Network Working Group C. Brown
Request for Comments: 2427 Consultant
STD: 55 A. Malis
Obsoletes: 1490, 1294 Ascend Communications, Inc.
Category: Standards Track September 1998
Multiprotocol Interconnect over Frame Relay
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This memo describes an encapsulation method for carrying network
interconnect traffic over a Frame Relay backbone. It covers ASPects
of both Bridging and Routing.
Systems with the ability to transfer both the encapsulation method
described in this document, and others must have a priori knowledge
of which virtual circuits will carry which encapsulation method and
this encapsulation must only be used over virtual circuits that have
been eXPlicitly configured for its use.
Acknowledgments
This document could not have been completed without the support of
Terry Bradley of Avici Systems, Inc.. Comments and contributions
from many sources, especially those from Ray Samora of Proteon, Ken
Rehbehn of Visual Networks, Fred Baker and Charles Carvalho of Cisco
Systems, and Mostafa Sherif of AT&T have been incorporated into this
document. Special thanks to Dory Leifer of University of Michigan for
his contributions to the resolution of fragmentation issues (though
it was deleted in the final version) and Floyd Backes and Laura
Bridge of 3Com for their contributions to the bridging descriptions.
This document could not have been completed without the expertise of
the IP over Large Public Data Networks and the IP over NBMA working
groups of the IETF.
1. Conventions and Acronyms
The keyWords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [16].
All drawings in this document are drawn with the left-most bit as the
high order bit for transmission. For example, the drawings might be
labeled as:
0 1 2 3 4 5 6 7 bits
+---+---+---+---+---+---+---+
+---------------------------+
flag (7E hexadecimal)
+---------------------------+
Q.922 Address*
+-- --+

+---------------------------+
: :
: :
+---------------------------+
Drawings that would be too large to fit onto one page if each octet
were presented on a single line are drawn with two octets per line.
These are also drawn with the left-most bit as the high order bit for
transmission. There will be a "+" to distinguish between octets as
in the following example.
--- octet one ------ octet two ---
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+--------------------------------------------+
Organizationally Unique
+-- +--------------------+
Identifier Protocol
+-----------------------+--------------------+
Identifier
+-----------------------+
The following are common acronyms used throughout this document.
BECN - Backward Explicit Congestion Notification
BPDU - Bridge Protocol Data Unit
C/R - Command/Response bit
DCE - Data Communication Equipment
DE - Discard Eligibility bit
DTE - Data Terminal Equipment
FECN - Forward Explicit Congestion Notification
PDU - Protocol Data Unit
PTT - Postal Telephone & Telegraph
SNAP - Subnetwork Access Protocol
2. IntrodUCtion
The following discussion applies to those devices which serve as end
stations (DTEs) on a public or private Frame Relay network (for
example, provided by a common carrier or PTT. It will not discuss
the behavior of those stations that are considered a part of the
Frame Relay network (DCEs) other than to explain situations in which
the DTE must react.
The Frame Relay network provides a number of virtual circuits that
form the basis for connections between stations attached to the same
Frame Relay network. The resulting set of interconnected devices
forms a private Frame Relay group which may be either fully
interconnected with a complete "mesh" of virtual circuits, or only
partially interconnected. In either case, each virtual circuit is
uniquely identified at each Frame Relay interface by a Data Link
Connection Identifier (DLCI). In most circumstances, DLCIs have
strictly local significance at each Frame Relay interface.
The specifications in this document are intended to apply to both
switched and permanent virtual circuits.
3. Frame Format
All protocols must encapsulate their packets within a Q.922 Annex A
frame [1]. Additionally, frames shall contain information necessary
to identify the protocol carried within the protocol data unit (PDU),
thus allowing the receiver to properly process the incoming packet.
The format shall be as follows:
+---------------------------+
flag (7E hexadecimal)
+---------------------------+
Q.922 Address*
+-- --+

