# wrong size computation

Hi folks,
As regards the way bro deals with the number of bytes transferred for
each connection, it seems that bro DOES NOT keep a variable in which
incrementally stores the sum of each packet size for all the packets
involved in that very connection, but instead does a certain computation
(i wonder how...) involving only the first and the last packet in the
connection... Am I correct?
I'm asking this question because I've found something very strange.
In bro's conn.log file there are lines like this:

Dec 1 00:22:53 1.058870 A B http 49331 80 tcp 886477697 ? RSTOS0 L

(yes it's correct, 800MB in 1 second) but if I look at the trace, this
is what I see:

A B
49331 --> 80 (SYN) Seq=0,Ack=0
49331 <-- 80 (ACK) Seq=0,Ack=0
49331 --> 80 (RST) Seq=0,Ack=188164531

(Only 3 packets transferred...)

Of course there's some bug in these hosts, but bro should not be
misleaded in computing the amount of bytes transferred inside a
connection. Could someone explain me what's happening here?

Best regards,

Vincenzo

As regards the way bro deals with the number of bytes transferred for
each connection, it seems that bro DOES NOT keep a variable in which
incrementally stores the sum of each packet size for all the packets
involved in that very connection, but instead does a certain computation
(i wonder how...) involving only the first and the last packet in the
connection... Am I correct?

Yes, for TCP connections Bro computes connection sizes based on TCP sequence numbers.

I'm asking this question because I've found something very strange.
In bro's conn.log file there are lines like this:

Dec 1 00:22:53 1.058870 A B http 49331 80 tcp 886477697 ? RSTOS0 L

(yes it's correct, 800MB in 1 second) but if I look at the trace, this
is what I see:

A B
49331 --> 80 (SYN) Seq=0,Ack=0
49331 <-- 80 (ACK) Seq=0,Ack=0
49331 --> 80 (RST) Seq=0,Ack=188164531

(Only 3 packets transferred...)

Of course there's some bug in these hosts, but bro should not be
misleaded in computing the amount of bytes transferred inside a
connection. Could someone explain me what's happening here?

This is weird. Could you please send me the trace file?

I think there are two separate issues here:

1. How Bro comes up with the 800MB number and whether the ack sequence in the RST packet should be taken into computation. We need to look at the trace to find out.

2. In general, Bro connection sizes can be incorrect in weird cases, and if you want accurate numbers in your measurements, my suggestions (and what we did in our measurements) are: (a) if you have all packets in your traces, there's a Bro script (Vern?) to sample packets very efficiently to estimate connection size, even in the case the sequence numbers wrap around a number of times, which then can be used to validate the connection size; (b) if all you have are SYN, FIN, and RST packets, you can make some assumption about the bandwidth of the link to check if the connection size makes sense.

Ruoming

Here it is a sample... it is just an handshake but if you run

\$bro -r to-brolist_anonym.pcap brolite

you will see in the conn.log file the BIG computation mistake...

Cheers,
Vincenzo

Ruoming Pang (庞若鸣) wrote:

to-brolist_anonym.pcap (212 Bytes)