Introduction to TCP

1
Introduction to TCP

• A first look at the sockets API for
  connection-oriented client/server application
  programs

2
Benefits of TCP

• Communication is transparently reliable
• Data is delivered in the proper sequence
• An application programmer does not need to worry
  about issues such as
• Lost or delayed packets
• Timeouts and retransmissions
• Duplicated packets
• Packets arriving out-of-sequence
• Flow and congestion control

3
Overview
application layer
process A
process B
transport layer
port P
port Q




reliable TCP byte-stream
connection
T C P
T C P
network layer
I P
I P
unreliable IP datagrams
4
Transport-Layers duties

• Create the illusion of a reliable two-way
  point-to-point connection linking a client
  application with a server application
• Manage the error-control mechanism
• Manage the flow-control mechanism
• Manage the connections persistence
• Manage the connections shutdown

5
Interaction overview
The server application
socket()
bind()
The client application
socket()
listen()
bind()
connect()
3-way handshake
accept()
read()
write()
data flow to server
write()
read()
data flow to client
close()
close()
4-way handshake
6
What is a connection?

• An applications socket is connected if it has
  a defined pair of socket-addresses
• An IP-address and port-number for the host
• An IP-address and port-numbet for the peer

hopper.usfca.edu
hrn23501.usfca.edu


138.202.192.14
138.202.171.14
port 80
port 53124
2-way data stream
138.202.171.14
138.202.192.14
port 53124
port 80
classroom workstation
USFs web-server
7
Layout of TCP header
32-bits
destination port address
source port address
sequence number
acknowledgment number
Header Length
window size
F I N
S Y N
R S T
P S H
A C K
U R G
reserved
urgent pointer
checksum
options and padding
8
Sequence number
segmented stream of 1000 bytes
data (256 bytes)
data (256 bytes)
data (256 bytes)
data (232 bytes)
ISN
ISN 256
ISN 512
ISN 768
The sequence number field defines the number
being assigned to the first byte of data
contained in this segment. During connection
setup, each party to the connection uses a
random number generator to get the value it will
assign to the first byte of data it will
transmit, called its initial sequence number
(ISN). Thereafter, the sequence number in each
succeeding segment will equal the sequence number
used in the prior segment plus the number of
data bytes in that prior segment. By this
scheme the receiver can arrange all the incoming
data bytes in the proper order, even if some
segments happen to arrive out-of-order.
9
Acknowledgment number

• This field holds the number of the byte that the
  source of this segment is expecting to receive
  next from its connection partner
• This fields value is meaningful only when this
  segments ACK control flag bit is set

9, 8, 7
6, 5 ?
4, 3, 2, 1
sender
receiver
ACK 5
10
TCP Header Length

• Like IP headers, the TCP Headers length is
  expressed in multiples of 32-bits its at least
  5 (i.e., 20-bytes) if there arent any TCP
  Options included in the TCP header
• The amount of DATA in a TCP packet can be
  calculated from the IP Headers Total Length
  field, minus the number of bytes that comprise
  these two headers (IP header TCP header)

IP header
TCP header
DATA
Total Length (in bytes)
11
Control flags
5 4 3 2 1 0
URG
ACK
PSH
R S T
SY N
FIN
Legend FIN Terminate the connection
SYN Synchronize sequence numbers RST
Reset the connection PSH Push the data
ACK The value in the acknowledgement field is
valid URG The value in the urgent pointer
field is valid
12
Establishing the connection
timeline
server application (passive)
client application (active)
SYN J
SYN K, ACK J1
ACK K1
The 3-way Handshake
13
Exchanging data
timeline
server application (passive)
client application (active)
PSHACK
ACK
PSHACK
ACK
A typical Request and Reply transaction
14
Connection shutdown
timeline
server application (passive)
client application (active)
ACKFIN
ACK
ACKFIN
ACK
The 4-way Handshake
15
3-way handshake
SYN
ACK SYN
ACK
16
request-and-reply
ACKPSH
ACK
ACKPSH
ACKFIN
17
4-way handshake
ACKFIN
ACK
ACKFIN
ACK
18
TCP Timers