+---------------------------+
Control (UI = 0x03)
+---------------------------+
Pad (when required) (0x00)
+---------------------------+
NLPID
+---------------------------+
.
.
.
Data
.
.
+---------------------------+
Frame Check Sequence
+-- . --+
(two octets)
+---------------------------+
flag (7E hexadecimal)
+---------------------------+
* Q.922 addresses, as presently defined, are two octets and
contain a 10-bit DLCI. In some networks Q.922 addresses
may optionally be increased to three or four octets.
The control field is the Q.922 control field. The UI (0x03) value is
used unless it is negotiated otherwise. The use of XID (0xAF or
0xBF) is permitted and is discussed later.
The pad field is used to align the data portion (beyond the
encapsulation header) of the frame to a two octet boundary. If
present, the pad is a single octet and must have a value of zero.
Explicit directions of when to use the pad field are discussed later
in this document.
The Network Level Protocol ID (NLPID) field is administered by ISO
and the ITU. It contains values for many different protocols
including IP, CLNP, and IEEE Subnetwork Access Protocol (SNAP)[10].
This field tells the receiver what encapsulation or what protocol
follows. Values for this field are defined in ISO/IEC TR 9577 [3]. A
NLPID value of 0x00 is defined within ISO/IEC TR 9577 as the Null
Network Layer or Inactive Set. Since it cannot be distinguished from
a pad field, and because it has no significance within the context of
this encapsulation scheme, a NLPID value of 0x00 is invalid under the
Frame Relay encapsulation. Appendix A contains a list of some of the
more commonly used NLPID values.
There is no commonly implemented minimum maximum frame size for Frame
Relay. A network must, however, support at least a 262 octet
maximum. Generally, the maximum will be greater than or equal to
1600 octets, but each Frame Relay provider will specify an
appropriate value for its network. A Frame Relay DTE, therefore,
must allow the maximum acceptable frame size to be configurable.
The minimum frame size allowed for Frame Relay is five octets between
the opening and closing flags assuming a two octet Q.922 address
field. This minimum increases to six octets for three octet Q.922
address and seven octets for the four octet Q.922 address format.
4. Interconnect Issues
There are two basic types of data packets that travel within the
Frame Relay network: routed packets and bridged packets. These
packets have distinct formats and therefore, must contain an
indicator that the destination may use to correctly interpret the
contents of the frame. This indicator is embedded within the NLPID
and SNAP header information.
For those protocols that do not have a NLPID already assigned, it is
necessary to provide a mechanism to allow easy protocol
identification. There is a NLPID value defined indicating the
presence of a SNAP header.
A SNAP header is of the form:
+--------------------------------------------+
Organizationally Unique
+-- +--------------------+
Identifier Protocol
+-----------------------+--------------------+
Identifier
+-----------------------+
The three-octet Organizationally Unique Identifier (OUI) identifies
an organization which administers the meaning of the Protocol
Identifier (PID) which follows. Together they identify a distinct
protocol. Note that OUI 0x00-00-00 specifies that the following PID
is an Ethertype.
4.1. Routed Frames
Some protocols will have an assigned NLPID, but because the NLPID
numbering space is limited, not all protocols have specific NLPID
values assigned to them. When packets of such protocols are routed
over Frame Relay networks, they are sent using the NLPID 0x80 (which
indicates the presence of a SNAP header) followed by SNAP. If the
protocol has an Ethertype assigned, the OUI is 0x00-00-00 (which
indicates an Ethertype follows), and PID is the Ethertype of the
protocol in use.
When a SNAP header is present as described above, a one octet pad is
used to align the protocol data on a two octet boundary as shown
below.
Format of Routed Frames
with a SNAP Header
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 pad 0x00
+---------------+---------------+
NLPID 0x80 Organization-
+---------------+
ally Unique Identifier (OUI)
+-------------------------------+
Protocol Identifier (PID)
+-------------------------------+

Protocol Data

+-------------------------------+
FCS
+-------------------------------+
In the few cases when a protocol has an assigned NLPID (see Appendix
A), 48 bits can be saved using the format below:
Format of Routed NLPID Protocol
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 NLPID
+---------------+---------------+
Protocol Data
+-------------------------------+
FCS
+-------------------------------+
When using the NLPID encapsulation format as described above, the pad
octet is not used.
In the case of ISO protocols, the NLPID is considered to be the first
octet of the protocol data. It is unnecessary to repeat the NLPID in
this case. The single octet serves both as the demultiplexing value
and as part of the protocol data (refer to "Other Protocols over
Frame Relay for more details). Other protocols, such as IP, have a
NLPID defined (0xCC), but it is not part of the protocol itself.
Format of Routed IP Datagram
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 NLPID 0xCC
+---------------+---------------+
IP Datagram
+-------------------------------+
FCS
+-------------------------------+
4.2. Bridged Frames
The second type of Frame Relay traffic is bridged packets. These
packets are encapsulated using the NLPID value of 0x80 indicating
SNAP. As with other SNAP encapsulated protocols, there will be one
pad octet to align the data portion of the encapsulated frame. The
SNAP header which follows the NLPID identifies the format of the
bridged packet. The OUI value used for this encapsulation is the
802.1 organization code 0x00-80-C2. The PID portion of the SNAP
header (the two bytes immediately following the OUI) specifies the
form of the MAC header, which immediately follows the SNAP header.
Additionally, the PID indicates whether the original FCS is preserved
within the bridged frame.
Following the precedent in RFC1638 [4], non-canonical MAC
destination addresses are used for encapsulated IEEE 802.5 and FDDI
frames, and canonical MAC destination addresses are used for the
remaining encapsulations defined in this section.
The 802.1 organization has reserved the following values to be used
with Frame Relay:
PID Values for OUI 0x00-80-C2
with preserved FCS w/o preserved FCS Media
------------------ ----------------- ----------------
0x00-01 0x00-07 802.3/Ethernet
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A FDDI
0x00-0B 802.6
In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2,
identifies Bridge Protocol Data Units (BPDUs) as defined by
802.1(d) or 802.1(g) [12], and the PID value 0x00-0F identifies
Source Routing BPDUs.
A packet bridged over Frame Relay will, therefore, have one of the
following formats:
Format of Bridged Ethernet/802.3 Frame
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 pad 0x00
+---------------+---------------+
NLPID 0x80 OUI 0x00
+---------------+ --+
OUI 0x80-C2
+-------------------------------+
PID 0x00-01 or 0x00-07
+-------------------------------+
MAC destination address
: :

+-------------------------------+
(remainder of MAC frame)
+-------------------------------+
LAN FCS (if PID is 0x00-01)
+-------------------------------+
FCS
+-------------------------------+
Format of Bridged 802.4 Frame
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 pad 0x00
+---------------+---------------+
NLPID 0x80 OUI 0x00
+---------------+ --+
OUI 0x80-C2
+-------------------------------+
PID 0x00-02 or 0x00-08
+---------------+---------------+
pad 0x00 Frame Control
+---------------+---------------+
MAC destination address
: :

