Services
Connections
UDP
Transport Layer
Mahalingam Ramkumar
Mississippi State University, MS
September 29, 2014
Ramkumar
TL
Services
Connections
UDP
Outline
1
Services
2
Connections
TCP Header
TCP Transmission Policy
Timer Management
3
User Datagram Protocol
Ramkumar
TL
Services
Connections
UDP
Transport Layer Services
Provides service to application layer
by making the network layer easier to use
Service can be connection oriented or connectionless
Example TCP (connection oriented) UDP (connectionless)
over IP.
Port numbers are access points for transport service
Ramkumar
TL
Services
Connections
UDP
Connection Oriented Service
Reliable delivery of data
In the right order
Acknowledged delivery
Hide implementation details from top layer
Ramkumar
TL
Services
Connections
UDP
Interface Provided to Application Layer
listen(), accept()
connect()
send()
recv()
close()
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Connection Establishment in TCP
Client sends connection request
Server acknowledges
Issues due to unreliable lower layer?
Packets can get lost, delayed, duplicated
FLAGS and seq/ack numbers (for counting bytes
sent/received) on TCP header.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
SYN/ACK/FIN flags, seq/ack number
SYN flag set: sequence number is the starting sequence
number
SYN is not set: sequence number is the byte count for first
data byte included the packet
ACK flag set when acknowledgement number is valid;
indicates next byte expected
FIN set for closing a connection
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Three-Way Handshake
SYN and ACK flags; seq and ack numbers
Both sender and receiver choose initial sequence number
Connection request (SYN, seq no x) from initiator (client)
SYN-ACK (SYN, seq y , ACK, x + 1) response (from server)
ACK (ACK, y + 1) from client.
SYN — SYN/ACK — ACK sequence.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Four-Way Handshake
Two unidirectional connections
What if one side is not ready to disconnect (not finished
sending)?
Four-way handshake
A → B:
B → A:
B → A:
A → B:
FIN
ACK, ack number (next byte expected + 1)
FIN
ACK, ack number (next byte expected + 1)
FIN — ACK, FIN — ACK (four packets) or
FIN — ACK/FIN — ACK (three packets)
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Sequence and ACK numbers
To keep track of application data bytes sent/received
Client C , server S
C → S : seq 1+ packet with 1000 application bytes (1000
byte payload)
S → C : ack 1001 (the next byte I am expecting from you is
1001).
C → S : seq 1001+ packet with 500 application bytes
S → C : ack 1501 (the next byte expected is 1501).
C → S : seq 1501+ packet with 200 application bytes.
S → C : ack 1501 (the next byte expected is 1501 probably
because it did not receive the third packet from the client).
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Both ways at the same time!
C → S : seq 1 + 100 payload bytes
S → C : seq 1 ack 101, 1500 payload bytes
C → S : seq 101 ack 1501, 200 payload bytes
S → C : seq 1501 ack 301, 1500 payload bytes
C → S : ack 3001 (no payload)
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
ACK/SEQ numbers Offset by starting sequence numbers
C , qc , S, qs
C → S : SYN, seq qc
S → C : SYN, seq qs , ACK, ack qc + 1
C → S : ACK, ack qs + 1 (completes 3-way handshake)
C → S : seq qc + 1 + 100 payload bytes
S → C : seq qs + 1 ack 101, +1500 payload bytes
C → S : seq qc + 101 ack qs + 1501, +200 payload bytes
S → C : seq qs + 1501 ack qc + 301, +1500 payload bytes
C → S : ack qs + 3001 (no payload)
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
SYN/ACK Flags with SEQ/ACK numbers
→
SYN, seq(qc )
← SYN, seq(qs ), ACK, ack(qc + 1)
→ seq(qc + 1), ACK, ack(qs + 1)
→ seq(qc + 1), ACK, ack(qs + 1), 100 data bytes
← seq(qs + 1), ACK, ack(qc + 101), 1500 data bytes
→ seq(qc + 101), ACK, ack(qs + 1501)
← seq(qs + 1501), ACK, ack(qc + 101), 1400 data bytes
→ seq(qc + 101), ACK, ack(qs + 2901), FIN
← seq(qc + 2901), ACK, ack(qc + 102), 1200 data bytes
→ ACK, ack(qs + 4101)
← seq(qs + 4101), ACK, ack(qc + 102), FIN
→ ACK, ack(qs + 4102)
CLIENT
SERVER
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
All together
connect()
→
SYN, seq(qc )
accept()
.
