51

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 length IPv4 header. Also, you cannot fragment IPv6 packets along the path, the way you can for IPv4, because packet fragmentation is resource intensive. Not ...


25

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. What's left are logic errors in the packet itself. But almost all transport protocols based on IPv6, like TCP or UDP, also have error checking to catch ...


10

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 only applies to IPv4. IPv6 has eliminated the checksum, instead it relies on protocols above and below it to do any error checking.


8

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 of the IEEE 802.3 standard. For you trivia buffs, it's 0x82608EDB.


8

If Layer-2 has a checksum, shouldn't this validation be enough? Why we are having checksum validation in different OSI Layers? Simply put, different layers of the OSI model have checksums so you can assign blame appropriately. Suppose there is a webserver running on some system (assume TCP port 80, i.e. OSI Layer 4) Suppose there is a software error ...


7

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. message digest hash algorithm digital fingerprint randomization function


7

Hardware offloading feature 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 usually low to very low - there's little benefit gained from offloading but there's still a risk of buggy hardware/drivers. That probably drove the work sheet'...


6

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 limited by available computing power at the time. It gives you 99.9984% assurance that your data has not been corrupted by single bit errors. The developers assumed ...


6

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 Clause 3.2.9 - complement first 32 bits, run the polynomial, complement the result and transmit most significant to least significant bit - this is reverse the ...


5

ICMP and ICMPv6 are two different protocols. For ICMPv6 checksum callulation see RFC2463 section 2.3 and the documents reffered there.


5

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 checksum itself is corrupted? The calculated value by the receiver is certain to fail the match. If checksum and data are corrupted, t(as given in the first answer)...


5

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 own way of detecting errors in a frame). Any frame that does not pass the FCS test is dropped, the payload never reaching the network layer. IPv4 has a header ...


4

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 because it takes no measures to ensure that the data is actually delivered in the event of problems on the underlying network. It also takes no measures to ensure ...


4

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, but you can search for tcp offloading on Server Fault and Super User to learn about the problems you may encounter, and how to disable the feature if necessary.


3

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 and take corrective action. Since then: Multidrop networks have been replaced with switched networks, which generally do not forward packets with bad ...


3

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 - likewise, the transport layer passes L3 and L4 information upward to the application layer. Information isn't passed as the original packet but through data fields ...


3

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 constant, the protocol (TCP is Internet Protocol number 6.) From RFC 793, p17. +--------+--------+--------+--------+ | Source Address | +------...


3

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 corruption, it means that 1 in 4 billion packets gets accidentally accepted even though it contains corrupted data. Considering that one packet is typically about ...


3

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} //fill the destination IP //0x0020 is the packet length of ICMPv6 //fill 3bytes of zero and 0x3a is the type of ICMPv6, so we have //0x00, 0x003a unsigned short ...


3

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 segment payloads. The source TCP calculates the checksum as it creates a segment, and the destination TCP calculates the checksum when it receives a segment. A ...


2

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.


2

It's hard to research because jumbo frames don't exist on the internet. (Internet2, maybe, but that's not a global, public network.) The primary industry use for jumbo frames is large bulk data transfers -- i.e. SAN infrastructure. Use of jumbo on a network connected to the internet is asking for problems -- PMTUd should allow it to work, but there are ...


2

Personaly i have never seen them used in a real setting so i am wondering if i am just watching the wrong networks, or if jumboframes are generally not used (and then: why? Are devices unable to handle jubo frames still an issue today?). Jumbo frames are practically non-existent on the internet at large; however, many companies make heavy use of jumbo ...


2

Allow me to quote Mr. Jeff Doyle Like EGP, BGP forms a unique, unicast-based connection to each of its BGP-speaking peers. To increase the reliability of the peer connection, BGP uses TCP (port 179) as its underlying delivery mechanism. The update mechanisms of BGP are also somewhat simplified by allowing the TCP layer to handle such duties as ...


2

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 possible values and recalculating the checksum. However, educational purposes, hacking challenges and such are off-topic here.


2

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 pseudo header and payload combinations that will resolve to the same checksum. It is a one-way function. There is no completely reliable way to communicate over a ...


2

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 calculated as valid", and might well do so differently from one version to another.


2

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 routers between the source and destination do not need to calculate the checksum to see if the packet header is corrupt, not do they need to recalculate the ...


2

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" field in the IPv6 header? This would mean that the TCP checksum depends on the extension headers because the value used for checksum calculation would be 6 ...


2

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 recalculated at every router it goes through, because the Timestamp data will change? Yes, see RFC 791.


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