Protocol Tables

The x-kernel runtime environment always includes a protocol table. For each protocol, this table defines a numbering space for identifying other protocols. The x-kernel builds a table of protocol numbers by reading configuration files at boot time, and provides an operation for protocols to query this table (see section 3.3). This frees protocols from embedding explicit protocol numbers in the protocol code itself.

If you have not configured any non-standard protocols in your graph, you can safely use one of the default protocol tables in /usr/xkernel/etc and may skip to section 13.1.1.

The following is an example configuration file defining a protocol table:

#
# prottbl
#
# This file describes absolute protocol id's 
# and gives relative protocol numbers for those
# protocols which use them
#
# x-kernel v3.2

eth         1    
{
        ip         x0800
        arp        x0806
        rarp       x8035
        #
        # ethernet types x3*** are not reserved
        #
        blast      x3001
}
ip          2
{
        icmp       1
        tcp        6
        udp        17
        #
        # IP protocol numbers n, 91 < n < 255, are unassigned
        #
        blast      101
}
arp         3
rarp        4
udp         5
tcp         6
icmp        7
blast       8
sunrpc      9

Each protocol has an entry of the form:

name idNumber [ {hlp1 relNum1 hlp2 relNum2 ... }]

where the idNumber uniquely identifies each protocol.

There are two ways for a protocol to define its protocol number space. The first technique uses an explicit numbering scheme; the protocol explicitly indicates which protocols may be configured above it and the relative number that should be used for each higher-level protocol. Use of this numbering scheme is indicated by the presence of the optional hlp list after the protocol's idNumber.

The second technique uses an implicit numbering scheme; the protocol does not explicitly name its allowed upper protocols, but will implicitly use each protocol's unique idNumber as its relative protocol number. BLAST, for example, employs this technique and would use SUNRPC's idNumber 9 as its protocol number relative to BLAST. Protocol ID numbers are 4-byte quantities in the x-kernel, so protocols using implicit numbering have a 4-byte field in their headers for the upper protocol number.

Implicit numbering clearly allows more flexibility in how protocols may be composed. As flexible composability is one of the goals of the x-kernel, all new protocols written in the x-kernel should use this implicit numbering scheme.

The x-kernel is distributed with two useful configuration files for building protocol tables, located in /usr/xkernel/etc: prottbl.std and prottbl.nonstd. The first of these two files, prottbl.std contains the standard protocol numbers; e.g., TCP is known as protocol ``6'' relative to IP, IP is known as protocol ``x0800'' relative to the ethernet, and so on. This file should be used when there is no danger of interfering with the standard protocol suite on the platform you are using. The second file, prottbl.nonstd, uses non-standard protocol numbers. You will want to use this file when there is a danger of interfering with the platform's native protocol stack; e.g., both the x-kernel and the native stack are running and getting packets diverted their way by a packet filter.

All x-kernels must load at least one protocol table, and all protocols configured in a kernel must have an entry in the protocol table. If you are writing a new protocol, you will probably want to define an auxiliary protocol table which assigns a temporary idNumber to your new protocol and then configure your kernel to read in both the system table and your auxiliary table (see section 12.1 for configuration examples.) If you need to configure your new protocol above an existing protocol which uses explicit numbering, you can augment the table for the existing protocol as in this example:

#
# Auxiliary protocol table 
#

yap	1000

ip	2 	{
        yap	200
}

Here, we define our new protocol YAP and indicate that it has protocol number 200 relative to IP. Note that when augmenting the tables of other protocols, the idNumber of the other protocol must match its number in the system file.

The x-kernel runs consistency checks on the protocol tables and will give error messages and abort in the presence of inconsistencies.

ROM options

Protocol table configuration files can be specified in either graph.comp (see section 12) or in a ROM file (see section 13.2). The ROM file format is:

prottbl FILENAME1
prottbl FILENAME2
...

In both cases, the named protocol tables are loaded at run-time. Platforms for which loading protocol tables at run-time is impractical (e.g., inside the Mach kernel) provide platform-specific mechanisms to support protocol tables (see, for example, section 17).

By default, the protocol table allows multiple upper protocols to use the same protocol number relative to a single lower protocol. To restrict this behavior, so that each upper protocol must have a unique relative protocol number, the unique_hlps ROM option may be used:

prottbl unique_hlpson