----- Original Message -----
From: "ronnie sahlberg" <ronniesahlberg@xxxxxxxxx>
To: "Michael Richardson" <mcr@xxxxxxxxxxxxxxxxxxxxxx>
Cc: "LEGO" <luis.ontanon@xxxxxxxxx>; "Ethereal development"
<ethereal-dev@xxxxxxxxxxxx>; <ntar-workers@xxxxxxxxxxx>
Sent: Sunday, June 26, 2005 8:27 PM
Subject: Re: [ntar-workers] Re: [Ethereal-dev] Re: NTAR - PCAP next
generationdump file formatimplementation
Can any of these things be written without seeking backwards in the
file? Can we make sure that the value 0, or some other nonsense value
can be inserted to mean, "I didn't calculate that", and let a
post-processor worry about fixing the actual values in.
I agree with the implied meaning above that a no seeking backward
solution is absolutely required so that one can pass a capture through
a pipe.
Yes, of course.
At the moment NTAR fully supports reading and writing from stdin/stdout. As
a matter of facts, the only difference (apart from some settings to putt
stdin/stdout in binary mode on windows) is the support (or lack of it) for
seek operation. And at the moment backward seeks are actually disabled even
on normal files, due to the hell of having a portable fseek working on large
files. I'm working on it...
Number of bytes until the next marker should be possible to implement
without seeking in the file. Just make a new marker once every 1Mbyte
into the file and if there is not enough space left to fit the next
packet to write, just add some padding data until the next marker.
Uh, we should think of where to put such padding. pcap-ng does not specify
any additional "random" padding (apart from the one used to align everything
to 32bits).
In general, I would prefer not to have such "hacks" in the file format, as
much as I'd like not to have these marker blocks required. The application
should be We already have the interface description block that is needed in
every section...
Then one marker would always know that the next marker starts 1 MB
from now and no seeking of the file required.
The number of packets until next marker and the timestamps can be left
as zero indicating "i didnt calculate that" and be left for a
postprocessing tool to calculate if required.
If the bytes until next marker points to beyong the end of the file
this would just mean : no more markers.
Uhm... I'm not sure this will work: a file can contain multiple sections,
either because the app writing the file created multiple sections, or simply
because the user cat'ted two trace files together. A "blind" jump of 1MB
ahead will result in an error (we will jump somewhere in the next section).
A solution is to compute the effective length of "marked" packet blocks
making use of the section length (this length is saved in the section header
block), but this approach does not work all the times, since this length is
written on file only if the underlying file descriptor support backward
seeks (i.e. it will not work with pipes).
Have a nice day
GV
On 6/27/05, Michael Richardson <mcr@xxxxxxxxxxxxxxxxxxxxxx> wrote:
-----BEGIN PGP SIGNED MESSAGE-----
>>>>> "Gianluca" == Gianluca Varenni <gianluca.varenni@xxxxxxxxx> writes:
Gianluca> In my opinion the marker should contain the following
informations
Gianluca> - number of packets up to the next "marker block"
Gianluca> - number of bytes up to the next "marker block"
Gianluca> - timestamp of the first and last packet up to the next
"marker block"
Gianluca> - offset to the next "marker block"
Gianluca> - other??
Can any of these things be written without seeking backwards in the
file? Can we make sure that the value 0, or some other nonsense value
can be inserted to mean, "I didn't calculate that", and let a
post-processor worry about fixing the actual values in.
- --
] Michael Richardson Xelerance Corporation, Ottawa, ON |
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] mcr @ xelerance.com Now doing IPsec training, see |net
architect[
] http://www.sandelman.ca/mcr/ www.xelerance.com/training/ |device
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