++++++++++++++++++++++++++++++++++++++
<!-- WSDG Chapter Dissection -->
++++++++++++++++++++++++++++++++++++++
    
[[ChapterDissection]]

== Packet dissection

[[ChDissectWorks]]

=== How it works

Each dissector decodes its part of the protocol, and then hands off
decoding to subsequent dissectors for an encapsulated protocol.

Every dissection starts with the Frame dissector which dissects the packet
details of the capture file itself (e.g. timestamps). From there it passes the
data on to the the lowest-level data dissector, e.g. the Ethernet dissector for
the Ethernet header. The payload is then passed on to the next dissector (e.g.
IP) and so on. At each stage, details of the packet will be decoded and
displayed.

Dissection can be implemented in two possible ways. One is to have a dissector
module compiled into the main program, which means it's always available.
Another way is to make a plugin (a shared library or DLL) that registers itself
to handle dissection.

There is little difference in having your dissector as either a plugin or
built-in. On the Windows platform you have limited function access through the
ABI exposed in 'libwireshark.def', but that is mostly complete.

The big plus is that your rebuild cycle for a plugin is much shorter than for a
built-in one. So starting with a plugin makes initial development simpler, while
the finished code may make more sense as a built-in dissector.

[NOTE]
.Read README.developer
====
The file 'doc/README.developer' contains detailed information about implementing
a dissector. In many cases it is more up to date than this document.
====

[[ChDissectAdd]]

=== Adding a basic dissector

Let's step through adding a basic dissector. We'll start with the made up "foo"
protocol. It consists of the following basic items.

* A packet type - 8 bits, possible values: 1 - initialisation, 2 - terminate, 3 - data.

* A set of flags stored in 8 bits, 0x01 - start packet, 0x02 - end packet, 0x04 - priority packet.

* A sequence number - 16 bits.

* An IPv4 address.

[[ChDissectSetup]]

==== Setting up the dissector

The first decision you need to make is if this dissector will be a
built-in dissector, included in the main program, or a plugin.

Plugins are the easiest to write initially, so let's start with that.
With a little care, the plugin can be made to run as a built-in
easily too so we haven't lost anything.

.Dissector Initialisation.
====
----
#include "config.h"

#include <epan/packet.h>

#define FOO_PORT 1234

static int proto_foo = -1;


void
proto_register_foo(void)
{
    proto_foo = proto_register_protocol (
        "FOO Protocol", /* name       */
        "FOO",      /* short name */
        "foo"       /* abbrev     */
        );
}
----
====

Let's go through this a bit at a time. First we have some boilerplate
include files. These will be pretty constant to start with.

Next we have an int that is initialised to +$$-1$$+ that records our protocol.
This will get updated when we register this dissector with the main program.
It's good practice to make all variables and functions that aren't exported
static to keep name space pollution down. Normally this isn't a problem unless your
dissector gets so big it has to span multiple files.

Then a +#define+ for the UDP port that carries _foo_ traffic.

Now that we have the basics in place to interact with the main program, we'll
start with two protocol dissector setup functions.

First we'll call +proto_register_protocol()+ which registers the protocol. We
can give it three names that will be used for display in various places. The
full and short name are used in e.g. the "Preferences" and "Enabled protocols"
dialogs as well as the generated field name list in the documentation. The
abbreviation is used as the display filter name.

Next we need a handoff routine.

.Dissector Handoff.
====
----
void
proto_reg_handoff_foo(void)
{
    static dissector_handle_t foo_handle;

    foo_handle = create_dissector_handle(dissect_foo, proto_foo);
    dissector_add_uint("udp.port", FOO_PORT, foo_handle);
}
----
====

What's happening here? We are initialising the dissector. First we create a
dissector handle; It is associated with the foo protocol and with a routine to
be called to do the actual dissecting. Then we associate the handle with a UDP
port number so that the main program will know to call us when it gets UDP
traffic on that port.

The stardard Wireshark dissector convention is to put +proto_register_foo()+ and
+proto_reg_handoff_foo()+ as the last two functions in the dissector source.

Now at last we get to write some dissecting code. For the moment we'll
leave it as a basic placeholder.

.Dissection.
====
----
static void
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    col_set_str(pinfo->cinfo, COL_PROTOCOL, "FOO");
    /* Clear out stuff in the info column */
    col_clear(pinfo->cinfo,COL_INFO);
}
----
====

This function is called to dissect the packets presented to it. The packet data
is held in a special buffer referenced here as tvb. We shall become fairly
familiar with this as we get deeper into the details of the protocol. The packet
info structure contains general data about the protocol, and we can update
information here. The tree parameter is where the detail dissection takes place.

