Question

You are an Internet service provider; your client hosts connect directly to your routers. You know some hosts are using experimental TCPs and suspect some may be using a "greedy" TCP with no congestion control. What measurements might you make at your router to establish that a client was not using slow start at all? If a client used slow start on startup but not after a timeout, could you detect that?

Short answer

Monitor packet patterns, interval timings, and RTT to detect absence of slow start. Look for abnormal congestion window adjustments and post-timeout behavior for slow start non-compliance.

Step-by-step solution

- Identify TCP Connection Patterns

Monitor the packets being transmitted from the hosts. Look for the characteristic exponential growth pattern in the number of packets that indicates the use of TCP slow start. If a host's packet transmission ramps up linearly or remains constant, it might be an indication that slow start is not being used.

- Measure Packet Intervals

Measure the intervals between consecutive packet transmissions from the hosts. In TCP slow start, intervals should decrease as more packets are sent. If the intervals remain constant or do not exhibit the expected pattern of decrease, it suggests that there is no slow start operation.

- Analyze Round-Trip Time (RTT)

Track the Round-Trip Time for the packets sent to and received from the hosts. During slow start, the RTT might initially be higher due to network congestion and then stabilize. Consistent or random RTT values that do not follow this trend may indicate bypassing slow start.

- Check for Congestion Avoidance Phase

Once the initial slow start phase is over, TCP moves to the congestion avoidance phase. Monitor the congestion window (cwnd) size for each host. A client not using slow start may show irregular increases in cwnd rather than the expected additive increase.

- Detect Timeout Behavior

Observe the transmission behavior of the hosts after a timeout. TCP typically restarts with slow start after a timeout. If the packets do not show the exponential growth pattern typically associated with slow start after a timeout, it's likely that the host is not adhering to slow start protocols post-timeout.

Key concepts

TCP slow start

TCP slow start is a congestion control algorithm designed to prevent network congestion. When a TCP connection starts, it begins with a small congestion window (cwnd), typically set to the size of a single Maximum Segment Size (MSS). The cwnd then grows exponentially with each acknowledgment received. This means that for every acknowledged packet, the cwnd doubles until it reaches a threshold (ssthresh). After reaching this threshold, TCP transitions to the congestion avoidance phase.

To detect if a host is skipping TCP slow start, monitor the initial packet transmission patterns. Look for exponential growth in packet numbers; a lack of this exponential increase may indicate that the host is not using slow start.

packet transmission patterns

Packet transmission patterns provide critical insight into whether TCP slow start is in use. During the slow start phase, the number of packets sent by the host should initially grow exponentially.

Here are specific steps to monitor packet transmission patterns:

• Count the number of packets that are sent consecutively from the host.
• Track the pattern in which these packets are sent over time.
• If the number increases quickly and exponentially, TCP slow start is likely being used.
• If the increase is linear or constant, slow start might not be in operation.

Regular monitoring of these patterns can help in identifying any deviations from the expected TCP behavior.

Round-Trip Time (RTT) analysis

Round-Trip Time (RTT) is the duration a packet takes to travel from the sender to the receiver and back. Analyzing RTT can help in understanding network performance and congestion.

During the TCP slow start phase, the RTT may initially be high due to network congestion but should stabilize as congestion decreases.

• Track the RTT for packets sent and received.
• Observe how these times change over the connection duration.
• Look for an initial higher RTT that then stabilizes; this is typical during slow start.
• Consistent or erratic RTT values that do not follow this pattern could indicate the host is bypassing slow start.

Effective RTT analysis requires continuous monitoring and data logging.

congestion window (cwnd) monitoring

Monitoring the congestion window (cwnd) provides insights into TCP’s flow control. The cwnd determines how many packets a sender can transmit before needing an acknowledgment.

During slow start, the cwnd should grow exponentially until it reaches a threshold known as ssthresh. After this, TCP enters the congestion avoidance phase where cwnd grows additively.

• Regularly monitor the cwnd size for each TCP connection.
• Look for exponential growth patterns during the initial stage.
• After reaching ssthresh, the growth should become linear.
• Irregular increases in cwnd size could indicate that the host is bypassing TCP slow start protocols.

Accurate cwnd monitoring helps ensure compliance with TCP congestion control principles.

timeout behavior in TCP

Timeouts play a crucial role in TCP’s recovery mechanism. When a timeout occurs, TCP assumes packet loss due to congestion and thus, reduces the cwnd to its initial value, restarting the slow start phase.

To detect non-compliance post-timeout:

• Monitor the packet transmissions after a timeout event.
• Look for the expected exponential growth pattern indicative of slow start.
• Absence of this pattern suggests that the host is neglecting the slow start protocol after timeouts.

Observing timeout behavior can indicate whether a client adheres to proper TCP congestion control after network disruptions.