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Introducing TCP & UDP Internet Transport Layers (C) Herbert Haas 2005/03/11 TCP/IP Protocol Suite BootP Application SMTP HTTP FTP Telnet DNS SNMP etc. DHCP ( M I M E ) Presentation Routing Protocols Session OSPF BGP UDP TCP

  1. Introducing TCP & UDP Internet Transport Layers (C) Herbert Haas 2005/03/11

  2. TCP/IP Protocol Suite BootP Application SMTP HTTP FTP Telnet DNS SNMP etc. DHCP ( M I M E ) Presentation Routing Protocols Session OSPF BGP UDP TCP RIP EGP Transport (User Datagram (Transmission Control Protocol) Protocol) IP (Internet Protocol) Network ICMP ARP RARP IP Transmission over Link ATM IEEE 802.2 X.25 FR PPP RFC 1483 RFC 1042 RFC 1356 RFC 1490 RFC 1661 Physical 2005/03/11 2

  3. TCP/UDP and OSI Transport Layer 4 Layer 4 Protocol = TCP (Connection-Oriented) Layer 4 Protocol = UDP (Connectionless) IP Host A IP Host B TCP/UDP Connection (Transport-Pipe) 4 4 M M Router 1 Router 2 2005/03/11 3

  4. TCP Facts (1) � Connection-oriented layer 4 protocol � Carried within IP payload � Provides a reliable end-to-end transport of data between computer processes of different end systems � Error detection and recovery � Sequencing and duplication detection � Flow control � RFC 793 4 (C) Herbert Haas 2005/03/11

  5. TCP Facts (2) � Application's data is regarded as continuous byte stream � TCP ensures a reliable transmission of segments of this byte stream � Handover to Layer 7 at "Ports" � OSI-Speak: Service Access Point 5 (C) Herbert Haas 2005/03/11

  6. Port Numbers � Using port numbers TCP (and UDP) can multiplex different layer-7 byte streams � Server processes are identified by Well known port numbers : 0..1023 � Controlled by IANA � Client processes use arbitrary port numbers >1023 � Better >8000 because of registered ports 6 (C) Herbert Haas 2005/03/11

  7. Registered Ports � For proprietary server applications � Not controlled by IANA only listed in RFC 1700 � Examples � 1433 Microsoft-SQL-Server � 1439 Eicon X25/SNA Gateway � 1527 Oracle � 1986 Cisco License Manager � 1998 Cisco X.25 service (XOT) � 6000-6063 X Window System 7 (C) Herbert Haas 2005/03/11

  8. TCP Communications Server Host A Host B Server-Proc 1 Server-Proc 2 Client-Proc Client-Proc WWW POP3 Port 4711 Port 7312 Port 80 Port 110 TCP (80 / 110) TCP (4711) TCP (7312) IP (10.1.1.9) IP (10.1.1.1) IP (10.1.1.2) DA:10.1.1.9 DA:10.1.1.9 DP:80 DP:110 SA:10.1.1.1 SA:10.1.1.2 SP:4711 SP:7312 8 (C) Herbert Haas 2005/03/11

  9. Sockets � Server process multiplexes streams with same source port numbers according source IP address � (PortNr, SA) = Socket � Each stream ("flow") is uniquely identified by a socket pair 9 (C) Herbert Haas 2005/03/11

  10. TCP Communications Server Host A Host B Connection 2: Connection 1: Socket: 10.1.1.9 : 80 Socket: 10.1.1.9 : 80 Socket: 10.1.1.9 : 7312 Socket: 10.1.1.1 : 4711 Server-Proc 1 Client-Proc Client-Proc WWW Port 4711 Port 7312 Port 80 TCP (80) TCP (4711) TCP (7312) IP (10.1.1.9) IP (10.1.1.1) IP (10.1.1.2) DA:10.1.1.9 DA:10.1.1.9 DP:80 DP:80 SA:10.1.1.1 SA:10.1.1.2 SP:4711 SP:7312 10 (C) Herbert Haas 2005/03/11

  11. TCP Communications Server Host Connection 2: Connection 1: Socket: 10.1.1.9 : 80 Socket: 10.1.1.9 : 80 Socket: 10.1.1.9 : 7312 Socket: 10.1.1.2 : 4711 Server-Proc 1 WWW Client-Proc 1 Client-Proc 2 Port 80 Port 4711 Port 7312 TCP (80) TCP (4711 / 7312) Connection 1: Connection 2: Socket: 10.1.1.9 : 80 Socket: 10.1.1.9 : 80 Socket: 10.1.1.2 : 4711 Socket: 10.1.1.9 : 7312 IP (10.1.1.9) IP (10.1.1.2) DA:10.1.1.9 DA:10.1.1.9 DP:80 DP:80 SA:10.1.1.2 SA:10.1.1.2 SP:4711 SP:7312 11 (C) Herbert Haas 2005/03/11

  12. TCP Header 0 4 8 12 16 20 24 28 32 Source Port Number Destination Port Number Sequence Number Acknowledgement Number Header U A P R S F Reserved Window Size R C S S Y I Length G K H T N N TCP Checksum Urgent Pointer Padding Options (variable length) PAYLOAD 12 (C) Herbert Haas 2005/03/11

  13. TCP Header (1) � Source and Destination Port � 16 bit port number for source and destination process � Header Length � Multiple of 4 bytes � Variable header length because of options (optionally) 13 (C) Herbert Haas 2005/03/11