For now we'll do the minimum we can get away with. In the first line we set the
text of this to our protocol, so everyone can see it's being recognised. The
only other thing we do is to clear out any data in the INFO column if it's being
displayed.

At this point we should have a basic dissector ready to compile and install.
It doesn't do much at present, other than identify the protocol and label it.

In order to compile this dissector and create a plugin a couple of support files
are required, besides the dissector source in 'packet-foo.c':

* 'Makefile.am' - The UNIX/Linux makefile template.

* 'Makefile.common' - Contains the file names of this plugin.

* 'Makefile.nmake' - Contains the Wireshark plugin makefile for Windows.

* 'moduleinfo.h' - Contains plugin version information.

* 'moduleinfo.nmake' - Contains DLL version info for Windows.

* 'packet-foo.c' - Your dissector source.

* 'plugin.rc.in' - Contains the DLL resource template for Windows.

You can find a good example for these files in the interlink plugin directory.
'Makefile.common' and 'Makefile.am' have to be modified to reflect the relevant
files and dissector name. 'moduleinfo.h' and 'moduleinfo.nmake' have to be
filled in with the version information. Compile the dissector to a DLL or shared
library and copy it into the plugin directory of the installation.

[[ChDissectDetails]]

==== Dissecting the details of the protocol

Now that we have our basic dissector up and running, let's do something with it.
The simplest thing to do to start with is to just label the payload.
This will allow us to set up some of the parts we will need.

The first thing we will do is to build a subtree to decode our results into.
This helps to keep things looking nice in the detailed display. Now the
dissector is called in two different cases. In one case it is called to get a
summary of the packet, in the other case it is called to look into details of
the packet. These two cases can be distinguished by the tree pointer. If the
tree pointer is NULL, then we are being asked for a summary. If it is non NULL,
we can pick apart the protocol for display. So with that in mind, let's enhance
our dissector.

.Plugin Packet Dissection.
====
----
static void
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{

    col_set_str(pinfo->cinfo, COL_PROTOCOL, "FOO");
    /* Clear out stuff in the info column */
    col_clear(pinfo->cinfo,COL_INFO);

    if (tree) { /* we are being asked for details */
        proto_item *ti = NULL;
        ti = proto_tree_add_item(tree, proto_foo, tvb, 0, -1, ENC_NA);
    }
}
----
====

What we're doing here is adding a subtree to the dissection.
This subtree will hold all the details of this protocol and so not clutter
up the display when not required.

We are also marking the area of data that is being consumed by this
protocol. In our case it's all that has been passed to us, as we're assuming
this protocol does not encapsulate another.
Therefore, we add the new tree node with +proto_tree_add_item()+,
adding it to the passed in tree, label it with the protocol, use the passed in
tvb buffer as the data, and consume from 0 to the end (-1) of this data.
ENC_NA ("not applicable") is specified as the "encoding" parameter.

After this change, there should be a label in the detailed display for the protocol,
and selecting this will highlight the remaining contents of the packet.

Now let's go to the next step and add some protocol dissection. For this step
we'll need to construct a couple of tables that help with dissection. This needs
some additions to the +proto_register_foo()+ function shown previously.

Two statically allocated arrays are added at the beginning of
+proto_register_foo()+. The arrays are then registered after the call to
+proto_register_protocol()+.

.Registering data structures.
====
----
void
proto_register_foo(void)
{
    static hf_register_info hf[] = {
        { &hf_foo_pdu_type,
            { "FOO PDU Type", "foo.type",
            FT_UINT8, BASE_DEC,
            NULL, 0x0,
            NULL, HFILL }
        }
    };

    /* Setup protocol subtree array */
    static gint *ett[] = {
        &ett_foo
    };

    proto_foo = proto_register_protocol (
        "FOO Protocol", /* name       */
        "FOO",      /* short name */
        "foo"       /* abbrev     */
        );

    proto_register_field_array(proto_foo, hf, array_length(hf));
    proto_register_subtree_array(ett, array_length(ett));
}
----
====

The variables +hf_foo_pdu_type+ and +ett_foo+ also need to be declared somewhere near the top of the file.

.Dissector data structure globals.
====
----
static int hf_foo_pdu_type = -1;

static gint ett_foo = -1;
----
====

Now we can enhance the protocol display with some detail.

.Dissector starting to dissect the packets.
====
----
   if (tree) { /* we are being asked for details */
        proto_item *ti = NULL;
        proto_tree *foo_tree = NULL;

        ti = proto_tree_add_item(tree, proto_foo, tvb, 0, -1, ENC_NA);
        foo_tree = proto_item_add_subtree(ti, ett_foo);
        proto_tree_add_item(foo_tree, hf_foo_pdu_type, tvb, 0, 1, ENC_BIG_ENDIAN);
    }   
----
====

Now the dissection is starting to look more interesting. We have picked apart
our first bit of the protocol. One byte of data at the start of the packet
that defines the packet type for foo protocol.

