29

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. There are also fragmentation attacks, so many businesses drop fragments. What you are confusing is something like TCP segmentation, which is very different than ...


8

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, Internet Protocol says: If the Don't Fragment flag (DF) bit is set, then internet fragmentation of this datagram is NOT permitted, although it may be discarded. This ...


8

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 endpoint. For example, if the transmitting side of a TCP connection believes the path MTU to be 1500 bytes, it will send packets of that size. If the real path ...


6

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 (it can be whole or fragmented). (according to RFC 791, page 24)


6

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 networks to communicate with one another. The design accommodates networks of diverse physical nature; it is independent of the underlying transmission technology ...


6

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 packet size. For example. the original serial WAN connections could have maximum packet sizes of greater than 4000 bytes, but ethernet specifies a maximum ...


6

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 from your description: sockets are stream-oriented. The stack will handle the fragmenting/defragmenting for you. I'm not sure why you're concerned.


6

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 segment is lost in the network, the entire segment has to be retransmitted. If IP fragmentation occurs, the segment will be split into multiple fragments. The ...


5

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 that enables specialized fragmented packet-handling behavior. In general, noninitial fragments that match the Layer 3 statements (protocol, source address, ...


5

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 as an alternative, and there was a draft RFC (A TCP transport for the Internet Key Exchange draft-ietf-ipsecme-ike-tcp-01), which expired. The problem you ...


5

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 is not marked DF), by the IP process at the exit interface of the router. The TTL of the resulting packet fragments will be the same as if the packet were not ...


5

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 will be transparent to your application. Packets can get fragmented in the path, then the destination host will reassemble them at IPv4 before passing the packet ...


4

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 containing fragments. The details are in Internet Protocol RFC 760 section 2.2, but in brief the first one has the beginning of the packet including the TCP header,...


4

(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 20, and a NAT/CEF/flow entry will be created for it and all subsequent fragments will not go through the ACL -- it's part of an established flow that's already ...


4

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/IPsec are only responsible for creating a secure transport. Not a reliable one. Imagine transferring VOIP through an IPsec/IKE tunnel. VOIP largely (and ...


4

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) are fragmented by the first hop router, and they remain fragmented upon leaving the second hop router. Device B is responsible to reassemble the fragment into ...


4

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 flag off you can just send packets as large as possible on your local link, and routers along the path will fragment if/when necessary. You therefore as a ...


4

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 octets passing through a router to a network with an MTU of 2600 you would get: 20 octet IPv4 header and 2576 octet payload 20 octet IPv4 header and 1924 octet ...


4

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, and an ICMP message will (normally) be sent back to the sender.


4

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 create a new packet with index 1. (the router that perform this new fragmentation is totally unaware of the existence of the other fragment). You now have 2 ...


3

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 though. Many firewalls will defragment packets because it's difficult to do effective firewalling on fragments. For instance, in the diagram below assume ...


3

The IP fragmentation and reassembly is described by the RFCs. You must fragment on 64-bit boundaries. There are RFCs dealing with this, and other sites which will describe the fragmentation and reassembly process in depth; you can do a search to find them Start with RFC 791, INTERNET PROTOCOL: To fragment a long internet datagram, an internet protocol ...


3

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 already fragmented packets, you normally see something like this: original sends (say) 4001 byte packet through something which fragments it into three: 2000, 2000, ...


3

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 answers for the final fragments B2 and C4 below. Assuming 20-byte headers throughout, a 2600-byte MTU will make maximum packets of 2596-bytes and a 1400-byte ...


3

Answer is (1) (TTL of received datagram minus 1) This is not a hypothetical case, by the way.


3

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 not care about fragmentation. They pass things on exactly as they receive them. (unless it's the source of the fragmentation.) As such, the router will be ...


3

It takes place when the IP packet is being handed off to the link level, and as you say, depends on the MTU. Each link would normally have its own MTU. The procedure is pretty clearly laid out in the Internet Protocol RFC on page 23, and begins: If the total length is less than or equal the maximum transmission unit then submit this datagram to the next ...


3

I used the following configuration: Standard MTU on A and B; MTU lowered to 500 on the router (B side); /proc/sys/net/ipv4/ip_no_pmtu_disc set to 1 on the sender (A). And then the sender sticks to 1500 byte segments, and the router fragments.


3

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 required) is built and the TCP segment is inserted into that 2000 byte IP packet. This packet will be fragmented on IP layer (layer-3) so it can be transmitted ...


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