TCP - Transmission Control Protocol

© Peter R. Egli 2017
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TCP - Transmission Control Protocol indigoo.com
Peter R. Egli
INDIGOO.COM
INTRODUCTION TO TCP, THE INTERNET'S
STANDARD TRANSPORT PROTOCOL
TCPTRANSMISSION CONTROL
PROTOCOL

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Contents
1. Transport layer and sockets
2. TCP (RFC793) overview
3. Transport Service Access Point (TSAP) addresses
4. TCP Connection Establishment
5. TCP Connection Release
6. TCP Flow Control
7. TCP Error Control
8. TCP Congestion Control RFC2001
9. TCP Persist Timer
10. TCP Keepalive Timer – TCP connection supervision
11. TCP Header Flags
12. TCP Header Options
13. TCP Throughput / performance considerations
14. A last word on „guaranteed delivery“

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1. Transport layer and sockets
 The transport layer is made accessible to applications through the socket layer (API).
The transport layer runs in kernel space (Operating System) while application processes run
in user space.
Upper Layers
Application
Network Layer IP
Data Link Layer
Physical LayerOSI Layer 1
OSI Layer 2
OSI Layer 3
OSI Layer 4
OSI Layer > 4
Transport Layer TCP/UDP
Socket = API
Kernel
Space (OS)
User Space
(application)TSAPs
(sockets)
DL Frames
IP Packet
TCP Segment
Bits
Data units:
Application
message

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2. TCP (RFC793) overview
 TCP is a byte stream oriented transmission protocol:
N.B.: The size of application data chunks (data units passed over socket interface)
may be different on the sending and receiving side; the segments sent by TCP may
again have different sizes.
 TCP error control provides reliable transmission (packet order preservation, retransmissions
in case of transmission errors and packet loss).
 TCP uses flow control to maximize throughput and avoid packet loss.
 Congestion control mechanisms allow TCP to react and recover from network congestion.
TCP
App
300
500
150
100
700
150
TCP
App
210
600
140
Application
writes
TCP Segments
Socket
interface
Socket
interface
Application
reads

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3. Transport Service Access Point (TSAP) addresses (1/3)
 TSAPs are identified by 16bit port numbers. 65536 port numbers are available (0...65535, but
0 is never used).
IP Header
TCP Header

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 Each TSAP (TCP port) is bound to at most 1 socket, i.e. a TCP port can not be opened
multiple times.
IP 192.168.1.1
TCP
Data Link
Physical Link
IP 213.4.3.2
TCP
Data Link
Physical Link
Application
process
TSAP 1208
(port number)
NSAP
Data Link Layer
Physical Layer
Network Layer
Transport Layer
Application Layer
192.168.1.131452213.4.3.2 1208
213.4.3.21208192.168.1.1 31452
Source
Port
Source
IP
Dest.
Port
Dest.
IP
Server
process
TSAP 31452
(port number)
NSAP

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ICANN (former IANA) Well-known ports [1, 1023] (Standard ports, e.g. POP3 110)
Registered ports [1024, 49151]
Dynamic/private ports [49152, 65535]
BSD Reserved ports [1, 1023]
Ephemeral ports [1024, 5000]
BSD servers [5001, 65535]
SUN solaris 2.2 Reserved ports [1, 1023]
Non-priviledged ports [1024, 32767]
Ephemeral ports [32768, 65535]
ICANN: Internet Corporation for Assigned Names and Numbers
IANA: Internet Assigned Numbers Authority
BSD: Berkeley Software Distribution
Ephemeral means „short-lived“ (not permanently assigned to an application).
 There are different definitions for the port ranges:

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4. TCP Connection Establishment (1/4)
 A TCP connection is established with 3 TCP packets (segments) going back and forth.
 3-way handshake (SYN, SYN ACK, ACK) is used to
synchronize sequence and acknowledge numbers; after 3-way
handshake the connection is established.
 Host1 performs an „active open“ while host2 does a „passive
open“.
 A connection consists of 2 independent half-duplex
connections. Each TCP peer can close its (outgoing) TCP
half-duplex connection any time independently of the other.
 A connection can remain open for hours, days, even months
without sending data, that is there is no heartbeat poll
mechanism!
 The Ack-number is the number of the next byte expected by
the receiver.
The SYN occupies 1 number in the sequence number space
(even though a SYN segment usually does not carry user data)!

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 In case of a call collision only 1 connection is created.
Usually Internet applications are client/server where clients do active open and servers passive
open (thus no connection establishment collisions possible).
In peer-2-peer applications collisions are possible; e.g. BGP (Internet backbone routing) where
BGP speakers establish TCP connections with each other.

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Host 1 Host 2
SYN (SEQ=x)
RST
Demo: Telnet connection to closed port (telnet <server IP> 12345).
 If no server is listening on the addressed port number TCP rejects the connection
and sends back a RST (reset) packet (TCP segment where RST bit = 1).

