ARS Packet Description system

  This document describes the APD format. APD is a way to describe TCP/IP
  packets, and it is used in high level functions of the ARS library.
  The general format is the following:

    layer_type{field_1=value_1,field_2=value_2,...,field_n=value_n}

  more layers can be combined using the "+" simbol. Example:

    ip{dst=192.168.1.2}+udp{sport=53,dport=53}+data{file=./dns.packet}

  You don't need to specify fields that ARS can guess. For example
  if you don't specify checksums they will be correctly generated
  in the process of packet compilation.

  AVAILABLE LAYERS
  ~~~~~~~~~~~~~~~~

  A layer type is one of the following:

    ip            IP header
    ipopt.eol        IP option EOL
    ipopt.nop        IP option NOP
    ipopt.sec        IP option Security
    ipopt.sid        IP option Stream ID
    ipopt.lsrr        IP option Loose Source Routing
    ipopt.ssrr        IP option Strict Source Routing
    ipopt.rr        IP option Record Route
    ipopt.ts        IP option Timestamp
    udp            UDP header
    tcp            TCP header
    tcpopt.eol        TCP option END (EOL)
    tcpopt.nop        TCP option NOP
    tcpopt.mss        TCP option Max Segment Size
    tcpopt.wscale        TCP option Window Scale
    tcpopt.sackperm        TCP option Selective ACK permitted
    tcpopt.sack        TCP option Selective ACK
    tcpopt.echoreq        TCP option Echo Request
    tcpopt.echoreply    TCP option Echo Reply
    tcpopt.ts        TCP option Timestamp
    icmp            ICMP header
    data            Generic Data

  Different fields are defined for different layer types:

  IP FIELDS:    DESCRIPTION:        POSSIBLE VALUE:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  saddr        Source address        192.168.1.2, or www.yahoo.com
  daddr        Destination address    192.168.1.2, or www.yahoo.com
  ihl        IP header len        numerical value
  ver        IP version        numerical value
  tos        Type of Service        numerical value
  totlen    IP tot len        numerical value
  id        IP packet ID        numerical value
  fragoff    IP fragment offset    numerical vaule
  mf        More Fragment        0 or 1
  df        Dont Fragment        0 or 1
  rf        Reserved Frag. bit    0 or 1
  ttl        Time to Live        numerical value
  proto        ip protocol field    numerical value
  cksum        ip checksum        numerical value

  UDP FIELDS:    DESCRIPTION:        POSSIBLE VALUE:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  sport        Source port        numerical value
  dport        Destination port    numerical value
  len        UDP len field        numerical value
  cksum        UDP checksum        numerical value

  TCP FIELDS:    DESCRIPTION:        POSSIBLE VALUE:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  sport        Source port        numerical value
  dport        Destination port    numerical value
  seq        TCP sequence number    numerical value
  ack        TCP acknowledge number    numerical value
  x2        TCP reserved bits    numerical value
  off        TCP header size        numerical value
  flags        TCP flags        FSRPAUXY (see the example)
  win        TCP window        numerical value
  cksum        TCP checksum        numerical value
  urp        TCP urgent pointer    numerical value

  ICMP FIELDS:    DESCRIPTION:        POSSIBLE VALUE:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  type        ICMP type        numerical value
  code        ICMP code        numerical value
  cksum        ICMP cksum        numerical value
  id        ICMP echo ID        numerical value
  seq        ICMP echo sequence nr  numerical value
  gw        ICMP gateway        192.168.1.2 or www.yahoo.com
  unused    ICMP 32bit id/seq/gw    numerical value

  DATA FIELDS:    DESCRIPTION:        POSSIBLE VALUE:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  file        Data file        /etc/passwd
  str        A string        hello world! (no escaping available)
  uint32    A 32bit integer        numerical value
  uint24    A 24bit integer        numerical value
  uint16    A 16bit integer        numerical value
  uint8        A 8bit integer        numerical value

  Other layer types fields aren't still implemented, anyway
  most of this contains sane defaults, (like IP record route option
  and so on).

  You can specify numerical values as hex, octal and decimal numbers.

  Decimail:    10
  Hex:        0xA
  Octal:    012

  Examples
  ~~~~~~~~

  /* Just an ICMP echo request */
  ip{saddr=1.2.3.4,daddr=www.yahoo.com}+icmp{type=8,code=0}\
  +data{str=hello world}

  /* An ICMP destination unreachable with the quoted UDP packet */
  ip{saddr=1.2.3.4,daddr=5.6.7.8}+icmp{type=3,code=3}\
  +ip{saddr=www.yahoo.com,daddr=1.2.3.4}+udp{sport=53,dport=53}\

  /* A TCP packet with the SYN flag set */
  ip{saddr=1.2.3.4,daddr=5.6.7.8}+tcp{flags=S,dport=80,sport=10}