I made some editorial comments about Section 5.1 the other day. I have a
substantive comment now, and a text proposal to pull it all together.
Comment: the statement
"For a = 0, A MAY be 0 to
allow for the provisioning of the system ports."
just below Figure 2 is illogical, since if a = 0, the value A does not
exist. In fact, a = 0 returns us to the naive approach in the second
paragraph of Section 5.1. Hence I propose alternative text for the
section. Note two changes to Figure 2: removed extra bit 16 and changed
the title.
Proposed text:
5.1. Port Mapping Algorithm
The port mapping algorithm is used to represent the port numbers
allocated to a given CE if the CE has been given a shared IPv4 address.
The simplest way to allocate ports is to allocate one range of
consecutive ports to each CE sharing an address. The Port-Set Identifier
(PSID) is an index over the sequence of allocated ranges, starting from
zero. In this case, assuming that the number of port numbers in each
allocated range is a power of 2, the set of port numbers allocated to a
given CE can be represented using a notation similar to CIDR [RFC4632],
but with 16 bits instead of 32. For example, assuming a range size of
256 port numbers, the first 256 ports are
represented as port prefix 0.0/8, the last 256 ports as 255.0/8. In
hexadecimal, 0x0000/8 (PSID = 0) and 0xFF00/8 (PSID = 0xFF).
It can be seen from the example that the lowest PSID values allocate
ports in the system port range [I-D.ietf-tsvwg-iana-ports] to one or
more CEs. To avoid this, the lowest PSID values (0x00 to 0x03 in the
example) should not be used.
A more sophisticated approach allocates ports in a series of blocks,
where each CE sharing an address receives one range from each block.
This can be represented as shown in Figure 2.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-------+-----------+----------+
Ports in | A | PSID | M |
the CE port set | > 0 | |any value |
+-------+-----------+----------+
|a bits | k bits | m bits |
Figure 2: Three-Part Decomposition of Allocated Port Numbers
A Indexes the blocks of port numbers. The lowest block number
(A = 0) and possibly others contain the system port numbers,
and hence should not be used if port numbers 0-1023 are to
be avoided.
a Width of the field containing the block index A. Indirectly
a determines the block size, by the formula:
Block size = 65536 / 2^a
a = 0 brings us back to the simple (single block) model
described above, where the PSID value rather than the value
of A must be restricted to avoid the system ports. The value
of a is optionally provisioned as part of the mapping rule.
The default value for a is 4, to place the start of the PSID
on a nibble boundary. This leaves ports in the range 0-4095
unallocated.
PSID Indexes the position of the allocated range for a given CE
within each block. In the more general model, where A > 0,
PSID values can begin at 0 without causing system ports to
be allocated.
k Width of the field containing the PSID. The number of CEs
sharing the same IPv4 address is given by 2^k, except when
a = 0 and the lowest PSID values are excluded.
M Indexes a specific port number within a range determined by
the values of A and the PSID.
m Width of the port index field. The number of ports in each
allocated range is given by 2^m.
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