第五章补充b 点到点协议简介.ppt

Additional Materials for Ethereal Network Analysis Example from New York Times Point-to-Point Protocol Analysis by using Ethereal Network Analyzer Example Ethereal windows Top pane: frame sequence Middle pane: Encapsulation Middle pane: Encapsulation Middle pane: Encapsulation PPP: Point-to-Point Protocol Data link protocol for point-to-point lines in Internet Router-router; dial-up to router 1. Provides Framing and Error Detection Character-oriented HDLC-like frame structure 2. Link Control Protocol Bringing up, testing, bringing down lines; negotiating options Authentication: key capability in ISP access 3. A family of Network Control Protocols specific to different network layer protocols IP, OSI network layer, IPX (Novell), Appletalk PPP Applications PPP used in many point-to-point applications Telephone Modem Links 30 kbps Packet over SONET 600 Mbps to 10 Gbps IP→PPP→SONET PPP is also used over shared links such as Ethernet to provide LCP, NCP, and authentication features PPP over Ethernet (RFC 2516) Used over DSL PPP Frame Format PPP Frame PPP uses similar frame structure as HDLC, except Protocol type field Payload contains an integer number of bytes PPP uses the same flag, but uses byte stuffing Problems with PPP byte stuffing Size of frame varies unpredictably due to byte insertion Malicious users can inflate bandwidth by inserting 7D 7E Byte-Stuffing in PPP PPP is character-oriented version of HDLC Flag is 0x7E Control escape 0x7D Any occurrence of flag or control escape inside of frame is replaced with 0x7D followed by original octet XORed with 0x20 PPP Example PPP Phases Home PC to Internet Service Provider 1. PC calls router via modem 2. PC and router exchange LCP packets to negotiate PPP parameters 3. Check on identities 4. NCP packets exchanged to configure the network layer, e.g. TCP/IP ( requires IP address assignment) 5. Data transport, e.g. send/receive IP packets 6. NCP used to tear do