JP2024049593A - TCP communication device, method and program - Google Patents

Abstract

[Problem] To provide a TCP communication device capable of setting an appropriate retransmission threshold according to the communication situation. [Solution] A TCP communication device that receives packets transmitted in sequence from a transmitting side as a receiving side is characterized by executing a first process 2, 3 for calculating the degree of change in the arrival order of packets from each of a predetermined number of packets most recently received, and a second process 4 for determining a retransmission threshold based on a statistical value of the degree of change in the arrival order of the predetermined number of packets, so that the transmitting side can determine whether to retransmit a packet when the number of times the transmitting side has received duplicate ACKs from the receiving side reaches the retransmission threshold. [Selected Figure] Figure 1

Description

The present invention relates to a TCP communication device, method, and program.

Non-patent document 1 specifies fast retransmission for TCP (Transmission Control Protocol). Normally, if TCP receives three duplicate ACKs (acknowledgements), it will retransmit without waiting for a timeout.

"RFC 5681 - TCP Congestion Control", September 2009

However, in conventional technology, the retransmission threshold is set to a fixed value, such as three times, and appropriate processing is not possible in the following situations:

In the 6G era, it is expected that the capacity of single-user communications will increase (100Gbps class). As the capacity of communications increases, it is expected that multipath communications will be used to secure bandwidth. However, the more multipath communications are used, the greater the risk of packets being sent out of order.

However, as mentioned above, TCP normally assumes that a loss has occurred if it receives three duplicate ACKs, and performs a retransmission, and depending on the type of congestion control algorithm, reduces the congestion window size. In other words, even if three packets overtake a packet, it is considered a loss. As the number of communication paths is increased to four, five, etc. in order to achieve the speed requirements for large-volume communication, this type of retransmission process can easily occur.

On the other hand, if the retransmission decision threshold (i.e., the threshold for determining how many duplicate ACKs are required to perform a retransmission) is set to a value that is too large, retransmissions when actual loss has occurred will be delayed, creating the risk of application communication being halted for long periods of time.

Furthermore, in the case of a multipath configuration such as "Cellular + Cellular," the asymmetry is expected to be smaller (there will be less difference in delay between routes) compared to a "Cellular + WiFi (registered trademark)" configuration, which further increases the risk of mis-ordering, making countermeasures essential.

In view of the problems with the conventional technology described above, the present invention aims to provide a TCP communication device, method, and program that can set an appropriate retransmission threshold according to the communication conditions.

To achieve the above object, the present invention is characterized in that a TCP communication device that receives packets transmitted in sequence from a transmitting side as a receiving side executes a first process of calculating the degree of change in the arrival order of packets from each of a predetermined number of packets most recently received, and a second process of determining a retransmission threshold based on a statistical value of the degree of change in the arrival order of the predetermined number of packets, so that the transmitting side can determine whether to retransmit a packet when the number of times the transmitting side has received duplicate ACKs from the receiving side reaches the retransmission threshold. The present invention is also characterized in that it is a method and a program corresponding to the device.

According to the present invention, it is possible to set an appropriate retransmission threshold according to the communication conditions.

1 is a functional block diagram of a TCP communication device according to an embodiment. FIG. 1 is a configuration diagram of a communication system according to an embodiment. 1 is a flowchart of an operation of a TCP communication system according to an embodiment. FIG. 13 is a diagram showing an explanatory example in the form of a table.

Figure 1 is a functional block diagram of a TCP communication device according to one embodiment. The TCP communication device 10 includes a TCP function unit 1, an out-of-order distance calculation unit 2, an out-of-order distance management unit 3, a retransmission threshold calculation unit 4, a retransmission packet determination unit 5, a retransmission threshold rewriting unit 6, a retransmission flag setting unit 7, and a communication processing unit 8. The TCP communication device 10 may be a mobile terminal such as a smartphone, which performs network communication via a wireless access network.

The TCP functional unit 1 is responsible for normal TCP functions such as connection management, congestion control, retransmission control, and ACK transmission. The out-of-order distance calculation unit 2, out-of-order distance management unit 3, retransmission threshold calculation unit 4, retransmission packet judgment unit 5, retransmission threshold rewriting unit 6, and retransmission flag setting unit 7 each provide additional functions to the TCP functional unit 1 in this embodiment, as will be described in detail later.

The communication processing unit 8 is responsible for all processes required for the TCP communication device 10 to communicate in the TCP/IP hierarchical model, other than the transport layer processes performed by the TCP functional unit 1, and is responsible for the processes of the lower network interface layer and internet layer, and the higher application layer. The communication processing unit 8 can realize each of these layers using any existing method.

Figure 2 is a configuration diagram of a communication system according to one embodiment. The communication system 20 includes a sending node 10S and a receiving node 10R. Both the sending node 10S and the receiving node 10R have the configuration of the TCP communication device 10 in Figure 1 as a functional block configuration, but in the following explanation, as shown in Figure 2, the side that transmits data using some application such as email is referred to as the sending node 10S (abbreviated as the "sending side"), and the side that receives the data is referred to as the receiving node 10R (abbreviated as the "receiving side").