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Connection 1: 208.1.2.3 / 3544 / 177.44.4.2 / 80
Connection 2: 208.1.2.3 / 3545 / 177.44.4.2 / 80
Connection 3: 17.6.5.4 / 37659 / 177.44.4.2 / 80
TCP port = 3544
208.1.2.3
Host 1
TCP port = 37659
17.6.5.4
TCP port = 80
177.44.4.2
TCP port = 3545
208.1.2.3
Host 1
 A TCP connection is identified by the quadruple source/destination IP address and
source/destination port address.
If only one of the addresses of this quadruple is different the quadruple identifies a different
TCP connection.
Host 2
Host 3

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5. TCP Connection Release (1/2)
 The 2 half-duplex connections are closed independently of each other.
 Each host closes “its” half-duplex connection independently of the other
(that means the closing of the 2 unidirectional connections is unsynchronized).
 Host1 does an active close while host2 a passive close.
 Connection closing is a 4-way handshake and not a 3-way handshake since
the closing of the half-duplex connections is independent of each other.
 Half-close: only one half-duplex connection is closed (still traffic in the other direction).
 FIN segments occupy 1 number in the sequence number space (as do SYN segments)!
Application calls close()
Host 1 Host 2
FIN
FIN ACK
Half-duplex connection Host1  Host2 closed.
Half-duplex connection Host2  Host1 still open.
FIN
FIN ACK
Both half-duplex connections closed.
Both half-duplex
connections established
Deliver EOF (End Of File) to application.
Application calls close()

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Host 1 Host 2
FIN
ACK
FIN
ACK
Both half-duplex
connections establishedNormal 4-way
close:
Host 2Host 1
FIN
FIN, ACK
ACK
Both half-duplex
connections established3-way
close:
Host 1 Host 2
FIN FIN
ACKACK
Both half-duplex
Simultaneous
close:
Half-
close:
Few applications use half-close,
e.g. UNIX rsh command:
#rsh <host> sort < datafile
This command is executed remotely.
The command needs all input from
Host1. The closing of the connection
Host1 Host2 is the only way to
tell Host2 that it can start executing
the command.
The output of the command is sent
back to Host1 through the still
existing half-duplex connecion
Host2  Host1.
Host 1 Host 2
FIN
ACK of FIN
Data
ACK (Data)
Both half-duplex
FIN
ACK of FIN
5. TCP Connection Release (2/2)
 Different scenarios as to how both half-duplex connections are closed:

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TCP Header
Window size
advertisment
TCP fields involved
in flow control
ISN Initial
Sequence Number
6. TCP Flow Control (1/10)
 Sliding window mechanism:
 Unlike lock-step protocols, TCP allows
data burst for maximizing throughput.
 The receiver advertises the size of receive buffer.
 The sequence and acknowledge numbers are
per byte (not per segment/packet).
 The receiver‘s ack number is the number of the
next byte it expects; this implicitly acknowledges
all previously received bytes. Thus acks are cumulative,
Ack=X acknowledges all bytes up to and including
X-1.

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1
3-way handshake for connection establishment. Through corresponding socket calls (indicated by ‚socket()..‘) host ‚A‘ and ‚B‘
open a TCP connection (host ‚A‘ performs an active open while host ‚B‘ listens for an incoming connection request). Host ‚A‘
and ‚B‘ exchange and acknowledge each other the sequence numbers (ISN Initial Sequence Number). Host ‚A‘ has a receive
buffer for 6000 bytes and announces this with Win=6000. Host ‚B‘ has a receive buffer for 10000 bytes and announces this with
Win=10000. Note that the SYNs occupy 1 number in the sequence number space thus the first data byte has Seq=ISN+1.
2
Application Process (AP) ‚A‘ writes 2KB into TCP. These 2KB are stored in the transmit buffer (Tx buffer). The data remains
in the Tx buffer until its reception is acknowledged by TCP ‚B‘. In case of packet loss TCP ‚A‘ has the data still in the
Tx buffer for retransmissions.
3
TCP ‚A‘ sends a first chunk of 1500 bytes as one TCP segment. Note that Seq=1001 = ISN+1. These 1500 bytes are stored
in TCP ‚B‘s receive buffer.
4
TCP ‚A‘ sends a chunk of 500 bytes. Again, these 500 bytes are stored in TCP ‚B‘s receive buffer (along with the previous
1500 bytes). The sequence number is Seq=2501 = previous sequence number + size of previous data chunk.
Note that the initial 2KB data are still in TCP ‚A‘s Tx buffer awaiting to be acknowledged by TCP ‚B‘.
5
TCP ‚B‘ sends an acknowledge segment acknowledging successful reception of 2KB. This is indicated through Ack=3001
which means that TCP ‚B‘ expects that the sequence number of the next data byte is 3001 or, in other words, the sequence
number of the last data byte successfully received is 3000. Upon reception of the acknowledge TCP ‚A‘ flushes the Tx buffer.
6
AP ‚B‘ reads out 2KB with 1 socket call. Application ‚B‘ may have called ‚receive()‘ much earlier and only now
TCP ‚B‘ (the socket interface) unblocked the call and returned the chunk of 2KB data.
The 2KB data are deleted from host ‚B‘s Rx buffer.
7
TCP ‘B’ sends a pure window update segment to signal to TCP ‘A’ that the receive window size is now 10000 again (Rx buffer).
Note that a real TCP implementation would not send a window update if the Rx buffer still has reasonable free capacity.
A real TCP implementation would wait for more data to arrive, acknowledge this next data and together with the acknowledge
segment also signal the new receive window size. Only when the Rx buffer’s capacity falls below a threshold it is advisable to
send a TCP segment merely updating the window size.

