As you can see in first figure, what does that Raw packet data mean? What is the difference between normal tcp packet in figure 2?
FYI, I'm using Wireshark 2.2.0.
Basically when you are capturing packets on an interface you have an associated link type to it (ethernet, 802.11, 802.15.4, etc).
Raw packet is used when you dont have any, the first bytes captured are directly the IPv6 or IPv4 header.
Raw IP; the packet begins with an IPv4 or IPv6 header, with the "version" field of the header indicating whether it's an IPv4 or IPv6 header.
From http://www.tcpdump.org/linktypes.html
Related
I couldn't understand why UDP header has 'length' field, and why it is needed?
If the reason is to know where the 'application message(L5 data)' begins in the segment, it can just be gotten from 'UDP data - UDP header length(it is already known value)'.
Because UDP can be transmitted over another protocol than IP.
And also because UDP transmits datagram messages with a length (udp length) which can be sent over multiple IP fragmented packets.
Source: https://notes.shichao.io/tcpv1/ch10/
The UDP header length field is the length of the UDP header plus the UDP data. It is indeed redundant since this length can be calculated from the IP header total length field where the UDP datagram length is the IP total length minus the IP header length.
UDP uses message stream NON FIFO as communication model between the sender and receiver. If the size is not mentioned then it will not be possible to decipher the message at rxr.
Say m1m2m3 is sent then each message to be notified so that u can trace back the message.
Regards
I have a protocol that exactly follows ipv6 header format and is encapsulated in udp for transmission. Is there any way to make wireshark decode the udp data as an ip packet?
There's a protocol called "Teredo Tunneling" (which see) which encapsulates/tunnels IPV6 in UDP packets (using UDP port 3544).
If your capture is of a UDP conversation using UDP port 3544 on one end of the conversation, then Wireshark should automatically recognize the conversation as Teredo (i.e., IPV6 over UDP) and properly decode the IPV6 payload in the UDP data.
If your capture is using a different UDP port, do the following to specify that a particular UDP conversation is to be decoded as Teredo:
Right click on a UDP frame you wish to decode as Teredo.
Select "Decode As" in the right-click menu.
Select the "Transport" tab in the "Decode As" pop-up window.
Select Teredo in the right-hand "decode as" column.
Click "OK".
Often when I'm using WireShark, I don't really need to capture the contents of a frame. I'm essentially just making sure that some traffic is flowing, and maybe checking a few flags (FIN ACK etc). Yet the default setting is to capture everything the (filtered) NIC sees, which can fill up my HD pretty fast.
Is there a setting where you can just see the summary lines, without capturing the contents?
I don't really need to capture the contents of a frame ... I'm essentially just making sure that some traffic is flowing, and maybe checking a few flags (FIN ACK etc)
So you need only the TCP header (and all the packet data that goes before the TCP header). A typical IPv4 header is 20 bytes long, as is a typical TCP header, so, on an Ethernet, you would typically only need to capture the first 54 bytes of the packet. For IPv6, the typical header is 40 bytes long, so that, on an Ethernet, you would typically only need the first 74 bytes. However, the IPv4 and TCP headers might have options, and the IPv6 header might have extension headers, so capturing 68 bytes for IPv4 or 96 bytes for IPv4-or-IPv6 might be better
For other networks, you'd have to adjust that value based on the link-layer header length. For 802.11 when not in monitor mode, you'll probably get "fake Ethernet" headers, so the values that are used for Ethernet will work; for 802.11 in monitor mode, you might have a "radiotap" header or some other "radio metadata" header, so you'd have to look at some captures on your machine to see how big the 802.11 header + the radio metadata header would be.
Once you know the "snapshot length" you should use, you can specify it in the "Limit each packet to [ ... ] bytes" field of the interface options in Wireshark 1.8 and later or in the "Capture Options" dialog prior to 1.8.
Wireshark will still show what packet details it can given the limited amount of packet data that it captured, so you won't see only the summary lines. You will, however, get less data per packet, saving disk space.
