Questions tagged [layer4]
For questions about OSI Layer 4 (transport layer).
147
questions
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1answer
24 views
Nature of TCP Timeouts
Are timeouts in TCPs inherently part of sequence numbers, or is it an entirely different component of a TCP packet?
I understand that timeouts provide some level of reliabilty to a data stream, in ...
8
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5answers
1k views
Does the destination port change during TCP three-way handshake?
I'm learning networking programming in C and there is a question bothers me a lot, does the destination port change during TCP three-way handshake? Let's say I have a cilent application running on ...
1
vote
1answer
41 views
Does the first TCP FIN packet (active close) goes with just only FIN bit set?
Suppose we have established TCP connection. The active close process assumes the FIN packet been sent to trigger TCP connection closing.
But I can't figure out, is this first (active close) FIN packet ...
1
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1answer
59 views
Is there any other network protocol than tcp/udp that works with ports?
Do you know at least one protocol (any OSI layer) that, like tcp and udp, works with ports ?
Thank you
2
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1answer
104 views
Why is delayed segment prevention (TIME_WAIT) in TCP only done for one side?
One of the reasons for TIME_WAIT is to prevent old packets/duplicates, coming in late, to screw up subsequent connections on the same (src ip, src port, dst ip, dst port) quadruplet.
That makes sense, ...
43
votes
7answers
15k views
Does UDP do anything at all?
It's my understanding that TCP has logic for ensuring reliable communication, but UDP just naively sends information along the channel set up for it using IP and things in lower layers.
Does UDP ...
2
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2answers
116 views
What will happen if a server receives a duplicate SYN for an existing connection?
If a server receives a fresh SYN packet for a connection that is already established, what should it do?
I have already seen What will happen at server side if it received 2 SYN packet from the same ...
1
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2answers
141 views
Is this UDP diagram wrong?
I was just wondering if anyone can tell me why all UDP diagrams I have seen list a request from the server to the client as the first point of communication? Take a look at the following diagram for ...
-1
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2answers
121 views
What is the window size of TCP pipelined sending?
We know that instead of sending a packet and waiting a RTT then sending another (stop and wait), TCP uses pipelined sending as picture below shows:
So, my question is, what's the maximum number of ...
2
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1answer
112 views
Do both sides need to agree on MSS value?
By definition, The TCP Maximum Segment Size (MSS) is the maximum amount of data that a host is willing to accept in a single TCP/IPv4 datagram. And MSS is one of the parameters which is negotiated ...
0
votes
2answers
47 views
Diffie helman Agreement on Prime Number and Generator
So, to start DH, the two parties need to agree upon a prime number and Generator, correct? How do they do do that without someone listening in?
Where does the loop end/start?
Also, Does it matter if a ...
0
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1answer
243 views
TCP Handshaking Meaning of RST
I am trying to implement TCP handshaking but receive RST instead of ACK:
No. Time Source Destination Protocol Length Info
62 24.622012890 192.168.0.147 ...
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0answers
15 views
Understanding OSI communication model from Electronics Perspective [duplicate]
I have gone through the OSI communication model and the associated levels.
But I do not understand on how is this implemented in the Hardware level. Or let me put it like this.
Suppose we have a USB ...
0
votes
1answer
421 views
TCP Checksum Calculation
I recently started with networking and tcp/ip.
The RFC 793 document tells the checksum is made of:
pseudo header+tcp header+tcp payload+zeroes(if not divisible by 16).
So I have to take 16bit ...
4
votes
1answer
90 views
Which exact bytes are transmitted while TCP zero window probing?
I read the post about TCP zero window probe and it seems that such a probing segments transmit some payload.
But I can't figure out which exactly bytes are sent in probing segments? Is it one (or more)...
1
vote
2answers
139 views
Can TCP stand for “Transport Control Protocol”
I know it might sound like a silly question, but afaik TCP stands for transmission control protocol. However I found a couple of websites using the term Transport Control Protocol.
So I was wondering ...
1
vote
1answer
72 views
How does triple handshake avoid establishing connection for duplicate packets(coming later down the line)
The idea behind triple handshake seems to be for duplicates to not cause issues.
The way I understand it host sends a sequence x to server. Server acknowledges it and sends it's own sequence y. Host ...
