4 // Fraunhofer Institute for Open Communication Systems (FOKUS)
5 // Competence Center NETwork research (NET), St. Augustin, GERMANY
6 // Stefan Bund <g0dil@berlios.de>
8 // This program is free software; you can redistribute it and/or modify
9 // it under the terms of the GNU General Public License as published by
10 // the Free Software Foundation; either version 2 of the License, or
11 // (at your option) any later version.
13 // This program is distributed in the hope that it will be useful,
14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 // GNU General Public License for more details.
18 // You should have received a copy of the GNU General Public License
19 // along with this program; if not, write to the
20 // Free Software Foundation, Inc.,
21 // 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
23 /** \mainpage libPackets: How to define and use a new Packet Type
25 This howto will introduce the facilities needed to define a new packet type. As example, the
26 \c GREPacket type is defined.
28 \section howto_newpacket_start Getting started
30 Before starting with the implementation, we look at the specification of the GRE packet. This is
31 found in <a href="http://tools.ietf.org/html/rfc2784">RFC 2784</a> in Section 2.1:
34 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
35 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
36 |C| Reserved0 | Ver | Protocol Type |
37 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
38 | Checksum (optional) | Reserved1 (Optional) |
39 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
42 Using this protocol definition, we can decide the first important question: Whether the packet
43 header is fixed size or dynamically sized. As we see above, the header incorporates optional
44 fields. Therefore it must be dynamically sized. The RFC further details, that if the \a Checksum
45 \a Present bit \a C is set, both \a Checksum and \a Reserved1 are present, otherwise they must
48 Another information we take from the RFC is, that the \a Protocol \a Type is used to define the
49 type of payload which directly follows the GRE header. This value is an <a
50 href="http://www.iana.org/assignments/ethernet-numbers">ETHERTYPE</a> value. To allow the packet
51 library to automatically parse the GRE payload data, we need to tell the packet library which
52 ETHERTYPE represents which packet type. This association already exists in form of the
53 senf::EtherTypes registry. Our GRE packet will therefore utilize this registry.
55 To summarize, we have gathered the following information:
57 \li The GRE packet header is a dynamically sized header.
58 \li The GRE packet header utilizes the senf::EtherTypes registry for next-header selection
60 \section howto_newpacket_parser Implementing the GRE Parser
62 The next step in creating a new packet type is to implement the parser. The parser is
63 responsible for turning a bunch of bytes into an interpreted header with specific fields. The
64 parser will later be constructed with an iterator (pointer) to the first byte to be interpreted
65 as a GRE header and will provide member functions to access the header fields. You can implement
66 these members manually but the SENF library provides a large set of helper macros which simplify
67 this task considerably.
70 struct GREParser : public senf::PacketParser
72 # include SENF_PARSER()
76 SENF_PARSER_FINALIZE(GREParser);
80 This is the standard skeleton of any parser class: We need to inherit senf::PacketParser and
81 start out by including either \ref SENF_PARSER() or \ref SENF_FIXED_PARSER(). Which, depends on
82 whether we define a fixed size or a dynamically sized parser. As \c GREParser is dynamically
83 sized, we include \ref SENF_PARSER().
85 After the fields are defined, we need to call the \ref SENF_PARSER_FINALIZE() macro to close of
86 the parser definition. This call takes the name of the parser being defined as it's sole
89 This is already a valid parser, albeit not a very usable one since it defines no fields. We now
90 go back to define the parser fields and begin with the simple part: Those fields which are
94 SENF_PARSER_BITFIELD ( checksumPresent, 1, bool );
95 SENF_PARSER_SKIP_BITS ( 12 );
96 SENF_PARSER_BITFIELD ( version, 3, unsigned );
97 SENF_PARSER_BITFIELD ( protocolType, 16, unsigned );
100 This is a direct transcript of the field definition above. There are quite a number of macros
101 which may be used to define fields. All these macros are documented in '\ref
104 This is a correct \c GREPacket header definition but we can optimize a little bit: Since the \a
105 protocolType field is aligned on a byte boundary, instead of defining it as a bitfield, we can
106 define it as a UInt16 field:
109 SENF_PARSER_BITFIELD ( checksumPresent, 1, bool );
110 SENF_PARSER_SKIP_BITS ( 12 );
111 SENF_PARSER_BITFIELD ( version, 3, unsigned );
113 SENF_PARSER_FIELD ( protocolType, senf::UInt16Parser );
116 Whereas \ref SENF_PARSER_BITFIELD can define only bit-fields, \ref SENF_PARSER_FIELD can define
117 almost arbitrary field types. The type is specified by passing the name of another parser to
118 \ref SENF_PARSER_FIELD.
120 It is important to understand, that the accessors do \e not return the parsed field value. They
121 return another \e parser which is used to further interpret the value. This is the inherent
122 recursive nature of the SENF packet parsers. This allows to define wildly complex header formats
123 if needed. Of course, at some point we need the real value. This is, what the so called
124 <em>value parsers</em> do: They interpret some bytes or bits and return the value of that field
125 (not a parser). Examples are the bitfield parsers returned by the accessors generated by
126 SENF_PARSER_BITFIELD (like senf::UIntFieldParser) or the senf::UInt16Parser.
128 What happens in the above macros? Most of the macros define an accessor for a specific field: \a
129 checksumPresent() or \a protocolType(). They also manage a <em>current Offset</em>. This value
130 is advanced according to the field size whenever a new field is defined (and since this parser
131 is defined as a dynamically sized parser, this offset is not a constant, it is an expression
132 which calculates the offset of a field depending on the preceding data).
