EEC-484/584 Computer Networks Lecture 7 Wenbing Zhao [email protected] (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer

EEC-484/584EEC-484/584Computer NetworksComputer Networks

Lecture 7Lecture 7

Wenbing ZhaoWenbing Zhao

[email protected]@ieee.org(Part of the slides are based on Drs. Kurose & (Part of the slides are based on Drs. Kurose &

RossRoss’’s slides for their s slides for their Computer Networking Computer Networking book)book)

22

Spring Semester 2009Spring Semester 2009 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao

OutlineOutline

• Sliding window protocols

• UDP

• TCP– Segment header structure– Connection management

33


Sliding Window ProtocolsSliding Window Protocols

• Full-duplex: Use same connection for data in both directions (AB and BA)

• Interleave data and ack packets– B piggybacks its ack for A’s packet onto B’s next packet– Savings of header in separate ack packet

• If B sends data infrequently, use timeout to determine when B should send ack in separate ack packet

• Each packet contains sequence number in ranges 0..2n-1 (for n-bit sequence numbers)

44


Sending WindowSending Window• Start from empty and grow to a maximum size• Within sending window, packets sent but not acked

– Sender must keep those packets for possible retransmission– If max window size = w, need w buffers

0 1 2 3 4 5 6 7

1st outstanding packet

Last packetsent

0 1 2 3 4 5 6 7

A new packet sent (if send window allows)

Sent window enlarges when more packet is sent

When new packet arrives from application layer, it is given

next highest sequence number, and upper edge of

window is incremented

55


Sending WindowSending Window

0 1 2 3 4 5 6 7

3ack

0 1 2 3 4 5 6 7

Sent window shrinks when the ack corresponding to the 1st outstanding packet Is received

When ack arrives from receiver, lower edge of window is incremented

In general, ack is cumulative

66


Receiving WindowReceiving Window

• List of consecutive sequence numbers of packets that receiver is permitted to accept

• When packet with (seq num = lower edge of window) arrives– Packet is passed to higher layer– Ack is generated– Window slid down by 1 (remains same size as was initially)

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

3

77


Reliable Data Transfer:Reliable Data Transfer:Sliding Window ProtocolsSliding Window Protocols

• A One-Bit Sliding Window Protocol• A Protocol Using Go Back n• A Protocol Using Selective Repeat

88


One-Bit Sliding Window ProtocolOne-Bit Sliding Window ProtocolA sliding window of size 1, with a 3-bit sequence number

Initially After first packet sent

After first packet received

After first ack received

99


Sliding Window Protocols: PipeliningSliding Window Protocols: Pipelining

• Problem of one-bit sliding window protocol:– Sender blocks till receives acks

• Solution: pipelining– Allow sender to send up to w packets before blocking

• With pipelining, if packet in middle is lost or damaged, what to do with the packets following it ?

• Solution: two strategies– Go Back n - all the packets following it are discarded– Selective repeat – nack the lost/damaged packet and

retransmit that packet

1010


Pipelining and Error RecoveryPipelining and Error Recovery

Go back n: Effective receiver window size is 1

Packets discarded

1111


Pipelining and Error RecoveryPipelining and Error Recovery

Selective repeat

Can you think of an alternative to go-back-n and selective-repeat?

Packets buffered

1212


Go Back Go Back nn

• Sender– Stores all packets in output buffer– Must get acks in order in which packets are sent

• Receiver– Discards all packets following lost or damaged one

• Works well – If transmission errors rare and few retransmissions– If lot of traffic in both directions

1313


Selective RepeatSelective Repeat

• Receiver accepts and buffers packets following lost or damaged packets

• Both sender and receiver maintain windows– Sender’s window starts at 0, grows to MAX SEQ– Receiver’s window fixed at MAX SEQ

• Receiver has buffer reserved for each seq num in its window

0 1 2 3 4 5 6 7

1414


Selective RepeatSelective Repeat• When packet arrives, receiver checks if seq num in

window• If so and if not already received, this packet is accepted

and stored• If all lower numbered packets delivered, this packet is

delivered as well

0 1 2 3 4 5 6 7

5

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

3

1515


Selective RepeatSelective Repeat

• If no reverse traffic before timer goes off, separate ack is sent

• When receiver suspects error, sends NAK back to sender (request for retransmission)

• Two circumstances that trigger NAK– Damaged packet arrives– Packet other than expected one arrives, suspect expected one is

lost

• Receiver sends only one NAK for packet expected

1616


UDP: User Datagram ProtocolUDP: User Datagram Protocol

• “No frills,” “bare bones” Internet transport protocol• “Best effort” service, UDP segments may be:

– Lost– Delivered out of order to app

• Connectionless:– No handshaking between UDP sender, receiver– Each UDP segment handled independently of

others

1717


Why is There a UDP?Why is There a UDP?

• No connection establishment (which can add delay)

• Simple: no connection state at sender and receiver

• Small segment header

• No congestion control: UDP can blast away as fast as desired

1818


UDPUDP• Often used for streaming

multimedia apps– Loss tolerant– Rate sensitive

• Other UDP uses– DNS– SNMP

• Reliable transfer over UDP: add reliability at application layer

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,

includingheader

1919


UDP ChecksumUDP Checksum

Sender:• treat segment contents as

sequence of 16-bit integers• checksum: addition (1’s

complement sum) of segment contents

• sender puts checksum value into UDP checksum field

Receiver:• compute checksum of

received segment• check if computed checksum

equals checksum field value:– NO - error detected– YES - no error detected. But

maybe errors nonetheless?