+-------------------------------+
(remainder of MAC frame)
+-------------------------------+
LAN FCS (if PID is 0x00-02)
+-------------------------------+
FCS
+-------------------------------+
Format of Bridged 802.5 Frame
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 pad 0x00
+---------------+---------------+
NLPID 0x80 OUI 0x00
+---------------+ --+
OUI 0x80-C2
+-------------------------------+
PID 0x00-03 or 0x00-09
+---------------+---------------+
pad 0x00 Frame Control
+---------------+---------------+
MAC destination address
: :

+-------------------------------+
(remainder of MAC frame)
+-------------------------------+
LAN FCS (if PID is 0x00-03)

+-------------------------------+
FCS
+-------------------------------+
Format of Bridged FDDI Frame
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 pad 0x00
+---------------+---------------+
NLPID 0x80 OUI 0x00
+---------------+ --+
OUI 0x80-C2
+-------------------------------+
PID 0x00-04 or 0x00-0A
+---------------+---------------+
pad 0x00 Frame Control
+---------------+---------------+
MAC destination address
: :

+-------------------------------+
(remainder of MAC frame)
+-------------------------------+
LAN FCS (if PID is 0x00-04)

+-------------------------------+
FCS
+-------------------------------+
Format of Bridged 802.6 Frame
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 pad 0x00
+---------------+---------------+
NLPID 0x80 OUI 0x00
+---------------+ --+
OUI 0x80-C2
+-------------------------------+
PID 0x00-0B
+---------------+---------------+ -------
Reserved BEtag Common
+---------------+---------------+ PDU
BAsize Header
+-------------------------------+ -------
MAC destination address
: :

+-------------------------------+
(remainder of MAC frame)
+-------------------------------+

+- Common PDU Trailer -+

+-------------------------------+
FCS
+-------------------------------+
Note that in bridge 802.6 PDUs, there is only one choice for the PID
value, since the presence of a CRC-32 is indicated by the CIB bit in
the header of the MAC frame.
The Common Protocol Data Unit (CPDU) Header and Trailer are conveyed
to allow pipelining at the egress bridge to an 802.6 subnetwork.
Specifically, the CPDU Header contains the BAsize field, which
contains the length of the PDU. If this field is not available to
the egress 802.6 bridge, then that bridge cannot begin to transmit
the segmented PDU until it has received the entire PDU, calculated
the length, and inserted the length into the BAsize field. If the
field is available, the egress 802.6 bridge can extract the length
from the BAsize field of the Common PDU Header, insert it into the
corresponding field of the first segment, and immediately transmit
the segment onto the 802.6 subnetwork. Thus, the bridge can begin
transmitting the 802.6 PDU before it has received the complete PDU.
One should note that the Common PDU Header and Trailer of the
encapsulated frame should not be simply copied to the outgoing 802.6
subnetwork because the encapsulated BEtag value may conflict with the
previous BEtag value transmitted by that bridge.
Format of BPDU Frame
+-------------------------------+
Q.922 Address
+-------------------------------+
Control 0x03
+-------------------------------+
PAD 0x00
+-------------------------------+
NLPID 0x80
+-------------------------------+
OUI 0x00-80-C2
+-------------------------------+
PID 0x00-0E
+-------------------------------+

BPDU as defined by
802.1(d) or 802.1(g)[12]

+-------------------------------+
FCS
+-------------------------------+
Format of Source Routing BPDU Frame
+-------------------------------+
Q.922 Address
+-------------------------------+
Control 0x03
+-------------------------------+
PAD 0x00
+-------------------------------+
NLPID 0x80
+-------------------------------+
OUI 0x00-80-C2
+-------------------------------+
PID 0x00-0F
+-------------------------------+

Source Routing BPDU


+-------------------------------+
FCS
+-------------------------------+
5. Data Link Layer Parameter Negotiation
Frame Relay stations may choose to support the Exchange
Identification (XID) specified in Appendix III of Q.922 [1]. This
XID exchange allows the following parameters to be negotiated at the
initialization of a Frame Relay circuit: maximum frame size N201,
retransmission timer T200, and the maximum number of outstanding
Information (I) frames K.
A station may indicate its unwillingness to support acknowledged mode
multiple frame operation by specifying a value of zero for the
maximum window size, K.
If this exchange is not used, these values must be statically
configured by mutual agreement of Data Link Connection (DLC)
endpoints, or must be defaulted to the values specified in Section
5.9 of Q.922:
N201: 260 octets
K: 3 for a 16 Kbps link,
7 for a 64 Kbps link,
32 for a 384 Kbps link,
40 for a 1.536 Mbps or above link
T200: 1.5 seconds [see Q.922 for further details]
If a station supporting XID receives an XID frame, it shall respond
with an XID response. In processing an XID, if the remote maximum
frame size is smaller than the local maximum, the local system shall
reduce the maximum size it uses over this DLC to the remotely
specified value. Note that this shall be done before generating a
response XID.
The following diagram describes the use of XID to specify non-use of
acknowledged mode multiple frame operation.
Non-use of Acknowledged Mode Multiple Frame Operation
+---------------+
Address (2,3 or 4 octets)