1
vote
1answer
153 views
Do sequence and acknowledgment numbers treat 3-Way Handshake differently?
1- Just as the message ID that is assigned to each message in PAR, the "sequence number" is a means of providing reliability in a transfer of a byte stream. So sequence numbers are assigned to every ...
4
votes
2answers
488 views
Is a datagram from an upper network layer converted 1:1 to one of the lower layer?
I know datagrams from upper layers are converted to those of lower layers by adding metadata, but I cannot seem to find this information anywhere. Which one of the below is correct?
A single TCP ...
1
vote
2answers
575 views
Sequence numbers of segments in three way handshakes of TCP connection establishment or termination
I was trying to understand TCP connection establishment and termination steps from Forouzan's book.
In third step of three way handshake of connection establishment, it says following
The client ...
1
vote
1answer
66 views
Is TCP aware of MAC adress
lets say I would like to make connection between client and server on port 80. Is 3 way handshake aware of server MAC address? I mean: client send SYN data packet to specific IP:port and MAC. In ...
3
votes
1answer
720 views
When TCP sender window shrinks?
I was learning TCP from Forouzan's book. It explains sender side window as follows:
In that it defines size of receiver window as follows:
rwnd = receiver buffer size - number of bytes waiting to ...
1
vote
1answer
102 views
ports and security
I'm thinking about how the "port" operates for the network connections and tried reading the CCNA cert. guide to understand the details, however, I need to clarify doubts:
does occupying all the TCP/...
1
vote
1answer
67 views
Finding MSS for a remote connection that uses tunnelling
I'm working on a codebase where we're trying to test network health by verifying if we can ping the remote site having payload = MSS without any packet fragmentation (using -df flag).
Typically, for ...
0
votes
0answers
20 views
how OSI and TCP/IP layers work? [duplicate]
I am really confused by how the layers these model work. I'm not asking about what are their functions. I read that transport layer provides end-to-end of all packet delivery, and networking also ...
0
votes
1answer
262 views
How does browser decide which destination port to use?
Suppose I have a Vanilla machine and this is the first time I am connecting it to Internet. There is no Cache at any level. I access a website through browser by typing example.com (no http//: or ...
2
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2answers
638 views
Why do transport layer do data chunking. If there is fragmentation in Network Layer
I just curiously wanted to know about the question regarding L4, L3 Data Chunks, IP Fragmentation :
Data chunking happened at Layer 4(Transport, TCP), and
Fragmentation at Layer 3(Network, ipv4)
...
1
vote
1answer
426 views
Why server cannot send data in TCP three-way handshaking?
In the three-way handshaking, only from the ACK packet we can send data.
Why cannot the server send data with SYN-ACK packet? I understated why a client cannot send data to a server (preventing, for ...
2
votes
3answers
622 views
What's the use of an acknowledgement number without the ACK flag set?
Isn't it redundant if the ACK flag in the header isn't also on?
Do you ever mix an acknowledgement with a normal message - send data and acknowledge a message in the same TCP segment?
I feel like ...
1
vote
1answer
530 views
TCP Three Way Handshake?
I have the following query about the tcp handshake.
To establish a connection the server must send a packet with SYN flag to 1
Then the server replies with a SYN,ACK flag to 1 , and afterwards the ...
2
votes
2answers
208 views
If an old FIN is delivered, will TCP terminate the new connection?
Two programs use a TCP connection (3-way handshake) to open, communicate, terminate the connection and open a new connection. If a FIN message sent to shut down the first connection is duplicated and ...
5
votes
4answers
7k views
Can anyone explain to me the difference between the PSH and URG flags in TCP segment?
According to me when PSH flag is set, it indicates that data will have to be sent immediately to the wire and on receiving side the data has to be pushed to the application immediately.
When URG flag ...
4
votes
1answer
153 views
Is an RST only segment ever legitimate?
Is a TCP segment with RST but no ACK ever legitimately encountered today?
While some kernels emit such an RST when a SYN segment is sent "illegitimately" from themselves (e.g., nmap half-syn, scapy, ...
2
votes
1answer
166 views
Of what layer is socket a concept?
In network layer, IP address is a concept.
In transport layer, port is a concept. Is socket a concept of transport layer? If not, what is the corresponding concept to socket in transport layer? Of ...
