SDTP, PPP Serial Data Transport Protocol

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Protocol suite: PPP.
Protocol type:PPP network layer protocol.
PPP protocol:0x0049.
MIME subtype:
Working groups: pppext, Point-to-Point Protocol Extensions.

The SDTP protocol is used for synchronous serial data compression over a PPP link.

Before any SDTP packets may be communicated, PPP must reach the Network-Layer Protocol phase, and the SDTP Control Protocol must reach the Opened state.

The maximum length of the SDTP datagram transmitted over a PPP link is limited only by the negotiated Maximum-Frame-Size and the maximum length of the Information field of a PPP encapsulated packet. Note that if compression is used on the PPP link, this the maximum length of the SDTP datagram may be larger or smaller than the maximum length of the Information field of a PPP encapsulated packet, depending on the particular compression algorithm and protocol used.

RFC 1963:

This document describes a new Network level protocol (from the PPP point of view), PPP Serial Data Transport Protocol, that provides encapsulation and an associated Serial Data Control Protocol (SDCP) for transporting serial data streams over a PPP link. This protocol was developed for the purpose of using PPP's many features to provide a standard method for synchronous data compression. The encapsulation uses a header structure based on that of the ITU-T Recommendation V.120.

This document is a product of the TR30.1 ad hoc committee on compression of synchronous data. It represents a component of a proposal to use PPP to provide compression of synchronous data in DSU/CSUs.

In addition to providing support for multi-protocol datagrams, the Point-to-Point Protocol (PPP) has defined an effective and robust negotiating mechanism that can be used on point to point links. When used in conjunction with the PPP Compression Control Protocol and one of the PPP Compression Protocols, PPP provides an interoperable method of employing data compression on a point-to- point link.

This document provides a PPP encapsulation for serial data, specifying a transport protocol, PPP Serial Data Transport Protocol (PPP-SDTP), and an associated control protocol, PPP Serial Data Control Protocol (PPP-SDCP). When these protocols are added to above mentioned PPP protocols, PPP can be used to provide compression of serial data on a point-to-point link.

This first edition of PPP-SDTP/SDCP covers HDLC-like synchronous serial data and asynchronous serial data. It does this by using a terminal adaption header based on that of ITU-T Recommendation V.120. Support may be added in the future for other synchronous protocols as the marketplace demands.

The V.120 terminal adaption header allows transported data frames to be split over several packets, supports the transport of DTE port idle and error information, and optionally supports the transport of DTE control state information.

In addition to the V.120 Header, fields can be added to the packet format through negotiation to provide support for features not included in the V.120 header. The extra fields are: a Length Field, which is used to distinguish packets in compound frames, and a Port field, which is used to provide multi-port multiplexing capability. The protocol also allows reserved bits in the V.120 header to be used to transport non-octet aligned frames and to provide a flow control mechanism.

To provide these features, PPP-SDTP permits a single frame format to be selected from several possible formats by using PPP-SDCP negotiation. The terminal adaption header can be either fixed length or variable length, to allow either simplicity or flexibility.

The default frame format places the terminal adaption header at the end of the packet. This permits optimal transmitter timelines when user frames are segmented and compression is also used in conjunction with this protocol.


CCITT Recommendation V.120 (09/92), "Support by an ISDN of Data Terminal Equipment with V-Series Type Interfaces with Provision for Statistical Multiplexing", 1993.


[RFC 1963] PPP Serial Data Transport Protocol (SDTP).


Obsolete RFCs:

Description Glossary RFCs Publications Obsolete RFCs