Unit V computer network notes for study.

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1. UNIT V TRANSPORT LAYER 1

2. Transport Layer Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers . Ensures that the data units are delivered error free. Ensures that data units are delivered in sequence. Ensures that there is no loss or duplication of data units. Provides connectionless or connection oriented service .

3. Transport Layer Provide logical communication between application processes running on different hosts Run on end hosts Sender: breaks application messages into segments , and passes to network layer Receiver: reassembles segments into messages, passes to application layer Multiple transport protocol available to applications Internet: TCP and UDP application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

4. Transport Layer Responsibility Process to process delivery End-to-end Connection between hosts Multiplexing and Demultiplexing

5. PROCESS PROCESS-TO-PROCESS DELIVERY The transport layer is responsible for process process-to-process process delivery delivery—the delivery of a packet, packet, part of a message, message, from one process to another process

6. PROCESS PROCESS-TO-PROCESS DELIVERY Data Link Layer requires the MAC address of source-destination hosts to correctly deliver a frame Network layer requires the IP address for appropriate routing of packets In a similar way Transport Layer requires a Port number to correctly deliver the segments of data to the correct process amongst the multiple processes running on a particular host.

7. PROCESS PROCESS-TO-PROCESS DELIVERY

8. PROCESS PROCESS-TO-PROCESS DELIVERY Process-to-process delivery needs two identifiers, IP address and the port number, at each end to make a connection . The combination of an IP address and a port number is called a socket address. The client socket address defines the client process uniquely just as the server socket address defines the server process uniquely.

9. UDP: User Datagram Protocol I n TCP/IP protocol suite , using IP to transport datagram (similar to IP datagram) . Allow s a application to send datagram to other application on the remote machine . Delivery and duplicate detection are not guaranteed.

10. UDP: Characteristics End-to-End : an application sends/receives data to/from another application. Connectionless : Application does not need to pre-establish communication before sending data; application does not need to terminate communication when finished. Message-oriented : application sends/receives individual messages (UDP datagram), not packets. Best-effort : same best-effort delivery semantics as IP. I.e. message can be lost, duplicated, and corrupted. Arbitrary interaction : application communicates with many or one other applications. Operating system independent : identifying application does not depend on O/S

11. UDP: Datagram Format Source Port - 16 bit port number Destination Port - 16 bit port number . Length (of UDP header + data) - 16 bit count of octets UDP checksum - 16 bit field . if 0, then there is no checksum, else it is a checksum over a pseudo header + UDP data area UDP uses a pseudo-header to verify that the UDP message has arrived at both the correct machine and the correct port

12. UDP: Encapsulation and Layering UDP message is encapsulated into an IP datagram IP datagram in turn is encapsulated into a physical frame for actually delivery.

13. Transmission Control Protocol (TCP) Connection oriented Explicit set-up and tear-down of TCP session Stream-of-bytes service Sends and receives a stream of bytes, not messages Reliable, in-order delivery Checksums to detect corrupted data Acknowledgments & retransmissions for reliable delivery Sequence numbers to detect losses and reorder data Flow control Prevent overflow of the receiver’s buffer space Congestion control Adapt to network congestion for the greater good

14. TCP: Reliable Delivery Acknowledgments from receiver Positive: “okay” or “ACK” Negative: “please repeat that” or “NACK” Timeout by the sender (“stop and wait”) Don’t wait indefinitely without receiving some response … whether a positive or a negative acknowledgment Retransmission by the sender After receiving a “NACK” from the receiver After receiving no feedback from the receiver

15. TCP: Reliable Delivery Checksum Used to detect corrupted data at the receiver …leading the receiver to drop the packet Sequence numbers Used to detect missing data ... and for putting the data back in order Retransmission Sender retransmits lost or corrupted data Timeout based on estimates of round-trip time Fast retransmit algorithm for rapid retransmission

16. TCP : Segment S R Send SYN Receive SYN SYN_SEND Time out SYN_RCV Time out Receive SYN_ACK Send SYN_ACK Send ACK Receive ACK Connection Established Data Transfer TCP Connection Established S R Send FIN Receive FIN SYN_WAIT Time out Receive ACK Send ACK Receive FIN Send FIN Close_wait Timeout Close Time out TCP Connection Terminate Send ACK Receive ACK

17. TCP Segments Byte 0 Byte 1 Byte 2 Byte 3 Byte 0 Byte 1 Byte 2 Byte 3 Host A Host B Byte 80 Byte 80

18. TCP Segments Byte 0 Byte 1 Byte 2 Byte 3 Byte 0 Byte 1 Byte 2 Byte 3 Host A Host B Byte 80 TCP Data TCP Data Byte 80 Segment sent when: Segment full (Max Segment Size), Not full, but times out, or “Pushed” by application.

19. TCP Segments IP packet No bigger than Maximum Transmission Unit (MTU) E.g., up to 1500 bytes on an Ethernet TCP packet IP packet with a TCP header and data inside TCP header is typically 20 bytes long TCP segment No more than Maximum Segment Size (MSS) bytes E.g., up to 1460 consecutive bytes from the stream IP Hdr IP Data TCP Hdr TCP Data (segment)

20. TCP: Initial Sequence Number (ISN) Sequence number for the very first byte E.g., Why not ISN is 0? Practical issue IP addresses and port #s uniquely identify a connection Eventually, though, these port #s do get used again … and there is a chance an old packet is still in flight … and might be associated with the new connection So, TCP requires changing the ISN over time Set from a 32-bit clock that ticks every 4 microseconds … which only wraps around once every 4.55 hours! But, this means the hosts need to exchange ISN

21. TCP : Sequence Numbers Host A Host B TCP Data TCP HDR ISN (initial sequence number) Sequence number = 1 st byte ACK sequence number = next expected byte TCP Data TCP HDR

22. TCP : Segment

23. TCP segment structure S R Send SYN Receive SYN SYN_SEND Time out SYN_RCV Time out Receive SYN_ACK Send SYN_ACK Send ACK Receive ACK Connection Established Data Transfer TCP Connection Established S R Send FIN Receive FIN SYN_WAIT Time out Receive ACK Send ACK Receive FIN Send FIN Close_wait Timeout Close Time out TCP Connection Terminate Send ACK Receive ACK

24. TCP : Segment

25. TCP : three-way handshaking

26. Stream Control Transmission Protocol Process-to-Process Communication Multiple Streams Multihoming Full-Duplex Communication Connection-Oriented Service Reliable Service Message-oriented

27. Stream Control Transmission Protocol UDP: Message-oriented, Unreliable TCP: Byte-oriented, Reliable SCTP Message-oriented, Reliable Other innovative features Association, Data transfer/Delivery Fragmentation, Error/Congestion Control

28. Stream Control Transmission Protocol Multiple Streams If one of the streams is blocked, the other streams can still deliver their data.

29. Stream Control Transmission Protocol Multi-Homing Two  fundamental  concepts  in  SCTP: Endpoints   (communicating   parties) Associations   ( communicating   relationships )      SCTP   Associations  allows   multiple  IP  addresses  for  each  end   point.

30. TCP Flow Control Endpoints identified by < src_ip , src_port , dest_ip , dest_port > Network Transport Application P1 P2 P3 P4 P6 P7 P5 Host 1 Host 2 Host 3 Unique port for each applicatio n Applications share the same network Server applications communicate with multiple clients

31. TCP Flow Control R eceive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate app process may be slow at reading from buffer sender won’t overflow receiver’s buffer by transmitting too much, too fast flow control

32. TCP Flow Control Each side: Notifies the other of starting sequence number ACKs the other side’s starting sequence number Client Server SYN < SeqC , 0> SYN/ACK < SeqS , SeqC+1> ACK <SeqC+1, SeqS+1> Why Sequence # +1? Important TCP flags (1 bit each) SYN – synchronization, used for connection setup ACK – acknowledge received data FIN – finish, used to tear down connection

33. TCP Flow Control Either side can initiate tear down Other side may continue sending data Half open connection shutdown() Acknowledge the last FIN Sequence number + 1 What happens if 2 nd FIN is lost? Client Server FIN < SeqA , * > ACK < * , SeqA+1> ACK Data FIN < SeqB , *> ACK <*, SeqB+1>

34. TCP Flow Control Each side of the connection can send and receive Different sequence numbers for each direction Client Server Data (1460 bytes) Data/ACK (730 bytes) Data/ACK (1460 bytes) Seq. Ack. Seq. Ack. 1 23 23 1461 1461 753 753 2921 Data and ACK in the same packet 23 1

35. TCP Flow Control Problem: how many packets should a sender transmit? Too many packets may overwhelm the receiver Size of the receivers buffers may change over time Solution: sliding window Receiver tells the sender how big their buffer is Called the advertised window For window size n , sender may transmit n bytes without receiving an ACK After each ACK, the window slides forward Window may go to zero!

