Internet & Intranet Security

13.2 LAYER 2 FORWARDING PROTOCOL

Historically, the first layer 2 forwarding/tunneling protocol was the Layer 2 Forwarding (L2F) protocol originally developed and proposed by Cisco Systems. It addressed two areas of standardization:

• The encapsulation of layer 2 frames (i.e., PPP frames) within the L2F protocol. Each L2F frame, including an L2F header and a payload, is then encapsulated and sent within an IP packet or a UDP datagram, respectively. Contrary to more recent layer 2 forwarding/tunneling protocol proposals, the L2F protocol does not take into account the use of cryptography to protect the confidentiality of the encapsulated layer 2 frames.

• The connection management for the layer 2 tunnel (i.e., how the tunnel is initiated and terminated).

Both areas are specified in RFC 2341 [5]. According to this specification, the L2F protocol uses the well-known UDP port 1701 (for both source and destination ports).

Because the L2F protocol is only of historical value,^[3] we do not delve into the technical details of the L2F protocol specification in this book. You may refer to the referenced RFC document if you are interested in history (or if you are an administrator in charge of installing and configuring an implementation of the L2F protocol).

^[3]Note that the category of the referenced RFC document is "historic."