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Overview of TCP/IP Protocols

Overview of TCP/IP Protocols. Computer Network Programming. Announcement. Homework #2 is assigned. You can find it on the course web page: http://www.cs.bilkent.edu.tr/~korpe/cs424.html due date is: March 6, Wednesday, beginning of class.

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Overview of TCP/IP Protocols

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  1. Overview of TCP/IP Protocols Computer Network Programming

  2. Announcement • Homework #2 is assigned. You can find it on the course web page: http://www.cs.bilkent.edu.tr/~korpe/cs424.html • due date is: March 6, Wednesday, beginning of class. • If you want to copy the relevant parts of the TCP/IP Illustrated book, let me know and I can lend you my book. You may need this to obtain information about TCP/IP protocols. Textbook does not cover the protocols.

  3. ICMP - Internet Control Message Protocol • ICMP protocol is used to communicate errors and other conditions that require attention. • It is used both by computers and routers. • ICMP messages are usually acted on by either IP layer or higher layer protocol (TCP or UDP) • Some ICMP messages cause errors to be returned to the user process.

  4. ICMP Messages IP Header ICMP Message 20 bytes 1 1 byte 2 type code checksum Content (depends on the type and code fields) ICMP Message

  5. ICMP • The messages are grouped into two: • Error messages • A host receiving an error message never replies back with an other error message • Query messages • A host receiving a query message may reply back with an error message.

  6. Some ICMP Messages ICMP Timestamp request and reply ICMP Port unreachable error ICMP echo request/reply ping command uses ICMP echo request message to check whether a remote host is alive or not. Type(3) Code(3) checksum Unused (0) IP Header(with options) + 8 bytes of original IP datagram

  7. Specific ICMP Messages Type 0 Echo reply 3 destination unreachable 4 source quench 5 redirect 8 echo request 9 router advertisement 10 router solicitation 11 time exceeded 12 parameter problem 13 timestamp request 14 timestamp reply 15 information request 16 information reply 17 address mask request 18 address mask reply

  8. Destination unreachable Type code 3 0 network unreachable 1 host unreachable 2 protocol unreachable 3 port unreachable 4 fragmentation needed but don’t fragment bit set 5 source route failed 6 destination network unknown 7 destination host unknown 8 source host isolated 9 destination network administratively prohibited 10 destination host administratively prohibited 11 network unreachable for TOS 12 host unreachable for TOS 13 communication administratively prohibited by filtering 14 host precedence violation 15 precedence cutoff in effect

  9. Transport Layer • Responsible from demultipling the incoming packets into different higher layer applications • Reliable delivery • Flow control • Congestion Control

  10. Applications and Transport layer Interaction application1 application2 applicationN Programming Interface Socket API TCP UDP Transport Layer Socket API provides programming interface to the applications The applications operate directly over transport layer (TCP or UDP) and use their services using the socket programming interface. Hence we should know the details of the transport layer well in order to program correctly and efficiently.

  11. Simple datagram oriented transport protocol Provides no reliability UDP: User Datagram Protocol IP Datagram UDP datagram IP Header UDP Header UDP Data 20 bytes 8 bytes - Applications are using the UDP protocol by socket interface that is provided by the Operating system. - A UDP datagram that is sent by the application is put into an exactly one IP datagram.

  12. UDP Header 16 bit source port number 16 bit destination port number 8 bytes 16 bit UDP length 16 bit UDP checksum Data (if any) Length field is the size of UDP header + UDP data

  13. Port numbers identify the sending process (application) and receiving application • Hence port numbers are used for demultiplexing at the destination to different applications • Checksum is used to integrity check the UDP datagram if it is corrupted on the way or not • it is end-to-end checksum. Intermediate nodes don’t touch to it.

  14. UDP Checksum UDP pseudo header Source IP address (32 bit) Dest IP address (32 bit) zero protocol UDP length Source port dest port UDP header UDP length UDP checksum data data Pad byte(0) Pad byte is used if the size of data is odd number. UDP checksum is computed over the fields shown above.

  15. Size of an UDP datagram Theoretically: sizeof max UDP datagram is sizeof max IP datagram - sizeof IP header - sizeof UDP header 65535 - 20 - 8 = 65537. However, some operating systems limit the size by the socket API: ~to around 8192 bytes.

  16. TCP: Transmission Control Protocol • Connection oriented • applications using TCP should establish a TCP connection before application data is transferred over the connection. • Reliable delivery • Byte steam oriented • Provides Flow Control • Provides Congestion Control

  17. Byte stream oriented • A stream of 8 bit bytes are exchanged across the TCP connection between two applications • There are no record markers inserted by TCP into the byte stream • If application writes 10 bytes followed 50 bytes, the receiver will not be able to tell what sizes the write operation was at the sender. • The bytes stream received by the receiving application is the same (no reordering) with the stream that the sender sent.

