Questions tagged [layer4]

For questions about OSI Layer 4 (transport layer).

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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
31 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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NetFlow to determine users on IPTV

tl;dr: I want to know if there is a possibility to determine how many users are streaming from specific IP addresses? I want to know if there is a software which filters NetFlow data from Cisco ...
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2answers
65 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
57 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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136 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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38 views

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
309 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
131 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
572 views

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
62 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
51 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
48 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
32 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
374 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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27 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
425 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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57 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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2k 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
4k 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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114 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
96 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
379 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
1k 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
211 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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74 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
232 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
622 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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1answer
235 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
76 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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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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3k 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
431 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
59 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
3k 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
4k 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
794 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 ...
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2answers
1k views

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
1k 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
462 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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924 views

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
625 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
272 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
5k views

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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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
624 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" ...