IP over HIPPI
Extracted elements (26)
Camp-on must be used in the initial 64-bit connection attempt because without it a rejection is ambiguous: it could mean the Destination lacks Cable B or is simply busy. Camp-on removes this ambiguity, allowing the Source to correctly interpret a rejection as a capability limitation rather than transient congestion.
The 68-burst connection limit was chosen to allow transmission of one maximum-sized packet or a reasonable number of smaller packets per connection, ensuring each Source gets a fair share of a common Destination's bandwidth and preventing high-volume Sources from monopolizing it.
The MTU of 65,280 bytes was selected to allow the IP packet to fit in one 64 KB buffer with up to 256 bytes of overhead (current overhead is 40 bytes, leaving 216 bytes of room for expansion).
The optional header format features from RFC 1374 (optional short burst first, D1 fill bytes, nonzero D2 offset) were removed to make the header format non-variable. Since no RFC 1374 implementations used these options, the change eliminates complexity without interoperability impact.
In a mixed 32-bit/64-bit HIPPI LAN, a 64-bit Source connecting to a 32-bit Destination must request with W=0 and not use Cable B. A 64-bit Destination must examine the W bit in the received I-field and use or ignore Cable B accordingly.
Support of IEEE 802.2 XID, TEST and Type 2 PDUs is not required on HIPPI; Destinations that receive these PDUs may either ignore them or respond correctly according to IEEE 802.2 requirements.
The ARP description was separated from IP encapsulation and moved to an Informational memo due to insufficient implementation experience. All existing RFC 1374 IP encapsulation implementations are believed compliant with this memo's requirements; no interoperability problems are anticipated from the header simplification.
A Destination SHALL reject all connections if its network interface is not initialized. Once initialized, a Destination may reject connections only for incorrect I-field parity or invalid I-field contents; it SHALL accept a connection immediately when buffers are available.
A Source may hold a connection for no more than 68 HIPPI bursts across all packets in that connection. The Source SHALL make a best effort to ensure connection durations do not exceed this limit, checking burst count after each packet.
A Source SHALL NOT make consecutive connection requests without Camp-on to the same Destination while those requests are being rejected, in order to prevent circumventing the switch's fair-share arbitration mechanism.
Destinations SHALL accept all packets that arrive during a connection and SHALL NOT abort a connection (deassert CONNECT) simply because too many bursts were received. Transient buffer shortages must never cause rejected connections.
Each HIPPI interconnection between a node and a switch SHALL consist of a pair of HIPPI links, one in each direction. If a link supports the 1600 Mb/s option (Cable B), the node's HIPPI-PH implementation SHALL also support 32-bit operation and SHALL be able to select or deselect the 1600 Mb/s option at each connection.
Source rules: Do not assert REQUEST until a packet is ready; transmit bursts as quickly as READYs permit; deassert REQUEST immediately when no packet is available or the last packet of the connection has been sent.
The HIPPI-FP D1_AreaSize SHALL be 3 (64-bit words); no D1 fill is permitted. Leading short bursts are not permitted; any short burst must be the last burst of the packet. Nonzero values for D2_Offset are not permitted.
The I-field Locally Administered bit (bit 31) SHALL be zero. The Double_Wide (W) bit SHALL be set when Source Cable B is connected and a 64-bit connection is desired. Path Selection SHALL be 00, 01, or 11 (binary); source route mode uses bits 23–00 as a 24-bit route, logical address mode uses 12-bit addresses.
The IEEE 802.2 LLC header SHALL begin in the first byte of the HIPPI-FP D2_Area. SSAP and DSAP SHALL each contain 0xAA. CTL SHALL contain 0x03 (Unnumbered Information). SNAP Organization Code SHALL be zero; EtherType SHALL be set per Assigned Numbers (IP=0x0800, ARP=0x0806, RARP=0x8035).
The MTU for HIPPI-SC LANs is 65,280 bytes (the IP packet size including IP header, excluding lower-layer overhead). This is a fixed value required so that Destination buffer sizes can be determined.
ULP-id (8 bits) SHALL contain 4. D1_Data_Set_Present SHALL be set. Start_D2_on_Burst_Boundary SHALL be zero. Reserved bits SHALL be zero. D2_Size SHALL contain the byte count of the IEEE 802.2 LLC PDU and SHALL NOT exceed 65,288.
Camp-on (I-field bit 24) requests the HIPPI-SC switch to queue the connection rather than reject it when the Destination is busy. With Camp-on, rejection indicates an error; without Camp-on, rejection is ambiguous between a busy Destination and an error condition.
Channel Data Rate Discovery: a 64-bit Source discovers path capability by attempting a 64-bit Camp-on connection. Rejection means the Destination or path lacks Cable B; a subsequent 32-bit connection success confirms 32-bit-only capability. The HIPPI-LE Double_Wide bit hints at 64-bit capability on the return path.
Path MTU Discovery per RFC 1191 may be used without modification by HIPPI nodes to discover MTU restrictions on paths between HIPPI-SC LANs and other networks. Gateways between HIPPI-SC LANs and other network types should implement RFC 1191.
Security issues are not discussed in this memo.
The 48-bit Universal LAN MAC Address (ULA) in HIPPI-LE fields follows IEEE 802.1A canonical bit order: ULA byte 0 occupies bits 15–8 (with U/L bit at bit 9 and I/G bit at bit 8), bytes 1–5 follow in order across bits 7–0 and the subsequent 32-bit word.
The HIPPI-LE header (words 2–7) carries: FC (3 bits, SHALL be zero), Double_Wide (1 bit), Message_Type (4 bits, values 0–4), Destination_Switch_Address (24 bits, right-justified), Source_Switch_Address (24 bits, right-justified), Reserved (16 bits), Destination_IEEE_Address (48 bits), LE_Locally_Administered (16 bits), and Source_IEEE_Address (48 bits).
The HIPPI packet structure for IP datagrams is: HIPPI-FP header (8 bytes), HIPPI-LE header (24 bytes), IEEE 802.2 LLC/SNAP header (8 bytes), IP datagram, and trailing fill bytes to round up to a multiple of 8 bytes. The total overhead is 40 bytes.
The I-field (32 bits) in logical address mode encodes: Locally Administered (bit 31), Reserved (bits 30–29), Double_Wide/W (bit 28), Direction/D (bit 27), Path Selection/PS (bits 26–25), Camp-on/C (bit 24), Source Address (bits 23–12), and Destination Address (bits 11–0, containing the Switch Address).