.
.
← SYN, seq(qs ), ACK, ack(qc + 1)
→ seq(qc + 1), ACK, ack(qs + 1)
send()
recv()
→ seq(qc + 1), ACK, ack(qs + 1), 100 data bytes
recv()
← seq(qs + 1), ACK, ack(qc + 101), 1500 data bytes send()
→ seq(qc + 101), ACK, ack(qs + 1501)
← seq(qs + 1501), ACK, ack(qc + 101), 1400 data bytes
shutdown(WR)
→ seq(qc + 101), ACK, ack(qs + 2901), FIN
recv() ← seq(qc + 2901), ACK, ack(qc + 102), 1200 data bytes
→ ACK, ack(qs + 4101)
close()
← seq(qs + 4101), ACK, ack(qc + 102), FIN
close()
→ ACK, ack(qs + 4102)
CLIENT
SERVER
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Header
32 Bits
Source port
Destination port
Sequence number
Acknowledgement number
U A P R S F
R C S S Y I
G K H T N N
TCP
header
length
Window size
Checksum
Urgent pointer
Options (0 or more 32-bit words)
Data (optional)
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Header Fields
Source port and destination port
Sequence Number - chosen randomly for the first packet
(connection establishment). In subsequent packets containing
data, it indicates the offset of the first (application data) byte
included in the packet.
Acknowledgment number indicates next byte expected (offset
by the sequence number).
A’s ack. to B offset by B’s initial sequence number
Header length (4 bits) - in number of 32 bit words. Actually
indicates start of application data.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Header - FLAGS
Six unused bits
Six flags, URG, ACK, PSH, RST, SYN, FIN
PSH, URG are not used a lot in practice (and therefore the
urgent pointer too)
SYN used for connection establishment
FIN used for disconnect
ACK is used during connection setting-up, acknowledgement
of data received, and during connection-tearing-up
RST used to indicate rejection of connection
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Header - Window Size
16 bits.
Window size is the number of bytes the receiver is willing to
accept.
Window size in A’s TCP packet to B says the maximum
window size B can use for sending data to A (and vice-versa)
The receiver determines the window size for each one-way
connection (A to B and B to A)
Window size is dynamic.
Window size 0 indicates “stop sending data - I’m swamped”
Could later send another packet with a larger window size to
resume sending.
If A has advertised a window size of 500 and the last
acknowledged byte is 1000, B cannot send beyond byte 1500
to A.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Header - Checksum
Checksum of header, data and a ”conceptual pseudo-header”
Includes Source and Dest IP addresses, TCP segment length,
IP protocol field.
32 Bits
Source address
Destination address
00000000
Protocol = 6
Ramkumar
TCP segment length
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Checksum - Some Finer Points
TCP segment does not include length
IP layer (in the final destination) passes this info to TCP
layer, along with the entire TCP packet
TCP layer is also provided with the Source and Dest IP
addresses by the IP layer
All three needed to validate checksum
Violates protocol hierarchy
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Header - Options
Specify maximum TCP payload - used very frequently.
SRP instead of GBN - possibility of including NACKs before
sending ACK - RFC 1106
Scaling Window size
For high bandwidth / high delay (or both) 64-kB window is
not sufficient.
On a T3 line 12msec to output 64kB
If RTT is 50 msec, nothing can be sent for 38 msec.
RFC 1323
Allows left shift of window size upto 14 bits - max window size
230 .
