CDN MTU Optimization and Path MTU Discovery: Avoiding Fragmentation

Back in 2019 we were chasing a phantom 1.8% rebuffer rate in our Southeast Asia PoPs that didn't correlate with any CPU, bandwidth, or cache hit anomaly. After two weeks of tcpdump captures and unhappy calls with a transit provider in Singapore, the culprit turned out to be a single upstream router with a 1476 byte MTU silently dropping ICMP "need to fragment" messages. Clients behind PPPoE saw our 1500-byte packets vanish, TCP timed out, and the player stalled. That incident taught me that in CDN networking, MTU optimization isn't a set-and-forget interface setting - it's an active engineering discipline.

Why 1500 Bytes Will Still Break Your CDN Edge

Most engineers still treat 1500 bytes as the safe Ethernet default, and for a bare-metal edge server with no overlay, it usually is. But a modern CDN edge is never bare-metal. By the time a byte leaves your cache server it has likely picked up encapsulation you didn't account for in your original design: VXLAN adds 50 bytes, GRE adds 24, an MPLS label stack adds 4 bytes per label, and if you're doing anycast with IPsec between shield and edge, you're looking at another 50-73 bytes depending on cipher suite. I’ve audited PoPs where the effective MTU shrank to 1420 bytes before the packet even hit the NIC driver, yet the origin and edge servers were still happily advertising 1500. That mismatch is where packet fragmentation begins, and it’s brutal for performance.

When a router has to fragment, you're not just paying for the frag header overhead. You're forcing the receiving stack to allocate reassembly buffers, wait for the slowest fragment, and discard the entire datagram if one fragment is lost. On high-throughput video delivery, that reassembly cost shows up as microbursts of CPU softirq pressure that won't appear in your average load graphs. We've measured fragmentation-induced retransmits adding 40-120ms of tail latency on 4K segments, which is enough to blow past your ABR controller's deadline. And if you're load balancing across multiple ECMP paths - which most CDNs do - fragmented packets can hash differently than unfragmented ones, so they don't even follow the same path, as we covered in our deep dive on ECMP Routing in CDN Networks: Equal-Cost Multi-Path Traffic Distribution.

On Linux, you can't rely on interface MTU alone to tell you what's happening on the wire. I always start with a DF-bit ping sweep from the edge itself to a sample of eyeball prefixes, because it shows you where PMTUD would fail before you turn it on:

# Don't trust 'ip link show' - probe the actual path with DF set
# Sweep MTU from 1500 downward, record where ICMP Type 3 Code 4 disappears

for mtu in 1500 1472 1420 1400 1280; do
  echo "=== Probing MTU $mtu ==="
  ping -M do -s $((mtu - 28)) -c 3 -W 2 203.0.113.50
  # -M do = prohibit fragmentation, -s = payload size (28 = IP+ICMP headers)
  # No response + no ICMP error = PMTUD black hole
done

# On the CDN cache, verify what the kernel currently thinks
ip route get 203.0.113.50 | grep mtu
ss -i 'dst 203.0.113.50'  # check tcp mss and pmtu cache entries

How Path MTU Discovery Breaks in the Real World

Classic Path MTU Discovery (PMTUD) as defined in RFC 1191 and RFC 8201 for IPv6 is elegant in theory. The sender sets the Don't Fragment (DF) bit, a router with a smaller MTU drops the packet and sends back an ICMP Fragmentation Needed (Type 3, Code 4) with its MTU, and the sender lowers its packet size. In practice, that ICMP message is treated as hostile by half the internet. Enterprise firewalls, overly strict iptables rules, and some cloud security groups filter all ICMP by default. We see roughly 12-15% of IPv4 paths from our U.S. edge to residential eyeballs filter PTB (Packet Too Big) messages entirely. That's a PMTUD black hole - the sender keeps retransmitting large packets, never learns it needs to fragment, and the connection hangs until it times out or falls back.

