As per Relevance of the word addresses, we have this rfc below:
Network Working Group M.
Request for Comments: 2225 Com21, Inc
Category: Standards Track J.
Obsoletes: 1626, 1577 Newbridge Networks, Inc
April 1998
Classical IP and ARP over
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1998). All Rights Reserved
Table of
1. ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. ACKNOWLEDGMENT . . . . . . . . . . . . . . . . . . . . . . . 2
3. CONVENTIONS . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 3
5. IP SUBNETWORK CONFIGURATION . . . . . . . . . . . . . . . . . 6
5.1 Background . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2 LIS Configuration Requirements . . . . . . . . . . . . . . 7
5.3 LIS Router Additional Configuration . . . . . . . . . . . . 8
6. IP PACKET FORMAT . . . . . . . . . . . . . . . . . . . . . . 8
7. DEFAULT VALUE FOR IP MTU OVER ATM AAL5 . . . . . . . . . . . 9
7.1 Permanent Virtual Circuits . . . . . . . . . . . . . . . . 9
7.2 Switched Virtual Circuits . . . . . . . . . . . . . . . . . 9
7.3 Path MTU Discovery Required . . . . . . . . . . . . . . . . 11
8. LIS ADDRESS RESOLUTION SERVICES . . . . . . . . . . . . . . . 11
8.1 ATM-based ARP and InARP Equivalent Services . . . . . . . . 11
8.2 Permanent Virtual Connections . . . . . . . . . . . . . . . 12
8.3 Switched Virtual Connections . . . . . . . . . . . . . . . 12
8.4 ATMARP Single Server Operational Requirements . . . . . . . 13
8.5 ATMARP Client Operational Requirements . . . . . . . . . . 14
8.5.1 Client ATMARP Table Aging . . . . . . . . . . . . . . . . 16
8.5.2 Non-Normal VC Operations . . . . . . . . . . . . . . . . 17
8.5.3 Use of ATM ARP in Mobile-IP Scenarios . . . . . . . . . . 17
8.6 Address Resolution Server Selection . . . . . . . . . . . . 17
8.6.1 PVCs to ATMARP Servers . . . . . . . . . . . . . . . . . 18
8.7 ATMARP Packet Formats . . . . . . . . . . . . . . . . . . . 18
Laubach & Halpern Standards Track [Page 1]
RFC 2225 IP and ARP over ATM April 1998
8.7.1 ATMARP/InATMARP Request and Reply Packet Formats . . . . 18
8.7.2 Receiving Unknown ATMARP packets . . . . . . . . . . . . 20
8.7.3 TL, ATM Number, and ATM Subaddress Encoding . . . . . . . 20
8.7.4 ATMARP_NAK Packet Format . . . . . . . . . . . . . . . . 21
8.7.5 Variable Length Requirements for ATMARP Packets . . . . . 21
8.8 ATMARP/InATMARP Packet Encapsulation . . . . . . . . . . . 22
9. IP BROADCAST ADDRESS . . . . . . . . . . . . . . . . . . . . 23
10. IP MULTICAST ADDRESS . . . . . . . . . . . . . . . . . . . . 23
11. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 23
12. MIB SPECIFICATION . . . . . . . . . . . . . . . . . . . . . 24
13. OPEN ISSUES . . . . . . . . . . . . . . . . . . . . . . . . 24
14. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . 24
15. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . 26
APPENDIX A - Update Information . . . . . . . . . . . . . . . . 27
FULL COPYRIGHT STATEMENT . . . . . . . . . . . . . . . . . . . . 28
1.
This memo defines an initial application of classical IP and ARP
an Asynchronous Transfer Mode (ATM) network environment configured
a Logical IP Subnetwork (LIS) as described in Section 5. This
does not preclude the subsequent development of ATM technology
areas other than a LIS; specifically, as single ATM networks grow
replace many Ethernet local LAN segments and as these networks
globally connected, the application of IP and ARP will be
differently. This memo considers only the application of ATM as
direct replacement for the "wires" and local LAN segments
IP end-stations ("members") and routers operating in the "classical
LAN-based paradigm. Issues raised by MAC level bridging and
emulation are beyond the scope of this paper
This memo introduces general ATM technology and nomenclature
Readers are encouraged to review the ATM Forum and ITU-TS (
CCITT) references for more detailed information about
implementation agreements and standards
2.
The authors would like to thank the efforts of the IP over
Working Group of the IETF. Without their substantial, and
contentious support, of the Classical IP over ATM model, this
memo would not have been possible. Section 7, on Default MTU,
been incorporated directly from Ran Atkinson's RFC 1626, with
permission. Thanks to Andy Malis for an early review and
for rolc and ion related issues
Laubach & Halpern Standards Track [Page 2]
RFC 2225 IP and ARP over ATM April 1998
3.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
document are to be interpreted as described in RFC 2119 [20].
4.
The goal of this specification is to allow compatible
interoperable implementations for transmitting IP datagrams and
Address Resolution Protocol (ATMARP) requests and replies over
Adaptation Layer 5 (AAL5)[2,6].
This memo specifies the stable foundation baseline operational
which will always be available in IP and ARP over
implementations. Subsequent memos will build upon and refine
model. However, in the absence or failure of those extensions
operations will default to the specifications contained in this memo
Consequently, this memo will not reference these other extensions
This memo defines only the operation of IP and address
over ATM, and is not meant to describe the operation of ATM networks
Any reference to virtual connections, permanent virtual connections
or switched virtual connections applies only to virtual
connections used to support IP and address resolution over ATM,
thus are assumed to be using AAL5. This memo places no
or requirements on virtual connections used for other purposes
Initial deployment of ATM provides a LAN segment replacement for
1) Local area networks (e.g., Ethernets, Token Rings and FDDI).
