Computer Networking: A Top-Down Approach, CH3

TRANS: Transport Layer Services

TRANS: Provide logical communication between processes on different end systems; Break app messages into segments

TRANS: Transport Layer vs. Network Layer

TRANS: Network layer - logical communication between hosts
Transport layer - logical communication between processes

TRANS: 2 Transport Layer Protocols

TRANS: TCP, UDP

TRANS: Multiplexing

TRANS: Handle data from multiple sockets, add transport header

TRANS: Demultiplexing

TRANS: Use header info to deliver received segments to correct socket

TRANS: Connectionless demultiplexing

TRANS: When creating datagram, specify destination IP and port number. When receiving UDP segment, direct to socket with that port number. Datagrams with same destination port number with different source IP address/port number directed to same socket at destination.

TRANS: Connection-oriented demultiplexing

TRANS: TCP socket identified by source IP address and port number and destination IP address and port number. Receiver uses all four values to direct segment to socket. Server may support simultaneous TCP sockets (threading).

TRANS: What does UDP stand for?

TRANS: User Datagram Protocol

TRANS: Describe UDP

TRANS: no frills, bare bones, best effort service
Segments may be lost or out of order
Connectionless
Used in streaming multimedia, DNS, SNMP
Add reliability at application layer

TRANS: Why is there UDP?

TRANS: no connection (reduces delay)
simple (no connection states)
small header size
no congestion control (fast as desired)

TRANS: 4 UDP header values

TRANS: Source port
Destination port
Length
Checksum

TRANS: Goal of Checksum

TRANS: error detection

TRANS: How to calculate checksum

TRANS: complement sum of 16-bit segment headers

TRANS: rdt 1.0

TRANS: underlying channel reliable
no bit errors
no packet loss

TRANS: rdt 2.0

TRANS: underlying channel may flip bits
error detection: checksum
feedback: ACK, NAK

TRANS: Stop-and-wait

TRANS: sender send one packet, then waits for receiver to respond

TRANS: rdt 2.1

TRANS: handles garbled ACK/NAK
Sequence numbers

TRANS: rdt 2.2

TRANS: NAK-free
Receiver include sequence number of ACKed packet

TRANS: rdt 3.0

TRANS: underlying channel can lose packets (data, ACKs)
sender waits for ACK
retransmit if no ACK received in time
sequence number handles retransmission duplicates
requires countdown timer

TRANS: Sender Utilization

TRANS: Fraction of time sender busy sending
(N*L/R) / (RTT + L/R), where N is number of packets sent at a time

TRANS: Pipelining

TRANS: Sender allows multiple, in-flight, yet-to-be-ACKed packets
Range of sequence numbers must be increased
Buffering at sender or receiver
Increases sender utilization

TRANS: 2 Pipelined Protocols

TRANS: Go-Back-N
Selective Repeat

TRANS: Describe TCP

TRANS: Point-to-point (one sender, one receiver)
Reliable, in-order byte stream
Pipelined
Full duplex data (bidirectional data flow)
Connection-oriented
Flow controlled

TRANS: MSS

TRANS: maximum segment size

TRANS: 16 TCP segment headers

TRANS: Source port
Dest port
Options
Unused
6 flags - UAPRSF
Checksum
Receive window
Urg data pointer
Sequence number
Acknowledgement number
Header length

TRANS: Exponential Weighted Moving Average

TRANS: EstimatedRTT = (1-a) * EstimatedRTT + a*SampleRTT
(a = 0.125)

TRANS: Timeout Interval

TRANS: Timeout Interval = EstimatedRTT + 4*DevRTT

TRANS: SampleRTT deviation from EstimatedRTT

TRANS: DevRTT = (1-b) *DevRTT + b*|SampleRTT - EstimatedRTT|
(b = 0.25)

TRANS: TCP cumulative ACK

TRANS: ACK the packet you expect next

TRANS: TCP fast retransmit

TRANS: if sender receives 3 ACKs for same data (triple duplicate ACK), resend unacked segment with smallest sequence number

TRANS: TCP flow control

TRANS: Receiver advertises free buffer space in TCP header, sender limits unacked data to receivers rwnd value, guarantees receive buffer will not overflow

TRANS: TCP 3-way handshake

TRANS: 1) Client sets sequence number and set SYN, sends to server 2) Server sets sequence number and set SYN, ACKS clients information 3) Receives ACKS, ACKS server information with data

TRANS: TCP Closing Connections

TRANS: 1) Client sets FIN, sends with sequence number 2) Server ACKS 3) Client waits for server to close 4) Server sets FIN, sends with sequence number 5) Client ACKS 6) Client waits 2 * max segment lifetime

TRANS: TCP Congestion Control

TRANS: Additive Increase, Multiplicative Decrease

TRANS: Additive Increase

TRANS: Increase window by 1 MSS every RTT until loss detected

TRANS: Multiplicative Decrease

TRANS: Cut window in half after loss

TRANS: TCP sending rate

TRANS: window/RTT

TRANS: TCP Slow Start

TRANS: When connection begins, increase rate exponentially until first lost event

TRANS: TCP Reaction to Loss

TRANS: Loss indicated by timeout - window set to 1, then slow start to threshold, then grows linearly
RENO: Loss indicated by triple duplicate ACK - cut window to half then grow linearly
TAHOE: set window to 1after timeout or triple duplicate ACK

TRANS: When exponential increase switch to linear

TRANS: when window gets to 1/2 its value before timeout

TRANS: TCP throughput

TRANS: 3/4 window/RTT

TRANS: Transport Layer Protocols

TRANS: UDP/TCP