giao thức. Không giống như Ethernet, có kích thước khung hình tối thiểu và các đơn vị truyền tải tối đa (MTU) cho Token Ring không phải là một số được xác định, nhưng phụ thuộc vào các yếu tố như tỷ lệ bit và giữ thời gian mã thông báo. MTUs Token Ring phức tạp hơn bởi sự hiện diện của cây cầu định tuyến nguồn Token Ring. | Chapter 4 Point-to-Point Protocol PPP 69 Protocol lllllll IIIIIII 0XC0-23 Code Identifier Length Peer ID Length Peer ID Password Length Password IIIIIII 1 IIIIIII IIIIIII IIIIIII IIIIIII mini IIIIIII IIIIIII IIIIIII IIIIIII num num num num Figure 4-3 The structure of the PAP Authenticate-Request message Password Length A 1-byte field that indicates the size of the Password field in bytes. Password A variable-sized field that contains the password of the calling peer. Figure 4-4 shows the PAP Authenticate-Ack and Authenticate-Nak messages. The following are the fields in the Authenticate-Ack and Authenticate-Nak messages Code For an Authenticate-Ack message the value of the Code field is set to 2. For an Authenticate-Nak message the value of the Code field is set to 3. Identifier A 1-byte field that is set to the value of the Identifier field in the corresponding Authenticate-Request message. Length A 2-byte field that indicates the size of the PAP message in bytes. Message Length A 1-byte field that indicates the size of the Message field in bytes. Message A variable-sized field that contains a message for the calling peer. The Message field is not used by Windows. Some PPP implementations display the message text to the user who is connecting. 70 Part I The Network Interface Layer Capture 04-02 in the Captures folder on the companion CD-ROM contains an example of a PAP authentication. CHAP CHAP is a more secure authentication protocol described in RFC 1994 which uses a challenge-response exchange of messages to validate that the calling peer has knowledge of the user s password. The password itself is never sent. Although more secure than PAP CHAP does not provide mutual authentication. The calling peer authenticates to the answering peer but the answering peer does not authenticate to the calling peer. Without mutual authentication a calling peer is unable to determine whether it is calling a valid answering peer. When the use of CHAP is negotiated during phase 1
