Method and apparatus for segmenting a data packet

[0001] The present invention relates to data communications, and more particularly, to data communications in which multiple service classes are supported.

[0002] The Internet is a collection of interconnected networks, all of which use the Internet Protocol (IP). The connections between these networks can be used to support a wide range of applications including, for example, electronic mail, file transfer, electronic commerce, downloading of web site information, and voice over IP. Different types of IP services may require different qualities of service. Quality of service (QoS) is the level of assurance that the network can meet a particular application's service requirements. From a technical perspective, quality of service can be characterized by several performance criteria such as availability, throughput, setup time, percentage of successful transmissions, etc., and can be measured in terms of bandwidth, packet loss, delay, and jitter. In an IP header, one of the fields typically corresponds to a traffic class, which enables different types/classes of traffic to be differentiated from others. A higher level traffic class corresponding to a higher quality of service may be given a higher priority than a lower level traffic class with a lower QoS. For example, real-time applications such as voice might be given a higher priority than other non-real-time applications such as e-mail.

[0003] In IP networks that support multiple quality of service classes, there may be situations when one traffic class with stricter delay requirements is multiplexed with another traffic class with less strict delay requirements. For example, voice traffic has strict delay requirements while certain types of data traffic typically has less strict delay requirements. In such a situation, even though the voice traffic has priority over the data traffic, the delay of voice packets is nonetheless influenced by the size of the data packets. At the start of transmission of a large data packet, a voice packet cannot be sent until transmission of that large data packet is finished. For example, if the size of the data packet is one kilobyte and the transmission rate is 64 kbps, the next voice packet to be transmitted in the multiplexed transmission may be delayed by as much as 125 milliseconds.

[0004] Accordingly, the way in which IP packets are sized or the way in which IP packets are fragmented/segmented for transmission may affect delay and other service parameters. One example algorithm for fragmenting packets is the point-to-point protocol (PPP) multilink protocol (MP) described in an IETF RFC written in 1990 by K. Sklower et al. entitled, The PPP Multilik Protocol W). The RFC indicates that systems implementing the multilink procedure are not required to segment packets, although segmentation may be performed. Segmenting longer, lower priority data packets may prevent transmission delays of a voice packet on a multiplexed transmission link. However, no segmenting algorithm is described in the RFC.

[0005] One simple approach to segmenting data packets for multilink procedures, (as well as for other procedures), is to divide the packet into segments of equal size, with all of the segments having the maximum segmentation size possible. More than likely, some portion of the packet smaller than the maximum segmentation size will make up the last packet segment. Unfortunately, this relatively simple segmentation procedure does not take into account how it impacts transmission delays for both the voice and data.

[0006] Consider the example in FIG. 1 which shows the output of a voice buffer or queue when there is no data traffic to be transmitted over a multiplexed link A first voice burst includes four packets separated by a very brief idle period from a second voice burst that contains five voice packets. A considerably longer idle period separates the second and third voice bursts, the third voice burst having five data packets, etc. FIG. 2 illustrates a situation where data traffic is interspersed in the transmission with the voice traffic. The timing of transmission of the original voice burst when there is no data traffic is illustrated in dotted blocks for comparison to the multiplexed transmission. The data packet is segmented into four segments, with segments 1, 2, and 3 having the same segment size, and the last segment 4 having a much smaller size.

[0007] As can be seen in FIG. 2A, the five packets in the second voice burst are uniformly delayed by the time it takes to transmit the first data packet segment minus the duration of the first idle period. If the idle period were longer, the delay in transmitting the voice packets in the second burst would be shorter. Segment 1 of the data packet could have increased the delay of the corresponding burst by a value uniformly distributed between 0 and the segment size. This is the case for all other segments. In other words, large data segments are more likely to cause longer voice delays.

