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Why does one say IP fragmentation is bad and to be avoided when in reality data always needs to be fragmented for MTU compatibility?

Fragmentation is resource intensive in a router, and it slows packet forwarding. Today, we use PMTUD to determine the smallest MTU in the path so that packets are properly sized prior to sending. ...
Ron Maupin's user avatar
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9 votes
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What's the DF flag mean in IP packet header?

The IPv4 DF flag means that an intermediate host (router) cannot fragment the packet if necessary, and it would then need to drop the packet and can send an ICMP message stating that. RFC 791, ...
Ron Maupin's user avatar
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8 votes

Why does one say IP fragmentation is bad and to be avoided when in reality data always needs to be fragmented for MTU compatibility?

IP packet level fragmentation occurs when the transmitting side is not properly aware of the MTU of the path. This results in worse performance than if the packets are sized correctly already at the ...
jpa's user avatar
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7 votes

Why does one say IP fragmentation is bad and to be avoided when in reality data always needs to be fragmented for MTU compatibility?

IP fragmentation can cause excessive retransmission at the TCP level. TCP transmits information as a series of segments, and these are the units of acknowledgement and retransmission as well. If a TCP ...
Barmar's user avatar
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6 votes
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How does a router behave when some of the fragments of a packet is lost?

There's no mechanism to request a fragment be resent. The entire packet cannot be reassembled, so the entire packet will have to be resent. This is why Fragmentation Is Bad(tm). Routers typically do ...
Ricky's user avatar
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6 votes
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What is 576 bytes and 68 byte chunk when it comes to MTU?

68 bytes is the minimum size of IPv4 datagram every device must be able to forward without further fragmentation. 576 bytes is the minimum size of IPv4 datagram every device has to be able to receive (...
Fuki's user avatar
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6 votes
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Calculating Fragmentation offset doubt

Edit: Since you completely changed the question (very bad form), I will attempt to answer the new question. You are still very confused about how fragmentation works. With an original payload of 4500 ...
Ron Maupin's user avatar
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6 votes
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Ip fragmentation and packet headers

The Fragmentation and reassembly section of the IPv4 Wikipedia article explains it quite well: Fragmentation and reassembly Main article: IP fragmentation The Internet Protocol enables ...
JFL's user avatar
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6 votes
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Confusion about data fragmentation/MTUs - why was it introduced in the first place?

One of the two basic functions for IPv4 is packet fragmentation (the other is addressing). IP is designed to send packets from one network to another network. Each network can have a different maximum ...
Ron Maupin's user avatar
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6 votes

Is it possible get fragmented if the packet size is less then MTU?

YES. If the MTU somewhere along the path is smaller than your packet size, it will be fragmented. This may not apply in your simple network, but it's possible in the real world. One thing isn't clear ...
Ron Trunk's user avatar
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6 votes
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In IPv4 Fragmentation, should the first block be necessarily the closest smaller multiple to 8

The Fragment Offset field is three bits smaller than the Total Length field. 2^3 = 8 That means that you must use a multiple of eight. Typically you will use the largest multiple of eight that fits ...
Ron Maupin's user avatar
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5 votes
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How does a Cisco ACL handle fragments

You are not denying fragments. Cisco has an Access Control Lists and IP Fragments document that specifically deals with this problem. ACLs and Fragmented Packets ACLs have a fragments keyword ...
Ron Maupin's user avatar
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5 votes
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How are ping times computed

I'm not sure why you would think fragmentation would double the time. If a router fragments a packet, it sends both fragments sequentially. The host on the other end will get the fragments, reassemble ...
Ron Maupin's user avatar
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5 votes
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TTL used during fragmentation

The TTL of a packet is decremented as the packet is processed by the IP process upon reception in the router. The packet gets fragmented, if necessary to traverse the exit network (assuming the packet ...
Ron Maupin's user avatar
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5 votes
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why ip protocol use fragment offset instead of index number?

