8.7.1 ATMARP/InATMARP Request and Reply Packet Formats . . . . 188.7.2 Receiving Unknown ATMARP packets . . . . . . . . . . . . 208.7.3 TL, ATM Number, and ATM Subaddress Encoding . . . . . . . 208.7.4 ATMARP_NAK Packet Format . . . . . . . . . . . . . . . . 218.7.5 Variable Length Requirements for ATMARP Packets . . . . . 218.8 ATMARP/InATMARP Packet Encapsulation . . . . . . . . . . . 229. IP BROADCAST ADDRESS . . . . . . . . . . . . . . . . . . . . 2310. IP MULTICAST ADDRESS . . . . . . . . . . . . . . . . . . . . 2311. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 2312. MIB SPECIFICATION . . . . . . . . . . . . . . . . . . . . . 2413. OPEN ISSUES . . . . . . . . . . . . . . . . . . . . . . . . 2414. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . 2415. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . . 26APPENDIX A - Update Information . . . . . . . . . . . . . . . . 27
FULL COPYRIGHT STATEMENT . . . . . . . . . . . . . . . . . . . . 281. ABSTRACT
This memo defines an initial application of classical IP and ARP in
an Asynchronous Transfer Mode (ATM) network environment configured as
a Logical IP Subnetwork (LIS) as described in Section 5. This memo
does not preclude the subsequent development of ATM technology into
areas other than a LIS; specifically, as single ATM networks grow to
replace many Ethernet local LAN segments and as these networks become
globally connected, the application of IP and ARP will be treated
differently. This memo considers only the application of ATM as a
direct replacement for the "wires" and local LAN segments connecting
IP end-stations ("members") and routers operating in the "classical"
LAN-based paradigm. Issues raised by MAC level bridging and LAN
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 (formerly
CCITT) references for more detailed information about ATM
implementation agreements and standards.
The authors would like to thank the efforts of the IP over ATM
Working Group of the IETF. Without their substantial, and sometimes
contentious support, of the Classical IP over ATM model, this updated
memo would not have been possible. Section 7, on Default MTU, has
been incorporated directly from Ran Atkinson's RFC 1626, with his
permission. Thanks to Andy Malis for an early review and comments
for rolc and ion related issues.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 .
The goal of this specification is to allow compatible and
interoperable implementations for transmitting IP datagrams and ATM
Address Resolution Protocol (ATMARP) requests and replies over ATM
Adaptation Layer 5 (AAL5)[2,6].
This memo specifies the stable foundation baseline operational model
which will always be available in IP and ARP over ATM
implementations. Subsequent memos will build upon and refine this
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 resolution
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 channel
connections used to support IP and address resolution over ATM, and
thus are assumed to be using AAL5. This memo places no restrictions
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 be
able to connect islands of ATM networks. In 3), public or private
ATM Wide Area networks will be used to connect IP routers, which in
turn may or may not connect to local ATM networks. ATM WANs and LANs
may be interconnected.
Private ATM networks (local or wide area) will use the private ATM
address structure specified in the ATM Forum UNI 3.1 specification
 or as in the ATM Forum UNI 4.0 specification . This
structure is modeled after the format of an OSI Network Service
Access Point Address (NSAPA). A private ATM address uniquely
identifies an ATM endpoint.
Public networks will use either the address structure specified in
ITU-TS recommendation E.164 or the private network ATM address
structure. An E.164 address uniquely identifies an interface to a
The characteristics and features of ATM networks are different than
those found in LANs:
o ATM provides a Virtual Connection (VC) switched environment. VC
setup may be done on either a Permanent Virtual Connection (PVC)
or dynamic Switched Virtual Connection (SVC) basis. SVC call
management signalling is performed via implementations of the UNI
3.1 protocol [7,9].
o Data to be passed by a VC is segmented into 53 octet quantities
called cells (5 octets of ATM header and 48 octets of data).
o The function of mapping user Protocol Data Units (PDUs) into the
information field of the ATM cell and vice versa is performed in
the ATM Adaptation Layer (AAL). When a VC is created a specific
AAL type is associated with the VC. There are four different AAL
types, which are referred to individually as "AAL1", "AAL2",
"AAL3/4", and "AAL5". (NOTE: this memo concerns itself with the
mapping of IP and ATMARP over AAL5 only. The other AAL types are
mentioned for introductory purposes only.) The AAL type is known
by the VC end points via the call setup mechanism and is not
carried in the ATM cell header. For PVCs the AAL type is
administratively configured at the end points when the Connection
(circuit) is set up. For SVCs, the AAL type is communicated
along the VC path via UNI 3.1 as part of call setup establishment
and the end points use the signaled information for
configuration. ATM switches generally do not care about the AAL
type of VCs. The AAL5 format specifies a packet format with a
maximum size of (64K - 1) octets of user data. Cells for an AAL5
PDU are transmitted first to last, the last cell indicating the
end of the PDU. ATM standards guarantee that on a given VC, cell
ordering is preserved end-to-end. NOTE: AAL5 provides a non-
assured data transfer service - it is up to higher-level
protocols to provide retransmission.
o ATM Forum signaling defines point-to-point and point-to-
point Connection setup [9, 19.] Multipoint-to-multipoint not yet
specified by ITU-TS or ATM Forum.