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
PSH and URG
PSH - a way for applications to request TCP layer to stop
buffering packets and just send the data out.
A set PSH flag indicates to the receiver that the packet
should be delivered immediately to the application above.
URG and Urgent pointer. Very similar to push conceptually.
But can specify a particular byte offset in a TCP packet that
needs to be attended to urgently - like a software interrupt.
Urgent pointer indicates offset.
Once again, at the receiver side, the TCP layer delivers the
data to the application ASAP and indicates the offset.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Connection Establishment
Case
(b) - only one connection is established!
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Connection Release
Each simplex connection released independently
1
2
3
4
5
(Assume B → A ACK x; A → B ACK y were last two ACK
packets exchanged between A and B).
A → B, FIN, seq x
B → A, ACK x + 1
B → A, FIN, seq y
A → B, ACK y + 1
2 and 3 can be combined, B → A, ACK, FIN.
Timeout when ACKs are not received
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
PSH Flag
PSH flag is actually used a lot in telnet
How many bytes are exchanged between A and B for sending
one character?
For each character, 20 bytes IP header + 20 bytes TCP
header + 1 = 41 bytes!
Each way
Two ACKs each way if we type real slow - 40 + 40 = 80 bytes
Total of 162 bytes for sending a single character
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Window Management
Client A, server B
Client has reserved a 10000 byte buffer for the connection
Client indicates this by setting window size 10000 in the SYN
packet
Server B allocates 4000 bytes. Indicates this by setting
window size 4000 in the SYN-ACK packet.
Window size indicates how many bytes beyond the last
acknowledged byte can be sent.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Window Management ...
After connection establishment, A sends 1000 bytes to B
B stores the bytes in the buffer - has only 3000 bytes of free
buffer.
B sends an ACK with window size 3000.
A sends another 3000 bytes
B sends ack with window size 0
A waits.
B’s TCP layer passes the data to the application layer. Buffer
is now empty. B sends a new “window advertisement” to A
with window size 4000.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Window Management
Application
does a 2K
write
Sender
Receiver
Receiver's
buffer
0
4K
Empty
2K S
EQ =
0
2K
ACK = 2048 WIN = 2048
Application
does a 2K
write
2K
SEQ = 2048
Full
Sender is
blocked
ACK =
ACK =
4096 W
Application
reads 2K
IN = 0
048
IN = 2
4096 W
2K
Sender may
send up to 2K
1K
SEQ =
4096
Ramkumar
1K
TL
2K
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Silly Window Syndrome
Receiver's buffer is full
Application reads 1 byte
Room for one more byte
Header
Header
1 Byte
Window update segment sent
New byte arrives
Receiver's buffer is full
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Specifications
Apart from the header protocol, TCP specifies very little
Example, defaults
Everything else is implementation dependent
Some of them may ignore PSH. Most do not support URG
TCP does not specify when ACKs should be sent or how
many bytes to buffer.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Congestion Control
Hosts realize that there is congestion when packets are not
acknowledged
Could occur due to two reasons
1
2
Low capacity receiver at the other end
Network congestion
The first is handled during the connection establishment
process - using window sizes
The second?
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Congestion Window
Every sending host chooses an additional window size
Congestion Window
Initially set to the maximum packet size the host can handle
(MTU 1500 for Ethernet based hosts).
The window size chosen is the minimum of Congestion window
WC and the advertised window size by the other side Wa .
First packet sent is up to MTU size
Assume Wa is very large
If successful in sending first packet (ack received before timer
goes off) congestion window size doubled to 2 MTUs
On success, doubled to 4 MTU and so on till failure happens then window size is halved.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Congestion Window
Failure is detected if 3 duplicate ACKs received or time out
occurs.
Additional parameter, threshold Window size, initially set to
64 kB
Exponential increase, only till the threshold is released.
After this increase is linear, for example 64 MTU to 65, 66
and so on.
