Because the Layer 2 protocols depend heavily on the features originally specified for PPP, it is worth examining this protocol more closely. PPP was designed to send data across dial-up or dedicated point to- point connections. PPP encapsulates IP, IPX, and NetBEUI packets within PPP frames, and then transmits the PPP-encapsulated packets across a point-to-point link. PPP is used between a dial-up client and an NAS.
There are four distinct phases of negotiation in a PPP dial-up session. Each of these four phases must complete successfully before the PPP connection is ready to transfer user data.
Phase 1: PPP link Establishment
PPP uses Link Control Protocol (LCP) to establish, maintain, and end the physical connection. During the initial LCP phase, basic communication options are selected. During the link establishment phase (Phase 1), authentication protocols are selected, but they are not actually implemented until the connection authentication phase (Phase 2). Similarly, during LCP a decision is made as to whether the two peers will negotiate the use of compression and/or encryption. The actual choice of compression and encryption algorithms and other details occurs during Phase 4.
Phase 2: User Authentication
In the second phase, the client PC presents the user's credentials to the remote access server. A secure authentication scheme provides protection against replay attacks and remote client impersonation. A replay attack occurs when a third party monitors a successful connection and uses captured packets to play back the remote client's response so that it can gain an authenticated connection. Remote client impersonation occurs when a third party takes over an authenticated connection. The intruder waits until the connection has been authenticated, and then traps the conversation parameters, disconnects the authenticated user, and takes control of the authenticated connection.
Most implementations of PPP provide limited authentication methods, typically Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), and Microsoft Challenge Handshake Authentication Protocol (MSCHAP).
1.Password Authentication Protocol (PAP): PAP is a simple, clear-text authentication scheme. The NAS requests the user name and password, and PAP returns them in clear text (unencrypted). Obviously, this authentication scheme is not secure because a third party could capture the User s name and password and use it to get subsequent access to the NAS and all of the resources provided by the NAS. PAP provides no protection against replay attacks or remote client impersonation once the user's password is compromised.
2.Challenge-Handshake Authentication Protocol (CHAP): CHAP is an encrypted authentication mechanism that avoids transmission of the actual password on the connection. The NAS sends a challenge, which consists of a session ID and an arbitrary challenge string, to the remote client. The remote client must use the MD5 one-way hashing algorithm to return the user name and an encryption of the challenge, session ID, and the client's password. The user name is sent without hash.
CHAP is an improvement over PAP because the clear-text password is not sent over the link. Instead, the password is used to create an encrypted hash from the original challenge. The server knows the client's clear-text password and can, therefore, replicate the operation and compare the result to the password sent in the client's response. CHAP protects against replay attacks by using an arbitrary challenge string for each authentication attempt. CHAP protects against remote client impersonation by unpredictably sending repeated challenges to the remote client throughout the duration of the connection.
Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP).
MS-CHAP is an encrypted authentication mechanism very similar to CHAP. As in CHAP, the NAS sends a challenge, which consists of a session ID and an arbitrary challenge string, to the remote client. The remote client must return the user name and an MD4 hash of the challenge string, the session ID, and the MD4-hashed password. This design, which manipulates a hash of the MD4 hash of the password, provides an additional level of security because it allows the server to store hashed passwords instead of clear-text passwords. MS-CHAP also provides additional error codes, including a password expired code, and additional encrypted client-server messages that permit users to change their passwords. In MS-CHAP, both the Client and the NAS independently generate an initial key for subsequent data encryption by MPPE. Therefore, MS-CHAP authentication is required to enable MPPE-based data encryption.
During phase 2 of PPP link configuration, the NAS collects the authentication data, and then validates the data against its own user database or against a central authentication database server, such as one maintained by a Microsoft® Windows NT® Primary Domain Controller (PDC) or a Remote Authentication Dial-in User Service (RADIUS) server.
Phase 3: PPP Callback Control
The Microsoft implementation of PPP includes an optional callback control phase. This phase uses the Callback Control Protocol (CBCP) immediately after the authentication phase. If configured for callback, both the remote client and NAS disconnect after authentication. The NAS then calls the remote client back at a specified phone number. This provides. an additional level of security to dial-up networking. The NAS allows connections from remote clients physically residing at specific phone numbers only.
Phase 4: Invoking Network Layer Protocol(s)
Once the previous phases have been completed, PPP invokes the various Network Control Protocols (NCPs) that were selected during the link establishment phase (Phase I) to configure protocols used by the remote client. For example, during this phase the IP Cont' 01 Protocol (IPCP) can assign a dynamic address to the dial-in user. In the Microsoft implementation of PPP, the compression control protocol is used to negotiate both data compression (using MPPC) and data encryption (using MPPE) because both are implemented in the same routine.
Once the four phases of negotiation have been completed, PPP begins to forward data to and from the two peers. Each transmitted data packet is wrapped in a PPP header which is removed by the receiving system. If data compression was selected in phase 1 and negotiated in phase 4, data is compressed before transmission. If data encryption is selected and negotiated, data is encrypted before transmission.
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