Introduction to PPPoE Protocol (part)

PPPOE (PPP over Ethernet)
PPPoE adds the Ethernet header to the front of the standard PPP message.

1.PPPOE frame Format
Destination_addr source_addr ether_type PAYLOAD CHECKSUM
6 6 2 1

PAYLOAD:
VER TYPE CODE session_id LENGTH PLAYLOAD1
0001 0001 8BIT 2 2

PLAYLOAD1:
Tag_type tag_length Tag_value ...

Ether_type:
0x8863 Discovery Stage
0x8864 PPP Session Stage
CODE:
0x00 PPP Session Stage
0x09 PPPOE Active Discovery Initiation (PADI) packet
0x07 PPPOE Active Discovery offer (pado) packet
0x19 PPPOE Active Discovery Request (PADR) packet
0x65 PPPOE Active Discovery session-confirmation (PADS) PACKET
0xa7 PPPOE Active Discovery Terminate (PADT) packet
Tag_types:
0x0000 end_of_list
0x0101 Service-name
0x0102 Ac-name
0x0103 Host_uniq
0x0104 Ac-cookie
0x0105 vendor-specific
0x0110 Relau-session-id
0x0201 Service-name-error
0x0202 Ac-system-error
0x0203 Generic-error

The working principle of PPPoE Protocol
Two stages: Discovery phase, session stage.

The discovery phase is divided into 4 steps, with the primary purpose of establishing a unique PPPoE SessionID.
1. Client broadcast packet (PADI): Purpose: Client looking for server. Implementation: The client sends the CODE:0X09,SESSION-ID=0X0000, the label Type field value is 0x0101, the address 0xffffffffffff;
2. Server collection (PADI) (PAD0): Purpose: Server feedback client. Implementation: The service side sends the code:0x07,session-id=0x0000, the label Type field value is 0x0102 (as well as one or more servers), address client;
3. Client Select Server (PADR): purpose; multi-server preference. Implementation: The client sends the CODE:0X19,SESSION-ID=0X0000, the client does not receive the PAD0, sends the PAD1 again, and adds the long wait time. Address client;
4. Service-side Session confirmation (PADS): Purpose: To start a PPP session. Implementation: Service-side send code:0x65,session-id= unique. After the client receives pads confirmation, start the session phase.

User Terminal ADSL
Hair---------PADI-------->
Present <--------PAD0---------
Step---------Padr-------->
Segment <--------PADS---------
<------PPP Data------> Session Phase
<--------Padt--------> Session Release
Session phase:
PPDT can be sent at any time after the session is established, terminating the PPPoE session. Code:0xa7,session-id.


Code Analysis:
typedef struct PPPOECONNECTIONSTRUCT {
int discoverystate; /* Where We are in discovery * *
int discoverysocket; /* RAW socket for Discovery frames * *
int sessionsocket; /* Raw socket for session frames * *
unsigned char Myeth[eth_alen]; /* My MAC address * *
unsigned char Peereth[eth_alen]; /* Peer ' s MAC address * *
uint16_t session; /* Session ID * *
Char *ifname; /* Interface Name * *
Char *servicename; /* Desired service name, if any/*
Char *acname; /* Desired AC name, if any *
int synchronous; /* Use synchronous PPP * *
int Usehostuniq; /* Use host-uniq Tag * *
int printacnames; /* Just Print AC names * *
int skipdiscovery; /* SKIP Discovery * *
int nodiscoverysocket; /* Don ' t even open discovery socket * *
int killsession; /* Kill Session and Exit * *
FILE *debugfile; /* Debug file for dumping packets * *
int Numpados; /* Number of Pado packets received * *
Pppoetag cookies; /* We have to send this if we get it * *
Pppoetag Relayid; * Ditto * *
int padshaderror; /* If PADS had an error tag * *
int discoverytimeout; /* Timeout for Discovery Packets * *
} pppoeconnection;