As of now CTS codecs supports 2 methods for securing Media traffic: SRTP and DTLS, in this section we are going to describe DTLS protocol and the way it works with CTS units. This article refers what is needed to enable DTLS it does not cover other security protocols used by CUCM, CTMS or CTS, to obtain more information consult TelePresence security guide in CCO.
http://www.cisco.com/en/US/docs/solutions/Enterprise/Video/telepresence.html#wp41946
DTLS
Datagram Transport Layer Security (DTLS) provides communication security for datagram protocols. DTLS is based on
Transport Layer Security (TLS) protocol. This datagram-compatible version of the protocol is specifically designed to be similar to TLS with the minimal amount of changes needed to fix problems created by the reordering or loss of packets. There are two main areas that unreliability creates problems for TLS:
- The traffic encryption layer does not allow individual packets to be decrypted, there are two inter-record dependencies:
- Cryptographic context is chained between records
- A Message Authentication Code (MAC) that includes a sequence number provides anti-replay and message reordering protection, but the sequence numbers are implicit in the records
- The handshake layer breaks if messages are lost because it depends on them being transmitted reliably for these two reasons:
- The handshake is a lockstep cryptographic handshake requiring messages to be transmitted and received in a defined order, causing a problem with potential reordering and message loss
- Fragmentation can be a problem because the handshake messages are potentially larger then any given datagram
The first problem caused by the inter-packet dependencies can be solved by using a method employed in the
Secure Internet Protocol (IPsec) by adding explicit state to each individual record.
To solve the issue of packet loss DTLS employs a simple retransmission timer. Figure 1 below illustrates the basic concept. The client is expecting to see the HelloVerifyRequest message from the server. If the timer expires then the client knows that either the ClientHello or the HelloVerifyRequest was lost and retransmits.
In TelePresence solution it does not matter the number of codecs in the system only primary codecs will perform dTLS handshake, IP Phones are not associated in the process. In every TelePresence call two dTLS handhsakes occur, one for each stream (video and audio)
DTLS reuses almost all the protocol elements of TLS with minor but important modifications for it to work properly with datagram transport,
A TLS client initiates the handshake by sending the ClientHello message. This message contains the TLS version, a list of algorithms and compression methods that the client will accept and a random nonce used for anti- replay.
The server responds with The Server- Hello contains the server’s choice of version and algorithms and a random nonce. The Certificate contains the server’s certificate chain. The ServerHelloDone is sim- ply a marker message to indicate that no other messages are forthcoming. In more complicated handshakes other messages would appear between the Certificate and the ServerHelloDone messages.
Then send the ChangeCipherSpec message to indicate that it is changing to the newly negotiated protection suite.
