![]() SSL and TLS are protocols for securing data flows at Layer 4 of the OSI model. Windows Server 2003 and Windows 2000 Server include L2TP/IPsec servers. Microsoft has provided a free L2TP/IPsec VPN client for Windows 98, ME and NT since 2002, and ships an L2TP/IPsec VPN client with Windows XP, 2000, 2003 and Vista. L2TP/IPsec combines L2TP's tunnel with IPsec's secure channel, which allows for easier secure Internet Key Exchange than pure IPsec. There are several open-source implementations of L2TP for Linux. L2TP can carry PPP sessions within its tunnel. This provides a tunnel, but no security or authentication. L2TP combines ideas from PPTP and L2F, an older protocol developed by Cisco Systems Inc., to create a data link layer protocol. Microsoft has addressed this issue by enforcing password strength policies in its operating systems, but Schneier and Mudge still recommend IPsec rather than PPTP for secure VPNs as inherently safer. Schneier, with "Mudge" of L0pht Heavy Industries, found and published security flaws in Microsoft PPTP in 1998 Microsoft quickly fixed these issues with MS-CHAPv2 and MPPE, and Schneier and Mudge published an analysis confirming the improvements in 1999, but they pointed out that the security of Microsoft PPTP still depended on the security of each user's password. However, as implemented by Microsoft, it has not always been the most secure of the secure VPNs. PPTP has been very popular, especially on Windows systems, because it is widely available, free and easy to set up. The company has also included PPTP servers in all its server products since Windows NT 4.0. Microsoft has included PPTP clients in all versions of Windows since Windows 95 OSR2 PPTP clients are in Linux, Mac OS X, Palm PDA devices and Window Mobile 2003 devices. ![]() Many vendors supply IPsec VPN servers and clients. IPsec is supported in Windows XP, 2000, 2003 and Vista in Linux 2.6 and later in Mac OS X, NetBSD, FreeBSD and OpenBSD in Solaris, AIX and HP-UX and in VxWorks. ![]() Some security experts, for instance, Bruce Schneier of Counterpane Internet Security Inc., have considered IPsec the preferred protocol for VPNs since the late 1990s. IPsec has a set of cryptographic protocols for two purposes: securing network packets and exchanging encryption keys. IPsec, or IP security, is a standard for encrypting and/or authenticating IP packets at the network layer. Secure VPNs can use IPsec with encryption, IPsec with Layer 2 Tunneling Protocol (L2TP), SSL 3.0 or Transport Layer Security (TLS) with encryption, Layer Two Forwarding (L2F) or Point-to-Point Tunneling Protocol (PPTP). ![]() MPLS is beginning to replace ATM and frame relay to implement trusted VPNs for large corporations and service providers. (Layer 1 is the physical layer Layer 3 is the network layer.) MPLS emulates some properties of a circuit-switched network over a packet-switched network, and operates at a layer often referred to as "2.5" that is intermediate between the data link and the network. The major technologies used for implementing trusted VPNs over IP networks are ATM circuits, frame-relay circuits and Multiprotocol Label Switching (MPLS).ĪTM and frame relay operate at the data link layer, which is Layer 2 of the OSI model. Over the years, implementations of trusted VPNs have moved from raw private circuits leased from telecommunications vendors to private IP network circuits leased from Internet providers. ![]()
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