The +proto_item_add_subtree()+ call has added a child node
to the protocol tree which is where we will do our detail dissection.
The expansion of this node is controlled by the +ett_foo+
variable. This remembers if the node should be expanded or not as you move
between packets. All subsequent dissection will be added to this tree,
as you can see from the next call.
A call to +proto_tree_add_item()+ in the foo_tree,
this time using the +hf_foo_pdu_type+ to control the formatting
of the item. The pdu type is one byte of data, starting at 0. We assume it is
in network order (also called big endian), so that is why we use +ENC_BIG_ENDIAN+.
For a 1-byte quantity, there is no order issue, but it is good practice to
make this the same as any multibyte fields that may be present, and as we will
see in the next section, this particular protocol uses network order.

If we look in detail at the +hf_foo_pdu_type+ declaration in
the static array we can see the details of the definition.

* 'hf_foo_pdu_type' - The index for this node.

* 'FOO PDU Type' - The label for this item.

* 'foo.type' - This is the filter string. It enables us to type constructs such
as +foo.type=1+ into the filter box.

* 'FT_UINT8' - This specifies this item is an 8bit unsigned integer.
This tallies with our call above where we tell it to only look at one byte.

* 'BASE_DEC' - Tor an integer type, this tells it to be printed as a decimal
number. It could be hexdecimal (BASE_HEX) or octal (BASE_OCT) if that made more sense.

We'll ignore the rest of the structure for now.

If you install this plugin and try it out, you'll see something that begins to look
useful.

Now let's finish off dissecting the simple protocol. We need to add a few
more variables to the hfarray, and a couple more procedure calls.

.Wrapping up the packet dissection.
====
----
...
static int hf_foo_flags = -1;
static int hf_foo_sequenceno = -1;
static int hf_foo_initialip = -1;
...

static void
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    gint offset = 0;

    ...

    if (tree) { /* we are being asked for details */
        proto_item *ti = NULL;
        proto_tree *foo_tree = NULL;

        ti = proto_tree_add_item(tree, proto_foo, tvb, 0, -1, ENC_NA);
        foo_tree = proto_item_add_subtree(ti, ett_foo);
        proto_tree_add_item(foo_tree, hf_foo_pdu_type, tvb, offset, 1, ENC_BIG_ENDIAN);
        offset += 1;
        proto_tree_add_item(foo_tree, hf_foo_flags, tvb, offset, 1, ENC_BIG_ENDIAN);
        offset += 1;
        proto_tree_add_item(foo_tree, hf_foo_sequenceno, tvb, offset, 2, ENC_BIG_ENDIAN);
        offset += 2;
        proto_tree_add_item(foo_tree, hf_foo_initialip, tvb, offset, 4, ENC_BIG_ENDIAN);
        offset += 4;
    }
    ...
}

void
proto_register_foo(void) {
    ...
        ...
        { &hf_foo_flags,
            { "FOO PDU Flags", "foo.flags",
            FT_UINT8, BASE_HEX,
            NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_foo_sequenceno,
            { "FOO PDU Sequence Number", "foo.seqn",
            FT_UINT16, BASE_DEC,
            NULL, 0x0,
            NULL, HFILL }
        },
        { &hf_foo_initialip,
            { "FOO PDU Initial IP", "foo.initialip",
            FT_IPv4, BASE_NONE,
            NULL, 0x0,
            NULL, HFILL }
        },
        ...
    ...
}
...
----
====

This dissects all the bits of this simple hypothetical protocol. We've
introduced a new variable offsetinto the mix to help keep track of where we are
in the packet dissection. With these extra bits in place, the whole protocol is
now dissected.

==== Improving the dissection information

We can certainly improve the display of the protocol with a bit of extra data.
The first step is to add some text labels. Let's start by labeling the packet
types. There is some useful support for this sort of thing by adding a couple of
extra things. First we add a simple table of type to name.


.Naming the packet types.
====
----
static const value_string packettypenames[] = {
    { 1, "Initialise" },
    { 2, "Terminate" },
    { 3, "Data" },
    { 0, NULL }
};   
----
====

This is a handy data structure that can be used to look up a name for a value.
There are routines to directly access this lookup table, but we don't need to
do that, as the support code already has that added in. We just have to give
these details to the appropriate part of the data, using the +VALS+ macro.