The communication system 20 of this embodiment improves the method of determining whether an "ACK is due to out-of-order" or "ACK is due to loss," making it possible to prevent unnecessary retransmissions. For this reason, the sending side and receiving side each have the following functions:

As shown in FIG. 1, the receiving side has the following functions 1R to 4R.
<1R> The receiving side has an out-of-order distance calculation unit 2, which counts up the out-of-order distance [unit: packet]. Here, the out-of-order distance is a value that indicates how many packets a packet has arrived late, derived from the maximum sequence number of packets that have already arrived and the sequence number of the arriving packet.

<2R> The receiving side has an out-of-order distance management unit 3, which manages the out-of-order distances of the most recent n packets. That is, while the out-of-order distance calculation unit 2 calculates the out-of-order distance for each arriving packet sequentially, the out-of-order distance management unit 3 stores the calculated out-of-order distances for the most recent n arriving packets. n is set in advance as a predetermined number.

＜3R＞ The receiving side has a retransmission threshold calculation unit 4, and periodically derives a retransmission threshold (i.e., the value for how many duplicate ACKs should trigger a retransmission) based on the statistical value of the out-of-order distance for n packets managed by the out-of-order distance management unit 3, and notifies the sending side.

The method for determining the retransmission threshold will be described later, but it may be determined in various ways, such as a percentile guarantee determination method, a method based on the average value, or a method based on the median value.
<4R> The receiving side has a retransmission packet determination unit 5, which has the function of checking the "resend flag" included in the option area of the TCP header to determine whether the received packet is a retransmission packet or not.

As shown in FIG. 1, the transmitting side has the following functions 1S and 2S.
<1S> The transmitting side has a retransmission threshold rewriting unit 6, and rewrites the currently set retransmission threshold based on the retransmission threshold received from the receiving side. When the transmitting side receives the same number of duplicate ACKs as the retransmission threshold, it performs retransmission processing.

<2S> The sending side has a retransmission flag setting unit 7, and when sending a retransmission packet, it sets the "retransmission flag" in the options area of the TCP header to 1, so that the receiving side can determine whether the received packet is a retransmission packet or not.

The reason for setting the resend flag is as follows. If resent packets are included in the calculation of out-of-order distance, the degree of out-of-order will not be calculated correctly. For this reason, a "resend flag area" is included in the optional field of the TCP header, allowing the receiving node to determine whether a packet is a resent packet or a packet that has been out of order.

Figure 3 is a flowchart of the operation of a TCP communication system 20 according to one embodiment.

In step S1, the sender starts sending data to the receiver as part of some application process such as e-mail, establishes a connection with the receiver through processing by the TCP functional unit 1, divides the transmission data into packets, adds the source port, destination port, sequence number, acknowledgment number, etc. as TCP headers, and sends each packet sequentially to the receiver. Also in step S1, the receiver establishes a connection through processing by the TCP functional unit 1, and starts receiving the transmitted packets.

In step S2, the receiving side determines whether a new packet that was started to be transmitted in step S1 has been received. If a new packet has been received, the receiving side proceeds to step S3. If a new packet has not been received, the receiving side returns to step S2 and waits for a new packet to be received.

In step S3, the receiving side determines whether or not it is necessary to send a duplicate ACK for the packet received in step S2. If it is necessary, the process proceeds from step S3 to step S4 to send a duplicate ACK from the receiving side to the sending side, and then proceeds to step S5. If it is not necessary, the process proceeds from step S3 to step S5.

In step S5, the receiving side updates the retransmission threshold using each of its units 2 to 5 (updating to the latest retransmission threshold calculated by retransmission threshold calculation unit 4), transmits it to the transmitting side, and then returns to step S2. Note that the transmission timing of step S5 can be preset to an appropriate timing, and thus may be transmitted as needed every time a packet is received, or may be transmitted once for multiple packets (e.g. 100 packets), or may be transmitted when the value is updated.

The processing performed by each of units 2 to 5 on the receiving side in step S5 is explained in detail below.

The retransmission threshold calculation unit 4 may determine the retransmission threshold using various statistics of the n out-of-order distances d stored in the out-of-order distance management unit 3 in the following manner.
● A method that determines the distance based on the standard of "preventing unnecessary retransmissions at the Xth percentile" ● A method that determines the distance based on the average value of out-of-order distances or the deviation from the average value ● A method that determines the distance based on the median value of out-of-order distances or the deviation from the median value

Here, in case packets happen to repeatedly arrive in order, a lower limit may be placed on the retransmission threshold to prevent it from being set to 0, 1, etc. Similarly, an upper limit may be placed on the retransmission threshold to prevent the time until retransmission from becoming extremely long.

The out-of-order distance d can be calculated as follows:
d = (seq_num_max - seq_num) / segment_size
Here, seq_num_max is the maximum sequence number among the n most recently arrived packets, seq_num is the sequence number of the most recently arrived packet, and segment_size is the segment size (the data length of the packet).

