Abstract:
Nodes in a datagram network are arranged to route datagrams in dependence on a batch start time parameter contained in the header of datagrams belonging to a batch. A network operator is thus able to offer a service that is connection-oriented at the batch level, without necessarily being connection-oriented at the message level. This is advantageous to the network operator since it allows the packet flow generated by the user to be re-routed during the transmission of the message. The user can arrange those datagrams which are required to be delivered from the network in the same order as they are supplied to the network into batches. In that way, the user is not adversely affected by route changes made by the network operator in order to improve the performance of the datagram network.

Description:
This application is the US national phase of international application PCT/GB01/01384 filed 28 Mar. 2001 which designated the U.S. 
     BACKGROUND 
     1. Technical Field 
     According to an exemplary embodiment of the present invention, there is provided a method of operating a datagram network node having at least one input channel and a plurality of output channels, said method comprising the steps of: 
     2. Related Art 
     It is now common for messages (whether they be telephone conversations or data file transfers) to be divided into packets before transmission across a communications network. This is advantageous to the operator of the communications network because packets from one message can be interspersed with packets from another message. That enables a more efficient utilisation of the available network resources. 
     Some packet networks offer the user a connectionless service. This means that a user using the network to transfer packets cannot rely on the network to output the packets in the same order he or she puts them in. In applications such as data transfer between two computers, the burden this places on the user is found to be acceptable. This is because computers are often supplied with networking software which controls the computer to attach a sequence number to each outgoing packet when it acts as a sender and to order received packets in accordance with their sequence number when it acts as a receiver. 
     Other packet networks offer the user a connection-oriented service. Here, the network can be relied on to output the packets derived from a given message in the same order as that in which they are supplied to the network. In practice, in order to offer such a service it is found necessary to send every one of the packets derived from a given message along the same route. Most telecommunications networks provide a connection-oriented service and this requirement for persistent routes reduces a telecommunications network operator&#39;s ability to manage the traffic traversing its network. Over recent years, computer data has provided an increasing proportion of telecommunications traffic. As explained above, data transfer between computers does not require a connection-oriented service—hence a telecommunications network that can take advantage of this and offer a connection-oriented service only to those that require it is becoming increasingly desirable. 
     BRIEF SUMMARY 
     According to the present invention, there is provided a method of operating a datagram network node having at least one input channel and a plurality of output channels, said method comprising the steps of:
         forwarding a leading subset of a batch of datagrams in accordance with a stored extant route entry comprising an indication of a datagram-carried route identifier and an associated extant output channel;   storing a new route entry comprising an indication of said datagram-carried route identifier and an associated new output channel;   forwarding one or more datagrams in accordance with said new route entry;   subsequently identifying one or more datagrams as members of a trailing subset of said batch of datagrams; and   forwarding members of said trailing subset in accordance with said extant route entry.       

