IP Network - TCP

 

 

 

TCP stands for Transmission Control Protocol. This may be one of the most common protocols in relation to any IP network. For example, most of the application layer protocol we are commonly using these days , like HTTP, HTTPS, SMTP, POP3, IMAP, SSH, FTP, Telnet ect, are typically encapsulated in TCP packet.

 

TCP is a kind of session protocol which requires a special procedure to establish the connnection between the client and server and also requires a special procedure to release the connection between them.

 

 

TCP Establish

 

To begin the communication using TCP, the client and server has to go through a special sequence to establish the connection and this sequence goes as shown below. It is done by 3 steps. That's why this procedure is often called 3 way handshaking or 3 way negotiation.

 

 

Step (1) : Client send a TCP SYN Packet to server to request connection establishment. At this step, the client assign a specific sequence number (this is called as ISN) as the initial (first) sequence number. This is not zero, it is the number generated by a special algorithm in such a way that it cannot easily been guessed by any intruder. As you see in the following example, the real initial sequence number is randomly (specially) assigned as in (C), but in wireshark it is labeled to be 0 as shown in (A).  Also, this initial packet has SYN flag is set to be '1'.

 

Transmission Control Protocol, Src Port: 42206 (42206), Dst Port: sip (5060), Seq: 0, Len: 0

    Source port: 42206 (42206)

    Destination port: sip (5060)

    [Stream index: 0]

    Sequence number: 0    (relative sequence number) <=== (A)

    Header length: 40 bytes

    Flags: 0x002 (SYN)

        000. .... .... = Reserved: Not set

        ...0 .... .... = Nonce: Not set

        .... 0... .... = Congestion Window Reduced (CWR): Not set

        .... .0.. .... = ECN-Echo: Not set

        .... ..0. .... = Urgent: Not set

        .... ...0 .... = Acknowledgment: Not set

        .... .... 0... = Push: Not set

        .... .... .0.. = Reset: Not set

        .... .... ..1. = Syn: Set <=== (B)

            [Expert Info (Chat/Sequence): Connection establish request (SYN): server port sip]

                [Message: Connection establish request (SYN): server port sip]

                [Severity level: Chat]

                [Group: Sequence]

        .... .... ...0 = Fin: Not set

    Window size value: 65535

    [Calculated window size: 65535]

    Checksum: 0x9d0f [validation disabled]

        [Good Checksum: False]

        [Bad Checksum: False]

    Options: (20 bytes), Maximum segment size, SACK permitted, Timestamps,

                         No-Operation (NOP), Window scale

        Maximum segment size: 1440 bytes

            Kind: MSS size (2)

            Length: 4

            MSS Value: 1440

        TCP SACK Permitted Option: True

            Kind: SACK Permission (4)

            Length: 2

        Timestamps: TSval 4294945900, TSecr 0

            Kind: Timestamp (8)

            Length: 10

            Timestamp value: 4294945900

            Timestamp echo reply: 0

        No-Operation (NOP)

            Type: 1

                0... .... = Copy on fragmentation: No

                .00. .... = Class: Control (0)

                ...0 0001 = Number: No-Operation (NOP) (1)

        Window scale: 6 (multiply by 64)

            Kind: Window Scale (3)

            Length: 3

            Shift count: 6

            [Multiplier: 64]

 

0000  00 30 64 0b 6c 69 00 00 91 05 fa 92 86 dd 60 00   .0d.li........`.

0010  00 00 00 28 06 40 20 01 00 00 00 00 00 02 00 00   ...(.@ .........

0020  00 00 00 00 00 01 20 01 00 00 00 00 00 02 00 00   ...... .........

0030  00 00 00 00 00 02 a4 de 13 c4 0c f3 f8 fa <== (C) ................

0040  00 00 a0 02 ff ff 9d 0f 00 00 02 04 05 a0 04 02   ................

0050  08 0a ff ff ac 6c 00 00 00 00 01 03 03 06         .....l........

 

 

Step (2) : Server accept the SYN request and send ACK and SYN in a single packet to the Client. At this step, the Server assign a specific sequence number (this is called as ISN) as the initial (first) sequence number. This is not zero, it is the number generated by a special algorithm in such a way that it cannot easily been guessed by any intruder. As you see in the following example, the real initial sequence number is randomly (specially) assigned as in (E), but in wireshark it is labeled to be 0 as shown in (A).