+---------------+
Control 0xAF
+---------------+
format 0x82
+---------------+
Group ID 0x80
+---------------+
Group Length (2 octets)
0x00-0E
+---------------+
0x05 PI = Frame Size (transmit)
+---------------+
0x02 PL = 2
+---------------+
Maximum (2 octets)
Frame Size
+---------------+
0x06 PI = Frame Size (receive)
+---------------+
0x02 PL = 2
+---------------+
Maximum (2 octets)
Frame Size
+---------------+
0x07 PI = Window Size
+---------------+
0x01 PL = 1
+---------------+
0x00
+---------------+
0x09 PI = Retransmission Timer
+---------------+
0x01 PL = 1
+---------------+
0x00
+---------------+
FCS (2 octets)

+---------------+
6. Address Resolution for PVCs
This document only describes address resolution as it applies to
PVCs. SVC operation will be discussed in future documents.
There are situations in which a Frame Relay station may wish to
dynamically resolve a protocol address over PVCs. This may be
accomplished using the standard Address Resolution Protocol (ARP) [6]
encapsulated within a SNAP encoded Frame Relay packet as follows:
+-----------------------+-----------------------+
Q.922 Address
+-----------------------+-----------------------+
Control (UI) 0x03 pad 0x00
+-----------------------+-----------------------+
NLPID 0x80 SNAP Header
+-----------------------+ OUI 0x00-00-00 + Indicating
ARP
+-----------------------+-----------------------+
PID 0x0806
+-----------------------+-----------------------+
ARP packet
.
.
.
+-----------------------+-----------------------+
Where the ARP packet has the following format and values:
Data:
ar$hrd 16 bits Hardware type
ar$pro 16 bits Protocol type
ar$hln 8 bits Octet length of hardware address (n)
ar$pln 8 bits Octet length of protocol address (m)
ar$op 16 bits Operation code (request or reply)
ar$sha noctets source hardware address
ar$spa moctets source protocol address
ar$tha noctets target hardware address
ar$tpa moctets target protocol address
ar$hrd - assigned to Frame Relay is 15 decimal
(0x000F) [7].
ar$pro - see assigned numbers for protocol ID number for
the protocol using ARP. (IP is 0x0800).
ar$hln - length in bytes of the address field (2, 3, or 4)
ar$pln - protocol address length is dependent on the
protocol (ar$pro) (for IP ar$pln is 4).
ar$op - 1 for request and 2 for reply.
ar$sha - Q.922 source hardware address, with C/R, FECN,
BECN, and DE set to zero.
ar$tha - Q.922 target hardware address, with C/R, FECN,
BECN, and DE set to zero.
Because DLCIs within most Frame Relay networks have only local
significance, an end station will not have a specific DLCI assigned
to itself. Therefore, such a station does not have an address to put
into the ARP request or reply. Fortunately, the Frame Relay network
does provide a method for oBTaining the correct DLCIs. The solution
proposed for the locally addressed Frame Relay network below will
work equally well for a network where DLCIs have global significance.
The DLCI carried within the Frame Relay header is modified as it
traverses the network. When the packet arrives at its destination,
the DLCI has been set to the value that, from the standpoint of the
receiving station, corresponds to the sending station. For example,
in figure 1 below, if station A were to send a message to station B,
it would place DLCI 50 in the Frame Relay header. When station B
received this message, however, the DLCI would have been modified by
the network and would appear to B as DLCI 70.
~~~~~~~~~~~~~~~
( )
+-----+ ( ) +-----+
-50------(--------------------)---------70-
A ( ) B
-60-----(---------+ )
+-----+ ( ) +-----+
( )
( ) <---Frame Relay
~~~~~~~~~~~~~~~~ network
80

