Improvement in TCP Performance

IMPROVEMENT IN TCP PERFORMANCE

The protocols to improve the performance of TCP are:

Link-layer protocols

There have been several proposals for reliable link-layer protocols. The two main classes of techniques employed by these protocols are: error correction (using techniques such as forward error correction (FEC)), and retransmission of lost packets in response to automatic repeat request (ARQ) messages. The link-layer protocols for the digital cellular systems in the U.S. — both CDMA and TDMA — primarily use ARQ techniques. While the TDMA protocol guarantees reliable, in-order delivery of link-layer frames, the CDMA protocol only makes a limited attempt and leaves it to the (reliable) transport layer to recover from errors in the worst case.

The AIRMAIL protocol employs a combination of FEC and ARQ techniques for loss recovery. The main advantage of employing a link-layer protocol for loss recovery is that it fits naturally into the layered structure of network protocols. The link-layer protocol operates independently of higher-layer protocols (which makes it applicable to a wide range of scenarios), and consequently, does not maintain any per-connection state. The main concern about link-layer protocols is the possibility of adverse effect on certain transport-layer protocols such as TCP.

Indirect-TCP (I-TCP) protocol

This was one of the early protocols to use the split-connection approach. It involves splitting each TCP connection between a sender and receiver into two separate connections at the base station — one TCP connection between the sender and the base station, and the other between the base station and the receiver. In our classification of protocols, ITCP is a split-connection solution that uses regular TCP for its connection over wireless link. I-TCP, like other split-connection proposals, attempts to separate loss recovery over the wireless link from that across the wireline network, thereby shielding the original TCP sender from the wireless link.

However, as experiments indicate, the choice of TCP over the wireless link results in several performance problems. Since TCP is not well-tuned for the lossy link, the TCP sender of the wireless connection often times out, causing the original sender to stall. In addition, every packet incurs the overhead of going through TCP protocol processing twice at the base station (as compared to zero times for a non-split-connection approach), although extra copies are avoided by an efficient kernel implementation.

Another disadvantage of this approach is that the end-toend semantics of TCP acknowledgments is violated, since acknowledgments to packets can now reach the source even before the packets actually reach the mobile host. Also, since this protocol maintains a significant amount of state at the base station per TCP connection, handoff procedures tend to be complicated and slow.

The Snoop Protocol

The snoop protocol introduces a module, called the snoop agent, at the base station. The agent monitors every packet that passes through the TCP connection in both directions and maintains a cache of TCP segments sent across the link that have not yet been acknowledged by the receiver. A packet loss is detected by the arrival of a small number of duplicate acknowledgments from the receiver or by a local timeout.

The snoop agent retransmits the lost packet if it has it cached and suppresses the duplicate acknowledgments. In classification of protocols, the snoop protocol is a link-layer protocol that takes advantage of the knowledge of the higher-layer transport protocol (TCP). The main advantage of this approach is that it suppresses duplicate acknowledgments for TCP segments lost and retransmitted locally, thereby avoiding unnecessary fast retransmissions and congestion control invocations by the sender.

The per-connection state maintained by the snoop agent at the base station is soft, and is not essential for correctness. Like other link-layer solutions, the snoop approach could also suffer from not being able to completely shield the sender from wireless losses.

Selective Acknowledgments

Since standard TCP uses a cumulative acknowledgment scheme, it often does not provide the sender with sufficient information to recover quickly from multiple packet losses within a single transmission window. Several studies have shown that TCP enhanced with selective acknowledgments performs better than standard TCP in such situations. SACKs were added as an option to TCP by RFC 1072. However, disagreements over the use of SACKs prevented the specification from being adopted, and the SACK option was removed from later TCP RFCs. Recently, there has been renewed interest in adding SACKs to TCP.

Two of the more interesting proposals are the TCP SACKs Internet Draft and the SMART scheme. The Internet Draft proposes that each acknowledgment contain information about up to three non-contiguous blocks of data that have been received successfully. Each block of data is described by its starting and ending sequence number. Due to the limited number of blocks it is best to inform the sender about the most recent blocks received.

An alternate proposal, SMART, uses acknowledgments that contain the cumulative acknowledgment and the sequence number of the packet that caused the receiver to generate the acknowledgment (this information is a subset of the three-blocks scheme proposed in the Internet Draft). The sender uses these SACKs to create a bitmask of packets that have been successfully received. This scheme trades off some resilience to reordering and lost acknowledgments in exchange for a reduction in overhead to generate and transmit acknowledgments.