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Use of ISO CLNP in TUBA Environments
draft-ietf-tuba-clnp-05

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This is an older version of an Internet-Draft that was ultimately published as RFC 1561.
Author David M. Piscitello
Last updated 2013-03-02 (Latest revision 1993-11-16)
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draft-ietf-tuba-clnp-05
IETF                                                       Page 1
November 15, 1993        CLNP for TUBA             Internet Draft

              Use of ISO CLNP in TUBA Environments

                  <draft-ietf-tuba-clnp-05.txt>

              David M. Piscitello
                    Bellcore
                  dave@sabre.bellcore.com

                       Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its
Areas, and its Working Groups. Note that other groups may also
distribute working documents as Internet Drafts.

Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time.  It is not appropriate to use
Internet Drafts as reference material or to cite them other than
as a "working draft" or "work in progress."

Please check the Internet Draft abstract listing contained in the
IETF Shadow Directories (cd internet-drafts) to learn the current
status of this or any other Internet Draft.

This Internet-Draft 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]).  This draft document will be submitted to the RFC editor as
a protocol specification. Distribution of this memo is unlimited.
Please send comments to dave@eve.bellcore.com.

                            Abstract

This document 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

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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/NCSL) and David
Oran (DEC) for their assistance in composing this text.

                           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 RFC 791, RFC 792 [7], 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.

[Editor's 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

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ISO CLNP and associated routing protocols are available via
anonymous FTP from merit.edu, and may be found in the directory
/pub/ISO/IEC.

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.

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

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Function                | ISO CLNP              | DOD IP
------------------------|-----------------------|-----------------------
Header Length           | indicated in octets   | in 32-bit words
Version Identifier      | 1 octet               | 4 bits
Lifetime (Time to live) | 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 (assigned #)
Options                 | Security              | Security
                        | Priority              | Precedence bits in TOS
                        | 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

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

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these messages is described in Section 5.

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.

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                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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          Note that each tick mark represents one bit position.

                        Figure 4-1. CLNP for TUBA

Note 1: For illustrative purposes, Figure 4-1 depicts 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
        happen to use a odd length link (e.g., 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,

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and to bound the maximum CLNP datagram lifetime. [Like IP, the
colloquial usage of TTL in CLNP is as a coarse hop-count.]

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.

[Editor's Note: If the Fragmentation Permitted field is set to
the value O, 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).

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:

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  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
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

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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

        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.

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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).

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.

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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 lenth>, 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 RFC1108 [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 PROTO field in the source address; if the value of
PROTO indicates the CLNP client is TCP or UDP, the security
option described in RFC1108 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.

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+--------+--------+--------+--------+--------+------//-----+---
|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  Quality 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 internet-draft
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 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).

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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.

+----------+----------+-----------+
| 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).

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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.

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

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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

[Editor's 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 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.

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

[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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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.

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):

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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)
        [ED. There are error code points available in the ER type
             code block that can be used to identify these message types.]

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

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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

Figure 5-1 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 5-1. Pseudo-header

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

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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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7.  REFERENCES

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

[2]     Callon, R., TCP/UDP over Bigger Addresses (TUBA), Request for
        Comments 1347, Network Information Center, SRI
        International, Menlo Park, CA, May 1992.

[3]     Postel, J., Transmission Control Protocol (TCP). Request for
        Comments 793, Network Information Center, SRI
        International, Menlo Park, CA, 1981 September.

[4]     Postel, J., User Datagram Protocol (UDP). Request for Comments 768,
        Network Information Center, SRI International, Menlo Park, CA.

[5]     Postel, J., Internet Protocol (IP). Request for Comments 791,
        Network Information Center, SRI International, Menlo Park,
        CA, 1981 September.

[6]     Chapin, L., ISO DIS 8473, Protocol for Providing the
        Connectionless Network Service. Request for Comments 994,
        Network Information Center, SRI International, Menlo Park, CA,
        1986 March.

[7]     Postel, J.  Internet Control Message Protocol. Request for
        Comments 792, Network Information Center, SRI International,
        Menlo Park, CA  1981 September.

[8]     Braden, R.,ed.  Requirements for Internet hosts - communication layers
        Request for Comments 1122. Network Information Center, SRI
        International, Menlo Park, CA, 1989 October.

[9]     Hagens, R. and C. Wittbrodt, CLNP Ping, Request for Comments
        1139, Network Information Center, SRI International, Menlo
        Park, CA. 1993 May.

[10]    Sklower, K., <checksum implemenation article from CCR>

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

[12]    Callon, R.,  NSAPA Guidelines for the Internet,
        Request for Comments RFC 1237, Network Information Center, SRI
        International, Menlo Park, CA.

[13]    Piscitello, D., Assignment of System Identifiers for TUBA/CLNP
        hosts, Internet-drafts (draft-tuba-sysids-01.txt).

[14]    ISO/IEC 9542:1988/PDAM 1. Information Processing Systems -- Data
        Communications -- End System to Intermediate System Routeing
        Exchange Protocol for use in conjunction with the protocol for

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        providing the connectionless-mode network service -- Amendment 1:
        Dynamic Discovery of OSI NSAP Addresses by End Systems.

[15]    Reynolds, J., and J. Postel, Assigned Numbers.
        Request for Comments 1340, Network Information Center, SRI
        International, Menlo Park, CA. 1992 July.

[16]    Kent, S., Security Option for IP,
        Request for Comments 1108, Network Information Center, SRI
        International, Menlo Park, CA.

[17]    Almquist, P., Type of Service in the Internet
        Protocol Suite. Request for Comments 1349, Network Information
        Center, SRI International, Menlo Park, CA.

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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:

   o?+ modulo 255 arithmetic;

   o?+ 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:

         o?+ c0 <- c0 + Bi

         o?+ c1 <- c1 + c0

  C.  Calculate X, Y as follows:

         o?+ X <- (L - 8)(c0 - c1) modulo 255

         o?+ 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:

  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

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

         o?+ c0 <- c0 + Bi

         o?+ 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