Patent Application
20080002575
Transmission Control Protocol (TCP) provides a variety of mechanisms that enable senders and receivers to tailor a connection to the capabilities of the different devices participating in a connection and the underlying network. For example, TCP enables a receiver to specify a receive window that the sender can use to limit the amount of unacknowledged data sent. For example, during connection setup, the receiver can set a window value in a TCP segment header that corresponds to an amount of buffer space reserved for receiving connection data. The sender may maintain a send window to ensure that the amount of unacknowledged data does not exceed the receive window.
As another example, TCP permits a receiver to specify a maximum segment size header option that identifies the largest TCP segment that the receiver can handle. Again, a sender may size outgoing segments to conform to the receiver's declared maximum segment size limit.
In addition to these receiver reported capabilities, TCP uses a technique known as a “slow start” to prevent a sender from overwhelming a network when a connection begins. The slow start uses a congestion window that limits unacknowledged data transmissions to an amount that is, at least initially, much smaller than the receive window. For example, a congestion window may initially be set to one maximum segment size. While starting at a relatively small value, the congestion window can grow exponentially and soon reach the size of the receive window. For example, the congestion window size may be incremented by the maximum segment size for each non-duplicate (“new”) ACK received.
At any time, congestion may be detected by a TCP sender, for example, by receiving duplicate ACKs, detecting transmission time-outs of data sent but unacknowledged, or other techniques. To cure this congestion, a TCP sender can reduce its congestion window in an attempt to reduce traffic in a congested network. In addition, the sender can determine a slow start threshold that slows growth of the congestion window when the window reaches a size that may have overwhelmed the underlying network. After congestion detection, the congestion window can again grow exponentially until the congestion window reaches the slow start threshold. Thereafter, the sender can adjust the congestion window in smaller, more linear growth. For example, a sender may increment the congestion window by [(maximum segment size/congestion window)*maximum segment size] for each non-duplicate ACK received. Computing the division of maximum segment size by the congestion window, however, can be a very expensive operation in terms of compute cycles. Instead of this division,
As shown, the code 120 also represents the slow start threshold 102 as an integer value, “ssthread_factor” that represents a ratio of a slow start threshold 102 to the maximum segment size. Again, this division need not be implemented, but is implied by the handling of the “ssthread_factor”. As shown, when the congestion window factor (“cwnd_factor”) exceeds the ssthresh_factor 122, the code increments a congestion window count (“cwnd_count”) that represents a count of non-duplicate ACKs received since a count reset. When the congestion window count (“cwnd_count”) equals the congestion window factor 126, the congestion window factor is incremented by 1, the count is reset, and the congestion window is determined 130 to be an amount increased by the maximum segment size. The code 120 dispenses with largely insignificant fractional increases in the congestion window until an entire maximum segment size adjustment occurs. Since TCP typically sends data in maximum segment size segments when enough data is ready to be sent, an increase of the congestion window by a fraction of the maximum segment size does not alter the number of segments transmitted within the window. Thus, the technique avoids an expensive division while operating in an equivalent manner to an implementation that performs the division.
Generally, TCP implementations maintain state information for a connection in a set of state data known as a TCB (Transmission Control Block). The state data is generally read and updated for each segment transmitted and received, for example, to update the next expected sequence number, adjust a send window, etc. The code in
The implementation of
As shown, a TCP sender may respond 160 to detected congestion. In this case, a slow start threshold factor is determined 162. For example, a slow start threshold may be computed by floor (½ [(min (congestion window size, receive window size)]/maximum segment size). Alternately, such a computation may be determined based on a representation of the congestion window and receive window as factors as described above (e.g., floor (½ [min (congestion window factor, receive window factor)])). In addition, the sender may 164 reduce the congestion window factor and/or congestion window size to permit the underlying network to recover.
The techniques describe above can be implemented in a variety of ways and in different environments. For example, the techniques may be implemented with a network processor. As an example,
Both processors 200, 250 may be coupled to or include Media Access Devices (MACs) to receive network data (e.g., a wireless or Ethernet MAC). In both processors 200, 250, TCP processing may be offloaded to one or more of the cores. Such cores may not feature division instructions. In such circumstances the congestion management techniques can save substantial compute cycles over techniques that implement division as repeated subtractions and/or shifting. For example, implementing the techniques described above can potentially reduce congestion window computations from 235 cycles to 15 cycles. This low-cycle consumption can, for example, help a processor attain 10-Gigabit/s wire-speed processing.
While
Techniques described above can be implemented in a wide variety of circuitry (e.g., ASICs (Application Specific Integrated Circuits), PGA (Programmable Gate Arrays), and so forth). The term circuitry as used herein includes digital circuitry and analog circuitry. The circuitry design may be generated by encoding logic described above in a hardware description language (e.g., Verilog or VHDL). The circuitry may also include programmable circuitry. The programmable circuitry may include a processor that operates on computer programs, disposed on computer readable storage mediums.
Other embodiments are within the scope of the following claims.