  18. Reliability • Application data is broken into TCP segments. Each TCP segment is sent using an IP datagram. • The receiver acknowledges the receipt of the segments if it receives it. • If the sender does not receive an acknowledgment, it resends the segment. • Segments can arrive out of order to the receiver. The receiver reorders them before giving to the application.

  19. Flow Control • Each end of TCP connection has finite amount of buffer space to store the received segmenta. • The receiving TCP only allows the other end to send as much data as the receiver has buffer for. • This prevents a fast host from taking all the buffers on a slower host.

  20. Congestion Control • If there is congestion on the intermediate nodes (routers), that the packets will be lost. • The sender TCP detect this, and reacts to the losses by decreasing the sending rate (congestion window size). • Thereby the load on the intermediate routers are reduced and traffic can continue to flow. • When the congestion is over, TCP reacts to it by increasing the sending rate (congestion window size)

  21. TCP Header TCP segment IP Header TCP Header TCP data 20 bytes 20 bytes IP datagram 16 bit source port number 16 bit destination port number 32 bit sequence number 20 bytes 32 bit acknowledgment number header length Reserved 6 bits U R G A C K P S H R S T S Y N F I N 16 bit window size 16 bit TCP checksum 16 bit urgent pointer Options (if any) Data (if any)

  22. Fields • Port numbers are used to identify the sending and receiving application. • The two values along with the source and destination IP addresses are uniquely identify a TCP connection • Sequence number identifies the byte in the stream of data from the sending TCP to the receiving TCP that the first byte of data in this segment represents • TCP numbers each byte with a sequence number. • Wraps around to 0 after reach 2^32 - 1. • SYN flag is turned on when a new connection is being established and the sequence number will contain the initial sequence number (ISN) chosen for this connection. SYN flag consumes 1 sequence number. Hence the first byte of data will be assigned a sequence number which is equal to ISN + 1.

  23. Fields • Acknowledgement (ack) number field contains the next sequence number that the sender of ack expects to receive. This is therefore the sequence number plus 1 of the last successfully received byte of data. • ACK field is turned on when the ack number field contains a valid number. It is usually always turned on. • Header length gives the length of the header in 32-bit words (including the options). • Window size is used for flow control. It indicates the number of bytes that the receiver is willing to accept. • Checksum covers the TCP Header and TCP Data. • Urgent pointer is used when the sender wants to send urgent data.

  24. Flags • URG: urgent pointer is valid • ACK: acknowledgement number is valid • PSH: the receiver should pass this data to the application as soon as possible • RST: reset the connection • SYN: synchronize sequence numbers to initiate a connection. • FIN: the sender is finished sending data.

  25. TCP connections • A TCP connection is full-duplex: data can flow in each direction independently. • Therefore sequence number for each direction needs to be synchronized and maintained. • Window size for each direction should be maintained and used for flow control • Popular applications that use TCP are: telnet, ftp, http, electronic mail (smtp).

  26. TCP Connection Establishment and Termination Three-way handshake The server must be prepared to accept an incoming connection. This is done by calling socket, bind and listen and it is called passive open. The client issues an active open by calling connect. This causes a SYN segment to be sent to the server The server must acknowledge the client’s SYN and must also send its SYN. The client must acknowledge the server’s SYN.

  27. client server socket connect (blocks) (active open) socket, bind, listen accept(blocks) SYN i ACK i+1, SYN j connect returns ACK j+1 accept returns read (blocks)

  28. Connection Termination One application calls close first and we say that it performs active close. This end’s TCP sends a FIN segment which means that it finished sending data. The other end that receives FİN performs passive close. The received FIN is acknowledged by TCP. The receipt of the FIN is also passed to the application as an end-of-file. Sometime later the application that received the end-of-file will close its socket. This causes its TCP to send a FIN. The TCP on the system that receives this final FIN acknowledges the FIN.

  29. server client close active close FIN M Passive close read returns 0 ack M+1 close FIN N ack N+1

  30. Example • Establish a telnet connection to the discard port on machine aspendos.cs.bilkent.edu.tr • telnet aspendos.cs.bilkent.edu.tr discard • to find out the port number corresponding to the discard service look to the file /etc/services • And quit from telnet without sending any data. • This will cause establishment of a TCP connection to aspendos and termination of the TCP connection without sending any data

