TTL Expiration

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

TTL = 2

TTL = 1

-►

-►

fS Pi

<-

-<-

ICMP

ICMP

Time

Time

Exceeded

Exceeded

TTL = 254

TTL = 255

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

MPLS Label IPv4 Packet

ICMP Time Exceeded TTL = 255

ICMP Time Exceeded TTL = 255

Ingress LSR LSR LSR Egress

LSR LSR

ICMP Time Exceeded TTL = 248

ICMP Time Exceeded TTL = 251

ICMP Time Exceeded TTL = 251

ICMP Time Exceeded TTL = 251

ICMP Time Exceeded TTL = 252

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.

NOTE Chapter 13, "Troubleshooting MPLS Networks," has a more detailed description of what happens when the TTL expires. Tracerouting—which relies on expiring TTL to function properly—in an MPLS network is explained there, too.

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