Lab 2: Admin details

• Lab 2 is out!

• Due: o November 4th, 12 noon

o November 6th, 12 noon (90% cap)

• Assignment and starter code online

• Start early!On average, took students 14 hours to complete Lab 2 last

year

Functionality

Functionality

Write an application that:

• Takes in data from one host

Functionality

Write an application that:

• Takes in data from one host

• Turns those data into packets

Functionality

Write an application that:

• Takes in data from one host

• Turns those data into packets

• Transmits those packets reliably and in-order to the other host

Example

Beware the appearance of simplicity

Why the Internet isn't just "tubes"

• A packet can be corrupted

• A packet can be delayed

• A packet can be dropped

• A packet can be duplicated

corrupted delayed dropped duplicated

corrupted delayed dropped duplicated

Please transfer $100 from my bank account to Phil's

corrupted delayed dropped duplicated

Please transfer $100 from my bank account to Phil's

Please transfer $14837135 from my bank account to Phil's

corrupted delayed dropped duplicated

corrupted delayed dropped duplicated

Please deposit $100 into my account

Please transfer $100 from my bank account to Phil's

corrupted delayed dropped duplicated

Please deposit $100 into my account

Please transfer $100 from my bank account to Phil's

Please deposit $100 into my account

Please transfer $100 from my bank account to Phil's

corrupted delayed dropped duplicated

corrupted delayed dropped duplicated

Please transfer $100 from my bank account to Phil's

corrupted delayed dropped duplicated

Please transfer $100 from my bank account to Phil's

corrupted delayed dropped duplicated

corrupted delayed dropped duplicated

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Tools for ordered, reliable streams

• Checksums

• Sequence numbers

• Acknowledgments

• Timeouts

cksums seq nos acks timeouts

cksums seq nos acks timeouts

Hash:

cksum(data) integer

cksums seq nos acks timeouts

Hash:

cksum(data) integer

cksum("a") 1

cksums seq nos acks timeouts

Please transfer $100 from my bank account to Phil's

Please transfer $14837135 from my bank account to Phil's

cksums seq nos acks timeouts

Please transfer $100 from my bank account to Phil's

Please transfer $14837135 from my bank account to Phil's

Tag packet with checksum before sending

cksums seq nos acks timeouts

Please transfer $100 from my bank account to Phil's

Please transfer $14837135 from my bank account to Phil's

Tag packet with checksum before sending

Please transfer $100 from my bank account to Phil's

Please transfer $14837135 from my bank account to Phil's

cksum("Please ...") cksum("Please ...")

cksums seq nos acks timeouts

Note:

cksum("Please transfer ... $100 ...")

cksum("Please transfer ... $14837135 ...")

cksums seq nos acks timeouts

Note:

cksum("Please transfer ... $100 ...")

cksum("Please transfer ... $14837135 ...")

Please transfer $100 from my bank account to Phil's

Please transfer $14837135 from my bank account to Phil's

cksum("Please ...") cksum("Please ...")

cksums seq nos acks timeouts

Monotonically increasing number appended to packet

cksums seq nos acks timeouts

Monotonically increasing number appended to packet

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

cksums seq nos acks timeouts

Monotonically increasing number appended to packet

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Seq no: 152 Seq no: 152

Seq no: 152

cksums seq nos acks timeouts

Monotonically increasing number appended to packet

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Seq no: 152 Seq no: 152

Seq no: 152

cksums seq nos acks timeouts

Monotonically increasing number appended to packet

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Seq no: 152 Seq no: 152

Seq no: 152

cksums seq nos acks timeouts

Monotonically increasing number appended to packet

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Please transfer $100 from my bank account to Phil's

Seq no: 152 Seq no: 152

Seq no: 152Sender: Last sequence number sent (and last sequence number ack-d)

Receiver: Last sequence number received

cksums seq nos acks timeouts

Receiver acknowledges received packets

If sender does not receive ack, retransmits

cksums seq nos acks timeouts

Receiver acknowledges received packets

If sender does not receive ack, retransmits

Please transfer $100 from my bank account to Phil's

Seq no: 152

cksums seq nos acks timeouts

Receiver acknowledges received packets

If sender does not receive ack, retransmits

Please transfer $100 from my bank account to Phil's

Seq no: 152

Please transfer $100 from my bank account to Phil's

Seq no: 152

Please transfer $100 from my bank account to Phil's

Seq no: 152

cksums seq nos acks timeouts

Receiver acknowledges received packets

If sender does not receive ack, retransmits

Please transfer $100 from my bank account to Phil's

Seq no: 152

Please transfer $100 from my bank account to Phil's

Seq no: 152

Please transfer $100 from my bank account to Phil's

Seq no: 152

Ack no: 153

Duplicate

Corrupt

Delay

Drop

Checksums

Sequence numbers

Acknowledgments

Timeouts

Stop and wait

If you have a Sender and a Receiver:

• Sender sends a data message.

• Sender waits until receiver acknowledges receptiono If Sender receives ack, sends its next available

packeto If Sender does not receive ack, the Sender re-

transmits the data that had not been acknowledged

Beware the appearance of simplicity

Difficulties

• Conceptual

• The "little" things

Conceptual: Event-based code

Conceptual: Event-based code

• What are the events?

• What parts of code map to events?

• How to respond to events?

• How track state in? When transition between states?

Events

• Receive data packet

• Receive ack packet

• Input goes from empty to having data

• Output goes from full to having room

• Timer expires

• Create a new connection

------

• Receive shutdown from opposite end

• Receive shutdown from input

Conceptual: Event-based code

• What are the events?