+-----+

C

+-----+
Figure 1
Lines between stations represent data link connections (DLCs).
The numbers indicate the local DLCI associated with each
connection.
DLCI to Q.922 Address Table for Figure 1
DLCI (decimal) Q.922 address (hex)
50 0x0C21
60 0x0CC1
70 0x1061
80 0x1401
For authoritative description of the correlation between DLCI and
Q.922 [1] addresses, the reader should consult that specification.
A summary of the correlation is included here for convenience. The
translation between DLCI and Q.922 address is based on a two byte
address length using the Q.922 encoding format. The format is:
8 7 6 5 4 3 2 1
+------------------------+---+--+
DLCI (high order) C/REA
+--------------+----+----+---+--+
DLCI (lower) FECNBECNDE EA
+--------------+----+----+---+--+
For ARP and its variants, the FECN, BECN, C/R and DE bits are
assumed to be 0.
When an ARP message reaches a destination, all hardware addresses
will be invalid. The address found in the frame header will,
however, be correct. Though it does violate the purity of layering,
Frame Relay may use the address in the header as the sender hardware
address. It should also be noted that the target hardware address,
in both ARP request and reply, will also be invalid. This should not
cause problems since ARP does not rely on these fields and in fact,
an implementation may zero fill or ignore the target hardware address
field entirely.
As an example of how this address replacement scheme may work, refer
to figure 1. If station A (protocol address pA) wished to resolve
the address of station B (protocol address pB), it would format an
ARP request with the following values:
ARP request from A
ar$op 1 (request)
ar$sha unknown
ar$spa pA
ar$tha undefined
ar$tpa pB
Because station A will not have a source address associated with it,
the source hardware address field is not valid. Therefore, when the
ARP packet is received, it must extract the correct address from the
Frame Relay header and place it in the source hardware address field.
This way, the ARP request from A will become:
ARP request from A as modified by B
ar$op 1 (request)
ar$sha 0x1061 (DLCI 70) from Frame Relay header
ar$spa pA
ar$tha undefined
ar$tpa pB
Station B"s ARP will then be able to store station A"s protocol
address and Q.922 address association correctly. Next, station B
will form a reply message. Many implementations simply place the
source addresses from the ARP request into the target addresses and
then fills in the source addresses with its addresses. In this case,
the ARP response would be:
ARP response from B
ar$op 2 (response)
ar$sha unknown
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA
Again, the source hardware address is unknown and when the response
is received, station A will extract the address from the Frame Relay
header and place it in the source hardware address field. Therefore,
the response will become:
ARP response from B as modified by A
ar$op 2 (response)
ar$sha 0x0C21 (DLCI 50)
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA
Station A will now correctly recognize station B having protocol
address pB associated with Q.922 address 0x0C21 (DLCI 50).
Reverse ARP (RARP) [8] works in exactly the same way. Still using
figure 1, if we assume station C is an address server, the following
RARP exchanges will occur:
RARP request from A RARP request as modified by C
ar$op 3 (RARP request) ar$op 3 (RARP request)
ar$sha unknown ar$sha 0x1401 (DLCI 80)
ar$spa undefined ar$spa undefined
ar$tha 0x0CC1 (DLCI 60) ar$tha 0x0CC1 (DLCI 60)
ar$tpa pC ar$tpa pC
Station C will then look up the protocol address corresponding to
Q.922 address 0x1401 (DLCI 80) and send the RARP response.
RARP response from C RARP response as modified by A
ar$op 4 (RARP response) ar$op 4 (RARP response)
ar$sha unknown ar$sha 0x0CC1 (DLCI 60)
ar$spa pC ar$spa pC
ar$tha 0x1401 (DLCI 80) ar$tha 0x1401 (DLCI 80)
ar$tpa pA ar$tpa pA
This means that the Frame Relay interface must only intervene in the
processing of incoming packets.
In the absence of suitable multicast, ARP may still be implemented.
To do this, the end station simply sends a copy of the ARP request
through each relevant DLC, thereby simulating a broadcast.
The use of multicast addresses in a Frame Relay environment, as
specified by [19], is presently being considered by Frame Relay
providers. In time, multicast addressing may become useful in
sending ARP requests and other "broadcast" messages.
Because of the inefficiencies of emulating broadcasting in a Frame
Relay environment, a new address resolution variation was developed.
It is called Inverse ARP [11] and describes a method for resolving a
protocol address when the hardware address is already known. In
Frame Relay"s case, the known hardware address is the DLCI. Support
for Inverse ARP is not required to implement this specification, but
it has proven useful for Frame Relay interface autoconfiguration.
See [11] for its description and an example of its use with Frame
Relay.
Stations must be able to map more than one IP address in the same IP
subnet (CIDR address prefix) to a particular DLCI on a Frame Relay
interface. This need arises from applications such as remote access,
where servers must act as ARP proxies for many dial-in clients, each
assigned a unique IP address while sharing bandwidth on the same DLC.
The dynamic nature of such applications result in frequent address
association changes with no affect on the DLC"s status as reported by
Frame Relay PVC Status Signaling.
As with any other interface that utilizes ARP, stations may learn the
associations between IP addresses and DLCIs by processing unsolicited
("gratuitous") ARP requests that arrive on the DLC. If one station
(perhaps a terminal server or remote access server) wishes to inform
its peer station on the other end of a Frame Relay DLC of a new
association between an IP address and that PVC, it should send an
unsolicited ARP request with the source IP address equal to the
destination IP address, and both set to the new IP address being used
on the DLC. This allows a station to "announce" new client
connections on a particular DLCI. The receiving station must store
the new association, and remove any old existing association, if
necessary, from any other DLCI on the interface.
7. IP over Frame Relay
Internet Protocol [9] (IP) datagrams sent over a Frame Relay network
conform to the encapsulation described previously. Within this
context, IP could be encapsulated in two different ways.
1. NLPID value indicating IP
+-----------------------+-----------------------+
Q.922 Address
+-----------------------+-----------------------+
Control (UI) 0x03 NLPID 0xCC
+-----------------------+-----------------------+
IP packet
.
.
.
+-----------------------+-----------------------+
2. NLPID value indicating SNAP
+-----------------------+-----------------------+
Q.922 Address
+-----------------------+-----------------------+
Control (UI) 0x03 pad 0x00
+-----------------------+-----------------------+
NLPID 0x80 SNAP Header
+-----------------------+ OUI = 0x00-00-00 + Indicating
IP
+-----------------------+-----------------------+
PID 0x0800
+-----------------------+-----------------------+
IP packet
.
.
.
+-----------------------+-----------------------+
Although both of these encapsulations are supported under the given
definitions, it is advantageous to select only one method as the
appropriate mechanism for encapsulating IP data. Therefore, IP data
shall be encapsulated using the NLPID value of 0xCC indicating IP as
shown in option 1 above. This (option 1) is more efficient in
transmission (48 fewer bits), and is consistent with the
encapsulation of IP in X.25.
8. Other Protocols over Frame Relay
As with IP encapsulation, there are alternate ways to transmit
various protocols within the scope of this definition. To eliminate
the conflicts, the SNAP encapsulation is only used if no NLPID value
is defined for the given protocol.
As an example of how this works, ISO CLNP has a NLPID defined (0x81).
Therefore, the NLPID field will indicate ISO CLNP and the data packet
will follow immediately. The frame would be as follows:
+---------------------------------------------+
Q.922 Address
+----------------------+----------------------+
Control (UI) 0x03 NLPID 0x81 (CLNP)
+----------------------+----------------------+
remainder of CLNP packet
.
.
+---------------------------------------------+
In this example, the NLPID is used to identify the data packet as
CLNP. It is also considered part of the CLNP packet and as such, the
NLPID should not be removed before being sent to the upper layers for
processing. The NLPID is not duplicated.
Other protocols, such as IPX, do not have a NLPID value defined. As
mentioned above, IPX would be encapsulated using the SNAP header. In
this case, the frame would be as follows:
+---------------------------------------------+
Q.922 Address
+----------------------+----------------------+
Control (UI) 0x03 pad 0x00
+----------------------+----------------------+
NLPID 0x80 (SNAP) OUI - 0x00 00 00
+----------------------+

