TCP与ECN机制：优势、挑战与应用分析

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本文档深入探讨了TCP/IP协议中显式拥塞通知（ECN）机制的应用及其对TCP性能的影响。首先，作者提出了新的指导原则，针对TCP如何处理ECN机制，例如源抑制报文（Source Quench packets）和数据包头中的ECN字段。这些机制旨在通过在IP数据包头中设置ECN位来指示网络拥塞，而不是仅仅依赖丢包作为信号。作者利用NS2（Network Simulator 2）进行模拟研究，模拟环境中使用了修改后的随机早期检测（Random Early Detection, RED）路由器，它们在检测到拥塞时会将ECN位置为1。同时，模拟的TCP版本是经过调整的 Reno 实现，它不仅可以响应丢包，也能识别到ECN位的变化，从而做出相应的拥塞控制策略。这种设计允许TCP在遇到轻微拥塞时就采取措施，而非等待大量数据包丢失后才启动重传。实验结果显示，ECN机制的一个显著优点是减少了不必要的丢包，从而降低了来自低带宽、延迟敏感的TCP连接的数据包延迟。这在流量密集的网络环境中尤其有价值，因为它避免了因过度的丢包而产生的额外延迟。此外，对于局域网（LAN）等场景，TCP重传定时器的粗粒度可能导致其在快速变化的网络条件下效果受限。ECN机制在这个情况下表现出了更好的适应性，因为它能够更早地感知并调整流量。论文还讨论了一些关于TCP/IP网络中特定ECN机制的实施问题，包括可能遇到的技术挑战以及如何优化它们以提高网络效率。这包括了如何确保TCP客户端和服务器正确识别和响应ECN信号，以及如何在不同的硬件和操作系统环境下实现兼容性。这篇论文提供了对TCP/IP网络中ECN机制的深入理解，强调了其在拥塞控制中的优势，并提出了实用的建议，对于网络设计者、系统管理员和研究者来说，具有重要的参考价值。通过模拟研究和实证分析，论文不仅理论性强，而且为改进TCP性能和网络效率提供了实用的方法论。

longer time scales, to its response to a dropped packet as

an indication of congestion.

Over smaller time scales (e.g., one or two round trip

times), TCP’s response to ECN can be less conservative

than its response to a dropped packet as an indication of

congestion. In Tahoeand Reno implementations of TCP,

after a packet has been dropped the TCP source stops

sending for a time period on the order of a round trip

time (half a round trip time for Reno implementations),

allowing network queues to dissipate somewhat. This

delay is not necessary as a response to an ECN, which

does not indicate a queue overﬂow.

For TCP, the receipt of a single ECN (e.g., a single

Source Quench packet, or a single packet with the ECN

bit set) should trigger a response to congestion. This

is unlike the DECbit congestion control scheme, where

the source responds to congestion only if at least half

of the packets in the last window had the ECN bit set

[RJ90]. The decision to allow a single ECN message to

trigger a response to congestion requires a minimum of

overhead. In addition, because the gateway does not set

the ECN ﬁeld in every arriving packet when the average

queue size is too high, the gateway can use probabilis-

tic algorithms to inform particular sources of congestion

[FJ93]. Because the probability that a connection is no-

tiﬁed of congestion is proportional to that connection’s

share of the bandwidth at the congested gateway, these

probabilistic algorithms reduce global synchronization

and improve fairness.

TCP should react to an ECN at most once per

roundtrip time. The TCP source should ignore succeed-

ing ECNs if the source has reacted to a previous ECN

or to a dropped packet in the last roundtrip time. This

also means that if, immediately after reacting to an ECN,

the TCP source receives three duplicate ACKs indicat-

ing a dropped packet, the TCP source should not repeat

the reduction of the congestion window; the packet was

probably dropped before the source reduced its window

in response to the ECN.

TCP should follow the existing algorithms for send-

ing data packets in response to incoming ACKs. The

response to an ECN does not trigger the sending of any

new (or retransmitted) data packets.

TCP should follow the normal procedure after the

timeout of a retransmit timer. That is, after a retransmit

timer timeout the TCP source should slow-start and re-

transmit the dropped packet. However, the TCP source

should not decrease the slow-start threshold ssthresh if

it has been decreased within the last roundtrip time.

5 Implementing ECN in our simula-

tor

In this section we describe the implementation of TCP’s

response to ECN in our simulator. This implementation

of ECN was made to a version of TCP that incorpo-

rates the Fast Recovery congestion control algorithm in

Reno TCP (4.3-reno BSD TCP), as well as the Slow-

Start, Congestion Avoidance, and Fast Retransmit algo-

rithms in the earlier Tahoe TCP (4.3-tahoe BSD TCP

[J88, S94]).

For the simulations in this paper the RED gateways

were given an option to set the ECN bit in the packet

header, rather than dropping the packet, as an indication

of congestion when the buffer had not yet overﬂowed.

When the TCP receiver receives a data packet with the

ECN bit set in the packet header, the receiver sets the

ECN bit in the next outgoing ACK packet.

First we brieﬂy describe the Slow-Start, Congestion

Avoidance, Fast Retransmit, and Fast Recovery algo-

rithms in TCP. There are two phases to the window-

adjustment algorithm. The connection begins in slow-

start phase, and the current congestion window cwnd is

doubled each roundtrip time until the congestion win-

dow reaches the slow-start threshold ssthresh. Then the

congestion-avoidance phase is entered, and the conges-

tion window is increased by roughly one packet each

roundtrip time. The congestion window is never allowed

to increase to more than the receiver’s advertised win-

dow, which we refer to as the “maximum window”.

In addition to using retransmit timers to detect lost

packets, the source uses the Fast Retransmit procedure

to discover a packet loss. If three duplicate ACK packets

are received acknowledging a previously-acknowledged

data packet, the source infers that a packet has been

dropped.

In the Tahoe version of TCP, the source reacts to a

packet loss by setting the slow start threshold to half

the congestion window, decreasing the congestion win-

dow to one packet, and entering the slow-start phase. In

contrast, with Reno’s Fast Recovery algorithm the TCP

source does not slow-start after inferring that a packet

has been dropped. Instead, the TCP source effectively

waits half a round trip time and halves the congestion

window. The source retransmits the dropped packet and

uses incoming duplicate ACKs to clock additional out-

going packets. The FastRecovery algorithm isdescribed

in more detail in [S94].

The description of the Fast Recovery algorithm in [S94] is quite

good, and is the only complete publicly-available description of this

algorithm that we are aware of. However, we have a caution to readers.

First, the earlier printings (prior to the 4th) do not distinguish between

the Fast Retransmit and the Fast Recovery algorithms. Second, in

the description of the window increase algorithm in the congestion

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