[0008] To better understand how segmentation affects the delay of the segmented packet, examine two other segmentation options. FIG. 2B shows a situation where the size of Segment 3 is increased and the size of Segment 2 is decreased by the same amount of bytes. Although the delay of the third voice burst is reduced, the overall delay of the data packet remains the same. FIG. 2C shows that when the size of the last segment is increased, the delay of the data packet decreases. Thus, the delay of the last segment corresponds to the delay of the complete packet and depends on the size of the last segment, assuming the packet size does not change. Accordingly, in to achieve lower delay for the data packet, the last segment should be as large as possible. For lower voice delay, the largest data packet segments should be as small as possible.

[0009] FIG. 3 shows a graph where the size of the last segment and the size of the largest segment for a 1013-byte data packet is plotted as a function of maximum segmentation size in the case where a maximum segmentation algorithm is used. Using this maximum segmentation algorithm is not optimal because the last segment ends up being smaller than the maximum size. Indeed, in some situations, the last segment is very small which corresponds to a longer, worst case delay of data packets.

[0010] More formally, if the size of the last segment is denoted by L, the packet size by P, and the bit rate of the multiplexed link by C, the time needed to transmit the last data packet segment is L/C. The time needed for transmission of the data packet is (T₁T₂), where T₁ is the time when the sum of idle times between voice bursts is equal to (PL)/C, and T₂L/C. Therefore, the delay of the data packet is minimal if the size of last segment is maximized.

[0011] Based on these recognitions, two general rules are employed to characterize how the segmentation size of data packets influences delays in the voice/data multiplex transmission. First, a worst case delay increase of higher priority voice packets is reduced when the size of the largest data segment is reduced. Second, the delay of low priority data packets is reduced when the size of the last data segment, (not the size of the largest segment), is increased relative to the size of the other segments of the data packet.

[0012] The efficiency of transmitting lower priority data traffic along with higher priority traffic is improved by segmenting a data packet in such a way so as to reduce transmission delay of the higher priority traffic. The data packet is segmented so that all its segments, including the last segment, are approximately the same size. The segment size is set smaller than a maximum permitted segment size. However, there may be reasons not to set that size too small. For example, because each includes a protocol header, the total overhead of the data packet transmission is proportional to the number of segments. To reduce such overhead, the number of segments should preferably be kept to the minimum number allowed by the maximum permitted segment size. Thus, it is desirable (though not necessary) to set the segment size as small as design parameters allow in order to reduce transmission delay of the higher priority traffic but at the same time not increase overhead associated with segment headers. Because the last segment is set at the same size or a larger size than the other segments, delay in transmitting the data packet is also reduced. The last segment may be sized as large as practical to minimize the transmission delay of the data packet. Once segmented, the data packet segments are transmitted along with the higher priority traffic.

[0013] One example, non-limiting implementation employs a relatively simple algorithm. Initially, an overall size of the data packet to be transmitted is determined. First and second segment sizes are determined for the data packet. The first and second segment sizes are determined to reduce the delay in transmitting the higher priority traffic, transmitting the data packet, or both. The data packet is segmented into plural segments at the first segment size and a last segment at the second segment size. The higher priority traffic is multiplexed along with the data packet segments. The first segment size is smaller than the maximum allowed segment size, and all of the data packet segments except the last segment are the same first segment size. Although the last segment may be the same size as the first segment size, the second segment size is preferably larger than the first segment size.

[0014] The examples above, described with two priority levels of traffic, may be applied to multiplexed transmissions with three or more traffic priority levels. Two detailed, non-limiting, examples of how to implement the basic segmenting algorithm are described below. In general, the first example emphasizes delay reduction for higher priority traffic, while the second example emphasizes delay reduction for the lower priority data packet segments. However, both the first and second segmentation examples achieve delay reductions for both the high priority and data packet traffic.

[0015] The foregoing and other objects, features, and advantages of the present invention may be more readily understood with reference to the following description taken in conjunction with the accompanying drawings.

