Advanced SNA/IP : A Simple SNA Transport Protocol
Carrying IBM network traffic through the Internet
Behl, W., Sterling, B., and W. Teskey, "Advanced SNA/IP : A Simple SNA Transport Protocol", RFC 1538, DOI 10.17487/RFC1538, October 1993, <https://www.rfc-editor.org/info/rfc1538>.
@misc{rfc1538,
series = {Request for Comments},
number = 1538,
howpublished = {RFC 1538},
publisher = {RFC Editor},
doi = {10.17487/RFC1538},
url = {https://www.rfc-editor.org/info/rfc1538},
author = {W. Behl and B. Sterling and W. Teskey},
title = {{Advanced SNA/IP : A Simple SNA Transport Protocol}},
pagetotal = 10,
year = 1993,
month = oct,
} RFC text
Contents
Network Working Group W. Behl
Request for Comments: 1538 McDATA Corporation
Category: Informational B. Sterling
McDATA Corporation
W. Teskey
I/O Concepts
October 1993
Advanced SNA/IP : A Simple SNA Transport Protocol
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
This RFC provides information for the Internet community about a
method for establishing and maintaining SNA sessions over an IP
internet. While the issues discussed may not be directly relevant to
the research problems of the Internet, they may be interesting to a
number of researchers and implementors. Any questions or comments
relative to the contents of this RFC may be sent to the following
Internet address: snaip@mcdata.com.
Table of Contents
1. Introduction.................................................. 2
2. Motivation and Rationale...................................... 2
3. SNA/IP Protocol Specification................................. 3
3.1 Glossary..................................................... 3
3.2 Conventions and Assumptions.................................. 3
3.3 The Protocol................................................. 3
3.3.1 Connection Establishment................................... 3
3.3.2 Data Transfer.............................................. 5
3.3.3 Connection Termination and Loss............................ 6
3.3.4 Session Data Flow.......................................... 7
3.3.5 State Transition Table for the Initiating Node............. 8
4. LLC to SNA/IP Conversion...................................... 8
5. Performance................................................... 8
6. VTAM Definition............................................... 9
7. Acknowledgments............................................... 9
8. References.................................................... 9
9. Security Considerations....................................... 10
10. Authors' Addresses........................................... 10
11. Disclaimer................................................... 10
1. Introduction
Advanced SNA/IP suggests a method for the transmission of SNA session
data over an IP network. This memo documents the SNA/IP protocol as
implemented in the McDATA LinkMaster(R) 6200 Network Gateway, McDATA
LinkMaster(R) 7100 Network Controller, and I/O Concepts X-Direct
TN3270 Server.
Advanced SNA/IP differs from other protocols designed to enable
routing of SNA session traffic over an IP network. SNA/IP was
originally designed for implementation in peripheral network nodes
like SNA gateways and downstream nodes (DSNs). It is the authors'
view, however, that SNA/IP could also be implemented in intermediate
network nodes like routers as the base for an LLC to IP subnet
gateway or data link switch function.
2. Motivation and Rationale
The token-ring media access control (MAC) protocol 802.5 and logical
link control (LLC) protocol 802.2 were the first set of LAN protocols
used to provide a reliable and connection-oriented data link service
for SNA sessions in a LAN environment.
McDATA's experience with transporting SNA over 802.5 networks led to
an 802.3/802.2 (Ethernet) based variation. As prospective customers
were introduced to these Ethernet products, the question of
routability arose. Network administrators, accustomed to working
with Ethernet networks and the IP-based protocols, required an IP
routable solution. McDATA's "SNA over Ethernet" products were
bridgeable, but were not routable.
SNA sessions require a reliable and connection-oriented data link.
TCP running over IP provides a reliable and connection-oriented
transport service and has the added benefit of being routable. It
seemed the UDP and TCP protocols could be used in place of 802.2 Type
I and Type II levels of service used in traditional SNA token-ring
implementations. Advanced SNA/IP was created as a result of these
observations.