That's why we moved entirely to Packetization Layer PMTUD (PLPMTUD) per RFC 8899. Instead of relying on ICMP, PLPMTUD probes the path with progressively larger packets at the transport layer and watches for acknowledgments or loss. TCP's implementation in Linux via tcp_mtu_probing does this without any ICMP dependency. It doesn't care if a middlebox ate the PTB message; if a 1500-byte probe isn't ACK'd but a 1240-byte one is, the stack converges on the smaller value. For CDN networking at scale, this is the only reliable approach for both IPv4 and IPv6, especially since IPv6 routers are forbidden to fragment at all. The HTTP Working Group Specifications now assume PMTUD issues are handled below HTTP, but if you haven't configured them, your HTTP/2 and HTTP/3 streams will inherit the loss. We also had to account for how our backbone encapsulation interacts with these probes, which ties directly into how we use MPLS in CDN Backbone Networks: Label Switching for Predictable Performance and more recently Segment Routing for CDN Traffic Engineering: Source-Routed Path Selection - every label or SRH you push reduces the payload MTU your probes can validate.

The trick is enabling PLPMTUD without causing probe storms. On a cache serving 80k concurrent connections, aggressive MTU probing on every socket will create measurable overhead. Here is the baseline sysctl profile we've standardized on after testing across 40 PoPs, with IPv4 and IPv6 tuned together:

# /etc/sysctl.d/99-cdn-pmtud.conf - PLPMTUD for high-concurrency CDN edge
# Tested on kernel 5.15+ with Cubic and BBR

# Enable PLPMTUD for TCP (0=disabled, 1=enabled when ICMP black hole detected, 2=always)
net.ipv4.tcp_mtu_probing = 2
net.ipv4.tcp_base_mss = 1240      # conservative starting point for probing
net.ipv4.tcp_mtu_probe_floor = 1000

# IPv6 equivalent - even more critical since fragmentation is end-to-end only
net.ipv6.route.mtu_expires = 600  # re-probe every 10 min, not default 10 min? we keep 600s
# Disable accepting bogus small MTUs from forged ICMP (MTU security)
net.ipv4.tcp_min_snd_mss = 536
net.ipv6.conf.all.disable_ipv6 = 0

# For QUIC/UDP (HTTP/3) - ensure DPLPMTUD in user space, kernel can't do it for UDP
# Validate your quiche/nginx-quic sets max_udp_payload_size >= 1350 and enables PMTUD
sysctl -p /etc/sysctl.d/99-cdn-pmtud.conf
ip route flush cache

Probing, Jumbo Frames, and What the Numbers Actually Show

There’s a persistent myth that enabling jumbo frames (9000 bytes) on the edge automatically improves goodput. Inside the data center fabric between cache SSD and NIC, yes, 9000 MTU reduces per-packet overhead, fewer interrupts, and we’ll take that win all day for cache-to-cache fill. But I’ve seen teams push 9000 MTU to the internet-facing interface and then wonder why throughput collapses over transit. No major Tier 1 supports end-to-end jumbo frames to eyeballs, and advertising 9000 with DF set guarantees PMTUD will have to downsize every single flow. The right model is a split MTU domain: 9000 internally on the spine-leaf, 1500 (or more accurately, 1472-1480 after encapsulation) externally, and let PLPMTUD negotiate the delta. Don't forget that HTTP/3 over QUIC adds its own overhead - the IETF recommends a default of 1350 bytes for QUIC datagrams for a reason, covered well in the performance guidance on web.dev Articles.

We benchmarked three MTU strategies on identical edge hardware (100 Gbps Mellanox ConnectX-6, BBR, 32k concurrent 10MB segment fetches) to quantify the tradeoff. The test was from our Chicago PoP to synthetic clients behind varying access MTUs - clean 1500, PPPoE 1492, and tunneled 1280 - to simulate real eyeball diversity. The point wasn't maximum theoretical goodput, it was stable goodput without packet fragmentation or PMTUD failures. The table below shows the 24-hour median results that drove our global default.

Tuning the Edge: A Practical PMTUD and MSS Clamping Implementation

Knowing PMTUD is fragile doesn't help unless you ship a config that survives it. On our CDN edge fleet, we don't rely on classic ICMP-based PMTUD alone — you'll get burned. The pragmatic baseline is RFC 4821 PLPMTUD for TCP plus MSS clamping for everything that tunnels. We've standardized on enabling PLPMTUD with a conservative base MTU and letting the stack probe up, rather than starting at 1500 and fragmenting down.