[0016] FIG. 1 illustrates the output of the voice queue in FIG. 1 when data packet traffic is not present in the data queue;

[0017] FIGS. 2A-2C illustrate the output of the scheduler 20 in FIG. 1 when data traffic is multiplexed with the voice for different packet segment sizes;

[0018] FIG. 3 is a graph illustrating segment size versus maximum segmentation size for both the last segment and the largest/maximal segment in a segmentation approach where the maximum segmentation size is employed;

[0019] FIG. 4 is a simplified block diagram of a communications system where bursts and data packets are multiplexed over a transmission channel and in which the present invention may be employed;

[0020] FIG. 5 illustrates an Optimized Segmentation routine in accordance with an example embodiment of the present invention;

[0021] FIG. 6 illustrates details of one example segmentation algorithm;

[0022] FIG. 7 is a flowchart illustrating details of a second example segmentation algorithm;

[0023] FIG. 8 is a graph illustrating segment sizes of last and maximal segments for segmentation algorithms;

[0024] FIG. 9 illustrates an example apparatus for implementing a segmentation algorithm;

[0025] FIG. 10 is a graph illustrating voice delay as a function of a maximum segmentation size; and

[0026] FIG. 11 is a graph illustrating data delay as a function of maximum segment size.

[0027] In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular embodiments, procedures, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. For example, while the example embodiment described below relates to voice traffic and low priority data traffic, the present invention may be applied to any types of traffic in a communications system that offers communication services with different qualities of service, priorities, etc.

[0028] In some instances, detailed descriptions of well-known methods, interfaces, devices, and signaling techniques are omitted so as not to obscure the description of the present invention with unnecessary detail. Moreover, individual function blocks are shown in some of the figures. Those skilled in the art will appreciate that the functions may be implemented using individual hardware circuits, using software functioning in conjunction with a suitably programmed digital microprocessor or general purpose computer, using an application specific integrated circuit (ASIC), and/or using one or more digital signal processors (DSPs).

[0029] The present invention may be employed in any communications system where different classes/types/priorities of traffic are multiplexed onto or otherwise share a transmission channel. Consider the simplified, example communications system 10 shown in FIG. 4 that includes a transmit side 12 which sends packet information over a channel to a receive side 14. The transmit side 12 includes a voice queue 16 for storing higher priority voice bursts/packets and a data queue 18 for storing lower priority data packets. A scheduler 20 receives the higher priority voice bursts as well as the lower priority data packets and employs a segmenter 22 for segmenting data packets received from the data queue 18 before multiplexing data packet segments received from the segmenter 22 with the voice bursts before transmission over the channel. The receive side includes a demultiplexer and reassembler 24 which extracts the voice packets and directs them to a voice output as well as reassembles the packets segments into a data packet and directs that reassembled data packet to a data output.

[0030] Optimal segmentation is determined for a data packet of a lower priority or traffic class when information from a higher priority class is transmitted along with segments of the lower priority data packet. Such segmentation may also be employed when there are three or more different traffic classes/priority levels QoS's. However, for simplicity, and not for limitation, the following description employs the example of two traffic classes characterized as higher priority traffic, e.g., voice, and lower priority traffic, e.g., data.

[0031] Although various optimal segmentation algorithms with specific procedures are described below, the present invention is not limited to the details of a particular segmentation algorithm. Rather, the present invention follows two guidelines. First, to reduce delay of a higher priority traffic class, the largest packet size of the lower priority data packet should be reduced. Ideally, the largest packet size should be reduced as small as possible. However, in practice, this goal may be limited to ensure that the number of segments is not unduly increased. As explained above, if the number of segments is larger then needed, the overhead for the packet increases because a segment header is needed for each segment. Thus, it is preferred (but not necessary) that the packet be segmented into as few segments as the maximum segment size allows while minimizing the size of each of those segments in that minimum number. Second, the delay of the lower priority traffic is reduced when the size of the last data segment is increased. Ideally, the last data segment size should be increased as large as possible or practical within design constraints. Thus, both high priority and low priority traffic delays may be reduced if the last segment size is larger than the other segments, and the other segments have approximately the same relatively small size. If the number of segments is determined as the smallest value allowed by the maximum allowed segment size, the largest segment is as small as possible when all of the segments are of equal size.