 

Transmission Control Protocol, Src Port: sip (5060), Dst Port: 42206 (42206), Seq: 0, Ack: 1,Len: 0

    Source port: sip (5060)

    Destination port: 42206 (42206)

    [Stream index: 0]

    Sequence number: 0    (relative sequence number) <== (A)

    Acknowledgment number: 1    (relative ack number) <== (B)

    Header length: 40 bytes

    Flags: 0x012 (SYN, ACK)

        000. .... .... = Reserved: Not set

        ...0 .... .... = Nonce: Not set

        .... 0... .... = Congestion Window Reduced (CWR): Not set

        .... .0.. .... = ECN-Echo: Not set

        .... ..0. .... = Urgent: Not set

        .... ...1 .... = Acknowledgment: Set <== (C)

        .... .... 0... = Push: Not set

        .... .... .0.. = Reset: Not set

        .... .... ..1. = Syn: Set <== (D)

            [Expert Info (Chat/Sequence): Connection establish acknowledge (SYN+ACK):

                                                                              server port sip]

                [Message: Connection establish acknowledge (SYN+ACK): server port sip]

                [Severity level: Chat]

                [Group: Sequence]

        .... .... ...0 = Fin: Not set

    Window size value: 8192

    [Calculated window size: 8192]

    Checksum: 0x4037 [validation disabled]

        [Good Checksum: False]

        [Bad Checksum: False]

    Options: (20 bytes), Maximum segment size, No-Operation (NOP), Window scale,

                                                              SACK permitted, Timestamps

        Maximum segment size: 1440 bytes

            Kind: MSS size (2)

            Length: 4

            MSS Value: 1440

        No-Operation (NOP)

            Type: 1

                0... .... = Copy on fragmentation: No

                .00. .... = Class: Control (0)

                ...0 0001 = Number: No-Operation (NOP) (1)

        Window scale: 8 (multiply by 256)

            Kind: Window Scale (3)

            Length: 3

            Shift count: 8

            [Multiplier: 256]

        TCP SACK Permitted Option: True

            Kind: SACK Permission (4)

            Length: 2

        Timestamps: TSval 73513, TSecr 4294945900

            Kind: Timestamp (8)

            Length: 10

            Timestamp value: 73513

            Timestamp echo reply: 4294945900

    [SEQ/ACK analysis]

        [This is an ACK to the segment in frame: 119]

        [The RTT to ACK the segment was: 0.000129000 seconds]

 

0000  00 00 91 05 fa 92 00 30 64 0b 6c 69 86 dd 60 00   .......0d.li..`.

0010  00 00 00 28 06 80 20 01 00 00 00 00 00 02 00 00   ...(.. .........

0020  00 00 00 00 00 02 20 01 00 00 00 00 00 02 00 00   ...... .........

0030  00 00 00 00 00 01 13 c4 a4 de 18 c3 b3 cb <==(E)  0c f3   ................

0040  f8 fb <== (F) a0 12 20 00 40 37 00 00 02 04 05 a0 01 03   .... .@7........

0050  03 08 04 02 08 0a 00 01 1f 29 ff ff ac 6c         .........)...l

 

 

Step (3) : Now the client send ACK to the Server for the SYN packet. You see here both Sequence Number and Acknowledgement number got incremented by 1.

 

Transmission Control Protocol, Src Port: 42206 (42206), Dst Port: sip (5060), Seq: 1, Ack: 1, Len: 0

    Source port: 42206 (42206)

    Destination port: sip (5060)

    [Stream index: 0]

    Sequence number: 1    (relative sequence number) <== (A)

    Acknowledgment number: 1    (relative ack number) <== (B)

    Header length: 32 bytes

    Flags: 0x010 (ACK)

        000. .... .... = Reserved: Not set

        ...0 .... .... = Nonce: Not set

        .... 0... .... = Congestion Window Reduced (CWR): Not set

        .... .0.. .... = ECN-Echo: Not set

        .... ..0. .... = Urgent: Not set

        .... ...1 .... = Acknowledgment: Set <== (C)

        .... .... 0... = Push: Not set

        .... .... .0.. = Reset: Not set

        .... .... ..0. = Syn: Not set

        .... .... ...0 = Fin: Not set

    Window size value: 1350

    [Calculated window size: 86400]

    [Window size scaling factor: 64]

    Checksum: 0xdab5 [validation disabled]

        [Good Checksum: False]

        [Bad Checksum: False]

    Options: (12 bytes), No-Operation (NOP), No-Operation (NOP), Timestamps

        No-Operation (NOP)

            Type: 1

                0... .... = Copy on fragmentation: No

                .00. .... = Class: Control (0)

                ...0 0001 = Number: No-Operation (NOP) (1)

        No-Operation (NOP)

            Type: 1

                0... .... = Copy on fragmentation: No

                .00. .... = Class: Control (0)

                ...0 0001 = Number: No-Operation (NOP) (1)

        Timestamps: TSval 4294945901, TSecr 73513

            Kind: Timestamp (8)

            Length: 10

            Timestamp value: 4294945901

            Timestamp echo reply: 73513

    [SEQ/ACK analysis]

        [This is an ACK to the segment in frame: 120]

        [The RTT to ACK the segment was: 0.016912000 seconds]

 

0000  00 30 64 0b 6c 69 00 00 91 05 fa 92 86 dd 60 00   .0d.li........`.

