56
votes
Accepted
Why does the IPv6 header not include a checksum?
One of the ideas around IPv6 was to speed up packet forwarding. To that end, several decisions were made. For example, the IPv6 header was greatly simplified and is a fixed length, unlike the variable ...
- 97.7k
27
votes
Why does the IPv6 header not include a checksum?
Because it's redundant.
All the common link-layer protocols, like Ethernet or WiFi, have their own error checking and error correction mechanisms, so physical transmission errors are already unlikely....
- 370
10
votes
Accepted
Must the checksum field of the IPv4 header always be updated?
Since the router changes the IPv4 header (it decrements the TTL), it needs to calculate a new value for the checksum, otherwise subsequent devices receiving the packet will think it is damaged.
This ...
- 97.7k
8
votes
Accepted
In Cyclic Redundancy Check, how does the receiver knows what the generator polynomial is?
It's true that you have to know it in advance in order to calculate the CRC.
The correct polynomial depends on the application of the CRC. For Ethernet, for example, the CRC-32 polynomial is part ...
- 66.1k
8
votes
Accepted
If TCP is a reliable data transfer method, then how come its checksum is not 100% reliable?
No checksum is 100% reliable. The more reliable the error detection, the more computational power you need. It's a tradeoff between reliability and speed/processing power.
The TCP checksum was ...
- 66.1k
8
votes
Does it make sense to generally "Disable TCP Checksum Offload"?
Hardware offloading features may have bugs but they are generally beneficial. I only deactivate them on certain NICs or vendors which do have problems.
However, on a workstation/PC the network load is ...
- 79.8k
7
votes
Accepted
Difference between IP checksum and TCP checksum
As your data travels up the network stack, it may be subject to error detection at each layer of the stack.
For example, ethernet has the FCS for its frames (other data link protocols may have their ...
- 97.7k
7
votes
Accepted
What is the difference between "Message Digest" and Checksum?
A checksum is the general term used. A checksum can range from a check digit (parity bit) to a complex output string. Different checksums (examples below) can be chosen depending on the application.
...
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7
votes
Accepted
Calculate the FCS number from a frame ethernet
The FCS is a CRC over all fields (except the FCS) with the polynomial
G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
with the procedure detailed in IEEE 802.3 ...
- 79.8k
5
votes
Accepted
Can the checksum bits be corrupted?
To strictly answer the questions:
Can the checksum itself be corrupted? Certainly, the noise sources have no idea whether they are corrupting data bits or checksum bits
What happens if only the ...
- 16.1k
4
votes
Is TCP checksum verification also offloaded to NICs?
There are some NICs that perform offloading, if configured to do so by the OS and drivers. This is not universal, and it can cause problems.
Host configurations and applications are off-topic here, ...
- 97.7k
4
votes
Accepted
Why are checksums used if they're flawed?
When transmitting TCP over IPv4 over Ethernet, there are three levels where checksum (or CRC) is used:
Ethernet has 32-bit CRC called frame check sequence (FCS). This is very reliable: for random ...
- 465
4
votes
If TCP is a reliable data transfer method, then how come its checksum is not 100% reliable?
So in the networking book by Kurose, we define UDP as a unreliable method because it might not detect errors and therefore transfer a corrupted packet
No, we define UDP as an unreliable protocol ...
- 12.8k
3
votes
Accepted
Why is the FCS not included in most ethernet frames?
In the original old fashioned 10Mb/s Ethernet, you could usually ask the NIC driver to deliver packets to you, even if the checksum was incorrect. You might then be able to identify a problematic host ...
- 1,558
3
votes
How does the Transport Layer construct the Pseudo-Header for TCP checksum?
The pseudo-header doesn't really exist - it is only temporarily created to calculate the checksum.
Within the IP stack, the network layer passes the L3 information upward to the transport layer - ...
- 79.8k
3
votes
Accepted
How does the Transport Layer construct the Pseudo-Header for TCP checksum?
The TCP pseudo header has only information which was used to create the original connection (source and destination IP addresses), a length (which is available to the upper level) and a well-known ...
- 16.1k
3
votes
Calculating checksum for ICMPv6
Thanks for Jens's help. After having look for RFC2463. I pre-append the following data for the pseudo header.
unsigned short src_ip[8] = {0} //fill the source IP
unsigned short dst_ip[8] = {0} //...
- 373
3
votes
Accepted
TCP Checksum Calculation
Think about it. A 16-bit checksum only has 65,536 unique values, so it will be fairly common to repeat checksum values on multiple segments. What you have is not going to be unusual with different ...
- 97.7k
3
votes
Accepted
Is the OPTIONS field included in IP checksum calculation?
I am confused as to whether the OPTIONS are included in IP checksum calculation at the source?
The IPv4 options are part of the header, so they're included in checksum calculation.
If yes, is it ...
- 79.8k
3
votes
Accepted
UDP checksum calculation for checksum results in the value zero
A UDP checksum value of zero indicates that the checksum option isn't used (checksum value is not to be verified). Therefore, a calculated checksum of zero is replaced by all ones to indicate that ...
- 79.8k
2
votes
Accepted
IP Header value
RFC 791, Internet Protocol, Section 3, shows the header format. The length field is 2 bytes (16 bits), not 4 bits. It explains that this allows for 65,535 octets.
- 97.7k
2
votes
Can I find port number with the TCP checksum?
You can't reconstruct missing packet elements from the checksum. It's for error detection, not correction.
If just one byte/word/longword is missing it could be reconstructed, e.g. by just trying all ...
- 79.8k
2
votes
Can the checksum bits be corrupted?
If the TCP checksum is corrupted, then it will not match the TCP pseudo header and payload. There should only be one checksum that matches the pseudo header and payload, but there are multiple TCP ...
- 97.7k
2
votes
Accepted
Does it make sense to generally "Disable TCP Checksum Offload"?
Checksum offload is generally quite reliable. The last NIC I knew of that had a bug around this was a 100 Mbps Sun hme interface, which had a problem with UDP checksum offloads for IPv6 only. That ...
2
votes
Does it make sense to generally "Disable TCP Checksum Offload"?
I've just spent about 2 days debugging and figuring how to workaround a problem that seems to have come down to buggy TCP offloading with linux and Intel ethernet drivers/chipsets in Intel NUCs. ...
- 121
2
votes
Wireshark udp checksum errorI
Can you see if the UDP checksum is 0 in the packets?
If so, it means "no checksum sent", which is valid for UDP on IPv4.
Wireshark might care to display "it's zero" differently from "sent and ...
- 16.1k
2
votes
Difference between IP checksum and TCP checksum
Routers only check the IPv4 header checksum. If the header is corrupted the packet is dropped. Payload or higher-layer errors are not detected here. IPv6 even drops the header checksum and leaves that ...
- 79.8k
2
votes
IPv6 Extension Headers and UDP/TCP/ICMPv6 Checksums
Should the checksum for IPv6 packets (UDP, TCP, ICMPv6, etc.) change
if extension headers are present?
IPv6 packets do not have checksums. That was one of the improvements made for IPv6, so that the ...
- 97.7k
2
votes
IPv6 Extension Headers and UDP/TCP/ICMPv6 Checksums
If I understand you question correctly, you want to ask the following:
Is the "next header" field in the "pseudo-header", which is used for TCP checksum calculation, identical to the "next header" ...
- 2,291
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