Abstract:
A method of transmitting packets from the source edge router through the label switch router to the destination edge router, comprising the steps of:
       assigning different protocol type indicators at the source edge router to user MPLS packets and to non-user MPLS packets of at least one additional protocol type;   at the label switch router, forwarding MPLS packets received from the source edge router or another label switch router in such a manner as to preserve the protocol type indicator of the packet transport protocol of each received MPLS packet;   at the destination edge router, recognizing the protocol type indicator of the transport protocol of the MPLS packets received from the label switch router, and segregating the user MPLS packets from non-user MPLS packets. Preferably, user and non-user MPLS packets are assigned the same MPLS label and sent via the same label switched path. Corresponding enhanced MPLS network is also provided.

Description:
FIELD OF THE INVENTION 
     The invention relates to communications networks, and in particular, to a packet network and a method for carrying multiple packet streams within a single label switched path. 
     BACKGROUND OF THE INVENTION 
     A commonly used standard packet transport protocol for the transmission of data packets over fiber links is POS, where POS is an acronym for “PPP Over SONET”, PPP is an acronym for “Point-to-Point Protocol”, and SONET is an acronym for “Synchronous Optical NETwork”. 
     PPP provides an encapsulation for variable length packets transmitted from a sending terminal to a receiving terminal. PPP is described in more detail in the IETF (internet engineering task force) documents RFC 1661 and RFC 1662. POS provides the adaptation of a PPP packet to meet the requirements of the SONET standard. POS is described in more detail in the IETF document RFC 2615. 
     Several other fiber transport protocols exist for transmitting data packets over fiber links. For example, the proposed Gigabit Ethernet WAN (Wide Area Network) standard (10 Gigabit Ethernet Technology Overview White Paper, http://www.10gea.org/10GEA_Whitepaper — 0901.pdf) provides similar capabilities for carrying data packets as does the POS standard. 
     MPLS (Multiprotocol Label Switching) is a protocol used in high speed data packet networks to provide efficient routing and switching of packets. In an MPLS network, packets are assigned a label (by a label edge router) and forwarded along a label switched path (LSP) where each label switch router (LSR) makes forwarding decisions based on the contents of the label. One of the capabilities of MPLS is the ability to create end-to-end circuits with specific performance characteristics. The MPLS architecture is described in IETF document RFC 3031. The MPLS label mechanism is described in IETF document RFC 3032. 
     The Internet protocol (IP) is the most common networking protocol providing end-to-end user packet networks. MPLS networks are used to build high capacity and high performance backbone networks linking IP networks. 
     While MPLS and POS protocols provide the basis for building fiber based packet networks which can forward IFP user packets, there is also a requirement in such networks to provide OAM&amp;P (Operations, Administration, Maintenance, and Provisioning) capabilities which permit the operator of the network to interrogate and control the operation of the network. 
     As part of the OAM&amp;P functionality, it is advantageous in high performance networks to be able to monitor network performance in real time in order to detect any deterioration of the expected performance. This is especially important in MPLS networks where the minimum performance of a user connection may be specified in a service level agreement between the network operator and the user. Performance parameters of interest include packet loss, end-to-end packet delay, and delay variation. 
     One method of determining network performance is the collection of statistics by the network nodes. This method can provide summary or detailed packet loss information, but is not appropriate for monitoring delay parameters. This method also requires a great deal of processing of all packets if detailed (per connection) information is to be gathered, and does not lend itself to real time monitoring. 
     Another method is to send test packets through the network. The disadvantage of this method is that test packets must either have different labels (in an MPLS network) or different IP destination addresses, in order to be distinguishable from the user data. If they have different labels, they require additional network resources (labels are a limited resource) and it would be difficult to guarantee that the test traffic will be subject to exactly the same degradation as the user traffic. If the method is based on different IP destination addresses, additional processing in the forwarding path is required, leading to greater expense or lower throughput. 
     MPLS provides the general capability of inserting more than one label in each packet (known as a label stack). This capability could be used to provide an additional label to differentiate between the user data stream from the OAM&amp;P packets, but at the expense of the additional label (an increase in packet overhead), and the additional label insertion and decoding step in the edge routers (additional processing in the forwarding path). 
