Network Working Group D. Piscitello

Network Working Group D. Piscitello
Request for Comments: 1561 Core Competence
Category: Experimental December 1993

Use of ISO CLNP in TUBA Environments

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Abstract

This memo specifies a profile of the ISO/IEC 8473 Connectionless-mode
Network Layer Protocol (CLNP, [1]) for use in conjunction with RFC
1347, TCP/UDP over Bigger Addresses (TUBA, [2]). It describes the
use of CLNP to provide the lower-level service expected by
Transmission Control Protocol (TCP, [3]) and User Datagram Protocol
(UDP, [4]). CLNP provides essentially the same datagram service as
Internet Protocol (IP, [5]), but offers a means of conveying bigger
network addresses (with additional structure, to aid routing).

While the protocols offer nearly the same services, IP and CLNP are
not identical. This document describes a means of preserving the
semantics of IP information that is absent from CLNP while preserving
consistency between the use of CLNP in Internet and OSI environments.
This maximizes the use of already-deployed CLNP implementations.

Acknowledgments

Many thanks to Ross Callon (Wellfleet Communications), John Curran
(BBN), Cyndi Jung (3Com), Paul Brooks (UNSW), Brian Carpenter (CERN),
Keith Sklower (Cal Berkeley), Dino Farinacci and Dave Katz (Cisco
Systems), Rich Colella (NIST/CSL) and David Oran (DEC) for their
assistance in composing this text.

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Conventions

The following language conventions are used in the items of
specification in this document:

* MUST, SHALL, or MANDATORY -- the item is an absolute
requirement of the specification.

* SHOULD or RECOMMENDED -- the item should generally be
followed for all but exceptional circumstances.

* MAY or OPTIONAL -- the item is truly optional and may be
followed or ignored according to the needs of the
implementor.

1. Terminology

To the extent possible, this document is written in the language of
the Internet. For example, packet is used rather than "protocol data
unit", and "fragment" is used rather than "segment". There are some
terms that carry over from OSI; these are, for the most part, used so
that cross-reference between this document and RFC 994 [6] or ISO/IEC
8473 is not entirely painful. OSI acronyms are for the most part
avoided.

2. Introduction

The goal of this specification is to allow compatible and
interoperable implementations to encapsulate TCP and UDP packets in
CLNP data units. In a sense, it is more of a "hosts requirements"
document for the network layer of TUBA implementations than a
protocol specification. It is assumed that readers are familiar with
STD 5, RFC 791, STD 5, RFC 792 [7], STD 3, RFC 1122 [8], and, to a
lesser extent, RFC 994 and ISO/IEC 8473. This document is compatible
with (although more restrictive than) ISO/IEC 8473; specifically, the
order, semantics, and processing of CLNP header fields is consistent
between this and ISO/IEC 8473.

[Note: RFC 994 contains the Draft International Standard version of
ISO CLNP, in ASCII text. This is not the final version of the ISO/IEC
protocol specification; however, it should provide sufficient
background for the purpose of understanding the relationship of CLNP
to IP, and the means whereby IP information is to be encoded in CLNP
header fields. Postscript versions of ISO CLNP and associated routing
protocols are available via anonymous FTP from merit.edu, and may be
found in the directory /pub/ISO/IEC.

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3. Overview of CLNP

ISO CLNP is a datagram network protocol. It provides fundamentally
the same underlying service to a transport layer as IP. CLNP provides
essentially the same maximum datagram size, and for those
circumstances where datagrams may need to traverse a network whose
maximum packet size is smaller than the size of the datagram, CLNP
provides mechanisms for fragmentation (data unit identification,
fragment/total length and offset). Like IP, a checksum computed on
the CLNP header provides a verification that the information used in
processing the CLNP datagram has been transmitted correctly, and a
lifetime control mechanism ("Time to Live") imposes a limit on the
amount of time a datagram is allowed to remain in the internet
system. As is the case in IP, a set of options provides control
functions needed or useful in some situations but unnecessary for the
most common communications.

Note that the encoding of options differs between the two protocols,
as do the means of higher level protocol identification. Note also
that CLNP and IP differ in the way header and fragment lengths are
represented, and that the granularity of lifetime control (time-to-
live) is finer in CLNP.

Some of these differences are not considered "issues", as CLNP
provides flexibility in the way that certain options may be specified
and encoded (this will facilitate the use and encoding of certain IP
options without change in syntax); others, e.g., higher level
protocol identification and timestamp, must be accommodated in a
transparent manner in this profile for correct operation of TCP and
UDP, and continued interoperability with OSI implementations. Section
4 describes how header fields of CLNP must be populated to satisfy
the needs of TCP and UDP.

