week8 lec1-bscs1
DESCRIPTION
Computer NetworksTRANSCRIPT
Chapter 3Transport Layer
Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 Connection Oriented
HandshakeSend 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
TCP Segment Structure
source port # dest port #
application data (variable length)
sequence number
acknowledgement number Receive window
Urg data pointerchecksumFSRPAU
headlen
notused
Options (variable length)
URG: urgent data
ACK?
PSH?
RST, SYN, FIN:connection estab(setup, teardown
commands)
Used for flow control
Used in implementinga reliable data transfer
Same as in UDP
To negotiate maximum
segment size etc.
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
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 rulesTwo choices
o The receiver discards out of order segmentso Keeps out of order bytes and waits for missing bytes to
fill
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
Seq=42, ACK=79, data = ‘C’
Seq=79, ACK=43, data = ‘C’
Seq=43, ACK=80
Usertypes‘C’
host ACKsreceipt of echoed‘C’
host ACKsreceipt of‘C’, echoesback ‘C’
time
Piggybacked
TCP Reliable Data Transfer
TCP creates reliable data transfer service on top of IP’s unreliable service
TCP uses single retransmission timer, even if there are multiple unacknowledged segments (RFC 2988)
Uses cumulative acknowledgements Retransmissions are triggered by:
Timeout Duplicate Acks ( will see shortly)
Initially consider simplified TCP sender: Ignore Duplicate Acks Ignore Flow control
TCP Sender Events:
(1) Data Rcvd from Application Layer
Create segment with sequence number.
Sequence number is byte-stream number of first data byte in segment.
Start timer if not already running.
(2) Timeout: Retransmit segment Restart timer
(3)Ack Received: If acknowledges
previously unACked segments Update what is
known to be ACKed Start timer if there
are outstanding segments
TCP Sender(Simplified)
NextSeqNum = InitialSeqNum SendBase = InitialSeqNum
loop (forever) { switch(event)
event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data)
event: timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer
event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer }
} /* end of loop forever */
TCP: Retransmission Scenarios
Host A
Seq=100, 20 bytes data
ACK=100
timePremature Timeout
Host B
Seq=92, 8 bytes data
ACK=12
0
Seq=92, 8 bytes data
Seq=
92
tim
eout
ACK=12
0
Host A
Seq=92, 8 bytes data
ACK=100
losstim
eout
Lost ACK Scenario
Host B
X
Seq=92, 8 bytes data
ACK=10
0
time
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
TCP Retransmission Scenarios Host A
Seq=92, 8 bytes data
ACK=100
losstim
eout
Cumulative ACK scenario
Host B
X
Seq=100, 20 bytes data
ACK=12
0
time
SendBase= 120
TCP ACK Generation [RFC 1122, RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq #. All data up toexpected seq # already ACKed
Arrival of in-order segment withexpected seq #. One other Inorder segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq. # .Gap detected
TCP Receiver action
Delayed ACK. Wait up to 200ms for next in order segment. If no next segment, send ACK
Immediately send single cumulative ACK, ACKing both in-order segments
Immediately send duplicate ACK, indicating seq. # of next expected byte
Fast Retransmit
Time-out period often relatively long: long delay before resending lost packet
Detect lost segments via duplicate ACKs. Sender often sends many segments back-to-back If segment is lost, there will likely be many duplicate
ACKs.
If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost: Fast Retransmit: resend
segment before timer expires
Host A
tim
eout
Host B
time
X
resend 2nd segment
Resending a Segment after Triple Duplicate ACK
event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } else { increment count of duplicate ACKs received for y if (count of duplicate ACKs received for y ==3) { resend segment with sequence number y }
Fast Retransmit Algorithm:
Fast Retransmit
Go-Back N or Selective Repeat GBN Protocol
TCP acknowledgements are cumulative and out of order segments are not individually acked.
Difference Buffering of correctly received packets GBN transmits all packets after the lost packet
TCP only the missing packet Selective Repeat
Buffering of out of order packets TCP SACK (RFC 2018) allows to ACK out of order
packets selectively rather than cumulatively Difference
Timer with every packet
Hybrid of Go-Back N and Selective Repeat