     Therefore there is a need for the development of the enhanced network and method of transmitting the data through the network, which would provide additional capabilities without using additional resources in the network. 
     SUMMARY OF THE INVENTION 
     An objective of the invention is to provide a method and an enhanced network, which would provide carrying multiple packet streams within a single label switched path. 
     According to one aspect of the invention there is provided a multi-protocol label switching (MPLS) packet network, having at least one source edge router, at least one label switch router, and at least one destination edge router connected by transmission links, and using a packet transport protocol providing a protocol type indicator of the transported packet, the network comprising: 
     means for assigning different protocol type indicators for user MPLS packets and non-user MPLS packets of at least one additional protocol type; 
     means at the source edge router for transmitting the non-user MPLS packets; 
     means at the label switch router for forwarding MPLS packets received from the source edge router or from another label switch router in such a manner as to preserve the protocol type indicator of the packet transport protocol of each received MPLS packet; 
     means at the destination edge router for recognizing the protocol type indicator of the transport protocol of the MPLS packets received from the label switch router and means for segregating the user MPLS packets from non-user MPLS packets. 
     Preferably, the means at the source edge router for transmitting the non-user MPLS packets of the additional protocol type comprises a means for transmitting the non-user MPLS packets of said additional protocol type with the same MPLS labels as user MPLS packets. Accordingly, the means for segregating the user MPLS packets from non-user MPLS packets comprises a means for segregating, based on said protocol type, MPLS packets received with the same MPLS label. Conveniently, the source edge router further comprises a means for sending non-user MPLS packets to the destination edge router, using the same label switched path as for the user MPLS packets. 
     The means for transmitting non-user MPLS packets may comprise means for transmitting signalling frames, OAM&amp;P (operations, administration, maintenance and provisioning) frames or other non-user types of frames between the edge routers. Conveniently, the network further comprises a means for monitoring said label switched path by using said OAM&amp;P frames. 
     Beneficially, the source edge router comprises processing means for generating non-user MPLS packets, and the destination router comprises processing means for receiving and analyzing received non-user MPLS packets. 
     The described enhanced network may use one of the following transport protocols: Point-to-point over SONET (POS), Gigabit Ethernet or Internet Protocol (IP). 
     According to another aspect of the invention there is provided a method for transmitting packets in an MPLS packet network comprising at least one source edge router, at least one destination edge router and at least one label switch router connected by transmission links and using a packet transport protocol providing a protocol type indicator of the transported packet, the method of transmitting packets from the source edge router through the label switch router to the destination edge router, comprising the steps of: 
     assigning different protocol type indicators at the source edge router to user MPLS packets and to non-user MPLS packets of at least one additional protocol type, 
     at the label switch router, forwarding MPLS packets received from the source edge router or another label switch router in such a manner as to preserve the protocol type indicator of the packet transport protocol of each received MPLS packet; and 
     at the destination edge router, recognizing the protocol type indicator of the transport protocol of the MPLS packets received from the label switch router, and segregating the user MPLS packets from non-user MPLS packets. 
     Advantageously, the step of transmitting the traffic comprises transmitting the non-user MPLS packets of said additional protocol type with the same MPLS labels as user MPLS packets. Correspondingly, the step of segregating the user MPLS packets from non-user MPLS packets comprises segregating, based on said protocol type, MPLS packets having the same MPLS label. Conveniently, the method provides transmission of non-user packets from the source edge router to the destination edge router using the same label switched path as for the user MPLS packets. The method can be applied for the transmission of different types of traffic (MPLS packets) such as IP traffic, OAM&amp;P (operations, administration, maintenance and provisioning) traffic or signalling traffic. 
     According to another aspect of the invention there is provided an edge router for an multi-protocol label switching (MPLS) network, including the edge router and at least one label switch router connected by transmission links and using different protocol type indicators of the transported packets for user MPLS packets and non-user MPLS packets of at least one additional protocol type, the router comprising: 
     means for transmitting the non-user MPLS packets; 
     means for recognizing the protocol type indicator of the transport protocol of the MPLS packets received from the label switch router; and 
     means for segregating the user MPLS packets from non-user MPLS packets. 
     Beneficially, the means for transmitting the MPLS packets comprises a multiplexer for multiplexing user and non-user MPLS packets and assigning same MPLS label to the user and non-user packets. Conveniently, the edge router may be used as the source edge router. 