.Adding Names to the protocol.
====
----
   { &hf_foo_pdu_type,
        { "FOO PDU Type", "foo.type",
        FT_UINT8, BASE_DEC,
        VALS(packettypenames), 0x0,
        NULL, HFILL }
    }
----
====

This helps in deciphering the packets, and we can do a similar thing for the
flags structure. For this we need to add some more data to the table though.

.Adding Flags to the protocol.
====
----
#define FOO_START_FLAG 0x01
#define FOO_END_FLAG        0x02
#define FOO_PRIORITY_FLAG   0x04

static int hf_foo_startflag = -1;
static int hf_foo_endflag = -1;
static int hf_foo_priorityflag = -1;

static void
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    ...
        ...
        proto_tree_add_item(foo_tree, hf_foo_flags, tvb, offset, 1, ENC_BIG_ENDIAN);
        proto_tree_add_item(foo_tree, hf_foo_startflag, tvb, offset, 1, ENC_BIG_ENDIAN);
        proto_tree_add_item(foo_tree, hf_foo_endflag, tvb, offset, 1, ENC_BIG_ENDIAN);
        proto_tree_add_item(foo_tree, hf_foo_priorityflag, tvb, offset, 1, ENC_BIG_ENDIAN);
        offset += 1;
        ...
    ...
}

void
proto_register_foo(void) {
    ...
        ...
        { &hf_foo_startflag,
            { "FOO PDU Start Flags", "foo.flags.start",
            FT_BOOLEAN, 8,
            NULL, FOO_START_FLAG,
            NULL, HFILL }
        },
        { &hf_foo_endflag,
            { "FOO PDU End Flags", "foo.flags.end",
            FT_BOOLEAN, 8,
            NULL, FOO_END_FLAG,
            NULL, HFILL }
        },
        { &hf_foo_priorityflag,
            { "FOO PDU Priority Flags", "foo.flags.priority",
            FT_BOOLEAN, 8,
            NULL, FOO_PRIORITY_FLAG,
            NULL, HFILL }
        },
        ...
    ...
}
...
----
====

Some things to note here. For the flags, as each bit is a different flag, we use
the type +FT_BOOLEAN+, as the flag is either on or off. Second, we include the flag
mask in the 7th field of the data, which allows the system to mask the relevant bit.
We've also changed the 5th field to 8, to indicate that we are looking at an 8 bit
quantity when the flags are extracted. Then finally we add the extra constructs
to the dissection routine. Note we keep the same offset for each of the flags.

This is starting to look fairly full featured now, but there are a couple of
other things we can do to make things look even more pretty. At the moment our
dissection shows the packets as "Foo Protocol" which whilst correct is a little
uninformative. We can enhance this by adding a little more detail. First, let's
get hold of the actual value of the protocol type. We can use the handy function
+tvb_get_guint8()+ to do this. With this value in hand, there are a couple of
things we can do. First we can set the INFO column of the non-detailed view to
show what sort of PDU it is - which is extremely helpful when looking at
protocol traces. Second, we can also display this information in the dissection
window.

.Enhancing the display.
====
----
static void
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
    guint8 packet_type = tvb_get_guint8(tvb, 0);

    col_set_str(pinfo->cinfo, COL_PROTOCOL, "FOO");
    /* Clear out stuff in the info column */
    col_clear(pinfo->cinfo,COL_INFO);
    col_add_fstr(pinfo->cinfo, COL_INFO, "Type %s",
             val_to_str(packet_type, packettypenames, "Unknown (0x%02x)"));

    if (tree) { /* we are being asked for details */
        proto_item *ti = NULL;
        proto_tree *foo_tree = NULL;
        gint offset = 0;

        ti = proto_tree_add_item(tree, proto_foo, tvb, 0, -1, ENC_NA);
        proto_item_append_text(ti, ", Type %s",
            val_to_str(packet_type, packettypenames, "Unknown (0x%02x)"));
        foo_tree = proto_item_add_subtree(ti, ett_foo);
        proto_tree_add_item(foo_tree, hf_foo_pdu_type, tvb, offset, 1, ENC_BIG_ENDIAN);
        offset += 1;
    }
}
----
====

So here, after grabbing the value of the first 8 bits, we use it with one of the
built-in utility routines +val_to_str()+, to lookup the value. If the value
isn't found we provide a fallback which just prints the value in hex. We use
this twice, once in the INFO field of the columns -- if it's displayed, and
similarly we append this data to the base of our dissecting tree.

[[ChDissectTransformed]]

=== How to handle transformed data

Some protocols do clever things with data. They might possibly
encrypt the data, or compress data, or part of it. If you know
how these steps are taken it is possible to reverse them within the
dissector.

As encryption can be tricky, let's consider the case of compression.
These techniques can also work for other transformations of data,
where some step is required before the data can be examined.

