anchor.diff:          pkt = STLPktBuilder(pkt = base_pkt/pad,
                               vm = vm)
   ----
  -<1> Define struct with two dependent variables: tuple.ip, tuple.port 
  -<2> Write tuple.ip variable to `IPv4.src` field offset
  -<3> Write tuple.port variable to `UDP.sport` field offset. You should set UDP.checksum to zero  
  -
  -
  -.Pcap file output 
  +<1> Defines a struct with two dependent variables: tuple.ip, tuple.port
  +<2> Writes the tuple.ip variable to `IPv4.src` field offset.
  +<3> Writes the tuple.port variable to `UDP.sport` field offset. Set UDP.checksum to 0.
  +// Hanoch: add how to set UDP.checksum to 0
  +
  +
  +.Output - pcap file 
   [format="csv",cols="1^,2^,1^", options="header",width="40%"]
   |=================
   pkt,Client IPv4,Client Port
  
positive.code: 
  The following are the main traffic profile formats. Native is the preferred format. There is a separation between how the traffic is defined and how to control/activate it. The API/Console/GUI can load a traffic profile and start/stop/get a statistic. Due to this separation it is possible to share traffic profiles.
  
  .Traffic profiles formats
  [cols="1^,1^,10<", options="header",width="80%"]
  |=================
  | Profile Type       | Format | Description  
  | Native             | Python | Has the most flexibility. Any format can be converted to native using `stl-sim` using --native option
  | HLT                | Python | Uses HLT arguments 
  | YAML               | YAML   | It is the common denominator traffic profile. We suggest not to use it by human as it is not possible to compose packet using scapy. it is used to move a profile between GUI and Console or API. It can be converted to native using the stl-sim using --native switch 
  |=================
  
  
  
negative.code:                          mode = STLTXSingleBurst( pps = 1, total_pkts = 100) )
  
  ----
  <1> Defines a variable size of 2 bytes.
  <2> Writes the stream variable label with a shift of 12 bits, with a 20-bit MSB mask. Cast the stream variables of 2 bytes to 4 bytes.
  <3> Change the second MPLS header.
  
  
  ==== Tutorial: Field Engine, Random packet size 
  
  The following example demonstrates varies the packet size randomly, as follows:
  1. Defines the template packet with maximum size.
  2. Trims the packet to the size you want.
  3. Updates the packet fields according to the new size. 
  
  *file*:: link:{github_stl_path}/udp_rand_len_9k.py[stl/udp_rand_len_9k.py]
  
  