wireshark/doc

$Id: README.tvbuff,v 1.2 2000/05/17 04:34:20 gram Exp $

TVBUFFs and Exceptions

This document describes the changes made to the Ethereal dissector routines
in post-0.8.8. All protocol dissectors need to be modified, but can be modified
one at a time. During this transition time, this document will stand apart
from 'README.developer'. Once all the protocol dissectors are converted to
use the new tvbuff routines, the information in this document will be merged
into 'README.developer'.

While Ethereal does a grand job of dissecting frames that are complete,
it has done only a mediocre job of dissecting partial frames. Frames can
be incomplete for two reasons: the user used a capture length which is smaller
than the MTU of the interface (which is the default behavior of tcpdump, BTW),
or the frame on the wire is corrupted. In either case, Ethereal should
gracefully handle the incomplete frame.

With the aid of two C preprocessor macros,  BYTES_ARE_IN_FRAME() and
IS_DATA_IN_FRAME(), dissector authors are supposed to check that the data
they are trying to read from the frame actually exists. Some dissectors
used these macros diligently, and others not. In the end we realized that
depending on human diligence would get us nowhere and that a programmed solution
would be necessary.

The idea was to encapsulate the byte array which represented the data in the
frame with a "class" that would check the enforce limits regarding the boundaries
of the byte array. In the event of an improper data access, it is not enough
to return an error condition since we knew that it would be impractical to
check this error flags after every data access. Instead, we needed to implement
exceptions in Ethereal. Other languages (Java, C++, Python) have exceptions, but
we had to introduce an exception library and some magic C preprocess macros
to implement them in C.

The encapsulating class is called a "tvbuff", or "testy, virtual(-izable) buffer".
They are testy in that they get mad when an attempt is made to access data beyond
the bounds of their array. In that case, they throw an exception.

They are virtualizable in that new tvbuff's can be made from other tvbuffs, while
only the original tvbuff may have data. That is, the new tvbuff has virtual data.

There are three types of tvbuffs, defined by an enum in tvbuff.h.

A TVBUFF_REAL_DATA contains a guint8* that points to real data.
The data is allocated by the user and is contiguous, since is an array of guint8's.

A TVBUFF_SUBSET has a backing tvbuff. The TVBUFF_SUBSET is a "window"
through which the program sees only a portion of the backing tvbuff. 

A TVBUFF_COMPOSITE combines multiple tvbuffs sequentually to produce
a larger byte array.


The top-most dissector, dissect_packet(), creates a TVBUFF_REAL_DATA that points
the frame's data. As each dissector completes its portion of the protocl analysis,
it is expected to create a new tvbuff of type TVBUFF_SUBSET which contains
the payload portion of the protocol (that is, the bytes that are relevant to
the next dissector).

The syntax for creating a new TVBUFF_SUBSET is:

next_tvb = tvb_new_subset(tvb, offset, length, reported_length)

Where:
	tvb is the tvbuff that the dissector has been working on. It can be
	a tvbuff of any type.

	next_tvb is the new TVBUFF_SUBSET.

	offset is the byte offset of 'tvb' at which the new tvbuff should start.
	The first byte is the 0th byte.

	length is the number of bytes in the new TVBUFF_SUBSET. A length argument
	of -1 says to use as many bytes as are available in 'tvb'.

	reported_length is the number of bytes that the current protocol says
	should be in the payload. A reported_length of -1 says that the protocol
	doesn't say anything about the size of its payload.

The tvb_new_subset() function will throw an exception if the offset/length pair
go beyond the boundaries of 'tvb'.


The tvbuff is an opaque structure. It's definition is in tvbuff.c, not tvbuff.h, so
you can't easily access its members. You must use one of the provided accessor methods
to retrieve data from the tvbuff. All accessors will throw an exception if
an attempt is made to read beyond the boundaries of the data in the tvbuff.

If reported_length is set, then if the attempt to access data goes beyond reported_length,
a ReportedBoundsError exception is thrown.

Otherwise, if an attempt to access data beyond the bounds of the tvbuff is made,
a BoundsError exception is thrown.

The accessors are:


Single-byte accessor:

guint8  tvb_get_guint8(tvbuff_t*, gint offset);

Network-to-host-order access for shorts (guint16), longs (guint24), and
24-bit ints:

guint16 tvb_get_ntohs(tvbuff_t*, gint offset);
guint32 tvb_get_ntohl(tvbuff_t*, gint offset);
guint32 tvb_get_ntoh24(tvbuff_t*, gint offset);

Little-Endian-to-host-order access for shorts (guint16), longs (guint24), and
24-bit ints:

guint16 tvb_get_letohs(tvbuff_t*, gint offset);
guint32 tvb_get_letohl(tvbuff_t*, gint offset);
guint32 tvb_get_letoh24(tvbuff_t*, gint offset);


Copying memory:
guint8* tvb_memcpy(tvbuff_t*, guint8* target, gint offset, gint length);
guint8* tvb_memdup(tvbuff_t*, gint offset, gint length);


Pointer-retrieval:
/* WARNING! This function is possibly expensive, temporarily allocating
 * another copy of the packet data. Furthermore, it's dangerous because once
 * this pointer is given to the user, there's no guarantee that the user will
 * honor the 'length' and not overstep the boundaries of the buffer.
 */ 
guint8* tvb_get_ptr(tvbuff_t*, gint offset, gint length);

The reason that tvb_get_ptr() have to allocate a copy of its data only occurs
with TVBUFF_COMPOSITES. If the user request a pointer to a range of bytes that
spans the member tvbuffs that make up the TVBUFF_COMPOSITE, the data will have
to be copied to another memory region to assure that all the bytes are contiguous.


Modifications to the Dissectors

The dissector prototype will now be:

void/gboolean	dissector(tvbuff_t*, packet_info*, proto_tree*)

The packet_info struct now has the frame_data struct that used to be passed
to each dissector.  Additionally, packet_info has a char* called 'current_proto'.
The first thing a dissector should do is set pinfo->current_proto to point
to a string referring to the name of the protocol (use the same name that appears
in the COL_PROTO column, if possible). If an exception jumps us out of a dissector,
dissect_packet() will use pinfo->current_proto to report which dissector encountered
an exception.

The packet_info struct also has a tvbuff_t* called 'compat_top_tvb'. This points
to the same tvbuff_t that dissect_packet() creates. This is useful for creating
a tvbuff (TVBUFF_SUBSET) inside a dissector which itself does not use tvbuffs.
Once all the dissectors are converted to use tvbuffs, 'compat_top_tvb' will be removed.

A dissector that uses tvbuffs can call another dissector that does not. This code
snippet shows how:

	tvbuff_t	*next_tvb;
	const guint8	*next_pd;
	int		next_offset;

	....

	next_tvb = tvb_new_subset(tvb, offset_of_next_protocol, -1);
	tvb_compat(next_tvb, &next_pd, &next_offset);

That is, next_pd and next_offset will get assigned values relative to the start of
the byte array, not relative to the tvbuff. This function, tvb_compat(), is only
useful while the dissectors are in transition; once all dissectors are converted,
this function can be removed.


Exceptions

The exception module from Kazlib was copied into the Ethereal tree. A header file
"exceptions.h" was created which defines C preprocess macros that make the usage
of the exception functions easier. The exception routines in Kazlib have a lot of
features, but in Ethereal we only need a subset of those features, so the macros
hide the complexity of the Kazlib calls, and try to emulate the syntax of languages
which have native support for exceptions.