Packetization Layer Path MTU Discovery

Terry Slattery
Principal Architect

I recently became aware of RFC 4821, Packetization Layer Path MTU Discovery, dated March 2007.  One of the problems with the traditional Path MTU discovery mechanism is that it relies on the return of ICMP code for “fragmentation needed and DF set” (also called the Datagram Too Big message in RFC 1191).  In today’s networks, ICMP messages are often filtered out as a way to hide network infrastructure (see RFC2923).  Network security folks have learned to let these packets through so that Path MTU continues fo function.  But what about alternative mechanisms so that security concerns can be addressed?

RFC 4821 describes a way in which the packetization layer (layer 4 in the OSI model, or TCP in the Internet Protocol model) increases packet sizes and uses packet loss to detect the maximum packet size that will transit a given path.  The lower layers are not used to determine the MTU.  The RFC mentions an interesting case: an inconsistently configured multi-channel link or multipath topology, each with an inconsistent MTU.  Some packets make it through at a high MTU and others fail due to the link(s) with a lower MTU.  The protocol monitors packet loss to detect inconsistent multi-channel MTU configurations.

One of the interesting limitations with this protocol is its use with UDP and applications in which there is minimal (or no) feedback on which base the modification of the outoing MTU.

I just checked Cisco’s web site and didn’t find anything there that referenced the use of this protocol.   A few google searches didn’t turn up anyone who had implemented this mechanism.  Fortunately, there isn’t a chicken-and-egg problem here in that there is no dependency between multiple systems to implement it.  I’d like to know if anyone has implemented it and how well it works.  Unfortunately, the RFC doesn’t mention any reference implementations.  If you know of one, Please post a comment to let everyone know where it is and how well it works.



Re-posted with Permission 

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under


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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.