Questions tagged [checksum]

For questions about checksum in relation to networking. For instance, you want to verify an image on a router and for that you need to initialize a checksum sequence.

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Understanding IE Content - calculate checksum

I'm trying to understand article about calculate probe request 6-byte footprint. this what it say: First, a 6-byte footprint is initialized to zero (00: 00: 00: 00: 00: 00). The IEs of each Wi-Fi ...
3 votes
3 answers
14k views

Difference between IP checksum and TCP checksum

If an error on a packet occurs on a router, who does detect the error, IP checksum or TCP checksum? I have understood that the IPv4 checksum detects errors only inside the IP header and the TCP ...
1 vote
2 answers
170 views

Calculating Checksum for ICMPv6 Packet

I have a packet that has a checksum of 0xffff, and a calculated checksum of 0x0000. I know in UDP, a checksum of 0x0000 is not allowed, and is instead replaced with 0xffff, thus the packet should be ...
0 votes
1 answer
276 views

Why UDP checksum is optional for IPv4?

We know that IPv4 header has checksum for header not for data .Therefore for checking data error we need checksum in transport layer. Suppose we're using UDP in layer 4. So if I remove the checksum ...
0 votes
1 answer
372 views

UDP checksum calculation for checksum results in the value zero

I read UDP checksum from Wikipedia that "If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as the one's complement (all 1s) as a zero-value checksum ...
6 votes
2 answers
16k views

Does it make sense to generally "Disable TCP Checksum Offload"?

In a work sheet for PCs that we deliver to customers, I found instructions to always "Disable TCP Checksum Offload" on the NICs. Via those NICs, the PCs are connected to the customer's LAN, and it is ...
1 vote
1 answer
369 views

Is the OPTIONS field included in IP checksum calculation?

The source host wants to estimate the time it takes for the datagram to go from one router to another. Therefore, the router uses the timestamp option in IP with Timestamp Flag = 1. I am confused as ...
0 votes
2 answers
640 views

IPv6 Extension Headers and UDP/TCP/ICMPv6 Checksums

Should the checksum for IPv6 packets (UDP, TCP, ICMPv6, etc.) change if extension headers are present? For example, will a UDP packet without a Hop-By-Hop extension header have a different checksum ...
36 votes
3 answers
16k views

Why does the IPv6 header not include a checksum?

It is stated in Wikipedia that an IPv6 header does not include a checksum. What are the reasons that were behind this decision?
0 votes
1 answer
735 views

TCP Checksum Calculation

I recently started with networking and tcp/ip. The RFC 793 document tells the checksum is made of: pseudo header+tcp header+tcp payload+zeroes(if not divisible by 16). So I have to take 16bit ...
0 votes
0 answers
33 views

What is wrong with this CRC calculation? [duplicate]

I have gone through a few documents to understand how to check that an ethernet frame is valid. I do the following calculation, but I can see it is incorrect because it does not result in a crc value ...
4 votes
2 answers
2k views

How does the Transport Layer construct the Pseudo-Header for TCP checksum?

THE TCP Psuedo Header needs sender and destination IP Address and protocol type. All of this informations is present in the network layer/IP header. Since the transport layer cannot access the data ...
1 vote
1 answer
1k views

UDP checksum calculation on improperly padded packets

Suppose I'm receiving IPv4 UDP packets whose payload is less than 18 octets, so when they are transmitted over Ethernet they have some trailing padding. Between the packets' source and my receiver ...
1 vote
2 answers
2k views

How does the TCP/IP stack handle UDP checksum errors?

RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, Section 8.1 Upper-Layer Checksums, states the UDP checksum is mandatory and any IPv6 receivers must discard UDP packets containing a zero ...
5 votes
2 answers
4k views

Calculating checksum for ICMPv6

Referring http://www.faqs.org/rfcs/rfc1071.html, I try to deduce the c code for calculating the checksum for ICMPv6. But check with the checksum parse by wireshark, I find that the answer got from my ...
0 votes
1 answer
404 views

Wireshark udp checksum errorI

I sniffing udp communication with 2 instance of wireshark , 1: 2.0.0 2: 2.2.1 . In version 2.0.0 I get valid udp.checksum , but with 2.2.1 I always get udp checksum = 0000 . Why is that? those ...
5 votes
1 answer
758 views

In Cyclic Redundancy Check, how does the receiver knows what the generator polynomial is?

