Re: Making my own IDS... how to detect packet loss ?

[Paul Palmer <paul_palmer () us ibm com>]

The short answer is that you correlate the SEQ# from one side of the 
connection with the ACK# you see on the other side of the connection. If 
you are seeing SEQ# from after the lost data and the other host is 
acknowledging receipt past the lost data, it is highly unlikely that you 
will ever see the lost data come across the wire.

Paul




From:
Jonathon <thejunkjon () gmail com>
To:
focus-ids () securityfocus com
Date:
07/16/2009 12:58 PM
Subject:
Making my own IDS... how to detect packet loss ?



I am writing my own IDS.  I have a packet sniffer + TCP reassembler
that I've written.  For each stream I detect, I have to keep some
state + a buffer of the packets for the stream.  However, a problem
that I've run into is that sometimes packets could get lost (meaning I
do not capture them, but they were actually sent/received between two
hosts on the network).  If I do drop the packets, my current
implementation gets all messed up since I never end up freeing the
resources I've allocated for that stream.  So, lost packets seem to be
the bain of my current implementation.

My question is (assuming all TCP streams), how can I determine whether
a packet is lost by just looking at packets that I currently have in
my buffer?

The packets could come in any order so just because I receive a packet
with seq #N, doesn't mean that a packet with seq #N-1 was sent.

I hope this is the appropriate mailing list for my question.

Thanks
J

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A guide to understanding SSL certificates, how they operate and their application. By making use of an SSL certificate 
on your web server, you can securely collect sensitive information online, and increase business by giving your 
customers confidence that their transactions are safe.
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