Network Technology 1 5th - Transport Protocol. Mario Lombardo -

Transcription

1 Network Technology 1 5th - Transport Protocol Mario Lombardo - lombardo@informatik.dhbw-stuttgart.de 1

2 overview Transport Protocol Layer realizes process to process communication data unit is called a segment typical protocols are tcp (rfc 793,...) or udp (rfc 768) provides multiplex and error detection 2

3 comparison UDP user datagram protocol process to process communication error detection TCP transmission control protocol process to process communication error detection reliable data transfer correct data order flow control fair network use 3

4 application mutliplex addressing by ports enables a multiplex on a host 2^16 different ports port# <1024 are privileged ports (admin/root only) port# <49151 are IANA registered Internet Assigned Numbers Authority (IANA) port examples 21/tcp ftp 25/tcp smtp 80/tcp http 443/tcp https 110/tcp pop3 4

5 application mutliplex (cont.) Who decides which port to be used? destination ports are typical bound by server type source ports are typical allocated dynamical (exception: ftp-data) 5

6 user datagram protocol defined in rfc 768 no handshake best effort transfer multiplex advantages direct connection easy handling (no state) slim - just 8 bytes overhead no regulation of transfer rate 6

7 user datagram protocol (cont.) reasons to choose udp realtime applications periodical transmission (routing for example) non critical transfer when the application takes care for realibility multicast/broadcast applications 7

8 user datagram protocol (cont.) reasons to choose udp realtime applications periodical transmission (routing for example) non critical transfer when the application takes care for realibility multicast/broadcast applications reason to avoid udp no flow control (overall) possible data loss on bottle-necks 8

9 user datagram protocol (cont.) segment structure of udp header size is 8 bytes 2 byte source port 2 byte destination port 2 byte length (segment size incl. header) 2 byte checksum (ones complement, rfc 1072) 9

10 implementing a reliable protocol definition of a reliable protocol no changes within data no data loss respect order of data 10

11 implementing a reliable protocol (cont.) rdt a reliable protocol - error detection (by checksum) 11

12 implementing a reliable protocol (cont.) rdt a reliable protocol enhancement: compensate possible data errors requirement: ARQ protocol (automatic repeat request) - error detection (by checksum) - data acknowledge (with ARQ) 12

13 implementing a reliable protocol (cont.) rdt a reliable protocol issue: ACK/NAK can be damaged too possible solutions - enhance ACK/NAK by error correcting data - sender asks for ACK if no reply - sender retransmits the non-ack ed data 13

14 implementing a reliable protocol (cont.) rdt a reliable protocol (sender) enhancement: retransmission of data requirement: sequence number - error detection (by checksum) - data acknowledge (with ARQ) 14

15 implementing a reliable protocol (cont.) rdt a reliable protocol (receiver) enhancement: retransmission of data requirement: sequence number - error detection (by checksum) - data acknowledge (with ARQ) 15

16 implementing a reliable protocol (cont.) rdt a reliable protocol (sender) enhancement: possible loss of a segment requirement: timer - error detection (by checksum) - data acknowledge (with ACK) - timeout 16

17 using sequence numbers flow chart for various scenarios no loss segment loss 17

18 using sequence numbers flow chart for various scenarios ACK loss fast timeout 18

19 stop and wait operation 19

20 stop and wait operation (cont.) disadvantage of stop and wait operation high latency impact channels capacity is unused without pipelining with pipelining 20

21 pipelining algorithms sliding window algorithms are used go back to N a buffer for the sender required a global timer required selective repeat a buffer on both sites required a timer for each segment required note: there are always both sites sending and receiving 21

22 setting the tcp timeout finding the optimal timeout is not trivial timeout has to be aligned to RTT timeout too long long delay due to late identification of data losses timeout too fast false positive timeouts heavy network usage 22

23 the tcp segment header a tcp segment stores header information as follows: 32 bit port# (source and destination) 32 bit sequence# absolute address of payload in the stream 32 bit acknowledge# absolute address within the stream, that is expected next 16 bit window size (for flow control) 4 bit length of header (amount of 32 bit words) 6 bit flags ACK acknowledge SYN, RST, FIN connection management PSH forward immediate (instead of caching) URG urgent data (+ 16 bit pointer) n * 32 bit options 23

24 the tcp segment header (cont.) 24

25 sample tcp connection example tcp session (during connection) 25

26 flow control flow control prevents the flooding of the receivers buffer a tcp participant tells its partner his own buffer space 26

27 ACK=y+1, seq=x+1 tcp connection management 3-way handshake client server SYN=1, seq=x SYN=1, ACK=x+1, seq=y 27

28 tcp connection management connection end client server FIN ACK FIN timed wait ACK 28

29 tcp connection lifecycle client site 29

30 tcp connection lifecycle server site 30

31 network overload Host A λ in : original data λ out Host B unlimited shared output link buffers 31

32 network overload (cont.) Issues due to network overload: packet loss (buffer overrun on the routing device) delay (queue in buffers) Possible solutions: end to end flow control network assisted flow control 32

33 tcp slow start strategy - progressive probing of the channel capacity - double the sending window on success (below threshold) - increment sending window on success (over threshold) on data loss - set the threshold to the half value of the last window size - set the sending window to the initial value of 1 usage - on tcp setup - on retransmission - on long idle time 33

34 tcp slow start (cont.) 34

35 tcp slow start (cont.) Host A Host B one segment RTT two segments four segments time 35

36 thank you questions? 36