Abstract:
A method for establishing a hierarchical network with Backbone Provider Bridges includes: dividing the network into a plurality of layers, and partitioning each layer into at least one domain; each domain assigning a service tag for itself; a local domain sending a mapping request including a user type ID to a further domain in the same layer; the further domain in the same layer returning a service tag corresponding to the user type ID to the local domain; the local domain establishing and saving a mapping relationship; and edge devices of each domain learning MAC addresses. Further, a system and device having the same are also provided in the disclosure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2006/003637, filed Dec. 28, 2006, which claims priority to Chinese Patent Application No. 200610081150.5, filed May 18, 2006, both of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to network communication technology, and more particularly, to a method and system for establishing a hierarchical network with provider backbone bridges, and to a method and device using the system for data transmission. 
     BACKGROUND 
     Media Access Control (MAC) address is a unique number for identifying a network adapter after it is manufactured. Provider Backbone Bridges (PBB) (also known as Mac-in-Mac), together with QinQ (a transmission technology using an inner-layer 802.1Q tag encapsulated with an outer-layer 802.1Q tag), make it possible for the whole transmission network (Layer 2 network) to base on the Ethernet architecture, thereby allowing Metropolitan Area Network (MAN) and even Wide Area Network (WAN) to be realized by Ethernet technology from the access layer through the aggregation layer to the backbone layer. Thus, PBB is a crucial trend in Ethernet development, because Ethernet is of low cost and easy to use. With the advance of Mac-in-Mac technology and carrier Ethernet related features, such as the improvement on Quality of Service (QoS) and Operation Administration and Maintenance (OAM), MAN Ethernet and WAN Ethernet will be developed rapidly and Mac-in-Mac technology will get wider and wider applications. 
     Mac-in-Mac technology, capable of solving the scalability problem of Ethernet, is mainly applied to provider backbone network, and also applied to similar situations, such as the access layer and the aggregation layer. Mac-in-Mac differs from QinQ mainly in that: QinQ requires that all of the nodes learn a user&#39;s MAC address, which imposes a strict requirement on the nodes; while for Mac-in-Mac it only needs to maintain a user&#39;s address at the edge nodes. 
       FIG. 1  illustrates a typical example of applying Mac-in-Mac in networking. Here, the Mac-in-Mac technology is used in the backbone network. This means the backbone edge nodes support Mac-in-Mac, while the intermediate nodes are not required to support Mac-in-Mac. The intermediate nodes can be ordinarily switches supporting 802.1Q. Network-Provider Edge (N-PE) supports both Mac-in-Mac and QinQ, while the access layer only supports QinQ. The backbone layer and the access layer perform Spanning Tree Protocol (STP), respectively, to create their respective forwarding trees. 
     During the transmission of a user packet in a network, the detailed process is as follows: 
     After a user accesses a User Provider Edge (U-PE), different Virtual Local Area Network tags (designated as S-VLAN) are assigned to the user, according to the service type or the user type. There are 4094 total tags for S-VLAN. The user packet is encapsulated by means of QinQ, and then goes through the following transformation: 
     
       
         
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                 C-DA 
                 C-SA 
                 C-TAG 
                 DATA 
                 C-FCS 
               
             
          
           
               
                 
                           
                 
               
             
          
           
               
                   
                 C-DA 
                 C-SA 
                 S-TAG 
                 C-TAG 
                 DATA 
                 FCS 
               
               
                   
               
               
                 The symbols are defined as follows: 
               
               
                 C-DA: customer destination MAC address; 
               
               
                 C-SA: customer source MAC address; 
               
               
                 C-TAG: customer 802.1Q tag; 
               
               
                 S-TAG: service 802.1Q tag 
               
               
                 DATA: customer data; 
               
               
                 C-FCS: customer check sum; and 
               
               
                 FCS: Frame check sum. 
               
