BGP MTU Discovery.

What is MTU and Why is it Important?

The Maximum Transmission Unit (MTU) is the maximum size, in bytes, that a packet can have when being transmitted over a network link. If a packet is larger than the MTU of a link, it must be fragmented into smaller packets, which can lead to inefficiencies and increased latency. In some cases, if the "Don't Fragment" (DF) bit is set, oversized packets will be dropped, leading to transmission failures.

In a BGP environment, where routing information is exchanged between routers across different networks, ensuring that packets are transmitted within the MTU limits of each link is crucial. This is where BGP MTU discovery comes into play.

What is BGP MTU Discovery?

BGP MTU Discovery is the process of dynamically determining the optimal MTU size for BGP packets being transmitted between two BGP peers. It ensures that the packets are transmitted without the need for fragmentation, thereby optimizing the flow of routing information and preventing potential transmission issues.

How Path MTU Discovery Works:

1 Initial Transmission with Maximum MTU: The sender starts by assuming the maximum possible MTU size (typically the MTU of the local network). It sends packets with the "Don't Fragment" (DF) bit set, indicating that routers along the path should not fragment these packets.

2 Router Handling and MTU Checks: As the packet traverses through various networks, each router checks the packet's size against its own MTU. If a router encounters a packet that is too large to be forwarded through the next link, it drops the packet instead of fragmenting it.

3 ICMP "Fragmentation Needed" Message: When a router drops a packet due to the MTU size, it sends an ICMP "Fragmentation Needed and DF set" message back to the sender. This message contains the MTU size of the next link, which is the largest packet size that can be transmitted without fragmentation.

4 Adjusting Packet Size: Upon receiving the ICMP message, the sender reduces the packet size to the specified MTU and retransmits the data. This process continues until the sender successfully finds the smallest MTU size along the entire path.

5 Stable Communication: Once the correct MTU is discovered, the sender can continue to transmit packets without fragmentation, ensuring efficient data delivery.

Configuring MTU Discovery for a Single BGP Neighbor

Device(config)# router bgp 45000

Device(config-router)# address-family ipv4 unicast

Device(config-router-af)# neighbor 192.168.1.1 remote-as 45000

Device(config-router-af)# neighbor 172.16.1.1 activate

Device(config-router-af)# no neighbor 172.16.1.1 transport path-mtu-discovery

Benefits of Path MTU Discovery

1. Prevention of Packet Fragmentation

• Efficiency: MTU Discovery helps avoid packet fragmentation by ensuring that packets are transmitted at a size that all network links can handle without needing to be broken into smaller fragments. Fragmentation can lead to inefficiencies as it requires additional processing and reassembly at the destination.
• Reduced Latency: Fragmented packets can increase latency due to the need for reassembly, and smaller fragments might take different paths, causing out-of-order delivery. By preventing fragmentation, MTU Discovery ensures that packets reach their destination faster and in the correct order.

2. Improved Transmission Reliability

• Avoiding Packet Drops: When the "Don't Fragment" (DF) bit is set in a packet, it will be dropped if it exceeds the MTU of any link along its path. MTU Discovery dynamically adjusts the packet size to avoid exceeding the MTU, thus preventing packet drops and ensuring reliable transmission.
• Fewer Retransmissions: By avoiding packet drops, MTU Discovery reduces the need for retransmissions, which can slow down the network and increase congestion.

3. Optimized Network Performance

• Better Throughput: With the optimal MTU size discovered, data packets can flow through the network more smoothly, leading to better overall throughput. Larger packets can carry more data, reducing the overhead associated with packet headers.
• Consistency Across Networks: MTU Discovery ensures that the MTU size is compatible across all networks in the path, leading to consistent and predictable network performance.

4. Simplified Network Management

• Automated Adjustment: MTU Discovery automates the process of adjusting packet sizes, reducing the need for manual configuration and minimizing the risk of human error in setting MTU sizes.
• Easier Troubleshooting: By preventing fragmentation and packet drops, MTU Discovery can simplify troubleshooting, as network issues related to MTU mismatches are less likely to occur.

5. Enhanced Security

• Preventing Attack Vectors: Fragmented packets can sometimes be used to evade security measures like intrusion detection systems (IDS). By preventing fragmentation, MTU Discovery reduces the attack surface related to these types of exploits.

6. Better Compatibility with Modern Networks

• Support for IPv6: MTU Discovery is especially important in IPv6 networks, where fragmentation is not handled by routers but by the source host. Ensuring that packets are sized correctly from the start is crucial for maintaining network efficiency in IPv6 environments.

Challenges and Considerations:

• ICMP Blocking: Some networks may block ICMP messages, which are essential for PMTUD. This can prevent the sender from receiving necessary feedback about the correct MTU size, leading to potential communication issues.
• Black Hole Networks: If ICMP messages are blocked, packets that are too large may simply disappear without notification, creating a "black hole" where data is lost without explanation.
• PMTUD in IPv6: In IPv6, routers do not fragment packets. The sender must perform PMTUD to determine the correct packet size, making PMTUD even more critical in IPv6 environments.

Conclusion

MTU Discovery is a crucial process that plays a vital role in optimizing network communication. By allowing devices to determine the optimal packet size for transmission, MTU Discovery enhances efficiency, reduces the risk of packet loss, and ensures smoother, more reliable network performance. Despite the challenges, its benefits make it an indispensable tool in the arsenal of network administrators, ensuring that data is transmitted swiftly and without unnecessary overhead.

Sources

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/configuring-bgp-neighbor-session-options.pdf

https://documentation.nokia.com/acg/23-10-2/books/classic-cli-part-i/c145-pmtud.html

https://blog.ipspace.net/kb/Internet/PMTUD/20-mtu-discovery/

https://support.hpe.com/techhub/eginfolib/networking/docs/switches/5130ei/5200-3942_l3-ip-svcs_cg/content/483572483.htm