2) Local-area backbones between existing (non-ATM) LANs
3) Dedicated circuits or frame relay PVCs between IP routers
NOTE: In 1), local IP routers with one or more ATM interfaces will
able to connect islands of ATM networks. In 3), public or
ATM Wide Area networks will be used to connect IP routers, which
turn may or may not connect to local ATM networks. ATM WANs and
may be interconnected
Private ATM networks (local or wide area) will use the private
address structure specified in the ATM Forum UNI 3.1
[9] or as in the ATM Forum UNI 4.0 specification [19].
structure is modeled after the format of an OSI Network
Access Point Address (NSAPA). A private ATM address
identifies an ATM endpoint
Laubach & Halpern Standards Track [Page 3]
RFC 2225 IP and ARP over ATM April 1998
Public networks will use either the address structure specified
ITU-TS recommendation E.164 or the private network ATM
structure. An E.164 address uniquely identifies an interface to
public network
The characteristics and features of ATM networks are different
those found in LANs
o ATM provides a Virtual Connection (VC) switched environment.
setup may be done on either a Permanent Virtual Connection (PVC
or dynamic Switched Virtual Connection (SVC) basis. SVC
management signalling is performed via implementations of the
3.1 protocol [7,9].
o Data to be passed by a VC is segmented into 53 octet
called cells (5 octets of ATM header and 48 octets of data).
o The function of mapping user Protocol Data Units (PDUs) into
information field of the ATM cell and vice versa is performed
the ATM Adaptation Layer (AAL). When a VC is created a
AAL type is associated with the VC. There are four different
types, which are referred to individually as "AAL1", "AAL2",
"AAL3/4", and "AAL5". (NOTE: this memo concerns itself with
mapping of IP and ATMARP over AAL5 only. The other AAL types
mentioned for introductory purposes only.) The AAL type is
by the VC end points via the call setup mechanism and is
carried in the ATM cell header. For PVCs the AAL type
administratively configured at the end points when the
(circuit) is set up. For SVCs, the AAL type is
along the VC path via UNI 3.1 as part of call setup
and the end points use the signaled information
configuration. ATM switches generally do not care about the
type of VCs. The AAL5 format specifies a packet format with
maximum size of (64K - 1) octets of user data. Cells for an AAL
PDU are transmitted first to last, the last cell indicating
end of the PDU. ATM standards guarantee that on a given VC,
ordering is preserved end-to-end. NOTE: AAL5 provides a non
assured data transfer service - it is up to higher-
protocols to provide retransmission
o ATM Forum signaling defines point-to-point and point-to
point Connection setup [9, 19.] Multipoint-to-multipoint not
specified by ITU-TS or ATM Forum
An ATM Forum ATM address is either encoded as an NSAP form
EndSystem Address (AESA) or is an E.164 Public-UNI address [9,
19]. In some cases, both an AESA and an E.164 Public UNI
are needed by an ATMARP client to reach another host or router
Laubach & Halpern Standards Track [Page 4]
RFC 2225 IP and ARP over ATM April 1998
Since the use of AESAs and E.164 public UNI addresses by
are analogous to the use of Ethernet addresses, the notion
"hardware address" is extended to encompass ATM addresses in
context of ATMARP, even though ATM addresses need not
hardware significance. ATM Forum NSAP format addresses (AESA
use the same basic format as U.S. GOSIP OSI NSAPAs [11]. NOTE
ATM Forum addresses should not be construed as being U.S.
NSAPAs. They are not, the administration is different,
fields get filled out are different, etc. However, in
document, these will be referred to as NSAPAs
This memo describes the initial deployment of ATM within "classical
IP networks as a direct replacement for local area
(Ethernets) and for IP links which interconnect routers,
within or between administrative domains. The "classical" model
refers to the treatment of the ATM host adapter as a
interface to the IP protocol stack operating in a LAN-based paradigm
Characteristics of the classical model are
o The same maximum transmission unit (MTU) size is the default
all VCs in a LIS. However, on a VC-by-VC point-to-point basis
the MTU size may be negotiated during connection setup using
MTU Discovery to better suit the needs of the cooperating pair
IP members or the attributes of the communications path. (
to Section 7.3)
o Default LLC/SNAP encapsulation of IP packets
o End-to-end IP routing architecture stays the same
o IP addresses are resolved to ATM addresses by use of an
service within the LIS - ATMARPs stay within the LIS. From
client's perspective, the ATMARP architecture stays faithful
the basic ARP model presented in [3].
o One IP subnet is used for many hosts and routers. Each
directly connects two IP members within the same LIS
Future memos will describe the operation of IP over ATM when
networks become globally deployed and interconnected
The deployment of ATM into the Internet community is just
and will take many years to complete. During the early part of
period, we expect deployment to follow traditional IP
boundaries for the following reasons
Laubach & Halpern Standards Track [Page 5]
RFC 2225 IP and ARP over ATM April 1998
o Administrators and managers of IP subnetworks will tend
initially follow the same models as they currently have deployed
The mindset of the community will change slowly over time as
increases its coverage and builds its credibility
o Policy administration practices rely on the security, access
routing, and filtering capability of IP Internet gateways: i.e.,
firewalls. ATM will not be allowed to "back-door" around
mechanisms until ATM provides better management capability
the existing services and practices
o Standards for global IP over ATM will take some time to
and deploy
This memo details the treatment of the classical model of IP
ATMARP over ATM. This memo does not preclude the
treatment of ATM networks within the IP framework as ATM
globally deployed and interconnected; this will be the subject
future documents. This memo does not address issues related
transparent data link layer interoperability
5. IP SUBNETWORK
5.1
In the LIS scenario, each separate administrative entity
its hosts and routers within a LIS. Each LIS operates
communicates independently of other LISs on the same ATM network
In the classical model, hosts communicate directly via ATM to
hosts within the same LIS using the ATMARP service as the
for resolving target IP addresses to target ATM endpoint addresses
The ATMARP service has LIS scope only and serves all hosts in
LIS. Communication to hosts located outside of the local LIS
provided via an IP router. This router is an ATM endpoint
to the ATM network that is configured as a member of one or
LISs. This configuration MAY result in a number of disjoint
operating over the same ATM network. Using this model hosts
differing IP subnets MUST communicate via an intermediate IP
even though it may be possible to open a direct VC between the two
members over the ATM network
By default, the ATMARP service and the classical LIS routing
MUST be available to any IP member client in the LIS
Laubach & Halpern Standards Track [Page 6]
RFC 2225 IP and ARP over ATM April 1998
5.2 LIS Configuration
The requirements for IP members (hosts, routers) operating in an
LIS configuration are
o All members of the LIS have the same IP network/subnet number
address mask [8].
