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
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3. What’s a Protocol? An agreed upon convention for communication.
Protocols must be formally defined and unambiguous
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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
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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
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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
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11. Physical Layer Responsibility: Issues:
transmission of raw bits over a communication channel.
Issues:
mechanical and electrical interfaces
time per bit
distances
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12. Data Link Layer - Data Link Control
Responsibility:
provide an error-free communication link
Issues:
framing (dividing data into chunks)
header & trailer bits
addressing
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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.
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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.
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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
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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.
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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
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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
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19. The Session Layer Responsibilities:
establishes, manages, and terminates sessions between applications.
service location lookup
Many protocol suites do not include a session layer.
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20. The Presentation Layer
Responsibilities:
data encryption
data compression
data conversion
Many protocol suites do not include a Presentation Layer.
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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”
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22. Layering & Headers Each layer needs to add control information.
Typically prefixed to the data before passing on to the lower layer.
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23. Headers DATA Process Process Transport H DATA Transport Network H H
Data Link H H H
DATA
Data Link
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24. Example Headers Physical: no header Data Link:
address of the receiving endpoints
address of the sending endpoint
length of the data
checksum
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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
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26. Connecting Networks Repeater: physical layer Bridge: data link layer
Router: network layer
Gateway: network layer and above.
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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
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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
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29. Router Copies packets from one network to another.
Makes decisions about what route a packet should take (looks at network headers).
ROUTER
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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
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31. Encapsulation Example
Gateway
Gateway
Provides service connectivity even though intermediate network does not support protocols.
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32. Translation Translate from green protocol to brown protocol Gateway
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33. Encryption/Decryption
Encryption gateway
Secure
Network
Encryption/Decryption
Gateways GW ?
Insecure Network
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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.
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36. Connection-Oriented Service
establishment of a logical connection between two processes.
transfer data
terminate connection.
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37. Connectionless Service
Sends independent messages.
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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.
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39. Error Control Some services require error detection.
Checksums provide a simple error detection mechanism.
Error control sometimes involves notification and retransmission.
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40. Flow Control
Flow control prevents the sending process from overwhelming the receiving process.
Flow control can be handled in a variety of ways.
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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.
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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.
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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.
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44. End-to-End
Process A
Process B
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45. Hop-by-Hop
Process A
Process B
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46. Buffering Buffering can provide more efficient communications.
Buffering is most useful for byte stream services.
Process A
Send
Buffer
Recv.
Buffer
Process B
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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.
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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)
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49. The IP in TCP/IP IP is the network layer
packet delivery service (host-to-host).
translation between different data-link protocols.
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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.
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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.
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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.
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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).
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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
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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
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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.
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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
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59. IP6 Address 3FFE: 0B00: 1234: 0000: 0001 128 bits 16 bits
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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
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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.
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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.
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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
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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.
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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.
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67. ICMP Message Types Echo Request Echo Response Destination Unreachable
Redirect
Time Exceeded
Redirect (route change)
more ...
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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.
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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.
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70. Ports Host A Host B Process Process Process Process Process Process
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71. UDP Datagram Format Source Port Destination Port Length Checksum Data
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72. Sockets
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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.
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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.
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75. TCP Connection-oriented Reliable Full-duplex Byte-Stream
Transmission Control Protocol :
Connection-oriented
Reliable
Full-duplex
Byte-Stream
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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.
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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.
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78. Connection establishment
Connection establishment phase is required
Ensures that the receiving process is available and to synchronize sequence numbers, etc.
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79. TCP State Diagram
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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.
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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).
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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.
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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.
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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).
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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
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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
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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
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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
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89. Sequence Number Initial Sequence Number (ISN) is randomly generated.
What if ISN is not random?
You can hijack and kill arbitrary connections!
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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.
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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
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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).
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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
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94. TCP three-way handshake
Flags src dst seq ack
SYN
SYN-ACK
ACK
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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
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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