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8 AP ‚A‘ writes 4KB into TCP. Shortly after application ‚B‘ writes 2KB into TCP.
9 TCP ‚A‘ sends a chunk of 1500 bytes as one TCP segment. Seq = 3001 = last sequence number + size of last data segment.
TCP ‚B‘ sends a chunk of 1500 bytes as one TCP segment. Seq = 4001 = ISN + 1 (since it is the first TCP segment with data).
Win = 8500 = Rx buffer size – size of data in buffer. Ack = 4501 = sequence number of last received segment + size of data.
TCP ‚A‘ deletes the acknowledged 1500 bytes from the Tx buffer (they are successfully received by TCP ‚B‘, even though
not necessarily received by application ‚B‘; thus these 1500 do not need to be kept in the Tx buffer for retransmissions and can
be deleted).
10
AP ‚A‘ writes another 2KB into TCP ‚A‘. These 2 KB are stored in TCP ‚A‘s Tx buffer along with previous 2.5KB of data.
Around the same time AP ‚B‘ reads 1KB of data from its socket. These 1KB are immediately freed from the Rx buffer to make
room for more data from TCP ‚A‘.
11
TCP ‚B‘ sends a chunk of 500 bytes (Seq = 5501 = last sequence number + data size of last segment). The window update in
this segment indicates that the Rx buffer has room for 9500 bytes.
12
TCP ‚A‘ sends a segment with 1000 bytes. TCP ‚B‘ writes this data into its Rx buffer there joining the previous 500 byte.
Shortly after AP ‚B‘ reads 1KB from its socket interface leaving 500 byte of data in the Rx buffer.13
14
TCP ‚A‘ sends a segment with 1000 bytes. TCP ‚B‘ writes this data into its Rx buffer there joining the previous 500 byte.
Shortly after AP ‚B‘ reads 1KB from its socket interface leaving 500 byte of data in the Rx buffer.
Thereupon TCP ‚A‘ sends an acknowledgment segment with Ack= 4001 + sizes of last 2 received data segments. This Ack
segments makes TCP ‚B‘ delete the 2KB in its Tx buffer since these are no longer needed for possible retransmissions.
15 TCP ‚A‘ sends a segment with 1500 bytes. TCP ‚B‘ writes this data into its Rx buffer there joining the previous 500 byte.
Shortly after AP ‚B‘ reads 2KB from its socket interface thus emptying the Rx buffer.

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16
TCP ‚B‘ sends an acknowledge segment acknowledging successful reception of all data so far received.
This leaves 2KB of unsent data in TCP ‚A‘s Tx buffer.
Since no data is in the Rx buffer the receive window size is at its maximum again (Win=10000).
Around the same time AP ‚A‘ reads out 2KB from the Rx buffer.
17 TCP ‚A‘ sends a segment with 1500 bytes. TCP ‚B‘ writes this data into its Rx buffer.
18
TCP ‚A‘ sends a last data segment with 500 bytes. TCP ‚B‘ writes this data into its Rx buffer there joining the previous 1500
bytes. Shortly after that AP ‚B‘ reads out 2KB and thus empties the Rx buffer.
19 TCP ‚B‘ sends an acknowledgment segment that acknowledges all data received from TCP ‚A‘.
20
AP ‚A‘ is finished with sending data and closes its socket (close()). This causes TCP ‚A‘ to send a FIN segment in order to
close the half-duplex connection ‚A‘‘B‘.
TCP ‚B‘ acknowledges this FIN and closes the connection ‚B‘‘A‘ with a FIN.
Note well that FINs also occupy 1 number in the sequence number space. Thus the acknowledgment sent back by TCP ‚B‘
has Ack=previous sequence number in ‚A‘s FIN segment + 1.
Legend:
CTL = Control bits in TCP header (URG, ACK, PSH, RST, SYN, FIN); if a bit is listed its value is 1
Seq = Sequence number
Win = Window size
Ack = Acknowledgement number
Data = Size of application data (TCP payload)