0010  00 00 00 20 06 40 20 01 00 00 00 00 00 02 00 00   ... .@ .........

0020  00 00 00 00 00 01 20 01 00 00 00 00 00 02 00 00   ...... .........

0030  00 00 00 00 00 02 a4 de 13 c4 0c f3 f8 fb <== (D) 18 c3   ................

0040  b3 cc <== (E) 80 10 05 46 da b5 00 00 01 01 08 0a ff ff   .....F..........

0050  ac 6d 00 01 1f 29                                 .m...)

 

 

TCP Release

 

 

 

TCP Flags

 

The type and the property of a TCP packet is determined by various flags located in the TCP header. Some of the important flags are listed below.

Flag

Meaning

Description

URG

Urgent Pointer

Indicate "This packet contains Urgent data pointed by Urgent Ponter". I request the reciever to set high priority for the data

ACK

ACKnowledgement

Acknowledge the successful receipt of the data

PSH

PUSH

Request the receiver to PUSH (Transfer) this data to higher layer as soon as it recieves regardless of the reciever buffer status

RST

RESET Indicate some error (e.g, unexpected packet) occurred and Abort the connection.

SYN

SYNchronisation

Indicate "This is a packet for Synchronization (TCP Establishment) process"

FIN

FINish Indicate "This is a packet for Finish (TCP Release) process"

 

 

TCP Window Size

 

In TCP header, there is a important field named as Window Size as shown below.

 

 

What does this mean ?

It indicate the amount of space (in byte) left in the receiving buffer. Basically it notifies the other party "I have this much of space in my receiving buffer, so you can send that amout of data to me at maximum". In other words, "Don't send me the data more than that".

What if this window size is set to be 0 ? What does it mean ?

It means "I have no space to receive any further data. So don't send me any data to me now".

 

As you may guess, this window size would be very important indicator for traffic flow control.. so is used as very important indicator for troubleshooting various TCP related issues.

 

There are various ways to change this window size adaptively and followings are some of the web posts that may be helpful for you.

 

TCP Window Scale

 

TCP Window Size is represented by a 16 bit number. It means that the maximum window size that can be represented is 64 K. What if we need to represent a window size larger than 64 K ? It cannot be reprented by the 16 bit number. To overcome this restriction, they come out with an option field called 'Window Scale'. If 'Window Scale' field is set, the real window size is calculated by as follows.

 

Real Window Size (Calculated Window Size) = Window Size Value x 2^(Window Scale in bits)

 

Window Scale can be set independantly for Client and Server.

Window Scale for Client can be set by SYN packet as shown below.

 

 

Window Scale for Server can be set by SYN,ACK packet as shown below.

 

 

 

MSS (Maximum Segment Size)

 

MSS represent the maximum size of a TCP packet. This size can be configured differently on Client and Server and the size information is exchanged by [SYN] and [SYN,ACK] packet as shown below. One thumb rule to set the MSS is to set it to be (MTU - 40) bytes. MTU is a kind of maximum packet size for each network card. So if you set MSS too large and the whole IP packet gets larger than MTU, it should be fragmented into multiple packets to make it fit to physical packet limitation and it may influence negatively on TCP throughput.

 

 

 

Just for your reference, you can figure out MTU of each of your network card by using following command if you are using Windows.

 

 

 

Example 1

 

File : Network_IP_TCP_Example01

 

Packet (1) ---------------------------------------------------------------------------------------------

 

 

Packet (2) ---------------------------------------------------------------------------------------------

 

 

Packet (3) ---------------------------------------------------------------------------------------------

 

 

Packet (4) ---------------------------------------------------------------------------------------------

 

 

Packet (5) ---------------------------------------------------------------------------------------------

 

 

Packet (6) ---------------------------------------------------------------------------------------------

 

 

Packet (14) ---------------------------------------------------------------------------------------------

 

 

 

Packet (15) ---------------------------------------------------------------------------------------------

 

 

Packet (16) ---------------------------------------------------------------------------------------------

 

 

 

Packet (17) ---------------------------------------------------------------------------------------------

 

 

 

References

 

[1] Understanding Asymmetric Links, Bu ers and Bi-Directional TCP Performance

[2] TCP SEQUENCE & ACKNOWLEDGEMENT NUMBERS - SECTION 2