1
vote
1answer
62 views
How MPTCP closed connection?
how MPTCP connection with more than one subflow closed? what would happen to MPTCP connection if a client decide to close one of the subflows? it would be very kind of you if you explain it step by ...
0
votes
1answer
42 views
Can a ACK or SEQ Number exceed a range and crash the NIC?
I was watching the TCP Stream out of pure curiosity of my FTP Server and saw that the ACK and SEQ number increases by one with each sequence being transmitted successfully. I am not sure, but ...
11
votes
3answers
3k views
acknowledgment by TCP does not guarantee that the data has been delivered
In RFC 793 there is a part about the acknowledgment of TCP segments:
When the TCP transmits a segment containing data, it puts a copy on a
retransmission queue and starts a timer; when the ...
3
votes
1answer
444 views
How can a UDP Client and a packet sniffer run on the same machine?
In my understanding, if you have a UDP socket client that is bound to a particular port, then a second UDP client on the same machine can't bind to the same port.
If this is the case, then how can a ...
1
vote
1answer
71 views
Question about output of 'lsof -i' and UDP being connectionless
I'v got a simple setup:
Server (IP 10.0.0.33) listening on port 7770 using the command:
$ netcat -lup 7770
And a client (IP 10.0.0.2) that communicates with the server using:
$ nc -u 10.0.0.33 ...
4
votes
2answers
1k views
How does the Transport Layer construct the Pseudo-Header for TCP checksum?
THE TCP Psuedo Header needs sender and destination IP Address and protocol type. All of this informations is present in the network layer/IP header. Since the transport layer cannot access the data ...
1
vote
0answers
69 views
Processing received SACK information in case of reordering
I would like to understand how TCP receiver in established state should treat SACK information of out-of-order and fully within receive window segments,
where rcv_nxt < seq_no < rvc_nxt + ...
8
votes
4answers
8k views
Why is a TCP Socket identified by a 4 tuple?
Newbie to networking here. I'm reading the Computer Networking (3rd edition) book, and in section 3.2 they are discussing multiplexing / demultiplexing for both UDP and TCP.
In the UDP protocol, a ...
9
votes
1answer
8k views
How can a TCP window size be allowed to be larger than the maximum size of an ethernet packet?
I know that TCP window sizes can be scaled to over 64KB, but looking at an ethernet packet datagram, such as this one:
it looks like a layer 2 packet is limited in size to be much smaller than that. ...
1
vote
1answer
152 views
Questions about TCP packets
I am currently in college learning about networks, and I just wanted to make sure that I understood part of the TCP correctly.
The question is "What does TCP use to reassemble packets in the correct ...
2
votes
1answer
141 views
TCP connection question
Letās say I have 4 ethernet interfaces getting IP addresses via DHCP.
I make a TCP connection to a TCP server via one of the IP addresses on a certain ethernet interface. After the connection is ...
1
vote
1answer
917 views
Sender and Receiver timers in GBN, SR and TCP?
So basically for each of the GBN, SR and TCP's retransmission method, i have 2 question:
1.Is there a Timeout timer for EACH packet or one timeout timer for all of the packets?
2.Is there a ...
4
votes
3answers
4k views
At which layer does router operate?
I have studied from various sources that router is a layer 3 device. It does routing based on Network layer (layer 3) header, but I have a doubt.
Network address translation (NAT) is a feature of ...
1
vote
1answer
521 views
TCP close 2-way handshake with tcpdump package
Well, I got a pcap package that uses the tcpdump command. In this pcap, the TCP closes in 2-way handshake!
Questionsļ¼
In this picture, can anyone determine who initiated the signal to
close the ...
3
votes
2answers
125 views
Why is ARQ (Automatic Repeat reQuest) named so?
I can't make sense of a naming for Automatic Repeat reQuest.
The idea is that ARQ is a method for transmitting data without negative ACKs, so standard ARQ flow is:
Sender sends a packet
If sender ...
2
votes
2answers
738 views
Send TCP request to closed port, In which layer will the request be discarded
Client send TCP packet to server to a closed portļ¼ ie 80.
In which layer will the sync packet be discarded?
I guess for Client, the request will timeout, for server it should discard the request ...
