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TCP/IP TCP/IP Internal.

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1 TCP/IP TCP/IP Internal

2 Learning outcome Application layer Transport layer Internet layer
TCP/IP Learning outcome Application layer HTTP, FTP, TELNET, POP3, SMTP, IMAP, DNS protocols Transport layer TCP and UDP TCP and UDP segment Opening and closing connections Flow control Reliable data transmission Internet layer IP , ICMP, ARP and RARP IP datagram Routing

3 Learning outcome cont’d
TCP/IP Learning outcome cont’d Before we have explained how Each layer adds header information to the block of data passed to it from the previous layer And these headers are interpreted and removed by corresponding layer at the receiving end In this Chapter We will look in details at the header information constructed at the transport and internet layers We will also show how this information is used

4 Reading List for this chapter
TCP/IP Reading List for this chapter Joe Casad, Teach Teach Yourself TCP/IP, chs. 4-6 or William Buchanan, Mastering the Internet, Ch. 4 or Julian Moss, “understanding TCP/IP” (parts 2-4, October 1997-March 1998) and Liam Proven, “understanding TCP/IP” [details of IP addressing] ( January 2001)

5 Network-specific protocols (e.g. Ethernet, Token-ring, FDDI, ATM)
TCP/IP TCP/IP Protocol Suite is a four-layered protocol suite. The location of the important protocols within the TCP/IP layers is showed below OSI layers TCP UDP IP Internet layer ICMP Network interface Network-specific protocols (e.g. Ethernet, Token-ring, FDDI, ATM) Transport layer SMTP HTTP TELNET FTP SNMP DNS RTP Application layer IMAP POP 5

6 The suite of Protocols for TCP/IP

7 TCP/IP suite The application layer Handles high-level protocols, issues of representation, encoding, and dialog control.  The TCP/IP combines all application-related issues into one layer, and assures this data is properly packaged for the next layer. FTP, HTTP, SMNP, DNS ... Format of data, data structure, encode … Dialog control, session management …

8 Application Protocols
TCP/IP suite Application Protocols Protocols Role Ports HTTP Hyper Text Transfer Protocol browser and web server communication client browser connects to HTTP server client browser send a request to the HTTP server HTTP server reacts by sending a response HTTP server disconnects 80 FTP File transfer protocol allow people anywhere on the Internet to log in and download whatever files they have placed on the FTP server, or upload other files. Port 20 for data channel and 21 for control channel 20, 21

9 Application Protocols
TCP/IP suite Application Protocols Protocols Role Ports DNS Domain Name System provides translation between host name and IP address DNS messages are carried using UDP on port 53 53 TELNET Remote login 23

10 Application Protocols (cont’d)
TCP/IP suite Application Protocols (cont’d) Protocols Role Ports POP3 Post Office Protocol 3 The point of POP3 is to fetch from the remote mailbox and store it on the user’s local machine to read later. Downloaded s are then deleted from the server. 110 IMAP Internet Message Access Control Retrieve s retaining on the server and for organizing it in folders on the serve 143 SMTP Sending Sending s Establish TCP connection to port 25 of the destination machine / server Start sending message 25

11 The transport layer Transport layer Transport protocols
TCP/IP suite The transport layer Transport layer Transport protocols UDP TCP TCP AND UDP segments

12 Transport Protocols in the Internet
TCP/IP suite Transport Protocols in the Internet The Internet supports 2 transport protocols UDP - User Datagram Protocol datagram oriented unreliable, connectionless No acknowledgment simple unicast and multicast useful only for few applications, e.g., multimedia applications used a lot for services network management (SNMP), routing (RIP), naming (DNS), etc. TCP - Transmission Control Protocol stream oriented reliable, connection-oriented complex only unicast used for most Internet applications: web (HTTP), (SMTP), file transfer (FTP), terminal (TELNET), etc.

13 User Datagram Protocol
TCP/IP User Datagram Protocol UDP Header UDP Data Datagram Header Datagram Data Area UDP Header UDP Data Frame Header Frame Data Area Datagram Header Datagram Data Area UDP Header UDP Data

14 User Datagram Protocol
TCP/IP User Datagram Protocol Source port (optional - zero if not used) Length - Count of octets including header and data (minimum is 8) Checksum (optional - zero if not used) UDP Source Port UDP Destination Port UDP Message Length UDP Checksum Data . . .

15 User Datagram Protocol
TCP/IP User Datagram Protocol IP checksum does not include data UDP checksum is only way to guarantee that data is correct UDP checksum includes pseudo-header Pseudo Header UDP Header UDP Data

16 UDP Pseudo-Header Source IP Address Destination Address Zero Protocol
TCP/IP UDP Pseudo-Header Source IP Address Destination Address Zero Protocol UDP Length UDP Source Port UDP Destination Port UDP Message Length UDP Checksum Data . . .

