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Prabhaker Mateti (ack: Many many sources …)

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1 Prabhaker Mateti (ack: Many many sources …)
TCP/IP Refresher Prabhaker Mateti (ack: Many many sources …)

2 TCP/IP ? TCP = Transmission Control Protocol IP = Internet Protocol
Almost always includes other protocols: UDP, User (Unreliable) Datagram ICMP, Internet Control Message ARP, Address Resolution Mateti, TCP/IP Refresher

3 What’s a Protocol? An agreed upon convention for communication.
Protocols must be formally defined and unambiguous Mateti, TCP/IP Refresher

4 Layers TCP UDP ICMP other IP layer Physical Physical
UDP ICMP other IP layer Physical   Physical The relative heights indicate the level of functionality. Mateti, TCP/IP Refresher

5 Unix is a Layered System
Applications Libraries System Calls Kernel Mateti, TCP/IP Refresher

6 Layers The routines/methods of Layer N will not call Layer N+1.
The routines/methods of Layer N typically do call the same layer methods. The routines/methods of Layer N typically do call Layer N-1 methods. Mateti, TCP/IP Refresher

7 DoD model: Four Layers Network Access Layer: Delivery over physical media in use. Internet Layer: Delivery across different physical networks that connect source and destination machines. Host-to-Host Layer: Connection rendezvous, flow control, retransmission of lost data, etc. TCP and UDP protocols are in this layer. Process Layer: User-level functions, such as SMTP, FTP and rlogin. Mateti, TCP/IP Refresher

8 OSI Reference Model Seven Layers 7. Application 6. Presentation
5. Session 4. Transport 3. Network 2. Data Link 1. Physical Mateti, TCP/IP Refresher

9 TCP/IP & OSI In OSI reference model terminology -the TCP/IP protocol suite covers the network and transport layers. TCP/IP can be used on many data-link layers (can support many network hardware implementations). Mateti, TCP/IP Refresher

10 Process Layer Transport Layer Network Layer Data-Link Layer Process
TCP UDP Transport Layer ICMP, ARP & RARP IP Network Layer 802.3 Data-Link Layer Mateti, TCP/IP Refresher

11 Physical Layer Responsibility: Issues:
transmission of raw bits over a communication channel. Issues: mechanical and electrical interfaces time per bit distances Mateti, TCP/IP Refresher

12 Data Link Layer - Data Link Control
Responsibility: provide an error-free communication link Issues: framing (dividing data into chunks) header & trailer bits addressing Mateti, TCP/IP Refresher

13 The Data Link Layer - The MAC sub layer
Medium Access Control (MAC) - needed by multi-access networks. MAC provides DLC with “virtual wires” on multi-access networks. Mateti, TCP/IP Refresher

14 Ethernet: A Data-Link Layer
IEEE 802.3 Variety of physical layers. Multi-access (shared medium). Interface has a unique 6-byte hardware address. (E.g. 00-D0-09-E ) The broadcast address is all 1’s. Addresses are assigned to vendors by a central authority. Mateti, TCP/IP Refresher

15 An Ethernet Frame Preamble Destination Address Source Len CRC DATA 8 bytes 6 6 2 0-1500 4 Preamble is a sequence of alternating 1’s and 0’s used for synchronization. CRC is Cyclic Redundancy Check Mateti, TCP/IP Refresher

16 Ethernet Addressing Each NIC looks at every frame and inspects the destination address. If the address does not match the hardware address of the interface or the broadcast address, the frame is discarded. Some NICs can be programmed to recognize multicast addresses. Mateti, TCP/IP Refresher

17 The Network Layer Responsibilities: Issues:
path selection between systems (routing). subnet flow control. fragmentation & reassembly translation between different network types. Issues: packet headers virtual circuits Mateti, TCP/IP Refresher

18 The Transport Layer Responsibilities: Issues:
provides virtual end-to-end links between peer processes. end-to-end flow control Issues: headers error detection reliable communication Mateti, TCP/IP Refresher

19 The Session Layer Responsibilities:
establishes, manages, and terminates sessions between applications. service location lookup Many protocol suites do not include a session layer. Mateti, TCP/IP Refresher

20 The Presentation Layer
Responsibilities: data encryption data compression data conversion Many protocol suites do not include a Presentation Layer. Mateti, TCP/IP Refresher

21 The Application Layer Responsibilities: Issues:
anything not provided by any of the other layers Issues: application level protocols appropriate selection of “type of service” Mateti, TCP/IP Refresher

22 Layering & Headers Each layer needs to add control information.
Typically prefixed to the data before passing on to the lower layer. Mateti, TCP/IP Refresher

23 Headers DATA Process Process Transport H DATA Transport Network H H
Data Link H H H DATA Data Link Mateti, TCP/IP Refresher