  31. Tcp segments on the wire No. Time Source Destination Protocol Info connection establishment starts here 1 0.000000 pckorpe aspendos.cs.bilkent.edu.tr TCP 3271 > discard [SYN] Seq=3835219300 Ack=0 Win=16384 Len=0 2 0.001260 aspendos.cs.bilkent.edu.tr pckorpe TCP discard > 3271 [SYN, ACK] Seq=250150548 Ack=3835219301 Win=8760 Len=0 3 0.001299 pckorpe aspendos.cs.bilkent.edu.tr TCP 3271 > discard [ACK] Seq=3835219301 Ack=250150549 Win=17520 Len=0 connection termination starts here 4 8.758093 pckorpe aspendos.cs.bilkent.edu.tr TCP 3271 > discard [FIN, ACK] Seq=3835219301 Ack=250150549 Win=17520 Len=0 5 8.758580 aspendos.cs.bilkent.edu.tr pckorpe TCP discard > 3271 [ACK] Seq=250150549 Ack=3835219302 Win=8760 Len=0 6 8.767515 aspendos.cs.bilkent.edu.tr pckorpe TCP discard > 3271 [FIN, ACK] Seq=250150549 Ack=3835219302 Win=8760 Len=0 7 8.767604 pckorpe aspendos.cs.bilkent.edu.tr TCP 3271 > discard [ACK] Seq=3835219302 Ack=250150550 Win=17520 Len=0 The data length of all segments is zero. These segments contain only TCP header (no data).

  32. TCP state transition diagram starting point CLOSED appl:passive open send: nothing appl:active open send: SYN LISTEN recv:SYN, send:SYN,ACK appl:close or timeout recv:RST simultaneous open recv:SYN send:SYN, ACK SYN_SENT SYN_RCVD recv:SYN,ACK send:ACK recv:ACK send:nothing ESTABLISHED CLOSE_WAIT Recv:FIN send:ACK appl:close send:FIN Data transfer state appl:close send:FIN recv:FIN send:ACK FIN_WAIT1 CLOSING simultaneous close recv:ACK send:nothing LAST_ACK recv:ACK send:nothing recv:ACK send:nothing recv:FIN, ACK send:ACK passive close recv:FIN send:ACK FIN_WAIT2 TIME_WAIT active close

  33. TIME_WAIT state • Duration of this state is 2MSL (maximum segment life). MSL is the maximum amount of data that an IP datagram can live in Internet (since we have a TTL field in the IP header). • MSL value is recommended to be 2 Minutes but some implementations use 30 seconds. • Hence TCP can wait in this state 1-4 minutes. • Two reasons for having this state: • to implement TCP’s full duplex connection termination reliably: the sender of last ACK should wait in order to resend the ACK if the receiver did not receive it. • To allow old duplicate segments to expire in the network. We don’t allow a new connection with same port numbers and IP address in this state.

  34. TCP Data Flow • TCP Interactive data flow • the user interacts with a server on the remote machine, hence the response time should be very low. • telnet, rlogin etc. • TCP Bulk Data Flow • ftp, http, etc. • Throughput is more important than response time.

  35. TCP Interactive data flow • I opened a telnet session to aspendos.cs and I just typed: • ls local.cshrc • Every character that I typed is sent as a different TCP segment and the character is also echoed back with an other TCP segment. • If there is no data to send in one direction, the ACKs are delayed around 200ms.

  36. How telnet works Screen Telnet server shell Telnet client network (tcp connection) Terminal driver Terminal driver keyboard echo (pseudo-terminal driver pair) Computer you use Remote computer You type input to the keyboard (you type commands). The input is transferred over the network to the shell though telnet server. The the terminal driver under the shell echos back every character you type so that you can see what you type on the screen. Every charcter is also given to the shell. When you hit enter, shell executes the command and gives the output to the screen over the network.

  37. TCP Interactive data Flow 77 44.357790 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data …m 78 44.358610 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data …m 79 44.523858 pckorpe aspendos.cs.bilkent.edu.tr TCP 4982 > telnet [ACK] Seq=1092462175 Ack=316432462 Win=16796 Len=0 80 44.533723 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … o 81 44.534499 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … o 82 44.661754 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … r 83 44.662574 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … r 84 44.701764 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … e 85 44.702544 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … e 86 44.733765 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … <space> 87 44.734574 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … <space> 88 44.917750 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … l 89 44.918568 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … l 90 45.024578 pckorpe aspendos.cs.bilkent.edu.tr TCP 4982 > telnet [ACK] Seq=1092462180 Ack=316432467 Win=16791 Len=0 91 45.045736 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … o 92 45.046502 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … o 93 45.109786 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … c 94 45.110592 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … c 95 45.173747 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … a 96 45.174554 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … a 97 45.229797 pckorpe aspendos.cs.bilkent.edu.tr TELNET Telnet Data … l 98 45.230626 aspendos.cs.bilkent.edu.tr pckorpe TELNET Telnet Data … l 99 45.425157 pckorpe aspendos.cs.bilkent.edu.tr TCP 4982 > telnet [ACK] Seq=1092462184 Ack=316432471 Win=16787 Len=0

