📄 rfc1613.txt
字号:
Network Working Group J. Forster
Request for Comments: 1613 G. Satz
Category: Informational G. Glick
cisco Systems, Inc.
R. Day
JANET
May 1994
cisco Systems X.25 over TCP (XOT)
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Table of Contents
1. Introduction....................................................1
2. Conventions.....................................................2
3. Relationship Between XOT and X.25...............................2
4. Overall Packet Format...........................................3
4.1 XOT Header....................................................4
5. TCP Connection, Port Number, and Logical Channel Numbers (LCNs).4
6. XOT Packets.....................................................5
6.1 Virtual Circuit Setup and Clearing............................5
6.2 Data and Flow Control.........................................6
6.3 Interrupt, and Reset Packets..................................8
6.4 Restart, DTE Reject, Diagnostics, and Registration............8
6.5 PVC Setup.....................................................8
7. Acknowledgments................................................12
8. Security Considerations........................................12
9. References.....................................................12
10. Authors' Addresses.............................................13
1. Introduction
It is sometimes desirable to transport X.25 over IP internets. The
X.25 Packet Level requires a reliable link level below it and
normally uses LAPB. This memo documents a method of sending X.25
packets over IP internets by encapsulating the X.25 Packet Level in
TCP packets.
TCP provides a reliable byte stream. X.25 requires that the layer
below it provide message semantics, in particular the boundary
between packets. To provide this, a small (4-byte) XOT header is
used between TCP and X.25. The primary content of this header is a
Forster, Satz, Glick & Day [Page 1]
RFC 1613 X.25 Over TCP (XOT) May 1994
length field, which is used to separate the X.25 packets within the
TCP stream.
In general, the normal X.25 protocol packet formats and state
transition rules apply to the X.25 layer in XOT. Exceptions to this
are noted.
2. Conventions
The following language conventions are used in the items of
specification in this document:
o MUST, SHALL, or MANDATORY -- This item is an absolute
requirement of the specification.
o SHOULD or RECOMMEND -- This item should generally be followed
for all but exceptional circumstances.
o MAY or OPTIONAL -- This item is truly optional and may be
followed or ignored according to the needs of the implementor.
In some places in this document, there is parenthetical material
labeled "DISCUSSION". This material is intended to give
clarification and explanation of the preceding text.
3. Relationship Between XOT and X.25
When a networking device (a host, router, etc.) has an X.25 engine
(i.e., protocol implementation), that engine may be connected to
interface(s) running LAPB, and/or to logical interface(s) running LLC
or XOT/TCP/IP. In general, the XOT layer itself is not at all
sensitive to what kind of packets the X.25 engine passes to it.
However, to improve interoperability between separate
implementations, this document in some cases does specify behavior of
the X.25 engine.
While this document primarily discusses XOT from the perspective of
switching X.25 traffic (i.e., connecting an X.25 Virtual Circuit
between the local X.25 interfaces of two networking devices), this
should not prevent a host from offering X.25 connectivity using XOT.
The various X.25 standards may call a given packet type by a
different name according to the assigned DTE/DCE role of the
interface that originated the packet. XOT is intended to be
insensitive to the DTE/DCE role of the local interfaces at either end
of an XOT TCP connection, so, for this document, the following terms
are interchangeable unless stated otherwise:
Forster, Satz, Glick & Day [Page 2]
RFC 1613 X.25 Over TCP (XOT) May 1994
o Call, Call Request and Incoming Call
o Call Confirm, Call Accepted and Call Connected
o Clear, Clear Request and Clear Indication
o Clear Confirm, DTE Clear Confirmation and DCE Clear Confirmation
o Data, DTE Data and DCE Data
o Interrupt, DTE Interrupt and DCE Interrupt
o Interrupt Confirm, DTE Interrupt Confirmation and
DCE Interrupt Confirmation
o RR, DTE RR and DCE RR
o RNR, DTE RNR and DCE RNR
o REJ, Reject and DTE REJ
o Reset, Reset Request and Reset Indication
o Reset Confirm, DTE Reset Confirmation and DCE Reset Confirmation
o Restart, Restart Request and Restart Indication
o Restart Confirm, DTE Restart Confirmation and
DCE Restart Confirmation
4. Overall Packet Format
The entire encapsulated packet has the following format:
---------------------------------
| |
| IP Header |
| |
---------------------------------
| |
| TCP Header |
| |
---------------------------------
| |
| XOT Header |
| |
---------------------------------
| |
| X.25 Packet |
| |
---------------------------------
RFC convention is that a packet format is represented graphically
with the data sent first above the data sent later. This convention
is followed in this document, and therefore, while we refer to X.25
being transported over TCP, we draw the packet format with the X.25
portion of the packet lower on the page than the TCP portion.
Forster, Satz, Glick & Day [Page 3]
RFC 1613 X.25 Over TCP (XOT) May 1994
4.1 XOT Header
The XOT header has the format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version (2 octets)
The version number of the XOT protocol is encoded in the first
two octets. The version number MUST be 0. Other values of
this field are reserved for future use. If a value other than
0 is received, then the TCP connection MUST be closed.
Length (2 octets)
The length of the X.25 packet is encoded in the second two
octets. Values must be legal X.25 packet lengths. If the
Length field has an illegal value, then the TCP connection MUST
be closed.
5. TCP Connection, Port Number, and Logical Channel Numbers (LCNs)
A separate TCP connection MUST be used for each X.25 virtual circuit.
All connections MUST be made to TCP port number 1998. This port
number is an IANA Registered Port Number registered by cisco Systems;
cisco has designated it for use by XOT.