24 Example Headers Physical: no header Data Link:
address of the receiving endpoints address of the sending endpoint length of the data checksum Mateti, TCP/IP Refresher

25 Network layer header - examples
protocol header checksum source network address destination network address protocol suite version type of service length of the data packet identifier fragment number time to live Mateti, TCP/IP Refresher

26 Connecting Networks Repeater: physical layer Bridge: data link layer
Router: network layer Gateway: network layer and above. Mateti, TCP/IP Refresher

27 Repeater Copies bits from one network to another
Does not look at any bits Allows the extension of a network beyond physical length limitations REPEATER Mateti, TCP/IP Refresher

28 Bridge Copies frames from one network to another
Can operate selectively - does not copy all frames (looks at data-link headers). Extends the network beyond physical length limitations. BRIDGE Mateti, TCP/IP Refresher

29 Router Copies packets from one network to another.
Makes decisions about what route a packet should take (looks at network headers). ROUTER Mateti, TCP/IP Refresher

30 Gateway Operates as a router Data conversions above the network layer.
encapsulation - use an intermediate network translation - connect different application protocols encryption - could be done by a gateway Mateti, TCP/IP Refresher

31 Encapsulation Example
Gateway Gateway Provides service connectivity even though intermediate network does not support protocols. Mateti, TCP/IP Refresher

32 Translation Translate from green protocol to brown protocol Gateway
Mateti, TCP/IP Refresher

33 Encryption/Decryption
Encryption gateway Secure Network Encryption/Decryption Gateways GW ? Insecure Network Mateti, TCP/IP Refresher

34 Hardware v. Software Repeaters are typically hardware devices.
Bridges can be implemented in hardware or software. Routers and gateways are typically implemented in software so that they can be extended to handle new protocols. Many workstations can operate as routers or gateways. Mateti, TCP/IP Refresher

35 Modes of Service connection-oriented vs. connectionless sequencing
error-control flow-control byte stream vs. message based full-duplex vs. half-duplex. Mateti, TCP/IP Refresher

36 Connection-Oriented Service
establishment of a logical connection between two processes. transfer data terminate connection. Mateti, TCP/IP Refresher

37 Connectionless Service
Sends independent messages. Mateti, TCP/IP Refresher

38 Sequencing Sequencing provides support for an order to communications.
A service that includes sequencing requires that messages (or bytes) are received in the same order they are sent. Mateti, TCP/IP Refresher

39 Error Control Some services require error detection.
Checksums provide a simple error detection mechanism. Error control sometimes involves notification and retransmission. Mateti, TCP/IP Refresher

40 Flow Control Flow control prevents the sending process from overwhelming the receiving process. Flow control can be handled in a variety of ways. Mateti, TCP/IP Refresher

41 Byte Stream vs. Message Byte stream implies an ordered sequence of bytes with no message boundaries. Message oriented services provide communication service to chunks of data called datagrams. Mateti, TCP/IP Refresher

42 Full- v. Half-Duplex Full-Duplex services support the transfer of data in both directions. Half-Duplex services support the transfer of data in one direction. Mateti, TCP/IP Refresher

43 End-to-End v. Hop-to-Hop
Service modes, flow control and error control can be Either between endpoints of the communication. Or between consecutive nodes on the path between the endpoints. Mateti, TCP/IP Refresher

44 End-to-End Process A Process B Mateti, TCP/IP Refresher

45 Hop-by-Hop Process A Process B Mateti, TCP/IP Refresher

46 Buffering Buffering can provide more efficient communications.
Buffering is most useful for byte stream services. Process A Send Buffer Recv. Buffer Process B Mateti, TCP/IP Refresher

47 Addresses Physical Layer: no address necessary
Data Link Layer: address must be able to select any host on the network. Network Layer: address must be able to provide information to enable routing. Transport Layer: address must identify the destination process. Mateti, TCP/IP Refresher

48 Broadcasts Broadcast = sending a message from one host to all other hosts on the network. A special address called the “broadcast address” is created. Some popular network services are based on broadcasting (YP/NIS, rup, rusers) Mateti, TCP/IP Refresher

49 The IP in TCP/IP IP is the network layer
packet delivery service (host-to-host). translation between different data-link protocols. Mateti, TCP/IP Refresher

50 IP Datagrams IP provides connectionless, unreliable delivery of IP datagrams. Connectionless: each datagram is independent of all others. Unreliable: there is no guarantee that datagrams are delivered correctly or at all. Mateti, TCP/IP Refresher

51 IP Addresses The address must include information about what network the receiving host is on. This makes routing feasible. IP addresses are not the same as the underlying data-link (MAC) addresses. Mateti, TCP/IP Refresher