What basically needs to happen here, is to identify the data that needs
conversion, take that data and transform it into a new stream, and then call a
dissector on it. Often this needs to be done "on-the-fly" based on clues in the
packet. Sometimes this needs to be used in conjunction with other techniques,
such as packet reassembly. The following shows a technique to achieve this
effect.

.Decompressing data packets for dissection.
====
----
    guint8 flags = tvb_get_guint8(tvb, offset);
    offset ++;
    if (flags & FLAG_COMPRESSED) { /* the remainder of the packet is compressed */
        guint16 orig_size = tvb_get_ntohs(tvb, offset);
        guchar *decompressed_buffer = (guchar*)g_malloc(orig_size);
        offset += 2;
        decompress_packet(tvb_get_ptr(tvb, offset, -1),
                tvb_length_remaining(tvb, offset),
                decompressed_buffer, orig_size);
        /* Now re-setup the tvb buffer to have the new data */
        next_tvb = tvb_new_child_real_data(tvb, decompressed_buffer, orig_size, orig_size);
        tvb_set_free_cb(next_tvb, g_free);
        add_new_data_source(pinfo, next_tvb, "Decompressed Data");
    } else {
        next_tvb = tvb_new_subset_remaining(tvb, offset);
    }
    offset = 0;
    /* process next_tvb from here on */
----
====

The first steps here are to recognise the compression. In this case a flag byte
alerts us to the fact the remainder of the packet is compressed. Next we
retrieve the original size of the packet, which in this case is conveniently
within the protocol. If it's not, it may be part of the compression routine to
work it out for you, in which case the logic would be different.

So armed with the size, a buffer is allocated to receive the uncompressed data
using +g_malloc()+, and the packet is decompressed into it. The +tvb_get_ptr()+
function is useful to get a pointer to the raw data of the packet from the
offset onwards. In this case the decompression routine also needs to know the
length, which is given by the +tvb_length_remaining()+ function.

Next we build a new tvb buffer from this data, using the
+tvb_new_child_real_data()+ call. This data is a child of our original data, so
calling this function also acknowledges that. One procedural step is to add a
callback handler to free the data when it's no longer needed via a call to
+tvb_set_free_cb()+. In this case +g_malloc()+ was used to allocate the memory,
so +g_free()+ is the appropriate callback function. Finally we add this tvb as a
new data source, so that the detailed display can show the decompressed bytes as
well as the original.

After this has been set up the remainder of the dissector can dissect the buffer
next_tvb, as it's a new buffer the offset needs to be 0 as we start again from
the beginning of this buffer. To make the rest of the dissector work regardless
of whether compression was involved or not, in the case that compression was not
signaled, we use +tvb_new_subset_remaining()+ to deliver us a new buffer based
on the old one but starting at the current offset, and extending to the end.
This makes dissecting the packet from this point on exactly the same regardless
of compression.

[[ChDissectReassemble]]

=== How to reassemble split packets

Some protocols have times when they have to split a large packet across
multiple other packets. In this case the dissection can't be carried out correctly
until you have all the data. The first packet doesn't have enough data,
and the subsequent packets don't have the expect format.
To dissect these packets you need to wait until all the parts have
arrived and then start the dissection.

[[ChDissectReassembleUdp]]

==== How to reassemble split UDP packets

As an example, let's examine a protocol that is layered on top of UDP that
splits up its own data stream. If a packet is bigger than some given size, it
will be split into chunks, and somehow signaled within its protocol.

To deal with such streams, we need several things to trigger from. We need to
know that this packet is part of a multi-packet sequence. We need to know how
many packets are in the sequence. We also need to know when we have all the
packets.

For this example we'll assume there is a simple in-protocol signaling mechanism
to give details. A flag byte that signals the presence of a multi-packet
sequence and also the last packet, followed by an ID of the sequence and a
packet sequence number.

----
msg_pkt ::= SEQUENCE {
    .....
    flags ::= SEQUENCE {
        fragment    BOOLEAN,
        last_fragment   BOOLEAN,
    .....
    }
    msg_id  INTEGER(0..65535),
    frag_id INTEGER(0..65535),
    .....
}
----

.Reassembling fragments - Part 1
====
----
#include <epan/reassemble.h>
   ...
save_fragmented = pinfo->fragmented;
flags = tvb_get_guint8(tvb, offset); offset++;
if (flags & FL_FRAGMENT) { /* fragmented */
    tvbuff_t* new_tvb = NULL;
    fragment_data *frag_msg = NULL;
    guint16 msg_seqid = tvb_get_ntohs(tvb, offset); offset += 2;
    guint16 msg_num = tvb_get_ntohs(tvb, offset); offset += 2;

    pinfo->fragmented = TRUE;
    frag_msg = fragment_add_seq_check(tvb, offset, pinfo,
        msg_seqid, /* ID for fragments belonging together */
        msg_fragment_table, /* list of message fragments */
        msg_reassembled_table, /* list of reassembled messages */
        msg_num, /* fragment sequence number */
        tvb_length_remaining(tvb, offset), /* fragment length - to the end */
        flags & FL_FRAG_LAST); /* More fragments? */
----
====

We start by saving the fragmented state of this packet, so we can restore it
later. Next comes some protocol specific stuff, to dig the fragment data out of
the stream if it's present. Having decided it is present, we let the function
+fragment_add_seq_check()+ do its work. We need to provide this with a certain
amount of data.