o All members within a LIS are directly connected to the
network
o All members of a LIS MUST have a mechanism for resolving
addresses to ATM addresses via ATMARP (based on [3]) and
versa via InATMARP (based on [12]) when using SVCs. Refer
Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo
o All members of a LIS MUST have a mechanism for resolving VCs
IP addresses via InATMARP (based on [12]) when using PVCs.
to Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo
o All members within a LIS MUST be able to communicate via ATM
all other members in the same LIS; i.e., the Virtual
topology underlying the intercommunication among the members
fully meshed
The following list identifies the set of ATM specific parameters
MUST be implemented in each IP station connected to the ATM network
o ATM Hardware Address (atm$ha). The ATM address of the
IP station
o ATMARP Request Address list (atm$arp-req-list): atm$arp-req-
is a list containing one or more ATM addresses of
ATMARP servers located within the LIS. In an SVC environment
ATMARP servers are used to resolve target IP addresses to
ATM address via an ATMARP request and reply protocol.
servers MUST have authoritative responsibility for
ATMARP requests of all IP members using SVCs located within
LIS
A LIS MUST have a single ATMARP service entry configured
available to all members of the LIS who use SVCs
In the case where there is only a single ATMARP server within
LIS, then all ATMARP clients MUST be configured identically to
only one non-null entry in atm$arp-req-list configured with the
address of the single ATMARP service
Laubach & Halpern Standards Track [Page 7]
RFC 2225 IP and ARP over ATM April 1998
If the IP member is operating with PVCs only, then atm$arp-req-
MUST be configured with all null entries and the client MUST not
queries to either address resolution service
Within the restrictions mentioned above and in Section 8,
administration MUST decide which server address(es) are
for atm$arp-req-list
By default, atm$arp-req-list MUST be configured using the MIB [18].
Manual configuration of the addresses and address lists presented
this section is implementation dependent and beyond the scope of
document; i.e., this memo does not require any specific
method. This memo does require that these addresses MUST
configured completely on the client, as appropriate for the LIS
prior to use by any service or operation detailed in this memo
5.3 LIS Router Additional
It is RECOMMENDED that routers providing LIS functionality over
ATM network also support the ability to interconnect multiple LISs
Routers that wish to provide interconnection of differing LISs
be able to support multiple sets of these parameters (one set
each connected LIS) and be able to associate each set of
to a specific IP network/ subnet number. In addition, it
RECOMMENDED that a router be able to provide this multiple
support with a single physical ATM interface that may have one
more individual ATM endpoint addresses. NOTE: this does
necessarily mean different End System Identifiers (ESIs) when
are used. The last octet of an NSAPA is the NSAPA Selector (SEL
field which can be used to differentiate up to 256 different LISs
the same ESI. (Refer to Section 5.1.3.1, "Private Networks" in [9].)
6. IP PACKET
Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation
described in [2]. LLC/SNAP encapsulation is the default
format for IP datagrams
This memo recognizes that other encapsulation methods may be
however, in the absence of other knowledge or agreement, LLC/
encapsulation is the default
This memo recognizes that end-to-end signaling within ATM may
negotiation of encapsulation method on a per-VC basis
Laubach & Halpern Standards Track [Page 8]
RFC 2225 IP and ARP over ATM April 1998
7. DEFAULT VALUE FOR IP MTU OVER ATM AAL
Protocols in wide use throughout the Internet, such as the
File System (NFS), currently use large frame sizes (e.g., 8 KB).
Empirical evidence with various applications over the
Control Protocol (TCP) indicates that larger Maximum
Unit (MTU) sizes for the Internet Protocol (IP) tend to give
performance. Fragmentation of IP datagrams is known to be
undesirable [16]. It is desirable to reduce fragmentation in
network and thereby enhance performance by having the IP
Transmission Unit (MTU) for AAL5 be reasonably large. NFS
to an 8192 byte frame size. Allowing for RPC/XDR, UDP, IP, and
headers, NFS would prefer a default MTU of at least 8300 octets
Routers can sometimes perform better with larger packet sizes
most of the performance costs in routers relate to "packets handled
rather than "bytes transferred". So, there are a number of
reasons to have a reasonably large default MTU value for IP over
AAL5.
RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which
larger than 8300 octets but still in the same range [1]. There is
good reason for the default MTU of IP over ATM AAL5 to be
from IP over SMDS, given that they will be the same magnitude
Having the two be the same size will be helpful in
and will also help reduce incidence of IP fragmentation
Therefore, the default IP MTU for use with ATM AAL5 shall be 9180
octets. All implementations compliant and conformant with
specification shall support at least the default IP MTU value for
over ATM AAL5.
7.1 Permanent Virtual
Implementations which only support Permanent Virtual Circuits (PVCs
will (by definition) not implement any ATM signalling protocol.
implementations shall use the default IP MTU value of 9180
unless both parties have agreed in advance to use some other IP
value via some mechanism not specified here
7.2 Switched Virtual
Implementations that support Switched Virtual Circuits (SVCs)
attempt to negotiate the AAL CPCS-SDU size using the ATM
protocol. The industry standard ATM signalling protocol uses
different parts of the Information Element named "AAL Parameters"
exchange information on the MTU over the ATM circuit being setup [9].
The Forward Maximum CPCS-SDU Size field contains the value over
path from the calling party to the called party. The
Laubach & Halpern Standards Track [Page 9]
RFC 2225 IP and ARP over ATM April 1998
Maximum CPCS-SDU Size Identifier field contains the value over
path from the called party to the calling party. The ATM
specifies the valid values of this identifier as 1 to 65535
inclusive. Note that the ATM Forum's User-to-Network-Interface (UNI
signalling permits the MTU in one direction to be different from
MTU in the opposite direction, so the Forward Maximum CPCS-SDU
Identifier might have a different value from the Backwards
CPCS-SDU Size Identifier on the same connection
If the calling party wishes to use the default MTU it shall
include the "AAL Parameters" information element with the
values for the Maximum CPCS-SDU Size as part of the SETUP message
the ATM signalling protocol [9]. If the calling party desires to
a different value than the default, it shall include the "
Parameters" information element with the desired value for
Maximum CPCS-SDU Size as part of the SETUP message of the
Signalling Protocol. The called party will respond using the
information elements and identifiers in its CONNECT message
[9].
