Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface Chapter 10 SRC 192. 168.140.103 Figure 10-12 DST 192.168.140.25 Using the VPN 255.255.255.2 tunnel. 139.64.1.20 192.168.140.103 Public Network Intranet Tunnel Switch Intranet SRC 255.255.255.2 DST 192.168.1.20 Media Tunnel SRC 192.168.140.103 DST 192.168.140.25 case of an insecure network between the GGSN and the intranet switch (for example, the Internet network), a security protocol like IPSec can be used. The four existing tunneling protocols are the PPTP, L2F, L2TP, and IPSec: PPTP, L2F, and L2TP are L2TP; they are connectivity-oriented protocols that are limited in supporting PPP connections. This makes them unavailable for LAN-to-LAN connections. PPTP is a client-initiated protocol that doesn't require special ISP service; it creates a tunnel toward the destination switch using an IP in IP encapsulation and a generic routing encapsulation header (GRE). L2F and L2TP do not require any software on the client; they are implemented at the ISP side. A tunnel is created from the ISP Network Access Server (NAS) or the ISP LAC to the L2F or LNS of the destination switch. L2TP is a synthesis of PPTP and L2F protocols. IPSec is a Layer 3 Tunneling Protocol; it is a security-oriented protocol. It provides security protocols, including an AH and an encapsulation security header (for encryption), SAs to provide security for a connection data, key management, encryption, and an authentication algorithm. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface SGSN-to-GGSN (G) and GGSN-to-PDN (G) Interface PDP Context SGSN Role The SGSN receive