As per Relevance of the word hardware, we have this rfc below:
Network Working Group T.
Request for Comments: 1490 Wellfleet Communications, Inc
Obsoletes: 1294 C.
Wellfleet Communications, Inc
A.
Ascom Timeplex, Inc
July 1993
Multiprotocol Interconnect over Frame
Status of this
This RFC specifies an IAB standards track protocol for the
community, and requests discussion and suggestions for improvements
Please refer to the current edition of the "IAB Official
Standards" for the standardization state and status of this protocol
Distribution of this memo is unlimited
This memo describes an encapsulation method for carrying
interconnect traffic over a Frame Relay backbone. It covers
of both Bridging and Routing. Additionally, it describes a
fragmentation procedure for carrying large frames over a frame
network with a smaller MTU
Systems with the ability to transfer both the encapsulation
described in this document, and others must have a priori
of which virtual circuits will carry which encapsulation method
this encapsulation must only be used over virtual circuits that
been explicitly configured for its use
Comments and contributions from many sources, especially those
Ray Samora of Proteon, Ken Rehbehn of Netrix Corporation, Fred
and Charles Carvalho of Advanced Computer Communications and
Sherif of AT&T have been incorporated into this document.
thanks to Dory Leifer of University of Michigan for his
to the resolution of fragmentation issues and Floyd Backes from
and Laura Bridge from Timeplex for their contributions to
bridging descriptions. This document could not have been
without the expertise of the IP over Large Public Data
working group of the IETF
Bradley, Brown & Malis [Page 1]
RFC 1490 Multiprotocol over Frame Relay July 1993
1. Conventions and
The following language conventions are used in the items
specification in this document
o Must, Shall or Mandatory -- the item is an
requirement of the specification
o Should or Recommended -- the item should generally
followed for all but exceptional circumstances
o May or Optional -- the item is truly optional and may
followed or ignored according to the needs of
implementor
All drawings in this document are drawn with the left-most bit as
high order bit for transmission. For example, the dawings might
labeled as
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+
+---------------------------+
| flag (7E hexadecimal) |
+---------------------------+
| Q.922 Address* |
+-- --+
| |
+---------------------------+
: :
: :
+---------------------------+
Drawings that would be too large to fit onto one page if each
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
transmission. There will be a "+" to distinguish between octets
in the following example
Bradley, Brown & Malis [Page 2]
RFC 1490 Multiprotocol over Frame Relay July 1993
|--- 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
BPDU - Bridge Protocol Data
C/R - Command/Response
DCE - Data Communication
DE - Discard Eligibility
DTE - Data Terminal
FECN - Forward Explicit Congestion
PDU - Protocol Data
PTT - Postal Telephone &
SNAP - Subnetwork Access
2.
The following discussion applies to those devices which serve as
stations (DTEs) on a public or private Frame Relay network (
example, provided by a common carrier or PTT. It will not
the behavior of those stations that are considered a part of
Frame Relay network (DCEs) other than to explain situations in
the DTE must react
The Frame Relay network provides a number of virtual circuits
form the basis for connections between stations attached to the
Frame Relay network. The resulting set of interconnected
forms a private Frame Relay group which may be either
interconnected with a complete "mesh" of virtual circuits, or
partially interconnected. In either case, each virtual circuit
uniquely identified at each Frame Relay interface by a Data
Connection Identifier (DLCI). In most circumstances, DLCIs
strictly local significance at each Frame Relay interface
The specifications in this document are intended to apply to
switched and permanent virtual circuits
Bradley, Brown & Malis [Page 3]
RFC 1490 Multiprotocol over Frame Relay July 1993
3. Frame
All protocols must encapsulate their packets within a Q.922 Annex
frame [1,2]. Additionally, frames shall contain
necessary to identify the protocol carried within the protocol
unit (PDU), thus allowing the receiver to properly process
incoming packet. The format shall be as follows
+---------------------------+
| flag (7E hexadecimal) |
+---------------------------+
| Q.922 Address* |
+-- --+
| |
+---------------------------+
| Control (UI = 0x03) |
+---------------------------+
| Optional Pad (0x00) |
+---------------------------+
| NLPID |
+---------------------------+
| . |
| . |
| . |
| Data |
| . |
| . |
+---------------------------+
| Frame Check Sequence |
+-- . --+
| (two octets) |
+---------------------------+
| flag (7E hexadecimal) |
+---------------------------+
* Q.922 addresses, as presently defined, are two octets
contain a 10-bit DLCI. In some networks Q.922
may optionally be increased to three or four octets
The control field is the Q.922 control field. The UI (0x03) value
used unless it is negotiated otherwise. The use of XID (0xAF
0xBF) is permitted and is discussed later
The pad field is used to align the remainder of the frame to a
octet boundary. There may be zero or one pad octet within the
field and, if present, must have a value of zero
The Network Level Protocol ID (NLPID) field is administered by
Bradley, Brown & Malis [Page 4]
RFC 1490 Multiprotocol over Frame Relay July 1993
and CCITT. It contains values for many different protocols
IP, CLNP and IEEE Subnetwork Access Protocol (SNAP)[10]. This
tells the receiver what encapsulation or what protocol follows
Values for this field are defined in ISO/IEC TR 9577 [3]. A
value of 0x00 is defined within ISO/IEC TR 9577 as the Null
Layer or Inactive Set. Since it cannot be distinguished from a
field, and because it has no significance within the context of
encapsulation scheme, a NLPID value of 0x00 is invalid under
Frame Relay encapsulation. The Appendix contains a list of some
the more commonly used NLPID values
There is no commonly implemented minimum maximum frame size for
Relay. A network must, however, support at least a 262
maximum. Generally, the maximum will be greater than or equal
1600 octets, but each Frame Relay provider will specify
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
the opening and closing flags assuming a two octet Q.922
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
There are two basic types of data packets that travel within
Frame Relay network: routed packets and bridged packets.
