TTL Processing in the Platform Label Space Overview

JunosE MPLS TTL processing is compliant with RFC 3443. The details of TTL processing vary with the tunnel model that is configured for TTL processing, pipe or uniform.

To keep backward compatibility with earlier JunosE releases, you do not use the mpls tunnel-model command to configure the tunnel model for TTL processing, That command is used instead to configure the tunnel model for EXP bits processing. The default tunnel model varies between TTL and EXP processing; for EXP processing, the default tunnel model is pipe, while for TTL processing the default tunnel model is uniform.

You can issue the no mpls ip propagate-ttl command to change the TTL processing tunnel model from the default uniform model to the pipe model. Issue the no mpls ip propagate-ttl local command to set the tunnel model to pipe for locally originated packets. Issue the no mpls ip propagate-ttl forwarded command to set the tunnel model to pipe for forwarded packets.

TTL Processing on Incoming MPLS Packets

The flow chart on Figure 51 illustrates TTL processing on incoming MPLS packets. On a transit LSR or an egress LER, MPLS pops one or more labels and can push one or more labels. The incoming TTL of the packet is determined by the configured TTL processing tunnel model.

When all of the following conditions are met, the incoming TTL is set to the TTL value found in the immediate inner header:

If any of those conditions is not met, then the incoming TTL is set to the TTL value found in the outermost label. In all cases, the TTL values of any further inner labels are ignored.

When an IP packet is exposed after MPLS pops all the labels that should be popped, MPLS passes the packet to IP for further processing, including TTL checking. When the uniform tunnel model for TTL processing is in effect, MPLS sets the TTL value of the IP packet to the incoming TTL value that was just set. In other words, the TTL value is copied from the outermost label to the IP packet. When the pipe model for TTL processing is in effect, the TTL value in the IP header is left unchanged.

If an IP packet is not exposed by the label popping, then MPLS performs the TTL validation. If the incoming TTL is less than 2, the packet is dropped. If innermost packet is IP, an ICMP packet is built and sent. If the TTL does not expire and the packet needs to be sent out, the outgoing TTL is determined by the rules for outgoing MPLS packets.

Figure 51: TTL Processing on Incoming MPLS Packets

TTL Processing on Incoming MPLS Packets

TTL Processing on Outgoing MPLS Packets

The flow chart on Figure 52 illustrates TTL processing on outgoing MPLS packets.

Rules for Processing on an LSR

On an LSR—where an MPLS packet is label-switched after processing on the line module—the TTL value in the swapped-to label is decremented by 1 from the incoming TTL value when the swapped-to label is not implicit-null. When the swapped-to label is implicit-null (for example, in a PHP configuration), the inner or exposed header's TTL is either left unchanged (when the forwarded option for the mpls ip propagate-ttl command has been configured) or is decremented by 1 from the incoming TTL value. If MPLS needs to push more labels, it sets the TTL for each label according to the following LER rules, because for those labels the router effectively is an ingress LER.

Rules for Processing on an LER

On an LER, when the packet is a locally originated IP packet, MPLS copies the TTL of all pushed MPLS labels from the IP header when the local option for the mpls ip propagate-ttl command has been configured. When the no mpls ip propagate-ttl local command has been configured, MPLS sets the TTL to 255.

When the packet is a forwarded IP or MPLS packet, MPLS copies the TTL of all pushed labels from the inner IP or MPLS header when the forwarded option for the mpls ip propagate-ttl command has been configured. When the no mpls ip propagate-ttl forwarded command has been configured, MPLS sets the TTL for these pushed labels to 255.

When the packet is neither IP nor MPLS, such as a Martini packet, MPLS sets the TTL of all pushed labels to 255.

Figure 52: TTL Processing on Outgoing MPLS Packets

TTL Processing on Outgoing MPLS Packets

MPLS Rules for TTL Expiration

MPLS takes the following actions when the TTL in a MPLS label of a received MPLS packet expires:

  1. A TTL-expired ICMP packet is constructed.
  2. The destination address of ICMP packet is set to the source address of the IP packet that was encapsulated in the MPLS packet.
  3. The source address of ICMP packet is set to the router ID of the router on which the TTL expired.
  4. The first 128 bytes of the MPLS packet including the IP payload encapsulated in the MPLS packet are copied into the payload of the ICMP packet, followed by the entire label stack of the original packet.

The ICMP packet is label-switched to the end of the LSP. From that location, the packet is forwarded back to the source of the IP packet. This behavior enables IP trace-route to work even when the LSR in the middle of the LSP does not have an IP route to the source address of the IP packet.

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