📄 rfc2760.txt
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FACK is expected to improve performance in all environments outlined in section 2. Since it is better able to sustain its self-clock than TCP Reno, it may be considerably more attractive over long delay paths.3.3.2.2.5 Possible Interaction and Relationships with Other Research Both SACK based loss recovery algorithms described above (the fast recovery enhancement and the FACK algorithm) are similar in that they attempt to effectively repair multiple lost segments from a window of data. Which of the SACK-based loss recovery algorithms to use is still an open research question. In addition, these algorithms are similar to the non-SACK NewReno algorithm described in section 3.3.1, in that they attempt to recover from multiple lost segments without reverting to using the retransmission timer. As has been shown, the above SACK based algorithms are more robust than the NewReno algorithm. However, the SACK algorithm requires a cooperating TCP receiver, which the NewReno algorithm does not. A reasonable TCP implementation might include both a SACK-based and a NewReno-based loss recovery algorithm such that the sender can use the most appropriate loss recovery algorithm based on whether or not the receiver supports SACKs. Finally, both SACK-based and non-SACK-based versions of fast recovery have been shown to transmit a large burst of data upon leaving loss recovery, in some cases [Hay97]. Therefore, the algorithms may benefit from some burst suppression algorithm.3.3.3 Explicit Congestion Notification3.3.3.1 Mitigation Description Explicit congestion notification (ECN) allows routers to inform TCP senders about imminent congestion without dropping segments. Two major forms of ECN have been studied. A router employing backward ECN (BECN), transmits messages directly to the data originatorAllman, et al. Informational [Page 16] RFC 2760 Ongoing TCP Research Related to Satellites February 2000 informing it of congestion. IP routers can accomplish this with an ICMP Source Quench message. The arrival of a BECN signal may or may not mean that a TCP data segment has been dropped, but it is a clear indication that the TCP sender should reduce its sending rate (i.e., the value of cwnd). The second major form of congestion notification is forward ECN (FECN). FECN routers mark data segments with a special tag when congestion is imminent, but forward the data segment. The data receiver then echos the congestion information back to the sender in the ACK packet. A description of a FECN mechanism for TCP/IP is given in [RF99]. As described in [RF99], senders transmit segments with an "ECN- Capable Transport" bit set in the IP header of each packet. If a router employing an active queueing strategy, such as Random Early Detection (RED) [FJ93,BCC+98], would otherwise drop this segment, an "Congestion Experienced" bit in the IP header is set instead. Upon reception, the information is echoed back to TCP senders using a bit in the TCP header. The TCP sender adjusts the congestion window just as it would if a segment was dropped. The implementation of ECN as specified in [RF99] requires the deployment of active queue management mechanisms in the affected routers. This allows the routers to signal congestion by sending TCP a small number of "congestion signals" (segment drops or ECN messages), rather than discarding a large number of segments, as can happen when TCP overwhelms a drop-tail router queue. Since satellite networks generally have higher bit-error rates than terrestrial networks, determining whether a segment was lost due to congestion or corruption may allow TCP to achieve better performance in high BER environments than currently possible (due to TCP's assumption that all loss is due to congestion). While not a solution to this problem, adding an ECN mechanism to TCP may be a part of a mechanism that will help achieve this goal. See section 3.3.4 for a more detailed discussion of differentiating between corruption and congestion based losses.3.3.3.2 Research [Flo94] shows that ECN is effective in reducing the segment loss rate which yields better performance especially for short and interactive TCP connections. Furthermore, [Flo94] also shows that ECN avoids some unnecessary, and costly TCP retransmission timeouts. Finally, [Flo94] also considers some of the advantages and disadvantages of various forms of explicit congestion notification.Allman, et al. Informational [Page 17] RFC 2760 Ongoing TCP Research Related to Satellites February 20003.3.3.3 Implementation Issues Deployment of ECN requires changes to the TCP implementation on both sender and receiver. Additionally, deployment of ECN requires deployment of some active queue management infrastructure in routers. RED is assumed in most ECN discussions, because RED is already identifying segments to drop, even before its buffer space is exhausted. ECN simply allows the delivery of "marked" segments while still notifying the end nodes that congestion is occurring along the path. ECN is safe (from a congestion control perspective) for shared networks, as it maintains the same TCP congestion control principles as are used when congestion is detected via segment drops.3.3.3.4 Topology Considerations It is expected that none of the environments outlined in section 2 will present a bias towards or against ECN traffic.3.3.3.5 Possible Interaction and Relationships with Other Research Note that some form of active queueing is necessary to use ECN (e.g., RED queueing).3.3.4 Detecting Corruption Loss Differentiating between congestion (loss of segments due to router buffer overflow or imminent buffer overflow) and corruption (loss of segments due to damaged bits) is a difficult problem for TCP. This differentiation is particularly important because the action that TCP should take in the two cases is entirely different. In the case of corruption, TCP should merely retransmit the damaged segment as soon as its loss is detected; there is no need for TCP to adjust its congestion window. On the other hand, as has been widely discussed above, when the TCP sender detects congestion, it should immediately reduce its congestion window to avoid making the congestion worse. TCP's defined behavior, as motivated by [Jac88,Jac90] and defined in [Bra89,Ste97,APS99], is to assume that all loss is due to congestion and to trigger the congestion control algorithms, as defined in [Ste97,APS99]. The loss may be detected using the fast retransmit algorithm, or in the worst case is detected by the expiration of TCP's retransmission timer. TCP's assumption that loss is due to congestion rather than corruption is a