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ffrdp is a fast reliable protocol base on udp + arq + fec.
License: GNU Lesser General Public License v3.0
This project forked from rockcarry/ffrdp
ffrdp is a fast reliable protocol base on udp + arq + fec.
License: GNU Lesser General Public License v3.0
ffrdp 是一个基于 udp + arq + fec 的快速可靠协议 (500 行代码实现了完整的 udp + arq + fec,简洁高效。github 上目前还找不到更简洁的) livedesk + fanplayer 使用 ffrdp 实现了 100ms 以内低延迟的 wifi 无线音视频实时传输 rto 计算: 初始: rtts = rttm; rttd = rttm / 2; 迭代: rtts = (1 - alpha) * rtts + alpha * rttm; // alpha = 1 / 8 rttd = (1 - beta ) * rttd + beta * abs(rttm - rtts); // beta = 1 / 4 正常: rto = rtts + r * rttd; 超时: rto = 1.5 * rto; 帧定义: data frame: data_full frame: 0x00 seq0 seq1 seq2 data ... data_short frame: 0x01 seq0 seq1 seq2 data ... data_fec2 frame: 0x02 seq0 seq1 seq2 data ... fec_seq0 fec_seq1 data_fec3 frame: 0x03 seq0 seq1 seq2 data ... fec_seq0 fec_seq1 data_fec4 frame: 0x04 seq0 seq1 seq2 data ... fec_seq0 fec_seq1 ... ... data_fec32 frame: 0x3E seq0 seq1 seq2 data ... fec_seq0 fec_seq1 ack frame: 0x40 una0 una1 una2 mack0 mack1 mack2 rwnd query frame: 0x41 data_full frame 为不带 fec 的 data 长帧 data_short frame 为不带 fec 的 data 短帧 data_fecN 为每 N 帧带一个 fec 帧(N >= 2 && N <= 32) 协议特点: 选择重传、快速重传、非延迟 ACK、UNA + MACK、非退让流控、FEC 前向纠错 协议说明: seq una 长度为 24bit,recv_win_size 为 16bit ack 帧包含了 una, mack 和 rwnd size 信息 mack 24bit 是一个 bitmap, 包含了 una 之后,但又已经被 ack 的帧号 query 命令用于查询 ack fec_seq 长度为 16bit 用于 FEC 例如:una: 16, mack: 0x000003 这个应答代表 ack 1 1 1 1 0 1 1 0 0 0 0 seq 12 13 14 15 16 17 18 19 20 21 22 ... 这些帧已经被接收方收到并应答 una+mack 的方式被用于选择重传和快速重传 FEC 说明: 采用异或方式实现 FEC 针对 full frame 即帧长度为 MTU 的帧,进行 FEC 纠错 data frame 的最后两个字节用作 FEC 的 seq. rockcarry 2020-9-1
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