1. ZyXEL Communications Corporation
PPP Introduction
ZyXEL Communications Corporation

2. PPP in a communication system diagram

3. What is PPP ? Point-to-Point Protocol
Provide a method for transporting multi-protocol datagrams over point-to-point link.
PPP is generally viewed as a successor of Serial Line IP (SLIP).
PPP is designed for simple links which transport packets between two peers. These links provide full-duplex simultaneous bi-directional operational operation, and are assumed to deliver packets in order. It is intended that PPP provide a common solution for easy connection of a wide variety of hosts, bridges and routers.
PPP is comprised of three main components:
1. A method for encapsulating multi-protocol datagram.
2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data link connection.
3. A family of Network Control Protocol (NCP) for establishing and configuring different network layer protocol.

4. PPP Frame Flag Address Control 01111110 11111111 00000011
Protocol information Padding
16 bits * *
PPP in HDLC-like framing
- Physical layer requirement: full-duplex
- Data Link layer : ISO HDLC frame structure.
Flag (PPP frame delimiter) : 7E
Address: Always FF (1 byte)
Control: Always 03 (1 byte)
Protocol: Discuss later. (2 byte)
The Address/Control/Protocol field make up the PPP header (4 byte). Depending on the PPP negotiation, these fields may be compressed. Address Field Compression (AFC) and Control Field Compression (CFC) compress those two fields. Together, this compression is referred to as ACFC, in which case, these two bytes will not be seen. There is also compression available for the Protocol field (PFC) which reduces the length from 2 bytes to 1 byte.
Data: The data field stores the information of the PPP frame. In general, the maximum length of this field is 1500 bytes. However, in the case of bridging, the size of this field may be up to 1524 bytes to account for MAC address.
CRC: 2 bytes or 4 bytes. Prestige support 2 bytes option.
CRC Flag Inter-frame fill
16/32 bits or next Address

5. PPP Phase Diagram UP OPENED Dead Establish Authenticate FAIL FAIL
Link Dead:
- Physical-layer not ready.
- Link begins and ends with this phase. When an external event, such as carrier detection, indicates that the physical-layer is ready to be used, PPP will proceed to Link Establish Phase.
Link Establish Phase:
- Used to establish the connection through an exchange of Configured packets.
- Only configuration options which are independent of particular network-layer are configured by LCP.
- Only configure-ack has been sent and receive will enter LCP opened state.
Authentication Phase:
- Not mandatory.
- Should take place as soon as possible after LCP is in open state.
Network Layer Protocol Phase:
- Each network layer protocol must be separately configured by appropriate Network Control Protocol (NCP)
- NCP may be opened and closed at any time.
Link Terminate
- Used to close the link through exchange of Terminate packet.
Terminate Network
CLOSING
DOWN

6. PPP Protocol field and Protocol Pipe
The different protocols on each level are identified by the 2 byte protocol field. The PPP communication pipe is built from the bottom up. The LCP negotiation first, then authentication, then the network protocol level. Finally, the protocol specified application are built on top of this pipe.

7. PPP Data field The data field can be further broken down:
CODE: The CODE field identifies what kind of request or response you are making. There are 4 types of request and for each of these, there can be different response.
Request Response
Configure ACK/NAK/Reject
Terminate ACK
Echo Reply
Discard Code Reject
A list of CODE can be found in the appendix.
ID: Used to identify the PPP frame (1 byte)
LEN: Length of the data field. The Data portion of the field is actually the value of LEN-4
Option: The data field can be broken into specific options.
Type: Can be found in the appendix.
LEN:
Data:

8. LCP Packet format example
Length
C021
Code ID
Length
Data
Code:
- 0x01 Configure-Request
- 0x02 Configure-Ack
- 0x03 Configure-Nak
- 0x04 Configure-Rej
Code Packet Type
Vendor Specific [RFC2153]
Configure-Request
Configure-Ack
Configure-Nak
Configure-Reject
Terminate-Request
Terminate-Ack
Code-Reject
8 * Protocol-Reject
9 * Echo-Request
* Echo-Reply
* Discard-Request
* Identification
* Time-Remaining
Reset-Request [RFC1962]
Reset-Reply [RFC1962]

9. LCP Packet - example C021 01 ID Length Option Options:
Type Configuration Option
Vendor Specific [RFC2153]
Maximum-Receive-Unit
Async-Control-Character-Map
Authentication-Protocol
Quality-Protocol
Magic-Number
DEPRECATED (Quality-Protocol)
Protocol-Field-Compression
Address-and-Control-Field-Compression
FCS-Alternatives [RFC1570]
Self-Describing-Pad [RFC1570]
Numbered-Mode [RFC1663]
DEPRECATED (Multi-Link-Procedure)
Callback [RFC1570]
DEPRECATED (Connect-Time)
Type Length Data ...

