TCP stands for Transmission Control Protocol and is one of the core protocols of the Internet Protocol Suite. TCP complements the Internet Protocol (IP), and therefore the entire suite is commonly referred to as TCP/IP.

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28
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8answers
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Why would a server not send a SYN/ACK packet in response to a SYN packet

Lately, we've become aware of a TCP connection issue that is mostly limited to mac and Linux users who browse our websites. From the user perspective, it presents itself as a really long connection ...
86
votes
7answers
128k views

How to forcibly close a socket in TIME_WAIT?

I run a particular program on linux which sometimes crashes. If you open it quickly after that, it listens on socket 49201 instead of 49200 as it did the first time. netstat reveals that 49200 is in ...
5
votes
5answers
14k views

Server not sending a SYN/ACK packet in response to a SYN packet

Using iptraf, tcpdump and wireshark I can see a SYN packet coming in but only the ACK FLAG is set in reply packet. I'm running Debian 5 with kernel 2.6.36 I've turned off window_scaling and ...
17
votes
4answers
86k views

Which is the default TCP connect timeout in Windows?

Which is the default TCP connect timeout in Windows? There is a registry key to configure it or it's set dynamically?
5
votes
5answers
5k views

Many ISP's is block port 25, how do I choose an alternative port?

I am building an application that will be acting as a combined MUA/MTA on different networks. However, many of the networks are with ISP's that block port 25 for SMTP. Therefore I would also like to ...
3
votes
1answer
16k views

Enabling system wide TCP keepalives on a Windows system

Can I change/add the KeepAliveTime and related registry variables to make a Windows machine send TCP keepalive packets on an idle connection? I want to know if anyone has successfully got this ...
21
votes
5answers
55k views

What is the best way to transfer a single large file over a high-speed, high-latency WAN link?

This looks related to this one, but it's somewhat different. There is this WAN link between two company sites, and we need to transfer a single very large file (Oracle dump, ~160 GB). We've got full ...
41
votes
11answers
37k views

What is the difference between UDP and TCP?

My router has two protocols (and a "both" option) that I can select when setting up port forwarding: UDP and TCP. What is the difference between these two protocols and when would you select one over ...
19
votes
4answers
2k views

Slow Transfers over Distance

From our NY Datacenter, transfers to locations that are farther away are having poor performance. Using speed test to test various locations, we can saturate our 100 mbit uplink to Boston and ...
3
votes
2answers
7k views

how to calculate packet loss from a binary TCPDUMP file

Our connection to just one remote server on port 80 over the internet is not working properly. (From time to time it is working and sometimes not) It must be some kind of packet loss, because from ...
20
votes
1answer
2k views

Under what circumstances is TCP-over-TCP performing significantly worse than TCP alone (2014)?

Many admins keep perpetuating - on ServerFault and elsewhere - how bad of an idea TCP-over-TCP is, e.g. in VPNs. That even the slightest packet loss will make one suffer from at least severe ...
45
votes
8answers
378k views

What does “TTL expired in transit” mean on a ping attempt?

We get the message “TTL expired in transit” when we try to ping to a server in a different network segment. When we run tracert, 4 ip addresses repeat themselves indefinitely: 14 60 ms 59 ms ...
29
votes
5answers
10k views

Can TCP and UDP packets be split into pieces?

Can TCP packets arrive to receiver by pieces? For example, if I send 20 bytes using TCP protocol, can I be 100% sure that I will receive exactly 20 bytes at once, not 10 bytes then another 10 bytes ...
10
votes
2answers
11k views

How can I tune the initial TCP retransmit timeout?

The initial TCP RTO value of 3s is too long for most LAN-based applications. How can I tune it lower? Is there a sysctl?
8
votes
3answers
8k views

What are the ramifications of setting tcp_tw_recycle/reuse to 1?

I set both tcp_tw_recycle/reuse to 1 in my configuration file. What are the ramifications of doing this? If a tcp socket is re-used, does that pose a security risk? i.e. 2 different connections both ...
6
votes
3answers
3k views

How do high traffic sites service more than 65535 TCP connections?

If there is a limit on the number of ports one machine can have and a socket can only bind to an unused port number, how do servers experiencing extremely high amounts (more than the max port number) ...
4
votes
3answers
2k views

Why is it 3-way-handshake and not more?