             
          
         
       
     
     When the packet arrives at N-PE, the address of the related backbone bridge edge node may be found, according to the customer destination address. Accordingly, Backbone Destination Address (B-DA) and Backbone Source Address (B-SA) are inserted in the packet. Meanwhile, Service Instance TAG (I-TAG) and Backbone VLAN TAG (B-TAG) fields are inserted in the packet. The I-TAG, as an inner tag, provides a Service Instance ID (SID) having at least 20 bits, for indicating various service instances. B-TAG, as an outer tag, indicates the various service types provided by the backbone provider (such as audio, video, and so on), or indicates different domains or different service providers. According to B-DA, B-SA, and B-TAG, the backbone nodes perform forwarding, which is actually equivalent to establishing a tunnel. 
     There are two ways to insert B-TAG and I-TAG. 
     1. S-TAG mode: A S-TAG of a packet is first identified, and then the S-TAG is mapped to an I-TAG that is found by searching an internal mapping table. Because the function of I-TAG is same as that of S-TAG, the transformed packet no longer carries S-TAG, and the format of the transformed packet is as follows: 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 B-DA 
                 B-SA 
                 B-TAG 
                 I- 
                 C-DA 
                 C-SA 
                 C- 
                 DATA 
                 B- 
               
               
                   
                   
                   
                 TAG 
                   
                   
                 TAG 
                   
                 FCS 
               
               
                   
               
             
          
         
       
     
     2. Transparent mode: it neither processes the S-TAG of the packet, nor performs the related mapping. All of the packets are assigned with the same I-TAG and B-TAG for transmission. The user packet (including S-TAG) is transmitted transparently without any modification. The format of the packet is shown as follows: 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 B- 
                 B- 
                 B- 
                 I- 
                 C- 
                 C- 
                 S- 
                 C- 
                 DATA 
                 FCS 
                 B- 
               
               
                 DA 
                 SA 
                 TAG 
                 TAG 
                 DA 
                 SA 
                 TAG 
                 TAG 
                   
                   
                 FCS 
               
               
                   
               
             
          
         
       
     
