 1
- type:74, instance: 0
  
  The system returns ((2,1),(2,1),(74,0))
- alc.gtp2c.get(ie_type, instance_id)
  
  The system returns a tuple of str. Each str is the IE data of specified (ie-type, instance-id). If the IE is a grouped IE, then the str is also the content of whole grouped IE. If the specified (ie_type, instance_id) does not exist, then return (); if the IE length=0, then return ‟” for this IE instance.
- alc.gtp2c.set(ie_type, instance_id, ie_tuple)
  
  This function removes all instances of (ie_type, instance_id) and inserts a list of new IE instances. Each element in an ie_tuple is a string, represent one instance of IE to be inserted. For each inserted IE instance, the type is ie_type, the instance is instance_id.
- alc.gtp2c.clear(ie_type, instance_id)
  
  This function removes all existing instances of (ie_type, instance-id).
- alc.gtp2c.str_to_groupedIE (parent_ie_str)
  
  Parses the parent_ie_str and return a tuple of embedded IEs. Parent_ie_str is a string of data field of parent IE; each element of returned tuple is a tuple of (ie_type, ie_instance, ie_data), which represent one embedded IE. For example, with the following code:
  - ie_str = alc.gtp2c.get(93,0)[0]
  - ie_tuple = alc.gtp2c.str_to_groupedIE(ie_str)
  The above code parses the first instance of the bearer context with an instance=0. Assume the bearer context looks like that shown in Figure: Bearer context example:
  
  Figure: Bearer context example
  
  Then, the ie_tuple is
  
  ((5,0,’\x05’),(87,5’\x9f\xfe\x50\x02\x25\x0a\xbe\xac\xfe’),(80,0,’\x04\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00’))
  
  An exception is raised if the system cannot parse the parent_ie_str into a ie_tuple.
- alc.gtp2c.groupedIE_to_str(grouped_ie_tuple)
  
  Converts the grouped_ie_tuple into a str and return the str (see the description of alc.gtp2c.str_to_groupedIE for the format grouped_ie_tuple). An exception is raised if the conversion fails. The following is an example:
  - ie_str = alc.gtp2c.get(93,0)[0]
  - ie_tuple = alc.gtp2c.str_to_groupedIE(ie_str)
  - fteid = list(ie_tuple[1])
  - fteid[1]=0
  - ie_str = alc.gtp2c.groupdedIE_to_str((ie_tuple[0],tuple(ftied),ie_tuple[2]))
  - alc.gtp2c.set(93,0,(ie_str,))
  With the previous example packet, the code changes the instance of the embedded F-TEID to 0.

Be aware that the alc.gtp2c.str_to_groupedIE (parent_ie_str) and alc.gtp2c.groupedIE_to_str(grouped_ie_tuple) do not check whether the input parameter is a grouped IE. The user should verify that the input parameter is a grouped IE before using these two functions.