packets have distinct formats and therefore, must contain
indicator that the destination may use to correctly interpret
contents of the frame. This indicator is embedded within the
and SNAP header information
For those protocols that do not have a NLPID already assigned, it
necessary to provide a mechanism to allow easy
identification. There is a NLPID value defined indicating
presence of a SNAP header
A SNAP header is of the form
+--------------------------------------------+
| Organizationally Unique |
+-- +--------------------+
| Identifier | Protocol |
+-----------------------+--------------------+
| Identifier |
+-----------------------+
All stations must be able to accept and properly interpret both
Bradley, Brown & Malis [Page 5]
RFC 1490 Multiprotocol over Frame Relay July 1993
NLPID encapsulation and the SNAP header encapsulation for a
packet
The three-octet Organizationally Unique Identifier (OUI)
an organization which administers the meaning of the
Identifier (PID) which follows. Together they identify a
protocol. Note that OUI 0x00-00-00 specifies that the following
is an Ethertype
4.1. Routed
Some protocols will have an assigned NLPID, but because the
numbering space is so limited, not all protocols have specific
values assigned to them. When packets of such protocols are
over Frame Relay networks, they are sent using the NLPID 0x80 (
indicates a SNAP follows) followed by SNAP. If the protocol has
Ethertype assigned, the OUI is 0x00-00-00 (which indicates
Ethertype follows), and PID is the Ethertype of the protocol in use
There will be one pad octet to align the protocol data on a two
boundary as shown below
Format of Routed
with
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
|Control 0x03 | pad 0x00 |
+---------------+---------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x00-00 |
+-------------------------------+
| Ethertype |
+-------------------------------+
| Protocol Data |
+-------------------------------+
| FCS |
+-------------------------------+
In the few cases when a protocol has an assigned NLPID (
appendix), 48 bits can be saved using the format below
Bradley, Brown & Malis [Page 6]
RFC 1490 Multiprotocol over Frame Relay July 1993
Format of Routed NLPID
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
|Control 0x03 | NLPID |
+---------------+---------------+
| Protocol Data |
+-------------------------------+
| FCS |
+-------------------------------+
The NLPID encapsulation does not require a pad octet for alignment
so none is permitted
In the case of ISO protocols, the NLPID is considered to be the
octet of the protocol data. It is unnecessary to repeat the NLPID
this case. The single octet serves both as the demultiplexing
and as part of the protocol data (refer to "Other Protocols
Frame Relay for more details). Other protocols, such as IP, have
NLPID defined (0xCC), but it is not part of the protocol itself
Format of Routed IP
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
|Control 0x03 | NLPID 0xCC |
+---------------+---------------+
| IP Datagram |
+-------------------------------+
| FCS |
+-------------------------------+
4.2. Bridged
The second type of Frame Relay traffic is bridged packets.
packets are encapsulated using the NLPID value of 0x80
SNAP. As with other SNAP encapsulated protocols, there will be
pad octet to align the data portion of the encapsulated frame.
SNAP header which follows the NLPID identifies the format of
bridged packet. The OUI value used for this encapsulation is
802.1 organization code 0x00-80-C2. The PID portion of the
header (the two bytes immediately following the OUI) specifies
form of the MAC header, which immediately follows the SNAP header
Additionally, the PID indicates whether the original FCS is
within the bridged frame
The 802.1 organization has reserved the following values to be
with Frame Relay
Bradley, Brown & Malis [Page 7]
RFC 1490 Multiprotocol over Frame Relay July 1993
PID Values for OUI 0x00-80-C
with preserved FCS w/o preserved FCS
------------------ ----------------- ----------------
0x00-01 0x00-07 802.3/
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A
0x00-0B 802.6
In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2,
identifies bridged protocol data units (BPDUs) as defined
802.1(d) or 802.1(g) [12].
