ICMP type 5, Redirect

Protocol suite:	TCP/IP.
Protocol type:	Transport layer control protocol.
Base protocol:	ICMP, Internet Control Message Protocol.
Host implementation:
Router implementation:	Mandatory.
Links:	IANA: ICMP parameters.

The ICMP Redirect message is used to notify a remote host to send data packets on an alternative route. A host SHOULD NOT send an ICMP Redirect message. Redirects SHOULD only be sent by gateways.

The IP address of the gateway and the internet header plus the first 8 bytes of the original datagram's data is returned to the sender. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first 64 data bits of the original datagram's data.

This message is not generated in response to a datagram destined for a multicast address.

RFC 792, page 13:

The gateway sends a redirect message to a host in the following situation. A gateway, G1, receives an internet datagram from a host on a network to which the gateway is attached. The gateway, G1, checks its routing table and obtains the address of the next gateway, G2, on the route to the datagram's internet destination network, X. If G2 and the host identified by the internet source address of the datagram are on the same network, a redirect message is sent to the host. The redirect message advises the host to send its traffic for network X directly to gateway G2 as this is a shorter path to the destination. The gateway forwards the original datagram's data to its internet destination.

For datagrams with the IP source route options and the gateway address in the destination address field, a redirect message is not sent even if there is a better route to the ultimate destination than the next address in the source route.

RFC 816, page 3:

The ICMP "redirect" message indicates that the gateway to which the host sent the datagram is no longer the best gateway to reach the net in question. The gateway will have forwarded the datagram, but the host should revise its routing table to have a different immediate address for this net.

RFC 1349, page 9:

The ICMP Redirect message also includes a code, which specifies the class of datagrams to which the Redirect applies. There are currently four codes defined: 0 -- redirect datagrams for the network. 1 -- redirect datagrams for the host. 2 -- redirect datagrams for the type of service and network. 3 -- redirect datagrams for the type of service and host.

Host Implementation:

RFC 816, page 2:

The ICMP redirect message indicates that the gateway to which the host sent the datagram is no longer the best gateway to reach the net in question. The gateway will have forwarded the datagram, but the host should revise its routing table to have a different immediate address for this net.

RFC 1122, pages 40 and 41:

A host receiving a Redirect message MUST update its routing information accordingly. Every host MUST be prepared to accept both Host and Network Redirects and to process them.

RFC 1122, page 48:

To efficiently route a series of datagrams to the same destination, the source host MUST keep a "route cache" of mappings to next-hop gateways. A host uses the following basic algorithm on this cache to route a datagram; this algorithm is designed to put the primary routing burden on the gateways.

If the route cache contains no information for a particular destination, the host chooses a "default" gateway and sends the datagram to it. It also builds a corresponding Route Cache entry.

If that gateway is not the best next hop to the destination, the gateway will forward the datagram to the best next-hop gateway and return an ICMP Redirect message to the source host.

When it receives a Redirect, the host updates the next-hop gateway in the appropriate route cache entry, so later datagrams to the same destination will go directly to the best gateway.

Since the subnet mask appropriate to the destination address is generally not known, a Network Redirect message SHOULD be treated identically to a Host Redirect message; i.e., the cache entry for the destination host (only) would be updated (or created, if an entry for that host did not exist) for the new gateway.

RFC 1812, page 57:

The ICMP Redirect message is generated to inform a local host that it should use a different next hop router for a certain class of traffic.

Router Implementation:

RFC 1122, page 41:

A Redirect message SHOULD be silently discarded if the new gateway address it specifies is not on the same connected (sub-) net through which the Redirect arrived, or if the source of the Redirect is not the current first-hop gateway for the specified destination.

RFC 1812, page 57:

A router MAY ignore ICMP Redirects when choosing a path for a packet originated by the router if the router is running a routing protocol or if forwarding is enabled on the router and on the interface over which the packet is being sent.

