When I set UIP_STATISTICS=1, I can use uip_stat.udp.sent to know how many UDP packets I have send, but it does not include the number of packets forwarded. So how can I get it?
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I am trying to ping a host and capture the packets using Wireshark. I need to view the results in ICMP so I can view the IP addresses, but Wireshark keeps capturing the packets in ICMPv6 and as a result I can only see MAC addresses. Is there a way to change that?
I've tried looking in the options of my capture interfaces and didn't really find anything helpful. I'm not sure if it's a Wireshark issue or an issue with my system.
I made a tcpdump and captured packets, the configured MTU is 2140. I am analysing pcap files using Wireshark.
According to the configured MTU the expected maximum size of the packets should be 2154 (2140 bytes +14 ethernet header bytes). But I see packets of size greater than 2154 (ex 9010 bytes), On analyzing I found that these packets are generated on the machine where I made tcpdump (let's say A) and have the destination to another machine (let's say B). I expect a packet to be fragmented before it is sent to another host. I found some explanations online that says tcpdump captures packets before NIC breakdown, though this seems to be a valid explanation but it seems to contradict in my case because on machine A, I received packets of size greater than 2154 from B. Any thoughts, on why machine A is sending and receiving packets greater than configured MTU.
What you are seeing is most likely the result of TCP Segment Reassembly Offloading. This is a feature available on some network cards with matching drivers.
The idea is that the reassembly of many of the TCP segments is handled in the NIC itself. This turns out to be pretty effective in reducing overhead on the CPU/OS side since the network driver need only handle, perhaps, 1 "packet" out of 10, seeing just one large packet, rather than receiving and reassembling all 10.
You can read more about it here.
Updated answer
If your packet is UDP
This behaviour is normal. but there is not much you can do to see the individual packets on the end machines. The UDP packet is broken down into MTU compliant packets and reassembled at the Link layer, usually by specific hardware. This is too low to to be captured by Wireshark/pcap.
If you want to capture the individual broken down packets, you have to do this on an intermediate machine/network card, for example a gateway between the two hosts, because the original UDP packet is not reassembled until it reaches its final destination. Note : this gateway can be virtual.
See notes.shichao.io/tcpv1/ch10
Leaving this here in case someone with the same problem comes...
If your packet is TCP :
It sounds like Wireshark is reassembling packets for you. This is often the default for TCP streams. You can change this by richt-click on a stream -> Protocol Preferences -> Allow subdissectors to reassemble TCP.
i'm trying to do the following:
a) Set Contiki in Promiscuous Mode.
b) Then retrieve all UDP and RPL packets send, not only to current node but also between two other nodes within communication range.
I have the following code:
NETSTACK_RADIO.set_value(RADIO_PARAM_RX_MODE, 0);
simple_udp_register(&unicast_connection, 3001,
NULL, 3000, receiver);
where receiver is an appropriate callback function. I am able to collect UDP packets send to the current node, but still unable to receive packets send between other nodes in my communication range.
Setting the RADIO_PARAM_RX_MODE only controls which packets the radio driver filters out. There are multiple layers in an OS network stack, of which the radio driver is only the first one. The next ones are RDC and MAC, which still filter out packets addressed to other nodes, and there is no API to disable that.
The solution is to either modify the MAC to disable dropping of packets not addressed to the local mode or write your own simple MAC. The latter is what the Sensniff (Contiki packet sniffer) does - see its README and source code. By the way, if you just want to log all received packets, just use Sensniff!
When I enable Wireshark capture on my laptop, the application becomes slow because it captures all the packets. I am running about 100 Mbps of traffic with different packet sizes.
Let's say, I only need to capture rtp,sip packets and ignore the other UDP, TCP, DHCP etc.
How can I do it using Wireshark?
Note: I know the filter option to view only the packets I need, but the request is to only capture rtp packets
From the Wireshark SIP wiki page:
Capture Filter
You cannot directly filter SIP protocols while capturing. However, if you know the UDP or TCP or port used (see above), you can filter on that one.
For help with writing capture filters, refer to the pcap-filter man page.
I want to send broadcast message to all peers in my local network. Message is a 32 bit integer. I can be sure, that message will not me fragmented, right? There will be two options:
- peer will receive whole message at once
- peer will not receive message at all
Going further, 4 bytes is maximum size of data, that can be sent in one UDP datagram? I use Ethernet based network in 99%.
IPv4 specifies a minimum supported MTU of 576 bytes, including the IP header. Your 4 byte UDP payload will result in an IP packet far smaller than this, so you need not fear fragmentation.
Furthermore, your desired outcome - "peer will receive whole message at once or peer will not receive message at all" is always how UDP works, even in the presence of fragmentation. If a fragment doesn't arrive, your app won't recieve the packet at all.
The rules for UDP are "The packet may arrive out-of-order, duplicated, or not at all. If the packet does arrive, it will be the whole packet and error-free.". ("Error-free" is obviously only true within the modest limits of the IP checksum).
Ethernet packets can be up to around 1500 bytes (and that's not counting jumbo frames). If you send broadcast messages with a payload of only 4 bytes, they shouldn't get fragmented at all. Fragmentation should only occur when the packet is larger than the Maximum Transmission Unit (so about 1500 bytes over Ethernet).