A packet bridged over Frame Relay will, therefore, have one of
following formats
Format of Bridged Ethernet/802.3
+-------------------------------+
| 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 |
+-------------------------------+
Bradley, Brown & Malis [Page 8]
RFC 1490 Multiprotocol over Frame Relay July 1993
Format of Bridged 802.4
+-------------------------------+
| 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 |
+-------------------------------+
Bradley, Brown & Malis [Page 9]
RFC 1490 Multiprotocol over Frame Relay July 1993
Format of Bridged 802.5
+-------------------------------+
| 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 |
+-------------------------------+
Bradley, Brown & Malis [Page 10]
RFC 1490 Multiprotocol over Frame Relay July 1993
Format of Bridged FDDI
+-------------------------------+
| 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 |
+-------------------------------+
Bradley, Brown & Malis [Page 11]
RFC 1490 Multiprotocol over Frame Relay July 1993
Format of Bridged 802.6
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
| Control 0x03 | pad 0x00 |
+---------------+---------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+ --+
| OUI 0x80-C2 |
+-------------------------------+
| PID 0x00-0B |
+---------------+---------------+ -------
| Reserved | BEtag |
+---------------+---------------+
| BAsize |
+-------------------------------+ -------
| 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
value, since the presence of a CRC-32 is indicated by the CIB bit
the header of the MAC frame
The Common Protocol Data Unit (CPDU) Header and Trailer are
to allow pipelining at the egress bridge to an 802.6 subnetwork
Specifically, the CPDU Header contains the BAsize field,
contains the length of the PDU. If this field is not available
the egress 802.6 bridge, then that bridge cannot begin to
the segmented PDU until it has received the entire PDU,
the length, and inserted the length into the BAsize field. If
field is available, the egress 802.6 bridge can extract the
from the BAsize field of the Common PDU Header, insert it into
corresponding field of the first segment, and immediately
the segment onto the 802.6 subnetwork. Thus, the bridge can
transmitting the 802.6 PDU before it has received the complete PDU
One should note that the Common PDU Header and Trailer of
encapsulated frame should not be simply copied to the outgoing 802.6
Bradley, Brown & Malis [Page 12]
RFC 1490 Multiprotocol over Frame Relay July 1993
subnetwork because the encapsulated BEtag value may conflict with
previous BEtag value transmitted by that bridge
Format of BPDU
+-------------------------------+
| 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] |
| |
+-------------------------------+
4. Data Link Layer Parameter
Frame Relay stations may choose to support the
Identification (XID) specified in Appendix III of Q.922 [1].
XID exchange allows the following parameters to be negotiated at
initialization of a Frame Relay circuit: maximum frame size N201,
retransmission timer T200, and the maximum number of
Information (I) frames K
A station may indicate its unwillingness to support acknowledged
multiple frame operation by specifying a value of zero for
maximum window size, K
If this exchange is not used, these values must be
configured by mutual agreement of Data Link Connection (DLC
endpoints, or must be defaulted to the values specified in
5.9 of Q.922:
Bradley, Brown & Malis [Page 13]
RFC 1490 Multiprotocol over Frame Relay July 1993
N201: 260
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
T200: 1.5 seconds [see Q.922 for further details
If a station supporting XID receives an XID frame, it shall
with an XID response. In processing an XID, if the remote
frame size is smaller than the local maximum, the local system
reduce the maximum size it uses over this DLC to the
specified value. Note that this shall be done before generating
response XID
The following diagram describes the use of XID to specify non-use
acknowledged mode multiple frame operation
Bradley, Brown & Malis [Page 14]
RFC 1490 Multiprotocol over Frame Relay July 1993
Non-use of Acknowledged Mode Multiple Frame
+---------------+
| 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
+---------------+
| 0x01 | PL = 1
+---------------+
| 0x00 |
+---------------+
| 0x09 | PI = Retransmission
+---------------+
| 0x01 | PL = 1
+---------------+
| 0x00 |
+---------------+
| FCS | (2 octets
| |
+---------------+
6. Fragmentation
Fragmentation allows the exchange of packets that are greater
the maximum frame size supported by the underlying network. In
Bradley, Brown & Malis [Page 15]
RFC 1490 Multiprotocol over Frame Relay July 1993
case of Frame Relay, the network may support a maximum frame size
small as 262 octets. Because of this small maximum size, it
recommended, but not required, to support fragmentation
reassembly
Unlike IP fragmentation procedures, the scope of Frame
fragmentation procedure is limited to the boundary (or DTEs) of
Frame Relay network
The general format of fragmented packets is the same as any
encapsulated protocol. The most significant difference being
the fragmented packet will contain the encapsulation header.
is, a packet is first encapsulated (with the exception of the
and control fields) as defined above. Large packets are then
up into frames appropriate for the given Frame Relay network and
encapsulated using the Frame Relay fragmentation format. In
way, a station receiving fragments may reassemble them and then
the reassembled packet through the same processing path as a
that had not been fragmented
Within Frame Relay fragments are encapsulated using the SNAP
with an OUI of 0x00-80-C2 and a PID of 0x00-0D. Individual
will, therefore, have the following format
+---------------+---------------+
| Q.922 Address |
+---------------+---------------+
| Control 0x03 | pad 0x00 |
+---------------+---------------+
| NLPID 0x80 | OUI 0x00 |
+---------------+---------------+
| OUI 0x80-C2 |
+---------------+---------------+
| PID 0x00-0D |
+---------------+---------------+
| sequence number |
+-+-------+-----+---------------+
|F| RSVD |offset |
+-+-------+-----+---------------+
| fragment data |
| . |
| . |
| . |
+---------------+---------------+
| FCS |
+---------------+---------------+
The sequence field is a two octet identifier that is
Bradley, Brown & Malis [Page 16]
RFC 1490 Multiprotocol over Frame Relay July 1993
every time a new complete message is fragmented. It allows
of lost frames and is set to a random value at initialization
The reserved field is 4 bits long and is not currently defined.
must be set to 0.
