29
votes
Accepted
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. ...
- 98.8k
9
votes
Accepted
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, ...
- 98.8k
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 ...
- 356
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 ...
- 436
6
votes
Accepted
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 ...
- 31.8k
6
votes
Accepted
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 (...
- 344
6
votes
Accepted
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 ...
- 98.8k
6
votes
Accepted
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 ...
- 19.5k
6
votes
Accepted
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 ...
- 98.8k
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 ...
- 67.1k
5
votes
Accepted
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 ...
- 98.8k
5
votes
Accepted
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 ...
- 98.8k
5
votes
Accepted
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 ...
- 98.8k
5
votes
Accepted
Why does IKE use UDP instead of TCP?
Originally, UDP was chosen over TCP because of its lower latency and processing requirements. Also, if ISPs followed the IETF standards, this wouldn't be an issue. There has been a movement to add TCP ...
- 98.8k
5
votes
Accepted
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 ...
- 19.5k
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 ...
- 98.8k
5
votes
Accepted
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 ...
- 82.7k
4
votes
Accepted
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 ...
- 98.8k
4
votes
Why does IKE use UDP instead of TCP?
IKE is mean to encapsulate other communication. If that "other communication" requires all the L4 error correction that TCP offers, than the "other communication" should use TCP.
In this way, IKE/...
- 15k
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 ...
- 31.8k
4
votes
Accepted
IP Fragmentation
As the diagram shows, fragmentation happens along the path, as needed. It is up to the end-device to reassemble any fragments.
Notice in the drawing how the colored boxes (representing IP packets) ...
- 98.8k
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 ...
- 6,720
4
votes
Accepted
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 ...
- 16.2k
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 ...
- 2,346
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, ...
- 19.5k
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 ...
- 98.8k
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 ...
- 8,826
3
votes
Accepted
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 ...
- 16.2k
3
votes
Calculating Fragmentation offset doubt
I don't disagree for a second with the description in previous answer, but as I assumed the 4500 packet was the whole length -- as that is what is compared to the MTU -- my arithmetic gives different ...
- 16.2k
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