Download Transmission Control Protocol (TCP)

TCP/IP
TCP/IP Protocol Suite (1)
Physical layer
Data-link layer –PPP, ARP, RARP
Network layer – IP, ICMP, IGMP, BootP
Transport layer _ TCP, UDP, RTP
Application layer – http, smtp, ftp
TCP/IP Protocol Suite (2)
Point-to-Point Protocol (PPP): a link layer
protocol used in the Internet
Address Resolution Protocol (ARP): IP
address  Ethernet address
Reverse Address Resolution Protocol (RARP):
Ethernet address  IP address
Bootstrap Protocol (BOOTP): function is
similar to RARP, but using UDP messages,
and was extended to DHCP (Dynamic Host
Configuration Protocol)
TCP/IP Protocol Suite (3)
Internet Control Message Protocol (ICMP) :
monitor or test the Internet
Internet Group Management Protocol (IGMP) :
manage the membership of IP multicast
groups
Real-time Transport Protocol (RTP): provides
end-to-end network transport functions
suitable for applications transmitting real-time
data
TCP/IP Protocol Suite (4)
http: HyperText Transfer Protocol
smtp: Simple Mail Transfer Protocol
ftp: File Transfer Protocol
Internet Protocol (IP)
Addressing
Routing
Fragmentation and Reassembly
Quality of Service
Multiplexing and Demultiplexing
Addressing
Need unique identifier for every host in
the Internet (analogous to postal
address)
IP addresses are 32 bits long
Hierarchical addressing scheme
Conceptually …

IPaddress =(NetworkAddress,HostAddress)
Address Classes
Class A
0 netId
hostId
7 bits
24 bits
Class B
1 0
netId
14 bits
hostId
16 bits
Class C
11 0
netId
21 bits
hostId
8 bits
IP Address Classes (contd.)
Two more classes


1110 : multicast addressing
1111 : reserved
Significance of address classes?
Why this conceptual form?
Addresses and Hosts
Since netId is encoded into IP address,
each host will have a unique IP address
for each of its network connections
Hence, IP addresses refer to network
connections and not hosts
Why will hosts have multiple network
connections?
Special Addresses
hostId of 0
hostId of all
All 1’s
netId of 0
Loopback
:
1’s:
:
:
:
network address
directed (distant) broadcast
limited (local) broadcast
this network
127.0.0.0
Dotted decimal notation: IP addresses are written as four
decimal integers separated by decimal points, where each
integer gives the value of one octet of the IP address.
Dotted decimal notation
11001010, 00100110, 01000000, 00000010
202.38.64.2
Exceptions to Addressing
Subnetting



Splitting hostId into subnetId and hostId
Achieved using subnet masks
Useful for?
Supernetting (Classless Inter-domain Routing
or CIDR)



Combining multiple lower class address ranges
into one range
Achieved using 32 bit masks and max prefix
routing
Useful for?
Examples
Subnetting


192.168.1.0/24 – class C network
192.168.1.64/26 and 192.168.1.128/26 – 2
subnetworks with upto 62 stations each!
Supernetting


192.168.2.0/24 and 192.168.3.0/24 – 2
class C networks
192.168.2.0/23 – 1 super network with
upto 510 stations!!
Weaknesses
Mobility
Switching address classes
Notion of host vs. IP address
IP Routing
Direct


If source and destination hosts are connected
directly
Still need to perform IP address to physical
address translation. Why?
Indirect


Table driven routing
Each entry: (NetId, RouterId)
 Default router
 Host-specific routes
IP Routing Algorithm
RouteDatagram(Datagram, RoutingTable)
Extract destination IP address, D, from the
datagram and compute the netID N





If N matches any directly connected network address deliver
datagram to destination D over that network
Else if the table contains a host-specific route for D, send
datagram to next-hop specified in table
Else if the table contains a route for network N send
datagram to next-hop specified in table
Else if the table contains a default route send datagram to
the default router specified in table
Else declare a routing error
Routing Protocols
Interior Gateway Protocol (IGP)


