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RFC 2661 http://tools.ietf.org/html/rfc2661

The L2TP Protocol is used for integrating multi-protocol dial-up services into existing Internet Service Providers Point of Presence (hereafter referred to as ISP and POP, respectively). This protocol may also be used to solve the "multilink hunt-group splitting" problem. Multilink PPP, often used to aggregate ISDN B channels, requires that all channels composing a multilink bundle be grouped at a single Network Access Server (NAS). Because L2TP makes a PPP session appear at a location other than the physical point at which the session was physically received, it can be used to make all channels appear at a single NAS, allowing for a multilink operation even when the physical calls are spread across distinct physical NASs.

The format of the L2TP packet is shown in the following illustration:

8 16 32 bits
T L X X S X O P X X X X VER Length
Tunnel ID SESSION ID
Ns Nr
AVP (bytes +)
L2TP packet structure

T
The T bit indicates the type of message. It is set to 0 for data messages and 1 for control messages.

L
When set, this indicates that the Length field is present, indicating the total length of the received packet. Must be set for control messages.

X
The X bits are reserved for future extensions. All reserved bits are set to 0 on outgoing messages and are ignored on incoming messages.

S
If the S bit is set, both the Nr and Ns fields are present. S must be set for control messages.

O
When set, this field indicates that the Offset Size field is present in payload messages. This bit is set to 0 for control messages.

P
If the Priority (P) bit is 1, this data message receives preferential treatment in its local queuing and transmission. LCP echo requests used as a keepalive for the link, for instance, are generally sent with this bit set to 1. Without it, a temporary interval of local congestion could result in interference with keepalive messages and unnecessary loss of the link. This feature is only for use with data messages. The P bit has a value of 0 for all control messages.

Ver
The value of the ver bit is always 002. This indicates a version 1 L2TP message.

Length
Overall length of the message, including header, message type AVP, plus any additional AVP's associated with a given control message type.

Tunnel ID
Identifies the tunnel to which a control message applies. If an Assigned Tunnel ID has not yet been received from the peer, Tunnel ID must be set to 0. Once an Assigned Tunnel ID is received, all further packets must be sent with Tunnel ID set to the indicated value.

Call ID
Identifies the user session within a tunnel to which a control message applies. If a control message does not apply to a single user session within the tunnel (for instance, a Stop-Control-Connection-Notification message), Call ID must be set to 0.

Nr
The sequence number expected in the next control message to be receivec.

Ns
The sequence number for this data or control message.

Data messages have two additional fields before the AVP as follows:

Offset size (16 bits) Offset pad (16 bits)
Additional fields in L2TP payload message

Offset size
This field specifies the number of bytes past the L2TP header at which the payload data is expected to start. It is recommended that data thus skipped be initialized to 0s. If the offset size is 0, or the O bit is not set, the first byte following the last byte of the L2TP header is the first byte of payload data

Vulnerabilities for this protocol (from CVE)

CVE ID Protocol Source Port Targetport

TCP/IP Protocols:

AHARP/RARPATMPBGP-4COPSDCAPDHCPDNS
DVMRPEGPEIGRPESPFANPFingerFTPHSRP
HTTPICMPICMPv6IGMPIGRPIMAP4IPIPDC
IPv6ISAKMPL2FL2TPLDAPMARSMobile IPNARP
NetBIOS/IPNHRPNTPOSPFPIMPOP3PPTPRadius
RIP2RIPng for IPv6RLOGINRSVPRTSPRUDPS-HTTPSCTP
SLPSMTPSNMPSOCKS V5TACACSTALITCPTELNET
TFTPUDPVan JacobsonVRRPWCCPX-WindowXOT