The final bit is a one bit field set to 1 on the last fragment
set to 0 for all other fragments
The offset field is an 11 bit value representing the logical
of this fragment in bytes divided by 32. The first fragment must
an offset of zero
The following figure shows how a large IP datagram is fragmented
Frame Relay. In this example, the complete datagram is
into two Frame Relay frames
Bradley, Brown & Malis [Page 17]
RFC 1490 Multiprotocol over Frame Relay July 1993
Frame Relay Fragmentation
+-----------+-----------+
| Q.922 Address |
+-----------+-----------+
| Ctrl 0x03 | pad 0x00 |
+-----------+-----------+
|NLPID 0x80 | OUI 0x00 |
+-----------+-----------+
| OUI 0x80-C2 |
+-----------+-----------+ +-----------+-----------+
|ctrl 0x03 |NLPID 0xCC | | PID 0x00-0D |
+-----------+-----------+ +-----------+-----------+
| | | sequence number n |
| | +-+------+--+-----------+
| | |0| RSVD |offset (0) |
| | +-+------+--+-----------+
| | | ctrl 0x03 |NLPID 0xCC |
| | +-----------+-----------+
| | | first m bytes of |
| large IP datagram | ... | IP datagram |
| | | |
| | +-----------+-----------+
| | | FCS |
| | +-----------+-----------+
| |
| | +-----------+-----------+
| | | Q.922 Address |
| | +-----------+-----------+
| | | Ctrl 0x03 | pad 0x00 |
+-----------+-----------+ +-----------+-----------+
|NLPID 0x80 | OUI 0x00 |
+-----------+-----------+
| OUI 0x80-C2 |
+-----------+-----------+
| PID 0x00-0D |
+-----------+-----------+
| sequence number n |
+-+------+--+-----------+
|1| RSVD |offset (m/32) |
+-+------+--+-----------+
| remainder of IP |
| datagram |
+-----------+-----------+
| FCS |
+-----------+-----------+
Fragments must be sent in order starting with a zero offset
ending with the final fragment. These fragments must not
Bradley, Brown & Malis [Page 18]
RFC 1490 Multiprotocol over Frame Relay July 1993
interrupted with other packets or information intended for the
DLC. An end station must be able to re-assemble up to 2K octets
is suggested to support up to 8K octet re-assembly. If at any
during this re-assembly process, a fragment is corrupted or
fragment is missing, the entire message is dropped. The upper
protocol is responsible for any retransmission in this case.
that there is no reassembly timer, nor is one needed. This
because the Frame Relay service is required to deliver frames
order
This fragmentation algorithm is not intended to reliably handle
possible failure conditions. As with IP fragmentation, there is
small possibility of reassembly error and delivery of an
packet. Inclusion of a higher layer checksum greatly reduces
risk
7. Address
There are situations in which a Frame Relay station may wish
dynamically resolve a protocol address. Address resolution may
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
+-----------------------+ OUI = 0x00-00-00 +
| |
+-----------------------+-----------------------+
| PID = 0x0806 |
+-----------------------+-----------------------+
| ARP packet |
| . |
| . |
| . |
+-----------------------+-----------------------+
Where the ARP packet has the following format and values
Data
ar$hrd 16 bits Hardware
ar$pro 16 bits Protocol
ar$hln 8 bits Octet length of hardware address (n
Bradley, Brown & Malis [Page 19]
RFC 1490 Multiprotocol over Frame Relay July 1993
ar$pln 8 bits Octet length of protocol address (m
ar$op 16 bits Operation code (request or reply
ar$sha noctets source hardware
ar$spa moctets source protocol
ar$tha noctets target hardware
ar$tpa moctets target protocol
ar$hrd - assigned to Frame Relay is 15
(0x000F) [7].
ar$pro - see assigned numbers for protocol ID number
the protocol using ARP. (IP is 0x0800).
ar$hln - length in bytes of the address field (2, 3, or 4)
ar$pln - protocol address length is dependent on
protocol (ar$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
significance, an end station will not have a specific DLCI
to itself. Therefore, such a station does not have an address to
into the ARP request or reply. Fortunately, the Frame Relay
does provide a method for obtaining the correct DLCIs. The
proposed for the locally addressed Frame Relay network below
work equally well for a network where DLCIs have global significance
The DLCI carried within the Frame Relay header is modified as
traverses the network. When the packet arrives at its destination
the DLCI has been set to the value that, from the standpoint of
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
received this message, however, the DLCI would have been modified
the network and would appear to B as DLCI 70.