Errors detected during the processing of a CLNP datagram MAY be
reported using CLNP Error Reports. Implementations of CLNP for TUBA
environments MUST be capable of processing Error Reports (this is
consistent with the 1992 edition (2) of the ISO/IEC 8473 standard).
Control messages (e.g., echo request/reply and redirect) are
similarly handled in CLNP, i.e., identified as separate network layer
packet types. The relationship between CLNP Error and Control
messages and Internet Control Message Protocol (ICMP, [7]), and
issues relating to the handling of these messages is described in
Section 5.

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Table 1 provides a high-level comparison of CLNP to IP:

Function | ISO CLNP | DOD IP
----------------------|------------------------|-----------------------
Header Length | indicated in octets | in 32-bit words
Version Identifier | 1 octet | 4 bits
Lifetime (TTL) | 500 msec units | 1 sec units
Flags | Fragmentation allowed, | Don't Fragment,
| More Fragments | More Fragments,
| Suppress Error Reports | <not defined>
Packet Type | 5 bits | <not defined>
Fragment Length | 16 bits, in octets | 16 bits, in octets
Header Checksum | 16-bit (Fletcher) | 16-bit
Total Length | 16 bits, in octets | <not defined>
Addressing | Variable length | 32-bit fixed
Data Unit Identifier | 16 bits | 16 bits
Fragment offset | 16 bits, in octets | 13 bits, 8-octet units
Higher Layer Protocol | Selector in address | Protocol
Options | Security | Security
| Priority | TOS Precedence bits
| Complete Source Route | Strict Source Route
| Quality of Service | Type of Service
| Partial Source Route | Loose Source Route
| Record Route | Record Route
| Padding | Padding
| <defined herein> | Timestamp

Table 1. Comparison of IP to CLNP

The composition and processing of a TCP pseudo-header when CLNP is
used to provide the lower-level service expected by TCP and UDP is
described in Section 6.

[Note: This experimental RFC does not discuss multicasting.
Presently, there are proposals for multicast extensions for CLNP in
ISO/IEC/JTC1/SC6, and a parallel effort within TUBA. A future
revision to this RFC will incorporate any extensions to CLNP that may
be introduced as a result of the adoption of one of these
alternatives.]

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4. Proposed Internet Header using CLNP

A summary of the contents of the CLNP header, as it is proposed for
use in TUBA environments, is illustrated in Figure 4-1:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........Data Link Header........ | NLP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Header Length | Version | Lifetime (TTL)|Flags| Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dest Addr Len | Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PROTO field | Src Addr Len | Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Source Address | Reserved | Data Unit Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Offset | Total Length of packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (see Table 1) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note that each tick mark represents one bit position.

Figure 4-1. CLNP for TUBA

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Note 1: For illustrative purposes, Figure 4-1 shows Destination
and Source Addresses having a length of 19 octets,
including the PROTO/reserved field. In general, addresses
can be variable length, up to a maximum of 20 octets,
including the PROTO/reserved field.

Note 2: Due to differences in link layer protocols, it is not
possible to ensure that the packet starts on an even
alignment. Note, however, that many link level protocols
over which CLNP is operated use a odd length link
(e.g., IEEE 802.2). (In Figure 4-1, the rest of the CLNP
packet is even-aligned.)

The encoding of CLNP fields for use in TUBA environments is as
follows.

4.1 Network Layer Protocol Identification (NLP ID)

This one-octet field identifies this as the ISO/IEC 8473 protocol; it
MUST set to binary 1000 0001.

4.2 Header Length Indication (Header Length)

Header Length is the length of the CLNP header in octets, and thus
points to the beginning of the data. The value 255 is reserved. The
header length is the same for all fragments of the same (original)
CLNP packet.

4.3 Version

This one-octet field identifies the version of the protocol; it MUST
be set to a binary value 0000 0001.

4.4 Lifetime (TTL)

Like the TTL field of IP, this field indicates the maximum time the
datagram is allowed to remain in the internet system. If this field
contains the value zero, then the datagram MUST be destroyed; a host,
however, MUST NOT send a datagram with a lifetime value of zero.
This field is modified in internet header processing. The time is
measured in units of 500 milliseconds, but since every module that
processes a datagram MUST decrease the TTL by at least one even if it
process the datagram in less than 500 millisecond, the TTL must be
thought of only as an upper bound on the time a datagram may exist.
The intention is to cause undeliverable datagrams to be discarded,
and to bound the maximum CLNP datagram lifetime. [Like IP, the
colloquial usage of TTL in CLNP is as a coarse hop-count.]

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Unless otherwise directed, a host SHOULD use a value of 255 as the
initial lifetime value.

