What is IP address "overlapping" within the context of NAT ?

network internet

IP address overlapping refers to a situation where two locations that want to interconnect are both using the same IP address scheme. This is not an unusual occurrence; it often happens when companies merge or are acquired. Without special support, the two locations will not be able to connect and establish sessions. The overlapped IP address can be a public address assigned to another company, a private address assigned to another company, or can come from the range of private addresses as defined in RFC 1918. Private IP addresses are unroutable and require NAT translations to allow connections to the outside world. The solution involves intercepting Domain Name System (DNS) name-query responses from the outside to the inside, setting up a translation for the outside address, and fixing up the DNS response before forwarding it to the inside host. A DNS server is required to be involved on both sides of the NAT device to resolve users
wanting to have connection between both networks. NAT is able to inspect and perform address translation on the contents of DNS “A” and “PTR” records, as shown in Using NAT in Overlapping Networks.

What is Service Provider PAT Port Allocation Enhancement for RTP and RTCP?

network internet

The Service Provider PAT Port Allocation Enhancement for RTP and RTCP feature ensures that for SIP, H.323, and Skinny voice calls. The port numbers used for RTP streams are even port numbers and the RTCP streams are the next subsequent odd port number. The port number is translated to a number within the range specified conforming to RFC-1889. A call with a port number within the range will result in a PAT translation to another port number within this range. Likewise, a PAT translation for a port number outside this range will not result in a translation to a number within the given range.

Difference between IP Fragmentation and TCP Segmentation

tcp ip model

     IP fragmentation occurs at Layer 3 (IP) and that TCP segmentation occurs at Layer 4 (TCP). IP Fragmentation takes place when packets that are larger than the Maximum Transmission Unit (MTU) of an interface  are sent out this interface. These packets will have to be either fragmented, or discarded when they are sent out the interface. If the Don’t Fragment (DF) bit is not set in the IP header of the packet, the packet will be fragmented. If the DF bit is set in the IP header of the packet, the packet is dropped and an ICMP error message indicating the next-hop MTU value will be returned to the sender. All the fragments of an IP packet carry the same Ident in the IP  header, this allows the final receiver to reassemble the fragments into the original IP packet. Please see Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC for more information.   TCP Segmentation takes place when when an application on an end station is sending data. The application data is broken into what TCP considers the best-sized chunks to send. This unit of data passed from TCP to IP is called a segment. TCP segments are sent in IP datagrams. These IP datagrams can then become IP Fragments as they pass through the network and encounter lower MTU links than they can fit through.

   TCP will first segment this data into TCP segments (based on TCP MSS value) and will add the TCP header and pass this TCP segment to IP. Then IP will add an IP header to send the packet to the remote end host. If the IP packet with the TCP segment is larger than the IP MTU on an outgoing interface on the path between the TCP hosts then IP will fragment the the IP/TCP packet in order to fit. These IP packet fragments will be reassembled on the remote host by the IP layer and the complete TCP segment (that was originally sent) will be handed to the TCP layer. The TCP layer has no idea that IP had fragmented the packet during transit. NAT can deal with IP fragments but it does not deal with TCP Segments.