• What parts of code map to events?

• How to respond to events?

• How track state in? When transition between states?

Events

• Create a new connection

rel_create

• Receive data packet

• Receive ack packet

rel_recvpkt

• Input goes from empty to having data

rel_read

• Output goes from full to having room

rel_output

• Timer expires

rel_timer

Conceptual: Event-based code

• What are the events?

• What parts of code map to events?

• How to respond to events?

• How track state in? When transition between states?

Events

• Create a new connection

rel_create

• Receive data packet

• Receive ack packet

rel_recvpkt

• Input goes from empty to having data

rel_read

• Output goes from full to having room

rel_output

• Timer expires

rel_timer

Timer expires

cksums seq nos acks timeouts

Receiver acknowledges received packets

If sender does not receive ack, retransmits

Please transfer $100 from my bank account to Phil's

Seq no: 152

Please transfer $100 from my bank account to Phil's

Seq no: 152

Please transfer $100 from my bank account to Phil's

Seq no: 152

Timer expires

Timer expiration maps to retransmits

When do we retransmit?

Timer expires

Timer expiration maps to retransmits

When do we retransmit?

• Have unack-ed packet outstanding

• We sent unack-ed packet > TIMEOUT ago

Receive data packet

Receive data packet

• Sanity checks

Receive data packet

• Sanity checkso Does its checksum match?o Are you expecting the sequence number?o Is it the right length?o Has the other end already closed its output stream?

Receive data packet

• Sanity checkso Does its checksum match?o Are you expecting the sequence number?o Is it the right length?o Has the other end already closed its output stream?

• Action takeno Should you send ack?o Should you pass payload to output?

Receive data packet

• Sanity checkso Does its checksum match?o Are you expecting the sequence number?o Is it the right length?o Has the other end already closed its output stream?

• Action takeno Should you send ack?o Should you pass payload to output?

Conceptual: Event-based code

• What are the events?

• What parts of code map to events?

• How to respond to events?

• How track state in? When transition between states?

Ticky-tacky bugs

Computing checksum

htons/htonl/ntohs/ntohlstrcpy

Receive data packet

• Sanity checkso Does its checksum match?o Are you expecting the sequence number?o Is it the right length?o Has the other end already closed its output stream?

• Action takeno Should you send ack?o Should you pass payload to output?

cksum

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno; /* Only valid if length > 8 */

char data[500];

};

typedef struct packet packet_t;

cksum

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno; /* Only valid if length > 8 */

char data[500];

};

typedef struct packet packet_t;

Provided a cksum function in code:

uint16 cksum(const void* data, int len)

cksum

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno;

char data[500];

};

typedef struct packet packet_t;

Provided a cksum function in code:

uint16 cksum(const void* data, int len)

cksum

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno;

char data[500];

};

typedef struct packet packet_t;

Provided a cksum function in code:

uint16 cksum(const void* data, int len)

How do you know what to fill in for cksum field before performing the cksum?

cksum

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno;

char data[500];

};

typedef struct packet packet_t;

Provided a cksum function in code:

uint16 cksum(const void* data, int len)

How do you know what to fill in for cksum field before performing the cksum?

Zero cksum field before performing cksum on packet

Ie: pkt.cksum = 0; pkt.cksum = cksum( <some cast> pkt, len);

Host order/network order

How do I write the number 512?

Host order/network order

How do I write the number 512?

Hint: 512 = 2^9

Host order/network order

How do I write the number 512?

Hint: 512 = 2^9

Binary: 1000000000

Hex: 200

Host order/network order

How do I write the number 512?

Hint: 512 = 2^9

Binary: 1000000000

Hex: 200

MSB LSB

Host order/network order

MSB LSB

Should a machine store this as or ?

How do I write the number 512?

Hint: 512 = 2^9

Binary: 1000000000

Hex: 200

How do I write the number 512?

Hint: 512 = 2^9

Binary: 1000000000

Hex: 200

Host order/network order

MSB LSB

Should a machine store this as or ?

Little-endian

Big-endianNetwork byte order

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno;

char data[500];

};

typedef struct packet packet_t;

Host order/network order

?

?

Host order/network order

struct packet {

uint16_t cksum;

uint16_t len;

uint32_t ackno;

uint32_t seqno;

char data[500];

};

typedef struct packet packet_t;

?

?

htons htonl ntohs ntohl

htons ntohs htonl ntohl

If your host is little-endian:

ntohs( ) htons( )

If your host is big-endian

ntohs( ) htons( )

Example

recvd_pkt.seqno = ntohl(recvd_pkt.seqno);

Example

recvd_pkt.seqno = ntohl(recvd_pkt.seqno);

Design goal:

Limit the places you use ntohl, ntohs, htons, htonl to as few places as possible.

Testing

● Test interoperability with reference solution● Test interoperability with classmates● use test options to simulate bad network connectivity

○ --drop○ --duplicate○ --corrupt○ --delay

Summary

What the network can do to packets

Primitives for reliability

• Checksums

• Sequence numbers

• Acknowledgments

• Timeouts

Event-based code

• What are events?

• How do they map to code?

• How do you respond to them?

• How do you transition between states?

Gotchas

• cksum

• nthos htons ntohl htonl

Words of advice

Start early!

Start by reading/thinking/desiging

Test your implementation thoroughly

Start early!

Figure out when to call conn_input

Start early!