4.5 Flags

Three flags are defined. These occupy bits 0, 1, and 2 of the
Flags/Type octet:

0 1 2
+---+---+---+
| F | M | E |
| P | F | R |
+---+---+---+

The Fragmentation Permitted (FP) flag, when set to a value of one
(1), is semantically equivalent to the "may fragment" value of the
Don't Fragment field of IP; similarly, when set to zero (0), the
Fragmentation Permitted flag is semantically equivalent to the "Don't
Fragment" value of the Don't Fragment Flag of IP.

[Note: If the Fragmentation Permitted field is set to the value 0,
then the Data Unit Identifier, Fragment Offset, and Total Length
fields are not present. This denotes a single fragment datagram. In
such datagrams, the Fragment Length field contains the total length
of the datagram.]

The More Fragments flag of CLNP is semantically and syntactically the
same as the More Fragments flag of IP; a value of one (1) indicates
that more segments/fragments are forthcoming; a value of zero (0)
indicates that the last octet of the original packet is present in
this segment.

The Error Report (ER) flag is used to suppress the generation of an
error message by a host/router that detects an error during the
processing of a CLNP datagram; a value of one (1) indicates that the
host that originated this datagram thinks error reports are useful,
and would dearly love to receive one if a host/router finds it
necessary to discard its datagram(s).

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4.6 Type field

The type field distinguishes data CLNP packets from Error Reports
from Echo packets. The following values of the type field apply:

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| flags | 1 | 1 | 1 | 0 | 0 | => Encoding of Type = data packet
+---+---+---+---+---+---+---+---+
| flags | 0 | 0 | 0 | 0 | 1 | => Encoding of Type = error report
+---+---+---+---+---+---+---+---+
| flags | 1 | 1 | 1 | 1 | 0 | => Encoding of Type = echo request
+---+---+---+---+---+---+---+---+
| flags | 1 | 1 | 1 | 1 | 1 | => Encoding of Type = echo reply
+---+---+---+---+---+---+---+---+

Error Report packets are described in Section 5.

Echo packets and their use are described in RFC 1139 [9].

4.7 Fragment Length

Like the Total Length of the IP header, the Fragment length field
contains the length in octets of the fragment (i.e., this datagram)
including both header and data.

[Note: CLNP also may also have a Total Length field, that contains
the length of the original datagram; i.e., the sum of the length of
the CLNP header plus the length of the data submitted by the higher
level protocol, e.g., TCP or UDP. See Section 4.12.]

4.8 Checksum

A checksum is computed on the header only. It MUST be verified at
each host/router that processes the packet; if header fields are
changed during processing (e.g., the Lifetime), the checksum is
modified. If the checksum is not used, this field MUST be coded with
a value of zero (0). See Appendix A for algorithms used in the
computation and adjustment of the checksum. Readers are encouraged to
see [10] for a description of an efficient implementation of the
checksum algorithm.

4.9 Addressing

CLNP uses OSI network service access point addresses (NSAPAs); NSAPAs
serve the same identification and location functions as an IP
address, plus the protocol selector value encoded in the IPv4
datagram header, and with additional hierarchy. General purpose

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CLNP implementations MUST handle NSAP addresses of variable length up
to 20 octets, as defined in ISO/IEC 8348 [11]. TUBA implementations,
especially routers, MUST accommodate these as well. Thus, for
compatibility and interoperability with OSI use of CLNP, the initial
octet of the Destination Address is assumed to be an Authority and
Format Indicator, as defined in ISO/IEC 8348. NSAP addresses may be
between 8 and 20 octets long (inclusive).

TUBA implementations MUST support both ANSI and GOSIP style
addresses; these are described in RFC 1237 [12], and illustrated in
Figure 4-2. RFC 1237 describes the ANSI/GOSIP initial domain parts
as well as the format and composition of the domain specific part. It
is further recommended that TUBA implementations support the
assignment of system identifiers for TUBA/CLNP hosts defined in [13]
for the purposes of host address autoconfiguration as described in
[14]. Additional considerations specific to the interpretation and
encoding of the selector part are described in sections 4.9.2 and
4.9.4.

+-------------+
| <-- IDP --> |
+----+--------+----------------------------------+
|AFI | IDI | <-- DSP --> |
+----+--------+----+---+-----+----+-----+---+----+
| 47 | 0005 |DFI |AA |Rsvd | RD |Area |ID |Sel |
+----+--------+----+---+-----+----+-----+---+----+
octets | 1 | 2 | 1 | 3 | 2 | 2 | 2 | 6 | 1 |
+----+--------+----+---+-----+----+-----+---+----+

Figure 4-2 (a): GOSIP Version 2 NSAP structure.

