📄 rfc2427.txt
字号:
| |
+-------------------------------+
| (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.
Brown & Malis Standards Track [Page 12]
RFC 2427 Multiprotocol over Frame Relay September 1998
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 |
+-------------------------------+
Brown & Malis Standards Track [Page 13]
RFC 2427 Multiprotocol over Frame Relay September 1998
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.
Brown & Malis Standards Track [Page 14]
RFC 2427 Multiprotocol over Frame Relay September 1998
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.
Brown & Malis Standards Track [Page 15]
RFC 2427 Multiprotocol over Frame Relay September 1998
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$pln is 4).
Brown & Malis Standards Track [Page 16]
RFC 2427 Multiprotocol over Frame Relay September 1998
ar$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.
Brown & Malis Standards Track [Page 17]
RFC 2427 Multiprotocol over Frame Relay September 1998
DLCI to Q.922 Address Table for Figure 1
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -