One document matched: draft-martinsen-tram-stuntrace-01.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<?rfc tocompact="yes"?>
<?rfc tocdepth="3"?>
<?rfc tocindent="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc comments="yes"?>
<?rfc inline="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<rfc category="std" docName="draft-martinsen-tram-stuntrace-01"
ipr="trust200902">
<front>
<title abbrev="stun trace">STUN Traceroute</title>
<author fullname="Paal-Erik Martinsen" initials="P.E" surname="Martinsen">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Philip Pedersens Vei 22</street>
<code>1325</code>
<city>Lysaker</city>
<region>Akershus</region>
<country>Norway</country>
</postal>
<email>palmarti@cisco.com</email>
</address>
</author>
<author fullname="Dan Wing" initials="D." surname="Wing">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>California</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>dwing@cisco.com</email>
</address>
</author>
<date />
<workgroup>TRAM</workgroup>
<abstract>
<t>
After a UDP protocol such as RTP determines a network path is
experiencing problems, a traceroute is often useful to
determine which router or which link is contributing to the
problem. However, operating system traceroute commands follow
a different path than the actual UDP flow which complicates
troubleshooting. A superior method is shown which is
absolutely path-congruent with the UDP protocol itself, works
on IPv4 and IPv6, and does not require administrative
privileges on most operating systems.
</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>
Traceroute <xref target="RFC1393"/> is a simple tool available
on most operating systems and is popular to debug the network
by simply getting round-trip time along each hop to a remote
IP address. More advanced tools, such as MTR, provide more
metrics such as packet loss and round trip time to each hop
over several seconds or minutes.
</t>
<t>
To simplify network debugging when dealing with bi-directional
real time media it is often useful to get as much information
as possible regarding the network path. In this specification
probe packets are sent using the same 5-tuple where (S)RTP
media is flowing. This will provide the most accurate results,
as probe packets sent on a different 5-tuple may take another
path due to Equal-Cost Multipath (ECMP, <xref
target="RFC2992"/>), policy-based routing, and similar
techniques.
</t>
<t>
To avoid those problems, the probe packets need to be sent
from the same socket and with the same DiffServ code point the
normal (S)RTP media packets. As shown in <xref
target="sec_implementation_details" />,
most operating systems can pass the ICMP "Time to Live
Exceeded" error to the application, so the application can
perform the diagnostics over that network path.
</t>
<t>
This specifications uses STUN <xref target="RFC5389"/> packets
as probes. STUN packets are designed to be multiplexed
together with RTP <xref target="RFC3550"/> (and <xref
target="RFC3711">SRTP</xref>) and are unlikely to cause any
"problems" for the (S)RTP receiver. To differentiate each hop
count, classic traceroute uses different UDP port numbers
(e.g., TTL=1 uses UDP port 55001, TTL=2 uses UDP port 55002,
etc.). The mechanism described here uses the same UDP port
number (so that the trace is path-congruent with the (S)RTP
packets), and uses different length UDP packets to
differentiate each hop count (e.g., TTL=1 uses length 501,
TTL=2 uses length 502, etc.).
</t>
<t>
Using a technique based on ICMP replies avoids a forklift
upgrade of the network to provide host applications with
useful information. ICMP is already supported in most network
and application stacks.
</t>
<t>
Additional network characteristics like MTU and bandwidth
availability can be discovered by using <xref
target="I-D.petithuguenin-behave-stun-pmtud"/> and
<xref target="I-D.martinsen-tram-turnbandwidthprobe"/>.
</t>
</section>
<section anchor="notation" title="Notational Conventions">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in <xref target="RFC2119"/>.
</t>
</section>
<section title="Overview of Operation">
<t>
An application using (S)RTP to send and receive media like
audio and video following the guidelines in <xref
target="RFC4961"/> uses symmetric send and receive ports. The
application opens one socket that it uses to both send and
receive media on.
</t>
<t>
It is important to note that the functionality described here
can be done on most OSes without any administrative
privileges.
</t>
<t>
<xref target="fig_overview"/> depicts the various components
needed for this to work. The application opens up its media
socket as it would in normal cases where media is to be sent
and received. It also opens up a ICMP socket or installs an
error listener on the media socket.