+-------------+
|<-- IDP --> |
+----+--------+----------------------------------+
|AFI | IDI | <-- DSP --> |
+----+--------+----+---+-----+----+-----+---+----+
| 39 | 840 |DFI |ORG|Rsvd | RD |Area |ID |Sel |
+----+--------+----+---+-----+----+-----+---+----+
octets | 1 | 2 | 1 | 3 | 2 | 2 | 2 | 6 | 1 |
+----+--------+----+---+-----+----+-----+---+----+

Figure 4-2 (b): ANSI NSAP address format for DCC=840

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Definitions:
IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
DFI DSP Format Identifier
AA Administration Authority
ORG Organization Name (numeric form)
Rsvd Reserved
RD Routing Domain Identifier
Area Area Identifier
ID System Identifier
Sel NSAP Selector

4.9.1 Destination Address Length Indicator

This field indicates the length, in octets, of the Destination
Address.

4.9.2 Destination Address

This field contains an OSI NSAP address, as described in Section 4.9.
It MUST always contain the address of the final destination. (This is
true even for packets containing a source route option, see Section
4.13.4).

The final octet of the destination address MUST always contain the
value of the PROTO field, as defined in IP. The 8-bit PROTO field
indicates the next level protocol used in the data portion of the
CLNP datagram. The values for various protocols are specified in
"Assigned Numbers" [15]. For the PROTO field, the value of zero (0)
is reserved.

TUBA implementations that support TCP/UDP as well as OSI MUST use the
protocol value (1Dh, Internet decimal 29) reserved for ISO transport
protocol class 4.

4.9.3 Source Address Length Indicator

This field indicates the length, in octets, of the Source Address.

4.9.4 Source Address

This field contains an OSI NSAP address, as described in Section 4.9.

The final octet of the source address is reserved. It MAY be set to
the protocol field value on transmission, and shall be ignored on
reception (the value of zero MUST not be used).

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4.10 Data Unit Identifier

Like the Identification field of IP, this 16-bit field is used to
distinguish segments of the same (original) packet for the purposes
of reassembly. This field is present when the fragmentation permitted
flag is set to one.

4.11 Fragment Offset

Like the Fragment Offset of IP, this 16-bit is used to identify the
relative octet position of the data in this fragment with respect to
the start of the data submitted to CLNP; i.e., it indicates where in
the original datagram this fragment belongs. The offset is measured
in octets; the value of this field shall always be a multiple of
eight (8). This field is present when the fragmentation permitted
flag is set to one.

4.12 Total Length

The total length of the CLNP packet in octets is determined by the
originator and placed in the Total Length field of the header. The
Total Length field specifies the entire length of the original
datagram, including both the header and data. This field MUST NOT be
changed in any fragment of the original packet for the duration of
the packet lifetime. This field is present when the fragmentation
permitted flag is set to one.

4.13 Options

All CLNP options are "triplets" of the form <parameter code>,
<parameter length>, and <parameter value>. Both the parameter code
and length fields are always one octet long; the length parameter
value, in octets, is indicated in the parameter length field. The
following options are defined for CLNP for TUBA.

4.13.1 Security

The value of the parameter code field is binary 1100 0101. The length
field MUST be set to the length of a Basic (and Extended) Security IP
option(s) as identified in RFC 1108 [16], plus 1. Octet 1 of the
security parameter value field -- the CLNP Security Format Code -- is
set to a binary value 0100 0000, indicating that the remaining octets
of the security field contain either the Basic or Basic and Extended
Security options as identified in RFC 1108. This encoding points to
the administration of the source address (e.g., ISOC) as the
administration of the security option; it is thus distinguished from
the globally unique format whose definition is reserved for OSI use.
Implementations wishing to use a security option MUST examine the

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PROTO field in the source address; if the value of PROTO indicates
the CLNP client is TCP or UDP, the security option described in RFC
1108 is used.

[Note: If IP options change, TUBA implementations MUST follow the new
recommendations. This RFC, or revisions thereof, must document the
new recommendations to assure compatibility.]

The formats of the Security option, encoded as a CLNP option, is as
follows. The CLNP option will be used to convey the Basic and
Extended Security options as sub-options; i.e., the exact encoding of
the Basic/Extended Security IP Option is carried in a single CLNP
Security Option, with the length of the CLNP Security option
reflecting the sum of the lengths of the Basic and Extended Security
IP Option.

