Transport LayerTCP and UDP

IS250Spring 2010

chuang@ischool.berkeley.edu

John Chuang 2

Network Layers

Application (layer 7): specific to application need

Transport (layer 4): end-to-end delivery, congestion and flow control

Network (layer 3): addressing, routing

Data Link (layer 2): framing, error detection

Physical (layer 1): bits (0/1), voltages, frequencies, wires, pins, …

IP

TCP, UDP

HTTP, FTP, NNTP, SMTP,

telnet, ...

coax, twisted pair, fiber,

wireless, ...

Ethernet

FDDI, SONET

Wi-Fi

John Chuang 3

TCP/IP ModelAppl

Transport

Network

Link

Network

Link

Network

Link

Appl

Transport

Network

Link

Host A Host BRouter 1 Router 2

end-to-end

point-to-point

point-to-point

end-to-end

John Chuang 4

Transport Layer

Functions1.Addressing (ports)2.Data integrity (error detection)3.Reliable data transport4.Flow control5.Congestion control

Protocols- Transmission Control Protocol (TCP)

- Reliable data transport (1, 2, 3, 4, and 5)

- User Datagram Protocol (UDP)- Unreliable data transport (1 and 2 only)

John Chuang 5

TCP Segment FormatBit 0 Bit 31

Data (variable length)

Data

Header

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)

Hdr Len(4) Flags (6) Window Size (16)

Options (if any) PAD

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)

Like the IPv4 header, TCP header is also 20 bytes long without options

John Chuang 6

UDP Datagram Format0 3116

Data

UDP Header

Source Port Number (16) Destination Port Number (16)

UDP Checksum (16)Message Length (16)

UDP is considered light-weight :- low overhead; no connection setup- used for real-time applications (don’t need retransmission)

John Chuang 7

TCP Maximum Segment Size

Note that TCP segment header does not include segment size field

Instead, Sequence Number field is used to identify location of segment in the TCP byte stream (more on SEQ later)

Sender constructs segments so that they do not need to be fragmented at the network layer

MSS = MTU - IP header length - TCP header length

Maximum Segment Size

Maximum Transmission Unit

20 bytes without Options 20 bytes without Options

John Chuang 8

Transport Layer Functions

1. Addressing (ports)2. Data integrity (error

detection)3. Reliable data transport4. Flow control5. Congestion control

John Chuang 9

L4 Addressing

TCP/UDP ports identify processes on a host

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)Hdr Len

(4) Flags (6) Window Size (16)

Options (if any) PAD

Bit 0 Bit 31

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)TCP Header

0 3116

UDP Header

Source Port Number (16) Destination Port Number (16)

UDP Checksum (16)Message Length (16)

John Chuang 10

Ports

Multiple processes can run on a single host - all processes share a single IP address- each process talks/listens via a different port

128.32.226.87; port 21128.2.14.60; port 50001

128.32.226.87; port 80128.2.14.60; port 50002

ftp client

http client

ftp server

http server

Note: IP cannot distinguish ftp packets from http packets (they have the same source and destination IP addresses)

John Chuang 11

Ports

The port numbers are divided into three ranges: - Well known ports (0-1023)- Registered ports (1024-49151)- Dynamic and/or private ports (49152 – 65535)

Some “well known ports”- ftp (21); ssh (22); telnet (23); smtp (25); finger (79); http (80)

- Assigned by Internet Assigned Numbers Authority (http://www.iana.org/numbers.html)

John Chuang 12

Transport Layer Functions

1. Addressing (ports)2. Data integrity (error

detection)3. Reliable data transport4. Flow control5. Congestion control

John Chuang 13

Data Integrity

TCP/UDP checksums cover entire segment/datagram

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)Hdr Len

(4) Flags (6) Window Size (16)

Options (if any) PAD

Bit 0 Bit 31

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)TCP Header

0 3116

UDP Header

Source Port Number (16) Destination Port Number (16)

UDP Checksum (16)Message Length (16)

