rfc1841.txt
字号:
Network Working Group J. Chapman
Request For Comments: 1841 Cisco Systems, Inc.
Category: Informational D. Coli
Cisco Systems, Inc.
A. Harvey
Cisco Systems, Inc.
B. Jensen
Cisco Systems, Inc.
K. Rowett
Cisco Systems, Inc.
September 1995
PPP Network Control Protocol for LAN Extension
Status of Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
Telecommunications infrastructure is improving to offer higher
bandwidth connections at lower cost. Access to the network is
changing from modems to more intelligent devices. This informational
RFC discusses a PPP Network Control Protocol for one such intelligent
device. The protocol is the LAN extension interface protocol.
Table of Contents
1.0 Introduction ........................................... 3
1.1 LAN Extension Interface Topology ..................... 4
1.2 LAN Extension Interface Architecture ................. 5
1.3 LAN Extension Interface Protocol ..................... 6
2.0 LAN Extension Interface Protocol Control Packets........ 8
2.1 Startup Options ...................................... 8
2.2 Remote Command Options ............................... 14
2.3 Conditions for Sending PPP-LEX Packet ................ 17
3.0 Filter Protocol Type ................................... 18
3.1 LEX_RCMD_REQUEST - Filter Protocol Type............... 19
3.2 Response Packets - Filter Protocol Type............... 21
4.0 Filter MAC Address ..................................... 22
4.1 LEX_RCMD_REQUEST - Filter MAC Address ................ 23
4.2 Response Packets - Filter MAC Address................. 25
Chapman, et al Informational [Page 1]
RFC 1841 LAN Extension Interface Protocol September 1995
5.0 Set Priority ........................................... 27
5.1 LEX_RCMD_REQUEST - Set Priority ...................... 27
5.2 Response Packets - Set Priority ...................... 29
6.0 Disable LAN Extension Ethernet Interface ............... 30
6.1 LEX_RCMD_REQUEST - Disable LAN Extension
Ethernet Interface ................................... 31
6.2 Response Packets - Disable LAN Extension
Ethernet Interface ................................... 32
7.0 Enable LAN Extension Ethernet Interface ................ 33
7.1 LEX_RCMD_REQUEST - Enable LAN Extension
Ethernet Interface ................................... 33
7.2 Response Packets - Enable LAN Extension
Ethernet Interface ................................... 34
8.0 Reboot LAN Extension Interface Unit .................... 35
8.1 LEX_RCMD_REQUEST - Reboot LAN Extension Interface
Unit ................................................. 35
8.2 Response Packets - Reboot LAN Extension
Interface Unit ....................................... 36
9.0 Request Statistics ..................................... 37
9.1 LEX_RCMD_REQUEST - Request Statistics ................ 37
9.2 LEX_RCMD_ACK - Request Statistics .................... 39
9.3 LEX_RCMD_NAK/LEX_RCMD_REJ - Request Statistics ....... 44
10.0 Download Request ...................................... 45
10.1 LEX_RCMD_REQUEST - Download Request ................. 46
10.2 Response Packets - Download Request.................. 48
11.0 Download Data ......................................... 49
11.1 LEX_RCMD_REQUEST - Download Request ................. 49
11.2 Response Packets - Download Data .................... 51
12.0 Download Status ....................................... 52
12.1 LEX_RCMD_REQUEST - Download Status .................. 53
12.2 LEX_RCMD_ACK - Download Status ...................... 54
12.3 LEX_RCMD_NAK/LEX_RCMD_REJ - Download Status ......... 56
13.0 Inventory Request ..................................... 56
13.1 LEX_RCMD_REQUEST - Inventory Request ................ 57
13.2 LEX_RCMD_ACK - Inventory Request .................... 58
13.3 LEX_RCMD_NAK/LEX_RCMD_REJ - Inventory Request ....... 61
14.0 LAN Extension Interface Protocol Data Packets ......... 62
14.1 Frame Format ........................................ 62
14.2 Summary Field Descriptions........................... 63
Chapman, et al Informational [Page 2]
RFC 1841 LAN Extension Interface Protocol September 1995
NOTES ...................................................... 65
REFERENCES ................................................. 65
SECURITY CONSIDERATIONS .................................... 66
AUTHORS' ADDRESSES ......................................... 66
1.0 Introduction
An increasing number of corporations allow their employees to
telecommute to work due to local government regulations on traffic
and air pollution. Additionally, many businesses are run out of
internetworked home offices and small branch offices. With these
changes in the workplace, more people and businesses require Internet
access from small LANs.
Today, routers serve the LAN-to-LAN traffic using high-speed WAN
links such as leased lines, ISDN, or Frame Relay. This new breed of
Internet users from home offices and small branch offices may have a
different, less network-literate skill set than those connecting up
to the Internet today. These new users need an alternative to the
complex and hard-to-configure routers currently employed for
connectivity. One such alternative is a LAN extension interface unit.