     By operating the node to forward a trailing subset of a batch of datagrams received after a routing update over the same channel as a leading subset received before the routing update, the likelihood of the datagram network re-ordering the packets in said batch is reduced. Furthermore, the user is offered a continuous range of service between a connection-oriented service to an almost connectionless service. A connection-oriented service might be obtained by placing every packet derived from a given message in the same batch, an almost connectionless service might be obtained by placing only two consecutive packets from the message in each batch. 
     In preferred embodiments of the present invention, said datagrams in said one or more batches include a time parameter which is substantially equal to the time of generation of the first datagram of said batch, said new route entry has a start time associated with it; and said identification step involves identifying datagrams having a time parameter which precedes the indicated start time as belonging to a trailing subset of one of said one or more batches of datagrams. 
     By identifying only that a received datagram belongs to a trailing subset whose transmission started before the start time of the new route, the need to examine a batch identifier that would otherwise have to be carried in the packet is obviated. This means that individual batch identifiers need not be stored at the network node or in packets which are members of the batch. 
     Other aspects of the present invention are set out in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There now follows a description of specific embodiments of the present invention. The embodiments are described by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  shows an internetwork which operates in accordance with a first embodiment of the present invention; 
         FIGS. 2A ,  2 B and  2 C are flow charts illustrating the operation of a packet source which operates in accordance with a first embodiment of the present invention; 
         FIG. 3  shows a pair of entries in a routing table stored in one of the routers of the internetwork of  FIG. 1 ; 
         FIG. 4  is a flow chart illustrating the forwarding process carried out by one or more of the routers of  FIG. 1 ; 
         FIG. 5  illustrates an example of a packet sequence received by a router in the internetwork of  FIG. 1 ; 
         FIG. 6  shows one of the packets of  FIG. 5  in more detail; 
         FIG. 7  shows another of the packets of  FIG. 5  in more detail; and 
         FIG. 8  shows how the packets of  FIG. 5  are handled by a router operating in accordance with the forwarding process of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       FIG. 1  shows an internetwork comprising a southern Local Area Network (LAN) N 2 , a Wide Area Network (WAN) N 1 , a north-eastern LAN N 3 , a north-western LAN N 4  and links L 4 , L 5 , L 6  therebetween. The southern LAN N 2  is connected via a southern outer link L 4  to the WAN N 1 . The WAN N 1  is connected by a north-eastern outer link L 5  to north-eastern LAN N 3  and by a north-western outer link L 6  to north-western LAN N 4 . 
     The WAN N 1  comprises a southern core router C 1 , a northern core router C 2  and an Frame Relay network N 5  that interconnects them. The southern core router C 1  is connected to the northern end of the southern outer link L 4 . The northern core router C 2  is connected to the southern ends of the north-eastern outer link L 5  and the north-western outer link L 6 . 
     The Frame Relay network N 5  includes a southern core switch S 1 , an eastern core switch S 2  and a northern core switch S 3 . A southern intermediate link L 7  connects the southern core switch S 1  to the southern core router C 1 . A northern intermediate link L 8  connects the northern core switch S 3  to the northern core router C 2 . A first inner link L 9  interconnects the southern core switch S 1  to the northern core switch S 3 . Eastern core switch S 2  is connected to the southern core switch S 3  and the northern core switch S 1  by second inner link L 10  and third inner link L 11  respectively. 
     As will be understood by those skilled in the art, the Frame Relay network N 5  is configured to provide a direct Permanent Virtual Connection (PVC) P 1  between the southern core switch S 1  and the northern core switch S 3 , which direct PVC uses the first inner link L 9 . The Frame Relay network N 5  is further configured to provide an indirect PVC P 2  between the same switches S 1 ,S 3 , which indirect PVC P 2  utilises second inner link L 10 , second core switch S 2  and third inner link L 11 . Each of the PVCs is configured to provide a 2 Mbits −1  service across the Frame Relay Network N 1 . 
     The southern LAN N 2  comprises a LAN E 1  that operates in accordance with the IEEE 802.3 standard and two devices connected thereto, namely a video workstation H 1  and southern router R 1 . The video workstation H 1  includes a video card which is connected to a video camera  10 . The network software in video workstation H 1  differs from normal TCP/IP networking software in a way that will be described in relation to 
       FIG. 3  below. Southern router R 1  is connected to the southern end of the southern outer link L 4 . 
     The north-eastern LAN N 3  comprises a LAN E 2  that operates in accordance with the IEEE 802.3 standard and two devices connected thereto, namely a personal computer H 2  and north-eastern router R 2 . North-eastern router R 2  is connected to the northern end of the north-eastern outer link L 5 . 
     The north-western LAN N 4  comprises a wireless LAN E 3  which operates in accordance with the IEEE 802.11 standard and which interconnects a wireless IP router R 3  and a mobile handset H 3 . The mobile handset H 3  is arranged to be able to provide a video display to its user on screen  12 . The wireless IP router R 3  is connected to the northern end of the north-western outer link L 6 . 
     In the example described, the internetwork ( FIG. 1 ) is configured so the interfaces between elements of the outer networks N 2 , N 3 , N 4  have the IPv4 addresses set out in Table 1 below: 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Interface 
                 IP address 
                 Interface 
                 IP address 
                 Interface 
                 IP address 
               
               
                   
               
             
             
               
                 H1 to E1 
                 204.20.74.21 
                 H2 to E2 
                 204.12.241.10 
                 H3 to E3 
                 204.12.245.2 
               
               
                 E1 to R1 
                 204.20.74.1 
                 E2 to R2 
                 204.12.241.1 
                 E3 to R3 
                 204.12.245.1 
               