If the called party receives a SETUP message containing the "
CPCS-SDU Size" in the AAL Parameters information element, it
handle the Forward and Backward Maximum CPCS-SDU Size Identifier
follows
a) If it is able to accept the ATM MTU values proposed by the
message, it shall include an AAL Parameters information
in its response. The Forward and Backwards Maximum CPCS-SDU
fields shall be present and their values shall be equal to
corresponding values in the SETUP message
b) If it wishes a smaller ATM MTU size than that proposed, then
shall set the values of the Maximum CPCS-SDU Size in the
Parameters information elements equal to the desired value in
CONNECT message responding to the original SETUP message
c) If the calling endpoint receives a CONNECT message that does
contain the AAL Parameters Information Element, but
corresponding SETUP message did contain the AAL
Information element (including the forward and backward CPCS-
Size fields), it shall clear the call with cause "AAL
cannot be supported".
d) If either endpoint receives a STATUS message with
"Information Element Non-existent or Not Implemented" or
"Access Information Discarded", and with a diagnostic
Laubach & Halpern Standards Track [Page 10]
RFC 2225 IP and ARP over ATM April 1998
indicating the AAL Parameters Information Element identifier,
shall clear the call with cause "AAL Parameters cannot
supported."
e) If either endpoint receives CPCS-SDUs in excess of the
MTU size, it may use IP fragmentation or may clear the call
cause "AAL Parameters cannot be supported". In this case,
error has occurred either due to a fault in an end system or
the ATM network. The error should be noted by ATM
management for human examination and intervention
If the called endpoint incorrectly includes the Forward and
Maximum CPCS-SDU Size fields in the CONNECT messages (e.g.,
the original SETUP message did not include these fields) or it
these fields to an invalid value, then the calling party shall
the call with cause "Invalid Information Element Contents".
7.3 Path MTU Discovery
The Path MTU Discovery mechanism is Internet Standard RFC 1191 [17]
and is an important mechanism for reducing IP fragmentation in
Internet. This mechanism is particularly important because
subnet ATM uses a default MTU sizes significantly different
older subnet technologies such as Ethernet and FDDI
In order to ensure good performance throughout the Internet and
to permit IP to take full advantage of the potentially larger
datagram sizes supported by ATM, all router implementations
comply or conform with this specification must also implement the
Path MTU Discovery mechanism as defined in RFC 1191 and clarified
RFC 1435 [14]. Host implementations should implement the IP Path
Discovery mechanism as defined in RFC 1191.
8. LIS ADDRESS RESOLUTION
8.1 ATM-based ARP and InARP Equivalent
Address resolution within an ATM LIS SHALL make use of the
Address Resolution Protocol (ATMARP) (based on [3]) and the
ATM Address Resolution Protocol (InATMARP) (based on [12]) and
defined in this memo. ATMARP is the same protocol as the
protocol presented in [3] with extensions needed to support
resolution in a unicast server ATM environment. InATMARP is the
protocol as the original InARP protocol presented in [12] but
to ATM networks. All IP stations MUST support these protocols
updated and extended in this memo. Use of these protocols
depending on whether PVCs or SVCs are used
Laubach & Halpern Standards Track [Page 11]
RFC 2225 IP and ARP over ATM April 1998
8.2 Permanent Virtual
An IP station MUST have a mechanism (e.g., manual configuration)
determining what PVCs it has, and in particular which PVCs are
used with LLC/SNAP encapsulation. The details of the mechanism
beyond the scope of this memo
All IP members supporting PVCs are required to use the Inverse
Address Resolution Protocol (InATMARP) (refer to [12]) on those
using LLC/SNAP encapsulation. In a strict PVC environment,
receiver SHALL infer the relevant VC from the VC on which
InATMARP_Request or response InATMARP_Reply was received. When
ATM source and/or target address is unknown, the corresponding
address length in the InATMARP packet MUST be set to zero (0)
indicating a null length, and no storage be allocated in the
packet, otherwise the appropriate address field should be filled
and the corresponding length set appropriately. InATMARP
format details are presented later in this memo
Directly from [12]: "When the requesting station receives
In[ATM]ARP_Reply, it may complete the [ATM]ARP table entry and
the provided address information. NOTE: as with [ATM]ARP
information learned via In[ATM]ARP may be aged or invalidated
certain circumstances." IP stations supporting PVCs MUST re-
ATMARP table entries as part of the table aging process. See
Section 8.5.1 "Client ATMARP Table Aging".
If a client has more than one IP address within the LIS and if
PVCs, when an InATMARP_Request is received an InATMARP_Reply MUST
generated for each such address
8.3 Switched Virtual
SVCs require support from address resolution services for
target IP addresses to target ATM endpoint addresses. All members
the LIS MUST use the same service. This service MUST
authoritative responsibility for resolving the ATMARP requests of
IP members within the LIS
ATMARP servers do not actively establish connections. They depend
the clients in the LIS to initiate connections for the
registration procedure and for transmitting ATMARP requests.
individual client connects to the ATMARP server using a point-to
point LLC/SNAP VC. The client sends normal ATMARP request packets
the server. The ATMARP server examines each ATMARP_Request
Laubach & Halpern Standards Track [Page 12]
RFC 2225 IP and ARP over ATM April 1998
the source protocol and source hardware address information of
sending client and uses this information to build its ATMARP
cache. This information is used to generate replies to any
requests it receives
InATMARP_Request packets MUST specify valid address information
ATM source number, ATM target number, and source protocol address
i.e., these fields MUST be non-null in InATMARP_Request packets
This memo defines the address resolution service in the LIS
constrains it to consist of a single ATMARP server. Client-
interaction is defined by using a single server approach as
reference model
This memo recognizes the future development of standards
implementations of multiple-ATMARP-server models that will extend
operations as defined in this memo to provide a highly
address resolution service
8.4 ATMARP Single Server Operational
A single ATMARP server accepts ATM calls/connections from other
end points. After receiving any ATMARP_Request, the server
examine the source and target address information in the packet
make note of the VC on which the ATMARP_Request arrived. It will
this information as necessary to build and update its ATMARP
entries
For each ATMARP_Request, then
1. If the source IP protocol address is the same as the target
protocol address and a table entry exists for that IP address
if the source ATM hardware address does not match the table
ATM address and there is an open VC associated with that
entry that is not the same as the VC associated with
ATMARP_Request, the server MUST return the table
information in the ATMARP_Reply, and MUST raise a "duplicate
address detected" condition to the server's management.
