[SIPForum-discussion] Max forwards geting added by proxies

Thu Jan 24 17:35:40 UTC 2013

Gottfried, Hal F

If outbond proxy adds it no problem.
(What i mean here outbond is my first proxy to UAC when Messages are sent
out)
 If intermediate proxies(after Outbond proxy for outgoing request)  start
adding Max header feilds, then i feel it can be problem.
This is beacuse i feel , if Max header feild is lost at Nth proxy then this
proxy may add it with recomended values .
I feel this may be cumulative  if proxies other than outbond proxies can
add it.
Please correct be if iam ignorant.
Regards
Vijay

On Thu, Jan 24, 2013 at 10:27 PM, Gottfried, Hal F <
hal.gottfried at verizon.com> wrote:

> Vijay – ****
>
> ** **
>
> The UAC shouldn’t be adding much to the header as that really is just the
> final train station if you will. When the call moves through a proxy device
> the MAX-FORWARDS field is modified.  Your question regarding the
> intermediate proxies is unclear; it is common practice for people often
> refer to an intermediate SIP proxy (and registration) server as the SIP
> server, however if you read RFC 3263 (locating SIP servers) it specifically
> says “SIP end systems are called user agents, and *intermediate elements
> are known as proxy servers*”* *meaning technically that any device that
> passes SIP traffic on from a UAC would be a proxy server.  So then I would
> believe the answer to your question would be yes.****
>
> ** **
>
> The only SIP Server (types) I am aware of are:****
>
> ** **
>
> **·         ***Proxy server*—Receives SIP messages and forwards them to
> the next SIP server in the network. The proxy server is an intermediate
> device that receives SIP requests from a client and then forwards the
> requests on behalf of the client. Proxy servers can provide functions such
> as authentication, authorization, network access control, routing, reliable
> request retransmission, and security. ****
>
> ** **
>
> **·         ***Redirect server*—Provides the client with information
> about the next hop or hops that a message should take. The client then
> contacts the next hop server or UAS directly. ****
>
> ** **
>
> **·         ***Registrar server*—Processes requests from UACs for
> registration of their current location. Registrar servers are often located
> near a redirect or proxy server. ****
>
> ** **
>
> The registrar server does not handle routing of the call so the
> MAX-FORWARDS would not be adjusted there. ****
>
> ** **
>
> If a proxy server is used, the caller UA sends an INVITE request to the
> proxy server. The proxy server determines the path and then forwards the
> request to the callee.****
>
> ** **
>
> [image:
> http://www.cisco.com/en/US/i/000001-100000/40001-45000/42501-43000/42871.jpg]
> ****
>
> ** **
>
> The callee responds to the proxy server, which in turn forwards the
> response to the caller.****
>
> ** **
>
> [image:
> http://www.cisco.com/en/US/i/000001-100000/40001-45000/42501-43000/42872.jpg]
> ****
>
> ** **
>
> The proxy server forwards the acknowledgments of both parties. A session
> is then established between the caller and callee. Real-Time Transfer
> Protocol (RTP) is used for the communication between the caller and the
> callee.****
>
> ** **
>
> [image:
> http://www.cisco.com/en/US/i/000001-100000/40001-45000/42501-43000/42873.jpg]
> ****
>
> ** **
>
> If you mean a redirect server when you speak of proxies then no, it will
> not update as these servers too are not proxies and do not actually route,
> but simply provide message, in this scenario the caller UA sends an INVITE
> request to the redirect server. The redirect server contacts the location
> server to determine the path to the callee, and the redirect server sends
> that information back to the caller. The caller then acknowledges receipt
> of the information.****
>
> ** **
>
> The caller then sends a request to the device indicated in the redirection
> information (which could be the callee or another server that will forward
> the request). If that request is sent to a server that is going to forward
> the request that is *another *proxy* *that will modify the MAX-FORWARDS
> field. ****
>
> ** **
>
> Once the request reaches the callee, it sends back a response (200 OK),
> and the caller acknowledges the response (ACK). RTP is used for the
> communication between the caller and the callee.  ****
>
> ** **
>
> If you are talking about something different, something I’ve not mentioned
> please clarify for example a SIP trace route.****
>
> ** **
>
> Back to MAX-FORWARDS, as I explained earlier the field is used to count
