By which fields of TCP packet, we can tell if it is a continuation of a previous TCP packet?
To be part of the same TCP stream, TCP segments must belong to the same session .i.e have the same source IP, source port, destination IP and destination port.
Once two segments are from the same stream, they can be ordered by the sequence number field in the TCP header. The sequence number of the next segment should be equal to the sequence number of the previous segment plus the number of bytes in the previous segment.
So if we have a segment with seq# 1000 and a payload of 200 bytes, the next one should have the number 1200.
TCP: Sequence number, Acknowledgement number.
In Wireshark, you can Right Click on a packet and choose Follow to follow TCP or HTTP stream. It should help seeing how there values change from packet to packet.
Related
I have read that if a packet in TCP does not have a valid sequence number, then the packet will be dropped by the receiving side.
But what if the packet have a valid sequence number but have an invalid ACK number, will the packet also be dropped by the receiving side?
There's really no such thing as an "invalid" acknowledgement number. If you mean that the number doesn't fall within the window (the advertised memory buffer of the host), that's a different question.
If the ACK isn't within the window, the host will respond with a reset to indicate that it has no notion of there being a connection with these "keys", where the keys are source, destination, source port, destination port, sequence numbers.
I am building an app where my phone frequently send data to my server. Since I would be using my mobile data, I was wondering how much data it cost to set up (and tear down?) a TCP connection to my server.
TCP Three-way handshake
Device 1 sends its TCP sequence number and maximum segment size to Device 2.
Device 2 responds by sending its sequence number and maximum segment size to Device 1.
Device 1 acknowledges receipt of the sequence number and segment size information.
Each packet is composed of an IP header and data (payload). In this case, the data section contains TCP. The TCP header contains various fields including the source and destination ports, sequence and acknowledgment numbers, window size, TCP flags, urgent pointer, and reserved bits.
Like the IP header, the TCP header may also contain options. (Note that TCP options and IP options are two different things.) Because the TCP options change the length of the TCP header, the length is set in the header.
IPv4 header is five 4-byte chunks, or 20 bytes total.
TCP typically usually uses 24 bytes of the header for handshake (first two packets) and about 20 for normal packet transmission.
Maximum Segment Size (MSS): 4 bytes
Window Scale (WSCALE): 3 bytes
Timestamp (TS): 10 bytes
No Operation (NOP): 1 byte
Selective Acknowledgment Permitted (SackOK): 2 bytes
Selective Acknowledgment Data: 10 bytes (plus 8 bytes for each additional pair of sequence numbers)
Terminating a Connection
Even though establishing a connection using 3-way handshake requires only 3 packets to be transmitted, tearing down one requires 4!
In the first frame the client sends a FIN that is accompanied by an ACK. The FIN parameter is set, it will inform the server that it has no more data to send.
The response (2nd frame) would be simply the server acknowledging the FIN sent from the client.
Even though TCP has established connections between the two computers, the connections are still independent of one another. Therefore, the server will also transmit a FIN to the client.
You guessed it right ... the client would ACK the FIN of the server in the last forth packet.
The offset of each of the frames is typically 20 bytes.
To sum it up.
Establishing a connection: ~ 128-136 bytes
Tearing down a connection: ~ 160 bytes
If you plan to use TLS / SSL handshake, this is estimated to be between 4.5k-6.5k.
Note: Please also take a look at TCP/IP Header Compression
Sources:
Inside the TCP Handshake
Explanation of the Three-Way Handshake via TCP/IP
Studying Normal Traffic, Part Three: TCP Headers | Symantec Connect
I'm filtering packets with libpcap with a filter like "tcp src localhost". It filters all the packets whose source is localhost (my host).
When localhost doesn't receive a TCP confirmation of an already sendt packet, localhost will forward the packet.
Not all the packets filtered by libpcap will arrive to its destination, and I need to identify when a packet is a "forwarded packet". Is there any way with libpcap to identify a forwarded packet?
By my understanding, you're looking for TCP retransmissions. These can be found by display fitters in wireshark after capturing. These two should help you:
Retransmitted packets can be found through the display filter tcp.analysis.retransmission (more such filters).
When the receiver gets an out-of-order packet (usually indicates lost packet), it sends a ACK for the missing seq number. This is a duplicate ACK and these can be found by using tcp.analysis.duplicate_ack (details).
I have a question on how TCP_ACK works when the original packet are fragmented.
For example, original packet size is 1,500*N bytes and MTU is 1,500. Then, the packet will be frgmented into (approximately) N packets.
In this case, how does the receiver sends TCP_ACK to the sender?
I checked with wireshark, it seems that the receiver sends TCP_ACK for every two fragmented packet. Is it right?
Could you give me some refereces for this or explanation?
Thanks.
IP layer on the receiver stack reassembles all the IP fragments into a single TCP segment before handing the packet over to TCP. Under normal conditions, TCP should send only one ACK for the entire TCP segment. The ACK # would be the next expected SEQ # as usual.
Wireshark and Network Monitor provide filters for this but I want to know how can I infer whether a packet is a TCP Keep-Alive or Keep-Alive Ack by looking at the header or payload.
A TCP keep-alive packet is an ACK with the sequence number set to one less than the
current sequence number for the connection.
Here's what Wireshark says about a keep-alive ACK:
Set when all of the following are true:
The segment size is zero.
The window size is non-zero and hasn’t changed.
The current sequence number is the same as the next expected sequence number.
-The current acknowledgement number is the same as the last-seen acknowledgement number.
The most recently seen packet in the reverse direction was a keepalive.
The packet is not a SYN, FIN, or RST.