3. SNA/IP Protocol Specification
3.1. Glossary
Data Link Switching (DLSw) - This is best described as a routing
protocol used for the conversion of LLC-based SNA sessions to an IP
form. The initial version of the DLSw protocol is documented in the
informational RFC 1434 [1].
Downstream Node (DSN) - An SNA Physical Unit (PU) type 2.0 or 2.1
device connected to the SNA network via a LAN (802.5, 802.3, etc.) as
opposed to an SDLC, X.25, or channel connection.
SNA Gateway - A device that provides a data link control (DLC)
conversion function for SNA PU type 5 (host) devices and LAN-
attached DSNs.
Subnet SNA Gateway - A device connected to both a traditional SNA
token-ring segment and an IP network that performs local termination
of the LLC connections, a mapping function of source address to
destination IP address, and a conversion (switching) function of LLC
to IP.
3.2. Conventions and Assumptions
Frame formats are shown starting with the IP header. Other headers
will, of course, appear in the actual frames sent, but these headers,
and the numbers of them, will vary across MAC types.
It is assumed the reader is familiar with both the standard SNA
protocol (to the extent it applies to SNA Gateway and DSN functions)
and the base set of TCP/IP protocols. Where practical, the reader is
asked to refer to appropriate SNA and TCP/IP documentation.
3.3. The Protocol
Conceptually, there are three phases to the Advanced SNA/IP protocol:
the Connection Establishment phase, the Data Transfer phase, and the
Connection Termination phase.
3.3.1. Connection Establishment
Connection Establishment involves the exchange of logical XID packets
between the connecting end nodes and culminates in the establishment
of a TCP connection. This process is similar to the IBM-specified
Test, XID, SABME and UA exchange used to establish a Type II 802.2
connection for SNA traffic [2]. In place of the 802.2 Type I
messages, SNA/IP defines the following set of UDP datagrams:
Logical Null XID
Use: Sent by an initiating node (such as a DSN) when the
connection to another SNA node is desired.
The Logical Null XID communicates the sending node's
desire to negotiate connection parameters. Once those
parameters are established, the Logical Null XID
communicates the sender's TCP port to which a connection
is to be made.
Format:
------------------------------------
| IP Header | UDP Header | 0xBF |
------------------------------------
Source IP address: The IP address of the initiating
node.
Destination IP address: The IP address of the partner SNA
node.
Source UDP Port: Must match the TCP port number to be
used in the eventual TCP connection.
Destination UDP Port: A known port on the partner node
that expects SNA/IP datagrams.
XID Request
Use: Sent in response to a Logical Null XID and requests the
receiving node to send a Logical SNA XID datagram.
Format:
------------------------------------
| IP Header | UDP Header | 0xBF |
------------------------------------
The source and destination IP and UDP port numbers follow,
logically, from those provided in the Logical Null XID
datagram.
The format of the XID Request and Logical Null XID are the
same. The two types are distinguished by the roles assumed by
the two nodes. In current implementations, the DSN initiates
the XID exchange by sending the Logical Null XID. The SNA
Gateway responds with the XID request.
Logical SNA XID
Use: Sent in response to an XID Request and in the context of
SNA XID negotiation.
Format:
----------------------------------------------------
| IP Header | UDP Header | 0xBF | SNA XID data |
----------------------------------------------------
For PU 2.0 nodes, the SNA XID data consists of a Format 0 XID
[3].
For PU 2.1 nodes, the SNA XID data consists of a Format 3 XID
[3].
A typical Connection Establishment data flow appears below.
Node 1 Node 2
Logical Null XID ------------------------->
<------------------------ XID Request
Logical SNA XID -------------------------->
<------------------------ TCP SYN
TCP SYN ACK ----------------------------->
<------------------------ TCP ACK
Note: The source UDP port of the Logical Null XID equals the
destination TCP port of the TCP SYN segment.
Retries of the Logical Null XID by the initiating node should occur
periodically until an XID Request is received in reply. The frequency
of the retries is left up to the implementor. The lower bound on the
retry timer should be more than the expected round trip time for a
packet on the network.