17 Transport Control Protocol
TCP/IP Transport Control Protocol

18 TCP/IP TCP Lingo When a client requests a connection, it sends a “SYN” segment (a special TCP segment) to the server port. SYN stands for synchronize. The SYN message includes the client’s ISN. ISN is Initial Sequence Number.

19 More... TCP/IP Every TCP segment includes a Sequence Number that refers to the first byte of data included in the segment. Every TCP segment includes a Request Number (Acknowledgement Number) that indicates the byte number of the next data that is expected to be received. All bytes up through this number have already been received.

20 And more... There are a bunch of control flags:
TCP/IP And more... There are a bunch of control flags: URG: urgent data included. ACK: this segment is (among other things) an acknowledgement. RST: error - abort the session. SYN: synchronize Sequence Numbers (setup) FIN: polite connection termination.

21 And more... MSS: Maximum segment size (A TCP option)
TCP/IP And more... MSS: Maximum segment size (A TCP option) Window: Every ACK includes a Window field that tells the sender how many bytes it can send before the receiver will have to throw it away (due to fixed buffer size).

22 Client Server SYN 1 ISN=X time SYN 2 ISN=Y ACK=X+1 3 ACK=Y+1
TCP Connection creation Client Server SYN ISN=X 1 SYN ISN=Y ACK=X+1 2 time ACK=Y+1 3 TCP 3-way handshake

23 TCP 3-way handshake 1 Client: “I want to talk, and I’m starting with byte number X+1”. Server: “OK, I’m here and I’ll talk. My first byte will be called number Y+1, and I know your first byte will be number X+1”. Client: “Got it - you start at byte number Y+1”. 2 3

24 TCP Data and ACK Once the connection is established, data can be sent.
Each data segment includes a sequence number identifying the first byte in the segment. Each segment (data or empty) includes a request number indicating what data has been received.

25 TCP Fast Retransmit Another enhancement to TCP congestion control
Idea: When sender sees 3 duplicate ACKs, it assumes something went wrong The packet is immediately retransmitted instead of waiting for it to timeout

26 Figure 6.12 Fast Retransmit
TCP Fast Retransmit Fast Retransmit Based on three duplicate ACKs Figure 6.12 Fast Retransmit

27 TCP Fast Retransmit Example
Sender Receiver MSS = 1K ACK of new data ACK = 2048 WIN = 31K 1K SEQ=2048 1K SEQ=3072 Duplicate ACK #1 ACK = 2048 WIN = 30K 1K SEQ=4096 Duplicate ACK #2 1K SEQ=5120 ACK = 2048 WIN = 29K Fast Retransmit occurs (2nd packet is now retransmitted w/o waiting for it to timeout) Duplicate ACK #3 ACK = 2048 WIN = 28K 1K SEQ=6144 1K SEQ=2048 ACK = 2048 WIN = 27K ACK = 7168 WIN = 26K

28 Buffering Keep in mind that TCP is (usually) part of the Operating System. It takes care of all these details asynchronously. The TCP layer doesn’t know when the application will ask for any received data. TCP buffers incoming data so it’s ready when we ask for it.

29 TCP Buffers Both the client and server allocate buffers to hold incoming and outgoing data The TCP layer takes care of this. Both the client and server announce with every ACK how much buffer space remains (the Window field in a TCP segment).

30 Send Buffers The application gives the TCP layer some data to send.
The data is put in a send buffer, where it stays until the data is ACK’d. it has to stay, as it might need to be sent again! The TCP layer won’t accept data from the application unless (or until) there is buffer space.

31 ACKs A receiver doesn’t have to ACK every segment (it can ACK many segments with a single ACK segment). Each ACK can also contain outgoing data (piggybacking). If a sender doesn’t get an ACK after some time limit it resends the data.

32 TCP Segment Order Most TCP implementations will accept out-of-order segments (if there is room in the buffer). Once the missing segments arrive, a single ACK can be sent for the whole thing. Remember: IP delivers TCP segments, and IP is not reliable - IP datagrams can be lost or arrive out of order.

33 Termination The TCP layer can send a RST segment that terminates a connection if something is wrong. Usually the application tells TCP to terminate the connection politely with a FIN segment.

34 FIN Either end of the connection can initiate termination.
A FIN is sent, which means the application is done sending data. The FIN is ACK’d. The other end must now send a FIN. That FIN must be ACK’d.

