tcp & udp - protocol details yen-cheng chen ycchen@ncnu.edu.tw

TCP & UDP- Protocol Details

Yen-Cheng Chenycchen@ncnu.edu.tw

Port Number

Divided into three ranges: Well Known Ports: 0 ~ 1023 Registered Ports: 1024 ~ 49151 Dynamic and/or Private Ports: 49152 ~ 6553

5

http://www.iana.org/assignments/port-numbers

Source Port Destination Port

Length Checksum

Data

16 bits 16 bits

UDP Header RFC 768

Checksum

Checksum is the 16-bit one's complement of the one's complement sum of a pseudo headerpseudo header of information from the IP header, the UDP headerUDP header, and the datadata.

Source Address

Destination Address

00000000

32 bits

Protocol (17) UDP Length

Pseudo Header

Protocol Number

http://www.iana.org/assignments/protocol-numbers

How to Ensure the Delivery of UDP Messages

Time OutRetry

TCP Segment (TCP PDU)

Source port (16 bits)Destination port (16 bits)Sequence number (32 bits)Acknowledgment number (32 bits)Data Offset (4 bits)Reserved (6 bits)Flags (6 bits) : URG, ACK, PSH, RST, SYN, FIN

Window (16 bits)Checksum (16 bits)Urgent Pointer (16 bits)Options (variable)

RFC 793

TCP Header

Sequence / Acknowledgment Number

Sequence Number: 32 bits The sequence number of the first data octet in

this segment (except when SYN is present). If SYN is present the sequence number is the initial sequence number (ISN) and the first data octet is ISN+1.

Acknowledgment Number: 32 bits If the ACK control bit is set this field contains the

value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always sent.

(Seq:X, Ack:Y, Data: a bytes)

(Seq:Y, Ack:X+a)

(Seq:X+a, Ack:Y, Data: b bytes)

(Seq:Y, Ack:X+a+b)

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Unidirectional Transmission

(Seq:20000, Ack:100, Data: 1000 bytes)

(Seq:100, Ack:21000)

(Seq:21000, Ack:100, Data: 750 bytes)

(Seq:100, Ack:21750)

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Example - Unidirectional

(Seq:X, Ack:Y, Data: a bytes)

(Seq:Y, Ack:X+a, Data: m bytes)

(Seq:X+a, Ack:Y+m, Data: b bytes)

(Seq:Y+m, Ack:X+a+b, Data: n bytes)Sen

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Receiv

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Bi-directional Transmission

(Seq:X+a+b, Ack:Y+m+n, Data: c bytes)

(Seq:25000, Ack:1000, Data:800 bytes)

(Seq:1000, Ack:25800, Data:500 bytes)

(Seq:25800, Ack:1500, Data: 1000 bytes)

(Seq:1500, Ack:26800, Data: 700 bytes)Sen

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Receiv

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(Seq:26800, Ack:2200, Data: 600 bytes)

Example – Bi-directional

Window (AdvertisedWindow)Window: 16 bits The number of data octets beginning wit

h the one indicated in the acknowledgment field which the sender of this segment is willing to accept.

The sender is limited to having no more than a value of AdvertisedWindowAdvertisedWindow bytes of unacknowledged data in any given time.

Sliding Window

Flow/Congestion Control

Slow Start Fast Retransmit – Duplicate ACK Fast Recovery

Implementation Tahoe, Reno, Vegas

Source Destination

…

Slow Start

Fast RetransmitPacket 1

Packet 2

Packet 3

Packet 4

Packet 5

Packet 6

Retransmit

packet 3

ACK 1

ACK 2

ACK 2

ACK 2

ACK 6

ACK 2

Sender Receiver

Data Offset (Header Length)

Data Offset: 4 bits The number of 32 bit32 bit words in the

TCP Header. This indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits long.

FlagsControl Bits: 6 bits (from left to right):URG: Urgent Pointer field significant ACK: Acknowledgment field significant PSH: Push Function RST: Reset the connection SYN: Synchronize sequence numbersFIN: No more data from sender

Urgent Pointer

Urgent Pointer: 16 bits This field communicates the current value

of the urgent pointer as a positive offset from the sequence number in this segment.

The urgent pointer points to the sequence number of the octet following the urgent data. This field is only be interpreted in segments with the URG control bit set.

Options / Paddings

Options Maximum Segment Size

Padding to ensure that the TCP header ends

and data begins on a 32 bit boundary.

Connection Progresses

11 States: LISTEN, SYN-SENT, SYN-RECEIVED,

ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and CLOSED.

CLOSEDCLOSED

LISTENLISTEN

SYN_RCVDSYN_RCVD SYN_SENTSYN_SENT

ESTABLISHEDESTABLISHED

CLOSE_WAITCLOSE_WAIT

LAST_ACKLAST_ACKCLOSINGCLOSING

TIME_WAITTIME_WAIT

FIN_WAIT_2FIN_WAIT_2

FIN_WAIT_1FIN_WAIT_1

Passive openPassive open CloseClose

Send/Send/ SYNSYN

SYN/SYN + ACKSYN/SYN + ACK

SYN + ACK/ACKSYN + ACK/ACK

SYN/SYN + ACKSYN/SYN + ACK

ACKACK

CloseClose /FIN/FIN

FIN/ACKFIN/ACKCloseClose /FIN/FIN

FIN/ACKFIN/ACKACK + FIN/ACK

ACK + FIN/ACK

Timeout after two Timeout after two segment lifetimessegment lifetimes

FIN/ACKFIN/ACK

ACKACK

ACKACK

ACKACK

CloseClose /FIN/FIN

CloseClose

CLOSEDCLOSED

Active openActive open /SYN/SYN

TCP Connection States (1)

LISTEN - waiting for a connection request SYN-SENT - waiting for a matching connection request after having sent a connection request.SYN-RECEIVED - waiting for a confirming connection request acknowledgment after having both received and sent a connection request.ESTABLISHED - an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.

TCP Connection States (2)

FIN-WAIT-1 - waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.FIN-WAIT-2 - waiting for a connection termination request from the remote TCP.CLOSE-WAIT - waiting for a connection termination request from the local user.CLOSING - represents waiting for a connection termination request acknowledgment from the remote TCP.

TCP Connection States (3)LAST-ACK - waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request). TIME-WAIT - waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. CLOSED - no connection state at all.

Basic 3-Way Handshake for Connection Synchronization

(Seq:X, Syn.)

(Seq:Y, Ack:X+1, Ack.,Syn.)

(Seq:X+1, Ack:Y+1, Ack.)

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Data Transmission

Example -Connection Synchronization

Closing a Connection (Seq:X, Ack:Y. Ack.,Fin.)

(Seq:Y, Ack:X+1, Ack..)

Req

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Resp

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(Seq:Y, Ack:X+1, Ack.,Fin.)

(Seq:X+1, Ack:Y+1. Ack.)

Example – Closing a Connection

MSL: Maximum Segment Lifetime

TCP Pseudo Header