               
                 R1 to L4 
                 204.00.35.1 
                 R2 to L5 
                 204.02.01.2 
                 R3 to L6 
                 204.02.02.2 
               
               
                 L4 to C1 
                 204.00.35.2 
                 L5 to N1 
                 204.02.01.1 
                 L6 to C1 
                 204.02.02.1 
               
               
                   
               
             
          
         
       
     
     Within the WAN N 1 , the southern core router C 1  is configured to have two IP addresses assigned to the southern intermediate link L 7 . One of those IP addresses is associated with the PVC P 1 , the other is associated with the PVC P 2 . The northern core router C 2  is equivalently configured. In the present example, the IP addresses assigned to those interfaces are as set out in Table 2 below. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Interface 
                 IP address 
                 Interface 
                 IP address 
               
               
                   
                   
               
             
             
               
                   
                 C1 to P1 
                 204.00.01.1 
                 C2 to P1 
                 204.00.01.2 
               
               
                   
                 C1 to P2 
                 204.00.02.1 
                 C2 to P2 
                 204.00.02.2 
               
               
                   
                   
               
             
          
         
       
     
     The video workstation H 1  is arranged to receive video data from the video camera  10  and process it to provide encoded video data. The encoded video data is arranged into 1.25 Kbyte blocks. The video data encoding process is such that a 20 ms video frame which differs substantially from its predecessor produces ten 1.25 Kbyte blocks of encoded video data. Frames which generate ten blocks of encoded video data are hereinafter referred to as key frames—key frames might, for example, result from video camera  10  panning across the scene in front of it. The ten blocks produced in response to a key frame are referred to hereinafter as a key frame block group. In contrast to key frames, a 20 ms frame which differs only in minor details from the previous frame generates only one 1.25 Kbyte block of data. Such frames are referred to herein as ordinary frames and the blocks of data that encode them as ordinary blocks. The video data encoding process places a sequence number in each block and also labels the fifth ordinary block of any series of consecutive ordinary blocks as a quiet period indicator block. 
     The video workstation H 1  is further arranged to respond to requests for live video data from devices attached to the internetwork ( FIG. 1 ) by operating an encoded video data packet generation process ( FIGS. 2A ,  2 B and  2 C). 
     On receipt of a request for video data (step  200 ) the workstation H 1  first finds whether the device requesting the video data requires a quasi-connection-oriented service or not (step  202 ). In the present embodiment this is achieved by noting the TCP/UDP port number on which the request arrives. If a quasi-connection-oriented service is required then the workstation H 1  operates in accordance with a special packet generation process ( FIG. 2B ). If a quasi-connection-oriented service is not required then the workstation H 1  instead operates in accordance with a normal packet generation process ( FIG. 2C ). 
     The special packet generation process ( FIG. 2B ) is carried out once every 10 ms (step  210 ). The process begins with the retrieval (step  212 ) of a 1.25 Kbyte block of data produced by a video data encoding process. Thereafter, the workstation H 1  finds (step  214 ) whether that 1.25 Kbyte block is a quiet period indicator block. If it is, then a quasi_connection_reset_time parameter is updated with the current time (step  216 ). In either case, control next passes to a special header generation step (step  218 ). 
     The special IP header generation step (step  218 ) generates a packet header which is constructed substantially in accordance with the Internet Protocol version 4 (Ipv4) but which has an additional 32-bit field at the end of the header. Such fields are known as ‘option’ fields in the art. Some options have already been standardised. However the option in the present case is new and is referred to herein as a ‘QCO Option’. The first byte of the thirty-two bits contains a value identifying the option as a QCO Option, the last three bytes provide a quasi-connection reset time field. The quasi_connection_reset_time set in the most recent update step (step  216 ) is written into the quasi-connection reset time field. That time is expressed as the number of hundredths of seconds that had elapsed in the current day (Greenwich Mean Time) at the time that the quasi_connection_reset_time parameter was last set (in the present description all times will be represented in 24-hour time-of-day notation). 
     The special header thus generated is appended to the 1.25 Kbyte block of video data to form a special IP packet (step  220 ). As soon as that packet is formed it is transmitted onto the LAN E 1  (step  222 ). 
     It will be seen that the special IP packet generation process ( FIG. 2B ) generates a packet containing around 1.25 Kbytes of data every 10 ms. This leads to the transmission of data onto the LAN E 1  at a rate of around 1 Mbits −1 . 