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<CTL=SYN, ACK><Ack=1001><Seq=4000><Win=10000>
<CTL=SYN><Ack=0><Seq=1000><Win=6000>
<CTL=ACK><Ack=4001><Seq=1001><Win=6000>
socket()... socket()...
send(2KB)
<Data=1500B>
<Data=500B>
<Data=0 (no data)>
Rx Buffer
Tx Buffer2K
Tx Buffer
Rx Buffer2K
Tx Buffer
Rx Buffer
2K
Tx Buffer
Rx Buffer
1.5
Rx Buffer
Tx Buffer
2K
Rx Buffer
Tx Buffer
2K
Rx Buffer
Tx Buffer
Tx Buffer
Rx Buffer
1
2
3
4
5
TCP ‚A‘AP ‚A‘ TCP ‚B‘ AP ‚B‘

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2K
1.5
4K
Rx Buffer
Tx Buffer
Tx Buffer
Rx Buffer
receive(2KB)
2K
Rx Buffer
Tx Buffer
Tx Buffer
Rx Buffer
<Data=0 (no data)>
Rx Buffer
Tx Buffer
send(2KB)
Tx Buffer
Rx Buffer
send(4KB)
2K
4K
Tx Buffer
Rx Buffer
Rx Buffer
Tx Buffer
<Data=1500B>
<Data=1500B>
6
7
8
9
10
1.5
2.5
Tx Buffer
Rx Buffer
2K
1.5
Rx Buffer
Tx Buffer

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2K
0.50.5
2K
0.50.5
4.5K
send(2KB) receive(1KB)
2K
Rx Buffer
Tx Buffer
1.5
Tx Buffer
Rx Buffer
4.5K
2K
Rx Buffer
Tx Buffer
2KTx Buffer
Rx Buffer
<Data=500B>
4.5K
Rx Buffer
Tx Buffer
2K
Tx Buffer
Rx Buffer
<Data=1000B>
4.5K
Tx Buffer
Rx Buffer
Rx Buffer
Tx Buffer
receive(1KB)
<Data=1500B>
<Data=0 (no data)>
1K
receive(2KB)
11
12
13
14
1K
4.5K
2K
Tx Buffer
Rx Buffer
Rx Buffer
Tx Buffer
15
2K

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1.5
16
2.0
Tx Buffer
Rx Buffer
<Data=0 (no data)>
Rx Buffer
Tx Bufferreceive(2KB)
2K
2.0
Tx Buffer
Rx Buffer
Rx Buffer
Tx Buffer
<Data=1500B>2.0
Tx Buffer
Rx Buffer
Rx Buffer
Tx Buffer
<Data=500B>
<CTL=ACK><Seq=6001><Ack=9001><Win=10000>
<Data=0 (no data)>
Rx Buffer
Tx Buffer
Tx Buffer
Rx Buffer
receive(2KB)
2K
<CTL=FIN, ACK><Ack=6001><Seq=9001><Win=6000>
<CTL=FIN,ACK><Ack=9002><Seq=6001><Win=10000>
17
18
19
20
close()
close()

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 Sliding window (TCP) versus lock-step protocol:
1. Lock-step protocol: sender must wait for Ack before sending next data packet.
2. Sliding window: sender can send (small) burst before waiting for Ack.
Lock-step protocol:
Sender Receiver
Data
Ack
Data
Ack
Data
Ack
Sliding window protocol:
Sender Receiver
Ack
Data
Data
Data
Data
Data
Data
Ack
Buffer
Sender
blocked
Sender
blocked
Sender
blocked

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 Sliding window mechanism:
Window size, acknowledgments and sequence numbers are byte based, not segment based.
1 2 3 4 5 6 7 8 9 10
Sending application
writes (to socket)
Sending application
writes (to socket)
Sending application
writes (to socket)
The lower window edge is
incremented as bytes are acknowledged;
it is initialized to ISN+1.
The upper window edge is
incremented by the number in window field.
It is initialized to ISN+1 + advertised window.
Send window
Bytes waiting
to be
acknowledged
Bytes
Time or
sequence number
Bytes waiting to be sent
in send buffer
Bytes sent and acknowledged Can be
sent
anytime

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 TCP layer and application interworking:
ACK and window size advertisments are „piggy-backed“ onto TCP segments.
TCP sender and receiver „process“ on a host are totally independent of each other in terms
of sequence and acknowledge numbers.
TCP ‘A’AP ‘A’
AP writes
AP reads
6
7
8
Tx Buffer
Sender
Process
3
Rx Buffer
Receiver
Process
TCP ‘B’ AP ‘B’
AP writes
AP reads
5
6
Rx Buffer
Receiver
Process
4
5
6
Tx Buffer
Sender
Process
TCP connection =
2 half-duplex
connections

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 Delayed acknowledgments for reducing number of segments:
The receiver does not send Acks immediately after the receipt of an (error-free) packet but
waits up to ~200ms/500ms if a packet is in the send buffer (depends on host OS).
If so it „piggybacks“ the Ack onto the transmit packet; if no transmit packet is available the
receiver sends an Ack latest after ~200ms/500ms.
Demo: Telnet connection with Echo:
 Without ‘delayed ack’ 4 segments per character (character, echoed character, 2 acks).
 With ‘delayed ack’ 2 segments (character, ack).
Sender Receiver
Data
Data + ACK
Up to 500ms
With delayed acks:
Data
Sender Receiver
Data
Ack
Ack
Without delayed acks:

