Transport Layer - Mississippi State Universityweb.cse.msstate.edu/~ramkumar/TLayerFeb18.pdf ·...

AL-TL InterfacesTransport Layer

TCPUDP

Transport Layer

Mahalingam RamkumarMississippi State University, MS

February 28, 2018

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TCPUDP

Sockets

Interfaces for networked processes/applications

A socket is identified by a protocol (TCP/UDP), IP address,and port number

A socket is actually the interface between an application andthe transport layer

TCP: assured delivery in the right order

UDP: best effort for each datagram. no ACKs

UDP does not provide much on top of IP. IP layer identifieshosts. UDP header has port number info to identify process inthe host.

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Sockets

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UDP Client and Server

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TCP Client and Server

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Listening and Connected Sockets

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TCPUDP

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

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TCPUDP

Connection Oriented Service

Reliable delivery of data

In the right order

Acknowledged delivery

Hide implementation details from top layer

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TCPUDP

Interface Provided to Application Layer

listen(), accept()

connect()

send()

recv()

close()

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Transport Ports

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Port Inversion

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TCP HeaderCongestion ControlTimer 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 bytessent/received) on TCP header.

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Interpretation of SYN/ACK/FIN flags, seq/ack number

If SYN flag set: sequence number is the starting sequencenumber

If SYN is not set: sequence number is the byte count for firstdata byte included the packet

If ACK flag is set acknowledgement number is valid; indicatesnext byte expected

FIN set for closing a connection

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Three-Way Handshake

For establishing a connection

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.

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3-Way Handshake

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Four-Way Handshake

For tearing down two unidirectional connections

What if one side is not ready to disconnect (not finishedsending)?

Four-way handshake

A→ B: FINB → A: ACK, ack number (next byte expected + 1)B → A: FINA→ B: ACK, ack number (next byte expected + 1)

FIN — ACK, FIN — ACK (four packets) or

FIN — ACK/FIN — ACK (three packets)

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4-Way Handshake

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Diving TCP Payload

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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 (1000byte payload)

S → C : ack 1001 (the next byte I am expecting from you is1001).

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 probablybecause it did not receive the third packet from the client).

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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)

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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)

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SYN/ACK Flags with SEQ/ACK numbers

CLIENT SERVER

→ 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)

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All together

CLIENT SERVER

→ 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)

connect()accept()

.

.

.

send() recv()

shutdown(WR)

send()recv()

recv()

close()

close()

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TCP Header

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TCP Header Fields

Source port and destination port

Sequence Number - chosen randomly for the first packet(connection establishment). In subsequent packets containingdata, it indicates the offset of the first (application data) byteincluded in the packet.

Acknowledgment number indicates next byte expected (offsetby 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. Actuallyindicates start of application data.

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TCP Header - FLAGS

Six unused bits

8 flags, 6 old ones URG, ACK, PSH, RST, SYN, FIN + 2 new(CWR,ECE)

PSH, URG are not used a lot in practice (and therefore theurgent pointer too)

SYN used for connection establishment

FIN used for disconnect

ACK is used during connection setting-up, acknowledgementof data received, and during connection-tearing-up

RST used to indicate rejection of connection

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TCP Header - Window Size

16 bits.

Window size is the number of bytes the receiver is willing toaccept.

Window size in A’s TCP packet to B says the maximumwindow size B can use for sending data to A (and vice-versa)

The receiver determines the window size for each one-wayconnection (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 toresume sending.

If A has advertised a window size of 500 and the lastacknowledged byte is 1000, B cannot send beyond byte 1500to A.

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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

0 0 0 0 0 0 0 0 Protocol = 6 TCP segment length

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Checksum - Some Finer Points

TCP segment does not include length

IP layer (in the final destination) passes this info to TCPlayer, along with the entire TCP packet

TCP layer is also provided with the Source and Dest IPaddresses by the IP layer

All three needed to validate checksum

Violates protocol hierarchy

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TCP Header - Options

Specify maximum TCP payload - used very frequently.

SRP instead of GBN - possibility of including NACKs beforesending ACK - RFC 1106

Scaling Window size

For high bandwidth / high delay (or both) 64-kB window isnot sufficient.On a T3 line 12msec to output 64kBIf RTT is 50 msec, nothing can be sent for 38 msec.RFC 1323Allows left shift of window size upto 14 bits - max window size230.

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PSH and URG

PSH - a way for applications to request TCP layer to stopbuffering packets and just send the data out.

A set PSH flag indicates to the receiver that the packetshould 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 thatneeds to be attended to urgently - like a software interrupt.

Urgent pointer indicates offset.

Once again, at the receiver side, the TCP layer delivers thedata to the application ASAP and indicates the offset.

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PSH Flag

PSH flag is actually used a lot in telnet

How many bytes are exchanged between A and B for sendingone character?

For each character, 20 bytes IP header + 20 bytes TCPheader + 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

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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 SYNpacket

Server B allocates 4000 bytes. Indicates this by settingwindow size 4000 in the SYN-ACK packet.

Window size indicates how many bytes beyond the lastacknowledged byte can be sent.

