Lecture Objectives. Lecture 07 Mobile Networks: TCP in Wireless Networks. Agenda. TCP Flow Control. Flow Control Can Limit Throughput (1)

Transcription

1 Lecture Objectives Wireless and Mobile Systems Design Lecture 07 Mobile Networks: TCP in Wireless Networks Describe TCP s flow control mechanism Describe operation of TCP Reno and TCP Vegas, including congestion avoidance (congestion control), slow start, and fast retransmission and recovery mechanisms Describe performance problems of TCP in wireless networks Summarize proposed schemes to overcome performance limitations of TCP in wireless networks Mobile Networks: TCP in Wireless Networks 2 Agenda TCP overview Flow control Congestion avoidance, slow start, and retransmission TCP Reno and TCP Vegas TCP in wireless networks Solutions to TCP performance problems in wireless networks TCP Flow Control TCP inherently supports flow control to prevent buffer overflow at the receiver Useful for fast sender transmitting to slower receiver Receiver advertises a window (wnd) in acknowledgements returned to the sender Sender cannot send more than wnd unacknowledged bytes to the receiver Src Dest Limits amount of data that destination must buffer Mobile Networks: TCP in Wireless Networks 3 Mobile Networks: TCP in Wireless Networks 4 TCP Flow Control Example Flow Control Can Limit Throughput (1) Sender Receiver wnd = bytes 500 bytes Let rtt be the round-trip time, i.e., the time from sending a segment until an acknowledgement (ACK) is received Let t = wnd/b be the time to transmit a full window of data, where b is link bandwidth wnd = 200 Sender Receiver 200 bytes wnd = 500 rtt t wnd bytes 500 bytes Mobile Networks: TCP in Wireless Networks 5 Mobile Networks: TCP in Wireless Networks 6

2 Flow Control Can Limit Throughput (2) For a link with a high delay-bandwidth product (rtt b), the flow control window can limit throughput for the connection In this case, t rtt Throughput is wnd/rtt Sender Receiver TCP Congestion Avoidance Congestion avoidance (control) was added to TCP in an attempt to reduce congestion inside the network A much harder problem Requires the cooperation of multiple senders Must rely on indirect measures of congestion Implemented at sender rtt t wnd bytes Src Dest Attempts to reduce buffer overflow inside the network Mobile Networks: TCP in Wireless Networks 7 Mobile Networks: TCP in Wireless Networks 8 Recent History of TCP TCP has been improved over the years More robust estimates of round-trip time Faster recovery from packet loss Congestion avoidance and improvements TCP Reno Developed by Van Jacobsen in 1990 Improvement to TCP Tahoe (1988) Added fast recovery and fast retransmit TCP Vegas Developed by Brakmo and Peterson in 1995 New congestion avoidance algorithm TCP Operation Flow control (already discussed) Congestion avoidance Introduce a congestion window (cwnd), in addition to flow control window (wnd) Need to manage size of congestion window Slow start Aggressively grow congestion window until congestion is detected In Reno, aggressively reduce rate when invoked Loss detection and retransmission Fast retransmission and recovery Less severe adjustment congestion window size Mobile Networks: TCP in Wireless Networks 9 Mobile Networks: TCP in Wireless Networks 10 Congestion Avoidance: TCP Reno (1) TCP can maintain a congestion window size, cwnd, at the sender Sender can transmit up to minimum of cwnd and wnd bytes TCP Reno uses packet loss as an indicator of network congestion Most packet loss occurs due to congestion at intermediate routers since IP has no congestion control mechanism Packet losses due to bit errors are rare TCP Reno is reactive with respect to congestion Responds to loss of packets indicated by timeout or duplicate ACKs Mobile Networks: TCP in Wireless Networks 11 Congestion Avoidance: TCP Reno (2) When packet loss occurs, congestion window size is reduced Due to timeout: cwnd = 1 and enter slow start Due to duplicate ACKs: cwnd = cwnd/2 + 3 segment_size Congestion window size is increased when data is successfully acknowledged Slow start Slow start active if cwnd ssthresh (threshold) During slow start, congestion window increased by segment_size for every ACK received opens the window exponentially Congestion avoidance cwnd = cwnd + (1/cwnd) + segment_size/8 for every ACK received additive growth in window size (about one segment every round trip time) Mobile Networks: TCP in Wireless Networks 12

