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Un échange ICMP vu de près.

La manip qui suit est destinée à éclaircir un peu les idées sur l'empilement des protocoles.

Elle est effectuée sur une machine Linux connectée en PPPoE sur un modem SpeedTouch Home (Accès ADSL).

L'interface Ethernet connectée au Modem est Eth1. le client rp-pppoe assure la connexion ppp au dessus. Ceci fait apparaître une interface supplémentaire: ppp0
Autrement dit, ppp0 est une interface PPP qui fonctionne au dessus de Eth1
Voici un extrait de ce que donne "ifconfig" :
[root@gw root]# ifconfig
....
eth1 Lien encap:Ethernet HWaddr 00:60:8C:50:F0:DF
inet adr:10.0.0.10 Bcast:10.0.0.255 Masque:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
....
ppp0 Lien encap:Protocole Point-à-Point
inet adr:217.128.147.4 P-t-P:217.128.147.1 Masque:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1492 Metric:1
....
Deux "sniffers", l'un sur Eth1 (couche Ethernet), l'autre sur ppp0 (couche PPPoE), vont observer simultanément un "ping" sur l'habituel ftp.oleane.net.

Voici ce que l'on obtient:

Sur Ethernet (Eth1)

Sur PPPoE (ppp0)

 
Frame 4 (106 on wire, 106 captured)
    Arrival Time: Nov 30, 2001 16:32:12.060281
    Time delta from previous packet: 3.750210 seconds
    Time relative to first packet: 8.636959 seconds
    Frame Number: 4
    Packet Length: 106 bytes
    Capture Length: 106 bytes
Ethernet II
    Destination: 00:02:3b:00:4f:7d (Redback_00:4f:7d)
    Source: 00:60:8c:50:f0:df (3Com_50:f0:df)
    Type: PPPoE Session (0x8864)
PPP-over-Ethernet Session
    Version: 1
    Type: 1
    Code: Session Data
    Session ID: 218c
    Payload Length: 86
Point-to-Point Protocol
    Protocol: IP (0x0021)
Internet Protocol, Src Addr: 217.128.147.4 
                   Dst Addr: 195.25.12.28 
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00 
                            (DSCP 0x00: Default; ECN: 0)
      0000 00..=Differentiated Services 
                               Codepoint:Default (0)
      .... ..0.=ECN-Capable Transport (ECT): 0
      .... ...0=ECN-CE: 0
    Total Length: 84
    Identification: 0x0000
    Flags: 0x04
        .1.. = Don't fragment: Set
        ..0. = More fragments: Not set
    Fragment offset: 0
    Time to live: 64
    Protocol: ICMP (0x01)
    Header checksum: 0xfeee (correct)
    Source: 217.128.147.4
    Destination: 195.25.12.28
Internet Control Message Protocol
    Type: 8 (Echo (ping) request)
    Code: 0 
    Checksum: 0xf320 (correct)
    Identifier: 0x0e10
    Sequence number: 00:00
    Data (56 bytes)
0000  7c a6 07 3c 87 e9 00 00 08 09 0a 0b 0c 0d 0e 0f
0010  10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
0020  20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
0030  30 31 32 33 34 35 36 37                         
Frame 5 (106 on wire, 106 captured)
    Arrival Time: Nov 30, 2001 16:32:12.122676
    Time delta from previous packet: 0.062395 seconds
    Time relative to first packet: 8.699354 seconds
    Frame Number: 5
    Packet Length: 106 bytes
    Capture Length: 106 bytes
Ethernet II
    Destination: 00:60:8c:50:f0:df (3Com_50:f0:df)
    Source: 00:02:3b:00:4f:7d (Redback_00:4f:7d)
    Type: PPPoE Session (0x8864)
PPP-over-Ethernet Session
    Version: 1
    Type: 1
    Code: Session Data
    Session ID: 218c
    Payload Length: 86
Point-to-Point Protocol
    Protocol: IP (0x0021)
Internet Protocol, Src Addr: 195.25.12.28
                   Dst Addr: 217.128.147.4
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00 
                            (DSCP 0x00: Default; ECN: 0)
      0000 00..=Differentiated Services 
                               Codepoint:Default (0)
      .... ..0.=ECN-Capable Transport (ECT): 0
      .... ...0=ECN-CE: 0
    Total Length: 84
    Identification: 0xf960
    Flags: 0x00
        .0.. = Don't fragment: Not set
        ..0. = More fragments: Not set
    Fragment offset: 0
    Time to live: 248
    Protocol: ICMP (0x01)
    Header checksum: 0x8d8d (correct)
    Source: 195.25.12.28 (195.25.12.28)
    Destination: 217.128.147.4 (217.128.147.4)
Internet Control Message Protocol
    Type: 0 (Echo (ping) reply)
    Code: 0 
    Checksum: 0xfb20 (correct)
    Identifier: 0x0e10
    Sequence number: 00:00
    Data (56 bytes)
0000  7c a6 07 3c 87 e9 00 00 08 09 0a 0b 0c 0d 0e 0f
0010  10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
0020  20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
0030  30 31 32 33 34 35 36 37
 
