Network Working Group A. Fuqua Request for Comments: 2043 IBM Category: Standards Track October 1996
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.
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.
1. Introduction
1.1 Specification of Requirements
1.2 Terminology
2. A PPP Network Control Protocol for SNA
3. Sending SNA PIUs and NLPs
3.1 Sending SNA XID or FID2 PIUs over LLC
3.2 Sending HPR Network Layer Packets (NLPs)
3.3 Other Considerations
SECURITY CONSIDERATIONS
REFERENCES
ACKNOWLEDGEMENTS
CHAIR'S ADDRESS
AUTHOR'S ADDRESS
PPP has three main components:
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).
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.
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.
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.
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].
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.
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 | | | |
-+----------+--------+--------+------------------+-
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 issues are not discussed in this memo.
[1] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, 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.
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.
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
Questions about this memo can also be directed to:
Andrew M. Fuqua
International Business Machines Corporation
EMail: fuqua@vnet.ibm.com