     The normal packet generation process ( FIG. 2C ) is similar to the special packet generation process ( FIG. 2B ) save that it does not involve a check to find whether the current 1.25 Kbyte block is a quiet period indicator block and the consequent updating of the quasi_connection_reset_time parameter. Also, the header generation step (step  230 ) and the packet generation step (step  232 ) differ in that the generated header does not include a QCO option field. 
     Both packet generation processes produce one packet per block of encoded video data. Packets carrying quiet period indicator blocks are referred to a quiet period indicator packets, packets carrying key frame initial blocks are referred herein as key frame initial packets and the group of packets corresponding to a key frame block group are referred to herein as a key frame packet group. 
       FIG. 3  shows an excerpt from the routing table stored in southern core router C 1 . Each routing table entry comprises a destination IP address parameter, a route number parameter, a route update time parameter, a next hop IP address parameter and a physical output port parameter. The routing table formation software is similar to conventional routing table formation software but enables the operator of the router C 1  to make one or more further routing table entries for one or more selected destination addresses. On making such further routing table entries, the operator also provides a route update time parameter to be associated with the entry. The software controls the router C 1  to automatically rank entries for a given destination address on the basis of their route update time parameter. The entry having the earliest route update time has its route number parameter set to 1, the next earliest has its route number set to 2 and so on. If the number of routing table entries for that destination address has already reached a predetermined maximum number (5 say), then the router is arranged to delete the routing table entry whose route number parameter is set to one and decrement the route number of each of the other stored routes for that destination by one. 
     Those skilled in the art will be able to generate suitable routing table formation software for the router C 1 . 
     As will be understood by those skilled in the art, the value in the destination IP address field in  FIG. 3  applies to all destination addresses in the range 204.12.240.0 to 204.12.255.255—i.e. any addresses whose first twenty bits are identical to the first twenty bits of the address 204.12.240.0. This has the result that packets addressed to either a device attached to the north-eastern network N 4  or a device attached to the north-western network N 3  are routed using only the routing table entries illustrated in  FIG. 3 . 
     The southern core router C 1  is arranged to route packets received at C 1  by carrying out a routing process illustrated by the flow chart of  FIG. 4 . 
     On a packet being received (step  400 ) the router C 1  first finds (step  402 ) whether the QCO Option is present in the header of the packet. 
     If the QCO option is present (i.e. the packet is a special packet), then a counter n is initialised to a value (step  408 ) equal to the number of routing entries currently stored for the destination address found in the packet. Thereafter, the quasi-connection reset time is read from the packet header and compared to the route update time associated with route number n. If the quasi-connection reset time is later than the route update time then the packet is forwarded (step  412 ) on the basis of that route. 
     If the quasi-connection reset time is earlier than the route update time then a first check is carried out to find whether the route associated with the route update time is in fact the only routing entry for the packet (step  414 ). If it is the only entry, then the packet is forwarded on the basis of that entry (step  412 ). 
     If, on the other hand, one or more other routing entries exist for the packet&#39;s destination address then a searching process (steps  416 ,  410 ,  414 ) is carried out to find the routing table entry whose route update time most closely precedes the quasi-connection reset time found in the header of the special packet. 
     Each round of the searching process begins by decrementing the counter n by 1 (step  416 ). Thereafter, the route update time associated with route number n is compared to the quasi-connection reset time. If the route update time for that route number precedes the quasi-connection reset time then the routing table entry whose route update time most closely precedes the quasi-connection reset time has been found and the packet is forwarded on the basis of that routing table entry (step  412 ). Otherwise, the current round of the searching process continues with a check (step  414 ) that the counter has not reached one (i.e. that other routing table entries for this destination remain to be searched). If no such routing table entries exist, then there is no routing entry having a route update time that precedes the quasi-connection reset time, so the packet is forwarded on the basis of the route having the oldest route update time available (step  412 ). 
     If the counter is still greater than one then a further round of the searching process described above is carried out. 
