The Transport Layer. Internet solutions. Nixu Oy PL 21. (Mäkelänkatu 91) Helsinki, Finland. tel fax.

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1 The Transport Layer Nixu Oy PL 21 (Mäkelänkatu 91) Helsinki, Finland tel fax info@nixu.fi

2 OVERVIEW User Datagram Protocol Transmission Control Protocol Mowgli XTP WAP Multicast Copyright 2000 Nixu Oy 2/36 The Transport Layer

3 Transport Layer Protocols Provide services to the applications protocols Reliable datagram or byte stream delivery Additional addressing to the network layer host addresses Provide a sharing of work load Copyright 2000 Nixu Oy 3/36 The Transport Layer

4 UDP UDP = User Datagram Protocol Defined in RFC-768 UDP packet syntax SOURCE PORT LENGTH DATA Port is a 16-bit application identifier number. DESTINATION PORT UDP CHECKSUM Checksum is calculated over both the header and the data. UDP checksum is optional. Copyright 2000 Nixu Oy 4/36 The Transport Layer

5 UDP UDP datagram is encapsulated into an IP datagram. IP header UDP header UDP data Unreliable datagram-oriented transportation layer protocol offers little extra functionality besides port numbers simple, fast, light-weight, easy to implement Applications using UDP: DNS, Radius, NTP, SNMP Copyright 2000 Nixu Oy 5/36 The Transport Layer

6 A UDP (DNS) Session Snoop 23 $ dig a ;; got answer: ;; QUESTIONS: ;; tapas.nixu.fi, type = A, class = IN ;; ANSWERS: tapas.nixu.fi A ;; AUTHORITY RECORDS: nixu.fi NS ns2.tele.fi. nixu.fi NS ns.nixu.fi. nixu.fi NS ns.tele.fi. ;; ADDITIONAL RECORDS: ns2.tele.fi A ns.nixu.fi A ns.tele.fi A ns.tele.fi A ;; Total query time: 88 msec ;; FROM: mole.nixu.fi to SERVER: ;; MSG SIZE sent: 31 rcvd: riku@mole $ Copyright 2000 Nixu Oy 6/36 The Transport Layer

7 DNS Query, Ethernet Header ETHER: Packet 1 arrived at 11:19:24.80 ETHER: Packet size = 73 bytes ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) Copyright 2000 Nixu Oy 7/36 The Transport Layer

8 DNS Query, IP Header IP: Version = 4 IP: Header length = 20 bytes IP: Type of service = 0x00 IP: xxx.... = 0 (precedence) IP: = normal delay IP: = normal throughput IP: = normal reliability IP: Total length = 59 bytes IP: Identification = IP: Flags = 0x4 (do not fragment) IP: Fragment offset = 0 bytes IP: Time to live = 255 seconds/hops IP: Protocol = 17 (UDP) IP: Header checksum = 7e65 IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi IP: No options Copyright 2000 Nixu Oy 8/36 The Transport Layer

9 DNS Query, UDP Header UDP: Source port = UDP: Destination port = 53 (DNS) UDP: Length = 39 UDP: Checksum = E34A Copyright 2000 Nixu Oy 9/36 The Transport Layer

10 DNS Query, Data 0: f12c b80 0e t.,..;...e. 16: 003b 8b ff11 7e65 c2c : b e34a 000a v...5.'.j... 48: e tapas.nix 64: u.fi... From now on only relevant portions of the headers will be displayed Copyright 2000 Nixu Oy 10/36 The Transport Layer

11 DNS Reply Headers ETHER: Packet size = 217 bytes ETHER: Destination = 0:0:3b:80:e:93, ETHER: Source = 8:0:20:74:f1:2c, Sun ETHER: Ethertype = 0800 (IP) IP: Total length = 203 bytes IP: Flags = 0x4 (do not fragmnet) IP: Protocol = 17 (UDP) IP: Header checksum = 8ed6 IP: Source address = , jalopeno.nixu.fi IP: Destination address = , mole.nixu.fi UDP: Source port = 53 UDP: Destination port = UDP: Length = 183 UDP: Checksum = AD48 Copyright 2000 Nixu Oy 11/36 The Transport Layer

12 DNS Reply Data 0: b80 0e f12c ;... t.,..e. 16: 00cb 7a ff11 8ed6 c2c : b5 00b7 ad48 000a v..5...h... 48: e tapas.nix 64: c0 0c u.fi some reply data deleted : 087b db00 04c1 d f.{... Copyright 2000 Nixu Oy 12/36 The Transport Layer