* The tvb buffer we are dissecting.

* The offset where the partial packet starts.

* The provided packet info.

* The sequence number of the fragment stream. There may be several streams of
  fragments in flight, and this is used to key the relevant one to be used for
  reassembly.

* The +msg_fragment_table+ and the +msg_reassembled_table+ are variables we need
  to declare. We'll consider these in detail later.

* msg_num is the packet number within the sequence.

* The length here is specified as the rest of the tvb as we want the rest of the packet data.

* Finally a parameter that signals if this is the last fragment or not. This
  might be a flag as in this case, or there may be a counter in the protocol.

.Reassembling fragments part 2
====
----
    new_tvb = process_reassembled_data(tvb, offset, pinfo,
        "Reassembled Message", frag_msg, &msg_frag_items,
        NULL, msg_tree);

    if (frag_msg) { /* Reassembled */
        col_append_str(pinfo->cinfo, COL_INFO,
                " (Message Reassembled)");
    } else { /* Not last packet of reassembled Short Message */
        col_append_fstr(pinfo->cinfo, COL_INFO,
                " (Message fragment %u)", msg_num);
    }

    if (new_tvb) { /* take it all */
        next_tvb = new_tvb;
    } else { /* make a new subset */
        next_tvb = tvb_new_subset(tvb, offset, -1, -1);
    }
}
else { /* Not fragmented */
    next_tvb = tvb_new_subset(tvb, offset, -1, -1);
}

.....
pinfo->fragmented = save_fragmented;
----
====

Having passed the fragment data to the reassembly handler, we can now check if
we have the whole message. If there is enough information, this routine will
return the newly reassembled data buffer.

After that, we add a couple of informative messages to the display to show that
this is part of a sequence. Then a bit of manipulation of the buffers and the
dissection can proceed. Normally you will probably not bother dissecting further
unless the fragments have been reassembled as there won't be much to find.
Sometimes the first packet in the sequence can be partially decoded though if
you wish.

Now the mysterious data we passed into the +fragment_add_seq_check()+.

.Reassembling fragments - Initialisation
====
----
static GHashTable *msg_fragment_table = NULL;
static GHashTable *msg_reassembled_table = NULL;

static void
msg_init_protocol(void)
{
    fragment_table_init(&msg_fragment_table);
    reassembled_table_init(&msg_reassembled_table);
}   
----
====

First a couple of hash tables are declared, and these are initialised in the
protocol initialisation routine. Following that, a +fragment_items+ structure is
allocated and filled in with a series of ett items, hf data items, and a string
tag. The ett and hf values should be included in the relevant tables like all
the other variables your protocol may use. The hf variables need to be placed in
the structure something like the following. Of course the names may need to be
adjusted.

