Jumbo frame

In computer networking, jumbo frames are Ethernet frames with more than 1500 bytes of payload, the limit set by the IEEE 802.3 standard. Conventionally, jumbo frames can carry up to 9000 bytes of payload, but variations exist and some care must be taken using the term. Many Gigabit Ethernet switches and Gigabit Ethernet network interface cards can support jumbo frames. Some Fast Ethernet switches and Fast Ethernet network interface cards can also support jumbo frames. 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. 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. 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 and have a positive effect on end-to-end TCP performance. 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. 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. IEEE 802.1AB-2009 and IEEE 802.3bc-2009 added LLDP discovery to standard Ethernet for maximum frame length (TLV subtype 4). 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). Larger frames are more likely to suffer undetected errors with the simple CRC32 error detection used in Ethernet frames — a larger amount of data increases the probability that several errors cancel each other out. Consequently, additional mechanisms have been developed to improve error detection on higher network layers. IETF solutions for adopting jumbo frames avoids data integrity reduction of the service data unit through use of the Castagnoli CRC polynomial being implemented within the SCTP transport (RFC 4960) and iSCSI (RFC 7143). Selection of this polynomial was based upon work documented in the paper '32-Bit Cyclic Redundancy Codes for Internet Applications'. 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.