Popular Topics in General Networking. Which of the following retains the information it's storing when the system power is turned off? Submit ». Solutions Inc. This person is a verified professional. Verify your account to enable IT peers to see that you are a professional.
Ya its called ping ; But you pose an interesting and difficult question, you can find some detailed information using wireshark. Try firing up Wireshark and run your program to see what kind of packet loss you are getting.
David A. Holder Nov 17, at UTC. Ya its called ping ; Real smart I'll fire up Wireshark and check what it sees. I just can't quite follow The following cisco docs helped me to get into the subject. Holder Nov 18, at UTC. If any of the six fragments are dropped because of a congested link, the complete original datagram has to be retransmitted, which means that six more fragments will have to be created. If this link drops one in six packets, then the odds are low that any NFS data can be transferred over this link, since at least one IPv4 fragment would be dropped from each NFS byte original IPv4 datagram.
Firewalls that filter or manipulate packets based on Layer 4 L4 through Layer 7 L7 information in the packet might have trouble processing IPv4 fragments correctly. If the IPv4 fragments are out of order, a firewall might block the non-initial fragments because they do not carry the information that would match the packet filter. This would mean that the original IPv4 datagram could not be reassembled by the receiving host.
If the firewall is configured to allow non-initial fragments with insufficient information to properly match the filter, then a non-initial fragment attack through the firewall could occur.
Also, some network devices such as Content Switch Engines direct packets based on L4 through L7 information, and if a packet spans multiple fragments, then the device might have trouble enforcing its policies.
Contrary to popular belief, the MSS value is not negotiated between hosts. The sending host is required to limit the size of data in a single TCP segment to a value less than or equal to the MSS reported by the receiving host. Originally, MSS meant how big a buffer greater than or equal to bytes was allocated on a receiving station to be able to store the TCP data contained within a single IPv4 datagram.
This TCP segment could be as large as 64K the maximum IPv4 datagram size and it could be fragmented at the IPv4 layer in order to be transmitted across the network to the receiving host.
Scenario 1 illustrates the way MSS was first implemented. In order to assist in avoiding IPv4 fragmentation at the endpoints of the TCP connection, the selection of the MSS value was changed to the minimum buffer size and the MTU of the outgoing interface - The hosts will then compare the MSS size received against their own interface MTU and again choose the lower of the two values.
Scenario 2 illustrates this additional step taken by the sender in order to avoid fragmentation on the local and remote wires. Notice how the MTU of the outgoing interface is taken into account by each host before the hosts send each other their MSS values and how this helps to avoid fragmentation.
In Scenario 2, fragmentation does not occur at the endpoints of a TCP connection because both outgoing interface MTUs are taken into account by the hosts. Packets can still become fragmented in the network between Router A and Router B if they encounter a link with a lower MTU than that of either hosts' outbound interface.
TCP MSS as described earlier takes care of fragmentation at the two endpoints of a TCP connection, but it does not handle the case where there is a smaller MTU link in the middle between these two endpoints. PMTUD was developed in order to avoid fragmentation in the path between the endpoints. It is used to dynamically determine the lowest MTU along the path from a packet's source to its destination. Other protocols do not support it.
Here is an example of an ICMP "fragmentation needed and DF set" message that you might see on a router after the debug ip icmp command is turned on:. Even when this information was supplied, some hosts ignore it. It is used by hosts in order to arrive more quickly at a reasonable value for the send MSS and as shown in the image.
PMTUD is done continually on all packets because the path between sender and receiver can change dynamically. The TCP client sends small packets and the server sends large packets. The client's packets are small less than bytes and will not trigger PMTUD because they do not require fragmentation to get across the MTU link. Scenario 4 shows an asymmetric routing example where one of the paths has a smaller minimum MTU than the other. Asymmetric routing occurs when different paths are taken to send and receive data between two endpoints.
The client, on the other hand, will never receive an ICMP "Destination Unreachable" message with the code that indicates "fragmentation needed and DF set" because Router A does not have to fragment packets when it sends them to the server through Router B.