.Reassembling fragments - Data
====
----
...
static int hf_msg_fragments = -1;
static int hf_msg_fragment = -1;
static int hf_msg_fragment_overlap = -1;
static int hf_msg_fragment_overlap_conflicts = -1;
static int hf_msg_fragment_multiple_tails = -1;
static int hf_msg_fragment_too_long_fragment = -1;
static int hf_msg_fragment_error = -1;
static int hf_msg_fragment_count = -1;
static int hf_msg_reassembled_in = -1;
static int hf_msg_reassembled_length = -1;
...
static gint ett_msg_fragment = -1;
static gint ett_msg_fragments = -1;
...
static const fragment_items msg_frag_items = {
    /* Fragment subtrees */
    &ett_msg_fragment,
    &ett_msg_fragments,
    /* Fragment fields */
    &hf_msg_fragments,
    &hf_msg_fragment,
    &hf_msg_fragment_overlap,
    &hf_msg_fragment_overlap_conflicts,
    &hf_msg_fragment_multiple_tails,
    &hf_msg_fragment_too_long_fragment,
    &hf_msg_fragment_error,
    &hf_msg_fragment_count,
    /* Reassembled in field */
    &hf_msg_reassembled_in,
    /* Reassembled length field */
    &hf_msg_reassembled_length,
    /* Tag */
    "Message fragments"
};
...
static hf_register_info hf[] =
{
...
{&hf_msg_fragments,
    {"Message fragments", "msg.fragments",
    FT_NONE, BASE_NONE, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment,
    {"Message fragment", "msg.fragment",
    FT_FRAMENUM, BASE_NONE, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment_overlap,
    {"Message fragment overlap", "msg.fragment.overlap",
    FT_BOOLEAN, 0, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment_overlap_conflicts,
    {"Message fragment overlapping with conflicting data",
    "msg.fragment.overlap.conflicts",
    FT_BOOLEAN, 0, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment_multiple_tails,
    {"Message has multiple tail fragments",
    "msg.fragment.multiple_tails",
    FT_BOOLEAN, 0, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment_too_long_fragment,
    {"Message fragment too long", "msg.fragment.too_long_fragment",
    FT_BOOLEAN, 0, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment_error,
    {"Message defragmentation error", "msg.fragment.error",
    FT_FRAMENUM, BASE_NONE, NULL, 0x00, NULL, HFILL } },
{&hf_msg_fragment_count,
    {"Message fragment count", "msg.fragment.count",
    FT_UINT32, BASE_DEC, NULL, 0x00, NULL, HFILL } },
{&hf_msg_reassembled_in,
    {"Reassembled in", "msg.reassembled.in",
    FT_FRAMENUM, BASE_NONE, NULL, 0x00, NULL, HFILL } },
{&hf_msg_reassembled_length,
    {"Reassembled length", "msg.reassembled.length",
    FT_UINT32, BASE_DEC, NULL, 0x00, NULL, HFILL } },
...
static gint *ett[] =
{
...
&ett_msg_fragment,
&ett_msg_fragments
...
----
====

These hf variables are used internally within the reassembly routines to make
useful links, and to add data to the dissection. It produces links from one
packet to another, such as a partial packet having a link to the fully
reassembled packet. Likewise there are back pointers to the individual packets
from the reassembled one. The other variables are used for flagging up errors.

[[TcpDissectPdus]]

==== How to reassemble split TCP Packets

A dissector gets a +tvbuff_t+ pointer which holds the payload
of a TCP packet. This payload contains the header and data
of your application layer protocol.

When dissecting an application layer protocol you cannot assume
that each TCP packet contains exactly one application layer message.
One application layer message can be split into several TCP packets.

You also cannot assume that a TCP packet contains only one application layer message
and that the message header is at the start of your TCP payload.
More than one messages can be transmitted in one TCP packet,
so that a message can start at an arbitrary position.

This sounds complicated, but there is a simple solution.
+tcp_dissect_pdus()+ does all this tcp packet reassembling for you.
This function is implemented in 'epan/dissectors/packet-tcp.h'.

.Reassembling TCP fragments
====
----
#include "config.h"

#include <epan/packet.h>
#include <epan/prefs.h>
#include "packet-tcp.h"

...

#define FRAME_HEADER_LEN 8

/* This method dissects fully reassembled messages */
static int dissect_foo_message(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
{
    /* TODO: implement your dissecting code */
    return tvb_length(tvb);
}

/* determine PDU length of protocol foo */
static guint get_foo_message_len(packet_info *pinfo, tvbuff_t *tvb, int offset)
{
    /* TODO: change this to your needs */
    return (guint)tvb_get_ntohl(tvb, offset+4); /* e.g. length is at offset 4 */
}

/* The main dissecting routine */
static int dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data)
{
    tcp_dissect_pdus(tvb, pinfo, tree, TRUE, FRAME_HEADER_LEN,
                     get_foo_message_len, dissect_foo_message, data);
    return tvb_length(tvb);
}

...
----
====

As you can see this is really simple. Just call +tcp_dissect_pdus()+ in your
main dissection routine and move you message parsing code into another function.
This function gets called whenever a message has been reassembled.

The parameters tvb, pinfo, treeand dataare just handed over to
+tcp_dissect_pdus()+. The 4th parameter is a flag to indicate if the data should
be reassembled or not. This could be set according to a dissector preference as
well. Parameter 5 indicates how much data has at least to be available to be
able to determine the length of the foo message. Parameter 6 is a function
pointer to a method that returns this length. It gets called when at least the
number of bytes given in the previous parameter is available. Parameter 7 is a
function pointer to your real message dissector. Parameter 8 is a the data
passed in from parent dissector.

[[ChDissectTap]]

=== How to tap protocols

Adding a Tap interface to a protocol allows it to do some useful things.
In particular you can produce protocol statistics from the tap interface.

A tap is basically a way of allowing other items to see whats happening as
a protocol is dissected. A tap is registered with the main program, and
then called on each dissection. Some arbitrary protocol specific data
is provided with the routine that can be used.