52 IP Addresses Includes a network ID and a host ID.
A Network ID is assigned to an organization by a global authority ( ) Host IDs are assigned locally by a system administrator. Mateti, TCP/IP Refresher

53 IP Addresses A single NIC is assigned one IP address.
A host may have multiple NICs, and therefore multiple host addresses. Hosts that share a network all have the same IP network address (the network ID). Mateti, TCP/IP Refresher

54 Subnet Addresses NetID SubnetID HostID
An organization can subdivide it’s host address space into groups called subnets. The subnet ID is generally used to group hosts based on the physical network topology. It is possible to have a single wire network with multiple subnets. NetID SubnetID HostID Mateti, TCP/IP Refresher

55 IP4 Addresses A B C D NetID HostID 10 110 1110 Multicast Address
NetID 10 110 1110 Multicast Address HostID A B C D 8 bits Class Mateti, TCP/IP Refresher

56 IP Addresses An IP broadcast address has a host- ID of all 1’s.
An IP address that has a host ID of all 0’s is called a network address and refers to an entire network. Mateti, TCP/IP Refresher

57 IP Addresses v. MAC Addresses
IP Addresses are not recognized by NIC. The process of finding the MAC address of a host given the IP address is called Address Resolution. The process of finding out the IP address of a host given a hardware address is called Reverse Address Resolution. Mateti, TCP/IP Refresher

58 IPv6 addresses Addresses are scoped
Address is 128 bits long (16 bytes) Addresses are written in hexadecimal Addresses can be abbreviated 3FFE:0B00:0000:0000:0000:0000:0000:0001 3FFE:0B00::0001 3FFE:B00::1 There is no broadcast addresses, only multicast. Loopback address is ::1 Addresses are scoped Link-local, site-local, global Mateti, TCP/IP Refresher

59 IP6 Address 3FFE: 0B00: 1234: 0000: 0001 128 bits 16 bits
Mateti, TCP/IP Refresher

60 IP4-Compatible IP6 Address
80 bits of 0s followed by 16 bits of 0s, followed by a 32 bit IP4 Address: 0000 IP4 Address 80 bits 16 bits 32 bits Mateti, TCP/IP Refresher

61 ARP ARP is a broadcast protocol. Each host checks the request against its own host addresses - the matched one responds. Hosts remember the hardware addresses of others. ARP protocol specifies that the receiving host should also remember the IP and hardware addresses of the sending host. Mateti, TCP/IP Refresher

62 Services provided by IP
Connectionless Delivery (each datagram is treated individually). Unreliable (delivery is not guaranteed). Fragmentation / Reassembly (based on hardware MTU). Routing. Error detection. Mateti, TCP/IP Refresher

63 Destination IP Address
IP Datagram VERS HL Fragment Offset Fragment Length Service Datagram ID FLAG TTL Protocol Header Checksum Source IP Address Destination IP Address Options (if any) (TCP) Data 1 byte Mateti, TCP/IP Refresher

64 IP Datagram Fragmentation
Fragmentation can happen when datagrams are forwarded through a network for which they are too big. IP specifies that datagram reassembly is done only at the destination (not on a hop-by-hop basis). If any of the fragments are lost the entire datagram is discarded (and an ICMP message is sent to the sender). Mateti, TCP/IP Refresher

65 ICMP (Internet Control Message Protocol)
ping ICMP uses IP to deliver messages. ICMP messages are usually generated and processed by the IP layer, not the user process. Mateti, TCP/IP Refresher

66 ICMP If packets arrive too fast the receiver discards excessive packets and sends an ICMP message to the sender (SOURCE QUENCH). If an error is found (header checksum problem, say) the packet is discarded and an ICMP message is sent to the sender. Mateti, TCP/IP Refresher

67 ICMP Message Types Echo Request Echo Response Destination Unreachable
Redirect Time Exceeded Redirect (route change) more ... Mateti, TCP/IP Refresher

68 UDP (User Datagram Protocol)
UDP is a transport protocol Uses IP to deliver datagrams Connectionless, Unreliable, Minimal UDP uses ports to provide communication services to individual processes. Mateti, TCP/IP Refresher

69 Ports Port : an abstract destination point.
Ports are identified by a positive 16-bit integer. Operating systems provide some mechanism that processes use to specify a port. Mateti, TCP/IP Refresher

70 Ports Host A Host B Process Process Process Process Process Process
Mateti, TCP/IP Refresher

71 UDP Datagram Format Source Port Destination Port Length Checksum Data
Mateti, TCP/IP Refresher

72 Sockets Mateti, TCP/IP Refresher

73 Sockets An active socket is connected to a remote active socket. Closing the connection destroys the active sockets at each endpoint. A passive socket is not connected, but rather awaits an incoming connection, which will spawn a new active socket. Mateti, TCP/IP Refresher