Keep in mind that a fragmented packet may be further fragmented: A packet is fragmented once, you now have two fragments, index 0 and 1. Now packet0 is further fragmented. So you keep index 0 and ...
JFL's user avatar
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5 votes

Is it possible get fragmented if the packet size is less then MTU?

Can the network devices on the path fragment my packet for a reason? If it is IPv4, and the DF bit is clear, then yes, your packets can be fragmented in the path. The fragmentation and reassembly ...
Ron Maupin's user avatar
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5 votes
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How can a router know the MTU of the next router?

How does router B know that the MTU of router C is 1400 bytes? The MTU is a property of a network segment/subnet (more specifically of a link), not of a device/node which can have many interfaces ...
Zac67's user avatar
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4 votes

How does a TCP segment fit into a smaller IP packet?

How is the fragmentation handled here? We have to distinguish two cases: Case 1: Layer-2 does not allow a frames of such a length In this case an IP packet of 2000 bytes size (or whatever size is ...
Martin Rosenau's user avatar
4 votes
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How does a TCP segment fit into a smaller IP packet?

After the routing decision is made for a given packet, it is scheduled to go out of a particular interface. If the packet is too big for the MTU of the link, it is sent as two or more IP packets ...
jonathanjo's user avatar
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4 votes

cisco ACL no counter

(This came up on DSLR recently.) It has to do with the way traffic is processed. Since the first fragment carries the full layer-4 information, it is not handled as a fragment. So, it will match rule ...
Ricky's user avatar
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4 votes
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Cisco IP Fragments ACL strange behavior

Apparently, this will treat an initial fragment the same as a non-fragment. The initial fragment will get through, but subsequent fragments in the same packet will be dropped. See Access Control ...
Ron Maupin's user avatar
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4 votes

Is Path MTU Protocol more beneficial for IPv6

IPv4 packets have a Don't-Fragment (DF) flag which indicates whether routers on the path are allowed to perform fragmentation when the packet doesn't fit the MTU of the next link. If you leave the DF ...
Sander Steffann's user avatar
4 votes
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Why the IPv4 need Don't fragment Flag?

If frame is bigger than MTU and have don't fragment bit set then it will drop the packet. Discarding router will send back to sender ICMP message Fragmentation Needed (Type 3, Code 4) which contains ...
kubn2's user avatar
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4 votes

What's the DF flag mean in IP packet header?

The first one, it instructs routers on the path to the destination to not fragment the packet. So if the packet is to be sent through an interface with a MTU too small, the packet will be dropped, ...
JFL's user avatar
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4 votes

Does the MSS (Maximum Segment Size) prevent IP Fragmentation in all cases?

Does the MSS (Maximum Segment Size) prevents IP Fragmentation in all cases? No. What prevents in-path IP fragmentation is to use the IPv4 DF flag, or use IPv6. If an IPv4 packet does not have the DF ...
Ron Maupin's user avatar
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4 votes

How can a router know the MTU of the next router?

To add to Zac67's answer: why router B pre-fragment packets to send before router C Fragmentation is expensive in terms of system ressources on routers, and should be avoided. Routers and Firewalls ...
Marc 'netztier' Luethi's user avatar
3 votes
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When do IP packets get reassembled if fragmentation occurs in a tunnel?

I've read that fragmented IP packets "always" become reassembled at their ultimate destination, e.g. the recipient host. That was the original intent of the Designers of IP, it's not so true nowadays ...
Peter Green's user avatar
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3 votes
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IPv4 fragmentation - overlapping fragments

Overlapping fragments shouldn't occur for "normal" traffic. They are usually a sign of someone trying to circumvent some security system. You are right that there are different ways to reassemble ...
Sander Steffann's user avatar
3 votes
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Understanding how IP Reassembly Process works

You have them all correct. 5th fragment: remember that the routers can change their settings at any time, and the fragmentation limit can change from one packet to the next. When you fragment ...
jonathanjo's user avatar
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