For bare-metal Linux edges (kernel 5.10+), that means tcp_mtu_probing=1 not 2. Mode 2 is too aggressive and will badly inflate retransmits on lossy eyeball networks. We set tcp_base_mss to 1420 to account for typical PPPoE + VLAN + VXLAN overhead, and we clamp MSS to 1380 on our transit routers and virtual appliances. If you're running Network Function Virtualization in CDN Infrastructure: Virtual Appliances at Scale, that clamping has to happen in the vSwitch datapath — doing it only on the host NIC leaves your Geneve-encapsulated overlay traffic with a phantom 1500 that'll black-hole 3-5% of clients behind carrier-grade NAT.

Here's the hardened template we roll to every PoP. It gives you PLPMTUD, sane TCP probe intervals, and an iptables rule that saves you from GRE/IPsec tunnels dropping large GETs. Don't forget to mirror this in your Software-Defined Networking for CDN: Programmable Traffic Control controller so new routes inherit it automatically — we've seen a single unclamped peering router tank cache-fill throughput by 40% for 6 hours:

# /etc/sysctl.d/90-cdn-mtu.conf - CDN Edge MTU Hardening
net.ipv4.tcp_mtu_probing = 1
net.ipv4.tcp_base_mss = 1420
net.ipv4.tcp_mtu_probe_floor = 1000
net.ipv4.tcp_min_snd_mss = 512
net.ipv6.conf.all.mtu = 1420

# MSS clamping for tunneled traffic (run on edge + origin shield)
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
  -j TCPMSS --clamp-mss-to-pmtu
# For VXLAN/Geneve overlays with 50-80B overhead
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
  -o vxlan0 -j TCPMSS --set-mss 1380

# Verify: tracepath -n 203.0.113.50 / ping -M do -s 1472 -c 4 203.0.113.50

Advanced Patterns: Anycast, Overlay Tunnels, and Hunting Black Holes

At scale, your biggest MTU headaches aren't individual links — they're your CDN Backbone Topology Design: Ring, Mesh, and Hierarchical Architectures. Anycast makes PMTUD state ephemeral by design. A client probing for MTU to PoP-A may complete its handshake via PoP-B after a route flap, and the cached PMTU value is gone. That's why we disable long-lived PMTU caching on anycast frontends and rely on per-connection PLPMTUD instead. For your backbone, where you control both ends, just do the opposite: enable 9000-byte jumbo frames end-to-end (actual MTU 8972 after 28B ICMP/IP overhead), validate with 8972-byte ping -M do tests, and you'll see 12-18% better origin shield throughput on large objects due to fewer interrupts and better GRO/GSO batching.

When things break, they break silently — the classic PMTUD black hole. The symptom is always the same: small objects (under ~1400 bytes) load fine, but 50KB+ assets, video segments, or cache fills hang after the first 1-2 seconds, then timeout. Don't start with logs. Start with packet captures. Run tcpdump 'icmp or icmp6' on the edge and look for the absence of "Fragmentation Needed" or "Packet Too Big" messages. If your client sends a 1500-byte packet and you never send or receive an ICMP PTB, something in the middle is filtering it. We've found that 92% of our black holes in the last year were not firewalls, but misconfigured CPE and CGNAT that rate-limit ICMP to 1 pps.

Your troubleshooting workflow should be: 1) tracepath and mtu discovery from both ends to spot asymmetry, 2) test with curl --tcp-mss 1200 vs default to confirm MTU sensitivity, and 3) temporarily lower the origin's interface MTU to 1280 (the IPv6 minimum) — if the hang disappears, you've confirmed fragmentation. Long term, fix it with MSS clamping on the tunnel endpoints and set your DID/QUIC max_udp_payload_size to 1350, not 1500. For QUIC/HTTP/3, remember UDP doesn't get kernel PMTUD for free; you must implement DPLPMTUD in userspace or you'll fragment at the IP layer and lose 5-10% of that traffic to middlebox drops.

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