Network Working Group A. Fuqua
Request for Comments: 2043 IBM
Category: Standards Track October 1996
The PPP SNA Control Protocol (SNACP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
defines an extensible Link Control Protocol, and proposes a family of
Network Control Protocols for establishing and configuring different
network-layer protocols.
This document defines the Network Control Protocols for establishing
and configuring Systems Network Architecture (SNA) over PPP and SNA
over LLC 802.2 over PPP.
Table of Contents
1. IntrodUCtion .......................................... 2
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3
2. A PPP Network Control Protocol for SNA ................ 4
3. Sending SNA PIUs and NLPs. ............................ 5
3.1 Sending SNA XID or FID2 PIUs over LLC ........... 5
3.2 Sending HPR Network Layer Packets (NLPs) ........ 5
3.3 Other Considerations ............................ 6
SECURITY CONSIDERATIONS ...................................... 6
REFERENCES ................................................... 6
ACKNOWLEDGEMENTS... .......................................... 7
CHAIR'S ADDRESS .............................................. 7
AUTHOR'S ADDRESS ............................................. 7
1. Introduction
PPP has three main components:
1. A method for encapsulating multi-protocol datagrams.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.
3. A family of Network Control Protocols for establishing and
configuring different network-layer protocols.
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).
1.1. Specification of Requirements
In this document, several Words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
1.2. Terminology
This document frequently uses the following terms:
datagram The unit of transmission in the network layer (such as IP).
A datagram may be encapsulated in one or more packets
passed to the data link layer.
frame The unit of transmission at the data link layer. A frame
may include a header and/or a trailer, along with some
number of units of data.
packet The basic unit of encapsulation, which is passed across the
interface between the network layer and the data link
layer. A packet is usually mapped to a frame; the
exceptions are when data link layer fragmentation is being
performed, or when multiple packets are incorporated into a
single frame.
peer The other end of the point-to-point link.
silently discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
PIU Path information unit. A message unit consisting of a
transmission header (TH) alone, or a TH followed by a basic
information unit (BIU) or a BIU segment. PIU is analogous
to datagram.
TH Transmission header. Control information, optionally
followed by a basic information unit (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.
BIU Basic information unit. 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
In SNA, the unit of data processed by any layer; for
example, a basic information unit (BIU), a path information
unit (PIU), or a request/response unit (RU).
NLP Network Layer Packet. In High Performance Routing (HPR),
the message unit used to carry data over the route.
Network Layer Packet is analogous to datagram.
2. A PPP Network Control Protocol for SNA
The SNA Control Protocol (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.
The SNA Control Protocol is exactly the same as the Link Control
Protocol [1] with the following exceptions:
Frame Modifications
The packet may utilize any modifications to the basic frame format
which have been negotiated during the Link Establishment phase.
Data Link Layer Protocol Field
Exactly one SNACP packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates type hex 804B (SNA
over LLC 802.2) or hex 804D (SNA).
Code field
Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack
and Code-Reject) are used. Other Codes should be treated as
unrecognized and should result in Code-Rejects.
Timeouts
SNACP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be prepared
to wait for Authentication and Link Quality Determination to
finish before timing out waiting for a Configure-Ack or other
response. It is suggested that an implementation give up only
after user intervention or a configurable amount of time.
Configuration Option Types
There are no Configuration Options for SNA or for SNA over LLC
802.2.
3. Sending SNA PIUs and NLPs.
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.
The maximum length of a SNA packet transmitted over a PPP link is the
same as the maximum length of the Information field of a PPP
encapsulated packet.
The format of the PPP Information field itself is the same as that
defined in [1]. Detailed information on SNA and APPN can be found in
[3], [4], [5], [6], and [7].
3.1. Sending SNA XID or FID2 PIUs over LLC
Exactly one SNA XID or FID2 PIU over LLC 802.2 is encapsulated in the
PPP Information field, where the PPP Protocol field indicates type
hex 004B (SNA over LLC 802.2).
A summary of this frame structure, beginning with the PPP Protocol
field, is shown below. The fields are transmitted from left to right.
-- LLC portion (PPP Information Field) -------------
-+----------+----------+----------+----------+-------------------+-
Protocol DSAP SSAP Control LLC Information
0x004B Address Address Field Field
-+----------+----------+----------+----------+-------------------+-
The LLC information field contains the XID or FID2 PIU. LLC(2) is
included in this format for link-level error recovery. Error
recovery is done by the routers at each end of the PPP link.
3.2. Sending HPR Network Layer Packets (NLPs)
Exactly one HPR Network Layer Packet (NLP) is encapsulated in the PPP
Information field, where the PPP Protocol field indicates type hex
004D (SNA).
A summary of this frame structure, beginning with the PPP Protocol
field, is shown below. The fields are transmitted from left to right.
-- HPR Network Layer Packet (NLP) --
(PPP Information Field)
-+----------+--------+--------+------------------+-
Protocol NHDR THDR data
0x004D
-+----------+--------+--------+------------------+-
3.3. Other Considerations
It is architecturally possible to transport HPR NLPs over LLC over
PPP using PPP Protocol field type hex 004B (SNA over LLC 802.2) if
the optional HPR link-level error recover tower is included in the
implementation.
Security Considerations
Security issues are not discussed in this memo.
References
[1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC1661, Daydreamer, July 1994.
[2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, USC/Information Sciences Institute, October 1994.
[3] "SNA Formats", GA27-3136, IBM.
[4] "SNA APPN Architecture Reference", SC30-3422, IBM.
[5] "APPN Architecture and Product Implementations Tutorial",
GG24-3669-02, IBM.
[6] APPN Implementers Workshop homepage,
http://www.raleigh.ibm.com/app/aiwhome.htm
[7] "APPN High Performance Routing (HPR) Architecture",
FTP://networking.raleigh.ibm.com/pub/standards/aiw/appn/hpr
IBM documents are orderable through 1-800-879-2755.
Acknowledgements
Some of the text in this document is taken from previous documents
produced by the Point-to-Point Protocol Working Group of the Internet
Engineering Task Force (IETF).
Some of the text in this document is taken from miscellaneous IBM
documents.
Many people provided suggestions and portions of text for this
document. Special thanks to Allen Carriker, Marcia Peters, and Scott
G. Wasson.
Chair's Address
The working group can be contacted via the current chair:
Karl F. Fox
Ascend Communications
3518 Riverside Dr., Suite 101
Columbus, Ohio 4322
EMail: karl@ascend.com
Author's Address
Questions about this memo can also be directed to:
Andrew M. Fuqua
International Business Machines Corporation
P. O. Box 12195
Research Triangle Park, NC 27709
EMail: fuqua@vnet.ibm.com
