Before any FZA packets may be communicated, PPP must reach the Network-Layer Protocol phase.
When the Compression Control Protocol (CCP) has reached the Opened state, and FZA is negotiated as the primary compression algorithm, the PPP Protocol field indicates type hex 00FB (link compressed datagram), or type hex 00FD (compressed datagram).
The maximum length of the FZA datagram transmitted over a PPP link is the same as the maximum length of the Information field of a PPP encapsulated packet.
The FZA packets require the negotiation of the Self-Describing- Padding Configuration Option  at LCP Link Establishment.
Reliability and Sequencing
The FZA algorithm expects a reliable link, as described in "PPP Reliable Transmission" .
FZA expects the packets to be delivered in sequence.
The maximum expansion of Gandalf FZA is 2:1. However, typical expansion on pre-compressed data is 1.01:1. Expanded data is sent to maintain the integrity of the compression history.
When the expansion exceeds the size of the peer's Maximum Receive Unit for the link, the expanded packet is sent in multiple PPP frames. The compressed data contains an indication of the end of the original packet.
A summary of the Gandalf FZA packet format is shown below. The fields are transmitted from left to right.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPP Protocol | Compressed Data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
One or two octets. The PPP Protocol field is described in the Point-to-Point Protocol Encapsulation .
Type 00FD is used when the PPP multilink protocol is not used, and/or "inside" a multilink bundle. Type 00FB is used "outside" multilink, to compress independently on individual links of a multilink bundle. This value MAY be compressed when LCP Protocol-Field-Compression is negotiated.
One or more octets. The compressed PPP encapsulated packet(s).
Prior to compression, the uncompressed data begins with the original PPP Protocol number. This value MAY be compressed when LCP Protocol-Field-Compression is negotiated.
The original Protocol number is followed by the original Information field. The length of the original Information field before compression MUST NOT exceed the link Maximum Receive Unit (MRU).
PPP Link Control Protocol packets MUST NOT be sent within compressed data.