TCP Segment

TCP segment

TCP Segment

Telematics EngineeringUniversitat Politecnica de Catalunya (UPC)

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TCP Segment

TCP segment

Outline

1 TCP segment

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TCP Segment

TCP segment

TCP Segment I

• It is the PDU (Protocol Data Unit) in TCP applications.• Used in:

• Connections establishment / release.• Data transfer.• Data recognition.• Window size announcement...

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TCP segment

TCP Segment II

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TCP segment

Ports

TCP Socket: IPs,Ps,IPd,PdMux/Demux

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TCP segment

Window

TCP provides an end to end Sliding-Window like flowcontrol (explained below).

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TCP segment

Sequence Numbers I

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TCP segment

Sequence Numbers II

• Sequence numbers of 32 bits.

• TCP transmits the byte stream splited into segments.

• Each byte of the flow is identified by a unique sequencenumber.

• This number is reused cyclically (wrap around).

• Sequence numbers refer to bytes in ”flight“.

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TCP segment

Sequence Numbers III

• The sequence number of the first flow’s byte israndomly chosen at the start of the connection (InitialSequence Number).

• In the following segments, reveals the first packed byteinto the byte stream.

• In ACKs indicates the next byte that is expected to bereceived.

• TCP generates a RESET when an impossiblesequence number is received.

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TCP segment

Other Fields I

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Other Fields II

• Header Length in 32 bits word.• The header size is variable as it may contain options.

• Code Bits (6 bits):• SYN: Synchronism. Indicates the start of the

connection.• FIN: Reveals the end of the connection.• RST: Reset of the connection.• PSH: Force the delivery of information.• ACK: Valid recognition field.• URG: Valid urgent pointer field.

• Urgent Data Pointer: position where urgent data ends.

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Urgent Mode

• It provides a way to send ”offtopic“ data.• Another parallel flow (the flow ”urgent“).• To do so:

• An end can mark the data as urgent.• TCP must notify to the receiver process, and the

application must make sense of it.

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TCP segment

Checksum I

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TCP segment

Checksum II

• UDP like.• However, in this case, the checksum is mandatory.• Check the integrity of TCP data, header and

pseudo-header taking keeping in mind:

• Protection against faulty segments due to routingerrors.

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Options

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TCP segment

MSS Option

• MSS (Maximum Segment Size)• Aims to inform the other end of the maximum Bytes a

TCP segment can contain.• This option can be sent within the SYN segment. Why

MSS? Efficiency!• Small Packets:

• Inefficiency.• Large Packages:

• Fragmentation.• The loss of fragment implies the retransmission of the

entire segment.

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Timestamp Option

• Allows the transmitter to include time reference in eachsegment.

• The receiver echoes the timestamp.• By this option, the sender can estimate the segment

round trip time (RTT).• Very useful in precise timeout estimation.

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