[0032] Referring now to one example procedure entitled Optimize Segmentation (block 30) shown in FIG. 5, the overall size of the lower traffic class packet to be transmitted is determined (block 32). One or more segment sizes is then determined to reduce the delay of the higher priority traffic and/or the lower priority traffic (block 34). These first two steps define the basic optimized segmentation methodology in accordance with a general example embodiment of the present invention. However, additional steps are shown in FIG. 5 that may be desirably performed. For example, in block 36, the same segment size is selected for most segments of the data packet from the lower traffic class, with that segment size being preferably as small as practical within other system design constraints. In addition, a larger segment size is selected for the last segment to reduce the delay of the lower priority traffic data packet. The packet is segmented accordingly, and the segments are transmitted with the higher priority traffic packets in multiplexed fashion (e.g., similar to the example shown in FIG. 2) over a communications channel (block 36). One example multiplex communication environment is that described above for networks that support multiple service classes and employ the PPP multilink protocol. However, any protocol that segments packets may be used, such as segmentation at the IP level.

[0033] Using the procedures described above, the traffic delay associated with a system that carries two or more traffic classes is decreased both for the higher and lower priority traffic. Furthermore, this reduced traffic delay is easy to implement using a variety of relatively simple segmentation algorithms, two examples of which are described below.

[0034] Any segmentation algorithm that follows the guidelines set forth above may be employed. A first example, non-limiting segmentation algorithm is now described that segments the data packet into equal size segments. A predetermined segment size parameter, e.g., a maximum segment size (MSS) defined for the lower priority traffic class, is used to calculate a number of segments n. More formally, the number of segments $n=\mathrm{ceil}\left[\frac{P}{\mathrm{MSS}}\right],$

[0035] where ceil denotes rounding to the next larger integer if $\frac{P}{\mathrm{MSS}}$

[0036] results in an integer plus a remainder. A number of large segments n₁mod(P, n), where mod denotes the remainder of the division $\frac{P}{n}.$

[0037] A number of small segments n₂nn₁. The size of one or more large segments $S_1=\mathrm{ceil}\left[\frac{P}{n}\right]$

[0038] and the size of the small segments $S_2=\mathrm{floor}\left[\frac{P}{n}\right],$

[0039] where floor denotes rounding to the next smaller integer if $\frac{P}{n}$

[0040] results in an integer and a remainder. The first n₂ segment is small, and the remaining n₂ segments are large. The difference between large and small segments is one byte. It may be desirable in some situations to set the first segment size to a size larger than the maximum segment size that will be applied on a packet. In this case, two different maximum segment sizes will be used. The first segment will be set to a predetermined value, and a segmentation algorithm is applied to the rest of the packet.

[0041] FIG. 6 illustrates a Segment 1 routine (block 40) illustrated in flowchart format in accordance with the first segmentation algorithm. The size of the packet P is determined (block 42). The number of segments n is determined for the lower priority packet in accordance with $n=\mathrm{ceil}\left[\frac{P}{\mathrm{MSS}}\right],$

[0042] where MSS is the maximum segment size set to reduce high priority traffic delay (block 44). A number of larger segments n, mod(P, n) and a number of smaller segments n₂nn₁ are calculated (block 45). Segment sizes Sand S₂ are determined in accordance with the equations: $S_1=\mathrm{ceil}\left[\frac{P}{n}\right]$

[0043] for large segment(s) and $S_2=\mathrm{floor}\left[\frac{P}{n}\right]$

[0044] for small segment(s) (block 46). The data packet is segmented using the segment sizes S₁ and S₂ (block 48). The first n₁ packet segments are set to size S₁. The last n₂ segments are set to size S₂.

[0045] A second example, non-limiting segmentation algorithm is described in a Segment 2 routine (block 50) shown in flowchart form in FIG. 7. Again, the size of the packet P is determined (block 52). The number of segments n is determined for the packet in accordance with $n=\mathrm{ceil}\left[\frac{P}{\mathrm{MSS}}\right],$

[0046] where MSS is the maximum segment size set to reduce high priority traffic delay (block 54). The size of the segment S(i) is determined in accordance with the following $S\left(i\right)=\{\begin{array}{c}\begin{array}{c}\mathrm{if}1in-2\mathrm{then}\mathrm{ceil}\left[\frac{P-\mathrm{MSS}}{n-1}\right]\\ \mathrm{if}i=n-1\mathrm{then}P-\left(n-2\right)\mathrm{ceil}\left[\frac{P-\mathrm{MSS}}{n-1}\right]\end{array}\\ \mathrm{if}i=n\mathrm{then}\mathrm{MSS}\end{array}$

[0047] (block 56). The data packet is segmented using the size relationships for S(i) set forth in block 56 (block 58).