Bradley, Brown & Malis [Page 20]
RFC 1490 Multiprotocol over Frame Relay July 1993
~~~~~~~~~~~~~~~
( )
+-----+ ( ) +-----+
| |-50------(--------------------)---------70-| |
| A | ( ) | B |
| |-60-----(---------+ ) | |
+-----+ ( | ) +-----+
( | )
( | ) <---Frame
~~~~~~~~~~~~~~~~
80
|
+-----+
| |
| C |
| |
+-----+
Figure 1
Lines between stations represent data link connections (DLCs).
The numbers indicate the local DLCI associated with
connection
DLCI to Q.922 Address Table for Figure 1
DLCI (decimal) Q.922 address (hex
50 0x0C21
60 0x0CC
70 0x1061
80 0x1401
If you know about frame relay, you should understand
correlation between DLCI and Q.922 address. For the uninitiated
the translation between DLCI and Q.922 address is based on a
byte address length using the Q.922 encoding format. The
is
8 7 6 5 4 3 2 1
+------------------------+---+--+
| DLCI (high order) |c/r|ea
+--------------+----+----+---+--+
| DLCI (lower) |FECN|BECN|DE |EA
+--------------+----+----+---+--+
For ARP and its variants, the FECN, BECN, C/R and DE bits
assumed to be 0.
When an ARP message reaches a destination, all hardware
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RFC 1490 Multiprotocol over Frame Relay July 1993
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
address. It should also be noted that the target hardware address
in both ARP request and reply, will also be invalid. This should
cause problems since ARP does not rely on these fields and in fact
an implementation may zero fill or ignore the target hardware
field entirely
As an example of how this address replacement scheme may work,
to figure 1. If station A (protocol address pA) wished to
the address of station B (protocol address pB), it would format
ARP request with the following values
ARP request from
ar$op 1 (request
ar$sha
ar$spa
ar$tha
ar$tpa
Because station A will not have a source address associated with it
the source hardware address field is not valid. Therefore, when
ARP packet is received, it must extract the correct address from
Frame Relay header and place it in the source hardware address field
This way, the ARP request from A will become
ARP request from A as modified by
ar$op 1 (request
ar$sha 0x1061 (DLCI 70) from Frame Relay
ar$spa
ar$tha
ar$tpa
Station B's ARP will then be able to store station A's
address and Q.922 address association correctly. Next, station
will form a reply message. Many implementations simply place
source addresses from the ARP request into the target addresses
then fills in the source addresses with its addresses. In this case
the ARP response would be
ARP response from
ar$op 2 (response
ar$sha
ar$spa
ar$tha 0x1061 (DLCI 70)
ar$tpa
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RFC 1490 Multiprotocol over Frame Relay July 1993
Again, the source hardware address is unknown and when the request
received, station A will extract the address from the Frame
header and place it in the source hardware address field. Therefore
the response will become
ARP response from B as modified by
ar$op 2 (response
ar$sha 0x0C21 (DLCI 50)
ar$spa
ar$tha 0x1061 (DLCI 70)
ar$tpa
Station A will now correctly recognize station B having
address pB associated with Q.922 address 0x0C21 (DLCI 50).
Reverse ARP (RARP) [8] will work in exactly the same way.
using figure 1, if we assume station C is an address server,
following RARP exchanges will occur
RARP request from A RARP request as modified by
ar$op 3 (RARP request) ar$op 3 (RARP request
ar$sha unknown ar$sha 0x1401 (DLCI 80)
ar$spa undefined ar$spa
ar$tha 0x0CC1 (DLCI 60) ar$tha 0x0CC1 (DLCI 60)
ar$tpa pC ar$tpa
Station C will then look up the protocol address corresponding
Q.922 address 0x1401 (DLCI 80) and send the RARP response
RARP response from C RARP response as modified by
ar$op 4 (RARP response) ar$op 4 (RARP response
ar$sha unknown ar$sha 0x0CC1 (DLCI 60)
ar$spa pC ar$spa
ar$tha 0x1401 (DLCI 80) ar$tha 0x1401 (DLCI 80)
ar$tpa pA ar$tpa
This means that the Frame Relay interface must only intervene in
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
through each relevant DLC, thereby simulating a broadcast
The use of multicast addresses in a Frame Relay environment
presently under study by Frame Relay providers. At such time
the issues surrounding multicasting are resolved,
Bradley, Brown & Malis [Page 23]
RFC 1490 Multiprotocol over Frame Relay July 1993
addressing may become useful in sending ARP requests and
"broadcast" messages
Because of the inefficiencies of broadcasting in a Frame
environment, a new address resolution variation was developed. It
called Inverse ARP [11] and describes a method for resolving
protocol address when the hardware address is already known.
Frame Relay's case, the known hardware address is the DLCI.
Inverse ARP for Frame Relay follows the same pattern as ARP and
use. That is the source hardware address is inserted at
receiving station
In our example, station A may use Inverse ARP to discover
protocol address of the station associated with its DLCI 50.