+---------------------------------------------+
PID 0x8137
+---------------------------------------------+
IPX packet
.
.
+---------------------------------------------+
9. Bridging Model for Frame Relay
The model for bridging in a Frame Relay network is identical to the
model for remote bridging as described in IEEE P802.1g "Remote MAC
Bridging" [13] and supports the concept of "Virtual Ports". Remote
bridges with LAN ports receive and transmit MAC frames to and from
the LANs to which they are attached. They may also receive and
transmit MAC frames through virtual ports to and from other remote
bridges. A virtual port may represent an abstraction of a remote
bridge"s point of access to one, two or more other remote bridges.
Remote Bridges are statically configured as members of a remote
bridge group by management. All members of a remote bridge group are
connected by one or more virtual ports. The set of remote MAC bridges
in a remote bridge group provides actual or *potential* MAC layer
interconnection between a set of LANs and other remote bridge groups
to which the remote bridges attach.
In a Frame Relay network there must be a full mesh of Frame Relay VCs
between bridges of a remote bridge group. If the frame relay network
is not a full mesh, then the bridge network must be divided into
multiple remote bridge groups.
The frame relay VCs that interconnect the bridges of a remote bridge
group may be combined or used individually to form one or more
virtual bridge ports. This gives flexibility to treat the Frame
Relay interface either as a single virtual bridge port, with all VCs
in a group, or as a collection of bridge ports (individual or grouped
VCs).
When a single virtual bridge port provides the interconnectivity for
all bridges of a given remote bridge group (i.e. all VCs are combined
into a single virtual port), the standard Spanning Tree Algorithm may
be used to determine the state of the virtual port. When more than
one virtual port is configured within a given remote bridge group
then an "extended" Spanning Tree Algorithm is required. Such an
extended algorithm is defined in IEEE 802.1g [13]. The operation of
this algorithm is such that a virtual port is only put into backup if
there is a loop in the network external to the remote bridge group.
The simplest bridge configuration for a Frame Relay network is the
LAN view where all VCs are combined into a single virtual port.
Frames, such as BPDUs, which would be broadcast on a LAN, must be
flooded to each VC (or multicast if the service is developed for
Frame Relay services). Flooding is performed by sending the packet to
each relevant DLC associated with the Frame Relay interface. The VCs
in this environment are generally invisible to the bridge. That is,
the bridge sends a flooded frame to the frame relay interface and
does not "see" that the frame is being forwarded to each VC
individually. If all participating bridges are fully connected (full
mesh) the standard Spanning Tree Algorithm will suffice in this
configuration.
Typically LAN bridges learn which interface a particular end station
may be reached on by associating a MAC address with a bridge port.
In a Frame Relay network configured for the LAN-like single bridge
port (or any set of VCs grouped together to form a single bridge
port), however, the bridge must not only associated a MAC address
with a bridge port, but it must also associate it with a connection
identifier. For Frame Relay networks, this connection identifier is
a DLCI. It is unreasonable and perhaps impossible to require bridges
to statically configure an association of every possible destination
MAC address with a DLC. Therefore, Frame Relay LAN-modeled bridges
must provide a mechanism to allow the Frame Relay bridge port to
dynamically learn the associations. To accomplish this dynamic
learning, a bridged packet shall conform to the encapsulation
described within section 4.2. In this way, the receiving Frame Relay
interface will know to look into the bridged packet to gather the
appropriate information.
A second Frame Relay bridging approach, the point-to-point view,
treats each Frame Relay VC as a separate bridge port. Flooding and
forwarding packets are significantly less complicated using the
point-to-point approach because each bridge port has only one
destination. There is no need to perform artificial flooding or to
associate DLCIs with destination MAC addresses. Depending upon the
interconnection of the VCs, an extended Spanning Tree algorithm may
be required to permit all virtual ports to remain active as long as
there are no true loops in the topology external to the remote bridge
group.
It is also possible to combine the LAN view and the point-to-point
view on a single Frame Relay interface. To do this, certain VCs are
combined to form a single virtual bridge port while other VCs are
independent bridge ports.
The following drawing illustrates the different possible bridging
configurations. The dashed lines between boxes represent virtual
circuits.
+-------+
------------------- B
/ -------
/ / +-------+
/
+-------+/ +-------+
A ------- C
-----------------------
+-------+ +-------+