Within an autonomous domain
RIP (distance vector protocol), OSPF (link
state protocol)
Exterior Gateway Protocol (EGP)


Across autonomous domains
BGP (border gateway protocol)
IP Fragmentation
The physical network layers of different
networks in the Internet might have
different maximum transmission units
The IP layer performs fragmentation
when the next network has a smaller
MTU than the current network
IP fragmentation
MTU = 1500
MTU=500
IP Reassembly
Fragmented packets need to be put
together
Where does reassembly occur?
What are the trade-offs?
Multiplexing
Web
Email
TCP
MP3
UDP
IP
IP datagrams
Web
Email
TCP
UDP
IP
IP datagrams
MP3
IP Header
Used for conveying information to peer
IP layers
Destn
Source
Application
Transport
IP
DataLink
Physical
Router
IP
DataLink
Router
IP
DataLink
Physical
Physical
Application
Transport
IP
DataLink
Physical
IP Header (contd.)
4 bit 4 bit hdr
version length
8 bit
TOS
16 bit identification
8 bit TTL
16 bit total length
3 bit
flags
8 bit protocol
13 bit fragment offset
16 bit header checksum
32 bit source IP address
32 bit destination IP address
Options (if any) (maximum 40 bytes)
data
Internet Protocol (IP): Recap
Addressing
Routing
Fragmentation and Reassembly
Quality of Service
Multiplexing and Demultiplexing
Transmission Control Protocol
(TCP)
Transmission Control Protocol
(TCP)
End-to-end transport protocol
Responsible for reliability, congestion
control, flow control, and sequenced
delivery
Applications that use TCP: http (web),
telnet, ftp (file transfer), smtp (email),
chat
Applications that don’t: multimedia
(typically) – use UDP instead
Ports, End-points, &
Connections
http ftp smtptelnet
TCP
UDP
IP Layer
Protocol ID
A1
A2
A3
Transport
Port
IP address
Thus, an end-point is represented by (IP address,Port)
Ports can be re-used between transport protocols
A connection is (SRC IP address, SRC port, DST IP
address, DST port)
Same end-point can be used in multiple connections
TCP
Connection Establishment
Connection Maintenance




Reliability
Congestion control
Flow control
Sequencing
Connection Termination
Fundamental Mechanism
data
data
ack
retx
data
ack
Simple stop and
go protocol
Timeout based
reliability (loss
recovery)
Multiple
unacknowledged
packets (W)
Sliding Window Protocol: 1 2 3 4 5 6 7 8 9 10 11 12 ….
Active and Passive Open
How do applications initiate a
connection?
One end (server) registers with the TCP
layer instructing it to “accept”
connections at a certain port
The other end (client) initiates a
“connect” request which is “accept”-ed
by the server
Reliability (Loss Recovery)
data
Sequence Numbers
TCP uses cumulative
Acknowledgments (ACKs)
ack

1
2
3 3
4 4
5
1
2
3 3
4
3
3
4


Next expected in-sequence packet
sequence number
Pros and cons?
Piggybacking
Timeout calculation


Rttavg = k*Rttavg + (1k)*Rttsample
RTO = Rttavg + 4*Rttdeviation
Congestion Control
Slow Start
 Start with W=1
 For every ACK,
W=W+1
Congestion Avoidance
Alternative: Fall to W/2 and start (linear increase)
congestion avoidance directly
 For every ACK,
 W = W+1/W
Congestion Control
(multiplicative decrease)
 ssthresh = W/2
 W = 1
Why LIMD? (fairness)
• W=1
• 100
10
• 1
1
• Problem? – inefficient
diff = 90
diff = 0
•
•
•
•
•
•
•
•
•
•
•
10
5
6
7
diff
diff
diff
diff
28
14
diff = 45
diff = 23.5
38.25
19.65
diff = 23.5
diff = 11.2
• W=W/2
100
50
51
52
..
73
37.5
..
61.75
30.85
..
=
=
=
=
90
45
45
45
Flow Control
Prevent sender from overwhelming the
receiver
Receiver in every ACK advertises the
available buffer space at its end
Window calculation

MIN(congestion control window, flow control window)
Sequencing
1
2
3 3
4
3
3
4
Byte
sequence
1 given to app
numbers
2 given to app
Loss
TCP receiver buffers
4 buffered (not given to app)
out of order
segments and
3 & 4 given to app
reassembles them
4 discarded
later
Starting sequence
number randomly
chosen during
connection
establishment
Connection Establishment &
Termination
Active open
SYN
Send connection
request
SYN+ACK
Server does passive open
Accept connection request
Send acceptance
ACK
DATA
Start connection
3-way handshake
used for connection
establishment
Randomly chosen
sequence number is
conveyed to the other
end
Similar FIN, FIN+ACK
exchange used for
connection
termination
TCP Segment Format
16 bit SRC Port
16 bit DST Port
32 bit sequence number
32 bit ACK number
HL resvd flags 16 bit window size
16 bit TCP checksum 16 bit urgent pointer
Options (if any)
Data
Flags: URG, ACK,
PSH, RST, SYN,
FIN
TCP Flavors
TCP-Tahoe

W=1 adaptation on congestion
TCP-Reno

W=W/2 adaptation on fast retransmit,
W=1 on timeout
TCP-newReno

TCP-Reno + fast recovery
TCP-Vegas, TCP-SACK
TCP Tahoe
Slow-start
Congestion control upon time-out or DUPACKs
When the sender receives 3 duplicate ACKs
for the same sequence number, sender infers
a loss
Congestion window reduced to 1 and slowstart performed again
Simple
Congestion control too aggressive
TCP Reno
Tahoe + Fast re-transmit
Packet loss detected both through timeouts,
and through DUP-ACKs
Sender reduces window by half, the ssthresh
is set to half of current window, and
congestion avoidance is performed (window
increases only by 1 every round-trip time)
Fast recovery ensures that pipe does not
become empty
Window cut-down to 1 (and subsequent slowstart) performed only on time-out
TCP New-Reno
TCP-Reno with more intelligence during fast
recovery
In TCP-Reno, the first partial ACK will bring
the sender out of the fast recovery phase
Results in timeouts when there are multiple
losses
In TCP New-Reno, partial ACK is taken as an
indication of another lost packet (which is
immediately retransmitted).
Sender comes out of fast recovery only after
all outstanding packets (at the time of first
loss) are ACKed
TCP SACK
TCP (Tahoe, Reno, and New-Reno) uses
cumulative acknowledgements
When there are multiple losses, TCP Reno
and New-Reno can retransmit only one lost
packet per round-trip time
What about TCP-Tahoe?
SACK enables receiver to give more
information to sender about received packets
allowing sender to recover from multiplepacket losses faster
TCP SACK (Example)
Assume packets 5-25 are transmitted
Let packets 5, 12, and 18 be lost
Receiver sends back a CACK=5, and
SACK=(6-11,13-17,19-25)
Sender knows that packets 5, 12, and
18 are lost and retransmits them
immediately
Other TCP flavors
TCP Vegas


Uses round-trip time as an earlycongestion-feedback mechanism
Reduces losses
TCP FACK

Intelligently uses TCP SACK information to
optimize the fast recovery mechanism
further
User Datagram Protocol (UDP)
Simpler cousin of TCP
No reliability, sequencing, congestion control,
flow control, or connection management!
Serves solely as a labeling mechanism for
demultiplexing at the receiver end
Use predominantly by protocols that do no
require the strict service guarantees offered
by TCP (e.g. real-time multimedia protocols)
Additional intelligence built at the application
layer if needed
UDP Header
Src Port
Length
Dst Port
Checksum
Length: length of header
+ data (min = 8)
Recap
TCP
Connection management
Reliability
Flow control
Congestion control
TCP flavors
UDP