RE: NAPT - NAT Port selection

["Bill Royds" <broyds () rogers com>]

If the destination IP address is different, a port forwarder on the receiving
end has your problem in reverse. It needs to translate the destination port on
one IP to a different port on the actual web server. As long as the combination
source_IP:Source_Port,Destination_IP:Destination_Port is unique, then you have a
unique TCP (or UDP) connection. 
So (from your example)
Source_IP     Destination_IP   Protocol    Source_Port    Destination_Port
 66.10.5.10    70.1.2.5              TCP            2000               80
 66.10.5.10    70.1.5.6              TCP            2000               80
A port forwarding load balance on destination IP 70.1.2.5 and destination
70.1.2.6 could translate incoming traffic by changing your source ports as it
goes through their load balancer's NAT. Internally their servers would see

Source_IP     Destination_IP   Protocol    Source_Port    Destination_Port
 66.10.5.10    192.168.20.5          TCP            32001              80
 66.10.5.10    192.168.20.5          TCP            32002              80

Which leaves the sockets still unique. Their load balancer has the
responsibility of ensuring uniqueness within their internal network, not you.

The limit of 64K is for source ports per sourceaddress:destination IP:port
triplet, not just per source address. How do you think AOL supports millions of
customers on a fairly limited IP source address space?

-----Original Message-----
From: firewall-wizards-admin () honor icsalabs com
[mailto:firewall-wizards-admin () honor icsalabs com] On Behalf Of
ravivsn () www rocsys com
Sent: Thursday, August 19, 2004 11:36 PM
To: firewall-wizards () honor icsalabs com
Cc: ravivsn () rocsys com
Subject: [fw-wiz] NAPT - NAT Port selection

Hi,
NAPT devices allow access to internet by internal machines having
private IP addresses using one or more public IP addresses.

We are vendors of security devices and these have NAPT feature
and we allow upto 63K TCP connections from internal machines at any
time. 63K limit comes from number of unique source ports that can be
assigned as NAT ports.

Our customer, who is one of  small ISPs, wanted to use these devices. This
ISP gives private IP addresses to their customers and using NAPT provides
internet access to its customers. The ISP has limited number of public IP
addresses and it wants us to increase the number of TCP connection for each
public IP address to go from 63K to very high number.

Internally, among developers, we discussed this issue and we came out with
one suggestion - Reusage of NAT port in multiple sessions, as long as
atleast one of 5 tuples is different - Since source IP is same (public IP
address), destination IP or destination port has to be different.

It means that, on the Internet side, it is possible to have following
(example):
 Source_IP     Destination_IP   Protocol    Source_Port    Destination_Port
 66.10.5.10    70.1.2.5              TCP            2000               80
 66.10.5.10    70.1.5.6              TCP            2000               80



One of our Engineers points out that, port forwarders on the receiving end
 might be assuming that  for a given source IP, source port would always
be different. His point is that, if both 70.1.2.5 and 70.1.5.6 belong to
same webhosting server and if a port-forwarder forwards traffic from
these two IP addresses to the same internal server, then internal server
might drop one connection.


Some of us feel that, this would be very rare condition and if it happens,
port forwarders are intelligent enough detect this and forward the
connection to next server and gracefullty shutdown  second connection.

I solicit your feedback on this.
     -  Is it good for NAPT device to use same NAT port for different
sessions, if they are  going to different destination (based on
Destination IP and Port)?  Do you see any problems associated with
this apart one mentioned above?
     - Any experiences?

Thanks in advance
Ravi



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