RFC 1812, pages 82 - 84:

Routers MUST NOT generate the Redirect for Network or Redirect for Network and Type of Service messages (Codes 0 and 2) specified in [RFC 792]. Routers MUST be able to generate the Redirect for Host message (Code 1) and SHOULD be able to generate the Redirect for Type of Service and Host message (Code 3).

If the directly connected network is not subnetted (in the classical sense), a router can normally generate a network Redirect that applies to all hosts on a specified remote network. Using a network rather than a host Redirect may economize slightly on network traffic and on host routing table storage. However, the savings are not significant, and subnets create an ambiguity about the subnet mask to be used to interpret a network Redirect. In a CIDR environment, it is difficult to specify precisely the cases in which network Redirects can be used. Therefore, routers must send only host (or host and type of service) Redirects.

A Code 3 message is generated when the packet provoking the redirect has a destination for which the path chosen by the router would depend (in part) on the TOS requested.

Routers that can generate Code 3 redirects MUST have a configuration option (which defaults to on) to enable Code 1 redirects to be substituted for Code 3 redirects. A router MUST send a Code 1 Redirect in place of a Code 3 Redirect if it has been configured to do so.

If a router is not able to generate Code 3 Redirects then it MUST generate Code 1 Redirects in situations where a Code 3 Redirect is called for.

Routers MUST NOT generate a Redirect Message unless all the following conditions are met:

The packet is being forwarded out the same physical interface that it was received from.

The IP source address in the packet is on the same Logical IP (sub)network as the next-hop IP address.

The packet does not contain an IP source route option.

The source address used in the ICMP Redirect MUST belong to the same logical (sub)net as the destination address.

A router using a routing protocol (other than static routes) MUST NOT consider paths learned from ICMP Redirects when forwarding a packet. If a router is not using a routing protocol, a router MAY have a configuration that, if set, allows the router to consider routes learned through ICMP Redirects when forwarding packets.

ICMP Redirect is a mechanism for routers to convey routing information to hosts. Routers use other mechanisms to learn routing information, and therefore have no reason to obey redirects. Believing a redirect which contradicted the router's other information would likely create routing loops.

On the other hand, when a router is not acting as a router, it MUST comply with the behavior required of a host.

RFC 2003, pages 6 and 8:

After an encapsulated datagram has been sent, the encapsulator may receive an ICMP message from any intermediate router within the tunnel other than the tunnel exit point. The action taken by the encapsulator depends on the type of ICMP message received. When the received message contains enough information, the encapsulator MAY use the incoming message to create a similar ICMP message, to be sent to the originator of the original unencapsulated IP datagram (the original sender). This process will be referred to as "relaying" the ICMP message from the tunnel.

The encapsulator MAY handle the ICMP Redirect messages itself. It MUST NOT not relay the Redirect to the sender of the original unencapsulated datagram.

ICMP type 5, Redirect message:

00	08	16
Type	Code	ICMP header checksum
IP address
IP header + the first 64 bits of the original datagram's data.

Type. 8 bits. Set to 5.

Code. 8 bits.
Specifies the reason for the error.

Code	Description
0	Network error.
1	Host error.
2	TOS and network error.
3	TOS and host error.

ICMP Header Checksum. 16 bits.
The 16-bit one's complement of the one's complement sum of the ICMP message, starting with the ICMP Type field. When the checksum is computed, the checksum field should first be cleared to 0. When the data packet is transmitted, the checksum is computed and inserted into this field. When the data packet is received, the checksum is again computed and verified against the checksum field. If the two checksums do not match then an error has occurred.

IP address. 32 bits.
The IP address of the gateway.

Internet Header + 64 bits of Original Data Datagram.
The internet header plus the first 64 bits of the original datagram's data. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first 64 data bits of the original datagram's data.

Glossary:

RFCs:

[RFC 792] INTERNET CONTROL MESSAGE PROTOCOL.