On Jun 18, 2007, at 2:49 AM, Kaushal Shriyan wrote:
what one means by saying dissector.
A dissector is a module in Wireshark (or in another network analyzer,
although they may use a term other than "dissector") that can look at
the raw data in a packet for a particular protocol and analyze it as a
set of fields - for example, the IPv4 header described in RFC 791:
http://www.ietf.org/rfc/rfc0791.txt
has, in the first byte, an IP version number of 4 in the upper 4 bits
and an IP header length (in units of 32-bit words) in the lower 4
bytes. The next byte has some information used to give information
about service quality requested for the packet, congestion
indications, and so forth, and then come two bytes, in host byte
order, giving the total length of the IP packet (header and data), and
so on and so forth. A "dissector" would show the values of all those
items as individual elements.
Also I am not able to understand "Packet Bytes" pane. what does the
hexadecimal number signifies,
The hexadecimal numbers in there are the raw byte values in the
packet. An Ethernet packet as sent by or received by a host, for
example, is just a sequence of bytes. The first 6 bytes are the
Ethernet address to which the packet is being sent, and the next 6
bytes are the Ethernet address from which the packet is being sent.
The next two bytes are either a packet length indication or a packet
type indication. A value of 0x08 0x00 is a packet type indication,
indicating that the packet is an IPv4 packet. If that's the case, the
bytes after the packet type indication are the bytes of an IPv4 header
followed by the IPv4 payload, which might be a TCP packet or a UDP
packet or....
Wireshark is an application that was designed under the assumption
that the user is at least somewhat familiar with the way network
packets are constructed (just as, for example, an oscilloscope is
designed under the assumption that the user is somewhat familiar with
the waveforms they're measuring, and a logic analyzer is designed
under the assumption that the user is somewhat familar with the
digital system they're analyzing); people unfamiliar with the way
network packets are constructed and network protocols work should
probably read a book about networking before using Wireshark, as,
otherwise, much of what Wireshark displays would mean nothing to them.
"TCP/IP Illustrated, Volume 1: The Protocols":
http://www.kohala.com/start/tcpipiv1.html
might be a good book for this.