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 Nagle‘s Algorithm for further optimization:
In interactive applications (Telnet) where single bytes come in to the sender from
the application sending 1 byte per TCP segment is inefficient (98% overhead).
When activated Nagle‘s algorithm sends only the first byte and buffers all subsequent bytes
until the first byte has been acknowledged. Then the sender sends all so far buffered bytes
in one segment. This can save a substantial number of TCP segments (IP packets) when the
user types fast and the network is slow.
Sender Receiver
ACK
With Nagle‘s algorithm:
AP
Data
Data
RTT
RTT Round Trip Time (time between sending a packet and receiving the response).
Sender Receiver
Without Nagle‘s algorithm:
AP
Data
Data
Data
Data
Data

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 Silly window syndrome:
This Problem occurs when the receiver reads out only small amounts of bytes from the receive
buffer and the sender sends relatively fast (and in large chunks). Then the flow-control
mechanism becomes inefficient and ruins TCP performance.
Solution: Clark‘s algorithm.
 The receiver only sends window updates when there is sufficient space in receive buffer
(sufficient = min(MSS, half buffer size)).
 The sender must not send „tynigrams“ (small segments).
IP packetPayloadTCPIP
TCP
segment (MSS)
MSS: Maximum Segment Size

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7. TCP Error Control (1/6)
 Out of sequence segments:
Out of sequence segments are not dropped but buffered for later use.
Sender Receiver
ACK=X+512, Win=3584
Seq=X
Seq=X+1024
Seq=X+512
SYN
SYN ACK
ACK, Win=4096
ACK=X+1536
The receiver does not throw away out-of-sequence
segment but waits a little bit for the outstanding
segment; the receiver acknowledges then all
bytes correctly (error-free, in order) received.

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 Lost packets cause retransmissions triggered by the expiry of the retransmission timer:
Host 1 Host 2
(1) SEQ = X
Both half-duplex
(1)
write (1) to RB1
ACK=X+512
1
Retransmission
timer stopped
(6) SEQ = X+2560(6)
6
network
temporarily
out of order
ACK=X+2560
6
7
8
(6) SEQ = X+2560
Retransmission
timer expires
ACK=X+3072
5 write (5) to RB
6 write (6) to RB
(6)
Retransmission
timer stopped
7
8
(7) SEQ = X+3072
7 write (7) to RB
(7)
7
8
ACK=X+3584
MSS: Max. segment
size (=512 in example).
Retransmission timer
stopped (not expired).
expired.
Packet Loss.
RB: Application’s
receive buffer

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 Lost packets / retransmissions, 3 acks („fast retransmit“):
Host 1 Host 2
ACK=X+512
(1) SEQ = X
Both half-duplex
1(1)
(2) SEQ = X+512
write (1) to RB1
(2)
1
2
(3) SEQ = X+1024
2
3
3
(4) SEQ = X+1536
2
3
4
3
4
ACK=X+512
(3)
(4)
(5) SEQ = X+2048
ACK=X+512
2
3
4
5
3
4
5
(5)
Retransmission
timer stopped
(2) SEQ = X+512 (retransmission)
3rd ACK
received
3
4
5
2
ACK=X+2560
Retransmission
timers stopped
write (2) (3) (4) (5) to RB
(6) SEQ = X+2560
6
(6)
6
MSS: Max. segment
expired.
Packet Loss.
RB: Application’s
receive buffer

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 Lost packets / retransmissions, lost ack:
Host 1 Host 2
(1) SEQ = X
Both half-duplex
write (1) to RB1
ACK=X+512
1
(1)
Retransmission
timer stopped
(6) SEQ = X+2560(6)
6
ACK=X+2560
ACK=X+3072
5 write (5) to RB
6 write (6) to RB
Retransmission
timer expires (6) SEQ = X+2560
6 discard (6); return ACK
ACK=X+3072
(6)
6
MSS: Max. segment
expired.
Packet Loss.
RB: Application’s
receive buffer
 TCP does not specify how often a specific segment has to be retransmitted (in case of
repeated packet loss of the same segment). Typical values are max. 5 or 8 retransmissions of the
same segment (if more retransmissions necessary connection is closed).

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Solution:
Constantly adjust RTO based on measurement of RTT. For each segment that was sent start a
(retransmission) timer.
RTO = RTT + 4 * D
where D is mean deviation as per Dnew=αDold + (1- α) * |RTT – M|
M = observed RTT value for a specific ACK
α = smoothing factor that determines how much weight is given to the old value (typ. α=7/8)
 Retransmission timer (RTO Retransmission Timeout) assessment:
Problem:
a. Single data link  RTO = (2 + e)*RTT where e<<2 (RTT rather deterministic).
b. Internet (multiple data links)  RTT varies considerably, also during lifetime of a TCP conn.