10. LCP Config-Request 15 DEPRECATED (Compound-Frames)
DEPRECATED (Nominal-Data-Encapsulation)
Multilink-MRRU [RFC1717]
Multilink-Short-Sequence-Number-Header [RFC1717]
Multilink-Endpoint-Discriminator [RFC1717]
Proprietary [KEN]
DCE-Identifier [SCHNEIDER]
Multi-Link-Plus-Procedure [Smith]
Link Discriminator for BACP [RFC2125]
LCP-Authentication-Option [Culbert]
Consistent Overhead Byte Stuffing (COBS) [Carlson]
Prefix elision [Bormann]
Multilink header format [Bormann]

11. Authentication- PAP SERVER( Answer) Client ( Originate ) PAP(C023)
Authenticate-Request
PAP(C023)
Authenticate-Ack
PPP defines an extensible Link Control Protocol, which allows negotiation of an Authentication Protocol for authenticating its peer before allowing Network Layer protocols to transmit over the link.
PAP provides a simple method for the peer to establish its identity using a 2-way handshake. This is done only upon initial link establishment.
After the Link Establishment phase is complete, an Id/Password pair is repeatedly sent by the peer to the authenticator until authentication is acknowledged or the connection is terminated.
PAP is not a strong authentication method. Passwords are sent over the circuit "in the clear", and there is no protection from playback.
| Code | Identifier | Length |
Code
The Code field is one octet and identifies the type of PAP packet.
PAP Codes are assigned as follows:
Authenticate-Request
Authenticate-Ack
Authenticate-Nak
Authenticate-Nak

12. Authentication- CHAP SERVER( Answer) Client ( Originate ) CHAP (C223)
Authenticate-Challenge ( challenge value)
CHAP (C223)
Authenticate-Response
Response value= Hash(ID+secret+challenge value)
The Challenge-Handshake Authentication Protocol (CHAP) is used to periodically verify the identity of the peer using a 3-way handshake. This is done upon initial link establishment, and MAY be repeated anytime after the link has been established.
1. After the Link Establishment phase is complete, the authenticator sends a "challenge" message to the peer.
2. The peer responds with a value calculated using a "one-way hash" function.
3. The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authentication is acknowledged; otherwise the connection SHOULD be terminated.
4. At random intervals, the authenticator sends a new challenge to the peer, and repeats steps 1 to [RFC 1994]
CHAP/MS-CHAP:
PPP AUTHENTICATION ALGORITHMS
A one octet field is used in the Challenge-Handshake Authentication
Protocol (CHAP) to indicate which algorithm is in use [RFC1994].
Number Name
Reserved [RFC1994]
Reserved [RFC1994]
Reserved [RFC1994]
Reserved [RFC1994]
Reserved [RFC1994]
CHAP with MD [RFC1994]
0x80 (128) MS-CHAP [Crocker]
Authenticate-Success
(check response value)
Authenticate-Fail

13. IPCP Code field is the same as LCP Options 8021 Code ID Length Option
0x02 IP-Compression-Protocol
0x03 IP -Address
PPP IPCP CONFIGURATION OPTION TYPES
Type Configuration Option
IP-Addresses (deprecated) [RFC1332]
IP-Compression-Protocol [RFC1332]
IP-Address [RFC1332]
Mobile-IPv [RFC2290]
Primary DNS Server Address [RFC1877]
Primary NBNS Server Address [RFC1877]
Secondary DNS Server Address [RFC1877]
Secondary NBNS Server Address [RFC1877]
8021
Code ID
Length
Option

14. IP packet encapsulate in PPP
0021
IP packet

15. PPP and TCP/IP Protocol stack
Applications
(SMTP, FTP, Telnet,
TCP IP
Serial Line
Interface
Modem
ISDN TA
CSU/DSU
PPP
LAN
Ethernet
Token Ring
AppleTalk

16. IP packet format Ver 45 IHL 00 Length Identifier Flag Fragment offset
Ver 45
IHL 00
Type of Service
Length
Identifier
Flag
Fragment offset
TTL
Protocol
Header Checksum
Source Address
Destination Address

17. UDP Header
Source Port
Destination Port
Length
Checksum
Data

18. TCP Header …... …... Destination Port Source Port Sequence Number
Source Port
Destination Port
Sequence Number
Ack Number
…...
…...