  38. TCP data flow receiver sender segment 1, seq=x, datasize=n ack = x+n segment 2, seq=x+n, datasize=m ack = x+n+m time

  39. Retransmissons receiver sender Assume segment gets lost start timer segment 1, seq=x, datasize=n ack = x+n ack would normally come at this time timer expires retransmit the same segment, star timer segment 1, seq=x, datasize=n ack = x+n stop timer transmit the next segment, start timer segment 2, seq=x+n, datasize=m time

  40. Retransmissons When a segment sent, we start a timer. If timer expires after some certain amount of time, we resend the segment. The timer value should be bigger than the round-trip-time (RTT), so that we don’t resend the segment before the ack comes. If duplicate segments are received at the destination, the destination TCP gives only one copy to the application. If we get an ack for the segment before timer expires, we stop the timer and send the next segment.

  41. Bulk Data Transfer If there is data available to send, TCP sends usually more than one segment at a time without getting acknowledments. This increases the throughput of data transfer operation. TCP is allowed to send as much segments as it is allowed by the advertised receive window of the receiver. But it is not a must for the sender to send that much data.

  42. Sliding Window offered window (advertised by the receiver) usable window 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ……. Can’t send until window moves Sent but not ACKed Sent and acknowledged Can send ASAP The window close at the left edge (moves to right) when we send data and receive the corresponding acknowledgements. The window opens at the right edge (moves right), allowing us to send more data. This happens when the receiving process reads the data from the TCP receive buffers, hence freeing up space in the TCP receive buffers.

  43. Example FTP transfer A file is transferred from mendelson.ceng.metu.edu.tr to pckorpe # time sender receiver 214 118.621016 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 215 118.814162 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363566136 Win=17520 Len=0 216 118.855262 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 217 118.861184 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 218 118.861239 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363569056 Win=17520 Len=0 219 118.890430 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 220 118.896816 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 221 118.896861 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363571976 Win=17520 Len=0 222 118.902655 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 223 118.902709 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363573436 Win=17520 Len=0 224 118.927645 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 225 118.933468 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 226 118.933507 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363576356 Win=17520 Len=0 227 118.939907 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 228 118.939975 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363577816 Win=17520 Len=0 229 118.945714 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 230 118.951776 mendelson.ceng.metu.edu.tr pckorpe FTP-DATA FTP Data: 1460 bytes 231 118.951827 pckorpe mendelson.ceng.metu.edu.tr TCP 1269 > ftp-data [ACK] Seq=891333048 Ack=2363580736 Win=17520 Len=0

  44. Example file transfer Mendelson pckorpe Segment 1, size = 1460, seq = 64676 ack = 66136 (64676 + 1460) Segment 2, size = 1460, seq = 66136 Segment 3, size = 1460, seq = 67596 ack = 69056 Segment 4, size = 1460, seq = 69056 Segment 5, size = 1460, seq = 70516 ……………... Time

  45. Acks • Usually acks are sent every two segment • Acks are cumulative: we acknowledge all the data that is received so far. • If we receive 2 segments, for example, we acknowledge them in one ACK packet.

  46. Application Protocols • DNS: domain name system • SNMP: simple network management protocol • SMTP: simple mail transfer protocol • HTTP: hypertext transfer protocol

  47. DNS You access a web page www.cnn.com from your browser. You don’t know the IP address of CNN web server. People don’t like to use IP address in their application, instead they use hostname or domain names which are more meaningful, easy to remember and user friendly then the IP addresses. Your browser needs to establish a TCP connection to www.cnn.com to fetch the requested page But TCP does not know anything about names like www.cnn.com. It only uses IP addresses. Hence your browser should supply to TCP the corresponding IP address for www.cnn.com. Your browser contact a domain name server to find out the IP address corresponding to domain name (or hostname) www.cnn.com That is why we need a protocol to talk to domain name servers in order to get the IP addresses corresponding to hostnames.

  48. DNS name space Unnamed root …... ….. arpa com edu gov tr ae umd zone (provide multiple name servers for the zone) ibm in-addr 139 zone www cs 179 www.ibm.com 10 oreo oreo.cs.umd.edu 6 6.10.179.139.in-addr.arpa (used for IP address to Name mapping)

  49. Characteristics of DNS • Hierarchical naming scheme • Delegation of authority for names • Distributed databases of name to address mappings • Each name authority must provide at least two name servers for their domain. One is primary, other is secondary Authoritive server Root server Resolver on a computer is responsible for contacting the name server for clients. zone:umd.edu zone: cs.bilkent.edu.tr Primary server Receives mappings Secondary server Disk file for name to IP addr mappings resolver /etc/resolv.conf file keeps the names of the name servers client

  50. DNS Message Format 0 16 31 identification flags mumber of questions number of answer RRs number of authority RRs number of additional RRs questions answers (variable number of resouce records) authority (variable number of resource records) additional information (variable number of resource records)

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