     In the described edge router, the means for segregating the user and non-user MPLS packets may comprise a demultiplexer, which provides segregation of said packets based on the assigned protocol type indicators. Such router can be used as the destination edge router. 
     According to yet another aspect of the invention there is provided a label switch router for a multi-protocol label switching (MPLS) network, including at least one edge router and the label switch router connected by transmission links and using different protocol type indicators of the transported packets for user MPLS packets and non-user MPLS packets of at least one additional protocol type, the label switch router comprising: 
     means for forwarding MPLS packets received from the edge router or from another label switch router in such a manner as to preserve the protocol type indicator of the
         packet transport protocol of each received MPLS packet.       

     The method and enhanced network providing sending of different type of MPLS traffic along same MPLS path, e.g. sending test traffic, allows a direct observation of the performance of a user&#39;s connection through the network. For example, when used for signalling, the method permits the establishment of a signalling path over an existing path through a network. This is a more efficient use of resources than if additional paths for signalling were established. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail with reference to the attached drawings, in which: 
         FIG. 1  shows an MPLS network of the prior art providing a data path between two IP networks; 
         FIG. 2  illustrates the format of a PPP packet of the prior art; 
         FIG. 3  shows an enhanced packet network of the embodiment of the invention, including enhanced label edge routers and an enhanced label switch router; 
         FIG. 4  shows the first enhanced label edge router of the enhanced packet network of  FIG. 3 ; 
         FIGS. 5A ,  5 B and  5 C illustrate formats of PPP packets for carrying IP data, OAM data and signalling data respectively in the network of  FIG. 3 ; 
         FIG. 6  shows a detailed structure of the ingress label demultiplexer of the enhanced label switch router; 
         FIG. 7  shows a detailed structure of the egress label multiplexer of the enhanced label switch router; and 
         FIG. 8  shows the second enhanced label edge router of the enhanced packet network of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an MPLS network of the prior art, illustrating an exemplary data path between two IP networks through an MPLS network. Packets originating in a first IP network  10 , and destined for a second IP Network  12  are transmitted through an MPLS network  14 . Only one direction of traffic flow is described, it being understood that traffic flow in the opposite direction is handled by similar means as the direction described. 
     The MPLS network includes a first label edge router (LER)  16  acting as a source edge router, at least one label switch router (LSR)  18 , and a second label edge router (LER)  22  acting as a destination edge router. The first IP network  10  is connected to the first LER  16  with link  24 . The first LER  16  is connected to the LSR  18  with link  26 . The LSR  18  is connected to the second LER  22  with link  20 , link  20  possibly extending through additional LSRs as indicated by the dashed line. The second LER  22  is connected to the second IP network  12  with link  28 . 
     The links  24  and  28  carry IP data packets encapsulated in a protocol suitable for the links  24  and  28 . Such protocols include Ethernet, ATM, POS and others. 
     The LERs form the edge of the MPLS network. The links  26  and  20  between LERs and LSRs are fiber links using a SONET signal format. Although other fiber link signal formats may also be used, such as a Gigabit Ethernet signal, the detailed description of the prior art and of the preferred embodiment of the invention will be based on SONET, specifically PPP over SONET (POS). 
     As is the nature of networks, an MPLS network would normally contain connections to additional IP networks, additional label edge routers, and additional label switch routers, but for clarity of the description, only a simple path from a first IP network to a second IP network is shown. 
       FIG. 2  illustrates the format of a typical PPP packet  50  of the prior art. 
     The PPP packet  50  includes a PPP header  52 , an MPLS label  54 , an IP header  56 , and IP user data  58 . The IP header  56  and the IP user data  58  together form an IP packet  60 . For transmission over a SONET link using the POS protocol, the packet undergoes additional transformation steps as detailed in the POS standard (RFC 2615) but not illustrated in  FIG. 2 . These steps include the generation of a Frame Check Sequence (FCS) to be appended to the packet, a byte stuffing operation, and scrambling, before insertion in the SONET framing structure. 