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 TCP Header Checksum Calculation:
TCP uses a 16-bit checksum for the detection of transmission errors (bit errors). The checksum
field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the
pseudo IP header, TCP header and data (the checksum field is initialized with zeroes before
the checksum calculation). By including the IP header in the checksum calculation TCP depends
on IP; it may not run on anything other than IP (this is a violation of the layering principle!).
Pseudo
header
Checksum calculated
over pseudo header,
TCP header and
data

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8. TCP Congestion Control RFC2001 (1/6)
 Congestion control is a control (feedback control) mechanism to prevent congestion
(congestion avoidance).
Problem (a): A fast network feeding a low capacity receiver (congestion in receiver).
Problem (b): A slow network feeding a high-capacity receiver (congestion in network).
Aggregate bandwidth
exceeds available
bandwidth of output link.
Packets dropped
by router 3.
Congestion can always occur (aggregate ingress
Bandwidth exceeds egress bandwidth).
Router 1
Router 2
Router 3

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Prior to 1988 TCP did not define a congestion control mechanism. TCP stacks just sent as
many TCP segments as the receiver’s buffer could hold (based on advertise window). With the
growth of the Internet router links became congested and thus buffers got full which resulted
in packet loss. Packet loss lead to retransmissions which in turn aggravated the congestion
problem.
 It became necessary to avoid congestion in the first place (it’s always better to
avoid a problem in the first place rather than cure it!).
 How to avoid congestion efficiently (ideas of Van Jacobson):
a. Sender can detect packet loss and interpret it as network congestion (this is in
most cases true; packet loss can however also be due to bit errors).
b. When a sender detects packet loss it should reduce the transmission rate quickly.
c. When there is no (more) congestion (no more packet loss) the sender can slowly
increase the transmission rate.
e. At the very beginning of a TCP session a sender does not know the maximum
possible transmission rate yet. Thus it should start slowly with sending segments
(„slow start“ procedure).
f. If a sender receives an ACK packet it knows that a segment has been received and
is no longer on the network (in transmission). This fact is used for the slow start
algorithm (see below).  This procedure is „self-clocking“.
 Network stability is achieved through quickly reacting to bad news (packet loss), moderately
reacting to good news (no packet loss, all segments sent are acknowledged).

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Rev. 3.70
 Congestion control window:
A congestion control window is used in addition to the flow control window (both windows
work in parallel).
The max. number of segments that can be sent = min(Ws, Wc).
Ws = Flow control window
Wc = Congestion control window
Heavily loaded networks  flow of segments controlled by Wc.
Lightly loaded networks  flow of segments controlled by Ws.
4 5 6 7 8 9 10
Bytes waiting to
be acknowledged
in send buffer
Bytes sent and acknowledged
Can be
sent
anytime
321

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Rev. 3.70
 Normal congestion control window procedure: Slow start phase:
Wc starts opening „slowly“ from 1 segment
until threshold (SST) reached (exponential
growth of Wc).
Transmission number:
0  1 segment sent
1  2 segments sent (burst)
2  4 segments sent (burst)
etc.
Congestion avoidance phase:
Wc is increased by 1/Wc (linear growth)
until next threshold reached.
Constant phase:
Wc remains constant.
32
16
8
4
2
12345
Slow start
phase
64
Congestion avoidance
phase
Constant phase
(Wc fully open)
RTT
Wc remains constant
Wc increases by 1 segment for each 32 ACKs received
Wc increases by 1 segment for each ACK received (exponential growth)
Wc
segments or kbytes
Slow start threshold
3710 20 30

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Rev. 3.70
 Light congestion (reception of 3 duplicate Acks):
On receipt of 3 duplicate ACKS Wc is
reduced by half.
32
16
8
4
2
12345
64
RTT
Wc
segments/kbytes
3710 20 30
First set of 3 duplicate
ACKs received
Second set of 3 duplicate
ACKs received
Wc=34
Wc=17
Fast Recovery

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Rev. 3.70
 Heavy congestion (no reception of Acks):
When retransmission timer expires Wc
is immediately reset to 1. From there
slow-start restarts normally.
32
16
8
4
2
12345
64
RTT
Wc
segments/kbytes
3710 20 30
First RTO
Second RTO
Third RTO

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Rev. 3.70
9. TCP Persist Timer (1/2)
 Problem: Possible deadlock situation in TCP flow control:
The receiver buffer is full thus the receiver sends an ACK with Win=0 (window is closed).
Thereafter data is read from the receiver buffer to the application. The receiver is ready again
to receive data and sends an ACK with Win>0 (window is re-opened). This segment however is
lost. Sender and receiver are now deadlocked: sender waits for the window to be re-opened,
the receiver waits for data.
Host 1 Host 2
Both half-duplex
ACK=X+2048, Win=0
3
4
5
6
4
5
6
write (3) to RB
ACK=X+2048, Win=512
sender and receiver
deadlocked
Packet Loss