13 TCP TCP = Transmission Control Protocol Defined in RFC-793 Connection-oriented, reliable, byte-stream service Application data is broken into segments, which are sent as IP datagrams. Features: Checksums, timeouts and flow control Segment reassembly in correct order, discarding duplicate packets Applications using TCP: SMTP, HTTP (WWW), NNTP (News),... Copyright 2000 Nixu Oy 13/36 The Transport Layer

14 TCP Segment Format source port number destination port number sequence number acknowledgment number hdrlen reserv. flags window size TCP checksum urgent pointer options (if any) data (if any) Ports identify source and destination applications. Sequence number identifies the first byte of the segment. Acknowledge number is the next expected sequence number for incoming data. Copyright 2000 Nixu Oy 14/36 The Transport Layer

15 TCP Segment Format Flags URG validates the urgent pointer. ACK acknowledges the synchronization. PSH marks that the data buffer should be flushed to the receiving application. Sometimes considered outdated. RST resets the connection. SYN marks the synchronization phase of connection. FIN ends connection. Copyright 2000 Nixu Oy 15/36 The Transport Layer

16 TCP Segment Format Window Size is the number of bytes the receiver is willing to accept Urgent pointer points to the last byte of urgent data. Urgent data feature is used, for example, when a user presses the interrupt key during a telnet session. Options contain often maximum segment size at the start of connection, options can be also used to discover path MTU or to set window scale factor. Copyright 2000 Nixu Oy 16/36 The Transport Layer

17 TCP Data Flow Receiver sends acknowledgment for each segment. If a packet gets lost, timeout will ensure it s retransmitted Sender packet 1 waiting for acknowledgment retransmission Packets in transit packet gets lost Receiver packet arrives acknowledgment sent Copyright 2000 Nixu Oy 17/36 The Transport Layer

18 TCP Data Flow Normally a sliding window technique Sender Packets in transit Receiver packet 1 packet 2 packet 3 waiting for ACK ACK packets 1 and 2 ACK packet 3 The window size is changeable, default size is around couple dozen kilobytes depending on the implementation. Copyright 2000 Nixu Oy 18/36 The Transport Layer

19 Establishing a TCP Connection The three-way handshake Client active open Packets in transit SYN SYN+ACK ACK Server passive open Copyright 2000 Nixu Oy 19/36 The Transport Layer

20 Closing a TCP Connection Client active close Packets in transit FIN Server ACK FIN passive close ACK There may be data between the middle ACK and FIN So called half-close is utilized by some applications. Client says I am done sending data but I still want to receive Copyright 2000 Nixu Oy 20/36 The Transport Layer

21 TCP shortcomings Window size Maximum window size 64 kb Max bandwidth = max window size / round trip time Error handling TCP assumes that lost packets are lost due to network congestion > Internet is built on high-quality fixed trunk connections and LANs The solution is to make the sending window smaller than maximum size On modern wireless networks data is often lost to radio errors > The correct solution would be to resend the same data again as soon as possible > Instead TCP starts to send slower Copyright 2000 Nixu Oy 21/36 The Transport Layer

22 A Snooped SMTP Session 24 $ telnet jalopeno 25 Trying Connected to jalopeno. Escape character is '^]'. 220-jalopeno.nixu.fi ESMTP Sendmail / ready at Sat, 5 Oct :24: ESMTP spoken here QUIT 221 jalopeno.nixu.fi closing connection Connection closed by foreign host. 25 riku@mole $ Copyright 2000 Nixu Oy 22/36 The Transport Layer

23 1. SYN Client->Server (Ethernet Header) ETHER: Packet 1 arrived at 10:58:0.62 ETHER: Packet size = 60 bytes ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) Copyright 2000 Nixu Oy 23/36 The Transport Layer

24 1. SYN Client->Server (IP Header) IP: Version = 4 IP: Header length = 20 bytes IP: Type of service = 0x00 IP: xxx.... = 0 (precedence) IP: = normal delay IP: = normal throughput IP: = normal reliability IP: Total length = 44 bytes IP: Identification = IP: Flags = 0x4 (do not fragment) IP: Fragment offset = 0 bytes IP: Time to live = 255 seconds/hops IP: Protocol = 6 (TCP) IP: Header checksum = 1188 IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi IP: No options Copyright 2000 Nixu Oy 24/36 The Transport Layer

25 1. SYN Client->Server (TCP Header) TCP: Source port = TCP: Destination port = 25 (SMTP) TCP: Sequence number = TCP: Acknowledgement number = 0 TCP: Data offset = 24 bytes TCP: Flags = 0x02 TCP: = No urgent pointer TCP: = No acknowledgement TCP: = No push TCP: = No reset TCP: = Syn TCP: = No Fin TCP: Window = 8760 TCP: Checksum = 0x17a1 TCP: Urgent pointer = 0 TCP: Options: (4 bytes) TCP: - Maximum segment size = 1460 bytes Copyright 2000 Nixu Oy 25/36 The Transport Layer