> the number of proxies’ it has traversed.  You can also use the VIA header
> field to prevent looking by having the proxy validate if its address is
> still in the VIA header.  I believe you may have asked about this once
> before too. ****
>
> ** **
>
> The VIA header will still be required, *as one proxy sends message to
> another proxy on application layer,* by creating a new packet (transport
> and IP layer), and thus sets the TTL to the initial high value instead of
> decrementing from received IP packet. Detecting loops using the VIA header
> may be very CPU Intensive, hence Max-Forwards allows you to have a fallback
> loop prevention.****
>
> ** **
>
> I hope this helps.****
>
> ** **
>
> ** **
>
> *Hal F. Gottfried*
>
> Sr Consultant, Contact Center Consulting & Services****
>
> ** **
>
> *From:* Vijay Kumar [mailto:vj.tech776 at gmail.com]
> *Sent:* Thursday, January 24, 2013 10:14 AM
> *To:* Vijay Badola
> *Cc:* Gottfried, Hal F; discussion at sipforum.org
>
> *Subject:* Re: [SIPForum-discussion] Max forwards geting added by proxies*
> ***
>
> ** **
>
> Vijay****
>
>
> There may be some ambiguty in RFC OK .****
>
> My second question is as below iam pasting from original mail in this mail
> chain****
>
> ** **
>
> ** **
>
> 2. As per 3261 if Max forward Is not added by UAC then SIP outbond proxy
> Must add Maxforward header feild.****
>
>     Can Max-forwarded be added by intermediate proxies as wel l???****
>
> ** **
>
> (If yes i feel the  purpose of Max forwards header feild  does not serve
> the purpose.)****
>
> ** **
>
> Regards****
>
> Vijay****
>
> On Thu, Jan 24, 2013 at 7:39 PM, Vijay Badola <Vijay.Badola at onmobile.com>
> wrote:****
>
> There are many places in RFCs that show the contradictory meaning. And
> this also looks as a part of that contradiction.****
>
>  ****
>
> Regards,****
>
> Vijay Badola****
>
>  ****
>
> *From:* discussion-bounces at sipforum.org [mailto:
> discussion-bounces at sipforum.org] *On Behalf Of *Vijay Kumar
> *Sent:* Wednesday, January 23, 2013 7:37 AM
> *To:* Gottfried, Hal F
> *Cc:* discussion at sipforum.org
> *Subject:* Re: [SIPForum-discussion] Max forwards geting added by proxies*
> ***
>
>  ****
>
> Hal F. Gottfried****
>
> Agreed, but my question is that why Proxy should add it.****
>
> If an outbound proxy gets a request from a UAC then it may add it  no
> problem.****
>
> Can intermediate proxy do this?****
>
> Regards****
>
> Vijay****
>
>  ****
>
> On Wed, Jan 23, 2013 at 4:59 AM, Gottfried, Hal F <
> hal.gottfried at verizon.com> wrote:****
>
> Vijay –****
>
> I believe the wordings of the RFC are slightly ambiguous when it comes to
> the use of the Max-Forwards field.  There was a time when this field was
> optional although now it has become mandatory. Max-Forwards limits the
> number of hops a request can make on the way to its destination. It
> consists of an integer, normally 70 that is decremented by one at each hop.
> If the Max-Forwards value reaches 0 before the request reaches its
> destination, it is rejected with a 483 (Too Many Hops) error response. ***
> *
>
>  ****
>
> The real use behind the Max-Forwards mechanism is in that it allows the
> preventing of a message from entering into an endless loop between a set of
> proxies.****
>
>  ****
>
> I hope this helps.****
>
>  ****
>
> *Hal F. Gottfried*****
>
> Sr Consultant, Contact Center Consulting & Services****
>
>  ****
>
> *From:* discussion-bounces at sipforum.org [mailto:
> discussion-bounces at sipforum.org] *On Behalf Of *Vijay Kumar
> *Sent:* Tuesday, January 22, 2013 3:25 AM
> *To:* discussion at sipforum.org
> *Subject:* [SIPForum-discussion] Max forwards geting added by proxies****
>
>  ****
>
> Agreed as per RFC 3261 Sec 8.1.1.6   " Max forward is Mandatory header
> feild in SIP Requests****
>
> UAC MUST insert it."****
>
>  ****
>
> But as per 16.6 ****
>
>  ****
>
> "If the copy does not contain a Max-Forwards header field, the proxy MUST
> add****
>
> one with a field value, which SHOULD be 70."****
>
>  ****
>
> My questions****
>
> 1.If MAx forwards is a Mandatory parameter then why should proxy add it.(
> If UAs are not able to add it). ****
>
>  ****
>
> 2. As per 3261 if Max forward Is not added by UAC then SIP outbond proxy
> Must add Maxforward header feild.****
>
>     Can Max-forwarded be added by intermediate proxies as wel l???****
>
>  ****
>
> (If yes i feel the  purpouse of Max forwards header feild  does not serve
> the purpose.)****
>
>  ****
>
> Enlighten me please****
>
> Thanks in Advance****
>
> Vijay****
>
>  ****
>
>  ****
>
> ** **
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