</t>
<t>
<figure anchor="fig_overview"><artwork><![CDATA[
POLL/ Network Node Network Node
SELECT
+-----+ | | |
| A |* ICMP +++ /+\++++++++++++|++++ <ICMP Reply> |
| L | SOCKET | | | ++++++++++++ |
| I | | | | + |
| C |* MEDIA ==|===|===========|=================+|====<(S)RTP>
| E | SOCKET --\-/------------|------------------X <STUN Probe>
+-----+ | | |(TTL expired)
====== Media Path
------ STUN Probes (on same 5 tuple as Media)
++++++ ICMP reply
]]></artwork></figure>
</t>
<t>
The application also need to listen on the sockets for any
incoming ICMP packets or socket error messages. This is
usually done with the socket calls select() or poll(). How to
actually receive the ICMP messages will vary from OS to
OS. See <xref target="sec_implementation_details"/> for
implementation details on various OSes.
</t>
<t>
Once the application have media running and is listening for
ICMP replies it can start sending probes to detect networks
nodes in the media path. This is done by sending STUN messages
and setting the TTL/MAX_HOP limit in the IPv4/IPv6
header. <xref target="sec_set_ttl"/> explains how to set this
on various platforms.
</t>
<t>
The STUN packet is sent on the same socket as the media packet
are sent and received on. Mixing (S)RTP and STUN is well known
behavior and should not cause any problems.
</t>
<t>
Along the path, every layer 3 network node (a.k.a. router)
decreases the IPv4 TTL or IPv6 HOP_LIMIT field. If the field
becomes 0 the network node responds with a ICMP error "Time to
Live Exceeded" (TTL Exceeded) or "Hop Limit Exceeded in
Transit" (Time Exceeded Message).
</t>
<t>
The application will receive a ICMP error in response to the
offending probe packet. The source IP address of the ICMP
packet will be the sending network node. This enables the
application to trace the path towards the destination. The
ICMP reply contains at least 8 bytes of the offending
packet. The IP fragment of the offending packet in the ICMP
reply can be used to determining if this ICMP reply actually
was a reply to an offending packet the application did send
out.
</t>
</section>
<section title="New STUN Attributes" anchor="new_attributes" >
<t>
This STUN extension defines the following new attribute:
</t>
<figure>
<artwork align="left">
<![CDATA[
0xXXX0: PATH-NODE-PROBE
]]></artwork>
</figure>
<section title="PATH-NODE-PROBE">
<t>
This attribute have a length of 8. Padding is needed to hit
the required STUN 32 bit STUN attribute boundary.
</t>
<t>
<figure anchor="hop_attr" title="PATH-NODE-PROBE Attribute">
<artwork align="left"><![CDATA[
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| HOP |
+-+-+-+-+-+-+-+-+
]]></artwork></figure></t>
<t>
The HOP field indicates what hop in the network path
(relative to the application) the application is trying to
learn the IP address of. This field should be set to the
same value as the TTL/HOP_LIMIT field in the IPv4/IPv6
header of the probe packet leaving the application. Note
that the TTL/HOP_LIMIT field in the IPv4/IPv6 header will
decrease as the packet traverses the path. The HOP field
in the attribute will remain unchanged.
</t>
<t>
This attribute is useful for clients when receiving the
whole offending IP packet in the ICMP reply. The attribute will
be reflected back in a STUN response if the remote
application supports is. This makes it easier to correlate
sent probe packets and ICMP responses.
</t>
</section>
</section>
<section title="Base Protocol Procedures">
<t>
The procedures are simple; send a probe packet that may or may
not trigger a reply from one of the nodes in the network path
and then listen and parse any incoming replies. The reply
might be an ICMP Time To Live Exceeded (from an intermediate
hop), a STUN response (from the (S)RTP peer), or any other
ICMP error message.
</t>
<section title="Forming STUN Packet Probes">
<t>
To reduce chances of a STUN traceroute probe being stopped
by various middle-boxes it is RECOMMENDED to use a STUN
binding request as described in ICE <xref
target="RFC5245"/>.
</t>
<t>
Since the STUN packet can traverse the whole media-path and
reach the remote peer it is RECOMMENDED the agent follows
the guidelines for sending connectivity checks defined in
ICE <xref target="RFC5245"/>. Adding a USERNAME
attribute and integrity protecting the STUN message enables
the remote peer to authenticate the STUN message and create
an appropriate response. If the remote peer is unable to
authenticate the STUN request it will not send any
response. Getting a response from the remote peer is useful
as it is an indication the probe have traveled the whole
network path.