The TCP connection MUST be created before the virtual circuit can be
established. The TCP connection MAY be maintained after the virtual
circuit has been cleared. Data MUST NOT be passed along with the TCP
SYN packet.
The Logical Channel Number (LCN) field in the X.25 header has no
significance and has arbitrary values. A corollary of this is that
there is no assignment of one side of the connection to be DTE and
another to be DCE.
DISCUSSION
Consider three devices A, B and C, where A and B both conduct XOT
sessions to C. It's possible that C could receive two calls with
the same LCN and, unless the X.25 engine could tell that they were
received on different logical (XOT) interfaces, here would a
danger of call collision (indeed a valid LCN on one interface may
Forster, Satz, Glick & Day [Page 4]
RFC 1613 X.25 Over TCP (XOT) May 1994
not even be valid on a different interface). It is therefore
necessary for C's X.25 engine to distinguish between the two
streams, but the LCN field is not sufficient to do this. The XOT
protocol design decision was to expect the XOT layer to
communicate the stream identification to the X.25 layer.
6. XOT Packets
For each X.25 packet received from the TCP connection to be sent out
a local interface, an XOT implementation MUST set the packet's
logical channel number to that used on the outgoing interface. For
the purposes of this RFC, a logical channel number is the 12 bit
field confusingly defined by the X.25 Recommendations as the high-
order 4 bit "logical channel group number" and low-order 8 bit
"logical channel number", where the latter phrase is used to refer to
both the aggregated 12 bits and the low-order 8 bits.
An XOT implementation SHOULD NOT modify the X.25 packet header
information received on a local interface to be transmitted over the
TCP connection.
An XOT implementation MUST modify the X.25 packet header information
as required for proper X.25 protocol operation for packets received
on a TCP connection to be transmitted over a local interface.
An XOT implementation MAY support connection between interfaces that
use different flow control modulos. If this feature is supported,
XOT MUST modify the packet General Format Identifier on all packets
received over the TCP connection to set the proper modulus
identifier.
6.1 Virtual Circuit Setup and Clearing
Once a TCP connection has been established, the X.25 Call packet is
sent by the XOT that initiated the TCP connection. Eventually a Call
Confirm or Clear packet is received, or the X.25 T11/T21 timeout
occurs or the XOT TCP connection is closed. The usual X.25 state
transitions are followed.
Any legal X.25 facilities from the family of X.25 protocols
(including but not limited to the 1980, 1984 and 1988 CCITT X.25
Recommendations) MAY be included in the Call, Call Confirm and Clear
packets. Receipt of an unknown or unsupported X.25 facility received
from the TCP connection SHOULD be ignored (i.e., not presented in the
packet sent out the local interface) or treated as an error as
defined by the X.25 standard implemented.
Forster, Satz, Glick & Day [Page 5]
RFC 1613 X.25 Over TCP (XOT) May 1994
To simplify end-to-end flow control, the packet size and window size
are always sent explicitly as facilities in the Call packet. The
Call packet MUST contain both Packet Size and Window Size facilities.
The Call Confirm packet MAY contain these facilities. The handling
of a Call received over a TCP connection that does not encode one or
both of the flow control facilities is a local matter--if the XOT
accepts such a Call, it MUST encode the missing flow control facility
values that apply to the connection in the returned Call Confirm
packet.
DISCUSSION
X.25 interfaces normally have a concept of network default values
for packet size and window size. It was thought that when
connecting diverse sites over a TCP/IP network this concept would
be difficult to achieve in practice. If there is no network
default, then each call must state the packet size and window
size. This is the reason for requiring the packet size and window
size facilities. It is expected that this can be achieved either
by the XOT layer itself, or by configuring the X.25 engine such
that there no network default on this interface.
After sending a Clear the TCP connection MAY be closed immediately
without waiting for the Clear Confirm. A Clear Confirm received on
the TCP connection MAY be silently discarded.
A packet with an invalid X.25 Packet Type Identifier (PTI) received
over the TCP connection before a Call has been received (i.e., while
in the P1 state) MUST be silently discarded.
6.2 Data and Flow Control
DISCUSSION
The implementation of X.25 flow control is a local matter, but
different implementation choices affect XOT behavior.
An XOT implementation may implement either end-to-end flow
control, where DATA, RR and RNR packets are sent over the TCP
connection as received over the local interface, or local flow
control, where flow control packets (RR, RNR and, if supported,
REJ) are sent on a VC according to local criteria, a complete
packet sequence of DATA packets may be fragmented or combined, and
data packet numbering normally has only local DTE-DCE
significance.
Existing implementations of XOT perform end-to-end flow control.
Data and flow control packets are simply transferred between the
Forster, Satz, Glick & Day [Page 6]
RFC 1613 X.25 Over TCP (XOT) May 1994
two local interfaces via the TCP connection, adjusting the X.25
header data as necessary for mixed modulo operation. This does
not preclude an XOT implementation that performs local flow
control, but interoperability requires that a local flow control
implementation conduct the XOT session such that a connecting
end-to-end flow control implementation receives Data packets of
the proper size and flow control fields with appropriate P(S) and
P(R) values.
An X.25 implementation that performs local flow control similarly
may set up a Call between two local interfaces where each logical
channel has its own packet and window sizes and Data packets must
be fragmented or collected between the interfaces and each
interface manages distinct packet sequence numbers; XOT operation
is simply an extension to this operation as a VC is connected
between the local interface and an XOT/TCP virtual interface, each
of which have distinct window and packet sizes.
An XOT that implements local flow control MUST send data packet
acknowledgements across the TCP connection for the DATA packets it
receives from the TCP connection, using the received packet numbers,
and MUST observe the maximum packet sizes agreed to across the TCP
connection.
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