Whenever failure occurs (say at N MTUs), threshold is halved
(in both TCP Tahoe and TCP Reno)
And WC set to N/2 MTU (TCP Tahoe, fast recovery) or 1
MTU (TCP Reno, slow recovery)
ICMP source quench packets (we will see this later in the
network layer) are also treated as time-outs for this purpose.
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Reno (Slow Recovery)
44
Timeout
40
Congestion window (kilobytes)
36
Threshold
32
28
Threshold
24
20
16
12
8
4
0
0
2
4
6
8
10
14
12
Transmission number
Ramkumar
TL
16
18
20
22
24
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
TCP Timers
Retransmission Timer - decides timeout
How? Need to estimate RTT. Chicken and Egg Problem?
Not really - use a large timeout initially!
T
0.3
T1
0.3
Probability
0.2
Probability
0.2
T2
0.1
0
0.1
0
30
40
10
20
Round-trip time (msec)
50
0
0
(a)
10
20
30
40
Round-trip time (msec)
(b)
Ramkumar
TL
50
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Estimating Round-Trip Time (RTT)
Jacobson’s algorithm
1
2
3
4
5
6
Time for receiving acknowledgement M
RTT = αRTT + (1 − α)RTT
α = 7/8. Timeout is βRTT . How do we choose β?
β should be based on the standard deviation of the RTT
estimate.
Estimate of standard deviation
D = αD + (1 − α) | RTT − M |.
Timeout = RTT + 4 × D
What happens when time outs occur? Is the ACK for the
original or for the re-transmission?
Making a wrong judgment here could affect the estimate very
badly!
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Karn’s Algorithm
Do not update RTT for segments that have been
retransmitted
Whenever timeout occurs double RTT
Keep doing it till timeout does not occur!
Used in most TCP implementations (TCP does not specify
what to do).
Ramkumar
TL
Services
Connections
UDP
TCP Header
TCP Transmission Policy
Timer Management
Two more Timers!
TCP uses 3 timers - retransmission, persistence and
keep-alive
Persistence Timer - for prevention of deadlock if window
size re-advertisement is lost.
When persistence times goes off, sender transmits a probe
(even though receiver had previously informed the sender to
stop sending by advertising window size 0), and receiver sends
the window size as response.
Keep-alive Timer - Connections may be idle for a long time
(telnet connections). On expiry of keep-alive timer a probe is
sent to detect if the other side is still alive. If so timer is
started again (else connection is terminated).
Ramkumar
TL
Services
Connections
UDP
UDP
Does very little over what IP does
Provides port numbers
No flow control, error control, retransmission
Applications have to do everything
32 Bits
Source port
Destination port
UDP length
UDP checksum
Ramkumar
TL
Services
Connections
UDP
UDP
Checksum similar to TCP
DNS
RPC (Remote procedure calls). eg gethostbyname()
The need for 3 packets for establishing connection and 3 or 4
for tearing them down makes TCP very expensive for many
applications
Especially when networks are often highly reliable.
Multicasting - TCP cannot be used.
UDP is used for RTP (real-time transport protocol)
Ramkumar
TL
Services
Connections
UDP
Transactional TCP
TCP has high overhead for scenarios where the transaction is
simple (small request, small response, like DNS)
UDP is efficient but highly insecure
Even servers that did not actually receive the query can
pretend to have received, and send a “response”
Transaction TCP is actually a different way of using TCP
Query and responses are sent with connection request packets
Three-way handshake simultaneously completes connection
establishment, acknowledged delivery of data and termination!
Ramkumar
TL
Services
Connections
UDP
Transactional TCP
Client
1
Server
1
SYN
SYN)
SYN, ACK(
3
Time
4
5
2
Server
SYN, reques
t,
FIN)
SYN, ACK(
3
ACK(SYN)
request
Time
t + FIN)
reply
FIN
6
7
8
ACK(FIN)
(a)
(b)
Ramkumar
TL
FIN
, reply, FIN
ACK(FIN)
FIN
ACK(reques
9
Client
2