An ATM Forum ATM address is either encoded as an NSAP form ATM
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 address
are needed by an ATMARP client to reach another host or router.
Since the use of AESAs and E.164 public UNI addresses by ATMARP
are analogous to the use of Ethernet addresses, the notion of
"hardware address" is extended to encompass ATM addresses in the
context of ATMARP, even though ATM addresses need not have
hardware significance. ATM Forum NSAP format addresses (AESA)
use the same basic format as U.S. GOSIP OSI NSAPAs . NOTE:
ATM Forum addresses should not be construed as being U.S. GOSIP
NSAPAs. They are not, the administration is different, which
fields get filled out are different, etc. However, in this
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 networks
(Ethernets) and for IP links which interconnect routers, either
within or between administrative domains. The "classical" model here
refers to the treatment of the ATM host adapter as a networking
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 for
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 Path
MTU Discovery to better suit the needs of the cooperating pair of
IP members or the attributes of the communications path. (Refer
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 ATMARP
service within the LIS - ATMARPs stay within the LIS. From a
client's perspective, the ATMARP architecture stays faithful to
the basic ARP model presented in .
o One IP subnet is used for many hosts and routers. Each VC
directly connects two IP members within the same LIS.
Future memos will describe the operation of IP over ATM when ATM
networks become globally deployed and interconnected.
The deployment of ATM into the Internet community is just beginning
and will take many years to complete. During the early part of this
period, we expect deployment to follow traditional IP subnet
boundaries for the following reasons:
o Administrators and managers of IP subnetworks will tend to
initially follow the same models as they currently have deployed.
The mindset of the community will change slowly over time as ATM
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 these
mechanisms until ATM provides better management capability than
the existing services and practices.
o Standards for global IP over ATM will take some time to complete
This memo details the treatment of the classical model of IP and
ATMARP over ATM. This memo does not preclude the subsequent
treatment of ATM networks within the IP framework as ATM becomes
globally deployed and interconnected; this will be the subject of
future documents. This memo does not address issues related to
transparent data link layer interoperability.
5. IP SUBNETWORK CONFIGURATION
In the LIS scenario, each separate administrative entity configures
its hosts and routers within a LIS. Each LIS operates and
communicates independently of other LISs on the same ATM network.
In the classical model, hosts communicate directly via ATM to other
hosts within the same LIS using the ATMARP service as the mechanism
for resolving target IP addresses to target ATM endpoint addresses.
The ATMARP service has LIS scope only and serves all hosts in the
LIS. Communication to hosts located outside of the local LIS is
provided via an IP router. This router is an ATM endpoint attached
to the ATM network that is configured as a member of one or more
LISs. This configuration MAY result in a number of disjoint LISs
operating over the same ATM network. Using this model hosts of
differing IP subnets MUST communicate via an intermediate IP router
even though it may be possible to open a direct VC between the two IP
members over the ATM network.
By default, the ATMARP service and the classical LIS routing model
MUST be available to any IP member client in the LIS.
5.2 LIS Configuration Requirements
The requirements for IP members (hosts, routers) operating in an ATM
LIS configuration are:
o All members of the LIS have the same IP network/subnet number and
address mask .
o All members within a LIS are directly connected to the ATM
o All members of a LIS MUST have a mechanism for resolving IP
addresses to ATM addresses via ATMARP (based on ) and vice
versa via InATMARP (based on ) when using SVCs. Refer to
Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.
o All members of a LIS MUST have a mechanism for resolving VCs to
IP addresses via InATMARP (based on ) when using PVCs. Refer
to Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.
o All members within a LIS MUST be able to communicate via ATM with
all other members in the same LIS; i.e., the Virtual Connection
topology underlying the intercommunication among the members is
The following list identifies the set of ATM specific parameters that
MUST be implemented in each IP station connected to the ATM network:
o ATM Hardware Address (atm$ha). The ATM address of the individual
o ATMARP Request Address list (atm$arp-req-list): atm$arp-req-list
is a list containing one or more ATM addresses of individual
ATMARP servers located within the LIS. In an SVC environment,
ATMARP servers are used to resolve target IP addresses to target
ATM address via an ATMARP request and reply protocol. ATMARP
servers MUST have authoritative responsibility for resolving
ATMARP requests of all IP members using SVCs located within the
A LIS MUST have a single ATMARP service entry configured and
available to all members of the LIS who use SVCs.
In the case where there is only a single ATMARP server within the
LIS, then all ATMARP clients MUST be configured identically to have
only one non-null entry in atm$arp-req-list configured with the same
address of the single ATMARP service.
If the IP member is operating with PVCs only, then atm$arp-req-list
MUST be configured with all null entries and the client MUST not make
queries to either address resolution service.
Within the restrictions mentioned above and in Section 8, local
administration MUST decide which server address(es) are appropriate
By default, atm$arp-req-list MUST be configured using the MIB .
Manual configuration of the addresses and address lists presented in
this section is implementation dependent and beyond the scope of this
document; i.e., this memo does not require any specific configuration
method. This memo does require that these addresses MUST be
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 Configuration
It is RECOMMENDED that routers providing LIS functionality over the
ATM network also support the ability to interconnect multiple LISs.
