Is the PMTU-D packet 1420 bytes greater than effective MTU 1396?

Is the PMTU-D packet 1420 bytes greater than effective MTU 1396?

I’ve followed some of the postings and also this one, http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a00804c8b9f.shtml. I had a SonicWALL and a Netscreen firewal before and both of them just worked fine.

What’s the maximum length of an IP packet?

IP supports a maximum length of 65,536 bytes for an IP packet, but most data-link layer protocols support a much smaller length, called a maximum transmission unit (MTU). Based on the supported MTU, it can be necessary to break up (fragment) an IP packet to transmit it across a particular data-link layer media type.

What’s the difference between IPv4 fragmentation and MTU?

The design of IPv4 accommodates MTU differences since it allows routers to fragment IPv4 datagrams as necessary. The receiving station is responsible for the reassembly of the fragments back into the original full size IPv4 datagram. IPv4 fragmentation involves to break a datagram into a number of pieces that can be reassembled later.

How does IPv4 fragmentation and path maximum transmission unit discovery work?

This document describes how IPv4 Fragmentation and Path Maximum Transmission Unit Discovery (PMTUD) work and also discusses some scenarios that involves the behavior of PMTUD when combined with different combinations of IPv4 tunnels.

What should the path MTU be for PMTUD?

A robust method for PMTUD that relies on TCP or another protocol to probe the path with progressively larger packets has been standardized in RFC 4821. A workaround used by some routers is to change the maximum segment size (MSS) of all TCP connections passing through links with MTU lower than the Ethernet default of 1500.

What happens if my MTU is smaller than my packet?

Then, any device along the path whose MTU is smaller than the packet will drop it, and send back an Internet Control Message Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message containing its MTU, allowing the source host to reduce its Path MTU appropriately.

How is the path MTU determined in ICMP?

The process is repeated until the MTU is small enough to traverse the entire path without fragmentation. If the Path MTU changes after the connection is set up and is lower than the previously determined Path MTU, the first large packet will cause an ICMP error and the new, lower Path MTU will be found.