35 App1 App2 FIN SN=X 1 ACK=X+1 2 ... FIN SN=Y 3 ACK=Y+1 4

36 TCP Termination 1 App1: “I have no more data for you”.
App2: “OK, I understand you are done sending.” dramatic pause… App2: “OK - Now I’m also done sending data”. App1: “Goodbye, It’s been real pleasure talking to you ” 2 3 4

37 TCP TIME_WAIT Once a TCP connection has been terminated (the last ACK sent) there is some unfinished business: What if the ACK is lost? The last FIN will be resent and it must be ACK’d. What if there are lost or duplicated segments that finally reach the destination after a long delay? TCP hangs out for a while to handle these situations.

38 Test Questions Why is a 3-way handshake necessary?
Who sends the first FIN - the server or the client? Once the connection is established, what is the difference between the operation of the server’s TCP layer and the client’s TCP layer?

39 TCP Features Connection-oriented Byte-stream app writes bytes
TCP sends segments app reads bytes Reliable data transfer Flow control: keep sender from overrunning receiver Congestion control: keep sender from overrunning network Application process W rite bytes TCP Send buffer Segment T ransmit segments Read Receive buffer

40 Segment Format

41 TCP Segment Fields Source & Destination Ports Sequence number
16 bit port identifiers for each packet (65536 ports) Sequence number The packet’s unique sequence ID Initial number selected at connection time Acknowledgement number The sequence number of the next packet expected by the receiver

42 TCP Segment Fields (cont’d)
Window size (flow control) Specifies how many bytes may be sent after the first acknowledged byte Checksum Checksums the TCP header and IP address fields Urgent Pointer Points to urgent data in the TCP data field Sender Data (SequenceNum) Acknowledgment + AdvertisedWindow Receiver

43 TCP Segment Fields (cont’d)
Header bits URG = Urgent pointer field in use ACK = Indicates whether frame contains acknowledgement PSH = Data has been “pushed”. It should be delivered to higher layers right away. RST = Indicates that the connection should be reset SYN = Used to establish connections FIN = Used to release a connection

44 TCP Congestion Window TCP introduces a second window, called the “congestion window” To determine how many bytes it may send, the sender takes the minimum of the receiver window and the congestion window Example: If the receiver window says the sender can transmit 8K, but the congestion window is only 4K, then the sender may only transmit 4K If the congestion window is 8K but the receiver window says the sender can transmit 4K, then the sender may only transmit 4K

45 Sliding Window Revisited
Sending application LastByteWritten TCP LastByteSent LastByteAcked Receiving application LastByteRead LastByteRcvd NextByteExpected

46 Internet Layer Best path determination and packet switching 3/28/2017

47 Internet Layer TCP UDP IP 802.3 Process Process Application Layer
Transport Layer ICMP, ARP & RARP IP Internet Layer 802.3 Data-Link Layer

48 IP Datagram The Internet layer defines A packet format
Addressing scheme And IP (Internet protocol) Ensures that any computer on the Internet has a unique IP The Internet layer adds an IP Header to a packet. A packet with an IP header is called: IP datagram Header Source IP address Destination IP address Payload size (actual data sent without header) And some other stuff…

49 Forwarding a Datagram Because datagrams are a connectionless communication, they are forwarded from node to node. At each step, the router (node) inspects the destination address of the datagram and forwards it to the appropriate interface.

50 Simple Datagram Forwarding

51 Datagram Forwarding with a Routing Table

52 Network Address From our subnetting discussion, we’ve already seen how the network address can be determined from the IP address and the netmask. & == With the network address, the router can determine the correct next hop.

53 Best-Effort Delivery Although IP makes the best-effort of datagram delivery, it does not guarantee proper handling of: Datagram duplication Delayed or out-of-order delivery Corruption of data Datagram loss Other protocol layers are responsible for error handling.

54 IP Datagram Header

55 IP Datagram Header (cont.)
Vers: version of IP (4 bits) Only 2 permitted 0100 for IPv4 and 0110 for IPv6 H. LEN: Header Length (4 bits) length of the header in 32 bit words. Service Type: Information about how data transmission is prioritised

56 IP Datagram Header(cont.)
Total Length (16 bits): Total length of the datagram, measured in octets, including header and data. Identification (16 bits): A value assigned to aid in assembly of fragments. Identification, Flags and fragment offset: These values allow datagrams to be fragmented and reassembled ant the destination. Time to Live (8 bits): Maximum time the datagram is allowed to exist in the system. Each router that handles the datagram decrements the TTL by 1. If the value is reaches 0 the datagram is discarded and an ICMP message is sent to the source host.

