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L25_reliable_data_transport

L25_reliable_data_transport - The Transport Layer...

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1 The Transport Layer: End-to-end Functions Lecture 25 6.02 Spring 2008 May 9, 2008 Reliable delivery using timeout + retransmission Stop-and-wait protocol Sliding window protocols Today Data Link Physical Ethernet, WiFi, … Framing, coding, [limited] rxmits, channel access Modulation/demodulation Application HTTP, FTP, SMTP, … Transport TCP, UDP End-to-End Layer Layering in the Internet Network IP Forwarding & routing (and addressing) Everything else! Reliability, integrity, ordering, jitter ctrl, congestion response, …
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2 Packets in a Best-Effort Network Lead a Rough Life Can be lost for any number of reasons • No current route • Current route not working properly • Queue overflow at switch (due to congestion) • Packet corruption (noise/interference), collisions • Uncorrectable errors detected by CRC • Queue overflow at receiving node Can arrive out-of-order And experience variable delays Transport Layer Transport layer “cleans up” best-effort behavior to present clean abstraction Common functions (not all needed for apps, so multiple transport layer choices) Reliability : At least once : each packet received at least once At most once • Exactly once Ordering • Data integrity (detect corruption) • Timeliness (“remove” variable latency) • Flow and congestion control: for performance
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3 Fundamental Mechanisms All loss recovery mechanisms employ redundancy Retransmissions • Re-send after timeout Forward Error Correction (FEC) • Code data to overcome some errors or loss “At Least Once” Transport Protocol: Take 1 Stop-and-Wait Each packet has a sequence number set by sender A “nonce”: always fresh, never reused Simple approx.: incrementing 32 or 64-bit number Receiver: upon receipts of packet k , send acknowledgment (ack) for k (“I got k ”) Sender: Upon ack k , send k +1. If no ack within timeout , then retransmit k
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