So I have trouble finding a source that describes whether the TCP Packet is the payload of the IP Datagram or vice versa. I imagine the TCP Packet must be the payload because presumably the router can divide the IP Datagram therefore splitting up the TCP Packet and then the final router would have to reassamble them. Am I right?
If by "payload" you're referring to the data that comes after an IP header, then TCP is the "payload" of an IP packet when receiving data, since it's an upper level protocol.
The proper term for networking is actually encapsulation though.
It basically works by adding on progressive layers of protocols as information travels down from the application to the wire. After transmission, the packets are re-assembled and then the packets are error checked, the headers are stripped off, and what you are referring to as the "payload" becomes the next chunk of information that is checked. Once all of the outer protocol layers are stripped off the server/client has the information that directly corresponds to what the application sent.
Tcp\IP are two important proctocols. Tcp is connection oriented, while IP is a connection-less protocol. IP stands for a logical address, which works as packet address. The source packet has destination address for its destination. Tcp works with this logical address and helps the packets to reach their destinations, and provides acknowledgement when packet reached to its destination.
Why is the Pseudo header prepended to the UDP datagram for the computation of the UDP checksum? What's the rational behind this?
The nearest you will get to an answer "straight from the horse's mouth", is from David P. Reed at the following link.
http://www.postel.org/pipermail/end2end-interest/2005-February/004616.html
The short version of the answer is, "the pseudo header exists for historical reasons".
Originally, TCP/IP was a single monolithic protocol (called just TCP). When they decided to split it up into TCP and IP (and others), they didn't separate the two all that cleanly: the IP addresses were still thought of as part of TCP, but they were just "inherited" from the IP layer rather than repeated in the TCP header. The reason why the TCP checksum operates over parts of the IP header (including the IP addresses) is because they intended to use cryptography to encrypt and authenticate the TCP payload, and they wanted the IP addresses and other TCP parameters in the pseudo header to be protected by the authentication code. That would make it infeasible for a man in the middle to tamper with the IP source and destination addresses: intermediate routers wouldn't notice the tampering, but the TCP end-point would when it attempted to verify the signature.
For various reasons, none of that grand cryptographic plan came to pass, but the TCP checksum which took its place still operates over the pseudo header as though it were a useful thing to do. Yes, it gives you a teensy bit of extra protection against random errors, but that's not why it exists. Frankly, we'd be better off without it: the coupling between TCP and IP means that you have to redefine TCP when you change IP. Thus, the definition of IPv6 includes a new definition for the TCP and UDP pseudo header (see RFC 2460, s8.1). Why the IPv6 designers chose to perpetuate this coupling rather than take the chance to abolish it is beyond me.
From the TCP or UDP point of view, the packet does not contain IP addresses. (IP being the layer beneath them.)
Thus, to do a proper checksum, a "pseudo header" is included. It's "pseudo", because it is not actaully part of the UDP datagram. It contains the most important parts of the IP header, that is, source and destination address, protocol number and data length.
This is to ensure that the UDP checksum takes into account these fields.
When these protocols were being designed, a serious concern of theirs was a host receiving a packet thinking it was theirs when it was not. If a few bits were flipped in the IP header during transit and a packet changed course (but the IP checksum was still correct), the TCP/UDP stack of the redirected receiver can still know to reject the packet.
Though the pseudo-header broke the separation of layers idiom, it was deemed acceptable for the increased reliability.
"The purpose of using a pseudo-header is to verify that the UDP
datagram has reached its correct destination. The key to
understanding the pseudo-header lies in realizing that the correct
destination consists of a specific machine and a specific protocol
port within that machine. The UDP header itself specifies only the
protocol port number. Thus, to verify the destination, UDP on the
sending machine computes a checksum that covers the destination IP
address as well as the UDP datagram. The pseudo-header is not
transmitted with the UDP datagram, nor is it included in the length."
E. Comer - Internetworking with TCP/IP 4th edition.
Pseudo IP header contains the source IP, destination IP, protocol and Total length fields. Now, by including these fields in TCP checksum, we are verifying the checksum for these fields both at Network layer and Transport layer, thus doing a double check to ensure that the data is delivered to the correct host.