conservative mechanism that prevents congestion collapse [Jac88,FF98]. Over satellite networks, however, as in many wireless environments, loss due to corruption is more common than on terrestrial networks. One common partial solution to this problem isAllman, et al. Informational [Page 18] RFC 2760 Ongoing TCP Research Related to Satellites February 2000 to add Forward Error Correction (FEC) to the data that's sent over the satellite/wireless link. A more complete discussion of the benefits of FEC can be found in [AGS99]. However, given that FEC does not always work or cannot be universally applied, other mechanisms have been studied to attempt to make TCP able to differentiate between congestion-based and corruption-based loss. TCP segments that have been corrupted are most often dropped by intervening routers when link-level checksum mechanisms detect that an incoming frame has errors. Occasionally, a TCP segment containing an error may survive without detection until it arrives at the TCP receiving host, at which point it will almost always either fail the IP header checksum or the TCP checksum and be discarded as in the link-level error case. Unfortunately, in either of these cases, it's not generally safe for the node detecting the corruption to return information about the corrupt packet to the TCP sender because the sending address itself might have been corrupted.3.3.4.1 Mitigation Description Because the probability of link errors on a satellite link is relatively greater than on a hardwired link, it is particularly important that the TCP sender retransmit these lost segments without reducing its congestion window. Because corrupt segments do not indicate congestion, there is no need for the TCP sender to enter a congestion avoidance phase, which may waste available bandwidth. Simulations performed in [SF98] show a performance improvement when TCP can properly differentiate between between corruption and congestion of wireless links. Perhaps the greatest research challenge in detecting corruption is getting TCP (a transport-layer protocol) to receive appropriate information from either the network layer (IP) or the link layer. Much of the work done to date has involved link-layer mechanisms that retransmit damaged segments. The challenge seems to be to get these mechanisms to make repairs in such a way that TCP understands what happened and can respond appropriately.3.3.4.2 Research Research into corruption detection to date has focused primarily on making the link level detect errors and then perform link-level retransmissions. This work is summarized in [BKVP97,BPSK96]. One of the problems with this promising technique is that it causes an effective reordering of the segments from the TCP receiver's point of view. As a simple example, if segments A B C D are sent across a noisy link and segment B is corrupted, segments C and D may have already crossed the link before B can be retransmitted at the linkAllman, et al. Informational [Page 19] RFC 2760 Ongoing TCP Research Related to Satellites February 2000 level, causing them to arrive at the TCP receiver in the order A C D B. This segment reordering would cause the TCP receiver to generate duplicate ACKs upon the arrival of segments C and D. If the reordering was bad enough, the sender would trigger the fast retransmit algorithm in the TCP sender, in response to the duplicate ACKs. Research presented in [MV98] proposes the idea of suppressing or delaying the duplicate ACKs in the reverse direction to counteract this behavior. Alternatively, proposals that make TCP more robust in the face of re-ordered segment arrivals [Flo99] may reduce the side effects of the re-ordering caused by link-layer retransmissions. A more high-level approach, outlined in the [DMT96], uses a new "corruption experienced" ICMP error message generated by routers that detect corruption. These messages are sent in the forward direction, toward the packet's destination, rather than in the reverse direction as is done with ICMP Source Quench messages. Sending the error messages in the forward direction allows this feedback to work over asymmetric paths. As noted above, generating an error message in response to a damaged packet is problematic because the source and destination addresses may not be valid. The mechanism outlined in [DMT96] gets around this problem by having the routers maintain a small cache of recent packet destinations; when the router experiences an error rate above some threshold, it sends an ICMP corruption-experienced message to all of the destinations in its cache. Each TCP receiver then must return this information to its respective TCP sender (through a TCP option). Upon receiving an ACK with this "corruption-experienced" option, the TCP sender assumes that packet loss is due to corruption rather than congestion for two round trip times (RTT) or until it receives additional link state information (such as "link down", source quench, or additional "corruption experienced" messages). Note that in shared networks, ignoring segment loss for 2 RTTs may aggravate congestion by making TCP unresponsive.3.3.4.3 Implementation Issues All of the techniques discussed above require changes to at least the TCP sending and receiving stacks, as well as intermediate routers. Due to the concerns over possibly ignoring congestion signals (i.e., segment drops), the above algorithm is not recommended for use in shared networks.3.3.4.4 Topology Considerations It is expected that corruption detection, in general would be beneficial in all environments outlined in section 2. It would be particularly beneficial in the satellite/wireless environment over which these errors may be more prevalent.Allman, et al. Informational [Page 20] RFC 2760 Ongoing TCP Research Related to Satellites February 20003.3.4.5 Possible Interaction and Relationships with Other Research SACK-based loss recovery algorithms (as described in 3.3.2) may reduce the impact of corrupted segments on mostly clean links because recovery will be able to happen more rapidly (and without relying on the retransmission timer). Note that while SACK-based loss recovery helps, throughput will still suffer in the face of non-congestion related packet loss.3.4 Congestion Avoidance3.4.1 Mitigation Description During congestion avoidance, in the absence of loss, the TCP sender adds approximately one segment to its congestion window during each RTT [Jac88,Ste97,APS99]. Several researchers have observed that this policy leads to unfair sharing of bandwidth when multiple connections with different RTTs traverse the same bottleneck link, with the long⌨️ 快捷键说明
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