</t>
<t>
When forming the STUN packet probe the agent SHOULD add the
PATH-NODE-PROBE attribute and MAY add a PADDING attribute as
described in <xref target="RFC5780"/> Section 7.6. The
PATH-NODE-PROBE attribute is useful for STUN servers
receiving the STUN probe and it can be used to correlate any
ICMP replies if the reply contains the complete offending
packet. Adding the PADDING attribute is useful for clients
that needs to have several outstanding probe packets on the
same 5-tuple. The length of the offending packet reported
back in any ICMP reply will make it possible to correlate
this to the correct probe.
</t>
<t>
The agent sending the STUN packet probe MUST store the
length of the UDP packet (as reported in the IP header)
containing the STUN probe.
</t>
<t>
Before sending the probe on the wire it is important to set
the appropriate TTL or HOP_LIMIT field in the IPv4 or IPv6
header before the packet is sent. How to do this on various
OSes are described in <xref target="sec_set_ttl"/>.
</t>
<t>
The probe MUST also be sent with the same DSCP value as the
(S)RTP packets. This is normally not a problem as the STUN
probes and (S)RTP packets are sent on the same socket.
</t>
</section>
<section title="Receiving a STUN Packet Probe" anchor="sec_rx_probe">
<t>
An agent that listens for STUN requests (a.k.a STUN server)
that receives a STUN request with a PATH-NODE-PROBE
attribute, MUST include a PATH-NODE-PROBE attribute with the
same value in the generated response.
</t>
<t>
Any PADDING attributes as defined in <xref target="RFC5780"/>
SHOULD be ignored by the STUN server.
</t>
</section>
<section title="Receiving ICMP Messages">
<t>
After an agent sends a STUN probe it must be ready to
receive a ICMP reply or a STUN reply. Details on how to do
this on various OSes are described in <xref
target="sec_rx_icmp"/>.
</t>
<!--
<t>
TODO: Terminology. Is "IP fragment of the offending probe
packet in the ICMP reply" clear and avoids confusion?
TODO: earlier we need to use the term "IP Fragment" to
explain that is the 64 bits (or whatever length) returned by
routers in their ICMP payloads. I think the paragraph below
is first time we use the term "IP Fragment". "IP fragment"
is confusing when used with ICMP because it isn't a
*fragmented IP packet*, which was my first interpretation of
the term "IP fragment"!
</t>
-->
<t>
To prevent ICMP spoofing attacks <xref target ="RFC5927"/>
<!--[TODO: cite Gont's work]-->, the received ICMP packet
MUST be validated by port number and length in the IP
fragment of the offending packet contained in the ICMP
payload. Port number validation checks that the port number
in the offending IP fragment of the probe packet contained
in the ICMP payload <!--[TODO: terminology problem
(fixed?)]--> corresponds to the (S)RTP media (and STUN
probe) 5-tuple. The length validation checks IP packet
length field in the IP fragment of the offending packet
received in the ICMP reply. This value MUST correspond to
any length stored when the agent sent the STUN probe. If
the agent uses the PADDING (Defined in <xref
target="RFC5780"/>) attribute to generate different length
on the STUN probes it is possible to have several
outstanding probes, thus speeding up the trace.
</t>
</section>
</section>
<section title="IPv4 and IPv6 Differences">
<t>
Core functionality is the same. In IPv6 the IPv4 TTL field is
renamed to HOP_LIMIT to better reflect what it actually
represent.
</t>
</section>
<section title="IANA Considerations">
<t>
The code-point for the new STUN attribute defined in this
specification is described in <xref target="new_attributes"/>.
</t>
</section>
<section anchor="security" title="Security Considerations">
<t>
ICMP messages does leak network topology, which is a
well-known threat to networks and mitigations have long
existed in routers and firewalls so that networks can be
configured to not leak this topology information beyond their
borders.
</t>
<t>
ICMP spoofing and DOS attack prevention exist in routers
deployed on the Internet today.
</t>
<t>
No new threats have been added in this specification.
</t>
</section>
<section anchor="ack" title="Acknowledgements">
<t>
Trond Andersen for actually implementing this and Wilson Chen for
helping out with different OS behavior testing.
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.3550"?>
<?rfc include="reference.RFC.3711"?>
<?rfc include="reference.RFC.4961"?>
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include="reference.RFC.5780"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.1393"?>
<?rfc include="reference.RFC.2992"?>
<?rfc include="reference.RFC.5927"?>
<?rfc include='reference.I-D.petithuguenin-behave-stun-pmtud'?>
<?rfc include='reference.I-D.martinsen-tram-turnbandwidthprobe'?>
<reference anchor="ICMPTest" target="https://github.com/palerikm/ICMPTest/">
<front>
<title>ICMP test github repo</title>
<author/>
<date/>
</front>
</reference>
</references>
<section title="Platform Implementation Details" anchor="sec_implementation_details">
<t>
This section provides examples and hint on how probe packets can
be sent and ICMP messages received on various OSes. For a
complete example please refer to <xref target="ICMPTest"/>.
