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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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how to calculate the udp checksum when the udp payload is and odd number e.g 45 bytes payload [duplicate]

I am new to networking and was learning about the UDP Protocol and i came through the term UDP checksum. I searched the internet spent so much time finding the correct way to calculate the UDP ...
Immad's user avatar
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1 vote
2 answers
858 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 ...
user avatar
1 vote
1 answer
2k 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 ...
S. M.'s user avatar
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1 answer
1k 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 ...
S. M.'s user avatar
  • 398
2 votes
1 answer
822 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 ...
Fabhi's user avatar
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1 vote
2 answers
1k 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 ...
thndrwrks's user avatar
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0 votes
1 answer
1k 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 ...
killertoge's user avatar
38 votes
3 answers
21k 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?
Paulo Tomé's user avatar
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0 answers
34 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 ...
DevShark's user avatar
  • 175
9 votes
3 answers
24k 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 ...
doppelfish's user avatar
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 ...
Ajitesh's user avatar
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1 answer
565 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 ...
pointers's user avatar
3 votes
3 answers
16k 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 ...
J.D.'s user avatar
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4 votes
2 answers
2k 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 ...
John P's user avatar
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5 votes
1 answer
3k 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 ...
Ninja Bug's user avatar
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0 votes
1 answer
111 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?
Matt Lyons-Wood's user avatar
1 vote
1 answer
834 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 ...
Nadax's user avatar
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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 ...
Corvus Corax's user avatar
5 votes
1 answer
4k 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 ...
davidb's user avatar
  • 249
6 votes
1 answer
14k 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 ...
Davide's user avatar
  • 175
3 votes
3 answers
6k 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, ...
0TTT0's user avatar
  • 139
0 votes
2 answers
169 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 ...
Tapinou's user avatar
3 votes
2 answers
2k 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) ...
Adam Zerner's user avatar
1 vote
1 answer
5k 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 ...
Castiel's user avatar
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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 ...
user_ABCD's user avatar
  • 634
6 votes
2 answers
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?
dulaj sanjaya's user avatar
5 votes
1 answer
817 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?
Romy's user avatar
  • 153
3 votes
1 answer
346 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 ...
Abo abdu's user avatar
5 votes
2 answers
5k 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 ...
Anakin Tung's user avatar
3 votes
2 answers
890 views

statistics on usage of jumbo Ethernet frames and size of jumbo frames

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 ...
user3802900's user avatar
1 vote
1 answer
575 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 ...
user2720323's user avatar
  • 1,611
5 votes
1 answer
9k 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, ...
user2720323's user avatar
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