Jumbo frame

Each Ethernet frame must be processed as it passes through the network. Processing the contents of a single large frame is preferable to processing the same content broken up into smaller frames, as this makes better use of available CPU time by reducing interrupts. This also minimizes the overhead byte count and reduces the number of frames needing to be processed.[3] This is analogous to physically mailing a packet of papers instead of several single envelopes with one sheet each, saving envelopes and cutting sorting time.

Jumbo frames gained initial prominence when Alteon WebSystems introduced them in their ACEnic Gigabit Ethernet adapters.[4] Many other vendors also adopted the size; however, jumbo frames are not part of the official IEEE 802.3 Ethernet standard.

Jumbo frames have the potential to reduce overheads and CPU cycles[5] and have a positive effect on end-to-end TCP performance.[6] The presence of jumbo frames may have an adverse effect on network latency, especially on low bandwidth links. The frame size used by an end-to-end connection is typically limited by the lowest frame size in intermediate links. 802.5 Token Ring can support frames with a 4464-byte MTU, FDDI can transport 4352 byte, ATM 9180 byte and 802.11 can transport 7935 byte MTUs. The IEEE 802.3 Ethernet standard only mandates support for 1500-byte MTU frames, 1518 byte total frame size (1522 byte with the optional IEEE 802.1Q VLAN/QoS tag).

The use of 9000 bytes as preferred payload size for jumbo frames arose from discussions within the Joint Engineering Team of Internet2 and the U.S. federal government networks.[7] Their recommendation has been adopted by all other national research and education networks. In order to meet this mandatory purchasing criterion, manufacturers have in turn adopted 9000 bytes as the conventional MTU size, with a jumbo frame size of at least 9018/9022 bytes (without/with IEEE 802.1Q field). Most Ethernet equipment can support jumbo frames up to 9216 bytes.[8]

IEEE 802.1AB-2009 and IEEE 802.3bc-2009 added LLDP discovery to standard Ethernet for maximum frame length (TLV subtype 4).[9] It allows frame length detection on a port by a two-octet field. As of IEEE 802.3-2015, allowed values are 1518 (only basic frames), 1522 (802.1Q-tagged frames), and 2000 (multi-tagged, envelope frames).[10]

Simple additive checksums as contained within the UDP and TCP transports have proven ineffective at detecting bus specific bit errors since these errors with simple summations tend to be self-canceling. Testing that led to the adoption of RFC 3309 simulated error injection against real data and demonstrated as much as 2% of these errors were not being detected.

Larger frames are more likely to suffer undetected errors with the simple CRC32 error detection used in Ethernet frames—as packet size increases it becomes more likely that multiple errors cancel each other out.[lower-alpha 1]

One IETF solution for adopting jumbo frames avoids data integrity reduction of the service data unit by performing an extra CRC at the next network protocol layer above Ethernet. SCTP transport (RFC 4960) and iSCSI (RFC 7143) use the Castagnoli CRC polynomial. The Castagnoli polynomial 0x1EDC6F41 achieves the Hamming distance HD=6 beyond one Ethernet MTU (to a 16,360-bit data word length) and HD=4 to 114,663 bits, which is more than 9 times the length of an Ethernet MTU. This gives two additional bits of error detection ability at MTU-sized data words compared to the Ethernet CRC standard polynomial while not sacrificing HD=4 capability for data word sizes up to and beyond 72 kbits.[12] Support of Castagnoli CRC polynomial within a general-purpose transport designed to handle data chunks, and within a TCP transport designed to carry SCSI data, both provide improved error detection rates despite the use of jumbo frames where an increase of the Ethernet MTU would otherwise have resulted in a significant reduction in error detection.

Some vendors include the headers in the size settings while others do not, that is either the maximum frame size (including frame headers, maximum layer-2 packet size) or the maximum transfer unit (MTU) (maximum layer 3 packet size excluding frame headers). Therefore, you might find that different values must be configured in equipment from different vendors to make the settings match.

A mixture of devices configured for jumbo frames and devices not configured for jumbo frames on a network has the potential to cause network performance issues.[13]

Jumbo frames can increase the efficiency of Ethernet and network processing in hosts by reducing the protocol overhead, as shown in the following example with TCP over IPv4. The processing overhead of the hosts can potentially decrease by the ratio of the payload sizes (approximately six times improvement in this example). Whether this is significant depends on how packets are processed in the host. Hosts that use a TCP offload engine will receive less benefit than hosts that process frames with their CPU.

Baby giant or baby jumbo frames are Ethernet frames that are only slightly larger than allowed by the IEEE Ethernet standards.[2] Baby giant frames are, for example, required for IP/MPLS over Ethernet to deliver Ethernet services with standard 1500 byte payloads. Most implementations will require non-jumbo user frames to be encapsulated into MPLS frame format which in turn may be encapsulated into a proper Ethernet frame format with EtherType values of 0x8847 and 0x8848.[16] The increased overhead of extra MPLS and Ethernet headers means that the support for frames up to 1600 bytes is required in Carrier Ethernet networks.[17]

Super jumbo frames (SJFs) are generally considered to be frames that have a payload size of over 9000 bytes. The relative scalability of network data throughput as a function of packet transfer rates is related in a complex manner [18] to payload size per packet. Generally, as line bit rate increases, the packet payload size should increase in direct proportion to maintain equivalent timing parameters. This however implies the scaling of numerous intermediating logic circuits along the network path to accommodate the maximum frame size required. As it has been a relatively difficult, and somewhat lengthy, process to increase the path MTU of high performance national research and education networks from 1500 bytes to 9000 bytes or so, a subsequent increase, possibly to 64,000 bytes for example, may take some time.

The main factor involved with an increase in the maximum segment size (MSS) is an increase in the available memory buffer size in all of the intervening persistence mechanisms along the path. The main benefit of this is the reduction of the packet rate, both at end nodes and intermediate transit nodes. As the nodes in general use reciprocating logic to handle the packets, the total number of machine cycles spent processing packet headers decreases as the average MSS of the packets increases given a certain total amount of data transmitted. This relationship becomes increasingly important as average network line bit rate increases to 10 gigabits per second, and above.

By making CPU load independent of frame size, large send offload (LSO) is an approach to eliminate the per-packet overhead that jumbo frames were designed to reduce.[19] Large receive offload (LRO), the inbound counterpart of large segment offload, does not quite eliminate per-packet overhead borne by the CPU. Therefore, jumbo frames remain beneficial for inbound traffic. Jumbo frames are also still useful from a bandwidth perspective, as they reduce the amount of bandwidth used for non-data overhead.

• Jumbogram – large packets for IPv6

• Jumbo Frames – Where to use it?
• Jumbo frames? Yes!, by Selina Lo, Alteon Networks, 2/23/1998 in NetworkWorld
• SJF Supercomputing 2005 Demo
• Pushing up the Internet MTU
• IEEE 802.3as Frame Expansion Task Force
• 32-Bit Cyclic Redundancy Codes for Internet Applications
• Need To Know: Jumbo Frames in Small Networks
• How to Jumbo Frames in Archlinux