Inverse ARP request would be as follows
InARP Request from A (DLCI 50)
ar$op 8 (InARP request
ar$sha
ar$spa
ar$tha 0x0C21 (DLCI 50)
ar$tpa
When Station B receives this packet, it will modify the
hardware address with the Q.922 address from the Frame Relay header
This way, the InARP request from A will become
ar$op 8 (InARP request
ar$sha 0x1061
ar$spa
ar$tha 0x0C21
ar$tpa unknown
Station B will format an Inverse ARP response and send it to
A as it would for any ARP message
8. IP over Frame
Internet Protocol [9] (IP) datagrams sent over a Frame Relay
conform to the encapsulation described previously. Within
context, IP could be encapsulated in two different ways
Bradley, Brown & Malis [Page 24]
RFC 1490 Multiprotocol over Frame Relay July 1993
1. NLPID value indicating
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | NLPID = 0xCC |
+-----------------------+-----------------------+
| IP Packet . |
| . |
| . |
+-----------------------+-----------------------+
2. NLPID value indicating
+-----------------------+-----------------------+
| Q.922 Address |
+-----------------------+-----------------------+
| Control (UI) 0x03 | pad 0x00 |
+-----------------------+-----------------------+
| NLPID = 0x80 | | SNAP
+-----------------------+ OUI = 0x00-00-00 +
| |
+-----------------------+-----------------------+
| PID = 0x0800 |
+-----------------------+-----------------------+
| IP packet |
| . |
| . |
| . |
+-----------------------+-----------------------+
Although both of these encapsulations are supported under the
definitions, it is advantageous to select only one method as
appropriate mechanism for encapsulating IP data. Therefore, IP
shall be encapsulated using the NLPID value of 0xCC indicating IP
shown in option 1 above. This (option 1) is more efficient
transmission (48 fewer bits), and is consistent with
encapsulation of IP in X.25.
9. Other Protocols over Frame
As with IP encapsulation, there are alternate ways to
various protocols within the scope of this definition. To
the conflicts, the SNAP encapsulation is only used if no NLPID
is defined for the given protocol
As an example of how this works, ISO CLNP has a NLPID defined (0x81).
Bradley, Brown & Malis [Page 25]
RFC 1490 Multiprotocol over Frame Relay July 1993
Therefore, the NLPID field will indicate ISO CLNP and the data
will follow immediately. The frame would be as follows
+---------------------------------------------+
| Q.922 Address |
+----------------------+----------------------+
| Control (0x03) | NLPID - 0x81 (CLNP) |
+----------------------+----------------------+
| remainder of CLNP packet |
| . |
| . |
+---------------------------------------------+
In this example, the NLPID is used to identify the data packet
CLNP. It is also considered part of the CLNP packet and as such,
NLPID should not be removed before being sent to the upper layers
processing. The NLPID is not duplicated
Other protocols, such as IPX, do not have a NLPID value defined.
mentioned above, IPX would be encapsulated using the SNAP header.
this case, the frame would be as follows
+---------------------------------------------+
| Q.922 Address |
+----------------------+----------------------+
| Control 0x03 | pad 0x00 |
+----------------------+----------------------+
| NLPID - 0x80 (SNAP) | OUI - 0x00 00 00 |
+----------------------+ |
| |
+---------------------------------------------+
| PID = 0x8137 |
+---------------------------------------------+
| IPX packet |
| . |
| . |
+---------------------------------------------+
10. Bridging Model for Frame
The model for bridging in a Frame Relay network is identical to
model for remote bridging as described in IEEE P802.1g "Remote
Bridging" [13] and supports the concept of "Virtual Ports".
bridges with LAN ports receive and transmit MAC frames to and
the LANS to which they are attached. They may also receive
transmit MAC frames through virtual ports to and from other
bridges. A virtual port may represent an abstraction of a
bridge's point of access to one, two or more other remote bridges
Bradley, Brown & Malis [Page 26]
RFC 1490 Multiprotocol over Frame Relay July 1993
Remote Bridges are statically configured as members of a
bridge group by management. All members of a remote bridge group
connected by one or more virtual ports. The set of remote MAC
in a remote bridge group provides actual or *potential* MAC
interconnection between a set of LANs and other remote bridge
to which the remote bridges attach
In a Frame Relay network there must be a full mesh of Frame Relay
between bridges of a remote bridge group. If the frame relay
is not a full mesh, then the bridge network must be divided
multiple remote bridge groups
The frame relay VCs that interconnect the bridges of a remote
group may be combined or used individually to form one or
virtual bridge ports. This gives flexibility to treat the
Relay interface either as a single virtual bridge port, with all
in a group, or as a collection of bridge ports (individual or
VCs).