     When the customer data arrives at the peer backbone node, B-DA, B-SA, B-TAG, and I-TAG fields are removed. If it is in the S-TAG mode, a S-TAG may be obtained by searching the mapping table with reference to the I-TAG, and then the packet is re-encapsulated. If it is in the transparent mode, TAG mapping is not required anymore. After the packet is processed, it is directly forwarded to a U-PE, according to C-DA. The U-PE subsequently removes S-TAG and forwards the packet to the user. 
     In practical applications, the backbone network may be very huge. To solve this problem, the backbone network is divided into different layers, each of which is further partitioned into different domains, and various B-TAGs are assigned for the respective domains in each different layer.  FIG. 2  shows a Mac-in-Mac hierarchical application in the related art, in which the backbone network is divided into two layers, i.e. a first layer and a second layer. The first layer is further divided into four domains A, B, C, and D, each of which has its own B-TAG. During the data forwarding among the different domains, it is required to replace the corresponding B-TAG, while keeping the I-TAG unchanged during the whole data transmission procedure. 
     The devices in  FIG. 2  are respectively: 
     BB: Backbone Provider Bridge; 
     PB: Provider Bridge (as defined by 802.1ad); 
     BB PB: Backbone Provider Bridge Edge, i.e. the edge devices between the backbone network and network provider; and 
     BB BB: Backbone Provider Bridge Layer Edge, i.e. the edge device between the backbone network and network provider layer. 
     The processing on a user packet is shown as follows: 
     At PB, the user packet is encapsulated in the form of QinQ. At BB PB, the S-TAG of the user packet is mapped to an I-TAG of Mac-in-Mac, which consists of a SID having at least 20 bits, and at last a B-TAG is encapsulated in the packet for transmission. At BB BB, the original B-TAG is mapped to a new B-TAG, and the packet is transmitted to the peer BB BB, and then transmitted to the peer BB PB via B-TAG mapping. The peer BB PB maps the I-TAG back to S-TAG and removes the Mac-in-Mac header, then transmits the packet to the peer PB finally. The peer PB processes and transmits the packet to the peer user. 
     In the technical solution of the related art, each domain of the various network layers cannot decide the allocation of I-TAG on its own. The I-TAG is globally valid in the backbone network and kept unchanged, therefore I-TAG cannot be extended anymore. 
     SUMMARY 
     The disclosure is to provide a method and system for establishing a hierarchical network with Backbone Provider Bridges, so as to realize a hierarchical network and enhance the scalability of the network. It also provides a method and device for transmitting data using said system, so as to realize data transmission in the hierarchical network system with Provider Backbone Bridges. 
     An embodiment of the disclosure provides a method for establishing a hierarchical network to support Provider Backbone Bridges. The method includes: 
     dividing the network into a plurality of layers, and partitioning each layer into at least one domain; 
     assigning, by each domain, a service tag for its local domain in its own way, according to a user type ID of a lower layer which accesses the local domain; 
     establishing and saving a mapping relationship between the service tag of the local domain and the service tags of the other domains in the same layer; and 
     learning, by an edge device of each domain, Media Access Control (MAC) addresses according to a predefined rule. 
     An embodiment of the disclosure provides a hierarchical network system with Provider Backbone Bridges, in which: 
     a network is divided into a plurality of layer, each layer including at least one domain, and each domain including an assigning module, a mapping module and an edge device having a MAC address forwarding table; 
     the assigning module is adapted to assign a service tag for its local domain in its own way according to a user type ID of a lower layer which accesses the local domain; and 
     the mapping module is adapted to establish and save a mapping relationship between the service tag of the local domain and the service tags of the other domains in the same layer. 
     An embodiment of the disclosure further provides a method for transmitting data in a hierarchical network system with Provider Backbone Bridges. The method includes: 
     receiving, by a domain in a local layer, a data packet from a domain in a lower layer at local side; encapsulating the data packet; and forwarding the encapsulated data packet to a domain in an upper layer at local side, until the encapsulated data packet arrives at a peer end in a top layer; and 
     upon receiving the data packet by the peer end in the top layer, de-encapsulating the data packet; configuring a corresponding service tag in the de-encapsulated data packet and forwarding the configured data packet to a domain in a lower layer, until the configured data packet arrives at a destination peer. 
     An embodiment of the disclosure further provides a device for transmitting data. The device includes: 
     an encapsulating module, adapted to encapsulate a data packet in a domain of its local layer upon receiving the data packet from a domain in a lower layer at local side, and forwarding the encapsulated data packet to a domain in an upper layer at local side, until the encapsulated data packet arrives at a peer end in a top layer; and 
     a de-encapsulating module, adapted to de-encapsulate the data packet upon the peer end in the top layer receiving the data packet, to configure a corresponding service tag in the de-encapsulated data packet, and to forward the configured data packet to a domain in a lower layer, until the configured data packet arrives at a destination peer. 
     In summary, according to the technical solutions proposed in the embodiments of the disclosure, each network layer only manages itself, and the sub-layers are isolated from each other, which not only simplifies the network management but also improves the network security. Hierarchical user access authority is realized through managing the tag allocation by the users in each layer on their own. Meanwhile, it breaks through the number restriction imposed by the existing VLAN, which significantly improves the networking scalability. As for each network layer, only the edge devices of the network layer have to learn MAC addresses, the intermediate nodes therein can be regular switches, thereby saving the cost of networking. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a typical Mac-in-Mac networking application; 
