Lecture 3: The Transport Layer: UDP and TCPshervin/courses/ceg4185/lecture… · ·...

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Prof. Shervin Shirmohammadi CEG 4185 3-1

Lecture 3:

The Transport Layer:

UDP and TCP

Prof. Shervin Shirmohammadi

SITE, University of Ottawa


The Transport Layer

• Provides efficient and robust end-to-end service to upper layers.

– Not all services need to be provided!

• Uses services provided by the network layer.

• End-to-end delivery of entire message.

Connection

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End-to-End Delivery

• Services are done from an end-to-end perspective

– Error control, flow control, …

• Connectionless or connection-oriented

…

Transport layer

Networking layer


Addressing• Service Access Point (SAP), also referred to as Ports at

the transport layer.

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Reliable Delivery

• Note that it is not compulsory for a transport layer protocol to provide such services.

• The idea is that if any of these services are to be provided in an end-to-end sense, then the transport layer is where you would implement them.


error checked

at networking

error checked

at networking

error checked

at networking

Error Control

• Error checking is done for the Transport

Layer PDU (T-PDU).

• Even though T-PDU is put into the N-PDU

as payload, the networking layer may or

may not provide error control for it.

– What does IP do?

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Sequence Control


Loss Control

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Duplication Control

What can cause duplication?


Flow Control• Controls the flow of information, again from an end-to-end perspective.

– Factors affecting this are the end host, but also the network.

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Multiplexing and Demultiplexing

Data Link Networking

Transport


App1 App2 App3

TCP UDP123 250 125

IP

App 3 dataSPort=125

data data data

App 2 dataSPort=250

App 1 dataSPort=123

TCP dataProtocol=TCP


UDP dataProtocol=UDP

to data Link layer

Multiplexing


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App1 App2 App3

TCP UDP560 440 1090

IP

App 3 dataDPort=1090

data data data

App 2 dataDPort=440

App 1 dataDPort=560



UDP dataProtocol=UDPFrom data Link layer

Demultiplexing



Connection

• Obviously required only if we’re talking

about a connection-oriented transport

protocol.

– Otherwise just send the packet and hope for the

best.

• Need two protocols really:

– connection establishment

– connection release

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User Datagram Protocol (UDP)• Defined in RFC 768.

• Connectionless service

• Multiplexing/Demultiplexing

• Error Detection (not really error control)

– Detection only, no recovery; and even the detection is optional

• No flow control

• No congestion control

• What does it do then?

• Used by SNMP, DNS, RTP, Multicasting.


Multiplexing/Demultiplexing

• Uses Port numbers

– Also used for addressing

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13: daytime 53: name server

69: TFTP 111:RPC

161 SNMP

ICANN Ranges

Internet Corporation for Assigned Names and Numbers

http://www.icann.org/


UDP Packet Format

• There is also a pseudo-header that is created

and computed. The pseudo-header does not

really exist as a header!

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Pseudoheader

• This is done to detect wrong delivery of the packet by the

Networking layer.


Checksum

One’s complement of the one’s complement sum modulo

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Multicasting

• The ability to send a packet to more than one

destination. (send once, receive at multiple

destinations).

• Uses the IP multicasting (class D) addresses.

• Interested parties must join a multicast group.

• Network infrastructure (routers, bridges, …)

must support IP Multicast for this to happen

• Mostly not supported on the Internet.


Transmission Control Protocol (TCP)

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TCP - Transmission Control Protocol

• Provides reliable end-to-end byte stream over unreliable IP subnet.

• Designed to dynamically adapt to properties of subnet (bandwidth, delay, loss) and for robust handling of failures.

• Initial definition in standard RFC 793. Updates in RFC 1122, RFC 1323, RFC 2001

• TCP accepts user data from local processes, splits into pieces no larger than 64K bytes (usually about 1500 bytes in practice), and sends each piece as a separate IP datagram. When IP datagrams arrive at the destination, TCP reconstructs the original byte stream.


