5.3.3.3 Precedence Handling For All Routers

A router (whether or not it employs precedence-ordered queue service):

1. MUST accept and process incoming traffic of all precedence levels normally, unless it has been administratively configured to do otherwise.

2. MAY implement a validation filter to administratively restrict the use of precedence levels by particular traffic sources. If provided, this filter MUST NOT filter out or cut off the following sorts of ICMP error messages: Destination Unreachable, Redirect, Time Exceeded, and Parameter Problem. If this filter is provided, the procedures required for packet filtering by addresses are required for this filter also.

   DISCUSSION

   Precedence filtering should be applicable to specific source/destination IP Address pairs, specific protocols, specific ports, and so on.

   An ICMP Destination Unreachable message with code 14 SHOULD be sent when a packet is dropped by the validation filter, unless this has been suppressed by configuration choice.

3. MAY implement a cutoff function that allows the router to be set to refuse or drop traffic with precedence below a specified level. This function may be activated by management actions or by some implementation dependent heuristics, but there MUST be a configuration option to disable any heuristic mechanism that operates without human intervention. An ICMP Destination Unreachable message with code 15 SHOULD be sent when a packet is dropped by the cutoff function, unless this has been suppressed by configuration choice.

   A router MUST NOT refuse to forward datagrams with IP precedence of 6 (Internetwork Control) or 7 (Network Control) solely due to precedence cutoff. However, other criteria may be used in conjunction with precedence cutoff to filter high precedence traffic.

   DISCUSSION

   Unrestricted precedence cutoff could result in an unintentional cutoff of routing and control traffic. In the general case, host traffic should be restricted to a value of 5 (CRITIC/ECP) or below; this is not a requirement and may not be correct in certain systems.

4. MUST NOT change precedence settings on packets it did not originate.

5. SHOULD be able to configure distinct precedence values to be used for each routing or management protocol supported (except for those protocols, such as OSPF, which specify which precedence value must be used).

6. MAY be able to configure routing or management traffic precedence values independently for each peer address.

7. MUST respond appropriately to Link Layer precedence-related error indications where provided. An ICMP Destination Unreachable message with code 15 SHOULD be sent when a packet is dropped because a link cannot accept it due to a precedence-related condition, unless this has been suppressed by configuration choice.

   DISCUSSION

   The precedence cutoff mechanism described in (3) is somewhat controversial. Depending on the topological location of the area affected by the cutoff, transit traffic may be directed by routing protocols into the area of the cutoff, where it will be dropped. This is only a problem if another path that is unaffected by the cutoff exists between the communicating points. Proposed ways of avoiding this problem include providing some minimum bandwidth to all precedence levels even under overload conditions, or propagating cutoff information in routing protocols. In the absence of a widely accepted (and implemented) solution to this problem, great caution is recommended in activating cutoff mechanisms in transit networks.

   A transport layer relay could legitimately provide the function prohibited by (4) above. Changing precedence levels may cause subtle interactions with TCP and perhaps other protocols; a correct design is a non-trivial task.

   The intent of (5) and (6) (and the discussion of IP Precedence in ICMP messages in Section [4.3.2]) is that the IP precedence bits should be appropriately set, whether or not this router acts upon those bits in any other way. We expect that in the future specifications for routing protocols and network management protocols will specify how the IP Precedence should be set for messages sent by those protocols.

   The appropriate response for (7) depends on the link layer protocol in use. Typically, the router should stop trying to send offensive traffic to that destination for some period of time, and should return an ICMP Destination Unreachable message with code 15 (service not available for precedence requested) to the traffic source. It also should not try to reestablish a preempted Link Layer connection for some time.

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   Next: 5.3.4 Forwarding of Link Layer Broadcasts

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   5.3.3.3 Precedence Handling For All Routers