The first and last of the three bullets here are uncommon and are usually the result of an error, but the middle bullet describes a common problem. A packet filter can block all ICMP message types except those that are "unreachable" or "time-exceeded.
An example of such a packet filter, implemented on a router is shown here. There are other techniques that can be used in order to help alleviate the problem of ICMP being completely blocked. Clear the DF bit on the router and allow fragmentation anyway This might not be a good idea, though. See Issues with IP Fragmentation for more information. In the next scenario, Router A and Router B are in the same administrative domain. A workaround for this situation is to clear the DF bit in both directions on Router B in order to allow fragmentation.
This can be done with policy routing. The result is that the TCP sender will send segments no larger than this value. This command effects traffic both inbound and outbound on interface serial0. IPv4 fragmentation issues have become more widespread since IPv4 tunnels have become more widely deployed. The reason that tunnels cause more fragmentation is because the tunnel encapsulation adds "overhead" to the size of a packet.
For example, the addition of Generic Router Encapsulation GRE adds 24 bytes to a packet, and after this increase, the packet might need to be fragmented because it is larger than the outbound MTU.
In a later section of this document, you will see examples of the kinds of problems that can arise with tunnels and IPv4 fragmentation. Some common reasons for the existence of these smaller MTU links are:.
This reduces the effective MTU of the Ethernet to - 8. This also reduces the effective MTU of the outgoing interface. In the next sections, the impact of PMTUD where a tunneling protocol is used somewhere between the two end hosts are studied.
Of the three previous cases, this case is the most complex and covers all of the issues that you might see in the other cases. A tunnel is a logical interface on a Cisco router that provides a way to encapsulate passenger packets inside a transport protocol.
It is an architecture designed to provide services in order to implement a point-to-point encapsulation scheme. Tunneling has these three primary components:. GRE - Cisco's multiprotocol carrier protocol. The packets shown in this section illustrate the IPv4 tunneling concepts where GRE is the encapsulation protocol and IPv4 is the transport protocol. The passenger protocol is also IPv4. In this case, IPv4 is both the transport and the passenger protocol.
This illustrates the fact that the carrier protocol can encapsulate multiple passenger protocols as shown in the image.
A network administrator might consider tunneling in a situation where there are two discontiguous non-IPv4 networks separated by an IPv4 backbone. If the discontiguous networks run DECnet, the administrator might not want to connect them together by configuring DECnet in the backbone. The administrator might not want to permit DECnet routing to consume backbone bandwidth because this could interfere with the performance of the IPv4 network. Tunneling encapsulates the DECnet packets inside IPv4, and sends them across the backbone to the tunnel endpoint where the encapsulation is removed and the DECnet packets can be routed to their destination via DECnet.
The endpoints use private addresses RFC and the backbone does not support routing these addresses. For the rest of the document, IPv4 is used as the passenger protocol and IPv4 as the transport protocol. Richard Lotz August 18, at a. Colin August 18, at p. Brandon Bennett August 29, at p. Nice discussion and it leaves me curious: How do servers cache their discovery of path MTU? Is on a per distant-end IP basis? Per socket? Nick March 18, at a. Great explanation, thanks!
Alex November 17, at a. Raghu December 29, at a. Diego Zamberlan May 13, at p. Anees June 17, at p. Hi , I was facing an issue where only two particular intranet sites were not opening on wireless but on wired it was working, there was no error just the page remains blank , the only resolution I could find was to reduce the mtu size on the user system but I am now looking for global resolutions, the wireless controllers are Motrola Symbol RFS , any suggestion would be very much appreciated.
Alex Dekker March 14, at a. The phrase " Richard M September 6, at a. Darshan April 14, at a. As you can see in the following screenshot, we started with , tested with , until finally gave a successful reply. That means the path MTU between our source and destination is somewhere between and bytes. The next step would be to move the size up between bytes until you get another successful reply. The routers along the path were still fragmenting bytes but not They considered to be too large to traverse the path.
But was the first proper packet size that returned a successful ICMP reply. The header size varies according to the transmission media.
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