Routers that wish to provide interconnection of differing LISs MUST
be able to support multiple sets of these parameters (one set for
each connected LIS) and be able to associate each set of parameters
to a specific IP network/ subnet number. In addition, it is
RECOMMENDED that a router be able to provide this multiple LIS
support with a single physical ATM interface that may have one or
more individual ATM endpoint addresses. NOTE: this does not
necessarily mean different End System Identifiers (ESIs) when NSAPAs
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 for
the same ESI. (Refer to Section 126.96.36.199, "Private Networks" in .)
6. IP PACKET FORMAT
Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as
described in . LLC/SNAP encapsulation is the default packet
format for IP datagrams.
This memo recognizes that other encapsulation methods may be used
however, in the absence of other knowledge or agreement, LLC/SNAP
encapsulation is the default.
This memo recognizes that end-to-end signaling within ATM may allow
negotiation of encapsulation method on a per-VC basis.
7. DEFAULT VALUE FOR IP MTU OVER ATM AAL5
Protocols in wide use throughout the Internet, such as the Network
File System (NFS), currently use large frame sizes (e.g., 8 KB).
Empirical evidence with various applications over the Transmission
Control Protocol (TCP) indicates that larger Maximum Transmission
Unit (MTU) sizes for the Internet Protocol (IP) tend to give better
performance. Fragmentation of IP datagrams is known to be highly
undesirable . It is desirable to reduce fragmentation in the
network and thereby enhance performance by having the IP Maximum
Transmission Unit (MTU) for AAL5 be reasonably large. NFS defaults
to an 8192 byte frame size. Allowing for RPC/XDR, UDP, IP, and LLC
headers, NFS would prefer a default MTU of at least 8300 octets.
Routers can sometimes perform better with larger packet sizes because
most of the performance costs in routers relate to "packets handled"
rather than "bytes transferred". So, there are a number of good
reasons to have a reasonably large default MTU value for IP over ATM
RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which is
larger than 8300 octets but still in the same range . There is no
good reason for the default MTU of IP over ATM AAL5 to be different
from IP over SMDS, given that they will be the same magnitude.
Having the two be the same size will be helpful in interoperability
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 this
specification shall support at least the default IP MTU value for use
over ATM AAL5.
7.1 Permanent Virtual Circuits
Implementations which only support Permanent Virtual Circuits (PVCs)
will (by definition) not implement any ATM signalling protocol. Such
implementations shall use the default IP MTU value of 9180 octets
unless both parties have agreed in advance to use some other IP MTU
value via some mechanism not specified here.
7.2 Switched Virtual Circuits
Implementations that support Switched Virtual Circuits (SVCs) MUST
attempt to negotiate the AAL CPCS-SDU size using the ATM signalling
protocol. The industry standard ATM signalling protocol uses two
different parts of the Information Element named "AAL Parameters" to
exchange information on the MTU over the ATM circuit being setup .
The Forward Maximum CPCS-SDU Size field contains the value over the
path from the calling party to the called party. The Backwards
Maximum CPCS-SDU Size Identifier field contains the value over the
path from the called party to the calling party. The ATM Forum
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 the
MTU in the opposite direction, so the Forward Maximum CPCS-SDU Size
Identifier might have a different value from the Backwards Maximum
CPCS-SDU Size Identifier on the same connection.
If the calling party wishes to use the default MTU it shall still
include the "AAL Parameters" information element with the default
values for the Maximum CPCS-SDU Size as part of the SETUP message of
the ATM signalling protocol . If the calling party desires to use
a different value than the default, it shall include the "AAL
Parameters" information element with the desired value for the
Maximum CPCS-SDU Size as part of the SETUP message of the ATM
Signalling Protocol. The called party will respond using the same
information elements and identifiers in its CONNECT message response
If the called party receives a SETUP message containing the "Maximum
CPCS-SDU Size" in the AAL Parameters information element, it shall
handle the Forward and Backward Maximum CPCS-SDU Size Identifier as
a) If it is able to accept the ATM MTU values proposed by the SETUP
message, it shall include an AAL Parameters information element
in its response. The Forward and Backwards Maximum CPCS-SDU Size
fields shall be present and their values shall be equal to the
corresponding values in the SETUP message.
b) If it wishes a smaller ATM MTU size than that proposed, then it
shall set the values of the Maximum CPCS-SDU Size in the AAL
Parameters information elements equal to the desired value in the
CONNECT message responding to the original SETUP message.
c) If the calling endpoint receives a CONNECT message that does not
contain the AAL Parameters Information Element, but the
corresponding SETUP message did contain the AAL Parameters
Information element (including the forward and backward CPCS-SDU
Size fields), it shall clear the call with cause "AAL Parameters
cannot be supported".
d) If either endpoint receives a STATUS message with cause
"Information Element Non-existent or Not Implemented" or cause
"Access Information Discarded", and with a diagnostic field
indicating the AAL Parameters Information Element identifier, it
shall clear the call with cause "AAL Parameters cannot be
e) If either endpoint receives CPCS-SDUs in excess of the negotiated
MTU size, it may use IP fragmentation or may clear the call with
cause "AAL Parameters cannot be supported". In this case, an
error has occurred either due to a fault in an end system or in
the ATM network. The error should be noted by ATM network
management for human examination and intervention.