36. TCP Flow Control Sequence Number Src . Port Acknowledgement Number Window Urgent Pointer Flags Checksum HL Packet Sent Dest . Port Src . Port Acknowledgement Number Window Urgent Pointer Flags Checksum HL Packet Received Dest . Port Sequence Number ACKed Sent To Be Sent Outside Window Window Must be buffered until ACKed

37. TCP Flow Control 1 2 3 4 5 6 7 5 6 7 Time Time TCP is ACK Clocked Short RTT  quick ACK  w indow slides quickly Long RTT  slow ACK  window slides slowly

38. TCP Flow Control ACK every packet Use cumulative ACK , where an ACK for sequence n implies ACKS for all k < n Use negative ACKs (NACKs), indicating which packet did not arrive Use selective ACKs (SACKs), indicating those that did arrive, even if not in order SACK is an actual TCP extension 38

39. TCP Flow Control The bursty traffic in the network results in congestion Traffic shaping reduces congestion and thus helps the carrier live up to its guarantees Traffic shaping is about regulating the average rate (and burstiness ) of data transmission

40. TCP Flow Control Traffic shaping controls the rate at which packets are sent (not just how many) At connection set-up time, the sender and carrier negotiate a traffic pattern (shape ) Two traffic shaping algorithms are: Leaky Bucket Token Bucket

41. The Leaky Bucket Algorithm The Leaky Bucket Algorithm used to control rate in a network. It is implemented as a single-server queue with constant service time. If the bucket (buffer) overflows then packets are discarded.

42. The Leaky Bucket Algorithm (a) A leaky bucket with water (b) a leaky bucket with packets .

43. The Leaky Bucket Algorithm The leaky bucket enforces a constant output rate regardless of the burstiness of the input. Does nothing when input is idle . The host injects one packet per clock tick onto the network. This results in a uniform flow of packets, smoothing out bursts and reducing congestion . When packets are the same size, the one packet per tick is okay. For variable length packets though, it is better to allow a fixed number of bytes per tick.

44. The Leaky Bucket Algorithm   Step - 1 : Initialize the counter to ‘n’ at every tick of clock. Step - 2 : If n is greater than the size of packet in the front of queue send the packet into the network and decrement the counter by size of packet. Repeat the step until n is less than the size of packet. Step - 3 : Reset the counter and go to Step - 1.

45. The Leaky Bucket Algorithm Let n = 1000 Packet =. 200 700 500 450 400 200 Since n > front of Queue i.e. n>200 Therefore, n= 1000-200 = 800 Packet size of 200 is sent to the network Packet=200 700 500 450 400 Now Again n > front of queue i.e. n > 400 Therefore , n= 800-400 = 400 Packet size of 400 is sent to the network Packet= 200 700 500 450 Since n < front of queue . There fore, the procedure is stop. And we initialize n = 1000 on another tick of clock. This procedure is repeated until all the packets is sent to the network .

46. Token Bucket Algorithm In contrast to the LB, the Token Bucket (TB) algorithm, allows the output rate to vary, depending on the size of the burst. In the TB algorithm, the bucket holds tokens. To transmit a packet, the host must capture and destroy one token. Tokens are generated by a clock at the rate of one token every  t sec. Idle hosts can capture and save up tokens (up to the max. size of the bucket) in order to send larger bursts later.

47. Token Bucket Algorithm

48. Token Bucket Algorithm TB accumulates fixed size tokens in a token bucket Transmits a packet (from data buffer, if any are there) or arriving packet if the sum of the token sizes in the bucket add up to packet size More tokens are periodically added to the bucket (at rate  t). If tokens are to be added when the bucket is full, they are discarded Does not bound the peak rate of small bursts, because bucket may contain enough token to cover a complete burst size Performance depends only on the sum of the data buffer size and the token bucket size

49. Choke Packet Choke packets are used for congestion and flow control over a network A choke packet is used in network maintenance and quality management Use to inform a specific node or transmitter that its transmitted traffic is creating congestion over the network . This forces the node or transmitter to reduce its output rate . he source node is addressed directly by the router, forcing it to decrease its sending rate  The source node acknowledges this by reducing the sending rate by some percentage.

50. What is Congestion? Load on the network is higher than capacity Capacity is not uniform across networks Modem vs. Cellular vs. Cable vs. Fiber Optics There are multiple flows competing for bandwidth Residential cable modem vs. corporate datacenter Load is not uniform over time 10pm, Saturday night = Heavy Load

51. Why is Congestion Bad? Results in packet loss Routers have finite buffers Internet traffic is self similar, no buffer can prevent all drops When routers get overloaded, packets will be dropped Practical consequences Router queues build up, delay increases Wasted bandwidth from retransmissions L ow network

52. The Danger of Increasing Load Knee – point after which Throughput increases very slow Delay increases fast In an M/M/1 queue Delay = 1/(1 – utilization) Cliff – point after which Throughput  0 Delay  ∞ Congestion Collapse Load Load Goodput Delay Knee Cliff Ideal point

53. Cong. Control vs. Cong. Avoidance Congestion Collapse Goodput Knee Cliff Load Congestion Avoidance: Stay left of the knee Congestion Control: Stay left of the cliff

54. Goals of Congestion Control Adjusting to the bottleneck bandwidth Adjusting to variations in bandwidth Sharing bandwidth between flows Maximizing throughput