         FIG. 2  is a schematic view of a Mac-in-Mac hierarchical application; 
         FIG. 3  is a flow chart of a method for establishing a hierarchical network with provider backbone bridges, according to an embodiment of the disclosure; 
         FIG. 4  is a flow chart of a method for establishing a hierarchical network with provider backbone bridges, according to another embodiment of the disclosure; 
         FIG. 5  is a schematic view of a hierarchical network system established by using the method based on provider backbone bridges, according to an embodiment of the disclosure; and 
         FIG. 6  is a flow chart of data transmission in a hierarchical network system established by using the method based on provider backbone bridges, according to an embodiment of the disclosure. 
         FIG. 7  is a flow chart of data transmission in a hierarchical network system according to another embodiment of the disclosure; 
         FIG. 8  is a flow chart of data transmission in a hierarchial network system according to another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, the disclosure will be explained in detail by reference to the accompanying figures and embodiments, which is not intended to limit the disclosure. 
     Embodiment One 
     As shown in  FIG. 3 , an embodiment of the method for establishing a hierarchical network with provider backbone bridges includes as follows: 
     Block  101 : A network, which supports Mac-in-Mac, is divided into several layers, and each layer is further partitioned into a plurality of domains. 
     Block  102 : Each of the domains assigns a service tag for its local domain in its own way, according to a user type ID of a lower layer which accesses the local domain, wherein the service tag is valid only in the local domain. For example, a local domain assigns a service tag  1  for itself, according to a user type ID (ID=A) of a lower layer which accesses the local domain, wherein the service tag  1  indicates a voice service. This local domain also assigns a service tag  2  for itself according to another user type ID (ID=B) which accesses the local domain, where the service tag  2  indicates a video service. 
     Block  103 : The local domain transmits a mapping request for a service tag, which includes the user type ID corresponding to the service tag, to a further domain in the same layer via a signaling protocol (such as GMPLS signaling protocol). For example, a mapping request for service tag  1  of the local domain is transmitted, and the mapping request includes the user type ID, i.e. ID=A, which corresponds to the service tag  1 . 
     Block  104 : After receiving the mapping request, the further domain finds a corresponding service tag in its own domain, according to the user type ID in the mapping request. For example, the corresponding service tag is service tag  5 , which represents a voice service as well. The service tag is then returned to the local domain via a mapping response. 
     Block  105 : Upon receiving the mapping response, the local domain establishes a mapping relationship between the service tag of the local domain and that of the further domain, and saves the mapping relationship in the local domain. For example, a mapping relationship between service tag  1  of the local domain and service tag  5  of a further domain is established and saved. 
     Block  106 : Edge devices of each domain learn the MAC addresses, according to a predefined rule. One rule is that the edge devices of each domain learn the MAC addresses of edge devices of each domain in the same layer and the MAC addresses of edge devices of each domain in the lower layer. Another rule is that the edge devices of each domain learn the MAC addresses of the edge devices of each domain in the same layer and the MAC addresses of all user terminals in the bottom layer. 
     Embodiment Two 
     As shown in  FIG. 4 , another embodiment of the method for establishing a hierarchical network with provider backbone bridges comprises the following steps: 
     Block  201 : A network supporting Mac-in-Mac is divided into several layers, and each layer is further partitioned into a plurality of domains. 
     Block  202 : Each of the domains (e.g. a local domain) assigns a service tag for its local domain in its own way, according to a user type ID of a lower layer which accesses the local domain, wherein the service tag is valid only in the local domain. 
     Block  203 : The local domain transmits a mapping request for a service tag, which includes the service tag and a user type ID corresponding to the service tag, to a further domain in the same layer via a signaling protocol (such as GMPLS signaling protocol). For example, the mapping request includes the service tag  2  (representing a video service) of the local domain and its corresponding user type ID=B. 
     Block  204 : After receiving the mapping request, the further domain finds a corresponding service tag in its own domain, according to the user type ID in the mapping request, and returns the corresponding service tag to the local domain via a mapping response. In the meantime, a further domain also establishes and saves a mapping relationship between the corresponding service tag and the service tag of the local domain. For example, after finding a service tag  7  that corresponds to the ID=B, a further domain establishes and saves the mapping relationship between the service tag  7  and the service tag  2  of the local domain. 
     Block  205 : Upon receiving the mapping response, the local domain establishes a mapping relationship between the service tag  2  of the local domain and the service tag  7  of a further domain, and saves the mapping relationship. 
