Week5 lec3-bscs1


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Week5 lec3-bscs1

  1. 1. Chapter 3 Transport Layer Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
  2. 2. Pipelined Reliable Data Transfer Protocols  Pipelining has the following consequences for reliable data transfer  Range of sequence numbers must be increased  Sender and receiver sides may have to buffer more than one packet.  Two basic approaches towards pipeline error recovery: Go-Back-N, Selective Repeat
  3. 3. Go-Back-N (GBN) Sender:  Sender is allowed to transmit multiple packets without waiting for an acknowledgement  Constrained to a certain maximum number N.  Base or send_base  Sequence number of oldest unacknowledged packet  Nextseqnum  Sequence number of next packet to be sent  The range of sequence numbers for transmitted but not acknowledged packets can be viewed as a window of size N.  This window slides forward as the protocol operates
  4. 4. Go-Back-N GBN sender must respond to three types of events  Invocation from above (rdt_send() is called):  If window is full, returns data to upper layer  Maintain synchronization mechanism  Receipt of an ACK:  ACK for packet with seq # n is taken as“Cumulative ACK”  More shortly in receiver  Time out event:  Sender has timer for oldest unacknowledged packet • If timer expires, retransmit all unacknowledged packets
  5. 5. Go-Back-N Receiver:  If a packet with seq # n is received correctly and is in order  ACK is sent and data is delivered to upper layers  For all other cases  Receiver discards the packet and resends ACK for most recently received in order packet  Packets are delivered one at a time to upper layers  If a packet k has been received and delivered, then all packets with seq # lower than k have also been delivered. r Receiver discards out of order packets  No receiver buffering  Need only remember expectedseqnum
  6. 6. GBN in action http://www.eecis.udel.edu/~amer/450/TransportApplets/GBN/GBNindex.html
  7. 7. Selective Repeat  Selective Repeat protocol avoid unnecessary retransmissions o Sender only retransmits packets that were lost or are in error o A window size N is used to limit the no of outstanding unacknowledged packets in the pipeline
  8. 8. Selective Repeat Sender:  Data received from upper layers o If window is full, returns data to upper layer o Maintain synchronization mechanism  Timeout o Each packet has its own timer o Single packet is retransmitted on timeout  ACK received: o Sender marked packet as received provided its in the window o Packets sequence no is equal to send_base, • The window base is moved forward to the unacknowledged packet
  9. 9. Selective Repeat Receiver:  Packets with sequence no in the window  Selective ACK is sent to the sender whether or not it is in order.  Out-of-order: buffer but send ACK for that packet  Deliver base plus buffered packets  Packets with sequence number below the window base  An ACK must be generated even though the packet has already been acknowledged by the receiver
  10. 10. Selective Repeat in Action http://www.eecis.udel.edu/~amer/450/TransportApplets/SR/SRindex.html
  11. 11. Selective Repeat: Dilemma  Finite range of sequence numbers Example:  Seq #‟s: 0, 1, 2, 3  Window size=3  Receiver sees no difference in two scenarios!  Incorrectly passes duplicate data as new in (a)  Window size of one less than the sequence number space does not work  Window size must be less than or equal to half the size of sequence no. space
  12. 12. TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581  Connection Oriented Handshake Send segments to each other to establish parameters of ensuing data transfer  Runs on the end systems  TCP connection is point to point (single sender and receiver) Does not support multicast (one sender many receivers)  TCP send buffer TCP grab a chunk of data from this buffer  MSS (Maximum Segment Size)  The maximum amount of data that can be grabbed and placed in a segment  TCP receive buffer
  13. 13. TCP Segment Structure URG: urgent data ACK? PSH? RST, SYN, FIN: connection estab (setup, teardown commands) Same as in UDP To negotiate maximum segment size etc. source port # dest port # sequence number acknowledgement number head not len used UAP R S F Receive window Urg data pointer checksum Options (variable length) Used in implementing a reliable data transfer Used for flow control application data (variable length) Find more out about OPTONS? (SEE RFC 853, RFC 1323)
  14. 14. TCP Sequence Numbers and ACKs Sequence Numbers:  Sequence nos. are over the stream of transmitted bytes and not over the series of transmitted segments  Sequence no. is the byte stream “number” of first byte in segment‟s data  Example:        Host A wants to send data to Host B File consisting of 500,000 bytes, MSS is 1,000 bytes First byte of stream is numbered zero TCP constructs 500 segments out of data stream First segment gets sequence number --- 0 Second segment gets sequence number----1000 Third segment gets sequence number------2000 and so on
  15. 15. TCP Sequence Numbers  Imagine a TCP connection is transferring a file of 6000 bytes. The first byte is numbered 10010. What are the sequence numbers for each segment if data is sent in five segments with the first four segments carrying 1,000 bytes and the last segment carrying 2,000 bytes?
  16. 16. TCP Sequence Numbers Solution: The following shows the sequence number for each segment: Segment 1  10,010 (10,010 to 11,009) Segment 2  11,010 (11,010 to 12,009) Segment 3  12,010 (12,010 to 13,009) Segment 4  13,010 (13,010 to 14,009) Segment 5  14,010 (14,010 to 16,009)
  17. 17. TCP ACKs Acknowledgement Numbers:  The acknowledgement no that hosts A puts in its segment is the sequence no of the next byte host A is expecting from host B.  Example  Host A receives all bytes numbered 0 through 535 from B  Host A puts 536 in the acknowledgment number field of the segment it sends to B  TCP acknowledges bytes up to first missing bytes in the stream  Cumulative Acknowledgement  How receiver handles out-of-order segments?  TCP RFCs do not impose any rules Two choices o The receiver discards out of order segments o Keeps out of order bytes and waits for missing bytes to fill
  18. 18. TCP Sequence Numbers and ACKs Example:  Host A sends a character to Host B, which echoes it back to Host A.  Starting Sequence no for client and server are 42 and 79. Piggybacking:  Acknowledgement of client to server data is carried by segment of server to client data Host A Host B User types „C‟ host ACKs receipt of echoed „C‟ host ACKs receipt of „C‟, echoes back „C‟ Piggybacked time
  19. 19. TCP Flow Control  Eliminate the possibility of sender overflowing receiver‟s buffer by transmitting too much, too fast.  Sender maintains a variable receive window – Gives the sender an idea of how much free space is available at receiver  Example  Host A is sending a large file to host B  Host B allocates a receive buffer and denotes size by RcvBuffer  LastByteRead The number of the last byte read from buffer by process in Host B  LastByteRcvd: The number of last byte that has been placed in buffer at B
  20. 20. TCP Flow Control  RcvWindow is set to the amount of spare room in the buffer  RcvWindow= RcvBuffer - [LastByteRcvd - LastByteRead]  Host B informs Host A about how much spare room it has in the connection buffer.  Places RcvWindow in the receive window field of every segment  Host A keeps track of two variables  LastByteSent and LastByteAcked
  21. 21. TCP Flow Control  LastByteSent – LastByteAcked – It is the amount of unacknowledged data that A has sent into the connection  LastByteSent – LastByteAcked RcvWindow – Keeping the unacknowledged data less than the value of RcvWindow  Suppose RcvWindow=0 – Host B advertises RcvWindow=0 to Host A – Suppose Host B has nothing to send to host A – TCP specification require Host A to send one byte of data Persistence Timer? Home Assignment