     IP data packets  60  are received over link  24  by the first LER ( 16 ) in  FIG. 1 . The first LER contains a label multiplexer  30  which transforms the IP data packets into PPP packets (PPP packet format  50 ) by the addition of the MPLS label  54 , and the PPP header  52 . The PPP header contains a protocol type identifier, which indicates the type of enclosed protocol; in the present case, the enclosed protocol is MPLS. The value of the protocol type identifier for MPLS is given in RFC 3032 as hexadecimal 0281 (MPLS unicast) or hexadecimal 0283 (MPLS multicast). 
     The first label edge router  16  outputs PPP packets in SONET format (POS) on its output link  26  to the label switch router  18 . The label switch router LSR  18  contains an ingress label demultiplexer  32 , a switching fabric  34 , and an egress label multiplexer  36 . 
     The LSR  18  receives PPP packets in SONET format (POS) from link  26 . After extracting the PPP packet from the SONET framing structure, the ingress label demultiplexer  32  checks the protocol type identifier in the PPP header which must indicate MPLS, and uses the incoming MPLS label to route the packet through the switching fabric  34 , the LSR further determines an outgoing MPLS label in accordance with the MPLS standards described in RFC 3031 and RFC 3032. The egress label multiplexer generates an outgoing PPP packet, which has a PPP header with the correct (MPLS) protocol type identifier, the outgoing MPLS label and a copy of the incoming IP data packet. The LSR  18  outputs the outgoing PPP packets in SONET format (POS) on its output link  20  to the second label edge router  22  or, as indicated by the dashed line  20  in  FIG. 3 , to one or more additional label switch routers. Each additional label switch routers in the path performs the same sequence of functions as the LSR  18 . 
     The second label edge router (LER  22 ) includes an egress label demultiplexer  38 . The LER  22  receives PPP packets in SONET format (POS) from the LSR  18  (or an additional LSR) over link  20 . The egress label demultiplexer  38  verifies and drops the PPP header and the MPLS label from the PPP packets received from the LSR in the MPLS network, and delivers the extracted IP data packets to the (second) IP network over link  28 . 
     The path taken by PPP packets starting at the ingress label multiplexer  30  in the first LER and ending at the egress label demultiplexer  38  in the second LER, where the packets use one set of assigned MPLS labels, is known as a label switched path (LSP  40 ). In typical usage, the traffic from a user in the first IP network to a user in the second IP network will be assigned to a dedicated LSP, different LSPs being assigned for different routes, different performance requirements, and other characteristics of the user traffic. 
     The description of the MPLS network of the prior art has been provided for illustrative purposes, and as a basis upon which the novel features of the invention are introduced to enhance the usefulness of a packet network based on the MPLS protocol. 
     The preferred embodiment of the enhanced packet network  114  of the invention is shown in  FIG. 3 . 
       FIG. 3  resembles the MPLS network of the prior art ( FIG. 1 ), but the label edge routers (first LER  16  and second LER  22 ) and the label switch router (LSR  18 ) of the prior art are replaced by enhanced label edge routers (first E-LER  116  and second E-LER  122 ) and an enhanced label switch router (E-LSR  118 ) of the invention. 
     The enhanced packet network  114  receives IP data packets from the first IP network over link  24 , and sends the routed IP data packets to the second IP network over link  28 , as in the prior art. The enhancement of the preferred embodiment of the invention is internal to the enhanced packet network  114 . In addition to providing user IP data packets over a given label switched path  140 , the enhanced packet network has the capability of sending additional packets (OAM frames and signalling frames) from the first E-LER  116  to the second E-LER  122  over the same label switched path  140  as the user IP data packets. 
     The first enhanced label edge router (E-LER)  116  is shown in detail in  FIG. 4 . It contains an enhanced label multiplexer  130 . One of the functions of the enhanced label multiplexer  130  is similar to the corresponding function of the label multiplexer  30  ( FIG. 1 ) of the prior art: the enhanced label multiplexer  130  receives user IP data packets  200  on link  24  and converts them to PPP packets  202 , to be sent out on link  126 . In addition, the enhanced label multiplexer  130  accepts OAM frames  204  and signalling frames  206  which are also converted to PPP packets  202 . 
       FIGS. 5A ,  5 B, and  5 C illustrate the format of the PPP packet  202  which is capable of carrying three types of data packets while using a common MPLS label  208 . 