In other words, if the data packets sent by the sender are assigned IDs of 1, 2, 3, ... in the order in which they were sent, the out-of-order distance d can be calculated by subtracting the ID value of the most recently arrived packet (say now) from the maximum value (say max) of the IDs of the n most recently arrived packets, and corresponds to a value that quantifies the difference in the arrival order of the most recently arrived packets (the degree to which the arrival order has been changed).
d=max-now

<Example of explanation>
For the sake of explanation, let us assume that n=9 and that the 9 packets are assigned IDs 1 to 9 from the start of transmission according to the order of transmission on the sending side. If the packets arrive in the order 1, 4, 2, 6, 9, 8, 7, 3, 5, the out-of-order distance d calculated sequentially is as shown in Figure 4.

In the example of the table in Figure 4, the out-of-order distance d is as follows: When ID = 4 arrives: d = 4 - 4 = 0
When ID = 2 arrives: d = 4 - 2 = 2
When ID = 6 arrives: d = 6 - 6 = 0
When ID = 5 arrives: d = 9 - 5 = 4

In the case of the example table of FIG. 4, when ID=5 arrives, the retransmission threshold t can be calculated as follows using various statistical values for n=9:
・To prevent unnecessary retransmissions at the 90th percentile, the retransmission threshold t = 5 (In the table above, if we allow up to 4 duplicate ACKs, we can prevent unnecessary retransmissions at the 90th percentile)
・If you use the average value (rounded up), then t = 2 (if the lower limit of the threshold is t_min, then if t_min ≧ 2, there is no change to the threshold)
・If you use the median, t = 1 (if the lower limit of the threshold is t_min, there is no change to the threshold if t_min ≧ 0)

As described above, according to the embodiment of the present invention, the number of unnecessary retransmission processes can be minimized by setting an appropriate retransmission threshold according to the communication situation. Various supplementary examples, alternative examples, additional examples, etc. will be explained below.

<About the hardware configuration and providing the present embodiment as a program>
Each function of the TCP communication device 10 is realized by the TCP communication device 10 being equipped with computer hardware such as a CPU and memory, and the CPU executing a computer program stored in the memory. A communication interface may be used for transmission and reception. The TCP communication device 10 may be configured using a general-purpose computer device, or may be configured as a dedicated hardware device.

The TCP communication device 10 of this embodiment can contribute to the development of infrastructure for information and communication technology. This makes it possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), which is to "build resilient infrastructure, promote sustainable industrialization, and foster innovation."

In steps S2 to S5 in Figure 3, the operation on the receiving side has been mainly explained, but the transmitting side also performs the processing of the retransmission threshold rewrite unit 6 and the retransmission flag setting unit 7 as described above depending on the information returned from the receiving side. When calculating statistics for n=9, etc. as shown in Figure 4, the retransmission packet decision unit 5 processes the statistics for n=9, etc., excluding the retransmission packets, as described above.

10...TCP communication device, 1...TCP function unit, 2...out-of-order distance calculation unit, 3...out-of-order distance management unit, 4...retransmission threshold calculation unit, 5...retransmission packet judgment unit, 6...retransmission threshold rewriting unit, 7...retransmission flag management unit

Claims (8)

In a TCP communication device that receives packets sequentially transmitted from a transmitting side as a receiving side,
a first process of calculating a degree of change in the packet arrival order from each of a predetermined number of packets most recently received;
and a second process of determining a retransmission threshold based on a statistical value of the degree of switching in the specified number of packets, so that the sending side can decide to retransmit a packet when the number of times the sending side has received duplicate ACKs from the receiving side reaches the retransmission threshold. The TCP communication device according to claim 1, further comprising a third process of transmitting the retransmission threshold to the sending side. The TCP communication device according to claim 1, characterized in that in the first process, an average value, a median value, or a percentile value is used as the statistical value. The TCP communication device according to claim 1, characterized in that in the first process, the degree of switching is calculated as a value obtained by subtracting the transmission order of the latest received packet from the transmission order of the latest of the predetermined number of packets. The TCP communication device according to claim 1, characterized in that in the first process and the second process, the predetermined number of packets is determined excluding retransmitted packets. The TCP communication device according to claim 1, characterized in that in the second process, the retransmission threshold is determined within a range of a predetermined upper limit and lower limit. In a TCP communication method in which a receiver receives packets sequentially transmitted from a sender,
a first step of calculating a degree of change in the packet arrival order from each of a predetermined number of packets recently received;
and a second step of determining a retransmission threshold based on a statistical value of the degree of switching in the predetermined number of packets, so that the sending side can determine whether to retransmit a packet when the number of times the sending side has received duplicate ACKs from the receiving side reaches the retransmission threshold. A program that causes a computer to function as a TCP communication device that receives packets transmitted sequentially from a sender as a receiver,
a first process of calculating a degree of change in the packet arrival order from each of a predetermined number of packets most recently received;
and a second process of determining a retransmission threshold based on a statistical value of the degree of switching in the specified number of packets, so that the transmitting side can determine whether to retransmit a packet when the number of times the transmitting side has received duplicate ACKs from the receiving side reaches the retransmission threshold.

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