If the called endpoint incorrectly includes the Forward and Backward
Maximum CPCS-SDU Size fields in the CONNECT messages (e.g., because
the original SETUP message did not include these fields) or it sets
these fields to an invalid value, then the calling party shall clear
the call with cause "Invalid Information Element Contents".
7.3 Path MTU Discovery Required
The Path MTU Discovery mechanism is Internet Standard RFC 1191 
and is an important mechanism for reducing IP fragmentation in the
Internet. This mechanism is particularly important because new
subnet ATM uses a default MTU sizes significantly different from
older subnet technologies such as Ethernet and FDDI.
In order to ensure good performance throughout the Internet and also
to permit IP to take full advantage of the potentially larger IP
datagram sizes supported by ATM, all router implementations that
comply or conform with this specification must also implement the IP
Path MTU Discovery mechanism as defined in RFC 1191 and clarified by
RFC 1435 . Host implementations should implement the IP Path MTU
Discovery mechanism as defined in RFC 1191.
8. LIS ADDRESS RESOLUTION SERVICES
8.1 ATM-based ARP and InARP Equivalent Services
Address resolution within an ATM LIS SHALL make use of the ATM
Address Resolution Protocol (ATMARP) (based on ) and the Inverse
ATM Address Resolution Protocol (InATMARP) (based on ) and as
defined in this memo. ATMARP is the same protocol as the ARP
protocol presented in  with extensions needed to support address
resolution in a unicast server ATM environment. InATMARP is the same
protocol as the original InARP protocol presented in  but applied
to ATM networks. All IP stations MUST support these protocols as
updated and extended in this memo. Use of these protocols differs
depending on whether PVCs or SVCs are used.
8.2 Permanent Virtual Connections
An IP station MUST have a mechanism (e.g., manual configuration) for
determining what PVCs it has, and in particular which PVCs are being
used with LLC/SNAP encapsulation. The details of the mechanism are
beyond the scope of this memo.
All IP members supporting PVCs are required to use the Inverse ATM
Address Resolution Protocol (InATMARP) (refer to ) on those VCs
using LLC/SNAP encapsulation. In a strict PVC environment, the
receiver SHALL infer the relevant VC from the VC on which the
InATMARP_Request or response InATMARP_Reply was received. When the
ATM source and/or target address is unknown, the corresponding ATM
address length in the InATMARP packet MUST be set to zero (0)
indicating a null length, and no storage be allocated in the InATMARP
packet, otherwise the appropriate address field should be filled in
and the corresponding length set appropriately. InATMARP packet
format details are presented later in this memo.
Directly from : "When the requesting station receives the
In[ATM]ARP_Reply, it may complete the [ATM]ARP table entry and use
the provided address information. NOTE: as with [ATM]ARP,
information learned via In[ATM]ARP may be aged or invalidated under
certain circumstances." IP stations supporting PVCs MUST re-validate
ATMARP table entries as part of the table aging process. See the
Section 8.5.1 "Client ATMARP Table Aging".
If a client has more than one IP address within the LIS and if using
PVCs, when an InATMARP_Request is received an InATMARP_Reply MUST be
generated for each such address.
8.3 Switched Virtual Connections
SVCs require support from address resolution services for resolving
target IP addresses to target ATM endpoint addresses. All members in
the LIS MUST use the same service. This service MUST have
authoritative responsibility for resolving the ATMARP requests of all
IP members within the LIS.
ATMARP servers do not actively establish connections. They depend on
the clients in the LIS to initiate connections for the ATMARP
registration procedure and for transmitting ATMARP requests. An
individual client connects to the ATMARP server using a point-to-
point LLC/SNAP VC. The client sends normal ATMARP request packets to
the server. The ATMARP server examines each ATMARP_Request packet
the source protocol and source hardware address information of the
sending client and uses this information to build its ATMARP table
cache. This information is used to generate replies to any ATMARP
requests it receives.
InATMARP_Request packets MUST specify valid address information for
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 and
constrains it to consist of a single ATMARP server. Client-server
interaction is defined by using a single server approach as a
This memo recognizes the future development of standards and
implementations of multiple-ATMARP-server models that will extend the
operations as defined in this memo to provide a highly reliable
address resolution service.
8.4 ATMARP Single Server Operational Requirements
A single ATMARP server accepts ATM calls/connections from other ATM
end points. After receiving any ATMARP_Request, the server will
examine the source and target address information in the packet and
make note of the VC on which the ATMARP_Request arrived. It will use
this information as necessary to build and update its ATMARP table
For each ATMARP_Request, then:
1. If the source IP protocol address is the same as the target IP
protocol address and a table entry exists for that IP address and
if the source ATM hardware address does not match the table entry
ATM address and there is an open VC associated with that table
entry that is not the same as the VC associated with the
ATMARP_Request, the server MUST return the table entry
information in the ATMARP_Reply, and MUST raise a "duplicate IP
address detected" condition to the server's management. The
table entry is not updated.