3.3.2. Data Transfer
There are no special packets defined for the Data Transfer phase.
Once the TCP connection is established, SNA Request Units (RUs) may
be exchanged between the two end nodes. The SNA session data appears
as TCP segment data. The only added SNA/IP requirement is that each
SNA message consisting of a Transmission Header (TH),
Request/Response Header (RH) and an optional Request/Response Request
Unit (RU) be preceded by a two octet length field. Examples of Data
Transfer frames are shown below.
-------------------------------------------------------
| IP Header | TCP Header | SNA Msg 1 len | SNA Msg 1 |
-------------------------------------------------------
----------------------------------------------
| IP Header | TCP Header | SNA Msg 1 cont'd ->
----------------------------------------------
--------------------------------
| SNA Msg 2 len | SNA Msg 2 |
--------------------------------
The length field is passed in big endian format. 0 is a valid length
value.
The format of the SNA Message pieces are as defined by SNA [3].
Reliable and sequential delivery of data is provided by the TCP
protocol [5,6].
3.3.3. Connection Termination and Loss
Either SNA node may, at any time, terminate the logical SNA
connection by issuing a TCP-level FIN segment. Dictates of the TCP
protocol apply to this termination process [5,6].
A connection is also terminated, though not as cleanly, if a TCP
Reset segment is sent by either SNA node.
Once a connection is terminated, a new connection may be established
by the process outlined in the Connection Establishment section. For
reconnections made to the LinkMaster 6200 gateway, the same UDP
source port must be used by the initiating node. This implies that
the same TCP port is used. This requirement stems from the fact the
gateway may not always be aware that a TCP connection has been
terminated. This would happen if the DSN became disabled prior to
sending a FIN or Reset segment. Under these circumstances, SNA host
resources remain allocated and a reconnection from a DSN, which the
host believes to already be in session, is not allowed. By requiring
the DSN to use the same port when reestablishing a connection, the
LinkMaster 6200 is able to recognize when a reset of the host
connection is required.
3.3.4. Complete Session Data Flow
Node 1 Node 2
Logical Null XID ------------------------->
(UDP Datagram)
Logical Null XID ------------------------->
(UDP Datagram)
<------------------------ XID Request
(UDP Datagram)
Logical SNA XID -------------------------->
(UDP Datagram)
<------------------------ TCP SYN
(TCP Message)
TCP SYN ACK ----------------------------->
(TCP Message)
<------------------------ TCP SYN
(TCP Message)
****************** Connection Established *******************
<------------------------ SNA ACTPU
(TCP Message)
SNA ACTPU Response --------------------->
(TCP Message)
<------------------------ SNA ACTLU
(TCP Message)
SNA ACTLU Response --------------------->
(TCP Message)
.
.
.
<------------------------ TCP FIN
(TCP Message)
TCP FIN ACK ------------------------>
(TCP Message)
<------------------------ TCP ACK
(TCP Message)
******************** Connection Closed *********************
Logical Null XID ----------------------->
(UDP Datagram)
.
.
.
.
3.3.5. State Transition Table for the Initiating Node
Transition State
Given State | No Conn | Null XID Sent | SNA XID Sent | Conn Estb
------------+---------+---------------+--------------+-----------
No | | Internal Act. | |
Connection | | Stimulus | |
| | ---> Sends | |
| | 1st Null XID | |
------------+---------+---------------+--------------+-----------
Null XID | | Internal | XID Request |
Sent | | Timer Event | Received |
| | ----> Resend | ----> Sends |
| | Null XID | SNA XID |
------------+---------+---------------+--------------+-----------
SNA XID | | Internal | SNA XID | Indication
Sent | | Timer Event | Received | that TCP
| | ----> Resend | ----> Send | connection
| | Null XID | SNA XID | is estb.
| | | |
------------+---------+---------------+--------------+-----------
Connection | Indica- | | | SNA
Established | tion | | | Session
| that | | | Data
| TCP conn| | |
| term. | | |
A gateway state transition table is not provided here because the
state transitions are dependent on the nature of the SNA host
interface (3172 Channel Protocol, 3174 Channel Protocol, SDLC, etc.).
