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Interconnecting Networks with TCP/IP

Santi Sa- Nguansup CCIE # 8615. Interconnecting Networks with TCP/IP. Objectives. Upon completion of this chapter you will be able to perform the following tasks: Identify the IP protocol stack, its protocol layer functions, and commonly used IP protocols

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Interconnecting Networks with TCP/IP

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  1. Santi Sa-Nguansup CCIE #8615 Interconnecting Networks with TCP/IP

  2. Objectives Upon completion of this chapter you will be able to perform the following tasks: • Identify the IP protocol stack, its protocol layer functions, and commonly used IP protocols • Identify IP address classes, IP addresses, IP subnet masks, IP network numbers, subnet numbers, and possible host numbers. • Configure IP addresses and subnet masks on a router interface and optionally configure a host table. • Interconnect the VLANs with a layer three device such as a router on a stick.

  3. Introduction to TCP/IP Host Host Internet TCP/IP • Early protocol suite • Universal

  4. TCP/IP Protocol Stack 7 5 6 Application Data 5 Presentation 4 Application 4 Session 3 Segment 3 Transport Transport 2 Packet 2 Internet Network 1 Frame 1 Data Link Data Link Bit Physical Physical

  5. Application Layer Overview File Transfer - TFTP * - FTP * - NFS E-Mail - SMTP Remote Login - Telnet * - rlogin * Network Management - SNMP * Name Management - DNS* Application Transport Internet Data Link *Used by the router Physical

  6. Overview of TCP Understanding TCP Behavior and Performance

  7. Transport Layer Overview Transmission Control Protocol (TCP) User Datagram Protocol (UDP) Connection-OrientedConnectionless Application Transport Internet Data Link Physical

  8. Overview of TCP Goals • Provide a high-level overview of TCP • Show important TCP header fields • Demonstrate session initiation and termination • Illustrate TCP Flow and Congestion Control • TCP Slow-Start • Data requests • Impact of delayed Acknowledgements • TCP response to data loss

  9. Overview of TCP Transmission Control Protocol (TCP) • Layer 4 - Transport • Full-duplex • Connection oriented • Reliable • TCP provide streams of application bytes • Provides end-to-end, in-order delivery of IP packets Sequence and Acknowledgement numbers point to the relative byte positions in data streams • TCP Protocol Data Units (PDUs) are called segments

  10. TCP Segment Format Bit 15 Bit 16 Bit 31 Bit 0 Destination port (16) Source port (16) Sequence number (32) Acknowledgement number (32) 20Bytes Reserved (6) TCP Flags (6) Window (16) Data Offset Checksum (16) Urgent Pointer (16) TCP Options (0 or 32 if any - Optional) Data (varies)

  11. TCP Segment Format

  12. Port Numbers RIP FTP TELNET SMTP DNS TFTP SNMP ApplicationLayer 520 21 23 25 53 69 161 Port Numbers TransportLayer TCP UDP

  13. TCP Segment Format Source port • Source port number. Destination port • Destination port number. Sequence number • The sequence number of the first data octet in this segment (except when SYN is present). If SYN is present, the sequence number is the initial sequence number (ISN) and the first data octet is ISN+1. Acknowledgment number • If the ACK control bit is set, this field contains the value of the next sequence number which the sender of the segment is expecting to receive. Once a connection is established, this value is always sent. Data offset • 4 bits. The number of 32-bit words in the TCP header, which indicates where the data begins. The TCP header (even one including options) has a length which is an integral number of 32 bits.

  14. TCP Segment Format Reserved • 6 bits. Reserved for future use. Must be zero. TCP Flags (Control bits) • 6 bits. The control bits may be (from right to left): • U (URG) Urgent pointer field significant. • A (ACK) Acknowledgment field significant. • P (PSH) Push function. • R (RST) Reset the connection. • S (SYN) Synchronize sequence numbers. • F (FIN) No more data from sender. Window • 16 bits. The number of data octets which the sender of this segment is willing to accept, beginning with the octet indicated in the acknowledgment field. Checksum • 16 bits. The checksum field is the 16 bit one’s complement of the one’s complement sum of all 16-bit words in the header and text. If a segment contains an odd number of header and text octets to be checksummed, the last octet is padded on the right with zeros to form a 16-bit word for checksum purposes. The pad is not transmitted as part of the segment. While computing the checksum, the checksum field itself is replaced with zeros. Urgent Pointer • 16 bits. This field communicates the current value of the urgent pointer as a positive offset from the sequence number in this segment. The urgent pointer points to the sequence number of the octet following the urgent data. This field can only be interpreted in segments for which the URG control bit has been set.

  15. TCP Segment Format: TCP Flags • TCP Flags: 8-bit field in TCP header • SYN bit: Synchronization bit in TCP Code Field Set to 1 by sender when initiating a TCP session. • Informs receiver to “synchronize” to SEQUENCE_Number • All TCP OPTIONS must be specified in SYN packet • ACK bit: Acknowledgement bit in TCP Code Field Set to 1 by receiver when acknowledging receipt of data. All TCP packets sent after initial SYN Packet have the ACK bit set to 1. • SuperAgent tracks these bits to calculate key performance indicators.

