Applications which operate as servers (such as HTTP, SMTP, and so on) or peer-to-peer applications can have difficulty when operating behind an S-NAPT because traffic from the Internet cannot reach the NAT without a mapping in place.
Different methods can be employed to overcome this, including:
Port forwarding
STUN support
Application Layer Gateways (ALG)
The 7750 SR supports all three methods following the best-practice RFC for TCP (RFC 5382, NAT Behavioral Requirements for TCP) and UDP (RFC 4787, Network Address Translation (NAT) Behavioral Requirements for Unicast UDP). Port Forwarding is supported on the 7750 SR to allow servers which operate on well-known ports <1024 (such as HTTP and SMTP) to request the appropriate outside port for permanent allocation.
STUN is facilitated by the support of Endpoint-Independent Filtering and Endpoint-Independent Mapping (RFC 4787) in the NAT device, allowing STUN-capable applications to detect the NAT and allow inbound P2P connections for that specific application. Many new SIP clients and IM chat applications are STUN capable.
Application Layer Gateways (ALG) allows the NAT to monitor the application running over TCP or UDP and make appropriate changes in the NAT translations to suit. The 7750 SR has an FTP ALG enabled following the recommendation of the IETF BEHAVE RFC for NAT (RFC 5382).
Even with these three mechanisms some applications still experience difficulty operating behind a NAT. As an industry-wide issue, forums like UPnP the IETF, operator and vendor communities are seeking technical alternatives for application developers to traverse NAT (including STUN support). In many cases the alternative of an IPv6-capable application gives better long-term support without the cost or complexity associated with NAT.