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
ICMP

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)