  14. TCP Header (2) � Sequence Number (32 Bit) � Number of first byte of this segment � Wraps around to 0 when reaching 2 32 -1) � Acknowledge Number (32 Bit) � Number of next byte expected by receiver � Confirms correct reception of all bytes including byte with number AckNr-1 14 (C) Herbert Haas 2005/03/11

  15. TCP Header (3) � URG-Flag � Indicates urgent data � If set, the 16-bit "Urgent Pointer" field is valid and points to the last octet of urgent data � There is no way to indicate the beginning of urgent data (!) � Applications switch into the "urgent mode" � Used for quasi-outband signaling 15 (C) Herbert Haas 2005/03/11

  16. TCP Header (4) � PSH-Flag � TCP should push the segment immediately to the application without buffering � To provide low-latency connections � Often ignored 16 (C) Herbert Haas 2005/03/11

  17. TCP Header (5) � SYN-Flag � Indicates a connection request � Sequence number synchronization � ACK-Flag � Acknowledge number is valid � Always set, except in very first segment 17 (C) Herbert Haas 2005/03/11

  18. TCP Header (6) � FIN-Flag � Indicates that this segment is the last � Other side must also finish the conversation � RST-Flag � Immediately kill the conversation � Used to refuse a connection-attempt 18 (C) Herbert Haas 2005/03/11

  19. TCP Header (7) � Window (16 Bit) � Adjusts the send-window size of the other side � Used with every segment � Receiver-based flow control � SeqNr of last octet = AckNr + window 19 (C) Herbert Haas 2005/03/11

  20. TCP Header (8) � Checksum � Calculated over TCP header, payload and 12 byte pseudo IP header � Pseudo IP header consists of source and destination IP address, IP protocol type, and IP total length; � Complete socket information is protected � Thus TCP can also detect IP errors 20 (C) Herbert Haas 2005/03/11

  21. TCP Header (9) � Urgent Pointer � Points to the last octet of urgent data � Options � Only MSS (Maximum Message Size) is used � Other options are defined in RFC1146, RFC1323 and RFC1693 � Pad � Ensures 32 bit alignment 21 (C) Herbert Haas 2005/03/11

  22. TCP 3-Way-Handshake ACK = ? SEQ = ? (idle) ACK = ? SEQ = 730 (random) A C K = ? S E Q = S 7 3 Y 0 N ACK = 731 SEQ = 400 (random) 0 0 4 = Q E S 1 3 7 = K K C C A A , N Y S ACK = 401 SEQ = 731 ACK=401 SEQ=731 ACK ACK = 731 ACK = 401 SEQ = 401 SEQ = 731 SYNCHRONIZED 22 (C) Herbert Haas 2005/03/11

  23. Sequence Number � RFC793 suggests to pick a random number at boot time (e.g. derived from system start up time) and increment every 4 µs � Every new connection will increments SeqNr by 1 � To avoid interference of spurious packets � Old "half-open" connections are deleted with the RST flag 23 (C) Herbert Haas 2005/03/11

  24. TCP Data Transfer ACK = 731 ACK = 401 SEQ = 401 SEQ = 731 A C K = 4 0 1 S E Q 2 = 0 7 3 B 1 y t e s ACK = 751 SEQ = 401 1 0 4 = Q E S 1 5 7 = K C s A e t y B 0 ACK = 401 SEQ = 751 ACK=401 SEQ=751 50 Bytes ACK = 801 SEQ = 401 1 0 4 = Q E S 1 0 8 = K C s A e t y B 0 ACK = 401 SEQ = 801 24 (C) Herbert Haas 2005/03/11

  25. TCP Data Transfer � Acknowledgements are generated for all octets which arrived in sequence without errors (positive acknowledgement) � Duplicates are also acknowledged (!) � Receiver cannot know why duplicate has been sent; maybe because of a lost acknowledgement � The acknowledge number indicates the sequence number of the next byte to be received � Acknowledgements are cumulative: Ack(N) confirms all bytes with sequence numbers up to N-1 � Therefore lost acknowledgements are no problem 25 (C) Herbert Haas 2005/03/11

  26. Cumulative Acknowledgement Data(13) Seq=10 Data(15) Seq=23 Ack = 23 Data(5) Seq=38 Ack = 38 Data(11) Seq=43 Ack = 43 Data(9) Seq=54 Ack = 54 Ack is lost Cumulative Ack Ack = 63 26 (C) Herbert Haas 2005/03/11

  27. Duplicate Acknowledgement Data(13) Seq=10 Data(15) Seq=23 Ack = 23 Data(5) Seq=38 Ack = 38 Data is lost Data(11) Seq=43 Repair Data(5) Seq=38 Duplicate Ack Ack = 38 Cumulative Ack Ack = 54 27 (C) Herbert Haas 2005/03/11

  28. TCP Retransmission Timeout � Retransmission timeout (RTO) will initiate a retransmission of unacknowledged data � High timeout results in long idle times if an error occurs � Low timeout results in unnecessary retransmissions � TCP continuously measures RTT to adapt RTO 28 (C) Herbert Haas 2005/03/11

  29. Retransmission ambiguity problem � If a packet has been retransmitted and an ACK follows: Does this ACK belong to the retransmission or to the original packet? � Could distort RTT measurement dramatically � Solution: Phil Karn's algorithm � Ignore ACKs of a retransmission for the RTT measurement � And use an exponential backoff method 29 (C) Herbert Haas 2005/03/11

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