The MPLS WG Archive

["Alexander Marhold" <alexander@marhold.at>]

Hello
I want to explain the mechanism a little bit more in detail

FRAME based general:
1. normally at the ingress LSR the IP-TTL is copied into the TTL-field of
the label header.
2. the label-TTL is decremented at each hop by 1
3. when it reaches 0 the labelled packet will be dropped and a
TTL-exceeded-ICMP message will be sent if possible
4. at the egress the label-TTL is copied back into the IP-TTL

CELL based general:
1. normally at the ingress LSR the IP-TTL is copied into the TTL-field of
the label header.
1a. the ingress LSR to the ATM-MPLS cloud either decrements the TTL by 1
1b		or by the amount of ATM_MPLS hops of the LSP ( this he knows from the
LSP-setup using downstream on demand label allocation and ordered
lsp-control label setup)
2. then the packet is chopped into cells
3. the egress reassembles the packet and copies the label ttl back into the
IP-TTL

if 1a is used then the ATM-MPLS cloud looks like a 1-hop network

special features:
a) on cisco LSRs there is the possibility to disable the copying of the
IP-TTL to the label-TTL and the label-TTL again starts at 255.
on the egress the smaller of the 2 TTL-values ( label and IP-header) will be
used for the IP-TTL
then for traceroute the MPLS cloud looks like a 1-hop (2-hop with PHP)
network

with best regards

Alexander

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     __/
    __/   Dipl.Ing. Alexander Marhold MBA
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  __/   Core & IP Services <Senior Consultant>
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-----Original Message-----
From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf Of
Asim.Khan@vf.vodafone.co.uk
Sent: Tuesday, August 21, 2001 6:22 PM
To: c-david.escobar@wcom.com
Cc: mpls@UU.NET
Subject: RE: Time to live


It stays the same. The routers only look at the label value not the IP
header. When the packet leaves MPLS domain then the normal operation would
apply.

Asim

-----Original Message-----
From: David Escobar [mailto:c-david.escobar@wcom.com]
Sent: 21 August 2001 17:19
To: mpls@UU.NET
Subject: Time to live


Hi;
What happens to the TTL field in the IP header when a labeled packet goes
through an LSP? Is it decreased at each router or stay the same?