table entry is not updated
2. Otherwise, if the source IP protocol address is the same as
target IP protocol address, and either there is no table
for that IP address, or a table entry exists for that IP
and there is no open VC associated with that table entry, or
the VC associated with that entry is the same as the VC for
ATMARP_Request, the server MUST either create a new entry
update the old entry as appropriate and return that table
information in the ATMARP Reply
Laubach & Halpern Standards Track [Page 13]
RFC 2225 IP and ARP over ATM April 1998
3. Otherwise, when the source IP protocol address does not match
target IP protocol address, the ATMARP server will generate
corresponding ATMARP_Reply if it has an entry for the
information in its ATMARP table. Otherwise, it will generate
negative ATMARP reply (ATMARP_NAK).
4. Additionally, when the source IP protocol address does not
the target IP protocol address and when the server receives
ATMARP_Request over a VC, where the source IP and ATM address
not have a corresponding table entry, the ATMARP server
create a new table entry for the source information
Explanation: this allows old RFC 1577 clients to register
this ATMARP service by just issuing requests to it
5. Additionally, when the source IP protocol address does not
the target IP protocol address and where the source IP and
addresses match the association already in the ATMARP table
the ATM address matches that associated with the VC, the
MUST update the table timeout on the source ATMARP table
but only if it has been more than 10 minutes since the
update. Explanation: if the client is sending ATMARP requests
the server over the same VC that it used to register its
entry, the server should examine the ATMARP request and note
the client is still "alive" by updating the timeout on
client's ATMARP table entry
6. Additionally, when the source IP protocol address does not
the target IP protocol address and where the source IP and
addresses do not match the association already in the
table, the server MUST NOT update the ATMARP table entry
An ATMARP server MUST have knowledge of any open VCs it has and
association with an ATMARP table entry, and in particular, which
support LLC/SNAP encapsulation. In normal operation, active
clients will revalidate their entries prior to the server
process taking effect
Server ATMARP table entries are valid for 20 minutes. If an
ages beyond 20 minutes without being updated (refreshed) by
client, that entry is deleted from the table regardless of the
of any VCs that may be associated with that entry
8.5 ATMARP Client Operational
The ATMARP client is responsible for contacting the ATMARP service
both initially register and subsequently refresh its own
information
Laubach & Halpern Standards Track [Page 14]
RFC 2225 IP and ARP over ATM April 1998
The client is also responsible for using the ATMARP service to
and revalidate ATMARP information about other IP members in the
(server selection overview is discussed in Section 8.6). As noted
Section 5.2, ATMARP clients MUST be configured with the ATM
of the appropriate server prior to client ATMARP operation
IP clients MUST register their ATM endpoint address with their
server using the ATM address structure appropriate for their
network connection: i.e., LISs implemented over ATM LANs
ATM Forum UNI 3.1 should register using Structure 1; LISs
over an E.164 "public" ATM network should register using Structure 2.
A LIS implemented over a combination of ATM LANs and public
networks may need to register using Structure 3.
based on this memo MUST support all three ATM address structures
See Section 8.7.1 for more details regarding the ATMARP
packet format
To handle the case when a client has more than one IP address
a LIS, when using an ATMARP server, the client MUST register
such address
For initial registration and subsequent refreshing of its
information with the ATMARP service, clients MUST
1. Establish an LLC/SNAP VC connection to a server in the
service for the purposes of transmitting and receiving
packets
NOTE: in the case of refreshing its own information with
ATMARP service, a client MAY reuse an existing
connection to the ATMARP service provided that the connection
previously used either to initially register its information
the ATMARP service or to refresh its information with the
service
2. After establishing a successful connection to the ATMARP service
the client MUST transmit an ATMARP_Request packet, requesting
target ATM address for its own IP address as the target
protocol address. The client checks the ATMARP_Reply and if
source hardware and protocol addresses match the
target hardware and protocol addresses, the client is
with the ATMARP service. If the addresses do not match,
client MAY take action, raise alarms, etc.; however,
actions are beyond the scope of this memo. In the case of
client having more than one IP address in the list, this
MUST be repeated for each IP address
Laubach & Halpern Standards Track [Page 15]
RFC 2225 IP and ARP over ATM April 1998
3. Clients MUST respond to ATMARP_Request and InATMARP_
packets received on any VC appropriately. (Refer to Section 7,
"Protocol Operation" in RFC 1293 [12].)
NOTE: for reasons of robustness, clients MUST respond
ATMARP_Requests
4. Generate and transmit address resolution request packets to
address resolution service. Respond to address resolution
packets appropriately to build/refresh its own client
table entries
5. Generate and transmit InATMARP_Request packets as needed
process InATMARP_Reply packets appropriately. InATMARP_
packets should be used to build/refresh its own client
table entries. (Refer to Section 7, "Protocol Operation"
[12].) If a client has more than one IP address within the
when an InATMARP_Request is received an InATMARP_Reply MUST
generated for each such address
The client MUST refresh its ATMARP information with the server
least once every 15 minutes. This is done by repeating steps 1
2.