+--------+--------+--------+--------+--------+---//----+-
|11000100|XXXXXXXX|01000000|10000010|YYYYYYYY| | ...
+--------+--------+--------+--------+--------+---//----+----
CLNP CLNP CLNP BASIC BASIC BASIC
OPTION OPTION FORMAT SECURITY OPTION OPTION
TYPE LENGTH CODE TYPE LENGTH VALUE
(197) (130)

---+------------+------------+----//-------+
... | 10000101 | 000LLLLL | |
-----+------------+------------+----//-------+
EXTENDED EXTENDED EXTENDED OPTION
OPTION OPTION VALUE
TYPE (133) LENGTH

The syntax, semantics and processing of the Basic and Extended IP
Security Options are defined in RFC 1108.

4.13.2 Type of Service

[Note: Early drafts recommended the use of IP Type of Service as
specified in RFC 1349. There now appears to be a broad consensus that
this encoding is insufficient, and there is renewed interest in
exploring the utility of the "congestion experienced" flag available
in the CLNP QOS Maintenance option. This RFC thus recommends the use
of the QOS Maintenance option native to CLNP.]

The Quality of Service Maintenance option allows the originator of a
CLNP datagram to convey information about the quality of service
requested by the originating upper layer process. Routers MAY use
this information as an aid in selecting a route when more than one
route satisfying other routing criteria is available and the

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available routes are know to differ with respect to the following
qualities of service: ability to preserve sequence, transit delay,
cost, residual error probability. Through this option, a router may
also indicate that it is experiencing congestion.

The encoding of this option is as follows:

+-----------+-----------+----------+
| 1100 0011 | 0000 0001 | 110ABCDE |
+-----------+-----------+----------+
CLNP QOS OPTION QOS FLAGS
TYPE (195) LENGTH

The value of the parameter code field MUST be set to a value of
binary 1100 0011 (the CLNP Quality of Service Option Code point).
The length field MUST be set to one (1).

Bits 8-6 MUST be set as indicated in the figure. The flags "ABCDE"
are interpreted as follows:

A=1 choose path that maintains sequence over
one that minimizes transit delay
A=0 choose path that minimizes transit delay over
one that maintains sequence
B=1 congestion experienced
B=0 no congestion to report
C=1 choose path that minimizes transit delay over
over low cost
C=0 choose low cost over path that
minimizes transit delay
D=1 choose pathe with low residual error probability over
one that minimizes transit delay
D=0 choose path that minimizes transit delay over
one with low residual error probability
E=1 choose path with low residual error probability over
low cost
E=0 choose path with low cost over one with low
residual error probability

4.13.3 Padding

The padding field is used to lengthen the packet header to a
convenient size. The parameter code field MUST be set to a value of
binary 1100 1100. The value of the parameter length field is
variable. The parameter value MAY contain any value; the contents of
padding fields MUST be ignored by the receiver.

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+----------+----------+-----------+
| 11001100 | LLLLLLLL | VVVV VVVV |
+----------+----------+-----------+

4.13.4 Source Routing

Like the strict source route option of IP, the Complete Source Route
option of CLNP is used to specify the exact and entire route an
internet datagram MUST take. Similarly, the Partial Source Route
option of CLNP provides the equivalent of the loose source route
option of IP; i.e., a means for the source of an internet datagram to
supply (some) routing information to be used by gateways in
forwarding the internet datagram towards its destination. The
identifiers encoded in this option are network entity titles, which
are semantically and syntactically the same as NSAPAs and which can
be used to unambiguously identify a network entity in an intermediate
system (router).

The parameter code for Source Routing is binary 1100 1000. The length
of the source routing parameter value is variable.

The first octet of the parameter value is a type code, indicating
Complete Source Routing (binary 0000 0001) or Partial Source Routing
(binary 0000 0000). The second octet identifies the offset of the
next network entity title to be processed in the list, relative to
the start of the parameter (i.e., a value of 3 is used to identify
the first address in the list). The offset value is modified by each
router using a complete source route or by each listed router using a
partial source route to point to the next NET.

The third octet begins the list of network entity titles. Only the
NETs of intermediate systems are included in the list; the source and
destination addresses shall not be included. The list consists of
variable length network entity title entries; the first octet of each
entry gives the length of the network entity title that comprises the
remainder of the entry.

4.13.5 Record Route

Like the IP record route option, the Record route option of CLNP is
used to trace the route a CLNP datagram takes. A recorded route
consists of a list of network entity titles (see Source Routing). The
list is constructed as the CLNP datagram is forwarded along a path
towards its final destination. Only titles of intermediate systems
(routers) that processed the datagram are included in the recorded
route; the network entity title of the originator of the datagram
SHALL NOT be recorded in the list.

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The parameter code for Record Route is binary 1100 1011. The length
of the record route parameter value is variable.

The first octet of the parameter value is a type code, indicating
Complete Recording of Route (0000 0001) or Partial Recording of Route
(0000 0000). When complete recording of route is selected, reassembly
at intermediate systems MAY be performed only when all fragments of a
given datagram followed the same route; partial recording of route
eliminates or "loosens" this constraint.