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Window Management ...

After connection establishment, A sends 1000 bytes to B

B stores the bytes in the buffer - has only 3000 bytes of freebuffer.

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. Bufferis now empty. B sends a new “window advertisement” to Awith window size 4000.

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Window Management

Application does a 2K write

Application does a 2K write

Application reads 2K

Sender is blocked

Sender may send up to 2K

Receiver's buffer

0 4K

2K

2K

Empty

Full

2K SEQ = 0

2K SEQ = 2048

1K SEQ = 4096

ACK = 2048 WIN = 2048

ACK = 4096 WIN = 0

ACK = 4096 WIN = 2048

2 K1K

Sender Receiver

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Silly Window Syndrome

Application reads 1 byte

Window update segment sent

New byte arrives

Header

Header

Receiver's buffer is full

Receiver's buffer is full

Room for one more byte

1 Byte

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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 howmany bytes to buffer.

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TCP Fast Retransmission

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TCP Retranmission Scenario 1

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TCP Congestion Control

Hosts realize that there is congestion when packets are notacknowledged

Could occur due to two reasons1 Low capacity receiver at the other end2 Network congestion

The first is handled during the connection establishmentprocess - using window sizes

The second?

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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 windowWC 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 timergoes 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.

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TCP Congestion Window

Failure is detected if 3 duplicate ACKs received or time outoccurs.

Additional parameter, threshold Window size, initially set to64 kB

Exponential increase, only till the threshold is released.

After this increase is linear, for example 64 MTU to 65, 66and 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 1MTU (TCP Reno, slow recovery)

ICMP source quench packets (we will see this later in thenetwork layer) are also treated as time-outs for this purpose.

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TCP Slow Start

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Tahoe and Reno

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TCP AIMD

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ECN - Explicit Congestion Notification

RFC 3168. ECN requires explicit cooperation between TCPand IP layers

Router experiencing congestion sets some ECN bits in the IPheader

Receiving host (at the other end of the TCP connection)echoes this to the sender in the header of a TCP ACKsegment

TCP sender reduces congestion window size by half, sets CWR(congestion window reduced) bit in the next TCP packet

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Explicit Congestion Notification

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TCP Timers

Retransmission Timer - decides timeout

How? Need to estimate RTT. Chicken and Egg Problem?

Not really - use a large timeout initially!

T T1 T20.3

0.2

0.1

00 10 20

Round-trip time (msec)

(a) (b)

Pro

babi

lity

0.3

0.2

0.1

0

Pro

babi

lity

30 40 50 0 10 20Round-trip time (msec)

30 40 50

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Estimating Round-Trip Time (RTT)

Jacobson’s algorithm1 Time for receiving acknowledgement M2 RTT = αRTT + (1− α)RTT3 α = 7/8. Timeout is βRTT . How do we choose β?4 β should be based on the standard deviation of the RTT

estimate.5 Estimate of standard deviation

D = αD + (1− α) | RTT −M |.6 Timeout = RTT + 4× D

What happens when time outs occur? Is the ACK for theoriginal or for the re-transmission?

Making a wrong judgment here could affect the estimate verybadly!

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TCPUDP


Karn’s Algorithm

Do not update RTT for segments that have beenretransmitted

Whenever timeout occurs double RTT

Keep doing it till timeout does not occur!

Used in most TCP implementations (TCP does not specifywhat to do).

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Two more Timers!

TCP uses 3 timers - retransmission, persistence andkeep-alive

Persistence Timer - for prevention of deadlock if windowsize re-advertisement is lost.

When persistence times goes off, sender transmits a probe(even though receiver had previously informed the sender tostop sending by advertising window size 0), and receiver sendsthe 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 issent to detect if the other side is still alive. If so timer isstarted again (else connection is terminated).

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UDP

Does very little over what IP does

Provides port numbers

No flow control, error control, retransmission

Applications have to do everything

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UDP

Checksum similar to TCP

DNS

RPC (Remote procedure calls). eg gethostbyname()

The need for 3 packets for establishing connection and 3 or 4for tearing them down makes TCP very expensive for manyapplications

Especially when networks are often highly reliable.

Multicasting - TCP cannot be used.

UDP is used for RTP (real-time transport protocol)

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TCPUDP

Transactional TCP

TCP has high overhead for scenarios where the transaction issimple (small request, small response, like DNS)

UDP is efficient but highly insecure

Even servers that did not actually receive the query canpretend 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 connectionestablishment, acknowledged delivery of data and termination!

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TCPUDP

Transactional TCP

Client Server

SYN

SYN, ACK(SYN) 2

6

2

7

8

3

1

345

9

1

ACK(SYN)

ACK(FIN)

Time

Client Server

SYN, request, FIN

ACK(FIN)

(a) (b)

SYN, ACK(FIN), reply, FIN

Timerequest

FIN

ACK(request + FIN)

FIN

reply

Ramkumar TL

Transport Layer - Mississippi State Universityweb.cse.msstate.edu/~ramkumar/TLayerFeb18.pdf ·...

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