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Questions tagged [layer4]

For questions about OSI Layer 4 (transport layer).

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Why do I observe a 4-way TCP handshake instead of an expected 3-way when establishing a connection? [duplicate]

I'm doing a simple HTTP GET query on a localhost from curl to a local webserver. After I did a tcpdump I noticed these packets to be sent each time a request is done: 11:11:27.537300 IP 127.0.0.1....
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1answer
236 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
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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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1answer
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Can anyone Explain me difference Between PSH and URG flag 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 ...
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1answer
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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
48 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
43 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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1answer
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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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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
281 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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1answer
25 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
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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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0answers
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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
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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
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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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1answer
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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
91 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
333 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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1answer
735 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
176 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
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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
145 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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1answer
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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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3answers
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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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1answer
173 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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1answer
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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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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
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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
417 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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1answer
55 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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1answer
2k 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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5answers
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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
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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
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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 ...
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2answers
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TCP streams/connections

I want to ask how TCP creates streams (and what exactly is a stream). In Wireshark for example, I see multiple streams to one server and I can not understand how/when TCP creates this streams from the ...
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3answers
808 views

Confusion regarding Port number

I may sound silly but I seem to be getting confused with what port numbers are used for. Someplace it's mentioned that port addresses are used for port translation using pat which i get. Somewhere it'...
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2answers
385 views

Are there any transport layer protocol without the concept of port?

Are there any transport layer protocol that does not have the concept of ports? If so what do they use instead? Also how does NAT work for such protocols?
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2answers
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Why do open ports give no response in networking?

Nmap places ports in open|filtered state when it is unable to determine whether a port is open or filtered. This occurs for scan types in which open ports give no response. The lack of response could ...
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2answers
558 views

How sliding window protocols handle duplicate packets caused by extreme network delays?

Let's say there is a network that sometimes delivers data with extreme delays. And there is a protocol that uses a "sliding window" algorithm for tracking packets acknowledgement. And imagine such a ...
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1answer
236 views

Does an EIM-NAT work across both UDP and TCP?

I've read RFC 4787, 5382 and 5128 and I just need a bit of clarification. Assume a NAT has Endpoint Independent Mapping behavior for both UDP and TCP. Although I understand that if a machine would ...
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1answer
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What's the difference between port number and protocol number?

Can someone explain it clearly. I know that port numbers are used by applications to differentiate which services you have requested, and routing protocols use protocol numbers in TCP header.
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5answers
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Is rerouting possible in UDP?

So I am trying to understand routing for TCP and UDP segments. I have a couple of things that I am confused about: In TCP, is there any specification on how many packets would be dropped before a ...
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1answer
2k views

RST after an out of order packet

I've encountered an force termination by a host, even after a successful (at least it seems to be successful; correct me if I'm wrong) FIN ACK close of connection. Packet trace at the moment is ...
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1answer
596 views

Where is the maximum packet size for TCP identified as 65,535?

In doing online searches, the value "65,535" is often referenced as the maximum packet size for TCP. Where is this stated? I ask because RFC 793 states, in relation to the "Maximum Segment Size" ...
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1answer
111 views

Why I don't see drops with a TCP flows in Linux

I wanted to observe the congestion window of a TCP flow so for that I have to nodes (using Linux namespaces) connected to each other through veth pairs and a bridge. To limit bandwidth to 10mbps I ...
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2answers
170 views

Hardware Implementation of Ports

What is the hardware implementation of a port and/or network socket? I understand that, at the lowest level, x86 uses opcodes such as IN and OUT when the data resulting from the instruction execution ...
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1answer
298 views

TCP and internal packet retransmission

When point A sends data to point B, it's delivered reliably over TCP. If the underlying channel loses data, the TCP stack on point B side automatically asks to re-transmit lost frames. Let's assume ...
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1answer
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What is the value of EstimatedRTT at first time?

Although this question might be marked as duplicated! I'm asking again because other question's answers like this couldn't help me. So please consider the case and don't mark it as duplicate and ...
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Question about the 7 layers of OSI Model [duplicate]

I just saw level 1-2-3-4 (from Physical to Transport) We have the following : The thing is: We know that at layer 3 there is the IP protocol that uses DNS Server to get the IP Address of the ...
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1answer
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TCP MSS over WiFi

The MTU for 802.11 is 2296 bytes. Does this mean that if TCP is used over 802.11, the MSS can only be 2296 - 40 = 2256? Can't one use a higher MSS which would then get fragmented over 802.11? In ...