     It will be realised that for special packets having a QCO option, the effect of the routing process shown in  FIG. 4  will be to forward those packets on the basis of the routing table entry whose route update time parameter most closely precedes the quasi-connection reset time parameter found in the QCO option field of the special packet. 
     If the QCO option is found not to be present (step  402 ) then a counter m is initialised to a value (step  420 ) equal to the number of routing entries currently stored for the destination address found in the packet. Thereafter, the current time (GMT) is compared to the route update time stored associated with route number m. If the time is now later than the route update time then the packet is forwarded (step  422 ) on the basis of that route. 
     If the route update time has not yet been reached then a first check is carried out to find whether the route associated with the route update time is in fact the only routing entry for the packet (step  426 ). If it is the only entry, then the packet is forwarded on the basis of that entry (step  424 ). 
     If, on the other hand, one or more other routing entries exist for the packet&#39;s destination address then a searching process (steps  428 ,  422 ,  426 ) is carried out to find the routing table entry with the most recent route update time. 
     Each round of the searching process begins by decrementing the counter m by 1 (step  428 ). Thereafter, the route update time associated with route number m is compared to the current time. If the route update time for that route number has already occurred then the routing table entry with the most recent route update time has been found and the packet is forwarded on the basis of that routing table entry (step  424 ). Otherwise, the current round of the searching process continues with a check (step  426 ) that the counter m has not reached one (i.e. that other routing table entries for this destination remain to be searched). If no such routing table entries exist, then there is no routing entry having a route update time that precedes the current time, so the packet is forwarded on the basis of the route having the closest route update time (step  424 ). 
     If the counter m is still greater than one then a further round of the searching process described above is carried out. 
     It will be realised that for normal packets, the effect of the routing process shown in  FIG. 4  will be to forward those packets on the basis of the routing table entry with the most recent route update time parameter. 
     A specific example of the performance of the internetwork ( FIG. 1 ) will now be described with reference to  FIGS. 5 ,  6 ,  7  and  8 . 
     For the purposes of this example it is assumed that the personal computer H 2  connected to the north-eastern LAN N 3  requests the video workstation H 1  to supply it with live video data representing what the video camera  10  is currently viewing. The personal computer H 2  is operating under control of networking software that uses the sequence number contained in the blocks of video data to re-order live video packets that arrive in the wrong order. Since the personal computer H 2  therefore does not require the packets to be delivered to it in the right order, the personal computer H 2  indicates in its request that it does not require a quasi-connection-oriented service from the internetwork. The video workstation receives the request and responds by carrying out the normal packet generation process ( FIG. 2C ). 
     At the same time, the user of mobile handset H 3  (connected to the north-western LAN N 4 ) requests the video workstation H 1  to supply it with the same live video data. In contrast to the personal computer H 2 , the mobile handset H 3  is arranged to carry out as little data processing as possible. By requesting a quasi-connection-oriented service from the internetwork ( FIG. 1 ) the data processing associated with re-ordering packets is obviated. Hence, the mobile handset H 3  requests a quasi-connection-oriented service. The video workstation receives the request and responds by carrying out the special packet generation process ( FIG. 2B ). 
     Once both video streams are being generated by the video workstation H 1 , the packets might arrive at the southern core router C 1  as illustrated in  FIG. 5 . It will be seen that the packets received at the router C 1  alternate between those belonging to a stream S 1  of special packets addressed to the mobile handset H 3  and those belonging to a stream S 2  of normal packets addressed to the personal computer H 2 . The time between packet arrivals is around 5 ms and the arrival of packets is shown over a time period that extends from around 19:48:03:65 until 19:48:04.05. 
     In the present example, it is assumed that each stream includes a key frame packet group (K 1 , K 2 ), the initial packets of which arrive at the southern core router at around 19:48:03.85. The key frame special packet group K 1  contains special packets A 1 , A 2 , A 3 , A 4 , A 5 , A 6 , A 7 , A 8 , A 9 , and A 10 . Each of those packets ( FIG. 6 ) has:
     a) a destination IP address field set to the IP address of the interface between the mobile handset H 3  and the wireless LAN E 3  (i.e. 204.12.245.2); and   b) a QCO option field containing a quasi-connection reset time parameter set to the transmission time of the quiet period marker packet I (19:48:03.75). (Note that, in the present case, the arrival time at the southern core router C 1  is assumed to be substantially simultaneous with the transmission time from the video workstation H 1 ).   