The second octet identifies the offset where the next network entity
title entry (see Source Routing) MAY be recorded (i.e., the end of
the current list), relative to the start of the parameter. A value
of 3 is used to identify the initial recording position. The process
of recording a network entity title entry is as follows. A router
adds the length of its network entity title entry to the value of
record route offset and compares this new value to the record route
list length indicator; if the value does not exceed the length of the
list, entity title entry is recorded, and the offset value is
incremented by the value of the length of the network entity title
entry. Otherwise, the recording of route is terminated, and the
router MUST not record its network entity title in the option. If
recording of route has been terminated, this (second) octet has a
value 255.

The third octet begins the list of network entity titles.

4.13.6 Timestamp

[Note: There is no timestamp option in edition 1 of ISO/IEC 8473, but
the option has been proposed and submitted to ISO/IEC JTC1/SC6.]

The parameter code value 1110 1110 is used to identify the Timestamp
option; the syntax and semantics of Timestamp are identical to that
defined in IP.

The Timestamp Option is defined in STD 5, RFC 791. The CLNP parameter
code 1110 1110 is used rather than the option type code 68 to
identify the Timestamp option, and the parameter value conveys the
option length. Octet 1 of the Timestamp parameter value shall be
encoded as the pointer (octet 3 of IP Timestamp); octet 2 of the
parameter value shall be encoded as the overflow/format octet (octet
4 of IP Timestamp); the remaining octets shall be used to encode the
timestamp list. The size is fixed by the source, and cannot be
changed to accommodate additional timestamp information.

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+--------+--------+--------+--------+
|11101110| length | pointer|oflw|flg|
+--------+--------+--------+--------+
| network entity title |
+--------+--------+--------+--------+
| timestamp |
+--------+--------+--------+--------+
| . |
.

5. Error Reporting and Control Message Handling

CLNP and IP differ in the way in which errors are reported to hosts.
In IP environments, the Internet Control Message Protocol (ICMP, [7])
is used to return (error) messages to hosts that originate packets
that cannot be processed. ICMP messages are transmitted as user data
in IP datagrams. Unreachable destinations, incorrectly composed IP
datagram headers, IP datagram discards due to congestion, and
lifetime/reassembly time exceeded are reported; the complete internet
header that caused the error plus (at least) 8 octets of the segment
contained in that IP datagram are returned to the sender as part of
the ICMP error message. For certain errors, e.g., incorrectly
composed IP datagram headers, the specific octet which caused the
problem is identified.

In CLNP environments, an unique message type, the Error Report type,
is used in the network layer protocol header to distinguish Error
Reports from CLNP datagrams. CLNP Error Reports are generated on
detection of the same types of errors as with ICMP. Like ICMP error
messages, the complete CLNP header that caused the error is returned
to the sender in the data portion of the Error Report.
Implementations SHOULD return at least 8 octets of the datagram
contained in the CLNP datagram to the sender of the original CLNP
datagram. Here too, for certain errors, the specific octet which
caused the problem is identified.

A summary of the contents of the CLNP Error Report, as it is proposed
for use in TUBA environments, is illustrated in Figure 5-1:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........Data Link Header........ | NLP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Header Length | Version | Lifetime (TTL)| 000 | Type=ER |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOTAL Length of Error Report | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dest Addr Len | Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PROTO field | Src Addr Len | Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address | Reason for Discard (type/len) |
| | 1100 0001 | 0000 0010 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason for Discard | Options... |
| code | pointer | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options |
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data |
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note that each tick mark represents one bit position.

Figure 5-1. Error Report Format

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5.1 Rules for processing an Error Report

The following is a summary of the rules for processing an Error
Report:

* An Error Report is not generated to report a problem
encountered while processing an Error Report.

* Error Reports MAY NOT be fragmented (hence, the
fragmentation part is absent).

* The Reason for Discard Code field is populated with one of
the values from Table 5-1.

* The Pointer field is populated with number of the first
octet of the field that caused the Error Report to be
generated. If it is not possible to identify the offending
octet, this field MUST be zeroed.

* If the Priority or Type of Service option is present in the
errored datagram, the Error Report MUST specify the same
option, using the value specified in the original datagram.

* If the Security option is present in the errored datagram,
the Error Report MUST specify the same option, using the
value specified in the original datagram; if the Security
option is not supported by the intermediate system, no Error
Report is to be generated (i.e., "silently discard" the
received datagram).

* If the Complete Source Route option is specified in the
errored datagram, the Error Report MUST compose a reverse of
that route, and return the datagram along the same path.