John Chuang 14

Transport Layer Functions

1. Addressing (ports)2. Data integrity (error

detection)3. Reliable data transport4. Flow control5. Congestion control

John Chuang 15

Reliable Data Transport

Connection (or Session) Management- Agreement on start and end of connection/session

Error Control- Data delivered without deletions, insertions, duplications, reordering

John Chuang 16

Connection Management

TCP establishes a session with ordered and bi-directional reliable delivery of bytes- Establishment:

- Informs receiving port of connection- Initializes packet sequence number (to a random number)

- Sets congestion and flow control state

- Teardown:- By either peer- Frees state and resources

John Chuang 17

A TCP SessionProcess A Process B

SYN

3-Way handshake to establish TCP session

time

SYN + ACK

Conversation

ACK

Teardown

Data + ACK

FIN

FIN + ACK

ACK

Data + ACK

Data + ACK

DataCan be merged into one

John Chuang 18

TCP Header Flags

Flags: URG, ACK, PSH, RST, SYN, FIN

Source Port # (16) Destination Port # (16)

Sequence Number (32 bits)

Acknowledgement Number (32 bits)

Hdr Len(4) Flags (6) Window Size (16)

Options (if any) PAD

Bit 0 Bit 31

Reserved (6)

TCP Checksum (16) Urgent Pointer (16)

John Chuang 19

Error Control

Original data stream: “I am here”

Can data be deleted? “I here” Can data be reordered? “here I am” Can data be duplicated? “I am am here”

Can non-data be inserted? “I am not here”

John Chuang 20

Reliable Delivery

Positive Acknowledgment with retransmission Sequence and acknowledgement numbers

Options (if any)

0 3116

Padding

Data

TCP Header

Source Port Number (16) Destination Port Number (16)

Sequence Number (32)

Acknowledgement Number (32)

Hdr Len(4)

Flags (6) Window Size (16)Reserved (6)

TCP Checksum (16) Urgent Pointer (16)

John Chuang 21

Reliable Delivery

Each packet has a sequence number (SEQ)- SEQ represents byte offset with respect to initial SEQ

- Duplicate packets can be detected and discarded- Out of order packets can be re-ordered

Each packet carries acknowledgment of received packet- ACK = sequence number of next byte expected by the receiver

Lost packet can be detected by missing ACK Lost packet can be retransmitted after a timeout period

John Chuang 22

A TCP SessionProcess A Process B

SYN (Seq=x)

3-Way handshake to establish TCP session

time

SYN (Seq=y; Ack=x+1)

Conversation

Data (Seq=x+1; Ack=y+1)

Teardown

Data (Seq=y+1; Ack=x+2)

FIN (Seq=x+3; Ack=y+3)

FIN (Seq=y+3; Ack=x+4)

ACK (Ack=y+4)

Data (Seq=y+2; Ack=x+3)

Data (Seq=x+2; Ack=y+2)

Notes: •Processes pick random initial values for x and y•SEQ incremented by one for illustrative purposes only (in practice, SEQ incremented by MSS)

John Chuang 23

ACK and Packet Retransmission

Process A Process B

time

Data (Seq=x+1)

Data (Seq=y+1; Ack=x+2)

ACK (Ack=x+3)

Data (Seq=x+2; Ack=y+2)T

imeo

ut

Data (Seq=x+2; Ack=y+2)

IP loses packet

TCP resends packet

John Chuang 24

ACK and Packet Retransmission

Process A Process B

time

Data (Seq=x+1)

Data (Ack=x+2)

ACK (Ack=x+3)

Data (Seq=x+2)T

imeo

ut

Data (Seq=x+2)

IP loses ACK

TCP resends packet

TCP resends ACK;drops duplicate

Data (Ack=x+3)

John Chuang 25

Adaptive Retransmission (Timeout)

Round trip time (RTT): elapsed time between sending of a TCP segment and the receipt of the corresponding ACK

EstRTT = (*EstRTT) + ((1- )*SampleRTT)Timeout = * EstRTT

In the original spec, suggested values for and are 0.9 and 2. In Jacobson/Karels algorithm, timeout is set adaptively to avoid spurious retransmissions