Introduction to PPP Sharon Katz

History Dial-up file transfer protocol Early 80`s: PC users begin to communicate through file transfer protocols: X-Modem Y-Modem Z-Modem Asymetric protocols In the unix world TCP/IP vs. file transfer protocols Telecommunication technologies:  Local networks: 3MB and 10MB ethernets.  Distance communication: Leased lines Primitive dial-up modems Dial-up networking

Using dial-up modems for packet-oriented- networking (I.e connecting to the internet) requires a data-link layer protocol widely-spread protocols: SLIP PPP History

SLIP - Serial Line Internet Protocol Defined in the early 80`s Purpose extending TCP/IP networking with dial-up serial connection Users unix users at the beginning and PC users later Advantages Very simple protocol Easy implementation

SLIP - Serial Line Internet Protocol Disadvantages Does not support error detction and correction Supports only IP (and not IPX/SPX etc`) Requires advance knowledge of the peer`s IP address Is not approved internet standard Does not provide any form of authentication

PPP – design principles Support multiple network protocols Link configuration Error detection Establishing network addresses Authentication Extensibility

PPP – a layered protocol PPP relies on another DLP – HDLC – to perform some basic operations After the initial handshake, PPP executes its own handshake PPP itself consists of two protocols: LCP – Link Control Protocol NCP – Network Control Protocol

HDLC Definition High-level Data Link Control, Bit-oriented link protocol published by ISO foundation for other protocols (examples ) Operations 1 st stage – frame formation 2 nd stage – medium-dependant frame transmission

3 variable-length fields Address – intended for multidrop links Control – specifies the type of message transmitted Information – content transferred by the top level application 1 fixed-length field Check HDLC – frame format addresscontrolinformationcheck HDLC frame

Establishing PPP communication requires transitioning a connection through several states Some of these operations are done by LCP and some by NCP PPP state machine

DeadEstablish Network terminateAuthenticate PPP state machine opened Success / None down closing fail up

PPP – frame format Protocol type of information in data/padding field (I.e: IP, IPCP, LCP, CHAP etc`) Data/Padding data for the upper layer protocols or PPP control packets dataprotocolflagaddrcontrol padding check PPP frame

LCP Purposes Link establishment Link maintenance Link termination Optional operations Link quality determination Authentication

LCP - packets There are 3 classes of LCP packets: Link configuration configure-request, configure-ack, configure-nak & configure-reject Link termination terminate-request & terminate-ack Link monitoring code-reject, protocol-reject, echo-request, echo-reply & discard-request

LCP – packet format Code – type of LCP packet (configure-ack etc`) ID – request-response matching ID Length – of the LCP packet Data – the LCP packet codelengthDataIDPPP frame

LCP Options MRU determination Magic number selection Authentication Protocol Escaped characters map

NCP Purpose Configuring the network layer protocol. There exists a separate NCP for each network layer protocol Negotiation process Same message formats, code numbers and state machines as LCP

IPCP – IP Control Protocol Purpose TCP/IP matching NCP Establishes, configures and terminates the TCP/IP network layer protocol Options IP-Compression protocol – I.e Van-Jacobson (VJ) compressed TCP/IP IP address – allows dynamic IP configuration DNS & NBNS address

IPCP – packet format data0x80210x7E0xFF0x03check PPP frame (protocol) IPCP datalengthCodeID

PPP – unsupported options Flow control Any PPP frame sent that overflows the receiver's buffer are lost Error correction PPP includes only Frame Check Sequence (CRC) Re-sequencing PPP assumes all frames, sent and received, retain their original intended order

Tunneling - definition The process of running one network protocol on top of another. Common use: VPN (Virtual Private Network) Tunneling method Extending the link between the HDLC driver and the rest of PPP over a separate network PPP tunneling protocols L2TP, L2F, PPTP & ethernet (PPPoE) Tunneling & PPP

THE MIDDLE Next: PPP security

Internet layered architechture Application Transport Network Data Link Physical Media, clocking and synchronization Data link operations (???) Forwarding and route discovery Port and traffic management End-user application

Data link protocol Defintion manages node-to-node transfer of data between two directly connected machines. Operations Error detection and correction (depends on the protocol) Addressing (in LANs) Frame-level synchronization between sender and receiver Flow control Maintaining awareness of link conditions

HDLC`s family HDLC LAPB (x. 25) LAPM (v. 42) LAPD (ISDN) LLC (LANs) SDLC (SNA) LAPX (teletex) ESF (T1) MTP-2 (SS7) LAPDm (GSM) LAPF (frame-relay) PPP (many)

PPP – a layered protocol Application Transport Network NCP LCP HDLC Physical Data Link PPP

PPP – design principles Support multiple network protocols Link configuration Error detection Establishing network addresses Authentication Extansibility Not limited to the internet and other TCP/IP networks, I.e: IPX/SPX and IPv6 Includes nagotiation mechanisms for establishing communication parameters betweens two PPP peers Detects datagrams corruption upon reception and discards such corrupted data Sets network addresses necessary for the datagrams routing Supports authentication between peers before useful communication begins Yet, maintains backward compatibility