  16. TCP Segment Format: TCP Flags • RESET bit: Abnormally terminates TCP session (for example, avoids normal tear down) • These are refused connection requests • SuperAgent tracks RESET responses from SYN requests on monitored servers • FIN bit: Normal termination of TCP session

  17. TCP Segment Format Options Options may be transmitted at the end of the TCP header and always have a length which is a multiple of 8 bits. All options are included in the checksum. An option may begin on any octet boundary. There are two possible formats for an option: • A single octet of option type. • An octet of option type, an octet of option length, and the actual option data octets. The option length includes the option type and option length, as well as the option data octets. The list of options may be shorter than that designated by the data offset field because the contents of the header beyond the End-of-Option option must be header padding i.e., zero. A TCP must implement all options. Data • TCP data or higher layer protocol.

  18. TCP Segment Format

  19. TCP Terminology MSS (Maximum Segment Size): • Indicates the largest amount of user data that can be sent in a single IP packet • MSS can be different in both directions • Effective MSS may be modified via Path Maximum Transmission Unit Discovery (PMTUD) • Typically 1460 bytes • Ethernet MTU is 1518 bytes: 1500 bytes data, 18 bytes header • IPv4 MTU is 1500 bytes: 1480 bytes data, 20 bytes header • TCP header is 20 bytes (when options not specified), but is not part of MSS; thus, start with the 1480 byte IP MTU and subtract the TCP 20 byte header and find an MSS of 1460 bytes

  20. TCP Terminology • RWND (Receive Window): Advertised by receiving host; largest amount of data that can be received. • Largest RWND without scaling enabled = 64KB • CWND (Congestion Window): State variable maintained by sending hosts representing the largest amount of data that can be sent – TCP’s perception of network congestion. • Active Window: Minimum of advertised RWND and CWND. Governs amount of unacknowledged data that can be transmitted by sender. • This is how much data will actually be sent without acknowledgement! • SSTHRESH (Slow Start Threshold): Variable used by congestion avoidance and slow-start algorithms. If CWND <= SSTHRESH TCP is in slow start; otherwise, TCP is in congestion avoidance

  21. TCP Terminology Active TCP Window: • Minimum of CWND and advertised RWND CWND=2 (2.92KB) RWND=64KB Active Window = 2.92KB CWND=60 (87.6KB) RWND=64KB Active Window = 64KB

  22. TCP Congestion Control • Increase transmission rate (window size), probing for usable bandwidth, until loss occurs • Increase CWND by 1 MSS for each ACK received until loss detected • Set SSThresh=CWND/2 after loss; reset CWND=1 or 2 • When CWND < SSThresh, sender in slow-start phase; window grows exponentially. • When CWND > SSThresh, sender in congestion-avoidance phase; window grows linearly, probing for network congestion state.

  23. The congestion window size begins at 2* and increments per ACK received in a manner called Slow Start TCP Congestion Algorithms When a loss occurs, TCP begins Slow Start again and sets the Slow Start threshold to half the window where the loss occurred When the SSThresh is reached, TCP goes into Congestion Avoidance mode, which results in a linear increase in window size Window Size (segments) TCP Window Size Increases *Microsoft congestion window starts at 2 Unix/Linux congestion window starts at 1 Time (ms)

  24. TCP Congestion Control • Implications: • Longer Network Round Trip Times (users farther from server) imply longer time to increase CWND. • Closer users may get faster response • WAN Accelerators mitigate this phenomenon • Applications that open TCP connections, send a couple of segments, then close don’t allow window to grow. • User Datagram Protocol users (e.g., VoIP or Video) have no such mechanism and can starve TCP.

  25. TCP Port Numbers Source Port Dest. Port … Telnet Z Host Z Host A Dest. port = 23.Send packet to my Telnet application. SP DP 1028 23 …

  26. 1 TCP Three Way Handshake/Open Connection Host A Host B Send SYN (seq=100 ctl=SYN) SYN received

  27. 1 2 TCP Three Way Handshake/Open Connection Host A Host B Send SYN (seq=100 ctl=SYN) SYN received Send SYN, ACK (seq=300 ack=101 ctl=syn,ack) SYN received

  28. 1 2 3 TCP Three Way Handshake/Open Connection Host A Host B Send SYN (seq=100 ctl=SYN) SYN received Send SYN, ACK (seq=300 ack=101 ctl=syn,ack) SYN received Established (seq=101 ack=301 ctl=ack) ACK received

  29. TCP Simple Acknowledgment Sender Receiver • Window size = 1

  30. TCP Simple Acknowledgment Sender Receiver Send 1 Receive 1 • Window size = 1

  31. TCP Simple Acknowledgment Sender Receiver Send 1 Receive 1 Send ACK 2 Receive ACK 2 • Window size = 1

  32. TCP Simple Acknowledgment Sender Receiver Send 1 Receive 1 Send ACK 2 Receive ACK 2 Send 2 Receive 2 • Window size = 1