     Block  206 : The edge devices of each domain learn the MAC addresses, according to a predefined rule. One rule is that the edge devices of each domain learn the MAC addresses of the edge devices of each domain in the same layer and the MAC addresses of the edge devices of each domain in the lower layer. Another rule is that the edge devices of each domain learn the MAC addresses of the edge devices of each domain in the same layer and the MAC addresses of all user terminals in the bottom layer. 
     Establishing the mapping relationship between the service tags in the above Embodiments One and Two can be realized by configuring a corresponding relationship between various I-TAGs under the help of the configuration schemes provided by the devices. 
     Embodiment Three 
       FIG. 5  illustrates an embodiment of a network system established by using the method based on provider backbone bridges, according to the disclosure. The system comprises three layers, i.e. a backbone provider network, a service provider network and a user network, each layer employing Mac-in-Mac technology. The edge devices of the backbone provider network are designated as B-PE; the edge devices of the service provider network are designated as S-PE, and the edge devices of the user network are designated as U-PE. Normal switches (supporting 802.1Q) are used inside each network layer. 
     There are two users, i.e. user  1  and user  2 , in the user network layer. User  1  is partitioned into two domains and user  2  is also partitioned into two domains, thus there are four domains in total. The two domains of user  1  (i.e. two branches) access two domains of the service provider  1  through S-PE 1  and S-PE 4 , respectively, and the two domains of user  2  (i.e. two branches) access two domains of the service provider  1  through S-PE 2  and S-PE 3 , respectively. 
     User  2  is further divided into two departments, i.e. department  1  and department  2 . Department  1  accesses the two domains of user  2  via U-PE 1  and U-PE 2 , respectively. Department  1  is divided into different VLANs for distinguishing different offices, and each office has a plurality of terminals. For example, there are two terminals PC 1  and PC 3  in the office represented by VLAN2.1.1, and two terminals PC 2  and PC 4  in the office represented by VLAN2.1.2. 
     In the user network layer, each of the four domains assigns a service tag for its local domain in its own way, respectively, and saves in its local domain the mapping relationship between its local domain and the other three domains. For example, the two domains of user  2  respectively assign service tags U-I-TAG- 1  and U-I-TAG- 2  according to their respective internal schemes, and then the mapping relationship between U-I-TAG- 1  and U-I-TAG- 2  are saved in the U-PE 1  and U-PE 2 , respectively. 
     In the service provider network layer, there are two service providers, i.e. service provider  1  and service provider  2 . Service provider  2  has only one domain, while service provider  1  is divided into two domains that access the backbone provider network via B-PE 1  and B-PE 2 , respectively. 
     In the service provider network layer, each of the three domains assigns a service tag for its local domain in its own way, respectively, and saves in its local domain the mapping relationship between its local domain and the other two domains. For example, the two domains of service provide 1 respectively assign service tags S-I-TAG- 1  and S-I-TAG- 2  according to their respective internal schemes, and the mapping relationship between S-I-TAG- 1  and S-I-TAG- 2  is saved in S-PE 2  and S-PE 3 , respectively. 
     The backbone provider assigns a service tag, such as B-I-TAG, in its internal way. 
     The above system realizes Mac-in-Mac hierarchical application without interference between each layer. 
     Embodiment Four 
       FIG. 6  illustrates a flow chart of data transmission performed in a hierarchical network system established with provider backbone bridges, according to an embodiment of the disclosure. This method is illustrated by taking the data transmission in the system of Embodiment Three (as shown in  FIG. 5 ) as an example. 
     PE device in the system of Embodiment Three identifies a layer 2 Mac-in-Mac header, which is inside the current Mac-in-Mac header. In other words, the PE device identifies a peer I-TAG in the layer 2 Mac-in-Mac header, finds a mapping relationship between the peer I-TAG and a local I-TAG in the same layer, and then replaces the peer I-TAG with the local I-TAG. Meanwhile, the PE device replaces the peer B-TAG in the layer 2 Mac-in-Mac header with a new B-TAG (a new B-TAG obtained based on the local I-TAG in the same layer) in the Mac-in-Mac. The PE device then transmits the replaced packet to a peer PE after recalculating the check sum. Moreover, edge devices of each layer in the system learn the MAC addresses of the edge devices of each domain in the same layer, and the MAC addresses of all user terminals in the bottom layer. For example, in the user network layer, U-PE 1  and U-PE 2  not only learn the MAC address of each other, but also learn the MAC addresses of the user terminals (i.e. PC 1  and PC 3 ) of the both sides in the bottom layer. In the service provider network layer, S-PE 2  and S-PE 3  learn the MAC address of each other as well as the MAC addresses of the user terminals of the both sides in the bottom layer. In the backbone provider network layer, B-PE 1  and B-PE 2  learn the MAC address of each other as well as the MAC addresses of the user terminals of the both sides in the bottom layer. Accordingly, when encapsulating a destination MAC address in the data packet according to the learnt MAC addresses, the MAC address of the egress edge device in the local domain is encapsulated as the destination address. Generally, a domain includes an ingress edge and an egress edge; and data packet enters into the domain via the ingress edge and is forwarded to other neighboring domains through the egress edge. In certain circumstances, the ingress edge and egress edge can be the same device. 
     The detailed procedure of packet forwarding in the case that terminal PC 1  accesses terminal PC 3  comprises: 
     Block  301 : a PC 1  packet is forwarded from terminal PC 1  to edge device U-PE 1  of the upper layer, i.e. the user network. The source address and destination address as well as a user tag are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-PC3+MAC-PC1+U-TAG+DATA+FCS,
 
