Summary

  • RFC 1385 proposed EIP as a new addressing and routing framework that retained an IPv4-shaped base header and made its extension appear to legacy nodes as an unknown IP option.
  • EIP separated 32-bit host numbers from variable-length network numbers, then staged the transition around upgraded border routers, DNS and temporary translation.

Published in November 1992, RFC 1385 explicitly presented EIP as an idea paper proposing a direction, not as a deployed architecture or an adopted Internet standard.

The cleverest field in EIP was one its new hosts were told not to trust. The four-bit Version field remained identical to IPv4 purely for compatibility; EIP implementations did not check it. IHL was retained for the same reason. A separate extension-length byte, not IPv4's IHL, determined how far the new header actually ran.

RFC 1385 preserved the first 20 bytes so thoroughly that source and destination addresses became source and destination host numbers. Each remained 32 bits, but was unique only inside its network. Network numbers moved into a variable extension together with room for other routing or addressing information. The total EIP header length was the extension length plus 20, permitting up to 276 bytes rather than the 60-byte ceiling expressible by IPv4 IHL.

The proposed EIP identifier was fixed at 0x8A. To an EIP-aware host, its position disclosed the new protocol. To an old IPv4 host or router, the extension was meant to resemble a new, unknown option and therefore be ignored. Within one network, the extension could be omitted entirely. During transition, unchanged subnet routers could continue treating the host number as an IPv4 address.

Compatibility was not free; it was concentrated at boundaries. Border routers would map existing network portions to new EIP network numbers and add the extension to outbound IPv4 packets. RFC 1385 argued that an EIP packet returning to an old host did not need reverse translation because the host would ignore the extension. DNS, meanwhile, would gain a new record for network numbers.

The migration plan followed the dependency graph. First choose an addressing scheme and assign network numbers. Then update backbone and border routers, DNS and translation. Upgrade key servers and other hosts afterward. Subnet routers came last. Translation could operate only while the reused 32-bit host numbers remained globally unique; exhaustion would end that bridge.

The paper reported experiments with 35 hosts in 30 countries and no visible performance difference under its setup. That was a bounded test, not proof that unknown options crossed every path or that EIP was deployed. In 1995, RFC 1752 recommended 128-bit SIPP as the basis for IPng. In 2012, RFC 6814 formally deprecated the EIP option, described it as superseded by IPv6 and moved RFC 1385 to Historic.

EIP's enduring interest lies in its compatibility wager: make the future legible to upgraded systems while keeping it safely boring to the past.

Sources