     The key frame normal packet group K 2  contains normal packets B 1 , B 2 , B 3 , B 4 , B 5 , B 6 , B 7 , B 8 , B 9 , and B 10 . Each of those packets ( FIG. 7 ) has a destination IP address field set to the IP address of the interface between the personal computer H 2  and the LAN E 2  (i.e. 204.12.241.10) but does not have a QCO option field. 
     Each of the normal packets B 1 , B 2 , B 3 , B 4 , B 5 , B 6 , B 7 , B 8 , B 9 , B 10  in the normal stream S 2  leads the corresponding packet A 1 , A 2 , A 3 , A 4 , A 5 , A 6 , A 7 , A 8 , A 9 , A 10  in the quasi-connection-oriented stream by 5 ms. The order of arrival of the packets of the key frame packet groups K 1 , K 2  at the southern core router is therefore B 1 , A 1 , B 2 , A 2 , B 3 , A 3 , B 4 , A 4 , B 5 , A 5 , B 6 , A 6 , B 7 , A 7 , B 8 , A 8 , B 9 , A 9 , B 10 , A 10 . 
     In the present example, it is assumed that, prior to or at 19:48:03.90, a second entry (see  FIG. 3 ) for destinations in the range 204.12.204.0 to 204.12.255.255 is added to the routing table, which additional entry has a route update time parameter which indicates the time 19:48:03.90. Such an entry might be added automatically or manually in order to cause data travelling from the southern core router C 1  to the northern core router C 2  to be carried along the PVC P 2  rather than the PVC P 1  after 19:48:03.90. This might be done because it is known that the link L 9  is to be withdrawn from service at 19:50, or it might be done in order to balance the load placed on different parts of the Frame Relay network N 5 . 
     The southern core router carries out the process of  FIG. 4  in relation to each of the received packets B 1 , A 1 , B 2 , A 2 , B 3 , A 3 , B 4 , A 4 , B 5 , A 5 , B 6 , A 6 , B 7 , A 7 , B 8 , A 8 , B 9 , A 9 , B 10 , A 10 . 
     For each of the packets A 1 , A 2 , A 3 , A 4 , A 5 , A 6 , A 7 , A 8 , A 9 , A 10  in the key frame special packet group K 1 , step  410  of  FIG. 4  will find that the route update time (19:48:03.90) for route number  2  falls after the quasi-connection reset time (19:48:03.75) contained in the QCO option field of the packet ( FIG. 6 ). Hence route number  2  will be disregarded and the packets will be forwarded on the basis of route number  1 —i.e. over PVC P 1 . 
     For each of the first five packets B 1 , B 2 , B 3 , B 4 , B 5  in the key frame normal packet group K 2 , step  422  of  FIG. 4  will find that the current time (19:48:03.85-.90) is before the route update time associated with route number  2  (i.e. that the route update time has not yet occurred). Hence, the first five packets B 1 , B 2 , B 3 , B 4 , B 5  will be forwarded over route number  1 —i.e. over PVC P 1 . 
     For each of the last five packets B 6 , B 7 , B 8 , B 9 , B 10  in the key frame normal packet group K 2 , step  422  of  FIG. 4  will find that the current time (19:48:03.90-.95) is after the route update time (19:48:03.90) associated with route number  2  (i.e. that the route update time has now occurred). Hence, the second five packets B 6 , B 7 , B 8 , B 9 , B 10  will be forwarded over route number  2 —i.e. over PVC P 2 . 
       FIG. 8  is an illustration of the packets queued for the intermediate link L 7  at 5 ms intervals from the moment of arrival of the fifth key frame special packet A 5 . There are two queues associated with the link L 7 , a first queue Q 1  for the PVC P 1 , and a second queue Q 2  for the PVC P 2 . 
     In this example, shortly before the arrival of the fifth key frame special packet A 5 , congestion has resulted in the first queue Q 1  storing the nine preceding packets B 1 , A 1 , B 2 , A 2 , B 3 , A 3 , B 4 , A 4 , B 5 . 
     As stated above the capacity of the PVCs P 1  and P 2  is 2 Mbits −1 , so around one packet can be forwarded from each queue every 5 ms. As the fifth key frame special packet A 5  arrives, the first packet B 1  of the key frame normal packet group K 2  is forwarded from the first queue Q 1 . 
     After 5 ms, the sixth packet of the B 6  of the key frame normal packet group K 2  arrives at the southern core router C 1  and is placed in the second queue owing to the route update time of route number  2  now having been reached. No other packets are awaiting transmission from the second queue Q 2 , so it (B 6 ) is forwarded immediately over the PVC P 2 . 
     The sixth packet A 6  of the key frame special packet group K 1  arrives 5 ms later. Unlike the previously received packet B 6 , A 6  is forwarded to the first queue Q 1  because the route update time is later than the quasi-connection reset time indicated in the QCO option field of the sixth key frame special packet A 6 . At the same time, the second packet B 2  of the key frame normal packet group K 2  is forwarded over the PVC P 1 . 
