Transport over Wireless Networks

Myungchul Kim

mckim@icu.ac.kr

• Introduction– Wireless networking technologies -> mobile Internet– Picocell, microcell, macrocell and global cell

networking in terms of bandwidth, latency, error rates, and range.

– Transport layer in micro- and macrocell wireless networks

– TCP• web traffic (http)• Reliability and congestion control

– Approaches for improving transport layer performance over wireless networks

• Link layer approaches: by using link layer changes• Indirect approaches: break the end-to-end semantics of TCP• End-to-end approaches: change TCP

• Overview of TCP– Connection management, flow control, congestion

control and reliability• Reliability

– Positive acknowledgement– Cumulative acknowledgement: ack number N

acknowledges all data bytes with seq number up to N-1– At the sender end, consider lost if no ack for that

segment is received.– Retransmission timeout timer (RTO)– RTO is important for the performance of TCP

• If the RTO value is too small, • If the RTO value is too large,

– RTO = RTT avg + 4 * RTT dev– TCP assumes all losses to be due to congestion.

• Congestion control– Receiver window: receiver’s buffering capacity

– Congestion window: available network capacity

– Slow start• The growth in congestion window size continues until it is

greater than the receiver window or some of the segments and/or their ACKs start to get lost.

– Congestion avoidance• Slow start threshold (ssthresh)

• Initially, ssthresh is set to 64 K bytes.

• Whenever a segment gets lost, ssthresh is set to half of the current congestion window

• If the segment loss is detected through duplicated ACKs, congestion window by half.

• If the segment lost through a time-out, congestion window is reset to one.

– Congestion control• Either delayed or lost segments

• Fast retransmit scheme: three or four duplicate ACKs -> cut down congestion window by half.

• TCP over wireless networks• Conventional cellular networks

• Between a mobile host in the wireless networks and a static host in the backbone Internet

– Random Losses• Packet error rates: 1% in microcell and up to 10% in macrocell

• TCP does not have any mechanisms to differentiate between congestion-induced losses and other random losses.

• If the loss probability is p, TCP’s throughput is proportional to 1/(root)p

– Large and varying delay• Low bandwidth -> large end-to-end delay

• Latency in the switching network

• Channel allocation: with voice traffic

• Asymmetry in channel access

• Unfairness in channel access

• Therefore, RTO is varying largely (ex: RTO 32 secs with RTT 1sec).

– Low bandwidth: affect TCP’s performance because of TCP’s bursty nature

– Path asymmetry• Low priority for the path from Mobile Host

• Channel capture effect

– Disconnections• Handoff

• Approaches to improve transport layer performance– Link layer enhancements

• Mask the characteristics of the wireless network by having special link layer mechanisms over the wireless link

• Transparent to the overlying transport protocol

• Buffering of packets at the base station and the retransmission of the packets that are lost due to errors on the wireless link

• Retain the end-to-end semantics of TCP

– The Snoop protocol• No change at either the sender or the receiver

• Change at the base station

• The random loss is detected by the receipt of duplicate TCP acknowledgement that arrive from the mobile host at the base station.

• At the base station, after receiving duplicate ACKs, suppresses the duplicate ACKs in addition of performing the retransmission.

• Fig 13.1

• In snoop, the module at the base station retransmits a lost packet when it observes duplicate ACKs and further suppresses the duplicate ACKs, preventing them from reaching the sender. Hence, the connection is less likely to experience fast retransmits (and associated window cut-downs)

• For bit error rate about 1.5*10-5, the performance of TCP by about a factor of 20

– Indirect protocols• Split the connection at the base station

• Two simultaneous connections

• The end-to-end semantics of the TCP are not followed

– I-TCP

– Fig 13.2

• Over wide-area wireless networks, I-TCP exceeds the performance of TCP by about 100% for different mobility scenarios.

– End-to-End protocols• Change at both the sender and the receiver

• Customized congestion control and reliability scheme

– Fig 13.3

• WCTP uses rate-based transmission at the source, interpacket separation at the receiver as the metric for congestion detection.

• Selective ACKs, no dependence on RTTs and RTOs

• Tunable ACK frequency

• Congestion control: rate-based transmission, congestion detection based on receiver interpacket separation, computation at the receiver, distinguishing between congestion- and noncongestion-related losses

• Reliability: no retransmission timeouts and tunable ACK frequency

• For packet error rates of around 4%, performance improvement of about 100%