TTL Behavior of Labeled Packets
Time To Live (TTL) is a well-known mechanism thanks to IP. In the IP header is a field of 8 bits that signifies the time that a packet still has before its life ends and is dropped. When an IP packet is sent, its TTL is usually 255 and is then decremented by 1 at each hop. If the TTL reaches 0, the packet is dropped. In such a case, the router that dropped the IP packet for which the TTL reached 0 sends an Internet Control Message Protocol (ICMP) message type 11 and code 0 (time exceeded) to the originator of the IP packet.
With the introduction of MPLS, labels are added to IP packets. This calls for a mechanism in which the TTL is propagated from the IP header into the label stack and vice versa. This ensures that packets do not live forever when entering and leaving the MPLS cloud, if there is a routing loop.
TTL Behavior in the Case of IP-to-Label or Label-to-IP
In MPLS, the usage of the TTL field in the label is the same as the TTL in the IP header. When an IP packet enters the MPLS cloud—such as on the ingress LSR—the IP TTL value is copied (after being decremented by 1) to the MPLS TTL values of the pushed label(s). At the egress LSR, the label is removed, and the IP header is exposed again. The IP TTL value is copied from the MPLS TTL value in the received top label after decrementing it by 1. In Cisco IOS, however, a safeguard guards against possible routing loops by not copying the MPLS TTL to the IP TTL if the MPLS TTL is greater than the IP TTL of the received labeled packet. If the MPLS TTL would be copied to the IP header, the smaller IP TLL value would be overwritten by a newer but higher value. If the IP packet would be injected into the MPLS cloud again—such as the result of a routing loop—the packet could live forever because the TTL would never reach 0. Figure 3-4 shows the default behavior of copying or propagating the TTL between the IP header and the MPLS labels and vice versa.
Figure 3-4 Propagation Behavior of TTL Between IP Header and MPLS Labels
TTL Behavior in the Case of Label-to-Label
If the operation that is performed on the labeled packet is a swap, the TTL of incoming label –1 is copied to the swapped label. If the operation that is performed on the labeled packet is to push one or more labels, the received MPLS TTL of the top label –1 is copied to the swapped label and all pushed labels. If the operation is pop, the TTL of the incoming label –1 is copied to the newly exposed label unless that value is greater than the TTL of the newly exposed label, in which case the copy does not happen. Figure 3-5 shows examples of TTL propagation in the case of Label-to-Label operation for a swap, push, and pop operation.
Figure 3-5 TTL Propagation in Label-to-Label Operation in the Case of a Swap, Push, and Pop Operation
The intermediate LSR does not change the TTL field in underlying labels or the TTL field in the IP header. An LSR only looks at or only changes the top label in the label stack of a packet.
When a labeled packet is received with a TTL of 1, the receiving LSR drops the packet and sends an ICMP message "time exceeded" (type 11, code 0) to the originator of the IP packet. This is the same behavior that a router would exhibit with an IP packet that had an expiring TTL. However, the ICMP message is not immediately sent back to the originator of the packet because an interim LSR might not have an IP path toward the source of the packet. The ICMP message is forwarded along the LSP the original packet was following.
Figure 3-6 shows a router sending the ICMP message "time exceeded" to the originator of the packet in the case of an IP network.
Figure 3-6 ICMP "Time Exceeded" Sent Back by a Router in an IP Network
Figure 3-7 shows an LSR forwarding the ICMP "time exceeded" message along the LSP of the original packet.
Figure 3-7 ICMP "Time Exceeded" Sent by a Router in an MPLS Network
The reason for this forwarding of the ICMP message along the LSP that the original packet with the expiring TTL was following is that in some cases the LSR that is generating the ICMP message has no knowledge of how to reach the originator of the original packet. Equally so, an intermediate LSR closer to the originator of the packet might not have that knowledge. One such case is a network with MPLS VPN. In this scenario, the P router does not have the knowledge to send back the ICMP messages to the originator of the VPN packet, because the P router does not have a route to directly return the ICMP message. (In general, the P routers do not hold the VPN routing tables.) Hence, the P router builds the ICMP message and forwards the packet along the LSP, in the hope that the ICMP message reaches a router at the end of the LSP that can return the packet to the originating routing. In the case of MPLS VPN, the ICMP message is returned by the egress PE or the CE that is attached to that PE, because these routers certainly have the route to correctly return the packet.
It is important that the P router—where the TTL expires—notes what the MPLS payload is. The P router checks whether the payload is an IPv4 (or IPv6) packet. If it is, it can generate the ICMP "time exceeded" message and forward it along the LSP. However, if the payload is not an IPv4 (or IPv6) packet, the P router cannot generate the ICMP message. Therefore, the P router drops the packet in all cases, except if it is an IPv4 (or IPv6) packet. A case in which the LSR drops a packet with the TTL expiring is AToM. The MPLS payload in the case of AToM is a Layer 2 frame and not an IP packet. Hence, if the TTL in the top label of an AToM packet expires at a P router, the only action that the P router can undertake is to drop the packet, because an IP lookup is not possible. The packet is also dropped if the payload is an IPv6 packet. However, if the P router runs newer Cisco IOS code—which understands the IPv6 protocol—that router can generate the ICMP IPv6 time exceeded packet. Whether the P router actually has an IPv6 route pointing to the originator of the packet is irrelevant. This is so because the ICMP message is always forwarded along the LSP of the packet with the expiring TTL.