SNACP, PPP SNA Control Protocol

Description Glossary RFCs Publications Obsolete RFCs


Protocol suite: PPP.
Protocol type:PPP network control protocol.
PPP protocol:0x804B, 0x804D.
MIME subtype:
Working groups: pppext, Point-to-Point Protocol Extensions.

SNACP is used for establishing and configuring the SNA over PPP and SNA over LLC 802.2 over PPP.

RFC 2043:

In order to establish communications over a point-to-point link, each end of the PPP link must first send LCP packets to configure and test the data link. After the link has been established and optional facilities have been negotiated as needed by the LCP, PPP must send SNACP packets to choose and configure the SNA network-layer protocol. Once SNACP has reached the Opened state, SNA datagrams can be sent over the link.

The link will remain configured for communications until explicit LCP or SNACP packets close the link down, or until some external event occurs (an inactivity timer expires or network administrator intervention).

SNACP is responsible for configuring, enabling, and disabling SNA on both ends of the point-to-point link. SNACP uses the same packet exchange mechanism as the Link Control Protocol (LCP). SNACP packets may not be exchanged until PPP has reached the Network-Layer Protocol phase. SNACP packets received before this phase is reached should be silently discarded.

Note that there are actually two SNA Network Control Protocols; one for SNA over LLC 802.2 and another for SNA without LLC 802.2. These SNA NCPs are negotiated separately and independently of each other.

There are no Configuration Options for SNA or for SNA over LLC 802.2.

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

PPP header SNACP header Data :::

SNACP header:

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Code Identifier Length
Data :::

Code. 8 bits.
Specifies the function to be performed.

0Vendor Specific. RFC 2153
2 Configure-Ack.
3 Configure-Nak.
4 Configure-Reject.
5 Terminate-Request.
6 Terminate-Ack.
7 Code-Reject.

Identifier. 8 bits.
Used to match requests and replies.

Length. 16 bits.
Size of the packet including the header.

Data. Variable length.
Zero or more bytes of data as indicated by the Length field.


BIU, Basic information unit.
(RFC 2043) In SNA, the unit of data and control information passed between half-sessions. It consists of a request/response header (RH) followed by a request/response unit (RU).

Message unit.
(RFC 2043) In SNA, the unit of data processed by any layer; for example, a BIU, a PIU, or a request/response unit (RU).

NLP, Network Layer Packet.
(RFC 2043) In High Performance Routing (HPR), the message unit used to carry data over the route. Network Layer Packet is analogous to datagram.

PIU, Path information unit.
(RFC 2043) A message unit consisting of a TH or a TH followed by a BIU or a BIU segment. PIU is analogous to datagram.

TH, Transmission header.
(RFC 2043) Control information, optionally followed by a BIU or a BIU segment, that is created and used by path control to route message units and to control their flow within the network.


[RFC 2043] The PPP SNA Control Protocol (SNACP).


Obsolete RFCs:

Description Glossary RFCs Publications Obsolete RFCs