2. Otherwise, if the source IP protocol address is the same as the
target IP protocol address, and either there is no table entry
for that IP address, or a table entry exists for that IP address
and there is no open VC associated with that table entry, or if
the VC associated with that entry is the same as the VC for the
ATMARP_Request, the server MUST either create a new entry or
update the old entry as appropriate and return that table entry
information in the ATMARP Reply.
3. Otherwise, when the source IP protocol address does not match the
target IP protocol address, the ATMARP server will generate the
corresponding ATMARP_Reply if it has an entry for the target
information in its ATMARP table. Otherwise, it will generate a
negative ATMARP reply (ATMARP_NAK).
4. Additionally, when the source IP protocol address does not match
the target IP protocol address and when the server receives an
ATMARP_Request over a VC, where the source IP and ATM address do
not have a corresponding table entry, the ATMARP server MUST
create a new table entry for the source information.
Explanation: this allows old RFC 1577 clients to register with
this ATMARP service by just issuing requests to it.
5. Additionally, when the source IP protocol address does not match
the target IP protocol address and where the source IP and ATM
addresses match the association already in the ATMARP table and
the ATM address matches that associated with the VC, the server
MUST update the table timeout on the source ATMARP table entry
but only if it has been more than 10 minutes since the last
update. Explanation: if the client is sending ATMARP requests to
the server over the same VC that it used to register its ATMARP
entry, the server should examine the ATMARP request and note that
the client is still "alive" by updating the timeout on the
client's ATMARP table entry.
6. Additionally, when the source IP protocol address does not match
the target IP protocol address and where the source IP and ATM
addresses do not match the association already in the ATMARP
table, the server MUST NOT update the ATMARP table entry.
An ATMARP server MUST have knowledge of any open VCs it has and their
association with an ATMARP table entry, and in particular, which VCs
support LLC/SNAP encapsulation. In normal operation, active ATMARP
clients will revalidate their entries prior to the server aging
process taking effect.
Server ATMARP table entries are valid for 20 minutes. If an entry
ages beyond 20 minutes without being updated (refreshed) by the
client, that entry is deleted from the table regardless of the state
of any VCs that may be associated with that entry.
8.5 ATMARP Client Operational Requirements
The ATMARP client is responsible for contacting the ATMARP service to
both initially register and subsequently refresh its own ATMARP
The client is also responsible for using the ATMARP service to gain
and revalidate ATMARP information about other IP members in the LIS
(server selection overview is discussed in Section 8.6). As noted in
Section 5.2, ATMARP clients MUST be configured with the ATM address
of the appropriate server prior to client ATMARP operation.
IP clients MUST register their ATM endpoint address with their ATMARP
server using the ATM address structure appropriate for their ATM
network connection: i.e., LISs implemented over ATM LANs following
ATM Forum UNI 3.1 should register using Structure 1; LISs implemented
over an E.164 "public" ATM network should register using Structure 2.
A LIS implemented over a combination of ATM LANs and public ATM
networks may need to register using Structure 3. Implementations
based on this memo MUST support all three ATM address structures.
See Section 8.7.1 for more details regarding the ATMARP Request
To handle the case when a client has more than one IP address within
a LIS, when using an ATMARP server, the client MUST register each
For initial registration and subsequent refreshing of its own
information with the ATMARP service, clients MUST:
1. Establish an LLC/SNAP VC connection to a server in the ATMARP
service for the purposes of transmitting and receiving ATMARP
NOTE: in the case of refreshing its own information with the
ATMARP service, a client MAY reuse an existing established
connection to the ATMARP service provided that the connection was
previously used either to initially register its information with
the ATMARP service or to refresh its information with the ATMARP
2. After establishing a successful connection to the ATMARP service,
the client MUST transmit an ATMARP_Request packet, requesting a
target ATM address for its own IP address as the target IP
protocol address. The client checks the ATMARP_Reply and if the
source hardware and protocol addresses match the respective
target hardware and protocol addresses, the client is registered
with the ATMARP service. If the addresses do not match, the
client MAY take action, raise alarms, etc.; however, these
actions are beyond the scope of this memo. In the case of a
client having more than one IP address in the list, this step
MUST be repeated for each IP address.
3. Clients MUST respond to ATMARP_Request and InATMARP_Request
packets received on any VC appropriately. (Refer to Section 7,
"Protocol Operation" in RFC 1293 .)
NOTE: for reasons of robustness, clients MUST respond to
4. Generate and transmit address resolution request packets to the
address resolution service. Respond to address resolution reply
packets appropriately to build/refresh its own client ATMARP
5. Generate and transmit InATMARP_Request packets as needed and
process InATMARP_Reply packets appropriately. InATMARP_Reply
packets should be used to build/refresh its own client ATMARP
table entries. (Refer to Section 7, "Protocol Operation" in
.) If a client has more than one IP address within the LIS
when an InATMARP_Request is received an InATMARP_Reply MUST be
generated for each such address.
The client MUST refresh its ATMARP information with the server at
least once every 15 minutes. This is done by repeating steps 1 and
An ATMARP client MUST have knowledge of any open VCs it has
(permanent or switched), their association with an ATMARP table
entry, and in particular, which VCs support LLC/SNAP encapsulation.
