Router using remote address resolution to enable bridge like data forwarding

• US 5,420,862 A
• Filed: 06/14/1991
• Issued: 05/30/1995
• Est. Priority Date: 06/14/1991
• Status: Expired due to Term

• Current Assignee: Enterasys Networks Incorporated (Extreme Networks, Inc.)
• Original Assignee: Digital Equipment Corporation (HP Inc.)
• Inventors: Perlman, Radia J.
• Primary Examiner(s): Olms, Douglas W.
• Assistant Examiner(s): Nguyen, Chau T.
• Application Number:
  US07/716,041
• Time in Patent Office: 1,446 Days
• Field of Search: 370/85.13, 370/85.14, 370/85.6, 370/94.1, 370/60, 370/60.1, 370/94.2, 370/92, 340/825.02
• US Class Current: 370/401
• CPC Class Codes: H04L 12/4604 LAN interconnection over a backbone network, e.g. Internet, Frame Relay

Abstract

A communications system is disclosed, having a first communications link, a second communications link, a first end station attached to said first communications link, a first packet forwarding apparatus attached to the first communications link, a second end station attached to the second communications link, and a second packet forwarding apparatus attached to the second communications link. Each packet forwarding apparatus routes packets it receives having destination address equal to a data link destination address of the apparatus, and bridges all other received packets. When the first end station wishes to send a packet to the second end station, it first transmits an ARP request message to learn the data link address of the second end station. The first apparatus receives the ARP (Address Resolution Protocol) request message, and determines that the end station for which a data link address is requested is attached to a remote communications link. The first apparatus requests the second apparatus to transmit an ARP request message to determine the second station'"'"'s address, and to relay the ARP response back to the first apparatus. When the first apparatus receives the ARP response, it forwards the response to the first end station. The first end station transmits subsequent packets to the second end station, using the data link address of the second end station as a data link destination address. These subsequent packets can be bridged by any intermediary apparatus between the first end station and the second end station.

Claims

1. A communications system having a first communications link and a second communications link, a first end station coupled with said first link, a second end station coupled with said second link,

a forwarding apparatus for forwarding a packet from said first link to said second link, said forwarding apparatus detecting a network layer header within said packet, said network layer header having a destination address, said destination address having a subnet field and a host field, said communications system comprising:

a forwarding mask, within said forwarding apparatus, said forwarding mask having a forwarding mask length, said forwarding mask for distinguishing said destination address into a first subnet address part and into a first host address part, said first subnet address part having a length equal to the length of said subnet field;

an end station mask, in said first end station, said end station mask having an end station mask length, for distinguishing said destination address into a second subnet address part and into a second host address part, said end station mask length less than said forwarding mask length to distinguish said second subnet address part as having a length less than the length of said subnet field;

said first end station and said second end station using said end station mask, responsive to said second subnet address part as having a length less than the length of said subnet field, to identify all end stations on said first link and said second link as being on a single link, and said forwarding apparatus using said forwarding mask to distinguish which of said first link or said second link to forward said data packet onto.

2. The communication system as in claim 1 further comprising:

means for said first end station to transmit a local Address Request Protocol (hereinafter ARP ) request message onto said first communications link, said local ARP request message requesting a data link address of a receiving end station, said receiving end station coupled with a third link;

means, in response to said local ARP request message, for said forwarding apparatus to send a remote ARP request message to a second forwarding apparatus, said second forwarding apparatus connected to said second link and said third link;

means for said forwarding apparatus to receive a remote ARP response message containing said data link address of said receiving end station from said second forwarding apparatus;

means, responsive to said remote ARP request message, for said forwarding apparatus to send a local ARP response message to said first end station.

3. The communications system as in claim 1, said forwarding apparatus further comprising:

means for operating as a bridge in response to a data link destination address in said data packet not equal to a data link address of said forwarding apparatus.

4. The apparatus in claim 3 wherein said means for operating as a bridge further comprises:

means for reading a data link layer header of said packet; and

means for forwarding said packet based on the contents of said data link layer header of said packet.

5. The communications system as in claim 1, said forwarding apparatus further comprising:

means for operating as a router in response to a data link destination address in said data packet equal to a data link address of said forwarding apparatus.

6. The apparatus as in claim 5 wherein said means for operating as a router further comprises:

means for reading a data link layer header of said packet;

means for reading a network layer header of said packet; and

means for forwarding said packet based on the contents of said data link layer header and said network layer header of said packet.

7. The communications system of claim 1 wherein said first communication link and said second communications link are local area networks (LANs).

8. A communications system having a forwarding apparatus, said forwarding apparatus coupled with a first link and a second link, for forwarding a packet from said first link to said second link, said forwarding apparatus detecting a network layer header on said packet, said network layer header having a destination address, said destination address having a subnet field and a host field, said communications system comprising:

said subnet field on said packet having a first sub-field and a second sub-field, said first sub-field equal for all end nodes on said first link and all nodes on said second link, a forwarding mask, within said forwarding apparatus, said forwarding mask having a forwarding mask length, for distinguishing said destination address into a first subnet address part and into a first host address part, said first subnet address part equal to said subnet field, and said first host part equal to said host field;

an end station mask, within a first end station coupled with said first link, said end station mask having an end station mask length, said end station mask for distinguishing said destination address into a second subnet address part and into a second host address part, said end station mask length less than said forwarding mask length, said second subnet address part equal to said first subfield of said subnet field;

said first end station using said end station mask to identify all end stations on said first link and said second link as being on a single link, and said forwarding apparatus using said forwarding mask to distinguish which of said first link or said second link a receiving end station addressed by said destination address is located on;

means, responsive to said greater length of said forwarding mask than said end station mask, for said first end station to transmit a local ARP request message onto said first link, said local ARP request message requesting a data link address of said receiving end station, said receiving end station connected to a third link that is not said first link or said second link;

means, responsive to said local ARP request message, for said forwarding apparatus to send a remote ARP request message to a second forwarding apparatus attached to said second link and said third link;

means, responsive to said remote ARP request message, for said second forwarding apparatus to send a second local ARP request message to said receiving end station;

means, responsive to said second local ARP request message, for said receiving end station to send a local ARP response message to said second forwarding apparatus, said second local ARP response message containing said data link address of said receiving end station;

means, responsive to said local ARP response message, for said second forwarding apparatus to send a remote ARP response message to said forwarding apparatus;

means, responsive to said remote ARP response message, for said forwarding apparatus to send a second local ARP response message to said first end station;

means, responsive to said second local ARP response message, for said first end station to receive a data link address of said receiving end station, and for said first end station to transmit a second packet containing said data link address of said receiving end station, and for said forwarding apparatus to operate as a bridge in response to said data link address of said receiving end station being contained in said second packet.

9. The communications system of claim 8 wherein said plurality of communications links are local area networks (LANs).

10. A communications system having a forwarding apparatus for forwarding a packet from a first link to a second link, said apparatus detecting a network layer header within said packet, said network layer header having a destination address, said communications system comprising:

a forwarding mask, within said forwarding apparatus, having a forwarding mask length, for distinguishing said destination address into a first subnet part and into a first host address part;

an end station mask, within an end station on said first link, having an end station mask length for distinguishing said destination address into a second subnet address part and into a second host address part;

said forwarding mask length greater than said end station mask length, said end station using said end station mask to identify all end stations on said first link and said second link as being on a single link such that said end station obtains the data link address of any one of said all end stations on said first link and said second link by transmitting a local ARP request packet onto said first link, and said apparatus using said forwarding mask to distinguish on which of said first link and said second link a second end station addressed by said network layer address is located on.

11. The communications system of claim 10 wherein said plurality of communications links are local area networks (LANs).

12. A forwarding apparatus for forwarding packets comprising:

a receiving means (1000) for receiving a packet from a first LAN;

a local ARP request detecting means (1020) for determining if said packet is a local ARP request packet;

a first determining means (1022), responsive to said packet being a local ARP request packet, for determining if said packet is requesting an address of an end station attached to said first LAN; and

a requesting means (1026), responsive to said packet being a local ARP request packet requesting the address of an end station not attached to said first LAN, for selecting a second LAN, and for requesting a second forwarding apparatus, said second forwarding apparatus attached to said second LAN, to both transmit a second local ARP request onto said second LAN and to relay a resulting ARP response packet to said forwarding apparatus.

13. The forwarding apparatus as in claim 12, further comprising:

a discarding means (1024) responsive to said first determining means determining said packet is a local ARP request packet requesting the address of an end station attached to said first LAN, for discarding said packet.

14. The forwarding apparatus as in claim 13, further comprising:

second determining means (1040) for determining if said packet is a remote ARP request;

caching means (1042) responsive to said packet being a remote ARP request, for keeping record of the packet in a cache of pending remote ARP requests; and

local request generating means (1049) responsive to said packet being a remote ARP request, for generating a local ARP request packet on a requested LAN indicated in said packet.

15. The forwarding apparatus as in claim 14 further comprising:

a local ARP response detecting means (1030) for determining if said packet is a local ARP response;

a cache checking means (1034) for determining if said cache of pending remote ARP requests cache contains a pending remote ARP request; and

a remote ARP response generating means (1038), responsive to said packet being a local ARP response and said cache of pending remote ARP requests containing a pending remote ARP request, for generating a remote ARP response packet.

16. The forwarding apparatus as in claim 15, further comprising:

a remote ARP response detecting means (1050) for determining if said packet is a remote ARP response packet; and

a local ARP response generating means (1052), responsive to said packet being a remote ARP response packet, for generating a local ARP response.

17. A communications system, comprising:

a first LAN and a second LAN, each end station on said first LAN and said second LAN having an associated network address, each said associated network address having a host field and a subnet field, each subnet field having a first subfield, the value of said first subfield being equal for all end stations on said first LAN and said second LAN;

a first end station coupled with said first LAN;

a second end station coupled with said second LAN;

a forwarding apparatus for forwarding a packet from said first LAN to said second LAN, said forwarding apparatus capable of detecting a network layer header on a data packet, said network layer header having a destination address;

means, within said forwarding apparatus, for distinguishing said destination address into a first subnet address part and into a first host address part, said first subnet address part equal to the value of said subnet field of said destination address, said first host address part equal to the value of said host field of said destination address, said forwarding apparatus using said first subnet address part to determine which of said first LAN or said second LAN an end station addressed by said destination address is located;

means, within said first end station and said second end station, for distinguishing said destination address into a second subnet address part and into a second host address part, said second subnet address part equal to said first subfield in said destination address, said first end station and said second end station using the value of said second subnet part to identify all end stations on said first LAN and said second LAN as being on a single LAN.

18. The communications system as in claim 17 further wherein

said forwarding apparatus forwards a packet based on a data link layer header of said data packet in the event that a data link destination address in said data link layer header of said data packet is not a data link address of said apparatus.

19. The communications system as in claim 17 further wherein said

forwarding apparatus forwards a packet based on a data link layer header and said network layer header of said packet in the event that a data link destination address in said data link header of said data packet is equal to a data link address of said forwarding apparatus.

20. A communications system comprising:

a first LAN;

a second LAN;

a forwarding apparatus for forwarding a first packet from said first LAN to said second LAN, said packet transmitted by an first end station on said first LAN, and said first packet containing a network layer destination address field value equal to a network layer address of a second end station on said second LAN;

first masking means within said forwarding apparatus for dividing said network layer destination address within said first packet into a first subnet part and a first host part, said first subnet part indicating that said packet is to be forwarded onto said second LAN; and

second masking means within said first end station on said first LAN for dividing said network layer destination address into a second subnet part and a second host part, said second subnet part indicating that all end stations on said first LAN and said second LAN are on a single LAN, such that said first end station obtains the data link address of said second end station by transmitting a local ARP request packet onto said first LAN.

21. The communications system as in claim 20, wherein said forwarding apparatus forwards:

a data packet based on a data link layer header of said data packet in the event that a data link destination address in said data link layer header of said data packet is not a data link address of said apparatus.

22. The communications system as in claim 20, wherein said forwarding apparatus forwards:

a data packet based on a data link layer header and said network layer header of said packet in the event that a data link destination address in said data link header of said packet is equal to a data link address of said apparatus.