4. LLC to SNA/IP Conversion
The use of Advanced SNA/IP to convert conventional token ring- based
SNA traffic to a routable form is both conceivable and practical.
While interesting, a discussion of this application falls outside the
context of this RFC. Very briefly, it can be said that an SNA/IP-
based "subnet SNA gateway" application could do many of the things
being discussed in the context of the DLSw specification [1].
5. Performance
The performance of SNA sessions running over an SNA/IP connection
will be affected by the bandwidth available on the network and by how
much traffic is on the network. SNA/IP is poised to take full
advantage of the prioritization and class of service enhancements
promised in the next generation of IP. Today, SNA/IP can take
advantage of router packet prioritization schemes based on port
number. SNA/IP also leaves intact the standard SNA class of service
prioritization protocol.
Performance measures taken at McDATA comparing the throughput of
SNA/IP and LLC across a single token-ring segment showed
approximately a 15 percent decrease in the maximum transactions per
hour (1500 bytes to the DSN, 50 bytes out to the host) for SNA/IP.
This decrease is well within the expected levels given the added
processing requirements of TCP/IP over LLC in the LinkMaster 6200 and
LinkMaster 7100 operating environments.
6. VTAM Definition
The host VTAM definition of SNA/IP downstream nodes is dependent on
the gateway implementation. Downstream nodes may appear as switched
major nodes connected to an XCA or as downstream nodes connected to a
PU 2.0 controller [4].
7. Acknowledgments
The authors wish to acknowledge that the definition of SNA/IP was a
collaborative effort involving many individuals ranging from
customers to sales and marketing personnel to engineers. Particular
thanks go to David Beal, Steve Cartwright, Tracey Floming, Audrey
McEwen, Mark Platte, Paul Schroeder, Chuck Weil, and Marty Wright,
who all played key roles in the development and testing of this
protocol and also in the editing of this RFC.
8. References
[1] Dixon, R., and D. Kushi, "Data Link Switching: Switch-to-Switch
Protocol", RFC 1434, IBM, March 1993.
[2] "Token-Ring Network Architecture Reference", IBM document #SC30-
3374-02.
[3] "Systems Network Architecture Formats", IBM document #GA27-3136-
12.
[4] "VTAM Resource Definition Reference", IBM document #SC31-6438-1.
[5] Comer, D., "Internetworking with TCP/IP Volume I", Prentice Hall
1991.
[6] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", STD 7, RFC 793, USC/Information
Sciences Institute, September 1981.
9. Security Considerations
This RFC does not address issues of security. SNA level security
procedures and protocols apply when SNA/IP is used as the transport.
10. Authors' Addresses
Wilfred Behl
310 Interlocken Parkway
Broomfield, Colorado 80021
Phone: 303-460-4142
Email: wil@mcdata.com
Barbara Sterling
310 Interlocken Parkway
Broomfield, Colorado 80021
Phone: 303-460-4211
Email: bjs@mcdata.com
William Teskey
2125 112th Ave. North East
Suite 303
Bellevue, WA 98004
Phone: 206-450-0650
Email: wct@ioc-sea.com
Note: Any questions or comments relative to the contents of this RFC
should be sent to snaip@mcdata.com. This address will be used to
coordinate the handling of responses.
11. Disclaimer
McDATA, the McDATA logo, and LinkMaster are registered trademarks of
McDATA Corporation. All other product names and identifications are
trademarks of their respective manufacturers, who are not affiliated
with McDATA Corporation.
AI ELI5
On this page
§1Introduction
This RFC documents a method for transporting Systems Network Architecture (SNA), IBM's proprietary network protocol, over IP networks. Traditionally, SNA sessions relied on token-ring networks and logical link control (LLC), a link-layer protocol designed for reliability and ordering. Advanced SNA/IP enables SNA nodes like gateways and downstream nodes to communicate via TCP/IP, making SNA routable across the Internet.