Goal: detect “errors” (e.g., flipped bits) in transmitted segment

2020


Internet Checksum ExampleInternet Checksum Example

• When adding numbers, a carryout from the most significant bit needs to be added to the result

• Example: add two 16-bit integers

1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

wraparound

sumchecksum

To know more: http://www.netfor2.com/udpsum.htmhttp://www.netfor2.com/checksum.html

2121


TCP: OverviewTCP: Overview• Full duplex data:

– Bi-directional data flow in same connection

– MSS: maximum segment size

• Connection-oriented: – Handshaking (exchange

of control msgs) init’s sender, receiver state before data exchange

• Flow controlled:– Sender will not

overwhelm receiver

• Point-to-point:– One sender, one receiver

• Reliable, in-order byte steam:– No “message boundaries”

• Pipelined:– TCP congestion and flow

control set window size

• Send & receive buffers

2222


TCP: OverviewTCP: Overview

• TCP connection is byte stream, not message stream, no message boundaries

• TCP may send immediately or buffer before sending• Receiver stores the received bytes in a buffer

2323


TCP Segment StructureTCP Segment Structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

A TCP segment must fit into an IP datagram!

2424


The TCP Segment HeaderThe TCP Segment Header

• Source port and destination port: identify local end points of the connection– Source and destination end points together identify the connection

• Sequence number: identify the byte in the stream of data that the first byte of data in this segment represents

• Acknowledgement number: the next sequence number that the sender of the ack expects to receive– Ack # = Last received seq num + 1– Ack is cumulative: an ack of 5 means 0-4 bytes have been

received

• TCP header length – number of 32-bit words in header

2525



• URG – indicates urgent pointer field is set• Urgent pointer – points to the seq num of the last byte in a

sequence of urgent data• ACK – acknowledgement number is valid• SYN – used to establish a connection

– Connection request: ACK = 0, SYN = 1– Connection confirm: ACK=1, SYN = 1

• FIN – release a connection, sender has no more data• RST – reset a connection that is confused• PSH – sender asked to send data immediately

2626



• Receiver window size –number of bytes that may be sent beyond the byte acked

• Checksum – add the header, the data, and the conceptual pseudoheader as 16-bit words, take 1’s complement of sum– For more info: http://www.netfor2.com/tcpsum.htm

http://www.netfor2.com/checksum.html

• Options – provides a way to add extra facilities not covered by the regular header– E.g., communicate buffer sizes during set up

2727


TCP Sequence Numbers and ACKsTCP Sequence Numbers and ACKs

Sequence numbers:– byte stream “number” of

first byte in segment’s data

ACKs:– seq # of next byte

expected from other side

– cumulative ACK

Host A Host B

Seq=42, ACK=79, data = ‘C’

Seq=79, ACK=43, data = ‘C’

Seq=43, ACK=80

Usertypes

‘C’

host ACKsreceipt

of echoed‘C’

host ACKsreceipt of

‘C’, echoesback ‘C’

timesimple telnet/ssh scenario

2828


TCP Connection ManagementTCP Connection Management

TCP sender, receiver establish “connection” before exchanging data segments

• Initialize TCP variables:– Sequence numbers– Buffers, flow control info (e.g. RcvWindow)

• Client: connection initiator Socket clientSocket = new

Socket("hostname","port number"); • Server: contacted by client Socket connectionSocket = welcomeSocket.accept();

2929



Three way handshake:

Step 1: client host sends TCP SYN segment to server

– specifies initial sequence number

– no data

Step 2: server host receives SYN, replies with SYN/ACK segment

– server allocates buffers

– specifies server initial sequence number

Step 3: client receives SYN/ACK, replies with ACK segment, which may contain data

3030



Three way handshake:• SYN segment is

considered as 1 byte• SYN/ACK segment is also

considered as 1 byte

client

SYN (seq=x)

server

SYN/ACK (seq=y, ACK=x+1)

ACK (seq=x+1, ACK=y+1)

connect accept

3131



Closing a connection:

client closes socket: clientSocket.close();

Step 1: client end system sends TCP FIN control segment to

server

Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN.

client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

3232



Step 3: client receives FIN, replies with ACK.

– Enters “timed wait” - will respond with ACK to received FINs

Step 4: server, receives ACK. Connection closed.

Note: with small modification, can handle simultaneous FINs

client

FIN

server

ACK

ACK

FIN

closing

closing

closed

tim

ed w

ait

closed

3333

ExerciseExercise

• A process at host A wants to establish a TCP connection with another process at host B. Assuming that host A chooses to use 1628 as the initial sequence number, and host B chooses to use 3217 as the initial sequence number for this connection, show the segments involved with the connection establishment process. You must include the following information for each such segment: (1) sequence number, (2) acknowledgement number (if applicable), (3) the SYN flag bit status, and (4) the ACK flag bit status.


Documents

EEC-484/584 Computer Networks Lecture 7 Wenbing Zhao [email protected] (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer