Advanced network accounting notes (III)

Transport layer

service

End system application Logical communication between

Network services —— Host to host
Transport services —— Process to process （ Finer particle size , Import port number ）

agreement

The communication between two peer-to-peer transport layer entities in the end system complies with the rule set

• TCP
  • There is a connection
  • reliable
  • Keep the order
• UDP
  • There is no connection
  • unreliable
  • No order

Reuse / Demultiplexing

Reuse

Source end （TCP / UDP） Entities collect data from multiple upper tier applications

Demultiplexing

The receiving end sets the data according to the port number （ combination IP Address ） To the appropriate application

working principle

IP & Port

• TCP： Four tuple
• UDP： Binary

UDP

The need for

• No need to be reliable , Delay sensitive real-time applications
• Need to be reliable but can't accept TCP Delay cost , Reliable based on application layer ——QUIK
• Low overhead ：8Byte Head
• Transactional applications ： One interaction complete

message

Connectionless datagram
Parity check

Reliable data transmission

Technical mechanisms

Protocol evolution

• The checksum
  • Positive confirmation
  • Reverse confirmation
• Serial number
  • repeat
  • Disorder
  • gap
• Test and resend / Timeout retransmission
  • The sliding window
  • Processing lost
• Positive acknowledgement mechanism

utilization

Stop waiting for Technology

Link bandwidth delay product （ Capacity ） High channel utilization is low

Pipeline technology

Send multiple messages continuously without the other party's confirmation PDU

• GBN（ Back off N）
  The send window is larger than 1, Accept window =1
  Can only receive in sequence , The sender only sets a single timeout , Error goes back to error PDU retransmission
• SR（ Choose to resend ）
  The send window is larger than 1, The acceptance window is larger than 1
  It can be received out of order , The sender of sends for each PDU Set the timeout , Retransmission timeout PDU

TCP

Service features

• Point to point
• Reliable sequence preserving byte stream
• The Conduit
• buffer
• full duplex
• Connection oriented
• flow control

Segment structure

• The sequence number is the offset of the first byte on the byte stream
• Negotiate the starting sequence number when establishing a connection , Random , The clock
• Confirmation is to receive the last byte of the sequence +1, And follow-up expectations

RTT Time estimates & Estimated retransmission timeout

Moving average
* Average round trip delay
* The standard deviation of the current round-trip delay sampling and average
RTO（ Over time retransmission ）$=flatallvalue+4\times markaccurateBad$

Reliable transmission

The fast retransmission

Receive three repetitions of a certain paragraph from the other party without timeout ACK

Four times

flow control

Purpose

Prevent flooding duif

methods

The feedback carries the receiving window size to the sender

Connection management

• establish
  3 The second handshake , Confirm the initial serial number , Prepare buffer
• Connect
  Resources to prepare , Control variable set , Prepare for communication
• Remove the connection
  symmetry , There is a problem between the two armies
• Connection state and transition

Congestion control

congestion

Too much data needs to be sent by the network core , It exceeds the processing capacity of some routers

performance

• Big delay , Excessive packet loss
• Overall, the throughput does not increase with the injection rate
• Accelerated deterioration

Purpose

• Return to a non congested state
• Try to transmit as fast as possible when there is no congestion （ Large throughput ）
• Little delay

methods

• End to end ：TCP
• Network assisted ：ATM Network flag and carry congestion information , Feedback to the host , upgrade TCP Of ECN

Traditional congestion control

Detect congestion

• Three redundancies ACK（ slight ）
• Overtime （ Small probability of miscalculation ）

Control mechanism

AIMD Slow start Conservative policy after timeout

Algorithm

• Tahoe——（ Timeout and ACK Redundant same processing ）
• Reno——（ Different treatment ）

follow-up

• New Reno
• Sack
• ECN
• CUBIC
• BBR