When a single virtual bridge port provides the interconnectivity
all bridges of a given remote bridge group (i.e. all VCs are
into a single virtual port), the standard Spanning Tree Algorithm
be used to determine the state of the virtual port. When more
one virtual port is configured within a given remote bridge
then an "extended" Spanning Tree Algorithm is required. Such
extended algorithm is defined in IEEE 802.1g [13]. The operation
this algorithm is such that a virtual port is only put into backup
there is a loop in the network external to the remote bridge group
The simplest bridge configuration for a Frame Relay network is
LAN view where all VCs are combined into a single virtual port
Frames, such as BPDUs, which would be broadcast on a LAN, must
flooded to each VC (or multicast if the service is developed
Frame Relay services). Flooding is performed by sending the packet
each relevant DLC associated with the Frame Relay interface. The
in this environment are generally invisible to the bridge. That is
the bridge sends a flooded frame to the frame relay interface
does not "see" that the frame is being forwarded to each
individually. If all participating bridges are fully connected (
mesh) the standard Spanning Tree Algorithm will suffice in
configuration
Typically LAN bridges learn which interface a particular end
may be reached on by associating a MAC address with a bridge port
In a Frame Relay network configured for the LAN-like single
port (or any set of VCs grouped together to form a single
port), however, the bridge must not only associated a MAC
with a bridge port, but it must also associate it with a
Bradley, Brown & Malis [Page 27]
RFC 1490 Multiprotocol over Frame Relay July 1993
identifier. For Frame Relay networks, this connection identifier
a DLCI. It is unreasonable and perhaps impossible to require
to statically configure an association of every possible
MAC address with a DLC. Therefore, Frame Relay LAN-modeled
must provide a mechanism to allow the Frame Relay bridge port
dynamically learn the associations. To accomplish this
learning, a bridged packet shall conform to the
described within section 7. In this way, the receiving Frame
interface will know to look into the bridged packet to gather
appropriate information
A second Frame Relay bridging approach, the point-to-point view
treats each Frame Relay VC as a separate bridge port. Flooding
forwarding packets are significantly less complicated using
point-to-point approach because each bridge port has only
destination. There is no need to perform artificial flooding or
associate DLCIs with destination MAC addresses. Depending upon
interconnection of the VCs, an extended Spanning Tree algorithm
be required to permit all virtual ports to remain active as long
there are no true loops in the topology external to the remote
group
It is also possible to combine the LAN view and the point-to-
view on a single Frame Relay interface. To do this, certain VCs
combined to form a single virtual bridge port while other VCs
independent bridge ports
The following drawing illustrates the different possible
configurations. The dashed lines between boxes represent
circuits
+-------+
-------------------| B |
/ -------| |
/ / +-------+
/ |
+-------+/ \ +-------+
| A | -------| C |
| |-----------------------| |
+-------+\ +-------+
\
\ +-------+
\ | D |
-------------------| |
+-------+
Since there is less than a full mesh of VCs between the bridges
this example, the network must be divided into more than one
Bradley, Brown & Malis [Page 28]
RFC 1490 Multiprotocol over Frame Relay July 1993
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
interconnection the three bridges (A, B, and C) into a single
port. This is an example of the LAN view configuration. The
group would also be a single virtual port which simply
bridges A and D. In this configuration the standard Spanning
Algorithm is sufficient to detect loops
An alternative configuration has three individual virtual ports
the first group corresponding to the VCs interconnecting bridges A,
and C. Since the application of the standard Spanning Tree
to this configuration would detect a loop in the topology,
extended Spanning Tree Algorithm would have to be used in order
all virtual ports to be kept active. Note that the second
would still consist of a single virtual port and the
Spanning Tree Algorithm could be used in this group
Using the same drawing, one could construct a remote bridge
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
C, and the VC connecting A and D are all bridge groups with a
virtual port
Bradley, Brown & Malis [Page 29]
RFC 1490 Multiprotocol over Frame Relay July 1993
11. Appendix
List of Commonly Used
0x00 Null Network Layer or Inactive
(not used with Frame Relay
0x80
0x81 ISO
0x82 ISO
0x83 ISO
0xCC Internet
List of PIDs of OUI 00-80-C
with preserved FCS w/o preserved FCS
------------------ ----------------- --------------
0x00-01 0x00-07 802.3/
0x00-02 0x00-08 802.4
0x00-03 0x00-09 802.5
0x00-04 0x00-0A
0x00-0B 802.6
0x00-0D
0x00-0E BPDUs as defined
802.1(d)
802.1(g)[12].
12. Appendix B - Connection Oriented procedures
This appendix contains additional information and instructions
using CCITT Q.933 and other CCITT standards for encapsulating
over frame relay. The information contained here is similar (and
some cases identical) to that found in Annex F to ANSI T1.617
by Rao Cherukuri of IBM. The authoritative source for
information is in Annex F and is repeated here only for convenience
The Network Level Protocol ID (NLPID) field is administered by
and CCITT. It contains values for many different protocols
IP, CLNP (ISO 8473) CCITT Q.933, and ISO 8208. A figure
a generic encapsulation technique over frame relay networks follows
The scheme's flexibility consists in the identification of
alternative to identify different protocols used either
- end-to-end systems
- LAN to LAN bride and routers
- a combination of the above
over frame relay networks
Bradley, Brown & Malis [Page 30]
RFC 1490 Multiprotocol over Frame Relay July 1993
Q.922
|
|
--------------------------------------------
| |
UI I
| |
--------------------------------- --------------
| 0x08 | 0x81 |0xCC | 0x80 |..01.... |..10....