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5.2 Comparison of ICMP and CLNP Error Messages

Table 5-1 provides a loose comparison of ICMP message types and codes
to CLNP Error Type Codes (values in Internet decimal):

CLNP Error Type Codes | ICMP Message (Type, Code)
----------------------------------|------------------------------------
Reason not specified (0) | Parameter Problem (12, 0)
Protocol Procedure Error (1) | Parameter Problem (12, 0)
Incorrect Checksum (2) | Parameter Problem (12, 0)
PDU Discarded--Congestion (3) | Source Quench (4, 0)
Header Syntax Error (4) | Parameter problem (12, 0)
Need to Fragment could not (5) | Frag needed, DF set (3, 4)
Incomplete PDU received (6) | Parameter Problem (12, 0)
Duplicate Option (7) | Parameter Problem (12, 0)
Destination Unreachable (128) | Dest Unreachable,Net unknown (3, 0)
Destination Unknown (129) | Dest Unreachable,host unknown(3, 1)
Source Routing Error (144) | Source Route failed (3, 5)
Source Route Syntax Error (145) | Source Route failed (3, 5)
Unknown Address in Src Route(146) | Source Route failed (3, 5)
Path not acceptable (147) | Source Route failed (3, 5)
Lifetime expired (160) | TTL exceeded (11, 0)
Reassembly Lifetime Expired (161) | Reassembly time exceeded (11, 1)
Unsupported Option (176) | Parameter Problem (12, 0)
Unsupported Protocol Version(177) | Parameter problem (12, 0)
Unsupported Security Option (178) | Parameter problem (12, 0)
Unsupported Src Rte Option (179) | Parameter problem (12, 0)
Unsupported Rcrd Rte (180) | Parameter problem (12, 0)
Reassembly interference (192) | Reassembly time exceeded (11, 1)

Table 5-1. Comparison of CLNP Error Reports to ICMP Error Messages

Note 1: The current accepted practice for IP is that source quench
should not be used; if it is used, implementations MUST
not return a source quench packet for every relevant packet.
TUBA/CLNP implementations are encouraged to adhere to these
guidelines.

Note 2: There are no corresponding CLNP Error Report Codes for the
following ICMP error message types:
- Protocol Unreachable (3, 2)
- Port Unreachable (3, 3)
[Note: Additional error code points available in the ER type
code block can be used to identify these message types.]

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6. Pseudo-Header Considerations

A checksum is computed on UDP and TCP segments to verify the
integrity of the UDP/TCP segment. To further verify that the UDP/TCP
segment has arrived at its correct destination, a pseudo-header
consisting of information used in the delivery of the UDP/TCP segment
is composed and included in the checksum computation.

To compute the checksum on a UDP or TCP segment prior to
transmission, implementations MUST compose a pseudo-header to the
UDP/TCP segment consisting of the following information that will be
used when composing the CLNP datagram:

* Destination Address Length Indicator

* Destination Address (including PROTO field)

* Source Address Length Indicator

* Source Address (including Reserved field)

* A two-octet encoding of the Protocol value

* TCP/UDP segment length

If the length of the {source address length field + source address +
destination address field + destination address } is not an integral
number of octets, a trailing 0x00 nibble is padded. If GOSIP
compliant NSAP addresses are used, this never happens (this is known
as the Farinacci uncertainty principle). The last byte in the
Destination Address has the value 0x06 for TCP and 0x11 for UDP, and
the Protocol field is encoded 0x0006 for TCP and 0x0011 for UDP. If
needed, an octet of zero is added to the end of the UDP/TCP segment
to pad the datagram to a length that is a multiple of 16 bits.

[Note: the pseudoheader is encoded in this manner to expedite
processing, as it allows implementations to grab a contiguous stream
of octets beginning at the destination address length indicator and
terminating at the final octet of the source address; the PROTOCOL
field is present to have a consistent representation across IPv4 and
CLNP/TUBA implementations.]

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Figure 6-1 illustrates the resulting pseudo-header when both source
and destination addresses are maximum length.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dest Addr Len | Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Destination Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (PROTO) | Src Addr Len | Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | (Reserved) | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP/TCP segment length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6-1. Pseudo-header

7. Security Considerations

ISO CLNP is an unreliable network datagram protocol, and is subject
to the same security considerations as Internet Protocol ([5], [8]);
methods for conveying the same security handling information
recommended for IP are described in Section 4.13.1, Security Option.

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8. Author's Address

David M. Piscitello
Core Competence
1620 Tuckerstown Road
Dresher, PA 19025

Phone: 215-830-0692
EMail: [email protected]

9. References

[1] ISO/IEC 8473-1992. International Standards Organization -- Data
Communications -- Protocol for Providing the Connectionless
Network Service, Edition 2.

