Questions tagged [layer4]

For questions about OSI Layer 4 (transport layer).

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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 ...
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1answer
57 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)...
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132 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 ...
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1answer
36 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 ...
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1answer
50 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 ...
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2answers
450 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 ...
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28 views

Why TCP is not in layer 5? [duplicate]

I read that TCP is in layer 4. But i also read that to send data with TCP i have to establish a session (in the handshaking phase). Does this mean that if we work only up to layer 4 TCP is useless (...
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2answers
146 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 ...
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1answer
55 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 ...
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1answer
194 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 ...
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1answer
90 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/...
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48 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 ...
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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 ...
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1answer
41 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 ...
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2answers
290 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) ...
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1answer
154 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 ...
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3answers
302 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 ...
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1answer
393 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 ...
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2answers
141 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 ...
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2answers
3k 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 ...
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79 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, ...
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1answer
76 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 ...
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1answer
52 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 ...
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39 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 ...
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2k 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 ...
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1answer
415 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 ...
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34 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 ...
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2answers
700 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 ...
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64 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 + ...
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4answers
5k 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 ...
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1answer
6k 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. ...
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132 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 ...
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1answer
101 views

TCP connection question

Let’s 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 ...
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1answer
601 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 ...
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2answers
2k 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 ...
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1answer
312 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! Questions: In this picture, can anyone determine who initiated the signal to close the ...
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2answers
104 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 ...
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2answers
411 views

Send TCP request to closed port, In which layer will the request be discarded

Client send TCP packet to server to a closed port, 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 ...
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801 views

Relationship between TCP window size and TCP congestion control

Am I correct that TCP receive window(which is a send window for the sender) has nothing to do with TCP CUBIC congestion control? TCP receive window is solely a property of TCP flow control?
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How is tcp/ip implemented

Well, first of all I am new to networking. And I have studied the layered structure of tcp/ip protocol stack. I know that the layered structure is an abstract model, but I didn't know how the ...
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322 views

Does 1-byte transmission for probe segment operate based on Timeout?

I wonder what ACK number would be if the receive buffer of receiver side is full. For example, Consider a case that sender transmits a segment whose SEQ is 800, and 100 bytes size. Because the receive ...
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82 views

Active Queue Management

Was reading through one of the papers on Active Queue Management.Just hit upon this paragraph, which I couldn't understand.Got confused at this part: https://queue.acm.org/detail.cfm?id=2209336 ...
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19k views

Does TCP open a new connection for every packet that is sent?

This might be a silly question but a me and few buddies have been discussing the potential limitations of TCP. We have an application that is going to listen for clients (think of a gateway) and route ...
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3answers
4k views

How does NAT decide which connections are inbound, and which are outbound?

I'm building a P2P application that needs to be able to access nodes behind a NAT. This NAT does not allow inbound connections, therefore the nodes outside the network cannot reach out to the nodes ...
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4answers
443 views

Port Numbers related doubt

Assume we have two browsers (Chrome and Firefox) running simultaneously, and both are requesting a web page. Lets assume Chrome is requesting www.facebook.com, and Firefox is requesting www.youtube....
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64 views

TCP Vegas: formula for expected number of backlogged packets

While reading Issues in TCP Vegas by Richard J. La, Jean Walrand and Venkat Anantharam (available online here), I stumbled across the following sentence (Part 3 - Rerouting, page 4): When the ...
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4k views

Is a TCP server limited to 65535 clients?

One might think that this places a hard limit on the number of clients that a single computer/application can maintain. One could be overseeing a web server, and be required to ensure that it can ...
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5k views

Why does NAT translate a source port?

I’m beginning to learn about nat and I was wondering why does NAT translate a source port? Doesn’t a port represent an application that’s requesting a service? So why must it be translated? Also what ...
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3answers
978 views

How does a router a layer 3 device translate port address in PAT, as it is a layer 3 device?

While I was reading about PAT i.e Port Address Translation.The router was configured to translate all the private IP addresses to one single public IP.Since router is a layer-3 device, it can do that ...
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3answers
1k views

Does the OSI model support connectionless communication in the transport layer? [closed]

In section 7.4.4.1.2 of ISO/IEC 7498-1:1994(E) it begins with: In connectionless-mode, the Transport Layer provides the following functions to support connectionless-mode transmission: But many ...