Why is our WiFi so slow?

Why is our WiFi so slow?

TCP over Wireles s PROF. MICHAEL TSAI 2016/6/3 TCP Congestion Contr ol (TCP Tahoe) Only ACK correctly received packets Congestion Window Size: Maximum number of bytes that can be s ent without receiving acknowledgement s. Larger window size == faster rate Three major mechanisms: 2 Slow start Congestion avoidance Fast retransmit

Slow start + congest ion avoidance Source Destination Congestion window size = 1 initially. Ssthresh: slow start threshold Slow start: For each received ACK (with a new seq. no. ), the window size is exponentially increased (e.g., doubled). (when the window size is smaller than ssthres h) Congestion avoidance: After reaching ssthresh, the window size is line arly increased.

3 Additive Increase Slow start + congest ion avoidance TCP calculates expected round trip time (RTT) and its deviation Timeout: wait time > expected RTT + 4 * deviation Timeout: it signals a packet loss due to c ongestion (?) Dropping ssthresh into half the current window or 2 (multiplicative decrease) Resetting its window size to 1 + enter slow start Double round trip time timer 4

When a timeout happens, TCP Tahoe doe s the following: Pseudocode 5 Initially: cwnd = 1; ssthresh = infinite; New ack received: if (cwnd < ssthresh) /* Slow Start*/ cwnd = cwnd + 1; else /* Congestion Avoidance */ cwnd = cwnd + 1/cwnd; Timeout: /* Multiplicative decrease */

ssthresh = cwnd/2; cwnd = 1; The big picture (with timeouts) Initially: cwnd = 1; ssthresh = infinite; New ack received: if (cwnd < ssthresh) /* Slow Start*/ cwnd = cwnd + 1; else cwnd /* Congestion Avoidance */ cwnd = cwnd + 1/cwnd; Timeout: Timeout

/* Multiplicative decrease */ ssthresh = cwnd/2; AIMD cwnd = 1; Timeout AIMD ssthresh Slow Start Slow Start

Time 6 Slow Start Fast Retransmit Duplicate ACK means that an out-of sequence segme nt was received segment 1 cwnd = 1 ACK 2 cwnd = 2 segment 2 segment 3

ACK 3 cwnd = 4 ACK 4 Notes: ACKs are for next expected 3 duplicate packet ACKs Packet reordering can caus e duplicate ACKs Window may be too small t o get enough duplicate AC Ks ACK 4 segment segment

segment segment 4 5 6 7 ACK 4 ACK 4 7 Resend a segment af ter 3 duplicate ACKs Big problems for TCP High bit error rates: up to 10^-5 bit error rate. Lots of lost packets due to bit errors.

Disconnections: Handoff: mobile devices move between base stations / access points. Mobility: mobile devices move in and out of range of the transmitter. Fading: signals blocked by buildings or other obstacles. Round trip time could significantly vary. (Indication of packet loss could be inaccurate) 8 Big problem: packet losses are not necessarily due to congestion! (the assumption of the original TCP) 9 Classification of So lutions Link layer Goal: improve the quality of the link layer, hide the losses from TCP Local problems should be solved locally

End-to-end Additional considerations are added to TCP to improve its performanc e Limited performance improvement No modification on the hosts is required Split-connection 10 Wireless usually the last link to the mobile host Isolate that wireless link with the rest of the path (wired) Two TCP connections, bridged at the wireless gateway Use regular TCP for wired links Use a special protocol designed specifically for that wireless link I-TCP Indirect TCP (I-TCP)

Access Point wireless transport Peter A. Steenkiste, CMCL, CMU Internet Standard TCP 28 11 Mobile host I-TCP and Mobility Indirect TCP (I-TCP) Access Point (1) Socket migration

and state transfer Peter A. Steenkiste, CMCL, CMU Access point (2) 29 12 Mobile Host Internet I-TCP Advantages No changes in the fixed network / hosts (TCP) Wireless transmission errors do not propagate to the wire-line network Simple and effective

Disadvantages 13 End-to-end semantics become less clear Higher end-to-end delay due to buffering and forwardi ng at the gateway Snooping TCP Transparent extension of TCP within the gateway Hides wireless losses from wired host Buffer packets sent to the mobile host Local retransmission: lost packets on the wireless link, for both directions, ar e retransmitted immediately by the mobile host or for eign agent Recognizes ACK in both directions and suppresses dup

licate ACKs 14 Wireless gateway snoops the packet flo w so that it can cover up signs of packet loss 15 Snooping TCP Snooping TCP Data transfer to the mobile host FA buffers data until it receives ACK from the MH FA detects packet loss via duplicated ACKs or timeout Data transfer from the mobile host FA detects packet loss on the wireless link via sequenc e numbers

FA answers directly with a NACK to the MH MH can now retransmit data with only a very short del ay Integration of the MAC layer 16 Similar mechanisms often exist in MAC TCP Snooping Snooping TCP Local Retransmission Wired Host Wireless Gateway Internet

Buffering of data End-to-end TCP connection 17 Mobile Host Snooping of ACKs

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