where the destination address is the MAC address of PC 3 , the source address is the MAC address of PC 1 , U-TAG is the user tag of VLAN2.1.1, DATA is the user data and FACS is the check sum.
 
     Block  302 : a U-PE 1  packet is forwarded from the edge device U-PE 1  to edge device S-PE 2  of the upper layer, i.e. the service provider network. The source address and destination address as well as the outer tag and the service tag U-I-TAG- 1  are encapsulated in the packet. The encapsulated packet has the following form:
 
MAC-S-PE2+MAC-U-PE1+U-B-TAG-1+U-I-TAG-1+PC1 packet+FCS,
 
where the destination address is the MAC address of the egress edge device of the local domain. In this example, the egress edge device of the local domain is the ingress edge device S-PE 2  of the upper service provider layer. As a result, the MAC address of S-PE 2  is taken as the destination MAC address. The source address is the MAC address of U-PE 1 , U-B-TAG- 1  is the outer tag for forwarding the packet from U-PE 1  to S-PE 2  in the Mac-in-Mac.
 
     Block  303 : a S-PE 2  packet is forwarded from the edge device S-PE 2  to an edge device B-PE 1  of the upper layer, i.e. the backbone provider network. The source address and destination address as well as the outer tag and the service tag S-I-TAG- 1  are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-B-PE1+MAC-S-PE2+S-B-TAG-1+S-I-TAG-1+U-PE1 packet+FCS;
 
where the destination address is the MAC address of an egress edge device of the local domain. For the purpose of simplification, as an example, the egress edge device of the local domain and the ingress edge device of the upper layer are overlapped with each other (the two edge devices may be the same device in a practical application). That is, the egress edge device of the local domain is B-PE 1 . Thus, the MAC address of B-PE 1  is taken as the destination MAC address. The source address is the MAC address of S-PE 2 , and S-B-TAG- 1  is the outer tag for forwarding the packet from S-PE 2  to B-PE 1  in the Mac-in-Mac.
 
     Block  304 : a B-PE 1  packet is forwarded from the edge device B-PE 1  to edge device B-PE 2  in the same layer. The source address and destination address as well as the outer tag and the service tag B-I-TAG are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-B-PE2+MAC-B-PE1+B-B-TAG+B-I-TAG+S-PE2 packet+FCS;
 
where the destination address is the MAC address of the egress edge device of the local domain, that is, the MAC address of B-PE 2 . The source address is the MAC address of B-PE 1 , and B-B-TAG is the outer tag for forwarding the packet from B-PE 1  to B-PE 2  in the Mac-in-Mac.
 