  33. TCP Simple Acknowledgment Sender Receiver Send 1 Receive 1 Send ACK 2 Receive ACK 2 Send 2 Receive 2 Send ACK 3 Receive ACK 3 • Window size = 1

  34. TCP Simple Acknowledgment Sender Receiver Send 1 Receive 1 Send ACK 2 Receive ACK 2 Send 2 Receive 2 Send ACK 3 Receive ACK 3 Send 3 Receive 3 • Window size = 1

  35. TCP Simple Acknowledgment Sender Receiver Send 1 Receive 1 Send ACK 2 Receive ACK 2 Send 2 Receive 2 Send ACK 3 Receive ACK 3 Send 3 Receive 3 Send ACK 4 Receive ACK 4 • Window size = 1

  36. 1 TCP Sequence and Acknowledgment Numbers Source Port Dest. Port Sequence # Acknowledgement # … I just sent #10. Source Dest. Seq. Ack. 1028 23 10 1

  37. 2 1 TCP Sequence and Acknowledgment Numbers Source Port Dest. Port Sequence # Acknowledgement # … I just sent #10. I just got #10, now I need #11. Source Dest. Seq. Ack. 1028 23 10 1 Source Dest. Seq. Ack. 23 1028 1 11

  38. 3 TCP Sequence and Acknowledgment Numbers Source Port Dest. Port Sequence # Acknowledgement # … I just sent #11. Source Dest. Seq. Ack. 1028 23 10 1 Source Dest. Seq. Ack. 23 1028 1 11 Source Dest. Seq. Ack. 1028 23 11 2

  39. 4 3 TCP Sequence and Acknowledgment Numbers Source Port Dest. Port Sequence # Acknowledgement # … I just sent #11. I just got #11, now I need #12. Source Dest. Seq. Ack. 1028 23 10 1 Source Dest. Seq. Ack. 23 1028 1 11 Source Dest. Seq. Ack. 1028 23 11 2 Source Dest. Seq. Ack. 23 1028 2 12

  40. TCP Windowing Sender Receiver

  41. TCP Windowing Sender Receiver Window size = 3 Send 1 Window size = 3 Send 2 Window size = 3 Send 3

  42. TCP Windowing Sender Receiver Window size = 3 Send 1 Window size = 3 Send 2 Window size = 3 Send 3 Packet 3 isDropped ACK 3 Window size = 2

  43. TCP Windowing Sender Receiver Window size = 3 Send 1 Window size = 3 Send 2 Window size = 3 Send 3 Packet 3 isDropped ACK 3 Window size = 2 Window size = 3 Send 3 Window size = 3 Send 4

  44. TCP Windowing Sender Receiver Window size = 3 Send 1 Window size = 3 Send 2 Window size = 3 Send 3 Packet 3 isDropped ACK 3 Window size = 2 Window size = 3 Send 3 Window size = 3 Send 4 ACK 5 Window size = 2

  45. TCP Bounce Diagram SYN SYN/ACK Delayed ACK Response Request Response ACK ACK FIN ACK ACK Client 10.10.10.4 TCP: 1024 Server 10.1.1.8 TCP: 80 RWND 64 KB CWND=2 (2x1.46 KB) CWND=3 (3x1.46 KB) 200 ms Request = Data that can fill three segments CWND=4 (4x1.46 KB)

  46. More information about TCP • RFC793 http://www.cis.ohio-state.edu/htbin/rfc/rfc793.html • RFC1146 http://www.cis.ohio-state.edu/htbin/rfc/rfc1146.html • RFC1072 http://www.cis.ohio-state.edu/htbin/rfc/rfc1072.html • This RFC has been replaced by RFC 1323. • The information on this page will be updated to suit the new RFC in the near future. • RFC1693 http://www.cis.ohio-state.edu/htbin/rfc/rfc1693.html • IETF RFC793 defines the Transmission Control Protocol (TCP). TCP provides a reliable stream delivery and virtual connection service to applications through the use of sequenced acknowledgment with retransmission of packets when necessary.

  47. Overview of UDP Understanding TCP Behavior and Performance

  48. UDP Segment Format Bit 0 Bit 15 Bit 16 Bit 31 Destination port (16) Source port (16) 8Bytes Length (16) Checksum (16) Data (if any) • No sequence or acknowledgment fields

  49. UDP Segment Format Source port • Source port is an optional field. When used, it indicates the port of the sending process and may be assumed to be the port to which a reply should be addressed in the absence of any other information. If not used, a value of zero is inserted. Destination port • Destination port has a meaning within the context of a particular Internet destination address. Length • The length in octets of this user datagram, including this header and the data. The minimum value of the length is eight. Checksum • The 16-bit one’s complement of the one’s complement sum of a pseudo header of information from the IP header, the UDP header and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets. Data • UDP data field.

  50. UDP Segment Format

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