41/51
Rev. 3.70
9. TCP Persist Timer (2/2)
 Solution: The sender sends a probe segments containing 1 single byte of data to invoke the
receiver to acknowledge previous bytes again.
Persist timer values are ascertained by a „exponential backoff algorithm“ that produces output
values from 5s (min.) to 60 (max.) seconds.
Host 1 Host 2
Both half-duplex
ACK=X+2048, Win=0
Persist
timer expires
3
4
5
6
4
5
6
write (3) to RB
ACK=X+2048, Win=512
(1)
Window Probe
SEQ=X+2048 (1 byte)
ACK=X+2049, Win=2048
SEQ=X+2049
Persist
timer stopped
7
Packet Loss

42/51
Rev. 3.70
10. TCP Keepalive Timer – TCP connection supervision (1/2)
 The keepalive timer is used to periodically check if TCP connections are still active.
Case 1: TCP connection is still alive (i.e. client is still alive and the connection open).
Host 1 (Client) Host 2 (Server)
Both half-duplex
Keep alive
timer expires
ACK=Y
Data Segment
2 hours
Keep Alive Probe
No data
SEQ=X-1
75s
ACK=X
Keep alive
timer stopped,
rearmed to 2h
2 hours
Server starts
keepalive timer

43/51
Rev. 3.70
10. TCP Keepalive Timer – TCP connection supervision (2/2)
 The keepalive timer is used to periodically check if TCP connections are still active.
Case 2: The TCP connection dead.
Host 1 (Client) Host 2 (Server)
Both half-duplex
Keep alive
timer expires
ACK=Y
Data Segment
2 hours
Keep Alive Probe 1
No data
SEQ=X-1
75s Keep alive
timer expires
75s Keep alive
timer expires
75s
Keep alive timer expires;
the connection is deleted
(not closed since
it is considered dead)
Keep Alive Probe 10
No data
SEQ=X-1
Keep Alive Probe 2
No data
SEQ=X-1
Keep Alive Probe 3
No data
SEQ=X-1

44/51
Rev. 3.70
11. TCP Header Flags (1/2)
 PUSH flag:
Problem: Segment size and time of transmit are determined by TCP (flow control, congestion
control, TCP does internal buffering). This is unsuitable for interactive applications like
X Windows, TELNET where data should be sent as soon as possible.
 Solution: With PUSH flag data can be expedited:
Sender: Send data immediately without further buffering.
Receiver: When receiving PUSH flag „pushes“ all buffered data to the
application (no further Buffering).
Sender Receiver
Data, PUSH=1
Ack
Data, PUSH=1
Ack
Send(data,PUSH)
Write immediately to
Application process
(no buffering in TCP)
Send(data,PUSH)
Write immediately to
Application process
(no buffering in TCP)

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Rev. 3.70
11. TCP Header Flags (2/2)
 URGENT flag:
This flag allows a sender to place urgent (=important) data into the send stream; the
urgent pointer points to the first byte of urgent data; the receiver can then directly jump to
the processing of that data; example: Ctrl+C abort sequence after other data.
Sender Receiver
Send(data)
URGENT=1 indicates that
the urgent pointer in the header is
valid;
The receiving application process
may then process urgent data
(e.g. abort command).
Send(data,URGENT)
Data, URGENT=1
TCP header:
URG=1
URGPTR=2219
Urgent
data
Seq.# = 2144Seq.# = 2219
TCP segment

46/51
Rev. 3.70
12. TCP Header Options (1/3)
 Window scale option:
Problem: High-speed networks with long delay; network (transmission path) then stores large
amounts of data („long fat pipe“).
E.g. 155Mbps line with 40ms delay  775Kbytes are underway in the transmission path
but the maximum window size is restricted to 65535 bytes (16 bit field).
Solution: With the window scaling option the window size can be increased (exponentially)
thus making the window large enough to „saturate“ the transmission path.
Lower window edge
incremented as bytes are acknowledged
initialized to ISN+1
Upper window edge
incremented by number in window field
initialized to ISN+1 + advertised window
Send window
Bytes waiting to
be acknowledged
Byte stream
in send buffer
Can be
sent
anytime
Required window size to fully utilize link bandwidth = Blink * Delaylink
e.g. long intercontinental fiberoptic line 155Mbps * 40ms = 775Kbytes
Max. window size = 65535 (16bit)
Bytes sent and
acknowledged
TCP Header
1=nop kind=3 len=3 shift count
Window size field remains unchanged,
but is scaled by window scale option
E.g. shift count=3:  Ws * 23 = Ws * 8
E.g. shift count=0:  Ws * 20 = Ws * 1

47/51
Rev. 3.70
 Timestamp option:
Problem: RTO calculation would require a recomputation for each TCP
segment sent. Some TCP implementations recompute the RTO only once per
window (less processing power required). This is ok for slower links where
the sliding window contains only few TCP segments. On faster links this
behavior leads to inaccurate RTO computations (and thus less-than-optimal
flow control). The TCP timestamp option allows to accurately ascertain the RTO.
Enable timestamp and window scale option on Windows:
Registry: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesTcpipParametersTcp1323Opts = 3
Sender Receiver
ACK=X+1024; TS=T1
SEQ=X; TS=T1
SYN; TS option request
SYN ACK; TS option ack
Option
negotiation
store TSrecent = T1SEQ=X+512; TS=T2 1
1
2
T1
RTT = T3 – T1
T2
SEQ=X+1024; TS=T4
write (1) (2) to RB
3
T4
T3
store TSrecent = T4
1=nop 1=nop kind=8 len=10
timestamp value
timestamp echo reply
Timestamp option in
TCP header