3 Congestion Window in TCP Reno Congestion Avoidance: TCP Vegas (1) G. Xylomenos, G. C. Polyzos, P. Mahonen, and M. Saaranen, TCP Performance Issues over Wireless Links, IEEE Communications Magazine, Vol. 39, No. 4, pp , April Mobile Networks: TCP in Wireless Networks 13 Sets congestion window size based on difference between the expected and actual data rates Goal is to control the number of outstanding bytes in queues in the network (i.e., the backlog in queue) Define cwnd: Current congestion window size rtt*: Minimum ( congestion-free ) round-trip time rtt: Actual (with congestion) round-trip time diff: Estimated backlog in queue α: low threshold for diff (want diff > α) β: high threshold for diff (want diff < β) diff = (cwnd/rtt* cwnd/rtt) rtt* Mobile Networks: TCP in Wireless Networks 14 Congestion Avoidance: TCP Vegas (2) Congestion Avoidance: TCP Vegas (3) Estimated backlog in queue (repeated here) diff = (cwnd/rtt* cwnd/rtt) rtt* TCP Vegas attempts to keep at least α bytes, but fewer than β bytes, in queue If diff < α, increase cwnd by 1 If diff > β, decrease cwnd by 1 Otherwise (α diff β), cwnd is not changed TCP Vegas provides a proactive response to congestion Congestion window changed gradually as observed backlog (delay) changes Mobile Networks: TCP in Wireless Networks 15 Throughput C Linearly Increasing Expected Throughput α/rtt β/rtt Actual Throughput Linearly Decreasing cwnd cwnd+β Window Size cwnd+α Mobile Networks: TCP in Wireless Networks 16 Slow Start Mechanism The goal of the slow start mechanism is to detect and avoid congestion as a connection begins or after a timeout Slow start threshold (sshtresh) set to half of cwnd when congestion is detected Slow start is active if cwnd ssthresh Initially, cwnd = 1 segment TCP Reno doubles the congestion window every round-trip time if no loss occurs TCP Vegas doubles the congestion window every other round-trip time if no loss occurs Loss Detection: TCP Reno Coarse-grain timeout indicates packet loss Sender starts a timer when TCP segment is sent Timeout occurs if ACK not received before timeout Retransmission occurs Slow start is invoked (big reduction in rate!) Three duplicate ACKs indicate packet loss Receiver required to send an ACK if it receives an out of order segment a segment may be out of order or lost Sender assumes loss when it receives three duplicate ACKs Fast retransmission and recovery mechanism retransmit the requested segment (which is presumed lost after three duplicate ACKs) without waiting for a timeout Congestion avoidance (smaller reduction in rate) Mobile Networks: TCP in Wireless Networks 17 Mobile Networks: TCP in Wireless Networks 18

4 Loss Detection: TCP Vegas TCP Reno Behavior Coarse-grain timeout mechanism Same as for TCP Reno Fine-grain timeout mechanism If a duplicate ACK is received and the round-trip time of the first unacknowledged segment exceeds the fine-grain timeout value, then segment loss is assumed and requested segment is retransmitted If a non-duplicate ACK is received after a retransmission and the round-trip time of the segment exceeds the fine-grain timeout value, then segment loss is assumed and retransmission occurs cwnd cwnd 1 Timeout SS Duplicate ACK CA cwnd cwnd/2 + 3 SS CA SS CA CA SS: Slow start CA: Collision avoidance time Mobile Networks: TCP in Wireless Networks 19 Mobile Networks: TCP in Wireless Networks 20 TCP Vegas Behavior TCP Reno Pros and Cons (1) cwnd SS time Converges more smoothly assuming sufficiently large buffers CA TCP Reno benefits Simple bandwidth estimation scheme Aggressive congestion avoidance mechanism ensures bandwidth when connected to TCP Vegas connections More widely deployed, probably due to its maturity and aggressiveness TCP Reno problems Constantly updates window size Can lead to periodic oscillation in window size Can lead to oscillation in round trip times, causing delay jitter and inefficient bandwidth utilization Can have many retransmissions of the same packets after a packet is dropped Mobile Networks: TCP in Wireless Networks 21 Mobile Networks: TCP in Wireless Networks 22 TCP Reno Pros and Cons (2) TCP Reno problems (continued) Connections with shorter round trip times can update congestion window sizes more quickly Such connections can received an unfair share of network capacity TCP Reno is biased against connections with longer delays TCP Vegas Pros and Cons TCP Vegas benefits Fair bandwidth estimation scheme Window update rate does not depend only on round-trip time as in TCP Reno Smooth sending rate and efficient link utilization when queue sizes are large (window stabilizes between α and β) TCP Vegas detects losses faster than TCP Reno and can recover from multiple drops more efficiently TCP Vegas problems Cannot compete with more aggressive TCP Reno connections Vegas may not stabilize if buffers are small, leading to behavior that is similar to that of TCP Reno Mobile Networks: TCP in Wireless Networks 23 Mobile Networks: TCP in Wireless Networks 24