• To achieve transparent reliability, the TCP
  subsystem maintains some internal timers
• One of these is the Retransmission Timer
• If a packet is sent, but its ACK does not arrive
  before this timer expires, then the packet will
  be retransmitted
• Of course, this could result in the receiver
  getting duplicate packets (if its a bit slow)

19
lost versus late
timeline
client application
server application
PSHACK
retransmit timeout
ACK
PSHACK
ACK arrives late
same PSH arrives twice
Busy server might be slow to acknowledge
20
piggyback

• To reduce traffic-flow when possible, TCP delays
  sending an immediate ACK for an arriving
  data-packet, in case the receiver might soon have
  some data of its own to send back in which case
  the ACK can piggyback on the outgoing data PSH
• This mechanism, of course, requires TCP to
  maintain a Delayed ACK timer

21
Delayed ACK senario
application layer
process A
write
read
transport layer
buffer for incoming data
Retransmit timer
Delayed ACK timer
buffer for outgoing data
port P


Keep Alive timer
Window Probe timer
T C P
to/from the IP layer
22
Window Size

• During the connection setup handshake, each
  host communicates to its partner a window size
  parameter, to let be known some information about
  its capacity for buffering packets
• It also conveys its MSS parameter (as a TCP
  header option) to inform its partner of its
  buffers Maximum Segment Size

23
MSS versus MTU

• A diagram shows the distinction between the
  protocols MSS (Maximum Segment Size) and the
  interfaces MTU (Maximum Transmission Unit) no
  TCP packets will be sent with a segment-size
  thats larger

MTU
MSS
datalink header
IP header
TCP header
packet DATA
FCS
24
Main TCP option types

• The TCP Header contains an options list,
  occupying from 0 to 11 longword values
• Each option is identified by an 8-bit type
• Type 0 End of the options list data follows
  this
• Type 1 No option used for alignment padding
• Type 2 Maximum Segment Size (MSS)
• Type 3 Window scaling option (WSOPT)
• Type 4 Selective Acknowledgments supported
• Type 5 Selective Acknowledgment (SACK)
• Type 8 Timestamp value and echo reply (TSOPT)

25
Option formats

• Option types 0 and 1 are single-bytes
• All other option types are at least 2-bytes, with
  the second byte containing the length

type 2
length 4
MSS value
type 3
length 3
WSOPT value
type 4
length 2
4 bytes
2 bytes
3 bytes
type 8
length 10
timestamp value
timestamp echo reply value
10 bytes
26
Looking at TCP options
HLEN
HLEN
HLEN
27
Wrapped sequences

• The TCP headers Sequence Number is a 32-bit
  value, initially chosen at random
• It could happen that a large number gets selected
  as an Initial Sequence Number and that a large
  amount of data gets sent, thus causing the 32-bit
  field to overflow
• So how does the receiver tell a wrapped
  sequence from a late-arriving segment?

28
Type 8 TSOPT

• The TCP timestamps have two purposes
• RTTM Round-Trip Time Measurement
• PAWS Protect Against Wrapped Sequences

16 bits
TYPE (8)
LENGTH (10)
Timestamp Value
Timestamp Echo Reply
29
The SACK option

• It conveys extended acknowledgment information
  from a receiver to a sender about gaps in the
  received data-stream

Type (5)
Length
Left Edge of first Block
no gaps here
Right Edge of first Block
2n8 bytes
. . .
Left Edge of n-th Block
no gaps here
Right Edge of n-th Block
32-bits
30
Demo programs

• We put tcpserver.cpp and tcpclient.cpp on our
  class website, so you can watch actual TCP
  packets being exchanged by using our nicwatch
  application (or some other packet-sniffer, e.g.,
  wireshark)
• We deliberately used loops which write to, or
  read from, sockets one-byte-at-a-time so that you
  can observe TCP buffering!