TCP uses the 3-way-handshake mechanism. I understand how it works pretty well but I wonder why it's not 4, or even 5-way? Is it because that's too much overhead and is not needed?
3
votes
3answers
68 views

Do clients typically implement failover/load-balancing on multiple A records?

Typically, load balancers like Amazon's Elastic Load Balancers use a DNS record set with multiple A records to provide multiple load balancer instances which can handle traffic to requesting endpoints:...
8
votes
2answers
8k views

Force forwarder DNS requests to TCP mode

I have set up a DNS-server on SLES10 (currently bind 9.6) on a multi-homed server. This server can be queried from all internal networks and delivers answers for all internal networks. We have two ...
5
votes
4answers
1k views

Why was the FTP protocol designed to use more than one port?

The question is in the title. The reason I wonder this is because I'm coding an tcp service and would like to explore some of the reasons as they may shed some light on my work.
2
votes
5answers
3k views

I am looking for a good load balancing (or reverse proxy) software for TCP and HTTP, any suggestions? [closed]

I am looking for a good load balancing (or reverse proxy) software for TCP and HTTP, any suggestions?
-1
votes
1answer
4k views

How do i view / enable kernel logs on an ec2 instance (amazon linux)?

I've got a amazon ec2 server that i'm playing around with TCP settings in the sysctl.conf to increase the number of concurrent TCP connections it can handle, and i want to be able to view the kernel ...
27
votes
3answers
98k views

“Possible SYN flooding” in log despite low number of SYN_RECV connections

Recently we had an apache server which was responding very slowly due to SYN flooding. The workaround for this was to enable tcp_syncookies (net.ipv4.tcp_syncookies=1 in /etc/sysctl.conf). I posted a ...
42
votes
9answers
19k views

Windows TCP Window Scaling Hitting plateau too early

Scenario: We have a number of Windows clients regularly uploading large files (FTP/SVN/HTTP PUT/SCP) to Linux servers that are ~100-160ms away. We have 1Gbit/s synchronous bandwidth at the office and ...
28
votes
2answers
26k views

What's the difference between Unix socket and TCP/IP socket?

It's from this answer: http://stackoverflow.com/questions/2482411/is-this-pdo-bug-fixed-now/2482424#2482424
10
votes
4answers
7k views

haproxy + stunnel + keep-alive?

I'd like to put stunnel in front of haproxy 1.4 to handle HTTPS traffic. I also need stunnel to add the X-Forwarded-For header. This can be achieved by the "stunnel-4.xx-xforwarded-for.diff" patches ...
16
votes
3answers
3k views

Troubleshooting Network Speeds — The Age Old Inquiry

I'm looking for help with what I'm sure is an age old question. I've found myself in a situation of yearning to understand network throughput more clearly, but I can't seem to find information that ...
15
votes
5answers
8k views

I'm designing a system to handle 10000 TCP connections per second, what problems will I run into?

I have a relatively new 8-core box running CentOS. I would like to develop a stats server that uses TCP. It's very simple, it accepts a TCP connection, increments a counter and closes the connection. ...
7
votes
3answers
7k views

TIME_WAIT connections not being cleaned up after timeout period expires

I am stress testing one of my servers by hitting it with a constant stream of new network connections, the tcp_fin_timeout is set to 60, so if I send a constant stream of something like 100 requests ...
3
votes
2answers
3k views

dmesg SYN flood on 80 sysctl -p

I am getting a synflood on my server port 80 and i cannot stop it. first i got tables full then i disabled iptables to find out that its a synflood netstat -n | grep :80 |wc -l #returns 1300 - 2000 ...
19
votes
4answers
16k views

What is the “in-the-wire” size of a ethernet frame? 1518 or 1542?

According to the table here, it says that MTU = 1500 bytes and that the payload part is 1500 - 42 bytes or 1458 bytes (<- this is actually wrong!). Now on top of that you have to add IPv4 and UDP ...
4
votes
2answers
2k views

Remote server hangs, gets stuck. How to debug?

I have an vps running on VmWare ESX with Ubuntu 8.04 LTS. It has been running smoothly for the past 3 months, however recently we've notices two strange bugs. a. The server hangs, today was second ...
8
votes
4answers
8k views

tool for splitting pcap files by TCP connection?

