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Point to Point Protocol (PPP)
Point to Point Protocol (PPP) protocol is utilized to establish a connection between your PC and
your ISP (Internet Service Provider). PPP is used on two points-connection only usually modem
to modem. This protocol accomplishes a few main tasks:
Checks the condition of the phone line.
Checks your password.
Provides your computer with an IP address valid for the duration of connection.
Establishes the configuration of packets (packet size, compression, etc).
Acknowledge one part that the other is closing the connection.
PPP operates by sending Request packets and waiting for Acknowledge packets that either
accept, reject or try to change the request.
Various protocols are utilized to accomplish different tasks which together form the PPP.
Link Control Protocol (LCP) checks among other things the condition of the line by ending a
large (4 byte-hex) randomly chosen number (Magic Number) which needs to be returned by the
receiving part.
Password Authentication Protocol (PAP) or Challenged Authentication Protocol (CHAP)
checks the password either sent in clear (PAP) or encrypted (CHAP).
Internet Protocol Control Protocol (IPCP) is utilized to assign an IP address to the PC, to
determine if compression will be used and to setup the configuration of the packets
encapsulated within PPP frames.
PPP PACKET FORMAT (example):
The fields are transmitted from left to right.
+----------+----------+----------+----------+---------------+----------+----------
| Flag | Address | Control | Protocol | Information | FCS | Flag |
| 0x7E | 0xFF | 0x03 | 16 bits | * | 16 bits | 0x7E |
+----------+----------+----------+----------+---------------+----------+----------
Here is a typical example of a PPP packet:
7E FF 7D 23 C0 21 7D 21 7D 22 7D 20 7D 38 7D 21 7D 24 7D 25 DC 7D 22 7D 26 7D 20
7D 20 7D 20 7D 20 7D 25 7D 26 29 23 BE 84 7D 27 7D 22 7D 28 7D 22 DF 7D 30 7E
Packet content:
Start7E
Address FF 03 FF 7D 23
Protocol (LCP) C0 21
Code (CONFREQ) 01 7D 21
Id 01 7D 21
Length 00 187D 20 7D 38
Type, Length, MRU 01 04 05 DC 7D 21 7D 24 7D 25 DC
Type, Length, ACCM 02 06 00 00 00 007D 22 7D 26 7D 20 7D 20 7D 20 7D 20
Type, Length, MAGIC 05 06 29 23 BE 847D 25 7D 26 29 23 BE 84
Type, Length, PFC 07 027D 27 7D 22
Type, Length, ACFC 08 027D 28 7D 22
FCS DF 10 DF 7D 30
Stop 7E
7E is the flag sequence.
7D is the escape sequence (any octet that follows is added 0x20).
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