</t>
<section title="Setting TTL or HOP_LIMIT on Probes" anchor="sec_set_ttl">
<t>
Setting the appropriate value in the IPv4 or IPv6 header is
the same for most platforms. Use
</t>
<t>
<figure><artwork><![CDATA[
setsockopt(sockHandle, IPPROTO_IP, IP_TTL, &sock_ttl,
sizeof(sock_ttl));
]]></artwork></figure>
</t>
<t>
for IPv4 or
</t>
<t>
<figure><artwork><![CDATA[ setsockopt(sockHandle,
IPPROTO_IPV6, IPV6_UNICAST_HOPS, &sock_ttl,
sizeof(sock_ttl));
]]></artwork></figure>
</t>
<t>
for IPv6.
</t>
<t>
Sending the probes on the same socket as media is flowing
requires the implementations to only set this when sending the
probe packet. Remember to set it back to initial value when
sending media. Most OSes seems to handle the setsockopt call
correctly and not set the value in the IP header of any
buffered packets.
</t>
</section>
<section title="Receiving ICMP Messages" anchor="sec_rx_icmp">
<section title="OS-X and iOS">
<t>
Creating a socket to listen for incoming ICMP messages can
be done as:
</t>
<t><figure><artwork><![CDATA[
icmpSocket=socket(config.remoteAddr.ss_family, SOCK_DGRAM,
IPPROTO_ICMP); <<<
]]></artwork></figure></t>
<t>
This is done in addition to the normal socket used to send
media on (RTP) and probes. (Yes, even if the probe are sent
on the media socket the ICMP reply will be on the ICMP
sockets..)
</t>
<t>
Code in the while(1) loop of poll would look something like:
</t>
<t><figure><artwork><![CDATA[
for(i=0;i<numSockets;i++){
if (ufds[i].revents & POLLIN) {
if(i == rtpSock){
//Handle "normal" data here.
}
if(i == icmpSock){//This is the ICMP socket
//Handle ICMP packets here.
}
}
}
]]></artwork></figure></t>
</section>
<section title="Linux and Android">
<t>
For unprivileged recipient of the ICMP messages an error
handler must be installed. This can be done like:
</t>
<t><figure><artwork><![CDATA[
setsockopt (config.sockfd, SOL_IP,
IP_RECVERR, &val, sizeof(val)) < 0);
]]></artwork></figure></t>
<t>
In the poll() section of the code something like this
needs to be there:
</t>
<t>
<figure><artwork><![CDATA[
struct msghdr msg;
if (ufds[dataSock].revents & POLLERR) {
if (recvmsg(sockfd, &msg, MSG_ERRQUEUE ) == -1) {
//Ignore for now. Will get it later..
continue;
}
//possible ICMP message
//use cmsg to read the structures in msg
}
]]></artwork></figure>
</t>
<t>
Failing to call rcvmsg seems to let the msg fall through
to the kernel. Looks like it will close down the socket
because of the received error. So be careful!
</t>
<t>
For application with the right administrative privileges
it is possible create a separate ICMP listen socket as
described in the previous section. The socket() call would
then look like:
</t>
<t>
<figure><artwork><![CDATA[
icmpSocket=socket(config.remoteAddr.ss_family, SOCK_RAW,
IPPROTO_ICMP); ]]></artwork></figure>
</t>
<t>
The poll() loop will be as described for OS-X and iOS. No
need for a error handler.
</t>
</section>
<section title="Windows">
<t>
The following code in select() or poll() will read and
detect any incoming ICMP messages on the send socket.
</t>
<t>
<figure><artwork><![CDATA[
if (FD_ISSET(sendsocket, &read_flags)) {
cc = recvfrom(sendsocket, receivepacket,
sizeof(receivepacket), 0,
(struct sockaddr *)&receiveaddr, (int*)&fromlen);
if (cc < 0 && GETERRORCODE == WSAENETRESET) {
//ICMP packet handling here
//Do:
//inet_ntoa(receiveaddr.sin_addr));
//to get the address of the router sending the
//ICMP reply
}
}
]]></artwork></figure>
</t>
</section>
</section>
</section>
</back>
</rfc>
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