The Service TCP Provides

• Connection-oriented

• Connection release

• Point-to-point communication

• Addressing, Multiplexing and Demultiplexing

• Complete reliability

• Full-duplex communication

• Reliable connection startup and shutdown– 3-way handshake

• Flow Control

• Congestion Control

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Port Numbers


Reserved TCP Port Numbers

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TCP Error Control

• Sources of error:

– Segment damaged in transit

– Segment fails to arrive

• Transmitter does not know of failure

• Receiver must acknowledge successful receipt

• Too many ACKs if sent for each segment

– Use cumulative acknowledgement

• Time out waiting for ACK triggers retransmission (retransmission timer)


Duplication Detection

• If ACK lost, segment is re-transmitted

– Causes duplication

• Receiver must recognize duplicates

• Duplicate received prior to closing connection

– Receiver assumes ACK lost and ACKs duplicate

– Sender must not get confused with multiple ACKs

– Sequence number space large enough to not cycle within maximum life of segment

• Duplicate received after closing connection

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Flow Control: Credit Allocation


Congestion

• Caused by too much traffic going through a network;

more than the network can handle.

• Results in Routers dropping packets.

• Causes time-outs: equally likely from lost messages due

to unreliable transmission media, as from congestion.

• Simply retransmitting a lost message makes congestion

worst.

– Why?

• There needs to be a way to control congestion.

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TCP Congestion Control

• RFC 1122, Requirements for Internet hosts

• Retransmission timer management

– Estimate round trip delay by observing pattern

of delay

– Set time to value somewhat greater than

estimate

– Simple average

– Exponential average

– RTT Variance Estimation (Jacobson’s

algorithm)


Window Management• Slow start

– awnd = MIN[credit, cwnd]

– Start connection with cwnd=1

– Increment cwnd (double) with ACK, to some threshold

– Increment cwnd by 1 with ACK after the threshold

• Dynamic windows sizing on congestion

– When a timeout occurs

– Set slow start threshold to half current window

• ssthresh=cwnd/2

– Set cwnd = 1 and slow start until cwnd=ssthresh

• Increasing cwnd by 1 for every ACK

– For cwnd >=ssthresh, increase cwnd by 1 for each RTT

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TCP Slow Start


TCP Data Transfer• Connection is a stream of bytes, not messages.

– Four messages of 512 bytes may be sent by TCP as any combination of pieces totaling 2048 bytes:

• 1 × 2048 bytes

• 2 × 1024 bytes

• 4 × 512 bytes

• 8 × 256 bytes

• ...

• When application sends data to TCP, it may not be sent immediately:

– TCP may decide to collect more data before transmission.

– TCP PUSH flag is used to request no transmission delay.

• TCP URGENT flag immediately sends all pending data, and

causes receiver to send an interrupt to its user.

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TCP Header


Control Field

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TCP Header Fields• Source and destination ports: 16 bit address of local port

(socket).

• Sequence and acknowledgment numbers:

– Every byte is numbered in a TCP stream.

– Acknowledgment number is next byte number expected.

– 32 bits each.

• Header length:

– Needed because options field can vary in length.

– Number of 32 bits words in header.

• URG: set to 1 if urgent pointer in use

– Pointer indicates offset from current sequence number at which urgent data ends.

– Meaning that the receiving program should be notified of its arrival as soon as possible.


TCP Header Fields (cont’d)• ACK: Set to 1 to indicate acknowledgment number is valid

– If 0, no acknowledgment in this segment.

• PSH: Set to 1 to indicate pushed data.

– Force delivery of bytes currently in the stream without waiting for buffer to fill.

• RST: Set to 1 to indicate reset.

– Host has become confused due to crash or for other reason.

– Also used to reject a connection, or refuse an invalid segment.

• SYN: used to establish connections.

– SYN = 1, ACK = 0 in connection request.

– SYN = 1, ACK = 1 in connection acceptance.

• FIN: set to 1 to indicate end of user data.

– Used to close connection.

– May continue to receive data.

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TCP Error Checking

• Checksum field provides error detection information for TCP segment header, plus the pseudo-header shown above.

• Checksum computation:

– Set checksum field to all zeros.

– Pad user data with extra 0 byte, if needed, so that user data has an even number of bytes.

– Add all 16 bit words in 1’s complement, and take 1’s complement of the sum.

– When receiver performs this computation, including checksum field, result should be 0.

TCP segment length00000000

Destination address

Source address

32 bits

Protocol = 6

TCP “Pseudo-header”


Additional Timer Issues• TCP persistence timer:

– Prevents deadlock when current window size is 0, and acknowledgment increasing window size is lost.

– When persistence timer expires, ask receiver for window size.

• TCP keep alive timer:

– Timer expires after long interval with no messages.

– On expiry, send a message to receiver asking “are you still there?”

– Connection terminated if no response.

• TCP close timer ensures all packets die on connection termination.

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