8.5.1 Client ATMARP Table Aging
Client ATMARP table entries are valid for a maximum time of 15
When an ATMARP table entry ages, an ATMARP client MUST invalidate the
table entry. If there is no open VC server associated with the
invalidated entry, that entry is deleted. In the case of an
invalidated entry and an open VC, the client MUST revalidate the
entry prior to transmitting any non address resolution traffic on
that VC; this requirement applies to both PVCs and SVCs. NOTE: the
client is permitted to revalidate an ATMARP table entry before it
ages, thus restarting the aging time when the table entry is
successfully revalidated. The client MAY continue to use the open
VC, as long as the table entry has not aged, while revalidation is in
In the case of an open PVC, the client revalidates the entry by
transmitting an InATMARP_Request and updating the entry on receipt of
In the case of an open SVC, the client revalidates the entry by
querying the address resolution service. If a valid reply is
received (e.g., ATMARP_Reply), the entry is updated. If the address
resolution service cannot resolve the entry (i.e., "host not found"),
the SVC should be closed and the associated table entry removed. If
the address resolution service is not available (i.e., "server
failure") and if the SVC is LLC/SNAP encapsulated, the client MUST
attempt to revalidate the entry by transmitting an InATMARP_Request
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 Operations
The specific details on client procedures for detecting non-normal VC
connection establishment or closures, or failed communications on an
established VC are beyond the scope of this memo. It is REQUIRED
however, that the client MUST remove the associated ATMARP entry for
a VC that fails to operate properly, as defined by the client, when
the client closes that VC, when it releases its resources for a VC,
or prior to any attempt to reopen that VC. This behavior
specifically REQUIRES that the client MUST refresh its ATMARP table
information prior to any attempt to re-establish communication to an
IP member after a non-normal communications problem has previously
occurred on a VC to that IP member.
8.5.3 Use of ATMARP In Mobile-IP Scenarios
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 term
connections with the ATMARP service. The absence of this VC will
permit a mobile node's registration, upon its return to the home
network, to immediately preempt the home agent's previous gratuitous
8.6 Address Resolution Server Selection
If the client supports PVCs only, the ATMARP server list is empty and
the client MUST not generate any address resolution requests other
than the InATMARP requests on a PVC needed to validate that PVC.
If the client supports SVCs, then the client MUST have a non-NULL
atm$arp-req-list pointing to the ATMARP server(s) which provides
ATMARP service for the LIS.
The client MUST register with a server from atm$arp-req-list.
The client SHALL attempt to communicate with any of the servers until
a successful registration is accomplished. The order in which client
selects servers to attempt registration, is a local matter, as are
the number of retries and timeouts for such attempts.
8.6.1 PVCs to ATMARP Servers
In a mixed PVC and SVC LIS environment, an ATMARP client MAY have a
PVC to an ATMARP server. In this case, this PVC is used for ATMARP
requests and responses as if it were an established SVC. NOTE: if
this PVC is to be used for IP traffic, then the ATMARP server MUST be
prepared to accept and respond appropriately to InATMARP traffic.
8.7 ATMARP Packet Formats
Internet addresses are assigned independently of ATM addresses. Each
host implementation MUST know its own IP and ATM address(es) and MUST
respond to address resolution requests appropriately. IP members
MUST also use ATMARP and InATMARP to resolve IP addresses to ATM
addresses when needed.
NOTE: the ATMARP packet format presented in this memo is general in
nature in that the ATM number and ATM subaddress fields SHOULD map
directly to the corresponding UNI 3.1 fields used for ATM
call/connection setup signalling messages. The IP over ATM Working
Group expects ATM Forum NSAPA numbers (Structure 1) to predominate
over E.164 numbers (Structure 2) as ATM endpoint identifiers within
ATM LANs. The ATM Forum's VC Routing specification is not complete
at this time and therefore its impact on the operational use of ATM
Address Structure 3 is undefined. The ATM Forum will be defining
this relationship in the future. It is for this reason that IP
members need to support all three ATM address structures.
8.7.1 ATMARP/InATMARP Request and Reply Packet Formats
The ATMARP and InATMARP request and reply protocols use the same
hardware type (ar$hrd), protocol type (ar$pro), and operation code
(ar$op) data formats as the ARP and InARP protocols [3,12]. The
location of these three fields within the ATMARP packet are in the
same byte position as those in ARP and InARP packets. A unique
hardware type value has been assigned for ATMARP. In addition,
ATMARP makes use of an additional operation code for ARP_NAK. The
remainder of the ATMARP/InATMARP packet format is different than the
ARP/InARP packet format.
The ATMARP and InATMARP protocols have several fields that have the
following format and values:
ar$hrd 16 bits Hardware type
ar$pro 16 bits Protocol type
ar$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$sha qoctets of source ATM number
ar$ssa roctets of source ATM subaddress
ar$spa soctets of source protocol address
ar$tha xoctets of target ATM number
ar$tsa yoctets of target ATM subaddress
ar$tpa zoctets of target protocol address
ar$hrd - assigned to ATM Forum address family and is
19 decimal (0x0013) .
ar$pro - see Assigned Numbers for protocol type number for
the protocol using ATMARP. (IP is 0x0800).
ar$shtl - Type and length of source ATM number. See
Section 8.7.4 for TL encoding details.
ar$sstl - Type and length of source ATM subaddress. See
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. Value
range is 0 or 4 (decimal). For IPv4 ar$spln is 4.
ar$thtl - Type and length of target ATM number. See
Section 8.7.4 for TL encoding details.
ar$tstl - Type and length of target ATM subaddress. See
Section 8.7.4 for TL encoding details.
ar$tpln - length in octets of the target protocol address. Value
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 address
ar$tha - target ATM number (E.164 or ATM Forum NSAPA)
ar$tsa - target ATM subaddress (ATM Forum NSAPA)
ar$tpa - target protocol address
8.7.2 Receiving Unknown ATMARP packets
If an ATMARP client receives an ATMARP message with an operation code
(ar$op) for which it is not coded to support, it MUST gracefully
discard the message and continue normal operation. An ATMARP client
is NOT REQUIRED to return any message to the sender of the
8.7.3 TL, ATM Number, and ATM Subaddress Encoding
The encoding of the 8-bit TL (type and length) fields in ATMARP and
In_ATMARP packets is as follows:
MSB 8 7 6 5 4 3 2 1 LSB
| 0 | 1/0 | Octet length of address |
bit.8 (reserved) = 0 (for future use)
bit.7 (type) = 0 ATM Forum NSAPA format
= 1 E.164 format
bit.6-1 (length) = 6 bit unsigned octet length of address
(MSB = bit.6, LSB = bit.1) Value
range is from 0 to 20 (decimal).
ATM addresses, as defined by the ATM Forum UNI 3.1 signaling
specification , include a "Calling Party Number Information
Element" and a "Calling Party Subaddress Information Element". These
Information Elements (IEs) SHOULD map to ATMARP/InATMARP source ATM
number and source ATM subaddress respectively. Furthermore, ATM
Forum defines a "Called Party Number Information Element" and a
"Called Party Subaddress Information Element". These IEs map to
ATMARP/InATMARP target ATM number and target ATM subaddress,
The ATM Forum defines three structures for the combined use of number
and subaddress :
ATM Number ATM Subaddress
Structure 1 ATM Forum NSAPA null
Structure 2 E.164 null
Structure 3 E.164 ATM Forum NSAPA
ATMARP and InATMARP requests and replies for ATM address structures 1
and 2 MUST indicate a null or unknown ATM subaddress by setting the
appropriate subaddress length to zero; i.e., ar$sstl.length = 0 or
ar$tstl.length = 0, the corresponding type field (ar$sstl.type or
ar$tstl.type) MUST be ignored and the physical space for the ATM
subaddress buffer MUST not be allocated in the ATMARP packet. For
example, if ar$sstl.length=0, the storage for the source ATM
subaddress is not allocated and the first byte of the source protocol
address ar$spa follows immediately after the last byte of the source
hardware address ar$sha in the packet.
Null or unknown ATM addresses MUST be indicated by setting the
appropriate address length to zero; i.e., ar$shtl.length and
ar$thtl.length is zero and the corresponding type field (ar$sstl.type
or ar$tstl.type) MUST be ignored and the physical space for the ATM
address or ATM subaddress buffer MUST not be allocated in the ATMARP
8.7.4 ATMARP_NAK Packet Format
The ATMARP_NAK packet format is the same as the received
ATMARP_Request packet format with the operation code set to ARP_NAK,
i.e., the ATMARP_Request packet data is exactly copied (e.g., using
bcopy) for transmission with the ATMARP_Request operation code
changed to ARP_NAK value.
8.7.5 Variable Length Requirements for ATMARP Packets
ATMARP and InATMARP packets are variable in length.
A null or unknown source or target protocol address is indicated by
the corresponding length set to zero: e.g., when ar$spln or ar$tpln
is zero the physical space for the corresponding address structure
MUST not be allocated in the packet.
For backward compatibility with previous implementations, a null IPv4
protocol address may be received with length = 4 and an allocated
address in storage set to the value 0.0.0.0. Receiving stations MUST
be liberal in accepting this format of a null IPv4 address. However,
on transmitting an ATMARP or InATMARP packet, a null IPv4 address
MUST only be indicated by the length set to zero and MUST have no
8.8 ATMARP/InATMARP Packet Encapsulation
ATMARP and InATMARP packets are to be encoded in AAL5 PDUs using
LLC/SNAP encapsulation. The format of the AAL5 CPCS-SDU payload
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 a
The OUI value of 0x00-00-00 (3 octets) indicates that the following
two-bytes is an EtherType.
The EtherType value of 0x08-06 (2 octets) indicates ARP .
The total size of the LLC/SNAP header is fixed at 8-octets. This
aligns the start of the ATMARP packet on a 64-bit boundary relative
to the start of the AAL5 CPCS-SDU.
The LLC/SNAP encapsulation for ATMARP/InATMARP presented here is
consistent with the treatment of multiprotocol encapsulation of IP
over ATM AAL5 as specified in  and in the format of ATMARP over
IEEE 802 networks as specified in .
