Computer network transport layer
This is inside the inverted binary checksum algorithm reference: https://blog.csdn.net/dingmin1860/article/details/48268927
- ARQ Automatic Repeat reQuest
- Continuous ARQ protocol
Introduction
Transport layer is a boundary level, for the above application, the following are oriented data transmission
According to the needs of different users -
Reliable, connection-oriented, TCP
Unreliable, non-connection-oriented, UDP. Fast, low transport costs.
port
# 16, 0-65535
0-1023 reserved
1024-5000 temporary
Universal Port Number
| FTP | 21/TCP |
| DNS | 53/UDP |
| HTTP | 80/TCP |
| SMTP | 25/TCP |
Why do we need a transport layer
The network layer has to provide transport services, unreliable, lost packets.
End host system needs its own reliable transport.
Several concepts
Connection-oriented service
Connectionless service
Reliable service
Feedback retransmission
Two transport layer protocol
- User Datagram Protocol user datagram protocol
- Transmission control protocol transmission control protocol
- Data transfer unit: packet
UDP
User Datagram Protocol user datagram protocol
Feature
- no connection
- It is the message transmission unit
- Small header
- Unreliable
- The recipient does not need to send an acknowledgment message
- It does not guarantee data integrity destination
- Reduce the communications burden on the network
UDP header format
Source Port (16 bits), destination port (16), and checksum (16), message length (16), the data
Port Number : 16, to allow different port number 65535
Checksum : Checksum range UDP data packet header + body + pseudo header
Message length : in bytes, UDP header + 8 bytes of the data portion, the maximum length of 65535
UDP checksum
Dummy header (12 bytes)
| 4 (bytes) | 4 | 1 | 1 | 2 |
|---|---|---|---|---|
| Source IP address | Destination IP address | 0 | 17 | UDP length |
Calculation range checksum: the UDP pseudo header header + data +
| 12 (bytes) | 2 | 2 | 2 | 2 | |
|---|---|---|---|---|---|
| Pseudo-header | Source Port | Destination port | Message length | Checksum | data |
The IP checksum calculation method: the binary one operation
UDP port numbers not only checked, also check the IP address
TCP
Transmission control protocol transmission control protocol
Feature
Connection-oriented
Three-way handshake to establish a connection agreement
To establish a connection on an unstable IP network
Client: Active open a network connection, a transmission section SNY, No. initialization sequence x
Server: passive open port, sending SNY y, and ACK x + 1
Client: transmitting ACK y + 1
reliable
Feedback retransmission mechanism
Three Strategies
- Stop waiting mode
It receives an ACK before sending the next packet
- Continuous mode of operation (depending on the receiving end there is no cache)
- Back manner (cumulative acknowledgment ACK) go back n
- Select the retransmission mode
ARQ mechanism: a sliding window
Timer
The sender does not receive an ACK within the predetermined time, the time out and resend the considered data packet.
Problems: receive duplicate messages.
The recipient can be removed by repeating the message packet sequence number.
Continuous ARQ protocol
- The sender may transmit a plurality of packets
- Receiver generally used cumulative confirmed by: sending an acknowledgment for the last packet arrive out of order
Sliding window sliding windows
Can only achieve continuous sliding window ARQ protocol, may also perform flow control (speed sender and recipient matches)
- The sender transmits data only in the "transmission window"
- Receiving side receives only the data in the "receive window"
Byte stream and
Sliding window
Confirm the number of bytes
Continue timer
A TCP connection as long as the party received the other side of zero window notification, starts the timer duration.
If the timer expires, A transmission zero window probe packets . After receipt of the other party, given the current window value.
flow control
The sender sending rate is not too fast, not only to let the recipient had time to receive (to prevent loss of data), nor to make network congestion.
TCP sliding window mechanism may be implemented in the control flow (both transmitting data in a consistent rate)
Congestion Control
Fundamental
Within a certain period of time, part of the demand exceeds the available resources can provide, network performance will deteriorate. ---> Congestion
Solutions
Increase resources and reduce demand
TCP Congestion Control
TCP header format
The minimum length of 20 bytes
UDP vs. TCP
Congestion Control vs. Flow Control
(Column outline, will complement the updated ~!)
Anti binary code checksum algorithm
Reference: https://blog.csdn.net/dingmin1860/article/details/48268927
First, IP, ICMP, UDP and TCP packet has a header checksum field, size is 16bit, the algorithm is essentially the same.
When transmitting data, the data packet in order to calculate the checksum. You should be as follows:
1, the checksum field set to 0;
2, as the data to be verified in units of 16-bit numbers, sequentially summing the binary one;
3, the result is stored in the checksum field
When data is received, the packet checksum calculation is relatively simple, as follows:
1, as the header portion of 16 bits in units of digits, sequentially summing the binary one, comprising a checksum field;
2, the calculated checksum and checking whether the result is 0;
3, if it is equal to 0, is divisible description, the checksum is correct. Otherwise, the checksum is wrong, protocol stacks to discard the packet.