Transport Layer - TCP

Transport Layer - TCP

TCP: A Closer Look Browser does not send HTTP-Request-PDUs

directly to the webserver application– The application layer programs are not

physically connected– Browser sends HTTP-Request-PDU to the user

PC’s transport layer process for delivery

Browser

TransportProcess

HTTP-Request-PDU

TCP: A Closer Look

Transport layer process stores the HTTP-Request-PDU Temporarily

Browser

TransportProcess

TransportProcess

TCP: A Closer Look

User PC transport process opens a connection to the webserver transport layer process

– This connection can be used to send several TCP-PDUs to handle a single HTTP request-response cycle in HTTP 1.0

Browser

TransportProcess

TransportProcess

TCP/IP: A Closer Look

The TCP header has a flags field with 1-bit SYN, ACK, and FIN fields.– SYN bit set to 1 to request connection– ACK bit set to 1 to acknowledge a received

TCP-PDU– FIN bit set to 1 to inform of closure

Flags Field

TCP-Header

TCP: A Closer Look Opening a connection takes 3 TCP-PDUs

– Second message asks to open a connection and also acknowledges the first SYN message

User PCTransportProcess

WebserverTransportProcess

SYN

SYN, ACK

ACK

TCP: A Closer Look Sending the HTTP-Request-PDU requires 2 TCP-PDUs

– Sends HTTP-Request-PDU in data field of a TCP-PDU– Receives an ACK TCP-PDU



TCP-PDU containingHTTP-Request-PDU

ACK

Delivering the HTTP-Response-PDU Transport layer process on the webserver

receives the TCP-PDU delivering the HTTP-Request-PDU– The transport process on the webserver

passes the HTTP-Request-PDU in the TCP-PDU data field to the webserver application program Webserver

Application

TransportProcess

HTTP-Request-PDU

Delivering the HTTP-Response-PDU Webserver application creates the HTTP-

Response-PDU– Webserver application passes the

HTTP-Response-PDU to the webserver transport layer process for delivery to the user PC transport layer process

WebserverApplication

TransportProcess

HTTP-Response-PDU

TCP: A Closer Look Fragmentation of HTTP-Response-PDU

– HTTP-Response-PDUs usually are large files

– TCP-PDUs are limited to a certain maximum segment size (MSS)

• Often only 512 bytes

– Typical HTTP-Response-PDU must be fragmented into multiple TCP-PDUs containing parts of the HTTP-response-PDU

HTTP-Response-PDU

TCP-PDU-3TCP-PDU-2TCP-PDU-1

A Closer Look at TCP Sample Fragmentation Calculation

– 20 KB Size of HTTP-Response-PDU.Varies. 20KB is only an example

– 20,480 octets 1,024 bytes per KB– 550 octets Maximum segment size (MSS).

Varies. 550 is only an example.– 530 octets Maximum data per TCP-PDU

(Header has 20 octets)– 38.6 Size of HTTP-Response PDU

divided by maximum data/PDU– 39 Number of TCP-PDUs needed.

(round up)

TCP: A Closer Look Sending the HTTP-Response-PDU takes many

TCP-PDUs– Each is acknowledged



Response TCP-PDU 1

ACK for 1

ACK for 2

Response TCP-PDU 2

TCP: A Closer Look Closing the Connection takes 4 TCP-PDUs

– Initiated by the webserver transport process when it finishes sending the last HTTP-Response-PDU fragment.



FIN

FIN

ACK

ACK

TCP Connections: Recap

For EACH HTTP request-response cycle…

– 3 TCP-PDUs to open the connection

– 2 TCP-PDU to send the HTTP request and get an acknowledgement

– N data TCP-PDUs to send the HTTP-Response-PDU and get acknowledgements

– 4 TCP-PDUs to close the connection

– For EACH HTTP request-response cycle at the application layer

TCP: Error Handling

TCP is reliable--it does error correction

– Each TCP-PDU has a sequence number that increases with each TCP-PDU a transport process sends

– ACKs acknowledge a specific TCP-PDU in the sequence

– If a TCP-PDU is not acknowledged, the other side retransmits it after awhile

– This adds TCP-PDUs beyond the ones we have seen

TCP: Why Connections? Opens, closes, and ACKs create overhead, so

undesirable Why do it?

– Allows sequence numbers, so that errors can be handled easily

– Creates sequence numbers, so that fragmentation can be handled easily

Overall, TCP is a high-overhead, highly reliable transport layer protocol that catches any errors at lower layers, giving “clean” service to the application programs

TCP-Header Fields in TCP Header

– Shows 32 bits on each row (0 to 31)– Note: start with zero

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 Port number designates and application program

– E.g, well-known port number for HTTP is 80– Other applications also have well-known port numbers






Bit 0 Bit 31

Reserved (6)


TCP-Header Sequence and Acknowledgement Numbers

– To be sure each TCP-PDU has arrived– To place arriving TCP-PDUs in order– To allow a specific TCP-PDU to be acknowledged






Bit 0 Bit 31

Reserved (6)


TCP-Header Flags

– 1 bit each– SYN, FIN, ACK are three of the flags






Bit 0 Bit 31

Reserved (6)


TCP-Header Window Size

– Number of bytes other side can send beyond the TCP-PDU being acknowledged

– Initially set to allow one TCP-PDU (slow start)






Bit 0 Bit 31

Reserved (6)


TCP-Header

TCP Checksum– To check for an error anywhere in the whole TCP-

PDU, including the data field






Bit 0 Bit 31

Reserved (6)


TCP-Header Options

– Rarely used– PAD set to bring options to 32-bit boundary






Bit 0 Bit 31

Reserved (6)


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Transport Layer - TCP