1. MTU
网络层单次最大传输单元，包括ip头，受限于数据链路层的最大栈大小。大的MTU减少传输开销，却增大网络延迟。
maximum transmission unit is the size of the largest protocol data unit (PDU) that can be communicated in a single network layer transaction.The MTU relates to, but is not identical to the maximum frame size that can be transported on the data link layer。
Larger MTU is associated with reduced overhead. Smaller MTU values can reduce network delay. A larger MTU brings greater efficiency because each network packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed.Large packets occupy a slow link for more time than a smaller packet, causing greater delays to subsequent packets.
Underlying data link and physical layers usually add overhead to the network layer data to be transported. For example, with Ethernet, the maximum frame size is 1518 bytes, 18 bytes of which are overhead (header and FCS), resulting in an MTU of 1500 bytes.
2.MSS
传输层中TCP的最大报文段字节数，不包括报头。可以设置为机器能处理的最大ip数据报大小减去ip报头和tcp报头。
The maximum segment size (MSS) is a parameter of the options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not count the TCP header or the IP header .
To avoid fragmentation in the IP layer, a host must specify the maximum segment size as equal to the largest IP datagram that the host can handle minus the IP and TCP header sizes.[2] Therefore, IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576[3] - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280[4] - 40 - 20).
Small MSS values will reduce or eliminate IP fragmentation, but will result in higher overhead.[5]
Each direction of data flow can use a different MSS.
For most computer users, the MSS option is established by the operating system.The value cannot be changed after the connection is established.
3. ARQ
自动重传请求（Automatic Repeat-reQuest，ARQ）是OSI模型中运输层的错误纠正协议之一。传统自动重传请求分成为三种，即停等式(stop-and-wait）ARQ，回退n帧（go-back-n）ARQ，以及选择性重传（selective repeat）ARQ。后两种协议是滑动窗口技术与请求重发技术的结合，由于窗口尺寸开到足够大时，帧在线路上可以连续地流动，因此又称其为连续ARQ协议。三者的区别在于对于出错的数据报文的处理机制不同。三种ARQ协议中，复杂性递增，效率也递增。
4. RTT
RTT（Round Trip Time）由三部分组成：链路的传播时间（propagation delay）、末端系统的处理时间、路由器缓存中的排队和处理时间（queuing delay）。
其中，前两个部分的值对于一个TCP连接相对固定，路由器缓存中的排队和处理时间会随着整个网络拥塞程度的变化而变化。所以RTT的变化在一定程度上反应了网络的拥塞程度。