An ATMARP client MUST have knowledge of any open VCs it
(permanent or switched), their association with an ATMARP
entry, and in particular, which VCs support LLC/SNAP encapsulation
8.5.1 Client ATMARP Table
Client ATMARP table entries are valid for a maximum time of 15
minutes
When an ATMARP table entry ages, an ATMARP client MUST invalidate
table entry. If there is no open VC server associated with
invalidated entry, that entry is deleted. In the case of
invalidated entry and an open VC, the client MUST revalidate
entry prior to transmitting any non address resolution traffic
that VC; this requirement applies to both PVCs and SVCs. NOTE:
client is permitted to revalidate an ATMARP table entry before
ages, thus restarting the aging time when the table entry
successfully revalidated. The client MAY continue to use the
VC, as long as the table entry has not aged, while revalidation is
progress
In the case of an open PVC, the client revalidates the entry
transmitting an InATMARP_Request and updating the entry on receipt
an InATMARP_Reply
Laubach & Halpern Standards Track [Page 16]
RFC 2225 IP and ARP over ATM April 1998
In the case of an open SVC, the client revalidates the entry
querying the address resolution service. If a valid reply
received (e.g., ATMARP_Reply), the entry is updated. If the
resolution service cannot resolve the entry (i.e., "host not found"),
the SVC should be closed and the associated table entry removed.
the address resolution service is not available (i.e., "
failure") and if the SVC is LLC/SNAP encapsulated, the client
attempt to revalidate the entry by transmitting an InATMARP_
on that VC and updating the entry on receipt of an InATMARP_Reply
If the InATMARP_Request attempt fails to return an InATMARP_Reply
the SVC should be closed and the associated table entry removed
If a VC with an associated invalidated ATMARP table entry is closed
that table entry is removed
8.5.2 Non-Normal VC
The specific details on client procedures for detecting non-normal
connection establishment or closures, or failed communications on
established VC are beyond the scope of this memo. It is
however, that the client MUST remove the associated ATMARP entry
a VC that fails to operate properly, as defined by the client,
the client closes that VC, when it releases its resources for a VC
or prior to any attempt to reopen that VC. This
specifically REQUIRES that the client MUST refresh its ATMARP
information prior to any attempt to re-establish communication to
IP member after a non-normal communications problem has
occurred on a VC to that IP member
8.5.3 Use of ATMARP In Mobile-IP
When an ATM LIS is used as the home network in a mobile-IP scenario
it is RECOMMENDED that the home agent NOT maintain long
connections with the ATMARP service. The absence of this VC
permit a mobile node's registration, upon its return to the
network, to immediately preempt the home agent's previous
registration
8.6 Address Resolution Server
If the client supports PVCs only, the ATMARP server list is empty
the client MUST not generate any address resolution requests
than the InATMARP requests on a PVC needed to validate that PVC
If the client supports SVCs, then the client MUST have a non-
atm$arp-req-list pointing to the ATMARP server(s) which
ATMARP service for the LIS
Laubach & Halpern Standards Track [Page 17]
RFC 2225 IP and ARP over ATM April 1998
The client MUST register with a server from atm$arp-req-list
The client SHALL attempt to communicate with any of the servers
a successful registration is accomplished. The order in which
selects servers to attempt registration, is a local matter, as
the number of retries and timeouts for such attempts
8.6.1 PVCs to ATMARP
In a mixed PVC and SVC LIS environment, an ATMARP client MAY have
PVC to an ATMARP server. In this case, this PVC is used for
requests and responses as if it were an established SVC. NOTE:
this PVC is to be used for IP traffic, then the ATMARP server MUST
prepared to accept and respond appropriately to InATMARP traffic
8.7 ATMARP Packet
Internet addresses are assigned independently of ATM addresses.
host implementation MUST know its own IP and ATM address(es) and
respond to address resolution requests appropriately. IP
MUST also use ATMARP and InATMARP to resolve IP addresses to
addresses when needed
NOTE: the ATMARP packet format presented in this memo is general
nature in that the ATM number and ATM subaddress fields SHOULD
directly to the corresponding UNI 3.1 fields used for
call/connection setup signalling messages. The IP over ATM
Group expects ATM Forum NSAPA numbers (Structure 1) to
over E.164 numbers (Structure 2) as ATM endpoint identifiers
ATM LANs. The ATM Forum's VC Routing specification is not
at this time and therefore its impact on the operational use of
Address Structure 3 is undefined. The ATM Forum will be
this relationship in the future. It is for this reason that
members need to support all three ATM address structures
8.7.1 ATMARP/InATMARP Request and Reply Packet
The ATMARP and InATMARP request and reply protocols use the
hardware type (ar$hrd), protocol type (ar$pro), and operation
(ar$op) data formats as the ARP and InARP protocols [3,12].
location of these three fields within the ATMARP packet are in
same byte position as those in ARP and InARP packets. A
hardware type value has been assigned for ATMARP. In addition
ATMARP makes use of an additional operation code for ARP_NAK.
remainder of the ATMARP/InATMARP packet format is different than
ARP/InARP packet format
Laubach & Halpern Standards Track [Page 18]
RFC 2225 IP and ARP over ATM April 1998
The ATMARP and InATMARP protocols have several fields that have
following format and values
Data
ar$hrd 16 bits Hardware
ar$pro 16 bits Protocol
ar$shtl 8 bits Type & length (TL) of source ATM number (q
ar$sstl 8 bits Type & length (TL) of source ATM subaddress (r
ar$op 16 bits Operation code (request, reply, or NAK
ar$spln 8 bits Length of source protocol address (s
ar$thtl 8 bits Type & length (TL) of target ATM number (x
ar$tstl 8 bits Type & length (TL) of target ATM subaddress (y
ar$tpln 8 bits Length of target protocol address (z
ar$sha qoctets of source ATM
ar$ssa roctets of source ATM
ar$spa soctets of source protocol
ar$tha xoctets of target ATM
ar$tsa yoctets of target ATM
ar$tpa zoctets of target protocol
Where
ar$hrd - assigned to ATM Forum address family and
19 decimal (0x0013) [4].
ar$pro - see Assigned Numbers for protocol type number
the protocol using ATMARP. (IP is 0x0800).
ar$shtl - Type and length of source ATM number.
Section 8.7.4 for TL encoding details
ar$sstl - Type and length of source ATM subaddress.