| | | | | |
Q.933 CLNP IP SNAP ISO 8208 ISO 8208
| | Modulo 8 Modulo 128
| |
--------------------
| | |
L2 ID L3 ID -------
| User | |
| specified | |
| 0x70 802.3 802.6
|
-------------------
|0x51 |0x4E | |0x4
| | | |
7776 Q.922 Others 802.2
For those protocols which do not have a NLPID assigned or do not
a SNAP encapsulation, the NLPID value of 0x08, indicating
Recommendation Q.933 should be used. The four octets following
NLPID include both layer 2 and layer 3 protocol identification.
code points for most protocols are currently defined in ANSI T1.617
low layer compatibility information element. There is also an
for defining non-standard protocols
Bradley, Brown & Malis [Page 31]
RFC 1490 Multiprotocol over Frame Relay July 1993
Format of Other
using Q.933
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
|Control 0x03 | NLPID 0x08 |
+---------------+---------------+
| L2 Protocol ID |
| octet 1 | octet 2 |
+-------------------------------+
| L3 Protocol ID |
| octet 2 | octet 2 |
+-------------------------------+
| Protocol Data |
+-------------------------------+
| FCS |
+-------------------------------+
ISO 8802/2 with user
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
layer 3 protocol
Note 2: Control field is two octets for I-format
S-format frames (see 88002/2)
Encapsulations using I frame (layer 2)
Bradley, Brown & Malis [Page 32]
RFC 1490 Multiprotocol over Frame Relay July 1993
The Q.922 I frame is for supporting layer 3 protocols which
acknowledged data link layer (e.g., ISO 8208). The C/R bit (T1.618
address) will be used for command and response indications
Format of ISO 8208
Modulo 8
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
| ....Control I frame |
+---------------+---------------+
| 8208 packet (modulo 8) Note 3 |
| |
+-------------------------------+
| FCS |
+-------------------------------+
Note 3: First octet of 8208 packet also identifies
NLPID which is "..01....".
Format of ISO 8208
Modulo 128
+-------------------------------+
| Q.922 Address |
+---------------+---------------+
| ....Control I frame |
+---------------+---------------+
| 8208 packet (modulo 128) |
| Note 4 |
+-------------------------------+
| FCS |
+-------------------------------+
Note 4: First octet of 8208 packet also identifies
NLPID which is "..10....".
13.
[1] International Telegraph and Telephone Consultative Committee
"ISDN Data Link Layer Specification for Frame Mode
Services", CCITT Recommendation Q.922, 19 April 1991.
[2] American National Standard For Telecommunications -
Services Digital Network - Core Aspects of Frame Protocol for
with Frame Relay Bearer Service, ANSI T1.618-1991, 18 June 1991.
Bradley, Brown & Malis [Page 33]
RFC 1490 Multiprotocol over Frame Relay July 1993
[3] Information technology - Telecommunications and
Exchange between systems - Protocol Identification in the
Layer, ISO/IEC TR 9577: 1990 (E) 1990-10-15.
[4] Baker, F., Editor, "Point to Point Protocol Extensions
Bridging", RFC 1220, ACC, April 1991.
[5] International Standard, Information Processing Systems -
Area Networks - Logical Link Control, ISO 8802-2: 1989 (E),
Std 802.2-1989, 1989-12-31.
[6] Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit Ethernet
for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT
November 1982.
[7] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
[8] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A
Address Resolution Protocol", STD 38, RFC 903,
University, June 1984.
[9] Postel, J. and Reynolds, J., "A Standard for the Transmission
IP Datagrams over IEEE 802 Networks", RFC 1042, USC/
Sciences Institute, February 1988.
[10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks
Overview and architecture", IEEE Standards 802-1990.
[11] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol",
RFC 1293, Wellfleet Communications, Inc., January 1992.
[12] IEEE, "IEEE Standard for Local and Metropolitan Networks:
Access Control (MAC) Bridges", IEEE Standard 802.1D-1990.
[13] PROJECT 802 - LOCAL AND METROPOLITAN AREA NETWORKS,
Standard 802.1G: Remote MAC Bridging, Draft 6, October 12, 1992.
14. Security
Security issues are not discussed in this memo
Bradley, Brown & Malis [Page 34]
RFC 1490 Multiprotocol over Frame Relay July 1993
15. Authors'
Terry
Wellfleet Communications, Inc
15 Crosby
Bedford, MA 01730
Phone: (617) 280-2401
Email: tbradley@wellfleet.
Caralyn
Wellfleet Communications, Inc
15 Crosby
Bedford, MA 01730
Phone: (617) 280-2335
Email: cbrown@wellfleet.
Andrew G.
Ascom Timeplex, Inc
Advanced Products Business
289 Great Road Suite 205
Acton, MA 01720
Phone: (508) 266-4500
Email: malis_a@timeplex.
Bradley, Brown & Malis [Page 35]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
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