+-------+
D
-------------------
+-------+
Since there is less than a full mesh of VCs between the bridges in
this example, the network must be divided into more than one remote
bridge group. A reasonable configuration is to have bridges A, B,
and C in one group, and have bridges A and D in a second.
Configuration of the first bridge group combines the VCs
interconnection the three bridges (A, B, and C) into a single virtual
port. This is an example of the LAN view configuration. The second
group would also be a single virtual port which simply connects
bridges A and D. In this configuration the standard Spanning Tree
Algorithm is sufficient to detect loops.
An alternative configuration has three individual virtual ports in
the first group corresponding to the VCs interconnecting bridges A, B
and C. Since the application of the standard Spanning Tree Algorithm
to this configuration would detect a loop in the topology, an
extended Spanning Tree Algorithm would have to be used in order for
all virtual ports to be kept active. Note that the second group
would still consist of a single virtual port and the standard
Spanning Tree Algorithm could be used in this group.
Using the same drawing, one could construct a remote bridge scenario
with three bridge groups. This would be an example of the point-to-
point case. Here, the VC connecting A and B, the VC connecting A and
C, and the VC connecting A and D are all bridge groups with a single
virtual port.
10. Security Considerations
This document defines mechanisms for identifying the multiprotocol
encapsulation of datagrams over Frame Relay. There is obviously an
element in trust in any encapsulation protocol - a receiver must
trust that the sender has correctly identified the protocol being
encapsulated. In general, there is no way for a receiver to try to
ascertain that the sender did indeed use the proper protocol
identification, nor would this be desired functionality.
It also specifies the use of ARP and RARP with Frame Relay, and is
subject to the same security constraints that affect ARP and similar
address resolution protocols. Because authentication is not a part
of ARP, there are known security issues relating to its use (e.g.,
host impersonation). No additional security mechanisms have been
added to ARP or RARP for use with Frame Relay networks.
11. Appendix A - NLPIDS and PIDs
List of Commonly Used NLPIDs
0x00 Null Network Layer or Inactive Set
(not used with Frame Relay)
0x08 Q.933 [2]
0x80 SNAP
0x81 ISO CLNP
0x82 ISO ESIS
0x83 ISO ISIS
0x8E IPv6
0xB0 FRF.9 Data Compression [14]
0xB1 FRF.12 Fragmentation [18]
0xCC IPv4
0xCF PPP in Frame Relay [17]
List of PIDs of OUI 00-80-C2
with preserved FCS w/o preserved FCS Media
------------------ ----------------- --------------
0x00-01 0x00-07 802.3/Ethernet
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A FDDI
0x00-0B 802.6
0x00-0D Fragments
0x00-0E BPDUs as defined by
802.1(d) or
802.1(g)[12].
0x00-0F Source Routing BPDUs
12. Appendix B - Connection Oriented Procedures
This Appendix contains additional information and instructions for
using ITU Recommendation Q.933 [2] and other ITU standards for
encapsulating data over frame relay. The information contained here
is similar (and in some cases identical) to that found in Annex E to
ITU Q.933. The authoritative source for this information is in Annex
E and is repeated here only for convenience.
The Network Level Protocol ID (NLPID) field is administered by ISO
and the ITU. It contains values for many different protocols
including IP, CLNP (ISO 8473), ITU Q.933, and ISO 8208. A figure
summarizing a generic encapsulation technique over frame relay
networks follows. The scheme"s flexibility consists in the
identification of multiple alternative to identify different
protocols used either by
- end-to-end systems or
- LAN to LAN bride and routers or
- a combination of the above.
over frame relay networks.
Q.922 control


--------------------------------------------

UI I Frame

--------------------------------- --------------
0x08 0x81 0xCC 0x80 ..01.... ..10....

Q.933 CLNP IP SNAP ISO 8208 ISO 8208
Modulo 8 Modulo 128

-------------------- OUI

L2 ID L3 ID -------
User
Specified
0x70 802.3 802.6

---------------------------
0x51 0x4E 0x4C 0x50

7776 Q.922 Others 802.2 User
Specified
For those protocols which do not have a NLPID assigned or do not have
a SNAP encapsulation, the NLPID value of 0x08, indicating ITU
Recommendation Q.933 should be used. The four octets following the
NLPID include both layer 2 and layer 3 protocol identification. The
code points for most protocols are currently defined in ITU Q.933 low
layer compatibility information element. The code points for "User
Specified" are described in Frame Relay Forum FRF.3.1 [15]. There is
also an escape for defining non-standard protocols.
Format of Other Protocols
using Q.933 NLPID
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 NLPID 0x08
+---------------+---------------+
L2 Protocol ID
octet 1 octet 2
+---------------+---------------+
L3 Protocol ID
octet 1 octet 2
+---------------+---------------+
Protocol Data
+-------------------------------+
FCS
+-------------------------------+
ISO 8802/2 with user specified
layer 3
+-------------------------------+
Q.922 Address
+---------------+---------------+
Control 0x03 NLPID 0x08
+---------------+---------------+
802/2 0x4C 0x80
+---------------+---------------+
User Spec. 0x70 Note 1
+---------------+---------------+
DSAP SSAP
+---------------+---------------+
Control (Note 2)
+-------------------------------+
Remainder of PDU
+-------------------------------+
FCS
+-------------------------------+
Note 1: Indicates the code point for user specified
layer 3 protocol.
Note 2: Control field is two octets for I-format and
S-format frames (see 88002/2)
Encapsulations using I frame (layer 2)
The Q.922 I frame is for supporting layer 3 protocols which require
acknowledged data link layer (e.g., ISO 8208). The C/R bit will be
used for command and response indications.
Format of ISO 8208 frame
Modulo 8
+-------------------------------+
Q.922 Address
+---------------+---------------+
....Control I frame
+---------------+---------------+
8208 packet (modulo 8) Note 3