[0048] FIG. 8 is a graph that shows the size of the last and largest segment for both algorithms, Segment 1 and Segment 2, depending upon the maximum segmentation size. Because the last segment is the largest one in both cases, minimal delays are incurred for both algorithms. The curve for the Segment 1 algorithm is below the curve of the Segment 2 algorithm. In other words, the largest and last segment size is smaller when using the Segment 1 algorithm as compared to the Segment 2 algorithm. Therefore, it can be expected that the Segment 1 algorithm outperforms the Segment 2 algorithm regarding voice delay. Regarding data delay, the Segment 2 algorithm outperforms the Segment 1 algorithm.

[0049] The present invention may be employed in any data transmitter. A simplified transmission apparatus 60 is shown in FIG. 9 for implementing the present invention. Multiple priority levels/traffic classes are shown 1, 2, 3, . . . X. Priority 1 traffic is shown directly input to a packet scheduler 62. Other lower priority level traffic classes input their packets into a respective segmenter 64 which segments the packet in accordance with a segmentation algorithm and stores the segments in a buffer 66. The output of the segment buffer 66 is provided to the scheduler 62. The scheduler 62 multiplexes the priority 1 traffic packets with packet segments from one or more of the other lower priority level traffic inputs and sends the combined information out on a physical channel/link, using an appropriate protocol.

[0050] Each segmenter 64, as shown for the priority x traffic, has its own maximum segment size MSS which can be used, for example, in either of the above-described, example segmentation algorithms to determine optimal segment sizes. While a single segmentation algorithm may be employed, it may be desirable in certain situations to have plural segmentation algorithms available and to select one. As will be described below, one segmentation algorithm may be more optimal for minimizing, delays of a higher priority traffic class, while the other algorithm is more optimal for mining delay of a lower priority traffic class. The segmenter 64 also may add a fragmentation protocol header, e.g., PPP/multiprotocol (MP) header, to each segment before sending the segment to the buffer if an PPP/MP protocol is used. These headers permit reconstruction of the data packet from received segments at the receive side.

[0051] To illustrate how the example first and second segment algorithms reduce delays when two different traffic classes are carried over the same link, a simulation was conducted for a low priority class with large packets that need to be segmented and a high priority voice class with potentially short voice packets. The simulation was based on a 1920 kbps link (E1) that carries multiplexed voice-data traffic. Each high priority voice source transmits 144-byte packet every 20 milliseconds, and each low priority data source transmits 1013-byte packet every 20 milliseconds. The traffic mix includes the traffic of one data source and 80 voice sources. Overhead introduced by segmentation was neglected during the simulations because it does not influence the comparison.

[0052] FIG. 10 shows for the simulation voice packet delay as a function of maximum segmentation size, and FIG. 11 shows data packet delay as a function of maximum segment size. From FIG. 10, it can be seen that voice packet delay as a function of maximum segmentation size is the best when segment algorithm 1 is applied. However, segment algorithm 2 still performed better than simply segmenting a data packet using the maximum segment size (MSS). In FIG. 11, it is apparent that the data delay is reduced the most if segment algorithm 2 is used. However, segment algorithm 1 is still better than the MSS method described in the background. Moreover, neither segmentation algorithm adds any additional complexity as compared to the MSS segmentation algorithm.

[0053] While the present invention has been described with respect to particular embodiments, those skilled in the art will recognize that the present invention is not limited to these specific exemplary embodiments. Different formats, embodiments, and adaptations besides those shown and described as well as many variations, modifications, and equivalent arrangements may also be used to implement the invention. Therefore, while the present invention has been described in relation to its preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention. Accordingly, it is intended that the invention be limited only by the scope of the claims appended hereto.