Traditionally, address resolution requests are broadcast to all
directly connected IP members within a LIS. It is conceivable in the
future that larger scaled ATM networks may handle ATMARP requests to
destinations outside the originating LIS, perhaps even globally;
issues raised by ATMARPing outside the LIS or by a global ATMARP
mechanism are beyond the scope of this memo.
9. IP BROADCAST ADDRESS
ATM does not support broadcast addressing, therefore there are no
mappings available from IP broadcast addresses to ATM broadcast
services. Note: this lack of mapping does not restrict members from
transmitting or receiving IP datagrams specifying any of the four
standard IP broadcast address forms as described in . 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 and
implementations that will extend the operations as defined in this
memo to provide an IP broadcast capability for use by the classical
10. IP MULTICAST ADDRESS
ATM does not directly support IP multicast address services,
therefore there are no mappings available from IP multicast addresses
to ATM multicast services. Current IP multicast implementations
(i.e., MBONE and IP tunneling, see ) will continue to operate
over ATM based logical IP subnets if operated in the WAN
This memo recognizes the future development of ATM multicast service
addressing by the ATM Forum. When available and widely implemented,
the roll-over from the current IP multicast architecture to this new
ATM architecture will be straightforward.
This memo recognizes the future development of standards and
implementations that will extend the operations as defined in this
memo to provide an IP multicast capability for use by the classical
11. SECURITY CONSIDERATIONS
Not all of the security issues relating to IP over ATM are clearly
understood at this time, due to the fluid state of ATM
specifications, newness of the technology, and other factors.
It is believed that ATM and IP facilities for authenticated call
management, authenticated end-to-end communications, and data
encryption will be needed in globally connected ATM networks. Such
future security facilities and their use by IP networks are beyond
the scope of this memo.
There are known security issues relating to host impersonation via
the address resolution protocols used in the Internet . No
special security mechanisms have been added to the address resolution
mechanism defined here for use with networks using IP over ATM.
12. MIB SPECIFICATION
Clients built to this specification MUST implement and provide a
Management Information Base (MIB) as defined in "Definitions of
Managed Objects for Classical IP and ARP Over ATM Using SMIv2" .
13. OPEN ISSUES
o Automatic configuration of client ATM addresses via DHCP  or
via ATM UNI 3.1 Interim Local Management Interface (ILMI)
services would be a useful extended service addition to this
document and should be addressed in a separate memo.
o ATMARP packets are not authenticated. This is a potentially
serious flaw in the overall system by allowing a mechanism by
which corrupt information may be introduced into the server
 Piscitello, D., and J. Lawrence, "The Transmission of IP
Datagrams over the SMDS Service", STD 52, RFC 1209, March 1991.
 Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
Layer 5", RFC 1483, July 1993.
 Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37, RFC
826, November 1982.
 Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
 Postel, J., and J. Reynolds, "A Standard for the Transmission
of IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
 CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII,
Geneva, 19-29 January 1993.
 CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23 September
- 2 October 1992.
 Braden, R., "Requirements for Internet Hosts -- Communication
Layers", STD 3, RFC 1122, October 1989.
 ATM Forum, "ATM User-Network Interface (UNI) Specification
Version 3.1.", ISBN 0-13-393828-X, Prentice-Hall, Inc., Upper
Saddle River, NJ, 07458, September, 1994.
 Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, August 1989.
 Colella, R., Gardner, E., and R. Callon, "Guidelines for OSI
NSAP Allocation in the Internet", RFC 1237, July 1991.
 Bradely, T., and C. Brown, "Inverse Address Resolution
Protocol", RFC 1293, January 1992.
 Bellovin, Steven M., "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Vol. 19, Issue 2,
pp. 32-48, 1989.
 Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC 1435, March 1993.
 Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
 Kent C., and J. Mogul, "Fragmentation Considered Harmful",
Proceedings of the ACM SIGCOMM '87 Workshop on Frontiers in
Computer Communications Technology, August 1987.
 Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
 Green, M., Luciani, J., White, K., and T. Kuo, "Definitions of
Managed Objects for Classical IP and ARP over ATM Using
SMIv2", RFC 2320, April 1998.
 ATM Forum, "ATM User-Network Interface (UNI) Specification
Version 4.0", ATM Forum specfication af-sig-0061.000,
ftp://ftp.atmforum.com/, July, 1996.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
15. AUTHORS' ADDRESSES
750 Tasman Drive
Milpitas, CA 95035
Newbridge Networks, Inc.
593 Herndon Parkway
Herndon, VA 22070-5241
APPENDIX A - Update Information
This memo represents an update to RFC 1577 and RFC 1626. The
following changes are included in this memo:
o Pointer to Classical MIB I-D for setting of variables
o Single ATMARP server address to ATMARP server list, configurable
via the MIB.
o RFC 1626 text replaces MTU section
o Client registration procedure from In_ATMARP to first
o Clarification of variable length ATMARP packet format
o Clarification of ARP_NAK packet format
o Clarification of InATMARP packet format for null IPv4 addresses
o Clarification on ATMARP registration and use of InATMARP_Reply
for clients having more than one IP address in a LIS
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