+-------------------------------+
FCS
+-------------------------------+
Note 3: First octet of 8208 packet also identifies the
NLPID which is "..01....".
Format of ISO 8208 frame
Modulo 128
+-------------------------------+
Q.922 Address
+---------------+---------------+
....Control I frame
+---------------+---------------+
8208 packet (modulo 128)
Note 4
+-------------------------------+
FCS
+-------------------------------+
Note 4: First octet of 8208 packet also identifies the
NLPID which is "..10....".
13. Appendix C - Modifications from RFC1490
RFC1490 has been widely implemented and used, and has been adopted
by the Frame Relay Forum in FRF.3.1 [15] and by the ITU in Q.933 [2].
This section describes updates to RFC1490 that have been made as a
result of this implementation and interoperability experience, and
which reflect current implementation practice.
Some language changes were necessary to clarify RFC1490. None of
these changes impacted the technical aspects of this document, but
were required to keep diagrams and language specific and consistent.
Specifics of these changes will not be listed here. Below are listed
those changes which were significant.
a) The requirement for stations to accept SNAP encapsulated protocols
for which a NLPID was available, was removed. RFC1490 indicated
that, if a protocol, such as IP, had a designated NLPID value, it
must be used. Later the document required stations to accept a
SNAP encapsulated version of this same protocol. This is clearly
inconsistent. A compliant station must send and accept the NLPID
encapsulated version of such a protocol. It MAY accept the SNAP
encapsulation but should not be required to do so as these frames
are noncompliant.
b) Fragmentation was removed. To date there are no interoperable
implementations of the fragmentation algorithm presented in RFC
1490. Additionally, there have been several suggestions that the
proposed mechanisms are insufficient for some frame relay
applications. To this end, fragmentation was removed from this
document, and has been replaced by the fragmentation specified in
FRF.12 [18].
c) The address resolution presented in RFC1490 referred only to PVC
environments and is insufficient for SVC environments. Therefore
the section title was changed to reflect this. Further work on
SVC address resolution will take place in the ION working group.
d) The encapsulation for Source Routing BPDUs was added, and the
lists in Appendix A were augmented.
e) The use of canonical and non-canonical MAC destination addresses
in the bridging encapsulations was clarified.
f) The Inverse ARP description was moved to the Inverse ARP
specification [11].
g) A new security section was added.
14. References
[1] International Telecommunication Union, "ISDN Data Link Layer
Specification for Frame Mode Bearer Services", ITU-T
Recommendation Q.922, 1992.
[2] International Telecommunication Union, "Signalling Specifications
for Frame Mode Switched and Permanent Virtual Connection Control
and Status Monitoring", ITU-T Recommendation Q.933, 1995.
[3] Information technology - Telecommunications and Information
Exchange between systems - Protocol Identification in the Network
Layer, ISO/IEC TR 9577: 1992.
[4] Baker, F., and R. Bowen, "PPP Bridging Control Protocol (BCP)",
RFC1638, June 1994.
[5] International Standard, Information Processing Systems - Local
Area Networks - Logical Link Control, ISO 8802-2, ANSI/IEEE,
Second Edition, 1994-12-30.
[6] Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit Ethernet Address
for Transmission on Ethernet Hardware", STD 37, RFC826, November
1982.
[7] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1700,
October 1994. See also: http://www.iana.org/numbers.Html
[8] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse
Address Resolution Protocol", STD 38, RFC903, June 1984.
[9] Postel, J., and J. Reynolds, "A Standard for the Transmission of
IP Datagrams over IEEE 802 Networks", RFC1042, February 1988.
[10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks:
Overview and architecture", IEEE Standard 802-1990.
[11] Bradley, T., Brown, C., and A. Malis, "Inverse Address
Resolution Protocol", RFC2390, September 1998.
[12] IEEE, "IEEE Standard for Local and Metropolitan Networks: Media
Access Control (MAC) Bridges", IEEE Standard 802.1D-1990.
[13] ISO/IEC 15802-5 : 1998 (IEEE Standard 802.1G), Remote Media
Access Control (MAC) Bridging, March 12, 1997.
[14] Frame Relay Forum, "Data Compression Over Frame Relay
Implementation Agreement", FRF.9, January 22, 1996.
[15] Frame Relay Forum, "Multiprotocol Encapsulation Implementation
Agreement", FRF.3.1, June 22, 1995.
[16] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[17] Simpson, W., "PPP in Frame Relay", RFC1973, June 1996.
[18] Frame Relay Forum, "Frame Relay Fragmentation Implementation
Agreement", FRF.12, December 1997.
[19] Frame Relay Forum, "Frame Relay PVC Multicast Service and
Protocol Implementation Agreement", FRF.7, October 21, 1994.
15. Authors" Addresses
Caralyn Brown
Consultant
EMail: cbrown@juno.com
Andrew Malis
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01886
Phone: (978) 952-7414
EMail: malis@ascend.com
16. Full Copyright Statement
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