Study of Middle-box Behavior on Network Layer Protocols

The home gateways typically act as middle-boxes between the internal network of a home user or small enterprise. These middle-boxes often perform various higher-layer functions such as traffic filtering, network address translation (NAT), advanced application layer operations and act as dynamic host configuration protocol (DHCP) server. While some of these functions such as DHCP are well standardized, some functions such as NAT have only been defined on a more abstract level and the exact operations have not been standardized. These more loosely defined functions are known to have undesired effects on normal protocol functions and hinder the development of new protocols and applications. Therefore, it is important to understand the various characteristics of different middle-boxes deployed all around the world to allow network engineers design protocols that can be deployed in realistic environments that typically include middle-boxes. In this paper, we perform an experimental study on a number of different home gateways focusing on the network layer (Layer 3) functionality, which is responsible for packet forwarding and routing. The experiments include tests on how the home gateways treat various IPv4 packet header fields, including IP options, Explicit Congestion Notification (ECN) and Differentiated Services Code Point (DSCP) fields, and how they treat broadcasted packets.