A LAN extension interface unit is a hardware device installed at
remote sites (such as a home office or small branch office) that
connects a LAN across a WAN link to a router at a central site. The
following sections introduce a LAN extension interface topology,
architecture, and protocol.
Chapman, et al Informational [Page 3]
RFC 1841 LAN Extension Interface Protocol September 1995
1.1 LAN Extension Interface Topology
Figure 1 shows the topology of LAN extension interfaces. The figure
shows two LAN extension interface units connected via a WAN link to a
central or "host router."
Figure 1 LAN Extension Interface Topology
-----------------------------------------
Router
-----------------------------------------
Virtual Interface Virtual Interface
123.123.78.1 123.123.89.1
.........................................
Serial 0 Serial n
-----------------------------------------
|<---- WAN Link ---->|
------------------ ------------------
| LAN Extension | | LAN Extension |
| Interface Unit | | Interface Unit |
------------------ ------------------
| |
------------------ ------------------
| |
------------ ------------
| End node | | End node |
------------ ------------
123.123.78.2 123.123.46.2
Each LAN extension interface unit maps to a virtual interface at the
host router. The virtual interface mirrors the characteristics of the
LAN extension interface unit. To the routing protocols, the virtual
interface looks just like a local interface, but with the bandwidth
of a serial line. The virtual interface keeps the state (up or down)
of the LAN extension interface unit, and identifies each LAN
extension interface unit by its MAC address.
A LAN extension interface protocol transfers MAC frames from the LAN
extension interface unit across the serial line to the host router.
At the termination point in the router, the router routes the
packets. This topology uses only one subnet per remote LAN rather
than two, as is the case when routers exist on both ends of a WAN
link. Figure 1 shows this subnetting structure. The IP addresses of
the virtual interfaces on the router are in the same subnet as the IP
addresses of the end nodes on the LAN of the LAN extension interface
unit. The LAN extension interface unit itself has no IP address.
Chapman, et al Informational [Page 4]
RFC 1841 LAN Extension Interface Protocol September 1995
LAN extension interface units resemble bridges, but with the
following distinct differences:
* LAN extension interface units always depend on a host router.
They cannot operate standalone or even back-to-back with other
LAN extension interface units.
* LAN extension interface units need not employ any spanning tree
algorithm.
* (LAN extension interface units transfer MAC frames across a
serial line (like bridges), but a router can either route or
bridge the LAN extension interface data packets.
1.2 LAN Extension Interface Architecture
Figure 2 shows the basic LAN extension interface architecture.
Figure 2 LAN Extension Interface Architecture
Router LAN Extension Interface
------------------- -------------------
| Network Layer | | MAC Layer |
------------------- -------------------
| |
------------------- -------------------
| |Virtual Interface| | Filters | |
| ------------------- ------------------- |
| | | |
| ------------------- ------------- |
| | PPP | | RCMD | |
| | | | Handler | |
| ------------------- ------------- |
| | | |
| ------------------- ------------------- |
| |Serial Interface | | PPP | |
| ------------------- ------------------- |
| | | |
| | ------------------- |
| | | Serial Interface| |
| | ------------------- |
| | WAN Link | |
| --------------------------------------- |
| |
| Outbound Inbound |
--------------> <---------------
Chapman, et al Informational [Page 5]
RFC 1841 LAN Extension Interface Protocol September 1995
In the inbound direction (from the remote LAN, to the LAN extension
interface unit, across the WAN link, to the host router), the LAN
extension interface unit can filter received frames to optimize WAN
utilization. The LAN extension interface unit can filter frames by
protocol type or by MAC address. Frames that pass through the LAN
extension interface filters go to the WAN protocol state machine. In
Figure 2, this state machine is PPP. The LAN extension interface unit
adds PPP encapsulation and forwards the packet to the router via the
WAN serial link.
Upon receiving the frame, the host router decapsulates the PPP header
and passes the packet to the virtual interface. From there the
virtual interface handles the packet like any packet received on a
local interface -- by routing or bridging the packet to another
interface, depending on configuration.
In the outbound direction (from the host router, across the WAN link,
to the LAN extension interface unit, to the LAN), the host router's
virtual interface builds the full MAC header, before adding PPP
encapsulation. The router then sends the packet across the WAN serial
link to the LAN extension interface unit. The LAN extension interface
unit strips the PPP header and forwards the packet directly onto the
LAN. The host router has already determined that the packet needs to
be forwarded to the LAN extension interface unit, hence there is no
need for additional filtering or processing at that end.
Embedded in the data stream is a control stream for configuring and
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