To create a tap, you first need to register a tap. A tap is registered with an
integer handle, and registered with the routine +register_tap()+. This takes a
string name with which to find it again.

.Initialising a tap
====
----
#include <epan/packet.h>
#include <epan/tap.h>

static int foo_tap = -1;

struct FooTap {
    gint packet_type;
    gint priority;
       ...
};

void proto_register_foo(void)
{
       ...
    foo_tap = register_tap("foo");
----
====

Whilst you can program a tap without protocol specific data, it is generally not
very useful. Therefore it's a good idea to declare a structure that can be
passed through the tap. This needs to be a static structure as it will be used
after the dissection routine has returned. It's generally best to pick out some
generic parts of the protocol you are dissecting into the tap data. A packet
type, a priority or a status code maybe. The structure really needs to be
included in a header file so that it can be included by other components that
want to listen in to the tap.

Once you have these defined, it's simply a case of populating the protocol
specific structure and then calling +tap_queue_packet+, probably as the last part
of the dissector.

.Calling a protocol tap
====
----
void dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
       ...
    fooinfo = ep_alloc(sizeof(struct FooTap));
    fooinfo->packet_type = tvb_get_guint8(tvb, 0);
    fooinfo->priority = tvb_get_ntohs(tvb, 8);
       ...
    tap_queue_packet(foo_tap, pinfo, fooinfo);
}
----
====

This now enables those interested parties to listen in on the details
of this protocol conversation.

[[ChDissectStats]]

=== How to produce protocol stats

Given that you have a tap interface for the protocol, you can use this
to produce some interesting statistics (well presumably interesting!) from
protocol traces.

This can be done in a separate plugin, or in the same plugin that is
doing the dissection. The latter scheme is better, as the tap and stats
module typically rely on sharing protocol specific data, which might get out
of step between two different plugins.

Here is a mechanism to produce statistics from the above TAP interface.

.Initialising a stats interface
====
----
/* register all http trees */
static void register_foo_stat_trees(void) {
    stats_tree_register("foo", "foo", "Foo/Packet Types",
        foo_stats_tree_packet, foo_stats_tree_init, NULL);
}

WS_DLL_PUBLIC_DEF const gchar version[] = "0.0";

WS_DLL_PUBLIC_DEF void plugin_register_tap_listener(void)
{
    register_foo_stat_trees();
}

#endif   
----
====

Working from the bottom up, first the plugin interface entry point is defined,
+plugin_register_tap_listener()+. This simply calls the initialisation function
+register_foo_stat_trees()+.

This in turn calls the +stats_tree_register()+ function, which takes three
strings, and three functions.

. This is the tap name that is registered.

. An abbreviation of the stats name.

. The name of the stats module. A $$'/'$$ character can be used to make sub menus.

. The function that will called to generate the stats.

. A function that can be called to initialise the stats data.

. A function that will be called to clean up the stats data.

In this case we only need the first two functions, as there is nothing specific to clean up.

.Initialising a stats session
====
----
static const guint8* st_str_packets = "Total Packets";
static const guint8* st_str_packet_types = "FOO Packet Types";
static int st_node_packets = -1;
static int st_node_packet_types = -1;

static void foo_stats_tree_init(stats_tree* st)
{
    st_node_packets = stats_tree_create_node(st, st_str_packets, 0, TRUE);
    st_node_packet_types = stats_tree_create_pivot(st, st_str_packet_types, st_node_packets);
}   
----
====

In this case we create a new tree node, to handle the total packets,
and as a child of that we create a pivot table to handle the stats about
different packet types.


.Generating the stats
====
----
static int foo_stats_tree_packet(stats_tree* st, packet_info* pinfo, epan_dissect_t* edt, const void* p)
{
    struct FooTap *pi = (struct FooTap *)p;
    tick_stat_node(st, st_str_packets, 0, FALSE);
    stats_tree_tick_pivot(st, st_node_packet_types,
            val_to_str(pi->packet_type, msgtypevalues, "Unknown packet type (%d)"));
    return 1;
}   
----
====

In this case the processing of the stats is quite simple. First we call the
+tick_stat_node+ for the +st_str_packets+ packet node, to count packets. Then a
call to +stats_tree_tick_pivot()+ on the +st_node_packet_types+ subtree allows
us to record statistics by packet type.

[[ChDissectConversation]]

=== How to use conversations

Some info about how to use conversations in a dissector can be found in the file
'doc/README.developer', chapter 2.2.

++++++++++++++++++++++++++++++++++++++
<!-- End of WSDG Chapter Dissection -->
++++++++++++++++++++++++++++++++++++++
    