Is there tool to split a packet capture file (in pcap format) into separate files for each TCP connection ? (other than a home grown shell script which probably needs to run twice over the capture...)...
8
votes
2answers
3k views

More than 65536 TCP connections on Linux

I'm stuck trying to setup more than 65536 outgoing TCP connections from a Linux (RedHat5) box. I have already configured both outgoing and accepting boxes to allow enough file descriptors. I don't ...
8
votes
2answers
3k views

Finding short-lived TCP connections owner process

Running tcpdump on local connections to an apache server, I found TCP connections being established and closed immediately every 2 seconds. How do I find which process is responsible for these? ...
7
votes
4answers
3k views

Automatically-reconnecting TCP tunnel

I have an unreliable network connection between two machines: sometimes active TCP connections get dropped for reasons beyond my control. I want to establish a reliable TCP connection between the two ...
7
votes
1answer
29k views

Why does WireShark think this frame is a TCP segment of a reassembled PDU

Please find a small pcap file here illustrating my problem. I have a three-way TCP handshake, followed by two FIX logons. (FIX is a protocol used in trading.) The first FIX logon (frame 4) is ...
7
votes
2answers
12k views

Reduce firewall rules by half - one iptables rule for tcp and udp

I have a number of iptables rules on my firewall that look like this: iptables -A zone_lan_forward -p tcp -d 1.2.3.0/24 -j ACCEPT iptables -A zone_lan_forward -p udp -d 1.2.3.0/24 -j ACCEPT Is ...
6
votes
2answers
10k views

How do I prevent TCP connection freezes over an OpenVPN network?

New details added at the end of this question; it's possible that I'm zeroing in on the cause. I have a UDP OpenVPN-based VPN set up in tap mode (I need tap because I need the VPN to pass multicast ...
6
votes
2answers
3k views

Duplicate incoming TCP traffic on Debian Squeeze

I have to test a homebrew server that accepts a lot of incoming TCP traffic on a single port. The protocol is homebrew as well. For testing purposes, I'd like to send this traffic both : - to the ...
3
votes
3answers
7k views

Easy way to “edit” the traffic coming from a tcp host (linux)

I need to make some small modification to incoming traffic from a known tcp host:port before the process handling the connection get the stream. For example, let 192.168.1.88 be a remote host which ...
2
votes
2answers
4k views

Can I test broadcast packets on a single machine?

I want to have a server sending broadcast UDP packets and two other clients, in the same machine, receiving them. Can I do that somehow? What IP address would I use? @gravyface gave me hope but I ...
6
votes
3answers
4k views

What does tcp_orphan_retries set to 0 mean?

Does setting tcp_orphan_retries to 0 mean there is no limit to retries, or does it mean that it won't retry at all?
4
votes
2answers
17k views

How to check TCP timeout in linux / macos?

I have a network problem on my MacOS that i need to troubleshoot. I know that TCP socket has internal timeout that will close connection if remote party is not responding (but no graceful disconnect ...
2
votes
2answers
2k views

socat connect-connect “proxy” two inbound TCP connections to expose a firewalled service?

I'm struggling a bit trying to see how to use socat (or a similar TCP port redirector) to expose a firewalled service on a PC. Here's the scenario: I have a server PC which will be acting as a ...
1
vote
5answers
3k views

Increasing bandwidth with multiple NICs

If I have two NICs in my system and both are connected to different ISP with different IP addresses, can I distribute my network traffic on them to increase my Bandwidth. I have already looked at ...
6
votes
4answers
5k views

apache webserver unresponsive with server-status showing all child processes waiting for connection

My setup: i have 3 nearly identical webserver machines serving the same high loaded dynamic website with simple load balancing over dns. The service has been working for over two years with the same ...
5
votes
3answers
681 views

Is SYN flooding still a threat?

Well recently I've been reading about different Denial of Service methods. One method that kind of stuck out was SYN flooding. I'm a member of some not-so-nice forums, and someone was selling a python ...
5
votes
2answers
5k views

Load Balancing long running TCP connections

I'm trying to research the best way to load balance long running TCP connections for the following scenario: We have multiple servers behind a redundant set of firewalls and clients establish long ...
5
votes
3answers
450 views

How to fine tune TCP performance on Linux with a 10Gb fiber connection

We have 2 Red Hat servers that are dedicated for customer speedtest. They both use 10Gb fiber connections and sit on 10Gb links. All network gear in between these servers fully support 10Gb/s. Using ...