     Block  305 : a B-PE 2  packet is forwarded from the edge device B-PE 2  to edge device S-PE 3  of a lower layer, i.e. the service provider network. When de-encapsulating the data packet, the service tag S-I-TAG- 1  is replaced with a service tag S-I-TAG- 2 , according to the mapping relationship between the service tag S-I-TAG- 1  and the service tag S-I-TAG- 2 . Moreover, the source address, destination address and outer tag are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-S-PE3+MAC-B-PE2+S-B-TAG-2+S-I-TAG-2+U-PE1 packet+FCS;
 
where the destination address is the MAC address of S-PE 3 , the source address is the MAC address of B-PE 2 , S-B-TAG- 2  is the outer tag for forwarding the packet from B-PE 2  to S-PE 3  in the Mac-in-Mac.
 
     Block  306 : a S-PE 3  packet is forwarded from the edge device S-PE 3  to an edge device U-PE 2  of a lower layer, i.e. the user network. When de-encapsulating the data packet, the service tag U-I-TAG- 1  is replaced with a service tag U-I-TAG- 2 , according to a mapping relationship between the service tag U-I-TAG- 1  and the service tag U-I-TAG- 2 . Moreover, the source address, destination address and the outer tag are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-U-PE2+MAC-S-PE3+U-B-TAG-2+U-I-TAG-2+PC1 packet+FCS;
 
where MAC-U-PE 2  is the destination address of the de-encapsulated data packet, i.e. the MAC address of U-PE 2 . MAC-S-PE 3  is the source address of the de-encapsulated data packet, i.e. the MAC address of S-PE 3 . U-B-TAG- 2  is the outer tag for forwarding the packet from S-PE 3  to U-PE 2  in the Mac-in-Mac.
 
     Block  307 : a U-PE 2  packet is forwarded from the edge device U-PE 2  to the terminal PC 3 , the content of the PC 1  packet is then obtained through de-encapsulation. 
     Block  308 : the terminal PC 3  de-encapsulates the U-PE 2  packet, i.e. PC 1  packet, and extracts the DATA received from the user PC 1 . 
     Embodiment Five 
     The disclosure also provides another embodiment of data transmission which is different from Embodiment Four. In this embodiment, when terminal PC 1  accesses PC 3  through packet forwarding, the MAC addresses learnt by the edge devices of each network layer are different from those in Embodiment Four. In specific, the edge devices of each network layer learn the MAC addresses of the edge devices of each domain in the same layer as well as the MAC addresses of those in the lower layer. For example, in the service provider layer, S-PE 2  and S-PE 3  learn the MAC address of each other as well as the MAC addresses of U-PE 1  and U-PE 2  in the lower layer. Accordingly, the destination address in the encapsulated data packet is also different. Specifically, the MAC address of the peer edge device in the same layer is encapsulated as the destination address. Thus, blocks  302  and  303  are replaced with the following blocks  402  and  403 . 
     Block  402 : A U-PE 1  packet is forwarded from the edge device U-PE 1  to edge device S-PE 2  of the upper layer, i.e. the service provider network. The source address and destination address as well as the outer tag and the service tag U-I-TAG-I are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-U-PE2+MAC-U-PE1+U-B-TAG-1+U-I-TAG-1+PC1 packet+FCS;
 
where the destination address is the MAC address of U-PE 2  of the peer domain in the same layer. The source address is the MAC address of U-PE 1 , and U-B-TAG- 1  is the outer tag for forwarding the packet from U-PE 1  to S-PE 2  in the Mac-in-Mac.
 
     Block  403 : A S-PE 2  packet is forwarded from the edge device S-PE 2  to edge device B-PE 1  of the upper layer, i.e. the backbone provider network. The source address and destination address as well as the outer tag and the service tag S-I-TAG- 1  are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-S-PE3+MAC-S-PE2+S-B-TAG-1+S-I-TAG-1+U-PE1 packet+FCS;
 
where the destination address is the MAC address of S-PE 2  of the peer domain in the same layer. The source address is the MAC address of S-PE 2 , and S-B-TAG- 1  is the outer tag for forwarding the packet from U-PE 1  to S-PE 2  in the Mac-in-Mac.
 