       FIG. 5A  is equivalent to  FIG. 2  showing the PPP packet  202  containing a user IP data packet  200  prefixed with MPLS label  208  and PPP header  210 . The protocol type identifier in the PPP header is hexadecimal 0281 or hexadecimal 0283 to indicate MPLS unicast and multicast respectively. 
       FIG. 5B  shows an OAM frame  204  being carried in the PPP packet  202 , identified by the protocol identifier hexadecimal 0E07. 
     In  FIG. 5C , a signalling frame  206  is carried in the PPP packet  202 , identified by the protocol identifier hexadecimal 0E01. 
     The enhanced LSR (E-LSR)  118  ( FIG. 3 ) contains the label switching functions of a conventional LSR of the prior art. It may be recalled that a conventional LSR uses the PPP header protocol identifier to verify that a PPP packet carries the MPLS protocol. The conventional LSR then uses the MPLS label for routing. 
     In the E-LSR, the label switching function is enhanced to also perform MPLS switching when the PPP header indicates certain other protocol types. In the preferred form of the E-LSR, new protocol identifier values hexadecimal 0E07 and hexadecimal 0E01 are recognized, and label switching is performed for PPP packets carrying the new protocol identifiers in the same way as if a PPP packet containing the MPLS unicast identifier (hexadecimal 0281) had been received. 
     In  FIG. 6  is illustrated a preferred form of the ingress label demultiplexer  132  of the enhanced LSR  118 . PPP over SONET packets are received over link  126 . The received PPP packet  250  includes a PPP header  252 , a first MPLS label  254 , and a payload packet  256 . The PPP header includes one of the recognized protocol type identifiers. The payload packet is either an IP data packet  200 , an OAM frame  204  or a signalling frame  206 , depending on the protocol type indicated in the associated PPP header, as was illustrated in  FIG. 5 . 
     The PPP header  252  is input to a protocol inspector  258 . The first MPLS label  254  is one of the inputs to a label processor  260 . The other input of the label processor is connected to an output  262  of the protocol inspector  258 . 
     The output of the ingress label demultiplexer  132  is a switch packet  264  which contains a switch header  266 , a second MPLS label  268 , and a copy of the payload packet  256 . The switch packet  264  is sent to the switch fabric  134  of the enhanced label switch router  118  (in  FIG. 3 ). 
     The switch header  266  serves a number of purposes related to the operation of the switch fabric as is customary in the design of packet switches. The switch header is composed of a number of fields (not shown in detail), which receive numerical values via line  272  from the protocol inspector  258  and via line  274  from the label processor  260 . 
     The second MPLS label  268  is output from the label processor  260 . The generation of MPLS labels is covered by the MPLS specifications. This function is unchanged from the prior art. 
     The protocol inspector  258  is responsive to the protocol type field in the PPP header  252  and has two outputs: 
     output  262 , connected to the label processor  260 , enables label processing whenever the PPP header contains a protocol type indicating that the packet contains an MPLS header, including the cases when either IP data packets, OAM frames, or signalling frames are contained in the payload of the PPP packet  250 ; 
     output  272  indicates which of the protocol types was received in the PPP header. This information is encoded in the switch header, and will be carried and switched with the switch packet  264  through the switch fabric  134 . 
     Output  274  of the label processor  260  contains the information required by the switch fabric  136  to route the switch packet  264  to the correct output. The form of this information is dependent on the switch fabric design, but generally is a fabric port address. 
     The egress label multiplexer  136  is illustrated in  FIG. 7 . The switch packet  264  received by the egress label multiplexer  136  from the switch fabric  134  is unchanged from the switch packet  164  transmitted into the switch fabric by the ingress label demultiplexer  132 . The switch header  266  is input to a PPP processor  280 . The output of the PPP processor is a PPP header  282 . The output of the egress label multiplexer  136  is a PPP packet  284  to be sent in SONET format (POS) on line  120 . 
     The PPP packet  284  has the PPP header  282 , a copy of the second MPLS label  268 , and a copy of the payload packet  256 . The PPP header  282  is generated by the PPP processor  280  from information contained in the switch header  266  which includes the encoded protocol type. The outgoing PPP packet  284  will be a copy of the incoming PPP packet  250  ( FIG. 6 ) with the exception of the MPLS label where the first MPLS label  254  has been replaced by the second MPLS label  268 , in accordance with the MPLS label switching mechanism. 