Must the receiver know the generator polynomial in order to detect the errors in the received data? So how does it know what generator polynomial the sender has used?
4 votes
2 answers
1k views

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 or transfer the data without the order so its ...
5 votes
1 answer
12k views

Calculate the FCS number from a frame ethernet

I've the following frame ethernet: 08 00 20 0A 70 66 08 00 20 0A AC 96 08 00 45 00 00 28 A6 F5 00 00 1A 06 75 94 C0 5D 02 01 84 E3 3D 05 00 15 0F 87 9C CB 7E 01 27 E3 EA 01 50 12 10 00 DF 3D 00 00 20 ...
5 votes
1 answer
2k views

What is the difference between "Message Digest" and Checksum?

As far as I understand Message Digest is a certain Hash function that is derived from the original payload, so that even if a single bit changes the Message Digest proves to be different, thus ...
0 votes
1 answer
105 views

Is TCP checksum verification also offloaded to NICs? [closed]

I understand that checksum calculation at the sender side is often offloaded to the NIC. Does this also happen for verification at the receiving end?
4 votes
1 answer
8k views

Why are there checksums for different OSI layers?

Can anyone clarify the question "why we need checksum in both layer-2 and layer-3 of OSI layer", Layer-2 would encapsulate the rest of OSI layers into a single unit. If Layer-2 has a checksum, ...
5 votes
1 answer
2k views

Must the checksum field of the IPv4 header always be updated?

When an IPv4 datagram comes to a router for forwarding, does it always update the value of the header checksum field?
3 votes
1 answer
273 views

IP Header value

We are given an IPv4 datagram with the following characteristics. The source IP address is 132.71.4.333 and destination is 132.71.4.54 Its data content is an unfragmented TCP message of 64 bytes (TCP ...
1 vote
1 answer
657 views

How does the UDP Checksum tell me if my data has changed?

currently I am having a Computer Networking cours and I am reading "Computer Networking a Top-Down Approach" At this point I am reading the Transport Layer part. And I have read about the checksum ...
3 votes
1 answer
3k views

Why is the FCS not included in most ethernet frames?

I turned on the calculatin of the FCS Checksum in wireshark. There wasn't a single correct checksum in the whole 500MB packet dump. I asume the reason for this is that the field is simply not used for ...
3 votes
3 answers
5k views

Can the checksum bits be corrupted?

If the data that are being encapsulated have the possibility of corruption while crossing over the wire, what is stopping the TCP checksum itself from being corrupted, and if it also can be corrupted, ...
0 votes
2 answers
125 views

Can I find port number with the TCP checksum?

So I got a frame where the port information is missing but I have everything else, how can I retrieve the port that where used in the frame ? I was thinking about the tcp checksum, but is it ...
3 votes
2 answers
1k views

Why are checksums used if they're flawed?

Consider an overly simple example: Say we want to calculate the checksum of "hi". Since h is the 8th letter of the alphabet, we'll say h=8, and similarly, i=9. So the checksum of hi is 8+9=17. 1) ...
1 vote
1 answer
4k views

Is there any way a UDP checksum could be inconclusive of error detection?

If I am receiving data through UDP, with a checksum that checks out to show no data loss, is there a possibility that the checksum missed some errors. My data transmitted is split into 24-bits chunks ...
3 votes
2 answers
807 views

statistics on usage of jumbo ethernet frames and size of juboframes

I am currently researching jumbo ethernet frames. There are a lot of articles, etc. out there that explain what jumboframes are and how to enable them, but there seem to be no statistics on how ...
1 vote
1 answer
505 views

Why do not we have checksum validation in the application Level (BGP , RIP)?

In BGP, TCP handles the checksum and BGP does not perform any checksum validation in the application level. How can we believe TCP must forward the data without errors? Similarly in RIP with UDP ...