48/51
Rev. 3.70
Host 1 Host 2
ACK=X+512
(1) SEQ = X
(2) SEQ = X+512
(3) SEQ = X+1024
(4) SEQ = X+1536
ACK=X+512
SACK={X,X+511}, {X+1024,X+1535}
(5) SEQ = X+512
(6) SEQ = X+1536
SYN; SACK-permitted option
SYN ACK
ACK
1=nop 1=nop kind=4 len=2
 SACK permitted option (selective retransmissions):
Receiver can request sender to retransmit specific segments (due to loss).
The SACK does not change the meaning of the ACK field; if the SACK option is not supported
by the sender TCP will still function (less optimally though in case of retransmissions).
kind=5 len relative origin
SACK-permitted option
SACK option
block size
Selective retransmissions of missing segments.
Packet Loss
Receiver signals contiguous blocks of successfully
received segments:
{X,X+511} = {left edge of data block, right edge of data block}

49/51
Rev. 3.70
13. TCP Throughput / performance considerations („goodput“) (1/2)
 TCP is delay sensitive!
max. throughput = Ws / RTT [Padhye 98]
This is not an exact formula and it is valid only for average RTT values.
The maximum throughput is bound by the window size Ws and decreases with increased
RTT (=delay). TCP as such is NOT suited for networks with long delays (e.g. satellite and
interplanetary links).
 TCP is not independent of the underlying network (as should be the case in theory)!
TCP was designed to run over wired networks (low Bit Error Rate BER, packet loss mostly
due to congestion). TCP performs badly on radio links (high BER, packet loss due to errors).
In case of packet loss on a radio link the sender should try harder instead slowing down.
Slowing down just further decreases the throughput.
On wired networks the sender should slow down in case of packet loss (caused by
congestion) in order to alleviate the problem.
How to handle TCP connection that spans a wired and a radio link?
 „Split TCP“: Effectively 2 separate TCP connections interconnected by the base station
passing TCP payload data between the connections. Each of the TCP connections is optimized
for their respective use. Example:
TCP accelerator devices for satellite links.

50/51
Rev. 3.70
13. TCP Throughput / performance considerations („goodput“) (2/2)
TCP adaptations for radio (satellite) links as per RFC2488, RFC3481:
a. Data link protocol:
Use data link protocols that do not do retransmissions and flow control or make sure that
retransmission mechanism and flow control of data link do not negatively affect TCP performance.
b. Path MTU discovery:
Path MTU discovery enables TCP to use maximum sized segments without the cost of
fragmentation/reassembly. Larger segments allow the congestion window to be more rapidly
increased (because slow start mechanism is segment and not byte based).
c. Forward Error Correction:
TCP assumes that packet loss is always due to network congestion and not due to bit
errors. On radio links (satellite, microwave) this is not the case – packet loss is primarily
due to bit errors. The congestion recovery algorithm of TCP is time consuming since it
only gradually recovers from packet loss. The solution is to use FEC to avoid false signals
as much as possible. FEC means that the receiver can detect and correct bit errors in the
data based on a FEC code.
d. Selective ACKs (SACK):
The TCP algorithms „Fast Retransmit“ and „Fast Recovery“ may considerably reduce TCP
throughput (because after a packet loss TCP gingerly probes the network for available
bandwidth; in a radio environment this is the wrong strategy). To overcome this use
selective ACKs (SACK) option to selectively acknowledge segments.
e. Window scale option:
Long delay in fast transmission links limits TCP throughput because large amounts of data are
underway but TCPs sliding window only allows up to 65536 bytes to be „in the air“.
To overcome this use the window scaling option thus increasing the size of the sliding window.

51/51
Rev. 3.70
14. A last word on „guaranteed delivery“
 TCP provides guaranteed transmission between 2 APs. That‘s it. There are
still zillions of reasons why things can fail! Thus it is still up to the application to
cater for application error detection and error handling (see RFC793 chapter 2.6 p.8).
Packet Loss
Host 1 Host 2
both half-duplex
TCP delivers data safely
and error-free between APs
(not less, not more)
Data gets lost/corrupted on its
way to the application process
(AP).
(1) SEQ = X
1
1
ACK=X+512
App
App
(1) SEQ = X
1
1
ACK=X+512
App
App
Data gets safely to the
application’s storage, but then is
overwritten (by some other
process, the app itself, hardware
failure ...).
(1) SEQ = X
1
1
ACK=X+512
App
App
The application starts a
transaction to store the data into
a database, but the transaction
fails.

TCP - Transmission Control Protocol

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