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2 bytes Special semantics: From 0xxx to 3xxx = network layer protocols From 8xxx to Bxxx = associated NCPs From 4xxx to 7xxx = upper protocols with no NCP From Cxxx to Fxxx = Linklayer Control protocols Internet Protocol Control Protocol (IPCP)Ĭhallenge Handshake Authentication Protocol (CHAP) not as flag)ġ1 3F 7D 5E 02 7D 5D 10 See RFC 1662 for bit-stuffing over bit synchronous links (versus byte-oriented links) Giuseppe Bianchi Receiver detects control escape And removes it restoring original byte as part of the frame (e.g. 0111.1110 flag is used as frame delimiter! Usual solution: stuffing via control escape octes: Send (“reserved” byte) as (control escape octet) + (reserved byte)XOR(0010.0000) Extra XOR (bit 5 complemented) for improved robustness Control escape octet 0111.1101 = 0x7d – of course NOW this is a reserved byte, too!
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PPP FRAME FLAG HOW TO
How to find the end of the Information? Locate flag, then remove FSC! Giuseppe Bianchiīyte stuffing Usual problem: how to send “reserved” bytes into information payload? E.g. May be removed (through suitable negotiation) over slow links Giuseppe Bianchiįrame Check Sequence 16 bits defauls May be negotiated to 32 through LCP Covers address, control, protocol, information (+ padding inside the information field, if employed) Flag Address Control 1 byte 1 byte 1 byte 01111110 11111111 00000011 Address & control NOT USEFUL (fixed values!!). Two consecutive flags = empty frame Just ignore flagĪddress No station address assigned! “all-station” address: 1111.1111 = 0xff Indeed: not necessary in a Point-to-point link!Ĭontrol field Fixed value: Unnumbered Information 0000.0011 = 0x03 i.e.
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Each frame starts & ends with reserved flag: 0111.1110 = 0x7e Just one flag required between consecutive frames flag What PPP does NOT provide Multi-point Limited to single pair of TX/RX peers Error correction, retransmission Provides only error checking via FCS Delivery in order Relies on orderly deliver from PHY Flow Control PPP transmits at maximum possible speed of the underlying PHY Giuseppe Bianchiįrame Format Flag Address Control 1 byte 1 byte 1 byte 01111110 11111111 00000011 IPCP = IP Control Protocol Giuseppe Bianchi A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols. PPP components Three main components: A method for encapsulating multi-protocol datagrams A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection. circuit switched telephone connection But also (semi)-permanent ADSL connection Multi-protocol support May support IP as well as other network-layer protocols (IPX, Appletalk, etc)
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PPP FRAME FLAG PC
PPP Protocol Stack Provides point to point link with error detection capabilities PC NAS Runs over a transparent full-duplex PHY Assumed to deliver packets in order branch offices) may be possible through a dedicated or dialon-demand WAN connection PPP: where (WAN) Interconnection of small networks (e.g. Authentication (CHAP, PAP)Īccesso diretto al nodo del fornitore di servizio Giuseppe BianchiĪccesso al nodo del fornitore di servizio mediante NAS (Network Access Server) del fornitore di accesso Giuseppe Bianchi PPP: where (access) Account management (RADIUS, DIAMETER) … disattended by 50+ RFCs … Recommended reading: PPP specification, RFC 1661 + 1662 (STD 51), July 1994 Giuseppe Bianchi Lecture 1.1: Point to Point Protocol (PPP) An introduction "the watchword for a point-to-point protocol should be simplicity" (RFC 1547, PPP requirements).