[2] Callon, R., "TCP/UDP over Bigger Addresses (TUBA)", RFC 1347,
Internet Architecture Board, May 1992.

[3] Postel, J., "Transmission Control Protocol (TCP)", STD 7, RFC
793, USC/Information Sciences Institute, September 1981.

[4] Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC 768,
USC/Information Sciences Institute, September 1981.

[5] Postel, J., "Internet Protocol (IP)", STD 5, RFC 791,
USC/Information Sciences Institute, September 1981.

[6] Chapin, L., "ISO DIS 8473, Protocol for Providing the
Connectionless Network Service", RFC 994, March 1986.

[7] Postel, J., "Internet Control Message Protocol (ICMP)", STD 5,
RFC 792, USC/Information Sciences Institute, September 1981.

[8] Braden, R., Editor, "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, Internet Engineering Task
Force, October 1989.

[9] Hagens, R., "An Echo Function for ISO 8473", RFC 1139, IETF-OSI
Working Group, May 1993.

[10] Sklower, K., "Improving the Efficiency of the ISO Checksum
Calculation" ACM SIGCOMM CCR 18, no. 5 (October 1989):32-43.

[11] ISO/IEC 8348-1992. International Standards Organization--Data
Communications--OSI Network Layer Service and Addressing.

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RFC 1561 CLNP in TUBA Environments December 1993

[12] Callon, R., Gardner, E., and R. Hagens, "Guidelines for OSI NSAP
Allocation in the Internet", RFC 1237, NIST, Mitre, DEC, July
1991.

[13] Piscitello, D., "Assignment of System Identifiers for TUBA/CLNP
Hosts", RFC 1526, Bellcore, September 1993.

[14] ISO/IEC 9542:1988/PDAM 1. Information Processing Systems -- Data
Communications -- ES/IS Routeing Protocol for use with ISO CLNP
-- Amendment 1: Dynamic Discovery of OSI NSAP Addresses by End
Systems.

[15] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340
USC/Information Sciences Institute, July 1992.

[16] Kent, S., "Security Option for IP", RFC 1108, BBN Communications,
November 1991.

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Appendix A. Checksum Algorithms (from ISO/IEC 8473)

Symbols used in algorithms:

c0, c1 variables used in the algorithms
i position of octet in header (first
octet is i=1)
Bi value of octet i in the header
n position of first octet of checksum (n=8)
L Length of header in octets
X Value of octet one of the checksum parameter
Y Value of octet two of the checksum parameter

Addition is performed in one of the two following modes:

* modulo 255 arithmetic;

* eight-bit one's complement arithmetic;

The algorithm for Generating the Checksum Parameter Value is as
follows:

A. Construct the complete header with the value of the
checksum parameter field set to zero; i.e., c0 <- c1 <- 0;

B. Process each octet of the header sequentially from i=1 to L
by:

* c0 <- c0 + Bi

* c1 <- c1 + c0

C. Calculate X, Y as follows:

* X <- (L - 8)(c0 - c1) modulo 255

* Y <- (L - 7)(-C0) + c1

D. If X = 0, then X <- 255

E. If Y = 0, then Y <- 255

F. place the values of X and Y in octets 8 and 9 of the
header, respectively

The algorithm for checking the value of the checksum parameter is as
follows:

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A. If octets 8 and 9 of the header both contain zero, then the
checksum calculation has succeeded; else if either but not
both of these octets contains the value zero then the
checksum is incorrect; otherwise, initialize: c0 <- c1 <- 0

B. Process each octet of the header sequentially from i = 1 to
L by:

* c0 <- c0 + Bi

* c1 <- c1 + c0

C. When all the octets have been processed, if c0 = c1 = 0,
then the checksum calculation has succeeded, else it has
failed.

There is a separate algorithm to adjust the checksum parameter value
when a octet has been modified (such as the TTL). Suppose the value
in octet k is changed by Z = newvalue - oldvalue. If X and Y denote
the checksum values held in octets n and n+1 respectively, then
adjust X and Y as follows:

If X = 0 and Y = 0 then do nothing, else if X = 0 or Y = 0 then the
checksum is incorrect, else:

X <- (k - n - 1)Z + X modulo 255

Y <- (n - k)Z + Y modulo 255

If X = 0, then X <- 255; if Y = 0, then Y <- 255.

In the example, n = 89; if the octet altered is the TTL (octet 4),
then k = 4. For the case where the lifetime is decreased by one unit
(Z = -1), the assignment statements for the new values of X and Y in
the immediately preceeding algorithm simplify to:

X <- X + 5 Modulo 255

Y <- Y - 4 Modulo 255

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