134 We now come to the optional fields. Since there are two fields which need to be disabled/enabled
135 together, we first need to define an additional sub-parser which combines those two
136 fields. After this parser is defined, we can use \ref SENF_PARSER_VARIANT() to add this parser
137 as an optional parser to the GRE header.
140 struct GREParser_OptFields : public senf::PacketParser
142 # include SENF_FIXED_PARSER()
144 SENF_PARSER_FIELD ( checksum, senf::UInt16Parser );
145 SENF_PARSER_SKIP ( 2 );
147 SENF_PARSER_FINALIZE(GREParser_OptFields);
151 This parser only parses the two optional fields of which the reserved field is just skipped. The
152 parser this time is a fixed size parser. We can now use this parser to continue the \c GREParser
156 SENF_PARSER_BITFIELD ( checksumPresent, 1, bool );
157 SENF_PARSER_SKIP_BITS ( 12 );
158 SENF_PARSER_BITFIELD ( version, 3, unsigned );
160 SENF_PARSER_FIELD ( protocolType, senf::UInt16Parser );
162 SENF_PARSER_VARIANT ( optionalFields, checksumPresent,
163 (senf::VoidPacketParser)
164 (GREParser_OptFields) );
167 For a variant parser, two things need to be specified: A selector and a list of variant
168 parsers. The selector is another parser field which is used to decide, which variant to
169 choose. In this simple case, the field must be an unsigned integer (more precisely a value
170 parser which returns a value which is implicitly convertible to \c unsigned). This value is used
171 as index into the list of variant types. So in our case, 0 is associated with
172 senf::VoidPacketParser whereas 1 is associated with \c
173 GREParser_OptFields. (senf::VoidPacketParser is a special empty parser which is used in a
174 Variant to denote cases in which the variant parser should not parse anything)
176 This parser will work, it is however not very safe and not very usable. If \a p is a GREParser
177 instance, than we access the fields via:
179 p.checksumPresent() = true;
181 p.protocolType() = 0x86dd;
182 p.optionalFields().get<1>().checksum() = 12345u;
185 There are two problems here:
186 \li accessing the checksum field is quite unwieldy
187 \li changing the checksumPresent() value will break the parser
189 The reason for the second problem lies in the fact, that the variant parser needs to be informed
190 whenever the selector (here \a checksumPresent) is changed since the variant parser must ensure,
191 that the header data stays consistent. In this example, whenever the checksumPresent field is
192 enabled, the variant parser needs to insert additional 4 bytes of data and remove those bytes,
193 when the checksumPresent field is disabled.
195 To fix this, we make the checksumPresent field read-only:
198 SENF_PARSER_BITFIELD_RO ( checksumPresent, 1, bool );
201 To change the checksumPresent value, we now need to use the variant parsers \a init member:
204 p.optionalFields().init<0>();
205 p.optionalFields().init<1>();
208 The first statements switches to the first variant and therefore in this case disables the
209 checksum field. The second statement will switch to the second variant and enable the checksum
212 This again is not very usable. So we complete the parser by providing simple additional members
213 which wrap these complicated calls. While doing this, we also mark the variant as a private
214 field so it is not directly accessible any more (since we now have the additional helpers which
215 are used to access the variant, we don't want anyone to mess around with it directly). Here the
219 struct GREParser_OptFields : public senf::PacketParser
221 # include SENF_FIXED_PARSER()
223 SENF_PARSER_FIELD ( checksum, senf::UInt16Parser );
224 SENF_PARSER_SKIP ( 2 );
226 SENF_PARSER_FINALIZE(GREParser_OptFields);
229 struct GREParser : public senf::PacketParser
231 # include SENF_PARSER()
233 SENF_PARSER_BITFIELD_RO ( checksumPresent, 1, bool );
234 SENF_PARSER_SKIP_BITS ( 12 );
235 SENF_PARSER_BITFIELD ( version, 3, unsigned );
237 SENF_PARSER_FIELD ( protocolType, senf::UInt16Parser );
239 SENF_PARSER_PRIVATE_VARIANT ( optionalFields_, checksumPresent,
240 (senf::VoidPacketParser)
241 (GREParser_OptFields) );
243 typedef GREParser_OptFields::checksum_t checksum_t;
244 checksum_t checksum() const
245 { return optionalFields_().get<1>().checksum(); }
247 void enableChecksum() const { optionalFields_().init<1>(); }
248 void disableChecksum() const { optionalFields_().init<0>(); }
250 SENF_PARSER_FINALIZE(GREParser);
254 Above code has one other twist we need to discuss: the \a checksum_t typedef. This is added as a
255 convenience to the user of this parser. The \c SENF_PARSER_* macros which define a field all
256 define some additional symbols providing further information about the field. Of these
257 additional symbols, the most important is <em>field</em><code>_t</code>, which is the (parser)
258 type returned by the field. This helps to work with a parser in more complex situations
259 (e.g. when using collection parsers) since it allows to access the parser type without exact
260 knowledge of this type (which may become quite complex if templates are involved) as long as the
261 field name is known. Since we provide an accessor for the \a checksum field, we also provide the
262 \a checksum_t typedef for this accessor.
264 The \c GREParser is now simple and safe to use. The only responsibility of the user now is to
265 only access \a checksum() if the \a checksumPresent() field is set. Otherwise, the behavior is
266 undefined (in debug builds, the parser will terminate the application with an assert).
273 // comment-column: 40
274 // c-file-style: "senf"
275 // indent-tabs-mode: nil
276 // ispell-local-dictionary: "american"
277 // compile-command: "scons -u doc"