Standard for the transmission of IP datagrams over IEEE 802 networks
obsoletes
- rfc-948 — Two methods for the transmission of IP datagrams over IEEE 802.3 networks
also
- std-43
Extracted elements (27)
ARP requests on 802.5 should first be sent as all-stations broadcasts (no RIF) to minimize all-ring traffic; if unsuccessful, all-routes broadcast is preferred over single-route broadcast because it provides fault tolerance in environments with multiple redundant bridges where spanning-tree failures might isolate paths.
SNAP is used instead of direct LSAP assignments so that existing EtherType codes (from Assigned Numbers) can be reused without modification on 802 networks. An Organization Code of 0 signals that the following 16 bits are a standard EtherType, avoiding the need for a parallel 802-specific type registry.
The RIF table is kept logically distinct from the ARP table (IP-to-48-bit-address mapping) to allow a generalized 802 implementation that does not waste ARP cache memory with RIF fields on non-802.5 networks, even if implementations may store them together for performance.
Ethernet and IEEE 802.3 may coexist on the same physical cable using different link protocols. Link-level broadcasts only reach hosts using the same link protocol. If mixed environments are necessary, an IP gateway should be used as if there are two distinct networks.
The Ethernet MTU allows a 1500-octet IP datagram, while the 802.3 MTU allows only 1492 octets due to the 8-octet LLC+SNAP overhead. These values are not equal and interoperating implementations must account for this difference.
802.2 Type 1 (connectionless) communication must be supported by all implementations and is the required baseline method. Type 2 (connection-oriented) may only be used by consenting systems after XID negotiation.
All 802.5 implementations are required to understand ARP/IP frames with no RIF (local ring) and frames with an empty RIF. Implementations supporting multi-ring source routing must store non-empty RIFs for future transmissions; others should gracefully ignore frames with non-empty RIFs.
All hosts must be able to communicate using the standard non-trailer encapsulation method. Trailer encapsulation may only be used by consenting systems on the same network.
All implementations are required to support IEEE 802.2 standard Class I service, including Unnumbered Information (UI) commands, XID commands and responses, and TEST commands and responses.
Gateway implementations must be prepared to accept full-length packets and fragment them when necessary. Hosts should accept full-length packets but must not send datagrams longer than 576 octets unless they have explicit knowledge the destination accepts them.
IP broadcasts on 802.5 networks must be sent as single-route broadcasts to prevent multiple copies of IP broadcast packets, which would have undesirable effects on protocols using IP.
IP precedence should not be mapped to 802.5 priority. All IP and ARP packets should be sent at the default 802.5 priority of 3.
The broadcast Internet address (host part all ones) should be mapped to the broadcast IEEE 802 address (all binary ones).
The mapping of 32-bit Internet addresses to 16-bit or 48-bit IEEE 802 addresses must be done via the dynamic discovery procedure of ARP.
When an IEEE 802 frame must be padded to meet minimum size requirements, padding octets of zero are appended. This padding is not part of the IP datagram and must not be included in the IP header total length field.
When an XID or TEST command is received, a response must be returned with destination and source addresses swapped, DSAP and SSAP swapped, and the poll/final bit preserved.
ARP parameter values for IEEE 802 networks: hardware type code (hrd) = 6, protocol type (pro) = 2048 (IP), hardware address length (hln) = 2 for 16-bit addresses or 6 for 48-bit addresses, protocol address length (pln) = 4, operation = 1 (request) or 2 (reply).
IEEE 802.5 supports three broadcast types indicated by the B field: 000 = non-broadcast (specific route), 100 = all-routes broadcast (global broadcast), 110 = single-route broadcast (limited broadcast, one copy per ring). All other values are reserved.
SNAP is used with Organization Code 0 to indicate that the following 16 bits are an EtherType code as listed in Assigned Numbers. This allows 802 networks to reuse existing EtherType assignments without new LSAP allocations.
The LF (Largest Frame) subfield in the 802.5 RC specifies the maximum MTU supported by all bridges on a route: 000=552 MAC/508 IP, 001=1064/1020, 010=2088/2044, 011=4136/4092, 100=8232/8188. Receivers must reject frames where LF is less than the MTU.
The Routing Information Indicator (RII) is the MSB of the 802.5 source address. RII=1 signals a RIF is present; RII=0 means no RIF. Implementations must reset RII to zero before passing source addresses to higher protocol layers to avoid confusion with the individual/group indicator in destination addresses.
When the 802.5 'address not recognized' indicator is set after transmission, the preferred recovery action is to delete the ARP cache entry and send a new ARP request. This allows graceful recovery from first-hop bridge/router failures and changed hardware addresses.
For IEEE 802.3 (10BASE5): MAC header+trailer totals 18 octets (6+6 destination/source addresses, 2-octet length field, 4-octet FCS). Maximum packet size is 1518 octets, yielding an IP MTU of 1492 octets (1518 - 18 MAC - 8 LLC+SNAP).
For IEEE 802.4: MAC header+trailer totals 17 octets (1 frame control, 6+6 addresses, 4-octet FCS). Maximum packet size is 8191 octets, yielding an IP MTU of 8166 octets (8191 - 17 - 8).
For IEEE 802.5 at 4 Mbps (9 ms token hold time): maximum packet size is 4508 octets. With 36-octet MAC header+trailer (including 18-octet RIF) and 8-octet LLC+SNAP, the IP MTU is 4464. A minimum of 2002 octets is recommended for all implementations.
IP datagrams and ARP are encapsulated using an 8-octet 802.2 LLC + SNAP header: DSAP=170, SSAP=170, Control=3 (Unnumbered Information), followed by a 3-octet Organization Code of 0 and a 2-octet EtherType (IP=2048, ARP=2054).
The 802.5 Routing Information Field (RIF) consists of a 2-octet Routing Control (RC) field containing B[3]:LTH[5]:D[1]:LF[3]:r[4] subfields, followed by 0 to 8 two-octet Route Designator (RD) fields. Total RIF length is 2–18 octets; only even values 2–30 are allowed for LTH.