9 - Transport Layer Protocols Flashcards
Encapsulation
Each layer has a PDU (Protocol Data Unit)
Header/Body
Each layer adds a header to the PDU
Transport Layer
Communcation between processes on different hosts
Transport Layer send side
Break app messages into segments (transport layer packets)
pass to net layer
Network Layer
Provides communication between hosts
Transport layer receive side
Reassemble segments into messages
Pass to app layer
Transport vs Network
Net: comms between hosts
Transport: comms between processes
TCP - Transmission Control Protocol
Reliable, in order delivery
- Congestion Control
- FLow Control
- Connection Setup
UDP - User Datagram Protocol
unreliable, unordered “best effort”
connectionless
- no handshaking
- each segment handled separately
Addressing Processes
IP Address - host
Port Num - Identifies process
eg HTTP 80, Mail 25
Multiplexing
At Send Host
- gathering data from multiple sockets, enveloping data with header. Collating messages to send
Demultiplexing
At Receiving host
- delivering received segments to correct socket
Non-Persistent vs Persistent HTTP
NON - at most one object sent over TCP
Persistent - Multiple objects over single TCP
UDP Checksum Sender
treat segment as seq of 16 bit ints
checksum: addition of contents (1’s complement sum)
put into UDP checksum field
UDP Checksum receiver
compute checksum
check if equals field value else error detected
SQN number
TCP Sequence number = first byte of data in segment.
bytes
initial is random
TCP is full duplex - numbering is independent in each direction
Acknowledgement Number
Sequence number of the next byte expected from the sender
cumulative
TCP connection handshake
1) client host sends TCP SYN seg to server
- specify init seq #
- no data
2) server replies with SYNACK seg
- allocates buffer
- specify server init seq #
3) client replies with ACK which may contain data
TCP Connection close
One side sends FIN=1 and the other ACKs
then second side sends FIN=1 and first ACKs
TCP FLow Control
Receiver has buffer sends the byte value of it in the Receive Window field.
Sender then makes sure that packets to send plus packets in transit will not overflow the buffer
As the receive window reduces, what happens to transmission rate?
Reduces
What happens when receive window reaches zero?
Comms stop as sender is not informed of free space.
Sender probes every few seconds with empty packets waiting for indication of space