5 TCP Reno versus TCP Vegas TCP Vegas generally outperforms TCP Reno in a homogeneous environment TCP Vegas achieves between 40% and 70% better throughput TCP Vegas has 20% to 50% of the losses compared to the TCP Reno Factors Slow-start and congestion avoidance have the greatest influence on throughput Congestion detection mechanism during congestion avoidance has only minor or negative effect on throughput Congestion detection mechanism may exhibit problems related to fairness among competing connections Agenda TCP overview Flow control Congestion avoidance, slow start, and retransmission TCP Reno and TCP Vegas TCP in wireless networks Solutions to TCP performance problems in wireless networks Mobile Networks: TCP in Wireless Networks 25 Mobile Networks: TCP in Wireless Networks 26 TCP Problems with Wireless Packet loss in wireless networks typically due to Bit errors due to wireless channel impairments Handoffs due to mobility Possibly congestion, but not often As we ve seen, TCP assumes packet loss is due to Congestion in the network Packet reordering, but not often In a wireless network, TCP congestion avoidance can be triggered by packet loss TCP s mechanisms do not respond well to packet loss due to bit errors or handoffs Performance of TCP-based applications can suffer More TCP Problems with Wireless Bursts of errors may occur due to low signal strength or longer period of noise More than one packet lost in TCP More likely to be detected as a timeout enter slow start! Delay is often very high Round-trip time can be very long and variable TCP s timeout mechanisms may not work well Problem exacerbated by link-level retransmission Links may be asymmetric Delayed ACKs in the slow direction can limit throughput in the fast direction Mobile Networks: TCP in Wireless Networks 27 Mobile Networks: TCP in Wireless Networks 28 Week 13 In-Class Laboratory Experiments to consider Influence of bit errors in the wireless channel on TCP performance TCP Reno versus TCP Vegas in this environment Interactions are relatively complex Typical studies use simulation, which provides a very controlled environment We re being a bit bold in trying to do experimental measurements There is no at-home exercise for this week You will be responsible for findings and observations on the final exam Agenda TCP overview Flow control Congestion avoidance, slow start, and retransmission TCP Reno and TCP Vegas TCP in wireless networks Solutions to TCP performance problems in wireless networks Mobile Networks: TCP in Wireless Networks 29 Mobile Networks: TCP in Wireless Networks 30

6 General Solution Approaches Link-layer approaches Split-connection approaches End-to-end approaches Link-Layer Protocols (1) Hide losses not due to congestion from the sender by making link appear to be more reliable Link-level automatic retransmission request (ARQ) Forward error correction (FEC) codes Hybrid ARQ and FEC Advantages Requires no change to existing sender behavior Matches layered protocol model Problem Interactions with TCP, e.g., fast retransmission by TCP can be triggered by delays due to link-level timeout and retransmission Mobile Networks: TCP in Wireless Networks 31 Mobile Networks: TCP in Wireless Networks 32 Link-Layer Protocols (2) Negative interactions with TCP can be reduced by making the link-level protocol TCP-aware Example: Snoop TCP Advantages Attempts to retransmit locally and suppress duplicate acknowledgements State is soft, so handoff is simplified Disadvantage May not completely shield TCP from the effects of mobility and the wireless link Split-Connection Protocols (1) Hide the wireless link entirely by terminating the TCP connection prior to the wireless link At the base station or access point Use a special protocol or regular TCP over the wireless link Example: Indirect TCP Problems Extra protocol overhead Violates end-to-end semantics of TCP Complicates handoff due to state information at the access point or base station where the protocol is split Mobile Networks: TCP in Wireless Networks 33 Mobile Networks: TCP in Wireless Networks 34 Split-Connection Protocols (2) End-to-End Protocols (1) Logical TCP Connection TCP AP Split Connection TCP* Make TCP sender aware that some losses are not due to congestion and, thus, avoid congestion control when not needed Use selective acknowledgement (SACKs) for finegrained error recovery SACK RFC SMART Use explicit loss notification (ELN) to distinguish between congestion and other losses Mobile Networks: TCP in Wireless Networks 35 Mobile Networks: TCP in Wireless Networks 36