Section 8.7.4 for TL encoding details
ar$op - The operation type value (decimal):
ATMARP_Request = ARP_REQUEST = 1
ATMARP_Reply = ARP_REPLY = 2
InATMARP_Request = InARP_REQUEST = 8
InATMARP_Reply = InARP_REPLY = 9
ATMARP_NAK = ARP_NAK = 10
ar$spln - length in octets of the source protocol address.
range is 0 or 4 (decimal). For IPv4 ar$spln is 4.
ar$thtl - Type and length of target ATM number.
Section 8.7.4 for TL encoding details
Laubach & Halpern Standards Track [Page 19]
RFC 2225 IP and ARP over ATM April 1998
ar$tstl - Type and length of target ATM subaddress.
Section 8.7.4 for TL encoding details
ar$tpln - length in octets of the target protocol address.
range is 0 or 4 (decimal). For IPv4 ar$tpln is 4.
ar$sha - source ATM number (E.164 or ATM Forum NSAPA
ar$ssa - source ATM subaddress (ATM Forum NSAPA
ar$spa - source protocol
ar$tha - target ATM number (E.164 or ATM Forum NSAPA
ar$tsa - target ATM subaddress (ATM Forum NSAPA
ar$tpa - target protocol
8.7.2 Receiving Unknown ATMARP
If an ATMARP client receives an ATMARP message with an operation
(ar$op) for which it is not coded to support, it MUST
discard the message and continue normal operation. An ATMARP
is NOT REQUIRED to return any message to the sender of
unsupported message
8.7.3 TL, ATM Number, and ATM Subaddress
The encoding of the 8-bit TL (type and length) fields in ATMARP
In_ATMARP packets is as follows
MSB 8 7 6 5 4 3 2 1
+-----+-----+-----+-----+-----+-----+-----+-----+
| 0 | 1/0 | Octet length of address |
+-----+-----+-----+-----+-----+-----+-----+-----+
Where
bit.8 (reserved) = 0 (for future use
bit.7 (type) = 0 ATM Forum NSAPA
= 1 E.164
bit.6-1 (length) = 6 bit unsigned octet length of
(MSB = bit.6, LSB = bit.1)
range is from 0 to 20 (decimal).
Laubach & Halpern Standards Track [Page 20]
RFC 2225 IP and ARP over ATM April 1998
ATM addresses, as defined by the ATM Forum UNI 3.1
specification [9], include a "Calling Party Number
Element" and a "Calling Party Subaddress Information Element".
Information Elements (IEs) SHOULD map to ATMARP/InATMARP source
number and source ATM subaddress respectively. Furthermore,
Forum defines a "Called Party Number Information Element" and
"Called Party Subaddress Information Element". These IEs map
ATMARP/InATMARP target ATM number and target ATM subaddress
respectively
The ATM Forum defines three structures for the combined use of
and subaddress [9]:
ATM Number ATM
-------------- --------------
Structure 1 ATM Forum NSAPA
Structure 2 E.164
Structure 3 E.164 ATM Forum
ATMARP and InATMARP requests and replies for ATM address structures 1
and 2 MUST indicate a null or unknown ATM subaddress by setting
appropriate subaddress length to zero; i.e., ar$sstl.length = 0
ar$tstl.length = 0, the corresponding type field (ar$sstl.type
ar$tstl.type) MUST be ignored and the physical space for the
subaddress buffer MUST not be allocated in the ATMARP packet.
example, if ar$sstl.length=0, the storage for the source
subaddress is not allocated and the first byte of the source
address ar$spa follows immediately after the last byte of the
hardware address ar$sha in the packet
Null or unknown ATM addresses MUST be indicated by setting
appropriate address length to zero; i.e., ar$shtl.length
ar$thtl.length is zero and the corresponding type field (ar$sstl.
or ar$tstl.type) MUST be ignored and the physical space for the
address or ATM subaddress buffer MUST not be allocated in the
packet
8.7.4 ATMARP_NAK Packet
The ATMARP_NAK packet format is the same as the
ATMARP_Request packet format with the operation code set to ARP_NAK
i.e., the ATMARP_Request packet data is exactly copied (e.g.,
bcopy) for transmission with the ATMARP_Request operation
changed to ARP_NAK value
8.7.5 Variable Length Requirements for ATMARP
ATMARP and InATMARP packets are variable in length
Laubach & Halpern Standards Track [Page 21]
RFC 2225 IP and ARP over ATM April 1998
A null or unknown source or target protocol address is indicated
the corresponding length set to zero: e.g., when ar$spln or ar$
is zero the physical space for the corresponding address
MUST not be allocated in the packet
For backward compatibility with previous implementations, a null IPv
protocol address may be received with length = 4 and an
address in storage set to the value 0.0.0.0. Receiving stations
be liberal in accepting this format of a null IPv4 address. However
on transmitting an ATMARP or InATMARP packet, a null IPv4
MUST only be indicated by the length set to zero and MUST have
storage allocated
8.8 ATMARP/InATMARP Packet
ATMARP and InATMARP packets are to be encoded in AAL5 PDUs
LLC/SNAP encapsulation. The format of the AAL5 CPCS-SDU
field for ATMARP/InATMARP PDUs is
Payload Format for ATMARP/InATMARP PDUs
+------------------------------+
| LLC 0xAA-AA-03 |
+------------------------------+
| OUI 0x00-00-00 |
+------------------------------+
| EtherType 0x08-06 |
+------------------------------+
| |
| ATMARP/InATMARP Packet |
| |
+------------------------------+
The LLC value of 0xAA-AA-03 (3 octets) indicates the presence of
SNAP header
The OUI value of 0x00-00-00 (3 octets) indicates that the
two-bytes is an EtherType
The EtherType value of 0x08-06 (2 octets) indicates ARP [4].
The total size of the LLC/SNAP header is fixed at 8-octets.
aligns the start of the ATMARP packet on a 64-bit boundary
to the start of the AAL5 CPCS-SDU
The LLC/SNAP encapsulation for ATMARP/InATMARP presented here
consistent with the treatment of multiprotocol encapsulation of
over ATM AAL5 as specified in [2] and in the format of ATMARP
IEEE 802 networks as specified in [5].
