Patent Publication Number: US-2011058559-A1

Title: Vlan data framing and transmission

Description:
TECHNICAL FIELD 
     This invention relates to methods for framing data for transmission in 
     VLAN data networks, and to methods of transmitting such data in such networks. 
     BACKGROUND ART 
     A virtual local area network, commonly known as a vLAN or as a VLAN, is a way of creating one or more independent logical networks within a physical network. A VLAN consists of a network of computers that behave as if connected to the same link layer network, even though they may actually be physically connected to different segments of a LAN. VLANs are configured through software rather than hardware, making them extremely flexible. One advantage of VLANs emerges when physically moving a computer to another location: it can stay on the same VLAN without the need for any hardware reconfiguration. Several VLANs can co-exist within a single physical network. 
     VLANs operate at the layer  2  (data link layer) of the OSI model. Users in a VLAN connect to the network via one or more layer  2  switches. These in turn connect to the wider network via distribution routers (or equivalent switches). Because the users in a given VLAN are logically on the same network, it is theoretically possible for there to be direct user-to-user communication without the need for routing or for the communication to pass through devices such as the distribution router with security function (e.g. firewalls). Such a communication path is inherently insecure and as such can be undesirable in networks to which the public have access. 
     The primary protocol currently used in configuring VLANs is IEEE  802 . 1 Q, which describes how traffic on a single physical network can be partitioned into VLANs by tagging each frame or packet with extra bytes to denote the virtual network to which the packet belongs. For Ethernet frames using Ethernet II framing,  802 . 1 Q sets the EtherType value in the Ethernet header to Tag Protocol ID (TPID) 0x8100, identifying this frame as an  802 . 1 Q frame. It then inserts extra bytes of Tag Control Information (TCI) after the TPID, followed by other bytes containing the frame&#39;s original EtherType. The TCI includes a VLAN ID field (VID) indicating the particular VLAN to which a frame belongs. Together the TPID and TCI are called the VLAN Tag. 
     Previously, direct user-to-user communication has been prevented by configuring the switches such that data originating with a user (upstream traffic) is assigned a first VID and that data with this VID is not transmitted directly to a user, only to a distribution router. The distribution routers are configured to assign a second, different VID to data for transmission to a user connected to a VLAN (downstream traffic). Essentially, the system creates two VLANS: one for upstream traffic, and one for downstream traffic. For example, EP1039698 discloses method and apparatus for grouping a node in multiple VLANs using a port based VLAN grouping paradigm. A unique frame tagging approach is used for explicit VLAN identification, wherein a packet transmitted by a node is explicitly associated one of multiple VLANs by way of the tag protocol identifier/virtual LAN identifier combination in the tag header inserted in the packet. 
     The use of the split VLAN described above leads to an asymmetric traffic flow. Where devices in the physical network have to interact with the data for routing, this is not necessarily a problem. However, problems can occur for physical devices that merely pass traffic on. It is therefore an object of the invention to provide a method of data framing that avoids the need to provide a split VLAN. 
     DISCLOSURE OF THE INVENTION 
     This invention provides a method of framing data for transmission to or from one of a number of users connected to a VLAN data network, comprising dividing the data into frames including a header and a payload, the header comprising a portion of one of two selectable values assigned according to whether the data is to be transmitted from or to a user, wherein the header comprises a VLAN tag including a Tag Protocol ID (TPID) portion and a Tag Control Information (TCI) portion, a first TPID value being assigned to data to be transmitted from the user and a second, different, TPID value being assigned to data to be transmitted to a user. 
     The TCI portion typically includes a VLAN ID field specifying the VLAN to which the frame belongs. 
     By assigning the TPID value to indicate upstream or downstream traffic, it is possible to assign the same VLAN ID to frames for transmission to or from the same user in a specific VLAN. 
     In one embodiment, the first TPID value is assigned for data transmitted from a user at a layer  2  switch to which the user is connected (upstream traffic) and the second TPID value is assigned for data transmitted to a user at a distribution router forming part of the network to which the user is connected downstream traffic. 
     The invention also provides a method of communicating data between first and second users connected via a VLAN, comprising: 
     sending data from the first user to a layer  2  switch to which it is connected for delivery to the second user; 
     framing the data according to a method as defined above so as to assign a first TPID value indicative of data being sent from a user; 
     forwarding the framed data to a distribution router in the network comprising the VLAN; 
     re-framing the data at a distribution router in the network comprising the VLAN according to a method as claimed in any preceding claim so as to assign a second TPID value indicative of data being sent to a user; 
     forwarding the re-framed data to a layer  2  switch to which the second user is connected; and 
     transmitting the data to the second user. 
     The first and second users can be connected to the same layer  2  switch or to different layer  2  switches. 
     Other changes can be made within the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic of an embodiment of a VLAN system to which the invention applies. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     In a simplified schematic data network shown in  FIG. 1 , a number of users A, B, C, . . . (i.e. computers) are connected to a network via respective layer  2  switches S 1 , S 2 , S 3  etc. The layer  2  switches S are connected to each other and to a network core (Internet) I via distribution routers D 1 , D 2  to form a loop L. The distribution routers D 1 , D 2  are also connected to each other. As will be appreciated, the actual physical structure of the network may be much more complex that shown here but the overall functional relationship between the various parts remains essentially the same. 
     A VLAN V is defined to include users A, B and C. As can be seen, user C is connected to a different layer  2  switch (S 2 ) to users A and B (S 1 ). Other users are either part of other VLANs or are directly connected to the network on their own. 
     Data for transmission over the VLAN includes a frame header which is attached to a data payload and provides information to allow its transmission on the network and delivery to the intended address. The header H will include a destination address (i.e. where the data is to be sent) portion DST, a source address (i.e. where the data originated) portion SRC, and an EtherType portion ET. For Ethernet frames using Ethernet II framing,  802 . 1 Q sets the EtherType value in the Ethernet header to Tag Protocol ID (TPID) 0x8100, identifying this frame as an  802 . 1 Q frame. It then inserts extra bytes of Tag Control Information (TCI) after the TPID, followed by other bytes containing the frame&#39;s original EtherType. The TCI includes a VLAN ID field (VID) indicating the particular VLAN to which a frame belongs. Together the TPID and TCI are called the VLAN Tag. The payload P follows this header. The basic structure of the header is: 
     [DST][SRC][ET(TPID=0x8100)(TCI(VID))][other header values]. 
     In order to prevent direct communication via the layer  2  switch(es) using the conventional technique discussed above, the first layer  2  switch assigns a first VID for upstream traffic. Any layer  2  switch receiving this data frame will only pass it upstream and so prevents direct communication with another user (who would be downstream of the switch). 
     When the data frame reaches the distribution router D upstream of the desired recipient, the router D amends the header to have a new VID indicative of downstream traffic. Thus, when the frame reaches the layer  2  switch S, it is passed on to the correct user according to the destination address. 
     As is discussed above, this use of two VIDs means that traffic is essentially running on two separate VLANs, but with asymmetric traffic flows. This invention provides a technique by which a single, logical VLAN can accommodate both upstream and downstream traffic. The invention allows assignment of different TPID values according to whether traffic is upstream or downstream. For example, an upstream header applied at the layer  2  switch can have the following structure: 
     [DST][SRC][ET(TPID=0x8100)(TCI(VID))][other header values] while the downstream header applied at the distribution router can have the structure: 
     [DST][SRC][ET(TPID=0x9100)(TCI(VID))][other header values]. 
     In both cases, the same VID is used, meaning that the assigned VLAN has both upstream and downstream traffic, unlike the asymmetric traffic flow of the conventional technique described above. 
     In the case of the present invention, the layer  2  switches respond to the different TPID values in a similar way to the different VID values of the conventional technique. Consequently, a layer  2  switch receiving a frame with a TPID value indicating upstream traffic (assigned by the originating layer  2  switch) will only forward the frame on towards the distribution router, whereas a layer  2  switch receiving a frame with a TPID value indicating downstream traffic (assigned by the distribution router) is permitted to pass this on to the addressed, directly-connected user (DST address). 
     Since the same VID (VLAN) is used for both upstream and downstream traffic, the number of separately available VLANS for a given system is twice what it would be using the conventional technique, thus providing greater capacity and flexibility. 
     Other changes can be made while remaining within the scope of the invention.