The TCP/IP Guide - PPP General Frame Format

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The Protocol field is the main “frame type” indicator for the device receiving the frame. For data frames this is normally the network-layer protocol that created the datagram, and for control frames, the PPP protocol that created the control message. In the case of protocols that modify data such as when compression (CCP) or encryption (ECP) are used, this field identifies the data as being either compressed or encrypted, and the original Protocol value is extracted after the Information field is decompressed/decrypted.

There are dozens of network-layer protocols and PPP control protocols, and a correspondingly large number of Protocol values The main PPP standard defines four ranges for organizing these values, as shown in Table 33.

**Table 33: PPP *Protocol* Field Ranges**
Protocol Field Range (hexadecimal)	Description
0000 - 3FFF	Encapsulated network layer datagrams that have an associated Network Control Protocol (NCP). In this case, control frames from the corresponding NCP use a Protocol field value that is computed by adding “8” to the first octet of the network layer Protocol value. For example, for IP the Protocol value is 0021, and control frames from the IP Control Protocol (IPCP) use Protocol value 8021. This range also includes several values used for specially-processed encapsulated datagrams, such as when compression or encryption are employed.
4000 - 7FFF	Encapsulated datagrams from “low-volume” protocols. These are protocols that do not have an associated NCP.
8000 - BFFF	Network Control Protocol (NCP) control frames that correspond to the network layer Protocol values in the 0000-3FFF range.
C000 - FFFF	Control frames used by LCP and LCP support protocols such as PAP and CHAP. Some miscellaneous protocol values are included here as well.

The standard also specifies that the Protocol value must be assigned so that the first octet is even, and the second octet is odd. So, for example, 0x0021 is a valid value but 0x0121 and 0x0120 are not. (The reason for this will become apparent shortly). There are also certain blocks that are reserved and not used.