26 1. SYN Client->Server (SMTP Data) SMTP: "" From now on only relevant portions of the headers will be displayed Copyright 2000 Nixu Oy 26/36 The Transport Layer

27 2. SYN+ACK Server->Client ETHER: Destination = 0:0:3b:80:e:93, ETHER: Source = 8:0:20:74:f1:2c, Sun ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , jalopeno.nixu.fi IP: Destination address = , mole.nixu.fi TCP: Source port = 25 TCP: Destination port = TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x12 (ACK, SYN) TCP: Options: (4 bytes) TCP: - Maximum segment size = 1460 bytes SMTP: "" Copyright 2000 Nixu Oy 27/36 The Transport Layer

28 3. ACK Client->Server ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi TCP: Source port = TCP: Destination port = 25 (SMTP) TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x10 (ACK) TCP: No options SMTP: "" Copyright 2000 Nixu Oy 28/36 The Transport Layer

29 4. Data Server->Client ETHER: Destination = 0:0:3b:80:e:93, ETHER: Source = 8:0:20:74:f1:2c, Sun ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , jalopeno.nixu.fi IP: Destination address = , mole.nixu.fi TCP: Source port = 25 TCP: Destination port = TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x18 (ACK, PSH) SMTP: "220-jalopeno.nixu.fi ESMTP Sendmail / ready" Copyright 2000 Nixu Oy 29/36 The Transport Layer

30 5. Reply Client->Server ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi IP: No options TCP: Source port = TCP: Destination port = 25 (SMTP) TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x10 (ACK) SMTP: " Copyright 2000 Nixu Oy 30/36 The Transport Layer

31 6. Data Client->Server ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi IPTCP: Source port = TCP: Destination port = 25 (SMTP) TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x18 (ACK, PSH) SMTP: "QUIT\r\n" Copyright 2000 Nixu Oy 31/36 The Transport Layer

32 7. Ack Server->Client (Containing Data) ETHER: Destination = 0:0:3b:80:e:93, ETHER: Source = 8:0:20:74:f1:2c, Sun ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , jalopeno.nixu.fi IP: Destination address = , mole.nixu.fi TCP: Source port = 25 TCP: Destination port = TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x18 (ACK, PSH) SMTP: "221 jalopeno.nixu.fi closing connection\r\n" Copyright 2000 Nixu Oy 32/36 The Transport Layer

33 8. Server Starts Closing ETHER: Destination = 0:0:3b:80:e:93, ETHER: Source = 8:0:20:74:f1:2c, Sun ETHER: Ethertype = 0800 (IP) IP: Protocol = 6 (TCP) IP: Source address = , jalopeno.nixu.fi IP: Destination address = , mole.nixu.fi TCP: Source port = 25 TCP: Destination port = TCP: Sequence number = TCP: Acknowledgement number = TCP: Data offset = 20 bytes TCP: Flags = 0x11 (ACK, FIN) SMTP: "" Copyright 2000 Nixu Oy 33/36 The Transport Layer

34 9. Client Acks Previous Data ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) IP: Protocol = 6 (TCP) IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi TCP: Source port = TCP: Destination port = 25 (SMTP) TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x10 (ACK) SMTP: "" Copyright 2000 Nixu Oy 34/36 The Transport Layer

35 10. Client Starts Closing as Well ETHER: Destination = 8:0:20:74:f1:2c, Sun ETHER: Source = 0:0:3b:80:e:93, ETHER: Ethertype = 0800 (IP) IP: Protocol = 6 (TCP) IP: Source address = , mole.nixu.fi IP: Destination address = , jalopeno.nixu.fi TCP: Source port = TCP: Destination port = 25 (SMTP) TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x11 (ACK, FIN) SMTP: "" Copyright 2000 Nixu Oy 35/36 The Transport Layer

36 11.Server Acks Closing ETHER: Destination = 0:0:3b:80:e:93, ETHER: Source = 8:0:20:74:f1:2c, Sun ETHER: Ethertype = 0800 (IP) IP: Flags = 0x4 (do not fragment) IP: Protocol = 6 (TCP) IP: Source address = , jalopeno.nixu.fi IP: Destination address = , mole.nixu.fi TCP: Source port = 25 TCP: Destination port = TCP: Sequence number = TCP: Acknowledgement number = TCP: Flags = 0x10 (ACK) SMTP: "" Copyright 2000 Nixu Oy 36/36 The Transport Layer

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