7 End-to-End Protocols (2) Indirect TCP: Overview Advantages Maintains end-to-end semantics of TCP Introduces no extra overhead at base stations for protocol processing or handoff Disadvantages Requires modified TCP May not operate efficiently, e.g., for packet reordering versus packet loss Fixed Wired Network Standard TCP Standard TCP TCP Proxy Wireless TCP* Mobile Indirect TCP (* Normal TCP or modified transport protocol) Mobile Networks: TCP in Wireless Networks 37 Mobile Networks: TCP in Wireless Networks 38 Indirect TCP: Handoff An access point or router can act as a Mobile IP foreign agent and as the TCP proxy for Indirect TCP (I-TCP) If the mobile host moves to a different foreign agent, a handoff is needed for Mobile IP If the mobile host moves to a different proxy, a handoff of the full TCP state is needed for I-TCP Buffered data Sequence numbers Port Indirect TCP: Advantages Does not require changes to TCP at the hosts in the fixed network Errors from the wireless link are corrected at the TCP proxy and, thus, do not propagate through the fixed network New protocol affects only a limited part of the Internet Optimizations possible over wireless link Variance in delay between proxy and mobile host may be small, permitting optimized TCP Opportunity for header compression, etc. Opportunity for a different transport protocol Mobile Networks: TCP in Wireless Networks 39 Mobile Networks: TCP in Wireless Networks 40 Indirect TCP: Disadvantages Loss of TCP s end-to-end semantics What happens if the proxy or the mobile host fails? Handoff overhead can be significant Overhead at the proxy for per packet processing (up to TCP and back down) Can be reduced by good design TCP proxy must be trusted Obvious opportunities for snooping and denial of service End-to-end IP-level privacy and authentication (e.g., using IPSec) must terminate at the proxy Indirect TCP: Wireless Transport I-TCP as originally proposed uses TCP as the wireless transport protocol Timeouts at the wireless sender may stall the original sender on the fixed network Selective acknowledgement protocols have been shown to provide better performance Better suited to wireless link with higher error rate Mobile Networks: TCP in Wireless Networks 41 Mobile Networks: TCP in Wireless Networks 42

8 Snoop TCP: Overview Provide reliable link layer that is TCP aware Snoop agent at the access point or foreign agent Buffers data at the ends of the links for retransmissions (instead of going back to TCP end points) Snoops on acknowledgements and filters duplicate acknowledgements Fixed Wired Network Standard TCP Snoop Agent Mobile Snoop TCP: Operation (1) Snoop agent monitors and buffers data sent from fixed network to mobile host Snoop agent monitors ACKs from the mobile host Can discard buffer data when acknowledged Can retransmit data when Delayed ACK, or Duplicate ACK Timeout can be relatively short leading to a fast retransmission Snoop Agent discards duplicate ACKs from mobile host Mobile Networks: TCP in Wireless Networks 43 Mobile Networks: TCP in Wireless Networks 44 Snoop TCP: Operation (2) Snoop agent discards duplicate data that has already been sent by the agent and acknowledged Snoop agent cannot generate ACKs that are sent back to the fixed host Unlike split-connection schemes, Snoop TCP preserves endto-end TCP semantics Snoop TCP: Reverse Direction Snoop monitors traffic from mobile host back to fixed host and detects missing segments A negative ACK (NACK) is sent immediately to the mobile host Mobile host can retransmit missing segment, hopefully in time to avoid a TCP timeout at the fixed host Mobile Networks: TCP in Wireless Networks 45 Mobile Networks: TCP in Wireless Networks 46 Snoop TCP: Advantages Preserves end-to-end TCP semantics Requires no changes in TCP for fixed hosts No changes in TCP are possible for the mobile hosts, but reverse direction traffic can benefit from changes at mobile host There is no need for handoff Automatic fallback to standard TCP No need to ensure that all foreign networks provide a Snoop agent Snoop TCP: Disadvantages Does not fully isolate wireless link errors from the fixed network Mobile host must be modified to handle NACKs for reverse (mobile to fixed) traffic Cannot snoop encrypted datagrams Cannot use with privacy Retransmission of data from agent not authenticated due to protection from replay attacks Cannot use with authentication Mobile Networks: TCP in Wireless Networks 47 Mobile Networks: TCP in Wireless Networks 48

9 Summary TCP is a complex protocol Minimal support from underlying protocols Indirect observation of network environment Large number of competing flows from different hosts Congestion avoidance is still a research issue TCP does not perform well in a wireless environment where packets are usually lost due to bit errors, not congestion Schemes have been proposed to address TCP performance problems Link-level recovery Split protocols End-to-end protocols Mobile Networks: TCP in Wireless Networks 49