     After another 5 ms interval, the seventh packet B 7  of the key frame normal packet group K 2  arrives at the router C 1 , is placed in the second queue Q 2 , and is immediately forwarded over the PVC P 2 . 
     If the delays over the PVCs P 1  and P 2  are equal, then it will be realised that packets will arrive at northern core router C 2  in the order that they were sent from southern core router C 1 . It will be seen that the packets of the key frame normal packet group K 2  have already become jumbled—the order of sending so far is first normal packet B 1 , sixth normal packet B 6 , second normal packet B 2 , seventh normal packet B 7 . This results in the personal computer H 2  receiving the packets in that order. 
     In contrast, the special packets of the key frame special packet group K 1  are all sent to the same queue and hence do not become jumbled and arrive at the mobile handset H 3  in the right order. 
     Around 50 ms after the last of the key frame packets A 1 -B 10  has arrived at the router C 1 , the router receives a special packet J which has a quasi-connection reset time parameter set to 19:48:04.00. It will be realised that this results from the block of video data carried in packet J being found to be the fifth consecutive ordinary block in step  214  ( FIG. 2 ) and hence the quasi_connection_reset_time being updated in step  216 . 
     Hence, the router operating in accordance with  FIG. 4  finds that the route update time for route number  2  precedes the quasi-connection reset time indicated in the header and hence forwards the packet along route number  2 —i.e. along the second PVC P 2 . 
     It will be realised that the change in route might still result in packet re-ordering. However, re-ordering of the ordinary packets will have a less deleterious effect on the video displayed by the mobile phone than would be caused by re-ordering of the key frame packets. 
     It will be seen how the first embodiment provides a quasi-connection-oriented service to the network user. By only updating the quasi-connection reset times at time when packet re-ordering is less likely to cause significant problems, the user is able to receive a service that provides a more ordered packet stream than a connectionless service. By enabling the network operator to re-route traffic flows during the lifetime of a traffic flow the network operator is better able to manage the flow of traffic around its network than it could were it offering a connection-oriented service. 
     A number of features of the above-described embodiment can be changed in order to provide alternative embodiments of the present invention. Possible changes include:
     i) Even datagrams that are not part of datagram streams that require a quasi-connection-oriented service might be formed as special packets, but with the time of transmission being the time of transmission of that particular datagram.   ii) the update to the quasi-connection reset time in the datagrams could be made in response to a signal from the network rather than independently thereof. This signal could be generated in good time before a change in the routing of traffic across the network takes place.   iii) the invention is, of course, useful in relation to many types of traffic other than video streams. In relation to streams of voice data packets, for example, the quasi-connection reset time could be updated each time that a voice activity detector judges the speaker to have been silent for more than 1 s, say.   iv) a maximum limit could be placed on the age of a routing table entry—once the route was that old the routing table entry would be deleted.   v) The routing updates could be those provided by known dynamic routing algorithms, with the routing processes in the node being altered such the previous route is stored rather than being substituted.   

     The section of the special packet stream extending between packet I and packet J in the above-described example can be regarded as a ‘virtual packet’. The creation of such ‘virtual packets’ enables a network to operate in a connection-oriented like mode of operation for a period that may be shorter than the duration of a session but longer than that of a single datagram. In other words, separate virtual packets are treated in a connectionless manner, while their component datagrams are treated in a connection-oriented manner. Therefore, simply by adjusting the size and duration of its virtual packets, an application will be able to create a dynamic mix of connectionless and connection-oriented modes of operation within a single session. In terms of the virtual packet concept, today&#39;s connectionless networks are a special case where the virtual packet is equivalent to a single datagram, whereas connection-oriented networks are a special case where the virtual packet is equivalent to a virtual circuit.