The protocol was originally implemented in products like the McDATA LinkMaster and I/O Concepts TN3270 servers. Unlike some earlier SNA-over-IP schemes, this design can work not just in peripheral network nodes but also in intermediate routers, making it viable for building IP-based SNA switches.
This explains how to send IBM SNA messages through the Internet. SNA was built for local networks, so this adds a wrapper that makes it work over the public Internet.
§2Why SNA Needed to Cross IP Networks
For years, McDATA and other vendors implemented SNA over token-ring (802.5) and later over Ethernet (802.3) using LLC. These work well within a local network, but LLC frames do not route across the Internet. As organizations installed IP networks and expected them to handle all traffic, the question arose: can SNA run over routable IP?
TCP running over IP provides the reliable, ordered delivery that SNA requires, plus the routing capability that LLC lacks. Advanced SNA/IP was created to replace LLC type I and type II services with UDP and TCP, giving SNA sessions the ability to traverse IP routers and the Internet itself.
Companies had SNA working on local networks but wanted it to cross the Internet. TCP is reliable and routable, so the designers used it to carry SNA conversations.
§3The Three Phases of Connection
The Advanced SNA/IP protocol divides a session into three phases. The first is Connection Establishment, which negotiates parameters between two SNA nodes using three UDP datagrams: a Logical Null XID sent by the initiating node (like a downstream node asking to connect), an XID Request from the partner (usually an SNA gateway), and a Logical SNA XID containing SNA-level configuration data. Once these exchanges agree on parameters, a TCP connection is established.
The second phase is Data Transfer. Once TCP is open, SNA messages flow as TCP segment data, each preceded by a two-octet (big-endian) length field. No special framing or SNA/IP protocol is added; the SNA message format is unchanged. TCP handles all reliability and ordering.
The third phase is Connection Termination. Either node may send a TCP FIN segment to cleanly close the session. If a FIN is not received (for example, if the downstream node fails), the gateway may not know the connection is gone; to handle this, reconnecting nodes must reuse the same TCP port so the gateway recognizes when a reset is needed.
First, the two computers exchange UDP messages to agree on how to talk. Then TCP opens. They send SNA messages down the TCP connection with a simple length header. Finally, one sends a close signal and TCP shuts down.
§4Packet and Message Structure
Connection establishment uses three types of UDP datagrams, each carrying just a one-byte marker (0xBF) or SNA XID data. The Logical Null XID and XID Request are identical in format; they differ only in direction and role (DSN initiates, gateway responds). Each includes the source and destination IP addresses and ports.
For data transfer, each TCP segment containing SNA data is prefixed with a two-octet length field in big-endian format. The length counts the SNA message (transmission header, request/response header, and optional data) but not the length field itself. A zero-length value is valid and permitted. The SNA message structure remains exactly as defined by the SNA standard; no conversion or modification occurs.
The source UDP port in the Logical Null XID must equal the destination TCP port used later, allowing the receiving node to know which TCP port to expect.
The setup messages are tiny UDP packets with just a flag. Once connected, each SNA message is preceded by a two-byte number telling TCP how big the message is.
§5Performance and Practical Considerations
Performance measurements comparing SNA/IP to native token-ring LLC showed approximately a 15 percent decrease in maximum transactions per hour (using a test with 1500-byte inbound messages and 50-byte outbound messages). McDATA attributed this to the added processing overhead of TCP/IP in their LinkMaster products compared to LLC in the same devices, not to an inherent limitation of the approach.
SNA/IP can take advantage of class-of-service prioritization already defined by the SNA protocol, allowing applications to request different service levels. It also works with router packet prioritization schemes based on port number, enabling operators to give SNA traffic priority if needed. Future IP technologies offering quality of service (then in development) would further improve SNA/IP performance.
Running SNA over TCP is a bit slower than using the old local network method, mostly because computers have to do more work. But if you need to cross the Internet, it is the trade-off you make.