Frame 1 (84 on wire, 84 captured)
    Arrival Time: Nov 30, 2001 16:32:12.059992
    Time delta from previous packet: 0.000000 seconds
    Time relative to first packet: 0.000000 seconds
    Frame Number: 1
    Packet Length: 84 bytes
    Capture Length: 84 bytes
Raw packet data
    No link information available
**
**
** Au niveau ppp, ce qu'il se passe en dessous
** interprété, le renifleur ne sait pas le faire,
** il se croit sur un "vrai" lien PPP
** parce qu'il écoute sur une interface PPP.
** Mais en regardant au niveau Ethernet
** Nous trouvons les informations relatives
** au protocole PPPoE
**
Internet Protocol, Src Addr: 217.128.147.4
                   Dst Addr: 195.25.12.28
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00 
                            (DSCP 0x00: Default; ECN:0)
      0000 00..=Differentiated Services 
                               Codepoint: Default (0)
      .... ..0.=ECN-Capable Transport (ECT): 0
      .... ...0=ECN-CE: 0
    Total Length: 84
    Identification: 0x0000
    Flags: 0x04
        .1.. = Don't fragment: Set
        ..0. = More fragments: Not set
    Fragment offset: 0
    Time to live: 64
    Protocol: ICMP (0x01)
    Header checksum: 0xfeee (correct)
    Source: 217.128.147.4 (217.128.147.4)
    Destination: 195.25.12.28 (195.25.12.28)
Internet Control Message Protocol
    Type: 8 (Echo (ping) request)
    Code: 0 
    Checksum: 0xf320 (correct)
    Identifier: 0x0e10
    Sequence number: 00:00
    Data (56 bytes)
0000  7c a6 07 3c 87 e9 00 00 08 09 0a 0b 0c 0d 0e 0f
0010  10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
0020  20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
0030  30 31 32 33 34 35 36 37                         
Frame 2 (84 on wire, 84 captured)
    Arrival Time: Nov 30, 2001 16:32:12.122954
    Time delta from previous packet: 0.062962 seconds
    Time relative to first packet: 0.062962 seconds
    Frame Number: 2
    Packet Length: 84 bytes
    Capture Length: 84 bytes
Raw packet data
    No link information available
**
**
**
**
** Et pour la réponse
** c'est la même chose
**
**
**
**
Internet Protocol, Src Addr: 195.25.12.28 
                   Dst Addr: 217.128.147.4
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00 
                           (DSCP 0x00: Default; ECN: 0)
      0000 00..=Differentiated Services 
                               Codepoint:Default (0)
      .... ..0.=ECN-Capable Transport (ECT): 0
      .... ...0=ECN-CE: 0
    Total Length: 84
    Identification: 0xf960
    Flags: 0x00
        .0.. = Don't fragment: Not set
        ..0. = More fragments: Not set
    Fragment offset: 0
    Time to live: 248
    Protocol: ICMP (0x01)
    Header checksum: 0x8d8d (correct)
    Source: 195.25.12.28 
    Destination: 217.128.147.4
Internet Control Message Protocol
    Type: 0 (Echo (ping) reply)
    Code: 0 
    Checksum: 0xfb20 (correct)
    Identifier: 0x0e10
    Sequence number: 00:00
    Data (56 bytes)
0000  7c a6 07 3c 87 e9 00 00 08 09 0a 0b 0c 0d 0e 0f
0010  10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
0020  20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
0030  30 31 32 33 34 35 36 37

Comme il est facile de le constater, on a bien de l'IP qui est transporté au dessus de PPP, lui même au dessus d'Ethernet. Si ce n'est la couche supplémentaire introduite par ce protocole, tout reste identique à ce que l'on observerait sur un réseau Ethernet "classique".