     It is worth noting that the PPP protocol type identifier in the outgoing PPP packet  284  is not (as in the prior art) just an MPLS protocol identifier, but is a copy of the protocol identifier present in the incoming PPP packet  250 . Both, incoming PPP packet  250  and outgoing PPP packet  284 , are instances of the PPP packet  202  whose format was described in  FIG. 5 . 
       FIG. 8  shows a block diagram of the second enhanced label edge router (E-LER  122  in  FIG. 3 ) including an enhanced label demultiplexer  138 . PPP over SONET packets are received from an enhanced label switch router, e.g. ELSR  118 , over link  120 . The PPP packets have the format of a PPP packet  202 . The enhanced label demultiplexer  138  has outputs generating three types of signal: OAM frames  204 , signalling frames  206 , and IP data packets  200 . The IP data packets  200  are output over line  28  to an IP network, e.g. the second IP network  12 , while the OAM and signalling frames are available for use within the second enhanced label edge router  122  itself. 
     In functional terms, the enhanced label demultiplexer  138  evaluates the protocol type identifier contained the PPP header  210  in the PPP packet  202  ( FIG. 5 ), strips the PPP header and the MPLS label, and generates the remaining packet as one of three types depending on the protocol type as illustrated in  FIG. 5 : a OAM frame  204  if the protocol type is hexadecimal 0E07; a signalling frame  206  if the protocol type is hexadecimal 0E01; or an IP data packet  200  if the protocol type is hexadecimal 0281 or hexadecimal 0283. 
     Summarizing the operation of the enhanced packet network  114  as a whole:
         the E-LER  116  receives user IP data packets from an IP network, e.g. the first IP network  10 , and inserts them into the label switched path (LSP)  140 ;   the E-LER  116  also has the ability to insert OAM frames and signalling frames in the same LSP  140 ;   the E-LSR  118 , and all other E-LSRs (not shown) which together provide the label switched path (LSP)  140 , forward packets with the PPP protocol type identifier unchanged;   the E-LER  122  extracts user IP data packets from the LSP  140  and forwards them to an IP network;   the E-LER  122  also has the ability to extract OAM frames and signalling frames from the same LSP  140 .       

     It should be noted that at the time of this application, the proposed additional protocol type identifiers (hexadecimal 0E07 and 0E01) have not been standardized. A number of protocol types, including the MPLS identifiers (hexadecimal 0281 and 0283), have been standardized for the standard PPP header, however the protocol type field contains a large number of unused identifier values, and the new values proposed here are to be understood as exemplary values which are compatible with the values already standardized. 
     The use of these new protocol types permits the shared use (multiplexing) of a single label switched path for a number of additional packet streams in addition to the user traffic. Additional packet streams may be generated and received by the edge routers in the enhanced packet network, or more specifically by control processors therein or thereto attached. 
     In terms of applications, the additional packet streams provide the edge routers in the enhanced packet network with the ability to insert test traffic in the label switched path of a user connection, without this test traffic reaching the user. 
     A further use of additional packet streams in the enhanced packet network is to provide a MPLS signalling facility where the label switch routers in the network provide a single semi-permanent path carrying an aggregate of traffic between two MPLS routers requiring a router-to-router signalling capability. Using the capability of the enhanced packet network of sharing an existing label switched path for signalling is a more efficient use of resources than if the routers had to establish additional label switched paths for signalling between them. 
     The concept of the enhanced network and the methods employed therein to create a shared use of a label switched path with SONET fiber links and the PPP over SONET protocol, is readily applied to the case where other transport protocols are used such as the proposed Gigabit Ethernet WAN protocol. The Gigabit Ethernet WAN protocol, provides a protocol type field which can be used in the same manner as the PPP protocol type identifier, to multiplex OAM and signalling frames over a single MPLS label switched path along with user IP data packets. 
     Other modifications to the method and enhanced network, using shared label switched paths for purposes other than OAM and signalling will be apparent to persons skilled in the art, without limiting the application of the invented method and system to other situations. 
     Although specific embodiments of the invention have been described in detail, it will be apparent to one skilled in the art that variations and modifications to the embodiments may be made within the scope of the following claims.