74 Sockets v. Ports A socket is not a port. A socket is associated with a port. This is a many-to-one relationship. Each port can have a single passive socket, awaiting incoming connections, and multiple active sockets, each corresponding to an open connection on the port. Mateti, TCP/IP Refresher

75 TCP Connection-oriented Reliable Full-duplex Byte-Stream
Transmission Control Protocol : Connection-oriented Reliable Full-duplex Byte-Stream Mateti, TCP/IP Refresher

76 Connection Four Numbers: Source IP Address, Source Port, Destination IP Address, Destination Port “connection is established”: Operating Systems of both source and destination hosts are maintaining “state information” re the connection. Mateti, TCP/IP Refresher

77 Connection-Oriented Connection oriented means that a virtual connection is established before any payload data is transferred. If the connection cannot be established the user program is notified. If the connection is ever interrupted the user program is notified. Mateti, TCP/IP Refresher

78 Connection establishment
Connection establishment phase is required Ensures that the receiving process is available and to synchronize sequence numbers, etc. Mateti, TCP/IP Refresher

79 TCP State Diagram Mateti, TCP/IP Refresher

80 Reliable Every transmission of data is acknowledged by the receiver.
If the sender does not receive ACK within a specified amount of time, the sender retransmits the data. ACK can be piggybacked on data. Mateti, TCP/IP Refresher

81 Byte Stream Stream means that the connection is treated as a stream of bytes. The user application does not need to package data in individual datagrams (as with UDP). Mateti, TCP/IP Refresher

82 Buffering TCP is responsible for buffering data and determining when it is time to send a datagram. It is possible for an application to tell TCP to send the data it has buffered without waiting for a buffer to fill up. Mateti, TCP/IP Refresher

83 Full Duplex TCP provides transport in both directions.
To the application program these appear as two unrelated data streams, although TCP can piggyback control and data communication by providing control information (such as an ACK) along with user data. Mateti, TCP/IP Refresher

84 TCP Ports Interprocess communication via TCP is achieved with the use of ports (just like UDP). UDP ports have no relation to TCP ports (different name spaces). Mateti, TCP/IP Refresher

85 TCP/UDP Ports Reserved Ports less than 1024: Only root can bind to these ports. Local Port of a process that requested the connection. Usually a random number, Remote Port: What application accepted the connection. Usually a known number. /etc/services. E.g., 80 for HTTP 143 for IMAP 443 for HTTP/SSL Mateti, TCP/IP Refresher

86 TCP Segments The chunk of data that TCP asks IP to deliver is called a TCP segment. Each segment contains: data bytes from the byte stream control information that identifies the data bytes Mateti, TCP/IP Refresher

87 TCP Segment Format Destination Port Options (if any) Data 1 byte
Source Port Sequence Number Request Number offset Res Control Window Checksum Urgent Pointer Mateti, TCP/IP Refresher

88 Sequence Number The “positional” number of the first data byte in this segment, except when SYN control flag is 1.  If SYN is 1 the sequence number is the initial sequence number (ISN). 32 bit unsigned integer Mateti, TCP/IP Refresher

89 Sequence Number Initial Sequence Number (ISN) is randomly generated.
What if ISN is not random? You can hijack and kill arbitrary connections! Mateti, TCP/IP Refresher

90 Acknowledgment Number
If the ACK control bit is set, this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always included. Mateti, TCP/IP Refresher

91 Control Bits URG: Urgent Pointer field significant PSH: Push Function
ACK: Acknowledgment field significant RST: Reset the connection SYN: Synchronize sequence numbers FIN: No more data from sender Mateti, TCP/IP Refresher

92 TCP v. UDP Q: Which protocol is better ? A: It depends on the application. TCP provides a connection-oriented, reliable byte stream service (lots of overhead). UDP offers minimal datagram delivery service (as little overhead as possible). Mateti, TCP/IP Refresher

93 TCP three-way handshake
Establishes a connection. A: “I would like to talk to you B.” A sends a SYN packet to B B: “Ok, let's talk.” B sends a SYN-ACK packet to A A: “Thanks for agreeing.” A sends ACK to B Mateti, TCP/IP Refresher

94 TCP three-way handshake
Flags src dst seq ack SYN SYN-ACK ACK Mateti, TCP/IP Refresher

95 Four-Way Handshake The Four-Way Handshake terminates a previously established connection: A to B: FIN B to A: ACK B to A: FIN A to B: ACK Mateti, TCP/IP Refresher

96 Connection Resetting Host X sends an RST packet resetting the connection if: Y requested a connection to a non-existent port P on host X, or For whatever reason (idle for a long time, or an abnormal condition, ...), the host X (client or the sever) wishes to close the connection. Resetting is unilateral. Mateti, TCP/IP Refresher

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