Laubach & Halpern Standards Track [Page 22]
RFC 2225 IP and ARP over ATM April 1998
Traditionally, address resolution requests are broadcast to
directly connected IP members within a LIS. It is conceivable in
future that larger scaled ATM networks may handle ATMARP requests
destinations outside the originating LIS, perhaps even globally
issues raised by ATMARPing outside the LIS or by a global
mechanism are beyond the scope of this memo
9. IP BROADCAST
ATM does not support broadcast addressing, therefore there are
mappings available from IP broadcast addresses to ATM
services. Note: this lack of mapping does not restrict members
transmitting or receiving IP datagrams specifying any of the
standard IP broadcast address forms as described in [8]. Members
upon receiving an IP broadcast or IP subnet broadcast for their LIS
MUST process the packet as if addressed to that station
This memo recognizes the future development of standards
implementations that will extend the operations as defined in
memo to provide an IP broadcast capability for use by the
client
10. IP MULTICAST
ATM does not directly support IP multicast address services
therefore there are no mappings available from IP multicast
to ATM multicast services. Current IP multicast
(i.e., MBONE and IP tunneling, see [10]) will continue to
over ATM based logical IP subnets if operated in the
configuration
This memo recognizes the future development of ATM multicast
addressing by the ATM Forum. When available and widely implemented
the roll-over from the current IP multicast architecture to this
ATM architecture will be straightforward
This memo recognizes the future development of standards
implementations that will extend the operations as defined in
memo to provide an IP multicast capability for use by the
client
11. SECURITY
Not all of the security issues relating to IP over ATM are
understood at this time, due to the fluid state of
specifications, newness of the technology, and other factors
Laubach & Halpern Standards Track [Page 23]
RFC 2225 IP and ARP over ATM April 1998
It is believed that ATM and IP facilities for authenticated
management, authenticated end-to-end communications, and
encryption will be needed in globally connected ATM networks.
future security facilities and their use by IP networks are
the scope of this memo
There are known security issues relating to host impersonation
the address resolution protocols used in the Internet [13].
special security mechanisms have been added to the address
mechanism defined here for use with networks using IP over ATM
12. MIB
Clients built to this specification MUST implement and provide
Management Information Base (MIB) as defined in "Definitions
Managed Objects for Classical IP and ARP Over ATM Using SMIv2" [18].
13. OPEN
o Automatic configuration of client ATM addresses via DHCP [15]
via ATM UNI 3.1 Interim Local Management Interface (ILMI
services would be a useful extended service addition to
document and should be addressed in a separate memo
o ATMARP packets are not authenticated. This is a
serious flaw in the overall system by allowing a mechanism
which corrupt information may be introduced into the
system
14.
[1] Piscitello, D., and J. Lawrence, "The Transmission of
Datagrams over the SMDS Service", STD 52, RFC 1209, March 1991.
[2] Heinanen, J., "Multiprotocol Encapsulation over ATM
Layer 5", RFC 1483, July 1993.
[3] Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit
Address for Transmission on Ethernet Hardware", STD 37,
826, November 1982.
[4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
July 1992.
[5] Postel, J., and J. Reynolds, "A Standard for the
of IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
February 1988.
Laubach & Halpern Standards Track [Page 24]
RFC 2225 IP and ARP over ATM April 1998
[6] CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII
Geneva, 19-29 January 1993.
[7] CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23
- 2 October 1992.
[8] Braden, R., "Requirements for Internet Hosts --
Layers", STD 3, RFC 1122, October 1989.
[9] ATM Forum, "ATM User-Network Interface (UNI)
Version 3.1.", ISBN 0-13-393828-X, Prentice-Hall, Inc.,
Saddle River, NJ, 07458, September, 1994.
[10] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, August 1989.
[11] Colella, R., Gardner, E., and R. Callon, "Guidelines for
NSAP Allocation in the Internet", RFC 1237, July 1991.
[12] Bradely, T., and C. Brown, "Inverse Address
Protocol", RFC 1293, January 1992.
[13] Bellovin, Steven M., "Security Problems in the TCP/IP
Suite", ACM Computer Communications Review, Vol. 19, Issue 2,
pp. 32-48, 1989.
[14] Knowles, S., "IESG Advice from Experience with Path
Discovery", RFC 1435, March 1993.
[15] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
March 1997.
[16] Kent C., and J. Mogul, "Fragmentation Considered Harmful",
Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers
Computer Communications Technology, August 1987.
[17] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[18] Green, M., Luciani, J., White, K., and T. Kuo, "Definitions
Managed Objects for Classical IP and ARP over ATM
SMIv2", RFC 2320, April 1998.
[19] ATM Forum, "ATM User-Network Interface (UNI)
Version 4.0", ATM Forum specfication af-sig-0061.000,
ftp://ftp.atmforum.com/, July, 1996.
Laubach & Halpern Standards Track [Page 25]
RFC 2225 IP and ARP over ATM April 1998
[20] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.
15. AUTHORS'
Mark
Com21, Inc
750 Tasman
Milpitas, CA 95035
Phone: 408.953.9175
FAX: 408.953.9299
EMail: laubach@com21.
Joel
Newbridge Networks, Inc
593 Herndon
Herndon, VA 22070-5241
Phone: 703.736.5954
FAX: 703.736.5959
EMail: jhalpern@Newbridge.
Laubach & Halpern Standards Track [Page 26]
RFC 2225 IP and ARP over ATM April 1998
APPENDIX A - Update
This memo represents an update to RFC 1577 and RFC 1626.
following changes are included in this memo
o Pointer to Classical MIB I-D for setting of
o Single ATMARP server address to ATMARP server list,
via the MIB
o RFC 1626 text replaces MTU
o Client registration procedure from In_ATMARP to
ATMARP_
o Clarification of variable length ATMARP packet
o Clarification of ARP_NAK packet
o Clarification of InATMARP packet format for null IPv4
o Clarification on ATMARP registration and use of InATMARP_
for clients having more than one IP address in a
Laubach & Halpern Standards Track [Page 27]
RFC 2225 IP and ARP over ATM April 1998
Full Copyright
Copyright (C) The Internet Society (1998). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implmentation may be prepared, copied,
andand distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet 