Reconsidering Fragmentation and Reassembly

重新考虑分裂和重组 互联网协议（IP）在过去15年中的成功，在此期间出现了几种新技术，这是由于它支持多样性。 对于在不同的网络上使用不同的最大数据包大小，IP的解决方案是将太大的数据包分成片段，并在目的地重新组装这些片段。 本项目重新考虑了与IP碎片化和重组相关的几个问题。 首先重新考虑重组。 目前的重组算法太慢。 如果片段以FIFO顺序到达，则可以使用一个简单的预期情况优化来将重组性能提高到每个片段38条指令;该项目的目标是在NetBSD UNIX内核中实现此优化。 该项目还引入了优雅中间重组（GIR）的新思想，这是对现有IP目的地和逐跳重组机制的推广。 在GIR中，片段在中间路由器处合并，以便在其传出接口上使用最大大小的数据包。 可以表明，GIR总是优于通常的逐跳重组的IP机制。 这个项目的一个目标是修改现有的IP路由器代码，以表明GIR可以经济地实现在路由器中的小处理和内存成本。 该项目还重新考虑了碎片化问题。 它示出，避免碎片有其自身的成本，在增加的数据包处理和/或往返延迟。 测量中描述的TCP性能提高后，打开碎片。 例如，在NetBSD下的以太网上，对于TCP使用536字节的段大小导致仅约6 Mb/s的吞吐量，而使用1460字节的段大小（没有分段）获得8.45Mb/s的吞吐量，并且使用16260字节的段大小和分段获得8.82Mb/s的吞吐量。 该项目还将研究简单的性能模型，这些模型可用于确定何时碎片化是有益的。 最后，该项目解决了分散的主要缺点。 一个主要的问题是，当一个片段丢失时，整个TCP数据段必须重新传输，从而导致减少或零有效吞吐量的损失。 针对这一问题，提出了一种新的机制-动态分段调整机制，即在数据丢失后分段大小动态减小。 模拟（使用修改后的NetBSD内核）将被用来显示动态段大小保持在合理的水平，即使在极端有损的条件下。 所有机制（重组优化、GIR和动态段大小调整）都是正交的，可以应用于TCP/IP以外的其他协议套件。 有关此项目状态的持续信息，请访问http://dworkin.wustl.edu/~varghese/FRAG/fraginfo.html

Reconsidering Fragmentation and Reassembly The Internet Protocol(IP)'s success for the last 15 years, during which several new technologies emerged, is due to its support for diversity. IP's solution to the use of different maximum packet sizes on different networks is to split packets that are too big into fragments, and to reassemble these fragments at the destination. This project reconsiders several issues related to fragmentation and reassembly IP. Reassembly is reconsidered first. The current reassembly algorithms are too slow. A simple expected case optimization can be used to improve reassembly performance to 38 instructions per fragment if the fragments arrive in FIFO order; a goal for this project is to implement this optimization in the NetBSD UNIX kernel. The project also introduces the new idea of Graceful Intermediate Reassembly (GIR), which is a generalization of the existing IP mechanisms of destination and hop-by-hop reassembly. In GIR, fragments are coalesced at an intermediate router in order to use the largest sized packets on its outgoing interface. It can be shown that GIR always outperforms the usual IP mechanism of hop-by-hop reassembly. A goal for this project is to modify existing IP router code to show that GIR can be implemented economically in routers with small processing and memory costs. The project also reconsiders fragmentation. It is shown that that avoiding fragmentation has costs of its own in terms of increased packet processing and/or round-trip delays. Measurements are described in which TCP performance improves after turning on fragmentation. For example, on Ethernet under NetBSD using a 536 byte segment size for TCP results in a throughput of only about 6Mb/s, whereas a throughput of 8.45Mb/s is obtained using a segment size of 1460 bytes (without fragmentation) and a throughput of 8.82Mb/s using a segment size of 16260 bytes with fragmentation. The project will also investigate simple performance models th at can be used to determine when fragmentation is beneficial. Finally, the project address the major disadvantages of fragmentation. A major problem is that when a fragment is lost, the entire TCP segment must be retransmitted, resulting in reduced or zero goodput under loss. A new mechanism --- dynamic segment sizing, in which the segment size is dynamically reduced after loss --- is introduced, which addresses this problem. Simulations (using a modified NetBSD kernel) will be used to show that dynamic segment sizing keeps the goodput at reasonable levels even under extremely lossy conditions. All mechanisms (reassembly optimization, GIR, and dynamic segment sizing) are orthogonal and can be applied to other protocol suites besides TCP/IP. Ongoing information about the status of this project can be found in http://dworkin.wustl.edu/~varghese/FRAG/fraginfo.html