     Accordingly, the destination address in the de-encapsulated data packet is also different, thus block  306  is replaced with the following block  406 : 
     Block  406 : A S-PE 3  packet is forwarded from the edge device S-PE 3  to edge device U-PE 2  of the lower layer, i.e. the user network. When de-encapsulating the data packet, the service tag U-I-TAG- 1  is replaced with the service tag U-I-TAG- 2 , according to the mapping relationship between the service tag U-I-TAG-I and the service tag U-I-TAG- 2 . Moreover, the source address, destination address and the outer tag are encapsulated in the packet. Thus, the encapsulated packet has the following form:
 
MAC-U-PE2+MAC-U-PE1+U-B-TAG-2+U-I-TAG-2+PC1 packet+FCS;
 
where the destination address is the MAC address of U-PE 2 . The source address is the MAC address of S-PE 3 . U-B-TAG- 2  is the outer tag for forwarding the packet from S-PE 3  to U-PE 2  in the Mac-in-Mac.
 
     The other blocks are the same as that of Embodiment Four, the detailed description of which is omitted here for simplification. 
     Embodiment Six 
     The disclosure also provides another embodiment of data transmission which is different from Embodiment Four. In this embodiment, when the terminal PC 1  accesses the terminal PC 3  through packet forwarding, in the transmission of data packet from the bottom layer to the upper layers at local side, and in the transmission of data packet within the top layer after it arrives at the top layer, I-TAG can be replaced in advance, according to the mapping relationships pre-established by the system, instead of replacing I-TAG in the transmission of the data packet from the top layer to the lower layers. Thus, blocks  304  and  305  are replaced with the following blocks  504  and  505 . 
     Block  504 : A B-PE 1  packet is forwarded from the edge device B-PE 1  to an edge device B-PE 2  in the same layer. The source address and destination address as well as the outer tag and the service tag B-I-TAG are encapsulated in the packet. Meanwhile, the I-TAG in layer 2 Mac-in-Mac header of the packet is replaced. Specifically, S-I-TAG- 1  is replaced with S-I-TAG- 2 , according to a pre-established relationship between S-I-TAG- 1  and S-I-TAG- 2 . The obtained packet has the following form:
 
MAC-B-PE2+MAC-B-PE1+B-B-TAG+B-I-TAG+new packet+FCS;
 
where the destination address is the MAC address of B-PE 2 , the source address is the MAC address of B-PE 1 , and B-B-TAG is the outer tag for forwarding the packet from B-PE 1  to B-PE 2  in the Mac-in-Mac. The new packet is:
 
MAC-B-PE1+MAC-S-PE2+S-B-TAG-1+S-I-TAG-2+U-PE 1  packet+FCS.
 
     Block  505 : A B-PE 2  packet is forwarded from the edge device B-PE 2  to edge device S-PE 2  of the lower layer, i.e. the service provider network. When de-encapsulating the data packet, the service tag is not replaced anymore, only the source address, destination address, and outer tag are encapsulated in the packet. The encapsulated packet has the following form:
 
MAC-S-PE3+MAC-B-PE2+S-B-TAG-2+S-I-TAG-2+U-PE1 packet+FCS;
 
where MAC-S-PE 3  is the destination address in the de-encapsulated data, i.e. the MAC address of S-PE 3 , MAC-B-PE 2  is the source address in the de-encapsulated data, i.e. the MAC address of B-PE 2 , S-B-TAG- 2  is the outer tag for forwarding the packet from B-PE 2  to S-PE 3  in the Mac-in-Mac.
 
     The other blocks are the same as those of Embodiment Four, the detailed of which is omitted here for simplification. 
     The above is just description of the preferred embodiments of the disclosure. Any variation and modification made by those skilled in the art within the scope of the technical solution of the disclosure are included in the scope of protection of the disclosure.