57 IP Datagram Header (cont.)
Type: Protocol (8 bits): Indicates which Transport Layer protocol the datagram is passed to. UDP or TCP Header Checksum (16 bits): Checksum is used to verify It is recomputed at each router hop. Source address (32 bits) Destination address (32 bits)

58 More about IP Routing Routing - the process of choosing a path over which to send packets Router - a computer that performs routing Routing is one of the Internet Protocol’s primary functions

59 IP Routing (cont’d) Criteria that could (ideally) be used to make routing decisions: Network characteristics Network topology Network load Datagram length Type of service requested in the datagram’s header IP routing software: Normally does not consider most of these factors Makes decisions based on fixed assumptions about shortest paths

60 Hosts vs. Routers Hosts make routing decisions
Hosts don’t typically transfer packets from one network to another Routers make routing decisions Routers typically transfer packets from one network to another

61 Direct vs. Indirect Delivery
Direct delivery - transmit datagram across a single physical network to the destination Indirect delivery - transmit datagram across multiple physical networks (with the aid of routers) to the destination How does a machine know which method of delivery to use?

62 Direct Delivery Map the destination IP address to a physical address
Encapsulate the datagram in a physical frame Send the frame over the physical network to the destination

63 Indirect Delivery Encapsulate the datagram in a frame
Choose a router on the physical network Send the frame to that router Router forwards the datagram on towards its final destination How does the host choose a router? How does the router forward the datagram?

64 The IP Routing Table Routing table - each machine stores information about destination networks and how to reach them Using only netid portion of the IP address keeps routing tables: Small Relatively stable

65 Next-Hop Routing

66 Next-Hop Routing (cont)
Routing table at machine M contains pairs (N,R) N is the IP address of a destination network R is the IP address of the “next” router (R and M must share a physical network) Routing table size: Depends on the number of networks in the internet Only grows when new networks are added

67 Properties of Next-Hop Routes
All traffic destined for a given network takes the same path Only the final router can determine whether a host exists or is operational Routes are not necessarily symmetric

68 The Internet Control Message Protocol
Abnormal normal communication among routers and hosts is sometimes necessary to: Report errors Handle abnormal conditions Update routing information ICMP

69 ICMP is for Error Reporting
Errors are reported to a datagram’s original sender It is the sender’s responsibility to take appropriate action

70 ICMP Message Format All ICMP messages begin with the same three fields: TYPE (1 octet) - identifies the message CODE (1 octet) - information about the subtype CHECKSUM (2 octets) - covers the ICMP message ICMP error messages always include the header and first 64 data bits of the datagram causing the problem

71 Mapping IP Addresses to Hardware Addresses (MAC)
IP Addresses are not recognized by hardware. If we know the IP address of a host, how do we find out the hardware address ? The process of finding the hardware address of a host given the IP address is called Address Resolution

72 ARP The Address Resolution Protocol is used by a sending host when it knows the IP address of the destination but needs the Ethernet (or whatever) address. ARP is a broadcast protocol - every host on the network receives the request. Each host checks the request against it’s IP address - the right one responds.

73 ARP (cont.) ARP does not need to be done every time an IP datagram is sent - hosts remember the hardware addresses of each other. Part of the ARP protocol specifies that the receiving host should also remember the IP and hardware addresses of the sending host.

74 ARP conversation not me HEY - Everyone please listen!
Will please send me his/her Ethernet address? not me Hi Red! I’m , and my Ethernet address is 87:A2:15:35:02:C3

75 Address Resolution Protocol (ARP)
Each device on a network maintains its own ARP table. A device that requires an IP and MAC address pair broadcasts an ARP request. If one of the local devices matches the IP address of the request, it sends back an ARP reply that contains its IP-MAC pair. If the request is for a different IP network, a router performs a proxy ARP. The router sends an ARP response with the MAC address of the interface on which the request was received, to the requesting host.

76 Reverse Address Resolution Protocol RARP
The process of finding out the IP address of a host given a hardware address is called Reverse Address Resolution Reverse address resolution is needed by diskless workstations when booting (which used to be quite common).

77 RARP conversation not me HEY - Everyone please listen!
My Ethernet address is 22:BC:66:17:01:75. Does anyone know my IP address ? not me Hi Red! Your IP address is

78 SUMMARY Transport layer Internet layer UDP TCP IP
Connectionless Unreliable transmission Less overheat TCP CONNECTION ORIENTED Reliable Transmission More overheat to deal with ack’s Internet layer IP Connectonless IP routing (next-hop using routing table) Unreliable ICMP (information control message protocol) ARP (IP to MAC) RARP (MAC TO IP)

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