55. General Approaches Do nothing, send packets indiscriminately Many packets will drop, totally unpredictable performance May lead to congestion collapse Reservations Pre-arrange bandwidth allocations for flows Requires negotiation before sending packets Must be supported by the network Dynamic adjustment Use probes to estimate level of congestion Speed up when congestion is low Slow down when congestion increases Messy dynamics, requires distributed coordination

56. TCP MN (S) Internet Host MN (R) Internet Transceiver Transceiver Router Router HA FA Home Network Foreign Network

57. I-TCP No changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol Optimized TCP protocol for mobile hosts Splitting of the TCP connection. How does your mobile phone work

58. I-TCP MN Internet Host MN Internet Wireless Transceiver Wireless Transceiver Router Router HA FA Home Network Foreign Network

59. I-TCP No changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol optimized TCP protocol for mobile hosts Splitting of the TCP connection. Internet hosts in the fixed part of the net do not notice the characteristics of the wireless part

60. I-TCP Advantage- Transmission errors on the wireless link do not propagate into the fixed network Simple to control, mobile TCP is used only for one hop, between a Foreign agent and a mobile host. Disadvantage- Loss of end-to-end semantics. Higher Latency High trust at foreign agent; end-to-end encryption impossible

61. Snooping TCP Buffering of packets sent to the mobile host. Lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission). The foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs. Changes of TCP only within the foreign agent

62. Snooping TCP Data transfer to the mobile host FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out. Fast retransmission possible, transparent for the fixed network. Data transfer from the mobile host FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH. MH can now retransmit data with only a very short delay.

63. M-TCP M-TCP splits as I-TCP does Unmodified TCP fixed network to supervisory host (SH) Optimized TCP SH to MH Supervisory host No caching, No retransmission Monitors all packets, if disconnection detects set sender window size to 0 sender automatically goes into persistent mode old or new SH reopen the window

64. M-TCP Advantage- Maintains semantics, supports disconnection, no buffer forwarding Disadvantage- Loss on wireless link propagated into fixed network.

65. Fast retransmit/fast recovery As soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose

66. Fast retransmit/fast recovery As soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose This forces the fast retransmit mode at the communication partners The TCP on the MH is forced to continue sending with the Half-of-window size and not to go into slow-start after registration

67. Fast retransmit/fast recovery S R X1 X2 X3 X3 X1’ X2 X3’X4 X4’X5 T1 T2 T3 X2’X3 X3’X4 X4 T4 T3 8 6 3 2 1 12

68. Transmission/time-out freezing TCP sends an acknowledgement only after receiving a packet. No packet exchange possible e.g., in a tunnel, disconnection due to overloaded cells or mux, with higher priority traffic. This forces the fast retransmit mode at the communication partners TCP disconnects after time-out completely

69. Transmission/time-out freezing S R X1 X2 X3 X4 X1’ X2 X3’X4 X4’X5 T1 T2 T4 T3 X2’X3

70. Transmission/time-out freezing S R X1 X2 X3 X4 X1’ X2 X3’X4 X4’X5 T1 T2 T4 T3 X2’X3 8 6 3 2 1 12

71. Transmission/time-out freezing S R X1 X2 X3 X3 X1’ X2 X3’X4 X4’X5 T1 T2 T3 X2’X3 X3’X4 X4 T4 T3 8 6 3 2 1 12

72. Transmission/time-out freezing If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver

73. Transmission/time-out freezing S R X1 X2 X3 X3 X1’ X2 X3’X4 X4’X5 T1 T2 T3 X2’X3 X3’X4 X4 T4 T3 8 6 3 2 1 12

74. Transmission/time-out freezing If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver Therefore, packet loss is not due to congestion, continue with current congestion window DO NOT USE SLOW-START

75. Transmission/time-out freezing MAC layer is often able to detect interruption in advance MAC can inform TCP layer of upcoming loss of connection TCP stops sending, but does now not assume a congested link MAC layer signals again if reconnected

76. Selective retransmission S R X1 X2 X3 X4 X1’ X2 X3’X4 X2 X4’X5 X2 T1 T2 T4 T3 X2’X3 X2 T2 8 6 3 2 1 12 X1 X2 X3 X4 X5

77. Selective retransmission S R X1 X2 X3 X4 X1’ X2 X3’X4 X2 X4’X5 X2 T1 T2 T4 T3 X2’X3 X2 T2 8 6 3 2 1 12 X1 X2 X3 X4 X5 X2 X6

78. Selective retransmission ACK n acknowledges correct and in-sequence receipt of packets up to n if single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth sender can now retransmit only the missing packets

79. Transaction oriented TCP ACK n acknowledges correct and in-sequence receipt of packets up to n if single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth sender can now retransmit only the missing packets